Scientists have struggled for years to make electricity from the sun's light at a cost competitive with power from coal or natural gas.
The challenges are formidable. But now they are close, using the sun's heat instead.
Last month, Acciona Energy, a Spanish company, opened a solar thermal installation spread across 160 hectares of desert outside Boulder City, Nevada, 40km southwest of Las Vegas. Called Nevada Solar One, it has 75km of trough-shaped mirrors, lined up in rows. Producing 64 megawatts, it is many times larger than the largest photovoltaic installations, which use the cells that are found in everything from rooftop panels to pocket calculators.
Acciona will not disclose the production costs at the thermal plant, which was subsidized by the Energy Department.
But according to the Solar Energy Industries Association, representing manufacturers of both photovoltaic and solar thermal systems, power from solar thermal electricity costs US$0.12 to US$0.14 a kilowatt-hour to produce, while power from solar cells costs US$0.18 to US$0.40 a kilowatt-hour.
The national average retail price of electricity is about US$0.11 a kilowatt-hour.
Photovoltaic cells, first made practical for the space program, are falling in price, but so are the thermal systems, which focus sunlight on a fluid-filled pipe to collect the heat.
"There's really this renaissance of solar thermal technology, and people are working very hard on how to reduce costs," said Martin Heming, the executive in charge of solar technology at Schott, the German company that made the collector pipe for Nevada Solar One.
Schott is developing a system that will use molten salt, rather than a liquid, to fill the pipes. Salt could absorb the same amount of heat or more without boiling, and it would stay at atmospheric pressure. The current system heats the pipe to 400oC, helped by small electric motors that change the angle of the mirrors during the day to face the sun.
The heated fluid runs through the pipes to a central power block, where it flows through a network of pipes bathed in ordinary water. The water is boiled and the steam drives a turbine, as in coal plants.
But a typical coal-burning plant produces about 600 megawatts, roughly 10 times more than Nevada Solar One does.
"The optimum economic size is probably even larger than Nevada Solar One," said Thomas Fair, executive for renewable energy at the two companies that are buying the solar plant's output, the Nevada Power Co and the Sierra Pacific Power Co. There is a lot of open desert land, he said.
A new generation of solar thermal plants is likely to use more efficient technology. Heming said that switching to sodium, for example, would allow for higher temperatures.
Less energy would leak out of the pipe, and electric output would rise.
Solar thermal power is also being applied in two other ways.
One is as a supplement to a combined-cycle gas plant. In those plants, natural gas is burned in a jet engine, turning a turbine shaft to make electricity.
The exhaust is used to boil water, and that steam is used to turn a second turbine, making more electricity.
In the solar thermal variant, heat from the sun is used to preheat the water that the exhaust gases will boil. Proponents say this system could get about one-sixth more work out of the natural gas as it operates at about 70 percent efficiency.
Most current plants operate with efficiencies in the range of 50 percent to 60 percent.
The World Bank is considering financial help for projects in Egypt and Morocco that would create such a hybrid plant.
The second application shows that not all solar thermal projects are large and high-tech.
An American start-up company, the Solar Turbine Group, founded by engineers at the Massachusetts Institute of Technology, received US$130,000 from the World Bank and is testing two prototypes in Lesotho, in southern Africa, that use simple components, including old car parts.
Four parabolic dishes, each 3m high and 1m across, focus heat on a black collector, to boil water.
The steam can be run through an old car air-conditioning unit, using the compressor backward, to convert heat energy into mechanical power. (Under a car's hood, the compressor takes mechanical power from the engine and uses it to squeeze the refrigerant down to a liquid, so it can give off its heat.)
In the solar turbine design, the mechanical power turns a generator, which can also be scavenged from a car. Or the mechanical power can be used to drive a refrigeration unit.
In production, the units will sell for US$5,000 and produce about 600 watts, said Sam White, a founder of the Solar Turbine Group. The price is about the same as for solar cells, but this system also creates steam for hot water.
Solar thermal may have another advantage: it may be easier to store energy by storing it as heat. That is important because solar production is strongest in the afternoon but ends long before peak demand ends. High temperatures persist when the sun is very low in the sky or below the horizon.
One possibility is to store large amounts of hot water or molten sodium to allow electricity production into the early evening hours. If utilities built solar plants instead of natural gas plants to meet peak demand, the value of the solar output would increase.
President William Lai (賴清德) recently attended an event in Taipei marking the end of World War II in Europe, emphasizing in his speech: “Using force to invade another country is an unjust act and will ultimately fail.” In just a few words, he captured the core values of the postwar international order and reminded us again: History is not just for reflection, but serves as a warning for the present. From a broad historical perspective, his statement carries weight. For centuries, international relations operated under the law of the jungle — where the strong dominated and the weak were constrained. That
The Executive Yuan recently revised a page of its Web site on ethnic groups in Taiwan, replacing the term “Han” (漢族) with “the rest of the population.” The page, which was updated on March 24, describes the composition of Taiwan’s registered households as indigenous (2.5 percent), foreign origin (1.2 percent) and the rest of the population (96.2 percent). The change was picked up by a social media user and amplified by local media, sparking heated discussion over the weekend. The pan-blue and pro-China camp called it a politically motivated desinicization attempt to obscure the Han Chinese ethnicity of most Taiwanese.
On Wednesday last week, the Rossiyskaya Gazeta published an article by Chinese President Xi Jinping (習近平) asserting the People’s Republic of China’s (PRC) territorial claim over Taiwan effective 1945, predicated upon instruments such as the 1943 Cairo Declaration and the 1945 Potsdam Proclamation. The article further contended that this de jure and de facto status was subsequently reaffirmed by UN General Assembly Resolution 2758 of 1971. The Ministry of Foreign Affairs promptly issued a statement categorically repudiating these assertions. In addition to the reasons put forward by the ministry, I believe that China’s assertions are open to questions in international
The Legislative Yuan passed an amendment on Friday last week to add four national holidays and make Workers’ Day a national holiday for all sectors — a move referred to as “four plus one.” The Chinese Nationalist Party (KMT) and the Taiwan People’s Party (TPP), who used their combined legislative majority to push the bill through its third reading, claim the holidays were chosen based on their inherent significance and social relevance. However, in passing the amendment, they have stuck to the traditional mindset of taking a holiday just for the sake of it, failing to make good use of
RSS