تجزیه و تحلیل عملکرد از یک رمان متمرکز فتوولتائیک ترکیب سیستم
|کد مقاله||سال انتشار||مقاله انگلیسی||ترجمه فارسی||تعداد کلمات|
|28081||2013||11 صفحه PDF||سفارش دهید||8665 کلمه|
Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Energy Conversion and Management, Volume 67, March 2013, Pages 186–196
In the present study, a novel Concentrating Photovoltaic Combined System (CPVCS) based on the spectral decomposing approach is introduced, modeled, tested experimentally and evaluated thermodynamically and economically. In this study, energy and exergy analyses of the system have been evaluated, economical analysis has been performed and the experimental results have been compared to data obtained by the control system. As a result, energy efficiencies of concentrator, vacuum tube and overall CPVCS have been determined to be 15.35%; 49.86%; and 7.3% respectively. Similarly the second law (exergy) efficiencies of concentrator, vacuum tube and overall CPVCS are 12.06%; 2.0%; and 1.16% respectively. The cost of energy production has been stated as 6.37 $/W and it is predicted that this value could be decreased by improving the system performance.
The conversion efficiency of photovoltaic cells is relatively low, usually in the range of 10–20% for commercially available silicon cells and up to 39% for more sophisticated multi-junction cells. More than half of the solar radiation, collected with considerable effort and investment, is converted to thermal energy and then emitted to the environment. A well known way to achieve a better overall efficiency is cogeneration: capturing the waste heat as well and using it as an additional energy product. This can be achieved with photovoltaic/thermal (PV/T) collectors that contain a heat exchanger behind the PV cells to collect the heat rejected from the cells . During the 1970s, the research on PV/T started, with the focus on PV/T collectors, with the main aim of increasing the overall energy efficiency . The conventional single semiconductor solar cells only convert photons of energy close to the semiconductor band gap effectively. Photons with less energy are not absorbed and their energy is totally wasted . A solar cell has its threshold photon energy corresponding to the particular energy band gap below which electricity conversion does not take place. Photons of longer wavelength do not generate electron–hole pairs but only dissipate their energy as heat in the cell. Common PV modules convert 4–17% of the incoming solar radiation into electricity, depending on the type of solar cells in use and the working conditions. In other words, more than 50% of the incident solar energy is converted to heat, not electrical energy. This may lead to extreme cell working temperature as much as 50 °C above the ambient environment. There can be two undesirable consequences: (i) a drop in cell efficiency (typically 0.4% per °C rise for c-Si cells) and (ii) a permanent structural damage of the module if the thermal stress remains for prolonged period . In PV/T system applications the production of electricity is the main priority, therefore it is necessary to operate the PV modules at low temperature in order to keep PV cell electrical efficiency at a sufficient level. This requirement limits the effective operation range of the PV/T unit for low temperatures . The co-generated heat is available at the relatively low temperature that can be achieved by flat plate collectors, usually about 40–60 °C. Keeping the temperature low is considered an advantage since the conversion efficiency of PV cells decreases with temperature. However, the collected thermal energy is suitable for domestic water heating or space heating, but it is inadequate for applications that require higher temperatures, such as absorption cooling. This limits the range of applications for PV/T systems. It also limits the practical system size, because the required collector area for water or space heating is usually much lower than the area requirement for electricity . Even today the conventional solar cell applications, compared with conventional electric power generation high cost is the biggest obstacle to solar cell applications becoming widespread. The combination of solar concentrators with PV modules is up to now the most viable method to reduce system cost, replacing the expensive cells with a cheaper solar radiation concentrating system. By concentrating, a large part of the expensive PV area is replaced by cheap mirror area, which is a way to reduce the payback time. This argument is the driving force behind CPV systems. CPVs present higher efficiency than the typical ones, but this can be achieved only when PV module temperature is maintained as low as possible  and . PVT and CPV systems have been investigated and discussed by many researchers for the last decade in the literature , , , , , , , , , , , , , , , , , , , ,  and . There are also many studies which employ Fresnel lenses for CPV systems in the literature , , , ,  and . Besides, some studies on the integration PVs and heat pumps were performed  and . There are very limited studies on concentrating photovoltaic thermal systems (CPVTs). Kribus et al.  presented and analyzed a novel miniature concentrating PV (MCPV) system. The system tested with a reflector of 0.95 m2 aperture area, under normal beam insolation of 900 W/m2. Most of thermal energy was removed by the coolant flow, but some is lost to the environment through the front and back surfaces. A miniature concentrator PV/thermal system producing about 140–180 W of electricity and an additional 400–500 W of heat was developed. Kribus et al.  also proposed simultaneous production of electrical and high grade thermal energy with a concentrating photovoltaic/thermal (CPVT) system operating at elevated temperature. CPVT collectors may operate at temperatures above 100 °C, and the thermal energy can drive processes such as refrigeration, desalination, and steam production. In this study, the performance and cost of a CPVT system with single