Concentrated Solar Power: Focusing the Sun

In the race to harness renewable energy, one technology combines a new-age focus on solar energy with the relatively old concept of steam-driven power plants. Known as Concentrated Solar Power (CSP), these systems rely on arrays of mirrors to focus the sun’s heat on a “working fluid,” which creates steam to drive a turbine. As we mentioned in a previous post, four distinct systems fall under the umbrella of CSP: parabolic troughs; Fresnel reflectors; dish Stirling; and solar power tower. Each has its own set of advantages and drawbacks, but as a whole they stand to contribute to growing renewable energy production in a big way.

Of the four types of CSP systems, the solar power tower is by far the most dramatic. Source: Solarpower.com.

Of the four types of CSP systems, the solar power tower is by far the most dramatic. Source: Solarpower.com.

Unlike photovoltaics (PV), CSP systems do not convert sunlight directly into electricity; rather, they convert it into intense heat. Dealing with energy in the form of heat has a number of benefits over direct conversion into electricity. The major advantage when looking to design a grid-scale renewable energy project is the simple fact that our ability to store excess heat is far more advanced than our ability to store excess electricity. CSP plants can employ a technology called Thermal Energy Storage (TES), whereby the sun is used to heat molten salt instead of water. The salt, rather than the sun directly, is then used to heat water, which creates the needed steam. It turns out that salt stays hotter much longer than water, and using molten salt as the middle-man allows the plant to continue generating electricity for over seven hours once the sun sets. Commercial deployment of TES is in its infancy, so the opportunities it already provides are particularly encouraging, all the more because three of the four CSP systems have demonstrated that they’re compatibile with it. Because of TES’s viability on a very large scale, Concentrated Solar Power offers enormous potential for energy storage in ways not even considered for PV (and not yet deployed on a commercial scale for wind, although big plans lie ahead).

NREL's map of CSP's potential in the US. Source: NREL.

NREL’s map of CSP’s potential in the US. Source: NREL.

But CSP is not without its drawbacks, and understanding them is key to determining CSP’s place in the diversified energy landscape of tomorrow. Given its dependence on long hours of intense sun, CSP is most effective in the Southwestern US. (Unlike PV, which is still viable in suboptimal regions, CSP really isn’t worth developing in places with less favorable conditions). Due to the fairly specific geographic range where CSP can be successful, these projects are often sited squarely in the fragile habitat of the threatened desert tortoise. For an industry making great strides on its reputation as environmentally friendly, this is no small hurdle. Already, renewable energy companies have spent tens of millions to protect the prehistoric relics.

In addition to ecological concerns, water issues in this arid region are being aggravated, as well. Current technology is very water intensive and the systems operating in the US today consume between 750 and 1,000 gallons of water per MWh, according to the Department of Energy. They rely on a process known as “wet cooling,” whereby water cools the system and then evaporates in a cooling tower, much like the process used at traditional fossil fuel and nuclear power plants. Since the regions with the greatest potential for CSP are also some of the most water-stressed, this degree of water consumption is a definite red flag. Fortunately, a promising alternative called “dry cooling” uses fans to cool steam pipes with air, completely eliminating the use of water as a coolant. Dry cooling reduces the efficiency of the power plant slightly, and ways to offset these losses are still being developed. This paper by the Congressional Research Service discusses at length CSP’s implications for water supply.

Of course, a major component of CSP’s success ultimately comes down to cost. The cost of electricity from each of the four systems varies, but in 2010 the cost from parabolic troughs (currently the most common system in the US) was between $0.12 and $0.18/kWh. In most cases, this is above grid parity – the point at which it costs the same as grid electricity derived from fossil fuels – but not far off. The Department of Energy’s SunShot Initiative aims to drive down the cost of solar electricity to $0.06/kWh by 2020, a price that is cost competitive in every state. Assuming these projections are on target and less water-intensive cooling technologies emerge, CSP appears poised to experience significant growth in the coming decade.

The Verdict: The future of renewable energy will rely not on a single, “perfect” technology but rather on a set of proven, cost-effective systems that take advantage of the differences in regional resources. CSP is one such system. Its ability to generate large amounts of renewable electricity and the potential to do so long after dark bode well for the industry and for the planet.

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