Air-cooled PV/T solar collectors with low cost performance improvements

Excess temperatures on installed photovoltaic (PV) modules lead to efficiency loss and PV cooling protects them from this undesirable efficiency drop. Both water and air have been used for PV cooling through a thermal unit attached to the back of the module yielding photovoltaic/thermal (PV/T) collector, but air is preferred due to minimal use of material and low operating cost despite its poor thermo-physical properties. This study investigates the performance of two low cost heat extraction improvement modifications in the channel of a PV/T air system to achieve higher thermal output and PV cooling so as to keep the electrical efficiency at acceptable level. The use of thin flat metal sheet suspended at the middle or finned back wall of an air channel in the PV/T air configuration are the suggested methods. A theoretical model is developed and validated against experimental data, where good agreement between the predicted results and measured data were achieved. The validated model was then used to study the effect of the channel depth, channel length and mass flow rate on electrical and thermal efficiency, PV cooling and pressure drop for both improved and typical PV/T air systems and their results were compared. Both experimental and theoretical results show that the suggested modifications improve the performance of the PV/T air system.     

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.     

Improved PV/T solar collectors with heat extraction by forced or natural air circulation

The photovoltaic (PV) cells suffer efficiency drop as their operating temperature increases especially under high insolation levels and cooling is beneficial. Air-cooling, either by forced or natural flow, presents a non-expensive and simple method of PV cooling and the solar preheated air could be utilized in built, industrial and agricultural sectors. However, systems with heat extraction by air circulation are limited in their thermal performance due to the low density, the small volumetric heat capacity and the small thermal conductivity of air and measures for heat transfer augmentation is necessary. This paper presents the use of a suspended thin flat metallic sheet at the middle or fins at the back wall of an air duct as heat transfer augmentations in an air-cooled photovoltaic/thermal (PV/T) solar collector to improve its overall performance. The steady-state thermal efficiencies of the modified systems are compared with those of typical PV/T air system. Daily temperature profiles of the outlet air, the PV rear surface and channel back wall are presented confirming the contribution of the modifications in increasing system electrical and thermal outputs. These techniques are anticipated to contribute towards wider applications of PV systems due to the increased overall efficiency.     

Review on recent approaches for hybrid PV/T solar technology

This paper reviews the recent hybrid photovoltaic thermal (PV/T) structural/geometrical topologies to highlight the state of the art on this form of collector approaches and designs for both liquid and air‐based systems. The review focuses on the development of the typical flat‐plate collector – as an essential part in the PV/T system – in terms of new concept and novel configurations and specifically on the design of these collectors that use air or liquid as a heat transfer medium and their ability to extract useful heat from the back surface of the PV panel. Different mechanisms of fluid flow either natural or forced are considered. Many different design configurations for hybrid PV/T collectors have been cataloged and evaluated. It is shown that at least 30 distinct configurations have been introduced in the literature in the last 5 years. The paper concludes with identifying the major factors that affect the performance of typical PV/T systems and lead to effective enhancement of the heat removal mechanisms thus improving the electrical and thermal solar conversion efficiencies. This paper should serve as a significant form of reference for any future development in the design of the PV/T concept. Copyright © 2016 John Wiley & Sons, Ltd.     

Advancement in solar photovoltaic/thermal (PV/T) hybrid collector technology

This article presents an overview on the research and development and application aspects for the hybrid photovoltaic/thermal (PV/T) collector systems. A major research and development work on the photovoltaic/thermal (PVT) hybrid technology has been done since last 30 years. Different types of solar thermal collector and new materials for PV cells have been developed for efficient solar energy utilization. The solar energy conversion into electricity and heat with a single device (called hybrid photovoltaic thermal (PV/T) collector) is a good advancement for future energy demand. This review presents the trend of research and development of technological advancement in photovoltaic thermal (PV/T) solar collectors and its useful applications like as solar heating, water desalination, solar greenhouse, solar still, photovoltaic-thermal solar heat pump/air-conditioning system, building integrated photovoltaic/thermal (BIPVT) and solar power co-generation.     

Thermal analysis of PV/T evaporator of a solar-assisted heat pump

The performance of photovoltaic/thermal (PV/T) solar collector had been studied theoretically and experimentally for some years. Air and water streams were used as the heat carriers for space heating or services hot water systems. The cooling effect allows the PV module to work at lower temperature and its PV efficiency is therefore improved. However, such an advantage diminishes when the solar irradiance is high. To improve the situation a new type of PV/T collector is proposed. It works as the evaporator of a heat pump, in that refrigerant evaporates in the tubing at the back of the flat-plate collector and the PV module is adhered to the front surface. Mathematical models were developed to simulate the complex energy conversion processes. Numerical analysis was then performed based on the distributed parameters approach. An experimental rig was also built to test its real performance. Our results showed that the PV/T evaporator had an overall efficiency in the range of 0.64–0.87, thermal efficiency 0.53–0.64 and PV efficiency 0.124–0.135. The simulation results were found in good agreement with the experiment measurements. Copyright © 2006 John Wiley & Sons, Ltd.     

