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• A novel Carnot Battery system with double waste heat source exploitation is studied. • Organic Rankine cycle, high-temperature heat pump and latent storage are used. • Parametric analysis and optimization are performed in steady-state conditions. • For the benchmark scenario, the roundtrip efficiency of 64 % was calculated. • Performance enhancements up to 174 % compared to ambient source unit were found. Carnot batteries are critical technologies for increasing the penetration of renewable energy sources because they enable suitable energy storage for long-term periods. The achievement of a high round-trip efficiency is an important point that determines the sustainability of the Carnot batteries. In this direction, the present work comes to investigate a Carnot battery with latent storage that exploits properly a waste heat stream. The examined unit includes a high-temperature heat pump, an organic Rankine cycle, a latent storage system and the proper heat exchanger for the waste heat stream exploitation. The novelty of the studied idea lies in the exploitation of the waste heat stream in two ways. Specifically, the waste heat stream feeds firstly the latent storage system with heat, and secondary it feeds the evaporator of the heat pump with heat input for enhancing the heat pump's coefficient of performance. This design makes it possible to achieve high round-trip efficiency values and also to exploit properly a waste heat stream. For the benchmark scenario with 100 kW electricity input, 350 kW waste heat input at 200 °C and 160 °C storage temperature, the round-trip efficiency was found at 64 %, the system's energy efficiency at 14.2 %, the system's exergy efficiency at 36.7 % and the produced electricity at 64 kW. [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]

Highlights • The use of nanofluids increases both the thermal and electrical performance. • The collector can operate up to 200 °C with oil and nanofluid. • At 50, 100 and 150 °C, the thermal efficiency was 45.0%, 46.8% and 39.7% respectively. • At 50, 100 and 150 °C, the electric efficiency was 5.8%, 6.6% and 3.9% respectively. • At 100 °C, the thermal and electric enhancements were 1.66% and 5.17% respectively. Abstract The objective of this paper is the investigation of a thermal photovoltaic concentrating collector operating with and without nanofluids. The use of nanofluid Syltherm 800/copper oxide with 5% volumetric concentration is examined and it is compared with the base fluid. The examined thermal photovoltaic has a parabolic concentrator and an inverted flat insulated receiver. The analysis is conducted with SolidWorks Flow Simulation which is able to perform the thermal, optical and flow analysis. According to the present study, the system is able to operate for inlet fluid temperatures up to 200 °C which is a promising result. Moreover, it is found that the use of nanofluid leads to enhancement both in thermal and electrical performance for all the examined cases (different combinations of flow rates and inlet temperatures). For an inlet temperature at 100 °C and flow rate 540 L/h with nanofluid, the thermal efficiency and the electrical efficiency were found at 46.84% and 6.60% respectively. For this case, the enhancements in the thermal, electrical, total and exergy efficiencies were found at 1.66%, 5.17%, 2.08% and 3.05% respectively. [ABSTRACT FROM AUTHOR]

The objective of this work is the investigation of a polygeneration system which is driven by solar energy and a biomass boiler. Parabolic trough solar collectors coupled to a storage tank are used in order to produce useful heat at high-temperature levels (∼350 °C). The system includes an organic Rankine cycle and a vapor compression cycle in order for electricity and cooling to be produced respectively. Moreover, useful heat is produced at two temperatures levels (50 °C and 150 °C) and so there are totally four useful energy outputs. The system is optimized in steady-state conditions and then the most suitable design is investigated in dynamic conditions for all the year period. According to the final results, the yearly energetic efficiency of the system is 51.26% while the yearly exergetic efficiency is 21.77%. The net present value is found 165.6 k€, the payback period 5.13 years, the internal rate of return 21.26%, while the yearly CO 2 avoidance to 125 tn. Finally, the results of this work indicate that the suggested configuration can provide various useful outputs with high efficiency and the total investment is characterized as viable due to the suitable financial index values. [ABSTRACT FROM AUTHOR]

