Search

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 compound parabolic concentrator is a promising technology for efficient solar irradiation exploitation at low- and medium-temperature levels. This collector type can be used in a series of applications, such as solar cooling, desalination, and industrial process heat applications. This work presents a novel compound parabolic concentrator that presents satisfying efficiency and low cost due to the use of flat glazing and not an evacuated tube receiver. More specifically, the goal of the present investigation is based on the energy and exergy analysis of a compound parabolic collector with flat glazing, which has a concentration ratio of 2.81. The collector is examined thermally and exegetically, aiming to calculate the efficiency of different operating inlet temperatures. Moreover, the solar unit is studied by a developed computational fluid dynamics model in the SolidWorks Flow Simulation tool. Emphasis is given to the calculation of the convection losses of the receiver tube with the internal air inside the collector. The heat convection coefficient is calculated, and the distribution of the thermal losses, convection, and radiation is presented. Furthermore, the temperature levels of the absorber, the cover glass, and the top thermal loss coefficient are found. The thermal efficiency of the solar unit was 77.4% for inlet temperature at 10 °C and 32.6% for inlet temperature at 110 °C. It was calculated that the maximum exergetic performance of the solar unit is 10.19% for operation at 90 °C, while the thermal efficiency for this case is 41.57%. Additionally, the temperature distributions for different cases are included in the present work. [ABSTRACT FROM AUTHOR]

An innovative parabolic trough concentrator coupled to a twin cavity receiver (PTC-TC) in evacuated tube conditions is investigated thermally and optically. The suggested design is compared with a PTC design with a flat receiver (PTC-F) in order to evaluate the efficiency of the proposed configuration. In the very first stages of the study, the optical efficiency was calculated for both collectors, and the optimum design was determined in the PTC-TC case. Then a mass flow rate independency procedure was conducted to ensure accurate results and to make a suitable comparison. The collectors were examined in a wide range of inlet temperatures ranging from 20 °C to 200 °C, and the thermal performance was calculated. Through the comparison process, it was revealed that the proposed collector appears to have higher thermal performance than the typical collector. In particular, there was a mean enhancement of approximately 8%, while the minimum enhancement was found to be greater than 5%. The simulation results regarding both configurations were verified through two models based on theoretical equations. In both geometries, the mean deviations in the verification procedure were lower than 5.6% in both the Darcy friction factor and the Nusselt number. The design and the simulations were conducted with the SolidWorks Flow Simulation tool. [ABSTRACT FROM AUTHOR]

The objective of this work is the integration of a linear cavity receiver in an asymmetric compound parabolic collector. Two different numerical models were developed; one for the conventional geometry and one for the cavity configuration. Both models were examined for inlet temperatures from 20 °C up to 80 °C, considering water as the operating fluid with a typical volume flow rate of 15 lt/h. Emphasis was given to the comparison of the thermal and optical performance between the designs, as well as in the temperature levels of the fluids and the receiver. The geometry of the integrated cavity receiver was optimized according to two independent parameters and two possible optimum designs were finally revealed. The optimization took place regarding the optical performance of the collector with the cavity receiver. The simulation results indicated that the cavity design leads to enhancements of up to 4.40% and 4.00% in the optical and thermal efficiency respectively, while the minimum possible enhancement was above 2.20%. The mean enhancements in optical and thermal performance were found to be 2.90% and 2.92% respectively. Moreover, an analytical solution was developed for verifying the numerical results and the maximum deviations were found to be less than 5% in all the compared parameters. Especially, in thermal efficiency verification, the maximum deviation took a value of less than 0.5%. The design and the simulations in the present study were conducted with the SolidWorks Flow Simulation tool. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In the present investigation, a solar parabolic trough concentrator (PTC) with a linear cavity was examined by using a developed numerical model. More specifically, a V‐shape cavity receiver was studied in this work as a new and alternative design. The use of a cavity in a PTC is a very promising idea because it leads to high efficiency and to low investment cost. Up today, the emphasis in the use of cavities is given in dish concentrators and so there is a need for investigation of cavities also in linear concentrators. Different dimensional and operational parameters of the PTC with a V‐shape cavity were considered. More specifically, the position, aperture area, and the tube diameter of the V‐shape cavity, solar irradiance, inlet temperature level, and mass flow rate were evaluated based on sensitivity analysis. The results revealed that the highest optical performance was attained when the cavity was placed at the PTC focal line. Moreover, it is found that there is an optimum cavity aperture area to determine the greatest thermal efficiency. For the optical errors of 5, 10, 15, 20, and 35 mrad, the optimum cavity aperture wide was calculated at 5, 5, 6, 7, and 8 cm, respectively, with the tracking error to be 0°. The highest thermal performance has found when the collector operates with low inlet temperature, high flow rate, high solar irradiation level, and small cavity tube diameter. [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]

