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Solar Hot Water Experiment


David Mack

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18 months ago Jen-in-Wellies wrote about her solar thermal hot water system. A few months later, while browsing eBay for solar panels for Belfast I came across someone selling two panels – one a photovoltaic panel (which for the past twelve months has been fitted to Belfast’s roof, keeping the batteries topped up), and one for solar hot water. This panel has been in my garage for the last year, but with the advent of Spring, and the enforced stay at home during the coronavirus lockdown, I decided to experiment with it.

 

Also lurking about at home was the old domestic copper hot water cylinder which I had replaced a couple of years ago as it was leaking around the seam between the sides and the domed top. I thought it would make a suitable stand-in for a boat calorifier. It is larger than a normal domestic tank, so substantially more water than a typical boat calorifier.

 

The panel and the cylinder were connected together with an assortment of pipe fittings and lengths of garden hose – basically whatever I had lying around or could bodge. As I didn’t have a suitable pump the system would have to work on gravity circulation, which meant putting the cylinder above the panel. So, taking advantage of the slope of the back garden, this was Version 1.

 

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The panel is on an east facing slope. The yellow/grey plastic pipe is a header tank (which needs to be topped up about hourly as the system is not completely watertight), and yes, you can wedge a push-fit plastic waste pipe elbow into a 1¼ bsp threaded iron fitting if you first trim the end with a craft knife, then use enough brute force and stixall. The staining down the tank side is from the leak when water was running down between the copper cylinder and the fibreglass wool insulation jacket (thrown away at the time as it was a sodden mess).

 

Once I had filled up the system and got it sufficiently leak tight, around 10 in the morning, the thermosyphon started almost immediately. On a bright sunny day, but with the air temperature quite cold, the short length of copper pipe below the T piece got so hot that you couldn’t keep your hand on it for too long. After a few hours, the tank was getting noticeably warm from water level (just below the leaking seam) to about half way between the coil connections. By 4.00 in the afternoon, as the sun was disappearing over the trees and the panel was about to be in shade, the upper ¾ of the tank was luke warm, so yes, I had heated a quantity of water (much more than a boat calorifier full), but not really to a useful temperature.

 

For Version 2 I thought about getting an increased height difference by moving the panel down the slope, and also by tilting it to face south to get more of the afternoon sun. It did involve a much longer length of hose, and some support to maintain a constant rising slope (just) from the upper panel connection towards the upper coil connection.

 

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Version 2 was not a success. Although the connection at the top of the panel was hot, the length of the pipe (and perhaps the almost flat gradient) meant that the hot water never got to the tank. The upper hose went from hot at the panel to no detectable warmth by about half way to the tank.

 

For Version 3 I reverted to having the tank close to the panel, but with the tank elevated, and the panel angled facing south. Yes, I should have built Version 3 closer to the garden tap – the hose for filling the tank only just reached! The rope was to hold the panel down, just in case the wind got up.

 

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This was much more successful! As soon as the panel pipework was filled, the copper pipe below the T piece was too hot to hold, and pretty soon after the whole T piece was almost as hot. By the time I had Version 3 completed it was too late in the day to get much heat into the tank, but over the next few days I topped the system up first thing in the morning, and by late afternoon the tank was almost hot enough for a shower from the top down to within a few inches of the bottom. And that with no insulation on the tank.

 

I am still wondering whether the rate of heat transfer is constrained by relying on a thermosyphon through ½” garden hose. So now I have a 12V circulation pump on order from eBay. I’ll see whether that makes any difference when the pump arrives.

 

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Just the copper cylinder without the panel needs testing for a fair comparison.

 

For an efficient system, insulate the cylinder and the pipes.  An old duvet will be enough for proof of concept.

 

The only reason I don't have one on my boat roof is because the 30' roof is a bit full of everything else!

 

I won't trade the PV solar for the water solar, but I need the coal and the wood and the plank and the shaft ... and I also need enough room to climb across to get to ladders.

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A few things strike me:

 

1) In a real boat installation the calorifier needs to be lower than the panel, meaning there needs to be an electric pump to push the hot water from the panel on the roof, down into the calorifier inside the cabin, so an PV panel is needed as well as the water-filled panel, to run the pump. (Or burn diesel to charge the batteries to run the pump which is self-defeating). 

