Session: 30-08 Heat Pumps

Paper Number: 129203

129203 - High Temperature Industrial-Scale CO2 Heat Pumps: Thermodynamic Analysis and Pilot-Scale Testing 

Today, industrial process heating above 150°C is primarily accomplished by burning fossil fuels. As the world works to reduce carbon emissions from fossil fuel combustion, decarbonization of industrial heating becomes imperative. At lower process temperatures, heat pumps provide an important mechanism to electrify process heating through traditional vapor compression cycles, and CO₂ transcritical heat pump cycles. However, higher temperatures are difficult to achieve with these technologies today. Many refrigerants used in vapor compression heat pump cycles today are limited to lower temperatures due to their thermal stability, and some have environmental risks due to ozone depletion, global warming potential, or long-term health risks due to their extreme persistence in the environment.

Some natural refrigerants, such as propane, butane and ammonia have flammability and/or health risks. Carbon dioxide is a non-flammable, low health risk refrigerant that has a long history in transcritical heat pump applications at small scales. It is thermally stable to very high temperature, and oil-free compressors at large scale are available, eliminating the thermal stability limitation of entrained lubrication oil. Thus CO₂-based heat pumps using turbomachinery can theoretically achieve high temperature operation, opening the potential to decarbonize industrial processes at temperatures exceeding today’s practical limits.

However, the low critical temperature of CO₂ (31°C) creates difficulties in achieving high levels of performance as the fluid thermophysical properties vary significantly in the temperature range over which high temperature heat pumps need to transfer heat into the heat sink. By using some of the concepts and techniques developed during sCO₂ power cycle development, novel CO₂-based heat pump cycles have been developed that take advantage of the variable specific heat capacity of CO₂ during the heat transfer process to the heat sink. Two classifications of heat sinks are considered. In the first, heat is transferred to a sensible enthalpy material, such as air or heat transfer fluid, in which the specific heat capacity of the heat sink material is essentially constant. The second heat sink is a constant-temperature phase change material, such as water/steam. The two heat sinks require substantially different, yet related heat pump cycle architecture to match well with the CO₂ characteristics over the relevant temperature and pressure range. Small-scale laboratory systems that simulate these cycles have been developed and tested.

Presenting Author: Timothy Held Echogen Power Systems

Presenting Author Biography: Dr. Timothy Held is the Chief Technology Officer at Echogen Power Systems in Akron, Ohio, where he is responsible for development, maturation, and transition to product of supercritical CO2 power cycles and energy storage systems. Prior to joining Echogen in 2008, Dr. Held was with GE Aviation for 13 years, where he held leadership positions in several combustor design and fuels technology teams. He received a BSAAE from Purdue University in 1987, and a Ph.D. in Mechanical and Aerospace Engineering from Princeton University in 1993. He has published several technical journal articles and book chapters, and holds 49 U.S. patents.

Authors:

Timothy Held Echogen Power Systems
Jason Miller Echogen Power Systems
Jason Mallinak Echogen Power Systems
Luke Magyar Echogen Power Systems