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Energy geostructures 41
The heating and cooling consumption of a building can also be determined by
well-established procedures found in standard heating, ventilation and air-conditioning
references (see, e.g. ASHRAE, 2009). The heating and cooling consumptions of
superstructures are a function of many factors, including the climate, construction
forms and orientations, materials used for the construction, the effects of sun and
shade, ventilation, lights and appliances, people’s presence and activity, and building
use and purpose (Narsilio et al., 2014).
For new constructions some of the previous factors may be designed in such a way
that heating and cooling consumptions are reduced and, if possible, balanced, so that
the ground source heat pump systems become more economical to install and operate
(Narsilio et al., 2014).
2.4.5 The coefficient of performance
The amount of external energy input to be supplied to heat pumps has to be kept as
low as possible to make the heat pump ecologically and economically desirable, the
heat pump efficiency becoming a crucial design parameter. The efficiency of heat
pumps can be characterised using the coefficient of performance, COP, which is a
device parameter that defines how many units of heat can be obtained using one unit
of electricity. The COP is defined as
Energy output after heat pump operation ½kW
COP 5 ð2:1Þ
Energy input for heat pump operation ½kW
The higher the COP, the lower the external energy input compared to the energy
output (e.g. useful heat). For example a COP of 4 means that from one unit of electri-
cal energy and three units of thermal energy (supplied, e.g. by the ground heat
exchanger), four units of usable energy are derived. Usually, geothermal heat pumps
have a COP in the range of 3.5 4. For economic reasons a value of COP $ 4 may
preferably be achieved (Brandl, 2006).
The efficiency of a heat pump is strongly influenced by intrinsic machine features
such as efficiency of internal heat exchangers and thermal losses, as well as by the dif-
ference between extracted and actually used temperature. A high user temperature
(inflow temperature to the heating system of the secondary circuit) and a low extrac-
tion temperature (due to a too low return-flow temperature) in the heat exchanger
(primary circuit) reduces its efficiency. To have a good efficiency the usable tempera-
ture in the building should not exceed 35 C 45 C and the extraction temperature in
the pipes should not fall below 0 C 5 C(Brandl, 2006). Besides, thermal properties
of the ground might vary considerably in freezing conditions and it is recommended
that excessive heat extraction causing ground freezing should be avoided (SIA-D0190,
2005).
