Modelling and Demonstration of a Bifacial PV Curtain for Building Applications

Abstract

Global energy demand is on the rise. According to the International Energy Agency (IEA), in 2018, the building sector alone constituted 28 % of all energy-related CO2 emissions. In an effort to reduce the carbon footprint of the building sector, the European Union has decreed that every new building from 2021 must be a nearly zero energy building where the low energy for such buildings must come from renewable sources. While this policy works well for new buildings, there needs to be also a solution developed for existing buildings or monumental buildings to be able to easily harness renewable energy, without the need for any major and costly renovation. The concept of a flexible cloth-based solar curtain as a plug and play solution for existing buildings has been explored in the current research, with particular focus on a bifacial (two-sided) design, to assess the significance of the indoor reflected daylight and artificial lights on the rear side power output of such a semi-transparent solar curtain. An optical model representing a basic semi-transparent curtain comprising 70 % cloth of roughly 62 % transparency (based on availability), and 30 % of bifacial solar photovoltaic (PV) minimodules, is prepared as a base case. To validate the optical model, it is replicated on a smaller scale and tested using an identical experimental setup in a controlled environment using a solar simulator. Four cases are prepared for this: the curtain in an empty white-walled room, the curtain in the same room but with some furniture, the curtain in a shorter room with a white rear wall and the curtain in a shorter room with a black rear wall. An analysis of simulation and experimental results shows near likeness between the two with an error margin of 3.3 % for the base case to 10.6 % for the black rear wall case. Slight non uniformity of optical property inputs used in the simulation model and an overestimation of experimentally measured power due to a relatively lower irradiance seen by the reference cell are attributed to this difference. After successful validation of the optical layout, the model is made to scale in simulations to forma PV curtain module of 538.44 Wp DC installed capacity. Using a north-east facing room as reference, an annual simulation is found to give a total DC energy yield of 188.45 kWh or 323 kWh/kWp with a bifacial gain of 8.64 %. 10 hours daily of artificial lighting inside the office is seen to increase the annual DC energy yield by 1.33 %. With an assumption that the curtain remains closed throughout sunshine hours (since it allows diffuse light to enter through its porous cloth), the power output of the curtain is seen to meet modest DC annual load profiles of 57.76 kWh in an office room for 95 % of the year with a 145 Wh lithium ion battery bank and of 48.86 kWh in a residential room for 92%of the year, but with a much higher 290 Wh battery capacity, due to a greater mismatch between generation and demand. A sensitivity analysis for room orientation shows that for a more optimal azimuth of 202.5° (180° being south), the same curtain gives 82%more annual DC yield compared to its current orientation, but at a much lower bifacial gain of 7.8 %. When analyzed for performance in different locations across the world, the curtain intuitively gives the best yield for locations in the southern hemisphere due to its orientation towards the north east. Bifacial gain is nevertheless, most significant for temperate locations where low average annual insolation makes the gain due to bifacial more significant. In cases where the room depth is reduced by half, yield is seen to improve by 2.1 %. Halving the amount of PV in the curtain, improves the bifacial gain by nearly 0.6 % but reduces the annual yield by 49 %. Conversely, doubling the PV in the curtain, results in a dip in bifacial gain by about 3.6 %, but an increase in annual DC power by 87 %. Although this is attractive, such a case must be treated with caution since the likelihood of leaving this curtain (that is now more opaque than semi-transparent) open during sunshine hours is higher. This can result in a lower power output than the base case as well as worse aesthetics. Finally, it is concluded that a bifacial curtain is a beneficial for ‘solarizing’ existing buildings with minimal renovation. The bifacial aspect is best exploited for rooms with sub-optimal azimuths. Nevertheless, to make the most of the PV curtain, it is advised that itmust be placed in shorter rooms with large windows and ideally facing the most optimal direction.