Search

Power system flexibility can be improved effectively, if the advantages of the peak shaving ability of molten salt solar tower power (STP) plant can be developed and utilized. In this paper, the heat transport and load response characteristics of the molten salt STP plant in the regulation process are studied, aiming at serving the development of the regulation method in the rapid peak regulation process. The steam generation system and turbine system models of a 50 MW STP station are established. Two load regulation methods and operation modes are used to investigate the inertia and delay effects of heat transfer and evaporation, as well as load response characteristics during the actual regulation operation, including steam generation system following (SF), turbine following (TF), constant pressure operation (CP), and sliding pressure operation (SP). The results show that when the plant load is reduced by 10% Pe at the rate of 3% Pe /min, the heat transfer of salt/water and the evaporation process of water in the whole regulation process have a 250 s delay compared with the load. The larger heat storage characteristics of the plant help to improve the load response speed. The load response speed of the plant is faster in SF mode. TF mode is beneficial to the safe operation of the plant. Considering the operational safety of thick-wall component, the recommended running mode is the collocation of SF and SP mode, TF and CP mode. The results show that the load change rate of 10% Pe /min is permittable under TF and CP modes. The conclusion of this paper lays a foundation for the formulation of a peak regulation strategy for molten salt STPs. • Dynamic model for STP plant as participated in grid peak-shaving is established. • Transient response characteristics of STP plant during real load regulation are analyzed. • Influences of load changing rate on SPT plant under SGS following control mode are compared. • Recommended operation mode and load adjusting rate of the 50MW el STP plant are given. [ABSTRACT FROM AUTHOR]

Deaerator is one of the most important equipment for steady state and dynamic operation of power plants. The deaerator energy storage utilization process is one of the most essential ways to enhance the variable load rate of power plants. The purpose of this study is to improve the dynamic simulation performance of the deaerator during unit load changes by constructing a more reasonable deaerator model, aiming to provide guidance for practical operations. In this paper, a thermal mass microelement algorithm is proposed for the heat transfer between droplets and steam in the deaerator, followed by segmental modeling of the deaerator. By comparing with the operation data of a power plant, the steady state operation error of the deaerator model is within 0.6 %. Subsequently, the unit load variation process is simulated and the dynamic variation accuracy of the model proposed in this paper is enhanced by 1–2 % compared to the lumped parameter model. The dynamic characteristics of the deaerator are obtained by simulating the step and ramp changes of the deaerator boundary conditions and the deaerator start-up process. In addition, during the simulation of condensate throttling, the maximum power of the unit using the deaerator model in this paper is 0.2–1.5 MW larger than that of the lumped parameter model. • A thermal mass microelement algorithm for heat exchange between droplets and steam in deaerator is proposed. • A more rational deaerator model is established. • Compared to the lumped parameter model, the proposed model enhances the accuracy of the dynamic simulation by 1–2 %. • The maximum power in the proposed model during condensate throttling is 0.2–1.5 MW greater than that in the lumped model. [ABSTRACT FROM AUTHOR]

Deaerator is one of the most important equipment for steady state and dynamic operation of power plants. The deaerator energy storage utilization process is one of the most essential ways to enhance the variable load rate of power plants. The purpose of this study is to improve the dynamic simulation performance of the deaerator during unit load changes by constructing a more reasonable deaerator model, aiming to provide guidance for practical operations. In this paper, a thermal mass microelement algorithm is proposed for the heat transfer between droplets and steam in the deaerator, followed by segmental modeling of the deaerator. By comparing with the operation data of a power plant, the steady state operation error of the deaerator model is within 0.6 %. Subsequently, the unit load variation process is simulated and the dynamic variation accuracy of the model proposed in this paper is enhanced by 1–2 % compared to the lumped parameter model. The dynamic characteristics of the deaerator are obtained by simulating the step and ramp changes of the deaerator boundary conditions and the deaerator start-up process. In addition, during the simulation of condensate throttling, the maximum power of the unit using the deaerator model in this paper is 0.2–1.5 MW larger than that of the lumped parameter model. • A thermal mass microelement algorithm for heat exchange between droplets and steam in deaerator is proposed. • A more rational deaerator model is established. • Compared to the lumped parameter model, the proposed model enhances the accuracy of the dynamic simulation by 1–2 %. • The maximum power in the proposed model during condensate throttling is 0.2–1.5 MW greater than that in the lumped model. [ABSTRACT FROM AUTHOR]

Organic Rankine cycle (ORC) is considered to be a good solution to the high-efficient recovery of low-grade heat for saving energy and reducing carbon emissions. As the core component, the heat exchanger plays a significant role in the thermo-economic performance evaluation of an ORC plant and its heat transfer correlations greatly influence the cycle performance. In this paper, 8 two-phase heat transfer correlations (4 flow boiling ones and 4 flow condensation ones) are utilized to design the plate-type heat exchangers, and a comparative study among different correlations is carried out. The result shows the designed evaporators are finitely affected by heat transfer correlations whereas the situation with the designed condensers is the opposite. The net output power is slightly influenced with a relative difference within 1.1% but the system's economic performance is significantly affected with the maximum relative difference of electricity production cost up to 11.2%. The environmental performance is hardly affected by correlations with the maximum relative difference of total greenhouse gas emission within 0.96%. This paper analyzed the performance variation of the ORC under different correlations and provided a guideline for designing heat exchangers and evaluating the off-design performance of ORC systems. • 8 two-phase heat transfer correlations are compared for designing the heat exchangers. • Designed condensers are affected by the heat transfer correlations more significantly. • Net output power is slightly influenced with a relative difference within 1.1%. • Electricity production cost is affected with a relative difference 11.2%. • Total carbon emissions exhibit minimal variation with a relative difference of only 0.96%. [ABSTRACT FROM AUTHOR]

Accurately predicting effective thermal conductivity is of great importance for the design and performance evaluation of emerging composites. In this paper, an efficient and implementation-friendly lattice Boltzmann (LB) scheme for predicting the effective thermal conductivity of 3D complex structures is proposed. The key innovation is that the optimum convergence parameter of the 3D thermal LB method is found, which enables the LB equation to converge to steady heat conduction equation with the fastest speed and without losing any accuracy. To deal with the thermal contact resistance between different components, an interface treatment scheme is derived. In comparison with the existing schemes, the present scheme enjoys several hundred times higher computational efficiency. By virtue of this LB scheme, the effective thermal conductivity of the reinforced composites with different dimensional fillers are systematically calculated, and a comprehensive machine learning model is developed. This work provides a powerful numerical tool for high-throughput simulations of the 3D representative volume elements with high thermal conductivity ratios and large grid numbers. It may facilitate the application of data-driven techniques in study of the thermal transport properties of emerging composite materials and structures. • A high-efficiency LB scheme for 3D thermal transfer simulations is proposed. • A treatment scheme of interface thermal resistance to TRT model is derived. • A random method is established to reconstruct RVE of composites with hybrid fillers. • A k eff machine learning model for composites with 0D, 1D and 2D fillers is developed. [ABSTRACT FROM AUTHOR]

Using metal hydride for hydrogen storage in stationary applications and for transportation is a promising technology due to its advantages of large hydrogen storage capacity, low pressure and low energy consumption. Combining the metal hydride reactor with PCM is expected to recover the heat generated during the hydrogen absorption and use it for hydrogen desorption, thus improving the energy efficiency of the system. This paper proposes a metal hydride reactor integrated with honeycomb fins and PCM to enhance heat transfer. Based on simulations, the results show that the achieved hydrogen storage capacity is 1.326 wt%, the gravimetric and volumetric storage densities are 0.411% and 14.76 kg of H 2 per m3, respectively, and the mean saturated rates are 1.222 × 10−3 g s−1 and 0.773 × 10−3 g s−1 for absorption and desorption processes. Compared with the reactor without fins, the mass and volume of the reactor using honeycomb fins are increased, resulting in a decrease in gravimetric and volumetric storage density, but a increase in reaction rate during hydrogen absorption and desorption processes. Based on this structure, we also propose a honeycomb fin reactor filled with sandwich PCM to further accelerate the heat transfer in the reaction process. Compare to the reactor with PCM only filled on the periphery of the honeycomb fins, the hydrogen absorption and desorption times are shortened by about 86.4% and 81.1%, respectively. In addition, different reactor structures are compared using multiple KPIs to provide relevant suggestions for the reactor optimization. The obtained research results can provide a reference for effective thermal management methods in MH storage systems. • A metal hydride reactor integrated with honeycomb fins and PCM was proposed. • The comparison of reactor without fins and the effects of fin size were studied. • The placement of PCM and MH was optimized. • Multiple KPIs were used to evaluate the hydrogen absorption and desorption processes. • The optimized reactor performed better than other forms of reactors. [ABSTRACT FROM AUTHOR]

