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The primary scope of the present work is the comprehensive investigation of a novel multigeneration system that exploits renewable energy sources, i.e. solar energy, geothermal energy, and biomass, and produces four useful outputs, i.e. electricity, heating, cooling, and green hydrogen. Solar irradiation is the primary energy source, which is exploited by parabolic trough collectors and powers a supercritical CO 2 regenerative Brayton cycle, while a biomass boiler serves as an auxiliary energy input. In addition, the whole installation includes a proton exchange membrane water electrolyzer, a water heater, as well as an absorption chiller, where a geothermal well is used for heat rejection. Apart from the thermal energy storage tank, which is coupled with the collectors, hydrogen production is used as a secondary energy storage method, as the excess produced renewable electricity feeds the electrolyzer. Consequently, the installation is examined parametrically, and optimized on steady-state conditions. Then, a dynamic simulation study is conducted for the climate data of Athens, Greece. According to the results, at the optimum operation point on steady-state conditions, the energy efficiency is determined at 37.89%, and the exergy efficiency at 20.86%. Moreover, the annual energy and exergy efficiencies are defined at 32.52%, and 18.51%, respectively. The economic evaluation analysis indicates that the payback period is about 11 years, while the internal rate of return, and the net present value are determined at 10.62% and 7,753,905 €, respectively. In addition, the system is estimated to save 3072.6 tons of CO 2 equivalent per annum. In conclusion, the proposed multigeneration system can address the various energy needs and recent environmental challenges, as a totally renewable, economically feasible, and environmentally friendly solution. • A multigeneration system that produces four useful outputs is examined. • The unit exploits three renewable sources, solar, biomass, and geothermal. • Both steady-state and dynamic analyses are carried out. • The annual energy and exergy efficiencies are defined at 32.52% and 18.51%. • The system payback period is found at 11 years, while it saves 3072.6 tn CO2-eq. [ABSTRACT FROM AUTHOR]

Energy retrofit solutions that concern a building's roof structure play a significant role in the enhancement of a building's thermal behaviour. This study investigates the integration of phase change materials (PCMs) with cool coatings (CCs) or thermochromic coatings (TCCs), namely, a PCM roof, a PCM-CC roof, and a PCM-TCC roof, as alternative and novel tactics for the simultaneous control of solar heat transfer and solar heat reflection. An energy simulation analysis with the DesignBuilder tool is conducted for a one-story residence and the climatic conditions of Athens. The simulation results indicate that, compared to the existing concrete roof construction, the PCM roof, PCM-CC, and PCM-TCC roof systems demonstrate energy savings that reach up to 13.55%, 16.04%, and 21.70%, respectively. The systematic analysis reveals that the increase in PCM's thickness leads to an increase in the total electricity savings of the buildings, but in the case of PCM-CC and PCM-TCC roof systems, they merely effect the cooling thermal loads. The mean phase transition temperature that favours the cumulative electricity savings is 28 °C in the case of PCM and PCM-TCC roof systems and 35 °C in the case of PCM-CC roof systems. The methodology of this study allows the design of efficient, integrated roof systems with advanced thermal and optical properties as energy retrofit solutions for Mediterranean climatic conditions. [ABSTRACT FROM AUTHOR]

The goal of this investigation is the thorough analysis and optimization of a solar-assisted heat pump heating unit for covering the space heating demand for a building in Athens, Greece. The novelty of the studied system is the use of a high-temperature heat pump that can operate with radiative terminal units, leading to high thermal comfort standards. The examined system includes flat-plate solar thermal collectors, an insulated thermal storage tank, auxiliary electrical thermal resistance in the tank and a high-temperature heat pump. The economic optimization indicates that the optimal design includes 35 m2 of solar thermal collectors connected with a storage tank of 2 m3 for facing the total heating demand of 6785 kWh. In this case, the life cycle cost was calculated at 22,694 EUR, the seasonal system coefficient of performance at 2.95 and the mean solar thermal efficiency at 31.60%. On the other hand, the multi-objective optimization indicates the optimum design is the selection of 50 m2 of solar field connected to a thermal tank of 3 m3. In this scenario, the life cycle cost was calculated at 24,084 EUR, the seasonal system coefficient of performance at 4.07 and the mean solar thermal efficiency at 25.33%. [ABSTRACT FROM AUTHOR]

