Exergetic performance assessment of a solar photovoltaic thermal (PV/T) air collector

In this paper, an attempt is made to evaluate the exergetic performance of a solar photovoltaic thermal (PV/T) air collector. A detailed energy and exergy analysis is carried out to calculate the thermal and electrical parameters, exergy components and exergy efficiency of a typical PV/T air collector. Some corrections are done on related heat loss coefficients. An improved electrical model is used to estimate the electrical parameters of a PV/T air collector. Further, a modified equation for the exergy efficiency of a PV/T air collector is derived in terms of design and climatic parameters. A computer simulation program is also developed 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. It is observed that the modified exergy efficiency obtained in this paper is in good agreement with the one given by the previous literature. It is also found that the thermal efficiency, electrical efficiency, overall energy efficiency and exergy efficiency of PV/T air collector is about 17.18%, 10.01%, 45% and 10.75% respectively for a sample climatic, operating and design parameters.     

A heat pipe photovoltaic/thermal (PV/T) hybrid system and its performance evaluation

Building-integrated photovoltaic/thermal (BIPV/T) system has been considered as an attractive technology for building integration. The main part of a BIPV/T system is PV/T collector. In order to solve the non-uniform cooling of solar PV cells and control the operating temperature of solar PV cells conveniently, a heat pipe photovoltaic/thermal (PV/T) hybrid system (collector) has been proposed and described by selecting a wick heat pipe to absorb isothermally the excessive heat from solar PV cells. A theoretical model in terms of heat transfer process analysis in PV module panel and introducing the effectiveness-number of transfer unit (?-NTU) method in heat exchanger design was developed to predict the overall thermal-electrical conversion performances of the heat pipe PV/T system. A detailed parametric investigation by varying relevant parameters, i.e., inlet water temperature, water mass flow rate, packing factor of solar cell and heat loss coefficient has been carried out on the basis of the first and second laws of thermodynamics. Results show that the overall thermal, electrical and exergy efficiencies of the heat pipe PV/T hybrid system corresponding to 63.65%, 8.45% and 10.26%, respectively can be achieved under the operating conditions presented in this paper. The varying range of operating temperature for solar cell on the absorber plate is less than 2.5 °C. The heat pipe PV/T hybrid system is viable and exhibits the potential and competitiveness over the other conventional BIPV/T systems.     

Economic and exergy analysis of alternative plants for a zero carbon building complex

The feasibility of zero carbon emission plants for heating, air conditioning and domestic hot water (DHW) supply, is analyzed, with respect to conventional plants, for a new residential building complex to be constructed, in Northern Italy. Two zero carbon plants are considered: the first is composed of air-to-water heat pumps for space heating and cooling, PV solar collectors, air dehumidifiers, thermal solar collectors and a wood pellet boiler for DHW supply; in the second, the air-to-water heat pumps are replaced by ground-coupled heat pumps. The conventional plant is composed of a condensing gas boiler, single-apartment air to air heat pumps, and thermal solar collectors. The economic analysis shows that both zero carbon plants are feasible, and that the air-to air heat pumps yield a shorter payback time. The exergy analysis confirms the feasibility of both plants, and shows that the ground coupled heat pumps yield a higher exergy saving.     

Evaluation of a PV-integrated building application in a well-controlled outdoor test environment

This paper presents the assessment of experimental data for electrical and thermal performance evaluation of photovoltaic (PV) systems integrated as cladding components into the building envelope, giving input to modelling and analysis work. From the experience gained in several EU research projects, an improved design for a common Test Reference Environment (TRE) has been developed. This specific design of the PV module and TRE makes it possible to study, through electrical and thermal energy flow analysis, the effect on electrical performance of using different materials for PV modules and the construction design of claddings. The results for a glass–glass PV module with forced ventilation are presented.     

Effect of building integrated photovoltaics on microclimate of urban canopy layer

Building integrated photovoltaics (BIPV) has potential of becoming the mainstream of renewable energy in the urban environment. BIPV has significant influence on the thermal performance of building envelope and changes radiation energy balance by adding or replacing conventional building elements in urban areas. PTEBU model was developed to evaluate the effect of photovoltaic (PV) system on the microclimate of urban canopy layer. PTEBU model consists of four sub-models: PV thermal model, PV electrical performance model, building energy consumption model, and urban canyon energy budget model. PTEBU model is forced with temperature, wind speed, and solar radiation above the roof level and incorporates detailed data of PV system and urban canyon in Tianjin, China. The simulation results show that PV roof and PV façade with ventilated air gap significantly change the building surface temperature and sensible heat flux density, but the air temperature of urban canyon with PV module varies little compared with the urban canyon of no PV. The PV module also changes the magnitude and pattern of diurnal variation of the storage heat flux and the net radiation for the urban canyon with PV increase slightly. The increase in the PV conversion efficiency not only improves the PV power output, but also reduces the urban canyon air temperature.     

