Firebricks have been part of modern technology for at least three thousand years. MIT researchers have found a way to use this old invention to play a key role in switching the world away from fossil fuels to carbon-free energy sources.
The researcher’s idea was to use the excess electricity produced when demand is low, such as from wind farms when strong winds are blowing at night, and use electric resistance heaters to convert electricity into heat. This new device would use the excess electricity to heat up a large mass of firebricks, which will retain heat for long periods of time if they are enclosed in an insulated casing. This heat could be used later for industrial processes or to feed generators that convert it back to electricity when power is needed.
While the technology is old, this potential usefulness is new and brought about by the rapid rise of intermittent renewable energy sources and the ways electricity prices are set. “Technologically, the system could have been developed in the 1920s, but there was no market for it then," says Charles Forsberg, a research scientist in MIT's Department of Nuclear Science and Engineering and lead author of a research paper describing the plan.
Forsberg says that the demand for industrial heat in the U.S. and most industrialized regions is larger than the total demand for electricity. Unlike the electricity demand, which varies and is often unpredictable, the demand for industrial heat is constant and provides an almost limitless market for the heat available from the firebrick system.
The system, named FIRES (Firebrick Resistance-heated Energy Storage), would raise the minimum price of electricity that is on the utility market. The current utility market can unpredictably plunge to almost zero when there is high production, like in the middle of a sunny day at a solar plant.
Electricity prices are decided 24 hours in advance and have a separate price for each one-hour segment of the day. This system is done through an auction between the producer and the distributors of power. The distributors determine how much power they think will be needed during each hour. The suppliers then bid based on their expected cost of producing the power. The prices can be low or high depending on the needs at any given time. At the end of the auctions the distributors figure out how many bids will be needed to meet the predicted demand and the price that will be paid to all of the suppliers is determined by the highest-priced bid of the accepted bids for the hour.
That system could lead to odd outcomes when power that is cheap to produce — like solar, wind and nuclear power — can supply enough to meet the demand. Then the price that the suppliers get for the power could be close to zero.
By diverting the excess output into thermal storage by heating a large amount of firebrick, then selling that heat directly or using it to drive turbines, FIRES could set a lower limit on the market price of electricity, which would lower the price of natural gas. This could help to make more carbon-free power sources more profitable, which would, in turn, encourage their expansion.
The fall of electricity prices is already happening due to the expansion of non-fossil energy, and it will continue to increase as renewable energy installations increase. When the amount of generating capacity provided by solar reaches about 15 percent of the total generating mix, or when wind power reaches about 30 percent of the total, building these installations can become unprofitable unless there is sufficient storage capacity to absorb the excess for later use.
The options for storing excess electricity are essentially limited to batteries or pumped hydroelectric systems. In contrast, the low-tech firebrick thermal storage system could cost anywhere from one-tenth to one-fortieth as much as either option.
The firebrick is a variant of ordinary bricks, made up of clays that are capable of withstanding higher temperatures, up to around 1,600 degrees Celsius or more. They are cheap to produce. High-temperature bricks like these have been found on sites that date back 3,500 years.
By varying the chemical composition of the clay, firebrick can be made with many properties. Bricks that are placed in the center of an assemblage could have high thermal conductivity so they can easily take in heat from the resistance heaters. The bricks could easily give up the heat to cold air being blown through the mass to carry away the heat for industrial use. The bricks used for the outer parts of the structure could have very low thermal conductivity. This creates an insulating shell to help retain the heat of the central stack.
The current limit on FIRES is the resistance heaters. The low-cost, reliable heaters that are currently on the market only reach 850 C. Eventually the bricks themselves could be made electrically conductive so they could act as low-resistance heaters on their own, producing and storing the heat. A promising material for the firebricks is silicon carbide, which is currently produced at massive scales.
Turning that heat block into electricity is a bigger, more technical challenge, so it is likely a next-generation version of the FIRES system. This is due to producing electricity with conventional turbines in natural gas power plants, which operate at a much higher temperature. While industrial process heat is usable at about 800 C, the turbines need compressed air heated to at least 1,600 C. Ordinary resistance heaters don’t reach those temperatures. The systems would also need an enclosing pressure vessel to handle the needed air pressure. The advantage would be great: Doubling the operating temperature would cut the cost in half of the heat produced.
The next step for Forsberg and the team will be to set up full-scale prototype units to prove the principles in real-world conditions. Forsberg believes that this will happen by 2020.
A paper on this research will be published this week in Electricity Journal.