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。     

A prototype photovoltaic/thermal system integrated with transpired collector

Building-integrated photovoltaic/thermal (BIPV/T) systems may be utilized to produce useful heat while simultaneously generating electricity from the same building envelope surface. A well known highly efficient collector is the open-loop unglazed transpired collector (UTC) which consists of dark porous cladding through which outdoor air is drawn and heated by absorbed solar radiation. Commercially available photovoltaic systems typically produce electricity with efficiencies up to about 18%. Thus, it is beneficial to obtain much of the normally wasted heat from the systems, possibly by combining UTC with photovoltaics. Combination of BIPV/T and UTC systems for building facades is considered in this paper - specifically, the design of a prototype façade-integrated photovoltaic/thermal system with transpired collector (BIPV/T). A full scale prototype is constructed with 70% of UTC area covered with PV modules specially designed to enhance heat recovery and compared to a UTC of the same area under outdoor sunny conditions with low wind. The orientation of the corrugations in the UTC is horizontal and the black-framed modules are attached so as to facilitate flow into the UTC plenum. While the overall combined thermal efficiency of the UTC is higher than that of the BIPV/T system, the value of the generated energy - assuming that electricity is at least four times more valuable than heat - is between 7% and 17% higher. Also, the electricity is always useful while the heat is usually utilized only in the heating season. The BIPV/T concept is applied to a full scale office building demonstration project in Montreal, Canada. The ratio of photovoltaic area coverage of the UTC may be selected based on the fresh air heating needs of the building, the value of the electricity generated and the available building surfaces.     

Aspects and improvements of hybrid photovoltaic/thermal solar energy systems

Hybrid photovoltaic/thermal (PV/T or PVT) solar systems consist of PV modules coupled to water or air heat extraction devices, which convert the absorbed solar radiation into electricity and heat. At the University of Patras, an extended research on PV/T systems has been performed aiming at the study of several modifications for system performance improvement. In this paper a new type of PV/T collector with dual heat extraction operation, either with water or with air circulation is presented. This system is simple and suitable for building integration, providing hot water or air depending on the season and the thermal needs of the building. Experiments with dual type PV/T models of alternative arrangement of the water and the air heat exchanging elements were performed. The most effective design was further studied, applying to it low cost modifications for the air heat extraction improvement. These modifications include a thin metallic sheet placed in the middle of the air channel, the mounting of fins on the opposite wall to PV rear surface of the air channel and the placement of the sheet combined with small ribs on the opposite air channel wall. The modified dual PV/T collectors were combined with booster diffuse reflectors, achieving a significant increase in system thermal and electrical energy output. The improved PV/T systems have aesthetic and energy advantages and could be used instead of separate installation of plain PV modules and thermal collectors, mainly if the available building surface is limited and the thermal needs are associated with low temperature water or air heating.     

Study of a new concept of photovoltaic–thermal hybrid collector

This work represents the second step of the development of a new concept of photovoltaic/thermal (PV/T) collector. This type of collector combines preheating of the air and the production of hot water in addition to the classical electrical function of the solar cells. The alternate positioning of the thermal solar collector section and the PV section permits the production of water at higher mean temperatures than most of existing hybrid collectors. These higher temperatures will allow the coupling of components such as solar cooling devices during the summer and obviously a direct domestic hot water (DHW) system without the need for additional auxiliary heating systems. In this paper, a simplified steady-state two-dimensional mathematical model of a PV/T bi-fluid (air and water) collector with a metal absorber is developed. Then, a parametric study (numerically and experimentally) is undertaken to determine the effect of various factors such as the water mass flow rate on the solar collector thermal performances. Finally, the results from an experimental test bench and the first simulation results obtained on full scale experiments are compared.     

