Utilisation of Ambient Air and Shallow Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 389

means of a throttling device to reabsorb the working medium. Utilisable heat is
thus generated in the absorber and the condenser.
The purity of a working medium after entering the heat pump circuit is impor-
tant for an efficient heat pump operation. It depends on the difference between the
boiling temperatures of the working pair. If a salt and a fluid (e.g. water/lithium-
bromide) are used, there is a big gap and the working medium water is available
in very pure condition. If ammonia and water are used, ammonia takes the role of
the working medium as it has a lower boiling point. Further components are inte-
grated here in order to ensure a high level of purity of the working medium in
spite of the small difference in the boiling points.
All in all, the absorption heat pump also absorbs heat at a low temperature level
(e.g. ambient air or shallow geothermal energy) in the evaporator. Drive energy
has to be used in the desorber and the solvent pump. The main input of drive en-
ergy in the absorber is done in the form of heat (i.e. "thermal compressor"). The
energy input to drive the solvent pump for pumping and the pressure increase of
the enriched liquid solvent is comparatively low.

Parameters. According to the first law of thermodynamics, the energy balance of

a compression heat pump is drawn in Equation (9.2). Q . Evap. describes the heat

flow to the evaporator, P Drive the compressor drive power and Q . Cond. the heat flow
delivered by the condenser.

& &

Q QP + =

(9.2) Evap . Drive Cond .

The efficiency of a heat pump can be quantified by a parameter similar to the
efficiency or the utilisation coefficient of other appliances. The efficiency or the
utilisation coefficients are generally defined as the ratio of "output" to "input".
Thus it is always below one.
This definition raises the issue of the level of "input" for heating the evaporator
of the heat pump. This is carried out from near-surface ground or ambient air in
this case. Amounts of heat that would largely be unutilised otherwise and are now
used by the heat pump are the "input". Therefore they are not considered when
calculating the energy parameter - like it is normally done for a system that exclu-
sively uses fossil fuels.
Thus the resulting parameters can be above "one" - as not the entire energy
utilised by the heat pump is balanced - (comparable to the "efficiency" or the
"utilisation coefficient"). For that reason special parameters are defined to de-
scribe the efficiency or the utilisation rate of a heat pump (i.e. the coefficient of
performance (COP), the (seasonal) performance factor (SPF) and the heating
rate). Additionally, as the reciprocal value of the COP and the performance factor
respectively, the input rate or the annual input rate (Table 9.1) are analysed. As
some of the terms are only used in German speaking countries, the German terms
are given in brackets. In the following only the two parameters COP and SPF used