Solar Thermal - I might have problems

[pharma]

Hi all,

I had Solar Thermal installed at the end of March this year and am disappointed with its production so far. I have 3 panels with a total net aperture of 3m2 (1m2 each panel) on a roof with no shade and directly facing South. In 7 months they have produced just 619Kwh of energy. From rough calculations, it looks like I will be lucky to produce 750Kwh by the first anniversary of the panels working.

I had a local company install the panels and have had a couple of problems with them already (solar pump not working correctly being the main one). Recently I checked the orientation of the panels and they are only at 24 degrees, whereas everything I read says that should be at 35 degrees for most parts of the UK (I live near London). Could that be the reason for the poor performance?

There are two key figures that I am struggling to comprehend and would be grateful if any experts could explain to me in simple-terms:

1. The Solar Trade Association suggests that I should receive ~ 1,100 Kwh of solar irradiation in one square metre surface. Does that translate to 3,300 Kwh of energy a year for my system? (seems a little high to me)

2. The installation manual of the panel manufacturer (a reputable brand) says I should obtain 586 Kwh / 1 square metre (taken using a 5m2 aperture in Germany). Is it fair for me to assume that my system should produce ~ 1,750Kwh a year (3m2 x 586) - less a little for being further North than the trial? (seems more reasonable to me than question 1.

I am keen to get to the bottom of this because at the moment I believe my system is missing out on ~1,000 Kwh a year, which over the 20 years of RHI equates to ~ £1,700.

Any help would be appreciated.

Thanks

[billt]

1. That is the gross energy falling on an area. The panels will be no more than 60-70% efficient in the best circumstances, overall they'll be less than 50% efficient. That brings the potential yield down to less than 1600kWhr a year.

Unfortunately there is no easy way to get an accurate assessment of system output as there are too many confounding factors; size of store, amount of water use, quality of pipe and store insulation, controller set up etc.

To give a comparison, my system has produced 4300kW since the middle of March, with an aperture area of 11.4 sq.m. That translates to about 1100kWhr for your size system. However, it is further north than you and the array is at a much steeper angle to try and extend the season.

The angle won't make a huge difference, less than 5%.

I'd say that you're system may be working at a bit less than optimum, but not by a huge amount.

[dhaslam]

I'm not sure how the radiation per square metre  is actually measured  but the summer figures quoted  by weather stations are usually   exaggerated because they take the full  180 degree +  angle that the sun moves through.  Part of the arc is behind the panels and some at too much of an angle an angle to produce any heat.       The other factor that is very variable is the temperature of the water being heated.   If water is drawn off fairly constantly during the day   the panels are  heating water that  is quite cold.  This doesn't matter on a sunny day but on dull days, which are the in the majority,   it makes a big difference. 


This  photograph of what used to be our local weather station shows  a circular hoop on the flat roof  on the left.    I presume that it is for reading solar radiation  but at that time of day the sun would be coming from the far left  corner of the picture.   
 
 

[forever green]

The recently published study by the EST showed that the majority of failings in a solar thermal system were the user not understanding what they have to do to get the best out of the system, ie only use back up heat source at the end of the day and just before hot water usage. any 'top up' heating AM will significantly reduce solar thermal output. even the biggest and best installed system will not work if other heat sources are allowed to interfere at all.

Also, how are you measuring the heat output, do you do it accurately with a flowmeter and dedicated flow and return sensors or is it just by extra sensor and averaged flowrate. this could make a huge difference

[pharma]

Thanks for the comments and suggestions.

billt - thanks for pointing out the efficiency issue - schoolboy error from me. Although I am curious about your final opinion that my system is probably working less than optimum, but not by a huge amount. Assuming the aperture area and output is linear, I agree that using figures from your system then I should be generating ~1,100kWh, yet I have actually only generated 620kWh - that’s not far short of 50% of what it should be producing which I believe is a huge amount - am I missing something?

forever green - I am very careful about heating the water conventionally; I even have the scars from the wife where she wants conventional on sooner! My kWh measurement is coming from the control panel as supplied by the manufacturer. However it was only on the third call out did the engineer realise that a wire was lose on the control panel and severely impacted the reading. They preferred to blame me for heating the water in the morning, which I have never done. I believe the system uses a flow and return heat comparison to calculate the output - which I would have thought is the best way to do it.

Many thanks for the feedback. I will contact the manufacturer and see what they say. Any other feedback or comments would be most appreciated.

All the best.

[martin]

I think the problems lie in the fact that you've got hold of some figures, and assumed that what hits the panels ends up in your tank - as others have explained, it is nowhere near that simple (which is why I try to avoid "figures" wherever possible - they can often mislead/bamboozle the unwary).
When designing a system, many factors should be taken into account - to my mind the ultimate aim should be to displace other heating for hot water for a large chunk of the year
Tank size and panel ratio is important (and should be chosen with projected usage in mind) - type of panels can make a hefty difference (vacuum tubes have advantages early and late in the season, effectively extending it)  - you need to size the tank according to the panel output - people often wrongly assume "bigger is better", whereas a smaller tank heated to higher temperatures is often best - big tanks can mean large quantities of tepid water that need further heating....
 I'm not quite sure how "RHI" enters into this - the financial "payback" time of a system depends on it's cost and the power source it displaces during the spring-autumn period - if you have immersion heaters and teenagers it could be as little as 3 years, if you are on mains gas it may take 15 years - any subsidy will drop those times considerably - the important thing is how quickly it repays it's embodied energy costs - which is in fact a very few years!
As others have said, I think the "problems" may well be measurement errors, and trying to extrapolate the figures incorrectly

[GavinA]

looks to be in the right sort of ball park to me. This below is a sap estimate for a standard 3m2 system facing south, no shading, 0-30deg roof angle.



Worth bearing in mind that unless you have a properly calibrated meter, then the chances of the generation figures being accurate are pretty slim. The inbuilt heat meter in the controller is only as accurate as the information supplied, and I doubt this information is particularly accurate.

[pharma]

Thank you for the comments. Point taken on applying pure mathematics on the return. So do you think it will make any noticeable difference if the panels were at 35 degrees rather than the current 24.5?

[dimengineer]

Thank you for the comments. Point taken on applying pure mathematics on the return. So do you think it will make any noticeable difference if the panels were at 35 degrees rather than the current 24.5?

No. You can do the sums - it will be less than 10% difference. Which to my mind is not "significant"