Performance evaluation of hybrid PV/thermal water/air heating system: A parametric study

In this paper, a thermal model of an integrated photovoltaic and thermal solar (IPVTS) water/air heating system has been developed. An analytical expression for the temperature of solar cell and water and an overall thermal efficiency of IPVTS system have been derived as a function of climatic and design parameters. Numerical computations have been carried out for composite climate of New Delhi for parametric studies. Four configurations, namely (a) unglazed with tedlar (UGT), (b) glazed with tedlar (GT), (c) unglazed without tedlar (UGWT) and (d) glazed without tedlar (GWT) have been considered. Comparison of the IPVTS system with water and air heater has also been carried out. It is found that the characteristic daily efficiency of IPVTS system with water is higher than with air for all configurations except GWT. It is also observed that an overall thermal efficiency of IPVTS system for summer and winter conditions is about 65% and 77%, respectively.     

A sensitivity study of a hybrid photovoltaic/thermal water-heating system with natural circulation

A flat-box aluminum-alloy photovoltaic and water-heating system designed for natural circulation was constructed. The hybrid photovoltaic/thermal (PV/T) collector was an integration of single-crystalline silicon cells into a solar thermal collector. The product was able to generate electricity and hot water simultaneously. Outdoor tests on an improved prototype were conducted in a moderate climate zone. Then dynamic simulation runs, using a validated numerical model, were performed. These included sensitivity tests with variations of the system water mass, PV cell covering factor, and front glazing transmissivity. The test results showed that the characteristic daily primary-energy saving could reach up to 65% for this system with a PV cell covering factor 0.63 and front glazing transmissivity of 0.83, when the hot water load per unit heat-collecting area exceeded 80 kg/m². The simulated results indicated that the higher the PV cell covering factor and the glazing transmissivity, the better the overall system performance. The effects were quantified.     

Energy and exergy analysis of photovoltaic–thermal collector with and without glass cover

In photovoltaic–thermal (PV/T) technology, the use of glass cover on the flat-plate hybrid solar collector is favorable to the photothermic process but not to the photovoltaic process. Because of the difference in the usefulness of electricity and thermal energy, there is often no straight forward answer on whether a glazed or unglazed collector system is more suitable for a specific application. This glazing issue was tackled in this paper from the viewpoint of thermodynamics. Based on experimental data and validated numerical models, a study of the appropriateness of glass cover on a thermosyphon-based water-heating PV/T system was carried out. The influences of six selected operating parameters were evaluated. From the first law point of view, a glazed PV/T system is found always suitable if we are to maximize the quantity of either the thermal or the overall energy output. From the exergy analysis point of view however, the increase of PV cell efficiency, packing factor, water mass to collector area ratio, and wind velocity are found favorable to go for an unglazed system, whereas the increase of on-site solar radiation and ambient temperature are favorable for a glazed system.     

Collector cum storage solar water heaters with and without transparent insulation material

The integrated collector–storage solar water heaters are less expensive and can offer the best alternative for domestic applications particularly to small families to meet hot water requirements. The top heat losses of such solar water heaters are quite high during the night and the temperature of stored hot water is considerably reduced unless covered with extra insulating cover in the evening which is a cumbersome job. The transparent insulation material widely used in Europe for space heating can also minimize top heat losses, if used in such solar water heaters. For this purpose, two units of solar collector cum storage water heaters have been designed to study the relative effect of TI for retaining solar heated hot water for a night duration. Both units were identical in all respects except one of them was covered with TIM. The theoretical exercise was carried out to evaluate design parameters of ISC which revealed total heat loss factor (U_L) 1.03 W/m² K with TIM glazed against 7.06 W/m² K with glass glazed. The TIM glazed has been found to be quite effective as compared to glass glazed SWH and yielded hot water at higher temperature by 8.5 to 9.5°C the next morning. The storage efficiency of such solar water heaters has been found to be 39.8% with TIM glazed as compared to 15.1% without TIM. The TIM glazing means not having to cover the ISC solar water heater with a separate insulator cover in the evening and thus makes its operation much simpler.     

