Mid-Summer solar data

Amazingly given that it is mid-summer and Wimbledon has just started the sun has actually been out for a good few days!   Some good data collection has been taking place and my rig has proved itself reliable at last.

Green is the PV, Red is the thermal.

21st of June, longest day!   Some big fluffy cloud banks passing over as you can tell from the big dips.   My fault, had family over for a BBQ.


22nd of June, pretty sunny day.   A little hazy just after lunch and then blue sky all afternoon.


23rd of June, a bit more cloud.   Didn’t mind as it was a Monday so stuck at work.


24th of June, cooler start with little clouds passing over from lunchtime giving the very sharp drops in the PV wattage.   Cooled off in the evening.


These show nicely the larger working angles of the evacuated tube solar thermal collector.   It starts to make heat best part of 2 hours before the PV cells really kick in.   But this makes sense as a cylinder will always present the same surface area no matter the angle you view it from.   A flat sheet doesn’t.

The drop off in the evening is actually the house getting in the way.   I could move the experiment to the house wall but someone might not like the noise of the pump and the potential for the glycol solution leaking again.

The PV panel has a collector area of roughly 800 sq cm. (25x35cm with ~10% removed for corners and gaps)   The solar thermal collector has an area of 290 sq cm.   36% the area of the PV.   But we see equal if not better peak power outputs from the thermal collector.

The average per day is much higher from the thermal collector.   Ranging from ~1.8x higher on a sunny day to ~2.5x higher on a more cloud strewn day.   So per sq cm of collector the efficiency is almost 7x higher than a silicon PV cell.

OK, not the most scientific test and I’m not entirely sure of the efficiency of my PV panel.   But if we assume its in the 12-14% range (its from China via ebay after all) it puts the evacuated tube in the 95% range.   Which is about right from my research.

If I was using hot fluid in the cooling circuit this efficiency would drop.   Most solar thermal systems aim for 70-90 deg C.   The fluid temps I’m dealing with are in the mid 30’s.   Mainly for safety as the experiment is a little Heath Robinson.