Tikalon Blog by Dev Gualtieri

Tikalon Header

Solar Nevada

June 6, 2011

One problem that affects all solar installations is that they generate only when the sun shines, so this solar energy must be stored to provide power at night and on cloudy days. In a previous article (Solar Salt, July 30, 2010), I described the Archimede solar thermal power plant at Priolo Gargallo, near Syracuse, Sicily , a solar power facility that works by heating a working fluid and then storing the energy in the fluid, itself.

The Archimede plant stores solar energy in the latent heat of fusion of low melting salts . This same technique was used in demonstration solar plants in the US Mojave desert , and in Spain.[1] The Mojave desert facility, called Solar Two could produce 10 MW of power.

Sodium chloride is not a useful salt in this application, since it melts at the relatively high temperature of 801 °C. One very useful salt is (NaNO₃)₅₀(KNO₃)₅₀, a eutectic mixture of sodium nitrate and potassium nitrate , which has the low melting point of 221 °C.[2] The Archimede plant uses the eutectic mixture.[3] Surprisingly, this molten salt is not corrosive to most metals and alloys, including stainless steels and ferrous alloys .[2]

The US is launching its own version of Archimede in Nevada .[4-7] Unlike Archimede, which uses linear parabolic reflectors concentrating solar energy onto pipes at their focus, the US solar plant being built by SolarReserve will use a multiplicity of flat mirrors (heliostats ) to direct solar energy to a central, tower-mounted collector (see figure).

SolarReserve solar collection plant

It's all done with mirrors.

The solar energy collection portion of the Crescent Dunes Solar Energy Project.

Modified image via SolarReserve Web Site .

SolarReserve was founded by scientists from the Rocketdyne division of United Technologies , and it has a worldwide exclusive license to UTC's molten salt power tower and heliostat technologies.[7] There are 17,500 heliostats in the system, and they will heat the salt to 1,050 °F (565 °C). The salt will then go through a heat exchanger to drive a steam -based turbine-generator . Cool, but molten, salt will be returned to the solar energy receiver at 550 °F (285 °C), comfortably above the melting point of the salt.

This solar farm, called the Crescent Dunes Solar Energy Project, will be built about 200 miles northwest of Las Vegas on federal land. The US government is providing a $737 million loan guarantee for the project.[5] This solar power plant is rated at 110-megawatts , which is supposedly enough to power 75,000 homes, but I think this estimate is on the high side. The Archimede plant is rated at just 5-megawatts.

Nevada senator, Harry Reid , states that the Crescent Dunes Project, located near the town of Tonopah, Nevada , will require 600 workers during construction, and it will have 45 employees when operational.[4] The project is designed to supply about 480,000 megawatt-hours annually. According to The New York Times , the average US home uses about 920 kilowatt-hours of electricity each month, or 11 megawatt-hours annually.[5]

Key parts of the solar energy conversion cycle are the tanks for molten salt storage. Only 2% of the energy is lost during storage.[8] The steam to electrical conversion is by the Rankine cycle , which is used also at coal-fired power plants .[8]

References: