Experimental studies on the development of a solar hybrid module with an aluminum microchannel evaporator

In recent years, the possibility to combine
photovoltaics (PV) and solar thermal collectors into one
hybrid module (PVT-module) has been increasingly
investigated. PVT-modules produce thermal and electrical
energy at the same time. As the efficiency of a photovoltaic
module decreases with temperature, the temperature of the
heat transfer media is often limited to about 30 °C and the
PVT-module is combined with a heat pump, which increases
the temperature on the “warm side”.
This paper deals with a PVT-module, which combines a
microchannel based evaporator of a CO2 (R744) heat pump
with a PV panel (PVT-direct). The PVT-direct overall
system is reduced to the refrigerant circuit due to the direct
refrigerant expansion in the PVT-module. A conventional
PVT heat pump system has an additional glycol-water
circuit. Since a pump and an additional heat exchanger for
the secondary cycle were omitted, the system has increased
efficiency. Due to lower module temperatures, the increase
of the photovoltaic efficiency is a further advantage of the
PVT-direct-module.
To prove the feasibility of the PVT-direct heat pump
system, the Cologne Institute for Renewable Energy (CIRE)
is developing and modeling a test facility for this purpose
within the research project “PVT-direkt”. Furthermore, a
functional PVT-direct-module with a microchannel based
evaporator was designed and built. Much importance has
been given to experimental studies under laboratory
conditions in order to investigate (1) the adjustment of the
functionality and layout of the PVT-direct-module for
characterizing the joining of brazed joints in aluminum
microchannel evaporators and (2) the influence of the
backside aluminum plate of the PVT-direct-module
regarding leakage currents and parasitic capacitances. The
overall results obtained in these experimental studies are
analyzed in this paper.