Solar water heating

[Woodfern]

I'm intrigued by what some of you clever people are doing with solar water heating . As a CCer with plenty of PVs, not having to run the engine or diesel heater in summer is appealing.

 

The best way of doing it seems to be to have the solar collector heat a coil in the calorifier. However I don't have a spare coil and the price of a new triple coil is offputting. Has anyone done it in a different way? I'm thinking of the following options:

 

(1) a solar pre-heater, where the collector directly heats potable water which is then either fed into the calorifier, or bypasses the calorifier, like this:

 

(2) a direct set up where the collector is tee'd off the cold input and hot output of the calorifier. Like this, plus some shut-off valves, and potentially a small separate hot water storage tank on the return of the solar collector:

I'd imagine the problem with both of these is you wouldn't get that much hot water except on the hottest of days because water is only passing through when you're drawing it. On houses people seem to get round this by having a storage tank about the collector, but obviously this isn't very practical on a boat!

 

So option (3) gets around the lack of a spare coil by having an external heat exchanger (which could be a cheap small single coil calorifier instead so it stores some)?:

 

 

 

Edited May 11, 2022 by Woodfern

[Jen-in-Wellies]

If your calorifier has an immersion heater, or a boss for one, then you can put in a solar coil in the immersion boss. It is possible to retrofit a suitable boss in a cauliflower, using an Essex Flange, but that is more work. If you are CC'ing and rarely on shore power, then an immersion heater will be largely redundant and this is probably the easiest way of converting it to solar thermal.

http://www.solarcoil.co.uk/products.html

I used a dual coil cauliflower. One heated by the stove back boiler, the other by the solar thermal system. I didn't bother with engine heating. There are even triple coil cauliflowers, but this means a new calorifier.

Jen

 

[Woodfern]

1 minute ago, Jen-in-Wellies said:

If your calorifier has an immersion heater, or a boss for one, then you can put in a solar coil in the immersion boss. It is possible to retrofit a suitable boss in a cauliflower, using an Essex Flange, but that is more work. If you are CC'ing and rarely on shore power, then an immersion heater will be largely redundant and this is probably the easiest way of converting it to solar thermal.

http://www.solarcoil.co.uk/products.html

I used a dual coil cauliflower. One heated by the stove back boiler, the other by the solar thermal system. I didn't bother with engine heating. There are even triple coil cauliflowers, but this means a new calorifier.

Jen

 

 

Thanks Jen. Do you find you miss having engine heating, for example in mild weather where you're not using the stove but can't get enough from the solar, and are running the engine to charge the batteries anyway?

[Jen-in-Wellies]

5 minutes ago, Woodfern said:

 

Thanks Jen. Do you find you miss having engine heating, for example in mild weather where you're not using the stove but can't get enough from the solar, and are running the engine to charge the batteries anyway?

Yes. It can be a pain some times. Summer days, where it rains all day are one. A few days in Spring and Autumn, where it is too hot for a stove, but not enough solar are another. I get round it by lighting the stove for a few hours and opening all the windows! I've had the current system for 15 years now and these disadvantages haven't driven me to change it.

I placed the cauliflower in the centre of the cabin, around 25' from the engine, to be in the optimum spot for the gravity circulation from the stove. This would have given very long pipe runs too and from the engine, so I decided not to do that.

Jen

[Jen-in-Wellies]

More to add. When I built my solar thermal system, photovoltaic solar panels were very expensive. Since then, the price has crashed through the floor. Solar thermal is more efficient per square foot at extracting energy and using it to heat water than solar PV feeding an immersion heater, but some people have such an excess of solar PV that they use the extra in summer to heat water. I doubt this works so well in spring and autumn, where there won't be such an excess of electricity after the rest of the boats needs are met. You have to make a decision on how to best use the limited roof space you have available.

That so few boaters have solar thermal is probably down to a couple of things:

A lack of ready to install systems. I had to cobble together mine from house parts, then spend the next fifteen years gradually replacing the bits that wore out, or were not suitable. This needs a certain minimum of engineering skill.

The other is that for most people, a combination of engine heating and cabin heating of the water is sufficient, plus immersion for those on a shore line in winter.

