Over 100 countries worldwide are building solar power plants and the installed capacity in 2010 has reached 40,000 MW. This is expected to grow 20% a year despite the problems that solar power production happens only in daylight hours and power output can also be affected by clouds. This intermittent nature of solar power needs some form of energy storage to even out such fluctuations. Use of storage batteries is impractical for the large solar power plants now being built.
A solution that has emerged from Rocketdyne, a United Technologies group company, is the use of molten salt to store sun energy for use beyond the daytime. The salt used is a mixture of Potassium and Sodium Nitrates in a 60:40 ratio. These salts are abundant and non-toxic. In fact, these salts are used as soil fertilizers. The principal reason for use of these molten salts is that they can be heated to temperatures over a 1000 degrees F without decomposition and they store thermal energy for long periods up to 15 hours. The cost of power production from molten salt is attractive at 5 cents per KWH. A pilot plant of 10 MW capacity was built and operated in the Mojave desert between 1994-98 to prove the technology. Power plants incorporating heliostats and thermal storage are termed Concentrated Solar Power (CSP) plants.
Rocketdyne has been a leader in the development of rocket engines for the US space program and has applied its knowledge of handling rocket fuels and advanced metallurgy for development of what they call “The Power Tower”. In molten salt power plants, the sunlight is reflected by mirrors (called heliostats) on to a tower that contains the molten salt. The radiated solar energy heats the salt to 1050 degrees F. The high temperature molten salt is pumped through heat exchangers to produce steam, that is then used to drive turbines for power production. The storage of sun energy in molten salt acts as the buffer to continue production of solar power, when photovoltaic generation stops in the night time or reduces due to overcast skies. The Power Tower technology has also been applied to fossil fuel burning power plants to hybridize them by adding solar steam production to replace a part of their fossil fuel usage. Another interesting application of this technology is to produce hydrogen with solar energy.
The major issue with CSP technology is the large land area needed for the heliostats. Many projects are under development around the world and the outcome of these projects could trigger even wider adoption of this technology.
1. The Rice Solar Energy project
Solar Reserve, which is also part of the United Technologies group, has received permission to start construction of a 150 MW solar CSP plant in the Sonoran desert, near Blythe in California. The construction, expected to start in the third quarter of 2011, is expected to be completed in a record 13 months. The solar field will have over 17,000 heliostats arranged in a circle, to reflect sunlight on a receiver tank of 100 feet height mounted on top of a 534 feet structure. The tall structure helps the heliostats to be angled steeper to save on land space. Even with this improvement, over 1500 acres of land will be needed for the solar mirrors. The project is being built on private land to avoid potential delays due to environmental clearances that impacted some of the projects built on government land. The project will use dry cooling to save on water usage.
The project cost will be between $ 750 and $ 850 million. Annual operating costs, however, will be low at $ 5 to $ 7 million.
2. Alcazar’s 50 MW solar thermal project
Solar Reserve in partnership with Preneal of Madrid has been awarded permission to build this 50 MW solar thermal power project in Spain, incorporating the molten salt tower technology. Construction is expected to start in 2011.
The project configuration is similar to that described above for the Rice project. The smaller capacity of the power plant needs fewer heliostats, some 4300 are to be installed. The land area needed is correspondingly lower at 270 acres. The tower height will be 100 meters.
This project is part of the Spanish government’s plans to meet the carbon reduction goals set for EU countries.
3. Cobra Energy solar power plant
The 250 MW solar thermal power plant, when constructed in Australia, could become the largest in the world. Most other plants being planned are 150 MW or smaller. The project is led by the Spanish group ACS which is a $ 20 billion conglomerate. ACS has built and is operating solar and wind power plants in Spain and in the Dominican Republic. For the Australian project, it is partnering Areva, the French nuclear energy company and a slate of Australian construction companies to bid for a large piece of the government’s solar flagships program to build 400 MW of solar power plants, which has $ 1.5 billion funding. The Cobra project cost is estimated at $ 1 billion.
This project also aims to use the molten salt heat storage technology. The project will be awarded by tender and there is some doubt if the recent economic problems will cause this project to be delayed. The project site is still to be selected from 3 on a shortlist, 1 in Victoria and 2 in Queensland.
4. Crescent Dunes Solar Energy project
Solar Reserve has received loan guarantees for $ 737 million to build this 110 MW CSP plant in northern Nevada in a configuration which could be identical to the Rice Solar project described above.
The replication of design would remove elements of risk from the project and drive overall costs lower.
5. Gemasolar Array
While the other projects listed here are in the award or construction phase, this plant of a more modest 19.9 MW capacity has gone into production in May 2011. This plant deploying the molten salt CSP technology achieved an important landmark, that of 24 hours uninterrupted power production for a full month. This was achieved in June 2011, just a month after the commissioning of the plant. This performance is an important proof for the efficacy of the molten salt heat storage technology and should encourage more of these plants to be built.
The Gemasolar plant is owned by Terrsol Energy, a joint venture between the Spanish construction company SENER and Masdar the futuristic zero emission UAE city. Masdar should be seeking to replicate this plant in the UAE.
The Gemasol project deploys 2650 heliostats spread over 185 acres that focus sunlight on a 140 meter tower.