Solar Thermal Power Generation
Abstract
Solar thermal power plants use the Sun as a heat source. In order to generate a high enough temperature for a power plant, solar energy must be concentrated. In a solar thermal power plant this is normally achieved with mirrors. Estimates for global solar thermal potential indicate that it could more than provide for total global electricity needs. There are three primary solar thermal technologies based on three ways of concentrating solar energy: solar parabolic trough plants, solar tower power plants, and solar dish power plants. The mirrors used in these plants are normally constructed from glass, although other techniques are being explored. Power plants of these types use solar heat to heat a thermodynamic fluid such as water in order to drive a thermodynamic engine; for water this will be a steam turbine. Solar thermal power plants can have heat storage systems that allow them to generate electricity beyond daylight hours.
Keywords
Solar thermal power; solar trough; solar tower; solar dish; thermal storage; heat engine; glass mirrors; solar pond; solar chimney
Solar thermal power generation technologies exploit the Sun as a heat source. A typical solar thermal plant captures the infrared radiation that falls on the Earth and uses it to heat a thermodynamic fluid in order to drive a heat engine. A solar thermal facility is able to exploit a small amount of the long-wavelength visible radiation too, and power-generating plants are able to use around 50% of the energy in sunlight that reaches the surface of the Earth. However, these types of power plants can only exploit direct radiation. Diffuse radiation is of no use because it cannot provide the necessary intensity.
The heat engine in a solar thermal plant is the device that converts heat energy into mechanical energy; it drives a rotating shaft, which turns a generator to produce electricity. The heat engine extracts energy from a thermodynamic fluid by means of a cycle through which the fluid passes and via a temperature gradient. The larger this temperature gradient, the more efficient the engine is at extracting energy. To create the temperature gradient there must be a hot source to heat the fluid on one side of the cycle and a cold source to cool it on the opposite side. The cold source is usually either air or water, while the hot source for a thermal power plant is the Sun.
The radiation that arrives directly from the Sun, without any reflection or scattering, will feel hot, but its intensity is relatively low. As a consequence it cannot, as it arrives, provide a sufficiently intense hot source for a thermal power plant. The temperature rise that can be achieved is simply too low for the heat engine to operate efficiently.1 In order to provide the intensity of radiation needed to heat a fluid to the temperature required to drive a heat engine efficiently, the solar radiation must be concentrated. This can be carried out using mirrors or lenses. For solar thermal power plants, mirrors are most commonly used; the main types of solar thermal power plants are defined by the way in which they concentrate solar energy.
The solar collection field comprises one of the major sections of a solar thermal power plant. The second major section is the energy conversion system. Some solar thermal power plants may also include a third section, for energy storage.
Global Solar Thermal Potential
As with solar energy potential in general, as discussed in Chapter 2, the potential for solar thermal generation is enormous. However, while solar photovoltaic power plants based on solar cells can be sited more or less anywhere, solar thermal power plants need a large area of land where there is good annual direct radiation and little cloud cover. This restricts their use but still leaves a vast exploitable resource.
Table 3.1 shows figures for this potential, by global region, for sites with high incident radiation intensity and limited land use restrictions. The aggregate global potential based on these figures is 12,150,000 TWh/year. This can be compared to total global electricity generation in 2013 of 23,322 TWh, according to the International Energy Agency (IEA).2 On this basis, solar thermal power generation could provide 520 times the electricity currently being produced.
Table 3.1
Regional Solar Thermal Potential
| Region | Potential Power Generation (1000 TWh/year) |
| North America | 1150 |
| South America | 1350 |
| Africa/Europe/Asia | 7350 |
| Pacific | 2300 |
| Total | 12,150 |
Source: SolarPACES 2009.3
The regional breakdown in Table 3.1 shows that in North America there is the potential to generate 1,150,000 TWh each year from solar thermal power plants, and in South America, 1,350,000 TWh. The three regions—Africa, Europe, and Asia—are grouped together in the analysis on which these figures are based; the total potential for these three regions is 7,350,000 TWh/year. For the Pacific region the total annual potential is estimated to be 2,300,000 TWh. It is important to bear in mind that this regional potential is often found in desert areas far from population centers, so power generated in these regions might have to be transported over long distances. Even so, with modern long distance power transmission technologies such as high-voltage direct current (HVDC) transmission, it would be feasible for the total world electricity supply to be provided from solar thermal power plants.
