DC immersion heater direct from PV

[Tom Richmond]

Just installed 6x260w PV panels, feeding a victron easysolar (50A MPPT charger) which charges my 24v 225AH battery bank. Been reaching float before lunch in recent sunny weather, got me wondering if I should harness the extra power for water heating? We have a 55l calorifier which has an AC immersion in, which is never used as our mooring is off grid.

My panels are vmp 30v and voc 37v, they are wired in series sets of two, doubling those values.

My question is if it will be ok to connect them directly to a 48v dc immersion element? 

I would use a relay, controlled by the programmable output of the easysolar to switch two of the three strings over to the immersion when the batteries reach float, leaving a pair of panels to keep the batteries topped up throughout the day, and provide any power used in the boat.

Second question, will 1000w of solar heat a 55l calorifier?

I guess I'd need a thermostat to switch off the immersion if/when it got to temp.

All thoughts/ideas welcome! Thanks in advance. 

[Jen-in-Wellies]

This was discussed very recently.

https://www.canalworld.net/forums/index.php?/topic/95912-heating-water/

Yes it can be done if you have lots of panels. It is very inefficient. Thermal solar, which I have on my boat, works very well. Lots of hot water for five months of the year, unless it is dull and raining.

Your idea of of using the solar controller to relay switch the output from some of the panels to the immersion seems sound and gets round the risk of accidentally draining batteries as they are never directly connected to the heater.

If it is really 1000W going to the heater, then it should heat up the cauliflower in the same time as a 1000w mains immersion connected to a shore line. This is a pretty standard boat thing, but I don't have one, so don't know how quick they are.

Full power from solar panels only happens on bright sunshine, mid summer with no clouds at midday. If any of these aren't there, then output falls dramatically.

Jen

Jen

Edited April 26, 2018 by Jen-in-Wellies

[Jen-in-Wellies]

9 hours ago, Tom Richmond said:

 

My panels are vmp 30v and voc 37v, they are wired in series sets of two, doubling those values.

My question is if it will be ok to connect them directly to a 48v dc immersion element? 

 

Would you be putting the four panels in to the 48V immersion in parallel, or two sets of two in series? In other words, would the open circuit voltage going in to the heater be 37V, or 74V? The reason I am asking is that the operating voltage of an immersion heater is set by its resistance. This restricts the current that can flow and hence the power for a particular voltage. Take the voltage beyond 48V and you risk overloading and damaging the heater. Running them at 37V open, 30V max power, will under run the heater and mean that it is putting less than 1KW in to the calorifier. The power in Watts used by a heater is V²/R, where V is the voltage and R the DC resistance of the heater. The resistance of the heater can be measured with a multimeter, but a 48V 1KW one should be around 2.3 ohms. A 48V heater run at 30V would only give out 390W. If you ran it at 60V it would be trying to put out 1500W in to the water and will have a (possibly very) short life. The power output from the heater is proportional to the square of the voltage, so deviations from the designed voltage have a dramatic effect.

You might be better off having two solar controllers, with the second having a voltage output matched to the immersion heater and use the first solar controller to relay switch two of the solar panel arrays to the second controller when the batteries go in to float. This sort of assumes that the solar controller can handle going in to a resistive load and not a battery without getting confused. There are high efficiency power supplies that will do variable input voltage in to fixed voltage output, but I don't know about ones for such high powers.

Apologies if you already know this stuff. I have a habit of going in to lecture mode!

Jen

Edited April 26, 2018 by Jen-in-Wellies

[nicknorman]

You don’t mention the power rating of the immersion. You have 1.5kw of solar and if you put the panels in series with a lowish power 48v heater you will get more than 60v across it which might result in a fairly short life. So choose a heater to match the panel output or a bit more, ie 1.5 to 2 kw if available.

[Jen-in-Wellies]

17 minutes ago, nicknorman said:

You don’t mention the power rating of the immersion. You have 1.5kw of solar and if you put the panels in series with a lowish power 48v heater you will get more than 60v across it which might result in a fairly short life. So choose a heater to match the panel output or a bit more, ie 1.5 to 2 kw if available.

He is talking about diverting four of the six panels in to the heater when the batteries go in to float, so 1kW, not 1.5, however power is dependent on resistance of the heater and the square of the voltage, so it gets more complicated. See my comment above.

Jen

[Sea Dog]

These discussions always leave me feeling that, whilst it seem fairly straightforward to have a big solar bank and use the excess in such ways, it's not at all as straightforward as it first appears. 

Particularly for those like me who cruise regularly but may sometimes wish to stop for a few days, I always draw the conclusion that best bang for buck is achieved by using the engine you already have for an hour to do the donkey work of the bulk charge (and provide a tank of hot water), then ensure you have sufficient solar to continue the charge to 100%.  After an hour of my alternator's efforts, my charge current is always down to about 30 amps - not a difficult place for even a relatively modest solar array to carry on from in the lighter months... and the hot water is already taken care of.  

