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37
ZEB
annual report 2014
In a net zero energy building, an nZEB, the
energy demand is supplied by renewable
energy sources installed on the building
itself. The use of solar energy is one of
the main strategies employed to reach an
energy balance. Buildings require energy
both in the form of heat and electricity during
operation, which can be provided by solar
thermal collectors and photovoltaic (PV)
modules (solar cells). However, in projects
with ambitious energy targets or limited
available area for installations, the two solar
technologies are sometimes competing for
the available space on the buildings’ roof and
facades.
Heat and electricity from the sun
A photovoltaic-thermal module, or PV/T
module as it is also called, is a combination
of PV module and a solar collector. A PV/T
module therefore generates electricity and
heat simultaneously, in one module.
While PV modules absorb much of the
incoming sunlight, they convert only about
15-20% of it to electricity. The rest of the
energy in the solar radiation is converted to
heat in the module. Since the efficiency of a
PV module decreases with increasing module
temperature (about -0.4%/°C), this is in fact a
double loss.
The idea of a PV/T module is to use air or
a liquid to transfer the excess heat away
from the PV cells. This increases the electric
efficiency of the cells, and makes it possible to
use the heat that would otherwise be wasted.
The combined energy output per area of a
PV/T module can therefore be higher than of
a pure PV or solar thermal module. From an
architectural point of view, a PV/T installation
can also provide a more uniform appearance
than separate installations of PV modules and
solar collectors.
Is a hybrid system better than two
separate ones?
A simulation study was performed to try to
answer this question, that is, to determine
whether an installation of PV/T would give a
higher energy output compared to an equally
sized installation of PV and solar thermal
collectors placed side by side.
The ZEB residential concept, a building model
developed at the ZEB Centre, was used for
the simulations. The building, which has a
very low energy demand, was simulated with
HYBRID SOLAR – BEST OF TWO WORLDS?
|
HYBRIDE SOLFANGERE – DET BESTE FRA TO
VERDENER?
It’s always sunny behind the clouds
Clara Good (NTNU)