Making use of excess solar power?

[MarcJ]

I should be getting my grubby hands on a second hand solar panel from work in the next few weeks. Probably a 120W one. I'll be fitting it to ease the reliance on marina power and it's an obvious answer to keeping the batteries topped up when on it's mooring (lake at the moment, but probably sea next year) when we're away from it for weeks at a time.

It's a 20ft Vivacity and a 12V 160ah bank of 2 x 6V batteries.

I looked up several threads last night, looking into regulators etc, but really I was wondering how you best use, safely, the excess power created.

From what I've read I could easily get a couple of small computer fans to run full time to keep air circulating, we haven't got a bilge pump but that sounds like it might be an idea (though I'd prefer to plug the leaks better!).

Is there a safe way of creating heat to keep the cabin nice and dry, whilst not risking an electrical fire?
The other question is a simple way of getting whatever heat source to only run when the batteries are fully charged. I've learned from previous posts that some regulators have an output for "the load" which you can set to turn on at a set voltage - say 13.7V, is this right?

One solution I thought might work is a timer switch (a cheap 240V one) plugged into an inverter to come on a couple of hours a day over winter. Only left for say a max of 4 weeks.

Any ideas or experiences gratefully received!!

 

[William_H]

If you are electronically minded you can build a simple circuit where current passes througha transistor when the voltage of the battery exceeds about 13.8 volts. This will mean that the battery is always charged and the excess current passes through the transistor. (and possibly a resistor). The transistor is effectively a resistor and will get hot with the excess power. t would be simple to connect a small fan in the circuit so that this heat is circulated around the boat. (If that is what you want.) PM me if you want a circuit
Regarding a timer http://www.altronics.com.au/index.asp?area=item&id=S0049 This little clock runs on its own battery (mine must hve done 5 years now on original battery) but will switch 12v circuits. It will give switch on and off times for each day or any day or days of of a week. I feel sure they would be available in UK. If not Altornics will post I guess about 30 squid possibly plus VAT. You could use the timer to switch on lights for security.
However regarding the 120w panel I think you will find it an inconvenient size ona 20fter.
What I have done at home is about 20 watts of solar panels into NiMh batteries about 24 claimed AH to one of these timers to LED lights around the house. Sort of a cross between security lights and Christmas lights. Means the yard is never really dark. good luck olewill

 

[MarcJ]

Many thanks Olewill for the ideas - just had a look at 12V timers yes they are available over here, probably a bit cheaper than your link.
I appreciate the thoughts about the large (!) panel on a wee boat, I'm hoping it will come in useful when we aren't hooked up to a posh marina. Anyway it's free, so I "have" to find a use for it!
Yes I could build a circuit, but don't really want to do that (haven't got time). Although that might be a safer bet than spending money on cheap chinese timers/fans/electronics on ebay!

 

[Seajet]

MarcJ,

I do like your idea of using the excess power, but I think Olewill is right, a 120W panel will be huge, I have a 30W job and it takes up most of my main hatch garage top area.

Personally I'd be too chicken to play with heaters when I'm not there but ventilation fans sound a good idea and will keep the cabin dry; an auto bilge pump is nice to have but shouldn't be running regularly to burn off power, unless there's a leak which needs sorting.

I knew someone with an old wooden boat who made an electronic counter to register whenever the pump came on, to keep tabs on how bad the leaks were !

 

[oldsaltoz]

Running a couple of computer fans should be no problem with Voltage Sensitive Relay (VSR). this can be used for running or swithing almost anything on and off depending on voltage.

Good luck and fair winds.

 

[webber63]

Hi MarcJ,
this device may help you (I bought it to turn on the Masthead light when the engine is on):
http://www.ebay.it/itm/DC12V-Auto-circuit-modifications-Voltage-comparator-lm393n-for-Remote-control-/160986663330?pt=LH_DefaultDomain_0&hash=item257b8d89a2

 

[MarcJ]

Thanks for the replies. Yes I'm too chicken to play with heaters that operate when I'm not there. Computer fans it is then!

 

[contessaman]

you can now buy 12v immersion heating elements that will screw into a calorifier in place of the standard 240v one. This would be an excellent way to dump excess energy from solar or wind sources. Though a calorifier might be a little excessive on a 20 foot boat, but then so is a 120w panel -thats going to be huge - like a metre square or something if not more. My 80w panel takes up the whole top of the wheelhouse on my motorsailer.

back to heat, I wouldnt be put off by fire risk. a given number of watts into something small will get it hot enough to ignite something for sure. but if the resistance wire/ element is large enough then it will dissipate the heat energy at a much lower and safer temperature. The only *problem* with solar powered heat is it just wont be very powerful. you'd be better off painting your deck black!