Experimental exergy analyses on fabricated parabolic solar collector with/without preheater and different collector materials

This is an experimental work, to get maximum performance by parabolic solar collector (PSC). Three different PSCs are opted with same dimensions for this experimental work; mirror, aluminium and preheater–aluminium collectors. Copper, aluminium, mild steel and brass are selected for various absorber pipe materials. By experimental analyses, fluid temperatures, convective heat transfer coefficients, overall heat loss coefficients, heat removal factors, collector efficiency factors, inlet exergies, outlet exergies, exergy gains and exergy efficiencies are obtained at a range of operating conditions for fabricated PSCs. Exergy analyses are conducted with various fluid flow rates and with different PSCs and then graphs are also generated for these analyses. After analyses, this work can be concluded as – the brass as absorber pipe material and preheater–aluminium sheet combination as PSC are the most excellent to achieve the most favourable performance from fabricated system. This combination gives maximum exergy efficiency which is near about 78%.     

A novel experimental method to find dust deposition effect on the performance of parabolic trough solar collectors

One main reason why the performance of a parabolic trough collector measured experimentally differs from the simulation results is inaccurate prediction of absorbed solar energy. The amount of absorbed energy of such systems mainly depends on optical properties of mirrors, absorber tube and transmissivity of glass cover of absorber tube. This study examines the thermal performances of three collectors during different periods regarding the weight and thickness of deposited dust. A transient computer code is prepared to estimate thermal performances of clean collectors and compare the results with dusty collectors. A special optical instrument is designed and constructed to measure the reflectivity of mirrors with respect to the thickness of dust deposited on the surface of the mirrors. Based on the measurements a new correlation is developed to find thermal performance of a parabolic collector for various dust thicknesses in comparison to a clean collector. Different cleaning schedules are proposed for the collectors’ field; and thermal performance and yearly solar energy absorption of collectors according to each cleaning schedule are also determined. The performance of a solar-fossil hybrid power plant is studied and the impact of different cleaning schedules on reducing fossil fuel consumption and CO₂ emission is investigated.     

Thermal performance analysis of the glass evacuated tube solar collector with U-tube

In this paper, based on the energy balance for the glass evacuated tube solar collector with U-tube, the thermal performance of the individual glass evacuated tube solar collector is investigated by analytical method. The solar collector considered in this study is a two-layered glass evacuated tube, and the absorber film is deposited in the outer surface of the absorber tube. The heat loss coefficient and heat efficiency factor are analyzed using one-dimensional analytical solution. And the influence of air layer between the absorber tube and the copper fin on the heat efficiency is also studied. The results show that the function relation of the heat loss coefficient of the glass evacuated tube solar collector with temperature difference between the absorbing coating surface and the ambient air is nonlinear. In the different ambient temperatures, the heat loss coefficient of the solar collector should be calculated by different expressions. The heat efficiency factor will be subject to influence of air layer between absorber tube and the copper fin. Specially, the influence is remarkable when the heat loss coefficient of the collector is large. When the synthetical conductance amounts to 5 W/m K, the solar collector efficiency decreases 10%, and the outlet fluid temperature decreases 16% compared with the case which the air thermal resistance is neglected. And the surface temperature of the absorbing coating increases 30 °C due to the effect of air thermal resistance. So the surface temperature of the absorbing coating is an important parameter to evaluate the thermal performance of the glass evacuated tube solar collector.     

Experimental energy and exergy analysis of a double-flow solar air heater having different obstacles on absorber plates

This paper presents an experimental energy and exergy analysis for a novel flat plate solar air heater (SAH) with several obstacles and without obstacles. For increasing the available heat-transfer area may be achieved if air is flowing simultaneously and separately over and under the different obstacle absorbing plates, instead of only flowing either over or under the different obstacle absorbing plates, leading to improved collector efficiency. The measured parameters were the inlet and outlet temperatures, the absorbing plate temperatures, the ambient temperature, and the solar radiation. Further, the measurements were performed at different values of mass flow rate of air and different levels of absorbing plates in flow channel duct. After the analysis of the results, the optimal value of efficiency is middle level of absorbing plate in flow channel duct for all operating conditions and the double-flow collector supplied with obstacles appears significantly better than that without obstacles. At the end of this study, the exergy relations are delivered for different SAHs. The results show that the largest irreversibility is occurring at the flat plate (without obstacles) collector in which collector efficiency is smallest.     

