Compact, High Efficiency CO2 Heat Pump for Shipboard Cooling

Cooling loads on modern warships are increasingly dominated by electronics and modern weapons systems. In contrast to traditional space cooling loads, these systems operate at higher temperatures to prevent condensation. In many applications it has become more energy efficient to operate shipboard chiller loops at these warmer temperatures, and use point-of-use heat pumps to provide lower temperatures for space cooling and dehumidification. These heat pumps must meet the cooling requirements with low power consumption, must present low hazard to personnel and the environment, and must meet other demanding Navy requirements for size, mass, emitted sound, and limited maintenance. To address this need, Creare proposes a compact, highly-efficient CO2 heat pump powered by a miniature, high-speed turbo-compressor. The heat pump design features a centrifugal compressor that is assisted by a two-phase expander on the same shaft to reduce power consumption, enabling an extremely high coefficient of performance (COP) to achieve solicitation targets. This technology builds upon decades of experience building similar maintenance-free high-speed compressors and expanders for challenging DoD and NASA applications. In Phase I, we proved the feasibility of this approach by: (1) conducting top-level system trades to assess system performance vs. size, mass, and development risk, (2) developing a detailed turbo-compressor design, (3) developing detailed designs for lightweight evaporator, condenser, and recuperative heat exchangers, and (4) assembling a detailed system CAD model to permit accurate estimates of size, mass, gas charge mass, and total power consumption. All values met Phase I solicitation requirements. We also (5) conducted experimental work to demonstrate the value of a two-phase expander to provide an assist to the compressor: we assembled a high-speed expander and evaluated its performance with inlet liquid CO2 at nominal design conditions for the heat pump and used results to validate parallel two-phase CFD models. Lab testing showed stable operation at design conditions, and recovery of more than 200W of shaft power. In Phase II we will design, fabricate, and assemble a full-scale CO2 heat pump; we will then deliver the integrated unit for future shipboard point-of-use cooling demonstration.