Winter is a different matter - but solar alone definitely isn't the answer then either.  

[nicknorman]

41 minutes ago, Jen-in-Wellies said:

He is talking about diverting four of the six panels in to the heater when the batteries go in to float, so 1kW, not 1.5, however power is dependent on resistance of the heater and the square of the voltage, so it gets more complicated. See my comment above.

Jen

Oh yes, I didn't notice the 4 panels thing. In which case a 1kw heater should be fine (the panels will rarely if ever put out 1kw). Yes I do understand the basic equations of ohms law and power, but the point is that if the panels can produce 1kw or less, and the heater is rated at 1kw, it is not possible for the heater to be overloaded / too much voltage. No need to consider V^2 etc. The down side is that when the panels are producing less than 1kw they will be pulled further down below Vmp but that isn't really a big deal. The fix would be to add another MPPT controller but at quite a considerable expense. At the very least, it can be tried without a controller and if found wanting, only then consider splashing out on another controller.

However, thinking about it, not sure another controller would work. They are designed to be connected to, and powered from, a battery. If they are connected to a resistive load without a battery they won't work I suspect.

Edited April 26, 2018 by nicknorman

[dor]

Whilst it might be seen to be adding another level of inefficient conversion, wouldn't the simplest way to do this be using the inverter to run the immersion.  If you really are getting close to 1KW of surplus energy, this should heat the calorifier in quite a short time - an hour maybe. The immersion element will have its own thermostat so will switch off anyway when up to temperature.

[nicknorman]

13 minutes ago, dor said:

Whilst it might be seen to be adding another level of inefficient conversion, wouldn't the simplest way to do this be using the inverter to run the immersion.  If you really are getting close to 1KW of surplus energy, this should heat the calorifier in quite a short time - an hour maybe. The immersion element will have its own thermostat so will switch off anyway when up to temperature.

Yes but the danger is the sun goes in and you then have a flat battery! Depending on the starting temperature of the water, it takes a lot more than 1kwh to heat a tank full. 55l from 20 C to 60C (ie gain of 40 C) is getting on for 3 hours. If the sun goes in, 3kwh is 250AH.

The advantage of the proposed way is that first you fully charge the batteries, and then the surplus is switched to heating the water. All automatic and making best use of the power regardless of how much of it there is.

Edited April 26, 2018 by nicknorman

[dor]

1 minute ago, nicknorman said:

Yes but the danger is the sun goes in and you then have a flat battery! Depending on the starting temperature of the water, it takes a lot longer than 1kwh to heat a tank full. 55l from 20 C to 60C (ie gain of 40 C) is getting on for 3 hours. If the sun goes in, 3kwh is 250AH.

Fair point.  Maybe fit a relay that is sensitive to the current/voltage being supplied.  But then that is adding another layer of complication.  It just seemed to me that with such a huge amount of surplus solar power there should be a simple way to dump it to water.  It's what some houses do with a big basement full of water to act as a power dump.

[nicknorman]

Just now, dor said:

Fair point.  Maybe fit a relay that is sensitive to the current/voltage being supplied.  But then that is adding another layer of complication.  It just seemed to me that with such a huge amount of surplus solar power there should be a simple way to dump it to water.  It's what some houses do with a big basement full of water to act as a power dump.

Well I suppose you could just heat up the large volume of water in the basement (aka the canal) and then bask in its warmth, a bit like those monkeys in Japanese hot springs!

[peterboat]

29 minutes ago, nicknorman said:

Yes but the danger is the sun goes in and you then have a flat battery! Depending on the starting temperature of the water, it takes a lot more than 1kwh to heat a tank full. 55l from 20 C to 60C (ie gain of 40 C) is getting on for 3 hours. If the sun goes in, 3kwh is 250AH.

The advantage of the proposed way is that first you fully charge the batteries, and then the surplus is switched to heating the water. All automatic and making best use of the power regardless of how much of it there is.

As well as being automatic it also saves wearing out an expensive inverter

[Tom Richmond]

Thank you all, particularly Jen in Wellies; well done for reading and understanding my OP and correcting others, quite refreshing.

1 hour ago, nicknorman said:

You don’t mention the power rating of the immersion.

No, because I have not spec'd the system. If I go ahead I will choose an appropriately rated heating element.

1 hour ago, Jen-in-Wellies said:

Would you be putting the four panels in to the 48V immersion in parallel, or two sets of two in series? In other words, would the open circuit voltage going in to the heater be 37V, or 74V?

