Abstract
In this work three different control strategies for the production of sanitary hot water by means of an
electric heat pump working with CO2 are investigated. The heat pump is a prototype, here modelled in
the vapour-compression software package IMST-ART. By simulating this model, the performance of the
heat pump is correlated to the boundary conditions and is scaled to different sizes, namely 1, 1.5, and 2
times larger than the reference system. After having chosen an application for which the load profile of
sanitary hot water during the year is known, these heat pumps are simulated in a TRNSYS16 model where
the production of sanitary hot water and the consumption are buffered by the presence of a tank. Key
parameter in guaranteeing comfort and good performance of the system is the stratification inside the
storage tank. The size of the tank necessary to keep a certain level of comfort at the user is then determined
through a parametric analysis for each size of the heat pump. The energetic performance is also
evaluated for each system in terms of seasonal performance factor. Then, the results obtained are compared
with a different system where the heat pump is equipped with an inverter and the circulation pump
follows a different control logic. The size of the tank and the seasonal performance factor are therefore
determined in this case too. Moreover, a night&day control logic is compared to these first two options
to have a baseline of comparison in terms of volume of storage needed to guarantee a same level of comfort
and performance. To provide information also on the running costs, a parametric analysis was run varying
the type of control, the heat pump and the tank sizes for different load profiles.
The results show that the size of the heat pump has a significant effect on the comfort of the user,
which usually leads to oversizing of the storage tank when the load profile is unknown. With regard to
this, the results obtained for the alternative control system show a 20% reduction of the volume of the
tank, given a certain level of comfort, and is therefore useful to reduce the size of the storage tank.
