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Pyrolysis process of coffee paper cup samples was investigated in a flow stream of nitrogen at different heating rates (10, 20, 30 and 40°Cmin−1), using thermo-analytical techniques. It was found that second pyrolysis stage can be described by Šesták-Berggren (SB) autocatalytic model, with mechanism function f(α)=α0.011(1−α)1.459. Based on analysis of SB kinetic exponents (designated by M and N), it was found that second pyrolysis stage is mainly controlled by chemical process, involving reactions with reaction-order (n) higher than unity. Applying specific statistical analysis, in order to obtain precise distribution of reactivity, the discrete (binomial) distribution has shown that there are two important areas of current distribution for corresponding energy outcomes, within the apparent activation energy as random variable. The first "concentration" area of energy outcomes corresponds to start of chain end depolymerization reaction forming levoglucosan at high enough temperature region, while second "concentration" area of energy outcomes includes occurrence of macro-radicals in liquid phase propagate with radical addition on unsaturated C-C bonds, with cross-linking and formation of small chemical species. It was found that further elevating of temperature (above 340°C), will leads to fact that rate of tar-forming reactions increases and formation of char decreases. [ABSTRACT FROM AUTHOR]

• Analysis of 12 decarbonisation scenarios via the electrification of space heating. • Innovative modelling approach that combines two highly detailed models. • Building stock and upstream supply sector of whole EU are covered. • Electrification via heat pumps leads to 11% lower system costs compared to hydrogen. • Heat pumps are cost-efficient in decentralised and in district heating systems. This paper analyses different levels and means of the electrification of space and hot water heating using an explorative modelling approach. The analysis provides guidance to the ongoing discussion on favourable pathways for heating buildings and the role of secondary energy carriers such as hydrogen or synthetic fuels. In total, 12 different scenarios were modelled with decarbonisation pathways until 2050, which cover all 27 member states of the European Union. Two highly detailed optimisation models were combined to cover the building stock and the upstream energy supply sector. The analysis shows that decarbonisation pathways for space and water heating based on large shares of heat pumps have at least 11% lower system costs in 2050 than pathways with large shares of hydrogen or synthetic fuels. This translates into system cost savings of around €70 bn. Heat pumps are cost-efficient in decentralised systems and in centralised district heating systems. Hence, heat pumps should be the favoured option to achieve a cost-optimal solution for heating buildings. Accordingly, the paper makes a novel and significant contribution to understanding suitable and cost-efficient decarbonisation pathways for space and hot water heating via electrification. The results of the paper can provide robust guidance for policymakers. [ABSTRACT FROM AUTHOR]

• Dual heat source high efficiency Organic Rankine cycle system. • Energy management strategies for complex power source vehicle systems. • Strategy parameter optimization and tuning for road condition adaptation. • Results of hybrid and integrated system selection for different road conditions. Developing hybrid electric vehicle architectures and Organic Rankine Cycle-based waste heat recovery for heavy commercial vehicles can improve engine efficiency and reduce energy consumption. Previous studies have focused on a single type of hybrid integrated with a simple Organic Rankine Cycle system, with engine efficiency to be further improved. In order to investigate the applicability of different hybrid configurations in different road conditions, this paper establishes series, parallel and 48 V mild hybrid simulation models based on SIMULINK software, as well as a model integrated with an efficient Organic Rankine cycle system, and operates them on highway and suburban road conditions respectively. The optimal selection of hybrid and integrated configurations under different road conditions is derived: the parallel hybrid and integrated system is selected for highway conditions. For suburban road conditions, hybrid only is selected in series, and 48 V mild hybrid configuration is selected after integrating the Organic Rankine Cycle system. The modelling of the waste heat recovery system and the derived hybrid selection basis provide a reference for energy saving and emission reduction of heavy-duty vehicles. [ABSTRACT FROM AUTHOR]

• A novel Stirling engine-based combined heat and power system model is developed. • Polytropic endothermic/exothermic processes are considered in Stirling submodels. • Constructal theory is employed to optimize the key components within the system. • A new comprehensive performance evaluation index for the system is constructed. • Effects of different types of regenerators on system performance are analyzed. Renewable energy-driven micro combined heat and power systems offer a clean and efficient solution for energy supply in residential and commercial buildings. This paper presents a novel model for a biogas-fired Stirling engine-based micro combined heat and power system, and establishes the mathematical relationships between geometric and operational parameters of components and the system's thermal-electric performance. To improve the accuracy of system performance prediction, the model meticulously considers the intricate heat-to-work conversion and thermal energy transfer processes, incorporating the finite heat transfer capacity and polytropic endothermic/exothermic processes in the heat exchangers of the prime mover. The reliability of the system model is verified, and constructal theory is introduced to optimize the critical geometric variables of thermal energy transfer components within the system based on the new model. The results demonstrate that compared to the performance before optimization, the electrical power output of the system increases by 142.8 %, while heat output efficiency and overall efficiency increase by 20.5 % and 10.5 %, respectively. This work provides a new approach and perspective for the mathematical modeling, performance assessment, and optimization design of Stirling engine-based micro combined heat and power systems. [ABSTRACT FROM AUTHOR]

• Heat extraction and storage models for deep borehole heat exchanger systems are presented. • The effects of heat storage on the heating power of deep borehole heat exchangers are quantified. • The thermal recovery of rock and soil under different heat storage conditions is revealed. • A novel design method for the threshold inlet water temperature of heat storage is proposed. The heating capacity of deep borehole heat exchanger (DBHE) systems gradually attenuates due to the cold accumulation of rock and soil during long-term operation. Therefore, heat storage in rock and soil is crucial to alleviate thermal attenuation and ensure stable operation. However, the mechanism and design methods for heat storage are unclear. In this paper, heat extraction and storage models for DBHE systems are established, the effects of heat storage on system operation are revealed, and a design method for the threshold inlet water temperature of heat storage is proposed. The results show that the fluid in the DBHE should inflow from the inner pipe to release more heat into the rock and soil when adopting heat storage. The heat storage power under fluid inflow from the inner pipe is 10.81 kW higher than that from the annular space under the same operating conditions. Increasing the inlet water temperature is more beneficial than increasing the flow rate to enhance the heat storage power. After 2880 h of operation, the heating power of the DBHE is increased by 3.44% for the next heating season when the storage flow rate is increased from 8 m3/h to 38 m3/h. Thus, a high heat storage inlet temperature with a low flow rate should be adopted during the heat storage period. Furthermore, the threshold inlet water temperature of heat storage is highly dependent on the depth of the DBHE and the geothermal gradient, with percentage contributions of 57.14% and 39.26%, respectively. The multiple linear regression model (R 2 = 0.972) has shown high predictive accuracy in the assessment of threshold inlet water temperatures, with a maximum relative error of −5.60%. The findings of this paper provide technical support for designing heat storage using DBHE systems. [ABSTRACT FROM AUTHOR]

• A two-layer framework is proposed for building HVAC and associated multi-energy consumption optimization. • 100% renewable complementarities are proposed for building multi-energy supplies. • A R-C thermodynamic network is formulated to model the building HVAC. • AC slow charging and DC fast charging types of EVs are considered and managed via a novel real-time supply–demand pricing. Building heating, ventilation, and air conditioning (HVAC) and associated energy consumption make up the more and more important part of the world, whose reduction provides a cost-effective path to the "dual carbon" goal. This paper proposes a two-layer management of HVAC-based multi-energy buildings under proactive demand response of fast/slow-charging electric vehicles (EVs). In this paper, the building HVAC is mathematically formulated via a R-C thermodynamic model, which coordinates with multi-energy converters and storages to form a 100% renewable building. The building management is a challenging optimization problem due to its severe constraints and strong spatio-temporal couplings. In the first layer, a day-ahead multi-energy dispatch is formulated to economically optimize the electrical, heat, gas energy carriers. In the second layer, alternating current (AC) slow charging and direct current (DC) fast charging types of EVs are considered and managed via a novel real-time supply–demand pricing mechanism. After acquiring the economical dispatch references in the first layer, the second layer implements a model predictive control (MPC)-based real-time scheduling to handle the multi-energy supply–demand fluctuations. The original two-layer optimization is further handled via mixed-integer linear program (MILP) reformulation for high-efficient solving. Comparisons have shown the advantageous performances of the proposed two-layer optimization over economics and practicability. Simulations results show that the overall system operating cost can be reduced by at most 3.01% with a higher operational flexibility in building management. [ABSTRACT FROM AUTHOR]

