| Sun-Dried Solar |
| Written by Jon-Erik Lappano, Editorial Assistant | ||
  Left: PS10 solar power-tower in Seville, Spain (source: afloresum); Right: parabolic trough system in Nevada (source: epants) Given the recent economic growth in cleantech innovation and a global enthusiasm for renewable energy, solar power is swiftly becoming a major force in the renewable energy sector. In particular, solar thermal power, also known as Concentrated Solar Power (CSP), is making headway, with CSP stations currently generating around 500 megawatts (MW) of power worldwide, a number that is guaranteed to grow significantly in the near future. According to The Economist, there are projects in development that will generate up to 12 gigawatts (GW) of power. The total estimated generating capacity of CSP could be enormous-- up to 11,000GW in America’s southwest alone. In terms of carbon reduction, the solar-producing regions of the world could collectively save over 2.1 billion tons of CO2 in 2050, according to a 2009 joint report by Greenpeace, Solar Paces, and the European Solar Thermal Electricity Association. The rapidly growing solar industry has sought out the perfect stomping grounds for large-scale power generation, and is moving swiftly to occupy them. The hot, flat, and sunny regions of the world are prime real estate for companies seeking to make solar the top player in the low-carbon energy boom. But these regions all come up short in one major criteria – water. Solar thermal power plants are water intensive. They use the heat of the sun to boil water into steam to run a turbine, and often require high volumes of water in their operation for cooling and facility maintenance. This is particularly problematic because the prime locations for solar power happen to be found in the desert regions of the planet. In Nevada, for example, proposed plans for two solar generators would deplete 1.3 billion gallons of water annually – equal to 20 per cent of the area's available water, according to a recent New York Times article. There are two predominant CSP designs currently being used on a large scale; power-towers, which use many directed mirrors called “heliostats” to focus sunlight onto a single point on a central tower; and parabolic-troughs, curved mirrors that focus sunlight onto a tube running along their length, heating the fluid inside it. The water problem is technology-based. Power towers use wet cooling methods, running large amounts of water through the system for the cooling and condensing of steam. Wet cooling requires about 500 gallons per megawatt hour (MWh). The parabolic-trough system – by far the most affordable and popular CSP technology in the market – requires even more water for cooling and for frequent washing of the mirrors, consuming around 800 gallons/MWh, according to a study by the Global Solar Thermal Energy Council. Collectively, parabolic-trough plants deplete billions of gallons of water per year from locations with already fragile aquifers. Still, the water consumption of CSP is not unprecedented in the energy sector. Coal, nuclear, and natural gas generators are also big water wasters, using about 500 gallons/MWh. With solar, there are proven solutions to the excessive water use that are readily available and awaiting implementation. The caveat: they cost a bit more, and are marginally less efficient. Dry cooling methods, where cool air is forced over pipes to condense the steam back into water, have been proven to eliminate water consumption by over 90 per cent. Increased cost depends on location, but ranges between 2 and 5 per cent higher, with a 3 per cent drop in energy output. For a less-significant loss, hybrid wet/dry cooling stations could eliminate water use between 50 and 85 per cent. In the Mojave desert, dry cooling is making headway. BrightSource Energy, a California-based solar thermal company is using air-cooled power-tower technology to generate power. Dish/engine models use the least amount of water, and have the highest conversion efficiency of all CSP technologies, according to Stirling Energy Systems. A dish/engine plant is made up of numerous parabolic dishes that focus sunlight onto a small engine on a central arm. This method only requires water to wash the mirror surfaces, and consumes a mere 20 gallons/MWh. The main limitation to the development of more water-friendly solar is financial, but cost is largely dependent on the ease in which government waves in water-intensive projects. In California, for example, there is an existing policy that prevents power plants from using drinking water for cooling, but a proposed bill in the California Legislature could soon change that. The bill would allow companies to draw from drinking water aquifers in the cooling process – which would have major implications for the water tables of local communities. Moreover, such depletion would have a negative effect on surrounding ecosystems. A lack of regulation might speed up the development and growth of renewable energy, and speed is important in the battle against cheap, high-carbon technologies, but it also does little to incentivize renewable energy companies to implement the most sustainable practices possible. Without careful steps forward, a renewable technology could lose its sustainable status, and get caught up in races to the bottom line. Solar power’s water problem should not hinder the development and adoption of such a promising and age-old renewable, but it should certainly spark the growth and investment in technologies that offer a solution. In a future where our food supplies and ecosystems will be stressed by climate-induced drought, water is a resource we should do our best not to drain.
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