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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]

The exploitation of solar irradiation in the building sector is a promising way to achieve decarbonization and reduce the operating costs of the building. The potential of solar energy in Greece is high and therefore this idea can lead to excellent results. In this direction, the goal of the present analysis is the detailed investigation of different solar thermal systems for domestic hot water production in the different climate zones of Greece. Four typical cities are studied in order to examine the climate zones A, B, C, and D, while three different solar thermal collectors coupled to insulated storage tanks are studied for every city. The simple flat plate collector, the advanced flat plate collector, and the evacuated tube collector are the selected solar systems for the present work. The climate data and the characteristics of the solar thermal systems follow the Greek Chamber regulations known as "KENAK". The analysis is conducted by following the well-known f-chart method and every design is optimized by using energy and financial criteria. The final design is determined by conducting a multi-objective evaluation methodology. It is notable to state that the collector slope and the collecting area are important parameters of this work, while the study case regards a building with 30 residents. According to the final results, the advanced flat plate collector is the best choice according to the multi-objective evaluation procedure. [ABSTRACT FROM AUTHOR]

The objective of this paper is the investigation of the annual performance of a solar power plant with linear Fresnel reflectors in the El-Oued region at Algeria. The solar collectors produce water steam that feeds a turbine to produce electricity. The System Advisor Model (SAM) tool is used for simulation. The mean net daily electricity production rate from 8:30 am to 5:30 pm is 48 MWe, and the respective annual production is 210,336 MWh/year. The mean daily optical efficiency of the solar field was close to 52%, while the mean thermal efficiency was about 39%. The net daily cycle efficiency is found to be 24%. The net capital cost of the examined system is $393 million, and the developer net present value is $47 million; the investor net present value is $15 million, the entire period of capital recovery is 11 years, and the levelized cost of electricity is 0.0382 $/kWh. The solar power plant leads to the yearly avoidance of 420,672 tons carbon dioxide emissions (operational cost savings of $6.1 million). Based on the obtained results, linear Fresnel reflectors can be used to achieve satisfying, energetic, financial, and environmental performance that can lead to sustainability. [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 solar collector (reflector and receiver) is the primary device being used in the concentrating solar power technologies for tapping the solar energy to meet various objectives. The performance of the solar collector is influenced by the type of reflector and receiver being selected, and its material also has significant impact. The choice of the heat transferring medium, storage material, receiver design and its performance parameter also influences the solar collector's performance. In this regard, a review has been carried out for these parameters and presented in this paper to highlight the challenges encountered, new methodologies identified and proposed, and future research needs by various researchers to improvise the thermal efficiencies of the different types of collectors used in the concentrating solar power technologies. [ABSTRACT FROM AUTHOR]

Abstract Parabolic trough collector (PTC) is the most established solar concentrating technology worldwide. The conventional parabolic trough collectors are used in various applications of medium and high-temperature levels. However, there are numerous studies which investigate alternative designs. The reasons for examining different PTC configurations regard the thermal efficiency increase, the reduction of the manufacturing cost and the development of more compact designs. The objective of this review paper is to summarize the existing alternative designs of PTC and to suggest the future trends in this area. Optical and thermal modifications are examined, as well as the use of concentrating thermal photovoltaic collectors. The optical modifications include designs with secondary concentrators, stationary concentrators and strategies for achieving uniform heat flux. The thermal modifications regard the use of nanofluids, turbulators and the use of thermally modified receivers with insulation, double-coating and radiation shields. The concentrating thermal photovoltaics are systems with flat or triangular receivers which can operate in low or in medium temperature levels with the proper alternative designs. It has been found that there are many promising choices for designing PTC with higher thermal performance and lower cost. The conclusions of this work can be used as guidelines for future trends in linear parabolic concentrating technologies. [ABSTRACT FROM AUTHOR]

