Utilisation of Ambient Air and Shallow Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 393

Heat can be withdrawn from ambient air in various ways. The most common way
is that ambient air flows directly around the evaporator of the heat pump that
withdraws heat. Generally, the evaporator is built as a finned tube heat exchanger
with bundles of tubes with fins on the air side running in parallel, through which a
refrigerant flows. The air flow rate of heat pump evaporators ought to be between
300 and 500 m 3 /kW of heat source capacity (evaporator capacity). The flow
velocity of the air through the heat exchanger should be below 2 m/s to prevent
the production of too much noise and an excessive use of electricity for the fan, an
integral part of the heat pump that has a negative effect on its COP /9-3/. If the air
at the heat exchanger's surfaces is cooled down to below 0 °C, the humidity con-
denses and settles as frost on the walls of the evaporator. This can occur even at
air entry temperatures below 6 °C. In order to prevent the evaporator from being
"blocked", it has to be defrosted occasionally under such operational conditions.
The related standstill time leads to losses in heating capacity, which result in a de-
cline in the SPF.
If no fan is used, the evaporators are called "silent evaporators". The ambient
air is then only moved through free convection, this leads to a lower heat transfer
coefficient. Because of the lower level of air-side heat transfer, such silent evapo-
rators thus need a larger surface. The advantage, however, is that they operate in
complete silence. In order to implement this form of evaporator, acceptance prob-
lems would probably have to be fought due to the large construction volume. De-
frosting a silent evaporator is also problematic.
Three types of utilisation for ambient air as a heat source can be differentiated
in general.
− Outdoor installation. Fig. 9.6 on the left shows this form of ambient air utilisa-
tion with heat pumps. Accordingly, the heat pump is entirely installed in the
open. The heat is transferred into the house by well-insulated tubes. A general
advantage of installing the heat pump in the open is the noise minimisation in-
side the house. Additionally, this form of installation only requires a small area
within the building. However, it has to be ensured that the heating tubes do not
cool down below 0 °C in order to avoid freezing.
− Split installation. One possible way of preventing the heating tubes from freez-
ing is the split installation. Here the evaporator of the heat pump is installed
outside the house (advantage: noise minimisation in the building) and the re-
maining heat pump is installed inside the house (Fig. 9.6 on the right). Both
parts of the heat pump are linked via the refrigerant tubes. Compared to the
outdoor installation, this form of installation requires more space inside the
building. The part of the heat pump installed on the inside, however, can be
hung on the wall, thus saving space. The split installation can also be used in
old buildings, as there is generally no major installation work required. All it
requires is breaking through some walls to lay the refrigerant tubes. The con-
densate of the air humidity at the evaporator has to be discharged in a way that
prevents icing below the evaporator.