Exergy analysis of integrated photovoltaic thermal solar water heater under constant flow rate and constant collection temperature modes

In this communication, an analytical expression for the water temperature of an integrated photovoltaic thermal solar (IPVTS) water heater under constant flow rate hot water withdrawal has been obtained. Analysis is based on basic energy balance for hybrid flat plate collector and storage tank, respectively, in the terms of design and climatic parameters. Further, an analysis has also been extended for hot water withdrawal at constant collection temperature. Numerical computations have been carried out for the design and climatic parameters of the system used by Huang et al. [Huang BJ, Lin TH, Hung WC, Sun FS. Performance evaluation of solar photovoltaic/thermal systems. Sol Energy 2001; 70(5): 443–8]. It is observed that the daily overall thermal efficiency of IPVTS system increases with increase constant flow rate and decrease with increase of constant collection temperature. The exergy analysis of IPVTS system has also been carried out. It is further to be noted that the overall exergy and thermal efficiency of an integrated photovoltaic thermal solar system (IPVTS) is maximum at the hot water withdrawal flow rate of 0.006 kg/s. The hourly net electrical power available from the system has also been evaluated.     

Optimization of a solar photovoltaic thermal (PV/T) water collector based on exergy concept

In this paper, the optimization of a solar photovoltaic thermal (PV/T) water collector which is based on exergy concept is carried out. Considering energy balance for different components of PV/T collector, we can obtain analytical expressions for thermal parameters (i.e. solar cells temperature, outlet water temperature, useful absorbed heat rate, average water temperature, thermal efficiency, etc.). Thermal analysis of PV/T collector depends on electrical analysis of it; therefore, five-parameter current–voltage (I–V) model is used to obtain electrical parameters (i.e. open-circuit voltage, short-circuit current, voltage and current at the point which has maximum electrical power, electrical efficiency, etc.). In order to obtain exergy efficiency of PV/T collector we need exergy analysis as well as energy analysis. Considering exergy balance for different components of PV/T collector, we obtain the expressions which show the exergy of the different parts of PV/T collector. Some corrections have been done on the above expressions in order to obtain a modified equation for the exergy efficiency of PV/T water collector. A computer simulation program has been developed in order to obtain the amount of thermal and electrical parameters. The simulation results are in good agreement with the experimental data of previous literature. Genetic algorithm (GA) has been used to optimize the exergy efficiency of PV/T water collector. Optimum inlet water velocity and pipe diameter are 0.09 m s⁻¹, 4.8 mm, respectively. Maximum exergy efficiency is 11.36%. Finally, some parametric studies have been done in order to find the effect of climatic parameters on exergy efficiency.     

一种新型全铝扁盒式PV/T热水系统

将单晶硅光伏电池与全铝扁盒式太阳能热水器集热板通过特殊工艺粘结起来,制成了一套自然循环式光伏光热一体化(PV/T)系统,在利用太阳能发电的同时提供热水。于04年7月-10月在合肥地区进行了室外实验,测试并讨论了该系统以不同水量和不同初始水温运行时的光电光热性能。结果表明,当m/Ac>80kg/m2时,这种PV/T热水系统的发电效率在10.15%左右,热效率在50%左右,光电光热总效率可以达到60%左右,光电光热综合性能效率可以达到70%左右。相对于单纯的光伏系统或自然循环式太阳能热水系统,这种PV/T热水系统具有占地面积小、综合效率高等优点。     

Thermal modeling of a combined system of photovoltaic thermal (PV/T) solar water heater

In this paper, an integrated combined system of a photovoltaic (glass–glass) thermal (PV/T) solar water heater of capacity 200 l has been designed and tested in outdoor condition for composite climate of New Delhi. An analytical expression for characteristic equation for photovoltaic thermal (PV/T) flat plate collector has been derived for different condition as a function of design and climatic parameters. The testing of collector and system were carried out during February–April, 2007. It is observed that the photovoltaic thermal (PV/T) flat plate collector partially covered with PV module gives better thermal and average cell efficiency which is in accordance with the results reported by earlier researchers.     

真空玻璃与单层玻璃盖板PV/T集热器性能的对比研究

提出一种真空玻璃盖板平板式PV/T集热器,建立了真空玻璃和单层玻璃盖板PV/T集热器的传热模型,并分别搭建了两种PV/T热水系统的实验平台进行模型的实验验证。预测结果与实验测量结果的均方根偏差（RMSD）在0.71% ~ 11.17%之间。利用数学模型模拟了真空玻璃与单层玻璃盖板PV/T集热器在合肥冬季的热、电性能,并比较了两者性能的差异。模拟结果表明真空玻璃盖板PV/T集热器的顶部热损失平均为22 W,而单层玻璃盖板PV/T集热器热损失平均为107 W。使用真空玻璃盖板能显著减少PV/T集热器的顶部热损失。真空玻璃盖板PV/T集热器相对单层玻璃盖板PV/T集热器的全天热效率提高了5.68%,二者分别为41.76%和36.08%,全天电效率分别为11.76%和12.79%,相差1.03%。     

