Solar Thermal vs Photovoltaic + Heat Pump - averaged over whole year (California / Bay Area)

eegz

Member
May 9, 2017
15
Los Altos, CA
Our Southern facing roof is covered in photovoltaic panels for household electricity and the Northern roof is covered in solar thermal panels for the pool.

Even though the thermal panels are on the north side, the roof gets enough sun to keep the pool warm for about 1/2 the year (shallow pitch, no shade).
The other 1/2 of the year, the thermal panels are doing nothing.

Given we get sunshine pretty much all year around, does anyone know if it would make sense to replace the solar thermal panels with photovoltaic cells powering a heat pump?
The 1/2 of the year where we're not using the pool, the photovoltaics would still be gathering electricity for the pump and other household uses and electricity is very expensive here (and will likely go up as PG&E continues to rack up fire bills).

I've read threads about heat pumps vs solar thermal and gas, but didn't see any that included the opportunity cost of losing half the roof that could be generating electricity the rest of the year.
 

cowboycasey

Gold Supporter
LifeTime Supporter
TFP Guide
Jul 3, 2013
4,535
Fletcher, OK
interesting... since you will be able to use the solar panels all the time and if you wanted you could turn off the heat pump to save power I think you will come out better using solar... They also have newer heat pumps that have variable speed to save energy and therefore save solar power when needed if run in the lower power mode.. :)

It says it pulls 2.4kw, I am sure that is at 6000 rpm mode so the lower 3500 mode is less than that, maybe 1.4 kw or so.....


Specs
60 HzSQ150VS
BTUs @6,000 RPM
(water temp/ambient air/relative humidity)
80/80/80115,000
BTUs @3,500 RPM
(water temp/ambient air/relative humidity)
80/80/8072,000
80/80/6366,000
80/50/6345,000
Coefficient of Performance
(water temp/ambient air/relative humidity)
80/80/809.3
80/80/638.6
80/50/635.0
RefrigerantR410A
Electrical
Heat or Heat & CoolH & C
kW Input2.4
Voltage/Hz/Phase
A – 208-230/60/1
A
Minimum/Maximum Fuse SizeA – 60
Minimum Circuit AmpacityA – 47
 
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scdaren

Gold Supporter
Bronze Supporter
May 20, 2018
397
Clovis, CA
We are in central California. We opted to pretty much cover our roof in pv panels, and use a heat pump for the pool. There are a lot of moving parts to figure out which is the best option -- the cost of the panels, how much electricity the heat pump uses, your overall heating needs, etc. etc. etc. I like that the PV panels offer a lot of flexibility on what that roof real estate gets used for. We also have an electric car, and are pretty heavy electric users.

So difficult question. But a couple of points I would make is that your panels on the north facing roof are not going to produce near to what your south-facing panels are going to. A good tool to determine that is PVWatts (PVWatts). Also, during that darker half of the year, your panels generally are not going to produce as much, especially the north-facing ones.

Anyway, you really need to crunch the numbers. I can give you some of my stats -- my heat pump is a Pentair 140 and uses 7500 watts of power (so 7.5kwh per hour), and it heats my 15,000 gal pool 1 degree per hour. I lose about 3 degrees overnight with a solar cover on, so I figure about 3 hours a day for the shoulder seasons when I'm heating. You also need to look at your PG&E rates, and what your time-of-use plan is, this usually involves looking at the actual tariffs to try to figure out.
 
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eegz

Member
May 9, 2017
15
Los Altos, CA
Thanks cowboycasey and scdaren!
That's a lot of detail with a good range (2.4kwh vs 7.5).

Our pool is about 25,000 gal so assuming worst case of 7.5kw at 5 hours instead of the 3 for the 15,000 gal pool scdaren mentioned would take a little less power than our whole south facing roof outputs in a fall day.
I suppose we could then cycle the water at night and could run at a lower RPM if we don't need to pump to the roof for the solar thermal.

Lots of complexity.