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Forum Index : Solar : pool solar heater - how powerful is mine?

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poida

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Joined: 02/02/2017
Location: Australia
Posts: 899
Posted: 03:17am 03 Nov 2020
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Today in Melbourne we have some sun. No, really. It's quite hot.
After repairing the solar heater after the cockies have completed
their vandalism for this Winter, it's time to see how it runs.

21 deg C water in the pool
25 deg C coming out of the heater
flow rate: fills a 20 litre bucket in 13 seconds.
water specific heat: 4180 Joules/kg.K

4 K (or deg C) x 4180 x 20 Litres / 13 seconds
is 25,700 Joules/second or Watts
It's a 25.7 kW heater? not bad.
This is 92.5MJ/hour

My street electric wholesaler gives me a fixed rate, $0.396/kWhr
so the roof solar heater is giving me $10.17 per hour equivalent heat.

This post will give anyone here thinking about roof solar heating for
a pool info on it's ROI.

Gas pool heaters are used, not electric anyway.
The bigest Hayward model H400FDAU uses 422 MJ gas per hour and outputs
96kW into the water according to the brochure.
I see the efficiency as
422E6 J/hour or 422E6/3600 Joules/second (Watts) = 117 kW input.
92/117 = 78% efficient.
Cost of gas for me here is 2.88c/MJ so it costs
2.88 x 422 cents/hour = 1,215 cents = $12.15/hour

$12.15/hr for 92kW or 3.7x the power of my rooftop heater.
So it's about $4/hour to run on gas for the equivalent all electric heater.

How about the heat pump type?
I chose the ELPI-17, a nominal 17kW unit.
From the brochure, 0.3kW to 3.02kW input gives a COP of 12.67 - 5.63
flat out it uses 3.02kW and gives 3.02 x 5.63 kW output into the water
or 17kW.
$0.396/kWhr x 3.02 = $1.20/hr for that 17kW.
for the 25.7kW rooftop capacity, it's $1.81/hr or about 1/2 the cost of a gas heater.

So that is why heat pump heaters are the way to go, if you can afford it.
wronger than a phone book full of wrong phone numbers
 
nickskethisniks
Senior Member

Joined: 17/10/2017
Location: Belgium
Posts: 252
Posted: 06:30am 03 Nov 2020
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25.7kWh?

Wow, how big is that? I've got a 3 by 4m solar waterpanel that's outputting about 7-8kW in full sun.
 
Davo99
Guru

Joined: 03/06/2019
Location: Australia
Posts: 727
Posted: 07:42am 03 Nov 2020
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  nickskethisniks said  25.7kWh?

Wow, how big is that?


I was thinking the same thing!
Is it a flat plate or a vac tube type?

I have a gas heater converted to waste oil. 200Kw and costs cents, if that, to run.
It is a bit hands on in that it does not automatically light and has no thermostat control.  Not really important to me as I don't mind a bit of hands on. I could automate it more but really only needs lighting and turning off.

I'll put it on a spa and use that for initial Heating and fast heating in winter.
In summer I will have enough PV power to run the regular heater. I Think that's only 5-6 Kw but enough to keep it warm.

In winter I want to use the spa as a heat bank and pump the water back through a HE to the house. Should hopefully only have to fire it every couple of days. I could do it less if I took the water temp higher but not sure what the fiberglass shell would take and don't want to damage it.


I have been reading lately that Direct solar water heating, Tubes I take it, are a lot more efficient than PV , Resistive I assume in that.
Anyone know the real numbers?
I was under the impression they are much on a par for given collector area.
 
nickskethisniks
Senior Member

Joined: 17/10/2017
Location: Belgium
Posts: 252
Posted: 09:30am 03 Nov 2020
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  Davo99 said  

I have been reading lately that Direct solar water heating, Tubes I take it, are a lot more efficient than PV , Resistive I assume in that.
Anyone know the real numbers?
I was under the impression they are much on a par for given collector area.


I've got mixed feelings about my system, we paid 12000€ for the solar flat panels, placed by professonals 800l tank, controller,.. . We thought we would have some subsidiary (+-3000€). Not...long story...bummer..
We have a big system were the floor heating and woodstove is also connected to the system. So it's also complicated to let everything interact with eachother on a good way.