[GUMPY]

I have solar water heating at home.

It uses a coil in the tank as you would expect. I would be very wary of directly heating potable water due to the high temperatures, once my tank is up to temperature I often see temperatures of 140degC in the panels on a sunny day.

You could run it through a three way valve to disconnect the engine and connect the solar thus using the engine coil. Mine has a static pressure of one bar when cold the pressure varies a lot with the temperature so you will also need a pressure vessel on the system to deal with this.

All in all its not a simple system and I think in hindsight if I had solar PV at the time I might have gone for a divert to feed the immersion instead

 

[GUMPY]

Another thought is that the Calorifier should be as big as you can get to maximize the storage of hot water, mine is 300 litres and uses the bottom coul for solar heating the whole tank and the top coil for the central heating only heating half a tank.

System is a Grant Solar one, maybe could be adapted for boat use.

 

[Jen-in-Wellies]

6 minutes ago, Loddon said:

Another thought is that the Calorifier should be as big as you can get to maximize the storage of hot water, mine is 300 litres and uses the bottom coul for solar heating the whole tank and the top coil for the central heating only heating half a tank.

System is a Grant Solar one, maybe could be adapted for boat use.

 

Good point. My cauliflower is 60l. Yesterday it reached  between 55 and 60C on solar. This morning, it had dropped to somewhere between 35 and 40C. Still plenty warm enough for a bath, or shower, but you have to plan your usage around the weather and how warm your water is likely to get, so bathing, or clothes washing on more marginal days fits around this, rather than when it is most convenient for you.

A big cauliflower is heavy. An extra kg for each litre of water inside. A massive calorifier needs to be planned in to the ballasting of your boat, or you could end up with a list, depending on where it is placed.

Edited May 11, 2022 by Jen-in-Wellies

[Woodfern]

  

Thanks both for the advice! Sounds as if options 1 and 2 might be dangerous to DIY as well as less effective...

 

Tbh, I had been thinking of upgrading the calorifier at some point anyway as I think there is space, and it doesn't really hold enough water for 2 people (especially if you shower as decadently as I do 😀) I fitted water filtration so I could use as much as I like, but now the limiting factor is how much hot you can store and for how long. I hadn't appreciated how much a new calorifier can be though. I will investigate to see if I have a boss for an immersion heater in the current one in the meantime.

 

I know many use the excess PV in the summer for water heating, but as you say this seems so inefficient I can't see it working except for in the hottest months. I was also hoping to be able to use the solar to at least partially heat during cold but sunny weather, cutting down on diesel use and helping the diesel heater get its heat to the radiators quicker.

Edited May 11, 2022 by Woodfern

[Jen-in-Wellies]

17 minutes ago, Loddon said:

I often see temperatures of 140degC in the panels on a sunny day.

I use open vented indirect for the solar panel coolant. This is simple and I just replace any coolant lost to boiling, or evaporation. When the calorifier reaches maximum safe temperature, the circulation pump is disabled and whatever coolant is within the panel will boil. It also means I can use antifreeze/inhibitor to protect the panel and pipe work and there is no need to drain it down completely to survive winter.

Edited May 11, 2022 by Jen-in-Wellies

[Tony1]

30 minutes ago, Woodfern said:

 

I know many use the excess PV in the summer for water heating, but as you say this seems so inefficient I can't see it working except for in the hottest months. I was also hoping to be able to use the solar to at least partially heat during cold but sunny weather, cutting down on diesel use and helping the diesel heater get its heat to the radiators quicker.

 

I'm not going to try to defend the inefficient method of using PV panels to heat water via an immersion heater, as there is little doubt Jen's method is better, but I can say that it works for more of the year than I expected it to. I have 1400w of panels, and at a very rough estimate I'd say I've probably been getting enough charge to heat a tank of water on most days since mid-March, and on every day since mid April- but that is subject to the mooring not being overlooked by trees etc.

On the poor solar days I have heated the water to say 40 degrees or so, which is still hot enough to shower or wash. 

One tank of water takes about 80Ah of charge to heat up via the immersion, and my panels can replace that charge in 2-3 hours on a late spring or early summer day.