• Carbon based composite phase change materials have broad application background. • 2D carbon based composite PCMs had higher thermal conductivity than others. • 3D carbon based material could both enhance the thermal conductivity and resist-leakage performance of PCMs. Energy storage plays a crucial role in saving energy and protecting the environment. The research on phase change latent heat storage materials has been in the forefront of the thermal storage research. However, the low thermal conductivity and the leakage during phase transition have limited the application of the phase change materials (PCMs). Carbon materials can be used to improve the properties of PCMs because of their excellent properties. Carbon materials have high thermal conductivity and excellent adsorption properties. Some carbon materials can improve the mechanical properties, electrical properties and flame retardant properties of PCMs. In this paper, performances and applications of one-dimensional carbon (carbon nanotubes, carbon fibers and graphite fibers) based composite PCMs, two-dimensional carbon (graphite, graphene, graphene oxide and exfoliated graphite nanoplates) based composite PCMs and three-dimensional carbon (expanded graphite, carbon based foam, graphene aerogel material and activated carbon) based composite PCMs were summarized and discussed. The advantages and disadvantages of the carbon materials with various dimensions in the application were also pointed out. References for thermal energy storage application were provided by this paper. [ABSTRACT FROM AUTHOR]

Highlights • Vertical fins contribute to melting by enhancing natural convection mechanism. • Extending fin length is more beneficial than increasing number of fins. • The cases with longer fins could have a higher overall heat transfer coefficient. • The compromise in energy density could be minor due to sensible energy gain. Abstract This paper investigates the effect of vertical fins, as an enhancement technique, on the heat transfer rate and energy density of a latent heat thermal energy storage system. This contributes with knowledge on the interaction of heat transfer surface with the storage material for optimizing storage capacity (energy) and power (heat transfer rate). For the assessment, numerical modeling is employed to study the melting process in a two-dimensional rectangular cavity. The cavity is considered heated isothermally from the bottom with surface temperatures of 55 ˚C, 60 ˚C or 70 ˚C, while the other surfaces are insulated from the surrounding. Aluminum and lauric acid are considered as fin/enclosure material and phase change material, respectively. Vertical fins attached to the bottom surface are employed to enhance the charging rate, and a parametric study is carried out by varying the fin length and number of fins. Thus, a broad range of data is provided to analyze the influence of fin configurations on contributing natural convection patterns, as well as the effects on melting time, enhanced heat transfer rate and accumulated energy. The results show that in addition to increasing the heat transfer surface area, the installation of vertically oriented fins does not suppress the natural convection mechanism. This is as opposed to horizontal fins which in previous studies have shown tendencies to reduce the impact of natural convection. This paper also highlights how using longer fins offers a higher rate of heat transfer and a better overall heat transfer coefficient rather than increasing the number of fins. Also, fins do not only enhance the heat transfer performance in the corresponding melting time, but also maintain similar total amount of stored energy as compared to the no-fin case. This paper discusses how this is the result of the enhanced heat transfer allowing a larger portion of sensible heat to be recovered. For example, in the case with long fins, the relative mean power enhancement is about 200% with merely 6% capacity reduction, even though the amount of PCM in the cavity has been reduced by 12% as compared to the no-fin case. Although the basis for these results stems from the principles of thermodynamics, this paper is bringing it forward with design consideration. This is because despite its importance for making appropriate comparisons among heat transfer enhancement techniques in latent heat thermal energy storage, it has not been previously discussed in the literature. In the end, the aim is to accomplish robust storage systems in terms of power and energy density. [ABSTRACT FROM AUTHOR]

Highlights • EnergyPlus is capable of accurately estimating the heat transfer in parked vehicles. • Spectrally selective glazing can cause an average temperature difference inside the cabin as high as 12.5 °C. • Solar-reflective opaque coating in combination with ChLC glazing can reduce the cabin air temperature up to 23.8 °C. Abstract Parked vehicles have the tendency to overheat quickly. This can lead to a negative impact on the thermal comfort of the driver and its passengers, as well as intensive use of air conditioning systems, and thus fuel consumption of the vehicle or, in the case of electric mobility, a reduced cruising range. In the search for effective measures to mitigate this effect, important guidance can be provided by the field of sustainable building design. On the one hand, inspiration can come from design strategies in terms of shapes and advanced cover materials, but this paper advocates that this can also pertain to the simulation-based design support tools that are used by building engineers. This paper first presents the results of a thermal soak test, and then uses this data to demonstrate the suitability of the building performance simulation tool EnergyPlus for predicting the thermal behavior of parked vehicles. This fit-for-purpose validated model is used to evaluate the performance of three overheating mitigation measures for two car models in two climates. The results show that spectrally selective glazing can reduce the cabin air temperature by 12.5 °C and when combined with solar reflective opaque surfaces, the reduction of cabin air temperature can reach 23.8 °C. Increased use of building performance simulation in the automotive domain can help to further optimize the overheating reduction potential of cars. [ABSTRACT FROM AUTHOR]

• Energy simulation of AIS using experimentally validated thermal resistances. • AIS evaluation at varying orientations, climate zones, and switching ratios. • Investigation of AIS switching and wall thermal conductivity/heat transfer. • Application of freeze timer to reduce the number of switches of AIS. Active insulation systems (AISs) in buildings are envelopes that integrate thermal insulation, thermal energy storage, and controls. Although different designs for AISs have been proposed in the literature, a comprehensive analysis of feasible AISs is lacking. This paper discusses the energy performance, peak demand reduction potential, and performance characteristics of an AIS that uses a concrete wall as thermal mass sandwiched between two solid-state thermal switches (STSs). These STSs change their thermal conductivity using an on/off metal switch to create or break a thermal bridge across the STS. This paper first describes the experimental setup, used to determine the ratio of thermal resistance during R-high (low thermal conductivity) and R-low (high thermal conductivity) states of the STSs. This ratio was then used in whole-building energy simulations to evaluate the performance of AIS walls across different climate zones with/without a freeze timer of 60 min. The timer was added to reduce the number of switches of STSs from one state to another, and hence the energy needed for these switches. Analysis of the switching frequency and interval of STSs, thermal conductivity of walls, impact of wall orientation, and heat transfer through the wall from the use of AIS at different climate zones/locations were performed. The simulation results show that the AIS can achieve energy savings ranging from ∼ 980 to 2,290 kWh in a single-family home with a floor area of ∼ 220 m2 compared with an IECC 2018 baseline. The energy savings was higher in dry climate zones which represent 17% of residential buildings in the United States, compared to humid or marine climate. [ABSTRACT FROM AUTHOR]

In the future due to continued integration of renewable energy sources, demand-side flexibility would be required for managing power grids. Building energy systems will serve as one possible source of energy flexibility. The degree of flexibility provided by building energy systems is highly restricted by power-to-heat conversion such as heat pumps and thermal energy storage possibilities of a building. To quantify building demand flexibility, it is essential to capture the dynamic response of the building energy system with thermal energy storage. To identify the maximum flexibility a building’s energy system can provide, optimal control is required. In this paper, optimal control serves to determine in detail demand flexibility of an office building equipped with heat pump, electric heater, and thermal energy storage tanks. The demand flexibility is quantified using different performance indicators that sufficiently characterize flexibility in terms of size (energy), time (power) and costs. To fully describe power flexibility, the paper introduces the instantaneous power flexibility as power flexibility indicator. The instantaneous power flexibility shows the potential power flexibility of TES and power-to-heat in any case of charging, discharging or idle mode. A simulation case study is performed showing that a water tank, a phase change material tank, and a thermochemical material tank integrated with building heating system can be designed to provide flexibility with optimal control. [ABSTRACT FROM AUTHOR]

The low-temperature cold start has become one of the leading technical bottlenecks limiting the large-scale commercial application of proton exchange membrane(PEM) fuel cells. This paper establishes the 1-D numerical model of the fuel cell stack to study the reactant gas, charge transfer, heat transfer, and water phase transition process. Then the critical parameters in the numerical model of the fuel cell stack are identified based on the cold start experiment data. The identification results show that the cold start model is consistent with the experimental results, and the model's output voltage and average temperature errors are under the acceptable range. Based on the numerical model, with the cold start time as the optimization objective and load current and initial membrane water content as the optimization variable, the cold start strategy is optimized under different ambient temperatures. The results show that the optimized current loading strategy can reduce the cold start time of the fuel cell stack and achieve a fast, successful cold start for −20 °C and − 25 °C conditions. The cold start time of the optimized cold start strategy is reduced by 42.2% compared with the strategy of step stair current under −20 °C. For the lower ambient temperature,-30 °C or below, to realize the successful cold start of fuel cell stacks, two aspects are needed: the redesign of the bipolar plate and endplate structure and the optimization of the cold start strategy. • The supercooled water phase change process is established based on JMAK theory. • Parameters identification and error analysis for the numerical model are conducted. • Initial membrane and segmented currents are optimized for the cold start strategy. [ABSTRACT FROM AUTHOR]