Experimental investigation and modeling of a direct-coupled PV/T air collector

Photovoltaic/thermal (PV/T) systems refer to the integration of photovoltaic and solar thermal technologies into one single system, in that both useful heat energy and electricity are produced. The impetus of this paper is to model a direct-coupled PV/T air collector which is designed, built, and tested at a geographic location of Kerman, Iran. In this system, a thin aluminum sheet suspended at the middle of air channel is used to increase the heat exchange surface and consequently improve heat extraction from PV panels. This PV/T system is tested in natural convection and forced convection (with two, four and eight fans operating) and its unsteady results are presented in with and without glass cover cases. A theoretical model is developed and validated against experimental data, where good agreement between the measured values and those calculated by the simulation model were achieved. Comparisons are made between electrical performance of the different mode of operations and it is concluded that there is an optimum number of fans for achieving maximum electrical efficiency. Also, results show that setting glass cover on photovoltaic panels leads to an increase in thermal efficiency and decrease in electrical efficiency of the system.     

太阳能光伏光热建筑一体化系统的研究

太阳能光伏光热一体化不仅能够有效降低光伏组件的温度,提高光伏发电效率,而且能够产生热能,从而大大提高了太阳能的转换效率。对光伏光热建筑一体化(BIPV/T)系统的两种主要模式:水冷却型和空气冷却型系统的工作原理和系统模型进行了理论介绍,详细说明了两种系统中热产品在家庭中的应用。并对目前研究情况下两个系统中存在的问题提出了改进方案。与常规建筑相比,光伏光热建筑减少了墙体得热,改善了室内空调负荷状况,提高了建筑节能效果。     

Performance optimization of a photovoltaic/thermal collector using microencapsulated phase change slurry

In a photovoltaic/thermal (PV/T) collector, a portion of absorbed solar energy is transformed into electrical energy, and the remaining part is transformed into thermal energy. Increasing waste heat collection and energy conversion rates are important to improve the performance of the PV/T collector. The utilization of microencapsulated phase change slurry (MPCS) in a PV/T collector to cool photovoltaic modules is an effective way, and electrical and thermal performances of the collector are improved. To investigate influences of operating parameters on performances of PV/T collector, numerical simulation is put into effect to analyze influences of the mass fraction of MPCS on the collector performance. The influences of MPCS mass flow rate and collector channel height on collector performances are also studied. When the flow rate is 0.005 kg/s and the channel height is 0.010 m, the PV/T collector obtains the best net efficiency with a MPCS mass concentration of 20 wt%. But electrical efficiency difference between 15 and 20 wt% is not obvious. With the growth in mass fraction, PV temperature drops more and more slowly because outlet fluid has not fully melt. Take PV/T collector performances into consideration, 15 wt% MPCS is a better choice to cool photovoltaic modules.     

An improved thermal and electrical model for a solar photovoltaic thermal (PV/T) air collector

In this paper, an attempt is made to investigate the thermal and electrical performance of a solar photovoltaic thermal (PV/T) air collector. A detailed thermal and electrical model is developed to calculate the thermal and electrical parameters of a typical PV/T air collector. The thermal and electrical parameters of a PV/T air collector include solar cell temperature, back surface temperature, outlet air temperature, open-circuit voltage, short-circuit current, maximum power point voltage, maximum power point current, etc. Some corrections are done on heat loss coefficients in order to improve the thermal model of a PV/T air collector. A better electrical model is used to increase the calculations precision of PV/T air collector electrical parameters. Unlike the conventional electrical models used in the previous literature, the electrical model presented in this paper can estimate the electrical parameters of a PV/T air collector such as open-circuit voltage, short-circuit current, maximum power point voltage, and maximum power point current. Further, an analytical expression for the overall energy efficiency of a PV/T air collector is derived in terms of thermal, electrical, design and climatic parameters. A computer simulation program is developed in order to calculate the thermal and electrical parameters of a PV/T air collector. The results of numerical simulation are in good agreement with the experimental measurements noted in the previous literature. Finally, parametric studies have been carried out. Since some corrections have been down on thermal and electrical models, it is observed that the thermal and electrical simulation results obtained in this paper is more precise than the one given by the previous literature. It is also found that the thermal efficiency, electrical efficiency and overall energy efficiency of PV/T air collector is about 17.18%, 10.01% and 45%, respectively, for a sample climatic, operating and design parameters.     

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.     

A photovoltaic/thermal (PV/T) collector with a polymer absorber plate. Experimental study and analytical model

A polymer solar heat collector was combined with single-crystal silicon PV cells in a hybrid energy-generating unit that simultaneously produced low temperature heat and electricity. The PV/T unit was tested experimentally to determine its thermal and photovoltaic performance, in addition to the interaction mechanisms between the PV and thermal energy systems. Thermal efficiency measurements for different collector configurations are compared, and PV performance and temperature readings are presented and discussed. An analytical model for the PV/T system simulated the temperature development and the performance of both the thermal and photovoltaic units.     