Based on how the solar has picked up since January, I'm expecting to have at least one tank of hot water per day until the end of August, and probably on most days after that until end of Sept, and then it will become very intermittent.  

So in summary I'd say the solar panel/immersion heater method is good for hot water on most days from April-Sept (inclusive), and on intermittent days in March and October. 

 

 

Edited May 11, 2022 by Tony1

[Jen-in-Wellies]

7 minutes ago, Tony1 said:

 

I'm not going to try to defend the inefficient method of using PV panels to heat water via an immersion heater, as there is little doubt Jen's method is better, but I can say that it works for more of the year than I expected it to. I have 1400w of panels, and at a very rough estimate I'd say I've probably been getting enough charge to heat a tank of water on most days since mid-March, and on every day since mid April- but that is subject to the mooring not being overlooked by trees etc.

On the poor solar days I have heated the water to say 40 degrees or so, which is still hot enough to shower or wash. 

One tank of water takes about 80Ah of charge to heat up via the immersion, and my panels can replace that charge in 2-3 hours on a late spring or early summer day.

Based on how the solar has picked up since January, I'm expecting to have at least one tank of hot water per day until the end of August, and probably on most days after that until end of Sept, and then it will become very intermittent.  

So in summary I'd say the solar panel/immersion heater method is good for hot water on most days from April-Sept (inclusive), and on intermittent days in March and October. 

 

 

Thanks for the real world numbers. More practical than I thought and similar to the performance I've been getting since March from an 8'x4' solar thermal panel. Combine that with Lithium batteries, which don't need complete recharging every few days, then it could well make more sense than solar thermal. Things have changed since I fitted out my boat. In the naughties, 1400W of solar PV panels would have bankrupted me! I could only afford 160W and that was expensive. Their price crash since then has changed the equation considerably.

Jen

Edited May 11, 2022 by Jen-in-Wellies

[Woodfern]

10 minutes ago, Tony1 said:

 

I'm not going to try to defend the inefficient method of using PV panels to heat water via an immersion heater, as there is little doubt Jen's method is better, but I can say that it works for more of the year than I expected it to. I have 1400w of panels, and at a very rough estimate I'd say I've probably been getting enough charge to heat a tank of water on most days since mid-March, and on every day since mid April- but that is subject to the mooring not being overlooked by trees etc.

On the poor solar days I have heated the water to say 40 degrees or so, which is still hot enough to shower or wash. 

One tank of water takes about 80Ah of charge to heat up via the immersion, and my panels can replace that charge in 2-3 hours on a late spring or early summer day.

Based on how the solar has picked up since January, I'm expecting to have at least one tank of hot water per day until the end of August, and probably on most days after that until end of Sept, and then it will become very intermittent.  

So in summary I'd say the solar panel/immersion heater method is good for hot water on most days from April-Sept (inclusive), and on intermittent days in March and October. 

 

 

 

That's interesting tony thanks! The big benefit with your method I suppose is you also have all that solar power to rely on for charging early/late in the year?

 

I don't have an immersion heater and only have 660W of solar (which is the max my MPPT can handle), so I think upgrading the PV and buying a new calorifier with immersion (or immersion element if my calorifier does have a boss for one) might bear the same cost as thermal... Something to think about though

Edited May 11, 2022 by Woodfern

[Tony1]

5 minutes ago, Woodfern said:

 

That's interesting tony thanks! The big benefit with your method I suppose is you also have all that solar power to rely on for charging early/late in the year?

 

I don't have an immersion heater and only have 660W of solar (which is the max my MPPT can handle), so I think upgrading the PV and buying a new calorifier with immersion (or immersion element if my calorifier does have a boss for one) might bear the same cost as thermal... Something to think about though

 

Yes, even in late Oct and Nov, when there's not enough solar power to heat water, you can still often run the other electrics, fridge etc using solar power alone.

But of course it gradually diminishes, and you find more and more often you have to run the engine to recharge, until by December and Jan you are running the engine almost every day. 

