Archives

I was thinking of installing a a solar panel on top of me Scion XB to power various things. I was wondering what size solar panel would it take to run the the ebh. Could I store the energy into a battery and use it whenever I want?

My car has a very large roof!!:rolleyes:

I think this has been brought up before and the answer is that its not cost effective. I'm sure it could be done with a battery system, using the solar panels to recharge them.. but your looking at several thousand dollars worth of equipment to do so. :(

Resistive loads are way down at the bottom of the list of candidates for slowler...eerr, excuse:D me...solar power. But, anything is possible. The rule of thumb for surface area, of a solar panel/array seems to be roughly 1 square foot per 10 watts. Your EBH is how many watts? IIRC mine is ~400 watts. That's just one thing to consider.

Randall

Not to overstate Randall's point, but the hard part is getting the right resistance.

Remember for highschool physics that V=IR (Volts = Current * Resistance). A simplified way to look at a block heater is a resister designed to offer resistance in exchange for heat. So if you look at my 300W block heater (where W = VI), that means that it's drawing...

300 = 120 * I
I = 300/120 = 30/12 = 2.something

So I'm drawing over two amps giving me a Resistance of....
R = V/I = 120/2.something = 55 or so...

So what you need is to serialize the heck out of the solar panels to to give you a real high V so your R stays at 55 or so.

So the problem comes in trying to pump 300Wh of energy into your block heater means high voltage DC, which gets kinda tricky. Most of the stuff you get is usually 12 or sometimes 24V. So at 24V and 500mA you get about 50R, but to get 300Wh you'd have to soak it for 20 hours.

Anyway, I don't know any good tricks around it. If you hooked 5 small 24V batteries in series, you'd get to 120VDC then you only need 2.something amps. Then you start shopping the batteries to get the right AmpHours...

Anyway... not impossible, but not off-the-shelf either.

11011011

Wow, that was some bizarre math to me. Let me try...

P = V*I => I = P/V = 300 w / 120v = 2.5A

P = I^2*R => R = P/I^2 = 48 ohms

So R = 48 ohms... this is a non-adjustable property of the heating element.

Because R is fixed, the power dissipated is proportional to the square of the current running through the element only. I think maybe you are discussing impedance matching the panels to the heater?

So if you have a 12v panel that supplies 500 ma, that is a 6 watt panel. If you connected this to the EBH, you would get I = 12v / 48 ohms = 250 ma of current. That is, you only get half of the power the panel is capable of supplying, 3w. Connect two in series and you get I = 24v / 48 ohms = 500 ma, or the full power from both panels for a total of 12 w.

Just trying to make some sense of what you said in my head since I had also considered this. You are right in that it would take an awful long time to develop 300 w-hr using 10-20 w worth of panels. I think the guy with the Prius in the other thread had ~250w worth of panels? That's much easier to work with but also more $$$.

I think the real benefit to running a EBH at low power would be to try to keep an "already-warm" block so during the day. Plug in EBH at night, drive to work, solar keeps it warm for the ride home.

And let's not get into how a resistive heating element represents a non-linear (not ohmic!) load. :)

Since the idea is to keep the block warm, it may be better to rig up a small solar thermal panel with a PV powered oil circulator pump.
PV panels convert only 10-19% of the energy to electric.
A good flat plate can convert 51-79% of the energy to heat.
The thing we want is heat, and not at a particularly high temperature.
Flat plate thermal panels are even more efficient when operating at lower temperature differentials.
Just my 2C