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The aim of the present work is the analysis of a solar-driven unit that is located on the non-interconnected island of Kythnos, Greece, that can produce electricity and green hydrogen. More specifically, solar energy is exploited by parabolic trough collectors, and the produced heat is stored in a thermal energy storage tank. Additionally, an organic Rankine unit is incorporated to generate electricity, which contributes to covering the island's demand in a clean and renewable way. When the power cannot be absorbed by the local grid, it can be provided to a water electrolyzer; therefore, the excess electricity is stored in the form of hydrogen. The produced hydrogen amount is compressed, afterward stored in tanks, and then finally can be utilized as a fuel to meet other important needs, such as powering vehicles or ferries. The installation is simulated parametrically and optimized on dynamic conditions, in terms of energy, exergy, and finance. According to the results, considering a base electrical load of 75 kW, the annual energy and exergy efficiencies are found at 14.52% and 15.48%, respectively, while the payback period of the system is determined at 6.73 years and the net present value is equal to EUR 1,073,384. [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]

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

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

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]

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

Solar 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]

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