Column: Diversification of heat-pump-related technologies by Kiyoshi Saito

Diversification of heat-pump-related technologies

To address the global requirements on halving greenhouse gas emissions, major scientific and technological challenges must be faced. In October 2020, Prime Minister of Japan Suga Yoshihide issued a general policy statement announcing Japan’s commitment to become a completely carbon-neutral society by 2050. For this purpose, the industry needs to radically change its business model and strategy. Japan’s government has declared that it will do its utmost to support the private sector in pursuing and implementing bold technological initiatives and innovations.

Energy-saving heat pump technologies have been widely applied in refrigeration, air conditioning, hot water supply, and industrial cold-hot process control systems. The production of heat pump systems is still expanding in Japan. More than 7 million domestic heat pump water heaters using environmentally friendly CO2 as a refrigerant have already been made commercially available. Even in the context of achieving net-zero emissions, heat pump technologies are expected to continue to spread as excellent energy-saving alternatives with positive economic impacts.

Heat pump technologies can also be used in humidity control applications. In summer, in humid countries like Japan and tropical climate regions, dehumidification improves comfort. In winter, when the humidity drops, controlling the humidity may help prevent colds and viral infections. Typically, a desiccant is used to control humidity. Desiccant-based air-conditioning control systems operate through a moisture adsorption and desorption process using solid or liquid desiccants. These systems require the simultaneous removal (for adsorption) and addition (for desorption) of heat to operate. A hybrid humidity control air conditioning system with extremely high performance can be obtained using a single heat pump for the cooling and heating process simultaneously. Additionally, this system can be driven efficiently even when significant ventilation is required. Therefore, it may be useful as a countermeasure for coronavirus infections.

Solid silica gel and liquid lithium chloride are commonly used as desiccants. Liquid desiccants are preferred for highly efficient cycles from a thermodynamics viewpoint. However, lithium chloride is highly corrosive and usually carried in dehumidified air, causing significant corrosion in metal building materials. That’s why liquid desiccant dehumidification systems with lithium chloride have almost disappeared from the market. To tackle this issue, our research group has developed a non-corrosive ionic fluid desiccant, focusing attention on liquid desiccants in air conditioning systems. We also developed a hybrid humidity control air-conditioning system combining the non-corrosive ionic fluid with a centrifugal chiller using water as a refrigerant. The developed hybrid system showed a COP of 7.9. Moreover, we developed a general-purpose energy system analysis simulator, "Energy flow + M". Our results show that annual CO2 reduction exceeded 40% when utilizing the hybrid humidity control air-conditioning system. The system developed in this work is being used for metro stations that have high humidity.

The applications for heat pump technologies are gaining in popularity. Although they are still in the developmental stage, their applications are expected to grow in many sectors.

KIYOSHI SAITO

Professor, School of fundamental science and engineering department of applied mechanics & aerospace engineering Waseda University, Japan