Reducing the optical end losses of a linear Fresnel reflector using novel techniques

The objective of this work is the investigation of some alternative ideas for enhancing the optical performance of a linear Fresnel reflector (LFR) with North-South orientation. More specifically, the examined methods aim to reduce the optical end losses which are crucial for the short LFR plants in particular during the winter period. The first studied idea is the extension of the receiver after the concentrator which is able to enhance the yearly mean incident angle modifier up to 50.3%. The second examined idea is the displacement of the receiver in order to eliminate the non-illuminated area at the beginning of the receiver. This idea proved that the mean yearly incident angle modifier can be enhanced up it 20.2% for a displacement equal to 20% of the concentrator length. The third examined idea is the hybrid design with the extended and displayed receiver in combined. This idea leads to intermediates enhancements compared to the previous cases but its advantage is the lower investment cost compared to the simple receiver extension case. The analysis is performed with a developed optical model in SolidWorks Flow Simulation and the yearly evaluation has been done for the location of Athens (Greece).Bodossaki Foundationhttp://www.elsevier.com/locate/solener2020-07-01hj2020Mechanical and Aeronautical Engineerin

Experimental investigation and parametric analysis of a solar thermal dish collector with spiral absorber

Solar-tracking dish collectors are a potential alternative to fossil fuels because of their high concentration ratios. Important considerations for solar collectors are manufacturing costs, complexity, efficiency, uniform flux distribution and working fluid selection. In this study, a simple, low-cost solar dish collector with a spiral absorber and lightweight structure is examined. Experiments were performed with water as working fluid where the volumetric flow rate, inlet and outlet temperatures, ambient temperature, air velocity and solar irradiation were measured. Experimental results were used to validate a numerical model developed in Engineering Equation Solver, where three working fluids (water, thermal oil and air) were considered in various operating conditions. According to the thermal analysis, water is the most appropriate working fluid for low-temperature applications and thermal oil the most appropriate for higher-temperature applications. The exergetic analysis, however, shows that air is the most appropriate for low-temperature applications and thermal oil the most appropriate for higher-temperature applications. The highest exergetic efficiency was observed for thermal oil with inlet temperature of 155 °C. The system can be feasible in areas with solar potential of more than 1600 kW h/m2 and where the cost of heating is more than 0.15 €/kW h.This paper is included in the research framework of Research Project: III42006 – Research and Development of Energy and Environmentally Highly Effective Polygeneration Systems Based on Renewable Energy Resources.The Ministry of Education, Science and Technological Development of the Republic of Serbia.http://www.journals.elsevier.com/applied-thermal-engineering/2018-07-05Mechanical and Aeronautical Engineerin

A Comparative Study of Solar-Driven Trigeneration Systems for the Building Sector

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

Polynomial Expressions for the Thermal Efficiency of the Parabolic Trough Solar Collector

The parabolic trough solar collector (PTC) is the most mature solar concentrating technology, and this technology is applied in numerous thermal applications. Usually, the thermal efficiency of the PTC is expressed with the aid of polynomial expressions. However, there is not a universal expression that is applied in all cases with high accuracy. Many studies use expressions with the first-degree polynomial, second-degree, or fourth-degree polynomial expressions. In this direction, this work is a study that investigates different expressions about the thermal efficiency of a PTC with a systematic approach. The LS-2 PTC module is examined with a developed numerical model in the Engineering Equation Solver for different operating temperatures and solar beam irradiation levels. This model is validated using experimental literature data. The found data are approximated with various polynomial expressions with up to six unknown parameters in every case. In every case, the mean absolute percentage error and the R2 are calculated. According to the final results, the use of the third power term leads to the best fitting results, as well as the use of the temperature difference term (ΔΤ), something that is new according to the existing literature. More specifically, the final suggested formula has the following format: “ηcol = a0 + a3∙ΔT3/Gb + b∙ΔΤ”. The results of this work can be used by the scientists for the optimum fitting of the PTC efficiency curves and for applying the best formulas in performance determination studies

Assessment of the thermal enhancement methods in parabolic trough collectors

Abstract Parabolic trough collector is an emerging solar technology for achieving the sustainability. Numerous studies have been focused on their performance evaluation and many techniques have been suggested for improving their thermal efficiency. The objective of this paper is to determine the impact of various thermal enhancement techniques on the thermal efficiency improvement of the PTC. The most usual techniques for increasing the thermal performance of parabolic trough collectors are the use of inserts, internal fins, metallic foams and the dimpled absorbers. A parametric analysis is conducted using different values of the thermal enhancement ratio (Nusselt number to the Nusselt number of the smooth absorber case). According to the final results, the thermal efficiency enhancement can reach up to 2% when the Nusselt number is about 2.5 times greater compared to the reference case and the inlet temperature is equal to 600 K. Moreover, in this case, the thermal losses are approximately 22% lower than in the respective reference case. For higher Nusselt number ratios, the thermal enhancement presents relatively small increase. This analysis is performed with a developed thermal model in Engineering Equation Solver (EES) which is validated with the literature results