Highlights • A secondary booster reflector is examined in a parabolic trough collector. • The optical enhancement is found to be higher in greater incident angles. • The optical enhancement is greater during the winter period. • The yearly enhancement is found to be 21.7% for Athens, Greece. • The thermal enhancement is more intense in higher operating temperatures. Abstract The objective of this work is the investigation of a novel booster reflector in a parabolic trough collector (PTC). More specifically, a vertical secondary booster reflector is located at the end of the trough in order to enhance the optical performance of the solar collector. This extra concentrator reduces the end optical losses of the PTC by reflecting back them in the receiver. Moreover, it acts as an extra reflecting area which captures extra solar beam irradiation in the cases with a non-zero incident angle. The collector is examined optically and thermally in order to determine both the optical and the thermal gain by the use of this novel idea. The overall analysis is conducted for steady-state conditions, as well as the yearly evaluation of the suggested idea is given. According to the final results, the use of the extra booster leads always to optical and thermal enhancement, especially in greater incident angles. The yearly optical enhancement, for a collector with a focal distance to length ratio of 0.236, is calculated at 21.7% and it is more intense in the months with greater incident angles. More specifically, it was calculated at 8.7% in June, 24.2% in September and 77.1% in December. The thermal analysis proved that the thermal enhancement is greater than the optical and it is higher when the system operates with great fluid temperature levels. [ABSTRACT FROM AUTHOR]

Solar concentrating solar thermal collectors are promising technologies for various applications which demand medium- and high-temperature levels. The objective of this work is to review the recent trends in the solar concentrating collectors and to give the emphasis on the performance enhancement methods which applied to the concentrating technologies. Optical and thermal enhancements methods are investigated for the following collector types: compound parabolic concentrator, parabolic trough collector, linear Fresnel reflector and solar dish concentrator. The emphasis is given to the utilization of nanofluids as working fluids because a lot of research has been focused on them in the last years. Moreover, the use of internal fins and inserts in the flow, the use of modified absorbers, as well as various optical design optimizations are included in this review. The final conclusions of this work clearly indicate the most effective enhancement methods in the concentrating solar collectors, as well as the future fields that have to be investigated. [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]

Parabolic trough collector is one of the most usual solar collectors for applications up to 400 °C. The thermal enhancement of this collector concentrates a lot of interest and various techniques are tested in order for the thermal efficiency to be maximized with a reasonable penalty in the pressure drop. The use of nanofluids as working fluids, as well as the use of flow turbulators, mainly inserts and internal fins or tube dimples are the main techniques which are examined. The objective of this work is to give a complete literature review of the existing studies on this domain and to present a numerical comparative analysis between the most usual thermal enhancement techniques. More specifically, the use of oil-based nanofluids with 6% CuO is compared with the use of internal rectangular fins in the absorber, while the combination of these techniques is also examined. The analysis is conducted with a validated CFD model in SolidWorks Flow Simulation for various fluid temperature levels. According to the final results, the use of nanofluids leads to 0.76% thermal efficiency enhancement, the use of internal fins to 1.10% and the combination of these techniques to 1.54%. Moreover, emphasis is given in the pressure drop of the examined cases and in the evaluation criteria which are used in every case. [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 use of solar dish thermal collectors is a promising choice for designing sustainable energy systems. The use of nanofluids is a new way for enhancing the thermal performance of solar collectors because of their improved thermal properties. The objective of this study is to investigate the use of mono and hybrid nanofluids in a solar dish collector in order to determine which kind of nanofluids leads to higher performance enhancements. The analysis is conducted with a developed thermal model in Engineering Equation Solver and the collector is studied thermally and exergetically. The examined hybrid nanofluid has as base fluid syltherm 800 with 1% Cu and 1% TiO2. Moreover, the examined mono nanofluids are the syltherm 800 with 2% Cu and syltherm 800 with 2% TiO2. The investigated solar dish collector has a spiral absorber and it is examined for inlet temperatures from 25 °C up to 300 °C with a flow rate of 200 L/h. According to the final results, the use of hybrid nanofluid leads to higher thermal efficiency enhancement compared to the mono nanofluids because of the higher increase in the Nusselt number in the flow. More specifically, the use of the hybrid nanofluids leads to 0.99% mean thermal efficiency enhancement compared to the pure oil case, while the use of Oil/Cu and Oil/TiO2 lead to 0.42% and to 0.56% mean thermal efficiency enhancement, respectively. Moreover, the exergy efficiency is found enhanced with the use of all nanofluids. The mean exergy efficiency enhancement is 1.21% with the hybrid nanofluid, while it is 0.73% with Oil/TiO2 and 0.53% with Oil/Cu. [ABSTRACT FROM AUTHOR]

