A growing number of solar power systems are being installed in Japan for residential use, helping to ease the environmental impact of producing energy for homes.

    Systems rated at three kilowatts (kw) or less used to be common in the past, but high-capacity systems that can produce more than five kw are beginning to be used. A wide variety of systems are already available, not only for homes, but also factories, lighthouses, distant islands, isolated deserts, other remote areas and satellites.

    Solar power systems are highly eco-friendly because they use the sun’s energy, a renewable resource, and emit no carbon dioxide. Support is growing for solar power as the world community becomes increasingly concerned about global warming and the conservation of nonrenewable resources. Amid an expanding global market for solar power systems, the Japanese government is providing strong support for continued refinement of photovoltaic technology.

    The photovoltaic process converts sunlight into electrical energy by taking advantage of the fact that silicon semiconductors generate electricity when they are exposed to light. A basic solar power system consists of photovoltaic modules, an inverter for converting direct current into alternating current and peripheral devices including a controller. Many photovoltaic modules are made of silicon materials, such as crystalline silicon (single-crystalline or multicrystalline) and amorphous silicon.

    The basic unit in a photovoltaic system is the cell. Silicon is crystallized to create a crystal column called an ingot, which is sliced thinly and processed into cells. Cells are arranged, interconnected, covered with tempered glass and packaged into a product called a module. There are many different sizes and shapes of panel-shaped modules for residential applications, varying roughly from 1.0 x 1.2 meters to half that size. Some are rectangular while others are triangular. A photovoltaic array is a set of modules arranged in a frame for mounting on a roof. The power rating of a system means the electric power generated by an array.

    Conversion Efficiency of 15.7%

    Kyocera Corporation has developed a residential solar power system with a conversion rating of 15.7%. This rating measures the system’s maximum power output divided by the total photovoltaic area of its modules (multicrystalline silicon). This is the highest rating in the world for a residential system. The cells themselves are actually rated at 17.7%, but resistance in the models’ electrodes and wiring lower the system’s conversion efficiency. Given that at the surface of the planet sunlight produces approximately one kw of energy per square meter, the conversion rating of 15.7% means 157 watts of electric power per square meter of module surface.

    Although Japanese homes generally have small roofs, they are large enough to accommodate three- to four-kilowatt systems. Accordingly, efficient systems that require relatively less surface area can be expected to enjoy greater popularity. Moreover, these systems should be easier to install and less costly than earlier-generation models.

    Newer systems will be even more efficient. A “concentration module” will track the sun with a special lens that concentrates sunlight in germanium cells that are 1.5 times more efficient than the silicon cells of modules in widespread use today. Recently, a 1.7 x 0.3 meter module produced about 150 watts of output with a 28.1% rating. Jointly developed by the New Energy and Industrial Technology Development Organization (NEDO), Sharp Corporation and Daido Metal Co, the new system is expected to be commercialized in 2005. The developers also hope to achieve a 40% rating and a module costing 100,000 yen per kw within the same year.

    Of course, the effectiveness of solar power systems depends on the amount of available sunlight, which varies depending on the region, the season, the time of day and the weather. It also depends on the inclination and direction of the roof mounting, as well as the rise in cell temperature.

    Taking all these conditions into account, current systems are generally expected to achieve 12% efficiency and generate nearly 1,000 kilowatt-hours (kwh) per available kw output per year. An average Japanese household with four members consumes some 4,500 kwh of electric power per year, which could be handled by a 4 to 5 kw system with a conversion efficiency of 15.7%. Consuming no fossil energy, the system would enable the family to produce the equivalent of 180 kgs less of carbon CO2 emissions and consume 243 liters less oil each year.

    Citizens Can Sell Surplus Electricity

    Another advantage of residential systems is that individuals can sell their surplus electricity to electric power companies at nearly the same rate as they would pay to purchase electric power. Solar power cells cannot store power, so when production is low (mornings, evenings, cloudy days, etc.) or not available (night), the shortfall must be offset with power purchased from an electric utility.

    The government hopes to see the cumulative capacity of solar power systems reach 4,820 mw, or the equivalent of five 1,000 mw-class nuclear power plants, by 2010. It is a most challenging goal considering that the estimated capacity was 637 mw at the end of 2002, according to international Energy Agency. The 2002 figure, by the way, accounted for 49% of the global total, compared to 277 mw in Germany and 212 mw in the United States.

    Attaining the goal will require installation costs to be lowered to a level comparable to that for household electric power charges. Besides making the systems more affordable, this would free the government from the need to offer consumers rebates to encourage them to buy solar power systems. In 1999, the total pretax cost of installing a home solar power system was 930,000 yen, or about $9,000 per kilowatt of rated power. This fell to 700,000 yen by 2004. With further reductions in equipment costs, it might be possible for the per-kilowatt cost to plunge below 500,000 yen (about $4,550).

    Meanwhile, power-generation costs remain high. Electricity from residential systems is about three times more expensive than electricity from public utilities (24 yen per kwh), while power from large systems is around five times more costly than the commercial rate (16 yen per kwh). Market development and overseas expansion, including international cooperation, depend on the cost of residential electricity falling to around the level of commercial electricity.

    But lower costs will require better power-generation and manufacturing technologies. New cells must feature more efficient energy conversion, thinner membranes, larger surface areas (up to 300 x 300 mm) and higher throughput, and they must be mass produced.

    In the meantime, efforts will continue to develop better technology for evaluating the performance and reliability of modules and systems, and for recycling and reusing power system components. Such efforts are being pursued by Sharp Corp, Kyocera Corp, Sanyo Electric and other Japanese manufacturers in collaboration with New Energy and Industrial Technology Development Organization (NEDO).

    Japan accounts for nearly 50% of the total solar cell production in the world and Japanese manufacturers dominate the global industry, exporting about 30% of their production. They are expected to continue leading field in the foreseeable future, including through offshore production.

    Outside Japan, global demand should expand at more than 20% per year, thanks to new incentives in California and other U.S. states, continued promotion in Germany and other EU members and demand generated by the Beijing Olympic Games in 2008. The future for solar power systems is bright.