effect absorption cooling was investigated in detail. The results showed that under a wide range of economic conditions, the combined solar cooling and power generation plant can be comparable to and sometimes even significantly better than, the conventional alternative. Again Kribus et al.  proposed a coupled system, comprised of a concentrating photovoltaic/thermal collector field and a multi-effect evaporation desalination plant. The combined system produces solar electricity and simultaneously exploits the waste heat of the photovoltaic cells to desalinate water. In this study, a detailed simulation was performed to compute the annual production of electricity and water. They indicated that the results indicate that the proposed coupled plant can have a significant advantage relative to other solar desalination approaches. There is a very crucial issue for the basis of PV, PVT and CPVT systems: Spectral effect. Today, the spectral effect in the use of solar cells is a parameter that is not considered in the vast majority of solar energy technologies. However, this feature is the basis of electricity generation with solar cells. Solar irradiance at shorter wavelengths is absorbed by solar cells. At long wavelengths, the radiation cannot be convert electricity and causes the excessive heat load on the cell material. This situation is related to spectral response range of the semiconductor material of the solar cell. Most of the solar radiation falling on cannot be converted into electrical energy due to spectral characteristics of solar cells. More than 80% remaining after conversion to solar energy to electrical energy transformed into heat is wasted to the environment . This excessive heat loss means energy loss for the conversion system. There is a very promising approach to prevent energy loss and to improve energy efficiency for concentrated solar energy technologies: Spectral decomposition. If solar radiation spectrum on the solar cell can be filtered to its operating range, almost all solar radiation will be converted into electrical energy and it will be possible to benefit from the full solar spectrum most effectively. The basic logic in PVT and CPVT systems is to use thermal energy due to solar radiation falling onto the cell via various systems and try to protect the cell from the negative effects of high temperature. However, if not all of the solar radiation but the section of on the spectral response range can be fallen on the cell, close to all of this energy can be converted to electrical energy. In other words, the part of the solar radiation can be utilized by solar cells most effectively is the part that spectrally separated before falling on the cell, in this case excessive heating on the solar cell is not in question. In the CPVT system which integrated with concentrators and filtered according to the spectral response range, almost of the solar radiation falling on the cell be converted to electrical energy as well as high-temperature radiation will be able utilized with high efficiency in thermal applications. In reviewing the literature on the subject, there is no existing system in which solar radiation is separated, extracted or evaluated spectrally before falling on the solar cells. In all systems studied all solar radiation is fallen on the solar cell the load of heat occurred in cell tried to be assessed by taking. This spectral decomposition approach which has been introduced and performed in the present study could also be applied to many different solar power generation systems. In this study, a novel Concentrating Photovoltaic Combined System (CPVCS), which has not been found open literature, is aimed to present the design and also evaluate its performance. It is possible to utilize the full solar spectrum and to produce both electricity and thermal power generation by CPVCS with spectral decomposition approach. Energy, exergy and economic analyzes of electricity and hot water production performance of CPVCS have been carried out. The concentrated solar energy has been separated by an optical device called “hot mirror” into visible and infrared (IR)–ultraviolet (UV) parts on the focal plane before coming on the solar cell. In this way, a combined system has been formed which benefits concentrated solar energy on the entire spectrum and integrates thermal power generation. In the present study, the visible part of concentrated solar energy has been used to produce electricity by a solar cell; while the IR and UV parts convert thermal energy, specifically solar water heating. Thermal energy obtained from the proposed system also could be employed for different purposes with minor modifications. It could be evaluated for not only solar water heating, but also it could be employed to heat space by a fan and air duct or to integrate a heat pump for heating and cooling applications. Furthermore, high-temperature of spectrally separated thermal energy obtained by CPVCS could also be considered for a solar water distillation system. In the present study, the scope of the research has been limited and only solar water heating has been investigated. The most distinctive characteristic of this spectral decomposition approach is being very applicable and adaptable to many different concentrated solar energy systems. In this experimental study, the power obtained by CPVCS (experimental system) and the power obtained from conventional PV module (control system) were compared through the energy, exergy and economic analyses. The first part of the study, a detailed analysis of the literature was presented related to PVT, CPV and CPVT. In the second part of the study, the theoretical modeling used in thermodynamic and economic analyzes was defined. In Section 3, elaborate explanations related to experimental design, materials and system components have been presented. The findings of theoretical analysis and experiments were discussed in the next section, and finally results obtained in the study and recommendations were presented.
