Civil Engineering Reference
In-Depth Information
flow channel's geometry and sizes and (3) suggesting the configuration of the
integrated PV/T panels including covers, PV cells and their connections.
Compared to the air-based system, the water-based system could improve the
electrical efficiency of the PVs and increase the solar heat energy utilisation.
However, the scope for improvement is severely limited due to some inherent
technical difficulties. Firstly, the water-based system remains continuously rising
temperature over the operating period which results in poor heat-removal effec-
tiveness and falling solar efficiency; secondly, additional heating prior to the heat
devices (to achieve required water temperature) would increase the complexity of
the system and reduce its efficiency. Furthermore, the freezing may be a problem
when the system operates at a cold climate region.
Refrigerant-based PV/T
Refrigerant-based PV/T is a recently emerging technology, and research into this
subject showed that the technology could significantly improve the solar utilisation
rate over the air- and water-based systems and therefore has potential to replace
the former two systems in near future. The system usually operates in conjunction
with a heat pump, and its performance is justified by the electrical and thermal
efficiencies of the PV/T modules and the COP of the PV/T heat pump system.
These parameters (efficiencies and COP) vary with the flow rate of the refrigerant,
its preset evaporation and condensation temperature/pressure, flow channels,
geometrical sizes and external climatic conditions. The most favourite configu-
ration and material are the solar cell encapsulation laminated on the front surface
of an aluminium alloy base plate, followed by the serpentine copper coils tightly
positioned in the X-shaped grooves of another aluminium fin plate, together with
insulation and outside frame forming up a unitary evaporator module (Ji et al.
2008
,
2009
). A diagram showing interrelation between solar efficiencies and the
operational parameters
is presented
in Fig.
23
(Zhao
et al.
2011
),
which
is
developed on the basis of the fixed geometrical conditions and PV type.
In overall, a typical refrigerant-based PV/T type can achieve maximum elec-
trical efficiency of around 10 % and thermal efficiency of around 65 % (Zhao
et al.
2011
). Researches related to refrigerant-based PV/T system usually focus on
(1) determining appropriate refrigerant type, flow rate, evaporation/condensation
temperature and pressure, (2) optimising refrigerant flow channel's geometrical
shape and sizes and (3) suggesting the configuration of the integrated PV/T and
heat pump including panel configuration, e.g. PV cells and combination between
PVs and refrigerant channels, and connection between the PV/T panels and the
heat pump.
Compared to the air- and water-based systems, the refrigerant-based system
could significantly improve the electrical efficiency of the PVs and increase the
solar heat energy utilisation. This initiative represents a step forward in Building
Integrated Photovoltaics (BIPV) cooling technology, but its practicality faces
many challenges: the refrigerant piping cycle needs a perfect seal in order to
maintain its higher (positive) or lower (negative) pressures at different sections and
prevent air being sucked into the system during operation, which is very difficult to