 

2) The very hot water David notes at the outlet from the solar is only that hot because of the minimal circulation. Once the water is being pushed around at a sensible rate, the panel water will assume nearly the same temperature (i.e. low) as the calorifier temperature. This paradoxically improves the thermal efficiency of the panel.

 

3) The effectiveness of a system that works well in sun is not gonna be the same on a normal summer's day when its pishing with rain.

 

4) The system will need antifreeze to avoid bursting the panel in frost. 

 

5) People design solar hot water systems from time to time but they never seem to get widely adopted, so I guess something always stops them working quite as well as hoped. I suspect they simply don't work at all in winter so people need a good alternative anyway, which tends to get used all year around rather than switching the solar ON in the summer. Just a guess though.

 

 

 

 

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11 hours ago, David Mack said:

I am still wondering whether the rate of heat transfer is constrained by relying on a thermosyphon through ½” garden hose. So now I have a 12V circulation pump on order from eBay. I’ll see whether that makes any difference when the pump arrives.

Your experiments seem to be coming on well. Looking forward to seeing more. Thermal syphoning without a pump is sometimes used in land based solar for heating of swimming pools, domestic water with the panels placed in a garden, rather than the roof. Not so useful for a boat, unless you have a long tug deck!  ?

My solar set up started producing useful hot water again since the weather changed as we went in to lock down. The calorifier exceeded 60C yesterday afternoon

1 hour ago, Mike the Boilerman said:

1) In a real boat installation the calorifier needs to be lower than the panel, meaning there needs to be an electric pump to push the hot water from the panel on the roof, down into the calorifier inside the cabin, so an PV panel is needed as well as the water-filled panel, to run the pump. (Or burn diesel to charge the batteries to run the pump which is self-defeating). 

See my comment above. The pump on mine is run from a small 20W PV panel. All that is required. Takes up about a square foot or two of roof space.

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2) The very hot water David notes at the outlet from the solar is only that hot because of the minimal circulation. Once the water is being pushed around at a sensible rate, the panel water will assume nearly the same temperature (i.e. low) as the calorifier temperature. This paradoxically improves the thermal efficiency of the panel.

Yep. There is a sweet spot of flow rate that optimises the energy extraction and cauliflower temperature.

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3) The effectiveness of a system that works well in sun is not gonna be the same on a normal summer's day when its pishing with rain.

Yes. on a summer day when it persists down all day I get effectively nowt from solar, both thermal and PV.

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4) The system will need antifreeze to avoid bursting the panel in frost. 

Yes. I use a combined propylene glycol antifreeze and inhibitor.

 

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5) People design solar hot water systems from time to time but they never seem to get widely adopted, so I guess something always stops them working quite as well as hoped. I suspect they simply don't work at all in winter so people need a good alternative anyway, which tends to get used all year around rather than switching the solar ON in the summer. Just a guess though.

. My new control electronics is making better use of the available sunshine in the spring and autumn. Mine has worked every summer and provided all my summer hot water since 2008. As it is a mix of adapted commercial and home made bits, rather than a developed boat solution, it has needed some work on my part to design, build, repair and update it. Some years it needs no work, others more. In winter the stove back boiler does all my hot water. It has suited my circumstances well, but I can design and build this sort of stuff and most boaters can't. No one has done a properly developed commercial system for boats in the way that PV now is and there aren't people who can fit them for those that don't want to DIY. Not convinced there is a market for it to be honest. It is only appropriate for a subset of the boating market.

I suspect the household market is similar. The systems just haven't been as well developed, due to their low volume and when they break down people just use the alternative that they have for water heating in the winter and rainy days. Try and get a plumber in to repair it and most wouldn't have a clue. Of the commercial bits in my system, the plumbing around the pump all had to be replaced after the plastic pneumatic push fit fittings they stupidly used all started leaking after a few years. The control electronics after the sensor aged and drifted. Only the panel is still the commercial part and on this a flexible connection hose failed over the winter that had to be replaced when I first started it up in the spring and all the antifreeze was dumped on to the roof! Another reason to use Propylene Glycol antifreeze. It is non toxic.

 

Jen

Edited by Jen-in-Wellies
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1 hour ago, Jen-in-Wellies said:

See my comment above. The pump on mine is run from a small 20W PV panel. All that is required. Takes up about a square foot or two of roof space.

