It is not uncomplicated to design s

It is not uncomplicated to design systems with different
technologies in a way that makes a reasonable comparison
possible. The systems for building A, A* and B2 were
dimensioned to get full solar thermal coverage during the
summer. All systems could instead have been dimensioned
to maximize Enet. However, the result of such an optimization
is that the thermal part of the system (solar collectors
or covered PV/T) should be as small as possible, that is, a
system of only PV modules. The exception is the system
with uncovered PV/T modules, where the thermal output
is an addition that is not at the sacrifice of the electric output.
The results from this study appear to support the idea
that it is more efficient to use PV in combination with heat
pumps for residential buildings, than it is to use solar thermal
collectors. If the ambition is only to reach a net zero
energy balance, the best choice would according to the
results presented here be a high efficiency PV system (building
C*), even though the thermal solar fraction is zero.
However, the building that is second closest to reaching
the nZEB criterion is the one with a combination of a
state-of-the-art PV system and solar thermal collectors
(building A*), which has the highest solar thermal fraction.

5.2. Energy performance of PV/T systems
The building with only uncovered PV/T modules (building
B1) is the third closest to reaching a net zero energy balance.
The thermal solar fraction for this building is only
9%, and the output from the PV/T modules during the
summer only reaches moderate temperatures. This means
that auxiliary heating is necessary for this building also
during the summer, which is not the case for the systems
with solar thermal collectors
The two PV/T systems analyzed here were based on the
system design of building A, where the solar modules and
the heat pump are connected in parallel, and the performance
could no doubt be improved with further system
adjustments. The seasonal performance factor (SPF) of
the heat pump is a sensitivity factor in the analysis. The
SPF of the heat pump is around 2.3 for the systems with
high solar thermal fraction (A, A* and B2), and 2.5 for
the other systems. The SPF is increased since the heat
pump is also operational during the summer, when the outdoor
temperatures are higher. The same heat pump was
used in all the systems and was dimensioned for the winter
load, when outdoor temperatures are colder. In particular,
the uncovered PV/T system would work better if it was
used for preheating, since the temperatures in the collectors
are not high enough, even during summer, to provide heating
of domestic hot water. In addition to the parallel connection
used here, Ille et al. (2014) analyzed systems with
PV/T modules and heat pumps in both series and parallel
connection. Their studies also included a system with the
option of feeding a borehole heat exchanger. With these
configurations, the temperature at which the collector
loop starts could be lowered and the operation times of
the PV/T modules extended. In such a system it may be
possible to utilize the very low temperature output during
winter to preheat the input to the heat pump. More
research is needed to analyze the usefulness of these systems
for Northern European conditions.
As Fig. 8 shows, there is a large difference in the temperature
of the thermal output from the uncovered and the
covered PV/T modules. The thermal energy output of the
covered PV/T system, as applied in building B2, is similar
to that of the solar thermal collectors, although a larger
area of PV/T modules are required to reach this temperature.
Even though the whole available area was used for
the installation of uncovered PV/T modules in building
B1, temperatures high enough for direct use for DHW
preparation were seldom reached The average operational
temperature during summer was 40 C, compared to
around 66 C for the other module types. Comparing the
energy yield and net energy demand of building A and
building B2 (Fig. 6 and Table 4), there seems to be a benefit
from replacing solar thermal collectors with covered PV/T
modules.