Mike's solar pool - August power output

[Mike N.]

Not the best August ever but the tubes still maintained the pool with no electrical heating for a fourth month. Total output for the month was 394.54kWh with a daily minimum of 4.22, average of 13.15 and maximum of 27.22.

It's tempting to equate that with nearly £40 saved, but of course had I been heating it electrically, I'd not have opted for a 40 degrees swim on two occasions  . Here's hoping for a good September.

Surface area of pool is 11.15 sq m, location is Essex, Lat 51.65, tubes are 60 x 58mm on a 30 degree roof, facing South, Ratio of active tube area to pool area is 56%, Ratio of panel area to pool area is 91%

Mike

[CeeBee]

Not the best August ever but the tubes still maintained the pool with no electrical heating for a fourth month. Total output for the month was 394.54kWh

Hi Mike. Just posted off to Ivan the August readings from the Resol attached to my 20x47mm tubes. Came to 147kWh in August 2008, though it's always been suspected of over-reading - not completely obvious what exactly the sensor in the panel is measuring, and the sensor on the 'return' to the panel is just poked under the insulation. Wouldn't take much error in either to affect readings, since we're looking at a small difference between two temperatures. Anyway, at least not completely in the wrong ballpark compared to yours.

August definately cr*appst month since March, though boiler still not been on for hot water.

[rob26440]

I don't have a pool but for comparison, here are my first year’s figures.

Attached (I hope) is doc showing the full year of "KWH's" as registered on the TDC3 for my 30*58mm panel on a S/W facing 40 deg sloping roof.  2 cylinders (216L & 186L).

August figures confirm the poor weather with numbers only slightly better than Feb and just a few short of the March total.  However, we only used 3 units of gas to keep the DHW up to temp.  So not bad, I suppose.

Rob.

[Mike N.]

Attached (I hope) is doc showing the full year of "KWH's" as registered on the TDC3 for my 30*58mm panel on a S/W facing 40 deg sloping roof.  2 cylinders (216L & 186L). August figures confirm the poor weather with numbers only slightly better than Feb and just a few short of the March total.  However, we only used 3 units of gas to keep the DHW up to temp.  So not bad, I suppose.
Rob.

Hi Rob

Are you in the UK? (Your figures are more than half mine for half the number of tubes). If so, I'm heartened by your graph showing that I might get free heat in at least March, April and May next year. (I didn't put my tubes up until May this year, so don't have any previous figures).

Mike

[dhaslam]

Rob,  the difference between the highest month (April 386) and the lowest (December 146), a ratio of 2.64,  is much smaller than the theoretical output would suggest.  Any idea why?

[rob26440]

Mike,

Are you in the UK? (Your figures are more than half mine for half the number of tubes).

Yes – in the UK – in Berkshire.  Plenty of decent heat March to May.  To improve performance, everything on the solar side is double lagged (triple in the loft) to minimise losses.  I find the Armaflex is good at withstanding the high temperatures but I’m not impressed with its insulation properties.  It feels warm even at fairly low temperatures.  So the Armaflex went on first then another layer or 2 of other insulation on top.  The TDC3 pump delta is set to 9/5. Also, the 15mm solar pipe work is only about 15 metres in total – with either 8 or 12 metres in use at any time (depends which of the 2 cylinders is being heated).  Pump is on the lowest setting and runs at about 8L/min (any lower and the flow setter produces a rushing noise).  I tried the pump speed control on the TDC3 for a month, and then turned it off.  I don't think it made any useful difference.  It lowered the flow speed to about 6.5L/min but the TDC3 emitted an annoying buzz when it was running with pump control.

Rob.

[rob26440]

dhaslam,

Rob,  the difference between the highest month (April 386) and the lowest (December 146), a ratio of 2.64,  is much smaller than the theoretical output would suggest.  Any idea why?

A bit of a delay in responding – while I had a think.  However, the simple answer is: “I don’t know”.  I hadn’t looked at it in that way.  I don’t have any scientific answers.  As a point of interest, what is the basis of the theoretical ratio?

I’ve struggled to find an explanation.  But I can provide words that might help to explain the actual ratio – but maybe some of the words will only add to the mystery.  Here goes….…

There was a change to the system between December and April.  At the end of February a second H/W cylinder (186L horizontal) was installed in the loft.  It is plumbed in series between the CW header tank and the main H/W cylinder.  During April, as the amount of sun hours increased, there were a few occasions when the system automatically switched to the second cylinder as a result of the primary cylinder reaching ~65C.  However, this action would result in the TDC logging more KWH (or whatever they really are) in April than if there had only been one cylinder, hence making the ratio larger than it would have been for a single cylinder.  This is because the panel temp does not need to be as high to meet the TDC3 delta to turn on the pump when heating the cooler, second cylinder.
 
Perhaps December might have had more sun than usual and April rather less, hence the closer ratio.  I don’t have data from any other year.

(February was the odd month for me. I thought it provided more KWH than expected.)

Comparisons are made more difficult because the system is not running in a controlled, static environment.  As well as the installation of the second H/W cylinder between Dec and April, general conditions such as the weather, and volume and timing of hot water usage, varied from day to day.  So I think the monthly KWHs are just that and do not provide reliable figures to make comaprisons to the theoretical ratio.  For example, if the hot water is used regularly throughout the day the temp of the water at the bottom of the cylinder, especially during the colder months, will tend to remain low. Thus more KWH will be registered.  In contrast to this, during June when we were on hol and less hot water was used (my son was at home for a part of the 2 weeks), under 12KWH/day were registered (only have an average) – despite the heat dump cutting in occasionally.  The panel had to be v. hot to add heat to the already hot water stored in the cylinders.  However, as soon as we started using the H/W again we were getting higher KWH registered in any one (good) day.  This is before summer deteriorated, of course!

A bit long winded but any other suggestions/comments/theories welcomed.

Rob.