 

That's good news how little power the pump needs. I was imagining it would need a decent sized 100w panel, in order to run a 20w pump at less then optimum sun conditions. But I gues its self balancing, as the power output from your PV drops, so does the need to circulate as the heat energy from the wet solar panel output also drops proportionally. What pump are you using, out of interest?  

 

1 hour ago, Jen-in-Wellies said:

Yes. on a summer day when it persists down all day I get effectively nowt from solar, both thermal and PV.

 

This surprises me. I still get significant output from my PV when it rains in summer. Certainly enough to keep the fridge and the rest of the boat powered up without having to run the engine or genny. THis is with 560W of panels.

 

On the primary circuit containing the antifreeze, you have a sealed system with expansion vessel I presume? How do you go about filling it with antifreeze? Or maybe you never need to.  

 

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2 minutes ago, Mike the Boilerman said:

 

That's good news how little power the pump needs. I was imagining it would need a decent sized 100w panel, in order to run a 20w pump at less then optimum sun conditions. But I gues its self balancing, as the power output from your PV drops, so does the need to circulate as the heat energy from the wet solar panel output also drops proportionally. What pump are you using, out of interest?  

 

 

This surprises me. I still get significant output from my PV when it rains in summer. Certainly enough to keep the fridge and the rest of the boat powered up without having to run the engine or genny. THis is with 560W of panels.

 

On the primary circuit containing the antifreeze, you have a sealed system with expansion vessel I presume? How do you go about filling it with antifreeze? Or maybe you never need to.  

 

My Sister has a house based solar hot water system and similar to Jen it uses a solar panel to power the pump, which means it's in effect self regulating,  suns out pumps running, it is remarkably efficient even on a sunny winters day the tank can be warm enough for a shower.

 

I could be wrong but I think her system doesn't contain antifreeze, the fluid just isn't in the panel when the pump isn't running, I am a bit woolly about this bit :)

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4 minutes ago, tree monkey said:

I could be wrong but I think her system doesn't contain antifreeze, the fluid just isn't in the panel when the pump isn't running, I am a bit woolly about this bit :)

 

I think it must have, but I've never had a really close look at one, they are always in dark and mucky lofts! Where would the fluid go if not in the panels, and how would they automatically refill and purge, would be the difficulties that I can't see simple solution for.

 

And another problem not address, is what happens if the pump fails in mid summer sun. The antifreeze fluid needs to either not boil in the panels in the sun, or the steam pressure needs handing somehow without losing the antifreeze.  In dunno how household systems manage this. 

 

 

 

 

 

 

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2 minutes ago, Mike the Boilerman said:

 

I think it must have, but I've never had a really close look at one, they are always in dark and mucky lofts! Where would the fluid go if not in the panels, and how would they automatically refill and purge, would be the difficulties that I can't see simple solution 

 

 

This is why I am said about the antifreeze because I am sure I asked her about a pump failure.

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1 minute ago, tree monkey said:

This is why I am said about the antifreeze because I am sure I asked her about a pump failure.

 

Even without antifreeze, the system would need to be strong enough to hold in steam pressure. Puzzling. 

 

 

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34 minutes ago, Mike the Boilerman said:

 

That's good news how little power the pump needs. I was imagining it would need a decent sized 100w panel, in order to run a 20w pump at less then optimum sun conditions. But I gues its self balancing, as the power output from your PV drops, so does the need to circulate as the heat energy from the wet solar panel output also drops proportionally. What pump are you using, out of interest?  

You almost don't need a controller if you drive the pump from a just big enough PV panel. I almost did this, but didn't like the idea of current flowing in the motor coils, but not enough to get it to turn. Worried about the long term effect.

Can't tell you what the pump is. I think it is a 24V one, being under run from a 19V open circuit solar panel. It came as part of the pump housing I bought, but there are no manufacturer name, or part numbers on the pump to identify it. I did measure the flow rate once, but would need to dig the nummbers out.

Quote

 

 

This surprises me. I still get significant output from my PV when it rains in summer. Certainly enough to keep the fridge and the rest of the boat powered up without having to run the engine or genny. THis is with 560W of panels.

 

On the primary circuit containing the antifreeze, you have a sealed system with expansion vessel I presume? How do you go about filling it with antifreeze? Or maybe you never need to.  