High-temperature heat pumps consist of a promising choice for substituting conventional boilers and producing hot water with temperature levels in the range of 70–80 °C. The use of high-temperature heat pumps makes it possible to exploit the existing radiators in existing buildings to achieve effective heating of the indoor space and keep thermal comfort at high standards. The goal of this work is the detailed investigation of a high-temperature heat pump, with the R1234ze(E) as the working medium for producing heating at 80 °C for space-heating purposes, in Greek climate conditions. The analysis was conducted at two locations, one in the south (Athens—Zone B) and one in the north (Thessaloniki—Zone C), in order to examine the performance of the study's technology in the most representative locations of Greece. The thermodynamic analysis of the heat pump was performed with a developed model in Engineering Equation Solver by exploiting real data from the compressor manufacturer and by practically using a gray box model. The heating thermal loads were calculated with a dynamic analysis using TRNSYS software. According to the final results, the high-temperature heat pump was beneficial when compared to a conventional oil boiler from energy, financial, and environmental points of view. The net savings were found at EUR 6448 for Athens and EUR 13,161 for Thessaloniki; the simple payback period resulted in 8.03 years for Athens and 6.11 years for Thessaloniki, while the CO2 emissions avoidance was found at 35% for Athens and 32% for Thessaloniki. Last but not least, the lifetime CO2 emissions reduction was calculated at 20,882 kgCO2 for Athens and 35,278 kgCO2 for Thessaloniki. [ABSTRACT FROM AUTHOR]

• An innovative in-line water heater solar thermal collector is proposed. • PCM function was validated through experimental results from literature (<5% deviations). • The collector was optically optimized and exceeded another one from literature by up to 30%. • The implementation of fins enhances significantly the performance of the collector. • The 8x8 plus geometry ensures 96% more useful output (2.78 kWh) than the no-fin one. In the present work, an innovative asymmetrical evacuated tube compound parabolic collector (CPC) with integrated Phase Change Material (PCM) is proposed as an in-line water heater. This solar collector was examined numerically as regards its thermal and optical operation. The optical efficiency was firstly optimized by modifying two independent design variables of the reflector. Then the optical performance was calculated for the finally selected geometry, considering 15 different transversal incident angles. As regards the thermal operation of the collector, the charging and discharging of PCM were investigated in detail. Four different internal fin arrangements were compared to the no-fin case and each other and the optimum one was revealed. The simulations were conducted for the city of Athens in Greece considering the typical clear day in July. It was considered that there is a charging period of five hours from 09:00 to 14:00 (solar time) and then a discharging one with a duration of one hour. For the thermal optimization, a volumetric flow rate of 4 L/min was selected during the discharging phase. The water temperature from the grid was considered to have a temperature of around 20 °C. The main parameters which were evaluated during charging and discharging were the useful power output, the outlet water temperature and the mean PCM temperature. For validation purposes, experimental results from a previous study were used. In the previous study, a PCM heat exchanger with water as the working fluid was examined experimentally. The same heat exchanger was designed in the present study for comparing the results. It was found that the present numerical results are very close to the experimental of the previous study with a mean deviation of 1.60 % and a maximum of 4.24 %. The design and the numerical simulations were conducted with SolidWorks Flow Simulation software. The final results show that the best design leads to 96 % higher daily useful heat production compared to the baseline case without fins. [ABSTRACT FROM AUTHOR]

The objective of this study is the investigation of different solar concentrating collectors for application in a trigeneration system. Parabolic trough collectors, linear Fresnel reflectors and solar dishes are the examined solar concentrating technologies in this work. The trigeneration unit includes an organic Rankine cycle coupled with an absorption heat machine that operates with LiBr/water. The analysis is performed throughout the year by using the weather data of Athens in Greece. The results of this work indicate that the selection of parabolic trough collectors is the best choice because it leads to the maximum yearly system energy efficiency of 64.40% and to the minimum simple payback period of 6.25 years. The second technology is the solar dish with the energy efficiency of 62.41% and the simple payback period of 6.95 years, while the linear Fresnel reflector is the less efficient technology with the energy efficiency of 35.78% and with a simple payback period of 10.92 years. Lastly, it must be stated that the thermodynamic investigation of the system is performed with a created model in Engineering Equation Solver, while the dynamic analysis is performed with a code in the programming language FORTRAN. [ABSTRACT FROM AUTHOR]

