Solar Heated Houses

Q: I've been contemplating solar heating for houses for a while, in fact, solar heating in all its forms. I have heard or read that solar thermal is economical as long as : (a) low temperature heat is needed (b) the heat is used as it is produced (no storage) (c) the system is large enough (to pay for the pumps, the piping , etc) I read somewhere that PV is already economical in some areas if you use roof-integrated panels and consider the money saved on shingles. I have also read somewhere that upgrading your windows will have a payback period of about 5 years. If you do it when you're building the house, though, and reduce the size of your furnace, high-efficiency windows can have payback periods of 6 months. The problem with heating a house with solar heat is that it needs the most heat when the least amount of solar energy is available. This makes your solar heating system much too large for the other 11 1/2 months of the year. I came to the conclusion that if heat from the summer could be stored until the winter, we might be able to have an economical solar system. One of the reasons for this, is that most solar systems have a 100% capacity back up system. If we are able to eliminate the backup system, the conditions for an economical solar heating system might change. Unfortunately, a storage tank large enough to carry heat from the summer to the winter is much larger than the storage tank needed for a house. The other money saving idea that I had was to integrate the collectors with the storage system. Collectors use a lot of materials and labour for the amount of heat that they produce. I hadn't heard of a batch heater by then, but I found out quickly with an internet search. How about a 1,000,000 gallon batch solar heater? (I'm *not* kidding.) (of course, this is a little too large for a house, but maybe a district heating system - Caddet has an article on a Solar District heating system built in Holland that had equivalent economics to a conventional district heating system) I like Nick Pine's solar closet. (I can't wait for some results.) Lastly, I read recently, maybe on this list, that the ground is the best heat storage. How about having a *large* geothermal storage in the ground - for a small community, with greenhouses sitting above the storage area? Of course, we'd need somewhere to collect the heat from...

A: I have been thinking along the same lines as you have. This kind of "seasonal" or "annual" storage system makes a tremendous amount of sense in climates where winters are cold and don't offer much sunshine (such as where I live in Ontario, Canada). My wife and I are currently experimenting with straw bale building, and it has occurred to me that straw bales might be an excellent match to the insulation needs of a large seasonal heat storage. I was thinking about a large pile of soil covered in straw bales (two layers would provide about R80 insulation value), with some light material to keep moisture out. Insulation underneath the pile would require some further thought. You may be right that a "district" heating system would make more sense. As the volume of the storage grows, the surface-to-volume ratio goes down, effectively reducing the percentage rate of heat loss. Hence, a bigger storage will work better than a smaller one. However, use of a phase-change material might have a very positive impact on heat loss from a long-term storage like this. I'd be interested in reading about that Dutch system you describe. Do you have a link to it?

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