Abstract: Warm temperature spikes represent one of the main spoilage causes of perishable good-stuffs. The development of packaging materials with thermal buffering properties represents a powerful solution to address the problems arising from an uncontrollable interruption during cold-chain logistic. Here, we propose the use of phase change material (PCM) composites for the design of cold storage packaging. Two different concentrations (25 and 50% w/w) of commercially available micro-encapsulated PCM were homogeneously dispersed in paper matrix via conventional negative filtration techniques. The possibility of obtaining composites with different latent heats in the 4–10°C range has been demonstrated via differential scanning calorimetry measurements. Heat transmission tests, simulating the heating processes typical of the removal from a cold room, were performed on a suitable multilayer configuration. The obtained materials show the ability to maintain the inner temperature for a duration up to 10-fold longer in time, when compared to a similar cellulose material with a thickness of 2cm. Experimental results have been numerically assessed by considering the material thermal parameters as homogeneous. Both experimental and computational approaches here discussed offer an easy way for the design of micro-encapsulated PCM–cellulose composite as building blocks in cold storage packaging design. [Copyright &y& Elsevier]

In this paper, enhancement for convection heat transfer of turbulent flow in a solar central receiver tube with porous medium and non-uniform circumferential heat flux was numerically investigated. A new method was introduced to build different porous medium configurations in a unified grid system. Four kinds of enhanced receiver tubes (ERTs) with different porous insert configurations were modeled to optimize the performance of ERT. Furthermore, parameters including filling ratio of porous medium, thermal conductivity ratio (thermal conductivity of porous medium versus that of working fluid), porosity and Reynolds number were analyzed. The results showed that ERT partially filled with porous medium has better heat transfer performance than that fully filled with porous medium. The configuration of porous insert for optimal thermal or thermo-hydraulic performance is interactively affected by all the parameters discussed in this paper. The thermal conductivity ratio is the most crucial parameter to the thermal or thermo-hydraulic performance of ERT. The value of thermal conductivity ratio should be greater than 100 to obtain a good thermo-hydraulic performance. The ERTs with horizontal cylindrical segment shaped porous inserts and hollow cylinder shaped porous inserts are proposed because they can obtain optimal thermal or thermo-hydraulic performance. [ABSTRACT FROM AUTHOR]

The two-phase expansion inside the expander could be divided into the non-equilibrium flash of superheated liquid and the expansion of the vapor. A higher flash rate would permit higher recovered expansion work. However, it is almost impossible to measure the flash rate inside the expansion chamber. In this paper, an equivalent expansion method s was proposed to study the non-equilibrium flash process for the two-phase expansion. The flash was controlled inside a constant volume flash chamber, which was connected to a vacuum tank to introduce and adjust the flashed vapor rate out of the flash chamber. The flash process inside the chamber was controlled under the working conditions similar to that inside the two-phase expander. By applying the equivalent expansion method, the flash rate could be measured easily. The experiments were carried out with an initial liquid height inside the chamber from 110 to 175 mm, and an initial liquid temperature from 110 to 130 °C with water as the working fluid. The instant flash rate and the P V diagram of the equivalent two-phase expansion were studied. Results showed that the instant flash rate increased and decreased sharply at first, and then the flash became gentle. Thus, the non-equilibrium flash of the two-phase expansion was divided into two stages of the rapid expansion process and the gradual expansion process. It was found that most of the flashed mass was generated during the rapid expansion process. Finally, a non-equilibrium flash model was proposed to simulate the flash process during the two-phase expansion process with the accuracy of ±3.7% predicting the pressure profile and the accuracy of ±2.7% predicting the expansion work. • Instant flash rate of two-phase expansion is obtained by equivalent expansion method. • The sharp jump stage of the instant flash rate was defined as the rapid flash. • Rapid flash is valuable to power recovery ratio considering the expander design. • The new non-equilibrium flash model predicted the flash process with high accuracy. [ABSTRACT FROM AUTHOR]

Latent heat storage allows efficient energy storage in systems with isothermal processes. The low thermal conductivity of cost-effective storage materials is the main challenge in the development of latent heat storage systems. Most of these systems developed so far use extended heat transfer surfaces to ensure sufficient heat transfer rates. The PCMflux concept described in this paper is based on the transport of the storage material across the heat transfer surface. The aim of this approach is to avoid the blockage of the heat transfer surfaces by solidified storage material. The paper gives an overview of the current development of the PCMflux concept including the theoretical analysis and the experimental proof-of-concept. [ABSTRACT FROM AUTHOR]

The integration temperature difference Δ T i considers the heat transfer routes, linking the heat transfer process with the thermodynamic behavior of heat exchangers. The first and second non-dimensional integration temperature differences are defined as Δ T i, h ∗ = Δ T i / T h, i and Δ T i, s ∗ = Δ T i / ( T h, i - T 0 ) respectively, where T h,i is the heat source temperature and T 0 is the environment temperature. This paper is the first to experimentally verify the significance of the non-dimensional integration temperature differences on organic Rankine cycle (ORC) systems. The first non-dimensional temperature difference is shown to have linear relationship with the revised entropy generation numbers ( N s ). With increases of the second non-dimensional integration temperature difference, the expander powers, system thermal and exergy efficiencies had parabola distributions. They simultaneously reached maximum at Δ T i, s ∗ = 0.282 , under which the vapor cavitation in the expander disappears and the exergy losses of heat exchangers are acceptable to elevate the expander efficiency. Beyond the optimal point, the ORC performance is worsened either due to the vapor cavitation in the expander, or due to the poor thermal matches in the evaporator and condenser. The second non-dimensional integration temperature difference comprehensively reflects the effects of heat source temperatures, heating powers and organic fluid flow rates and pressures, etc. It balances exergy destructions of various components to optimize the system. Thus, it can be an important parameter index to maximize the power or electricity output for a specific heat source. The usefulness of the integration temperature difference and the future work are discussed in the end of this paper. [ABSTRACT FROM AUTHOR]

Phase change materials (PCMs) are frequently and widely used in latent thermal energy storage (LTES) system and thermal management (TM) system due to their large latent heats and capabilities of maintaining nearly constant temperature. However, the performances of PCMs in LTES and TM systems are seriously limited by their low thermal conductivities and poor heat transfer performances, which consequently stimulates intensive experimental and theoretical investigations on improving the thermal conductivities and heat transfer performances of PCMs by using different methods. The present paper reviews the recent progresses of the investigations and applications of the composite PCMs with the enhanced performance. The focuses are placed on the composite PCMs fabricated by using the metal foams and carbon materials, which have been proved to be the most promising approaches for thermal conductivity and heat transfer promotion on PCMs. The fabrication processes of the composite PCMs are first introduced in the present paper followed by the thermal characterization. The measurement of the effective thermal conductivities of the composite PCMs is discussed in detail by comparing different measuring methods, and the theoretical models to predict the effective thermal conductivities of the composite PCMs are also presented. The mathematical models describing the phase change heat transfer characteristics of the composite PCMs which are very important for system modeling and design are also addressed and discussed in the present review. Furthermore, the applications of the composite PCMs to LTES and TM systems are introduced and summarized by illustrating the typical examples. It can be understood that the composite PCMs can effectively improve the performances of LTES and TM systems, which therefore calls for the further investigation in this research field. Finally, the future research topics are suggested. [ABSTRACT FROM AUTHOR]

Current aims regarding environmental protection, like reduction of fossil fuel consumption and greenhouse gas emissions, require the development of new technologies. These new technologies enable the production of renewable energy, which is both cleaner and more abundant in comparison to using fossil fuels for energy production. This necessity encourages researchers to develop new ways to capture solar energy, and if possible, store it for later use. In this paper, the Pavement Solar Collector (PSC), and its use to extract low temperature thermal energy, is studied. Such a system, which harvests energy by flowing water through a heat exchanger embedded in the pavement structure, could have a significant energy output since pavement materials tend to absorb large amounts of solar radiation. The main objective of this paper is to develop a modeling framework for the PSC system and validate it with a self-instructed experiment. Such a model will allow for a detailed parametric study of the system to optimize the design, as well as an investigation on the effect of aging (e.g. decreasing solar absorptivity) on the performance of the system. A long-term energy output of the system that is currently lacking is calculated based on results of the study on weather parameters. This newly acquired data could be the start of a comprehensive data set on the performance of a PSC, which leads to a comprehensive feasibility study of the system. [ABSTRACT FROM AUTHOR]