 

It is open vented with a header tank. If the pump doesn't work for whatever reason then the liquid in the collector will flash to steam in sunshine, which would spike the pressure in a sealed system. It could be done as a sealed system I'm sure, but I've kept it simple and top up the fluid as needed.

 

10 minutes ago, CV32 said:

Mine was designed as a drain back system and worked initially. Unfortunately, I suspect that the height difference to the roof of the boat doesn't give enough head to reliably open the check valve and let air in to the pipe to allow the coolant to drain back. A new check valve works OK for a week or so, then stays closed all the time. Could work for houses, especially with a couple of storeys between the tank and roof and with an inclined panel, but not on my boat system. Even the makers of my system said that using it without antifreeze would invalidate the warrantee if there was a freeze failure as the check valve is a single point of failure.

13 minutes ago, tree monkey said:

This is why I am said about the antifreeze because I am sure I asked her about a pump failure.

One system that was being heavily marketed around the time I did mine was all done in small bore soft plastic tube, including the collector. The idea was that this didn't need antifreeze as the tube could expand when frozen. No idea how reliable this was, or if the system is still available. I wasn't convinced myself at the time.

 

Jen

Edited by Jen-in-Wellies
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The basic setup interests me for my pool out here in France - using a small PV panel to run the pump to circulate from the pool > round the hot water solar > back to the pool. If it gives me a few degrees extra then its worth having a go !

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9 minutes ago, David Mack said:

A diagram may be more helpful.

 

Drainback-solar-thermal.jpg

Not sure this would work with a nearly horizontal panel, as on a boat roof.

Ah. Different from mine. Can't be bothered drawing a plumbing diagram as it doesn't work reliably on a boat anyway. As you say, horizontal collectors on a boat are a problem.

Jen

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I have replaced a pump on a domestic system that used small bore soft pipe, no antifreeze, direct to cylinder, no coil, the roof panel was part solar to drive the pump, there were 2 temp sensors on the cylinder, the pump was a diaphragm type and the house owner said the system worked well untill the pump failed, the pump was in the loft just above the main water tank, i did notice on replaceing the pump that the small bore pipe contained a lot of scale sediment.

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Nice garden, and well done. Some people stay at home doing nothing but moan during this lockdown. Getting out in the garden and doing some DIY science/engineering experiments should be compulsory for everybody.

 

...............Dave

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47 minutes ago, dmr said:

Nice garden, and well done. Some people stay at home doing nothing but moan during this lockdown. Getting out in the garden and doing some DIY science/engineering experiments should be compulsory for everybody.

 

...............Dave

Or if it’s raining watch project binky on YouTube. Great engineering with a hint of naked gun.

Edited by Jon57
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Slightly off topic and not really relevant for boats...  I have a PV solar system on my house and was looking to add a solar hot water system for hot water. Then I discovered the you could get a box of tricks that took the excess PV electricity and used it to power the immersion heater in my hot water cylinder. Today I got 4KWh of water heating which makes a quite a difference. No pumps no freezing no complex plumbing issues. Easy to fit been quite impressed so far 
 

 

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6 minutes ago, jonathanA said:

Slightly off topic and not really relevant for boats...  I have a PV solar system on my house and was looking to add a solar hot water system for hot water. Then I discovered the you could get a box of tricks that took the excess PV electricity and used it to power the immersion heater in my hot water cylinder. Today I got 4KWh of water heating which makes a quite a difference. No pumps no freezing no complex plumbing issues. Easy to fit been quite impressed so far 
 

 

Narrowboats just don’t have enough roof space for a kW or more of panels which is what you need to heat water by pv.  It just doesn’t work.

Edited by Chewbacka
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Yes self evidently which is why I never even hinted at such a solution afloat.

 

would be good if there was a similar system to divert excess PV power to the cauliflower immersion. My single 250W panel on the boat would make an impact on my cauliflower if I could use it usefully after charging the batts.  With typical boat PV installations being 500W+ I actually think your mistaken or don’t understand the sort of system I’m referring to.

 

heres the unit I have at home 

https://www.marlec.co.uk/news/new-solar-iboost-video/

 

 

Edited by jonathanA
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The most high tech.  experimental collector I've seen consisted of a sealed and evacuated glass cylinder half silvered on its long axis. Sealed within this cylinder was a smaller tube coated with platinum black through which the fluid to be heated was passed.
The silvering reflected and focused the sunlight onto the central tube while the platinum black coating significantly improved the energy transferred to the fluid.

