Regulating the use of certain refrigerants is part of the fight against global warming. And as the use of refrigerants with high global warming potential (GWP) becomes ever more restricted, companies developing heat-pumping devices must adjust their products. The Japanese companies Mitsubishi Heavy Industries Thermal Systems and MHI Bingshan Refrigeration are well on their way in that process.
Currently, they use R-134a in both centrifugal chillers and heat pumps, which is a high-GWP refrigerant. When moving to low-GWP substances several aspects are important: physical properties, stability, toxicity, flammability, and availability. The choice of refrigerant also depends on the characteristics of the equipment, such as capacity, compressor type and operating temperature. The substituting refrigerant must have the following properties:
- Environmental factors: Non-ozone depleting substance, GWP ≤ 100.
- Physical properties: Cycle efficiency equivalent to that of HFC refrigerants. Design pressure not excessively high.
- Low toxicity, and no or mild flammability.
- Availability: Low-GWP refrigerants have applications other than refrigeration and air-conditioning equipment, so there must be an adequate level of production and cost-effectiveness.
But since the requirements differ between cooling and heating applications, the same refrigerant might not be optimal as substitute in both cases. Therefore, the two cases are considered separately.
For large-capacity systems ranging from 1055 to 17581 kW, refrigerant R-1234ze(E) was selected to replace R-134a, partly since the physical properties are similar. Further, the new substance is mildly flammable and has low toxicity and it can be used both as foam and as an aerosol.For systems with lower capacity, another refrigerant was chosen, R-1233zd(E), with similar characteristics. Its main challenge is the specific gas volume, indicating that more volume is needed to use it in the system. But the companies found ways around this. Firstly, the compressor volume was reduced by designing it for a large gas flow rate. This made it necessary to optimize the design of parts of the equipment, by a computational fluid dynamics analysis. Secondly, with the help of thermo-fluid analysis, certain features of the shell-and-tube type heat exchangers were optimized. That allowed for a large reduction of the evaporator and condenser volumes, compared with the R-134a heat exchanger.
A centrifugal chiller with this component design was manufactured, and a verification test was carried out. This showed a 3 % improvement in performance for a 703-kW system. Moreover, despite a much higher gas specific volume of the refrigerant compared with R-134a, the installation area could be kept at about 105 % of the volume of R-134a throughout the range from 527 to 2461 kW.
Design of heat pump using low-GWP refrigerant
For heating equipment, the low-GWP refrigerant R-1336mzz(Z) is temporarily chosen. The aim is to develop a high temperature system capable of producing pressurized hot water at 200 °C with a COP of 3.5. But such a high temperature is not within reach with R-1336mzz(Z), so an exhaust heat recovery heat pump heating pressurized water to 160 °C was tested first.
An added challenge for heating applications is the lubricant oil. It must maintain its stability at high temperatures and be of the required temperature-dependent solubility in the refrigerant, viscosity, and other factors. Moving towards the 200 °C target, appropriate refrigerants and lubricant oils will be evaluated for their stability, safety and physical properties in parallel.
Ryosuke Suemitsu, Naoya Miyoshi, Yoshie Togano, Hiroyuki Yuki, Yoshinori Shirakata, Yasushi Hasegawa, Mitsubushi Heavy Industries Thermal Systems, Japan
Kenji Ueda, MHI Bingshan Refrigeration (Dalian) Co., Ltd., China