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Solar water heater - I don’t get it

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Sterling9250

New Member
Joined
Feb 14, 2024
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22
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Portugal
I live in Western Europe and I see solar water heaters all over the place. I don’t understand in any case why someone in their right mind would want one. Why wouldn’t one put in regular solar panels and use that energy to heat water, or whatever else one wants, instead of putting water lines through an attic and on a roof that could lead to problems down the road?
 
You only get 15-25% efficiency from a solar panel and then you lose some of it trying to change electricity back to heat. Direct solar water heating collection gives a much higher energy transfer. Or simply dark things in the sun get hot.
 
timselectric said:
It used to be more cost effective.
Not anymore.
And once the water is hot, the collector space is wasted.
Whereas PV can still produce usable energy.
I prefer versatility. I avoid things that can only be used for a single purpose. Especially if they take up valuable space.
Click to expand...
Space is also a premium in Europe. No need for a big honking water tank inside when it is on the roof. On the other hand, a HPWH will cool the house, which is a plus in the summer time (and negative in the winter time - direct vent in moderate climates works).
 
Solar hot water services have been around (in Australia at least) since the 1970's...
Solar gridtie PV became popular in Australia in the 1990's, but thermal solar was still far cheaper up until the late 2010's
A good thermal HWS will last for decades before needing replacement/repairs so there will likely be thermal HWS systems that have been up there for decades...

Where I used to live before buying this place, we had a thermal solar that was installed in 2001, and cost only $1200 at the time, where a PV system capable of running it was over $6000 at the time- so thermal only was installed (in fact we didn't put PV on until 2016, and it had only just paid itself off when I bought this place in 2020 (my eventual retirement home lol)
Our two decade old solar thermal HWS ran almost all year round with the electric booster element switched off- operating costs literally zero... With a 400L storage tank, it took basically almost a week of really bad weather for it to cool down enough that we needed to turn the booster on...
 
DIYrich said:
Space is also a premium in Europe. No need for a big honking water tank inside when it is on the roof. On the other hand, a HPWH will cool the house, which is a plus in the summer time (and negative in the winter time - direct vent in moderate climates works).
Click to expand...
I value roof space much higher than interior space.
But , that's just personal preference.
 
I remember reading installation costs were three times higher in US than in Europe. Typical water needs are also much smaller in Europe than US. A recent Spanish university study concluded that PV water heating was more cost effective. with low cost of panels. PV water heating is super cheap to do, just not in the US at this time.

Several Australians have told me they scraped solar hot water in favor of PV resistive.
 
It’s fairly common for the back country types to run a large black PVC tube, fill with water on a roof rack and plumb to a shower system or similar.
Doesn’t get much more cost effective than that.

It’s also entirely different than the residential panel based water heaters
 
G00SE said:
It’s fairly common for the back country types to run a large black PVC tube, fill with water on a roof rack and plumb to a shower system or similar.
Doesn’t get much more cost effective than that.

It’s also entirely different than the residential panel based water heaters
Click to expand...
Whistles innocently...
that is indeed what I am running, except mine is a 50m roll of black polypipe laying on the ground behind the shed, with the 400L tank inside the shed... (as the tank is higher than the polypipe, it doesn't even need a pump to circulate the water through the system, natural convection does it for me)
With the extreme heatwave conditions we had for the last month, the water was coming out of the kitchen tap in the shed literally almost at boiling point (I literally have been able to make cups of coffee straight from the tap...)
 
Bop said:
Whistles innocently...
that is indeed what I am running, except mine is a 50m roll of black polypipe laying on the ground behind the shed, with the 400L tank inside the shed... (as the tank is higher than the polypipe, it doesn't even need a pump to circulate the water through the system, natural convection does it for me)
With the extreme heatwave conditions we had for the last month, the water was coming out of the kitchen tap in the shed literally almost at boiling point (I literally have been able to make cups of coffee straight from the tap...)
Click to expand...
It’s a pretty awesome set up! I’m a fan for sure
 
DIYrich said:
If your house is burning, and the water tank ruptures, it will dump a lot of water on the fire. ...
Click to expand...
Only for rooftop tanks...
The one at the old place was a 400L tank at ground level and only the thermal panel was on the roof... (but that type needs a small pump (9w lol) to circulate the water, where the rooftop ones rely entirely on convection currents to do it)