Solar Thermal Technologies
As noted above, typical solar thermal power plants comprise several elements. There will be a collector field that collects and concentrates the heat energy arriving from the Sun. The concentrated heat energy is then used to raise the temperature of a heat collection fluid that is positioned at the point where the energy is concentrated or focused. This hot fluid may drive a heat engine directly or it may itself be used to heat a second thermodynamic fluid such as water, converting it to steam to drive a steam turbine. Once the heat has been extracted from the first fluid, it is returned to the solar field to be reheated.
There are three primary configurations that have been developed for solar thermal power plants. They all have common features, but each is normally considered a distinct type of solar thermal plant. The three, defined by the way in which they collect and concentrate sunlight, are called parabolic trough (or solar trough) power plants, solar tower power plants, and solar dish power plants. Two other solar thermal technologies, called solar ponds and solar chimneys (or sometimes, solar updraft towers), have also been developed, though neither has yet been deployed commercially on any scale.
In a solar trough power plant the solar collection field is made up of rows of reflective troughs, each with a parabolic cross section. The parabolic reflector focuses the sunlight to a line along the length of the trough; it is here that a tube carrying the heat absorbing fluid is placed. These plants have proved to be some of the most cost-effective solar thermal generating plants, and there are now a large number of such power stations in operation.
The solar dish power plant uses a parabolic dish reflector instead of a trough. The circular reflector collects all the light within its coverage area, and focuses it to a single point where the heat absorbing element of the plant is situated. This central focus allows a higher intensity to be achieved than is possible with a trough, and a consequent higher temperature to be reached. However, the design is limited by the size of the dish reflector that can be constructed. Individual solar dish plants are relatively small. Large numbers of these are needed to make a high-capacity generating plant.
The solar tower is, in a sense, a way around the problem of constructing a massive solar dish. This type of plant has a tower with a heat collector mounted on top. This tower is then surrounded by a field of flat plate (or slightly parabolic) reflectors, all mounted at ground level and all oriented so that sunlight reaching them is reflected onto the heat collector on top of the tower. This reflector field approximates a parabolic dish. As with the solar dish, this arrangement generates a high solar intensity at the collector and potentially provides for a high efficiency heat engine.
One of the drawbacks of solar energy is the fact that it is only available during daylight hours and, in the case of a solar thermal plant, it can only be used to produce electricity when a high enough level of direct radiation is available. In order to get around this, solar thermal power plants can be equipped with some form of thermal heat storage. In a plant with heat storage, the solar energy collected during daylight hours is used to heat the storage medium. Heat is then extracted from this medium using a second heat transfer cycle, and this heat is used to drive the heat engine. If the size of the heat store is large enough, it can hold heat energy sufficient to provide power for a full 24 hours each day.
Unconventional Solar Thermal Technologies
In addition to the three solar thermal generating plant configurations outlined above, there are two others that have either been proposed or tested: solar ponds and solar chimneys. The solar pond is a very low-technology approach to solar thermal power generation. It consists of a large pond filled with brine, which is heated by the Sun. As the pond warms, a temperature gradient develops between the upper and lower layers of the pond, and this temperature gradient is used to drive a small heat engine. The temperature gradient is never very large, and so the system depends on a low-temperature heat engine of special design, a type often used to extract energy from low-temperature geothermal resources.
The second unconventional approach is completely different. It uses a large greenhouse to heat air, which is then funneled towards a tall tower, thus creating a powerful updraft. The mass of moving air generated by this draft is used to drive wind turbines, which are placed in the path of the flowing air. Plants of this type have to be massive to become economically cost-effective, so while the concept has been proved at a pilot scale, no plant of commercial size has ever been built.
Solar Collectors
The majority of solar thermal power plants use mirrors to collect and focus solar energy. In most cases these mirrors are made from silvered glass. Glass is extremely durable and can be molded into the shapes necessary for the solar reflectors. Flat plate glass is cheap, but creating the parabolic shapes needed for solar reflectors adds to the cost. In addition, glass can be heavy, particularly with large area reflectors that must be physically strong enough to withstand any bending forces resulting from their own weight.
Alternatives to glass have been used in solar plants, but their use is relatively uncommon. Plastics such as silver-coated acrylic have been tested, and so has polished aluminum. Novel techniques for creating the required shapes, such as using a vacuum to shape a circular metal sheet or using arrays of flat sections to simulate a parabola, have also been used. However, glass has generally proved to be the most reliable and durable material for solar collectors.
