Technical CO₂ heat pump report by Mitsubishi Electric

In the December 2007 edition of its quarterly R&D magazine "ADVANCE", Mitsubishi Electric Corporation (Melco) presents the current status and future trends in heat pump technologies using the natural refrigerant CO₂ (R744). Focusing specifically on scroll and rotary compressors, as well as microchannel heat exchangers, the global R&D report also examines industry and legislative trends influencing the use of energy-efficient heating and cooling solutions. Given that hydrofluorocarbons (HFCs) in air-conditioners, water heaters or refrigerators are increasingly subject to use restrictions in both Asia and the EU, Melco announces to further intensify the development of CO₂ heat pump systems, known under the name of "Eco Cute" in Japan. According to the company report, Mitsubishi's "diahot" model has currently the largest share of the heat pump hot water heater market in Japan due to its high Coefficient of Performance (COP) of 4.9 and a new foam insulation increasing the heat insulation capacity of the tank unit by 15%.

The report lays special emphasis on two R&D activities carried out by Melco under the "New Energy and Industrial Technology Development Organization" (NEDO) project from fiscal year 2005-2007 to promote HFC-free technologies:

--image1--

• Double-stage compressor heat pump for cold regions: The former low COP and water-heating capacity of heat pumps working in cold districts were improved by a mechanism that includes the compression stroke divided into two stages and the refrigerant injection at intermediate pressure.
• Multi-air conditioner for buildings: This R&D project focused on achieving a high performance for air conditioners through the combination of a recovery function for cooling exhaust heat, an energy reutilization function with a power recovery expander and a gas cooler radiation mechanism.

Technical reports

Melco’s R&D report updates on technical challenges in the development of rotary compressors for CO₂, based on an improved conventional R410A refrigerant compressor. Moreover, it elaborates on wear-reducing technologies for single rotary compressors by coating the vane.

In a next article, Mitsubishi presents its prototype of a large-capacity R744 scroll compressor based on a mass-produced scroll compressor using R410A. Since the cooling capacity of CO₂ is higher than that of R410A, the stroke volume of the air-conditioning compressor could be reduced by one third. Mitsubishi is now working on the further improvement of the compressor performance using a quantitative loss analysis based on a simplified test. The last section of the technical report is dedicated to the prototype of a refrigerant-refrigerant microchannel heat exchanger which features a reduced size and improved performance through microchannel tubes joining the low- and high-termperature fluid flow.