• Various ideas for the optical enhancement of a linear Fresnel collector are studied. • The results are evaluated on a yearly basis for the climate of Athens – Greece. • The use of an extended receiver enhances the yearly performance up to 50.3%. • The use of a displaced receiver enhances the yearly performance up to 20.2%. • The combination of these methods enhances the yearly performance up to 48.7%. The objective of this work is the investigation of some alternative ideas for enhancing the optical performance of a linear Fresnel reflector (LFR) with North-South orientation. More specifically, the examined methods aim to reduce the optical end losses which are crucial for the short LFR plants in particular during the winter period. The first studied idea is the extension of the receiver after the concentrator which is able to enhance the yearly mean incident angle modifier up to 50.3%. The second examined idea is the displacement of the receiver in order to eliminate the non-illuminated area at the beginning of the receiver. This idea proved that the mean yearly incident angle modifier can be enhanced up it 20.2% for a displacement equal to 20% of the concentrator length. The third examined idea is the hybrid design with the extended and displayed receiver in combined. This idea leads to intermediates enhancements compared to the previous cases but its advantage is the lower investment cost compared to the simple receiver extension case. The analysis is performed with a developed optical model in SolidWorks Flow Simulation and the yearly evaluation has been done for the location of Athens (Greece). [ABSTRACT FROM AUTHOR]

In this work, two underfloor solar assisted heating systems without and with phase change materials (PCMs) are investigated energetically for a building of 100 m² floor area, which is situated in Athens, (Greece). The simulations are conducted with the commercial software TRNSYS 17. More analytically, flat plate collectors coupled to a storage tank are used while there is, in the first system, an auxiliary heater and in the second system, a heat pump, for supplying the extra heating demand when the solar potential is not sufficient. The PCM layer (BioPCM Q29/M91) is situated below the underfloor heating system, which operates with water, in order to increase the storage capacity. Moreover, this study compares the indoor temperature profiles of the building with and without a PCM layer on the floor and specifically in different cases by changing the area of the collectors and the thickness of the insulation layer. The results showed that the electrical energy consumption decreases on average 70% and 41% for the system with an auxiliary heater and for the system with heat pump respectively. Moreover, the application of the PCM layer on the floor in both systems gives an increase of the indoor temperature about 2°C into the limits of thermal comfort. [ABSTRACT FROM AUTHOR]

Highlights • In this study, a solar driven cooling system with absorption chiller is investigated. • The system is examined without and with phase change materials into radiant walls. • The best location of the PCM layer is found to be in the south wall. • The auxiliary energy consumption can be reduced from 30% up to 63%. • The application of the PCM layer can decrease the indoor temperature on average 0.6 °C. Abstract This work investigates energetically and financially a building solar cooling system with radiant walls which includes phase change materials (PCMs). The examined building has a floor area of 100 m2 and it is located in Athens (Greece). The solar cooling system includes evacuated tube collectors coupled to a single-effect absorption chiller for producing cold water for the building radiant walls. The use of PCM is examined in all the building's outer walls and different scenarios with PCM in only some walls are also investigated in details. Different combinations of collecting areas and storage tank volumes are examined in order to determine the optimum design for every scenario. The study is performed with the commercial software TRNSYS which uses an active layer to simulate the radiant wall. The most important calculated parameters of this work are the auxiliary energy consumption, the solar coverage and the indoor temperature of the building. The results indicate that the best location of PCM layer is in south wall with a reduction of the auxiliary energy 30%, an increase of the solar coverage 3.8% and a reduction of the total system cost of about 3%. [ABSTRACT FROM AUTHOR]

