Estimating operating cell temperature of BIPV modules in Thailand

Several models have been developed to estimate the operating cell temperatures of photovoltaic (PV) modules because they directly affect the performance of each PV module. In this study, two prediction models used most commonly, the nominal operating cell temperature (NOCT) model and the Sandia National Laboratory temperature prediction model (SNL), were investigated for their suitability in the prediction of PV module's temperatures for building integrated photovoltaic (BIPV) installation in the tropical climate conditions of Thailand. It was found that, in general, the SNL model tends to give better results of temperature prediction than those of the NOCT model. Nevertheless, both models are strongly over-biased in temperature predictions. The discrepancies of the predictions are basically caused by the dissimilarity of the BIPV installation and the standard installation as specified by the models, rather than the effect of differences in climatic conditions between the temperate and tropical zones. In the worst case, it was found that the highest value of the mean bias error (MBE) is +8 °C, or equivalent to +21% of the mean observed temperature, and the root mean square error (RMSE) is ±10 °C, or equivalent to ±24% of the mean observed temperature. However, although these errors were large, their effects on the accuracy of the final prediction of the electrical power output generated by the PV module over a long term would not be great. The error of the expected generated energy output would not be more than 6% of the averaged actual energy output, which is acceptable for most applications.     

Equilibrium thermal characteristics of a building integrated photovoltaic tiled roof

Photovoltaic (PV) modules attain high temperatures when exposed to a combination of high radiation levels and elevated ambient temperatures. The temperature rise can be particularly problematic for fully building integrated PV (BIPV) roof tile systems if back ventilation is restricted. PV laminates could suffer yield degradation and accelerated aging in these conditions. This paper presents a laboratory based experimental investigation undertaken to determine the potential for high temperature operation in such a BIPV installation. This is achieved by ascertaining the dependence of the PV roof tile temperature on incident radiation and ambient temperature. A theory based correction was developed to account for the unrealistic sky temperature of the solar simulator used in the experiments. The particular PV roof tiles used are warranted up to an operational temperature of 85 °C, anything above this temperature will void the warranty because of potential damage to the integrity of the encapsulation. As a guide for installers, a map of southern Europe has been generated indicating locations where excessive module temperatures might be expected and thus where installation is inadvisable.     

Effect of air gap on the performance of building-integrated photovoltaics

Ventilation of photovoltaic (PV) modules installed over or beside a building envelope can reduce the module temperature and increase the electrical conversion efficiency. A computational fluid dynamics (CFD) method has been used to assess the effect of the size of air gap between PV modules and the building envelope on the PV performance in terms of cell temperature for a range of roof pitches and panel lengths and to determine the minimum air gap that is required to minimise PV overheating. It has been found that the mean PV temperature and the maximum PV temperature associated with hot spots decrease with the increase in pitch angle and air gap. The mean PV temperature also decreases with increasing panel length for air gaps greater than or equal to 0.08 m whereas the maximum PV temperature generally increases with panel length. To reduce possible overheating of PV modules and hot spots near the top of modules requires a minimum air gap of 0.12–0.15 m for multiple module installation and 0.14–0.16 m for single module installation depending on roof pitches.     

Thermal aspects of c-Si photovoltaic module energy rating

Standard test conditions (STC) of photovoltaic (PV) modules are not representative of field conditions; PV module operating temperature often rises up to 30 °C above STC temperature (25 °C), causing a performance drop of 0.5%/°C for crystalline silicium modules. Normal operating cell temperature (NOCT) provides better estimates of PV module temperature rise. It has nevertheless to be measured; moreover NOCT wind speed conditions do not always fit field conditions. The purpose of this work is to model average PV module temperature at given irradiance levels as a function of meteorological parameters and PV module implementation. Thus, no empirical knowledge of PV module thermal behaviour is required for energy rating basing on irradiation distributions over irradiance levels.     

Influence of a building’s integrated-photovoltaics on heating and cooling loads

Building integrated photovoltaics (BIPV) has the potential to become a major source of renewable energy in the urban environment. BIPV has significant influence on the heat transfer through the building envelope because of the change of the thermal resistance by adding or replacing the building elements. Four different roofs are used to assess the impacts of BIPV on the building’s heating-and-cooling loads; namely ventilated air-gap BIPV, non-ventilated (closed) air-gap BIPV, closeroof mounted BIPV, and the conventional roof with no PV and no air gap. One-dimensional transient models of four cases are derived to evaluate the PV performances and building cooling-and-heating loads across the different roofs in order to select the appropriate PV building integration method in Tianjin, China. The simulation results show that the PV roof with ventilated air-gap is suitable for the application in summer because this integration leads to the low cooling load and high PV conversion efficiency. The PV roof with ventilation air-gap has a high time lag and small decrement factor in comparison with other three roofs and has the same heat gain as the cool roof of absorptance 0.4. In winter, BIPV of non-ventilated air gap is more appropriate due to the combination of the low heating-load through the PV roof and high PV electrical output.     