Individual metering and charging of heat and domestic hot water is one of the possibilities for reducing the energy consumption in existing multifamily buildings and, with this aim in mind, the EU-directive 2012/27/EU enforced the installation of individual heat consumption meters. Even though some experimental evaluation of energy savings that may be achieved in multifamily buildings with individual metering & charging systems can be found in the literature, the majority of these research pieces are focused on case studies or taking into consideration conditions related to cold climates, and there is still a lack of studies focused on evaluating its effects in more temperate climates that can be also found in Europe. Thus, in this paper, the potential of individual metering and charging of heat and hot water for saving energy in residential buildings in temperate climates is evaluated and quantified. To do so, a literature review on implementation of this system is carried out and presented firstly to get a better understanding of its implications on energy consumption in buildings. Afterwards, heating and hot water consumption data collected in a multifamily building where individual metering and charging system was implemented is evaluated in detail. With the aim of quantifying its effect on heating and hot water consumption, data corresponding to four complete heating seasons (two heating seasons prior to its implementation, and the two first heating seasons after implementing it) have been evaluated in detail, following a specific method described in the paper. Results show that individual metering and charging has brought a reduction of normalized energy consumption of 15–20% during the first two years after implementing it, and simple payback periods are around 10 years. These results confirm that individual metering and charging affects directly on user behaviour encouraging inhabitants to change their habits to reduce their energy consumption, and this effect is significant even in European temperate climates, such as the evaluated case study shows. [ABSTRACT FROM AUTHOR]

Financial issues have been always one of major priorities in scheduling of energy systems. Although these systems are able to serve several types of energy demands but generated emission by these systems is a challenging problem. Since improvement of each one of mentioned issues has negative effects on the other issue therefore a trade-off solution is necessary to be obtained between these issues. In this paper, a multi-objective model has been presented to satisfy both economic and environmental objectives of a hub energy system in the presence of demand response program. In the proposed paper, ε-constraint and max–min fuzzy satisfying methods have been employed to solve and select the trade-off solution. The main reason of implementation of demand response program is to reduce operation cost and improve environmental performance of hub energy system. In fact, demand response program transfers some percentage of load from peak periods to off-peak periods to flatten load curve which leads to reduction of cost and emission. A mixed-integer linear programming has been used to simulate the proposed model and general algebraic modeling system software has been utilized to solve it. A sample hub energy system containing renewable and non-renewable energy resources has been studied and comparison results are presented to validate efficiency of proposed techniques. [ABSTRACT FROM AUTHOR]

• Efficient operating range of the original district heating substation is below 1℃. • Proposed system formed by original components operates efficiently in whole winter. • Control strategies are based on the lowest primary return water temperature. • The proposed system has advantages in energy and economic performance. With the rapid development of heating technology, heating terminals are developing toward low temperatures and diversification, resulting in the simultaneous presence of different temperature demands in district heating systems. To reduce the primary return water temperature and satisfy the temperatures of different terminals, a dual-temperature district heating substation supplying dual temperatures of hot water with two absorption heat pumps and two heat exchangers has been proposed based on the design conditions. However, whether the dual-temperature district heating substation can operate and how to control this system during the entire heating period have not been explored. In this paper, the models for the substation with two absorption heat pumps and two heat exchangers and its subsystems were established to analyze and compare the operational conditions for the substation and its subsystems at different outdoor temperatures. By comparing the primary return water temperatures of all subsystems, control strategies and a combined system for efficient operation during the heating season were determined. Then a case study and analysis of the combined system were conducted under the proposed control strategies and compared with two other systems – two heat exchangers and two absorption heat exchangers – used in dual-temperature heating stations. Results show that 1) The original system with two absorption heat pumps and two heat exchangers could only operate when the outdoor temperature was −8–1℃. 2) The subsystem with a high-temperature absorption heat pump and two heat exchangers and the subsystem with two heat exchangers which could achieve efficient operation under outdoor temperature ranges of 1–7 °C and 7–11 °C respectively were considered in the combined system to broaden the operating range of the original system. 3) The combined system had the lowest return water temperature (23.8℃) and the highest coefficient of performance (6.60) during the entire heating season. The payback period of the combined system was 1.54 years. This study provides a reference for the efficient operation of dual-temperature district heating substations at different ambient temperatures during the heating season. [ABSTRACT FROM AUTHOR]

• Design of a non-intrusive device to intelligently control electric water heaters. • Development of a machine-learning tool to forecast residential hot water demand. • Details and results of a six-month pilot study where the device was implemented. • Heating demand reduced by an average of 26.43% and cost saving of 35.54% per device. • Device is cost-effective and can reduce the need for imported fossil-fuel generation. Water heating accounts for approximately 25% of household energy use in developed countries. Therefore, the optimal control of water heating through the deployment of intelligent residential Electric Water Heaters (EWH) brings significant benefits. This paper presents an innovative design and implementation of an easy-to-use device for intelligent residential water heating. The device relied upon machine learning techniques to forecast a consumer's hot water demand and optimize the operation of an EWH using a novel data collection process that relied on non-intrusive vibration data alone. The device was deployed in a six-month pilot project on the island of São Miguel, Portugal. The major difficulties were the novel use of vibration data to forecast the volume of hot water used and the uncertain behavior of the consumers. The challenges of only using vibration data were solved by careful data collection and artificial intelligence methods. To tackle the issue of uncertain consumer behavior, an innovative 'heat now' function was incorporated into the device to override the novel control framework. Results show that the device could accurately forecast hot water demand and optimally operate the EWH to meet this demand. The results showed an average reduction of 1.33 kWh/day per consumer, which equates to an average decrease of 35.5% in water heating costs. Calculations show that these devices can reduce the total energy used by 2832 kWh daily or 0.21% of total electricity generated. Furthermore, a fleet of these devices could reduce thermal generation by 0.37%, reducing emissions by 693.31 tons of CO 2 per year. The results from the consumer survey show that the device did not affect the consumer's comfort, validating the benefits and efficacy of the proposed device. Hence, the paper shows that a simple-to-use, novel device using an innovative forecasting algorithm based on non-intrusive vibration data brings numerous quantifiable benefits to actual consumers and electrical utilities. [ABSTRACT FROM AUTHOR]

Optimizing consumptions in the field of civil construction led to define energy labels for residential buildings. To calculate the building energy demand the EPgl was determined, i.e. the annual consumption per m 2 of primary energy. This paper examines the technical solutions useful to optimize the energy demands for heating during space-heating season and domestic hot water production (thanks to energy analysis softwares as MC11300 and TRNSYS) and, at the same time, to take into account the financial issues those interventions implied. The total inside heated surface of the building case study is 1204.00 m 2 , hence the inside heated volume is about 3250.80 m 3 . Besides the more traditional interventions concerning the building envelope and its systems, the paper examined the performance of a system obtained through the combination of a cogenerator (CHP) and a heat pump (HP), thus, substituting the conventional boilers of the buildings. CHP+HP solution increases the most the energy label of the building (from a D class with EPgl = 59.62 kW h m −2 year −1 , to an A class, with EPgl = 25.64 kW h m −2 year −1 ), determining an annual energy cost saving of 3,114 € year −1 , allowing to amortize installation costs (54,560 €) in a reasonable payback period, i.e. 15.4 years. This innovative solution in the residential sector can be realized through retrofit interventions on existing buildings, hence it leads the current dwelling towards nZEB with a remarkable benefits for the environment. [ABSTRACT FROM AUTHOR]

• Study of a novel combined open absorption heat pump and FlashME desalination. • Water and heat recovery from flue gas to reutilize it for seawater desalination. • Numerical and process modelling to analyze the system performance. • Open absorption cycle can recover 89.4% heat by operating at COP of 2.21. • Water recovery from FlashME is rated to be 9.19–37.17% by using recovered heat. Thermal desalination processes generally consume a huge amount of thermal energy and industries release a large amount of latent waste heat in flue gas exhausts into the environment. In order to efficiently utilize this waste heat and to improve the energy efficiency of desalination process, this paper presents a novel combined system based on the open cycle absorption heat pump and low-grade thermal energy driven FlashME desalination. The proposed combined system is designed to recover the latent waste heat from flue gas exhausts in a parallel double stage open absorption heat pump which is driven by multiple energy sources and efficiently utilizes the recovered low temperature heat in FlashME for freshwater production. The validated process model of the combined system is developed in Aspen Plus and a parametric assessment is carried out to analyze its energy performance with the impact of various independent operational parameters by using HCOOK-H 2 O as an absorbent solution. The simulation results of the parametric study show that the combined system is capable of recovering the waste heat with an efficiency of 89.42 % by operating at a thermal performance coefficient of 2.21 and utilizes this heat in the form of hot seawater at 70 °C to drive FlashME desalination process for freshwater production at the performance ratio of 3.05. The critical analysis of the results highlights that the heat recovery performance of the absorption cycle is strongly dependent on the inlet parameters of flue gas and spray solution. In contrast, the water recovery performance of FlashME is dependent on regeneration pressure such that the water recovery rate varies from 9.19 % to 37.17 % with a pressure variation between 23.23 and 51.39 kPa which raises the seawater temperature 60–79 °C before the flashing stage. Furthermore, the system is flexible enough to operate with two heat sources of different temperature levels by adjusting the regeneration load in separate regenerators. [ABSTRACT FROM AUTHOR]

