Energy Payback from solar cells

Energy Payback of Roof Mounted Photovoltaic Cells
by Colin Bankier and Steve Gale
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Abstract

The energy payback time of photovoltaic (PV) cells has been a contentious issue for more than a decade. Some studies claim that the joule content of the energy and materials that were put into the process of making the PV cell, will be equaled by the joule content of the electrical output of the cell within a few years of operation. Other studies claim that the useful electrical energy output of the PV cell will never exceed the total amount of useful energy contained within all the inputs of the manufacturing, installation and lifetime operating processes of the PV cell. These studies are often loosely referred to as measuring the energy "payback" of the PV cell. This study undertook a literature review to determine the key assumptions and considerations included in PV Life Cycle Analysis (LCA) modelling. In addition, other forms of modeling such as embodied energy (EE) analysis have also been considered. This review has concluded that the likely energy payback of a typical domestic sized rooftop grid connected PV cell is approximately four years. In addition, it was estimated that larger utility PV cell power stations would have a much longer energy payback period.



Previously published estimates for the energy requirements of present day crystalline silicon modules vary considerably. As noted in Alsema (2000), these differences can partly be explained by different assumptions for process parameters, but they mostly appear to arise from estimates for the silicon purification and the crystallisation process. The majority of silicon solar cells are made from off-spec material rejected by the micro-electronics industry, which introduces the question of whether to include process steps required for micro-electronics wafers in the energy requirements for the PV modules. In order to attempt to draw some conclusions as to the actual energy payback time of PV cells, several previous studies were reviewed. A summary of their findings is presented in table 1. These studies are all based on different assumptions, and evaluate different types of modules, and therefore cannot be directly compared. Some key assumptions of each study are shown. Please refer to the original articles for more detailed information.

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