Performance improvement of PV/T solar collectors with natural air flow operation

The electrical efficiency of a photovoltaic system drops as its operating temperature rises and PV cooling is necessary. The photovoltaic/thermal (PV/T) system is a relatively recent type of solar collector where a circulating fluid of lower temperature than PV module extracts heat from it, cooling the module to improve its output power while the solar pre-heated fluid provides sensible heat. In the present work, air cooling of a commercial PV module configured as PV/T air solar collector by natural flow is presented, where two low cost modification techniques to enhance heat transfer to air stream in the air channel are studied. The considered methods consist of thin metal sheet suspended at the middle or fins attached to the back wall of the air-channel to improve heat extraction from the module. A numerical model was developed and validated against the experimental data obtained from outdoor test campaigns for both glazed and unglazed PV/T prototype models studied. The validation results show good agreement between predicted values and measured data and thus could be used to study analytically the performance of these PV/T air collectors with respect to several design and operating parameters. The modified systems present better performance than the usual type and will contribute to better performance of integrated PV systems for natural ventilation applications in buildings, both space cooling and heating.     

Energy performance of water hybrid PV/T collectors applied to combisystems of Direct Solar Floor type

The integration of photovoltaic (PV) modules in buildings allows one to consider a multifunctional frame and then to reduce the cost by substitution of components. In order to limit the rise of the cell operating temperature, a photovoltaics/thermal (PV/T) collector combines a solar water heating collector and PV cells. The recovered heat energy can be used for heating systems and domestic hot water. A combination with a Direct Solar Floor is studied. Its low operating temperature level is appropriate for the operating conditions of the mono- or poly-crystalline photovoltaic modules which are selected in that study. However, for a system including a glass covered collector and localised in Mâcon area in France, we show that the annual photovoltaic cell efficiency is 6.8% which represents a decrease of 28% in comparison with a conventional non-integrated PV module of 9.4% annual efficiency. This is obviously due to a temperature increase related to the cover. On the other hand, we show that without a glass cover, the efficiency is 10% which is 6% better than a standard module due to the cooling effect.Moreover, in the case of a glazed PV/T collector with a conventional control system for Direct Solar Floor, the maximum temperature reached at the level of the PV modules is higher than 100 °C. This is due to the oversize of the collectors during the summer when the heating needs are null, i.e. without a heated swimming pool for example. This temperature level does not allow the use of EVA resin (ethylene vinyl acetate) in PV modules due to strong risks of degradation. The current solution consists of using amorphous cells or, if we do not enhance the thermal production, uncovered PV/T collector. Further research led to water hybrid PV/T solar collectors as a one-piece component, both reliable and efficient, and including the thermal absorber, the heat exchanger and the photovoltaic functions.     

Transient behaviour of collector/storage solar water heaters for generalised demand patterns

This communication presents a simple transient model for predicting the thermal performance of collector/storage solar water heaters for generalised demand patterns. These heaters consist of either (i) an insulated rectangular metallic tank whose top surface is blackened and suitably glazed (i.e. a built-in storage solar water heater) or (ii) an insulated open shallow tank with black bottom.inner sides and a glass plate at the surface in contact with the water (i.e. a shallow solar pond water heater). The time dependence of the water temperature for the withdrawal of hot water from the system at constant flow rates constantly or intermittently has been explicitly evaluated. Numerical results for the operation of the system in industrial and community service applications are discussed.     

非晶硅太阳能光伏/光热空气集热器性能对比实验研究

文章设计了新型非晶硅太阳能PV/T空气集热器,该空气集热器能够解决传统太阳能PV/T热水器在高温波动情况下,晶硅电池热应力大的问题,同时避免了冬季管道发生霜冻的现象。文章通过实验对比,分析了非晶硅太阳能PV/T空气集热器、单独非晶硅光伏电池和传统太阳能空气集热器的能量效率和[火用]效率的差异。分析结果表明:非晶硅太阳能PV/T空气集热器的平均热效率为45.70%,比传统太阳能空气集热器的平均热效率降低了约25.88%;当空气质量流量增大至0.048 kg/s时,非晶硅太阳能PV/T空气集热器中的非晶硅光伏电池的平均电效率高于单独非晶硅光伏电池,它们的平均电效率分别为4.70%,4.54%;非晶硅太阳能PV/T空气集热器的总[火用]效率高于传统太阳能空气集热器的热[火用]效率和单独非晶硅光伏电池的电[火用]效率,非晶硅太阳能PV/T空气集热器总[火用]效率最大值为7.14%。文章的分析结果为非晶硅太阳能PV/T空气集热器的推广提供了参考。     