Adiabatic compressed air energy storage (A-CAES) is regarded as a promising emission-free technology to facilitate the renewable energy integration, when a large amount of renewable energy is abandoned due to the difficulty of integrating fluctuating renewable energy into electricity grid systems all over the world. However, the temperature of discharged air from low pressure turbine is still high in the conventional A-CAES system and a considerable amount of energy is lost. So a modified A-CAES system is proposed to solve this problem in this paper. The proposed modified A-CAES system can simultaneously provide mechanical energy, heating energy and cooling power. This paper aims to make a comparative research of the two A-CAES systems from the technological point of view. The thermodynamic analyses including energy analysis and exergy analysis are evaluated by using steady-state mathematical models and thermodynamic laws. The results show that total exergy efficiency of the modified A-CAES system can be improved nearly 3% compared to the conventional A-CAES system in spite of lower round trip efficiency. Meanwhile, a parametric analysis is also carried out to evaluate the effects of several key parameters on the system performance of the two A-CAES systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new improved optimization method is proposed. • The standard deviation of the optimal solutions decision variables increased by 70%. • The implementation of the optimal solutions has been improved. • The requirements of overall planners and implementers are comprehensively considered. The multiobjective optimization of the system usually focuses on the optimization of the objective functions while ignoring the influence of decision variables on the implementation of the solution. This paper proposes a new improved optimization method by embedding the decision variable diversification mechanism in the optimization process, adopting the discretization mechanism in the multisource complementary heating model, and improving the search space. The new improved optimization method and the original method have similar performance in obtaining the Pareto front, and the hypervolume of the two algorithms differs by only 1.37% on average. The standard deviations of the decision variables in the optimal solutions obtained by the improved algorithm are increased by 70%, and it has a higher diversity of solutions in the decision space. The equipment capacity obtained by the improved algorithm is discretized, and avoids equipment with lower capacity which is beneficial to construction. In this paper, the optimal implementation solution is obtained through the selection of the objective functions by the overall planners and the construction preference of the solution implementers. In this way, the overall planners' requirements for energy conservation, emission reduction and economy, as well as the solution implementers' choice of implementation solutions can be comprehensively considered. In addition, this paper also obtains another optimal solution for adopting the carbon pricing method. [ABSTRACT FROM AUTHOR]

• The design and control of locally active heating system of the PEFC are developed. • Outlet heating compensates for the lowest current density region. • Inlet heating aggravates heterogeneity of current density distribution. • The direction of coolant flow should be actively monitored and frequently changed. A homogeneous current density distribution improves a fuel cell's performance and prolongs its service life. Effective cell structure designs and uniform compression during assembly could support this goal by ensuring a homogeneous reaction rate across the activation area. Due to the coupling of hydro-electro-thermal relationships, for instance, the concentration of reactants along the flow field decreases continuously as the electrochemical reaction proceeds, and the subsequent accumulation of liquid water leads to a low current density at the outlet. The effect of operating conditions, such as local heating, on the current density distribution requires further investigation. This paper studies the impact of local heating on polymer electrolyte fuel cell (PEFC) performance and analyses the effects on voltage by mapping the current density distribution across the active area. Local heating was supplied to the three regions of the electrode, namely, fuel inlet, central and outlet regions, with the latter exhibiting the best performance (in the activation, Ohmic and mass transport controlled region, the output voltage increases compared to no local heating corresponding to 1.28%, 2.17% and 2.46%, respectively). Here, we show that in all local heating cases, outlet heating can compensate for the lowest current density region with the largest current density increased by 91.10 mA cm−2 and achieves a more homogeneous current distribution, while inlet heating aggravates heterogeneity. This study provides practical guidance for optimal thermal management system development whereby the cooling channel design should be locally optimised for more uniform distributions of current density and temperature compared to heating the cell uniformly. [ABSTRACT FROM AUTHOR]

• Three new solar aided molten carbonate fuel cell-steam turbine-combined cooling, heating and power (MCFC-ST-CCHP) systems are proposed. • Thermal performances of different MCFC-ST-CCHP systems are studied and compared. • Solar aided MCFC-ST-CCHP systems have obvious advantages than reference system. • Effects of the direct normal irradiance (DNI) of solar energy on the best solar aided MCFC-ST-CCHP system (Scheme 2) are revealed. The efficient use of solar energy and fuel cells has become an important way to achieve the goal of "carbon neutral" power generation. This paper firstly proposes a new molten carbonate fuel cell-steam turbine-combined cooling, heating and power (MCFC-ST-CCHP) system. In order to further improve the performance of the MCFC-ST-CCHP system, three solar aided MCFC-ST-CCHP systems with different solar energy integration modes are proposed in this paper. Solar energy is used to drive the methane dry reforming reaction or replace part of the heating load of high pressure boiler drum or high pressure economizer in heat recovery steam generator. The performances of different systems are studied and compared. The results show that the energy efficiencies, the exergy efficiencies and the specific CO 2 emission rates of three systems are 65.89%, 63.77%, 480.79 g/kWh; 66.25%, 65.12%, 477.93 g/kWh; and 65.86%, 64.88%, 481.08 g/kWh, respectively. The best solar integration mode is to replace part of the heating load of high pressure boiler drum. The impact of solar irradiation intensity on new solar aided MCFC-ST-CCHP system with the best solar integration mode further investigated. The results show that when the direct normal irradiance (DNI) of solar energy changes throughout the day, the new system's solar heat collection efficiency, solar power generation and solar-to-electricity efficiency increase first and then decrease, and the specific CO 2 emission rate of the system decreases first and then increases. [ABSTRACT FROM AUTHOR]

Thermoacoustic engines convert heat energy into acoustic energy. Then, the acoustic energy can be used to pump heat or to generate electricity. It is well-known that the acoustic energy and therefore the exergetic efficiency depend on parameters such as the stack’s hydraulic radius, the stack’s position in the resonator and the traveling–standing-wave ratio. In this paper, these three parameters are investigated in order to study and analyze the best value of the produced acoustic energy, the exergetic efficiency and the product of the acoustic energy by the exergetic efficiency of a thermoacoustic engine with a parallel-plate stack. The dimensionless expressions of the thermoacoustic equations are derived and calculated. Then, the Particle Swarm Optimization method (PSO) is introduced and used for the first time in the thermoacoustic research. The use of the PSO method and the optimization of the acoustic energy multiplied by the exergetic efficiency are novel contributions to this domain of research. This paper discusses some significant conclusions which are useful for the design of new thermoacoustic engines. [Copyright &y& Elsevier]

Highlights • The largest solar district heating market worldwide has been presented. • Characters of solar district heating plants in Denmark were summarized. • Possibility of solar district heating in smart renewable energy system was shown. • Potential trend of solar district heating plants in Denmark was discussed. Abstract Large solar collector fields are very popular in district heating system in Denmark, even though the solar radiation source is not favorable at high latitudes compared to many other regions. Business models for large solar heating plants in Denmark has attracted much attention worldwide. Denmark is not only the biggest country in both total installed capacities and numbers of large solar district heating plants, but also is the first and only country with commercial market-driven solar district heating plants. By the end of 2017, more than 1.3 million m2 solar district heating plants are in operation in Denmark. Furthermore, more than 70% of the large solar district heating plants worldwide are constructed in Denmark. Based on the case of Denmark, this study reviews the development of large solar district heating plants in Denmark since 2006. Success factors for Danish experiences was summarized and discussed. Novel design concepts of large solar district heating plants are also addressed to clarify the future development trend. Potential integration of large solar district heating plants with other renewable energy technologies are discussed. This paper can provide references to potential countries that want to exploit the market for solar district heating plants. Policy-makers can evaluate the advantages and disadvantages of solar district heating systems in the national energy planning level based on the know-how and experiences from Denmark. [ABSTRACT FROM AUTHOR]

