[Strawbale] solar hot water - holding tank?

[Sherwood Botsford]

There are two ways to design a solar hot water system:
Low pressure:  At the top of the highest point in the system, the water 
pressure is only a few psi. Just enough to move the water back down.  In 
these systems the storage tank can be open or made from light weight 
material.  It's the easiest one to set up for drainback systems.  
However to get heat out of that water, you need some form of heat 
exchanger.  E.g. You put 200 feet of water line IN the storage tank, and 
use that as your supply to the your hot water system.

Household pressure:  In this system, the entire system is part of your 
household water system. 
Drainback is tricky.  But you don't need a heat exchanger.  A small pump 
circulates water from the storage tank to the solar panels whenever the 
panels are warmer than the tank.  When you need hot water, it's drawn 
off the top of the tank, and cold water goes into the bottom of the 
tank.  For this you need a tank that is sturdy enough to hold household 
pressure (30-60psi) plus a safety factor.

Now numbers:  A square foot of collector at right angles to the sun 
receives about 80 watts of light energy.  So 12 square feet collects a 
kilowatt.  In an hour it collects a kilowatt-hour.  A KwHr is about 3500 
btu..  However it's doesn't collect this all day.  The daily average is 
probably equal to about 4 hours of full noon.  So our 12 square foot 
collector is down to 14000 btu per day.  Now too, the system doesn't 
store all that heat.  Pulling a number out of the air, I'll guess that 
40% of the available energy gets into the storage tank in the basement. 
(Guess based on simple, low tech collector.)  That's 5600 btu.

Back to your stock watering trough.  If it's 8' x 2' x 2' it has a 
volume of roughly 32 cubic feet.  At 7.5 gal/cuft that's around 200 
gallons. (You aren't going to fill it to the rim, right?)  Two hundred 
gallons is 1600 lbs of water. So your 12 square feet warms the water up 
by about 4 degrees F. 

Montanagreenpower cites 1.5 gallons/square foot of collector.  In this 
example we have 16 gallons/square foot.  So in this case we are a bit 
undersized.  For a 200 gallon tank, you'd want something like 100-150 
square feet of collector.

Also:  Is 200 enough?  How often do you run out of hot water?  How many 
'tank fulls' of hot water do you go through during the day?  Do you get 
gray days without much sun?

If the tank is well enough insulated, then too big is ok.  It will take 
several days to get to its max temp, but will hold several days heat.

Since the tank may be 80-100 degrees above the room temp, it's worth 
putting more money into insulating it.  I'd think you'd want at least 
R40 around the tank.  Foam underneath it, fiberglass around it.  You 
have to figure out how to keep the insulation dry. At high temps the 
water will evaporate given half a chance, then condense inside your 
insulation.

Plan for the tank to leak 5 years from now.  Might now, but it's a good 
idea to be prepared.

Raftercat5 at aol.com wrote:
> I've been reading the posts about PEX and solar hot water systems,  etc.  
> We're in the building stage now of our SB house, and have  the  PEX imbedded 
> inside concrete slab (5-8" thick).  My husband is the tech  guy...I'm just 
> assisting where needed in the plumbing dept.  We're  wondering what would be the best 
> and most affordable tank to hold the water once  it's heated, since we're 
> going to be using a tankless hot water system back-up  for the solar.  We have 2 
> solar hot water panels (that are very heavy,  btw), and live in S. Carolina.  
> We toyed with the idea of using a stock  tank (like for cattle) and putting it 
> inside a home-made insulated box inside  the utility room, or investing in a 
> purchased one, which I saw can be well into  the $900+ range.  We thought if 
> we built a box to put the stock tank in, we  could use 2" of styrofoam board 
> inside the box, and also on the lid.  Would  that be sufficient?  TIA...
> - Kathy
>
>
>
> **************Start the year off right.  Easy ways to stay in shape.     
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