Solar Thermal Performance During Winter Months? Solar Thermal Forum

[Dan]

During the summer months I'm sure solar thermal performances are pretty decent, but what about the winter months?

 

[sparkypaul2]

Don't know from experience but know I have read where the colder winter months are actually a benefit, all other conditions equal to a warmer locale, because the efficiency of the panels goes down and thus output when temperature rises on the surfaces. I am sure there probably is an ideal temp vs available sunshine threshold but basically the extra sunshine logically would increase the ambient temperature surrounding the panels, which in a warm climate are already likely at a temperature where they are adversely effected for efficiency. Colder not so much and though the angle of sun detracts on available light energy striking the surface, it might be sufficient.

 

[SpecialistEnergy]

Don't know from experience but know I have read where the colder winter months are actually a benefit, all other conditions equal to a warmer locale, because the efficiency of the panels goes down and thus output when temperature rises on the surfaces. I am sure there probably is an ideal temp vs available sunshine threshold but basically the extra sunshine logically would increase the ambient temperature surrounding the panels, which in a warm climate are already likely at a temperature where they are adversely effected for efficiency. Colder not so much and though the angle of sun detracts on available light energy striking the surface, it might be sufficient.

This is true for Solar PV panels, but not accurate for solar thermal. Efficiency in solar thermal is defined by DeltaT, (difference between panel temp and ambient, and also solar fluid temp vs cylinder temp at the bottom of the cylinder, the 2 places where heat transfer occurs) and losses from the system will be reduced in cooler weather, but overall the performance in winter is much lower.

I have a data logging function on my system and if anyone wants the data I can pass it on, but my instinct is that December performance probably drops to about 20% of June / July on average.

 

[Worcester]

If you've got a csv fike of the data I'd be really interested to see that. PM it to me if you need to

We are seeing a resurgence of Solar Thermal on super efficent new houses were the heat pumps aren;t big enough to reheat a 250l+ DHW cylinder in a reasonable time.

A new 100sqm house often only needs a 3kW heat pump for space heating - that's 4+hours to reheat the cylinder from depleted.

 

[SpecialistEnergy]

Hi @Worcester - sorry totally forgot to do this. Is this any good? look at page 9:

https://www.withouthotair.com/synopsis10.pdf

 

[SpecialistEnergy]

Can do the CSV, but v busy ATM!

 

[Worcester]

Thanks @SpecialistEnergy , the book, makes sense, (common sense) unfortunately it may not reach as wide an audience as it should, and will probably be read by the already converted

When you have a mo, the csv would be most useful.

 

[Worcester]

Thanks @SpecialistEnergy , the book, makes sense, (common sense) unfortunately it may not reach as wide an audience as it should, and will probably be read by the already converted

When you have a mo, the csv would be most useful.

 

[SpecialistEnergy]

HI @Worcester I've emailed you through your website

 

[Worcester]

Thanks @SpecialistEnergy

Analysing 4 years of minute by minute daily data:

In summary a BIG system can do a maximum of 75% and should be able to do 65%, meeting 50% of your winter demand and 80-100% of the summer demand.
To do the 100% of the summer demand requires a thermal store 3 x the size of your normal DHW demand (1000 litres for a 300 litre DHW cylinder) without the thermal store it can still deliver 80% of the demand in the summer.

It generates 1/3 of its energy in the winter 6 months and 2/3 of its energy in the summer 6 months.

 

[SpecialistEnergy]

@Worcester have you ever read the Energy Saving Trust report "Here Comes the Sun" - a field trial of about 80 systems from memory? Its a pretty well written report, but of course focuses on the traditional British meanly sized hot water stores and does not cover best practice as carried out in the EU.

I seem to remember it's conclusions were pretty much "Make sure your customer REALLY understands not to turn the boiler on in the morning" (the single most important factor, alongside don't use electrically heating showers), and "Don't skimp on insulating the cylinder pipe work".

I also seem to remember that for the average UK system, not insulating the cylinder and surrounding pipe work meant you just about countered the heat loss - i.e a 4sqm flat plate will do about 1800kWh a year, and the cylinder can lose about the same if its not insulated well.

 

[SpecialistEnergy]

@Worcester I went through your analysis quickly, and if you average the high and low seasons across the 4 years, the ratio is almost exactly a 70/30 split as you say. I wonder if it wouldn't even be a little more pronounced on a system doing DHW only?

My store is stratified (a TiSUN, with the stratification unit on the side, really good design), and I also take heat out for a UFH circuit at low level which is left on constantly throughout winter - so I really am extracting every kWH out of the system I can, whereas most systems would struggle to raise cylinder temps much on poor days.

Thanks for collating into yearly files with that utility, I will have to remember that : )

If you are interested in modelling further, Polysun have an online simulator that is free, but you can't change the geographical location - great for testing out ideas though