Implementing heat transfer intensified techniques are now recognised as an efficient retrofit way of improving energy saving in heat exchanger networks (HENs). This not only increases heat recovery, but also prolongs exchanger operating time due to its effect on fouling mitigation. Compared with most of the existing work of HENs based on very simple assumptions for fouling effect, this paper addresses more accurate and complex fouling models reported recently (Yang et al., 2012). Due to the dynamic features of fouling, integration of dynamic equation of fouling rate is used to estimate fouling resistance at different operational times. The novelty of this paper is to present new insights to implementation of heat transfer intensified technologies for HEN retrofitting. It is the first study to implement hiTRAN ® (one commercial tube-insert technology) into heat exchangers to increase HEN heat recovery with the consideration of detailed exchanger performances including heat transfer intensifications, pressure drop constraints, and fouling mitigation. The overall retrofit profit is maximized based on the best trade-off among energy savings, intensification implementation costs, exchanger cleaning costs, and pump power costs. To solve such complex optimization problems, a new mixed-integer linear programming (MILP) model has been developed to consider fouling effects in retrofitting HENs with heat transfer intensification. An efficient iterative optimization approach is then developed to solve the MILP problem. In case studies, the new proposed approach is compared with the existing methods on an industrial scale problem, demonstrating that the new proposed approach is able to obtain more realistic solutions for practical industrial problems. [ABSTRACT FROM AUTHOR]

Radiation is the principal mode of heat transfer in a combustor. This paper presents a refined weighted sum of gray gases model for computational fluid dynamics modelling of conventional air–fuel combustion, which has greater accuracy and completeness than the existing gaseous radiative property models. This paper also presents new conversion-dependent models for particle emissivity and scattering factor, instead of various constant values in literature. The impacts of the refined or new models are demonstrated via computational fluid dynamics simulation of a pulverized coal-fired utility boiler. Although the refined gaseous radiative property model shows great advantages in gaseous fuel combustion modelling, its impacts are largely compromised in pulverized solid fuel combustion, in which particle-radiation interaction plays the dominant role in radiation heat transfer due to high particle loading. Use of conversion-dependent particle emissivity and scattering factor will not only change the particle heating and reaction history, but also alter the radiation intensity and thus temperature profiles in the furnace. For radiation modelling in pulverized fuel combustion, the priority needs to be placed on particle radiation and a proper description of particle emissivity and scattering factor is required. The refined gaseous radiative property model is still recommended for use in generic combustion modelling because of its inherent potential in improving the results, even though its advantages may be compromised by particle radiation in some cases. The gas and particle radiation modelling method and the conclusions presented in this paper are also applied to oxy-fuel combustion of pulverized fuels. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive review of the possible layout configurations of hybrid power plants based on the integration of solid oxide fuel cells (SOFC) and gas turbine (GT) technologies. SOFC/GT power plants have been investigated by using a plurality of approaches, such as: numerical simulations, experimental analyses, and thermo-economic optimizations. The majority of SOFC/GT hybrid systems are fed by methane, which is much cheaper and easier to manage than hydrogen. In fact, SOFC/GT systems use the capability of the fuel cell to internally perform the reforming process required to convert methane into hydrogen. The steam required to drive the reforming reaction can be supplied by the anode recirculated stream. Alternatively, such steam can be produced externally, by using the heat of the exhaust gases. In this case, steam can be used also for thermal purposes and/or for further system hybridization (e.g. Cheng cycle). The majority of the SOFC/GT power plants analyzed in literature are based on the pressurized arrangement, potentially able to ensure lower capital costs and higher efficiencies. Conversely, atmospheric plants are easier to manage, due to the possibility of operate the SOFC and the GT independently one of each other. The paper also investigates more complex SOFC/GT configurations, including: HAT turbines, IGCC SOFC/GT power plants, ORC cycles, etc. A detailed analysis of the SOFC/GT control strategies and part-load performance analyses is also presented, showing that such systems reach their best performance at nominal capacity, and are affected by significant reduction of the electrical efficiency in case of large variations of the load. Finally, the paper also presents a review of hybrid SOFC/GT power plants fed by alternative fuels, such as coal and biomass. [ABSTRACT FROM AUTHOR]

Ground (or geothermal) heat exchangers are attracting a great deal of attention as a way of using shallow geothermal energy. This paper provides not only a critical review but also a thorough introduction to the analysis of heat transfer by borehole and foundation pile ground heat exchangers, with an emphasis on different analytical models. The literature is reviewed in a time-scale framework because of the diversity of the time and space scales involved in the thermal processes of ground heat exchangers. We summarize, discuss, and evaluate major advances in this field, including heat-source models, short-time models, models for energy piles, in situ thermal-response tests, indoor sandbox experiments, and parameter estimation as an inverse problem. Of particular note is that the unit-step temperature response (i.e., G -function) of a ground heat exchanger with one U-shaped pipe is calculated; and six analytical models are compared: an infinite cylinder-source model, two infinite line-source models, two finite line-source models, and a composite-medium line-source model. This paper closes by identifying several unsolved problems that require solutions. [ABSTRACT FROM AUTHOR]

Parabolic trough solar collector (PTC) is one of the solar thermal energy applications, which focuses sunlight to heat the heat transfer fluid (HTF) circulating through the receiver. Researchers have put forward several heat transfer models for PTCs, basing on laws of conservation. This paper summarized the one-dimensional (1-D) mathematical models under different assumptions and details for PTCs. All the heat transfer processes were considered: convection within the absorber, in the annulus and between the glass and ambient; conduction through glass cover, absorber and support brackets; radiation in the annulus and from the glass to the sky. Moreover, a simple algorithm was adopted to make the control equations linear and solve easily. The difference in accuracy for diverse 1-D models were presented and analyzed on the basis of the experimental data from Sandia National Laboratories. The 1-D models with various details were different in accuracy and complexity. It can choose an appropriate 1-D model under a certain condition, which can be used in investigating the thermal performance of a PTC. In addition, the most accurate 1-D model presented in this paper were precise enough compared with the three-dimensional (3-D) model from other paper. The average difference of outlet temperature between the most accurate simulation in this paper and test data was 0.65 °C, however it was 2.69 °C between the 3-D model and experiment results. The reason may be that there were more assumptions for 3-D models than 1-D models, therefore the error was bigger. [ABSTRACT FROM AUTHOR]

Fuel cells offer many benefits for residential micro-cogeneration because of their high electrical efficiency, low emissions, and low heat-to-power ratio. However, the current design of fuel cell micro combined heat and power (CHP) systems for dwellings can be improved by better matching of the heat generated by the plant with the dwelling’s load profile and heat emitters. This paper explores the design options in building services using fuel cell micro-CHP and aims to bring the two research fields together in order to design superior integrated systems. Therefore a review of current literature on the field is performed and current available options for heat recovery from fuel cells, including the grades of heat available from different fuel cell process units are discussed. This is discussed in relation to the heat demand in a dwelling and the required temperature of the most suitable heat emitters for building services design. The paper motivates a methodology that considers all possible interconnections of plant in the design of fuel cell micro-CHP in dwellings and provides an example of how multi-objective optimisation can be used to identify promising system designs. [ABSTRACT FROM AUTHOR]

Latent heat thermal energy storage (LHTES) based on phase change material (PCMs) is an interesting solution to be used for mitigating the mismatch between energy demand and supply that affects various kinds energy systems. The advance of LHTES technology requires to overcome the limitations posed by the poor thermal conductivity of most of the PCMs. Thus, a large scale utilization of LHTES system strongly depends on the development of efficient heat transfer enhancement techniques. In this paper the use of tree shaped fins is proposed to enhance the performance of a shell-and-tube LHTES unit. The study is performed through a numerical model that takes into account of the thermal behaviour of the system. The geometry of Y-shaped fins with one and two bifurcations are optimized through the combined use of CFD modelling and response surface method. The results show that an increase of 24% of the system efficiency can be achieved by the optimized unit. Another relevant result arises from the analysis: the optimal fin design depends on the operating time of the LHTES unit. For short operating times Y-shaped fins with wide angles between branches are preferable. On the contrary, smaller angles are necessary for long operating times. To summarize, this work presents for the first time a shape optimization strategy for the improvement of a system involving transient operating conditions. This is a key feature of the paper and an innovative effort to tackle optimization problems involving energy systems that show unsteady state behaviour. [ABSTRACT FROM AUTHOR]

