Molten Salt Power Plants: A Cleaner, Better Future

There has recently been much controversy about the use of fossil fuels to produce energy. We need energy to heat our homes, power our industries, provide us with electricity and run our vehicles, but the problem is that 81% of it is still produced through the burning of fossil fuels. This process releases a lot of carbon dioxide, which contributes to the greenhouse effect and global warming. Luckily, over the past ten years engineers have been developing a cleaner way of producing energy with renewable sources via concentrated solar power (CSP) plants that use molten salt. There are already other clean methods on the market like solar, wind and tidal renewable energies, but the reason that this process has an edge over all the others hinges on the fact that it can store energy. Energy gathered via the CSP plants can be used at any time, even in the dark!  We are currently using lithium batteries to store energy in photovoltaic cells, but this method is not as efficient and cost-effective as the molten salt one will soon be.

A great thing about this new process is that it relies on a classic mechanism that has already been in use for over a century - a power plant - and would thus not involve much change in current technological practices. The only difference is that it requires sunlight and molten salt instead of fossil fuels. Here is how these CSP plants with molten salt work and to this effect, I am specifically going to talk about the CSP plant Gemasolar in the province of Seville in Spain, which produces enough electricity to power 25,000 homes.

This particular plant is set up in such a way that there is a tall central tower with a matte black receiver at the top, surrounded by 2,650 heliostats. These conventional mirrors reflect the light from the sun to the receiver at all time; this is quite a difficult feat to accomplish because the heliostats have to work in harmony with each other even though each points in a slightly different angle, in such a way that the receiver heats up from absorbing the concentrated solar thermal energy. Should the set-up be a fraction of a degree off, the system would stop being effective. Sodium and potassium nitrate salts are pumped from the cold tank, where the salt is originally held, into the receiver, where they are heated and reach up to 565 °C (1049 °F). They are then pumped down into the hot salt tank where they are stored in a molten state for later use, or converted directly into electricity via the heat engine for immediate use. In this way, the storage tank acts like a battery, as it stores energy which can later be used, even when the sun is not out. This way, electricity can be produced continuously for 24 hours a day. The thermal energy of the molten salt can be stored for up to a week. This is possible because the boiling point of salt is very high, 1,413 °C , and so we can store it at high temperatures without it evaporating. The properties of salt are also conducive to highly efficient heat exchange and thermal storage.

After the salt reaches the hot molten tank, the technology is exactly the same as with a normal old fashioned power plant. When electricity is needed, the hot molten salt is pumped into a conventional steam-generator to produce superheated steam that will then power a turbine/generator set just like in any coal, oil or nuclear power plant. Another advantage of using salt is that once the hot salt to create steam has been used, the now cooled molten salt is put back into the cold salt storage tank where it can be reused. This way, there are practically no wastes.

Using this technology is currently still more expensive than using traditional photovoltaics, and even though lithium batteries are getting more efficient, the CSP technology is developing faster and is predicted to overtake it by 2030. It is also becoming increasingly affordable. Figure 2 below, shows the power output of each CSP plant and its price per kWh. As some of these are still projects or plants under construction, the values are only approximate, but offer good estimates. On the table, we can clearly see that the power is increasing and that the price is decreasing. The newest project, Copiapo, is fully permitted and shovel ready. In 2019, its predicted year of inauguration, the power plant will deliver 260 megawatts of continuous 24-hour solar power, resulting in over 1,800 gigawatt-hours generated annually. The facility will operate at a capacity factor and availability percentage equal to that of a coal powered plant, while providing a highly competitive price – and with zero emissions! It will power 560,000 homes 24/7. There are other projects that are already in use like the Crescent Dunes in Nevada, which currently powers 75,000 homes, but the price is very steep compared to that of the project in Chile.