With all its virtues, solar energy has one drawback-it is not available all the times. In the language of power companies, it is ‘not dispatchable’ meaning thereby that you cannot rely on solar energy to meet an emergent demand. Solar energy plants are today able to produce huge amounts of electric power when the sun is shining. But this energy is not available for dispatch when the sun is not shining-that is unless we can store solar energy. Thus storage of energy is attracting a lot of research and development effort supported by business and government funding.
Advantages of Molten Salt Storage
Molten salt technology is one of the favorite techniques for both transfer and storage of solar energy. The advantages are:
- The molten salt mixture used is non-toxic and inert.
- Low (lowest?) capital cost among various energy storage systems.
- Cost competitive compared to new fossil fuel power projects.
- Storage is at atmospheric pressure-does not require high pressure tanks.
- Storage is at a temperature which can directly convert water to superheated steam.
- Conversion back to power is almost instant when desired.
- The system is scalable.
- The salt can double as both a storage medium, and as a transport medium from the CSP plant, and to the power generation plant. Two storage tanks suffice.
- The salt does not need replacement or topping up for over thirty years.
- Heat loss is less than 1 degree Celsius per day.
- The salt is usable as a good fertilizer at the ultimate decommissioning of the plant. No pollution.
How does it Work?
The ‘salt’ is a mixture of sodium nitrate and potassium nitrate which are both eco-friendly. Molten salt is circulated through pipes in a heat exchanger during the day providing heat to the salt. It is stored at atmospheric pressure in thermally insulated tanks at night (or when the sun is not present). The storage temperature is 566⁰C (1050⁰F). When supplementary power is required as during a demand peak, the molten salt is allowed to flow from the hot tank to a heat exchanger which heats water to super-heated steam. This steam drives a conventional steam turbine and generator to generate electricity. The cooler salt returns to the cooler tank. We can reclaim energy in this manner whenever we need it during day or night, without regard to the presence of the sun.
Why Not Store in Steam?
If we have to generate steam from the stored energy and then run a turbine, why not store the solar energy in steam directly? The reason is that to store energy in steam we need a much larger volume of steam tank, or compress it to very high pressures. High pressure has its problems of design, cost, and safety. Both make the project unfeasible. On the other hand, molten salt sores energy at atmospheric pressure which is much safer and convenient. At the same time, it needs comparatively a much smaller volume of tanks because no significant volume change is involved. Many companies including SolarReserve are already offering this technology for sale.