Estimation of photovoltaic module yearly temperature and performance based on Nominal Operation Cell Temperature calculations

The simulation of module temperature from Nominal Operation Cell Temperature (NOCT) is widely used to easily estimate module performance along the year. In this context, it is important to determine this parameter in a reliable way, as it is used to compare the performance of different module designs and can influence system predictions. At present there are several international standards that indicate the method to calculate NOCT in crystalline and thin-film terrestrial photovoltaic modules. This work presents the results obtained when applying these standards to different types of PV modules, including glass–glass and glass–tedlar structures, crystalline and thin-film technologies, and some special module designs for building integration applications. NOCT values so calculated have been used to estimate the yearly module temperature and performance for different orientations and tilted angles, analysing temperature influence in these estimations. Possible error sources that could bring about erroneous values of NOCT are also analysed.     

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。     

Practical application of building integrated photovoltaic (BIPV) system using transparent amorphous silicon thin-film PV module

An analysis has been carried out on the first practical application in Korea of the design and installation of building integrated photovoltaic (BIPV) modules on the windows covering the front side of a building by using transparent thin-film amorphous silicon solar cells. This analysis was performed through long-term monitoring of performance for 2 years. Electrical energy generation per unit power output was estimated through the 2 year monitoring of an actual BIPV system, which were 48.4 kWh/kWp/month and 580.5 kWh/kWp/year, respectively, while the measured energy generation data in this study were almost half of that reported from the existing data which were derived by general amorphous thin-film solar cell application. The reason is that the azimuth of the tested BIPV system in this study was inclined to 50° in the southwest and moreover, the self-shade caused by the projected building mass resulted in the further reduction of energy generation efficiency. From simulating influencing factors such as azimuth and shading, the measured energy generation efficiency in the tested condition can be improved up to 47% by changing the building location in terms of azimuth and shading, thus allowing better solar radiation for the PV module. Thus, from the real application of the BIPV system, the installation of a PV module associated with azimuth and shading can be said to be the essentially influencing factors on PV performance, and both factors can be useful design parameters in order to optimize a PV system for an architectural BIPV application.     

Numerical determination of adequate air gaps for building-integrated photovoltaics

The efficiency of photovoltaic (PV) devices is approximately inversely proportional to the cell temperature and the air gap of PV modules over or beside a building envelope can facilitate ventilation cooling of building-integrated photovoltaics. The effect of gap size on the performance of one type of PV module (with dimensions 1209 × 537 × 50 mm) in terms of cell temperature has been determined numerically for a range of roof pitches and panel lengths under two different settings of solar heat gains. It has been found that under constant solar heat gain, the air velocity behind PV modules due to natural convection in general increases with roof pitch angle. For a given location where solar heat gain varies with inclination from horizontal plane, however, the air velocity increases up to a pitch angle of about 60 degrees and then decreases with increasing roof pitch. The mean and maximum PV temperatures decrease with the increase in pitch angle and air gap. The mean PV temperature also decreases with increasing panel length for air gaps greater than or equal to 0.08 m, whereas the maximum PV temperature generally increases with panel length but decreases when the length of a roof-mounted panel increases from two modules to three modules and the air gap is between 0.1 and 0.11 m. Without adequate air circulation, overheating of PV modules would occur and hot spots could form near the top of modules with potential cell temperatures over 80 °C above ambient air temperature under bright sunshine.     

Available remodeling simulation for a BIPV as a shading device

A building integrated photovoltaic system as a shading device is used as an application and remodeling model. This study applies the simulation program SOLCEL and the computational fluid dynamics method to cases with solar irradiance components analysis and a ventilated double façade remodeling of the BIPV. For the validation of the theoretical work, experimental results of the Samsung Institute of Engineering and Construction Company building are used with a wind velocity of the weather data of Suwon area, Korea, where the real building is located. A photovoltaic system can be used only to generate electricity, but if a photovoltaic module can be used as an element of a double envelope, it could be more useful at the point of view of renewable energy usage and night insulation. Increase of PV module surface temperature is negative for power generation by installing PV module as an element of double envelope. A reasonable combination between renewable energy usage and power generation should be well analyzed for better usage of natural energy to design a BIPV.     

