Improving heat pump performance by stratified PCM storage

Residential heat pumps are usually combined with thermal energy
storage. Thermal storage decouples demand and supply and ena-
bles the heat pump to work at an efficient operation point constantly.
Standard storage devices are water tanks in which the water is
stratified from cold at the bottom to hot at the top. The stratification
ensures the efficient operation of the heat pump and is realized by
differences in density. However, these storage tanks require large
volumes due to the low volumetric energy densities. Furthermore,
keeping the stratification is difficult due to heat conduction and mix-
ing through water extraction and injection flows.
Phase change materials (PCM) can significantly increase the volu-
metric energy density and improve stratification. Polymers are
promising candidates for use as a PCM since melting temperatures
are higher and can be tailored by the polymer grade. For instance, semicrystalline cross-linked high-density polyethylene
(cHDPE) could enable temperature stratification and thus improve the performance of the entire heating system.

This work investigates the potential of polyethylene spheres with different cross-link densities in thermal storage tanks of
residential heat pump heating systems. The work is simulation-based and includes the following work packages:

• Transient modeling of a stratified water storage tank
• Transient modeling of a storage tank equipped with polyethylene spheres
• Combining a quasi-steady-state heat pump model with the storage models
• Optimization of polyethylene’s melting temperature as a function of position in the tank and in dependence of crosslinking
• Comparison of both storage systems by simulating typical demand profiles