The objective of this work is to compare and to evaluate the performance parabolic trough solar collector (PTC) and the linear Fresnel solar reflector (LFR) for the climate conditions of Athens, Greece. These collectors are examined under the same operating conditions and they are evaluated energetically, exergetically and financially. The analysis is conducted for all the year period using twelve typical days, one for every month. Performance curves for the solar systems are taken from the literature in order to estimate the collector performance for different operating temperature levels, solar irradiation level and solar angles. According to the final results, the mean yearly incident angle modifier for the PTC is about 83.73%, while for the LFR is 58.66%, at the examined climate conditions. It has been found that the mean yearly thermal efficiency at 350 °C is 52.25% for the PTC and 35.39% for the LFR, while the mean yearly electrical efficiency of the equivalent power production system is 16.11% with the PTC and 10.92% with the LFR. The simple payback period for process heat production was found close to 3–4 years, while for power production close to 8–9 years. Moreover, it is proved that the specific cost of the LFR has to be lower than 185 €/m 2 in order to be a more attractive choice financially compared to the PTC. [ABSTRACT FROM AUTHOR]

Solar cooling is one of the most promising technologies for covering the increasing needs of our society with a clean and sustainable way. This paper examines an ejector-absorption cooling system operating with the LiBr-H 2 O working pair which is driven by parabolic trough solar collectors. The main objective of this investigation is to optimize the total solar cooling system and to compare it with the respective optimum system with conventional absorption chiller and parabolic trough collectors. The generator temperature and pressure, as well as the pressure drop in the ejector, are the optimization parameters, while the maximization of the system performance is the objective function. According to the final results, the system with the ejector leads to higher COP (coefficient of performance) for the majority of the examined operating scenarios. Moreover, the optimized ejector-absorption system presents performance enhancement up to 60.9% compared to the conventional absorption system for the case with 12.5 °C evaporating temperature and 30 °C condenser temperature. For this optimum case, the system COP is about 1.01, the chiller COP is 1.65 and the system exergy efficiency is 4.76%. Generally, the system COP is ranged from 0.473 to 1.01 for the system with ejector, while the demanded specific collecting area from 0.987 to 2.11 m 2 /kW cooling . The analysis is performed with a developed model in Engineering Equation Solver which is validated with literature results. [ABSTRACT FROM AUTHOR]

The objective of this work is to present a hybrid Organic Rankine Cycle (ORC) driven by solar energy and waste heat. Parabolic trough collectors coupled to a storage tank feed the heat recovery system which also utilizes waste heat of low-grade temperature (150 °C – 300 °C). Four different working fluids (toluene, cyclohexane, MDM and n-pentane) are examined in the regenerative ORC. The system is designed properly in order the maximum electricity production to be produced with the highest possible exploitation of the waste heat source. The examined model is investigated under steady-state conditions with a developed thermodynamic model in EES (Engineering Equation Solver). According to the final results, toluene leads to the highest electricity production, with cyclohexane, MDM and n-pentane to follow. The electricity production with toluene is found to be ranged from 479 kW to 845 kW and the system efficiency from 11.6% to 19.7%, with waste heat source temperature variation from 150 °C to 300 °C respectively. Moreover, it is found that higher waste heat source temperature leads to a higher fraction of the waste heat input in the ORC for all the working fluids. [ABSTRACT FROM AUTHOR]

The objective of this work is to examine a solar ejector refrigeration system under various operating scenarios. Evacuated tube collectors of 100 m 2 collecting area coupled to a storage tank of 4 m 3 are selected for feeding the generator of the refrigerator. The system is examined for different evaporator temperatures from −10 °C to 10 °C and for different heat rejection temperatures from 30 °C to 50 °C. In every case, the system is optimized by selecting the optimum ejector design and the optimum generator temperature. Different refrigerants are examined as R123, R245fa, R600a and R134a, while the emphasis is given to the R141b. The innovation of this work is based on the systematic examination and optimization of a great range of operating scenarios giving a clear image for the system performance. According to the final results, the R141b is found to be the best candidate in all the operating scenarios. More specifically, the maximum system COP is found to be 0.234 when the system produces refrigeration at 10 °C and rejects heat to the environment at 30 °C. The optimum generator temperatures are found to be from 114 °C to 157 °C and the cooling capacity is ranged from 1.85 kW up to 23.39 kW. The analysis is performed with a developed model in EES (Engineering Equation Solver) under steady-state conditions. [ABSTRACT FROM AUTHOR]

