Utilisation of Ambient Air and Shallow Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 392

individual system elements are described and discussed in the following - to-
gether with resulting overall systems.

9.2.1 Heat source systems for ambient air utilisation

Air is generally available nearly everywhere as a heat source. It can supply a wide
range of required heat at very varying temperatures. In order to achieve the opti-
mum design, the seasonal and the daily course of ambient temperature and, if pos-
sible, also humidity that can supply latent heat through condensation, are required.
The utilisation of the heat source "ambient air", however, causes some specific
problems /9-2/.
− The low specific density (the water density to air density ratio is 1,000; i.e. wa-
ter density is 1,000 times higher than air density) and the specific heat capacity,
which is smaller by a factor of 4, requires large volumetric displacements and
thus large machines. If dimensioned too small, acoustic problems could occur.
Additionally, a suitable auxiliary system is required for the fans.
− Strongly fluctuating ambient temperatures during the heating season - very low
and very high temperatures occur very rarely and the ambient temperature is
likely to be in the average range of -3 to +11 °C in most countries e.g. in Eur-
ope - require a correspondingly high equipment input.
− In the case of heating buildings as the main form of utilisation, the substantial
difference between the heating capacity of the heat pump influenced by the
ambient temperature and the heating requirement of a building plays an addi-
tional role. The lower the ambient temperature, the higher the heating require-
ment of the house. Simultaneously, the temperature difference between the heat
source and the heat sink (high inlet temperature to the heat utilisation system
due to a high level of heating requirement of the house) increases. A higher
temperature difference leads to a lower heating capacity and a lower COP of
the heat pump (Fig. 9.5).

Heating capacity
of heat pump

Heat demand of
buildings

Fig. 9.5 Divergence between the heating capacity of heat pumps designed for ambient air
usage and the heating requirements of a building (see /9-2/)

Ambient temperature