Solar linear Fresnel reflector (LFR) is a promising concentrating technology, which presents important advantages such as the low investment cost, the reduced wind loads and the relatively low land utilization factor. The objective of this work is to investigate an innovative way for enhancing the thermal performance of LFR, especially at high temperatures. The utilization of nanofluid as heat transfer fluid is the investigated thermal enhancement method and more specifically the use of CuO nanoparticle dispersed on Syltherm 800 (6% volumetric concentration). The examined collector has total net aperture equal to 154 m 2 and concentration ratio of 58.36. The primary reflectors are curved mirrors, the secondary reflectors have compound parabolic shape and the receiver is an evacuated tube. The operation with nanofluid is compared to the operation with pure thermal oil for various inlet temperatures from 350 K up to 650 K and flow rate equal to 200 L/min. According to the final results, the maximum thermal efficiency enhancement with the nanofluid is close to 0.8%, while the pumping work demand is increased up to 50% with the nanofluid. Various criteria like the exergy efficiency, overall efficiency and entropy generation are applied in order to evaluate the nanofluid utilization properly. Finally, the operation with nanofluid is found to be beneficial, especially in high-temperature levels. The analysis is conducted with SolidWorks Flow Simulation with a validated model. [ABSTRACT FROM AUTHOR]

The objective of this paper is to investigate a solar cooling system with an absorption chiller driven by nanofluid based flat plate collectors. Pure water and water/Cu nanofluid (2% volumetric concentration) are the examined working fluids on the solar field and they are compared for steady-state conditions, as well as for daily operation. The system is optimized for steady-state conditions using a multi-objective procedure with energetic and exergetic criteria. The mean thermal efficiency enhancement by the use of nanofluids it is found close to 2.5%. The optimized system is further examined on a daily basis for operation with pure water and nanofluid. According to the final results, the use of nanofluids in the solar collector is able to enhance the exergetic performance of the system close to 4.0% and to increase the refrigeration production approximately 0.84% on a daily basis. The analysis is conducted with a developed model in Engineering Equation Solver. [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 investigate experimentally and numerically a solar dish collector and to examine the impact of possible improvements in its performance. The examined solar dish collector has a spiral absorber and it is lightweight, fact that makes the system to have low cost and to be sustainable. The experimental results are compared with the results of a developed numerical model written in EES (Engineering Equation Solver) and validation between the results is observed. The validated numerical model is used for further investigation of the solar collector for operation with Therminol VP1. In the first part of the investigation, the impact of flow rate on the thermal and the exergetic performance is examined, and finally 200 l/h was found as the best solution. The next part is the optical investigation of the collector and greater optical efficiencies are tested. It is found that optical performance of 75% leads to maximum exergetic efficiency of 22.49%, three times greater than 7.49% which corresponds to the present situation of 35% optical efficiency. In the last part of this study, the selective absorber is compared to the non-selective of the real system and it is found that the use of the first is vital for achieving high operating temperature levels. The results of this study can be used as guidelines for the future improvement of the present facility. [ABSTRACT FROM AUTHOR]

The main objective of this work is to investigate the utilization of nanofluids in the solar field in order to achieve higher system performance. An Organic Rankine Cycle (ORC) driven by solar parabolic trough collectors (PTCs) is the examined system. Four different nanoparticles are examined (Al 2 O 3 , CuO, TiO 2 and Cu) in the base fluid (Syltherm 800), as well as the pure thermal oil is examined as working fluid. The examined ORC is a regenerative cycle and four organic fluids are also tested (toluene, MDM, cyclohexane and n-pentane). In every combination between the organic fluid in ORC and working fluid (nanofluid) in the solar field, an optimization procedure is followed. The concentration of every nanoparticle and the pressure ratio (pressure in the turbine inlet to critical pressure) are the optimization parameters. According to the final results, toluene is the best organic fluid and CuO is the most suitable nanoparticle. The combination of these two working fluids leads to 167.05 kW electricity production and to 20.11% system efficiency with concentration 4.16%. The enhancement by the use of nanofluids is found up to 1.75% compared to the respective case with pure thermal oil and this result indicates that the use of them is able to improve the performance of solar driven ORCs. For the other nanoparticles and Toluene in the ORC, Cu, Al 2 O 3 and TiO 2 lead to 166.18 kW, 165.72 kW and 165.60 kW electricity productions respectively with optimum concentrations 3.98%, 2.51% and 2.57% respectively. [ABSTRACT FROM AUTHOR]

