Environmental Engineering Reference
In-Depth Information
Table 5.2.1
Recommendation of the collector type based on the type of demand.
Demand
Recommendation
Water
High temperature
Use glazed liquid collector. Also, an unglazed collector can be used if
PV/T has to be integrated to a heat-pump.
Low temperature
To meet only summer demand, use unglazed liquid collector. On the
other hand, to meet both summer and winter demands, use glazed
liquid collector; an unglazed collector can be chosen if PV/T has to be
integrated to a heat-pump.
Air
High temperature
Use glazed air collector or unglazed collector. Ventilated PV can be
used as a heat source if PV/T has to be integrated to a heat-pump.
Low temperature
Tomeet only summer demand or for the place receiving high irradiation
in winter, use unglazed air collector or ventilated PV. On the other hand,
to meet both summer and winter demands, use glazed air collector;
an unglazed collector can be a choice if PV/T has to be integrated to
a heat-pump.
ventilation during summer). Hence, the market for PV/T might even be larger than the
market for thermal collectors.
Depending on the application, the required thermal demand can be covered by
choosing appropriate PV/T system. There exist various forms of PV/T system which
depend on the type of PV module as well as its design, type of heat removal fluid
(water/glycol or air) and on the concentration of the incoming radiation.
The existing PV/T designs can be classified as:
(i)
Liquid PV/T collector
(ii)
Air PV/T collector
(iii)
Ventilated PV with heat recovery
(iv)
PV/T concentrator.
Irrespective of the type of collector, the absorber of each PV/T collector is provided
with a glass cover to reduce the thermal losses. If such a cover is present, the collector
is referred to as “glazed'', otherwise as “unglazed''.
Glazed collectors have less thermal losses, and hence they could higher collector
fluid temperatures. For medium to high temperature applications, this significantly
improves the annual thermal yield. However, glazed collectors result in high stagnation
temperatures that may be critical for certain types of PV encapsulant (risk of yellowing
and delamination) resulting in hot spots. In addition, bypass diodes may get overheated
due to the additional insulation. Reflection losses at the glazing further reduce electrical
performance. Increased temperature levels lower the electrical yield.
In summary, whether the collector should be glazed or not, it is important to find
a good balance (illustrated in Table 5.2.1) between the increased thermal yield on one
hand, and the reduction in electrical yield and the issues related to possible degradation
on the other hand.
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