Carbon Clawback in Solar Water Heating. Solar Parasistics. Coefficient of Performance: Energy & Carbon.

This is the data table which we were originally banned from publishing by the Solar Trade Association. I hope you enjoy reading it.

Data from “Side By Side Testing of Eight Solar water Heating Systems” DTI/ETSU by Martin and Watson, 2001. with “Split Water Runoff Scenario” using the 2.5 gas to electric carbon intensity weighting quoted in the report. For more about this report take a look at this video.

Who did the three main solar thermal technologies compare?

• Flat plates averaged 7% energy clawback and 18% carbon clawback.
• Evacuated tubes were even worse. They averaged 9% energy clawback and 23% carbon clawback.
• Solartwin had zero energy clawback and zero carbon clawback in its operation, proving that this simple technology is the least environmentally damaging, on the basis of there two important measures.

Clearly the carbon clawback of solar water heating can be reduced.

Five ways to reduce the levels of carbon clawback in solar water heating systems include:

1. Not choosing indirect evacuated tubes. It appears from the data that solar heating systems based on tubes are the least sustainable because they need stronger and more energy-thirsty pumps, perhaps because these sensitive technologies have rather small heat exchangers at the top of the tubes to remove heat from. So the fluid must rush past them as fast as possible.
2. Choosing more efficient pumps or lower powered pumps which simply consume less energy. It is notable that Zen (an import) was already doing this by the time of the study but the other, mainly UK made, systems seemed to be stuck in a timewarp. Ask your solar supplier if they use low power pumps. We, and a number of other solar heating suppliers do this.
3. Choosing variable speed pumps (flow modulating pumps) which drop their speed and power consumption at lower light levels or lower temperature differentials. Ask your solar supplier if they use modulating pumps. We, and a number of other solar heating suppliers do this.
4. Choosing water as a heat transfer fluid. This is 6 times less viscous (gloopy) than “normal” antifreeze so it requires less energy to pump. We, and a few other solar heating suppliers use water.
5. Choosing PV pumping. This gets rid of the nasty parasitics altogether. Again, this is what Solartwin does: cleaning up its operational environmental footprint. Which solar heater will you buy? Low carbon operation or Zero carbon operation…

It’s a great feeling to be free to publish this table!

We were ordered not publish brand by brand comparisons by the STA when we originally tried to do so (most board member companies at the time were suppliers of the tested systems). Then for good measure and presumably to show who was not top dog we were fined £1000 by the STA for resisting their censorship.

What you are reading are hard-won secrets: rights to disclose consumer-oriented information which (like Legionella safety) the solar thermal industry has sought to conceal using a ruthless campaign of exclusion, threats and coercion against whistleblowers.

It is hoped that the Energy Saving Trust report on solar water heating (publication expected later in 2011) will show that the solar thermal industry has improved dramatically on these figures and this it it trying to move on from its concealed but polluting past. The STA’s Chairman, Howard Johns,  even claimed that mains pumped systems were zero carbon until they were told not to do so by the ASA. One hopes that the rumoured pullout from the trials before it ended, by at least one evacuated tube supplier, was not linked in any way whatever to its emerging parasitics figures.

Should a minimum CoP (energy) of, say, at least 20 apply to state subsidised solar thermal systems? This kind of approach already happens for heat pumps.

I hope this is interesting. Feel free to tell your friends.