Large prefab SDH collectors: design and yields

Large prefab SDH collectors: design and yields

Collector size is key when designing solar thermal systems for use in district heating and industry. Roughly speaking, the larger the individual collectors, the easier it will be to install a system by crane and configure it to provide temperatures outside the usual range. In all, nine companies based in Germany, Austria and Finland have specialised in the manufacture of these prefabricated units with gross areas of 5 m2 to 16 m2, which makes the collectors big enough to meet the needs of customers operating in heat-intensive sectors (see the table at the end of this article). A few also offer roof-integrated systems in a variety of designs. In addition, each collector comes with a Solar Keymark certificate, which can be used to compare their performance.

Photo: Greenonetec

When designing systems that can supply thermal energy to district heating grids and industrial sites, the main focus is on how to cut costs, not how to provide more flexibility. The collectors that make up big solar thermal installations have been optimised to generate the most amount of heat at the highest possible temperatures. They are produced by several companies in Europe, namely German Viessmann, and Solvis, Austrian Greenonetec and Gasokol, and Finnish Savosolar.
Among the collectors offered by Greenonetec, Europe’s leading flat plate collector manufacturer, is a unit that can be mounted horizontally, as on a flat roof, to hide it from view – a favourite among owners of housing estates. In all, Greenonetec offers clients three types of large collectors, including double-glazed units, with outputs ranging from 3.5 kW to 9.5 kW.

Savosolar has a similar product range, offering both single- and double-glazed flat plate collectors. To reduce heat loss, the manufacturer has decided to place all collector connectors inside the insulated collector housing. In addition, it has designed a mounting system that allows two collectors to rest on the same support, reducing the time and expenses needed for their installation, Savosolar said.

Energy yields

Every collector manufactured by the companies discussed in this news article also comes with a Solar Keymark certificate. It includes yearly collector yields for three average collector temperatures for several locations in Europe which are calculated by the testing institutes based on measurement data. The results of these calculations can then serve as a basis for comparing collector performance. The following chart depicts, based on a mean temperature of 50 °C and 75 °C, the estimated specific annual yield the relevant collector could produce per square metre at the Würzburg site in Germany. Of note is that at a temperature of 75 °C, the specific yield of any of the 14 large prefab collectors selected in this scenario is, on average, 29 % lower than if the same unit were operated at 50 °C. If only single-glazed flat plate collector are considered, the average reduction in output is even higher, increasing to as much as 37 %. Anti-reflective coating on one or two sides of the glass cover also impacts the yield of this type of collector significantly, but this feature is not mentioned in the table.

The large, double-glazed flat plate collectors in the group performed well at 50 °C, providing yields in the range of their vacuum tube counterparts. Moreover, Savosolar (15DG) and Greenonetec (GK) actually outperformed the vacuum tube units produced by Akotec (Mega) and Viessmann (Vitosol), losing only to Ritter’s Aqua Plasma unit. But at 75 °C, the situation changes, with vacuum tube collectors offering much higher yields than double-glazed units.

Yield comparison of selected large prefab collectors. Source: Solar Keymark certificates
In-roof and rooftop installation
Another maker of large collectors is German SST Solar. Its focus, however, is on integrating them into buildings. SST offers both in-roof and rooftop units with a width of 2 to 8 metres and a height of 1 to 3 metres. Then there is Doma Solartechnik, a subsidiary of Swiss collector producer Ernst Schweizer. Thanks to its manufacturing facilities in Satteins, Austria, Doma’s range of products includes 25 roof-integrated collectors, additional rooftop units, and a collector specially suited for installation on flat roofs. In addition, the company can make collectors according to customer specifications.
Company Country Collector series Range of gross area per collector panel [m2] Collector type Absorber type Website
Akotec Germany Mega 12.99 heat-pipe vacuum tube full-copper strips
DOMA Austria Flex Alu 3.5 to 14.6 single-glazed flat plate laser welded Alu-copper fins
Gasokol Austria gevoSol 2.25 to 12 single-glazed flat plate full copper strips
Gasokol Austria powerSol 5.46 to 13.59 double-glazed flat plate full copper strips
Greenonetec Austria GK2002 5 to 10 single-glazed flat plate
Greenonetec Austria GK3003 8 to 13 double-glazed flat plate full-plate aluminium-copper
Ritter XL Germany XL 19/49 4.94 u-pipe vacuum tube aluminium absorber
Savosolar Finland 15 SG 15.96 single-glazed flat plate Aluminium Multi Port Extrusion Profile
Savosolar Finland 15 DG 15.96 double-glazed flat plate Aluminium Multi Port Extrusion Profile
Solvis Germany Fera 5.61 to 8.4 single-glazed flat plate full-plate aluminium-copper
SST Solar Austria ECO 6 to 24 single-glazed flat plate full copper absorber
Viessmann Germany Vitosol 100-F XL 7.91 to 13.17 single-glazed flat plate full-plate aluminium-copper
Viessmann Germany Vitosol 200-T SPX-5 5.05 to 10.3 heat-pipe vacuum tube n/a
Overview of manufacturers producing large prefab collectors of different types
Source: manufacturers’ information


Jens Peter Meyer

Jens Peter Meyer is a clean energy journalist and co-author of the German Solar Thermal Year Book 2021.