Highlights • A novel efficient braking energy recovery system for electric bus is proposed. • The models of blend braking and eddy-current heating are established. • The blend braking control strategy of braking & heating system are established. • The performance of system is tested by the simulation platform and vehicle tests. • The proposed system can offer higher regeneration efficiency. Abstract This paper proposes the eddy-current braking & heating system, which is a novel efficient braking energy recovery system for electric bus. The system is designed to solve two problems: one is the low recovery efficiency of regenerative braking energy, another is the driving range dramatic decline of electric bus when heating the bus cabin in winter. The system converts braking energy into heating energy directly by the principle of eddy-current braking and heating. It greatly improves the recovery efficiency of braking energy. The layout scheme of system is designed by considering the vehicle drive structure. The mathematical models of friction braking, regenerative braking, eddy-current braking & heating are established respectively. A blend braking control strategy considering braking strength, State of Charge, and supply air temperature is proposed. The energy-saving evaluation parameters of the system are presented. A simulation platform is built in the MATLAB/Simulink environment. The simulation and vehicle tests show that the calculated temperature and driving range agree well with the measured results and the models are validated. Compared with Positive Temperature Coefficient heating, the recovery efficiency of braking energy of the system increases from 23.8% to 65.3%, and the improvement rate of driving range reaches 81.4%. The system can satisfy the braking requirement of electric vehicle by blend braking control strategy. The system can effectively increase energy recovery of the regenerative braking. It can solve the reduced driving range problem of electric buses while heating in winter. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, an innovative concept is presented for a long-term energy storage system for house heating, using the absorption process. The solar energy is stored during summer through desorption and the heat is released during winter through absorption. The originality of this concept is to allow the solution to reach the crystallization point, which is usually avoided in the absorption refrigeration machines. The storage capacity and efficiency of seven absorption couples, CaCl2/H2O, Glycerin/H2O, KOH/H2O, LiBr/H2O, LiCl/H2O, NaOH/H2O and H2O/NH3, as a function of the temperature of absorption, temperature of evaporation, temperature of the solution before absorption and the presence of crystals in the storage tank have been studied in this paper. The appearance of crystals increases the storage capacity. The storage capacity increases with the temperature of evaporation and the temperature of the solution before absorption but decreases with the temperature of absorption. [Copyright &y& Elsevier]

Abstract: This paper addresses the unit commitment (UC) in multi-period combined heat and power (CHP) production planning under the deregulated power market. In CHP plants (units), generation of heat and power follows joint characteristics, which implies that it is difficult to determine the relative cost efficiency of the plants. We introduce in this paper the DRDP-RSC algorithm, which is a dynamic regrouping based dynamic programming (DP) algorithm based on linear relaxation of the ON/OFF states of the units, sequential commitment of units in small groups. Relaxed states of the plants are used to reduce the dimension of the UC problem and dynamic regrouping is used to improve the solution quality. Numerical results based on real-life data sets show that this algorithm is efficient and optimal or near-optimal solutions with very small optimality gap are obtained. [Copyright &y& Elsevier]

• The system engineering based on photovoltaic thermal heat pump was constructed. • A new idea for the energy-saving reconstruction of district heating was introduced. • The prediction model of system heating performance was established and optimized. • The equivalent electricity consumption of heating was defined. • The economic performance of energy-saving reconstruction was studied. Actively promoting the reconstruction of clean heating is the key direction of the "Planning for clean heating in winter in northern China (2017–2021)". The photovoltaic thermal heat pump system was introduced in this paper to improve the comprehensive energy efficiency and reduce the operation energy consumption of the heat source system, which provided a new idea for the energy saving reconstruction of clean district heating. In order to verify the feasibility of the energy saving reconstruction by photovoltaic thermal heat pump, following studies were carried out in this paper: First, the integrated energy system engineering based on photovoltaic thermal heat pump was constructed to study the heating performance of the system in winter. Subsequently, the prediction model of heating performance based on back propagation neural network was established and optimized to simulate the heating performance throughout the year. Then, the calculation model of district heating system was established and the evaluation method of system operation energy efficiency and economic performance based on "equivalent electricity consumption of heating" was introduced. Finally, a residential area of Dalian was taking as an example to study the economic performance of energy saving reconstruction of district heating system based on three conventional heat sources by photovoltaic thermal heat pump. The results showed that after the capacity expansion and energy saving reconstruction based on photovoltaic thermal heat pump were conducted, the payback period of energy saving reconstruction investment was 5 years. Besides, with the increase of the configuration ratio of photovoltaic in the system from 1 to 2, the payback period was further shortened to 3 years. [ABSTRACT FROM AUTHOR]

• Advanced climate-resilient energy retrofit technology reduces the use of non-renewable energy in the building sector. • Renewable District Heating as an optimal technology in deep energy renovation strategies. • Sustainable cities and buildings are feasible by Centralised District Heating system powered by biomass. • Key Performance Indicators (KPIs), to achieve nZEB performances established in the European legislations. This paper presents the implementation of a biomass-fuelled District Heating System (DHS), as a part of a deep energy renovation exercise to achieve a climate-resilient campus with minimum carbon dioxide emissions. The case study is carried out for the University of Valladolid, an average-sized university in Spain, with a continental weather climate. Prior to renovation, the different building blocks had a wide-ranging level of fossil fuel consumption for space heating and domestic hot water ranging between 60 and 430 kWh/m2·year. The application of this centralised heating system allows to achieve the minimum threshold for near zero-energy buildings (nZEB) of 100–120 kWh/m2, in accordance with the Spanish Standards. These values correspond to the maximum European indicators for offices in continental weather conditions. Results of this comprehensive study show that 15 out of the 19 buildings reached the nZEB target, due to the proposed strategy. The overall carbon dioxide emissions have dropped by 92.69% as compared to the original fossil-fuel powered boiler, thus bringing carbon dioxide emissions down to 1.57 kgCO 2 /m2·y. Therefore, it is shown that deep energy renovation strategies through renewable energy DHS have the potential of achieving nZEB for universities in continental weather conditions. [ABSTRACT FROM AUTHOR]

• Integration of the electrolyzer technology with a power-to-hydrogen and heat model into district heating systems. • Development of an advanced electrolyzer model implemented in the model predictive control to optimize electrolyzer performance. • Effective management of on–off cycles, power fluctuations, degradation costs, temperature, and efficiency of the electrolyzer. • Utilization of the waste heat recovered from the electrolysis process, achieving an average electrolyzer efficiency of 90%. District heating systems can utilize renewable energy sources and waste heat from industrial processes, making them a cost-effective heating solution. This paper integrates an electrolyzer technology into district heating system to improve energy efficiency and reduce greenhouse gas emissions by a power to hydrogen and heat model. To accurately predict the electrolyzer's electrochemical and thermal dynamic behavior, an advanced electrolyzer model is developed. The proposed model presents a dynamic approach implemented in model predictive control to optimize electrolyzer performance and reduce power fluctuations. By managing on–off cycles, degradation costs, temperature, and efficiency of the electrolyzer, the proposed model aims to harness waste heat generated during electrolysis, and store hydrogen in a hydrogen storage tank to manage renewable fluctuations. The district heating system utilizes electric, natural gas, and hydrogen boilers along with waste heat recovered from the electrolyzer to meet energy demands. The proposed model promotes sustainability and efficiency of district heating systems by utilizing renewable energy sources and effectively managing the electrolyzer. Case studies demonstrate the model's advantages and effectiveness, with results indicating that the optimal strategy utilizes recovered waste heat to satisfy approximately 10% of the heat demand, achieving an average electrolyzer efficiency of 90% and further enhancing sustainability. [ABSTRACT FROM AUTHOR]

• Detailed configuration of a CO 2 heat pump with parallel compression is described. • A complete CO 2 system layout for simultaneous heating and cooling is presented. • A validated mathematical model with FSD and VSD CO 2 compressors has been developed. • System performance of actual CO 2 system is evaluated over three months of operation. Carbon dioxide (CO 2) is a natural substance and an environmentally benign, safe and economical refrigerant that can be used for cooling and heating systems. Unlike a traditional reversible heat pump system that only works either in heating or cooling, the air-to-air transcritical CO 2 heat pump system reported in this paper is capable of providing simultaneous space heating and cooling through an Air Handling Unit (AHU) at any time. The system with cooling and heating capacities of 210 kW and 110 kW, respectively, was installed in a cinema complex in South Australia, and continuously operated from September to November in 2020. In this paper, the detailed configuration of the CO 2 heat pump system with parallel compression is described. A complete system layout, comprising of cooling, heating and shared circuits, is revealed for the first time. A mathematical model based on highly efficient Bitzer compressors has also been developed in this study. The experimental data recorded in an entire heating period of approximately one hour has been utilised for model validation and analysis. Through the validation, the simulated power consumption for the compressors with Variable Speed Drive (VSD) and Fixed Speed Drive (FSD) has been compared to the experimental data with acceptable agreements. The simulated discharge temperatures agree with the actual temperatures measured within 10%. Moreover, the latest system performance from September to November has been evaluated based on the heating and cooling loads and combined COPs. It was found that the combined COP was relatively stable at around 3 regardless of climate conditions. [ABSTRACT FROM AUTHOR]

