12v water heating setup

[noelex 77]

As others have indicated, the energy consumed when converting electricity into hot water is well known and easy to calculate.

As a practical indication, we have just over 1000w of solar and heat our shower water (and this is usually less than 30 litres) electrically on the better solar days. This depends a lot on the cruising ground time of year etc. At the moment we are above 55° N only two months after the winter solstice so using electricity from solar to heat shower water is not even remotely possible.

It is sensible to think of practical ways to use the excess power from solar production. It is a shame to see the solar regulator throwing away power. Heating water is great solution, but it is not practical on most boats to install enough solar (and/or wind/hydro) to make this feasible in all conditions. Therefore you either need to put up with cold showers (and who wants that) or have alternative means of generating hot water. On our boat this is primarily via our diesel heater. This is a nice compliment as this tends to be in use in poor solar conditions. We can also use our engine (via a large alternator) or propane. I don’t like cold showers .

[GILow]

Originally Posted by noelex 77

...I don’t like cold showers .

Amen to that!

[markcouz]

We regularly install domestic PV hot water systems in Johannesburg South Africa using 3x 330w PV panels into a 150L geyser (hot water cylinder for you guys) which delivers about 20C temperature rise on average on a sunny day - most of our days are sunny.

This experience leads me to believe that you will require the equivalent of 1x dedicated 330w (or thereabouts) PV panel into a 50L HWC to deliver a decent result.

Total PV requirement for your yacht will depend entirely on the balance of your electrical demand, if you are very frugal then maybe 1kWp may be sufficient

I will have 5x 370W panels which will include heating of a 50L HWC, and fully expect to spend a large portion of my day running around behind the Missus switching off unnecessary loads[emoji849]

[GILow]

Originally Posted by markcouz

We regularly install domestic PV hot water systems in Johannesburg South Africa using 3x 330w PV panels into a 150L geyser (hot water cylinder for you guys) which delivers about 20C temperature rise on average on a sunny day - most of our days are sunny.

This experience leads me to believe that you will require the equivalent of 1x dedicated 330w (or thereabouts) PV panel into a 50L HWC to deliver a decent result.

Total PV requirement for your yacht will depend entirely on the balance of your electrical demand, if you are very frugal then maybe 1kWp may be sufficient

I will have 5x 370W panels which will include heating of a 50L HWC, and fully expect to spend a large portion of my day running around behind the Missus switching off unnecessary loads[emoji849]

I think you’d agree that PV on a house produces a LOT more power than PV on a boat. I’d say at least double.

[markcouz]

Yup, shading and tilt angles are a mess on the boat.

[GILow]

Originally Posted by markcouz

This experience leads me to believe that you will require the equivalent of 1x dedicated 330w (or thereabouts) PV panel into a 50L HWC to deliver a decent result.

330 Watts will need 7 hours to get 50 litres from 20 to 60 degrees C. It is a pretty well accepted rule of thumb that you can expect only four hours equivalent full output from a panel on a boat. Based on this it would take two days to get to temperature, except, of course, the tank would cool down between heating sessions so you’d never make it and all you’d have is a big legionella factory.

Did you maybe mean to say 10 x 330 Watt panels by any chance?

[GordMay]

Originally Posted by GILow

330 Watts will need 7 hours to get 50 litres from 20 to 60 degrees C. It is a pretty well accepted rule of thumb that you can expect only four hours equivalent full output from a panel on a boat. Based on this it would take two days to get to temperature, except, of course, the tank would cool down between heating sessions so you’d never make it and all you’d have is a big legionella factory.
Did you maybe mean to say 10 x 330 Watt panels by any chance?

You’d require a roughly 1,760 Watt element to heat 30 litres of water from 10 (50F) to 60 (140F) Degrees C, in 1 Hour.
Your 300 Watt element would require roughly 6 hours to raise the 30 L of water temperature by 50 degrees C.
To heat water in 1 hour: kW = 30 x 4 x 50/3412 = 1.76

[JimMann]

Originally Posted by GILow

I don’t know where to start....

First, none of the numbers he quoted are remotely like what you stated. Volumes are different, start and finish temps are different... maybe some of the times are the same.

Next, he’s all over the place in his “testing” and I didn’t actually see anywhere where he tested the current and voltage being used.

Anyway, end of the day, you are not going to beat physics and you’ve just learned that YouTube is not a research tool. Consider all that a win.

Back to the drawing board, this time with the real numbers kindly provided by CF. You can still build this thing (the tank was probably the hardest part) you just need to adjust your expectations.

Hi,

Thanks for your explanation, I’ve spent some time looking into it and I understand the limitations now.

I can’t let a little thing Thermodynamics screw up my plans, so my new thought process is to put an additional AC 230v 1500w element in to get the water over the 60c mark when needed and to sterilise the tank periodically. I’ll power that off my inverter or generator and run the solar dump into the DC element for showering etc.

Any thoughts?