太阳能光伏光热一体化系统的实验研究

为提高太阳能的利用率同时得到可资利用的热水和电力，将小型贮能式光伏系统与家用平板型太阳能热水器结合起来，把光优电池组件层压在热水器的扁盒式铝合金集热板上，构成一套光优光热(PV／T)一体化系统，并在合肥地区进行了自然循环模式下的光电光热性能测试。实验结果表明，在晴朗或多云的天气条件下实验系统日平均热效率可达40％，日平均发电效率约9．5％，系统综合性能效率多在60％以上，比单独的光伏或热水系统效率有显著提高。     

Effects of auxiliary heater on annual performance of thermosyphon solar water heater simulated under variable operating conditions

A thermosyphon solar water heating system with electric auxiliary heater was simulated using the TRNSYS simulation program. Location of the auxiliary heater, inside the storage tank or connected in series between the system and the user, was studied using the TMY meteorological data for Los Angeles, California. Simulations were performed for two different water load temperatures (60 and 80°C) and for two types of daily hot water volumes (250 and 150 l). Four types of daily hot water consumption profiles were used in the present study, namely; the widely used Rand profile, continuous, evening and morning profiles. Also, the simulation is extended to cover the effects of thermal and optical properties of the flatplate collector and the volume of the storage tank. The results show that if water is drawn on a schedule corresponding to the Rand draw profile, the system operates with higher efficiency when the auxiliary heater is located in the storage tank than when the auxiliary heater is outside the storage tank. When operated with each of the other three draw schedules, however, better performance is achieved by locating the auxiliary heater outside the tank. The increase in solar fraction depends on the load profile and volume, temperature setting, as well as the quality of the collector and the storage tank volume. When the values of the parameters F_R(τα)_n and F_RU_L are changed from 0.8 and 16 kJ/h m²°C to 0.6 and 30 kJ/h m²°C, the solar fraction decreases by approximately 40–50%.     

Effect of hybrid nanofluids mixture ratio on the performance of a photovoltaic thermal collector

Solar photovoltaic-thermal (PV/T) collectors, are hybrid collectors used to convert solar radiation into usable thermal and electrical energy. Recently, the field of research on PV/T is has focused on improving the efficiency of the PV/T collector by replacing the conventional heat transfer fluids (HTFs) with nanofluids. This article investigates the effect of hybrid nanofluids mixture ratio on the useful energy and overall efficiency of a PV/T collector operating with Al₂O₃-ZnO water nanofluid as the HTF. Experiments to measure the thermophysical properties of the hybrid nanofluids were conducted for various temperatures, volume concentrations, and mixture ratios, furthermore, accurate correlation models were proposed. Metrological data and energy output readings collected from the PV solar farm at Cyprus International University were used to validate our model. The study observed that at the optimum mixture ratio (0.47 of Al₂O₃ in the hybrid), the electrical, thermal, and exergy efficiencies of the PV/T collector are 13.8%, 55.9%, and 15.13% respectively. Also, the cell temperature drops by 21% when the mass flow rate is 0.1 kg/s as compared to when it is 0.01 kg/s. Finally, the study concludes that by using the Al₂O₃-ZnO hybrid nanofluid an overall peak thermal efficiency of 91% can be attained, and this represents a 34% enhancement in the collector's performance when compared to water.     

Technoeconomic optimization of solar natural convection hybrid hot water systems

In this paper a techno-economic model for a hybrid domestic hot water system operating under natural convection mode is presented. Three modes of auxiliary energy supply viz.

• A electric heater fitted in the solar hot water tank.
• B electric heater fitted in a small water tank in series with the solar hot water tank, and
• C an instant electric heater fitted in the tap

have been considered. Taking into account the life and the capital and maintenance costs of the solar and electrical equipments, the cost of useful energy (Rs/kWh) has been calculated for different values of the collector area and the tank capacity, and thereby the optimum collector area and tank capacity (for a given demand), corresponding to minimum cost of useful energy, has been determined. From numerical calculations made for the climate of Delhi, India (a representative composite climate) corresponding to the two cases of monthly hot water demand viz. (i) constant monthly demand, and (ii) variable monthly demand, it is seen that case (C) is the most economic design of the hybrid hot water system; numerical calculations have also been made for this case corresponding to the climates of Srinagar and Madras (representing cold and hot climates). The effect of government subsidy on the optimized values of collector area, tank capacity and cost of useful energy has also been investigated for the climate of Delhi.     

