ASK THE EXPERTS: Solar Heat Storage

Beginner
In-Floor Radiant Loops
What are my options for storing solar-heated water for space heating purposes?

I am interested in solar collectors for space heating with storage (either concrete, rocks, or water), and was wondering if you could suggest anything. I am having trouble finding information and am wondering if it is a worthwhile plan to pursue. I find tons on solar water heating, but information on space heating with storage is limited. Thanks,

Michael Clark • via e-mail

Hello Michael, There were thousands of active solar heating systems with storage installed twenty or more years ago that are still operational today. Because of the high initial cost of many of these systems, they are dependent on tax incentives for their popularity.

I will only address thermal storage as it applies to active solar heating systems that I am familiar with, although there is a lot of overlap with some passive systems. Books from the early 1980s have some good information about storing solar energy, but they also have some info that has been proven wrong in the ensuing years.

Water has a relatively high specific heat, is inexpensive, easy to store, and easy to transport through piping. Because of this, it is the storage medium most often used with solar heating systems. The only big considerations are the tank and system design. I recommend that large, unpressurized tanks (more than 120 gallons) be constructed of fiberglass with a high-temperature liner, stainless steel, or polypropylene. Concrete and mild steel tanks have been successfully used, but they are thick and very heavy. Other materials like lower-density polyethylene and EPDM have proven to have very limited lifetimes in large tanks.

Concrete and brick floors are used for storage in radiant floor systems. Solar heating systems integrate nicely with radiant floor systems because of the low operating temperature and the built-in storage if the radiant tubing is embedded in concrete. Concrete and brick have specific heats of about 0.2—or one-fifth of the specific heat of water. Specific heat is a way to quantify any material’s heat content or ability to act as a thermal battery. Compared with water, the same volume of concrete or brick cannot hold as much heat at a given temperature, but because the weight of concrete is about triple that of water, the water will hold less than twice as much heat in the same volume.

Rock bins and eutectic salt (phase-change material) storage systems have rarely worked out for numerous reasons—please take my word that they have unforeseen problems after installation. Cinder blocks turned on their sides and capped with concrete have been used successfully as storage systems for air collectors, but this is not a widely used design.

I suggest that you make your search queries on the Web very specific when researching any of these different storage systems. If you can’t find anything there, you might search online booksellers for older solar heating books. Cheers,

Chuck Marken • AAA Solar

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