Parabolic trough collector (PTC) is the most mature concentrating technology which is used in a great variety of thermal applications. In this study, the daily performance of a PTC is investigated for all the year period. The module of Eurotrough parabolic trough collector is examined with various storage tanks from 3 m 3 to 15 m 3 and the collector performance is investigated in energetic and exergetic terms. An Input/Output method is applied for the daily evaluation of the solar thermal system, something innovative for concentrating solar collectors. According to the final results, different storage tank volumes lead to similar thermal outputs but to extremely different exergetic outputs because of the high dependence of the mean storage tank temperature with the storage tank volume. For the storage tank of 9 m 3 , the mean thermal efficiencies in December and in June are found to be 37.1% and 68.7% respectively, while the mean exergetic efficiencies 2.0% and 8.5% respectively. Moreover, it is proved that the Input/Output performance method in concentrating collectors has to take into consideration the solar incident angle at solar noon in order high accuracy to be achieved. [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 study is to compare energetically and financially a solar assisted heat pump heating system powered by flat plate collectors with a conventional air source heat pump system for twenty European cities. The examined buildings have 100 m 2 floor area and they are identical among the examined cases in order to perform a comparison under the same terms. Only the insulation thickness varies, examining two insulation scenarios with 4 cm and 8 cm insulation layer thickness. According to the final results, the electricity savings using the solar assisted heat pump system are ranged from 30% to 40%, indicating great sustainability in both insulation scenarios. The COP in the solar assisted systems is found close to 4 and for the conventional air source systems close to 2.5. The solar assisted heating system is proved to be also the financially optimum solution for the majority of the examined cities and especially in low insulation cases. More specifically, this system is proved to be ideal for cities with high solar potential and not extremely low heating loads. The simulation tool is TRNSYS and the analysis is performed for all the winter period. [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 efficient conversion of solar radiation into heat at high temperature levels requires the use of concentrating solar collectors. The goal of this paper is to present the optical and the thermal analysis of a parabolic dish concentrator with a spiral coil receiver. The parabolic dish reflector consists of eleven curvilinear trapezoidal reflective petals constructed by poly (methyl methacrylate) with silvered mirror layer and has a diameter of 3.8 m, while its focal distance is 2.26 m. This collector is designed with commercial software SolidWorks and simulated, optically and thermally in its Flow Simulation Studio. The optical analysis proved that the ideal position of the absorber is at 2.1 m from the reflector in order to maximize the optical efficiency and to create a relative uniform heat flux over the absorber. In thermal part of the analysis, the energetic efficiency was calculated approximately 65%, while the exergetic efficiency is varied from 4% to 15% according to the water inlet temperature. Moreover, other important parameters as the heat flux and temperature distribution over the absorber are presented. The pressure drop of the absorber coil is calculated at 0.07 bar, an acceptable value. [ABSTRACT FROM AUTHOR]

The integration of solar energy systems is crucial to achieving the global goals for sustainability as well as the reduction of greenhouse gases and pollutants emissions. In this direction, the development of multigeneration systems is very important as they can cover the increasing and numerous demands of modern society in a high-efficient way. In parallel, hydrogen is a promising alternative fuel that has gained attention recently because it can substitute conventional energy sources. So, the objective of the present work is the review, classification, discussion, and comparison of solar-driven multigeneration systems that include hydrogen production. This study is a novel one because it is focused on the integration of hydrogen generation methods into polygeneration systems, while existing publications present the useful outputs of the units, without specialization. In parallel, it meets the increasing need for the investigation of hydrogen energy potential. Apart from hydrogen, other common useful outputs are electricity, heating, cooling, domestic hot water, and freshwater. In this work, the plants are classified according to the hydrogen production method. In parallel, different types of solar devices, such as thermal collectors, photovoltaics, and thermal photovoltaics as well as other additional energy sources are also examined. So, numerous configurations are investigated comprehensively, which can produce a plethora of useful outputs, as well as include thermodynamic cycles, such as Rankine cycles, organic Rankine cycles, and other devices, such as absorption chillers, or desalination units. All the units are evaluated and compared in terms of energy, and exergy performance as well as economic viability, and hydrogen production rate. In general, solar tower-based and parabolic trough collector-based units are high-efficient energetically and exergetically, while concentrating photovoltaic thermal plants achieve high energy efficiency. Finally, suggestions for future research and development are made. • Solar-driven multigeneration systems with hydrogen production are investigated. • The systems are mainly classified according to the hydrogen generation methods. • Solar collectors, photovoltaics, and hybrid solar devices are examined. • Hybridization with auxiliary energy sources is also presented. • Solar towers, PTC, and CPVT are the most efficient systems. [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]

