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Heating water with limited solar


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Hello, I've been thinking about upgrading the solar (300 watts with 30A controller at present) to try and save engine running time whilst moored up. I've got room to either make it 500w total on the 30A controller or at a push 700w and change the controller to a 60A (Not sure if worth the cost). Anyway my question is about water heating. I need hot water daily. The calorifier has a 2kw immersion heater that the inverter will run (have tested briefly) off of battery power. 

 

Is planning to use this with solar foolish? A quick way to kill the batteries? I also have a webasto that I guess I could change the plumbing so it can select and only heat the hot water as don't want the radiators on wasting diesel in summer. Is this a better option? Or all rubbish and just run the engine for half hour etc. If I need to run it anyway for the hot water then maxing the solar out is pointless.

 

Just looking for experienced opinions before I venture further. Thanks

Edited by CanalRetentive
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16 minutes ago, David Mack said:

Having the option to run the webasto for hot water only is a no brainer. And better to use that than to have the engine running just for water heating (since solar will charge the batteries anyway).


Thanks, I was leaning towards that but wasn't sure. I'll just add another 200 watts of solar on the current controller and do some plumbing then :) 

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I think using the batteries to run an immersion heater is only viable if you have lithium batteries. 2kw is quite a big immersion heater for a boat, many have 1kw which makes it a bit more battery-friendly (but still not a good idea unless you have lithium). Generally a boat is either plugged into shore power in which case the immersion can be left on, or not. So there is no great need for a high powered immersion.

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6 minutes ago, nicknorman said:

I think using the batteries to run an immersion heater is only viable if you have lithium batteries. 2kw is quite a big immersion heater for a boat, many have 1kw which makes it a bit more battery-friendly (but still not a good idea unless you have lithium). Generally a boat is either plugged into shore power in which case the immersion can be left on, or not. So there is no great need for a high powered immersion.


Thanks, I was thinking of going down the lithium route but have put it off for now. Maybe when the current batteries give up the ghost I'll look into it again.

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Bear in mind that heating water with solar is not feasible from Nov to Feb, even if you have a lot of solar, so its not a year-round solution. But I tried it out anyway on that basis, as it would still save a lot of engine running hours during the warmer months.

 

I would also not want to run my diesel CH unit for hot water during the summer months, but that is very much a personal concern about how reliable they are when used for multiple hours each day over a long period, and the cost of having them serviced/repaired. I might be way off in having those nagging doubts, but I'd rather save the Eberspacher for those odd chilly mornings/evenings, when I need a short burst of warmth.  

 

But solar can work, and how much of the year it works for might depend on how many panels you have.

For example, I have been heating my water almost every day from the solar panels over the last few weeks, and even before that on an occasional basis (if there was enough sun). 

I have about 1400watts of panels, and at the moment that will recharge the batteries and also heat up a tank of water on most days by early afternoon, at which point I turn off the MPPTs. But it does depend on the mooring, and whether there are trees in the way, etc.

 

If I had 700 watts of panels, I'm guessing I would not have had enough power to heat my water back in March, say, but I would be able to do it now.

So yes, with your proposed 700 watts you will still get hot water for a significant part of the year- maybe mid April to end of Aug, and on odd days around the margins of those periods. 

On heavily overcast days, even in summer 700 watts might not give you quite enough power to heat the water properly, but it might get it just hot enough to shower in. 

 

But as Nick says, I think the key thing in my case is that I have 400Ah of lithium batteries, which are very good at soaking up solar charge, and I have a 1kw immersion heater plus a 2000w inverter- so the inverter can cope with running for say 50 mins to get the water hot- although even then its fan is on for a lot of that time, so its working fairly hard. And I do haev nagging doubts about working any of these systems hard on a daily basis- I'd much rather run them on an under-stressed basis.

 

But as you would expect, the 1kw inverter draws about 80amps from the batteries when its running, so if I were doing it again from scratch I'd have gotten at least another 100Ah of batteries, if not more.  

 

The panels themselves cost me less than £250, but the lithium batteries were a much bigger expense.

