There is a growing expectation for clean and inexhaustible solar energy as a renewable energy source indispensable for sustainable society and for preventing global warming. Photovoltaic power generation above all is a system which converts solar energy directly to electricity and, as it has no moving part like a turbine, its maintenance is easy and it can be applied in various scales and forms from pocket calculators to large-scale power plants.

Ever since the Sunshine Project which started in 1974 as a long-term national project after the first oil crisis, research and development efforts through industry-government-academia cooperation and governmental promotion policies effectively resulted in reducing the cost, and there has been rapid increase in production and installation in recent years. As can be seen in Fig. 1, our country is the largest producing country of solar cells in the world.

Although photovoltaic power generation system is an effective energy source in preventing global warming, a certain amount of energy is needed in manufacturing system components like photovoltaic cells and inverters, and naturally carbon dioxide (CO₂) is emitted in the process. The time needed to recover input energy and to reduce CO₂ emission during production is called energy payback time (EPT), and CO₂ payback time (CO₂PT) respectively. If these payback times are not sufficiently short compared to the life time of the system, it does not make sense at all as an energy producing technology. Life cycle assessment (LCA) which analyzes and evaluates these payback times are indispensably important in assessing the energy technology.

In fig.2 is shown the production process of polycrystalline silicon solar cells. In order to make accurate assessment of the input energy and the CO₂ emission in production, it is necessary to thoroughly investigate the input materials and to sum up the energy needed for fabrication at each process.

The payback time of photovoltaic power generation system is calculated from the ratio of the production energy and CO₂ emission at production of all system components like solar cells, and the annual electricity production and CO₂ reduction. As the former gradually decreases through development of new solar cells, improvement of production technology and expansion of production scale, and the latter increases along with improvement in conversion efficiency and efficient usage of the system, the payback time of photovoltaic power generation system, which is still in the middle of technological innovation, is rapidly shortening year by year. However, as the recent payback time value is not sufficiently known to the public, even now it is sometimes incorrectly stated that the payback time of photovoltaic power generation system is more than 10 years based on old data of over ten years ago.

The most recent EPT data published in our country (in the case of residential roof-top installations) states that EPT of polycrystalline silicon is 1.5 year, for amorphous silicon is 1.1 year, for compound thin film (CIS) is 0.9 year and CO₂PTs are 2.4, 1.5 and 1.4 year respectively^[2]. Please note, however, that with crystalline silicon, the above calculation was done with the new silicon manufacturing method presently being developed, and if calculated with the current method, the EPT is 2.0 years and CO₂PT is 2.7 years (Fig.2). Similar figures have been reported in Europe and the US. As the life time of solar cells is considered to be at least 20 to 30 years, both the EPT and CO₂PT based on the most recent data is sufficiently short, and a photovoltaic system is a good power generating system from the view point of LCA.

In the "New Energy Innovation Plan", which is one of the 4 pillars of the "New National Energy Strategy" made in May 2006 as the basic policy of the energy measures of our country, it is clearly stated that the cost of photovoltaic power generation will aim for a reduction to the level of conventional thermal power generation by 2030. In the long-term road map of research and development concerning photovoltaic power generation (PV2030) made in 2004, it is assumed that by 2030 the cumulative installation will be 100 GW (100 million kW) which will cover 10 % of the total electricity need.

In order to achieve these installation goals, it is indispensable to do research and development for further efficiency improvement of solar cells, cost reduction, and introduction of new system concepts that would allow expansion of application areas and installation sites. It is also important that, with the assessment of environmental effects with LCA introduced here, the latest information is constantly provided by continually investigating the results of new production technology introduction based on advancement in research and development and from expansion of production scale, in order to gain public understanding of photovoltaic power generation as a new energy technology.