A normal solar water heating system (domestic household 5persons?) for sanitary water would cost about 5000€ and after subsidiary about 2-2500€.

Mixed feelings with our system because when there is sun we have a lot of power, to much power, so it contribute to the floor heating.
And it's working like it should be. Some days it just heats the house on its own. We can expect 7 months/ year 100% without additional heating of the buffertank. I think the other months are roughly 50%.

However in clouded weather the panels are only maybe 30-40°C wich is not enough for sanitary water. Maybe for floor heating but that would make the system more complex and not worth the money and effort. Those days I would be more  with regular photovoltaic panels. I could've placed 8 panels more on the same surface and for the money spent on the system....

If I would redo the system, I would have place photovoltaic... I'm also off-grid..., but 3years ago I wasn't convinced I could stay off grid... once my kid(s)are becoming teenagers I think I need to get a grid connection.

Today I'm going to install 4extra solar panels
Winter is coming!
 
Warpspeed
Guru

Joined: 09/08/2007
Location: Australia
Posts: 3680
Posted: 11:35pm 03 Nov 2020
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Direct solar water heating works very well, provided reaching high temperatures are not a prime requirement.
Pool heating is the perfect application, but achieving higher efficiency at domestic hot water temperatures becomes much more difficult.

Probably the best way is to preheat the water with whatever solar is available, then feed the "warm" water into the main hot water heater, of whatever type that happens to be.

Even in mid winter with totally grey skies, you will still get a few degrees of pre warming, and that will stay warm for a very long time, as the losses through tank insulation will be very low.

At mid summer, the pre heater will probably boil, and the main hot water heater will require very little if any, additional energy input. A thermostatic mixing valve will be required to mix cold water with hot to prevent scalding injuries.  That is now a statutory requirement anyway.

At least in Melbourne, the most efficient system would be a mains pressure direct solar preheater, feeding into a normal natural gas powered storage water heater.

A photovoltaic hot water system might actually be pretty efficient where higher temperatures are required, especially if the entering water was pre warmed by direct solar.
Edited 2020-11-04 09:41 by Warpspeed
Cheers,  Tony.
 
Clockmanfr

Guru

Joined: 23/10/2015
Location: France
Posts: 344
Posted: 08:31am 04 Nov 2020
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I am in France, so pool heating is important for us to extend the pool use times.

In the end we opted for a Air Source Heat Pump that heated the water after it passed through the filter on the way into the pool.

It uses 3.2kW and supposedly gives 7.5kW output, cost was $1,700 US dollars.

I have sufficient AC coupling to power all the pool equipment.

Real figures......  from 20 degrees C, to 30 degrees C, with exterior ambient temps at approx. 20c  gives an increase over a 12 hour period of 3c so getting up to 30c takes about 3 to 4 days, depending on the time of year and the length of time the Sun is up and out.

When running the Heater is always putting out water at 2c above the actual pool temperature.

Trust this helps?

Here's a link to our pool and fun stuff.

https://www.fieldlines.com/index.php/topic,150204.msg1052752.html#msg1052752
Everything is possible, just give me time.

3 HughP's 3.7m Wind T's (12 years). 5kW PV on 3 Trackers, (8 yrs). 9kW PV AC coupled SH GTI's. OzInverter created Grid. 1300ah 48v.
 
nickskethisniks
Senior Member

Joined: 17/10/2017
Location: Belgium
Posts: 252
Posted: 10:55am 04 Nov 2020
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A practical example today @ 11.30:

Pump is running at 5l/min, and in/out is 30/40°C. That's about 3.4kW output.
Outside it is 9°C, clearsky, sun is far from 100% on the panels, you can see at the shading.
A solar panel 280W on the same angle without shading is giving 31V 5A01 --> 155W, 8 panels on the same area would bring me about 1240W.

So you see, the thermal panels are a lot more efficiënt in clear sky, the actual usefull thermal area is 11.1m² (Tisun Fi3/4), roof area is 3.01*4.01m.