In this paper, the intermittent experiment of a vertical ground source heat pump (GSHP) system is investigated and the corresponding geo-temperature variations are studied. The performance of the GSHP system under intermittent operation and the comparisons of different intermittent modes are presented in the paper. The parameters of soil backfill material, air temperature and inlet volume flow rate are also investigated. Experimental results suggest that, due to the recovery in ground thermal energy in intermittent time, the heat exchange rate and the operation performance coefficient (COP) of the heat pump increases, and the compressor power decreases in the successive working. But an insufficient soil recovery time leads to a rapid decline of the performance parameters and the soil temperature. The temperature transports faster under large soil thermal conductivity conditions and the soil temperature decreases more quickly and recovers more slowly with larger inlet flow rate and lower weather temperature for different soil thermal diffusivities. Through multiple nonlinear regression analysis, a curve formula can be fitted to predict the soil temperature variations under intermittent operation of the ground source heat pump in winter, Dalian. It can be found that the soil temperature increases at an exponential function with each factor. [ABSTRACT FROM AUTHOR]

In future smart thermal grids, decentralized prosumers shall supply each other over the network, optimized by an overarching management system. For this, the substation control must be able to directly process optimal power flow setpoints from the management, while considering the mutual influence of prosumers on the technical level. Addressing these requirements, this paper develops a control approach for bidirectional prosumer substations. Therefore, we identify the objectives and characteristics of the control problem, before relaxing it by reformulation. This enables to keep proportional–integral–derivative controllers (PIDs) for operating the pumps and valves as decentral actuators in the substations. We propose a control approach, exploring the combination of two key aspects: (1) The assignment of temperature objectives to actuators. (2) Weighted error functions that linearly combine temperature and power errors as input for the PIDs, allowing to handle multiple objectives with limited actuators. Through case studies, we validate the suitability of our proposed objective–actuator assignment and demonstrate the necessity and benefits of the weighted error functions. Like this, we conclude with a control approach for bidirectional prosumer substations that aims for the relevant temperature objectives, directly considers heat transfer setpoints, replicates and exploits the coupling within the network, effectively manages conflicting objectives, copes with prosumer interference, and is pragmatic for real-time operation. Our approach can be transferred to different network concepts, bridging the gap between decentralized field-level operation and the management level. • Develop control approach for bidirectional substations in thermal prosumer networks. • Propose weighted error function approach in combination with PID controllers. • Study suitability of different objective-to-actuator assignments for substations. • Bridge management and field level by processing power setpoints in substation. • Anticipate mutual influence of prosumers over the network on the technical level. [ABSTRACT FROM AUTHOR]

The quantification of the flow irreversibilities is crucial to developing future turbomachinery. Traditionally, design correlations and efficiencies are based on comparisons to isentropic relations to quantify the deviation from an "ideal" case. However, isentropic relations typically assume one-dimensional adiabatic flow, representing a significant departure from the actual situation in turbines operating with large levels of heat transfer. In this paper, the aerothermal losses and power generation in reversible processes are derived for three-dimensional compressible flow considering heat transfer effects. A new definition of the power potential in a reversible process is proposed that can be used to perform detailed budgeting of the losses. The proposed new loss framework can be adopted locally, which enables the precise identification of the loss source. New aerodynamic and aerothermal efficiency expressions are proposed to assess shaft power extraction in combination with heat exchange. The new method enables designers to identify the regions with high loss generation in steady and also in unsteady flow conditions, offering new avenues to evaluate the loss generation mechanisms during the design phase. Therefore, our new tool is essential to developing reduced-order models for the design of future fluid machinery. • Pioneering method for time-resolved loss production in compressible fluid machinery. • A novel approach based on volume integral compliant with heat flux. • First demonstration in transonic turbines with endwall heat flux. • Implementation suitable for experimental control volume assessment. • Proposed new correlations on thermal loss generation. [ABSTRACT FROM AUTHOR]

Wave energy resources have high, yet largely untapped potential as candidate generation technology. In this paper, we perform a data-driven analysis to characterize the impact of wave energy integration on bulk-scale power systems and market operations. Through data-driven sensitivity studies centered on an optimization-based production cost modeling formulation, our work characterizes the inflection point beyond which wave integration starts impacting power system operations, considering present day transmission infrastructure. Furthermore, our analysis also considers the joint effects of wave energy integration and system-wide transmission expansion. Finally, potential resilience scenarios such as wildfire-driven transmission contingencies and heat wave events are investigated, whereby the contributions of grid-integrated wave energy in alleviating the effects of the resilience events are analyzed. As our demonstration test bed, we consider a reduced-order network topology for the U.S. Western Interconnection with wave energy generation integrated at carefully selected sites across the coastal areas of Washington, Oregon, and northern California. Our results indicate that over a representative year of operations, wave energy integration systematically reduces locational marginal prices (LMPs) of energy and price volatility, especially during periods of high wave resource availability (winter months for the U.S. west coast). Average, maximum, and minimum of hourly LMPs over a typical year of operation was reduced by 2.95, 51.28, and 1.13 $/MWh respectively (over a baseline scenario with no wave energy integration), when the selected network model had a total of 5000 MW wave power installed capacity during the representative year of study. The effects of wave energy integration can remain localized with existing transmission infrastructure (identified to be most pronounced in the Pacific Northwest region in the example we studied). However, with concurrent transmission expansion, the impacts of wave energy integration are likely to have a higher geographical spread. Our results also indicate that wave energy may be able to assist power system operations during resilience events such as major transmission contingencies and heat wave events, although such benefits might be dependent on factors such as proximity of affected area to wave resources, availability of adequate resource potential and adequate transmission capacity. • Production cost model of a bulk power system with wave power is developed. • Sensitivity studies to infer impact of wave power on grid operations at varying capacities. • Wave power reduces electricity prices during periods of higher resource availability. • Wave power benefits can percolate inland with system-wide transmission expansion. • Wave power can have added benefits in resilience events like transmission outages. [ABSTRACT FROM AUTHOR]

The effective thermal conductivity (ETC) of composites is of great significance for simulating the macroscopic heat transfer process. In this paper, an extension of the morphology efficiency is proposed, which quantifies the influence of morphology on composite ETC, and is initially derived from two extreme models-series and parallel ETC bounds. The morphology efficiency obtained from different bound models are compared. It concluded that the morphology efficiency derived from Hashin and Shtrikman bound models is the best representative of the morphology of isotropic composite materials, which can conveniently reveal the optimal volume fraction of inclusion phase and thermal conductivity of the ideal thermal response. For composites with inverse Simple Cubic, Body Centered Cubic, Face-Centered Cubic and Kelvin lattice, the corresponding prediction model of ETC is proposed in a wide range of porosity and thermal conductivity. Among them, the ETC model of Kelvin cell and inverse Simple Cubic cell can well predict the high porosity foam and foams filled with phases with thermal conductivity ratio r in the range of 3 to 8, respectively. [Display omitted] • The best morphology factor is derived from Hashin–Shtrikman bounds. • The models for effective thermal conductivity (ETC) of regular lattices are proposed. • The ETC model of Kelvin cell can well predict the ETC of the high porosity foam. • The ETC model of SC cell can well predict the ETC of Al−Sn and Fe−Cu alloys. [ABSTRACT FROM AUTHOR]

Abstract: The Non-Inertive-Feedback Thermofluidic Engine (NIFTE) is a two-phase thermofluidic oscillator which, by means of persistent periodic thermal-fluid oscillations when placed across a steady temperature difference, is capable of utilising low-grade (i.e., low temperature) heat to induce a fluid motion. Two linearised models of the NIFTE are presented in this paper, both containing a description of the phase-change convective heat transfer that takes place between the working fluid and the heat exchangers. The first model (LTP) imposes a steady linear temperature profile along the surface of the heat exchangers; whereas the second model (DHX) allows the solid heat exchanger blocks to store and release heat dynamically as they interact thermally with the working fluid. In earlier work [Solanki R, Galindo A, Markides CN. Appl Therm Eng; [24]] it was found that these models predict the oscillation (i.e., operation) frequency of an existing NIFTE prototype pump well, but significantly overestimate its reported efficiency. Specifically, the LTP and DHX models predicted exergetic efficiencies 11 and 30 times higher than those observed experimentally, respectively. In the present paper, a dissipative thermal loss parameter that can account for the exergetic losses due to the parasitic, cyclic phase change and heat exchange within the device is included in both models in an effort to make realistic predictions of the exergetic efficiencies. The LTP and DHX models, including and excluding the thermal loss parameter, are compared to experimental data. It is found that the inclusion of the thermal loss parameter increases the predicted oscillation frequencies in the DHX model, but has a negligible effect on the frequencies predicted by the LTP model. A more significant effect is observed with respect to the efficiencies, whereby the inclusion of the thermal loss parameter leads to a greatly improved prediction of the exergetic efficiencies of the prototype NIFTE pump by both the LTP and DHX models, both in trend and approximate magnitude. From the results it is concluded that, on accounting for thermal losses, the DHX model achieves the best predictions of the key performance indicators of the NIFTE, that is, of the oscillation frequency and exergetic efficiency of the device. [Copyright &y& Elsevier]

