5.1. Solar energy and the nZEB defi

5.1. Solar energy and the nZEB definition
The results presented here do not give an unambiguous
answer to the question of which solar energy system is the
best choice for this type of building. Many factors influence
the answer, for example how the energy balance is calculated,
what the boundary conditions are, and if the goal
is to reach the lowest possible energy use or a lowest value
of delivered energy, primary energy, greenhouse gas emissions,
or other metric.
To reach a zero energy balance, the imported energy
should be balanced by the export of locally generated
renewable energy. Normally, the only form of energy
export that takes place in an nZEB building is that of electricity
to the grid (although e.g. thermal export to a district
heating network could be possible as well). This means that
an electricity source needs to be included in any building
where the aim is a net zero energy balance. In this case,
the choice of auxiliary heat source is also an important
factor, since e.g. the use of a heat pump means that only
electricity is imported and exported. This favors systems
with a high electricity output in the net energy balance
calculations.
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.