A Zero GWP Heat Pump and Distribution System for All-electric Heating and Cooling in California

Heat pumps are ideal for decarbonizing space heating in California’s moderate climate. Unlike boilers and furnaces, they are not reliant on fossil fuels, and they offer higher efficiency than electric resistance heaters. However, these systems depend on high global warming potential refrigerants that can be expensive to manufacture and are subject to regulations based on their environmental impact. These conditions present an avenue for deploying energy efficient natural refrigerant-based technologies in the effort to reach emission reduction goals, as well as a potential area for decarbonization through electrification. This project evaluated a first-ofits-kind ammonia-based heat pump with a carbon dioxide distribution system for multi-family or small commercial applications with 10- to 20-ton capacity. The design allows ammonia charge to be minimized while exploiting the attractive carbon dioxide heat transfer characteristics. The design and model efforts using commercially available components showed promising results with efficiency exceeding that of conventional products. However, the laboratory prototype evaluation revealed several technical challenges, which caused unstable system operation and low heating and cooling capacity. Several factors can be responsible for transience and underperformance, including control of an oversized ammonia compressor, compressor and pump speed on both ammonia and carbon dioxide sides, expansion valve positions, indoor unit fan speed, and system refrigerant charge. The combination of these factors made it difficult to ascertain which factor primarily caused the system issues. Even though the project will not be able to fulfill the originally planned field demonstrations, the laboratory evaluation accomplished a significant milestone for the industry by demonstrating supercritical carbon dioxide circulation with a high-pressure carbon dioxide pump for space heating. This demonstration is a substantial development for electrification and decarbonization efforts as the industry transitions to hazardous refrigerants that require secondary loops.