Storing energy with molten salt
One of the greatest challenges facing solar energy is the storage capacity required to continue generating electricity when it’s cloudy or the sun has set. The ability to accumulate heat in molten mineral salts is proving to be one of the most promising options to overcome this problem.
Systems for storing thermal energy offer concentrated solar a competitive advantage over other renewable energies, since they allow this efficient system to continue producing electricity at night or when it’s cloudy. Amongst the various thermal storage systems that currently exist, the use of molten salt has allowed progress towards more efficient concentrated solar plants.
Parabolic trough technology is currently the most well-known and commonly used system for concentrated solar plants. It uses synthetic oil as a heat-transfer fluid (HTF), which the solar field heats to a maximum temperature of 400ºC. However, these facilities have limitations related to the maximum temperature range that the HTF can work at, and the efficiency of their thermal-electric conversion.
This has resulted in tower technology gaining ground on parabolic troughs in recent years. Tower facilities can use steam as well as inorganic salts – mixtures of sodium nitrate and potassium nitrate – that allow the use of increased temperatures, in the 550ºC range, which offer a higher thermal-electric conversion efficiency.
” Thermal storage using molten salts is key to meeting the network’s energy demands.
With these two technologies, much like any other power plant, the energy produced by the plant must provide enough supply to match the electricity network’s demand. However, using the sun as its energy source, solar energy does not cover the period when demand is at its highest. In order to face up to this fluctuation in demand, the energy supplied by the power plant can be regulated with a thermal storage system.
Systems for storing thermal energy can help to provide stable production, extend the supply periods or increase electricity production with improved solar/electric conversion efficiency.
Selecting the ideal storage system depends on demand, the type of energy used and the location of the plant. The key reason that molten salt is so promising is the capacity that salts such as sodium nitrate or potassium nitrate have to preserve heat at high temperatures. When they heat to over 240 ºC, the salts melt and become liquid, and their temperature can increase to a maximum that will change based on the technology used. Storage temperatures for parabolic trough plants range between 280 and 400 °C, but can be above 550ºC for tower plants.
” Abengoa is at the cutting edge of developing technology for, building and operating concentrated solar plants that use heat-storage systems.
Due to the high level of competition on the market and the growing requirement for concentrated solar plants to incorporate heat storage, continued work is required to reduce costs and increase efficiency. The use of molten salt as a heat transfer fluid does exactly this, making plants more efficient and reducing costs.
Abengoa is at the cutting edge of developing technology for, building and operating concentrated solar plants that use heat-storage systems. For example, the power plant that Abengoa is developing in the Atacama Desert, in Chile, will store enough heat to supply up to 17.5 hours of electricity. This means that energy can be generated over night, using heat stored in salts, as well as during the day when it operates normally.
Abengoa has also implemented this energy storage system for one of the largest concentrated solar plants in the world, the Solana plant, in Arizona (United States) , which has a capacity of 280 MW, and can continue to generate electricity from stored heat energy for up to six hours.