The collector label is misleading

03.09.2015
Uwe Trenkner began his career in solar thermal as a project manager at the European Solar Thermal Industry Federation ESTIF in 2002. From 2005 to 2009, he served as secretary general of the association. He now works as a consultant for the solar thermal industry. He is also co-founder of the job platforms greenjobs.de and eejobs.de. (Photo: private)
Uwe Trenkner began his career in solar thermal as a project manager at the European Solar Thermal Industry Federation ESTIF in 2002. From 2005 to 2009, he served as secretary general of the association. He now works as a consultant for the solar thermal industry. He is also co-founder of the job platforms greenjobs.de and eejobs.de. (Photo: private)

The solar thermal industry is debating whether it should introduce its own efficiency label for solar collectors. Uwe Trenkner, who was secretary general of the European Solar Thermal Industry Federation ESTIF from 2005 to 2009, explains in a commentary why he rejects this idea.   

It has become clear to all parties involved that the energy label for heating systems and water heaters, which will become mandatory in September, hardly differentiates between different types of solar collectors: generally speaking, the impact on the overall result is too small. This actually makes sense: If a solar heating system in an old building covers 20 % to 30 % of the required energy, then a 10 % increase in the performance of the collector array only results in a 2 % to 3 % difference for the overall system. This will rarely be sufficient to raise the heating system to the next higher efficiency class.

In order to show customers that there are major differences between collectors, it might well make sense to have an energy label specifically for collectors. This begs the question: What defines a 'better' collector? Which collectors do we want to recommend to the customer? And what would a label need to look like in order to provide the right incentives?

Late last year, Stefan Abrecht presented a proposal for an energy label specifically for solar collectors in issue 12/2014 of Sonne Wind & Wärme. At the Intersolar in Munich, Ritter Solar was the first company to implement this proposal and marked its products with the 'SOLERG' label. However, it is now obvious that the label in its current form is misleading. The solar thermal industry should think twice before following this example. The label clearly provides the wrong incentives. Why is this so? At this point, it is important to ask ourselves what we want to achieve.

Solar yield is a central point

Here is the most important assumption first: Our goal is to promote the heating transition ("Wärmewende"). Anyone who wants the transition in the heating sector needs to reduce the consumption of conventional energy for heating purposes. In the context of solar thermal energy, this means that the largest possible share of conventional energy needs to be replaced by heat from the sun. Our products achieve this by maximising annual solar yields. This means that the label needs to focus on the solar yield.

Collector annual yield compared to efficiency class

The diagram clearly shows that the efficiency classes say little about the annual yield of the collector.  Data source: most recent collector overviews from Sonne Wind & Wärme. The selection of collectors is the same as described in the text.

Further assumptions: In most cases, the planner or installer will use a cost calculation or the desired solar coverage ratio to determine the size of the collector array. Whether three, four or five collectors are installed depends on the planner; it rarely depends on the available surface area. And further: If we want to promote the heating transition, we need to keep heat generation costs in mind. Solar thermal will be used primarily when it can provide heat at a low cost.

Is the main thing really an A++ rating?

Let us examine whether the SOLERG label fulfils these requirements in the form it was presented at the Intersolar: The clearest signal for the customer is given by the bar chart and the energy classes. They tell the customer: A++ collectors are preferable to B collectors. Unfortunately, the classes do not indicate whether a collector provides more or less solar energy. It simply indicates how efficiently it uses the collector area to generate the heat. Here is an example calculation  based on the classification proposed by Stefan Abrecht: The 'CPC 6 XL OEM' collector from Ritter Solar achieves an efficiency of 46.5 % at 50 °C, making it an A++ collector; the 'GK3102 – FL' from GREENoneTEC, on the other hand, only achieves 38.5 %, corresponding to an A+. However, the latter provides 4,813 kWh/a, which is almost six times as much solar heat as the supposedly better collector!

It is true that the GREENoneTEC model has a surface area of 10.06 m2, making it almost seven times as large as the Ritter model. But if we want the label to provide incentives to increase solar yields as much as possible, then the SOLERG label is a complete failure. It is also true that one would 'only' need to install six of the A++ collectors to achieve the same yield (8.7 m2). But do we seriously believe that an additional collector would be installed because of this? It is much more likely that the 1.4 m2 theoretically 'saved' would remain an empty roof area. The sunlight falling on that area would remain unused: The supposed advantage of the more efficient collector would fizzle.

Value for money is not the same as efficiency

Perhaps the classification could still be justified if the collectors with the best ratings also featured the lowest heat generation costs and therefore the best value for money. To find out whether this is the case, I evaluated the latest collector overviews provided by Sonne Wind & Wärme. They include 232 flat and 72 tube collectors. The annual yield from the Solar Keymark data sheet as well as a list price are provided for 58 of them. This allows to calculate their efficiency according to Stefan Abrecht's proposal as well as their heat generation costs. In the latter case, I divided the published list price by the heat yield over a period of 20 years. Only the four Germanstar collectors from EuroStar have been excluded from Diagram 2 because they alternated between being among the most efficient and among the least efficient collectors; this distorted the results significantly. The diagram is of course not an exhaustive overview of the market, but it does include collectors from a wide range of relevant companies and brands, such as Citrin, Junckers, Ritter, Solvis and Viessmann. It therefore provides a good indication of overall distribution.

Heat generation costs compared to efficiency class

The heat generation costs of collectors in the middle efficiency range tend to be the lowest.

On the one hand, Diagram 2 shows a wide spectrum of heating costs compared to the efficiency. On the other hand, it also shows very clearly that the lowest heating costs (0.02 to 0.03 EUR / kWh) are achieved exclusively by collectors in the medium efficiency range (B to A+, on a scale from D to A+++). The heating costs of the collectors in the A++ to A+++ range, however, are between 50 % and 130 % higher than those of the least expensive collectors (UFE Eco Star III blue and Junckers FCC-2).

For this reason, the verdict for the SOLERG label in its present form is scathing: It provides no incentive whatsoever for collectors with higher solar yields. And by focusing on efficiency, it actually promotes products that generally tend to have higher heat generating costs. The industry should not go along with this.

Uwe Trenkner

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