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Present state of R&D
R&D on PVT liquid modules started in the 1970's
and, with a dip during the 1980's, continued un-
til today. Most of the research is on thermal mod-
ule efficiency optimization, and to a lesser ex-
tent on systems studies and economical aspects,
while the number of studies on reliability aspects
and optical optimisation is very small. Since
2000, also a small number of life cycle assess-
ment (LCA) studies has been carried out.
PVT liquid collectors
A number of PVT prototypes have been con-
structed, such as a sheet-and-tube PVT collec-
tors (De Vries, 1998), channel type PVT collec-
tors (Zondag et al. 2003), PVT collectors with
plastic absorbers (Sandnes, 2002), thermosyphon
PVT collectors (Agarwal & Garg, 1994; Chow
et al. 2005) and ICS PVT collectors (Krauter,
2004). Glazed and unglazed PVT liquid collec-
tors were compared by Tripanagnostopoulos et
al. (2002). Often, the PVT prototypes were con-
structed by the connection of a commercial PV
laminate to a commercial solar thermal collector
A weak point often was the thermal resistance of
the connection, especially for unglazed collec-
tors (the thermal insulation due to the glazing
makes the collector less sensitive to the heat re-
sistance between the PV and the collector fluid).
Often, the performance of unglazed collectors
was severely degraded due to this internal ther-
mal resistance (Sudhakar and Sharon, 1994; Van
der Ree, 1996).
Glazed PVT collectors can produce high tem-
perature heat directly, and are therefore suitable
for direct heating of tap water. Various systems
studies on glazed PVT systems were carried out
(Zondag, 2001). In addition, a study on the stag-
nation temperature of a glazed PVT collector
was carried out by Zondag et al. (2002), while
LCA studies have been carried out by Frankl et
al. (2000) and Tripanagnostopoulos et al. (2005).
Unglazed PVT collectors have a lower output
temperature and are therefore often combined
with a heat pump in system studies (Ito et al.,
2004; Bakker et al., 2005; Nishikawa et al.,
1993; Gasner and Wen, 1982). However, the de-
pendence of unglazed PVT on the market pen-
etration of the heat pump may cause problems
for the marketing of unglazed PVT.
PVT air collectors
Only a few studies have been carried out into
PVT air collectors, which is probably due to the
limited market share of solar air heating. Per-
formance studies for single channel glazed and
unglazed PVT air collectors were determined
(Tripanagnostopoulos et al., 2002). Also LCA
studies have been carried out on these collec-
tors (Tripanagnostopoulos et al, 2004). Compari-
sons of single channel and dual channel PVT
air collectors were carried out by Sopian (1997)
and Hegazy (2000).
Ventilated PV with heat recovery
In the 1990's, with the coming of PV facades,
also the research into ventilated PV started. The
focus here was on the thermal efficiency of the
facade, ranging from the simple statement of
measured values to sophisticated flow
simulations and the establishment of effective cor-
relation functions for the heat transfer
(Brinkworth, 2000; Sandberg and Mosfegh, 1998;
Bazilian, 2001; Eicker, 2003) to module perform-
3. Present state of PVT market, certification and R&D
Photo: Grammer Solar
Photo: Solon - Bernd Litzenbur
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