The panels are currently wired in series pairs, so I would be looking to work with the system as it is. This would overdrive the heater, running it at 60v (the panels are 37v open circuit, but 30v under max power. I assume having a resistive load attached would reduce them to the lower voltage?)

However, 60v isn't too far off of the charge voltage for a 48v system, is it? My 24v bank charges at 28.8v, so I assume a 48v bank would 57.6v, and with different types of batteries that number might be higher? So my thinking (hoping) is that 60v would be within the tolerance of a 48v heating element, and not cause premature burning out?

As an alternative i could re-wire the panels in parallel sets of 2, voltage of 30v and underpower the element?

 

[Tom Richmond]

49 minutes ago, dor said:

Whilst it might be seen to be adding another level of inefficient conversion, wouldn't the simplest way to do this be using the inverter to run the immersion.  If you really are getting close to 1KW of surplus energy, this should heat the calorifier in quite a short time - an hour maybe. The immersion element will have its own thermostat so will switch off anyway when up to temperature.

 

1 hour ago, Sea Dog said:

These discussions always leave me feeling that, whilst it seem fairly straightforward to have a big solar bank and use the excess in such ways, it's not at all as straightforward as it first appears. 

Particularly for those like me who cruise regularly but may sometimes wish to stop for a few days, I always draw the conclusion that best bang for buck is achieved by using the engine you already have for an hour to do the donkey work of the bulk charge (and provide a tank of hot water), then ensure you have sufficient solar to continue the charge to 100%.  After an hour of my alternator's efforts, my charge current is always down to about 30 amps - not a difficult place for even a relatively modest solar array to carry on from in the lighter months... and the hot water is already taken care of.  

Winter is a different matter - but solar alone definitely isn't the answer then either.  

Thanks for your input guys, but your thoughts are not appropriate to my situation. 

1) My boat has no Engine. 

2) I am on a permanent mooring, without power (as indicated by the fact that I said 'Off Grid' in my OP).

3) I have massively oversized my PV array in order that it will provide most of my power even in winter. 

So, given the above, A) I have bucket loads of solar which will be creating excess power in summer. B) My batteries are a the most important factor here, so I will always want them to have priority over anything else (ie cold water+ charged batteries is fine, hot water + anything less than full batteries is not OK!) We have a morco gas heater as well (also mentioned in OP) 

[Flyboy]

10 minutes ago, Tom Richmond said:

 

Thanks for your input guys, but your thoughts are not appropriate to my situation. 

1) My boat has no Engine. 

2) I am on a permanent mooring, without power (as indicated by the fact that I said 'Off Grid' in my OP).

3) I have massively oversized my PV array in order that it will provide most of my power even in winter. 

So, given the above, A) I have bucket loads of solar which will be creating excess power in summer. My batteries are a the most important factor here, so I will always want them to have priority over anything else (ie cold water+ charged batteries is fine, hot water + anything less than full batteries is not OK!) We have a morco gas heater as well (also mentioned in OP) 

As you don't move, why not consider heating your water from a direct heating solar panel to supplement your battery/solar panel usage ?    

https://www.ebay.co.uk/itm/10-Evacuated-Tubes-Solar-Collector-of-Solar-Hot-Water-Heater-Vacuum-Tubes-new-/272311527493?_trksid=p2141725.m3641.l6368

[peterboat]

But he still has all that electric doing nowt! I have gone down the water heated solar in the past and it worked well but my next boat is going to be done the same as Toms, its a bit of wiring and a relay rather than pipes all over the place

[Tom Richmond]

13 minutes ago, Flyboy said:

As you don't move, why not consider heating your water from a direct heating solar panel to supplement your battery/solar panel usage ?    

https://www.ebay.co.uk/itm/10-Evacuated-Tubes-Solar-Collector-of-Solar-Hot-Water-Heater-Vacuum-Tubes-new-/272311527493?_trksid=p2141725.m3641.l6368

Yeah, I even added microbore pipes before lining the boat. But space on roof is limited and cost is significant. Even this ebay system would still require a pump, controller, pipes, antifreeze etc etc etc. 

As far as I can see the hardware and installation of the electrical route as suggested in OP is much less significant.

[nicknorman]

1 hour ago, Tom Richmond said:

Thank you all, particularly Jen in Wellies; well done for reading and understanding my OP and correcting others, quite refreshing.

No, because I have not spec'd the system. If I go ahead I will choose an appropriately rated heating element.

The panels are currently wired in series pairs, so I would be looking to work with the system as it is. This would overdrive the heater, running it at 60v (the panels are 37v open circuit, but 30v under max power. I assume having a resistive load attached would reduce them to the lower voltage?)

 

No it wouldn’t /can’t over drive the heater if the max solar power is no greater than the power rating of the heater. No it wouldn’t reduce to the lower voltage, it would reduce much further. As a maximum to 48v and most or all of the time, much lower.

