The main objective of this work is to develop a commercially feasible R744 HP system for the multi-family building sector for cold climates. The motivation behind this effort is the urgent need to bring into the markets, an efficient, low global warming potential air source heat pump system for cold-climates. It will aid in electrifying space heating, an essential strategy to decarbonize our society. We plan to address the efficiency of space heating for colder climates like the Northeast while being affordable and attractive to customers with existing gas or oil-powered systems. Our strategy aims to focus on optimizing the system using transcritical R744, this primarily means a revamp of various components, particularly the gas cooler. Gas coolers have been a challenge in terms of optimization with lower COPs than that of the more commonly used refrigerants. We have conducted an assessment of the viability of a staggered tubes design which allows for more efficient heat transfer between the R744 and air. This is quite important as the refrigerant operates at a much higher temperature (about 220–250°F) than seen in hydrofluorocarbon systems. Lowering the outlet temperature on the gas cooler side is a major step in providing a suitable design to be used in both residential and commercial buildings.

A Blackbox model has been designed, which allows for the simulation of different design options for optimization of the gas cooler design. A 3.5-ton capacity gas cooler with a COP above 2.5 has been designed to be built for lab testing conditions under simulated outdoor winter conditions.

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