The objective of this study is to present a detailed exergetic analysis of the commercial parabolic trough collector LS-2. A complete thermal model is developed in EES (Engineering Equation Solver) and it is validated with literature results. The solar collector is examined for operation with Therminol VP1 and air in order to examine the most representative liquid and gas working fluids. In the exergetic analysis, detailed presentation of the exergetic losses and the exergy destruction is given for various operation cases. More specifically, different combinations of flow rates and inlet temperature levels are tested for both working fluids and the results indicate the reasons for the exergetic reduction in every case. According to the final results, the global maximum exergetic efficiency for operation with air is 25.62% for an inlet temperature of 500 K and flow rate of 10,000 l/min, while for Therminol VP1 is 31.67% for 500 K and 100 l/min. Moreover, it is proved that the exergy destruction is more intense in the thermal oil case, while the exergetic losses are more important in the air case. The final results and conclusions clearly present the exergetic analysis of parabolic trough collector for a great range of operating conditions. [ABSTRACT FROM AUTHOR]

The objective of this work is to investigate the yearly performance an absorption-compression cascade refrigeration system driven by solar energy. Different operating scenarios are examined by testing different combinations of refrigeration and condensation temperatures. Evacuated tube collectors coupled to a storage tank are selected to be used for feeding the refrigeration system with the proper heat demand. LiBr-H 2 O and R134a are selected as working pair in the absorption cycle and refrigerant in the compression cycle respectively. The analysis is performed in energetic, exergetic, economic and environmental terms with a developed and validated model in EES (Engineering Equation Solver). According to the final results, the cascade system, in cases with low refrigeration temperatures is better economically while in higher refrigeration temperatures presents the greater percentage of electricity savings. On the other hand, higher heat rejection temperature leads both to higher economic indexes and to higher electricity savings. For the case of heat rejection at 40 °C and refrigeration at −20 °C, the electricity savings are found 53.98%, the IRR 6.6% and the payback period close to 14 years. [ABSTRACT FROM AUTHOR]

The objective of this study is to present and to examine a novel solar assisted mechanical compression refrigeration system. Evacuated tube collectors are used in order to partially thermally compress the refrigerant, after the mechanical compressor. This design aims to reduce the electricity consumption using a renewable energy source, creating a sustainable system. The suggested design is analyzed parametrically in order to investigate its energetic and exergetic behaviour in various operating conditions and it is examined with EES (Engineering Equator Solver) in steady state conditions. The temperature levels in the evaporator and in the condenser, as well as the thermal compression fraction (expressed with the pressure ratio parameter), are the examined parameters. The final results proved that the optimum values of the pressure after the mechanical compressor is the 75% of the maximum pressure and for this case, energy savings from 15% to 25% can be achieved. Moreover, the specific collecting area is found to be relatively low, close to 2 m 2 /kW for the optimum cases. The final results proved that the new design leads to energy savings in all the examined cases and especially in cases with higher evaporating temperature levels, a fact that makes it ideal for space cooling applications. [ABSTRACT FROM AUTHOR]

The objective of this study is to analyze and optimize a solar assisted gas turbine system. Parabolic trough collectors are used in order to supply a part of the demanded heat input, reducing the natural gas consumption and leading to an environmental friendly system. Emphasis is given in the pressure losses in the collector field loop which reduce the pressure level in the turbine inlet, leading to lower electricity production. In the first part of this study, the gas turbine and the collector field are investigated separately and in the second part, the solar assisted gas turbine is investigated parametrically and it is optimized. In the optimization part, a multi-objective optimization is performed by setting as goals the minimization of the collector area, of the fuel consumption and of the inversed electricity production. The final results proved that 1050 collector modules lead to 0.3389 kg/s natural gas consumption and to electricity production equal to 14.81 MW el . This optimum solution leads to 64% fuel savings with a 2.8% penalty on the produced electricity. The system is analyzed in steady state conditions with the Engineering Equator Solver (EES). [ABSTRACT FROM AUTHOR]

In this study, a solar cooling system of 100 kW is analyzed parametrically in dynamic basis for the city of Athens, Greece. The objective of this study is the design of a sustainable system, using energetic, exergetic and financial criteria. The examined system includes evacuated tube collectors, storage tank and a single stage absorption chiller operating with LiBr-H 2 O working pair. Different combinations of collecting areas and storage tank volumes are tested in order to determine the most suitable cases exergetically. These optimum cases are evaluated financially and finally the system with the higher financial indexes is selected as the most suitable. More specifically, the collecting area is analyzed from 150 m 2 to 600 m 2 and the storage tank from 6 m 3 to 16 m 3 . Finally, 450 m 2 of evacuated tube collectors with a 14 m 3 storage tank was proved to be the optimum solution financially with 15 years payback period and 67 k€ net present value. [ABSTRACT FROM AUTHOR]

In this study, the use of internal fins in parabolic trough collectors operating with gas working fluids is examined. Air, helium and carbon dioxide are the investigated working fluids, while Eurotrough ET-150 is the examined solar collector. The design and the simulation of this solar collector are performed with the commercial software Solidworks Flow Simulation. The internal fins lead to higher thermal efficiency but also to higher pressure losses; something very important for the solar fields of Concentrated Power Plants. Thus, the collector is examined energetically and exergetically in order to take into account the increase in the useful output with the simultaneous greater need of pumping power. Various fin lengths are examined and finally the fin of 10 mm was proved to be the most appropriate exergetically. In working fluid comparison, helium is the most efficient working fluid exergetically for all the examined cases. In the thermal efficiency comparison, helium performs better up to 290 °C, while carbon dioxide is the best choice in higher temperature levels. Moreover, the optimum mass flow rate for the helium was proved to be 0.03 kg/s and for the other working fluids the value of 0.015 kg/s seems to lead to the most satisfying results. [ABSTRACT FROM AUTHOR]

In this study a linear Fresnel solar collector with flat plate receiver is investigated experimentally and numerically with Solidworks Flow Simulation. The developed model combines optical, thermal and flow analysis; something innovative and demanding which leads to accurate results. The main objective of this study is to determine the thermal, the optical and the exergetic performance of this collector in various operating conditions. For these reasons, the developed model is validated with the respective experimental data and after this step, the solar collector model is examined parametrically for various fluid temperature levels and solar incident angles. The use of thermal oil is also analyzed with the simulation tool in order to examine the collector performance in medium temperature levels. The experiments are performed with water as working fluid and for low temperature levels up to 100 °C. The final results proved that this solar collector is able to produce about 8.5 kW useful heat in summer, 5.3 kW in spring and 2.9 kW in winter. Moreover, the operation of this collector with thermal oil can lead to satisfying results up to 250 °C. [ABSTRACT FROM AUTHOR]

The new technological trends are focused on the development of sustainable systems for covering the energy needs in buildings, using renewable energy sources and minimizing the carbon dioxide emissions. In this direction, the development of innovative trigeneration systems is an intelligent solution for creating energy efficient buildings which can satisfy their energy needs. In this paper, an innovative trigeneration system which uses low temperature level heat sources is analyzed and optimized. This system is consisted of an absorption heat pump and a steam turbine which consumes a part of the produced steam in the generator. This system is able to produce electricity, heating, cooling and hot water, covering the typical energy needs of buildings. Various scenarios are examined parametrically in order to present the system performance for various operation conditions. The final results proved that the exergetic efficiency is about 72% and the electrical output close to 9 kW, when a heat input of 100 kW is given. By selecting the proper parameters, the outputs of the system can be distributed according to the building energy demands, something very important for the sustainability of this system. The analysis is performed with the Engineering Equation Solver (EES). [ABSTRACT FROM AUTHOR]

The use of solar thermal energy for electricity production is a clean and sustainable way to cover the increasing energy needs of our society. The most mature technology for capturing solar energy in high temperature levels is the parabolic trough collectors (PTC). In this study, an Organic Rankine Cycle (ORC) coupled with PTC is analyzed parametrically in order to be optimized financially and energetically. The first step is the thermodynamic investigation of the ORC by using various working fluids. The second step is the energetic and financial investigation of the total system which includes the solar field, the storage tank and the ORC module. By testing many combinations of collecting areas and storage tank volumes, finally cyclohexane proved to be the most suitable working fluid for producing 1 MW el with PTC. Specifically, in the optimum situation a solar field of 25,000 m 2 with storage tank of about 300 m 3 leads to a payback period of 9 years and to an internal rate of return (IRR) equal to 13.46%. Moreover, an economic comparison for different commercial collectors is presented, with Eurotrough ET-150 being the financially optimum solution for this case study. [ABSTRACT FROM AUTHOR]

