The second most abundant renewable energy source in terms of availability is concentrated solar thermal power. In terms of availability on a global scale it is second only to wind power.
In select areas of the world concentrated solar energy is widely available year round. This source of energy is most suitable in areas where there is little cloud cover year round. It relies on the unimpeded intensity of the sun, just like a magnifying glass will only focus sunlight when there are no clouds blocking the sun.
Areas of the world that are most suitable for this type of power generation include the southwestern United States, South America, Australia, Africa, Spain, and the Middle East.
Concentrated solar thermal power generation works by focusing the sun’s energy on to a small area, much like a magnifying glass, resulting in localized high heat. This high heat can be utilized for electricity generation. The energy efficiency achieved by concentrated solar thermal energy can be as much as twice that of photovoltaics, which makes them a more attractive option. Concentrated solar thermal power plants are also less expensive to build than photovoltaic plants.
The primary technology used to generate power with concentrated solar energy is described below.
Parabolic Trough Design
This design uses long arrays of parabolic mirrors that are aligned in the north-south direction. These mirrors turn to face directly towards the sun as it moves across the sky from east to west, during the day. Thus, there is only one axis of rotation in the tracking mechanism.
In parabolic troughs the solar energy is focused along a line instead of at a point (as is the case for Power Towers and Stirling Dishes, discussed below). This means that the temperature at the focus is not as hot as it would be if the solar energy were focused to a point. Nevertheless, the temperature at the trough focus is about 400 degrees Celsius. This yields an efficiency of about 15%, meaning that 15% of the incident solar energy hitting the mirrors is converted to electricity. This efficiency is comparable to that achieved by photovoltaic solar cells.
A long tube is placed at the trough focus. The tube is filled with synthetic oil. As the oil circulates through the tube it heats up and is fed into a heat exchanger which transfers heat to steam turbine to generate electricity. The (cooler) oil then re-enters the tubes in the parabolic trough for reheating, and the cycle continues.
More advanced trough designs use molten salt as a method of thermal energy storage. The heated oil exchanges its heat with a salt medium which is then routed into insulated hot thermal storage tanks, for later use. When power is requested the system runs in “reverse” and the oil extracts heat back from the salt and then circulates through pipes that exchange heat with a steam turbine to produce electricity. The molten salt (which has lost some of its heat) then goes into insulated “cold” storage tanks.
The molten salt remains in the cold storage tanks until it’s circulated back out and heated up again by the oil, and the cycle repeats.
The additional heat transfer path between the oil and the steam turbine tends to reduce efficiency a bit since it introduces an additional pathway where energy can be lost. But the advantage of thermal storage is that energy can be generated during cloudy periods or at night time after the sun sets. This increases the reliability of solar energy by making it available when the sun isn’t shining. One can suitably size thermal storage tanks to provide a buffer for electricity generation when it’s needed at any time during the day.
The molten salt is typically a mixture of 60 percent sodium nitrate and 40 percent potassium nitrate. The salt melts at about 220 degrees Celsius.
For the Andasol plant in Spain, the cold storage tank salt temperature is 280 degrees Celsius. In the hot storage tank the salt temperature is 380 degrees Celsius. So it remains liquid throughout.
The thermal storage efficiency of the hot and cold thermal storage tanks is about 99% on an annual basis, meaning they lose only 1% of their stored energy to the surroundings, as a yearly average. The tanks can store heat energy for up to a week.
Molten salt is an ideal thermal storage medium because it is low-cost, non-flammable, and non-toxic. It is at atmospheric pressure at the high temperatures during operation. This means it can be easily transported through piping.
Using many heliostats (mirrors), the sun is focused onto a central tower using tracking mechanisms. The power tower can use this concentrated solar energy to heat up water directly and run a steam turbine to generate electricity. In more advanced designs, such as Solar Two (http://en.wikipedia.org/wiki/The_Solar_Project), heat from the power tower heats up molten salt to 565 degrees Celsius. The salt is then stored in large insulated tanks. This is similar in principal to parabolic trough energy storage described above.