Summary: Solar‐driven polygeneration systems are promising technologies for covering many energy demands with a renewable and sustainable way. The objective of the present work is the investigation of a trigeneration system, which is driven by solar‐dish collectors. The examined trigeneration system includes an organic Rankine cycle (ORC), which operates with toluene, and an absorption heat pump, which operates with LiBr/H2O. The absorption heat pump is fed with heat by the condenser of the ORC, which operates at medium temperature levels (120°C to 150°C). The absorption heat pump produces both useful heat at 55°C and cooling at 12°C. The ORC produces electricity, and it is fed by the solar dishes. The examined ORC is a regenerative cycle with superheating. The total analysis is performed with a developed model in Engineering Equation Solver (EES). The system is investigated parametrically for different ORC heat‐rejection temperatures, different superheating levels in the turbine inlet, and various solar‐beam irradiation levels. Furthermore, the system is investigated on a yearly basis for the climate conditions of Athens (Greece) and for Belgrade (Serbia). It is found that the yearly system energy and exergy efficiencies are 108.39% and 20.92%, respectively, for Athens, while 111.38% and 21.50%, respectively, for Belgrade. The values over 100% for the energy efficiency are explained by the existence of a heat pump in the examined configuration. For both locations, the payback period is found close to 10 years and the internal rate of return close to 10%. The final results indicate that the examined configuration is a highly efficient and viable system, which operates only with a renewable energy source. The present study investigates the use of solar‐dish concentrator for driving a trigeneration system for heating, cooling, and electricity production. The results of this work show yearly energy and exergy efficiencies around 120% and 20%, respectively, while the payback period is about 10 years. The analysis is conducted for two locations, Athens (Greece) and Belgrad (Serbia). [ABSTRACT FROM AUTHOR]

Solar energy utilization for covering the heating loads of buildings is an innovative and clean way which leads to lower electricity consumption. Passive heating systems are low-cost and simple technologies which are able to cover a great part of the space heating demand. Trombe wall is the most mature technology for capturing solar irradiation and transferring it inside the building in order to cover a part of the heating loads, especially the hours after the noon. In this study, an innovative Trombe wall with an extra window in the massive wall is suggested. This wall is compared with the conventional Trombe wall and the usual insulated wall in order to make a complete investigation. The examined walls are located in south direction, in order to maximize the impact of solar irradiation. According to the final results, the new Trombe wall is the most appropriate technology, creating warmer indoor profile than the other cases, especially the hours between noon and afternoon. The analysis in performed with the commercial software Solidworks Flow Simulation and the examined building is located in Athens, Greece. [ABSTRACT FROM AUTHOR]

This paper investigates numerically the deep renovation of a multi-family building in Greece to reduce dramatically its energy demand and also to incorporate renewable energy sources, rendering it a positive one; thus in position on an annual basis to offer net electricity to the grid. The examined building has 8 apartments of 75 m2 floor area each and is located in Moschato, a suburb of Athens in Greece. The goal of the present investigation is to determine the energy savings, but also to calculate the financial and environmental benefits through a life cycle analysis. The energy simulation of the building is conducted on annual basis by using a novel and detailed dynamic software tool (INTEMA.building), which is developed in the Dymola environment using the Modelica modeling language. This tool makes possible the detailed simulation of both passive and active systems in the building environment. Furthermore, it includes the control of the energy systems and can provide accurate enough results, encompassing detailed numerical models for the systems investigated, accounting for an adjustable time step of the dynamic analysis. According to the calculations, the proposed retrofitting scenario can achieve a reduction of the heating loads by 93% and of cooling loads by 78% respectively. The electrical demand for domestic hot water can be decreased by about 79%, while the electricity demand for appliances and lighting by about 60%. In terms of specific thermal needs, the specific heating demand can be reduced from 151.5 kWh/m2 down to 10.7 kWh/m2, while the cooling specific demand from 112.6 kWh/m2 to 24.4 kWh/m2. Moreover, it is calculated that the reduction in the primary energy demand after the renovation can be up to 88%, with the building providing around 5.3 MWh of net electricity to the grid on a yearly basis through a net-metering connection. Finally, the life cycle cost analysis indicated 622 k€ savings and specific CO 2 avoidance per renovated floor area in the range of 2.64 tons CO 2 /m2. • A new tool for dynamic simulations of buildings and energy systems is presented. • A holistic renovation scenario of a multi-family Greek building is investigated. • The primary energy reduction after the renovation was found at 88%. • The renovation payback period is 3.8 years and the specific lifecycle gains 1037€/m2. • The specific CO 2 avoidance was calculated at 2.64 tons CO 2 /m2. [ABSTRACT FROM AUTHOR]

