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Building integration issues
Building integration is applied successfully for
solar thermal collectors, and PVT collectors can
be integrated in a similar way.
Technical issues
Special integrated PVT absorbers are required,
for which the thermal resistance between PV and
collector fluid should be sufficiently small (es-
pecially for unglazed PVT). Leakage or freez-
ing may occur in case of faulty design.
Air PVT collector
The PVT air collectors are similar to a conven-
tional underflow air collector with a PV lami-
nate functioning as the top cover of the air chan-
nel. PVT air collectors have the important ad-
vantage over PVT liquid collectors that conven-
tional PV modules can be used, which reduces
the module costs relative to PVT liquid modules.
However, this benefit on module level may be
compensated by increased costs and lower an-
nual yields on systems level.
PVT air collectors can either be glazed or
unglazed.
Commercial issues
A problem for air collectors is the limited appli-
cation for hot air, especially during the summer
when most heat is available. This is reflected in
the installed area of air collectors, that is about
1,5% of the total installed solar collector area
(see Figure 8). In general, air collectors are
mostly applied if the users have a demand for
hot air, like in air heating systems and drying of
agricultural products.
Air heating systems are mainly designed to di-
rectly use the air for space heating. However,
the opportunity for this application depends di-
rectly on the market share of air heating sys-
tems, which is low in most countries. A niche
market is given by preheating of ventilation air
for large volume buildings (stores, sport halls,
schools, manufacturing halls,...) where tempera-
tures in the range of 15 to 25°C are desirable.
With the very same air systems, hot water prepa-
ration is often possible as well through an air/
water heat exchanger, which is done during the
summer season in order to increase the overall
performance of the system. Nevertheless, it is
difficult for these systems to compete with a liq-
uid collector, because of the cost and the lim-
ited efficiency of an air/water heat exchanger.
Other applications for hot air may be solar cool-
ing and drying processes.
Building integration issues
On the module level, a problem is the high air
volume flow required to obtain a good thermal
efficiency, and the corresponding items of large
diameter tubing, noise and fan losses. The large
tubing required may cause problems, especially
in retrofitting.
Technical issues
The application of air as a heat transport me-
dium has some advantages but also some big
disadvantages in comparison with water. To start
with the advantages:
· No freezing and no boiling of the collector
fluid.
· No damage if leakages occur.
The disadvantages are however rather severe:
· Low heat capacity and low heat conductiv-
ity, which result in a low heat transfer.
· Low density, which results in a high volume
transfer.
· High heat losses through air leakage
2. Overview of modules and systems
Figure 8. Installed solar thermal collector area
wordwide (end 2001) in GWp and m
2
collector area
(data from Weiss (2004), keynote lecture Eurosun
conference)
Photo: Grammer Solar
PVT Roadmap.p65
7/27/2006, 8:50 AM
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