Thanks guys

[GILow]

Originally Posted by JimMann

Hi,



Thanks for your explanation, I’ve spent some time looking into it and I understand the limitations now.



I can’t let a little thing Thermodynamics screw up my plans, so my new thought process is to put an additional AC 230v 1500w element in to get the water over the 60c mark when needed and to sterilise the tank periodically. I’ll power that off my inverter or generator and run the solar dump into the DC element for showering etc.



Any thoughts?



Thanks guys

Sounds workable, though you might also want to consider a diesel water heater in your options.

I did the numbers on my boat, and diesel came out a big winner in terms of running costs, ease of fuel storage/supply and overall ability to create a LOT heat which will be needed where I am going.

Setup cost is a bit eye watering, but I did manage to get my Webasto Thermotop for under AU$2K new, which helped.

But it is a bit of palaver to setup.

[Pizzazz]

Just use the right tool for the job. Excess solar power is best stored in batteries. Either increase your battery capacity or reduce your solar.

In general, heating and cooling is very energy intensive. You need a strong power source as in the sun (shower bags), a custom sun water hearing panels (you just circulate water through some panels, some developing countries have that, more efficient than going via electricity) or use diesel heat, ideally from the engine. If you want to take a shower, run the engine for 30 min and be done with it. Simple.

[JimMann]

Originally Posted by GILow

Sounds workable, though you might also want to consider a diesel water heater in your options.

I did the numbers on my boat, and diesel came out a big winner in terms of running costs, ease of fuel storage/supply and overall ability to create a LOT heat which will be needed where I am going.

Setup cost is a bit eye watering, but I did manage to get my Webasto Thermotop for under AU$2K new, which helped.

But it is a bit of palaver to setup.

This might be a project for next year but this year after whole load of super expensive welding work done (£45ah) and then a reasonable priced welder in redo all the work of the previous guy.. money is tight hence me trying Jerry rig my current water heating system.

I’d really like my hot water to be free so I’d really like to be find a way to balance it out with just solar / wind power and supplement when I have to but I guess I’ll realise my stupidly when I start cruising.


Question,

I don’t have a pressure relief valve, would an inline relief valve on the outlet pipe be suitable is that a daft idea?

Thanks

[GILow]

Originally Posted by Pizzazz

If you want to take a shower, run the engine for 30 min and be done with it. Simple.

Not exactly a nice way to prolong engine life. Just saying.

[GILow]

Originally Posted by JimMann

This might be a project for next year but this year after whole load of super expensive welding work done (£45ah) and then a reasonable priced welder in redo all the work of the previous guy.. money is tight hence me trying Jerry rig my current water heating system.

I’d really like my hot water to be free so I’d really like to be find a way to balance it out with just solar / wind power and supplement when I have to but I guess I’ll realise my stupidly when I start cruising.


Question,

I don’t have a pressure relief valve, would an inline relief valve on the outlet pipe be suitable is that a daft idea?

Thanks

I get it on the budget restraints. It’s an easier decision for me because the boat is also my home, but even then, I thought long and hard on this one.

Pressure relief valve, in theory, can go anywhere downstream of the check valve, but making it remote from the tank has a (very minor) potential to somehow be prevented from receiving the pressure to relieve. Yeah, I can’t think HOW that might actually happen, except some kind of really strange blockage or perhaps an insane plumbing mistake... but the then it’s a boat, weird things happen.

I made my system unpressurised, which solved that dilemma.

[cj88]

Originally Posted by JimMann

I’d really like my hot water to be free so I’d really like to be find a way to balance it out with just solar / wind power and supplement when I have to but I guess I’ll realise my stupidly when I start cruising.

Simple and low cost in the interim would be a solar shower or heating water on the stove while you save for your ideal solution.

[JimMann]

Originally Posted by noelex 77

As others have indicated, the energy consumed when converting electricity into hot water is well known and easy to calculate.

As a practical indication, we have just over 1000w of solar and heat our shower water (and this is usually less than 30 litres) electrically on the better solar days. This depends a lot on the cruising ground time of year etc. At the moment we are above 55° N only two months after the winter solstice so using electricity from solar to heat shower water is not even remotely possible.

It is sensible to think of practical ways to use the excess power from solar production. It is a shame to see the solar regulator throwing away power. Heating water is great solution, but it is not practical on most boats to install enough solar (and/or wind/hydro) to make this feasible in all conditions. Therefore you either need to put up with cold showers (and who wants that) or have alternative means of generating hot water. On our boat this is primarily via our diesel heater. This is a nice compliment as this tends to be in use in poor solar conditions. We can also use our engine (via a large alternator) or propane. I don’t like cold showers .

Thanks for your reply!

I also have 1000w plus wind to dedicate to this project, that’s why I thought it wouldn’t be an issue. I expected to have to supplement the power during the winter, my plan is to install a second AC 1500w element to run off the inverter or generator. I’m not too fussy when it’s coming to showering, I’ll just in the sea it’s all about keeping the missus happy

Thanks