Energy Vol. 23, No. 5, pp. 407–411, 1998  1998 Elsevier Science Ltd. All rights reserved Pergamon PII: S0360-5442(97)00102-3 Printed in Great Britain 0360-5442/98 $19.00 + 0.00 MATERIAL CONSTRAINTS FOR THIN-FILM SOLAR CELLS B. A. ANDERSSON, † C. AZAR, J. HOLMBERG and S. KARLSSON Institute of Physical Resource Theory, Chalmers University of Technology and Go ¨teborg University, S-412 96 Go ¨teborg, Sweden (Received 30 January 1997) Abstract —Harnessing solar energy by using photovoltaic cells has the potential to become a major CO 2 -free energy source. Materials requirements for the solar cells based on four types of thin-film photovoltaics have been estimated and compared with global reserves, resources and annual refining. The use of solar cells based on Cd, Ga, Ge, In, Ru, Se and Te as a major energy-supply technology has severe resource constraints. Other systems such as a-Si without Ge and crystalline silicon do not involve such constraints. For some of these metals, there is the risk of enhanced, environmentally deleterious concentrations in the ecosphere due to leakage from manufacturing, use or waste handling.  1998 Elsevier Science Ltd. All rights reserved 1. INTRODUCTION The present global energy supply is primarily obtained from the use of fossil fuels. Emissions of the greenhouse gas CO 2 will inevitably lead to a change in the radiative balance of the earth and introduce changes in climate. The changes resulting from the greenhouse effect are considered by many to pose one of the most serious global environmental problems, even though considerable scientific uncertainty remains about the magnitudes of changes. A possible solution to mitigation of the escalating greenhouse effect is basing the energy system on renewable energy resources. Of these, direct harnessing of solar energy by photovoltaic cells has the potential to become a major source of energy supplies in a sus- tainable society. Solar insolation to the earth contains approximately 10,000 times more energy than what is currently used globally. A solar cell area of 5 × 10 5 km 2 (covering less than 10% of the Sahara) would be sufficient to generate the present global annual primary energy supply of 100,000 TWh. Some promising solar cell technologies involve the use of chemical elements available from non- renewable and scarce resources. Several authors have studied environmental problems related to solar cells [1–8]. In some cases, resource scarcities were also noted. In this paper, we focus on resource and environmental constraints that a large-scale expansion of solar cells might encounter as a result of the use of scarce elements. 2. A RENEWABLE ENERGY SCENARIO BASED ON SOLAR CELLS We assume a solar cell scenario in which the expansion rate is 1000 (TWh/yr)/yr on average during the next century. The solar cells will then generate 100,000 TWh/yr in the year 2100 and remain at that level thereafter. This scenario corresponds to a mean power of approximately 11 TW or about 1.1 kW per capita for a global population of 10 billion people. This level of solar power generation would make a significant contribution to future global energy supplies. The present primary energy supply in the world is approximately 100,000 TWh/yr or around 2 kW per capita. In the OECD coun- tries, the mean energy use is about 6 kW per capita. Global primary energy requirements are expected to grow significantly over the next century as the population approximately doubles, the developing countries become industrialized, and the already rich countries become even richer. Thus, the assumed solar cell scenario alone is not likely to lead to stabilization of the atmospheric CO 2 concentration. The proposed scenario can be compared to the intermittent energy supplies assumed in the global † Author for correspondence. Fax: + 46 31 772 31 50; e-mail: frtba@fy.chalmers.se 407