The efficiency on test was shown to be high but manufacturing costs would have been prohibitive for all but special applications.

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9 hours ago, jonathanA said:

Slightly off topic and not really relevant for boats...  I have a PV solar system on my house and was looking to add a solar hot water system for hot water. Then I discovered the you could get a box of tricks that took the excess PV electricity and used it to power the immersion heater in my hot water cylinder. Today I got 4KWh of water heating which makes a quite a difference. No pumps no freezing no complex plumbing issues. Easy to fit been quite impressed so far 
 

 

You can do it this way. Thing is, the efficiency of PV panels at converting sunlight in to electricity is quite low. If you have excess solar PV electric on a very sunny day, then dumping it in to a hot water cylinder is a good use, rather than letting it go to waste. The alternative in a house is to sell excess back to the grid. Since the selling price is going to be less than the buying price, using it yourself to heat the hot water would be marginally more cost effective if you heat your hot water electrically anyway, but pennies. Some boaters with huge amounts of installed PV panels actually do use a calorifier immersion as a dump for excess solar electric. @peterboat, for example. A dedicated solar thermal panel is more roof space efficient on both a house and a boat. Don't know about relative installation cost, as PV panels have plummeted in price since I installed my solar thermal system.

7 hours ago, Man 'o Kent said:

The most high tech.  experimental collector I've seen consisted of a sealed and evacuated glass cylinder half silvered on its long axis. Sealed within this cylinder was a smaller tube coated with platinum black through which the fluid to be heated was passed.
The silvering reflected and focused the sunlight onto the central tube while the platinum black coating significantly improved the energy transferred to the fluid.

The efficiency on test was shown to be high but manufacturing costs would have been prohibitive for all but special applications.

There have been sealed evacuated glass collectors for a good while. A lower price point probably would sacrifice efficiency by not using platinum though! I considered them for my system around 2007. Their big disadvantage at the time was that if the pump failed and the collectors were in direct sunlight the heat would rapidly destroy them. Also uglier and more vulnerable to damage from vandals and canal infrastructure when cruising than flat panel collectors, so I discounted them. Not looked at them in years, so this may have changed.

 

Jen

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47 minutes ago, Jen-in-Wellies said:

You can do it this way. Thing is, the efficiency of PV panels at converting sunlight in to electricity is quite low. If you have excess solar PV electric on a very sunny day, then dumping it in to a hot water cylinder is a good use, rather than letting it go to waste. The alternative in a house is to sell excess back to the grid. Since the selling price is going to be less than the buying price, using it yourself to heat the hot water would be marginally more cost effective if you heat your hot water electrically anyway, but pennies. Some boaters with huge amounts of installed PV panels actually do use a calorifier immersion as a dump for excess solar electric. @peterboat, for example. A dedicated solar thermal panel is more roof space efficient on both a house and a boat. Don't know about relative installation cost, as PV panels have plummeted in price since I installed my solar thermal system.

There have been sealed evacuated glass collectors for a good while. A lower price point probably would sacrifice efficiency by not using platinum though! I considered them for my system around 2007. Their big disadvantage at the time was that if the pump failed and the collectors were in direct sunlight the heat would rapidly destroy them. Also uglier and more vulnerable to damage from vandals and canal infrastructure when cruising than flat panel collectors, so I discounted them. Not looked at them in years, so this may have changed.

 

Jen

Agreed

 

If you're on a grid connected system I would suggest using your generated electricity to heat hot water is inefficient, pump it back into the grid and get paid, solar hot water is a much better use of the available sun.

 

Obviously it's different is you are off grid

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@David Mack I'd be interested in what panel you've got. Looks much more slimline than mine!

 

We're very happy with our system. On sunny summer days gets plenty hot enough for laundry, showers etc, and provides useful initial heating on sunny spring and autumn days. No antifreeze in our system. We remove roof panel during winter, and just left with 2 pipe stubs that are capped off. Could probably be made more efficient with better pipe insulation. 

 

Initially looked at the standard thermal panels that use glass vacuum tubes, but decided they were too fragile for boat use so went with flat plate collector. More expensive, and less efficient, but in my opinion, better for boat use. Also most glass tube panels are not suitable for mounting flat - something to be aware of for boats. 

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