Solar-driven technologies for electricity production are very promising technologies for developing sustainable and viable energy units. The objective of this work is the energy and financial investigation of a solar-driven thermoelectric generator system. This system operates without concentrator; it absorbs the incident solar irradiation and it converts it partially to electricity. In this paper, a small module is investigated in details and the analysis is extended for a solar field of 100 m2. A numerical model is developed in Engineering Equation Solver for simulating the solar thermoelectric generator behavior and it is validated using experimental data from the literature. The maximum instantaneous efficiency is found close to 4.5% in the reference operating conditions, while the mean yearly efficiency for the climate conditions of Athens (Greece) is found at 3.2%. The yearly electricity yield is found to be 5414 kWh for a solar potential per area of 1695 kWh/m2. The parametric financial analysis proved that generally, the investment is viable and for a reasonable specific capital system cost of 1000 €/kW the payback period is about 4.5 years and the levelized cost of electricity is around 0.044 €/kWh. • A solar-driven thermoelectric generator is studied in energy and financial terms. • The system is investigated on a yearly basis for the location of Athens in Greece. • The yearly electrical efficiency is found at 3.2% and the electricity production 5414 kWh. • In the main scenario, the levelized cost of electricity is found at 0.0617 €/kWh. • The payback period is found to be about 4.5 years which indicates a viable investment. [ABSTRACT FROM AUTHOR]

This work investigated a trigeneration system for the building sector. Biomass is the utilized energy source which feeds an organic Rankine cycle for electricity production. Heating is extracted from the power cycle, especially with the proper heat exchanger after the turbine. A vapor compression heat pump is used for cooling production, and it is fed by work power from the turbine shaft. The total configuration was examined parametrically by investigating different superheating cases, pressure levels, and ambient temperatures. The analysis was energetic and financial for the climate conditions of Athens, Greece. For the case with a nominal cooling load of 10 kW, the system's yearly energy and exergy efficiencies were 48.68% and 24.86% respectively. The payback period was 5.02 years, the net present value was €118,000, and the internal rate of return was 21.57%. The yearly electrical, heating, and cooling production were 99.39, 91.17, and 22.67 MWh respectively, whereas the biomass energy input was 438 MWh on a yearly basis. [ABSTRACT FROM AUTHOR]

Energy consumption in the building sector is responsible for a very large amount of electricity consumption worldwide. The reduction of this consumption is a crucial issue in order to achieve sustainability. The objective of this work is to investigate the use of phase change materials (PCMs) in the building walls in order to reduce the heating and the cooling loads. The novelty of this work is based on the investigation of different scenarios about the position of the PCM layer in the south and the north walls. PCMs can improve the thermal performance and the thermal comfort of a building due to their ability to store large amounts of thermal energy in latent form and so to reduce the temperature fluctuations of the structural components, keeping them within the desired temperature levels. More specifically, this work presents and compares the heating loads, the cooling loads and the temperature distribution of a building in Athens (Greece), with and without PCMs in different positions in the south wall and in the north walls. The simulation is performed with the commercial software TRNSYS 17, using the TRNSYS component: type 1270 (PCM Wall). The results proved that the maximum energy savings per year were achieved by the combination of the insulation and the PCM layer in the north and south walls. More specifically, the reductions in the heating and the cooling loads were found to be 1.54% and 5.90%, respectively. Furthermore, the temperature distribution with the use of a PCM layer is the most acceptable, especially during the summer period. [ABSTRACT FROM AUTHOR]

The exploitation of low-grade heat sources that are available from existing thermal systems processes consists a promising concept for the production of energy. In this research, the integration of an Organic Rankine Cycle (ORC) unit on a supermarket transcritical CO 2 refrigeration system with ejectors is investigated, which is supplied with low-grade heat by the common discharge of the compressors. The optimization analysis proves that for different combinations of CO 2 temperature and pressure in the ORC evaporator, as well as for different ORC condensation temperatures, there is a different optimum ORC evaporation temperature that leads to the highest energy production. Concerning the annual performance analysis conducted for the city of Athens, Greece, operation with isopentane is found to provide the greatest energy production with 10.36 MWh annually, which corresponds to a 3.69% reduction in the refrigeration system's energy consumption. For operation under the ambient temperature of 40 °C, the corresponding reduction of the refrigeration system consumption due to the presence of ORC is 7.54%. Finally, the financial analysis reveals that toluene is the most advantageous solution, leading to a payback period of 3.9 years, with a net present value by the end of the project's lifetime equal to 34.32 k€. • Integration of an Organic Rankine Cycle to a transcritical CO 2 refrigeration system. • Optimization analysis of the system for operation with different working fluids. • Energetic and financial investigation for the case of Athens, Greece. • Isopentane leads to annual production of 10.36 MWh with 3.69% energy savings. • The lowest payback period is 3.9 years and it is achieved for operation with Toluene. [ABSTRACT FROM AUTHOR]

