Solar Thermal can be evaluated against a wide range of design criteria. The major one that i get asked though is it cost effective?
From an installer point of view – it is reliable, fairly straight forward to fit and with plenty of good reputable suppliers. Basically the product definitely works. Which is great as a typical pressurised solar thermal system can be up to 150 degC at times. Both the initial product quality and the installers’ understanding of the techniques and workmanship required to fit these successfully are both of a good standard in my experience. I am not going to discuss flat plate versus evacuated tubes here- the main difference i can see is visual impact versus compact size of each system. They both work well.
From a householder point of view (and probably that of government and policy makers) their main expectations in my experience is to see a reduction in their bills and a reduction in their carbon emissions.
I have solar thermal in my house – there is a undeniably a great feeling to having those ‘free’ hot solar showers – as technology I love it, however, is it cost effective?
Solar Thermal Performance
System performance is fairly well understood and with the widespread implementation of metering in the UK heating industry we can actually start to see actual in situ performance. This is important as factory efficiency levels are for idealised condition in test facilities. The MCS scheme and the RHI has driven up standards and training, the performance metering is only for non domestic installations which I think is a shame.
The delivered solar energy is a function of the input (solar energy landing on the panels) and the efficiency of the solar thermal panels at converting this to hot water energy.
The efficiency of a solar panel is high and there is a good amount of solar energy out there. Obviously facing your panels as South as possible and have them at say a 40 deg angle will maximise the solar input energy.
So why not cover your roof with solar? Basically you could but in peak sunny conditions you generate a lot of heat rapidly and that has to be stored into a water volume – generally this is your domestic hot water cylinder (as you always have a demand every day of the year). So cylinders for solar systems have to be calculated to hold enough “solar volume” as well as the occupants hot water needs. Basically your heating engineer has to “goldilocks” it- too little collector and it wont generate enough. Too much collector and you end up having too much heat generated on those summer days and this is bad news for the system (see below). Industry would say 35-50l minimum storage volume for every m2 of collector installed – thus a 2 panel system (4m2) might need 200l store minimum (but your better to have a calculation done!).
The max power (P) from a solar thermal system is given by the size of the panels (A) x solar irradiance energy (I) x optical efficiency (E) of the panel.
For a 3m2 collector panel in full sunshine (1000W/m2) with a panel efficiency of 80%:
P= 3 x 1000 x 0.8 = 2400W or 2.4kW. There are then further system losses, pump power energy etc but for simplicity lets assume the overall efficiency is say 40%- thus useful output is 1.2kW.
By comparison a typical cylinder would have a 3kW electrical immersion in it.
The limit is what to do with this energy – if you have a 200l store it will take 4-5 hours to charge at this rate and then it will ‘switch off’ – i.e. for safety reasons it will stop collecting energy. You could put in a bigger store but then the next day you would get the same solar input and your store is already ‘hot’. Sunny weather tends to come in periods of clustered sunny days.
Problems with Solar Thermal
- If the sun does not shine then there is no generation- so no free solar. Typically solar can produce around 50% of your hot water for a year. A good designer/installer might be able to get that to 60% over a year (maybe nudging 70%). The EST field trials in 2011 found an overall 60% saving in domestic hot water. The problem is that domestic hot water is only maybe 20% of a typical houses overall heating demand. So in effect you save around 10% off your heating bill. [In brand new eco housing or flats then domestic hot water demand may be more like 40-50% so the contribution is much more significant.]
- The cylinder can be a lot bigger than other heating setups- see boxes above.
- Some installers- generally those who are self installers or engineers tinkering with their own home system- can get the solar thermal to contribute to space heating and thus boost the solar effectiveness. However, solar tends to have this excess in the summer (and your heating is off). Also many of these systems (from the amateur DIY installer) would not pass a L8 legionnaires compliance check in my opinion. Just make sure you know what your doing or get a competent installer.
- A back up system is always needed- a good chunk of the time we have seen these switched to permanently ‘ON’ – thus the solar thermal (which is slow to react compared to the call for heat from the gas boiler say) never contributes to the system. Heating systems are often handed over without handover, instruction and people don’t know how to work their timers/ programmers etc. MCS is changing this but a lot of installs in new build or social hosing are non MCS.
