Solar Photovoltaic/Thermal (PVT) Test-bed

Toh Peng Seng1*, Jiang Fan2* and Goh Leag Huab a Grenzone Pte Ltd, Singapore

b School of Electrical and Electronic Engineering, Singapore Polytechnic, Singapore

Abstract. Solar photovoltaic/thermal (PVT) system that combines solar PV system with solar thermal system into one makes more effective use of solar energy when converting solar energy into both electrical energy and thermal energy. This paper addresses a solar PVT test-bed in Singapore where the different types of solar PVT systems developed in Singapore are installed and tested to investigate their performance under the tropical weather conditions.

Keywords: solar termal; photovoltaics, solar photovoltaic/thermal (PVT).

1. Introduction The conversion of solar energy to electrical or thermal energy has been practised for many years. The

most popular used solar energy collectors are flat plate solar panel for solar thermal system and flat plate solar PV module for solar electricity. Comparing to solar PV and solar thermal technology, hybrid solar photovoltaic/thermal (PVT) technology that produces both electricity and thermal energy under sunlight simultaneously is relatively new. A PVT collector, the most important component in a hybrid PVT system, is basically composed of solar PV cells/module mounted on top of heat absorber. The hybrid PVT collectors are divided in two groups[1-4]:

• Liquid PV/T collectors that normally produce both hot water and electricity. • Air PV/T collectors that are usually building-integrated by installing on facade or roof of

a building to provide warm ventilation air as well as electricity. In a tropical country like Singapore, water PVT system is more adequate for medium-temperature

domestic applications with temperature ranging from 45°C to 55°C. In applications, a water PVT collector can be divided into two categories, eg. (1). glazed PVT collector and (2). unglazed PVT collector. Fig. 1 illustrates the basic structures of glazed and unglazed PVT collectors. A glazed PVT collector with glass cover, can produce more thermal energy but has slightly lower electrical yield, while an unglazed PVT collector without glass cover produces less thermal energy but more electricity[5-6]. As seen in Fig. 1b, an unglazed PVT collector has simpler structure and consists of solar PV cells or PV module on top and thermal absorber below. It delivers relatively lower thermal energy due to more heat convection loss on its front surface and possesses lower total conversion efficiency than a glazed PVT collector[6-7]. Main advantages of an unglazed PVT collector over a glazed PVT collector are simpler structure, easier to fabricate and lower cost. The simplicity of low-cost unglazed PVT collector is balanced by generally lower thermal performance[8-10] due to high thermal resistance between liquid and PV cell (laminate layers, adhesive bond, irregularities in flatness of absorber, possible air traps or dry contacts, heat exchanger configuration); white

* Dr. Toh Peng Seng. Tel.: +65 65790560; fax: +65 65790561 E-mail address: tohpengseng@grenzone.com * Dr. Jiang Fan. Tel.: +65 68790629; fax: +65 67721974 E-mail address: jiagnfan@sp.edu.sg

2012 International Congress on Informatics, Environment, Energy and Applications-IEEA 2012

IPCSIT vol.38 (2012) © (2012) IACSIT Press, Singapore

gaps between PV cells in commercial PV modules results in reduced absorption of solar radiation at aperture area.

Fig. 1. Basic structure of liquid PVT collector: (a). glazed PVT collector and (b). unglazed PVT

collector With increasing concern about depletion of fossil fuels, high dependency of energy import and global

climate change, Singapore Economic Development Board(EDB) sets aside project funding for development of new solar PVT collectors and research on feasibility of PVT systems in tropical region. One of main tasks is to set up a test-bed to investigate the performance of new PVT collectors developed by local company. This paper presents performances of four different PVT systems installed at the test-bed.

2. Site for the PVT Test-Bed The solar PVT test-bed was set up on the rooftop of 5-storey teaching block in Singapore Polytechnic as

shown in the Fig.2. There are no any other obstacles on the site except a 10 meters high wind turbine. Fig. 3 depicts the shadows at point A of test-bed in a year. As can be seen from the figure, the site selected for PVT test-bed has no shadows all over a year except some afternoon time after 5pm between March and April, August and September respectively.

Fig. 2 Layout of the site selected for solar PVT test-bed

(a) (b)

Fig. 3 Measurment of shadows in situ: (a). shadow change in the first half year and (b). shadow change in the second half year

Owing to the negative temperature coefficients of PV cells, PV modules are normally installed in such a way that a certain air gap behind PV modules must be maintained to dissipate the heat of solar cells by natural ventilation. In a hybrid PV/T collector, the PV cells/modules are directly mounted on the top of thermal absorber. Although there is no air gap behind solar cells/module, the fluid flow beneath the PV cells will cool PV cells faster than natural air ventilation of a normal PV installation and improve the power output of PV cells. To explore the impact of cooling effect of PVT collector on PV cells, we also installed

two PV systems, one a-Si PV system and another mc-Si PV system, to compare the performance of PV cells of PV module and PVT collectors respectively.

To date, 3 PVT collectors developed by Grenzone Pte Ltd have been used to build 4 PVT systems to study the operational performance in tropical region. In addition, 2 PV systems were also installed in the test-bed for comparison of PVT system with normal PV system. The layout of PVT test-bed upon completion of installation is shown as in Fig. 4.

3. Structures and Operations of PVT Systems

Three different structures introduced in the test-bed are (1). a-Si stand-alone PVT system, (2) a-Si natural circulation grid-tied PVT system, and (3). a-Si & mc-Si forced circulation grid-tied PVT systems. The schematics of three types of PVT systems are illustrated in Figs. 5a, 5b and 5c respectively.

Fig. 4. New layout of the PVT test-bed after installation of all systems; 1). 93Wp unglazed a-Si PVT system ; 2).

768Wp unglazed a-Si syphon PVT system ; 3).1.532kWp unglazed a-Si PVT system ; 4).1.8kWp glazed mc-Si PVT system ; 5). 1.024kWp a-Si PV system and 6). 1.2kWp mc-Si PV system

(a) (b)

(c)

Fig. 5 The structures of three PVT systems in the test-bed: (a). Stand-alone forced circulation PVT system; (b). Grid-tied natural circulation PVT system and (c). grid-tied forced circulation PVT system

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