An Overview Of Concentrated Solar Power Systems

Concentrated solar power systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight, or solar thermal energy, onto a small area. Electricity is generated when the concentrated light is converted to heat, which drives a heat engine (usually a steam turbine) connected to an electrical power generator.

CSP is used to produce electricity usually generated through steam. Concentrated-solar technology systems use mirrors or lenses with tracking systems to focus a large area of sunlight onto a small area. The concentrated light is then used as heat or as a heat source for a conventional power plant (solar thermoelectricity). The solar concentrators used in CSP systems can often also be used to provide industrial process heating or cooling.

History

Even though the CSP technology is revolutionary within the field of renewable energy there is nothing new about the idea of concentrating solar power. The first mentioning of the use of concentrating solar power derives from ancient Greece, where Archimedes in 214-212 BC, as a defensive tactic, used bronze shields to concentrate the sun's rays onto invading Roman ships which, according to the myth, caught on fire.

The first documented use of concentrated solar power technology was in 1866 where Auguste Mouchout used parabolic troughs to heat water and produce steam to run the first solar steam engine. A series of inventors applied the technology in the following years. In 1912 in Meadi, Egypt, parabolic solar collectors were established in a small farming community by Frank Schuman, a Philadelphia inventor, solar visionary and business entrepreneur. The parabolic troughs were used for producing steam, which drove large water pumps, pumping 6000 gallons of water per minute to vast areas of arid desert land.

Professor Giovanni Francia (1911–1980) designed and built the first concentrated-solar plant, which entered into operation in Sant'Ilario, near Genoa, Italy in 1968. This plant had the architecture of today's concentrated-solar plants with a solar receiver in the center of a field of solar collectors. The plant was able to produce 1 MW with superheated steam at 100 bar and 500 °C. This plant has architectural similarities to modern plants with its central receiver surrounded by a field of solar collectors.

In 1982 the U.S. Department of Energy, along with an industry consortium began operating Solar One, a 10MW central-receiver demonstration project. The project established the feasibility of power tower systems. Four years later, in 1986, the world's largest solar thermal facility, located in Kramer Junction, California, was commissioned. The solar field contained rows of mirrors that concentrated the sun's energy onto a system of pipes circulating a heat transfer fluid. The heat transfer fluid was used to produce steam, which powered a conventional turbine to produce electricity.

Working

Concentrating solar power plants produce electric power by converting the sun’s energy into high-temperature heat using various mirror configurations. 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.

Concentrating solar power systems can be sized for village power (10 kilowatts) or grid-connected applications (up to 100 megawatts). 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. These attributes, along with world record solar-to-electric conversion efficiencies, make concentrating solar power an attractive renewable energy option in the Southwest and other sunbelt regions worldwide. There are four CSP technologies being promoted internationally. For each of these, there exist various design variations or different configurations. 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.

Current Technology

Concentrating technologies exist in four optical types, namely parabolic trough, dish, concentrating linear Fresnel reflector, and solar power tower. Parabolic trough and concentrating linear Fresnel reflectors are classified as linear focus collector types. Dish and solar tower as of the point focus type. Linear focus collectors achieve medium concentration (50 suns and over), and point focus collectors achieve high concentration (over 500 suns) factors.

Different types of concentrators produce different peak temperatures and correspondingly varying thermodynamic efficiencies, due to differences in the way that they track the sun and focus light. New innovations in CSP technology are leading systems to become more and more cost-effective.

Parabolic Trough

A parabolic trough is a type of solar thermal collector that is straight in one dimension and curved as a parabola in the other two, lined with a polished metal mirror. The sunlight which enters the mirror parallel to its plane of symmetry is focused along the focal lines, where objects are positioned that are intended to be heated.

A parabolic trough is made of a number of solar collector modules (SCM) fixed together to move as one solar collector assembly (SCA). A SCM could have a length up to 15 metres (49 ft 3 in) or more. About a dozen or more of SCM make each SCA up to 200 metres (656 ft 2 in) length. Each SCA is an independently-tracking parabolic trough.

A SCM may be made as a single-piece parabolic mirror or assembled with a number of smaller mirrors in parallel rows. Smaller modular mirrors require smaller machines to build the mirror, reducing cost. Cost is also reduced in case of the need of replacing a damaged mirror. Such damage may occur due to being hit by an object during bad weather.