• Deep direct-use projects were evaluated for heating, cooling and thermal storage. • Performance was compared with non-geothermal systems and other countries. • Subsurface, surface and financial conditions significantly impact feasibility. • Attractive levelized costs of energy found for heating, cooling and thermal storage. This paper investigates the techno-economic feasibility of geothermal deep direct-use for heating, cooling, and thermal energy storage in the United States. The six 2017–2019 deep direct-use projects funded by the U.S. Department of Energy are reviewed and evaluated using the simulation tool GEOPHIRES, and results are compared with prior studies, existing geothermal district heating systems in Europe and the United States, as well as non-geothermal centralized and non-centralized heating, cooling, and thermal energy storage systems. Analysis indicates that deep direct-use feasibility varies widely, depending on subsurface characteristics, system design, and financial conditions. Project base case levelized cost of heat values ranged between $13 and $350/MWh; key drivers lowering the levelized cost of heat include higher reservoir temperatures, shallower reservoir depths, higher well flow rates, higher utilization rates, lower drilling costs, and lower discount rates. Incentives such as grants and an investment tax credit resulted in small improvements in project economics. Base case levelized cost of heat values for four projects fell within the range of levelized cost values for existing systems in Europe and the United States, and were comparable to values found in previous studies, including the 2019 GeoVision study. The lowest-cost projects for district heating had comparable levelized cost of heat values to existing fossil-fuel-driven district heating systems, and lower values than decentralized heating with natural gas boilers or heat pumps. Chilled water production with absorption chillers driven by a high-quality geothermal resource obtained attractive levelized cost of cooling values, in line with values from typical centralized cooling production facilities and lower than domestic decentralized cooling with air-conditioning units. Also, thermal energy storage is a potential deep direct-use application, where levelized cost of storage values can be obtained that are comparable to those of other thermal storage techniques such as borehole thermal energy storage and hot water storage tanks. Other aspects of deep direct-use, which are not captured in a standard levelized cost of energy metric, include the ability to decarbonize the heating and cooling supply, and provide reliability and resiliency to the energy infrastructure. These attributes are gaining increased attention and improve overall project feasibility. Finally, this paper identifies technical, market, policy, and social barriers for deep direct-use development in the United States, and provides potential approaches for reducing these barriers. [ABSTRACT FROM AUTHOR]

• A method for sizing and optimizing storage and cogeneration units is proposed. • The method is demonstrated using the example of a coal-fired system. • The decision-support method addresses two planning horizons. • Operational and financial effects of integrating a thermal store are investigated. Thermal energy storage technologies are of great importance for the power and heating sector. They have received much recent attention due to the essential role that combined heat and power plants with thermal stores will play in the transition from conventional district heating systems to 4th and 5th generation district heating systems. This paper presents a novel decision support method for sizing and optimizing the operation of thermal energy storage units in combined heat and power plants. To achieve this goal, the method in this paper comprises three steps. The first step provides an approximation of the storage capacity based on the characterization of the thermal load. The second step extends the applicability of the method by enabling the evaluation of the hourly operation of the combined heat and power plant with thermal storage. The third step evaluates the long-term economic effects of retrofitting the combined heat and power plant with a heat storage option. The applicability of the method is illustrated using the example of a coal-fired combined heat and power plant and the study of two scenarios. The analysis of the scenarios shows that the utilization of the energy storage enhances the operational flexibility of the system by increasing the number of hours in which the combined heat and power plant operates at its maximum electrical output and, at the same time, reduces the thermal contribution of the heat-only boilers. The method developed in this work can be applied to carry out the financial analysis of an energy storage project. [ABSTRACT FROM AUTHOR]

Gas turbine combined cycle (GTCC) based combined cooling, heating and power (CCHP) or combined heating and power (CHP) system driven by natural gas is encouraged to set up for district heating/cooling demand in China due to the clean and efficient energy conversion. This paper presents a GTCC based CCHP system, which consists of a heavy-duty gas turbine, triple-pressure heat recovery steam generator (HRSG), steam turbines, heat exchanger and absorption chiller. Turbine inlet temperature (TIT) strategy and inlet guide vanes (IGV) strategy for the gas turbine are adopted to access the part load performance. The energy distribution and exergy destruction of the CCHP system are investigated under different gas turbine loads. The primary energy saving rate (PESR) and carbon dioxide emission rate (CDER) are set up to evaluate the system at various gas turbine loads and steam extraction ratios. The ability of shaving peak power for the system is investigated. The results show that IGV plays a role in increasing the steam turbine power output and reducing the exhaust heat in HRSG but causes more exergy destruction in the steam turbine expansion process. The PESR and CDER have been enhanced as the steam extraction ratio increases for the same gas turbine load. The IGV strategy reinforces the part-load performance of the CCHP system. For instances, the PESR has been enhanced from 0.2409 to 0.3108, and CDER has been strengthened from 0.8274 to 0.8465 by the IGV strategy at half of gas turbine load and without steam extraction. For the same heating load, both PESR and CDER are enhanced by the IGV strategy. The ability of supplying heating is deteriorating as the decrease in TIT. The ability of shaving peaking power is going to be deteriorated as heating load increases. For the small heating load, like 50 MW, the advantage of the IGV strategy is prominent, the PESR and CDER is advanced 5.81% and 1.48% respectively by the IGV strategy. [ABSTRACT FROM AUTHOR]

In low temperature heat sources driven cooling/heating absorption systems, there are many equipment combinations. Usually, there are contradictions between the equipment efficiency and their economy; for example, the high efficiency- solar collector usually has a high price. So, in this paper, ten absorption cooling/heating systems with different heat sources and different absorption technologies are investigated for technical-economic-thermal analysis. The heat sources are flat plate solar collector, evacuated U-pipe solar collector and middle-temperature high efficiency solar collector and waste heat. The absorption chillers are 1-effect LiBr absorption chiller, 2-effect LiBr absorption chiller, novel 1–2 effect LiBr absorption chiller and novel high efficiency-heat-recovery ammonia water absorption system. The results show that 2-effect absorption chiller with heat source of waste heat is the most optimized choice, which lowest payback period is about 1.9 years. The second choices are the 2-effect LiBr absorption system with novel middle-temperature solar collectors/natural gases and the novel ammonia water absorption system with waste heat sources. [ABSTRACT FROM AUTHOR]

This paper presents an optimization of a high temperature organic Rankine cycle (ORC) system providing the basis for optimal fluid selection and subsequent design parameters based on the working fluid selected. The working fluids used are m-xylene, propylcyclohexane and decane having high critical temperatures. The proposed system deals with the application of biomass due to the high content heat available during its combustion. The system is optimized through non-dominated sorting genetic algorithm (NSGA-II) by taking the prime operators such as; exergetic efficiency (η ex ) to extract maximum work and total heat transfer requirement (UA) to get a prediction of the heat transfer area and hence the cost of the system. The parameters subjected to constraints for optimization are evaporation pressure, degree of superheating and pinch point conditions at heat exchangers. The optimization results exhibit an increase of 22.9% for propylcyclohexane and 45.5% for decane in UA values, relative to m-xylene. Highest exergetic efficiency values for m-xylene among three working fluids further ensures its use in the system as the most viable option from both thermodynamic and economic aspect. Moreover, optimal evaporation pressure range is evaluated by taking the maximum and minimum of exergetic efficiency and UA value, respectively. Both objective functions show negative trend with increase in degree of superheating, with less significant drop. As the pinch point value increases, the UA value decreases showing significantly smaller areas of heat transfer and less cost, but with low exergetic efficiency, therefore, moderate pinch point condition of 8–10 °C is recommended. [ABSTRACT FROM AUTHOR]

This paper presents the exergy and exergoeconomic analyses of a typical combined cooling, heating and power (CCHP) system under off-design conditions. The exergoeconomic-related parameters and unit exergoeconomic cost of the flows are discussed using the exergy cost allocation method based on energy level (ECAEL). The absorption chiller is found to have an improved potential because of the highest relative cost difference and its continual increase with the decrease of the output power. According to the exergoeconomic factor analysis, the exergoeconomic performance of the turbine and combustor among all the components can be enhanced by decreasing the investment and destruction, respectively. Next, the unit energy costs of different products in the output power range (100–20%) including electricity, cooling and heating energy for users, are calculated. The results show that the electricity increases faster than that of other products from 0.537 to 1.077 Yuan/kWh. Finally, the sensitivity analyses for the unit energy cost of the products are presented with different influencing parameters, such as the natural gas price, service life and discount rate. This exergoeconomic analysis may provide guidance for evaluating the products in distributed energy systems for energy networks. [ABSTRACT FROM AUTHOR]

Very accurate second-order thermal models have been developed for the thermal simulation of Stirling engines in recent years. One of the last ones is the comprehensive polytropic model of Stirling engine called the CPMS model. The accuracy of the CPMS model was found to be sufficient for the nominal operation of a prototype Stirling engine known as the GPU-3 engine. Nevertheless, the accuracy of the CPMS model was drastically reduced at high rotational speeds of the engine. In this paper, power loss and pressure change due to the inertial force of the crank system were integrated into the CPMS thermal model in order to compensate inaccuracy of the CPMS model at high rotational speeds. Moreover, the effect of rotational speed on the gas temperature in heater and cooler was also incorporated. A precise model for evaluating the mechanical friction loss was also employed and compared with the simple frictional model of the simple frictional model used in the CPMS. The model was examined on the GPU-3 engine, and it was found that it has superior accuracy compared to the previous thermal model over the entire working regime of the GPU-3 engine. [ABSTRACT FROM AUTHOR]

