Chemisorption Heat Pump with High Energy Density Thermal Storage for Space and Water Heating
Space and water heating account for approximately half of the energy consumption in buildings in cold climates, contributing more than 4,000 MtCO2 of direct and indirect emissions. Electric heat pumps offer high-efficiency heating solutions; however, low ambient conditions present significant challenges to their adoption. Thermally driven adsorption heat pumps provide an energy-efficient, low global warming potential, and robust alternative for such operating conditions. This paper presents a proof-of-concept prototype reactor using magnesium chloride (MgCl2) and graphite for chemisorption heat pumps. Laboratory experiments show a composite energy density of ~1000 kJ/kg with a heat delivery temperature range of 45-70°C, making it suitable for space and water heating applications. The experimental facility can also be used to evaluate other materials. A four-bed configuration of a compression-assisted resorption heat pump is presented and analyzed using a thermodynamic cycle model. The system utilizes chemisorption reactions of metal halide salts (both low- and high-temperature salts) and ammonia, and the compressor varies the pressure, enabling the utilization of low-grade heat sources for system operation. The developed quasi-steady thermodynamic cycle model of the complete system allows the screening of different materials based on their overall system performance, using the coefficient of performance (COP) for cooling and heating, specific cooling power (SCP), and heating power (SHP), as well as the compression ratio (rp), as key parameters. Among the different materials evaluated using the model, the salt pair of ZnCl2- SrCl2 yields the lowest compressor power and the highest COP of ~0.8, but it has lower specific cooling and heating power. Finally, the electric COP of the proposed system is compared with a standard vapor compression heat pump to assess operational energy savings and economic viability. Ongoing work involves in-depth screening of different salt pairs for realistic operating conditions and development of a lab-scale system prototype.