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After applying a release agent and a waterproofing layer to a male mold, a worker encapsulates the liner and steel reinforcing frame in polyurethane spray foam. Completed tanks range in size from 5 by 5 by 4 feet to 8 by 8 by 20 feet.
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An 8-by-8-by-11-foot tank is delivered to its final resting place--a basement in this case, though tanks constructed for direct burial outdoors are also available.

In theory, liquid-based seasonal heat storage is simple enough: If you have more heat than you can use right away — in the form of warm-season output from solar thermal panels or a biomass boiler that burns a seasonally available fuel — you use it to raise the temperature of a big insulated container of water or some sort of water/antifreeze blend. Then, during the heating season, you tap into that reservoir and circulate the warm fluid to an air handler, conventional radiators, or radiant slab. When spring rolls around, you start recharging the tank with heat for the following season.

But while sunlight is free and water is notably cheap, stockpiling enough hot water to last for an entire heating season is neither. The history of solar heating is full of creative hippie stopgaps — from homemade plywood tanks to above-ground swimming pools and repurposed unused septic tanks — designed to provide adequate storage at a reasonable cost. Most fell short in reliability, volume, thermal performance, or all three.

Enter a Vermont-based company called Thermal Storage Solutions, which recently began marketing an exceptionally well-insulated and relatively lightweight heat storage tank made from polyurethane foam. Each tank is built upside-down on a plywood plug form that was sprayed first with a release agent and then with a 100-mil layer of a tough waterproofing material similar to that used on a truck-bed liner.

The tank itself is then sprayed with several lifts of 2-pound polyurethane foam, and a structural frame welded from steel tubing is lowered into place around it. Additional foam is sprayed onto the tank until the steel frame is fully encapsulated within 12-inch-thick foam walls, at which point the entire assembly is lifted from the form and the top finished. According to Thermal Storage Solutions vice president Bruce McGeoch, the resulting tank has an insulating value of about R-85 and can safely store water heated to as high as 205°F. “Boiling won’t hurt the material, but the tanks aren’t designed to be pressurized,” he says. “Our systems are designed so they never get that hot.”

The tanks aren’t cheap, though: Retail prices for above-ground tanks run from $7,000 to nearly $14,000 (direct-burial tanks are a bit more). A complete solar-thermal heating system for a 3,000-square-foot home would price out at about $25,000, McGeoch says, even after federal tax credits are factored in. (That price doesn’t include the cost of upgrading the home’s building envelope to reduce winter heating demand to a level that such a system can reasonably supply.) The company’s largest tank, for example, has a capacity of about 3 million Btu, or about 880 kilowatt hours. How many consumers are willing to invest tens of thousands of dollars to stockpile the equivalent of a hundred dollars’ worth of conventional electric heat per year? For now, at least, the customer base is likely to be confined to true alternative-energy believers who happen to have fairly deep wallets.