Diverting power from the solar panel to a load

[kevtherev]

OK
I want to use the power from the solar panel to heat 10 litres of water.
During the day, I see that the 100watt panel charges the batteries up quite quickly, the voltage then produced seems to be wasted in just maintenance charge.
I would like the charge controller to switch to some thing like a dump valve where it charges a capacitor that will supply a 20 amp submersible heating element?
...or simply once the batteries are at their peak a controller would switch to a load.

I'm sure this has been done but could someone tell me what would be required to do this...or even if this is more complicated than I am imagining it

[CJH]

What to do with excess 'green' power - it's a surprisingly difficult problem, not just in the T25 world. I visited a place that's been into solar and wind power for a long time - he's got so much solar and wind generation capacity that storing it is a big problem. Other than batteries he's experimented with compressing hydrogen and with pumping water from a low lake up to a higher one - both of these then release the stored energy when reversed.

So clearly neither of those is practical in your case, but I would have thought the simplest solution would be a second leisure battery, either connected in parallel or switched over when the first one is fully charged. Diverting the power to a water heater would be tricky I think, since you'd first have to automatically detect when the battery has switched to trickle charge and then you'd have to fool the solar charge controller into thinking the water heater is a discharged battery. I'm sure it could be done but I imagine it would take some bespoke circuitry.

[kevtherev]

Thanks CJ.
I would rather not carry another battery, simply put, there is no room But a good idea none the less.

[CJH]

Thanks CJ.
I would rather not carry another battery, simply put, there is no room But a good idea none the less.

I'll be interested to hear what others say about this - maybe Sterling have something in their product line.

As for not having room, I've put a second battery - a pukka starter battery - in the space behind the nearside tail light cluster and wired it in parallel to my leisure battery via a 0,1,2,1+2 switch.



I was very worried at first about having two dissimilar batteries charging in parallel on the ends of different cable runs, but in fact, after having them connected up to the solar charge controller for a few months they've equalised perfectly - always showing exactly the same voltage. It's given me almost 150Ah, a spare starter battery on board for if (when) I leave the headlights on by mistake, and it helps correct the lean!

[CJH]

Sorry - didn't mean to labour the point, I know it's not what you want.

[ninja.turtle007]

I can't remmebr the figures but I did the calculations and it turned out that there is no point in trying to heat 20l (or 10l) of water with a solar panel and 12v immersion heater. Joules per litre per 1*C

I can look back for the figures tomorrow if you want when I'm in front of the PC?

[kevtherev]

I guess your going to quote Kcals and Kwatt hours to raise 1kg of water 1 deg C.?
Yes I have seen all that and it is pointless if the water is at 0 deg.
However, if the water temp is already raised, it becomes less so.
I don't see raising the temperature of water from 30-35 deg to 40-45 beyond the realms
I just want to know if it is possible using the panel..
if it takes longer than 6hrs then I can forget it.

[CJH]

It seems feasible in that case. Feel free to correct any obvious blunders:

10 litres of water = 10kg
1 cal can raise 1g of water by 1 degree C, so to raise 10kg by 10 degrees you'll need 1 cal x 10000 x 10 = 100kcal
1kcal = 1.162Wh, so 100kcal = approx 120 Wh

So a 100W panel working at maximum output would theoretically take 1.2 hours to raise 10 litres of water by 10 degrees.

There'll be a lot of inefficiencies in there - heat loss from the container, less than 100W from the panel etc - but it seems possible to do it in less than 3 hours if you've got good sun on the panel.

I've no idea what the circuit would have to look like though. Would a manual switch be enough I wonder.

As a comparison, 120Wh is 9-10A from your battery for an hour. So a spare battery could do the job about 3 or 4 times before it would need recharging. It could do it after the sun's gone down too. Just saying.

[itchyfeet]

Why not just connect a 100w heater to the battery, the solar cell should keep up delivering this power just as your alternator would so your battery is still charged.
as for switch on id think thats the difficult bit maybe just a timer if your battery use is consistant?

[ghost123uk]

"We" know that these things =



Are rubbish (and oft quoted as a fire hazzard) but that is when they are being fed with a full compliment of amps via a fag lighter socket. Hard wired to the output of a 100W solar panel they would only be feeding off ~8 Amps absolute max. I have one somewhere (if I can find it) and coupled to my bench power supply, limited to say 7.5A (Kev, what real world figures in Amps do you get on a sunny day ?) I could actually test this "in the real world".

I did, as Chris did, start to do a little maths, but it's a bit complicated and anyway Chris's input covers it well enough

Mind you, bearing in mind latent heat of vaporisation and all that, (jeez, I remember that from school 45 years ago ) I reckon that 9 Ltrs of water at ~30 deg C would go to aprox the required temp if you tipped 1 Ltr of boiling water from your kettle into it

[CJH]

A couple of things to throw into the conversation. I saw this article that points out that converting energy into heat directly is an efficient process (wind energy was the subject of the article but I guess solar PV is similar), since inefficiencies in the electronics leading to losses through heat generation aren't a problem.

Ghost - do you have any idea what voltage latitude those immersion elements have? A raw solar panel can produce high teens of volts easily, so if that needs to go through a controller first to make it compatible with the heating element then I guess there's the issue of convincing the controller that the element is a flat battery, to make sure it's working at full capacity all the time. What sort of load would the heater look like compared to a battery?

[ghost123uk]

Re those heating elements. I reckon the issue would be the opposite way around. As in, I bet that given that resistive load, the panels "natural" output voltage (assuming panel connected directly to load) would drop below even 12 Volts as the load will be greater than the panel can supply. For this reason, if it where I trying this, I would wire that element up pre-controller, via a simple changeover switch. Off now to see if I can find that heating element I know I have, somewhere I rarely throw stuff like that away on the "you never know when" principle, and that is now for this (previously pretty useless) gadget.

[AdrianC]

What to do with excess 'green' power - it's a surprisingly difficult problem, not just in the T25 world. I visited a place that's been into solar and wind power for a long time - he's got so much solar and wind generation capacity that storing it is a big problem. Other than batteries he's experimented with ...pumping water from a low lake up to a higher one - both of these then release the stored energy when reversed.

Hmmm... Hightop van. Tank on roof. Waterwheel mounted on towbar...

Does anybody have the phone number of the patent office?

[ghost123uk]

What sort of load would the heater look like compared to a battery?

Sorry, forgot that bit. It would look like a seriously flat battery.

[marlinowner]

Or you could paint a jerry can black, fill it with water and stand it on the roof.