THE QUESTION OF SUN AND WATER

Concentrating Solar Power Commercial Application Study: Reducing Water Consumption of Concentrating Solar Power Electricity Generation; Report to Congress
July 2009 (U.S. Department of Energy/National Renewable Energy Laboratory)

SUMMARY
The future availability of water is a big question in and of itself but it is also a big question for energy producers. If worsening droughts associated with global climate change do not entirely impede access to adequate water, they will certainly make it a more precious – and therefore more expensive – commodity.

Will the added expense of water alter assumptions about which form of electricity generation is the best?

Concentrating Solar Power Commercial Application Study: Reducing Water Consumption of Concentrating Solar Power Electricity Generation; Report to Congress, from the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL), is aimed at answering that question as it pertains to solar power plant technologies.

Want to skip the details? Here’s the bottom line: The question of water puts more nails in the coffins of coal and nuclear as viable future power generators and offers yet more evidence that New Energy is the only sensible way to build.

In the details, the NREL report offers important suggestions about currently competing solar power plant concepts based on their water needs and what those needs mean to the cost of solar power plant-generated electricity.

Bottom line. (click to enlarge)

COMMENTARY
The report is in answer to a requirement by the Energy Independence and Security Act of 2007 for the Secretary of Energy to report to Congress on ways to cut the water use of concentrating solar power (CSP) systems.

The reason for requiring such a study is pretty obvious: The sun on the wide-open spaces of the U.S. Southwest is an energy asset the nation can no longer ignore. Concentrating solar power is the way to harvest that's sun generation. But some forms of CSP are water intensive and water in the Southwest is as precious as sun is abundant.

In late 2008, Congress extended the 30% investment tax credit (ITC), beginning in 2009, to the full cost of solar systems and made utilities and big power producers eligible for it for the first time. Other recession-easing incentives were also implemented. Despite the economic downturn, those incentives have proven too tempting to resist. Some solar power plants have started construction and many more are being planning.

There are now 400+ megawatts of installed CSP, some having performed at utility scale for more than 15 years. More than 4000 megawatts are in planning stages around the world.

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There are 4 main CSP technologies: parabolic troughs, linear Fresnel, power towers, and dish/engine.

Parabolic troughs are the most tested technology. Power tower technologies are beginning to be put into use in Spain and just being tested in the U.S. Linear Fresnel systems have agreements with utilities in California and pilot projects are under construction.

Parabolic troughs, linear Fresnel and power towers focus the sun to heat a liquid that flows to conventional Rankine steam cycle turbines like those heated by coal and natural gas combustion and nuclear energy. Steam cycle power plants require cooling to condense the steam and complete the cycle. It can be water cooling, air cooling or a combination.

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Dish/Stirling engine systems use curved mirroring to focus the sun’s heat to drive a small engine. The most common ones use Stirling cycle engines with hydrogen as the working fluid. They are air-cooled and only require water for mirror washing.

All 4 CSP designs use a small amount of water for mirror washing. The first three operate a steam cycle and, like fossil and nuclear plants, require water for steam makeup and, when they are water-cooled, a substantial amount of water for cooling. Though the use of the steam cycle gives the 3 water-consuming CSP technologies the same disadvantages it gives fossil and nuclear plants in terms of water use, it gives them 2 advantages: (1) Utility managers are familiar with the power generating system; and, (2) Storage systems can be integrated, allowing the dispatch of electricity as it is needed and the ability to produce electricity into the night.

Thermoelectric fossil fuel combustion and nuclear power plants are water-cooled by one of two methods: (1) Once-through cooling and (2) Recirculating evaporative cooling.

Once-through cooling withdraws large volumes (23,000 to 27,000 gallons per megawatt-hour) from a water source and returns it to the source at an elevated temperature. That causes further evaporative loss from the water source.

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Recirculating evaporative cooling withdraws a lesser amount (500 to 750 gallons per megawatt-hour) but evaporates most of the water directly. Once-through cooling ultimately consumes less water but is restricted in use because it potentially impacts the environment and aquatic habitat of the water source.

Air cooling blows steam cycle heat directly into the air. A fossil power plant using air cooling withdraws water only for the steam cycle and housekeeping uses and, therefore uses less than 10% of the water used by a water-cooled plant.

At present, water cooling is cheaper in fossil fuel, nuclear and CSP plants. Less capital is required for construction and it manages temperature variations more efficiently regardless of seasonal temperature fluctuations because water body temperature fluctuates less than air temperature. Air cooling is quite inefficient when it is hot because the air temperature is nearer the plant’s temperature and provides little cooling.

