Mirror mirror on the wall, what's the greatest energy source of all? The sun. Enough energy from the sun falls on the Earth everyday to power our homes and businesses for almost 30 years. Yet we've only just begun to tap its potential. You may have heard about solar electric power to light homes or solar thermal power used to heat water, but did you know there is such a thing as solar thermal-electric power? Electric utility companies are using mirrors to concentrate heat from the sun to produce environmentally friendly electricity for cities, especially in the southwestern United States. The southwestern United States is focusing on concentrating solar energy because it's one of the world's best areas for sunlight. The Southwest receives up to twice the sunlight as other regions in the country. This abundance of solar energy makes concentrating solar power plants an attractive alternative to traditional power plants, which burn polluting fossil fuels such as oil and coal. Fossil fuels also must be continually purchased and refined to use. Unlike traditional power plants, concentrating solar power systems provide an environmentally benign source of energy, produce virtually no emissions, and consume no fuel other than sunlight. About the only impact concentrating solar power plants have on the environment is land use. Although the amount of land a concentrating solar power plant occupies is larger than that of a fossil fuel plant, both types of plants use about the same amount of land because fossil fuel plants use additional land for mining and exploration as well as road building to reach the mines. Other benefits of concentrating solar power plants include low operating costs, and the ability to produce power during high-demand energy periods and to help increase our energy security—our country's independence from foreign oil imports. There are three solar thermal power systems currently being developed by U.S. industry: parabolic troughs, power towers, and dish/engine systems. Because these technologies involve a thermal intermediary, they can be readily hybridized with fossil fuel and in some cases adapted to utilize thermal storage. The primary advantage of hybridization and thermal storage is that the technologies can provide dispatchable power and operate during periods when solar energy is not available. Hybridization and thermal storage can enhance the economic value of the electricity produced and reduce its average cost. This paper provides an introduction on each of the three technologies, an overview of the technologies, their current status
II. WHY CONCENTRATING SOLAR POWER?
Economic Sustainability: The history of the Solar Electricity Generating Systems (SEGS) shows impressive cost reductions achieved up to now. Advanced technologies, mass production, economies of scale and improved operation will allow to reduce the solar electricity cost to a competitive level within the next 10 to 15 years. This will reduce the dependency on fossil fuels and thus, the risk of future electricity cost escalation. Hybrid solar-and-fuel plants, at favorable sites, making use of special schemes of finance, can already deliver competitively priced electricity today.
Environmental Sustainability: Life cycle assessment of emissions (bottom) and of land surface impacts of the concentrating solar power systems shows that they are best suited for the reduction of greenhouse gases and other pollutants, without creating other environmental risks or contamination. Most of the collector materials can be recycled and used again for further plants.
Social Sustainability: Their integration into the grid does not require any measures for stabilization or backup capacity. On the contrary, they can be used for these purposes, allowing for a smooth transition from today’s fossil fuel based power schemes to a future renewable energy economy. In sun-belt countries, CSP will reduce the consumption of fossil energy resources and the need for energy imports. The power supply will be diversified with a resource that is distributed in a fair way and accessible by many countries. Process heat from combined generation can be used for seawater desalination and help, together with a more rational use of water, to address the challenge of growing water scarcity in many arid regions. Thus, CSP will not only create thousands of jobs and boost economy, but will also effectively reduce the risks of conflicts related to energy, water and climate change.
