The potential benefits of solar energy, says Sudhakar Neti, seem almost as endless as the clear Arizona sky when they are compared with the cost, the pollution and the politics of fossil fuels.
The shining sun radiates about 1,000 watts (1 kW) of energy per square meter of land, says Neti, a professor of mechanical engineering and mechanics. Every hour, the sun diffuses more energy onto the earth than the entire human population uses in a year. And sunlight is not confined by political boundaries or geography.
Harnessing energy from the sun can be cleaner than extracting energy from coal, oil or natural gas, Neti adds, and solar energy is versatile. Solar electric systems use photovoltaic cells to convert sunlight directly into electricity. Solar thermal systems employ panels of mirrors to concentrate sunlight and convert it into heat, which then is used to drive turbines or engines to generate electricity.
Neti and four other Lehigh researchers recently received a $1.5-million grant from the U.S. Department of Energy to tackle one of the biggest obstacles to the wider use of solar thermal technology – its storage.
The three-year award will enable the group to study two materials whose phase changes (from solid to liquid and vice versa) are optimal for the storage and release of energy generated by solar thermal systems.
The multidisciplinary makeup of Neti’s group reflects the variety of challenges posed by solar thermal storage technology.
Wojciech Misiolek, professor of materials science and engineering, is an expert in metals and metal processing. John Chen, professor emeritus of chemical engineering, is renowned for his work in heat transfer. Alparslan Oztekin, associate professor of mechanical engineering and mechanics, is a specialist in numerical calculations, and Kemal Tuzla, professor of practice in chemical engineering, is an expert in the packed heat bed transfer technology that the group will utilize in its storage system.
High expectations
The proponents of solar energy hardly suffer from a lack of enthusiasm. The magazine Scientific American predicted a year ago that solar power could provide the U.S. with 70 percent of its electricity and more than a third of its total energy needs by 2050. Other observers predict a more modest increase in the portion of energy demand that will be met by solar and other forms of renewable energy.
In any case, the future of solar energy will depend in large part on cost and availability, says Neti. And these will require novel heat-transfer methods as well as new materials that enable solar facilities to store energy long enough so power can be generated on cloudy days and at night.
“We do not yet have the means of storing energy to make solar energy viable on a large scale,” says Neti. “Even in places like Arizona where sunshine is abundant, we need storage for the night.”
Two storage technologies now used by solar power plants are the pumping of compressed air into underground caverns and the use of insulated tanks filled with molten salt. But these are not capable of storing solar energy for more than a day.
A high dosage of zinc
Neti’s group believes encapsulated phasechange 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.
“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?
“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.
