Thoughts regarding running a low voltage DC immersion heating element directly from a solar panel?

[fourtytwo]

That sounds reasonable, other than I'm not sure what it would be that would 'catch fire' (but the thermostat contacts may well weld together. However ...

Emphasis for safety, certainly burns the material but it is non-combustible in theory, they don't weld shut so much as produce an intense arc trying to open. The arc continues until either the gap widens or the energy source goes away, if your unlucky neither happens and the arc does a lot of damage, emits loads of noxious smoke and gasses, pretty similar to a fire and if anything combustible is near enough becomes an actual fire!

If one is going to manufacture a 'converter' (which, as you say, ought not to be too difficult for someone with 'electronic skills'), would it not make sense for it to be a DC->AC 'converter' (a.k.a an 'inverter'), in which case the immersion's thermostat would be happy?

Kind Regards, John

It's an extra power stage to do the inversion so more losses & greater expense, better to use a thermostat to electronically shutdown the converter.

Just a thought... In terms of efficiency, I believe that solar thermal is more effective at water heating, even in winter, than solar pv...

Yes but...... It needs anti-freeze, a pump and a cylinder with multiple indirect coils, depends on your skills and how much money you want to spend. I just opted for simplicity (in my mind) plus I am/was an electronics engineer

 

[ericmark]

Just a thought... In terms of efficiency, I believe that solar thermal is more effective at water heating, even in winter, than solar pv...

My daughter has solar water heating in Turkey and it works well, but father-in-law had it in North Wales and best it could do was remove the chill. Two large panels with pumps and a controller all professionally fitted, we got another installer to re-do the power bleeding etc, still did not heat the domestic hot water, they had removed the immersion heater when the solar was fitted, and father-in-law noted a reduction is electric use, until smart meters fitted and then no hot water, pilot flame in boiler went out. The system looked the part, when he died new owners of his house ripped it all out.

If I look at my solar PV today it looks like a saw tooth, but when it does produce it goes direct into the system and can't reverse and send it back to solar panels, but with water a sensor has to detect the water is hot in the panels before starting the pump to circulate the water, or it can cool instead of heat the water.

If the panels are at ground level so you can use thermo syphon then a simple radiator painted black under a double glazed window would work, and in Turkey with a flat roof it was easy to bleed the panels at the top, all maintenance was easy, no need for scaffold and the like.

I note every time I drive to Welshpool one house with PV solar panels in their garden, easy to maintain and not over shadowed, but my garden has trees in it, and a hedge so would not get enough sun.

The wires to my solar panels need no lagging, and the system when sun lot shining uses very little energy, I would think the pumps on my father-in-laws solar panels used more power than the panels produced. He did live in Mold about as far north in Wales as you can get, had he lived down south they may have worked.

 

[JohnW2]

Emphasis for safety, certainly burns the material but it is non-combustible in theory, ....

The thermostat in entirely contained in a metal cylinder full of a large volume of water. I really can';t see that 'setting fire' to anything,(outside of the cylinder), can you?

they don't weld shut so much as produce an intense arc trying to open.

I agree, but welding of contacts does sometime happen and if it did, the water would boil, introducing a different safety issue.

... It's an extra power stage to do the inversion so more losses & greater expense, better to use a thermostat to electronically shutdown the converter.DC-DC SMPSUs work, isn't it?)

What sort of 'converter' are you talking about? Are you sure that it doesn't go through a stage of AC which it then turns back into DC (that's how most DC-DC converters work, isn't it?)? If so that that extra stage of 'converting back to DC' would not only invoke the thermostat issues but would also presumably also reduce efficiency, wouldn't it?

Kind Regards, John

 

[Harry Bloomfield]

Emphasis for safety, certainly burns the material but it is non-combustible in theory, they don't weld shut so much as produce an intense arc trying to open. The arc continues until either the gap widens or the energy source goes away, if your unlucky neither happens and the arc does a lot of damage, emits loads of noxious smoke and gasses, pretty similar to a fire and if anything combustible is near enough becomes an actual fire!

Run a suitably rated contactor for DC, from the immersion thermostat, switching the element on and off. Possibly use a suitably rated thyristor, to do the switching.