System performance studies on a photovoltaic/thermal (PV/T) air heating collector

A computer simulation model is presented for the analysis of a solar photovoltaic/thermal (PV/T) hybrid collector with air as heat transfer fluid and algorithm for making quantitative prediction regarding the performance of the system is described. Thermal efficiency curves for the solar PV/T hybrid collectors corresponding to various type of absorbers have been derived. In order to appreciate the model, numerical calculations have been made for evaluating the system performance corresponding to typical climate of Delhi, India     

基于相变微胶囊悬浮液的太阳能光伏热系统的研究进展

在太阳能光伏热系统中,光伏电池温度过高会导致太阳能发电效率下降。相变微胶囊悬浮液(MEPCMS)是一种潜热型功能性流体,将其作为冷却介质用于太阳能光伏热系统可以有效降低光伏电池温度,提高系统的能量利用率。针对相变微胶囊易泄露、导热性差等问题提出了改性方法,使其具有光热转换功能并提升了综合性能。基于性能评价指标分析了太阳能光伏热系统性能的影响因素。结果发现,流速、浓度和太阳辐照量是影响MEPCMS在太阳能光伏热系统中换热性能的关键因素。适当增加MEPCMS浓度和流速能提高工质的换热性能,在降低光伏板温度的同时增加太阳辐照量和系统热电产量,但需结合太阳辐照量大小合理匹配工质的浓度和流速。未来研究方向可集中在提升MEPCMS在太阳能光伏热系统中的换热性能、探究运行参数和太阳辐照量之间的匹配关系、优化集热器结构、利用其蓄热性解决太阳能间歇性等方面。     

Numerical study on photovoltaic/thermal systems with extended surfaces

The current study presents a novel and straightforward approach for simulating photovoltaic/thermal (PV/T) systems using the commercial computational fluid dynamics (CFD) solver ANSYS‐FLUENT. Instead of resolving the natural convection within the air gap between the PV and the glass cover, the effective thermal conductivity approach is implemented. Moreover, the solar radiation incident on the PV layer is directly included in the energy equation of the PV domain to evaluate the resultant power output and heat generation. The validity of these implications is proven by comparing predicted data with experimental data from the literature. Comparative results reveal a root‐mean‐square error of 7% and 2% for the PV temperature and the outlet air temperature, respectively. A comprehensive numerical analysis is also conducted for a PV/T system with and without finned surfaces. In the parametric study, the impacts of varying a number of design parameters, operating conditions, and weather data over a wide range are assessed. Results reveal that channel height and air velocity have the greatest impact on the overall efficiency and outlet air temperature of a PV/T system. An optimization study is also conducted using the response surface methodology to obtain optimal values of design parameters and operating conditions for this system. The highest overall efficiencies and outlet air temperatures are achieved in PV/T systems comprising narrow channel geometries, regardless of the operating conditions or weather data considered. Optimal conditions are achieved for a collector with a collector length of 1.5 m, a channel height of 1 cm, and an air velocity of 2.3 m/s. For the optimal design, overall efficiency and outlet temperature values are evaluated as 53.4% and 310.9 K, respectively. Sensitivity analyses also observe the impact of adding fins to the air channel, and it is concluded that the addition of fins improves the overall efficiency of the PV/T system by up to 19%. However, adding fins does not significantly affect the outlet air temperature; nor does it improve the overall efficiency of the PV/T system beyond a critical channel height.     

Feasibility study of a hybrid photovoltaic/thermal collector system in the climate of Jordan

This study is dedicated to investigating the feasibility of photovoltaic/thermal (PV/T) collectors' technology for application in Jordan. Simple parallel-plate collector configurations were simulated using COMSOL: rectangular fins, triangular fins, and wavy walls. The wavy-wall configuration was found the most efficient alternative in terms of heat transfer with respect to the pumping power and performance factor that took into account the comparison with a plain-wall parallel-plate collector. However, the performance of the plain-wall parallel plate preceded that of the wavy wall by increasing the Reynolds number and the water channel height. The plain-wall parallel-plate configuration was further investigated on HOMER as a 5 MW solar plant that provides energy to a 5-MW facility. One MW of its load is direct thermal load. Different solar plant designs were compared. The PV/T plant was found to be very much energy saving but not feasible due to its high initial cost. However, the PV/T plant was better than the PV when the cooling was not complete compromising on some electric energy in favor of heat generation. Further work on reducing the cost of the PV/T collector is required especially with regard to contact methods between the PV and the absorber plate and to the weight of the collector.