Concentrating Solar Power: Prospects and Challenges
When most people think about solar energy, they picture photovoltaic (PV) panels sitting on rooftops or perhaps in a large array on the ground. However, solar energy actually encompasses a variety of technologies. One group of solar technologies, called Concentrating Solar Power (CSP), uses the heat of the sun to boil water and make steam in order to turn a conventional steam turbine. Imagine a coal or natural gas power plant, but replace the burning of coal or natural gas with the thermal energy of the sun.
Unlike other solar technologies, CSP plants can produce electricity when the sun is not shining, for example when it is cloudy or at night. The thermal energy of the sun can be stored in large tanks of molten salt that can be used when needed to boil water and turn a steam turbine. This capability provides utilities and electric grid operators with dispatchable power – electricity on demand. For example, electricity production at a CSP plant can be shifted from peak sun in the afternoon to peak demand, which is late afternoon and evening in the summer and can be early morning and early evening in the winter.
CSP is a family of technologies. The two most commercial technologies are called “power towers” and “parabolic troughs.” With power towers, heliostats (mirrors that move on a two-dimensional axis) reflect sunlight towards the top of a tower where a receiver system is mounted. The heated fluid in the receiver is used to boil water and turn a turbine. In the case of parabolic troughs, parabolic mirrors track the sun and concentrate sunlight on a receiver tube placed on the focal line. The fluid in the receiver tube gets very hot as it moves though the solar field and is used to boil water and turn a turbine.
In the United States, 507MW of CSP plants are currently in operation. The oldest and, taken together, the largest are the SEGS parabolic trough plants, which were put into operation in California between 1985 and 1991. The nine plants that comprise the SEGS plants have a capacity of 354 MW. Five CSP plants are under construction in Arizona, California and Nevada, due to go online between 2013 and 2014. These new plants will have a total capacity of 1,321 MW. Two of the plants will have thermal energy storage, one for 6 hours and the other for 10 hours. As shown by an operating plant in Spain, it is possible to store enough thermal energy to produce electricity 24 hours a day. The amount of storage depends on the needs of the utility that agrees to buy the power.
CSP requires a high level of clear-sky solar radiation, called Direct Normal Irradiance (DNI), in order to produce electricity at the lowest price. In the US, high DNI can be found in Arizona, California, Colorado, Nevada, New Mexico, Texas and Utah. Outside of the US, this type of intense sun can be found in the Middle East, North Africa, South Africa, Spain, Chile, China, India, Australia and Mexico.
Within areas of high DNI, there are three requirements for building CSP plants: suitable land, transmission lines, and financing. CSP requires large tracts of relatively flat land with minimal environmental or cultural conflicts, such as endangered species or native artifacts. The Department of Interior has identified 17 solar energy zones (SEZs), totaling approximately 285,000 acres on public lands in six Southwestern states. Most of the identified SEZs, however, do not have available transmission lines or capacity to move the electricity to cities or other demand centers. Purchasing and building on private land remains an option, but in some states, large tracts of contiguous private land appropriate for CSP plants are difficult to find.
Transmission is one of the most complex challenges to developing CSP projects in the US. Numerous federal, state and tribal agencies are involved in permitting transmission lines, which often cross multiple jurisdictions, including private lands. In 2009, a Memorandum of Understanding was signed by 9 agencies that have a role in siting new transmission lines to improve coordination in the siting and permitting process. In addition, the Obama Administration established a Rapid Response Transmission Team (RRTT) with the goal of improving the federal government’s transmission permitting process on both federal and non‐federal lands. In the meantime, new projects will face a growing challenge to find suitable sites near existing transmission with available or upgradeable capacity.
Finally, large CSP projects are capital-intensive. Thus long-term, low-interest loans are necessary in order to make these projects financially viable. Since the global banking system collapsed in 2008, banks are only offering short-term loans for very limited amounts per loan, which is insufficient to finance these large power projects. The cost of CSP is coming down, but it is less developed than PV and has not yet come as far down the learning curve. Research and development can help bring down the cost of CSP, but it is also necessary to build new plants to decrease costs through improving construction methods, cultivating a trained workforce, and developing a competitive supply chain.
CSP with thermal energy storage is a unique solar-fueled power plant that can provide clean, domestic electricity when needed. CSP could enable even greater use of non-dispatchable renewables by adding stability to the electric grid, according to modeling by the National Renewable Energy Laboratory (NREL). While there are remaining challenges to siting, transmission and financing, the CSP plants coming online in the U.S. in the next two years will demonstrate the value of this unique clean energy technology.
The views expressed in these essays are those of their respective authors and do not necessarily reflect the views of Physicians for Social Responsibility.