Offtopic, the group of 4 panels I did yesterday, it took me more than a day to get those installed... Ones my aluminium bars were mounted, I discovered I had 40mm brackets instead of 35mm... So I had to adjust (file,saw,grinding...) them, it took me more than 2h for just that....
Installing solar panels on a roof like ours is a real pain...

I will try to make a new measurement in a few hours.

 
Davo99
Guru

Joined: 03/06/2019
Location: Australia
Posts: 727
Posted: 10:22am 05 Nov 2020
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  nickskethisniks said  
Offtopic, the group of 4 panels I did yesterday, it took me more than a day to get those installed...


I think this is a relevant point.

PV is a LOT easier to install than water heating panels.
Muc easier to run a wire than run Plumbing.  Sure it's a one off but there is also the aesthetic factor as well.

I put another 2.5 Kw of Panels on my fathers Shed roof today in about 90 min. Yeah, it's flat tin so pretty easy like that but all I had to do up top was lay the panels. run the cable and push it through the roof overlap.
This was adding on to an existing inverter but even putting on another one would not have taken an hour.  30 Min most likely and it can all be hidden. Doubling existing Panels would require much more time and expense.

Not disputing direct water can be more efficient but just saying there can be more to it. Given the better eating ability of direct at least for low heat applications, seems pool heating is ideal.

Don't know if anyone has ever done this but I used to cover my pool with black Builders plastic.  This gave 100% heating surface and also did a lot to stop heat loos through evaporation and also kept out leaves. Couple of months either side of summer this alone would heat the pool very well. In summer It would easily make the pool waay too hot.

There are different pool covers, some of them thermal insulative like Bubble wrap but just plain Builders plastic ( which I welded together with a soldering iron as I had a large pool ) added a lot of heat .  Dead cheap, very easy to install and kept the pool clean and rain water out so the chemical balance was maintained as well.

If using this with external heating, the water could be warmed really well and for a good time either side of the swimming months.
 
nickskethisniks
Senior Member

Joined: 17/10/2017
Location: Belgium
Posts: 252
Posted: 06:18pm 05 Nov 2020
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I didn't come to a new real measurement... I think the max difference was 15°C with 5.5l/min. My solar array on this roof was not doing more then 4000W (6720Wp). One thing I noticed was that the sun is lower when it's pointed straight to the panels. So there is actually not much gain after 12h.

Those 4 extra panels are really adding some kw's in this weather. So the washing machine was on all the time, even the dishwasher and dryer were joining for some time.
 
Davo99
Guru

Joined: 03/06/2019
Location: Australia
Posts: 727
Posted: 01:13pm 09 Nov 2020
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  nickskethisniks said   I think the max difference was 15°C with 5.5l/min.


If you are saying you got a 15oC temp rise with a 5.5L min flow rate, that's pretty impressive....  Especially from electric.

I have seen a lot of YT vid where people are trying to heat pools by throwing a coil of copper pipe on an open fiore. Makes me Cringe every time.  Sure they are normally above ground pools which are not that large but it also means the heat loos through the sides as well as the surface would be a LOT more than their pissweak heating could provide.
Just stuns me that these people can't see what a useless exercise this is or ever take the time to crunch some numbers and see what heating water takes!

I brought my "Utility" water heater down today. Going to fire it up tomorrow to boil some water to kill all the weeds that have again gone nuts thanks to the rain we have had. Was going to fire up the oil burner unit but I need to burn off a fair bit of power before the next meter read. Been over a Month with the solar turned off completely and still way ahead.  We have a more or less normal consumption when we don't have to heat and cool.

Hopefully I can burn off some of the power with boiling  500L or so of thermal weed killer.

  Quote  Those 4 extra panels are really adding some kw's in this weather. So the washing machine was on all the time, even the dishwasher and dryer were joining for some time.


Found that myself. Just  adding a small  amount of extra panels to a relatively large existing setup can make quite  the difference.
I put it down to my belief of waking things up early and keeping them going till late is where the real gains are rather than hitting the highest outright peak.

Just found that on the setup at my fathers I added to.  3 Kw to start, put on an extra 2.5 and didn't get relatively double the generation on similar day but  more than 3 times.  It's only a 4 KW inverter as well so again showing the benefits of overclocking.
 
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