Utilizing the waste heat of exhausted steam in power plant as a new type of heat source is a key direction to achieve both energy-saving and clean urban heating. In order to recycle all the exhausted steam waste heat of multiple turbine units in designing condition, the authors’ research group proposed a new cascade heating system with multi-heat source and applied it in several air cooling power plants in China. This paper focused on the system integration improving and operation strategy formulating, the main studies included: (1) Established the calculation model of system heating characteristics, and analyzed the problems about energy efficiency of system and safety of air cooled condenser in full operating conditions. (2) Introduced the “equivalent electricity of heating” as the evaluation method of comprehensive energy efficiency of the new co-generation heating system. (3) Improved the integration of heating system to further realize energy cascade utilization between turbine units. (4) Aiming at the improved heating system, the comprehensive optimization ideas of operational parameters in full operating conditions were put forward. By above methods, on one hand the “equivalent electricity of heating” of the new co-generation heating system is decreased by 16%, the comprehensive energy efficiency of system is improved, on the other the freezing risk of air cooled condenser is relieved, the operation safety of system is improved. [ABSTRACT FROM AUTHOR]

The preheating of the feedwater in a Regenerative Rankine Cycle power plant with solar thermal energy, termed Solar Aided Power Generation, is an efficient method to use low to medium temperature solar thermal energy. Here, we compared the use of medium temperature (200–300 °C) energy from concentrating solar collectors (e.g. parabolic trough collectors) to displace the extraction steam to high temperature/pressure feedwater heaters with that from low temperature (100–200 °C) non-concentrating solar collectors (e.g. evacuated tube collectors) to displace the extraction steam to low temperature/pressure feedwater heaters of the power plant. In this paper, the in terms of net land based solar to power efficiency and annual solar power output per collector capital cost of a Solar Aided Power Generation using concentrating and non-concentrating solar collectors has been comparted using the annual hourly solar radiation data in three locations (Singapore; Multan, Pakistan and St. Petersburg, Russia). It was found that such a power system using non-concentrating solar collectors is superior to concentrating collectors in terms of net land based solar to power efficiency. In some low latitude locations e.g. Singapore, using non-concentrating solar collectors even have advantages of lower solar power output per collector capital cost over using the concentrating solar collectors in an SAPG plant. [ABSTRACT FROM AUTHOR]

• A micro-CHP system integrating PVT collectors, a SE and energy storage is considered. • A transient model is developed to evaluate technoeconomic and environmental performance. • The primary energy savings and carbon emission reduction amount to 35% and 37%. • The discounted payback period of the system is 28 years. • The system has greater environmental benefits and a longer payback period than alternatives. This paper presents a comprehensive analysis of the energetic, economic and environmental performance of a micro-combined heat and power (CHP) system that comprises 29.5 m2 of hybrid photovoltaic-thermal (PVT) collectors, a 1-kW e Stirling engine (SE) and energy storage. First, a model for the solar micro-CHP system, which includes a validated transient model for the SE micro-CHP unit, is developed. Parametric analyses are performed throughout a year to evaluate the effects of key component sizes and operating parameters, including collector flow rate, storage tank size, SE micro-CHP flow rate, and battery capacity, on the energetic, economic and environmental performance of the proposed system using real hourly weather data, and thermal and electrical energy demand profiles of a detached house located in London (UK). The optimum component sizes and operating parameters are determined accordingly. The daily and monthly operating characteristics of the system are evaluated, and its annual performance is compared to those of a reference system (gas boiler plus grid electricity), as well as of other alternative solar-CHP systems including a PVT-assisted heat pump system and a standalone PVT system. The results indicate that the installation of such a system can achieve an annual electricity self-sufficiency of 87% and an annual thermal energy demand coverage of 99%, along with annual primary energy savings and carbon emission reduction rate of 35% and 37% relative to the reference system. Over 30 years of operation, the net present value (NPV) of the proposed system is £1990 and the discounted payback period is 28 years. The economics of the proposed system is very sensitive to utility prices, especially the electricity purchase price. Relative to the alternative solar systems, the proposed system offers greater environmental benefits but has a longer payback period. This implies that although the energy saving and emission reduction potential of the proposed system is significant, the initial/capital investment, especially of the SE CHP unit and the PVT collector array, are currently high, so efforts should focus on the cost reduction of these technologies. [ABSTRACT FROM AUTHOR]

• A combined cooling, heating and power system is proposed for waste heat recovery. • Thermo-economic analysis is conducted to study the performance of the system. • Single-objective optimization is conducted to maximize system exergy efficiency. • Multi-objective optimization is carried out. A novel combined cooling, heating and power system which consists of carbon dioxide power cycle, absorption chiller and heaters is proposed in this paper for waste heat recovery of gas turbine. In the proposed cogeneration system, the absorption chiller and heaters are driven by the residual energy of carbon dioxide power cycle and flue gas simultaneously. Detailed mathematic models are established to simulate and evaluate the system from the perspective of thermodynamics and economics. The results of parametric analysis indicate that there is an optimal value for the turbine inlet temperature, at which the exergy efficiency is maximized and the levelized cost of exergy is minimized. As the split ratio increases, the exergy efficiency and levelized cost of exergy of the proposed system first increase then decrease simultaneously. Single-objective optimization is carried out to maximize the exergy efficiency of the proposed system. The optimal results show that the exergy efficiency and levelized cost of exergy of the cogeneration system are 4.62% higher and 0.90 cent lower than that of standalone power cycle, respectively. Compared with the alternative cogeneration system in which only low-temperature flue gas or the waste heat of power cycle is used to drive the refrigeration subsystem and heating subsystem, the exergy efficiency of proposed system is enhanced by 2.89% and 1.3%, respectively. Finally, multi-objective optimization is carried out with exergy efficiency and levelized cost of exergy as objective functions. Pareto frontier is calculated and recommended point is given for the engineering practice. [ABSTRACT FROM AUTHOR]

• The feasibility of integrating large-scale heat pumps in the BTH region is studied. • Regional integrated planning is compared with independent planning. • EnergyPLAN is adopted to simulate the operation of heating and power systems. • The maximum economic feasible potential of heat pump penetration is identified. • Heat pumps earn energy and environmental benefits without increasing cost in 2030. Promoting cleaner heating is one of the key pathways towards future energy transition. One promising solution is to implement large-scale heat pumps into the existing heat supply system. This paper aims to investigate whether the integration of large-scale heat pumps can be the first feasible step for cities towards clean heating transition. As opposed to the traditional way of conducting independent energy planning within a single city/region, this paper presents a novel approach of employing integrated planning for the multiple neighboring regions to explore the advantages of interregional energy collaboration. The Beijing-Tianjin-Hebei (BTH) urban agglomeration is taken as a case study, which is the most polluted region in China caused by the coal-based heating system. By implementing a series of simulations for the heating and power systems in the EnergyPLAN tool for six predesigned future scenarios which consider different planning strategies and analysis focuses, this paper analyzes to what extent heat pumps can help to achieve energy and environmental improvement for the whole BTH region by 2030 while ensuring economic feasibility. This is done by creating three independent models for respective Beijing, Tianjin, and Hebei, and one integrated model for the whole BTH region. The results suggest that when guaranteeing economic feasibility, the integration of large-scale heat pumps can potentially result in at least 9.5% energy saving and 9.28% reduced CO 2 emission compared to the baseline. The integrated planning strategy can furthermore be more efficient in the urban agglomeration, which reduces 1.92% and 2.27% more energy and emissions in 2030 compared to the independent planning results. Based on cost analysis, the maximum economic feasible potential of heat pump penetration is identified as well. This paper can provide references for policymakers in the BTH region and the rest of Northern China, as well as present a principle of energy planning for cities on a general level. [ABSTRACT FROM AUTHOR]

The heat compensation unit with thermosyphon has been developed to eliminate the annual soil thermal imbalance of ground-coupled heat pump in heating-dominant buildings. But the issues on heating capacity deficiency at peak heating loads and on the high borehole investment are still unsolved in this non-coupled system. In this paper, a coupled operation of the heat compensation unit and ground-coupled heat pump system is proposed. That is, the heat compensation unit reheats the borehole outlet fluid and improves the temperature of the fluid entering the evaporator of heat pump during the heating season. By doing so, it can enhance the heating capacity at peak loads and reduce the number of boreholes required. The heat compensation required by the heat compensation unit in the non-heating season is also reduced, increasing the system efficiency. To demonstrate the effectiveness of the coupled system against the non-coupled one, the system models are built in TRNSYS to analyze the system reliability, efficiency and economy. Results show that, while the number of boreholes is reduced to 40%, the coupled system can maintain the soil thermal balance and meet the indoor heating demand. For the coupled system with different numbers of boreholes (40–100%), the seasonal average heating COPs of the ground-coupled heat pump and heat compensation unit are 4.39–4.70 and 3.67–3.80, respectively. The hourly heating capacity of the ground-coupled heat pump is also increased by 19–65%. The annual system COP of the coupled system is about 2.48–2.61, which is higher than that of 1.82–2.45 for non-coupled one. Compared to the conventional Boiler assisted ground-coupled heat pump system, the payback period of the coupled system is only 1 year. [ABSTRACT FROM AUTHOR]

