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Published on August 5th, 2011 | by John Farrell


When Picking Solar Power Options, It’s the Water, Stupid

August 5th, 2011 by  

Concentrating solar has promised big additions to renewable energy production with the additional benefit of energy storage — saving sun power for nighttime — but there’s a catch.  Most of the new power plants are big water users despite being planned for desert locations.

With solar PV prices dropping so rapidly, does concentrating solar still make sense?

Concentrating solar thermal power uses big mirrors to focus sunlight and make electricity.  Think: kids with magnifying glasses, but making power instead of frying ants.  The focused sunlight makes heat, the heat makes steam, and the steam powers a turbine to make electricity.  In “wet-cooled” concentrating solar power plants, more water is used to make power than in any other kind of power plant.  The following chart illustrates the amount of water used to produce power from various technologies.

Water consumption can be cut dramatically by using “dry-cooling,” but this change increases the cost per kilowatt-hour (kWh) of power generated from concentrating solar power (CSP).  In the 2009 report Juice from Concentrate, the World Resources Institute reports that the reduction in water consumption adds 2-10 percent to levelized costs and reduces the power plant’s efficiency by up to 5 percent.

Let’s see how that changes ILSR’s original levelized cost comparison between CSP and solar PV.  Here’s the original chart comparing PV projects to CSP projects, with no discussion of water use or energy storage.

To make the comparison tighter, we’ll hypothetically transform the CSP plants from wet-cooled to dry-cooled, adjusting the levelized cost of power.

Using the midpoint of each estimate from Juice from Concentrate (6-percent increase to levelized costs and 2.5-percent efficiency reduction), the change in the cost per kWh for dry-cooling instead of wet-cooling is small but significant.  For example, all three concentrating solar power projects listed in the chart are wet-cooled power plants.  With a 6% increase in costs from dry cooling and a 2.5% reduction in efficiency, the delivered cost of electricity would rise by approximately 1.7 cents per kWh.

The following chart, modified from an earlier post, illustrates the comparison.

With the increased costs to reduce water consumption, CSP’s price is much less competitive with PV.  A distributed solar PV program by Southern California Edison has projected levelized costs of 17 cents per kWh for 1-2 MW solar arrays, and that a group purchase program for residential solar in Los Angeles has a levelized cost of just 20 cents per kWh.

In other words, while wet-cooled CSP already struggles to compete with low-cost, distributed PV, using dry cooling technology makes residential-scale PV competitive with CSP.

But there’s one more piece: storage.


While Nevada Solar One was built without storage, the PS10 and PS20 solar towers were built with 1 hour of thermal energy storage.  Let’s see how that changes the economics.

To make the comparison comparable, we’ll add the cost of 1 hour of storage to our two PV projects, a cost of approximately $0.50 per Watt, or 2.4 cents per kWh.  The following chart illustrates a comparison of PV to CSP, with all projects having 1 hour of storage (Nevada Solar One has been removed as it does not have storage).

When comparing CSP with storage (and lower water use) to PV with battery storage, we have a comparison that is remarkably similar to our first chart.  Distributed PV at a commercial scale (1-2 MW) is still cheaper than CSP, but residential PV is more expensive.

Even though dry-cooled CSP competes favorably on price, it still uses much more water than PV.  That issue is probably why many solar project developers are switching from CSP to PV technology for their large-scale desert projects.

Concentrating solar thermal power had its moment of cost advantage a few years ago, but the rapid pace (and zero water use) of solar PV installations has quickly eroded even the energy storage advantage of CSP.

This post originally appeared on Energy Self-Reliant States, a resource of the Institute for Local Self-Reliance’s New Rules Project.

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About the Author

directs the Democratic Energy program at ILSR and he focuses on energy policy developments that best expand the benefits of local ownership and dispersed generation of renewable energy. His seminal paper, Democratizing the Electricity System, describes how to blast the roadblocks to distributed renewable energy generation, and how such small-scale renewable energy projects are the key to the biggest strides in renewable energy development.
Farrell also authored the landmark report Energy Self-Reliant States, which serves as the definitive energy atlas for the United States, detailing the state-by-state renewable electricity generation potential. Farrell regularly provides discussion and analysis of distributed renewable energy policy on his blog, Energy Self-Reliant States (energyselfreliantstates.org), and articles are regularly syndicated on Grist and Renewable Energy World.
John Farrell can also be found on Twitter @johnffarrell, or at jfarrell@ilsr.org.

  • Ian Eiloart

    I can’t see any of the images in this article.

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  • Ecobob

    very interesting. UK and Germany have poss desertec project, sea water could be desalinated in cooling process as very useful by-product near North Africa/Spain coastline. Spain and other countries have energy intensive desalination plants all over the place. I think PV prices have been falling substantially and both technologies are maturing. . .

  • Anonymous

    John, you’re using ‘first generation’ power costs from these plants. That could well be an inaccurate predictor of where prices will settle if/when the technology matures. (Think about what has happened and is still happening with the cost of PV.)

    Thermal might drop as low as PV is expected to drop. Down with the cost of wind.

    Then consider the value of electricity sold into afternoon/evening peak hour demand. It might turn out that these plants would spend the sunny hours storing heat and then crushing gas peakers after the sun fades. Those hours between when the sun weakens and before the night wind builds could provide an lucrative market for thermal solar, even with the added burden of air cooling….

  • Anonymous


  • Pingback: When Picking Solar Power Options, It's the Water, Stupid : TreeHugger()

  • Breath on the Wind

    Very Interesting stats and a great article. Water is not only a concern in Deserts but there are various predictions that suggest a world water shortage of fresh water by 2030: http://cleantechnica.com/2011/08/03/water-shortage-critical-for-non-wind-power/ Water useage and energy storage is probably why Brightsource has just come out with a new commercial product that includes both: http://cleantechnica.com/2011/08/05/that-other-solar-tower-technology/

    Photovoltaics don’t require water use for cooling and the coverings can be made to be relatively “self cleaning.” They also have the advantage of being modular. As soon as a few are up the project can start producing power. This must make funding much easier, but the efficiency of PV panels remains lower than CSP and to get the capacity factor higher, energy storage is required. Many of the energy storage options require turning electricity back to heat and this leaves CSP with an efficiency advantage. Long term, even seasonal storage may even be best served with chemical storage and MIT has a new method for converting light directly into chemical storage that may have a major future impact: http://cleantechnica.com/2011/07/29/21st-century-stable-thermal-energy-storage/

    With so many good options this may leave plants like the Arizona solar tower ( http://cleantechnica.com/2011/08/01/giant-arizona-solar-tower-high-energy-with-hot-air/ ) continuing to search for funding.

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