Molten Salt

Molten nitrate salt, though an excellent thermal storage medium, can be a troublesome fluid to deal with because o f its relatively hi gh freezing point (220oC/428°F). To keep the salt molten, a fairly complex heat trace system must b e employed. (Heat tracing is composed of electric wires attached to the outside surface of pipes. Pipes are kept warm by way of resistance heating.) Problems were experienced during the startup of Solar Two due to the improper installation of the heat trace. Though this problem has been addressed and corrected, research is needed to reduce the reliance on heat tracing in the plant. This could be accomplished by one or more of the following options: (1) develop a salt "anti-freeze" to lower the freezing point, (2) identify and/or develop components that can be "cold started' without preapplication of the heat trace, or (3) develop thermal management practices that are less reliant on heat trace. Within the Solar Two project, the third option will be explored. If it is unsuccessful, the other two options should be pursued. Also, valves can be troublesome in molten-salt service. Special packings must be used, oftentimes wit h extended bonnets, and leaks are not uncommon. Furthermore, freezing in the valve or packing can prevent it fro m operating correctly. While today's valve technology is adequate for molten-salt power towers, design improvement s and standardization would reduce risk and ultimately reduce O&M costs.

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