The objective of this study is to investigate the use of an alternative working pair in a solar absorption cooling system. LiCl-H 2 O is the new examined pair and it is compared energetically and exegetically with the conventional pair LiBr-H 2 O, which is the most usual in air-conditioning applications. The simplest solar cooling system is analyzed in order to focus in the comparison between these working fluids. Specifically, flat plate collectors, coupled with a storage tank, feed the single effect absorption chiller which produces 250 kW cooling at 10 °C. The two pairs are examined parametrically for various heat source temperature levels and for three ambient temperature levels (25 °C, 30 °C and 35 °C). The minimization of the collecting area, which means maximum exergetic efficiency, is the optimization goal in every case. The final results show that LiCl-H 2 O pair performs better in all cases by giving greater exergetic efficiency. More specifically, about 8% lower collecting area is required to cover the demanded cooling load with this working pair. Another interesting result is that the optimum heat source temperature for the LiCl-H 2 O is roughly lower than the respective for the LiBr-H 2 O. The system is analyzed in steady state with the commercial software Engineering Equator Solver (EES). [ABSTRACT FROM AUTHOR]

Using solar energy for space heating purposes consists an alternative way for substituting fossil fuel and grid electricity consumption. In this study, four solar assisted heat pump heating systems are designed, simulated and evaluated energetically and financially in order to determine the most attractive solution. The use of PV collectors with air source heat pump is compared to the use of FPC, PVT and FPC with PV coupled with a water source heat pump. A sensitivity analysis for the electricity cost is conducted because of the great variety of this parameter over the last years. The final results proved that for electricity cost up to 0.23 €/kW h the use of PV coupled with an air source heat pump is the most sustainable solution financially, while for higher electricity prices the coupling of PVT with an water source heat pump is the best choice. For the present electricity price of 0.2 €/kW h, 20 m 2 of PV is able to drive the air source heat pump with a yearly solar coverage of 67% leading to the most sustainable solution. Taking into account energetic aspects, the use of PVT leads to extremely low grid electricity consumption, fact that makes this technology the most environmental friendly. [ABSTRACT FROM AUTHOR]

• A solar-assisted pumped storage system with flat plate collectors is studied. • The novelty is based on the use of the solar thermal field for boosting the heat pump. • The solar-assisted system is compared in detail with an ambient source system. • The global optimum system presents a 68.48% power to power recovery ratio. • The simple payback period is around 7.8 years with 150 m2 collecting area. The objective of this work is the investigation of a solar-assisted pumped thermal energy storage system. The examined unit includes a solar field with flat plate collectors, a high-temperature heat pump, a latent storage system and an organic Rankine cycle. This system is fed by electricity from any renewable energy source (e.g., wind turbines or photovoltaic panels), and so the compressor of the heat pump is driven. The heat pump is fed with low-temperature heat from flat plate collectors, and it rejects heat of medium temperature to a latent storage system. The next step is exploiting the stored heat from an organic Rankine cycle for power production when there is a need for using electricity. The studied system is a "heat and power to power" system and can lead to high power recovery ratios. The analysis was conducted with a developed mathematical program in Engineering Equation Solver, and it was validated with numerical results from the literature. The optimum storage temperature has been found at 150 °C and the optimum temperature in the evaporator of the heat pump at 75 °C. In this case, the electricity recovery ratio (or power to power ratio) is found at 68.48%, the organic Rankine cycle efficiency at 18.45%, and the heat pump coefficient of performance at 3.704, while the collecting area is 150 m2. The use of a similar system without solar collectors but with the ambient as the heat source leads only to a 32.14% power recovery ratio. So, the specific gain of the used collecting area is 35.17 W/m2 which is an important amount of electricity. In the global optimum case, the simple payback period of the solar field investment is found to be at 7.8 years, which is a reasonable value and proves the financial viability of the examined idea. Moreover, this work proves that providing heat input of a relatively high temperature in the heat pump of the pumped thermal energy storage system is an excellent choice for recovering higher amounts of electricity. [ABSTRACT FROM AUTHOR]

