Authors: Ashima Charnalia, Jyotirmay Mathur, Rajan Rawal
Abstract:
The study focuses on ways to minimize interior lighting energy consumption (ILEC) in daytime use office buildings and proposes use of solar photovoltaic’s (SPVs) to suffice the remaining lighting energy requirement. The objective of the study is to attain interior lighting energy autonomy through Solar Photovoltaic’s. The study in whole focuses on low interior lighting energy consuming building designs whose interior lighting electricity loads are almost entirely met by grid connected roof top SPV systems. Energy Conservation Building Code (ECBC-2007-India) compliant hypothetical building models representative of office buildings in the hot-dry climate of Ahmedabad were modeled in Design Builder on Energy Plus platform. Fifty-four scenarios in all were examined for three-floor plate areas (500, 750 & 1000 m2). The scenarios were derived by keeping the carpet area constant of all the floor plates and varying building aspect ratios, orientation (1:1, 1:2 & 2:1), window wall ratio (40% & 60%) and applying external daylighting devices such as louvers and light shelves. From these scenarios, one building case for all the three different floor plates with the least ILEC was proceeded to integrate rooftop-grid SPV system (24KW SPV system for 500 m2, 36KW SPV system for 750 m2 and 48KW SPV system for 1000 m2 ) to arrive at potential energy generation figures. The study analyzed the two aspects – Energy and Economics, of the attempt. For first Energy analysis, the study analyzed annually to hourly interaction between the building’s ILEC and SPV energy generation. On annual basis, the study was able to illustrate a potential scenario to serve ILEC of 3.8 floors for 500 m2 floor plate, 3.4 floors for 750 m2 floor plate and 3.2 floors for 1000 m2 floor plate of the same building by the same roof top SPV system designed as per floor plate areas. These results were also due to interaction of energy imports and export with the utility grid but the system annually made no net demands on the utility grid to serve ILEC. This meant that ILEC of more than three floors of a building for all three-floor plate areas were demonstrated to be net zero by integration of SPV roof top system.
The results of three floor plates also demonstrated that with every increase of 250 m2 floor plate area, there is a percentage increase of 56.8% in annual ILEC, with percentage increase of 50% in annual roof top energy generation, with percentage increase of 62.5% in annual energy imports from the grid, with percentage increase of 46.5% in annual energy exports to the grid. This implies that with increasing the floor plate area as the core area increases, there is a percentage decrease of 32.8% in annual net surplus energy on site. The second analysis Economics part, the study projected the life period energy summary to obtain life period cost summary of the installed SPV systems, which demonstrated that rooftop SPV system is still very high capital incentive investment with payback period of 22-24 years, without considering any government subsidiaries. Overall, the study demonstrates a sustainable approach towards interior lighting energy use in the building sector by utilizing renewable solar energy source.