That said, even with lots of panels, by Oct you'll be running the engine every day or two anyway for hot water to wash/ shower etc, unless you use hot water from the stove to do that, as I sometimes did last year.  

All this is presupposing a CCing pattern of moving about once each week, so on most days you are not running the engine unless you need to for hot water or charging. If you cruise every other day then that will produce your hot water.

 

[Woodfern]

2 minutes ago, Tony1 said:

 

Yes, even in late Oct and Nov, when there's not enough solar power to heat water, you can still often run the other electrics, fridge etc using solar power alone.

But of course it gradually diminishes, and you find more and more often you have to run the engine to recharge, until by December and Jan you are running the engine almost every day. 

That said, even with lots of panels, by Oct you'll be running the engine every day or two anyway for hot water to wash/ shower etc, unless you use hot water from the stove to do that, as I sometimes did last year.  

All this is presupposing a CCing pattern of moving about once each week, so on most days you are not running the engine unless you need to for hot water or charging. If you cruise every other day then that will produce your hot water.

 

 

So you can rely on solar for (non-hot water related) power from say Feb to Nov? That does sound appealing. Could then just use the diesel heater to do hot water when the solar's not able to and only need to run the engine when stationary from around Dec-Jan

[Tony1]

28 minutes ago, Woodfern said:

 

So you can rely on solar for (non-hot water related) power from say Feb to Nov? That does sound appealing. Could then just use the diesel heater to do hot water when the solar's not able to and only need to run the engine when stationary from around Dec-Jan

 

I must apologise, I didnt make a note of the dates when the solar started to kick in properly, so I can only work from memory. 

Its a gradual thing as you would expect, there's no single cut-in date, it just begins to be more and more useful as February goes on.

I think I got enough solar to replace my daily 100-120Ah on about a third of the days in Feb, and about another third of the time I might have gotten a partial recharge from it, and the other third I got very little from it.u

The same pattern runs in reverse during mid-Oct to mid-Nov: some days there's enough solar, some days partly, and some almost none. That said, I spent a month of that period with high trees to my south last year, so I didnt give the solar a fair chance to show what it could do. 

As you  suggest, I use the Eberspacher for hot water on some winter days in preference to running the engine, but with the price of diesel as it now is, I do stop and think more about using the CH, and in fact I've been using the stove more than last year, partly because it was been a chillier Spring, but also because using coal to heat the saloon on chilly evenings seems much cheaper than using the CH. 

At £1.60 per litre for my last fill up, the CH has suddenly become much more expensive than coal, and I no longer put it on without thinking about it. 

The last couple of weeks have been an odd period- I've needed to light the stove on some evenings (and some mornings) because it was chilly, but its been sunny enough and the days are long enough that I could still heat the water tank from solar power. 

 

ETA: like you I started with 600watts of panels, and it was fine a lot of the time, but I did notice days when the incoming charge would just about be able to keep up with the electrical usage during the day, but there would be little or no surplus, so the SoC would not be increasing much. And even in June and July last year I had a few of those days where I had to run the engine (moored under trees, very cloudy days etc).

Increasing to 1400watts means that even on really bad days, there is enough solar charge that the batteries' SoC will always increase during the day (subject to the date ranges mentioned above).

Even now, with a steady drizzle and fairly heavy overcast sky, my panels are producing about 19 amps, so my daily power usage will be replaced in 5-6 hours, and there'll still be enough solar/daylight hours to heat up the water tank to washing temp. 

 

I dont want to advocate my approach over Jens solution, as her system is excellent and I dont know nearly enough to propose one system over the other. Please dont take my comments as me advocating for solar panels, its just describing what I have and how it performs.

But yes, with 1000 watts or more of panels you do get that bonus of running the fridge etc for maybe 9 months of the year. 

 

 

Edited May 11, 2022 by Tony1

[LadyG]

1 hour ago, Woodfern said:

 

So you can rely on solar for (non-hot water related) power from say Feb to Nov? That does sound appealing. Could then just use the diesel heater to do hot water when the solar's not able to and only need to run the engine when stationary from around Dec-Jan

Very optimistic, I have to monitor power from October to March, then freedom for six months. I expect to run engine at some time, for batteries, from Oct to March.

