The promise of solar energy: a low-carbon energy strategy for the 21st century.

• Author: Resch, Rhone

In an increasingly carbon-constrained world, solar energy technologies represent one of the least carbon-intensive means of electricity generation. Solar power produces no emissions during generation itself, and life-cycle assessments clearly demonstrate that it has a smaller carbon footprint from "cradle-to-grave" than fossil fuels.

Of the more than 10,000 terawatt-hours (TWh) of electricity generation produced by the countries of the Organization for Economic Cooperation and Development (OECD), solar currently accounts for just 8 TWh. Yet solar technologies, including photovoltaics, concentrating solar power and solar thermal constitute the fastest growing energy source in the world. With clear market signals from Governments, these low-carbon technologies could provide more than 30 per cent of the world's energy supply in aggregate by 2040.

Photovoltaics (PV) are perhaps the most well-known and fastest growing sector of solar technology. PV devices generate electricity directly from sunlight via an electric process that occurs naturally in certain types of material. Groups of PV cells are configured into modules and arrays, which can be used to power any number of electrical loads. PV energy systems have very good potential as a low-carbon energy supply technology. A September 2006 joint paper by scientists from Brookhaven National Laboratory, Utrecht University and the Energy Research Center of the Netherlands demonstrates that crystalline silicon PV systems have energy payback times of 1.5 to 2 years for South European locations and 2.7 to 3.5 years for middle-European, while thin film technologies have energy payback times in the range of 1 to 1.5 years in South Europe.

Accordingly, life-cycle carbon dioxide (C[O.sub.2]) emissions for PV are now in the range of 25 to 32 g/kWh. In comparison, a combined cycle gas-fired power plant emits some 400 g/kWh, while a coal-fired power plant with carbon capture and storage, about 200 g/kWh. Nuclear power emits 25 g/kWh on average in the United States; only wind power is better with a mere 11 g/kWh. For silicon technology, clear prospects for a reduction of energy input exist, and an energy payback of one year may be possible within a few years as silicon growth processes become more efficient. As a result, this could decrease the life-cycle C[O.sub.2] emissions to 15 g/kWh.

The global photovoltaic sector has been growing at an average of over 40 per cent in the last eight years, manufacturing...