 

[Harry Bloomfield]

The thermostat in entirely contained in a metal cylinder full of a large volume of water. I really can';t see that 'setting fire' to anything,(outside of the cylinder), can you?

The contacts of the thermostat, are in the upper part, where the terminals are located, rather than buried deep in the cylinder of water.

 

[fourtytwo]

What sort of 'converter' are you talking about? Are you sure that it doesn't go through a stage of AC which it then turns back into DC (that's how most DC-DC converters work, isn't it?)? If so that that extra stage of 'converting back to DC' would not only invoke the thermostat issues but would also presumably also reduce efficiency, wouldn't it?

This could get very technical but potential converters include buck, boost & sepic that all switch at high frequencies (10's or even 100's of Khz) but do not reverse the direction of current in the load. An arc does not have sufficient time to extinguish at these high frequencies nor is it encouraged to do so by the current reversal. It is also unusual to apply the output of such a converter directly to the load as the resulting EMC emissions would almost certainly be unlawful so the output has to be smoothed meaning the load never sees the true switching nature of the current. Isolated converters (that are not required in this application) do generate alternating current internally so they may use a transformer (for isolation) but again the frequencies are typically high and not applied to the load for the same reasons above and again even if they were the arc would not have time to extinguish. I have not found an easy to use reference but this link reburning shows the gas in the gap has a recovery time before full insulation is resumed, if the potential is re-applied in less than this time the arc will re-establish.

 

[JohnW2]

The contacts of the thermostat, are in the upper part, where the terminals are located, rather than buried deep in the cylinder of water.

Yes, on reflection, I supoose that uis inevitably true. However, I still think it pretty unlikely that arcing of the contacts would result in any ';fire'. However, as everyone has agreed, there are simple alternatives to having the thermostat switching DC.

Kind Regards, John

 

[JohnW2]

This could get very technical but .....

Thanks. That all makes sense.

However, it is ('electronically') simple enough to make DC-AC converters ("inverters") that have very low frequency *(50Hz if you wish) true AC outputs, even with something approaching a sine wave waveform - particularly if one is not concerned about size/space/weight.

Indeed, I have several 'boxes' that do just hat and, in my time, have constructed a good few. I presume that they don't present any appreciable EMC issues but I don't know about 'efficiency' (I'm sure you will tell me ) when one is using very low frequency transformers.

Mind you, another thing I don't know (but may be relevant) is how low the frequency has to be for arcs to be 'self-extinguishing' when breaking AC currents ... there may be some 'reasonable compromise' ("best of both worlds"?!) somewhere between 50 Hz and 'tens or hundreds of kHz).

Kind Regards, John

 

[fourtytwo]

So back to the OP's problem, I was simply trying to point out some potential pitfalls including the arcing of thermostat contacts handling DC power! Yes there are many possible conversion solutions and the thermostat contact problem should be born in mind, personally for significant powers (let's call it >1Kw) I would not consider a 50hZ transformer solution economic, actually all the iron and copper for even 500W is fairly ££££. It all depends on if the project is supposed to pay for itself in a reasonable time or is simply greenwashing lols

 

[JohnW2]

So back to the OP's problem, I was simply trying to point out some potential pitfalls including the arcing of thermostat contacts handling DC power! Yes there are many possible conversion solutions and the thermostat contact problem should be born in mind,

Yes, I understand that, and I'm sure he will have been gratefuykl for the valuable advice

personally for significant powers (let's call it >1Kw) I would not consider a 50hZ transformer solution economic, actually all the iron and copper for even 500W is fairly ££££.

Yes, I expected you to say that. However, as I asked, is there not a frequency appreciably (maybe considerably) above 50 Hz at which arcs would still be self-extinguishing?

It all depends on if the project is supposed to pay for itself in a reasonable time or is simply greenwashing lol

Very much so, and that's something I frequently remind people, particularly if they are anything like as old as me If there is a 'payback period' of, say, 6-10 years (another way of saying that one would be worse off for the next 6-10 years if one did it) then that is not necessarily a sensible approach ('gamble') for people 'beyond a certain age'!

There's also the issue, for people of any age, that equipment does not last for ever and the warranty will often expire fairly soon after one 'gets into profit', potentially putting one back to square one (and another X years of 'being worse off by doing it'!

Kind Regards, John