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Sputter system at the ISFH: The coating of the glass substrate with low-e coatings on a laboratory scale is essentially the same as the process used in the glass industry.
© ISFH
Expert interview
02.07.2010

Opening the window for more efficient flat-plate collectors

In recent years, window manufacturers have improved the thermal insulation in their glazing systems with low-emissivity coatings. With solar glass, on the other hand, special glass types and reflection-reducing coatings optimise the light transmission. Scientists at the Institut für Solarenergieforschung Hameln (ISFH) have now combined these two different principles. Nicole Ehrmann and Sebastian Föste are investigating how flat-plate collectors used for generating solar thermal energy can benefit from low-e coatings.

Ms. Ehrmann, low-e coatings have already been standard in the window sector for the last 15 years. Why has solar thermal research taken until now to discover this feature?

Ehrmann: Whereas windows need to minimise heat losses while ensuring sufficient light transmission, solar collectors require very high transmission for the entire solar radiation. If you were to install standard architectural double-glazing in a solar collector, this would achieve an optical efficiency of around 50%, whereas around 85% is reached with standard flat-plate collectors. In other words there is insufficient transmission of solar radiation, particularly for low-e coatings, which means that we need to develop special coatings.

The insulation glazing systems used in collectors are subjected to considerably greater loads than those used in windows. Significant here are the increased temperatures during operation and in the stagnation phases. For instance, maximum temperatures of 150 °C are reached on the lower low-e coated pane in collectors. This means that the structural design and choice of materials need to allow for aging effects and the thermo-mechanical loads created by the increase in pressure in the glass cavity. Therefore, a focus of our work is on investigating the long-term suitability for use.

How do low-emissivity coatings for collector glazing systems differ from those used for window glazing systems?

Ehrmann: With low-e coatings for window glazing systems, it is essential to achieve as low an emissivity as possible so that the transfer of heat radiation within the window cavity is inhibited to the greatest possible extent. A high emissivity for incident solar radiation is generally only desirable for the visible wavelength range.

When using low-e coatings in collector glazing systems, on the other hand, it is essential to maintain a high energy yield in the collector. The low emissivity requirements are less than for window glazing systems, as the heat loss paths are more complex with collectors. Here an emissivity value of 25% is entirely acceptable, whereas around 5% is usual for windows.

Temperature stability is essential and it is also desirable to have resistance against moisture loads. In order to meet all these requirements we are developing a multiple-coating system whose low-e coating is based on so-called TCO material (transparent conducting oxides; BINE editorial team). At the current stage of development we have achieved a solar transmittance of 85% with an emissivity of 30%.

Mr Föste, tell us about your day-to day research: What problems are you working on at the moment?

Föste: Since the usability of glazing systems is a prerequisite for their use in collectors, this is a focus of my work. We are therefore operating various test rigs in the laboratory and outdoors in order to apply temperature, UV, moisture and thermo-mechanical loads and to measure their effects in further analysis tests.

The heat transfer within the collector is also important for the structural design. In this regard we are investigating fill gases, the optimal spacing and the effects of thermal bridges by means of measurements and simulations.

Are market-ready coating systems already available to apply the transparent conducting oxides deployed by you?

Föste:We are currently only producing the low-e coatings developed by us on a laboratory scale – on a glass substrate that is 10 cm by 10 cm in size. The coating is applied using reactive and non-reactive sputtering. This process is deployed on a large scale in the glass industry. This will enable us to transfer our coating from the laboratory to the industrial scale without having to develop completely new types of systems.

You can already buy double-glazed flat-plate collectors filled with inert gas and with quadruple antireflective coatings. They are highly efficient – but also expensive. How economic are the prototypes developed by you?

Föste: We will have higher costs compared with single-glazed flat-plate collectors, particularly as a result of the second glass pane, the coating and edge seal. However, we expect the costs to be less than that for evacuated tube collectors. On the yield side, we also believe that we will end up between flat-plate and evacuated tube collectors, taking into consideration the desired temperature range above 80 °C.

The actual expected costs can only of course be determined in conjunction with existing production lines. These figures are not yet available.

Industrial entrepreneur or homeowner: For whom are flat-plate collectors with selectively coated high-performance glazing intended?

Föste: In principle for both. We believe there is considerable potential in applications in the process heat sector up to around 120 °C and for solar air-conditioning. When providing solar heating support with a high solar fraction, it is not the high operating temperature that is most significant but the expected yields gains in winter.

Further information

The German Federal Ministry of the Environment is funding the project, entitled ‘Basis for selectively coated high-performance glazing systems for flat-plate collectors’, as part of its Solarthermie2000plus programme. Those interested can obtain further information from the ISFH.

Contact

ISFH - Institut für Solarenergieforschung Hameln
Dipl.-Ing. Sebastian Föste
Am Ohrberg 1
31860 Emmerthal, Germany
Phone: +49 (0)5151-999-506
Email: foeste(at)isfh.de

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