Abstract: This paper introduced the concept, potential application and benefits relating to a novel solar photovoltaic/loop-heat-pipe (PV/LHP) heat pump system for hot water generation. On this basis, the paper reported the process and results of characterizing the performance of such a system, which was undertaken through dedicated thermo-fluid and energy balance analyses, computer model development and operation, and experimental verification and modification. The fundamental heat transfer, fluid flow and photovoltaic governing equations were applied to characterize the energy conversion and transfer processes occurring in each part and whole system layout; while the energy balance approach was utilized to enable inter-connection and resolution of the grouped equations. As a result, a dedicated computer model was developed and used to calculate the operational parameters, optimise the geometrical configurations and sizes, and recommend the appropriate operational condition relating to the system. Further, an experimental rig was constructed and utilized to acquire the relevant measurement data that thus enabled the parallel comparison between the simulation and experiment. It is concluded that the testing and modelling results are in good agreement, indicating that the model has the reasonable accuracy in predicting the system’s performance. Under the given experimental conditions, the electrical, thermal and overall efficiency of the PV/LHP module were around 10%, 40% and 50% respectively; whilst the system’s overall performance coefficient (COPPV/T ) was 8.7. Impact of the operational parameters (i.e. solar radiation, air temperature, air velocity, heat-pump’s evaporation temperature, glazing covers, and number of the absorbing heat pipes) to the performance of the system (in terms of efficiencies of the PV/LHP module and the system’s overall performance coefficient COPPV/T ) was investigated individually. The results indicated that lower solar radiation, lower air temperature, higher air velocity and smaller cover number led to enhanced electrical efficiency but reduced thermal efficiency of the module; whereas lower heat-pump’s evaporation temperature and larger number of heat absorbing pipes gave rise to both thermal and electrical efficiencies of the module. The research results would assist in developing a high efficient solar (space or hot water) heating system and thus contribute to realisation of the energy saving and associated carbon emission targets set for buildings globally. [Copyright &y& Elsevier]

Abstract: The closed greenhouse concept has been studied in this paper. The closed greenhouse can be considered as the largest commercial solar building. In principle, it is designed to maximize the utilization of solar energy by use of seasonal storage. In an ideal fully closed greenhouse, there is no ventilation window. Therefore, the excess heat must be removed by other means. In order to utilize the excess heat at a later time, long- and/or short-term thermal storage technology (TES) should be integrated. A theoretical model has been derived to evaluate the performance of various design scenarios. The closed greenhouse is compared with a conventional greenhouse using a case study to guide the energy analysis and verify the model. A new parameter has been defined in this paper in order to compare the performance of the closed greenhouse concept in different configurations – the Surplus Energy Ratio showing the available excess thermal energy that can be stored in the TES system and the annual heating demand of the greenhouse. From the energy analysis it can be concluded that SER is about three in the ideal fully closed greenhouse. Also, there is a large difference in heating demand between the ideal closed and conventional greenhouse configurations Finally, a preliminary thermo-economic study has been assessed in order to investigate the cost feasibility of various closed greenhouse configurations, like ideal closed; semi closed and partly closed conditions. Here, it was found that the design load has the main impact on the payback period. In the case of the base load being chosen as the design load, the payback period for the ideal closed greenhouse might be reduced by 50%. On the other hand, glazing type, ventilation ratio, and the closed area portion have a minor impact on the payback period. [Copyright &y& Elsevier]

Abstract: In certain building applications, the PV installation is extended to cover also south- or west-facing walls, taking care to circulate cooling air to the back of the panels. The cooling effect maintains a high conversion efficiency of the PV panels and the heated air may be exploited by the HVAC or service water heating system. Sizing and design of the double façade system is critical to its energetic performance. In this paper, the results from flow visualization and hot wire anemometry measurements performed on the basic structural module of a double-skin photovoltaic (PV/T) façade are discussed. The concept and its feasibility have been presented in a previous paper. The results of transient outdoor measurements with the testing device have been reported elsewhere. The air flow and turbulence field inside the cavity is analyzed in the present paper by means of indoor measurements with the testing device. The results are combined with CFD computations to calculate heat transfer coefficients and improve our understanding and modeling of the specific PV/T concept. [Copyright &y& Elsevier]

Abstract: Combustion in fired heaters, boilers and furnaces often accounts for the major energy consumption on industrial processes. Small improvements in efficiency can result in large reductions in energy consumption, CO2 emissions, and operating costs. This paper will describe some useful low cost modelling techniques based on the zone method to help identify energy saving opportunities on high temperature fuel-fired process plant. The zone method has for many decades, been successfully applied to small batch furnaces through to large steel-reheating furnaces, glass tanks, boilers and fired heaters on petrochemical plant. Zone models can simulate both steady-state furnace operation and more complex transient operation typical of a production environment. These models can be used to predict thermal efficiency and performance, and more importantly, to assist in identifying and predicting energy saving opportunities from such measures as: [•] Improving air/fuel ratio and temperature controls. [•] Improved insulation. [•] Use of oxygen or oxygen enrichment. [•] Air preheating via flue gas heat recovery. [•] Modification to furnace geometry and hearth loading. There is also increasing interest in the application of refractory coatings for increasing surface radiation in fired plant. All of the techniques can yield savings ranging from a few percent upwards and can deliver rapid financial payback, but their evaluation often requires robust and reliable models in order to increase confidence in making financial investment decisions. This paper gives examples of how low cost modelling techniques can be applied to improve confidence in implementing energy efficiency improvements whilst safeguarding manufacturing output and quality. [Copyright &y& Elsevier]

• Reviews the benefits of dimpled surfaces in energy related applications. • Discusses mechanisms of flow structure and enhancement. • Provides pros and cons of dimpled surfaces in thermal energy systems. • Presents suggestions for applications and future research. Recently, dimpled surfaces have found great attention due to their abilities in energy management and heat transfer enhancement, low weight, small values of pressure drop penalty, simple fabrication, and small maintenance costs. Many experimental and numerical studies are accomplished to investigate the potentials of dimpled surface technology in energy management of various thermal energy systems. This paper presents a comprehensive review about the developments and current status of this technology employed in different thermal energy systems including heat exchangers, mini/micro energy systems, two-phase flow energy systems, jet impingement cooling systems, and solar thermal energy systems. Moreover, comprehensive discussions concerning the flow structures and heat transfer behaviours in dimpled surfaces and different geometries and arrangements of dimples are provided. Finally, based on current status of studies in this field, some suggestions are suggested for future researches. The results showed that the main reasons for the convective heat transfer improvement achieved by dimples are flow reattachment, flow impingement, and upwash flow at the downstream region of the dimples, while the heat transfer can be reduced due to flow separation and recirculation in the upstream region of the dimples. [ABSTRACT FROM AUTHOR]

• A PVT collector with a roll-bond absorber is designed for rural home application. • Thermo-laminating is a novel and convenient method for PV-absorber connection. • The validated PVT model can simulate the roll-bond PVT performance accurately. • The model is available for predicting the dynamic performance of PVT systems. This paper mainly investigated the performance of a low-cost roll-bond water-type photovoltaic thermal (PVT) collector from both experimental and modelling approaches under the climatic conditions of Sichuan Basin of China. The design and manufacturing process of the PVT prototype adopted the available commercial PV panels for convenience and for reducing the manufacturing cost. The experiment measured all the main properties of the developed PVT module. The excellent heat transfer performance of its roll-bond absorber plate was also validated by outdoor steady-state tests. Then, the steady-state numerical models were used to simulate the performance of the PVT prototype in TRNSYS. By comparing the simulation and experimental results, two robust models were selected which have their own advantages in charactering the thermal and electrical properties of the PVT collector. Through validating the numerical models with experimental data, the deviation between the simulated and the measured values was further reduced. In addition, a complete photovoltaic thermal–domestic hot water (PVT-DHW) system with such PVT collectors was developed in TRNSYS, by which the annual system output and efficiency were predicted and analysed. [ABSTRACT FROM AUTHOR]