When power is requested the heated molten salt is circulated through pipes that exchange heat with a steam turbine to produce electricity. After exchanging heat with the steam turbine the (cooler) molten salt goes into a “cold” storage tank at a temperature of 290 degrees Celsius (ref: http://www.nrel.gov/csp/pdfs/34440.pdf).
The molten salt remains in the cold storage tanks until it’s circulated back out and heated up again in the power tower, and the cycle repeats.
The efficiency of a power tower is around 18-20%, more efficient than a parabolic trough system because the sun’s energy is focused to a point resulting in higher heat. Efficiency as high as 25% can be achieved with larger installations. Efficiency is somewhat proportional to the size of the installation.
Power tower installations are much less common than parabolic troughs. Although they tend to have higher efficiency they are a relatively new technology. However, parabolic trough plants are a fully mature technology with low technical and financial risk when it comes to developing plants in the near-term.
Solar Powered Stirling Engine
Another means to utilize the sun’s thermal energy is with a solar powered Stirling engine, which uses a parabolic dish to focus the sun’s heat onto a receiver. The figures below show a SunCatcher system produced by (the former) Stirling Energy Systems. A Stirling engine is placed near the focal point of a large parabolic dish, around 11 m in diameter. The dish is pointed directly at the sun using a tracking mechanism. This allows the sun’s energy to be continuously focused onto a receiver placed at the focal point. This receiver channels the heat energy into a Stirling engine which then generates electricity.
Below are images of Stirling dish systems produced by Solo Stirling GmbH (http://www.cleanergyindustries.com).
The Stirling engine is unique among heat engines in that all it needs to run is a hot and cold source. A working gas (usually hydrogen) is contained inside the engine at high pressure. This gas is cyclically compressed and expanded in the cold space and the hot space, respectively, to produce power.
In this case the hot source is the sun’s heat and the cold source is the ambient air. In the SunCatcher, the concentrated heat energy of the sun hitting the receiver can be as much as 800 degrees Celsius!
The efficiency is around 30%. This is currently the highest efficiency of any device that converts solar energy to electricity. It is comparable to the efficiency of coal-fired generating stations.
Each SunCatcher can generate an average of 25 kW of power. That is enough power for a dozen homes.
The SunCatcher is ideal for scalability in that it is modular, meaning that to get more power you simply add more units. This type of scalability is not as easy to implement for power towers, or parabolic troughs.
One drawback of Stirling dishes, despite their high efficiency, is that there is currently no practical means of energy storage (as is the case for parabolic trough and power tower systems). The difficulty is that Stirling dishes are modular, which means that they lose the advantage that large single installations have when it comes to energy storage. For a SunCatcher it would be impractical to set up a thermal storage system for each unit. It would be too bulky and would add considerable complexity to the design. Battery storage is a possibility but chances are it would also be too bulky to be practical, as well as very expensive.
Energy storage for Stirling dishes is currently in the R&D stages. One potential method involves using phase change materials to absorb and release thermal energy (this is known as using the latent heat of fusion, where a material changes from solid to liquid, and vice-versa) . This can greatly reduce the quantity of thermal storage material necessary relative to, say, molten salt. But complexities need to be overcome such as thermal stability, corrosion, and thermodynamic losses. Infinia Corporation (http://www.infiniacorp.com), a producer of Stirling dishes, is currently investigating phase change thermal energy storage.
It may be that Stirling dishes, in the absence of energy storage, will be better suited for reducing peak electrical demand during the day. This is often the case on warm summer days when many businesses and homes are running air conditioners.
However, it may be possible to store “excess” electricity generated during off-peak periods in pumped hydro storage facilities, for use when electrical demand is higher. As mentioned in A Guide To Satisfying Electrical Demand With Wind Power, Stirling dishes can store their off-peak energy production in pumped storage facilities.
Another possibility is to use Stirling dishes to produce hydrogen for aircraft and marine vessels, as explained in Alternative Fuel For Aviation And Marine Transportation. Their high efficiency may make them ideal for hydrogen production for transportation use, but less suitable for large-scale dispatchable power into the grid (in the absence of energy storage).
Nonetheless, due to their high efficiency, solar powered Stirling engines will be a major part of the energy mix, in the renewable energy future.