Summary: The objective of this work is the investigation of a novel solar‐driven cogeneration system, which combines both flat plate collectors (FPCs) and evacuated tube collectors (ETCs). This system includes an organic Rankine cycle in order to produce electricity and heat exchangers in order to produce useful heating at 50°C, which is a usual temperature level for domestic applications. The combination of FPCs and ETCs aims to reduce the investment cost and so to design a cost‐effective solar‐driven cogeneration system. The FPCs are located before the ETCs in order to work at lower temperature levels and the ETC at higher temperature levels, a design which provides optimum compatibility between temperature levels and solar thermal efficiency values. The system is examined energetically, exergetically, and financially. The power production is selected at 5 kW, while the heating production is studied from 5 kW up to 35 kW. According to the final results, it is found that in the typical case of 20‐kW heating production, the simple payback period of the system is around 11 years, while the energy and exergy efficiency at 16% and 4%, respectively. The analysis is conducted with a developed model in Engineering Equation Solver under steady‐state conditions. [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 • The use of nanofluids increases both the thermal and electrical performance. • The collector can operate up to 200 °C with oil and nanofluid. • At 50, 100 and 150 °C, the thermal efficiency was 45.0%, 46.8% and 39.7% respectively. • At 50, 100 and 150 °C, the electric efficiency was 5.8%, 6.6% and 3.9% respectively. • At 100 °C, the thermal and electric enhancements were 1.66% and 5.17% respectively. Abstract The objective of this paper is the investigation of a thermal photovoltaic concentrating collector operating with and without nanofluids. The use of nanofluid Syltherm 800/copper oxide with 5% volumetric concentration is examined and it is compared with the base fluid. The examined thermal photovoltaic has a parabolic concentrator and an inverted flat insulated receiver. The analysis is conducted with SolidWorks Flow Simulation which is able to perform the thermal, optical and flow analysis. According to the present study, the system is able to operate for inlet fluid temperatures up to 200 °C which is a promising result. Moreover, it is found that the use of nanofluid leads to enhancement both in thermal and electrical performance for all the examined cases (different combinations of flow rates and inlet temperatures). For an inlet temperature at 100 °C and flow rate 540 L/h with nanofluid, the thermal efficiency and the electrical efficiency were found at 46.84% and 6.60% respectively. For this case, the enhancements in the thermal, electrical, total and exergy efficiencies were found at 1.66%, 5.17%, 2.08% and 3.05% respectively. [ABSTRACT FROM AUTHOR]

The objective of this work is 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]

Highlights • The secondary concentrator of a linear Fresnel reflector is optimized. • Bezier polynomial parametrization is applied for the optimization of the concentrator. • The optimum design has 19.39% higher optical performance compared to the initial. • The optimum design leads to a relatively uniform heat flux over the absorber. • The thermal efficiency of the optimum design is enhanced close to 20.50% Abstract Linear Fresnel reflector (LFR) consists a promising solar concentrating technology for medium and high temperatures (150–400 °C). Usually, these collectors are designed with a secondary reflector in order to maximize the exploitation of the incident solar irradiation. The objective of this work is to optimize the secondary reflector of an LFR using Bezier polynomial parametrization which is something new in this domain. This parametrization is based on the movement of some control points (three in this case) in order to determine the optimum secondary reflector geometry. According to the final results, the optimum optical efficiency of the LFR is found to be 72.84%, while the optical efficiency in the initial design is 61.01%. Moreover, the optical efficiency is calculated for different solar angles and it is found that the optimized concentrator has superior performance compared to the initial design in all the cases. Furthermore, it is found that the optimum design leads to a relatively uniform heat flux distribution over the absorber periphery compared to the initial design. The last step is the determination of the thermal efficiency performance curve and it is found 20.50% mean thermal efficiency enhancement with the optimum design. [ABSTRACT FROM AUTHOR]

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. [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]

