Investigation into the effects of flow distribution on the photovoltaic performance of a building integrated photovoltaic/thermal solar collector

摘要

The conversion of solar energy into usable forms of energy such as electricity and heat is attractive given the abundance of solar energy and the numerous issues recently raised in the consumption of fossil fuels. Solar conversion technologies may generally be categorised as either photovoltaic or solar thermal types capable of converting incidental sunlight into electricity and heat respectively. The photovoltaic cell is able to transform incidental sunlight into electricity via the Becquerel effect, however, the single junction crystalline silicon solar cell, the predominant cell type in today’s photovoltaic market is only able to utilise a small portion (less than 20%) of incidental sunlight for this purpose. A majority of the remaining portion is absorbed much like a traditional solar thermal collector and sunk as heat by the cell, elevating its operating temperature. Given the negative effect of temperature on photovoltaic cell operation, where a linearly proportional drop in conversion efficiency with elevated temperature can be expected, photovoltaic conversion can be reduced significantly particularly in areas of high irradiance and ambient temperatures. Based on the intrinsic absorption characteristics of the photovoltaic cell, a third type of solar panel referred to as the hybrid photovoltaic thermal collector (PVT) collector has been developed where fluid channels running along the underside of the photovoltaic panel transfer heat away from the cells to minimise this detrimental effect. Furthermore, heat captured from the cells may then be used for space heating or domestic hot water improving the overall collector efficiency.In this study a unique building integrated PVT (BIPVT) collector is investigated consisting of an aluminium extrusion with structural ribs, fluid channels, and solar conversion materials. In order to evaluate this design, a mathematical model of the collector was developed in order to determine both thermal and electrical yield of the proposed design. The thermal analyses of the building integrated PVT collector in previous studies have generally adopted the approach applied to traditional solar thermal collectors where the distribution of coolant fluid flowing through the piping array is assumed uniform. For a conventional solar thermal collector this simplification may be reasonable under certain circumstances, however, given the temperature sensitivity of photovoltaic cells and their electrical connection scheme, this assumption may lead to significant modelling error. In order to further investigate this issue, a mathematical model has been developed to determine the photovoltaic yield of a BIPVT collector operating under non-homogeneous operating temperature as a result of flow maldistribution. The model is composed of three steps individually addressing the issues of 1) fluid flow, 2) heat transfer, and 3) the photovoltaic output of a BIPVT array. Fluid analysis was conducted using the finite element method in order to obtain the individual fluid channel flow rates. Using these values, a heat transfer analysis was then conducted for each module forming the BIPVT array to calculate the photovoltaic operating temperature for the constituent cells forming the array. During this step the finite difference method was utilised to approximate the fin efficiency of the building integrated collector, taking into account its irregular geometry. Finally the photovoltaic yield was calculated using a numerical approach which considered the individual operating temperature of the PV cells. During this step a new method was identified to determine the values of series and shunt resistances and also the diode constant required for the modelling of photovoltaic devices based on the multi-dimensional Newton-Raphson method and current-voltage equations expressed using the Lambert W-function. Experimentation was carried out to validate the new modelling methods.These models were combined to quantify the detrimental impact of flow mal-distribution on photovoltaic yield for a number of scenarios. In the case where flow uniformity was poorest, only a 2% improvement in photovoltaic yield was obtained in comparison to a traditional photovoltaic panel operating under the same environmental conditions. For the case where flow uniformity was optimal however, photovoltaic output was improved by almost 10%.This work has shown that the effects of poor flow distribution has the potential to have a substantial negative impact on the photovoltaic output of a building integrated solar collector especially given the variability in its physical geometry. The appropriate design of this technology should therefore consider the effects of this phenomenon. The methodology presented in this study can be used to approximate PV output for a BIPVT array with different array geometries and operating characteristics. Furthermore, the method to calculate solar cell modelling parameters developed in this study is not only useful for the analysis of hybrid PVT systems, but for the general analysis of photovoltaic systems based on crystalline silicon solar cells.