Scientists from two Chinese universities have modeled an integrated system in which hydrogen fuel cells power a hydrogen CO₂ heat pump with a phase-change material (PCM) storage unit. The simulation identified cathode inlet temperature as the key efficiency driver, and the hexadecane-filled tank was shown to cover a small household’s full daily hot water demand. The findings were published in the journal Results in Engineering.
Key findings:
- The system combines 30 PEM fuel cell stacks (600 cells total), a CO₂ heat pump, and a PCM tank containing 100 hexadecane-filled tubes.
- Raising the cathode inlet temperature from 2 to 34 °C increases hydrogen-to-heat efficiency by 29.4% and hydrogen-to-electricity efficiency by 60.97%.
- The PCM storage unit delivers 3.5 kWh of thermal energy — enough for 75 liters of hot water per day at a 40 °C temperature rise.
- At the same time, the heat pump’s coefficient of performance (COP) declines by 19.63%.
How the System Works
According to PV Magazine, a team from Dongguan University of Technology and Sun Yat-sen University recreated the installation in MATLAB. Thirty PEM fuel cell stacks of 20 cells each directly drive the heat pump compressor. The working fluid — carbon dioxide in a transcritical cycle — transfers heat to water in a gas cooler at a flow rate of 0.08 kg/s. The heated water then circulates through a tank containing 100 tubes, each 0.4 m long and 0.05 m in diameter, packed with hexadecane. This hydrocarbon melts at 18 °C and absorbs a large quantity of latent heat during the phase transition, releasing it back to the end user on demand.
After the gas cooler, the CO₂ passes through an expansion valve and evaporator, where it extracts heat from air or water at 19 °C. A liquid separator and internal heat exchanger prevent liquid CO₂ from reaching the compressor. The net result: the fuel cells supply electricity to drive the compressor, while the evaporator harvests additional low-grade heat from the ambient source.
What the Calculations Revealed
The authors independently varied the ambient temperature around the fuel cells and the anode and cathode inlet temperatures across a 2–34 °C range, holding the other two parameters at 25 °C. The air and water heat sources were fixed at 19 °C; water and PCM in the tank started at 5 °C, and each simulation run ended when the gas cooler outlet reached 30 °C.
Cathode inlet temperature proved to be the dominant variable. Increasing it from 2 to 34 °C raised hydrogen-to-heat efficiency from 28.46 to 36.83 kWh/kg (+29.4%) and hydrogen-to-electricity efficiency from 8.83 to 14.21 kWh/kg (+60.97%). The heat pump COP simultaneously fell by 19.63% — the paper does not explain the mechanism behind this trade-off.
To meet the daily requirement of 75 liters of hot water with a 40 °C temperature rise, the PCM tank must release roughly 3.5 kWh of heat — a level the modeled system comfortably achieves.
Relevance for Decentralized Energy
This configuration is attractive for two reasons. First, coupling a fuel cell with a CO₂ heat pump and a PCM buffer delivers both electricity and domestic hot water simultaneously — with no combustion inside the building. Second, the approach is well suited to off-grid or decentralized installations where grid independence is a priority.
Similar strategies for converting hydrogen into electricity are already being explored across Asia — for instance, in the UNIST project in South Korea — while useful context on high-efficiency heat utilization comes from research on direct heat-to-electricity conversion. Pairing such technologies with the measures outlined in the building energy efficiency roadmap could substantially reduce dependence on centralized district heating.
Frequently Asked Questions
Why CO₂ instead of conventional refrigerants?
Carbon dioxide as a refrigerant has zero ozone depletion potential and a far lower global warming impact than synthetic HFCs. In a transcritical cycle it efficiently delivers heat to water at high outlet temperatures, making it particularly well suited to domestic hot water applications.
What does hexadecane do in the storage tank?
Hexadecane is a phase-change material: it melts at 18 °C, absorbing a large amount of latent heat, and releases that heat when it solidifies again. This property allows the tank to meet a full day’s demand for 75 liters of hot water at a 40 °C temperature rise.
Sources: PV Magazine
