Exploring Concentrated Solar Power (CSP) Systems

Parabolic Trough Solar Systems

Concentrated Solar Power (CSP) systems use very different technology than photovoltaic systems. CSP systems use the sun as the "thermal heat" source as opposed to the photon energy of the sun as PV systems do. Most electric power plants use some form of fossil fuel as the thermal heat source to boil water into steam. The steam then spins a large turbine, which drives a generator to produce electricity.

The three main types of CSP systems are parabolic trough, power tower, and dish Stirling engine systems. All use the thermal energy from the sun to generate electricity. CSP systems are marketed mainly to utilities as they take up a fair amount of land and require mechanical maintenance. 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 (about 550 degrees Celsius or 1020 degrees Fahrenheit). 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.

Originally a special type of oil, called therminol, was used as the transfer fluid. Today, new designs are using a molten salt compound as the transfer fluid. The molten salt is a mixture of 60 percent sodium nitrate and 40 percent potassium nitrate, commonly called saltpeter. The molten salt can achieve a higher temperature and hold heat longer than the therminol. However, 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. Another huge advantage of the molten salt is that it can retain its heat for up to six hours when stored in specially designed storage tanks. This means 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.

Because the trough solar power approach uses conventional steam turbines, it is easy for the system to integrate seamlessly into the electrical grid. And having storage ability, it can compensate for moving cloud cover and other weather phenomenon. Trough solar systems typically run about a 20% sunlight to electricity conversion efficiency.

Because trough systems use conventional steam turbines, hybrid configurations of trough solar and natural gas turbines (oil and coal are possible too) can be built for continuous electrical power on rainy days and short winter days. Several hybrid systems are in development in the US and Europe.

As shown in the above schematic, CSP uses water to generate steam and also to clean the vast array of mirrors. While this is not a problem in most climes, it is an issue in desert areas (which have the most sunshine). All other types of electrical plants use water to drive steam turbines, and CSP is no different. This is only an issue in desert areas where water is at a premium.

In essence, concentrated solar power is basically a thermal application of mechanical engineering and thermodynamics, whereas photovoltaic power is an application of electrical engineering and semiconductors.

Solana Parabolic Trough Plant – Arizona

President Obama in 2010, announced that the US Government would provide a $1.45 billion loan guarantee through the Stimulus Act for the Solana CSP Plant in Gila Bend, AZ. The total cost is about $2 billion. The project was completed in 2013 and the 280 mega-watt plant is the largest CSP plant in the world. It is being built by Abengoa Solar, the leading Spanish CSP company, who will own the plant and operate it. Solana in Spanish means "sunny place".

The plant at full capacity supplies electricity for about 70,000 homes and uses the equivalent amount of water for about 4,000 homes. While this is not insignificant, the land was previously used for farming and the water use was about three to four times greater.

APS, the largest electrical utility in AZ, will buy all the electricity generated by the Solana Plant. Solana will also make use of molten salt storage for up to 6 hours of extra power to cover the peak hours of 4 pm to 7 pm in the evening especially in the summertime.

Solar Tower Systems

The first commercial solar tower system was built by Abengoa Solar of Spain and is referred to as PS10 at the Solucar Platform in the Spanish province of Seville. It began operation in March, 2007 and continues to this day. The sketch at the right illustrates how the sun's rays are directed to the top of the tower. The second picture to the right is that of PS10 at Solucar. Tower systems have three main components: ground heliostats, a tower, and a central receiver at the top of the tower.

The function of the heliostats (Helios in Greek means the sun) 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.

Dual-axis tracking allows the heliostat field to produce optimum power for more hours of the day than is possible with systems that rely on either fixed mounts or single axis tracking. The heliostats are composed of reflective glass mirrors, a supporting structure and mechanisms to orientate them. They are very large - 1300 square feet each and there is a grand total of 624 of them.

The centralized receiver is located in the upper section of the tower. The receiver is a "cavity" receiver composed of four vertical panels that are 18 feet wide by 39 feet tall. The panels are arranged in a semi-circle configuration and housed in a square opening 36 feet by 36 feet.

The receiver absorbs the sunlight from the heliostats and transfers the energy to a circulating fluid, usually molten salt. The salt is either stored for future use or it transfers its energy to saturated steam system at 250 degrees Celsius. The steam then drives a conventional turbine at the bottom of the tower.

The PS10 system stores up to one hour's worth of steam for additional electricity as required. On cloudy days the system can generate 15% of its total capacity by using an auxiliary natural gas turbine to generate its own electricity and help feed the main solar steam turbine with its waste energy which runs about 50%. The total efficiency from solar radiation to electricity is 17%. This is quite good recognizing that the efficiency of the steam cycle is only 27%. The tower supports the receiver which needs to be a certain height above the heliostat's level to avoid or reduce shade and other obstructions.

Ivanpah Plants – California

There are several "tower" solar projects proceeding in the US. The largest is Ivanpah in southeastern California located on 3,500 acres of federal land in the Mohave Desert. Ivanpah is a 377 mega-watt tower solar project being developed by BrightSource Energy. The complex will consist of three separate plants being built between 2010 and 2013. The first tower is shown at the right.

BrightSource has signed contracts to provide the power to PG&E and Southern California Edison. 392 mega-watts is enough energy to power 140,000 homes during peak hours of the day. The project is located near existing roads, transmission lines, and a natural gas power plant. The project will reduce carbon dioxide (CO2) emissions by more than 400,000 tons annually, which is the equivalent of taking more than 70,000 cars off the road.

BrightSource’s low impact heliostat layout is flexible, allowing the solar field to be built around the natural contours of the land and avoids areas of sensitive vegetation. In order to conserve scarce water resources, the technology employs an air-cooling system to recycle the steam back into water in a closed-loop system and then is reused to clean mirrors. By using air-cooling, BrightSource’s technology uses 90 percent less water than older technology trough plants with wet cooling. The Ivanpah Project was named the 2012 Energy Project of The Year by the CMAA Green Symposium.

Dish Stirling Systems

A dish Stirling system uses a much different technology than other concentrating technologies. It does not use water or steam except for a very small amount to wash the concentrators. The system still falls into the thermal CSP class of products as it uses the heat from the sun to drive a Stirling engine to generate electricity. The dish Stirling system is composed of a parabolic concentrator, a solar light receiver shaped like a cavity, and a Stirling Engine. The concentrator is a highly reflective mirror dish similar to a very large satellite dish.

The very concentrated sunlight (800 times normal) heats a working fluid in contact with the receiver to a temperature of approximately 650ºC. The thermal energy causes the cylinder piston engine to oscillate back and forth at 50 or 60 cycles per second. The piston moves a magnet back and forth inside a coil of wire which generates an AC current. The engine is air cooled by means of a radiator and a closed water-based coolant system.