Re: [Phys-l] water

[brian whatcott <betwys1@sbcglobal.net>]

On 11/7/2011 7:07 PM, Anthony Lapinski wrote:
> I am wondering if anyone can help with this, from our garden club:
>
> We're looking to keep 2 gallons max of water from freezing.  We do have a
> power outlet in the chicken house with an extension cord, but the closest
> grid power source is about 100' away.
>
> Could the water be heated with a solar cell grid? Not sure what area would
> be needed, or where we can buy the "most efficient" solar cells. Any help
> would be much appreciated.
I checked on the power of poultry water heater commercial offerings. At 
this water capacity
vendors offer 100 to 125 watt devices.
The solar constant for collectors is often taken as 700 watts/m^2.
Photovoltaic devices may offer rather low  efficiencies - let us say 4%
That would imply an area greater than 125 W X 1m^2/700W X 100/4 or 4.5 m^2

This scheme would suffer the usual problem for load-matching: the supply 
is most
abundant when least needed. Depending on Anthony's locale, it might need 
some
energy storage scheme in order to offer liquid water at daybreak.

I am reminded that there are hybrid cars which put water into a Dewar 
arrangement
in order to make available preheated oil for cold starts as an emission 
reduction element.

I suspect that a cost comparison might favor a buried cable to the mains 
supply, perhaps
 even a small bore heated water pipe to render the whole thing semi 
automatic.

Brian W