نتیجه گیری انگلیسی
In this study, a new CPVCS based on spectral decomposition approach was introduced, modeled, tested, compared to conventional PV module and evaluated thermodynamically and economically. Concentration of the sun on a small area, it is possible to produce the same amount or more electrical energy with less than the surface of a PV by the proposed system, also benefited as well as thermal energy. In light of the performance and economic analyses of the findings drawn from the results obtained in the study are listed below: a. The energy rate on the focal point was found to be 83.80 W in case of concentrating dish with a diameter of 60 cm aperture and 734.8 W/m2 solar radiation on the dish aperture. b. Radiative fluxes falling on solar cell in CPVCS system and reflected to vacuum tube respectively is 30.29 W and 34.24 W. c. Electrical power produced per solar cells in CPVCS system is 4.6 W and thermal power produced in vacuum tubes is 141.21 W. d. The energy efficiency of the concentrating dish, the vacuum tube and overall CPVCS system have been calculated respectively, 15.35%, 49.86%; as 7.3%. Especially, the energy efficiency of the concentrating dish needs to be improved to increase the overall energy efficiency. e. The exergy efficiencies of the concentrating dish, the vacuum tube and overall CPVCS system have been found respectively 12.06%, 2.0%; as 1.16%. The exergy efficiencies are found as considerably low; so the system needs to be improved potential for exergetically. f. For the system produces an electrical energy flux per unit area CPVCS 1783.94 W/m2, and the total energy flux per unit area produced 196.25 W/m2 total exergy flux was calculated as 13.19 W/m2. Electrical power generated by the control system has been determined as 213 W. Electrical energy flux generated per unit area of the control system was determined as 130.69 W/m2; the total exergy flux generated per unit area was calculated as 98.68 W/m2. g. Power obtained from a single of cell without concentration application is 3.6 W. Spectrally separated, and only uses a portion of the radiation from the solar power generated in cell is 4.6 W. Electrical power produced has an increase of 27.8% for each solar cell. This increase is expected to increase with the improvement of the system. h. The exergetic improvement potential of concentrating dish, the vacuum tube and solar cells are to be found as 426.66 W; 254.26 W and 21.77 W respectively. It seems to be great potential for concentrating dish to improve exergetically; however, the reason of these findings is that there is a large temperature difference between the ambient temperature and the focal temperature. In general, concentrating dishes has low exergy efficiency and a high exergetic improvement potential. i. In economic analysis, the payback period for CPVCS was found shorter than the payback period of control system. The energy production cost for the CPVCS system is estimated as $6.37/W and with improvements in the system this value is expected to be drawn down. j. Small paraboloidal dishes were located in the CPVCS system design. This dish has modular feature is easy to transport that does not require special equipment. Concentrating Photovoltaic Combined System (CPVCS) easily applied to the rooftops of buildings, the building could be an alternative to meet the need for thermal energy and electricity. In this system more electricity and heat energy is expected to be obtained when compared conventional PVT systems. Suggestions for the improvement of the CPVCS system are listed below: a. By minimizing concentrating the dish surface defects can be achieved the ratio of a higher reflectance. b. Shading effect can be minimized for the concentrating dish by employing proper size of solar cells. c. Increasing the number of the dish and minimizing the size of dish more power can be achieved in the same area and the same mechanical strength. CPVCS systems using spectral decomposing approach could be improve and some other research studies could be planned as a next step. Possible future studies could be proposed for researchers as below: a. By using Cassegrain-type or concave hot mirrors instead of flat mirrors in focus, different system configurations can be tested. b. Thermal power obtained from CPVCS system, can be integrated into different structures flat plate heating systems. c. Thermal energy obtained from CPVCS system – instead of the hot water- may be tried in air heating, cooling, the evaluation heat pump, and water distilling. d. By trying different dish concentrators with different diameters optimum performance experiments in can be made. e. CPVCS system performance can be compared to conventional PVT systems. f. Under different sky conditions, system performance can be evaluated with long-term solar radiation data. g. With different spectral character of concentrating dish materials, concentrator solar radiation collected in focus can be changed and performance analyses can be carried out. Global energy policies, energy supply security and a growing world population, have lead mankind to new and renewable energy sources. In order to meet energy requirements, all existing natural resources should be utilized most effectively. With using the CPVCS solar cells use of surface is decreased and thereby the cost is reduced. Concentrating Photovoltaic Combined System (CPVCS) easily applied to the rooftops of buildings, electrical and thermal energy and is offered as an alternative to meet requirements. The proposed system has an significant advantage is that the use of an existing solar cells efficiently. Namely, there are some studies to produce solar cells which are durable to very high temperatures for direct use with paraboloidal concentrator dishes. Considering the cost to these techniques and materials, the importance of using existing solar cells effectively is understood. The present system can be easily accessible with the spectral decomposition and the production of cheaper electricity produced by the solar cells themselves, concentrated solar thermal energy to be assessed for the purposes of heat energy. It is believed that the novel spectral decomposition approach introduced by this study has a great potential to apply to many different solar energy applications. Concentrating photovoltaic systems, spectral decomposition approach proposed in this study would help guide researchers and it is hoped to shed light on the energy efficiency issues.