PV panels and electric heater

yann

Well-known member
Hi all,

Looking for some experience and feedback on heating systems.
Even tough every piece of advice is welcome, Australian experience might be a little more accurate since we do have strong rebates on solar panels at the moment.

I do not have enough roof space to have a pool solar heating system and PV system to generate energy.

Has anybody installed a PV system to generate electricity, and powered an electric pool heating system with that free electricity?

Thanks
 

ajw22

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Jul 21, 2013
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What BTU electric pool heaters can you get down under? How many volts & amps do they require?
 

yann

Well-known member
I have no idea!
I heard someone in the neighbourhood did that and since the government is offering massive rebate and 0% interest on new solar instals, I thought I'd look into it.
My local pool $hop sells these:

Obviously a wide range of heaters and prices.
 

ajw22

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I thought you were thinking of resistive electrical heating and not heat pumps.

So you have a choice of 6 to 17 kw of heating.

Kw = BTU
6 = 20,500
9 = 30,700
13 = 44,000
17 = 58,000

9,600 gallons is 80,000 lbs water.

A 6kw heater will raise your pool temperature about 0.25F / hour
A 17.kw heater will raise your pool temperature about 0.75F / hour

How you power those HP with PV cells in above my pay grade.
 

yann

Well-known member
Yeah I would have imagined.

I thought you were thinking of resistive electrical heating and not heat pumps.

So you have a choice of 6 to 17 kw of heating.

Kw = BTU
6 = 20,500
9 = 30,700
13 = 44,000
17 = 58,000

9,600 gallons is 80,000 lbs water.

A 6kw heater will raise your pool temperature about 0.25F / hour
A 17.kw heater will raise your pool temperature about 0.75F / hour

How you power those HP with PV cells in above my pay grade.

Sorry mate, gallons pounds and oF make no sense to me....
What's BTU? Once I know that I'll be able to convert to metric (I already found the relationship between BTU and weight of water gives me temp increase)
 

ajw22

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1 BTU of heat raises 1 lb of water 1 degree F.

You will need to figure out how 1 kw of heat raises 1 liter of water how much C.

0.25F = 0.14C temperature rise per hour
0.75F = 0.41C temperature rise per hour
 
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yann

Well-known member
Ok. Formula would be
kW=volume in litres x 4 x temperature rise in degrees centigrade / 3412

Which gives me for the most powerful system
17.3=36500x4xt/3412
Or
T=3412x17.3/36500x4=0.4°C in 1h

And for the least powerful 0.14°C in 1 hour

Which means the energy required to heat up the pool by 1°C is the same, just faster with the more powerful system.
So the energy cost of heating up the pool by 1°C is the same with both systems, only faster with the 17.3kw

Correct?
 

yann

Well-known member
1 BTU of heat raises 1 lb of water 1 degree F.

You will need to figure out how 1 kw of heat raises 1 liter of water how much C.

0.25F = 0.14C temperature rise per hour
0.75F = 0.41C temperature rise per hour
I just saw your edit.
We must have been busy calculating at the same time.
Looks like our maths are still good after all these years :)
 

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JoyfulNoise

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There was a company here in Arizona that did just what you are describing - solar PV installation to run a lower power heat pump with any excess generated electricity going back into “the grid”. I wish I still had the white paper that the guy wrote (he also taught as adjunct faculty at the U of A) but the link is dead and his company is no longer in business. When federal and state subsidies existed, it made sense and the costs & efficiencies were on par with traditional solar heating. However, here in the US, the federal subsidies are gone and there’s not much else available from state and local companies (in fact, Tucson Electric is a little bit hostile to new solar installations) and so the cost/benefit no longer works out.

Strike while the iron is hot, I suppose ...
 

cfherrman

TFP Guide
May 10, 2017
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Hays, Kansas
In Kansas it's not very friendly to net metering, selling back to the grid, but any sunny place a simple grid tie is viable since the engery company is not involved and you use simple components to send some sun engery I to your house
 

nate

Active member
Jan 7, 2014
34
Sydney, Australia
heya - fellow Aus here... posted the same thing here: Resistive heater - free electricity

For reference, i pay 28c per kw/h for electricity, and I can see it back to the grid at 11c per kw/h.
So its all about consuming what i can, as you get little back from generating. I chose the most expensive (efficient) panels with micro inverters. It was $12k installed, and payback will be easy in 6years though savings.

2.3kw resistive (basically kettle)... gets me about 2deg a day from free excess solar.

What you posted is a head pump, MUCH more efficient, basically an air con unit
They draw 2kw of power, but produce the heating effect of 8kw+ So expect 4x times the heating.

Id only do resistive because power is free & its super cheap to setup.
Long term - for sure do a heat pump.
 

cfherrman

TFP Guide
May 10, 2017
2,527
Hays, Kansas
2kw power in will never net 8 kw heat out, never

A heat pump is more like 2kw heat in and 1.8 kw heat out (90% efficient) compared to resistance heating 2kw in 1 kw out (50% efficient)
 

JamesW

TFP Expert
Mar 2, 2011
24,207
Electric resistance heat is close to 100% efficient.

Heat pumps are typically better than 100%.

For example, Pentair's Ultratemp is listed as up to 580%.
 

PoolGate

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I think someone above nailed it. Calculate the solar savings reducing your overall electric bill as your pool will use a hefty amount. I would not plan on using solar to actively run your heater since your array won't produce enough power to do much. Are you able to get natural gas? There are also solar roof pool water heaters, which are a different animal. Water is pumped up into the panel for heating and returned to the pool. That may be a better system. In my experience there is no ROI on any solar power system. It sounds as though you guys get a better price for solar I looked for my own house and for a system that I owned it was approaching $40k. No other system configuration would allow net (selling access power back to the grid) production (the installer gets the net).
 

JamesW

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Mar 2, 2011
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Tesla sells a big battery that stores energy so that you can better utilise the available solar if you can't sell it back to the grid.

 

Donldson

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Jun 12, 2009
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NW Ohio
2kw power in will never net 8 kw heat out, never

A heat pump is more like 2kw heat in and 1.8 kw heat out (90% efficient) compared to resistance heating 2kw in 1 kw out (50% efficient)
James said it above, but thought I would jump in to elaborate.

With resistive heat you get virtually 100% efficiency. 2kw in to the heating coils, 2kw heat out. It's pure conversion from electricity to heat. Problem is that there is a lot of thermal mass in a body of water so it is not a very good way to handle things.

With a heat pump you aren't using the electricity to make heat, you are using it to draw heat out of the air and put it in the water. That is why it is more than 100% efficient, the heat isn't coming from the electricity it's coming from the air. You can see this with the massive amounts of heat that blows out of your AC compressor, several times more than if you had a resistive heater running with the same amount of electricity. The confusion just comes down to the terminology, since getting greater than 100% efficiency sounds impossible. However the heat is coming from somewhere, there is no free lunch, it's just not coming from the outlet.
 

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