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A high dosage of zinc
Neti’s group believes encapsulated phase-change materials (EPCMs) offer a more promising alternative. EPCMs can be designed to have high melting points with constant temperature during a phase change. Materials undergoing phase changes are capable of storing and releasing large quantities of energy as they change from solid to liquid and vice versa. These materials are now used in insulation, diving suits, cooling packs and other applications.
|A packed bed of zinc pellets absorbs solar thermal energy by day and transfers it to a heat exchanger.|
“In a solar thermal plant,” says Neti, “heat-transfer fluid is heated by solar collectors to 400–450 degrees C. This energy needs to be stored. You can store it passively in a large room filled with stones, heating the stones to store energy and reversing that process to get energy out. This has been used to date but with limits: Many good materials do not have sufficient thermal heat capacity, and this necessitates large piles of storage materials.
“We looked for a material that can change phase and thus store more energy. We settled on zinc. It is safe and nontoxic. It has a melting point of 420 degrees C., which is very good for our purposes.”
Neti’s group will conduct experiments on zinc pellets coated with nickel that range in diameter from 5 to 10 millimeters. The use of small spheres will expand the zinc’s total surface area and heat-transfer capabilities. The nickel, with a significantly higher melting point than zinc, will maintain its integrity, acting as a shield while the zinc changes phases, thus preserving the zinc’s optimum heat-transfer qualities.
“The encapsulated zinc pellets could conceivably cycle the changes of phase and store energy indefinitely,” says Misiolek, but a number of questions must first be answered.
“What is the optimum size for the pellets? Which size enables the most uniform heating? What is the optimum ratio of zinc and nickel? What is the best mixing process to use?
|At night, the pellets stop absorbing solar energy but continue to transfer it to a heat exchanger where it is converted to steam.|
“Also, what is the optimum thickness of the nickel? We’re going to be stacking thousands of pellets in the packed bed. We need to calculate the stress imposed on the bottom layer of the pellets so we have to determine how thick the coating should be to guarantee the safety of the process.”
There are other challenges: how to fabricate the zinc pellets cheaply and how best to coat the zinc with nickel.
“The goal is to find the best way of storing energy as it is being generated so it will be available for nighttime use,” says Misiolek, whose former graduate student Suradej Lorcharoensery successfully coated microparticles of iron with nickel several years ago as part of his doctoral dissertation.
After conducting lab experiments in a packed bed reactor, Neti’s group plans to design a full-scale thermal energy storage system that can be interfaced with an existing power plant and tested.
The researchers will also conduct tests on a second phase-change material, a eutectic mixture of magnesium and sodium chlorides. A eutectic substance is an alloy or mixture whose melting point is lower than that of any other combination of the same materials. The researchers plan to house the chloride mixture inside canisters of stainless steel. They are particularly interested in the steel’s ability to withstand high pressures during the heating and energy-storage process.