You might choose to praise Jen and ignore my contribution because I missed that you were only talking about 4 of the panels, but in fact all her talk of V^2 is irelevant and has confused you. But please feel free to make your own choice about whose advice to follow and whose to ignore,

Edited April 26, 2018 by nicknorman

[peterboat]

Right a friend of mine has 4 x 250watt panels, when his batteries are charged they go into series =120volts, and then heat his water using a standard 3KW ac immersion heater, he has a thermostat on on it to turn it off when hot. Its simple cheap and works, an immersion heater is a restive load and doesnt know whether its ac or dc being fed it

Edited April 26, 2018 by peterboat

[nicknorman]

1 hour ago, Tom Richmond said:

We have a morco gas heater as well (also mentioned in OP) 

Since we are being picky about what you did or didn't say in the OP, no, you didn't mention this in your OP. Not that is matters particularly.

[Sea Dog]

1 hour ago, Tom Richmond said:

 

Thanks for your input guys, but your thoughts are not appropriate to my situation. 

1) My boat has no Engine. 

2) I am on a permanent mooring, without power (as indicated by the fact that I said 'Off Grid' in my OP).

Ah, I get it, it's like an Agatha Christy where the killer no-one knew about pops out of the cupboard right at the end!

Silly us for not realising that "off grid" means a permanent mooring without power and a boat with no engine and not what those mistaken boaters who cruise and therefore don't have access to shore power thought it meant. 

Plus, the thing about such discussions is that some others with similar issues or interests also read these threads and benefit from shared opinion.

Perhaps you didn't mean to shoot us down in flames but, even if you did, Dor's inverter idea would still apply to your situation and it would give you the advantage of being able to use the 240V AC for other stuff when you don't need to use it for the immersion heater. I could be wrong I guess, but I thought the Victron EasySolar models have an inverter built in and you said you already had an AC immersion heater fitted.

Edited April 26, 2018 by Sea Dog
Autocorrect bolleaux

[Tom Richmond]

4 minutes ago, nicknorman said:

Since we are being picky about what you did or didn't say in the OP, no, you didn't mention this in your OP. Not that is matters particularly.

oops. sorry nicknorman. I wrote about the morco, but ended up editing it out as unnecessary clutter. 

1 hour ago, nicknorman said:

No it wouldn’t /can’t over drive the heater if the max solar power is no greater than the power rating of the heater. No it wouldn’t reduce to the lower voltage, it would reduce much further. As a maximum to 48v and most or all of the time, much lower.

So, are you saying that running a PV array of 72voc/60vmp into a 48v DC heating element would not burn out the element? This is the key question for me... 

[Tom Richmond]

4 minutes ago, Sea Dog said:

Ah, I get it, it's like an Agatha Christy where the killer no-one knew about pops out of the cupboard right at the end!

Silly us for not realising that "off grid" means a permanent mooring without power and a boat with no engine and not what those mistaken boaters who cruise and therefore don't have access to shore power thought it meant. 

Plus, the thing about such discussions is that some others with similar issues or interests also read these threads and benefit from shared opinion.

Perhaps you didn't mean to shoot us down in flames but, even if you did, Dor's inverter idea would still apply to your situation and it would give you the advantage of being able to use the 240V AC for other stuff when you don't need to use it for the immersion heater. I could be wrong I guess, but I thought the Victron EasySolar models have an inverter built in and you said you already had an AC immersion heater fitted.

Sorry if anyone felt 'shot down'. 

I do appreciate the various alternative ideas suggested, but am just indicating why they don't apply to me. Not having shore power my top priority is the condition of my batteries, so running anything through my batteries seems foolish to me. 

Yes, my Easysolar includes a 1600va inverter. But using it for water heating would mean it wasn't free if I wanted to use other devices. 

[WotEver]

5 hours ago, Jen-in-Wellies said:

Take the voltage beyond 48V and you risk overloading and damaging the heater. 

No you don’t, because the voltage source (PV) is current limited.

54 minutes ago, Tom Richmond said:

So, are you saying that running a PV array of 72voc/60vmp into a 48v DC heating element would not burn out the element? This is the key question for me... 

Yes he is. You have a theoretical maximum power source of (4x260) 1040 Watts. Resistive heaters are direct converters of power to heat. A 1kW heater will convert 1kW of electrical power to heat. Whether that’s 21A @ 48V or 40A at 25V it’s still only 1kW

Edited April 26, 2018 by WotEver
Clarity & correction

[WotEver]

2 hours ago, Tom Richmond said:

As an alternative i could re-wire the panels in parallel sets of 2, voltage of 30v and underpower the element?

That would be the worst approach as the heater would never demand the maximum from the panels.