26 July 2010
Building a robust, renewable-energy, electrical grid will require a potpourri of technologies that together must meet must-take, peak-, intermediate- and base-load needs. PV and wind provide power only when the resource is available making these technologies suited for must-take and peak-load generation. Yet electrical demands continue after the sun goes down—or there is no wind—when people return home from work. Thus, the ability to store energy is a key component for supplying intermediate- and base-load demands.
“Where the load is high and penetration of renewables is low, such as in California, storage is less important. However, in the future, when there is more variable generation from PV and wind renewables on the grid, the value of storage will increase significantly.” - Mark Mehos, NREL
Storing electrical energy is very difficult and expensive. But concentrating solar thermal (CST) can store thermal energy for later conversion into electricity. And, because CST powers steam turbine generators, CST can also be used in hybrid configurations with coal and natural gas for power at night or on cloudy days.
According to iSuppli Corp., photovoltaics (PV) is expected to grow by a factor of six from 2009 to 2014, but CST is projected to expand by a factor of 37. NREL estimates that by 2020 the cost of CST will drop to $0.054-.0621/kW-hour making it one of the more cost-effective renewable energy generation methods.
CST for utility grid generation isn’t new. Since the 1980s, Solar Electricity Generating Systems, or SEGS, which consists of nine CST hybrid (natural gas) plants with a combined capacity of over 350 MW, have been operating on over 1500 acres in the Mojave Desert, powering over 250,000 homes in California.
SEGS uses a tracking parabolic trough design—the most proven CST architecture. The highly reflective glass mirrors concentrate the sunlight onto a central tube filled with oil that heats to over 400°C. The oil passes through a series of heat exchangers to produce superheated steam that run the turbines.
Energy Storage in a Concentrating Solar Power (CSP) System
(This figure was prepared by the National Renewable Energy Lab
for the U.S. Department of Energy)
As natural gas prices plummeted in the 1990s, interest in CST halted. Recently interest in CST has experienced a resurgence. Solar Millenium AG is busy developing three Andasol plants in Spain, each with a capacity of 50 MW. Andasol 1 is the first parabolic trough power plant in Europe and also the largest solar power plant in the world. Andasol I and 2 are now online with Andasol 3 scheduled to be connected to the grid in spring 2011. Each uses a portion of the heat produced in the solar field to heat molten salt. When storing the heat, molten salt is pumped through an oil-salt heat exchanger from the “cold” tank (290°C) into the “hot” tank (390°C). At night or on cloudy days, the process is reversed, thus giving back the thermal energy to the oil circuit. The molten salt storage at full capacity can power the plant 7.5 hours at full-load.
Solar Millennium AG is developing three solar thermal parabolic trough power plants
in Southern Spain: Andasol 1, 2 and 3. Together the three Andasol power plants
provide approximately half a million people with environmentally friendly energy
(Photo courtesy of Solar Millennium AG / Paul-Langrock.de)
In the US, Spanish company Abengoa Solar is partnering with Arizona Public Service to build the next largest solar plant in the world. Expected to be completed in 2012, the 280 MW parabolic trough Solana plant in Gila Bend, Arizona will have a thermal storage reservoir that will keep the plant running for 6 hours with no sun.
SkyFuel offers an improvement on the traditional parabolic trough. Their non-glass reflectors consist of lightweight, slide-in reflective mirror panels made with ReflecTech Mirror Film. The mirror panels and the supporting aluminum space frame are lightweight, easy to manufacture, ship and deploy, significantly reducing construction time and costs.
BrightSource Energy’s first 100 MW commercial plant in the
Mojave Desert will consist of approximately 50,000 heliostats
(Image courtesy of BrightSource Energy)
Companies such as BrightSource Energy are touting the power tower as being more effective than parabolic troughs on a variety of fronts. Power tower systems use a field of heliostats (or mirrors) that individually track the sun and concentrate the sunlight onto a receiver at the top of a tower. The receiver is a traditional high-efficiency boiler that directly generates superheated steam of 550°C. Higher temperatures result in better efficiencies. And, the smaller, flat mirrors are simpler to manufacture, and are less costly to install than parabolic troughs.
BrightSource is currently building a solar power tower complex in the Mojave Desert. The 400 MW Ivanpah facility will consist of three separate plants providing power to PG&E and Southern California Edison. The first is scheduled to go online in 2012. In February 2010, BrightSource revised its plan to reduce the overall footprint by 12%. This was in response to concerns over environmental and wildlife impacts of the project.
While Ivanpah has no storage option, companies such as SolarReserve and Torresol Energy are demonstrating molten salt storage capability with power towers but on a smaller scale. Gemasolar, developed by Torresol Energy, will be a 17 MW thermosolar tower power plant in Spain. Its hot salt tank will operate at a temperature of over 500°C with 15 hours of storage capacity.
Yet another architecture being developed is the linear Fresnel reflector design, which consists of an array of nearly-flat reflectors to concentrate solar radiation onto elevated inverted linear receivers. SkyFuel’s Linear Power Tower is based on the linear Fresnel reflector but incorporates new materials and designs to achieve the high-temperature performance of power tower systems with thermal energy storage capabilities—and without the construction expense or practical and political difficulties of tall, brightly glowing towers.
At the end of 2009, 600 MW of solar thermal was running worldwide with another 500 MW under construction split fairly equally among the many technologies. Which technology will win out remains up for grabs. As Mark Mehos, Principal Program Manager of Concentrating Solar Power at NREL commented, “Where the load is high and penetration of renewables is low, such as in California, storage is less important. However, in the future, when there is more variable generation from PV and wind renewables on the grid, the value of storage will increase significantly. We are already hearing this from utilities outside of California.”
Written by Dr. Kathy Li Dessau, Contributing Editor - US