I am a light user, have back boiler, and a reasonably large battery bank. 3 to five days autonomy at best.

But I only have two X 275w panels, as otherwise cruising would be more difficult.

There is a "diminishing returns" factor, cost of the last few incoming amps is greater than the first few, in my case I'd need to pay for an electrician, another panel, two swivels struts, and a other controller, so it's not worthwhile , for me. If I was in a marina, I'd not bother so much about littering the roof, but tbh I'd rather pay for the shorepower and just have solar for the essentials.

Edited May 11, 2022 by LadyG

[Tony1]

2 minutes ago, LadyG said:

Very optimistic, I have to monitor power from October to March, then freedom for six months. I expect to run engine at some time, for batteries, from Oct to March

 

The truth is that solar is basically rubbish in Dec and Jan, and its pretty dodgy in Nov and Feb, but I guess how dodgy it is depends on your setup. 

I think having lithium batteries helps as they soak up the charge more readily than lead acid batteries seem to, and also in my case having 1400 watts of panels is a help.

There were some bright days even in mid-Feb when I got about 100Ah from the panels.

E.g. on moorings where the sides of the boat were facing south and it was worth tilting the panels up, and there were no trees or high bushes to the south. 

On days like that the output of the panels roughly doubled, and I got a full recharge, but those days were admittedly the exception, not the rule. 

If I'm remembering correctly, I think my panels gave me all the power I needed last Oct, on most of the days anyway. It was only as we got to the end of that month that I found I was having to run the engine on some days, but I moored in some tree-lined places around that time, which would have affected things a lot. 

 

 

[IanD]

3 hours ago, Jen-in-Wellies said:

Thanks for the real world numbers. More practical than I thought and similar to the performance I've been getting since March from an 8'x4' solar thermal panel. Combine that with Lithium batteries, which don't need complete recharging every few days, then it could well make more sense than solar thermal. Things have changed since I fitted out my boat. In the naughties, 1400W of solar PV panels would have bankrupted me! I could only afford 160W and that was expensive. Their price crash since then has changed the equation considerably.

Jen

I suspect the reason that people choose solar panels over solar thermal is that electrical power generation on a boat is a much bigger pain (and can be more expensive!) than heating, so if you're going to use the roof space for something cheap solar panels make more sense nowadays -- and any spare power can still be used for hot water heating. Lithium batteries do need a bigger upfront investment but more than pay for themselves over time, especially if this avoids having to run a generator for hours to get them up to 100% charge and equalised.

1 hour ago, Tony1 said:

 

The truth is that solar is basically rubbish in Dec and Jan, and its pretty dodgy in Nov and Feb, but I guess how dodgy it is depends on your setup. 

I think having lithium batteries helps as they soak up the charge more readily than lead acid batteries seem to, and also in my case having 1400 watts of panels is a help.

There were some bright days even in mid-Feb when I got about 100Ah from the panels.

E.g. on moorings where the sides of the boat were facing south and it was worth tilting the panels up, and there were no trees or high bushes to the south. 

On days like that the output of the panels roughly doubled, and I got a full recharge, but those days were admittedly the exception, not the rule. 

If I'm remembering correctly, I think my panels gave me all the power I needed last Oct, on most of the days anyway. It was only as we got to the end of that month that I found I was having to run the engine on some days, but I moored in some tree-lined places around that time, which would have affected things a lot. 

 

 

Average daily yield predicted by Victron for 1400W of panels in Birmingham...

 

[Markblox]

Just reading this thread and I also have 1.4Kw of solar, a 600Ah, 12V lithium system and have a duel element immersion (230 and 12V) and use a relay controlled from relay 1 on a Victron Cerbo to heat my water and agree completely with all the comments about a system of that size.  Works very well. 

[Tom and Bex]

How many and what size panels are people fitting for 1400w of solar? That seems an awful lot of roof space used. What about roof use during cruising? Pole and plank storage?

 

We've only got 500w solar at present, but considering upgrading. Part of our problem is our mooring is shaded by trees until mid to late morning. 

