Re ECF Project: 2008-09

Project Title: Environmental payback time analysis of building-integrated photovoltaic (BIPV) applications in Hong Kong

Applicant: The Hong Kong Polytechnic University (Dr. LU Lin)

Total Approved Grant: $406,000

Duration: 1/9/2009 to 31/5/2011

Project Status/Remarks: Completed

To measure the "sustainability" of BIPV applications in HK by introducing two environmental assessment indicators, i.e. energy payback time and greenhouse gas payback time.

Summary of the Findings/Outcomes:
This project investigates the "sustainability" of BIPV applications in HK by examining a series of environmental indicators of LCA, such as the energy payback time (EPBT), greenhouse-gas payback time (GPBT) and energy yield ratio (EYR). Five common solar cells, i.e. mono-crystalline, multicrystalline, amorphous silicon, CdTe thin film and CIGS thin film, were chosen for study. The life cycle energy requirement and greenhouse-gases emission of PV systems were obtained.

FORTRAN program was compiled to accurately simulate the hourly solar irradiation and hourly electricity power generation of PV modules.

BIPV system's thermal performance was measured. A related dynamic simulation model for heat transfer of BIPV wall was developed. PV façade can reduce the building heat gain (or cooling load) significantly which helps to reduce the building energy use. In summer, about 31 to 35 kWh space cooling loads would be reduced by integrating PV modules into the external envelope of building, and substantial energy saving for air conditioning is observed. If the COP of air conditioning was assumed to be 3.5, about 8.9 - 10 kWh electricity would be saved annually per unit square meter of PV module by reducing building heat gain, which results in further decreasing of the EPBT and GPBT of BIPV systems by 8% to 27%.

For different types of solar cells, EPBT of roof-top PV system is achieved as fellows, from 2.1 to 3.6 years, CdTe thin film has the best energy performance while silicon cells have the worst. For different orientations, the EPBT values results are totally different but far less than their life spans. The roof-top PV system gives the shortest, and east facing PV façade system provides the longest energy payback time. For different types of solar cells, GPBT of roof-top PV system is achieved, from 2.0 to 7.3 years. PV applications can help to reduce GHG emissions in Hong Kong significantly. 1 to 2 tons greenhouse gases can be reduced per square PV during its life cycle.

Software is developed for easily finding the detailed figures for different applications. With this software, engineers can easily calculated the PV systems' environmental indicators such as EPBT, GPBT, embodied energy demand, EYR, cooling load reduction, annual solar radiation d, annual energy output, etc., by considering different solar cell types, different orientations and inclined angles.

In a summary, BIPV application can not only generate and provide electricity at the point of use, but also reduce building energy use with the reduction of building heat gain or cooling load. BIPV applications in HK are truly sustainable with short energy payback time and greenhouse-gas emission payback time, around 2-3 years for roof-top type and 3-6 years for other vertical PV façade. Among the studied 5 types PV modules, CdTe thin film modules give the best environmental benefits such as the shortest EPBT and GPBT. Silicon PV systems perform the worst environmentally considering their energy and environmental performances during the life cycle.