In addition, V-type parabolic troughs exist which are made from 2 mirrors and placed at an angle towards each other.

Solar Power Tower

The solar power tower, also known as 'central tower' power plants or 'heliostat' power plants or power towers, is a type of solar furnace using a tower to receive the focused sunlight. It uses an array of flat, movable mirrors (called heliostats) to focus the sun's rays upon a collector tower (the target).

Early designs used these focused rays to heat water, and used the resulting steam to power a turbine. Newer designs using liquid sodium have been demonstrated, and systems using molten salts (40% potassium nitrate, 60% sodium nitrate) as the working fluids are now in operation. These working fluids have high heat capacity, which can be used to store the energy before using it to boil water to drive turbines. These designs also allow power to be generated when the sun is not shining.

Some concentrating solar power towers are air-cooled instead of water-cooled, to avoid using limited desert water. Flat glass is used instead of the more expensive curved glass. Thermal storage to store the heat in molten salt containers to continue producing electricity while the sun is not shining Steam is heated to 500 °C to drive turbines that are coupled to generators which produce electricity. Control systems to supervise and control all the plant activity including the heliostat array positions, alarms, other data acquisition and communication.

Linear Fresnel Reflector

A Linear Fresnel Reflector is another type of solar power collector. It uses flat mirrors as opposed to parabolic mirrors that are used in solar parabolic troughs. The basic principle remains the same with the mirrors collecting solar power which is then utilized to generate steam which in turn drives a turbine. This technology leads to the production of steam directly and do not use heat transfer fluid or other medium. The sunlight that is concentrated with the help of mirrors boils the water which is present in the receiver tubes thereby generating steam. No heat exchangers are used in this system.

One big disadvantage with this design is the shading effect of adjacent mirrors which can be encountered only by utilizing more ground space which increases costs.

Dish Stirling

A dish Stirling or dish engine system consists of a stand-alone parabolic reflector that concentrates light onto a receiver positioned at the reflector's focal point. The reflector tracks the Sun along two axes. The working fluid in the receiver is heated to 250–700 °C (482–1,292 °F) and then used by a Stirling engine to generate power. Parabolic-dish systems provide high solar-to-electric efficiency (between 31% and 32%) and their modular nature provides scalability.

Solar thermal enhanced oil recovery

Solar thermal enhanced oil recovery (abbreviated solar EOR) is a form of thermal enhanced oil recovery (EOR), a technique applied by oil producers to extract more oil from maturing oil fields. Solar EOR uses solar thermal arrays to concentrate the sun’s energy to heat water and generate steam. The steam is injected into an oil reservoir to reduce the viscosity, or thin, heavy crude thus facilitating its flow to the surface. Thermal recovery processes, also known as steam injection, have traditionally burned natural gas to produce steam. Solar EOR is proving to be a viable alternative to gas-fired steam production for the oil industry.

Types of solar EOR:

• Central tower originally designed for generating electricity, central tower, or power tower technology, uses a field of large tracking mirrors, called heliostats, to concentrate the sunlight on a boiler filled with water that rests on a central tower. The sun’s energy is reflected on the boiler to produce steam, which is used to turn a traditional turbine to create electricity. For EOR, the process ends at steam production. High-temperature steam made from demineralized water in the tower receiver passes through a heat exchanger, generating steam of lower temperature from high-contamination oilfield feed water at lower temperatures. The steam is fed into distribution headers which lead to injection wells, which convey steam into the oil-bearing formation.
• The enclosed trough architecture encapsulates the solar thermal system within a greenhouse-like glasshouse. The glasshouse creates a protected environment to withstand the elements that can negatively impact reliability and efficiency of the solar thermal system. Lightweight curved solar-reflecting mirrors are suspended within the glasshouse structure. A single-axis tracking system positions the mirrors to track the sun and focus its light onto a network of stationary steel pipes, also suspended from the glasshouse structure. Steam is generated directly, using oil field-quality water, as water flows from the inlet throughout the length of the pipes, without heat exchangers or intermediate working fluids. The steam produced is then fed directly to the field’s existing steam distribution network, where the steam is continuously injected deep into the oil reservoir.

