Editor’s note: “Storing That Power” is a seven-part series detailing technologies capable of reserving power obtained from renewable sources. Read each week to learn more about pumped hydroelectric, industrial-scale batteries, flow batteries, flywheels, compressed air energy storage, gravel batteries and molten salt.

In most cases, power storage systems are separate from power generation systems, and it doesn’t matter for storage whether the electricity was generated by wind or solar or some other technology. But molten salt is a different case. Molten salt is a method used for power collection in concentrating solar power systems, but its characteristics mean that it also acts as a storage medium that allows power to continue to be generated by a solar power facility even after the sun has set. Though less of a pure power storage solution than some of the other methods we have discussed in this series, it does allow a renewable power source to be operated in a much more constant fashion, and providing a steady and reliable supply of power to those who need it when they need it is the ultimate goal of all of these systems.

Molten salt absorbs solar energy as it is circulated through the collector troughs or through a focal point on a tower surrounded by a field of mirrors that concentrate the sun’s energy. Because of its high melting point, molten salt has a great deal of capacity to absorb and store heat before the material begins to vaporize.

By keeping the material in insulated storage tanks and maintaining a high temperature throughout the power cycle, molten salt systems do not rely on phase change of the heat transfer material (such as water being turned into steam in systems ranging from coal-burning to nuclear reactors to solar concentrating facilities), although the secondary heat transfer from the molten salt is still used to boil water and run the turbines that generate electricity. This also means that there is very little vapor pressure in the system, which makes it safer and easier to deal with circulating through the system. However, keeping the salt from freezing at night is one of the concerns that these systems have to deal with.

The molten salt is at such a high temperature (above 750 degrees F, or 400 degrees C) that it can readily transfer heat to boil water to generate electricity. And, with the extremely high temperatures that the salt reaches, it contains an incredibly large amount of energy which persists for hours or days within insulated storage tanks.

The salts being used currently in molten salt plants are a mixture of sodium and potassium nitrate. These materials are also widely used as fertilizers, so they are not particularly hazardous should there be a break in the system that causes a spill of the material.

Using high temperature molten salt systems also allows a concentrating solar power plant to operate for longer periods of time which makes it more able to be used as a base load power plant, rather than acting as an intermittent power plant whose production is unpredictable and varied. For power generation purposes, this reliability is important in balancing the needs of power consumers with the available power being generated.

The output of a photovoltaic panel can vary widely depending on whether it is being illuminated directly by the sun, or more diffuse and indirectly under cloudy conditions. But with the concentrating solar power system, variability is less of an issue because the temperature of the molten salt will remain very high even if a cloud passes across the sky for a short while. And even overcast conditions provide a great deal of solar energy which can be readily amplified by the concentrating mirrors in this type of facility.

In addition to the power storage benefits it provides, molten salt also makes a concentrating solar power plant more efficient by being able to absorb more energy than other transfer and collection materials. The oils used in some other facilities begin to break down as temperatures climb above a few hundred degrees, so the temperature of the material needs to be kept in check, and the full energy of the sun is not able to be captured. Molten salt operates at high temperature, and is able to withstand temperatures of over a thousand degrees without turning to vapor, so it can absorb and transfer more of the energy that can be collected. It is also more manageable throughout the power cycle because it does not produce high pressures withing the working medium as other materials do.

Pilot molten salt thermal solar plants have been tested and operated dating back to the 1990s. At present, a 110 megawatt solar thermal plant is under construction in Nevada and is expected to be completed late in 2013.

Main photo: Birds eye view of a molten salt reactor. Credit: Oak Ridge National Laboratory.