- Too much overheat in a sealed system will cause ‘stagnation’. Nearly all systems I see in the UK are sealed (pressurised) with a glycol fluid. The glycol allows a high heat transfer but also keeps the system from freezing in the winter . Thus stopping freezing and or boiling of the fluid in the loop and panels. However, prolonged high temperature in glycol causes it to break down and a thick sludge like material is formed which blocks flow and components. So we can see problems especially when say a homeowner goes away for 2–3 weeks in the summer in the middle of a heat wave.
- The systems do not like even small amounts of air ingress- this again degrades the glycol and causes maintenance problems. The best way to counter this is to have regular top ups of pressure and or a change of your glycol every 8-10 years. On the whole most people will need to pay £150 for an annual maintenance on their system, . If they don’t 9 times out of ten it will fail in a bad way anywhere from year 3 to 8 into the installation life- generally involving strip out of the system and replacement pumps etc- all of which costs more than the costs of paying for annual/ biannual regular maintenance. You see whole housing estates failing at much the same time – by then the housing developers have long left.
Solar Thermal Servicing Costs
I compared the servicing costs for 4 real life projects over 10 years. Bear in mind the service engineer has to make a profit and the glycol is more expensive than malt whisky!
So Client 1 has done a service and top up approx every 3 years with a repair and some glycol in year 8. Client 2 has had an annual check (in accordance with the recommendations) and a glycol change and flush in year 8. Client 3 did nothing then had a failure and a major repair and recharge in year 7. Client 4 again has biannual servicing (small top up) and minor repairs or recharges. The average bill for all clients was £85 if annualised over a 10 year period. The straight line in the graph is the average trend line. Statistically I would need say 10 systems to compare but we can anecdotally say there is a annual service cost.
Economics of Solar Thermal
Future Developments in Solar Thermal
- Given the scenario where it pays back is electrical heating – the answer may not be to use solar thermal! – you could fit use Solar PV with a thermal dump switch to charge the hot water during the day. It would cost less and you would get power as well as renewable heat – the advantage being power is valuable at all times and can be diverted to grid or other storage when the hot water is charged. There will be significantly less maintenance issues – not completely as the thermal switches have relays and thus can fail – but they are less heavy in the servicing requirements.
- Combined PV-T panels- these combined PV and thermal panels are an option- personally I think this would be for space limited applications, as a PV and a solar collector combined is not as efficient as the separate systems. Basically a thermal collector should be kept hot and a PV panel should be cool to maximise efficiency.
- Open loop or drain back systems. If you employ a fully open system or the ability to “drain back” the solar fluid (or freeze the panels on the roof – like Soltrophy) then you remove the maintenance requirements and change the economics substantially.
- Finally if you have the money then “Inter-seasonal storage” might be the answer. These systems typically use a phase change material (PCM) to use the increased heat capacity from latent as well as sensible heat. They are used more and more in commercial situations and we are starting to see a few domestic applications in the UK.
Is Solar Thermal cost effective?
In conclusion it is a marginal cost benefit. The upfront costs are fairly high and difficult to see how it can reduced- solar thermal has had a long development phase from the 1970s and the fixed costs of fitting panels on the roof (scaffold , roofer) and complex plumbing (good plumber) along with the kit prices explain the cost. It might be ~£1500 per kW installed.
As a comparison we are seeing Solar PV systems for much the same price if not a bit cheaper. Solar has so much more scale of investment and just seems to get cheaper and cheaper. With the addition of a thermal dump switch it can charge the cylinder for hot water via an electrical immersion anyway. You also have renewable power which is very valuable.
Where it might make good sense is if you have good communal schemes or switched on householders who can do regular maintenance at low cost, or use the Soltrophy system. Also if you have biomass you could also find using solar thermal in the summer months allows you to completely turn off you biomass in the summer. This compliments the biomass well, as short and small periods of running a biomass causes it to cycle in low load conditions, and generally this causes a high probability of clinker or tarring problems and can cost you a lot in maintenance. Generally your biomass service engineer can check and maintain your solar thermal at the same time as servicing your biomass so the economics are again improved.
If you have a heat pump they cope pretty well with low load conditions and can generally do your hot water needs throughout the year anyway.
Dan Gates
Building Services Engineer- Glasgow