An accurate method for the PV model identification based on a genetic algorithm and the interior-point method

Due to the PV module simulation requirements as well as recent applications of model-based controllers, the accurate photovoltaic (PV) model identification method is becoming essential to reduce the PV power losses effectively. The classical PV model identification methods use the manufacturers provided maximum power point (MPP) at the standard test condition (STC). However, the nominal operating cell temperature (NOCT) is the more practical condition and it is shown that the extracted model is not well suited to it. The proposed method in this paper estimates an accurate equivalent electrical circuit for the PV modules using both the STC and NOCT information provided by manufacturers. A multi-objective global optimization problem is formulated using only the main equation of the PV module at these two conditions that restrains the errors due to employing the experimental temperature coefficients. A novel combination of a genetic algorithm (GA) and the interior-point method (IPM) allows the proposed method to be fast and accurate regardless the PV technology. It is shown that the overall error, which is defined by the sum of the MPP errors of both the STC and the NOCT conditions, is improved by a factor between 5.1% and 31% depending on the PV technology.     

Flat roof integration of a-Si triple junction modules laminated together with flexible polyolefin membranes

In BIPV design (Building Integrated PV) with crystalline silicon (c-Si) solar cells, ventilation is important in order to keep cells as cool as possible. To allow good ventilation it is therefore generally preferable to mount the modules separated from the existing roof. In the case of sloped roofs, the modules are superimposed onto the existing roof and for flat roofs separated tilted mounting structures designed to withstand wind loads are used instead, but both are not real building integrations.In this paper we analyse the behaviour and the energy yield of a 15.36 kWp PV system based on flexible triple junction amorphous silicon modules laminated together with a single ply roofing system.The PV plant has been integrated on a flat roof of a professional school located south of Switzerland. A significant part of the data analysis is done in comparison with three small open-rack plants (reference plants) installed near the integrated plant.An important result was that the thermally insulated nearly horizontal modules showed temperatures higher than for modules mounted on an open-rack structure, especially for sunny days. This created higher power losses due to negative temperature coefficients. On the other hand, the higher temperature reached the level where the main degradation mechanism of a-Si modules could be reversed and better thermal annealing could be observed. This conclusion was arrived at after a direct performance comparison of the thermally insulated plant and the open-rack a-Si reference plant, which has the same module and orientation as the main plant.In order to better understand the thermally insulated nearly horizontal plant behaviour, we analysed and quantified the irradiation difference and optical losses with respect to a 20° tilted open-rack c-Si power plant. Optical losses for nearly horizontal modules were significant during the winter, partially affecting their low performance.As a main result, the final energy yield of the thermally insulated a-Si plant was almost comparable to a 20° tilted open-rack c-Si power plant, despite the lower irradiance and higher reflection losses with respect to the latter.Accordingly, compared to c-Si modules, the a-Si technology represents a better choice for thermal insulated BIPV.     

Pilot study on building-integrated PV: Technical assessment and economic analysis

Building-integrated photovoltaics (BIPV) is an innovative green solution that incorporated energy generation into the building façade with modification on the building material or architectural structure. It is a clean and reliable solution that conserves the aesthetical value of the architecture and has the potential to enhance the building's energy efficiency. Malaysia's tropical location has a high solar energy potential to be exploited, and BIPV is a very innovative aspect of technology to employ the available energy. Heriot-Watt University Malaysia (HWUM) has a unique roof design that could be utilized as an application of the BIPV system to generate electricity, reducing the carbon footprint of the facility. Eight BIPV systems of different PV technologies and module types and with capacities of 411.8 to 1085.6 kW were proposed for the building. The environmental plugin software has been integrated with a building geometry modelling tool to visualize and estimate the energy potential from the roof surface in a 3D modelling software. Additionally, detailed system simulations are conducted using PVSyst software, where results and performance parameters are analysed. The roof surface is shown to provide great energy potential and studied scenarios generated between 548 and 1451 MWh yearly with PR range from 78% to 85%. C-Si scenarios offer the best economical profitability with payback period of 4.4 to 6.3 years. The recommended scenario has a size of 1085.5 kW and utilizes thin-film CdTe PV modules. The system generates 1415 MWh annually with a performance ratio of 84.9%, which saves 62.8% of the electricity bill and has an estimated cost of 901 000 USD. Installation of the proposed system should preserve the aesthetical value of the building's roof, satisfy BIPV rules, and most importantly, conserves energy, making the building greener.     

