Phase change materials improve power plants

As global freshwater supplies shrink, the rejection of power plant heat using waterless Air Cooled Condensers (ACCs) is becoming increasingly essential. But many challenges remain to make ACCs work without sacrificing plant performance.

A few statistics illustrate the need for ACCs, say Sudhakar Neti, a professor of mechanical engineering and mechanics, and Carlos Romero, who directs Lehigh’s Energy Research Center:

• Of the fresh water used in the U.S., 41 percent is withdrawn by thermal power plants to condense plant steam into water so it can be reused in the boiler.
• According to ARPA-E (the Advanced Research Projects Agency-Energy of the U.S. Department of Energy), more than 1.1 billion people lack access to clean water.
• By 2025, the International Food Policy Research Institute forecasts that shortages in fresh water could cause global food prices to more than double.
• Only 1 percent of U.S. thermal power plants cool steam with direct dry ACCs. ACCs are more expensive than conventional once-through condensers and they cool steam less efficiently, especially in regions with high ambient air temperatures.

Neti and Romero are working with Advanced Cooling Technologies Inc. (ACT) of Lancaster, Pa., to improve the efficiency of ACCs with phase-change materials. PCMs absorb thermal energy as latent heat at a relatively constant temperature as they melt, and release thermal energy as they freeze. Neti showed in a recent DOE study that encapsulated PCMs can effectively store solar thermal energy at high temperatures.

The Lehigh-ACT group will develop a prototype and test several hydrated salts to determine each one’s energy storage capacity, thermal cycling performance and material compatibility.

“Our goal,” says Romero, “is to develop PCMs that take heat from the steam, store that heat during the day and then reject, or release, it at night, when air temperatures are lower.”

The project, supported by ARPA-E, also involves the University of Missouri and Evapco Inc., a manufacturing company specializing in heat transfer applications.

Story by Kurt Pfitzer