Costs

Concentrated Solar Power (CSP) costs could drop by 43 per cent during the next 10 years. The outlook is particularly positive, according to IRENA, for solar thermal technologies using molten salt: “The transition to the use of molten salt… means that a much higher cost reduction potential exists,” the report says. “By 2025, the use of molten salt …will raise operating temperatures and allow the storage medium requirements to be decreased by around half while maintaining the same number of hours of operation. As a result, the installed costs of the storage medium (per kWh-thermal) for a … system using molten salt … are around 50% lower compared to the 2015 reference.” The levelized cost of electricity or a measure of comparison for various power generation methods, for wind could fall by 35 per cent. However, the major winner in terms of lower costs is solar, including photovoltaic (PV) and concentrated solar power (CSP) applications, which could drop by as much as 59 per cent and 43 per cent respectively during the next 10 years.

Increasing economies of scale, more competitive supply chains and further technological improvements will continue reducing the costs of solar and wind power. The same factors will also boost the availability of these key renewable power sources at night and in varying weather conditions. With the right policies in place, the cost of electricity from solar and wind power technologies could fall by at least 26% and as much as 59% between 2015 and 2025, finds this cost-analysis report from the International Renewable Energy Agency (IRENA).

Future

A study done by Greenpeace International, the European Solar Thermal Electricity Association, and the International Energy Agency’s Solar Paces group investigated the potential and future of concentrated solar power. The study found that concentrated solar power could account for up to 25% of the world's energy needs by 2050.

Renewables, such as solar and wind are becoming increasingly competitive with fossil fuels, and in some countries, accounting for a larger (albeit still small) share of electricity production. Germany’s energy-transition (Energiewende) policy (published in 2010), for example, aims for a 55–60% share of renewable energy in gross electricity consumption by 2035.

CSP technology today:

Unlike solar PV panels, which collect sunlight and directly convert it into electricity, a CSP plant instead collects sunlight and converts it to heat. This thermal energy is then used to make steam to generate electricity. Today, there are basically four commercially available CSP technologies, which differ in the way they concentrate the sunlight.

The main advantage of a CSP plant is that it produces grid-friendly, dispatchable electricity. That means electricity is produced quickly to meet the demand — the same as any conventional steam-driven turbine power plant, he says. That’s because CSP plants being built today have the ability to store large amounts of heat using molten salt technology. The ability to store thermal energy instead of storing electricity (as may be done by PV and wind generation with battery storage). The electricity from a CSP is not only dispatchable, but it is dispatchable 24 hours per day.

Environmental impacts of Concentrated Solar power

Research has examined the potential local environmental impacts of the construction and use of large-scale CSP plants.

Land Use -Depending on their location, larger utility-scale solar facilities can raise concerns about land degradation and habitat loss. Total land area requirements vary depending on the technology, the topography of the site, and the intensity of the solar resource. Estimates for utility-scale PV systems range from 3.5 to 10 acres per megawatt, while estimates for CSP facilities are between 4 and 16.5 acres per megawatt.

Water Use - CSP plants that use wet-recirculating technology with cooling towers withdraw between 600 and 650 gallons of water per megawatt-hour of electricity produced. CSP plants with once-through cooling technology have higher levels of water withdrawal, but lower total water consumption (because water is not lost as steam). Dry-cooling technology can reduce water use at CSP plants by approximately 90 percent. However, the tradeoffs to these water savings are higher costs and lower efficiencies. In addition, dry-cooling technology is significantly less effective at temperatures above 100 degrees Fahrenheit.

Hazardous Materials - The PV cell manufacturing process includes a number of hazardous materials, most of which are used to clean and purify the semiconductor surface. These chemicals, similar to those used in the general semiconductor industry, include hydrochloric acid, sulfuric acid, nitric acid, hydrogen fluoride, 1,1,1-trichloroethane, and acetone. Thin-film PV cells contain a number of more toxic materials than those used in traditional silicon photovoltaic cells, including gallium arsenide, copper-indium-gallium-diselenide, and cadmium-telluride. If not handled and disposed of properly, these materials could pose serious environmental or public health threats. However, manufacturers have a strong financial incentive to ensure that these highly valuable and often rare materials are recycled rather than thrown away.

Effect on wildlife - Insects can be attracted to the bright light caused by concentrated solar technology, and as a result birds that hunt them can be killed by being burned if they fly near the point where light is being focused. This can also affect raptors who hunt the birds. Federal wildlife officials have begun calling these power towers "mega traps" for wildlife.