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Why RHI rules should be changed to incentivise solar thermal stores too

Posted by Stuart Elmes on 25 June 2014 at 10:45 am

The domestic Renewable Heat Incentive (RHI) was structured with the deliberate intention that the combination of solar thermal with other renewable heating technologies would be actively encouraged.  The thinking behind this is that solar thermal increases the average operating efficiency of the other RHI technologies when they’re used in a domestic setting. 

Heat pumps are at their least efficient when preparing high temperature domestic hot water and biomass boilers operate best when firing up in winter for extended periods to supply space heating. A solar water heating system providing high hot water coverage outside the space-heating season perfectly complements these other technologies. 

The way that the domestic RHI encourages combined systems is by effectively paying twice for the renewable energy provided to heat domestic hot water.  The householder is eligible for RHI payments for both the heat pump or biomass boiler and the solar heating system.  

Unfortunately, the wording of the legislation elsewhere has discouraged combined technologies because it completely rules out solar systems that could make even a theoretical contribution to space heating.

Why does this matter?

Well, a popular way to combine solar thermal with space heating is to use a thermal store, essentially a large hot water cylinder (typically from 500 litres to 1,000 litres) with heat inputs from both solar and the backup heating system and with outputs to domestic hot water and space heating.  Many different arrangements for a thermal store are possible (and manufacturers can speak at length about why their version is superior to everyone else's) but typically the body of water in the thermal store is heating system fluid (primary water) and domestic hot water is heated on-demand in a heat exchanger as it flows to the hot tap.  

As I mentioned before, both heat pumps and biomass heaters operate well when running continuously rather than cycling on and off, so charging a thermal store is a good technical solution that can improve the overall operating efficiency of the heat pump or biomass boiler.

When set up like this, the solar thermal systems can make a contribution to space heating as well as domestic hot water preparation, especially in spring and autumn where the days are still bright and there is a demand for space heating.

The degree to which this will occur will depend on how much solar panel area is installed.  

Where the designer is aiming for solar to cover mainly domestic hot water the panel array is smaller (typically in the range of 3  - 6 m2).  In this case there is a theoretical possibility that the solar energy will contribute to the space heating, although this contribution is likely to be rather small. 

Where the aim is for the solar to make a reasonable contribution to the space heating, the solar panel area installed is much larger (around 12-18 m2 for a domestic property).   These systems are not yet as common in the UK as those for only domestic hot water, but in more developed European markets such as Germany and Austria, so-called "solar combi systems" are very popular.

Unfortunately, the current domestic RHI legislation completely excludes solar thermal systems for anything other than ‘solely domestic hot water’.  An implementation of solar where there is even a theoretical possibility of the solar contributing towards space heating is completely excluded from the scheme.

The reasoning behind ruling out solar space heating was that the domestic RHI is “deemed” – the solar energy is not measured, instead it is estimated using an approved calculation and the calculation only works for domestic hot water.

However, by ruling out any solar installation that does not solely heat domestic hot water, the domestic RHI has made the combination of complementary renewable heating technologies such as solar and heat pumps less likely. Solar thermal has lower associated carbon emissions than any form of back up heater, so every unit of solar thermal heat that could be used, whether for space heating or domestic hot water reduces carbon emissions.

Configurations where the solar is offsetting a proportion of fossil fuel space heating are also excluded from the domestic RHI.

When installing biomass or heat pumps with a thermal store, the additional cost to add a solar coil into the store is very low, making the cost of adding solar thermal even more attractive.  The domestic RHI would provide greater value for money if it encouraged, rather than discouraged such systems.

How to Fix This

So how could the domestic RHI be changed to include solar space heating?  The Solar Trade Association recently made two suggestions to Government. 

First, it would be possible to use a heat meter to measure the solar input into a thermal store.  Solar space heating systems cost a fair bit more than solar systems aimed only at domestic hot water so the cost of a heat meter would be a relatively smaller proportion of the total.  Also, houses that can fit large thermal stores are relatively thin on the ground so it wouldn’t be too much of a cost for the scheme administrators to deal with the meter readings.

A second approach would be to allow solar space heating systems onto the scheme but to give RHI payments only for the domestic hot water energy provided, and calculate this with the current deeming method.  

The best outcome of all would be for it to be the choice of the homeowner which of the two to go for.  If they wanted the extra payments for space heating, then they would need to install a heat meter, otherwise they could claim for only the solar heat in their domestic hot water.

We impatiently await the government’s deliberations on this issue.

Photo credit: Solarblogger

More information

YouGen guide to RHI

YouGen guide to solar thermal

YouGen guide to biomass

YouGen guide to heat pumps

More from the blog

How to store heat in a thermal store (2009)

New solar thermal calculation for domestic RHI increases returns

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About the author:

Stuart Elmes is founder and CEO of Viridian Solar, a UK manufacturer of beautiful roof integrated solar PV panels and matching solar thermal panels.

If you have a question about anything in the above blog, please ask it in the comments section below.

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2 comments - read them below or add one

Sims Solar Ltd

Sims Solar LtdComment left on: 4 September 2014 at 6:25 pm

I've just sent an email to DECC on this. One poorly considered sentence in a 52p piece of legislation seams to have killed a very valid solution where domestic hot water heating is the primary role of solar thermal not space heating. Hopefully the legillation will be tweeked to make Solar Thermal and Thermal Stores an accepted solution still eligibgle for the RHI.

Since RHI payments are based on occupancy as much as anything I don't see there being a drive towards sizing solar thermal to provide space heating. In any case what do you use all that hot water for in the summer? Yes a pool but the RHI should not be available for that.


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ed1066Comment left on: 27 June 2014 at 5:14 pm

Hi Stuart

I like this proposal.  Good work by you and the STA.

My one concern would be that a Thermal Store may not be a good idea when there is a heat pump.  I am basing this on my feel for the technology rather than any direct information, so please correct me here where appropriate.  My thinking is that heat pumps are most effective with a small temperature rise, hence we want underfloor heating with relatively low circulating water temperatures and continuous use in winter.  Thus the building fabric itself is the heat store.  If the heat is concentrated into a Thermal Store the temperature must be higher, not least to enable a thermal gradient to take heat back out. 

I am worried that the loss of efficiency in the heat pump would reduce or eliminate the gains from solar thermal.

None of this applies to biomass boilers and Thermal Stores.

Finally, is there a chance that incentives could be introduced where solar thermal is used to recharge ground heat at the borehole style ground source heat pump installations? I've heard that it is often neccessary to avoid permafrost.

Thanks for the blog

Ed Atkinson

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