In my previous post of random thoughts, I led off with this: "We've got plenty of cold in the winter and hot in the summer. What we need is a way to store the former for the latter and the latter for the former." In other words, store the cold of the winter for use in the summer (saving the air conditioner), and store the heat of the summer for use in the winter (saving the furnace).

Capturing and storing heat or cold isn't that hard, and there's a readily available environmentally friendly storage medium: water. Water can be frozen in the winter and used for cooling in the summer, and heated in the summer (perhaps with solar collectors) and used for heat in the summer.

So.

How much water would we need?

Starting with cooling: Air conditioning capacity is often measured in "tons," where a "ton" of air conditioning is enough to freeze one ton of water in 24 hours (or, equivalently, the cooling from melting a ton of ice in 24 hours). In our house--which is fairly large--we have two central air conditioning units with a total capacity of 7.5 tons (we have a 5-ton and a 2.5-ton unit).

During the summer, we use the air conditioning for a total of perhaps 30 days, and usually for a duty cycle of no more than 50%. Actually, our usage is probably much less than this, since we usually only run the main (5-ton) unit, and often for only a few hours a day. But this is close enough for a back-of-the-envelope calculation.

7.5 tons 50% of the time for 30 days is (to within the precision of this estimate) a total of 100 tons of ice we'd need to store for the summer. It would have to be in a reasonably well insulated tank, with some sort of circulation system and outdoor heat exchanger to actually freeze the ice during the winter. Water and antifreeze would be an acceptable working fluid.

A ton of water is approximately a cubic meter, so we need an insulated tank with a capacity of about 100 cubic meters. For those who like feet, this is a tank about six feet deep, 30 feet long, and 15 feet wide. Clearly too big to put in the living room, but with some foresight, it could be buried under the foundation of the house before it is built (in our house, there is an ideal place underneath the garage which is right next to the utility room). There would need to be some access to the tank for maintenance, but the systems are frighteningly simple, and in practice you may never need to ever have access to the water/ice tank.

And heating: Natural gas is conveniently sold in units of therms, which are defined as 100,000 BTUs. Each year, we probably burn about 1,000 therms, or 100 million BTUs of gas (this includes hot water, cooking, and our gas clothes drier, but we can use our giant water tank for hot water as well as heating the house). A BTU is the amount of heat it takes to heat a pound of water by one degree F. If we heat our water tank to 210 degrees in the summer (just below the boiling point), and cool it to 110 degrees by the end of the winter (where it isn't too useful for heating anymore), we get a spread of 100 degrees.

So each pound of water can store 100 BTU for us, and we need a million pounds of water. That's 500 tons, or 500 cubic meters of water. Our hot water tank needs to be about six feet deep by 30 feet wide and 75 feet long (2 meters by 10 meters by 25 meters). That's about 20% of the size of an Olympic-sized swimming pool.

That's too big to fit under the garage, but it could still be buried under the foundation of the house (before the house is built, of course). As with the ice tank, you would need to be able to have access, but may never actually need to get in there since the systems are extremely simple.

Can we use the same tank for both heating and cooling? Maybe, but only if you use a heat pump, and you're going to lose some efficiency.

So the bottom line is this: A set of water tanks big enough to store hot for the winter and cold for the summer would not be impossibly large to build into an ordinary home. But it would be big enough that you'd need to bury them under the foundation of the house, and so you probably couldn't easily retrofit it to an existing home.