Innovative reheat approach of the TES system during the hotel’s standstill operation utilizing a CO₂ heat pump/chiller unit

The implementation of CO₂ as a refrigerant is considered one of the solutions to improve the overall building’s
energy performance, especially if the heat recovery concept is applied. In this paper, an innovative approach
to compensate for the thermal losses of the in-series stratified thermal energy storage (TES) tanks during
periods with insignificant Domestic Heating Water (DHW) demands is numerically investigated using
Modelica language. The approach adopts a CO₂ heat pump/chiller unit with two water-cooled gas coolers
and an air-cooled gas cooler in the case of cooling only is required. The flow leaving one of the middle tanks
is mixed with the flow leaving the first gas cooler and the mixture is heated up through the second gas cooler
to the heating setpoint and circulated back to the TES system. Results have shown improvements in the
thermal charging efficiency, while the system COP has reached 5.5 compared to 4.4 if the reheating approach
is not adopted. TIL, Modelica Buildings, and Modelica standards libraries were used to develop the model.
Keywords: Thermal Energy Storage, CO₂, Heat Pump, chiller, Modelica