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Vertical solar PV systems in tropical region

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With intense research and development of new solar PV technologies in recent years, improvement of conversion efficiencies and mass production has resulted in higher productivities and lower cost of solar PV modules. Dr. Jiang Fan senior lecturer at School of Electrical and Electronic Engineering, Singapore Polytechnic explores the potential of vertical Building Integrate Photovoltaic (BIPV) systems in tropical urban cities like Singapore.

Singapore, a tropical country located close to the equator (Latitude: 1°17´N, Longitude: 103°50’E), possesses an average daily irradiation of more than 4kWh/m2/day. Solar energy is a strong candidate in the renewable energy mix that the country can adopt.

Despite the steady increase of PV installations, Singapore has not overcome the issue of land scarcity due to high population density in the country. Its current PV installations are mainly located on the roof top of buildings. To explore the feasibility of harvesting more solar energy based on a building envelop, a study was conducted on the performance of four 1.116kWp a‐Si PV systems installed on the building vertical facades facing four different directions. The annual performance of the four vertical facade PV systems was investigated and compared with that of a 1.116kWp rooftop PV system that is tilted at 12° facing the due South to understand the energy production of different vertical PV systems in tropical region.

Solar Site Assessment in Situ

Prior to the design of a system PV system, the system designer needs to perform the solar site evaluation to obtain the information on solar energy profile in situ that includes the variation of sun path, distribution of partial shading and the monthly solar access in a year. As a result, the designer is able to estimate the system performance of the PV system to be installed at the location.

Solar radiation consists of three components: direct radiation, diffuse radiation and albedo. Direct radiation is the radiation that reaches the PV modules directly from the sun without any reflection and absorption, while both diffuse and albedo are reflected solar radiation from the sky and surrounding objects respectively. In application of solar energy, the direct and diffuse radiations are the two most important parameters to be assessed.

Solar assessment was conducted by use of the device ʺSuneyesʺ to investigate the sun path in the sky over the testing site. According to the features of solar radiation in Singapore, a horizontally installed PV system is more profitable than a vertical PV system because horizontal installation can capture more direct sun beam. However, Singapore is a tropical urbancity surrounded by the sea, diffuse radiation in the country is quite high due to intense cloud formation and high humidity. As diffuse radiation characterized as scattering light in the sky is not much affected by the position of the Sun, it can still be captured by vertically mounted PV modules.

PV Systems under Test

Each of the five PV systems UNISOLAR (ES‐124) and a triple junction a‐Si PV module consists of 20 triple junction a‐Si solar cells connected in series and each cell is composed of three separate p‐i‐n type a‐Si sub‐cells that is capable of capturing wider range of solar energy from the visible light spectrum of sunlight. The three different sub‐cells are cascaded in such a way that the top sub‐cell, the middle sub‐cell and the bottom sub‐cell can absorb and convert the blue light, green light and red light to electricity respectively.

The PV array of each system consists of one string with nine ES‐124 modules connected in series. The four vertical PV arrays mounted on the façades face to different orientations to investigate the operational performance of vertical PV installations. For comparison, another PV system was installed on the rooftop facing the South and tilted at 12°. The four facades for the vertical PV systems have no surrounding obstructers to cause shadow on the PV modules in a year, while the place for the rooftop PV system is shadow‐free from 8:30am to 6:00pm in a year.

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