Utilisation of Ambient Air and Shallow Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 406

Such heat sources are called concrete collectors, air wells or air registers. As air
has a very low heat capacity, comparatively large amounts of air have to be
moved. Lately, preheating and pre-cooling of intake air in tubes in the ground
(without heat pumps) have gained significance for the ventilation of buildings
with a low-energy and passive-energy standard.
Table 9.5 Designs and configurations of tubes for preheating of air in the ground
Types of design
Concrete tubes (can absorb humidity), PVC-tubes (low pressure decrease)
Tubes free in the ground, tubes insulated at the top, tubes flat underneath the foundation
Single tubes or tube registers
Types of operation
Fresh air is always transported through the tubes
Fresh air is only transported through the tubes if the outflow temperature is above the
ambient air
Fresh air is transported through the tubes every time the outlet temperature is below the
ambient temperature, for evaporators heat sources with a higher temperature are
always used (additional charging of the ground)
9.2.3 Heat pump
A heat pump - like any other technical system - consists of various system ele-
ments. In the following, they are primarily explained further for electrical com-
pression heat pumps, as they have the highest market share (e.g. /9-1/, /9-2/, /9-3/,
/9-12/). It can be differentiated by the heat exchanger utilised in the evaporator
and the condenser, the compressor, the expansion valve, the lubricant as well as
the working medium (refrigerant).
Heat exchangers. Heat exchangers are devices that transfer heat following the
temperature gradient between two or more substances. At the same time they en-
able a change of thermodynamic state of these substances (cooling, heating up,
evaporating, and condensing). For heat pumps, they are mainly used for the heat
transfer between the heat source and the heat pump (i.e. the evaporator) and be-
tween the heat pump and the heat sink (i.e. the condenser).
The size of the heat exchanger and thus the heat transfer surface is primarily
determined by the driving temperature difference (i.e. the gradient) between the
cooled heat source and the evaporation temperature at the evaporator or the con-
densation temperature and the temperature of the heated heat transfer medium in
the case of a condenser.
For a given capacity of the heat exchanger a small temperature difference (gra-
dient) requires a large heat transfer surface. In the opposite case, a large tempera-
ture difference needs a small heat transfer surface. In order to achieve a high heat
pump COP, the mean temperature gradient in the evaporator and the condenser
should be as small as possible. Thus the temperature difference between the heat