Feb 2001

Dear Solar Guy,

I am interested in using solar energy to heat my house. I am 
considering using an old satellite dish, polished, to concentrate
the heat. Can you give any advice on this project?

--------------------------------------------------------------

Hello Don, 

I am going to say some things that may surprise you. First, your old 
satellite dish, if polished and pointed at the sun, will generate extremely 
high temperatures at the focal point. Exactly how hot it would get 
depends on how good the focus is, but it might be well over a thousand 
degrees F. 

However, your parabolic reflector is a poor choice for collecting sun 
energy unless you absolutely need the extremely high temperatures. 
Recognize that temperature and heat are really different animals. 
Temperature is a measure of heat intensity, not heat itself. When
you concentrate solar energy, you do not actually create any more heat,
you just take what comes into a given area and squeeze it into a smaller
area, increasing the temperature. Aside from the fact that concentrating
doesn't actually increase the amount of heat, there is the fact that heat
losses increase dramatically as temperatures increase, so high temperature
collectors are gnerally less efficient that low temperature collectors.

If you want to heat your house, then you need a lot of heat 
at a fairly low temperature. The only way to get a lot of heat 
from the sun is to intercept a large area (full sunlight provides 
no more than 1000 watts per square meter). People who use solar 
collectors to heat houses usually employ fairly large areas 
of flat-plate collectors. But, I don't recommend that you do 
this. Read on... 

I will assume that you have already done the logical thing and 
checked your home's energy efficiency. Investment in insulation, 
weather stripping, caulk, storm windows, a setback thermostat, 
an efficient furnace, etc., will pay back quite rapidly, far 
quicker than any solar investment. 

Next, I recommend that you assess your windows. South facing 
windows with good sun exposure, on an energy efficient house, 
are what we call "passive solar design". It has been proven to 
be as effective for space heating as complex systems of collectors 
and energy storage. 

Next, if you still want to capture some solar energy, your best bet 
will be for domestic water heating. There are various commercial 
units available, and can be very effective. 

Finally, if you still want to heat your house with heat from solar
collectors, you will need to do some serious math to be sure the
whole thing will actually work, before you actually build anything.
You will need to estimate the heat needed by your house (given local
weather conditions and the specifics of your house), the heat loss
from your water storage tank  (and yes, it will need to be insulated)
and all other components, and then design a collection array large
enough to supply all of this heat. It will be a pretty big engineering
job.