Utilisation of Ambient Air and Shallow Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 391

The temperature difference between the heat source and the heating system (i.e.
heat utilisation system) has a considerable influence on the efficiency rate. Addi-
tionally, the refrigerant and the design of the heat pump play a role. With an in-
creasing temperature difference between the heat source and the heat utilisation
system, the efficiency rate of the heat pump decreases. This can also be derived
from the lg p-h -diagram (pressure-enthalpy-diagram) in Fig. 9.2. If the tempera-
ture difference between the evaporator and the condenser increases, the pressure
difference between those two points increases at the same time. Thus the com-
pressor has to overcome a higher pressure ratio and therefore has to contribute a
higher enthalpy difference or specific heat input. The enthalpy difference in the
condenser, however, remains almost the same if the pressure is increased. In order
to achieve a high COP, the temperature of the heat source ought to be as high as
possible and the flow rate in the heat utilisation system as low as possible.
Work rate. The efficiency of electrical heat pumps over a longer period of time is
described with the work rate. Here the discharged useful heat is compared to the
input of drive work. In addition to the drive work of the compressor, the energy
consumption of auxiliary components belonging to the heat pump (e.g. pumps)
plus losses through unsteady operation are considered. This enables the descrip-
tion of the system efficiency for a defined period of time (e.g. with the annual
work rate or seasonal performance factor (SPF) over the course of one year).
Whereas the COP is determined under given operational conditions (tempera-
tures), these conditions are defined by the practical operation within the heating
system. The work rate (mostly the annual work rate or seasonal performance fac-
tor) is thus more meaningful to describe the efficiency of heat pump systems.
Heat rate. For absorption heat pumps and heat pumps driven by combustion mo-
tors using natural gas, propane or diesel as their drive energy, the heat rate is used
instead of the efficiency rate. Instead of the annual work rate the annual heat rate
or SPF is provided. For the latter the utilisable energy is compared to the energy
content of the fossil energy carriers over a defined period of time (mostly one
year).
Considering the primary energy efficiency of electricity generation and distri-
bution, the annual work rate of electrically driven heat pumps can be compared
with the annual heat rate. For the English speaking countries heat rate and annual
heat rate are defined as COP and SPF.
9.2 Technical description
Systems using ambient air or shallow geothermal energy as heat source - not tak-
ing into account the heat distribution system (i.e. heat sink) in the building that is
similar for many common energy supply systems and will be not analysed further
- consist of the two main components: heat source system and heat pump. These