New type of photovoltaic module integrated with roofing material (highly fire-resistant PV tile)

A new type of photovoltaic (PV) module integrated with roofing material (a highly fire-resistant PV tile) has been developed. It offers many attractive features, such as a lower cost, simpler construction, better design, and greater fire resistance than previous modules, and it promises to help accelerate the use of PV modules in residential applications.     

A novel model for photovoltaic array performance prediction

Based on the I–V curves of a photovoltaic (PV) module, a novel and simple model is proposed in this paper to predict the PV module performance for engineering applications. Five parameters are introduced in this model to account for the complex dependence of the PV module performance upon solar-irradiance intensity and PV module temperature. Accordingly, the most important parameters, i.e. the short-circuit current, open-circuit voltage, fill factor and maximum power-output of the PV module, may be determined under different solar irradiance intensities and module temperatures. To validate the developed model, field measured data from one existing building-integrated photovoltaic system (BIPV) in Hong Kong was studied, and good agreements between the simulated results and the field data are found. This model is simple and especially useful for engineers to calculate the actual performances of the PV modules under operating conditions, with limited data provided by the PV module manufacturers needed.     

Implementation and verification of integrated thermal and electrical models for commercial PV modules

This paper presents a novel integrated photovoltaic (PV) model that simultaneously describes both electricity characteristics and thermal dynamics of a commercial PV module. Based on the thermal and electrical characteristics of commercial PV module available from the manufacturer datasheet, the proposed PV model is implemented on the Simulink environment and verified under the standard test condition (STC) and nominal operating cell temperature (NOCT) condition. The NOTC used to validate the thermal characteristics for a commercial PV module is first addressed. The cell temperatures and output electricity characteristics of a commercial PV module are evaluated through a series of experiments at real operating conditions. Comparing with the experiment results, the proposed model is validated and proved to have a great agreement with the output characteristics of a commercial PV module.     

Feasibility of photovoltaic – Thermoelectric hybrid modules

Outdoor performance of photovoltaic (PV) modules suffers from elevated temperatures. Conversion efficiency losses of up to about 25% can result, depending on the type of integration of the modules in the roof. Cooling of modules would therefore enhance annual PV performance. Instead of module cooling we propose to use the thermal waste by attaching thermoelectric (TE) converters to the back of PV modules, to form a PV–TE hybrid module. Due to the temperature difference over the TE converter additional electricity can be generated. Employing present day thermoelectric materials with typical figure of merits (Z) of 0.004 K⁻¹ at 300 K may lead to efficiency enhancements of up to 23% for roof integrated PV–TE modules, as is calculated by means of an idealized model. The annual energy yield would increase by 14.7–11%, for two annual irradiance and temperature profiles studied, i.e., for Malaga, Spain, and Utrecht, the Netherlands, respectively. As new TE materials are being developed, efficiency enhancements of up to 50% and annual energy yield increases of up to 24.9% may be achievable. The developed idealized model, however, is judged to overestimate the results by about 10% for practical PV–TE hybrids.     

BIPV: a real-time building performance study for a roof-integrated facility

Building integrated photovoltaic system (BIPV) is a photovoltaic (PV) integration that generates energy and serves as a building envelope. A building element (e.g. roof and wall) is based on its functional performance, which could include structure, durability, maintenance, weathering, thermal insulation, acoustics, and so on. The present paper discusses the suitability of PV as a building element in terms of thermal performance based on a case study of a 5.25?kW_p roof-integrated BIPV system in tropical regions. Performance of PV has been compared with conventional construction materials and various scenarios have been simulated to understand the impact on occupant comfort levels. In the current case study, PV as a roofing material has been shown to cause significant thermal discomfort to the occupants. The study has been based on real-time data monitoring supported by computer-based building simulation model.     

Modeling and coordinate control of photovoltaic DC building module based BIPV system

This paper presents the modeling method and coordinate control strategy for photovoltaic dc building module (PV-DCBM) based building integrated photovoltaic (BIPV) system. The PV-DCBM based BIPV system consists of plenty of PV-DCBMs and a centralized inverter which are coupled to the common dc bus in parallel. Each PV-DCBM is integrated with a PV building material to extract maximum power from it and then a centralized inverter is used to transfer the power to the grid. The PV-DCBM based BIPV system has some significant advantages for building integrated applications, such as individual MPPT, inherent data monitor, low cost and excellent expandability. A coordinate control strategy based on energy balance of the PV-DCBM based BIPV system is proposed to realize the individual control for each PV-DCBM and the centralized inverter. The accurate small-signal model of the PV-DCBM based BIPV system is built based on the proposed operation principle and a detailed design approach of the coordinate controller is proposed. Experimental results on the laboratory prototype verify the validity of the proposed modeling and coordinate control method.