Chinese scientists develop hydrogen-powered CO₂ heat pump with PCM storage

A research group in China has simulated the operation of a heating system combining a hydrogen fuel cell, a CO₂ heat pump and phase-change thermal storage. The study examined how ambient temperature and the fuel cell’s anode and cathode inlet temperatures affect hydrogen consumption, heat storage and system efficiency.

“Until now, the integration of a proton exchange membrane fuel cell (PEMFC)-driven CO₂ heat pump with a phase change material (PCM) tank has not been systematically investigated,” the researchers said. “In addition, the extent to which thermal boundary conditions influence the hydrogen-to-electricity-to-heat conversion pathway has remained insufficiently understood.”

The simulated system comprises 30 PEMFC stacks, each containing 20 cells, which supply electricity directly to the CO₂ heat pump compressor. The compressor adjusts its speed according to the fuel cells’ power output and sends hot CO₂ to a gas cooler, where it heats water circulating through the thermal storage tank at a flow rate of 0.08 kg/s.

The tank contains 100 hexadecane-filled tubes, each measuring 0.4 m in length and 0.05 m in diameter. Hexadecane is a PCM that absorbs heat as it melts at 18 C. After leaving the gas cooler, the CO₂ passes through an expansion valve and then an evaporator, where it extracts heat from either air or water at 19 C. A suction accumulator and internal heat exchanger help prevent liquid CO₂ from entering the compressor, completing the cycle.

The researchers conducted the simulations in MATLAB. They individually varied the ambient temperature around the fuel cell and the anode and cathode inlet temperatures from 2 C to 34 C, while holding the other two parameters at 25 C. The air and water heat sources were maintained at 19 C, while the water and PCM in the storage tank were initially set at 5 C. Each simulation ended when the gas cooler outlet water temperature reached 30 C.

“The results indicated that, compared with ambient temperature and anode inlet temperature, cathode inlet temperature exerted a stronger influence on the system performance indicators,” the researchers said.

They also noted that, as the cathode inlet temperature increased from 2 C to 34 C, the hydrogen-to-heat factor rose by 29.4%, from 28.458 kWh/kg to 36.825 kWh/kg, while the hydrogen-to-electricity factor increased by 60.97%, from 8.825 kWh/kg to 14.206 kWh/kg. Meanwhile, the system coefficient of performance (COP) fell by 19.63%, from 3.225 to 2.592.

For a target daily domestic hot water demand of 75 liters and a temperature increase of 40 C, the researchers estimated that the PCM tank would need to supply about 3.5 kWh of thermal energy. “As a result, the PCM tank proved capable of meeting the hot water demand,” they said.

The findings are presented in the study “Impact of thermal boundary conditions on hydrogen-driven CO₂ heat pump with PCM,” published in Results in Engineering. Scientists from China’s Dongguan University of Technology and Sun Yat-sen University contributed to the research.

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