Solar irradiation is a major contributor to renewable energy sources and its utilization is vital for designing a sustainable and green energy network. Greece is a country with high solar potential and thus the exploitation of solar irradiation in this country is an urgent need in order to reduce fossil fuel utilization. Moreover, the increase in solar energy exploitation will lead to the mitigation of environmental policy goals for cleaner energy production processes. To date, Greece exploits solar irradiation mainly with flat plate collectors for low-temperature heating applications and with photovoltaics, while there are no installations including concentrating solar systems although they are attractive technologies. The objective of the present review paper is to demonstrate and to discuss the present steps in the domain of solar concentrating systems in Greece, as well as to suggest future steps aiming to increase the solar energy contribution in this country. More specifically, this work includes studies about parabolic trough collectors, linear Fresnel reflectors, compound parabolic concentrators, solar dishes, solar tower and concentrating thermal photovoltaics. The existing situation regarding solar energy utilization in Greece is collaborated and predictions about future perspectives are provided. It is concluded that although much research interest has been devoted to installations in Greece, no real solar concentration systems are currently in operation. Moreover, the existing studies indicate that solar concentrating systems lead to sustainable and viable investments with acceptable payback period time in the range of 5–7 years. As a result, it is suggested that this is the time to proceed in the next step of the development of real solar concentrating installation in Greece. • All the solar concentrating systems for the Greek climate are included in this review. • Applications for power, refrigeration, heating and polygeneration are presented. • Up today, many theoretical studies have been conducted for Greek solar systems. • In the future, there is a need for real applications and experimental studies. • The examined cases are found to be viable for the climate conditions of Greece. [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]

The thermal enhancement of parabolic trough collectors is a critical issue and numerous ideas have been applied in the literature on this domain. The objective of this paper is to investigate some usual thermal enhancement techniques for improving the performance of evacuated and non-evacuated receivers of parabolic trough solar collectors. More specifically, the use of twisted tape inserts, perforated plate inserts, and internally finned absorbers are compared with the reference case of the smooth absorber. The analysis is conducted with a developed and validated thermal model in Engineering Equation Solver. The collector is investigated for a typical flow rate of 100 L/min and for inlet temperatures between 50 °C and 350 °C with Syltherm 800 as working fluid. According to the final results, the use of internally finned absorber leads to the highest thermal efficiency enhancement, which is up to 2.1% for the non-evacuated collector and up to 1.6% for the evacuated tube collector. The perforated plate inserts and the twisted tape inserts were found to lead to lower enhancements, which are up to 1.8% and 1.5%, respectively, for the non-evacuated collector, while they are up to 1.4% and 1.2%, respectively, for the evacuated collector. Moreover, the pressure drop increase with the use of the thermal enhancement methods is investigated and the use of internally finned absorber is found again to be the superior technique with the performance evaluation criterion to be ranged from 1.5 to 1.8 for this case. [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]

A low cost solar collector with a dish reflector and spiral absorber is examined in this work. This collector is investigated experimentally and numerically with a developed thermal model in the Engineering Equation Solver (EES). Numerical simulations are performed by the commercial software OptisWorks. The solar ray distribution inside these receiver geometries, including the helical coil used for the heat transfer fluid, was determined using this tool. The final results show that the thermal performance is about 34%, due to the high rate of thermal losses. After validating the numerical model, it is used for investigating the collector for various operating conditions. Three working fluids (Water, Therminol VP-1 and Air) are compared energetically and exergetically for various combinations of volumetric flow rates and operational temperature levels. The results proved that water is the most appropriate working fluid, among those investigated, as it is able to efficiently work at low temperature levels, while the thermal oil is the best at higher temperature values, according to thermal analysis. The exergetic analysis showed that air is the best choice in low temperatures and thermal oil in greater temperatures. Finally, an open receiver of a conical cavity shape with a helical tube was optically investigated, as a second strategy for enhancing the optical performance of the receiver. The results show that an average flux value of about 2.6 × 105 W/m2 was absorbed by the helical conical shape with an aperture area of 0.01606 m2. [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]

