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1. THERMAL SIMULATION AND EFFICIENCY OF A HERMETICALLY
SEALED FLAT PLATE COLLECTOR WITH A FULLY ADHESIVE
EDGE BOND
Summary
This research work deals with the thermal behaviour of a hermetically sealed flat-plate collector with a gas-
filled cavity between absorber and glazing. Published scientific work in this field is discussed and compared
to own laboratory testing and simulation results. A parameter study for gas-filled collectors with a fully
adhesive edge bond was conducted. Four different types of functional models were built and thoroughly tested
validating the simulation model. Impacts affecting the thermal efficiency such as the absorber deflection of
solar-collectors with a fully adhesive edge bond were analysed and evaluated. A system simulation was
conducted analysing the components temperatures and the fractional energy savings. Deduced by the
conducted research programme design guidelines for such types of collectors are given.
Keywords: solar thermal; absorber; gas-filled; edge bond; solar
1. Introduction
High thermal efficiencies in flat-plate collectors can be achieved by minimising the thermal heat loss. Over the
decades several measures were analysed to lower these heat losses. Since the upcoming of the high selective
absorber coating in the late 1990s the radiative heat loss from the absorber to the glazing has been significantly
minimised. The conductive heat loss of the absorber backside through the insulation to the ambient can be
adjusted by the insulation thickness. In few instances different approaches or materials other than mineral wool
are used to lower the backside losses [1]. However, a major part of the thermal losses is linked to the convective
top loss between absorber and glazing. It is therefore of interest to lower the convective heat transfer in a cost
effective manner and increase the thermal output of the solar-thermal collector.
The results of this paper are based on a thermal analysis of a new type of solar collector with a fully adhesive
edge bond and, thus, a gas tight cavity between absorber and glazing. A collector simulation model was
implemented and used for a parameter study. Based on the simulation studies different series of functional
models with varying parameters such as absorber geometry, gap spacing (distance between absorber and
glazing) or material pairing have been designed and tested. The thermal simulation model was validated by the
results collected in laboratory and outdoor testing. A system simulation was carried out analysing the fractional
energy savings and the temperature loads of the components. Finally, design guidelines are deduced for a gas-
filled flat-plate collector.
2. Methodology
Several approaches were followed to reduce the top heat loss [2]. In most cases some sort of structure was put
between the absorber and glazing dividing the cavity in several layers. [3] investigated the use of a transparent
foil between absorber and glass as a convection barrier. In 2013 the use of a low emissivity coated insulation
glazing unit as transparent cover was analysed [4]. An Israeli company is using a transparent honey comb
structure between absorber and glazing to prevent the convective heat loss. Similar design approaches are
discussed in [5], [6] or [7]. Even evacuated flat-plate collector designs were investigated [8] [9]. Compared to
a conventional collector design these measures are either resulting in a higher maintenance, higher collector
weight, a worsened optical efficiency or higher costs or respectively in a combination of these drawbacks.
However, much higher efficiency levels can be achieved.
In comparison to a conventional vented flat-plate collector a hermetically sealed flat-plate collector comes
along with several advantages. As a result of the adhesive edge bond between absorber and glazing
environmental contaminants such as moisture or dust have no negative effects on the absorber surface. The
adhesive edge bond is impermeable to gas allowing the use of a more suitable medium than air between
absorber and glazing. Beyond that no additional equipment such as a double glazing or spacer bars are needed
to lower the convective heat transfer. [10] [11] analysed a gas-filled cavity in a flat-plate collector in theory.
By simulation an improvement in the overall heat loss coefficient of more than 20 % was deduced. Even though

2. the simulation conclusions are at least in a similar range as the author’s own results the results are not reflecting
the outcomes of the functional models. Unfortunately, their results have not been validated by a functional
model. As a consequence the parameter variation such as the gap spacing was only simulated. However, own
collected data shows a significant deviation from the simulation models resulting in a much higher convective
heat loss than expected.
3. Results
In the course of this research programme the reason to the deviation was analysed. Furthermore, validated
simulation results were processed giving guidelines
designing a hermetically sealed flat-plate collector (Fig. 1).
Following these guidelines the thermal performance of the
last functional model was improved by more than 10 %
compared to the first types.
Different types of sheet pipe absorber were used in the
functional models as well as roll-bond absorbers. It was
concluded that the absorber deflection has a considerable
effect on the convective heat loss for low gap widths. The
results for small gap widths are contrary to the ones given in
the literature or simulation. The main reason to this is the
absorber deflection which is in turn affected by its
parameters such as the piping (harp, meander) or the initial
absorber shape.
As new materials – especially adhesives – are used
in this type of collector the long-term stability is of
interest. Throughout the system simulation and
during the outdoor testing insights concerning the
material stability were found. The occurring
temperature loads on the absorber and, thus, on the
adhesive were measured and simulated (Fig. 2).
These results can be used to modify the adhesive
which is usually found in insulation glazing units.
4. References
[1] Beikircher, T., 2013. Rückseitige Foliendämmung für Flachkollektoren. 22. Symposium Thermische Solarenergie
2013, Bad Staffelstein
[2] Platzer, W., 1988. Solare Transmission und Wärmetransportmechanismen bei transparenten Wärmedämmmaterialien,
Freiburg: Dissertation Universtät Freiburg.
[3] Beikircher, T., 2010. Hocheffizienter Flachkollektor mit Foliendämmung und Überhitzungsschutz für
Betriebstemperaturen von 70-100 °C. München: Abschlussbericht GME (FKZ: 0329280A)
[4] Foeste, S., 2013. Flachkollektor mit selektiv beschichteter Zweischeibenverglasung, Emmerthal: Dissertation
Universität Hannover
[5] Symons, G., 1984. Calculation of the transmittance-absorptance product of flat plate collectors with convection
suppression devices. Solar Energy, Band 33, pp. 637-640.
[6] Rommel, M. & Wagner, A., 1992. Application of transparent insulation materials in improved flat plate collectors and
integrated collector storages. Solar Energy, Band 49, pp. 371-380.
[7] Svendsen, S., 1989. Solar Collector Based on Monolithic Silica Aerogel. Proceedings ISES Solar World Congress.
[8] Benz, N., Beikircher , T. & Aghazedeh, B., 1996. Gas heat conduction in evacuated tube solar collector. Solar Energy,
Band 58, pp. 213-217.
[9] Buttinger, F., Beikircher, T., Pröll, M. & Schölkopf, W., 2010. Development of a new flat stationary evacuated CPC-
Collector for process heat applications. Solar Energy, Band 84, pp. 1166-1174.
[10] Vestlund, J., Rönnelid, M. & Dalenbäck, J.-O., 2009. Thermal performance of gas-filled flat plate solar collectors.
Solar Energy , Band 83, pp. 896-904.
[11] Vestlund, J., Rönnelid, M. & Dalenbäck, J.-O., 2012. Thermal and mechanical performance of sealed, gas-filled flat
plate solar collectors. Solar Energy, Band 86, pp. 13-25.
Fig. 1: Collector efficiency in dependence of the
gap width for a gas-filled and a vented solar
collector
Fig. 2: Simulation of the temperature loads on the
absorber of a gas-filled collector during dry stagnation
(12 months)