Investigation of the Environmentally-Friendly Refrigerant R152a for Air Conditioning Purposes

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

Evaluation of a solar driven trigeneration system with conventional and new criteria

Solar-driven trigeneration systems are able to cover all the building energy needs in heating, cooling and electricity using only the solar energy. The objective of this paper is to evaluate a solar trigeneration system under different criteria in order to make clear that there are numerous factors which have to be taken into consideration in the trigeneration system design. The examined system consists of parabolic trough solar collectors, a storage tank, an organic Rankine cycle and an absorption heat pump. The system is evaluated using different evaluation criteria which are associated with the energy, the exergy and the financial performance of the system. Moreover, a new criterion which takes into account the building energy needs is introduced and is investigated in different scenarios. The final results of this work clearly indicate that the optimisation of the solar driven trigeneration systems is depended on the desired design conditions and goals. Abbreviations: COP, Coefficient of performance; ECO, Economizer; EES, Engineering Equation Solver; EVAP, Evaporator; ORC, Organic Rankine Cycle; PTC, Parabolic trough collector; SC, Scenari

Financial Optimization of a Solar-Driven Organic Rankine Cycle

The objective of this work is the financial optimization of a solar-driven organic Rankine cycle. Parabolic trough solar collectors are used as the most mature solar concentrating system and also there is a sensible storage system. The unit is examined for the location of Athens in Greece for operation during the year. The analysis is conducted with a developed dynamic model in the program language FORTRAN. Moreover, a developed thermodynamic model in Engineering Equation Solver has been used in order to determine the nominal efficiency of the cycle. The system is optimized with various financial criteria, as well as with energy criteria. The optimization variables are the collecting area and the storage tank volume, while the nominal power production is selected at 10 kW. According to the final results, the minimum payback period is 8.37 years and it is found for a 160 m2 collecting area and a 14 m3 storage tank, while for the same design point the levelized cost of electricity is minimized at 0.0969 € kWh−1. The maximum net present value is 123 k€ and it is found for a 220-m2 collecting area and a 14-m3 storage tank volume. Moreover, the maximum system energy efficiency is found at 15.38%, and, in this case, the collecting area is 140 m2 and the storage tank volume 12 m3. Lastly, a multi-objective optimization proved that the overall optimum case is for a 160-m2 collecting area and a 14-m3 storage tank

A Realistic Approach of the Maximum Work Extraction from Solar Thermal Collectors

In this study, the maximum work extraction from the incident solar energy on solar thermal collectors is investigated by coupling solar collectors with a Carnot machine. A simplified thermal model for the solar collector performance is developed in which the radiation losses play a significant role. In every examined case, the optimum operating temperature that leads to maximum work extraction is calculated. The final results are presented parametrically, covering a great variety of real solar collectors. Moreover, the validation procedure of the developed model proves high accuracy. The results show that non-concentrating collectors should operate up to 400 K while concentrating collectors in higher temperature levels. More specifically, a parabolic trough collector can operate efficiently in temperature levels up to 850 K, while solar dish collectors can operate efficiently in temperature levels up to 1100 K. The results of this study can be exploited for the preliminary design and optimization of solar thermal systems. Moreover, a clear and realistic upper limit concerning the exergy production of solar irradiation with solar thermal collectors is given

Parametric Analysis of a Polygeneration System with CO2 Working Fluid

The objective of the present work is the investigation of a novel polygeneration system for power, refrigeration and heating production at two temperature levels. The present system uses CO2 as the working fluid, which is an environmentally friendly fluid. The total configuration is a combination of a transcritical refrigeration cycle coupled to a Brayton cycle with recompression, which is fed by a biomass boiler. The examined system, at nominal operating conditions, produces refrigeration at 5 °C, and heating at 45 °C and 80 °C. Additionally, the system can be converted into a trigeneration system where the two heating outputs are produced at the same temperature level. The system was studied parametrically by changing the following seven critical parameters: turbine inlet temperature, high pressure, medium pressure, heat exchanger effectiveness, refrigeration temperature, heat rejection temperature and high heating temperature. In nominal operating conditions, the system energy and exergy efficiencies were 78.07% and 26.29%, respectively. For a heat input of 100 kW, the net power production was 24.50 kW, the refrigeration production was 30.73 kW, while the low and high heating production was 9.24 kW and 13.60 kW, respectively. The analysis was conducted with a developed model in Engineering Equation Solver