I mean, if you took a simple mains heater like a little oil radiator, Its not going to know the difference between ac and dc so it will 'work' off the voltage of your panel - it just wont get very hot at all. bear in mind that if the panel is rated at 120w you will be unlikely to get even half this on even the best conditions in the UK. It would be easy to assume you might be getting something like 10a out of that panel but in reality you might see 4 on a good day and a couple of amps most of the time.


Still, its all good fun to play around and see what happens.

 

[contessaman]

meant to say, a computer fan whose bearing seizes with the salt air could just as easily start a fire as a properly thought out heat based load dump, could it not?

 

[Stemar]

meant to say, a computer fan whose bearing seizes with the salt air could just as easily start a fire as a properly thought out heat based load dump, could it not?

I'm open to correction, but I think most computer fans, at least decent ones, are brushless, and won't overheat if they seize up. If they did, gamers' computers would be bursting into flames all over the place as the fans get blocked by dust.

As for controlling stuff so it works only when the batteries are charging, a Fridgemate will do just that. http://www.jwmarine.co.uk/

 

[William_H]

Hi MarcJ,
this device may help you (I bought it to turn on the Masthead light when the engine is on):
http://www.ebay.it/itm/DC12V-Auto-circuit-modifications-Voltage-comparator-lm393n-for-Remote-control-/160986663330?pt=LH_DefaultDomain_0&hash=item257b8d89a2

That little device should be ideal for what you want. ie it is a VSR switch a load be it fans or resistor to dissipate power. You will be wanting to waste around 50 watts. That is about 1/20 the power of a small electric radiator. You could buy 8 x 10 watt resistors of 15 ohms clamp them between 2 pieces of Ali to spread the heat wired in parallel. Fit a fan if you want.
The VSR unfortunately might tend to hunt or switch of and on rapidly as the load applied reduces the voltage so turns it off. Perhaps OK once sun is really shining. Wortha fiddle. good luck olewill

 

[reeac]

If there's plenty of sunshine then natural ventilation should keep the interior dry. If there's no sunshine then the PV panels won't work. Keep it simple.
Sorry, I'm being a damp squib.

 

[geem]

If there's plenty of sunshine then natural ventilation should keep the interior dry. If there's no sunshine then the PV panels won't work. Keep it simple.
Sorry, I'm being a damp squib.

sell the big panel cos you wont have anywhere to put it. But a pair of £20w panels off ebay for £50+regulator for another £15. install dorades front and back of boat and let the breeze keep your boat ventilated/dry.

 

[webber63]

That little device should be ideal for what you want. ie it is a VSR switch a load be it fans or resistor to dissipate power. You will be wanting to waste around 50 watts. That is about 1/20 the power of a small electric radiator. You could buy 8 x 10 watt resistors of 15 ohms clamp them between 2 pieces of Ali to spread the heat wired in parallel. Fit a fan if you want.
The VSR unfortunately might tend to hunt or switch of and on rapidly as the load applied reduces the voltage so turns it off. Perhaps OK once sun is really shining. Wortha fiddle. good luck olewill

Hi William_H
I see what you mean but perhaps if the probe of the device is connected to the output of panels (before the regulator) it won't be affected by the voltage of batteries.

 

[MarcJ]

sell the big panel cos you wont have anywhere to put it. But a pair of £20w panels off ebay for £50+regulator for another £15. install dorades front and back of boat and let the breeze keep your boat ventilated/dry.

Yes the more I think about it, the more it seems the panel is going to swamp the top of the boat! Can't harm to have a play though! I won't be buying any bits until I've got the panel in my hands and can have a look at it in situ, as it were.... One 20W panel would probably do to keep the batteries topped up. As said above ventilation is the key.
Thanks again for all the ideas!

 

[ghostlymoron]

I too feel that 120w on a 20' boat is excessive even if you're getting it for free. 20w was enough on my 28'. Try looking at from the power required end not the 'what can i do with the power I've got' end. Panels are reasonably priced nowadays and you don't need an elaborate controller - a pmw is fine.