A zero GWP heat pump and distribution system for all-electric heating and cooling in California

Develop and demonstrate an advanced heat pump system that uses ammonia as the primary refrigerant and carbon dioxide is used both as a refrigerant and as a distribution fluid.

Electric Power Research Institute, Inc.


Palo Alto, CA

Recipient Location


Senate District


Assembly District



Amount Spent



Project Status

Project Update

The project team performed preliminary engineering calculations to determine design parameters and constraints and is currently refining the heat pump's design to minimize the number of components while realizing the required performance and cost characteristics. A prototype of the CO2 loop has been constructed and is undergoing preliminary performance testing.

The Issue

Heat pumps in the California climate are ideal substitutes for natural gas space heating. Unfortunately, the heat pumps available today use high global warming potential (GWP) refrigerants whose leakage into the atmosphere over time has the potential to offset any gains made by decarbonizing space heating. Currently, there is no commercially available technology that combines a reversible heat pump with near-zero GWP refrigerant at costs competitive with conventional refrigerants.

Project Innovation

The recipient is developing, testing and demonstrating an advanced heat pump system that will reduce energy demand for multi-family (MF) or small to medium commercial (SMC) applications, with 10 to 20 tons of refrigerating capacity, based on a reversible heat pump that uses ammonia (NH3) and carbon dioxide (CO2) in a unique way: NH3 is the primary refrigerant, while CO2 is used both as a refrigerant and as a distribution fluid, depending on operating mode.

Project Benefits

This project develops a heat pump to provide both heating and cooling that uses ammonia (NH3) as a primary refrigerant to exchange thermal energy between the outside air and the carbon dioxide (CO2) refrigerant in the distribution fluid loop. A heat pump using NH3 offers advantages over other low GWP refrigerants, such as hydrocarbons. NH3 is inexpensive, has superior efficiency properties, low flammability compared to hydrocarbons, and its odor makes it detectable for leaks. NH3 is not for direct use as a distribution fluid in commercial or residential systems and needs a secondary fluid, such as CO2 which has been demonstrated promising efficiency results and lower cost than traditional mixtures as a distribution fluid.. The heat pump system will use the ability of CO2 to transfer more heat per unit mass, thereby reducing piping sizes, installation, and operating costs.

Lower Costs


The advanced heat pump can provide energy savings over conventional heat pump systems. With full market penetration, the heat pump system could result in annual savings of $550M to building occupants. The system has the potential

Environmental & Public Health

Environmental Sustainability

The average annual refrigerant leakage is 10 percent. Since CO2 and NH3 are natural refrigerants with zero or very low GWP, this system has the potential of reducing or eliminating the impact on climate change due to system leaks

Greater Reliability


The advanced heat pump system could reduce the electricity peak cooling load, greatly benefitting a capacity-constrained system, especially in view of recent shutdowns of major transmission corridors due to concerns about fire pr

Increase Safety


Ammonia leaks are easy to detect, providing a self-alarming mechanism due to its pungent odor at levels less than 10 ppm (while R410A is odorless). Due to the high-pressure CO2 distribution loop, ammonia would not leak into the o

Key Project Members

Project Member

Andrea Mammoli



San Diego Gas & Electric Company


Optimized Thermal Systems, Inc.


Match Partners


San Diego Gas &amp


Electric Company


Southern California Edison


Southern Company Services, Inc.


Contact the Team