This paper evaluates the potential carbon dioxide emissions reduction from the implementation of electric energy storage to a combined power generation unit and an organic Rankine cycle relative to a conventional system that uses utility gas for heating and utility electricity for electricity needs. Results indicate that carbon dioxide emission reductions from the operation of the proposed system are directly correlated to the ratio of the carbon dioxide emission conversion factor for electricity to that of the fuel. The location where the system is installed also has a strong influence on the potential of the proposed system to save carbon dioxide emissions. Finally, it is shown that by using carbon emissions cap and trade programs, it is possible to establish a frame of reference to compare/exchange operational cost gains with carbon dioxide emission reductions/gains. [ABSTRACT FROM AUTHOR]

• A new cross-scale model for solid oxide fuel cell cogeneration system is proposed. • Overall constraints are obtained by considering internal and external processes. • Influences of multiple parameters on the system overall performance are analyzed. • Two optimization strategies for the optimal system operation are provided. The multi-scale, multi-component, multi-process, and multi-parameter characteristics increase intermediate variables and complexity of modeling and analysis of solid oxide fuel cell cogeneration systems. This paper applied the standard thermal resistance for constructing the heat current model to analyze the overall heat transfer performance of the external heat exchangers. On this basis, the research introduced the equivalent electric circuit for presenting the internal electrochemical process and then proposed the overall cross-scale modeling of the solid oxide fuel cell cogeneration system from the internal heat and mass transfer and electrochemical processes to the various external heat exchangers. Moreover, considering the internal and external multi-processes, the system's overall constraints were derived. The simulation results show that the total energy utilization rate of the solid oxide fuel cell cogeneration system is 79.12 %. Besides, the influences of water-to-carbon ratio, excess air coefficient, thermal conductances of each heat exchanger, and ambient temperature on the system performance were developed. The optimal operation parameters are given for improving the maximum net power generation of the system. Finally, the proposed cross-scale modeling method is feasible and convenient for analyzing and improving the solid oxide fuel cell cogeneration system. [ABSTRACT FROM AUTHOR]

• A heat pump coupled two-stage HDH system with waste heat recovery is proposed. • Performances of the proposed system with various working fluids are studied. • Effects of key operating parameters on system performance are investigated. • Performance comparison with previous studies are conducted. Humidification and dehumidification (HDH) desalination technology has attracted wide attention in the field of small-scale dispersed fresh water demand, because of its advantages such as simple configuration, low investment, and utilization of low-grade renewable energy and waste heat. In this paper, a heat pump coupled two-stage humidification-dehumidification desalination system with waste heat recovery is proposed, where the waste heat is used to heat the feed seawater of the first-stage humidifier, and the brine leaving the bottom is further heated by the condenser of heat pump and then taken as the feed of the second-stage humidifier, while the evaporator of heat pump is used to recover the heat of humid air from the first-stage dehumidifier. Performance evaluation of the proposed system with various working fluids are conducted regarding the fresh water production m pw , gained output ratio (GOR), recovery ratio (RR), specific entropy generation s tot , and unit cost of fresh water production Z pw , and parametric studies are performed to identify the key operating parameters. Results show that the largest entropy generation occurs in evaporator, followed by the first-stage humidifier and condenser. Taking R22 as the working fluids is corresponding to the largest m pw , while the lowest Z pw and s tot , and the highest GOR are obtained by R600. Higher spraying temperature of the first humidifier T 4 is conducive to improving the m pw and Z pw , but leading to a larger s tot. The GOR increases first and then decreases with the rise of T 4 , and the peak value is 4.57 at T 4 = 340.65 K. The mass flow rate ratio of the first stage MR 1 has significant effects on the system performance. When MR 1 is about 3.5, the m pw , GOR and RR reach the peak. However, the bottom value of Z pw is 8.74 $/m3 corresponding to the MR 1 of 2.2–2.5. With the increase of compression ratio (CR) of heat pump subsystem, the m pw , GOR and RR climb up first and then decrease, while Z pw and s tot get raised. [ABSTRACT FROM AUTHOR]

• Off-peak electricity driven chemisorption heat pump is developed for space heating. • Asynchronous start-stop control method is firstly developed for stable heat output. • Sorption bed composed of 42 reactors is specially designed for heat storage. • Heat pump can maintain stable performance at an evaporating temperature of −15 °C. • A lower heating cost throughout the whole heating season can be obtained. To guarantee clean heating of buildings in cold regions, an off-peak electricity driven chemisorption heat pump employing SrCl 2 /sulfurized expanded graphite composite sorbent is developed. However, the heat output temperature of chemisorption heat pump fluctuates greatly, up to 30 °C, which seriously restricts its popularization and application. In this paper, the above-mentioned problems are solved from two aspects of sorption bed structure design and system operation strategy. The sorption bed utilized for thermal energy storage is especially composed of several unit reactors, and the asynchronous start-stop control method of unit reactors is innovatively proposed. By controlling their start and stop time, the temperature fluctuation range of hot water that the heat pump outputs is significantly reduced. Simultaneously, to improve the energy efficiency of heat pump, small temperature difference fan coil units are adopted indoors. The results indicate that when ten reactors operate asynchronously, the output temperature of hot water fluctuates by as low as 1.4 °C. With four reactors in asynchronous operation, the temperature fluctuation range can be reduced to 3 °C, and 35 °C hot water enters the small temperature difference fan coil units to release heat to the indoor space. Additionally, the heating performance of the chemisorption heat pump is relatively stable in cold regions. When the evaporating temperature decreases from 5 °C to −15 °C, its energy efficiency is almost constant, about 1.47. Eventually, this chemisorption heat pump can shift electricity from peak periods to off-peak periods, and also saves 0.7 to 6.3 yuan per day in space heating cost in comparison to the conventional vapor-compression one. [ABSTRACT FROM AUTHOR]

• District heating system capacity and operation optimization. • Carbon and exergy taxing approach integrated with multi-objective optimization. • Natural gas boilers could be effectively pushed-out from the least-cost solution. • Allocation of CO 2 emissions in CHP units has great effect on the optimal solution. District heating systems are proven to be effective way of increasing energy efficiency, reducing environmental impact and achieving higher exergy efficiency. In research papers, district heating multi-objective optimization usually takes into account minimization of the total discounted cost and environmental impact, while exergetic objective function is rarely introduced. Most of the times, economic and ecological objective functions are studied as a single objective optimization problem through internalization of the cost related to carbon dioxide emissions tax. This paper presents novel approach since additional tax, related to exergy destruction, has been introduced. The influence of these two taxing systems on a single and multi-objective optimization results of district heating system has been carried out. Two approaches have been proposed. In the first one, multi-objective optimization has been used where objective functions were defined as economic and ecological or exergetic. In the second approach, single-objective optimization has been used where cost function also includes both carbon and exergy destruction tax. It has been shown that inclusion of carbon tax causes convergence of Pareto fronts after specific exergy destruction has been reached. On the other hand, if all technologies are available, increase of exergy tax doesn't reduce carbon dioxide emissions. The most important outcome of this paper is analysis of the impact of exergy tax on natural gas consumption in heat-only boilers. Acquired results show that exergy, together with carbon tax, can effectively reduce natural gas consumption in heat-only boilers. If there are no back-pressure CHP technologies available, these taxing systems can completely push out its consumption. Finally, the analyses with carbon emissions allocation in CHP units has also been carried out. Acquired results have shown that with increase of carbon tax, exergy efficiency of the system could be increased. [ABSTRACT FROM AUTHOR]

