Published in January 2013, Low Global Warming Potential (GWP) Refrigerants, Phase II: Defining the Configurations of Residential Air-Conditioning and Heat Pump Systems Using Hydrocarbons, Ammonia, Carbon Dioxide, and HFO-1234yf as Refrigerants and Meeting Previously Defined Safety Requirements, presents key design details of the most promising system configurations for these candidate low GWP refrigerants.
Safety requirements for carbon dioxide
Reviewed in the report are US safety standards published and maintained by engineering societies and organisations, most notably ASHRAE Standard 15 and UL Standards 484 and 1995.
According to the safety requirements provided in ASHRAE 15 and UL 1995, carbon dioxide systems are not limited in charge size or in their design. However, carbon dioxide systems must meet general strength requirements for all refrigerant-containing parts.
Carbon dioxide equipment configuration and key design parameters
AHRI’s proposed selected configuration for carbon dioxide is a 3-ton split-system air-conditioner and heat pump. To maximise efficiency, the proposed system will use a work-recovery expander instead of a thermostatic expansion valve to reduce the losses associated with the throttling expansion used in most conventional systems.
The guide design parameters regarding the sizing of components and relevant safety features are a follows:
- Capacity: 3RT
- System-type: direct split-system with indoor and outdoor units
- Target efficiency: 13 SEER; 7.7 HSPF1. According to AHRI, the low technical readiness of the design options discussed, may require further research and development to establish the viability of the work-recovery expander design.
- Refrigerant charge: 5.5 lbs
- High pressure mitigation: increased tube wall/diameter ratio
- Applicable standards: ASHRAE 15, UL 1995
- Design requirements: Design for high pressures, enhancements to address low efficiency of simple carbon dioxide cycles
- Low-efficiency mitigation: Work-recovery expander with generator
Key components of R744 AC/heat pump system
- Heat exchangers: Microchannel indoor coil, microchannel outdoor coil
- Compressor: Hermetic 3-ton rotary compressor
- Expansion device: Hermetic scroll expander/generator for work-recovery with power electronics and controls for conversion to 60Hz power while maintaining optimized high-side pressure.
- Additional refrigerant cycle features: Reversing valves to reverse refrigerant flow for heat pumping, refrigerant charge compensator, suction line accumulator
AHRI’s Low-GWP Alternative Refrigerants Evaluation Program
AHRI launched the low GWP Alternative Refrigerants Evaluation Program (AREP) to support industry’s research needs raised by the use of high GWP refrigerants. Since its launch the final test reports have been published for compressor calorimeter testing, system drop-in testing, soft-optimized system testing, and heat transfer testing of various refrigerants.