Anyone use a solar cover with an auto-cover?

[3dogowner]

In reading over my prior post, I found the most basic of mistakes - the surface area of my pool is only 112 square feet (8x14).
Thus assuming no wind and R value of cover of 0.5 - to calculate air temp when heater on constantly:
max heat output of my 4 kWh heater in BTUs = surface area x difference in pool and ambient air temperature divided by the R-value
88F - (13600/121*0.5) = 88-60.7 = 27.3F temp when heater turns on constantly...

So, over the last 66 days (mid-Nov thru mid-Jan, Seattle City Light bills are every 2 months...), everything else being equal, the additional cost of my pool heater + pump etc has been ~ $9.68 per day or ~88kWh per day. The average temp per the National Weather Service was 45-46F over that time period.

Since the max energy use from my 4kWh heater in 24 hrs would be 96 kwH per day, it clearly is not on all day. And given the recalculation on when the heater should be on constantly (worse case scenario), it hasn't been cold enough to trigger that event.

I am thinking about experimenting with 3 of the 2 inch thick 4 by 8 foot pink boards with an R value of 10 floated under the cover to see if I can save a cup of coffee per day.....at ~$100 total, even if it only cut my heat loss in half, it would pay for itself in just under 3 weeks.

 

[chem geek]

Thanks for keeping us updated. During the winter, I could really use an extra insulating cover since the automatic electric safety cover isn't very insulating at all. We've been keeping our pool heated during most months even when the weather was cold.

 

[gtemkin]

In reading over my prior post, I found the most basic of mistakes - the surface area of my pool is only 112 square feet (8x14).
Thus assuming no wind and R value of cover of 0.5 - to calculate air temp when heater on constantly:
max heat output of my 4 kWh heater in BTUs = surface area x difference in pool and ambient air temperature divided by the R-value
88F - (13600/121*0.5) = 88-60.7 = 27.3F temp when heater turns on constantly...

So, over the last 66 days (mid-Nov thru mid-Dec, Seattle City Light bills are every 2 months...), everything else being equal, the additional cost of my pool heater + pump etc has been ~ $9.68 per day or ~88kWh per day. The average temp per the National Weather Service was 45-46F over that time period.

Since the max energy use from my 4kWh heater in 24 hrs would be 96 kwH per day, it clearly is not on all day. And given the recalculation on when the heater should be on constantly (worse case scenario), it hasn't been cold enough to trigger that event.

I am thinking about experimenting with 3 of the 2 inch thick 4 by 8 foot pink boards with an R value of 10 floated under the cover to see if I can save a cup of coffee per day.....at ~$100 total, even if it only cut my heat loss in half, it would pay for itself in just under 3 weeks.

For the surface area you have, the foam boards sound manageable. Wish that were my case. I'd be looking at twenty 4' X 8' sheets!

 

[3dogowner]

I will make a run to HD this weekend and get the boards. Then I will do a 2 week on vs 2 week off comparison and report back the difference in power usage. The weather here is slow to change so that won't be an issue. Any thoughts on my test methods? Maybe go 3on 3off instead?

 

[3dogowner]

Well even though the news is all about record average temperature in Seattle (~50F) for the month of February, I went this morning and bought 6 of the 2" thick 4'x8' extruded foamular R-10 pink board from Dow Corning. I only had to trim about an inch of the end of one board which will lay at the end of the pool where the stairs are located. Except for some slight friction when the cover opens and closes between the vinyl and the insulation board, the fit is essentially perfect. The boards shift just slightly on the water surface when the cover moves, but that's it. The boards were ~$22 a piece. This provides coverage over ~85% of the pool surface. Good enough for now.
Photos are here: https://flic.kr/s/aHsk8tP5eq

I made a note of my electricity meter and will see what happens with and without the boards for the next 4 weeks (2 on, 2 off).

Great day in the pool - sunny, 72F in my microclimate. Love my microclimate. Officially 60F at my local wunderground station.:sunny:

 

[3dogowner]

Well it finally dropped down just below freezing last night. Seems like the pink boards are working well. For the first time ever, there was a layer of ice on the pool cover.
So, I'd say that so far this experiment is showing signs of being a success. Even without 100% surface area coverage (now at ~85%).
Removal each day for my swim time is certainly not difficult. I'm happy to have chosen the 4' width and not the 8' width boards.

 

[3dogowner]

Well the preliminary numbers are in, and I think I will have to halt it prematurely due to the outstanding early results.
For the 66 days in my last electricity bill (mid Nov - mid Jan) avg daily temp ~45F, with the pool heater set to maintain 88F and just an automatic cover, my daily electricity usage was 131.16 kWh/d.
For the 14 days I've had the pink board in place, averaging every other day use (same as before), I'm down to 52.45 kWh/d (I took a photo of my meter exactly 14 days ago noon today). This is still about 10 kWh/d more than the Nov-Jan time period last year, but considerably less than without the pink board.
This represents ~60% decrease in my electricity bill. So my pool energy cost is now only ~ 91 cents/day.
Of note, only ~85% of the water surface is covered with pink board.
A couple of recent cold nights I've noted ice stuck between the top surface of the pink boards and the cover putting more strain on the cover motor to open the cover. I'll take care in the future to watch out for this. I don't want the trade off to be lower bills now in exchange for an earlier than anticipated cover motor burn out.
B/c the boards don't absorb water, they are as easy to use today as they were on day 1.
The savings paid off the cost of the boards in exactly these first 14 days. I'd say this study is closed.

 

[gtemkin]

Funny you should update this today. I spent the day doing maintenance items on the pool and at one point I was gazing at the water trying to think of a way to make 20 foam boards easily accordion up and back into the water. I might have to by a few boards, some waterproof carpet tape, and some kind of hinging material to mockup a system. Congratulations on your success!

 

[3dogowner]

Frost on the cover. Melted where sun is hitting cover.
Also, yesterday I added some 4 inch strips of reflectix to the tops of the pink boards, about 18"s apart towards the middle of the boards where the cover contacts the boards. You can just make out the outline of some of these under the cover (above green hose are a couple). Seems to have decreased the friction somewhat, but no real way to measure except real life watching it in action. I will add picture as things defrost. Don't need to add unnecessary work to cover motor to break the frost/ice bonding to the pink board. ;-)