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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]

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

• 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]

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

Heat pumps are efficient and well-established technologies for providing the proper cooling load in the building sector. The objective of this work is the parametric investigation of a heat pump operating with the promising refrigerant R152a for different operating conditions. More specifically, the heat pump is studied for different ambient temperatures, different indoor temperatures and various compressor rotational speeds. The cooling capacity and the coefficient of performance (COP) are the most important parameters which calculated in every scenario. A detailed model is developed in Engineering Equations Solver (EES) and it is validated with literature data. According to the final results, the system can operate in nominal conditions with 5 kW cooling capacity and a COP equal to 6.46. It is found that the COP can be ranged from 4 to 12 and the cooling capacity, while the cooling capacity can reach up to 9 kW. Moreover, a regression equation about the performance of the system is suggested. The obtained results indicate that the use of the R152a leads to high performance and so it can be an environmentally friendly choice for the cooling systems. [ABSTRACT FROM AUTHOR]

In this study, a solar driven trigeneration system is investigated and optimized in energetic and exergetic terms. Parabolic Trough collectors (PTC) are used for supplying the demanded heat input in the present configuration which includes a storage tank, an Organic Rankine Cycle (ORC) module and an absorption heat pump operating with the LiBr-H 2 O working pair. The heat source from solar collectors drives the ORC which rejects heat to the generator of the absorption heat pump. This heat pump produces heating and cooling demand simultaneously, while electricity is produced from the ORC. The examined system is analyzed parametrically in steady state conditions with Engineering Equator Solver (EES) tool by keeping the collecting area equal to 1000 m 2 in all the examined cases. Eight different working fluids are tested in the ORC, while six extra design parameters are studied. The inlet temperature in the heat recovery system, the superheating, the maximum pressure in ORC, the heat rejection temperature of ORC, the evaporating temperature of the heat pump, as well as the solar beam irradiation are the examined parameters. According to the final results, toluene is the working fluid which leads to maximum exergetic output with n-octane and MDM to follow with 29.42%, 28.50% and 28.35% respectively. In the optimum case, the electricity production of the system is 177.6 kW el , while the cooling and the heating outputs are 398.8 kW and 972 kW respectively. [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]

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]

The objective of the present study is the parametric investigation of a ground source heat pump for space heating purposes with boreholes. The working fluid in the heat pump is CO2, and the geothermal field includes boreholes with vertical heat exchangers (U-tube). This study is conducted with a developed model in Engineering Equation Solver which is validated with data from the literature. Ten different parameters are investigated and more specifically five parameters about the heat pump cycle and five parameters for the geothermal unit. The heat pump's examined parameters are the high pressure, the heat exchanger effectiveness, the temperature level in the heater outlet, the flow rate of the geothermal fluid in the evaporator and the heat exchanger thermal transmittance in the evaporator. The other examined parameters about the geothermal unit are the ground mean temperature, the grout thermal conductivity, the inner diameter of the U-tube, the number of the boreholes and the length of every borehole. In the nominal design, it is found that the system's coefficient of performance is 4.175, the heating production is 10 kW, the electricity consumption is 2.625 kW, and the heat input from the geothermal field is 10.23 kW. The overall resistance of the borehole per length is 0.08211 mK/W, while there are 4 boreholes with borehole length at 50 m. The parametric analysis shows the influence of the ten examined parameters on the system's performance and on the geothermal system characteristics. This work can be used as a reference study for the design and the investigation of future geothermal-driven CO2 heat pumps. [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]

The utilization of solar irradiation in the building sector is vital to create sustainable systems. Trigeneration systems are highly efficient systems that usually produce electricity, heating and cooling which are the main energy needs in the buildings. The objective of this work is the energetic and financial investigation of three different solar-driven trigeneration systems that can be applied in buildings with high energy needs (e.g., hospitals or commercial buildings). The parabolic trough solar collector (PTC) is selected to be used because it is the most mature solar concentrating technology. The examined configurations practically are different combinations of organic Rankine cycle (ORC) with heat pumps. System 1 includes a PTC coupled to an ORC which feeds an absorption heat pump machine. System 2 includes a PTC which simultaneously feeds an ORC and absorption machine. System 3 includes a PTC which feeds an ORC and a heat exchanger for heating, while the ORC is fed with and electricity a vapor compression cycle for cooling production. The simple payback period of System 1 is 5.62 years and it is the lowest, with System 2 to have 7.82 years and System 3 to have 8.49 years. The energy efficiency of the three systems is 78.17%, 43.30% and 37.45%, respectively, while the exergy efficiency 15.94%, 13.08% and 12.25%, respectively. System 1 is the best configuration according to energy, exergy and financial analysis. This study is performed with developed thermodynamic models in Engineering Equation Solver and a dynamic model in FORTRAN. [ABSTRACT FROM AUTHOR]

This article presents a simulation study which focuses on the thermodynamic analysis of a solar-driven trigeneration system for heating, cooling, and electricity production. The system uses parabolic trough collectors operating with Therminol VP-1 for feeding an organic Rankine cycle operating with toluene and an absorption heat pump operating with a LiBr–H2O working pair. The collecting area is selected at 100 m2 and the storage tank at 4 m3. The system is studied parametrically in order to examine the impact of various parameters on the system energy efficiency, system exergy efficiency, electricity production, heating production, and cooling production in the simple payback period of the investment. The examined parameters are the following: solar beam irradiation level, solar beam irradiation angle, superheating degree in the turbine inlet, pressure level in the turbine inlet, heat source temperature level, generator temperature level, and the heat input in the generator. For the nominal case of a 15 kW generator input, the electricity production is 6.3 kW, the heating production 11.5 kW, and the cooling production 10.7 kW. The system energy efficiency is 40.7%, while the system exergy efficiency is 12.7%. The financial investigation of the investment proved that it is viable with the simple payback period to be 8.1 years in the nominal case and it can be reduced to 7.8 years with an optimization procedure. Lastly, it has to be said that the examined system is found to be a viable configuration which is an ideal choice for application in the building sector. The analysis was conducted under steady-state conditions with a model developed using Engineering Equation Solver (EES). [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 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]

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