Parametric study of various configurations of hybrid PV/thermal air collector: Experimental validation of theoretical model

In this paper, an attempt has been made to evaluate the overall performance of hybrid PV/thermal (PV/T) air collector. The different configurations of hybrid air collectors which are considered as unglazed and glazed PV/T air heaters, with and without tedlar. Analytical expressions for the temperatures of solar cells, back surface of the module, outlet air and the rate of extraction of useful thermal energy from hybrid PV/T air collectors have been derived. Further an analytical expression similar to Hottel–Whiller–Bliss (HWB) equation for flat plate collector has also been derived in terms of design and climatic parameters. Numerical computations have been carried out for composite climate of New Delhi and the results for different configurations have been compared. The thermal model for unglazed PV/T air heating system has also been validated experimentally for summer climatic conditions. It is observed that glazed hybrid PV/T without tedlar gives the best performance.     

Exergy analysis of domestic-scale solar water heaters

Solar water heater is the most popular means of solar energy utilization because of technological feasibility and economic attraction compared with other kinds of solar energy utilization. Earlier assessments of domestic-scale solar water heaters were based on the first thermodynamic law. However, this kind of assessment cannot perfectly describe the performance of solar water heaters, since the essence of energy utilization is to extract available energy as much as possible. So, it is necessary to evaluate domestic-scale solar water heaters based on the second thermodynamic law.No matter the technology process, from the property of energy utilization perspective, we can separate the technology process into three intimately related sub-procedures, namely conversion procedure, utilization procedure, and recycling procedure. An energy analysis entitled ‘Three Procedure Theory’ can be conveniently conducted as presented by Professor Hua Ben. Compared with other theories of energy analysis, three procedure theory exhibits great advantages. The utilization procedure puts forth requirement for the design of parameters in conversion procedure and sets up limits in the consideration of recycling procedure. Of course, under specific conditions, the utilization procedure also receives feedback from other procedures. Three procedure theory furnishes us a good platform to perform energy analysis.The study in this paper is based on three procedure theory. Exergy analysis is conducted with the aim of providing some methods to save cost and keep the efficiency of domestic-scale solar water heater to desired extent and at the same time figuring out related exergy losses. From this survey, it is shown that for an ordinary thermally insulated domestic-scale solar water heater, D_ju (exergy losses due to imperfectly thermal insulation in collector) and D_jR (exergy losses due to imperfectly thermal insulation in storage barrel) cannot be avoided. D_ku (exergy losses due to irreversibility in collector) is mainly caused by irreversibility of heat transfer and D_kR (exergy losses due to irreversibility in storage barrel) is dominated by the mixing of water at different temperature. D_ku acts as the driving force for the system while D_kR is of little contribution. A good design of storage barrel with little D_kR will go a long way in improving exergy efficiency. An equation for computing D_kR is presented. For the collector, which is the core of the domestic-scale solar water heater, a judicious choice of width of plate W and layer number of cover is necessary. We define collector exergy efficiency η_xc to be η_xc=E_xo/E_xu. The relation between collector exergy efficiency and width of plate together with layer number of cover is also analysed.     

Optimal design of orientation of PV/T collector with reflectors

Hybrid conversion of solar radiation implies simultaneous solar radiation conversion into thermal and electrical energy in the PV/Thermal collector. In order to get more thermal and electrical energy, flat solar radiation reflectors have been mounted on PV/T collector. To obtain higher solar radiation intensity on PV/T collector, position of reflectors has been changed and optimal position of reflectors has been determined by both experimental measurements and numerical calculation so as to obtain maximal concentration of solar radiation intensity. The calculated values have been found to be in good agreement with the measured ones, both yielding the optimal position of the flat reflector to be the lowest (5°) in December and the highest (38°) in June. In this paper, the thermal and electrical efficiency of PV/T collector without reflectors and with reflectors in optimal position have been calculated. Using these results, the total efficiency and energy-saving efficiency of PV/T collector have been determined. Energy-saving efficiency for PV/T collector without reflectors is 60.1%, which is above the conventional solar thermal collector, whereas the energy-saving efficiency for PV/T collector with reflectors in optimal position is 46.7%, which is almost equal to the values for conventional solar thermal collector. Though the energy-saving efficiency of PV/T collector decreases slightly with the solar radiation intensity concentration factor, i.e. the thermal and electrical efficiency of PV/T collector with reflectors are lower than those of PV/T collector without reflectors, the total thermal and electrical energy generated by PV/T collector with reflectors in optimal position are significantly higher than total thermal and electrical energy generated by PV/T collector without reflectors.