III. TECHNOLOGY OVERVIEW
Unlike solar (photovoltaic) cells, which use light to produce electricity, concentrating solar power systems generate electricity with heat. Concentrating solar collectors use mirrors and lenses using various mirror configurations to concentrate and focus sunlight onto a thermal receiver, similar to a boiler tube. The receiver absorbs and converts sunlight into heat. The heat is then transported to a steam generator or engine where it is converted into electricity. The heat is then channeled through a conventional generator. The plants consist of two parts: one that collects solar energy and converts it to heat, and another that converts heat energy to electricity. There are three main types of concentrating solar power systems: parabolic troughs, dish/engine systems, and central receiver systems. These technologies can be used to generate electricity for a variety of applications, ranging from remote power systems as small as a few kilowatts (kW) up to grid-connected applications of 200-350 megawatts (MW) or more. That is concentrating solar power systems can be sized for village power (10 kilowatts) or grid-connected applications. Some systems use thermal storage during cloudy periods or at night. Others can be combined with natural gas and the resulting hybrid power plants provide high-value, dispatchable power. The amount of power generated by a concentrating solar power plant depends on the amount of direct sunlight. Like concentrating photovoltaic concentrators, these technologies use only direct-beam sunlight, rather than diffuse solar radiation.
IV. SOLAR PARABOLIC TROUGH
The collector field consists of a large field of single-axis tracking parabolic trough solar collectors. The solar field is modular in nature and is composed of many parallel rows of solar collectors aligned on a north-south horizontal axis. Each solar collector has a linear parabolic-shaped reflector that focuses the sun’s direct beam radiation on a linear receiver located at the focus of the parabola. The collectors track the sun from east to west during the day to ensure that the sun is continuously focused on the linear receiver. A heat transfer fluid (HTF) is heated as it circulates through the receiver and returns to a series of heat exchangers in the power block where the fluid is used to generate high-pressure superheated steam. The superheated steam is then fed to a conventional reheat steam turbine/generator to produce electricity. The spent steam from the turbine is condensed in a standard condenser and returned to the heat exchangers via condensate and feed-water pumps to be transformed back into steam. Condenser cooling is provided by mechanical draft wet cooling towers. After passing through the HTF side of the solar heat exchangers, the cooled HTF is re-circulated through the solar field. Individual trough systems currently can generate about 80 megawatts of electricity. Trough designs can incorporate thermal storage—setting aside the heat transfer fluid in its hot phase—allowing for electricity generation several hours into the evening. Currently, all parabolic trough plants are "hybrids," meaning they use fossil fuel to supplement the solar output during periods of low solar radiation.
IV.a System Application, Benefits, and Impacts
Large-scale Grid Connected Power: The primary application for parabolic trough power plants is large-scale grid connected power applications in the 30 to 300 MW range. Because the technology can be easily hybridized with fossil fuels, the plants can be designed to provide firm peaking to intermediate load power.
Domestic Market: The primary domestic market opportunity for parabolic trough plants is in the areas where the best direct normal solar resources exist. However, other nearby places may provide excellent opportunities as well. With utility restructuring, and an increased focus on global warming and other environmental issues, many new opportunities such as renewable portfolio standards and the development of solar enterprise zones may encourage the development of new trough plants.
International Markets: With the high demand for new power generation in many developing countries, the next deployment of parabolic troughs could be in any arid regions in developing countries as they are ideally suited for parabolic trough technologies. India, Egypt, Morocco, Mexico, Brazil, Crete (Greece), and Tibet (China) have expressed interest in trough technology power plants. Many of these countries are already planning installations of combined cycle projects.
Least Cost Solar Generated Electricity: Trough plants currently provide the lowest cost source of solar generated electricity available. They are backed by considerable valuable operating experience. Troughs will likely continue to be the least-cost solar option for another 5-10 years depending on the rate of development and acceptance of other solar technologies.
Daytime Peaking Power: Parabolic trough power plants have a proven track record for providing firm renewable daytime peaking generation. Trough plants generate their peak output during sunny periods when air conditioning loads are at their peak. Integrated natural gas hybridization and thermal storage have allowed the plants to provide firm power even during non-solar and cloudy periods.
Environmental: Trough plants reduce operation of higher-cost, cycling fossil generation that would be needed to meet peak power demands during sunny afternoons at times when the most photochemical smog, which is aggravated by NO emissions from power plants, is produced. Economic: The construction and operation of trough plants typically have a positive impact on the local economy. A large portion of material during construction can generally be supplied locally. Also trough plants tend to be fairly labor-intensive during both construction and operation, and much of this labor can generally be drawn from local labor markets