 

 

Edited by Tony1
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3 minutes ago, Tony1 said:

 

 

As you would expect, the 1kw inverter takes about 80amps from the batteries when its running, so if I were doing it again from scratch I'd have gotten at least another 100Ah of batteries, if not more. 

 

The panels themselves cost me less than £250, but the lithium batteries were a much bigger expense.

 

 

 

I don't think that 80 amps includes inverter losses. It is more like 100 amps per 1Kw taking all inverters into account. The OP would be looking at 200 amps. Not a bright idea with lead acid batteries that take so long to charge however powerful the charge source.

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If you really want solar heating of water, then solar thermal is much more efficient than via photovoltaic panels.

Not many boats have it, but it can be done. It gets round the inefficiency of converting solar energy to electricity, then in to heat. You'll need a different type of panel and pumps and plumbing though. Mine gives hot water from roughly Spring equinox to Autumn equinox. Outside that, there is little to no contribution, but that is when the back boiler on the stove is doing its job.

 

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9 minutes ago, Tony Brooks said:

 

I don't think that 80 amps includes inverter losses. It is more like 100 amps per 1Kw taking all inverters into account. The OP would be looking at 200 amps. Not a bright idea with lead acid batteries that take so long to charge however powerful the charge source.

 

I should apologise for my ignorance of the detail on these issues- I tend to observe all of this electrical wizardry in much the same way as a mediaeval peasant would observe an iPhone in action. 

 

The 80-85 amps draw from the batteries is measured by the BMV712 monitor, so my assumption is that when power is being wasted by the inverter doing its inverting, that is taken into account as part of the 85 amps that are coming from the batteries.

 

And 85 amps, in electrical pixie terms, is a lot. 

 

 

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1 hour ago, nicknorman said:

I think using the batteries to run an immersion heater is only viable if you have lithium batteries. 2kw is quite a big immersion heater for a boat, many have 1kw which makes it a bit more battery-friendly (but still not a good idea unless you have lithium). Generally a boat is either plugged into shore power in which case the immersion can be left on, or not. So there is no great need for a high powered immersion.

 

That's exactly what I have and do, with the added wrinkle that I have a mains thyristor connected to the immersion heater.  Think a volume control for the immersion heater if you don't know what one is. (Not you Nick! I know you know.)

 

I can dial down the power of the immersion to whatever spare sunshine is available to not hammer the batteries too much.

 

 

 

 

 

 

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2 hours ago, CanalRetentive said:

Hello, I've been thinking about upgrading the solar (300 watts with 30A controller at present) to try and save engine running time whilst moored up. I've got room to either make it 500w total on the 30A controller or at a push 700w and change the controller to a 60A (Not sure if worth the cost). Anyway my question is about water heating. I need hot water daily. The calorifier has a 2kw immersion heater that the inverter will run (have tested briefly) off of battery power. 

 

 

Just a thought, but instead of buying a 60amp MPPT, why not just buy another 30 amp one and use two MPPTs at once? I use two of them and it seems to work fine.

I guess it does depend on how many panels you get, and whether the total power is divided roughly evenly between two discrete sets of panels. 

 

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16 minutes ago, Tony1 said:

 

I should apologise for my ignorance of the detail on these issues- I tend to observe all of this electrical wizardry in much the same way as a mediaeval peasant would observe an iPhone in action. 

 

The 80-85 amps draw from the batteries is measured by the BMV712 monitor, so my assumption is that when power is being wasted by the inverter doing its inverting, that is taken into account as part of the 85 amps that are coming from the batteries.

 

And 85 amps, in electrical pixie terms, is a lot. 

 

 

 

At a nominal 12V the current at 1000 amps (1 kW) is 83.3 amps so we can conclude that your inverter uses about 1.7 amps just to work itself. Some use far more and some less so to allow for all we say divide the appliance wattage by 10 find the current drawn from the batteries. doing that will ensure the vast majority of people get a very conservative answer (a higher current) which is what you need for battery calculations.

Edited by Tony Brooks
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1 minute ago, TheBiscuits said:

 

That's exactly what I have and do, with the added wrinkle that I have a mains thyristor connected to the immersion heater.  Think a volume control for the immersion heater if you don't know what one is. (Not you Nick! I know you know.)

