Advances in Energy Engineering (AEE) Volume 3, 2015 www.seipub.org/aee doi: 10.14355/aee.2015.03.001 Study on the Performance of Vertical Solar PV Systems in Tropical Region Lin Tong Shen*1, Jiang Fan2 School of Electrical and Electronic Engineering, Singapore Polytechnic 500 Dover Road, Singapore 139651 *1
LTS1394@hotmail.sg; 2jiangfan@sp.edu.sg Abstract This paper addresses the study on the annual performance of vertical solar PV modules installed on building facades under the tropical region. To evaluate the yearly energy yields of vertical PV systems facing different orientations, four 1.116kWp triple‐
junction amorphous silicon (a‐Si) PV systems were mounted on the four facades of two buildings facing the North, East, South and West respectively. For the comparison, another same a‐Si PV system was horizontally mounted on the rooftop and tilted at 12° facing the South. The operational data of the five PV systems in a year were monitored and collected to analyse their performance under the same weather conditions. This paper will present the analytic results of energy yields of the five PV systems which help to understand the impacts of orientation of a vertically installed PV system in tropical region on its energy production and explore the potential of vertical Building Integrate Photovoltaic(BIPV) systems in tropical urban cities like Singapore. Keywords Vertical Solar Photovoltaic (PV) System; Triple‐junction Amorphous Module; Energy Yield Introduction
With intense Research and Development of new solar PV technologies in recent years, improvement of conversion efficiencies and mass production has resulted in higher efficiencies and lower cost of solar PV modules. This contributes to reduce PV investment and installation cost drastically and impulse the rapid growth of global PV market. 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. The Singapore government has identified that the renewable energy industry, particularly the Solar PV industry, as a new emerging industry and R&D area which can boost its economy and commit to its environmental goals. By 2013, the country has installed about 15MWp solar PV systems to reduce the consumption of fossil fuel and CO2 emission as illustrated in Fig. 1 [1]. The figure shows the steady increase of both residential and non‐residential PV installations. The sharp and steady rise of non‐residential installations reflects the strong government commitment in adopting solar energy as an alternative source of energy. FIG. 1 INSTALLED GRID‐TIED CAPACITY OF SOLAR PHOTOVOLTAIC (PV) SYSTEMS IN SINGAPORE Despite of 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. 1 www.seipub.org/aee Advances in Energy Engineering (AEE) Volume 3, 2015 Although Singapore is trying to maximize its land use for solar PV installation, it is still insufficient for electricity generated by solar energy to meet the minimum peak demand. To explore the feasibility of harvesting more solar energy based on a building envelop, we conducted study on the performance of four 1.116kWp a‐Si PV systems installed on the building vertical facades facing four different directions. This paper presents not only the annual performance of the four vertical facade PV systems but also compare their results with that of a 1.116kWp rooftop PV system that is tilted at 12° facing the due South. The analytic results drawn in the paper will help to understand the energy production of different vertical PV systems in tropical region and explore the potential of vertical BIPV systems in tropical urban cities like Singapore. 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 absorbtion, 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. To understand the solar energy profile, solar assessment was conducted by use of the device ʺSuneyesʺ to investigate the sun path in the sky over the testing site. Fig. 2 presents the measurement results of sun path on the site. The Figure shows that the sun path in the region is symmetrically distributed along the sun path in Mar/Sept and the solar noon in Singapore is around 1pm. It can be also seen that the strong sunlight in a year occurs at high elevations within the square around the solar noon as shown in Fig. 2. The sunlight at high elevation reaches a horizontal plane at small incident angles, resulting in more solar energy falling on the collector surface, but it reaches a vertical panel at big incident angles resulting in less solar energy falling on the collector. 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 urban city surrounded by the sea, diffuse radiation in the country is quite high due to intense cloud formation and high humidity [2]. 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. As the cost of PV system reduces and becomes more competitive in coming years, the vertical facade PV systems will become applicable in Singapore. Dec Nov/Jan Jun
Aug/Apr
5pm 4pm
3pm 2pm 1pm 12nn
11am
10am
9am
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8am
Sep/Mar 6pm WEST
Oct/Feb
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FIG. 2 MEASUREMENT OF SUN PATH IN SITU Sys. facing ‘W’ Sys. facing ‘N’ 2 Advances in Energy Engineering (AEE) Volume 3, 2015 www.seipub.org/aee Sys. facing ‘S’ Sys. facing ‘E’ Sys. facing ‘N’ Sys. facing ‘W’ Rooftop Sys. facing ‘S’, tilted at 12 FIG. 3 VERTICALLY AND HORIZONTALLY INSTALLED PV SYSTEMS CONSISTING OF ES‐124 PV MODULES Configuration of PV Systems under Test
In order to understand the performance of the PV modules vertically installed in tropical regions, four 1.116kWp a‐
Si PV systems were designed and installed on the four facades of two teaching blocks to face the N‐E‐S‐W orientations respectively. For comparison in the study, another 1.116kWp PV system was installed on the rooftop of one teaching block with the tilted angle of 12° facing the due South. Fig. 3 illustrates the installation of five PV systems at two teaching blocks. The PV modules selected for the five PV systems are UNI‐SOLAR (ES‐124), a triple junction a‐Si PV module as specified in Table 1. Each 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 as shown in Fig. 4 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. FIG. 4 CROSS‐SECTION OF ES‐124 PV MODULE The PV array of each system consists of one string with nine ES‐124 modules connected in series. As seen in Fig. 3, 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 obstructure 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 3 www.seipub.org/aee Advances in Energy Engineering (AEE) Volume 3, 2015 6:00pm in a year as depicted in Fig. 5. The PV systems under testing are grid‐tied and connected to the 230V AC utility grid via a grid‐tied inverter (SMA SB1100) as shown in Fig. 6. The built‐in data logger inside the inverter was set up to acquire the operating electrical data for both DC and AC circuits and transmit those data to the central data logger via local area network (LAN) every five minutes. The central data logger also acquired the data of solar irradiance, ambient temperature, wind speed and direction through weather station which was installed horizontally near the PV system installed on the rooftop. FIG. 5 MEASUREMENT OF SHADOW ON THE SITE TABLE 1 TECHNICAL DATA OF SOLAR PV MODULE USED FOR THE EXPERIMENTS PV module brand ES‐124(triple junction a‐Si PV module) Array capacity (kWp)