• An accurate bilinear cavern model for compressed air energy storage is deduced. • The model represents the thermodynamic behavior of cavern air in three processes. • The heat transfer between cavern air and cavern wall is considered in the model. • An existing model and field measured data are used to verify the model accuracy. • Superiority of using the model in optimization problems is verified via comparison. Compressed air energy storage is suitable for large-scale electrical energy storage, which is important for integrating renewable energy sources into electric power systems. A typical compressed air energy storage plant consists of compressors, expanders, caverns, and a motor/generator set. Current cavern models used for compressed air energy storage are either accurate but highly nonlinear or linear but inaccurate. The application of highly nonlinear cavern models in power system optimization problems renders them computationally challenging to solve. In this regard, an accurate bilinear cavern model for compressed air energy storage is proposed in this paper. The charging and discharging processes in a cavern are divided into several real/virtual states. The first law of thermodynamics and ideal gas law are then utilized to derive a cavern model, i.e., a model for the variation of temperature and pressure in these processes. Thereafter, the heat transfer between the air in the cavern and the cavern wall is considered and integrated into the cavern model. By subsequently eliminating several negligible terms, the cavern model reduces to a bilinear model. The accuracy of the bilinear cavern model is verified via comparison with both an accurate nonlinear model and two sets of field-measured data. The bilinear cavern model can be easily linearized and is then suitable for integration into optimization problems considering compressed air energy storage. This is verified via comparatively solving a self-scheduling problem of compressed air energy storage using different cavern models. [ABSTRACT FROM AUTHOR]

• A design tool for energy conversion and noise characterization in outdoor ducts. • 2-D modeling and analysis for an outdoor duct exposed to solar radiation. • Definition, expression and unit for different types of noise due to various power systems in an outdoor duct. • Validation of design tool with experimental results. • Noise calculation examples and charts for various types of noise. The aim of this paper is to present a design tool with its guiding principles for energy conversion and noise characterization of an exterior rectangular duct often installed on building envelopes for pre-conditioning of fresh air. The energy conversion in an exterior rectangular duct is a function of solar irradiation, air gap width, mass flow rate and pressure, wall and air temperatures. A generalized two dimensional thermal analysis of an outdoor duct is presented by placement of surface and air nodes into two adjacent stacks of control volumes representing outer and inner walls of duct. A matrix solution procedure is adopted by constituting conjugate heat exchange of conduction, convection, radiation and ventilation heat transport. The rectangular duct model is built by a metallic exterior wall exposed to a steady heat flux generation due to solar heat gain and a well-insulated back panel as its counterpart wall. The model improves the results of traditional thermal models for the cases where: (i) there are sections of conjugate heat exchange; and (ii) stack effect due to thermal buoyancy necessitates 2-D nodal analysis for distant composite nodes. The design tool is supported with some numerical and experimental results of an airflow window with a photovoltaic (PV) solar wall installed in an outdoor test-room. Furthermore, some examples of noise characterization calculations are illustrated using devised noise measurement equations. [ABSTRACT FROM AUTHOR]

Highlights • 3 new finned copper foams are experimentally evaluated for heat dissipation. • Effects of copper foam porosity, height, and impinging gas flow rate are studied. • Finned copper foam is superior to finned heat sinks in thermal performance. • Finned copper foam can potentially replace finned heat sinks in narrow spaces. Abstract The rapid development of electronic devices has made it necessary to develop novel and innovative thermal management solutions. This paper experimentally investigated the heat transfer and flow characteristics of three new finned copper foam heat sinks subjected to the impingement cooling by rectangular slot jet and axial fan. The effects of heat sink height (H, 15, 30, 45, 60 mm), the pore density of the inserted copper foam (PPI, pore per inch including 10, 20, and 30) and the gas flow Reynolds number (Re , varying from 2053 to 12737), were systematically investigated. Two kinds of conventional finned heat sinks, with 8 and 22 fins but without copper foam, were also tested for comparison. Experimental results reveal that inserting copper foam positively improves the thermal performance of finned heat sinks subjected to jet impingement. In addition, the thermal performance of finned copper foams with 20 PPI and 30 PPI even exceeds that of a conventional finned heat sink with 22 fins at a low height such as 15 mm, showing a great potential to replace traditional finned heat sinks. However, inserting metal foams leads to a much larger pressure drop than those of conventional finned heat sinks. From this work, finned copper foams are characterized by a better heat transfer performance than a conventional heat sink with the same number of fins. Even with increased flow resistance, finned copper foam heat sinks still have application prospects in some limited and narrow spaces where pump power consumption is not the dominant consideration. [ABSTRACT FROM AUTHOR]

• A new attempt combining MPCM slurry and tree-shaped GHE was proposed. • Thermo-fluidic characteristics of MPCM slurry were investigated. • Comparisons between MPCM and water flowing through different tubes were conducted. • Influences of volume fractions on thermos-fluidic performance are studied. Taking into account the potential of the tree-shaped ground heat exchanger to reduce pressure losses and microencapsulated phase change material slurry to enhance heat transfer performance, a new attempt combining microencapsulated phase change material slurry with tree-shaped structure is proposed in this paper. Firstly, to obtain the optimal structure of tree-shaped ground heat exchanger, influences of structure parameters on thermo-fluidic performance of ground heat exchanger are explored. Based on this optimal structure, numerical simulations adopting Eulerian-Eulerian approach are conducted to study hydraulic and heat transfer performance of microencapsulated phase change material slurry flowing through horizontal tree-shaped structure under constant flux. Comparison studies between different working fluid: water and microencapsulated phase change material slurry and different types of tube: horizontal straight tube and Y tube (tree-shaped tube) are studied. The results indicate that the numerical results accord well with experimental results. For tree-shaped ground heat exchanger with bifurcation level of 1, the optimal pipe diameter ratio meets D 0 / D 1 = 2 3 / 7 , D 1 / D 2 = 1 and the optimal length ratio is L 0 / L 1 = 1. Furthermore, the combination of microencapsulated phase change material slurry and tree-shaped structure can efficiently enhance thermal performance and reduce pressure losses. Considering comprehensive performance of microencapsulated phase change material slurry, the optimal volume fraction is 12% for Y tube, whose overall performance is 38.9% higher than that of pure water flowing through straight tube. [ABSTRACT FROM AUTHOR]

• Sulfur is a promising storage medium for low-cost and high-temperature TES. • Experiments demonstrate the characteristics of sulfur heat transfer behavior. • Validated CFD model elucidates thermal charge/discharge performance of sulfur. • Empirical correlations of Nusselt number is developed. • Comparison diagram for vertical and horizontal tube configurations is developed. Elemental sulfur is a promising medium for moderate to high-temperature thermal energy storage (TES) systems due to its low cost and excellent chemical stability up to very high temperatures (1200 °C). Previous studies show that vertically-oriented tubes of isochorically contained thermal storage media (i.e., supercritical CO 2) can exhibit higher heat transfer rates than horizontal tubes. Storing thermal storage media in vertical tubes in a TES system also has some potential system-level advantages related to exergy capacity, operation and maintenance, and cost. This paper investigates the heat transfer behavior and performance of sulfur contained in vertically-oriented tubes between room temperature (25 °C) and 600 °C. Experimental and computational analyses show that the natural convection heat transfer behavior for sulfur in a vertically-oriented tube is strongly dependent on the sulfur viscosity, which varies greatly over the range of temperatures used in this study. Validated Nusselt number correlations for vertical tubes of lengths between 0.5 and 3 m and diameters between 5.5 and 21.2 cm are developed for use in parametric studies and designs. In comparison to the horizontally-oriented tube, the vertical tube can have better heat transfer performance with some ranges of tube length and diameter. Therefore, the selection of the tube orientation strongly depends on the tube dimensions and application needs. The results from the current study provide important quantitative and qualitative design bases for sulfur-based TES (SulfurTES) systems. [ABSTRACT FROM AUTHOR]

Highlights • Correlations for mass transfer performance were comprehensively reviewed. • Structure improvements are effective way to enhance system performance. • Metal corrosion is a serious issue needing to be overcome for LDCS. • Surface treatment and new technology are promising to improve performance. • Solution modification by surfactant and nanoparticle needs more study. Abstract Liquid desiccant cooling systems are considered a promising technology for accurate humidity control and high energy efficiency. The dehumidifier and the regenerator are the two main components in the system, and their performance directly determines the system performance. This paper is a comprehensive review of the empirical correlations for the determination of mass transfer coefficient and moisture effectiveness in both adiabatic and internal cooling/heating dehumidifier/regenerators, and it further discusses approaches to enhance their mass transfer performance. These methods include structural improvements, such as structural modification, ultrasound atomisation and membrane-based modules, and modification of liquid desiccants, such as the addition of surfactants and nanoparticles. Finally, a brief summary and some suggestions for future work are outlined and addressed. [ABSTRACT FROM AUTHOR]