• A nanofluid-based thermal PV coupled a heat pump is investigated. • This system produces space heating and net electricity. • The system is optimized with a multi-objective procedure. • Water/Cu nanofluid leads to 4.80% energy efficiency enhancement. • The heating production is increased by 5.75% with water/Cu nanofluid. The objective of this paper is to investigate a solar assisted heat pump which is driven by nanofluid-based hybrid PV collectors for space-heating and electricity production purposes. The use of nanofluids, as working fluids in the solar collector, enhances both the thermal and electrical performance in order to have higher useful outputs. Practically, the use of nanofluids enhances the heat transfer rates inside the flow and so higher thermal output are produced in the collector and the photovoltaic cell produces higher electricity because it is cooled efficiently by the nanofluids. Water/Cu and water/Al 2 O 3 are found to be the most efficient nanofluids and they are tested in the total system. The system is optimized for steady-state conditions and the optimum design is examined on a daily basis for the pure water case and the nanofluids cases. The optimization procedure is a multi-objective procedure which is conducted using energy, exergy and financial criteria. According to the final results, the use of water/Cu nanofluid is the best case which presents 4.80% higher energy system efficiency and 0.66% higher exergy system efficiency compared to the pure water case. The use of water/Al 2 O 3 leads to an enhancement of 4.34% and 0.19% for energy and exergy efficiency respectively. For the case with water/Cu, the heating production during the winter is found 7423 kWh th and the net electricity production 1068 kWh el. The analysis is performed with a developed thermal/thermodynamic model in Engineering Equation Solver. [ABSTRACT FROM AUTHOR]

Highlights • A conical and a spiral cavity receiver are examined for a solar dish collector. • The analysis is optical, thermal and exergetic for different operating conditions. • It is found that the conical design is more efficient than the spiral one. • The optical efficiency of the conical design is 1.38% higher than the spiral one. • The thermal efficiency enhancement with the conical design is 5.63% at 100 °C. Abstract The objective of this work is to compare two cavity receivers for a solar dish concentrator. The spiral and the conical cavities are investigated using a developed thermal model. The analysis is optical, thermal and exergetic for different operating temperatures and flow rates. The developed thermal model is combined with an optical tool in order to simulate properly the solar dish collector and it is validated for the case of the spiral absorber with experimental results. Every receiver is separated in the different coil and every coil is simulated separately in order to increase the model accuracy. Totally, 13 coils are used for the spiral design and 11 for the conical design. The location of the receiver in every case is optimized in order to achieve maximum optical efficiency. The results show that the conical design leads to a 1.38% increase in the optical efficiency due to the increased intercept factor. The thermal efficiency enhancement with the use of conical design is found to be 5.63% at 100 °C and 40.45% at 200 °C, while the exergy efficiency enhancement 6.67% at 100 °C and 42.06% at 200 °C. [ABSTRACT FROM AUTHOR]

The application of phase change materials (PCMs) in the building envelope can improve the thermal performance and the indoor thermal comfort conditions because they allow the proper storage or release of energy. In this work, three different solar assisted heat pump underfloor heating systems with and without the use of phase change material are designed, simulated and evaluated energetically for a building of 100 m 2 floor area. Different kinds of solar collectors (flat plate, photovoltaic, thermal photovoltaic) are coupled to a storage tank which feeds a heat pump for space heating purposes. The PCM is used on the underfloor heating system in order to increase the storage capacity and to avoid the operation of the heat pump during the peak electricity demand period. Furthermore, this work presents and compares the coefficient of performance, the electricity consumption, the solar coverage and the indoor temperature distribution of a building in Athens (Greece), with and without a PCM layer on the floor. More specifically, the PCM is placed on the underfloor heating system and different cases are examined by changing the collecting area, the collector type and the thickness of the insulation layer in the floor. The results prove that the use of the PCM layer on the underfloor heating system reduces the heating load by about 40%. The electricity consumption can be reduced between 42% and 67% using the solar-driven systems. It is also found that the system with the thermal PV consumes the lowest grid electricity. Moreover, the lowest simple payback period is found to be 9.61 years for the system with flat plate collectors without PCM. The simulation is conducted with TRNSYS for all the winter period. [ABSTRACT FROM AUTHOR]