1.116kWp String Numer 1 string with 9 modules Vmpp/Voc per string (V) Impp/Isc per string (A) 270.0/378.0 4.10/5.10 Array area (m2) 17.53 Weight (kg) 20.5 FIG. 6 SCHEMATIC CIRCUIT OF 1.116kWp a‐Si PV SYSTEMS Analyses on the Performances of Vertical PV Systems
The five PV systems were installed, tested and commissioned by end of Feb 2011. Since then, all systems have been operating reliably. This section will present the annual performance of four vertical PV systems based on the operational data from Mar 2011 to Feb 2012. Moreover, the outputs of four vertical systems are also compared with that of rooftop PV system to investigate the difference among different PV installations. The daily power curves of five systems presented in Fig. 7 help to explain the variation of power outputs of different systems from 7:10am to 6:40pm in a typical sunny day under the same weather conditions. It is apparently seen that the power generation of rooftop system can match the variation of solar irradiance closely throughout the day. With respect to the vertical PV systems, during the early morning period from 7:10am to 9:40am, the energy output of system facing the East followed the variation of solar irradiance closely, generating 4 Advances in Energy Engineering (AEE) Volume 3, 2015 www.seipub.org/aee more power than the other three vertical systems. After then, the power generated by the system facing the East decreased with increase of irradiance but it was still higher than other vertical counterparts. In the afternoon period from 2:10pm to 6:10pm, the system facing the West followed the change of irradiance closely producing the highest power among the four vertical systems. It is evident from the power curves that the PV system facing the West has the highest output power and the best conversion efficiency as compared to the other wall mounted PV systems. FIG. 7 DAILY POWER CURVES OF FIVE PV SYSTEMS ON 08 APR. 2011 Mar‐11 Feb‐12
FIG. 8 COMPARISON OF MONTHLY PERFORMANCE OF FIVE PV SYSTEMS Fig. 8 summarises the monthly energy yield of each PV system against the amount of solar irradiation from Mar 2011 to Feb 2012. Base on the chart, it is noticeable that the rooftop system has the highest monthly energy ranging from 73kWh in Jun 2011 to 106kWh in Feb 2012 and its average monthly energy yield is 90kWh. It can be found from the figure that among four vertical facade PV systems, the system facing the West presented the highest monthly energy production in 7 months of a year. Thanks to the symmetrical sun path in Singapore as presented in Fig. 2, the vertical system facing the South produced more electrical energy than that facing the North from Apr 2011 to Sept 2011, while the system facing the North generated more than the system facing the South from Oct 2011 to Feb 2012. FIG. 9 COMPARISON OF YEARLY PERFORMANCE OF FIVE PV SYSTEMS 5 www.seipub.org/aee Advances in Energy Engineering (AEE) Volume 3, 2015 To evaluate and compare the performances of different PV systems, we need to figure out the energy yields and performance ratios(PRs) of all five PV systems which are defined as follows [5‐8]. System Energy Yield 
system energy ouput (kWh)
installed power capacity (kW p )
and System Performance Ratio( PR)
system energy ouput (kWh)
installed power capacity ( kW p ) 
in  plane actual irradiation(kWh)
reference irradiance(kW )
The yearly energy yields and PRs of five PV systems have been calculated based on the data collected in a year and summarised in Fig. 9. Similar to the monthly system performance, the rooftop system has the best performance in yearly energy production (1090kWh), yearly PR (66%) and yearly energy yield (976kWh/kWp), followed by the vertical system facing the West, the South, the East and the North respectively. Among the four vertical systems, only the vertical system facing the West produced more than half of the energy generated by the rooftop PV system, while the others produced less than half of rooftop system. It is noticeable that the performance of the system facing the South is very close to that of the system facing the East. The results reveal that although the vertical PV systems mounted on a building facade does not generate as much energy as a rooftop mounted PV system, there still is a significant amount of solar energy that could be tapped on for a vertically mounted PV system in Singapore. Conclusion
Investigation on the field experimentation of vertically installed PV modules facing the N‐E‐S‐W orientations has been carried out in this paper. The analyses on the annual data were performed in order to find out the feasibility of vertical PV system applications in Singapore. The analytic results show that a rooftop PV system yields 1090kWh energy per year followed 560kWh/year, 458kWh/year, 451kWh/year and 406kWh/year produced by the four vertical systems facing the West, the South, the East and North respectively. A horizontal PV system in Singapore can produce about double energy output of a vertical PV system due to the high elevation Sun over the location. However, it is also concluded that a vertical PV system can still generate significant energy yield ranging from 37% to 51% of a rooftop PV system. With further drop of PV prices in the module and balance of system(BOS), the vertical PV systems may be considered as a feasible option of PV applications like Building Integrated Photovoltaic (BIPV) systems to help to save space and overcome the issue of land scarcity. ACKNOWLEDGEMENT
The authors would like to express their thanks to Economic Development Board (EDB), Singapore, for providing national research funding to the project and our thanks also go to Singapore Polytechnic for its support to the project. REFERENCES
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