太阳能平板集热器关键参数对其热性能的影响

为了提升太阳能在建筑能源供给体系中的比重,形式多样的太阳能集热设备及其系统在建筑领域得到了广泛应用。基于机理分析法,建立了具有单层玻璃盖板的管板式太阳能平板集热器的稳态传热模型。并且针对集热管间距、集热管内径、工质入口温度和工质质量流量等关键参数对集热器集热效率的影响特性进行了数值模拟与分析。结果表明,建立的该稳态传热模型是可行的;此外,在其余参数值保持不变的情况下,减小集热管间距或增加集热管内径均可使集热器瞬时效率增大;增大工质入口温度会导致集热器瞬时效率下降;而增大工质质量流量会提升集热器瞬时效率。这些结论对于太阳能平板集热器在太阳能建筑一体化的实际应用中,具有一定的参考作用。     

建筑一体化电热冷联产光伏组件能量特性研究

传统太阳能光伏或光热建筑一体化只能为建筑提供单一电能或热能。通过研究一种集成发电、集热、制冷3种功能的建筑一体化电热冷联产光伏组件,对其夏季工况下能量特性进行了实际检测。结果表明:白天,组件集热同时能有效降低光伏电池温度,组件工作温度高于环境温度约8~16℃,发电和集热效率分别为14.1%~13.7%和40.1%~15.7%;晴朗夜间,组件通过对流和辐射两种传热方式进行散热制冷,总制冷功率为26.0~268.5 W/m~2。电热冷联产光伏组件适合与热泵结合,为建筑提供所需能源。     

Performance and sustainability assessment of energy options for building HVAC applications

In this study, a building with a volume of 351 m³ and a net floor area of 117 m² is considered as a case study with the indoor and exterior air temperatures of 20 and 0 °C, respectively. For the heating applications, four options are studied with (1) a heat pump, (2) a condensing boiler, (3) a conventional boiler and (4) a solar collector, which are driven by renewable and non-renewable energy sources. An energy and exergy analysis is employed to assess their performances and compare them through energy and exergy efficiencies and sustainability index. Energy and exergy flows are investigated and illustrated. Also, the energetic and exergetic renewability ratios are utilized here along with sustainability index. The results show that overall exergy efficiencies of heat pump, condensing boiler, conventional boiler and solar heating systems are found to be 3.66, 3.31, 2.91, and 12.64%, while the sustainability index values for the four cases considered are calculated to be 1.039, 1.034, 1.030 and 1.144, respectively. So, solar collector-based heating system gives the highest efficiency and sustainability index values.     

空调用太阳能集热器适用性分析

计算了平板集热器、真空管集热器、热管真空管集热器和CPC真空管集热器的太阳能利用系数,分析了几种太阳能集热器在空调系统中的适用性。结果表明,太阳能空调(热泵)系统集热器的热性能随着气象条件、热源温度的变化而变化,具有不同热源温度要求的系统应选用不同的集热器。     

U型管式全玻璃真空管太阳集热器的热性能

在U型管式全玻璃真空管集热器能量平衡分析的基础上,推导了集热器热损系数、效率因子等性能参数计算公式,其理论计算结果与实验数据吻合良好。计算分析表明：真空管热损系数与吸热管温和环境温度之差是非线性关系,将两者的计算关系式按环境温度分段整理将使计算结果更接近实际;涂层发射比对集热器效率影响较大,降低涂层发射比是提高集热器效率的有效途径;采取适当的措施降低吸热管与肋片接触热阻后,采用U型管连接方式不会对热水系统集热器效率造成太大影响。     

Numerical simulation of the performance of an air heating solar collector

The effects of the thermal and hydrodynamics entry length for both laminar and turbulent flow regimes within an air-heating solar collector have been studied using a numerical procedure. Two internal arrangements were considered: (1) air passing on both sides of the absorber (parallel flow), and (2) use of finned and other augmented surfaces.

Results of the analysis showed that the temperatures of the cover plates for the parallel flow collector were lower than if the air had passed on only one side of the absorber plate (single flow). Consequently, the heat losses were reduced and the efficiency of the parallel flow collector was increased. Also the pumping power per unit energy gained by the collector for parallel flow mode was less than that for the single flow mode. By using a finned surface, the temperatures of the cover plates were lower than for single and parallel flow and consequently the heat losses were reduced and the efficiency of the finned surface collector was increased. But in this case, the pumping power per unit energy gained by the collector and operating costs also increased.     