Ground level obviously means far less weight on the roof, but needs a pump... (a 400L roofmount system has a 400l storage tank full of water AND the collector full of water up there- that comes to half a tonne of water up there alone (let alone the weight of the tank and collector...)- that roof needs to be SOLID to handle that... a groundmount like mine is only a couple of hundred kilos of water and panel combined...)
1708459851663.png
That's the east facing PV array on the left, and the thermal HWS panel facing north on the right (the west facing PV array is out of sight on the far side obviously...), the HWS storage tank is on the ground outside, just out of sight to the bottom right
 
Sterling9250 said:
I don’t understand in any case why someone in their right mind would want one.
Click to expand...
(y) agree... not now... too many complications and lack of ROI.

Why do you still see them? For the same reasons you see cars of the 1980's and 1990's having electric opening sun-roofs... because air-con wasn't cheaply available :) :cool:
 
timselectric said:
It used to be more cost effective.
Not anymore.
And once the water is hot, the collector space is wasted.
Whereas PV can still produce usable energy.
I prefer versatility. I avoid things that can only be used for a single purpose. Especially if they take up valuable space.
Click to expand...
They really don't take up a hell of a lot of space. And almost everyone uses/needs hot water.
Direct solar hot water panels will yeild about 2.5 times as many KWH as PV panels.
To avoid "wasting" collector space, use a large tank. I have 120 gallon storage capacity.
The cost of insulated water storage tanks is less that the cost of batteries and powerful inverters and direct electric water heating.
Consider the waste that occurs in a PV system when your hot water system turns on. The power requirement could exceed the capacity of your panels and batteries and all other devices combined and your system might have to use grid power.
 
SeaGal said:
(y) agree... not now... too many complications and lack of ROI.

Why do you still see them? For the same reasons you see cars of the 1980's and 1990's having electric opening sun-roofs... because air-con wasn't cheaply available :) :cool:
Click to expand...
It's a much quicker roi if sized properly and if you actually use hot water
 
SeaGal said:
(y) agree... not now... too many complications and lack of ROI.

Why do you still see them? For the same reasons you see cars of the 1980's and 1990's having electric opening sun-roofs... because air-con wasn't cheaply available :) :cool:
Click to expand...
Not many are installing them new (PV is now cheaper than thermal here at least), but there are systems literally 30-40 years old still running fine, and with the last of the common installs happening only a decade or so ago- expect to see most of those rooftop systems to be staying up there for decades to come yet...

With PV systems, there are two options- either using some of your gridtie PV power during the day (ie timer) or there are now 'mini inverters' that allow a HWS to use DC from the panels and these provide power to the HWS electric element- being AC they are effectively the same as running the element straight from the grid- but at no cost lol...

Using direct DC 'can' be done- but requires modifications to the control circuitry as the thermostat is not capable of switching high voltage DC directly with safety- the contacts could arc and weld together on switch off- and that could have disastrous results- especially if your HWS is one of the many (most) that you don't clear the overpressure valve by the manufacturers specs (every 3-6 months depending on the manufacturer)...

This has a good guide to the various types currently available

And the Mythbusters had a demo of what happens if your contacts weld together (likely on DC) and your overpressure valve is scaled shut...
mybusters exploding water tank
In fact that was so popular they did it several times...
500ft straight up...
Boom again...
 
McKravitts said:
They really don't take up a hell of a lot of space. And almost everyone uses/needs hot water.
Direct solar hot water panels will yeild about 2.5 times as many KWH as PV panels.
To avoid "wasting" collector space, use a large tank. I have 120 gallon storage capacity.
The cost of insulated water storage tanks is less that the cost of batteries and powerful inverters and direct electric water heating.
Consider the waste that occurs in a PV system when your hot water system turns on. The power requirement could exceed the capacity of your panels and batteries and all other devices combined and your system might have to use grid power.
Click to expand...
Like I said, it's a personal preference.
What might work for your location, would be wasteful for mine.
 
Electricity and hot water in Europe:
dualsun.com

DualSun SPRING

DualSun designs and manufactures the world’s first certified hybrid solar panel (PVT): the SPRING 2-in-1 panel.
dualsun.com dualsun.com

If you need more power than your roof can provde, it offloads some of your electricity requirements. Also cools panels for better production.
 
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