As
mentioned previously, the COP can be determined based on the
average outlet fluid temperature from the double pipe heat
exchanger. Thus, each heat injection scenario has an associated
heat extraction rate that will result in an overall identical average
outlet fluid temperature (i.e. constant COP). Maintaining a constant
COP will ensure that the ratio between the total heat provided by
the heat pump and the total electricity consumed by the heat pump
remains fixed. The constant COP scenarios summarized in Table 5,
allow for a deeper understanding of the effects of injecting solar
thermal energy because the geothermal resource is exploited very
similarly for each scenario. Naturally, the more thermal energy that
is injected into a geothermal resource, the more geothermal energy
should be available to be sustainably extracted. Table 5 demonstrates
that higher rates of heat injection during the summer
phases correspond to significantly improved heat extraction during
the winter phases. Moreover, it can be seen in Table 5 that the increase
in total heat provided begins to taper off above 1200 W of
solar thermal injection. By storing solar energy during summer
periods, the operational savings associated with the substitution of
natural gas/electricity has increased by $181.69/$2063.88 over a 15
year period, which is equivalent to a savings increase of 20.9% for
natural gas and 21.0% for electricity. Note that this is for a single
borehole system, and the savings for a network of vertical bores will
be considerable. This shows that storing solar heat into the ground
will provide the opportunity to increase the capacity of a
geothermal bore without increasing its depth, resulting in
increased savings.