The adequate planning of district energy systems is a key process to provide communities with proper heating and cooling networks. To find cost-effective planning solutions, this decision-making process is usually formulated as mathematical model, which is optimized to determine where and how many assets should be installed in the network. However, this optimization problem is becoming more complex as district energy systems evolve and become more elaborate. For the 5th generation district heating and cooling (5GDHC) networks, this planning framework comprises multiple building energy systems, a thermal and electrical network as well as central heating and cooling units. As a result, the optimization problem associated with these circumstances can easily become intractable as the number of elements of the network increases. To alleviate this tractability problem, in this paper, a Dantzig–Wolfe approach is devised to decompose a mixed-integer linear program into multiple subproblems (for every building) and a master problem (thermal and electrical network and central units). A realistic case study based on a 5GDHC system in Germany is considered. For this case study, it is demonstrated that the proposed decomposition approach yields the same results attained by the original not decomposed problem while achieving significant gains in terms of scalability and computational times. More specifically, the decomposition approach reduces the computational time for districts with more than 10 buildings by in average 94%. This corroborates the potential of the proposed approach to improve the computational efficiency of models that will deliver cost-effective investment plans for 5GDHC networks. • Design optimization model for 5th generation district heating and cooling networks. • Dantzig–Wolfe decomposition for a 5GDHC optimization model. • Validation of decomposition approach with full model formulation. • Performance test of decomposed models for large districts. • Analysis of shadow prices (heat and electricity) for a 5GDHC system. [ABSTRACT FROM AUTHOR]

• The flexibility of district heating-connected heat pumps was investigated. • The utilization of heat pump could help to electrify the district heating sector. • Heat pump technology resulted a viable solution to provide heat in district heating. • The heat pump flexibility could improve the electricity distribution system balance. • Exploiting the heat pump flexibility strongly increases the heat pump profitability. This paper investigates the Power-to-Heat energy conversion process carried out by heat pumps connected to a district heating network used to provide heat to the heating sector and, at the same time, to provide flexibility for the electricity sector. The aim of this work is to analyze from a techno-economic point of view the flexibility potential of this solution used to absorb the over-generation of Variable Renewable Energy Sources in the distribution system. A scenario, based on the electric distribution system and the district heating distribution system of the city of Turin, was created to carry out this type of study. The results showed that, flexible use of these Power-to-Heat systems can be exploited to shift part of electric loads in periods of renewable over-generation, with significant benefits from an economic point of view. Furthermore, the position of Power-to-Heat systems within an electricity distribution network has a significant impact; to make the most out of the flexibility of these plants, they should be placed in those areas of the network that present the greatest local over-generation of renewables. The use of heat pumps will be necessary for the energy transition thanks to the high conversion efficiency. However, in order to fully exploit all the benefit that this technology can offer its flexibility cannot be ignored. [ABSTRACT FROM AUTHOR]

• A coupled heat recovery system is proposed for air separation units. • A lab-scale prototype of the heat recovery system is presented. • Parametric analysis is performed to explore system performance. • Experimental investigations show good system stabilities and system performance. Power consumption of the compression process in air separation units can be significantly reduced by precooling the inlet air of the air compressors with an organic Rankine cycle and vapor compression cycle (ORC-VCR). To date, most of existing researches on compression heat recovery focus on thermodynamic modelling and performance analysis. This paper presents an experimental assessment of a compression heat recovery system based on a mechanically coupled ORC-VCR for air separation units. Experimental investigations on the stabilities of gas bearing fluid supply system and motor cooling system were conducted. Results indicate that both are capable of maintaining stable operations while achieving the designed functions. System performance tests were performed with various mass flow rates, pressure ratios of high-temperature and low-temperature evaporating pressure. The increase in the ratio of two evaporating pressures leads to a dramatic decrease in the system coefficient of performance (COP). A COP of 0.63, a cooling capacity of 14.2 kW at a refrigeration temperature of 14.6 °C were achieved at a rotation speed of 25822 rpm with full-load operation. This work experimentally demonstrates an effective compression heat recovery system based on ORC-VCR, which suggests the potential application of the compression heat recovery system in air separation units. [ABSTRACT FROM AUTHOR]

[Display omitted] • A CCHP system combined with solar thermal energy and TES capacity is proposed. • Primary energy saving rate is at least 11 percent higher than the reference systems. • The optimized TES capacity is economically founded. • The optimized operation strategy decreases the equivalent LCOE by 11.8%. The introduction of solar thermal energy and the thermal energy storage are effective methods for reducing the fossil fuel consumption and improving the operation performance of combine cooling, heating and power (CCHP) system. In this study, a CCHP system integrated with solar thermal energy and thermal energy storage is proposed. The thermal energy storage device, which plays the role of energy hub, absorbs the solar thermal energy form the parabolic trough collector and excess thermal energy in the flue gas and then releases the thermal energy when necessary. Transient model of the system is established and the corresponding performance of the proposed system in the typical days are compared with those of the separated system and the conventional CCHP systems. The primary energy saving rate of the proposed system in the typical days of summer, winter and transition seasons are found to be at least 11 percent higher than the other CCHP systems. The capacity of the system is then adjusted for the best economy and corresponding thermal energy storage device capacity is found. Considering the peak-flat-valley electricity price, the operation strategy is then optimized and the equivalent levelized cost of electricity is reduced by 14.3%. This paper provides a viable option for improving the CCHP system performance and economy in solar-abundant regions. [ABSTRACT FROM AUTHOR]

• Available energy losses and parameters in main components are analyzed in detail. • Hot regenerator's losses and parameters are the main factor deteriorating the efficiency. • The performance of Vuilleumier heat pump under different temperatures is studied. • The variations of COPh decrease with the cold temperature linearly and are small. Vuilleumier cycle heat pump (VHP) is a thermally driven heat pump and has huge potential for improving energy efficiency in residential heating. In this paper, performance of VHP is studied based on SAGE software. The errors between the experiment and simulation are in the range of −4.6 %∼4.5 % and −18.2 %∼7.4 % respectively for COP h and heating capacity. Based on developed model validated by experimental results, available energy losses (AEloss) are obtained and analyzed. Hot regenerator is the main component for AEloss and incomplete heat transfer is the main contribution for AEloss. Then, the effects of parameters on the performance are studied. And parameters are optimized. The results show that the improvement of COP h could be up to 3.6 %. Moreover, the performance and seasonal energy efficiency ratio of VHP in the cold region are carried out. The results show that seasonal COP could be up to 1.53 at −0.26 °C average ambient temperature. [ABSTRACT FROM AUTHOR]

The work was conducted to investigate pyrolysis characteristics and kinetics of low rank coals relating with coal structure by thermogravimetric analysis (TGA), the distributed activation energy model (DAEM) and solid-state 13 C Nuclear Magnetic Resonance (NMR). Four low rank coals selected from different mines in China were studied in the paper. TGA was carried out with a non-isothermal temperature program in N 2 at the heating rate of 5, 10, 20 and 30 °C/min to estimate pyrolysis processes of coal samples. The results showed that corresponding characteristic temperatures and the maximum mass loss rates increased as heating rate increased. Pyrolysis kinetics parameters were investigated by the DAEM using Miura integral method. The DAEM was accurate verified by the good fit between the experimental and calculated curves of conversion degree x at the selected heating rates and relatively higher heating rates. The average activation energy was 331 kJ/mol (coal NM), 298 kJ/mol (coal NX), 302 kJ/mol (coal HLJ) and 196 kJ/mol (coal SD), respectively. The curve-fitting analysis of 13 C NMR spectra was performed to characterize chemical structures of low rank coals. The results showed that various types of carbon functional groups with different relative contents existed in coal structure. The work indicated that pyrolysis characteristics and kinetics of low rank coals were closely associated with their chemical structures. [ABSTRACT FROM AUTHOR]

In the paper, a model for the simulation and the optimization of a novel solar trigeneration system is presented. The plant simulation model is designed to supply electricity, space heating or cooling and domestic hot water for a small residential building. The system is based on a solar field equipped with flat-plate photovoltaic/thermal collectors, coupled with a water-to-water electric heat pump/chiller. The electrical energy produced by the hybrid collectors is entirely supplied to the building. During the winter, the thermal energy available from the solar field is used as a heat source for the evaporator of the heat pump and/or to produce domestic hot water. During the summer, the heat pump operates in cooling mode, coupled with a closed circuit cooling tower, providing space cooling for the building, and the hot water produced by the collectors is only used to produce domestic hot water. For such a system, a dynamic simulation model was developed in TRNSYS environment, paying special attention to the dynamic simulation of the building, too. The system was analyzed from an energy and economic point of view, considering different time bases. In order to minimize the pay-back period, an optimum set of the main design/control parameters was obtained by means of a sensitivity analysis. Simultaneously, a computer-based Design of Experiment procedure was implemented, aiming at calculating the optimal set of design parameters, using both energy and economic objective functions. The results showed that thermal and electrical efficiencies are above 40% and 10%, respectively. The coefficient of performance of the reversible heat pump resulted above 4 for both heating and cooling modes. For the base case, a Simple Pay Back period of 5.36 years was found; such index decreases to 2.33 years in case a capital investment incentive of 30% is available. As expected, a decrease of the performance of the system was detected for weather conditions in which the availability of solar energy is scarce. The Design of Experiments analysis outlined that the appropriate selection of collector area is crucial in order to achieve a good profitability of the system under analysis. [ABSTRACT FROM AUTHOR]