Highlights • A detailed introduction and analysis of radiative cooling was reviewed. • The mathematical description of radiative cooling was reviewed and discussed. • The emitters' materials and radiative properties were compiled and analyzed. • The potential applications of radiative cooling were excavated. • Several recommendations on radiative cooling were presented. Abstract As a passive, effective, and renewable way of decreasing cooling energy requirements without power input, radiative cooling has attracted considerable attention in the field of energy-saving applications. Historically, radiative cooling was limited at nighttime because radiators with strong thermal radiation lack high reflectivity in the solar radiation band. With the recent technological advancements in radiators, such as the development of photonic radiators and metamaterials, the advantages of diurnal radiative cooling has been demonstrated. In this paper, the current state of the art in passive radiative cooling technology is reviewed and updated. First, the fundamental principles of radiative cooling, which comprise different mathematical and physical descriptions, are introduced. Then, the advanced materials and structures of various radiators, which are popular topics in radiative cooling, are presented. Furthermore, application developments in radiative cooling are also summarized and its prospects are preliminarily analyzed. This study provides a detailed introduction and analysis of radiative cooling technology, thereby serving as a key reference for promoting the development of radiative cooling utilization. [ABSTRACT FROM AUTHOR]

Highlights • A 3-D FEM and CFD model to simulate a hybrid PV-Thermal collector (PVT) is developed. • Several flat-box designs and materials for the absorber-exchanger are studied. • Flow-channel size or material seem not critical within the range of investigation. • Polymeric flat-box designs are expected to suffer lower strains than sheet and tube units. • Weight and investment cost reductions are achieved with polymeric PVT designs. Abstract In this paper, 26 alternative absorber-exchanger designs for hybrid PV-Thermal (PVT) solar collectors are proposed and compared against a reference-case, commercial sheet-and-tube PVT collector. The collectors involve different geometric design features based on the conventional sheet-and-tube configuration, and also on a newer flat-box structure constructed from alternative polymeric materials with the aim of maintaining or even improving heat transfer and overall (thermal and electrical) performance while achieving reductions in the overall weight and cost of the collectors. The main contributions of this research include: (i) the development and validation of a detailed 3-D computational finite-element model of the proposed PVT collector designs involving multi-physics processes (heat transfer, fluid dynamics and solid mechanics); (ii) results from comparative techno-economic analyses of the proposed PVT designs; and, (iii) further insights from thermal stress and structural deformation analyses of the proposed collectors, which are crucial for ensuring long lifetimes and especially important in the case of polymeric collectors. The results show that, in general, the flat-box designs (characterised by a thin absorber plate) are not sensitive to the flow-channel size or construction material, at least within the range of investigation. A PVT collector featuring a polycarbonate (PC) flat-box design with 3 × 2 mm rectangular channels appears to be a particularly promising alternative to commercial PVT collectors, achieving a slightly improved thermal performance compared to the reference case (with a 4% higher optical efficiency and 15% lower linear heat-loss coefficient), while also lowering the weight (by around 9%) and investment cost (by about 21%) of the collector. The structural analysis shows that the maximum von Mises stress experienced in the absorber-exchanger of the PC flat-box collector is considerably lower than that in the copper sheet-and-tube collector (<13% vs. 64% of the material's yield stress), which is attributed to the larger thermal expansion of the PC absorber-exchanger, that leads to lower stresses. Therefore, the proposed PC flat-box design is not expected to suffer higher strains than the commercially-available PVT collector. [ABSTRACT FROM AUTHOR]

Highlights • A novel heat pump water heater with a freezing latent heat evaporator was proposed. • The energy recovery of the novel system was 4.4 times that of the conventional one. • The novel system could save 60% of energy compared with the electrical water heater. • The heat transfer coefficient increased significantly at the moment of freezing. Abstract The temperature of domestic drain water is relatively high, but its flux is sometimes insufficient as a heating source. Thus, the conventional wastewater source heat pump water heater is incapable of meeting user heating demand given the limited sensible heat temperature difference in many situations. To address this problem, a novel in–house heat pump water heater with a freezing latent heat evaporator and assisted by domestic drain water was proposed. A prototype of the system was first constructed to determine the performance of the system. An experiment was then conducted, in which a parallel helical coil tube evaporator was designed and developed to extract freezing latent heat from domestic drain water. Experimental data was obtained in different experimental conditions. Results of the investigation show that the heat pump water heater with a freezing latent evaporator recovers 3.4 times more energy than the conventional wastewater source heat pump water heater, and saves 60% energy compared with the traditional electric water heater. Furthermore, an interesting result is that the heat transfer coefficient increased significantly at the moment the wastewater was just frozen. This paper provides a reference for the wastewater source heat pump water heater to extract freezing latent heat from domestic drain water. [ABSTRACT FROM AUTHOR]

Highlights • Estimation of U -value by means of artificial neural network. • The model developed does not need a data post-processing. • Representative results for all the building periods considered. Abstract Most of the existing building stock has a deficient energy behaviour. The thermal transmittance of façades is among those aspects which most affect this situation. In this paper, the calculation procedure with correction for storage effects from ISO 9869-1 was applied to the thermometric method to determine the U -value. Due to the need for determining the number and type of layers that compose the wall to apply the calculation, a multilayer perceptron has been developed to estimate the U -value. From the different model configurations suggested, the most adequate architecture was the one with 14 nodes in the hidden layer without making transformations in the input variables. Valid results have been obtained by the multilayer perceptron for the case studies analysed from different building periods, with deviations lower than 20% between the measured value and the expected one, varying the test duration according to the thermal resistance of the wall and the temperature variations. Furthermore, it is not necessary to carry out a data post-processing for the model, so this fact simplifies and hastens the calculation procedure. [ABSTRACT FROM AUTHOR]

Highlights • Thermochemical energy storage experiment is reported using focused solar simulator. • Local and integral energy transport and storage performance are both revealed. • Maximum reaction rate appears before focal point and reverse reaction is near the end. • Thermochemical efficiency has maximum with proper bed length and porosity of 0.45. • Insulation and coating improve thermochemical energy storage efficiency up to 34.8%. Abstract Steam methane reforming is suitable for thermochemical energy storage because of its large reaction enthalpy and high hydrogen content in reaction products. In this paper, heat transfer and storage performance of steam methane reforming in a tubular reactor heated by focused solar simulator is experimental demonstrated and numerically analyzed. According to experimental results, methane conversion remarkably decreases with inlet flow rate rising, while thermochemical energy storage efficiency first increases for more reactants and then decreases with methane conversion dropping. As incident energy flux rises, methane conversion increases with bed temperature rising, and the thermochemical energy storage efficiency reaches its maximum of 11.3% with central heat flux of 285.6 kW/m2. Three-dimensional transport and volumetric reaction model with concentrated energy flux boundary condition is established and validated, and local and integral energy transport and storage performance are further analyzed. Along flow direction, the maximum reaction rate appears before the focal point with maximum energy flux. The tendencies of methane conversion and thermochemical energy storage efficiency are very similar under different inlet conditions, and higher inlet temperature and appropriate steam to methane ratio benefit thermochemical energy storage. The structures of catalyst bed and reactor are critical important for thermochemical energy storage process. As bed length increases, the methane conversion and thermochemical energy storage efficiency first increase with the increase of positive reaction and then decrease with the increase of reverse reaction, and the optimal length is a little larger than focal spot diameter. When bed porosity is increased, the methane conversion and thermochemical energy storage efficiency first increases with the flow resistance decreasing and then decreases with catalyst amount decreasing, and optimal porosity is 0.45. Heat loss in heating side of bed region play major role in heat storage, and the thermochemical energy storage efficiency can be improved to 34.8% by using insulation and coating. [ABSTRACT FROM AUTHOR]

Highlights • A spray tower with upward spraying and downward gas flow (ST-UD) is proposed. • ST-UD has no restriction on the minimum droplet diameter and no drift eliminator. • A mathematical model was developed for the quick performance evaluation of ST-UD. • ST-UD has high heat and mass transfer performance and particle scavenging performance. • The impacts of operating parameters on thermal performance of ST-UD was evaluated. Abstract This paper presents the design and evaluation of a novel spray tower which called the reversibly used cooling tower with upward spraying and downward gas flow (RUCT-UD). Unlike conventional spray towers, RUCT-UD has no high-resistance drift eliminator and no restriction on the minimum droplet diameter, which gives it the potential to achieve better performance of heat and mass transfer and particle scavenging. A mathematical model was developed and validated by field experiments. This model is highly efficient in calculation due to the simultaneous consideration of rising and falling droplets. Using the model, the heat and mass transfer characteristics were investigated by a parametric study, which provides a theoretical basis for the tower design. After that, the performance of RUCT-UD was compared with that of other spray-type RUCTs. Results show that RUCT-UD is 45% shorter than RUCT-UU (the reversibly used cooling tower with upward spraying and upward gas flow) and RUCT-DU (the reversibly used cooling tower with downward spraying and upward gas flow), meanwhile it could realize a high thermal performance like RUCT-UU. The particle collection efficiency of PM 2.5 for RUCT-UD is 75.9%, which is higher than that of RUCT-UU (48.9%) and that of RUCT-DU (61.1%). The configuration of the proposed RUCT-UD might be applied to other spray towers for the performance improvement. [ABSTRACT FROM AUTHOR]