The objective of the present research work is to investigate a novel high-efficiency nanofluid in a solar dish concentrator by using the numerical model developed. The working fluids examined consisted of soybean oil-based MXene nanofluid of different concentrations (i.e. 0.025, 0.075 and 0.125 wt%) and pure soybean oil. The studied nanofluids yielded excellent thermal properties such as high thermal conductivity and heat capacity, which were two particular factors rendering them excellent candidates for solar thermal applications. The solar dish collector was evaluated for three different cavity receivers including cubical, hemispherical and cylindrical shapes. Then, thermal analysis was performed with a developed numerical model in steady-state conditions, which was validated by using experimental results. Meanwhile, the thermal properties of the oil-based nanofluid were described after the experiments. The analysis was parametric in nature, thereby studying the system performance on a daily basis. According to the analysis, the hemispherical cavity receiver led to maximum thermal efficiency with the nanofluid used. In particular, the daily mean thermal efficiency with nanofluid of 82.66% and the mean equivalent efficiency of 82.46% were achieved, while the mean enhancement was 0.6%. However, the enhancements were higher with the use of other cavities due to the higher thermal losses shown in such cases. Moreover, the equivalent efficiency proved that the increased pumping work due to the use of nanofluid could not overcome the thermal enhancement, thereby improving the overall performance of the solar collector. • Novel soybean oil-based MXene nanofluid is investigated in a solar dish collector. • The hemispherical cavity revealed as the best candidate for all examined working fluids. • The daily mean thermal efficiency with nanofluid was recorded as 82.66%. • Low volumetric flow rates in the developed nanofluid enhance the thermal efficiency. • The daily yield of the examined solar dish collector is 9.07 kWh. [ABSTRACT FROM AUTHOR]

The concentrating solar systems of linear Fresnel reflectors offer the ability to generate electricity from solar energy. The main objective of this paper is to perform a transient numerical simulation on a linear Fresnel solar reflector directed to produce superheated water steam for the power plants, in order to determine its efficiencies (optical and thermal), where the heat transfer fluid mass flow inside the copper absorber tube is 0.045 kg/s. The dimensions of the studied linear Fresnel solar collector are the same dimension as the modular system Nova-1 of NOVATEC SOLAR Company. The dimension of the studied model is 1D. For this purpose, El-Oued region has been designated as a study field. The twelve recommended average days for months have been selected for the study. This paper was based on a numerical solution using the finite differences method. MATLAB has been used as a software language. The optical efficiency of the LFR solar receiver has reached 53.7% where the thermal efficiency has reached 37.5%, while the pressure drop within the absorber pipe has reached 02 bars. As for superheated steam temperatures variations, it ranged between 233.35 °C and 263.75 °C, knowing that the water steam inlet temperature when entering the absorber tube is equal to 140 °C. As for the overall coefficient of heat loss, its mean value is 5.84 W/m2.°C. The results obtained are very encouraging to invest in this steam power plant, especially in the desert areas, which are rich in solar energy. Image 1 • Optical efficiency of the solar reflector obtained was 53.7% • Thermal efficiency of the solar reflector achieved was 37.5% • An annual electricity capacity of more than 2000 MW can be produced • This technique is 100% clean and has no negative environmental impact [ABSTRACT FROM AUTHOR]