Parabolic trough collector is one of the dominant emerging solar technologies for producing heat at high temperatures (usually 200–400 °C). The objective of this study is to investigate the thermal performance of internally finned absorbers. Twelve different fin geometries are examined and compared with the smooth absorber case for various operating scenarios. More specifically, the investigated internal fins have thicknesses 2 mm, 4 mm and 6 mm, while their lengths are 5 mm, 10 mm, 15 mm and 20 mm. The examined parameters for the evaluation of the internally finned absorbers are the thermal efficiency, the Nusselt number, the pressure losses, as well as the thermal enhancement index. According to the final results, higher fin thickness and length lead both to higher thermal performance and simultaneously to higher pressure losses. The impact of the length on the results is found to be more intense than the thickness. According to the thermal enhancement index, the case with 20 mm length and 4 mm thickness is found to be the optimum case. For this absorber, the increase in the thermal efficiency and the thermal enhancement index are found 1.27% and 1.483 respectively for 600 K inlet temperature, while the Nusselt number is proved to be 2.65 times greater than in the smooth case. [ABSTRACT FROM AUTHOR]

Among the solar concentrating technologies, parabolic trough collector (PTC) is the most mature and cost-effective technology for medium and high-temperature levels (150–400 °C). This paper investigates the utilization of internally finned absorbers in LS-2 PTC module for various operating conditions. Twelve different longitudinal fins are tested and compared with the smooth case. The analysis is performed with SolidWorks Flow Simulation, using a validated model by literature results. Generally, it is proved that both greater length and thickness lead to higher thermal enhancement and to higher pressure losses. Various methods are presented for evaluating together the thermal efficiency or Nusselt number enhancement versus the increase in pressure drop or in the friction factor. Taking into consideration four different criteria, the absorber with 10 mm fin length and 2 mm fin thickness is found to be the overall optimum case. For this case, the thermal efficiency is enhanced about 0.82%, the Nusselt number increase 65.8%, while the friction factor and the pressure losses are about the double compared to the smooth case. [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 analyze and optimize a solar assisted gas turbine system. Parabolic trough collectors are used in order to supply a part of the demanded heat input, reducing the natural gas consumption and leading to an environmental friendly system. Emphasis is given in the pressure losses in the collector field loop which reduce the pressure level in the turbine inlet, leading to lower electricity production. In the first part of this study, the gas turbine and the collector field are investigated separately and in the second part, the solar assisted gas turbine is investigated parametrically and it is optimized. In the optimization part, a multi-objective optimization is performed by setting as goals the minimization of the collector area, of the fuel consumption and of the inversed electricity production. The final results proved that 1050 collector modules lead to 0.3389 kg/s natural gas consumption and to electricity production equal to 14.81 MW el . This optimum solution leads to 64% fuel savings with a 2.8% penalty on the produced electricity. The system is analyzed in steady state conditions with the Engineering Equator Solver (EES). [ABSTRACT FROM AUTHOR]

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]

In this study, the thermal performance of a solar dish concentrator with different cavity receivers and nanofluids was experimentally studied. The first step in this work is a detailed literature review of the studies about the solar dish cavity receivers. The second step is the experimental investigation of a solar dish concentrator with a cubical cavity receiver using Al 2 O 3 /oil nanofluid and pure thermal oil. The last step of this work is the comparison of the found results with others from the literature in order to make an overall overview of cavity receivers. The finally presented results of the dish concentrator with different cavity receivers (including cubical, cylindrical, and hemispherical cavities) using different nanofluids (including Al 2 O 3 /oil, MWCNT/oil, and SiO 2 /oil nanofluids are reported based on a same experimental setup that was designed and built by authors of this research. The results reveal that the hemispherical and the cubical cavities are the most effective designs, while the cylindrical cavity presents lower performance. Moreover, it was found that the use of nanofluids always leads to thermal performance enhancement. More specifically, it was found that the mean thermal efficiency enhancement with the use of nanofluids is 12.90% with the hemispherical cavity, 5.84% with the cubical cavity and 1.44% with the cylindrical cavity. • A cubical cavity receiver was experimental investigated using Al 2 O 3 /oil nanofluid. • Performance comparison was conducted with pure oil and nanofluid. • Performance comparison of cubical, cylindrical, and hemispherical cavities was presented. • A detailed literature review related to the cavities and nanofluid was conducted. • The highest mean thermal efficiency enhancement was calculated equal to 12.90% using hemispherical cavity and nanofluid. [ABSTRACT FROM AUTHOR]