Concentrated Solar Power PlantsPublished on 05.30.2017
10 min read
Concentrated solar (CSP) plants concentrate the Sun's rays to produce extremely high temperatures, and in turn generate . They differ from photovoltaic (PV) solar plants, which directly convert sunlight to electricity using photosensitive cells.
© <br>MARWAN NAAMANI / AFP - Inaugurated in the Abu Dhabi desert in 2013, Shams 1 is one of the largest concentrated solar power plants in the world.
As the name suggests, concentrated solar power plants use mirrors to concentrate the sunlight and a transfer fluid to a high temperature. This fluid, which can be air, water, oil, molten salt or organic liquids such as butane or propane, heats a network of water, which produces steam and drives a turbine, thereby generating electricity.
Depending on the method used to concentrate the Sun's rays, there are several types of plant, including:
- Solar power towers: Hundreds or thousands of mirrors mounted on the ground, known as heliostats, direct the sunlight toward the top of a tower that can be over 200 meters tall. The heliostats rotate to follow the Sun's course throughout the day. At the top of the tower, a receiver, inside which flows the (usually molten salt) transforms the sunlight into high temperatures. These plants require intense sunlight and vast areas of land. An example of the technology can be found at the 392-megawatt Ivanpah plant in California, which uses 173,500 heliostats.
- Parabolic trough power plants (PTPPs): Parallel rows of long parabolic mirrors known as troughs concentrate the Sun's heat on an axial tube, containing a heat transfer fluid, placed above each trough. The mirrors rotate on a horizontal axis, which enables them to follow the Sun's course. The temperature of the transfer fluid can reach up to 500°C. As with solar power towers, parabolic trough power plants operate best across vast areas in regions with intense sunlight. This technology is notably used in the 100-megawatt Shams power plant in the United Arab Emirates, which harnesses 258,000 parabolic mirrors over 2.5 square kilometers, and in the 160‑megawatt Noor 1 power plant in Morocco, where 500,000 mirrors cover an area of 4.8 square kilometers, or the equivalent of 600 soccer fields.
- Linear Fresnel power plants: Long, flat mirrors, mounted horizontally on trackers on the ground, capture the Sun's energy, which is reflected onto a linear receiver tube fixed parallel above the mirrors. Inside this tube, a transfer fluid is heated to temperatures of up to 500°C. The technology is named after the early 19th century French scientist, Augustin Fresnel, who invented the thick lens mounted on lighthouse beacons. The flat mirrors are much less expensive than parabolic mirrors, but optical performance is lower. This technology is used in the 12-megawatt Alba Nova plant in Ghisonaccia, Corsica.
- Parabolic dish power plants: The large parabolic dish of mirrors mounted on a structure that tracks the sun continuously throughout the day looks a lot like a satellite dish. It focuses sunlight into a central point just above the dish. Here, it meets a Stirling engine1, which is powered by the rising pressure generated by a gas sealed inside a closed cylinder. Temperatures on the receiver can reach up to 1,000°C. The dishes are compact, independent units and therefore have a relatively small footprint. The performance of the plant is closely linked to the optical precision of the dish and the output of the Stirling engine. An example of the technology can be seen near Phoenix, Arizona, where, 60 parabolic dishes make up a 1.5-megawatt plant.
Useful Storage Capacity, at a Price
The costs related to concentrated solar power plants are significantly higher than for photovoltaic plants. The global average cost per megawatt hour (MWh) of concentrated solar energy is between $100 and $200, whereas it is between $40 and $50 on average for photovoltaic power.
However, the major advantage of concentrated solar lies in its energy storage capacity. Heat is much easier to store than electricity, particularly if molten salt is used as the transfer fluid. Molten salt can retain heat for between six and eight hours before it is transformed into electricity. Heat stored at midday, when external temperatures are highest, can therefore be fed into a power grid in early evening, when demand peaks.
The other restriction to the development of concentrated solar plants is the space they require. For this reason, the largest plants are situated in deserts, such as those in California and Arizona in the United States. The world's most powerful CSP facility, the Ivanpah plant located in California's Mojave desert, is a three-unit system covering approximately 1,400 hectares, with of almost 400 megawatts. In comparison, the Martigues gas-fired power plant near Marseille, France takes up just 42 hectares and has an installed capacity of 930 megawatts as well as far superior output efficiency.
- Invented in 1815 by the engineer Robert Stirling, the Stirling engine originally used air at ambient temperature to which the classic thermodynamic cycle was applied (compression, heating, expansion and cooling). It was designed to overcome the extremely high risk of boiler explosions in steam engines. Over the years, many different versions of the Stirling engine have been devised, using various gases ( and helium, in particular). Other than for solar power, the Stirling engine is used in niche applications such as military equipment, space exploration and research.