Solar power when the sun goes down

One of the most promising ways to generate renewable energy cleanly, says Sudhakar Neti, a professor of mechanical engineering and mechanics, is to convert sunlight into heat.

By employing reflectors, says Neti, engineers can concentrate the heat from sunlight to many times its power density and use it to heat water, other fluids or molten salts to temperatures exceeding 400 degrees Celsius—and soon,  800 degrees C. That is enough to produce high-energy fluids that are capable of running the turbines of very efficient power plants.

Solar thermal energy power plants are in use or under construction around the world, with the United States, Spain and China producing, or projected to produce, the most megawatts of solar power.

But the technology for storing solar thermal energy, Neti wrote recently in the ASME magazine Mechanical Engineering, does not yet enable solar power to generate electricity nearly as cost-effectively as fossil fuels do.

A coal-, gas- or oil-fired power plant can provide electricity at a steady rate, night or day, rain or shine. Solar thermal energy, by contrast, must be stored in a manner that allows it to retain its heat and generate electricity when the sun isn’t shining.

The magazine article, titled “Canned Heat,” was coauthored by D. Yogi Goswami of the University of South Florida and by Arun Muley and George Roe of Boeing Co. It was published as an ASME (American Society of Mechanical Engineers) Task Force on Thermal Energy Storage effort led by Neti, Goswami and Muley.

The article’s publication came as Neti and a team of Lehigh researchers concluded a four-year research project that demonstrated the effectiveness of using encapsulated phase-change materials (PCMs) to store solar thermal energy at high temperatures. The study was funded by the U.S. Department of Energy (DOE).

Greater heat transfer and energy retrieval

Besides Neti, the Lehigh research group included Profs. Kemal Tuzla and John Chen of chemical engineering, Prof. Wojciech Misiolek of materials science and engineering and Prof. Alparslan Oztekin of mechanical engineering and mechanics.

Students also played key roles in the project. They include James Blaney ’11, now a mechanical engineer with Air Products and Chemicals; Weihuan Zhao ’12G, now with Argonne National Laboratory; Ali F. Elmozughi and Laura Solomon, Ph.D. candidates in mechanical engineering; Ying Zheng, a Ph.D. candidate in chemical engineering; and Joseph C. Sabol, a Ph.D. candidate in materials science and engineering.

While solar energy is currently stored by heating water or salts and cooling them—a method called sensible heat exchange—the Lehigh group attempted to take advantage of the latent heat of materials while their phases change from solid to liquid and back.

Storing solar thermal energy in encapsulated PCMs, Neti and his coauthors wrote in Mechanical Engineering, has advantages over sensible heat storage techniques that are used in existing solar power plants. PCMs encapsulated in balls or pipes can “provide much more surface area for heat transfer and with proper design do not inhibit heat transfer during storage or retrieval of the energy.”

The Lehigh researchers experimented with several types of phase-change and encapsulation materials and built a specialized calorimeter for testing the PCMs while performing numerical analyses of heat transfer modes and phase-change durations.

A variety of melting points

In the Lehigh experiments, the group encapsulated sodium nitrate, a PCM, in a stainless steel container whose dimensions were optimized to achieve desirable heat transfer rates. The group first tested zinc as a PCM but switched after finding that the zinc reacted with the encapsulation materials such as stainless steel. The group also tested two other PCMs—magnesium chloride and sodium chloride and a eutectic mixture of the two. All these PCMs have melting points exceeding 300 degrees C.

“We developed methodologies, predicted numerically what can be done and validated this in the lab,” says Neti. “We showed with a calorimeter and with flow experiments that we could put energy into the encapsulated PCMs, gather it when we needed and be able to predict the amount.”

The researchers filled the capsules with phase-change materials that have different melting temperatures to achieve thermal energy storage around a range of temperatures providing for isothermal energy storage.

“That could allow for significant gains in the amount of usable energy that can be stored,” Neti and his colleagues wrote in Mechanical Engineering. “What’s more, research indicates that, with a PCM aligned with the system operating temperature and using an appropriate encapsulation, costs of storage can … meet [DOE’s] goal for capital costs of an energy storage system.”

The goal, says Neti, is to turn solar energy from an “opportunistic source of electricity” that is available only when the sun is shining into “something that’s dispatchable and reliable.”

“We want to save energy at the highest possible temperature,” says Neti, “so that it has a very high exergy, or available energy—that is, a high ability for us to use the heat, especially for the purpose of driving a turbine.”

The Lehigh researchers presented the results of their research recently at an Engineering Conferences International (ECI) Inc. conference in California. Neti and Trung Van Nguyen of the University of Kansas were conference co-chairs for the event, which was titled “Massive Energy Storage for the Broader Use of Renewable Energy Sources” and was sponsored by the National Science Foundation, MEMC Electronic Materials Inc., the Center for Energy Initiatives and the American Institute of Chemical Engineers.

At the ECI conference, Zhao presented the results of the group’s numerical studies, while Zheng presented the results of the air flow thermocline experiments.

Members of the Lehigh group have presented papers and led discussions in the last two years at ASME meetings in Denver, Houston, Puerto Rico and Washington, D.C., and also at the Innostock Conference in Spain that focused on energy storage and was sponsored by the University of Lleida and the Federation of European Heating, Ventilation and Air-Conditioning Associations.

Articles by group members have been published in Applied Thermal Engineering, Solar Energy, Renewable Energy and other journals.