 

I can dial down the power of the immersion to whatever spare sunshine is available to not hammer the batteries too much.

 

 

 

 

 

 

 

This is my next planned gadget, and for the same reason.

May I ask- what is the name of the one you use, and do you have a link to make sure I get the right one? 

 

Cheers

 

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47 minutes ago, Jen-in-Wellies said:

If you really want solar heating of water, then solar thermal is much more efficient than via photovoltaic panels.

Not many boats have it, but it can be done. It gets round the inefficiency of converting solar energy to electricity, then in to heat. You'll need a different type of panel and pumps and plumbing though. Mine gives hot water from roughly Spring equinox to Autumn equinox. Outside that, there is little to no contribution, but that is when the back boiler on the stove is doing its job.

 

I think I've seen a boat with thermal solar on his 'butty' - boat is between Calcutt Locks and Wigrams turn - looks an impressive setup. I think the boat is called 'Toast' ?

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33 minutes ago, TheBiscuits said:

 

That's exactly what I have and do, with the added wrinkle that I have a mains thyristor connected to the immersion heater.  Think a volume control for the immersion heater if you don't know what one is. (Not you Nick! I know you know.)

 

I can dial down the power of the immersion to whatever spare sunshine is available to not hammer the batteries too much.

But presumably that pulses the power between fully on and fully off, with a variable mark-space ratio, otherwise you will be wasting a lot of power. So the current from the inverter, and hence the current from the battery, is pulsing between zero and 85A or whatever, albeit that the average current is less. Is that any better for the inverter and batteries?

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 Solar is nice to have on the boat. I first installed solar panels in 1995 when they were rather more expensive. The latest ones are much bigger and much cheaper than those old BP 75w panels. 

 

One of the things which comes into the equation which people might not think about is that having large solar arrays on boats and relying on them for power and/or hot water can cause problems with where you moor. 

 

You might find that some places which would otherwise be really nice moorings are not appropriate due to lack of sun coverage. 

 

Be careful not to let the solar panels rule your behaviour. Yes they are good but not as nice as a really pleasant spot to moor up, specially on a hot sunny day. 

Edited by magnetman
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29 minutes ago, David Mack said:

But presumably that pulses the power between fully on and fully off, with a variable mark-space ratio, otherwise you will be wasting a lot of power. So the current from the inverter, and hence the current from the battery, is pulsing between zero and 85A or whatever, albeit that the average current is less. Is that any better for the inverter and batteries?

 

Well no.

 

At half voltage, the fixed resistance of the heating element gives quarter power, so it's only pulling around 21A from the batteries, or more usually directly from the solar array.  

 

The power used is "waste" sunshine anyway to total efficiency is irrelevant.  By the time it kicks in the immersion heater the battery banks are full and I dial in a voltage that isn't draining the batteries or at least not by much.

 

I was going to take some comparison readings for you but the immersion heater thermostat has switched it off so I can't.  Hot water and all battery banks fully charged so the MPPT has gone into lazy mode, pretty much just feeding the fridge ~5A.

 

I need to find something to do with this sunshine!

 

 

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1 hour ago, David Mack said:

But presumably that pulses the power between fully on and fully off, with a variable mark-space ratio, otherwise you will be wasting a lot of power. So the current from the inverter, and hence the current from the battery, is pulsing between zero and 85A or whatever, albeit that the average current is less. Is that any better for the inverter and batteries?

I did wonder about that myself, whether an inverter would be particularly happy with such a fast-fluctuating load. There will be a lot of smoothing of the DC so it doesn't translate into on / off of the dc supply, but even so... However Mr B's experience seems to suggest its OK.

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12 minutes ago, nicknorman said:

I did wonder about that myself, whether an inverter would be particularly happy with such a fast-fluctuating load. There will be a lot of smoothing of the DC so it doesn't translate into on / off of the dc supply, but even so... However Mr B's experience seems to suggest its OK.

 

Not sure if it's relevant, but I have a high-frequency inverter rather than half a tonne of transformer.  The magic smoke hasn't come out of it yet and I fitted the thyristor a year ago tomorrow.