• A solar-driven polygeneration unit with CO 2 as the working fluid is investigated. • Initial optimization in steady-state conditions and secondary dynamical analysis. • The payback period is found to be around 6.4 years and the total efficiency is 55%. • The use of an extra ORC can be used for enhancing electricity production. • The exergy efficiency with ORC is found 23.63%, while without 23.37%. The object of this study is the investigation of polygeneration plants utilizing solar energy in order to produce power products on the scale of a few MW. Parabolic trough collectors are used in order to exploit solar irradiation and to provide the generated heat to the polygeneration/trigeneration cycle in a high-temperature range (500–570 °C). The plants under consideration additionally consist of a storage tank, a Brayton cycle with s-CO 2 , a LiBr-H 2 O absorption chiller and also the installation of a suitable thermodynamic organic Rankine cycle (ORC) cycle is examined in order to utilize waste heat for further power generation. The systems are first investigated for operation under steady-state conditions and then for dynamic conditions on an annual basis. Based on the final annual results of the plants, the plant in which ORC was not used shows annual electrical, exergy and energy efficiencies equal to 18.33%, 23.37% and 54.91% respectively, while the plant in which ORC was used similarly shows 19.05%, 23.63% and 49.00%. According to economic analysis, the payback period was found at 6.37 years without ORC and 6.51 years with ORC. Taking into account the operational and techno-economic results of the plants it can be concluded that the plants produce satisfactory energy products ensuring also the economic viability of the investment. [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]

Highlights • A solar organic Rankine Cycle was considered using dish collector as the heat source. • The performance of three cavity receivers was modeled and compared. • A hemispherical cavity receiver was experimentally investigated. • Alumina/thermal oil was investigated as the solar working fluid. • The system was evaluated at different sizes and concentrations of nanoparticles. Abstract The objective of this work is the investigation of a solar-driven organic Rankine cycle with nanofluid-based solar dish concentrator. Three different cavity receivers are tested with the alumina/oil nanofluid in order to determine the most effective choice. More specifically, the examined cavities are hemispherical, cubical, and cylindrical, while the organic Rankine cycle is a non-regenerated system with superheating, R601 working fluid and heat rejection temperature at 310 K. The analysis is perfumed using the first and the second thermodynamic laws. The organic Rankine cycle is investigated for different pressure in the turbine inlet and different nanoparticle concentrations are also studied. According to the results, the thermal performance of the cavity and the electricity production are improved by decreasing alumina nanoparticle size, and by increasing the nanofluid concentration. Moreover, the optimum pressure in the turbine inlet was found at 2 MPa. Finally, the hemispherical cavity receiver has been found to be the best choice with cubical to be the second one and the cylindrical to be the less effective choice. [ABSTRACT FROM AUTHOR]

• A parabolic trough collector with a linear V-Shape cavity receiver was studied. • The V-Shape cavity receiver was coupled with an organic Rankine cycle system. • The solar organic Rankine cycle system was evaluated exergetically and financially. • The optimum turbine inlet temperature is 592 K and the optimum pressure 6 MPa. • The simple payback period was found close to 6.01 years for the large scale setup. In this research, a parabolic trough concentrator with linear V-Shape cavity receiver was studied as the heat source of an organic Rankine cycle system. The solar organic Rankine cycle system was evaluated under exergy and economic analyses. Thermal oil was used as the solar working fluid, and ethanol was used as the organic working fluid under different turbine inlet temperatures and turbine inlet pressure. The influence of different operational parameters, including solar radiation, mass flow rate, and inlet temperature of solar working fluid was investigated on the performance of the solar organic Rankine cycle system. It was found that exergy gain and exergy efficiency of the solar system improved with increasing solar radiation, increasing inlet temperature, and decreasing the flow rate of the solar working fluid. The highest organic Rankine cycle efficiency and total efficiency were found to be 35%, and 25% at turbine inlet temperature of 592 K and turbine inlet pressure of 6 MPa, respectively. Finally, the lowest levelized cost of electricity, and the lowest payback period were calculated equal to 0.0716 (€/kWh), and 8.79 (years) for the optimum condition of the developed solar organic Rankine cycle system, respectively. The present study is beneficial for improving the performance of the solar organic Rankine cycle systems with parabolic trough concentrators in a simple and convenient way and the development of solar thermal technologies. [ABSTRACT FROM AUTHOR]