Less waste heat from pump systems

A team of researchers from Nagoya University in Japan has developed a loop heat pipe (LHP) that can transport up to 10kW of heat without needing electricity.

This heat transport capability is the largest in the world. The group’s LHP is designed to contribute to energy savings and carbon neutrality in various fields, including industrial waste heat recovery, solar heat utilisation, electric vehicle (EV) thermal management, and data centre cooling. The findings are detailed in the International Journal of Heat and Mass Transfer.

The new LHP surpasses the previous largest loop heat pipe due to enhancements in the evaporator structure. These improvements led to an 18 per cent reduction in size, about a 60 per cent increase in heat transport capability, and a fourfold increase in heat transfer efficiency compared to the previous LHP developed by Nagoya University. LHPs have been used in manned space flights, electric vehicles, meteorological satellites, and home electronic appliances.

“This LHP is unprecedented in transporting such a large amount of heat without electricity, achieving the world’s largest non-electric heat transport,” Professor Hosei Nagano, a senior researcher involved in the project, said. “This eliminates the need for the electricity previously consumed by conventional mechanical pumps, allowing for near-perpetual heat transport without electricity.”

The EV industry is seeing a rising demand for energy-efficient cooling methods because companies are becoming aware of their carbon footprint. LHPs help EVs improve overall efficiency by providing cooling that does not require electricity, reducing the need for electrical power.

Shawn Somers-Neal, a graduate student involved in the project, said that maintaining the inverter temperature is crucial for optimal electric vehicle performance.

“Traditional cooling methods for inverters require energy, but our LHP maintains temperature without electricity. This leads to an increase in efficiency while handling the high heat loads needed in industry.”

In an LHP, a working fluid and a porous material called a wick transport heat efficiently over long distances. The wick draws the working fluid to the surface through capillary action. When heat is applied to the evaporator, the fluid on the wick’s surface absorbs the heat and turns it into vapour. This vapour travels to the condenser, releasing the heat and condensing it back into liquid. The liquid then returns to the compensation chamber, where it contacts the wick again, which draws it back to the surface and continues the cooling cycle.

The group enhanced the LHP wick section by making it thinner, longer and wider while preserving its high-quality porous properties. They also improved heat transport capabilities by narrowing the channels that allow the vapour to escape from the evaporator and adding additional channels on the sides, thereby increasing the total number of channels.

“The uniqueness of the loop heat pipe (LHP) is the shape, quality, and size of the wick, as well as its overall performance. Usually, when making larger wicks, the quality decreases, but the quality of this wick is similar to that of smaller wicks,” Professor Nagano said. “The wick has cores that help reduce the thickness, leading to less pressure drop and lower operating temperatures.”

During testing, the newly developed LHP demonstrated a heat transfer efficiency of more than four times that of existing LHPs. The design was so effective that it transported waste heat over 2.5m without power, using the capillary force generated by the wick. This set a record for non-power heat transport.

“This pioneering LHP technology is expected to revolutionise energy conservation and carbon neutrality across multiple fields, including factory waste heat recovery, solar heat utilisation, electric vehicle heat management, and data centre cooling,” Mr Somers-Neal said. “The effective saving of factory waste heat marks a significant step towards sustainable energy solutions.”

The study, ‘Experimental investigation of a 10kWw-class flat-type loop heat pipe for waste heat recovery,’ was published in the International Journal of Heat and Mass Transfer on 3 July 2024, at DOI:10.1016/j.ijheatmasstransfer.2024.125865.

Image: Nagano Lab, Nagoya University