There are two basic methods for generating electricity from solar power. The first method uses photovoltaic (PV) solar panels to generate electricity directly from sunlight. The second method is known as concentrating solar power (CSP) and converts sunlight into heat to produce steam, which is then fed through conventional steam-turbine generators to generate electricity.
A heliostat (from helios, the Greek word for sun, and stat, as in stationary) is a device that includes a mirror, usually a plane mirror, which turns so as to keep reflecting sunlight toward a predetermined target. CSP systems range from remote power systems as small as a few kilowatts up to grid-connected power plants of 100’s of megawatts (MW). Solar thermal utilities may use molten salts as heat transfer media. They are abundant, low cost, and remain as a water-like liquid through the range of temperatures needed (~290-570°C). Molten salt CSP plants use two storage tanks to balance heat capture and storage. The cold tank (~293°C) holds salts for heating up in the receiver, and the hot tank (~565°C) holds thermal energy after it has been captured.
In 2012, the global installed CSP capacity was about 2 GW (compared to 1.2 GW in 2010) with an additional 20 GW under construction or development. According to the International Energy Agency (IEA), solar energy, including both Concentrated Solar Power (CSP) and Photovoltaic could account for 25% of global electricity by 2050 and cover a third of global energy demand after 2060.
The molten salt must be kept at a temperature of about 290 degrees Celsius to keep it fluid as the salt freezes (becomes lumpy with solids) below 220 degrees C. This means that special care must be taken to ensure that the salt does not freeze in the field piping during the night.
Types of CSP Technologies
The three main types of CSP systems are parabolic trough, solar power tower, and dish Sterling engine systems. All use the thermal energy from the sun to generate electricity. Each has its own set of advantages, but as a whole they stand to contribute to growing renewable energy production in a big way.
The first type of CSP technology works through the use of parabolic troughs, long, curved mirrors that move to follow the path of the sun, and focus the sun’s heat onto a tube in front of the mirror. This dramatically increases the temperature of the heat-transfer material, which in turn boils water and creates steam that drives a generator. The parabolic trough technology is very mature and accounts for about 90% of the installed CSP base.
Trough systems collect the sun’s energy using long rectangular, parabolic mirror collectors. The trough field consists of a large array of these modular collectors. Many parallel rows of collectors span across the solar field, normally aligned on a north south axis. The mirrors mechanically rotate and follow the sun east to west, focusing sunlight on receiver tubes that run the length of the mirrors. The receiver tubes are positioned along the focal line of each parabolic mirror. The reflected condensed sunlight is very intense and heats a fluid flowing through the tubes to a very high temperature. The very hot fluid is then used to heat water to create steam for a conventional steam turbine generator to produce electricity.
The receiver tube is heated by the reflected sun’s rays which in turn heats up a transfer fluid as it circulates through the tubes. The receiver tube is a stainless steel tube with a special sunlight absorbing surface and is mounted inside an anti-reflective outer glass tube with a vacuum separating the two tubes.
Solar power towers, another type of CSP technology, were first used at experimental power plants in the California desert during the 1980s and 1990s; improved solar power towers are currently being developed for newer CSP plants. In these plants, a large array of flat mirrors (called heliostats) is focused on a central tower that contains the heat-transfer material. The transfer material is pumped into storage tanks that can contain the heat for up to a day. It is then passed through a heat exchanger, where it produces steam that drives the generators. The working fluid in the receiver is heated to 500–1000 °C
The function of the heliostats is to capture solar radiation from the sun and re-direct it to the central receiver. A heliostat rotates in two dimensions, east and west, and north and south, tracking the sun as it moves throughout the day and throughout the year. To accomplish this, each individual heliostat is guided by a computerized system which follows the sun and maximizes total energy output.
The mirrors on the solar heliostats have minimal reflection loss, so each heliostat reflects approximately its area in sunlight: about 1 kilowatt of heat per square meter. As a frame of reference, a typical electric space heater produces 1.5 kilowatts of heat. If one hundred heliostats, each with 2.2 square meters of mirrorarea, direct sunlight onto a single thermal receiver, the sunlight will be converted into 220 kilowatts of heat.
Engineers and scientists have recently developed a new form of CSP technology called the dish solar system. In this system, the mirrors are arranged in a parabolic shape, similar to that of a satellite dish, which focuses the heat onto a central receiver mounted above the center of the dish. The receiver contains an engine known as a Stirling engine that converts heat to mechanical power by compressing a cold fluid, which could be water or synthetic oil. The heating of the fluid causes it to expand through a turbine or a piston, which produces mechanical power. An electric generator or alternator then converts the mechanical power into electricity. Large scale electricity is produced by arranging several dishes into a larger array. Solar -dish systems provide high solar-to-electric efficiency (between 31% and 32%).
- One of the most fascinating aspects of concentrating solar power is the potential to supply plentiful of pollution-free electricity which is clean, reliable power from domestic renewable energy.
- Waste heat from steam turbines may be used to desalinate sea water. This could have a major impact in alleviating shortages of water in drier parts of the world; waste heat from electricity generation may also be used for air conditioning.
- Optimized solar field design to match utility peak generation demand requirements.
- CSP systems have more reliable and consistent power output than intermittent renewable resources such as photovoltaic (PV) solar and wind.
- Increased production capability through thermal energy storage and hybridization with fossil fuels and operate at high annual efficiencies and high load factor.
- Unlike solar photo-voltaic (PV), CSP uses only the direct component (DNI) of sunlight and provides heat and power only in regions with high DNI.
- The molten salt can retain its heat for up to six hours so that power is still available for up to six hours after the sun goes low on the horizon – enough to cover the period of peak electrical demand.
- The distinct advantage of CSP plants is that they can be designed to store energy that only adds about 5% to the total project cost, and provides up to 10 hours of storage capacity. This makes them more grid-friendly and reliable than PV solar plants.
- Another advantage of CSP is that at night or on extremely cloudy days, the conventional generators can be run on natural gas or petroleum, allowing the plant to continue to generate power when the sun is not shining.
- Unlike PV systems which are sensitive to high temperature that reduces efficiency, the concentrating power of the tower concept achieves very high temperatures, thereby increasing the efficiency at which heat is converted into electricity and reducing the cost of thermal storage, and due to the fairly specific geographic range where CSP can be successful.
- CSP systems typically run about 20% sunlight to electricity conversion efficiency.
- Heliostat is often referred to as a glass/metal heliostat. Alternative designs incorporate recent adhesive, composite, and thin film research to bring about materials costs and weight reduction. Some examples of alternative reflector designs are silvered polymer reflectors; glass fiber reinforced polyester sandwiches and aluminized reflectors.