Abstract Solar concentrating technologies can produce heat for applications such as solar heating, solar cooling, industrial processes, desalination and electric power generation. For a solar dish collector, various solar receivers and working fluids at different flow rates can be used in different applications. In this work, three different cavity receivers are investigated for application in a solar dish collector using either water or Behran oil. A numerical model is used in the analysis, which is validated with experimental results from a hemispherical cavity receiver using oil as working fluid. The model is applied to compare hemispherical, cylindrical and cubical receivers under the same operating conditions using either water or oil, at a volumetric flow rate of 100 ml/s and solar irradiance of 800 W/m2, in order to determine the most suitable cavity for a specific solar dish. The system is investigated for inlet temperatures ranging from 40 °C to 90 °C with water as working fluid, and from 40 °C to 300 °C with Behran oil as working fluid. Emphasis is placed on the calculation of useful heat production, as well as pressure drop which influences pumping power. The exergetic efficiency criterion and the overall efficiency criterion are used in order to evaluate the useful heat production and the pumping power simultaneously. The high exergetic efficiency of the hemispherical cavity with thermal oil at high temperatures makes this case a promising choice for high-temperature solar dish collector applications. Moreover, water is found to be the best candidate for low-temperature applications since it leads to the higher thermal efficiency with lower pumping power demand. Highlights • The thermal performance of three cavity receivers was modeled and compared. • Hemispherical, cylindrical, and cubical receivers were investigated. • Water and Behran thermal oil were used as the solar working fluid. • It was found that the best cavity was the hemispherical receiver. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • It was an experimental study of nanofluid in a solar dish collector. • A hemispherical cavity receiver was used as the dish absorber. • MWCNT/thermal oil nanofluid was tested as the solar working fluid. • Performance of the nanofluid was compared with the pure oil. • Two thermal efficiency models were suggested for the nanofluid and the pure oil. Abstract Nowadays, nanofluids are introduced as an effective technique for enhancing the thermal efficiency in solar collectors. In this work, the MWCNT/oil nanofluid with 0.8% nanoparticle mass fraction is investigated experimentally in a solar dish concentrator coupled to a hemispherical cavity receiver. The main objective of this research is to examine the effect of the nanofluid application for improving the solar thermal performance of the hemispherical cavity receiver. The results revealed that thermal efficiency of the hemispherical cavity receiver has averagely increased equal to 12.93%, using application of the MWCNT/oil nanofluid compared to the application of the basefluid. In the steady state periods, the average thermal efficiency of 76.23% and 67.50% were calculated for the hemispherical cavity receiver using the MWCNT/oil nanofluid and pure oil, respectively. It was conducted that the receiver heats gain and the thermal efficiency of the cavity receiver has a similar trend with the working fluid difference temperature. Two experimental models were suggested based on the overall performance versus T in - T amb I beam for the MWCNT/oil nanofluid and the pure oil. Finally, it is found that the use of the examined nanofluid is able to enhance the thermal performance of the examined collector about 13%. Also, in this study, group method of data handling (GMDH) artificial neural networks is applied in order to model the dependency of thermal efficiency and the cavity heat gain of the Hemispherical Cavity Receiver on the mentioned factors. [ABSTRACT FROM AUTHOR]

• Latest developments in solar thermal applications using nanofluids are reviewed • Low to medium temperature solar collectors are summarized • Improvements in solar collectors using nanofluids as working fluid is compiled • Application of solar collectors in buildings is reviewed The efficient exploitation of solar irradiation is one of the most encouraging ways of handling numerous environmental concerns. Solar collectors are suitable devices that capture solar irradiation and convert it into thermal energy and electricity. In the last years, the nanofluids used in solar thermal systems have been studied as a useful technique for enhancing the solar collectors' performance and establishing them as viable and highly efficient systems. The present review paper aims to summarize and discuss the most important numerical and experimental studies in nanofluid-based solar systems for application at low and medium temperature levels, while the emphasis on the fundamental physical phenomena that occur. In the first part, numerous numerical models and the principal physical phenomena affecting the heat transfer rate in the nanofluid have been analyzed. More specifically, the importance of different forces in nanofluid flows that exist in particulate flows such as drag, lift (Magnus and Saffman), Brownian, thermophoretic, Van der Waals, electrostatic double-layer forces are considered. Moreover, an overview of the thermophysical properties, physical models, heat transfer models, and evaluation criteria of nanofluids are included in this work. In the second part, which is the main part of this work, a comprehensive review is performed to gather and discuss the new advantages in the nanofluid-based solar collectors that operate at low and medium temperatures. More specifically, the examined solar systems are the flat plate collectors, the evacuated tube collectors, the direct absorption collectors, and the thermal photovoltaic systems, while the investigated applications are space-heating, space-cooling, household hot water production, desalination, industrial activities, and power generation. The aforementioned collectors and applications are the most usual in the real systems, indicating the importance of the present work. Moreover, the emphasis is given in the thermal, exergy, economic, and environmental evaluation of the studied systems, as well as in the discussion of the possible limitations of the use of nanofluids like the lack of long-term stability, the agglomeration of nanoparticles, and the increased pumping work due to the increased pressure drop. Finally, it is found that the nanofluid utilization usually enhances the collector efficiency up to 5%, while higher enhancements can be found in thermal photovoltaics. Moreover, it is concluded that there is a need to emphasize issues such as stability and the use of eco-friendly solar systems. Lastly, the field's future trends are highlighted, and a clear image of the present situation and the next steps in the field are given. [ABSTRACT FROM AUTHOR]