 

It never did quite what I wanted until I fitted lithium in February this year though.  It would use the very important final few Ah from the lead acids if I just left it running and it clouded over, so rather defeated the point of having it.

 

With lithium, I don't care if it uses half the stored power heating water as I'll get it back tomorrow.

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4 hours ago, TheBiscuits said:

At half voltage, the fixed resistance of the heating element gives quarter power, so it's only pulling around 21A from the batteries, or more usually directly from the solar array. 

That's fine. But then you are also dropping the same half voltage across your thyristor, which is also carrying 21A so that's over 120 Watts to be dissipated by the thyristor heatsink.

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1 hour ago, David Mack said:

That's fine. But then you are also dropping the same half voltage across your thyristor, which is also carrying 21A so that's over 120 Watts to be dissipated by the thyristor heatsink.

No that is not how a thyristor works. It is either on or off, it doesn’t “drop half the voltage across it” nor dissipate much power. It turns on at an adjustable phase angle of the ac supply, and off when the current momentarily falls to zero every 1/2 cycle. 

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4 hours ago, nicknorman said:

I did wonder about that myself, whether an inverter would be particularly happy with such a fast-fluctuating load. There will be a lot of smoothing of the DC so it doesn't translate into on / off of the dc supply, but even so... However Mr B's experience seems to suggest its OK.

 

 

Perhaps this is another one of those opportunities for you to design a product? In this case something that would do the job of safely controlling the amount of current given to the immersion heater? 

I'd certainly buy one, so there's one sale guaranteed. 

I do recall Ed Shiers being reluctant to fit one of these for me. It wasnt that he suspected it to be a risky setup, it was just that he didnt have enough experience of this particular tweak to be 100% certain that it was safe. 

 

But if myself and Mr Biscuits survive the summer in one piece and without even being singed, perhaps we can assume its reasonably safe?

 

What is certain is that both lithiums and solar panels are going to become more common, and in the summer months people will be looking for ways to use their surplus power. 

Like any normal person would, I have already worked up some preliminary designs for an electrically powered prod to repel overly aggressive swans, and my gerbil-executioner chair is in its final design stages.

But there is a part of me that thinks some of this surplus electricity might perhaps be used for other uses than killing small mammals. 

 

 

 

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40 minutes ago, Tony1 said:

 

 

Perhaps this is another one of those opportunities for you to design a product? In this case something that would do the job of safely controlling the amount of current given to the immersion heater? 

I'd certainly buy one, so there's one sale guaranteed. 

I do recall Ed Shiers being reluctant to fit one of these for me. It wasnt that he suspected it to be a risky setup, it was just that he didnt have enough experience of this particular tweak to be 100% certain that it was safe. 

 

But if myself and Mr Biscuits survive the summer in one piece and without even being singed, perhaps we can assume its reasonably safe?

 

What is certain is that both lithiums and solar panels are going to become more common, and in the summer months people will be looking for ways to use their surplus power. 

Like any normal person would, I have already worked up some preliminary designs for an electrically powered prod to repel overly aggressive swans, and my gerbil-executioner chair is in its final design stages.

But there is a part of me that thinks some of this surplus electricity might perhaps be used for other uses than killing small mammals. 

 

 

 

Power controllers are available, this is first stab M240_08036DI_Ver001_Einleger_Karton1 (farnell.com) 

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2 minutes ago, ditchcrawler said:

Power controllers are available, this is first stab M240_08036DI_Ver001_Einleger_Karton1 (farnell.com) 

 

That looks like a great solution, cheers. 

I'm hoping the other electrical experts will add some thoughts about this, because the truth is that over the next few years its something people will be looking for. 

 

 

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48 minutes ago, Tony1 said:

[snip]

What is certain is that both lithiums and solar panels are going to become more common, and in the summer months people will be looking for ways to use their surplus power. 

Like any normal person would, I have already worked up some preliminary designs for an electrically powered prod to repel overly aggressive swans, and my gerbil-executioner chair is in its final design stages.

But there is a part of me that thinks some of this surplus electricity might perhaps be used for other uses than killing small mammals. 

 

 

Electrocuting boaters who don't slow down when passing your moored boat? Lock-stealers? Late-night generator-runners?

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