 

As to solar thermal, we use the 2nd option, a direct system. Flat panel mounted on the roof, and using the direct system will heat the calorifier. It's plumbed in as your diagram, the main difference being that it uses a 12v differential controller to switch the pump instead of powering it direct from a solar panel. This means it doesn't waste the hot water if you have a tank of hot water after cruising etc. Fairly cheap and simple to set up, biggest problem was routing pipes up to the roof. Looks in diagram as though hot water drawn from the taps would bypass the tank, but this doesn't really happen in practice due to length of 10mm pipe  pump etc adding resistance to flow compared to just straight from the top of calorifier. Given a preference, I'd go with heating water via extra coil, but we don't have that option. Biggest downside to direct system is the need to drain down in winter (indirect systems can be filled with antifreeze). 

 

I do agree things have changed a lot in recent years, even since we bought our current boat 5 years ago. Not sure the extra effort to install solar thermal  is worth it now over having extra solar PV. Price drop of pv panels, combined with lithium batteries really does seem to have changed the balance and is a real game changer for off grid use. 

[Tony1]

47 minutes ago, Tom and Bex said:

How many and what size panels are people fitting for 1400w of solar? That seems an awful lot of roof space used. What about roof use during cruising? Pole and plank storage?

 

We've only got 500w solar at present, but considering upgrading. Part of our problem is our mooring is shaded by trees until mid to late morning. 

 

As to solar thermal, we use the 2nd option, a direct system. Flat panel mounted on the roof, and using the direct system will heat the calorifier. It's plumbed in as your diagram, the main difference being that it uses a 12v differential controller to switch the pump instead of powering it direct from a solar panel. This means it doesn't waste the hot water if you have a tank of hot water after cruising etc. Fairly cheap and simple to set up, biggest problem was routing pipes up to the roof. Looks in diagram as though hot water drawn from the taps would bypass the tank, but this doesn't really happen in practice due to length of 10mm pipe  pump etc adding resistance to flow compared to just straight from the top of calorifier. Given a preference, I'd go with heating water via extra coil, but we don't have that option. Biggest downside to direct system is the need to drain down in winter (indirect systems can be filled with antifreeze). 

 

I do agree things have changed a lot in recent years, even since we bought our current boat 5 years ago. Not sure the extra effort to install solar thermal  is worth it now over having extra solar PV. Price drop of pv panels, combined with lithium batteries really does seem to have changed the balance and is a real game changer for off grid use. 

 

In my case there are two sets of panels. The first set is mounted on the front half of the roof, starting about 2ft from the centre line ring (that bit of space helps to avoid fouling the centre line on the panels), and ending just before the chimney at the front end of the roof. 

The front set comprises four smallish panels totalling 660 watts, and they were there when I bought the boat. 

Those were great for running the fridge for most of the days during summer, except when under very thick tree cover, or heavily overcast sky, or both- but I wanted to try for solar hot water so I added a pair of panels mounted on the rear half of the roof, and those are rated at 375 watts each.

 

I left a 2ft space between those panels and the centre line ring, and that's worked out well. If I'd placed them much closer to the centre line, I think I'd be forever catching it under the panel edges.

That also leaves a space on the roof that will hold about 4-6 bags of coal, which is very handy in winter when the cratch is getting full.

 

Width is an issue though.

I cant safely walk the sides of the roof alongside the front panels- they cover the space almost completely. But they are about 7 years old, and more modern panels seem to give higher yields for a smaller surface area.

The rear panels are about 1 metre wide and I think 1.5m long.  One metre wide gives just enough room to walk past them on the roof, but very very carefully- and especially when the boat is bobbing a bit in a lock. 

You also need to leave a couple of feet of space for you to climb onto the roof, so you cant have the panels installed right up the rear end of the roof.

If I were starting over I'd sacrifice 20cm of width and go for the 80cm wide panels, which still give plenty of power, but which are much easier/safer to walk past. 

My boat is 50ft and tbh there's no space left on the roof, which is a pain as I'd have liked to fit a roof box for extra storage, but its all about compromises. 

In terms of performance, today was the first day in (I think) about 2 months when I didnt get enough solar to heat the water.