Making a Solar Powered Greenhouse Heater - Advice Needed


Postby Solentgreen » Mon Dec 29, 2014 3:36 pm

Hi, I am making a solar greenhouse heater..I have a TPS-102 2.4W @ 17.5 V solar panel. Now i want to use the solar panel to charge a 12v battery and then run a 12v fan heater from the battery. I want it to switch on for a few hours at night and switch on when temp goes below say 5C. I am looking to further purchase a charge controller, thermostat and timer. I am not greatly experienced in electronics and am not sure (yet) how its all going to fit together (planning on making a portable box with display of the various devices inside).Is there anything else I need to get ? I expect i will have to arrange some kind of switching charging and used with fan, not sure what to get for this. Cheers in advance.
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Postby ericmark » Wed Dec 31, 2014 1:14 am

Clearly you can only get out what you put in and so the fan heater would be very small can't see it working with that size panel.

However I will go though the idea. A solar panel will produce maximum power at varying voltages so we have a special switch mode controller which follows the voltage through the day giving the solar panel the best voltage to get maximum power. (Called MPPT)

Clearly the battery wants 12 volt or rather 13.2 - 14.8 volt to charge so the controller turns the power into it to high frequency AC and transforms it to a working voltage.

We hope to fully charge the battery in the limited time we have daylight so a simple trickle charge of 13.2 volts is not really good enough so there are two ways to quickly charge the battery.

The stage charger will start at maximum it can give until the voltage reaches around 14.8 this varies on type of battery. It then switches to monitoring current and once the current drops to a set level it then drops the voltage to 13.2 volt. However this monitoring of current means any draw from the battery can mean the battery is over charged.

The next generation charger is a pulse charger this measures how much the voltage decays after each pulse and uses that to control the charge rate and most wind and solar controllers use this method.

The problem is the controller uses power, this power does not alter much with size of unit so under a set size the power lost in using these units is more than the gains from using them. The cheapest Rutland HRS503 Regulator costs around £50 the Rutland HRSi Regulator around £77 and designed for 160W of solar panels + wind charger.

At 2.4W all the panel will do is stop the battery from self discharge I have one in my caravan for that it just ensures the battery does not get damaged over the winter but does not really charge the battery as such.

One could use a simple zener diode as used with old motor cycles to stop over charging but at 2.4W unless a very small battery hardly worth the effort. Likely the blocking diode is built in.

At 2.4W may just about power a small computer fan but not any heater.

EBERSPACHER and WEBASTO are well known for diesel cabin heaters which are electric controlled. These would allow for being electrical switched but the fan is still around 1.5A so over night it will need a 40 Ah battery to power it which clearly would not be recharged by next night with such a small panel. A all electric unit is more like 7.5A so looking at least 120 Ah even if only running for a short time. Also will need to be traction battery or at least a leisure battery a car battery is not suitable.

Personally I would use a watchmans red lamp of similar paraffin fueled they will run 24 hours. I would guess no draft or heater would be useless so any small paraffin lamp even naked flame. Even a large candle will give out more heat then you will get from that panel.
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