两种手段提升太阳能新风热利用效率的研究

严寒地区居住建筑面临较为严重的冬季新风不足问题,太阳能新风系统是改善室内空气品质的一种重要手段。研究目的:了解空气流道厚度降低和使用双层玻璃盖板对于提升基础型太阳能新风集热器效率的实际效果。方法:通过搭建太阳能新风集热器实验平台,自制单层、双层玻璃盖板太阳能新风集热器,对比研究空气流道厚度(10mm和30mm)以及不同玻璃盖板层数对集热器效率的影响,并对不同工况下的出口温度和逐时效率进行监测和分析。结果表明:改小集热器空气流道厚度之后平均集热效率上升了10.8%,使用双层玻璃盖板之后平均集热效率上升了7.5%。双层玻璃盖板在低温下能够进一步发挥保温优势,减少了玻璃盖板表面的热损失。结论:减小空气流道厚度,采用双层玻璃盖板具有明显的实际效果,太阳能新风系统具有很好的推广应用潜力。     

Performance of solar systems employing collectors with colored absorber

Flat plate solar collectors are of black appearance because of the color of the absorber, which is employed to maximize the absorption of solar spectrum. Generally, to avoid the monotony of the black color we can use collectors with absorbers of blue, red–brown, green or other color. These collectors are of lower thermal efficiency than that of the usual black type collectors, because of the lower collector absorptance, but they are of more interest to architects for applications on traditional or modern buildings. In this paper, applications of solar collectors with colored absorbers in a large hot water system suitable for multi-flat residential or office buildings, a house heating system, and an industrial process heat system are presented. The collectors are analyzed with respect to their performance and practical applications, aiming to give guidelines for their wider use on buildings. These systems are simulated on an annual basis at three different locations at different latitudes, Nicosia, Cyprus (35°), Athens, Greece (38°) and Madison, Wisconsin (43°). All simulations are carried out with TRNSYS. The results show that although the colored collectors present lower efficiency than the typical black type collectors, the difference in energy output depends on the absorber darkness. For a medium value of the coefficient of absorptance (α = 0.85), the colored collectors give satisfactory results regarding the drop of the amount of collected energy for the three locations (about 7–18%), compared to collectors with black absorbers (α = 0.95). This implies the use of proportionate larger collector aperture area to have the same energy output as that of typical black colored collectors. Additionally, the economic figures obtained for the systems investigated are very promising.     

浅析太阳能与住宅建筑一体化设计

太阳能集热器与住宅建筑一体化安装,是住宅节能化发展的必然趋势。其主要操作过程就是把集热器安装到建筑物中,以此来实现与建筑的完美结合。通过分析太阳能集热器的特点和使用要求,对集热器在各种类型建筑中的设置进行探讨。     

太阳能聚光光伏光热(CPV/T)系统研究

聚光光伏光热(CPV/T)系统将太阳电池与集热器相结合,通过聚光和跟踪太阳提高太阳辐照强度,增加太阳电池的单位面积发电量,集热器在冷却太阳电池的同时回收多余热能,获得电能和热能双重收益。本文研究聚光比、太阳电池效率、组件中电池片的有效覆盖率、热传导率等因素对系统性能的影响。     

Exergetic modeling and performance evaluation of solar water heating systems for building applications

Solar water heating (SWH) is a well-proven renewable energy technology and has been used in many countries of the world. The basic technology is straightforward, although there are a variety of various types of SWH systems. In the performance assessment of SWH systems, energy analysis (first law) method has been widely used, while the number of the studies on exergetic assessment is relatively low. The SWH system investigated consists of mainly three parts, namely a flat plate solar collector, a heat exchanger (storage tank) and a circulating pump. The main objectives of the present study are as follows, differing from the previously conducted ones: (i) to model and assess SWH systems using exergy analysis (second law) method as a whole, (ii) to investigate the effect of varying water inlet temperature to the collector on the exergy efficiencies of the SWH system components, (iii) to study some thermodynamic parameters (fuel depletion ratio, relative irreversibility, productivity lack and exergetic factor) and exergetic improvement potential, and (iv) to propose and present an exergy efficiency curve similar to the thermal efficiency curve for solar collectors. The system performance is evaluated based on the experimental data of the Izmir province, Turkey, which is given as an illustrative example. Exergy destructions (or irreversibilities) as well as exergy efficiency relations are determined for each of the SWH system components and the whole system. Exergy efficiency values on a product/fuel basis are found to range between from 2.02 to 3.37%, and 3.27 to 4.39% at a dead (reference) state temperature of 32.77 °C, which is an average of ambient temperatures at eight test runs from 1.10 to 3.35 p.m., for the solar collector and entire SWH system, respectively. An exergy efficiency correlation for the solar collector studied was also presented to determine its exergetic performance. It is expected that the model presented here would be beneficial to the researchers, government administration, and engineers working in the area of SWH systems for residential applications.