• Solar-biomass supercritical CO 2 trigeneration system is studied. • Energetic analysis and selection of optimal conditions for the proposed unit. • Assessment of financial viability for installation in refrigeration application. • Determination of performance results for annual operation in Athens climate. • Production of 185.59 MWh yearly with at least 519 k€ savings during the lifetime. The ever-growing increase of the energy demand required to satisfy the needs of society has aroused considerable interest regarding the investigation of polygeneration systems that employ renewable energy sources. In the present work, the concept of installing a supercritical CO 2 trigeneration system to cover the needs of a refrigerated warehouse in Athens with simultaneous production of power and heating is studied in terms of energy performance, and its financial benefits compared to the solution of an ejector refrigeration-only system are analyzed and assessed. The trigeneration system utilizes solar energy in parabolic trough collectors and biomass energy as the supplementary source, in order to increase the temperature of high-pressure CO 2 and produce power in the turbine which is coupled to a generator. Regarding the collectors, the installation of zero up to four solar modules that exploit the solar power is examined, while the rest energy input needed in order to achieve the selected operating conditions is supplied by a biomass boiler. Since all the trigeneration cases require the same amount of energy input to reach the desired conditions, the trigeneration system for every combination of solar modules is found to produce 185.59 MWh annually, while the refrigeration-only system is found to consume 172.94 MWh during a year. The power production efficiency, the combined power and refrigeration production efficiency and the total trigeneration efficiency for the selected nominal conditions are calculated 11.77 %, 66.09 % and 163.70 % respectively. The case of a trigeneration system with four solar collectors displays a payback period of 4.0 years compared to the refrigeration-only case, providing financial savings equal to 542 k€ for the duration of the project lifetime. The same case compared to that of trigeneration system using only biomass boiler and no solar collectors exhibits payback period of 13.5 years, achieving financial savings of 22.9 k€ for the considered lifetime of the project. [ABSTRACT FROM AUTHOR]

Refrigeration systems and heating/air conditioning units used in supermarkets are responsible for a great part of the annual electricity consumption of industrialized countries. Taking also into consideration the direct emissions of carbon dioxide to the environment due to leakage of refrigerants with high Global Warming Potential that are employed today for such applications, a step towards more environmentally friendly and less energy-consuming solutions should be made. In the current research, different topologies of an all-in-one R744 transcritical unit providing refrigeration, heating and air conditioning to a real Greek supermarket store (METRO S.A.) in Athens are studied, using the softwares Engineering Equation Solver and EnergyPlus. The different configurations include a state-of-the-art R744 system with parallel compression and multi-ejector block, the same system with the addition of a subcooling heat exchanger that utilizes available geothermal water, as well as the last described unit with the integration of adiabatic cooling configuration on the gas cooler. These solutions are investigated in terms of energy efficiency and environmental impact (total equivalent warming impact) and they are compared to an indirect arrangement employing R744 and R290, to conventional direct expansion solutions (with R448A or R404A) and to a heat pump employing R410A that covers the heating and air conditioning demands of the supermarket. The comparative energy analysis was separated into the refrigeration part, the heating and air conditioning part and the sum of them. The results indicated that for the climate of the store's location, the case including the subcooler and adiabatic cooling configuration, the case with only the subcooler and the case with neither the subcooler nor the adiabatic cooling configuration consume annually 20.9%, 17.6% and 8.7% less energy respectively than the combination of a refrigeration unit with R448A and a heat pump with R410A for the heating/cooling needs. • All-in-one CO 2 units for refrigeration, heating and air conditioning are studied. • Supermarket refrigeration units using CO 2 and conventional refrigerants are compared. • The use of adiabatic cooling and subcooler employing geothermal water is studied. • The most advanced CO 2 system consumes 20.9% less energy than the conventional system. • The most advanced CO 2 system emits 34.7% less eq.CO 2 tn than a conventional unit. [ABSTRACT FROM AUTHOR]