Large prefab SDH collectors: design and yields

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.

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

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	https://www.akotec.eu/
DOMA	Austria	Flex Alu	3.5 to 14.6	single-glazed flat plate	laser welded Alu-copper fins	https://www.domasolar.com/
Gasokol	Austria	gevoSol	2.25 to 12	single-glazed flat plate	full copper strips	https://www.gasokol.at/
Gasokol	Austria	powerSol	5.46 to 13.59	double-glazed flat plate	full copper strips	https://www.gasokol.at/
Greenonetec	Austria	GK2002	5 to 10	single-glazed flat plate		https://www.greenonetec.com/
Greenonetec	Austria	GK3003	8 to 13	double-glazed flat plate	full-plate aluminium-copper	https://www.greenonetec.com/
Ritter XL	Germany	XL 19/49	4.94	u-pipe vacuum tube	aluminium absorber	https://www.ritter-xl-solar.de/
Savosolar	Finland	15 SG	15.96	single-glazed flat plate	Aluminium Multi Port Extrusion Profile	https://savosolar.com/
Savosolar	Finland	15 DG	15.96	double-glazed flat plate	Aluminium Multi Port Extrusion Profile	https://savosolar.com/
Solvis	Germany	Fera	5.61 to 8.4	single-glazed flat plate	full-plate aluminium-copper	https://www.solvis.de/
SST Solar	Austria	ECO	6 to 24	single-glazed flat plate	full copper absorber	https://www.sst-solar.com/
Viessmann	Germany	Vitosol 100-F XL	7.91 to 13.17	single-glazed flat plate	full-plate aluminium-copper	https://www.viessmann.de/
Viessmann	Germany	Vitosol 200-T SPX-5	5.05 to 10.3	heat-pipe vacuum tube	n/a	https://www.viessmann.de/

Overview of manufacturers producing large prefab collectors of different types
Source: manufacturers’ information