Once the solar heat has been collected it must be absorbed by a heat transfer medium. This is carried out in a variety of ways. In solar trough plants a fluid is pumped through pipes that run along the focus of each trough. This fluid may be a high-temperature oil, or it can be water and steam. In other solar tower power plants a mixture of molten salts, or liquid sodium, have been utilized. It is also possible to use a pressurized gas. In some cases the heat transfer fluid is used directly in the heat engine of the plant, but often there is a secondary heat exchanger that transfers the heat from the transfer medium to the thermodynamic fluid. Both systems have their advantages.
Low-technology solar ponds do not use any form of concentration. Direct sunlight heats the pond without any additional measures. This means that only a modest temperature rise is possible, and efficiency is relatively low.
Energy Storage
Energy storage offers a means of extending the range of solar thermal power plants beyond the hours when heat from the Sun can be collected. Most solar thermal plants have a limited thermal inertia as a result of the volume of heat transfer fluid they contain (and its thermal mass), but this will not allow the plant to operate for more than a few minutes without heat input.
It is possible to build additional thermal storage capacity into plants in a number of ways. In solar tower power plants a common approach is to use a molten salt as the heat transfer and storage medium. Heat can be stored in large vessels full of the molten salt from which it is extracted by passing the salt through a heat exchanger to heat a thermodynamic fluid. Massive solid structures made from concrete, glass, or ceramics can also act as heat stores; these might provide high thermal storage density in the future. The solid/liquid or solid/gas phase change can also be exploited by using the latent heat needed to effect the phase change to store heat energy.
Some solar thermal power plants use a water/steam cycle as both the heat transfer and the thermodynamic fluid. In these plants it is possible to store energy in steam accumulators. This is not as efficient as some of the other methods available, but it has the advantage of simplicity and ease of interfacing with the plant.
According to the IEA, thermal storage is more efficient than the more common electrical energy storage technologies such as pumped storage hydropower.4 This suggests that further development of heat storage technologies would be economically effective. One way of improving efficiency is to raise the temperature at which heat is stored. However, this depends on being able to achieve higher temperatures when concentrating solar energy.
Energy Conversion and Heat Engines
Solar thermal power plants have used a range of heat engines to convert solar energy into electricity. For the low-technology solar pond, the most useful heat engine is probably an organic Rankine cycle engine. This can operate using a very small temperature difference between the hot and the cold source. The engine uses a low boiling point liquid so that the low temperature can be exploited, but in other respects the cycle is similar to that of a steam turbine.
Steam turbines are the most common means of extracting energy from a solar thermal plant. Some solar stations heat water directly to generate steam, but many use an intermediate heat transfer fluid—often an oil—which then heats the water through a heat exchanger. Direct systems are potentially more efficient, but they have proved to be more difficult to design. Meanwhile, the use of a steam cycle means that it is simple to add some form of supplementary heating, such as the addition of gas burners, to enable the plant to operate when solar heat is not available or is limited.
It is also possible to use the Brayton cycle gas turbine in a solar thermal plant. In this case the heat transfer medium must be a gas that can be heated to a high temperature and pressure. This can be achieved in solar tower plants and in solar dish plants where small gas turbines have been used as the heat engines. Solar dishes have also used another, novel engine, the Stirling engine. This is a closed-cycle engine with heat applied to its outside.
With any heat engine cycle it is important to have a cold source to remove heat from the engine. This is typically provided by air or water cooling. Many solar thermal power plants operate in arid desert regions where water is scarce, so they usually need to be able to use air cooling.
Electricity Transportation
Solar thermal generating potential is often concentrated in regions of high insolation, such as the southwestern United States or North Africa. These regions, often arid or desert, can in principle provide large volumes of electricity from the Sun. However, they are also often remote from the main population and demand centers, and so dedicated power transmission systems are likely to be needed if such resources are extensively exploited.
If solar thermal generating regions are developed in the future, then the power is likely to be transported using long HVDC transmission networks. For long-distance transmission, this type of system has lower energy losses than a conventional alternating current (AC) system. Most HVDC lines are point-to-point lines that typically carry power from a large power plant or agglomeration of power plants to a single converter station in a distant AC grid. These lines may be more flexible in the future, exploiting new technologies now being developed that provide the ability to build branching HVDC networks.
A system of this sort has been proposed to carry power from the southwestern states of the U.S. to the population centers of the northeast. A similar type of scheme has been suggested to transport power generated in North Africa across the Mediterranean Sea to provide energy for European countries. So far these projects remain no more than proposals, but they do offer a possible future means of mitigating global warming.