Can Solar Cells Ever Recapture the Energy Invested in their Manufacture?

The answer to the title question, which crops up with regularly in World Wide Web newsgroups and other fora, is known but not easily accessible to the public. This article reviews some of the research which support that answer (YES!).

The 1983 book by Hu and White [1 ] summarises the results from a 1977 Solarex study [ 2] which found an energy payback time of 6.4 years for the manufacture of solar modules using silicon cells of 12.5 per cent efficiency. In other words, these modules would need to operate for that time in order to produce as much energy as was invested in steps such as the reduction and refinement of the silicon, crystal growth, cell production and module construction. Hagedorn [ 3] presented in 1989 a study of the energy costs for photovoltaic power stations (including grid connection) of monocrystalline silicon (such as are made by BP Solar in Australia), polycrystalline silicon (such as are made by Solarex in Australia) and amorphous silicon solar cells.

They considered the three cell types under (a) actual 1989 German manufacturing conditions using manufacturer's reported energy consumption and (b) projected 1994 conditions.

For the 1994 cases they assumed that cell efficiencies, production volumes, the size of photovoltaic installations and production capacity all increased and that production technology slightly improved.

We first consider their results for monocrystalline silicon cells. For the 1989 situation they estimated that an input of 20.5 megaWatt hours (MWh) of conventional energy was required to produce each peak kilowatt* (kWp) of photovoltaic power station capacity, leading to a payback period of approximately 86 months. Under the 1994 conditions the embodied energy was found to be reduced to 12.2 MWh/kWp and the payback period to approximately 51 months. With polycrystalline technology the production energy was 20.0 MWh/kWp and the payback period was 84 months in 1989.

For the 1994 case, in addition to the changes mentioned above, the cell thickness was assumed to be reduced from 0.45 mm to 0.2 mm and the embodied energy and payback time reduced to 9.0 MWh/kWp and 38 months. For amorphous modules, figures of 13.3 MWh/kWp and 56 months were predicted to be able to be reduced to 7.5 MWh/kWp and 31 months. A more recent (1992) study [4 ] used data from commercial production lines for polycrystalline silicon and amorphous silicon cells.

They neglected the so-called "balance-of-system" components such as inverters and support structures and argued that their energy costs could be reduced to an insignificant level. The polycrystalline cell factory they considered used silicon waste from the electronics industry as its feedstock and there is no obvious methodology for the estimation of its energy content - What is the energy content of material which would otherwise be wasted? The manufacture of electronic grade silicon used approximately 200 kWh/kg while metallurgical grade silicon consumes only one tenth as much energy.

In choosing for their analysis a figure of 20 kWh/kg Palz and Zibetta argued that solar grade silicon had been produced elsewhere with energy content of less than 50 kWh/kg and that a reduction towards 20 kWh/kg was expected. The resulting payback times for the 12 per cent efficient polycrystalline modules was calculated to be in the range of 1.6 to 2.7 years, depending on the choice of European location in which they were used. The corresponding payback times for 6 per cent efficient amorphous modules was estimated to be 0.9 to 1.6 years.

The above summary shows that energy payback times for modules incorporating thick silicon cells are, at worst, of the order of six to seven years and possibly less than three years. Since warranty periods of 20 years are routinely offered on such modules[ ] it is clear that the embodied energy should be easily recovered.

However, it should be noted that the above payback periods assume that the modules are always operated at their maximum power points [5], as with a maximum power point tracker. It is also assumed that no photovoltaic power is wasted or dumped, as would sometimes occur in many stand-alone systems, such as those using battery storage. Grid-connected systems do not incur such losses. Photovoltaic modules used without maximum power point tracking and/or in stand-alone systems could have longer energy payback times than given here.

In addition to providing a clear answer to the title question studies such as those reviewed above help to explain the current research preoccupation with thin film cells of silicon and other semiconductors. The energy costs of the production of thick silicon wafers is such a large fraction of the total embodied energy that huge energy payback reductions are possible if cell thicknesses can be drastically reduced. In Australia, research into thin film silicon cells is proceeding at University of New South Wales (with Pacific Solar) and at the Australian National University.

* 1 kilowatt produced under standard sunlight conditions with the cell oriented to face the sun.

1. C. Hu and R. M. White, Solar Cells (McGraw-Hill, 1983), pp. 78 -80.
2. J. Lindmayer, M. Wihl and A. Scheinine, "Energy Requirement for the Production of Silicon Solar Arrays", Report SX/111/3, Solarex Corp., Rockville, Maryland, USA, October 1977.
3. G. Hagedorn, "Hidden Energy in Solar Cells and Photovoltaic Power Stations", Ninth European Photovoltaic Solar Energy Conference, 542 (1989).
4. W. Palz and H. Zibetta, in Yearbook of Renewable Energies 1992 (Eurosolar with Ponte Press, Bochum, Germany, 1992), p. 181.
5. A. Oakey, "Solar Panel Buyers Guide", Renew 57, Oct. - Dec. 1996. Submitted for publication to Renew January 1997.

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