Well, water has a high heat capacity and a good heat transfer rate. It’s also plentiful, inexpensive and non-toxic.
Great! That answers a lot of my questions but what is heat capacity.
Oh that! Heat capacity is the measure of the energy required to raise the temperature of a substance. For example it takes one BTU of heat energy to raise one pound of water one degree F. Water is a substance that has one of the highest heat capacities. An equal volume of water can hold about 2,000 times as much heat as an equal volume of air so water is an ideal low cost, safe substance for storing and transporting heat.
OK, OK I get the idea. Water is a substance of choice for storing and transferring heat, but doesn’t water freeze?
Yes water freezes, but freezing is not a problem with trickle down or drain back systems. Water simply drains back into the storage tank when the pump is turned off. There is no need for a pressure relief valve or check valve. The plumbing is simple and antifreeze is unnecessary.
OK, Let’s say that water is a great heat storage medium. How much water will it take to heat my house in Maine?
Generally speaking you’ll need at least one gallon of solar heated water for every square foot of living space. You’ll also need the entire surface area of a properly oriented steeply pitched roof to harvest solar heat.
Sounds good but I have a problem. My ranch roof faces East.
This is a problem. You’ll harvest less than half as much heat from an East facing roof as a South facing roof If you decide to add a second floor take the time to pitch the roof in the right direction.
OK if I have a 1000 sq ft roof pitched in the right direction what will be the value of my heat harvest per year?
Well this depends on a number of factors but let’s give the calculation a try. If you have 1000 sq ft solar roof and an average of 3 hrs of sunlight for 300 days you’ll have about 300x300x1000 BTU’s of available solar energy. At an efficiency of 50% you’ll be able to harvest 45,000,000 BTU’s of heat per year, or the heat energy equivalent found in 300 gallons of fuel oil. If we assume that the value of fuel oil is $3.33 the value of this much energy would be about $1000 per year. This might not sound great but as the value of fuel increases so will the value of the solar heating / hot water system. As long as we keep the initial cost down our investment will pay for itself in a short time.