CSP plants need water for cooling, for steam, for condensing steam, and for mirror washing. Yet the Southwest, where solar resources are ideal for CSP, has no extra water and importing or purifying water would be expensive. There are, however, ways to increase the water efficiency of CSP.

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New fossil and nuclear power plants use evaporative water cooling at ~500 gallons of water per megawatt-hour, as do solar power towers. A combined-cycle natural gas plant uses ~200 gallons per megawatt-hour. A water-cooled parabolic trough plant uses ~800 gallons per megawatt-hour, 2% for mirror washing. Dish/engine systems only require ~20 gallons per megawatt-hour for mirror washing.

When there are water limitations and environmental regulations, new fossil and nuclear plants and CSP power tower and trough facilities can use air cooling to cut water use dramatically. New fossil and nuclear plants with air cooling technology eliminate 90% of their water use. A dry-cooled parabolic trough plant in the Mojave Desert reduced output 5% per year and increased the cost 7-to-9% but eliminated 90% of its water use. In New Mexico, the cost increase was only 2% because daytime temperatures were not as high as in the Mojave.

The potential of the Southwest is incredible. (click to enlarge)

Different technologies lose different levels of output with air cooling. Average loss for CSP trough plants was 4.6% of the electricity output. Average power tower loss was only 1.3%. So many factors are part of the total output equation, from field size to local ambient temperatures, that conclusive comparisons are almost too generalized to be meaningful. Also relevant are local water conditions and the cost of peak demand electricity.

Other cooling systems are also used. There are hybrid wet/dry cooling systems which balance water use and output losses. Newer plants can use a parallel cooling system (PCS) that uses both air and water cooling and balances them more efficiently. A computer model for PCS in a parabolic trough CSP power plant showed 50% water savings with only a 1% drop in output or 85% water savings with a 3% output drop. PCS keeps the increased cost of electricity down to 5%, instead of the 7-to-9% cost increase in a purely air cooled CSP parabolic trough plant.

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Linear Fresnel CSP has not yet been evaluated. Dish/Stirling engine systems use almost no water except for mirror washing but are the newest and most untested form of CSP.

Clearly, air cooling and hybrid cooling systems are viable ways to reduce the water needs of CSP 80-to-90% while only increasing the costs of the electricity they generate by 2-to-10%. Air cooling and hybrid cooling systems also reduce the water needs of fossil fuel and nuclear power plants to comparable levels.

So, if water use is comparable, why not use Old Energies instead of experimenting with CSP technology? First, because Old Energy is dirty. Coal is filthy, the single biggest energy-generating contributor to global climate change. Nuclear represents a battery of dangers, from weapons proliferation to the radioactive waste for which nobody has provided a safe way disposal plan. But there’s a more important reason to choose CSP.

Not too many accidents with sun and wind. (click to enlarge)

To incorporate the air cooling and hybrid cooling systems, the Old Energy facilities have to be NEW. No NEW nuclear plant has been built in the U.S. since 1978 for a simple reason: It’s just too damned expensive and the risk of an accident, while remote, is just too potentially costly. And the public in the U.S. and Western Europe simply won’t stand for NEW coal plants anymore. Where new coal or nuclear plants manage to fight their way to the construction stage, they take huge amounts of capital and tie it up for 6-to-12 years or longer, invariably coming in far behind schedule and over budget.

Meanwhile, plans are being made and construction is underway for CSP installations all over the Southwest and in hot dry regions from the Maghreb of North Africa to Australia’s Great Outback to China’s empty western expanses. They go up in 1-to-2 years and promptly start paying back on the relatively available amounts of capital they tie up.

So, of course, the people who have the money are not putting it into coal and nuclear plants and are ever more interested in solar power plant technologies.

Adding all the costs, sun and wind are by far best buys. (click to enlarge)

QUOTES
- From the NREL study: “Peak power demand, particularly in California, Nevada and Arizona, is approaching system capacity and growing rapidly. It is expected that renewable energy sources will increasingly be tapped to meet market and regulatory demands. Many of the Southwestern states have established renewable portfolio standards (RPS) that encourage the development of technologies like CSP…”
From the NREL study: “Utilities are showing increasing interest in the deployment of concentrating solar power plants to meet the requirements of state renewable electricity standards…”