David Meiland was intrigued by something he’d read in a Breaktime post at Fine Homebuilding magazine in which a Minnesota builder discussed plans for a heat-storing layer of sand 4 ft. thick below the house slab.
PEX tubing would dump heat gathered by solar collectors into the insulated layer of sand in summer, and extract it during the winter. Although sand isn’t an ideal material for this type of system, it’s cheap and easy to work with.
Anyone care to comment? Meiland asks in a recent Q&A.
Sand is not the best option
This idea isn’t new, writes J Chesnut. While Chestnut has heard architects discuss it many times, and despite the FHB poster’s experiences, he finds the sand-bed approach too complicated.
“There are too many variables that can’t be modeled and analyzed as a system (as far as I know),” Chesnut says, “daily weather patterns, heat storage capacity of the bed, efficiency of capturing solar heat, the rate of thermal transfer, the heating design load, heat distribution, etc.
“This strikes me as a ‘shooting from the hip’ approach.”
Given the vagaries of a sand-bed design, the house would probably need a back-up heat source capable of meeting the entire design heat load, a redundant second system that only adds to construction costs.
“Intriguing idea,” adds GBA advisor Martin Holladay, “but the data don’t back up the theory. In short, when winter comes around, the sand isn’t warm enough to provide useful heat.
“Building an insulated sand bed costs money — and the yield on the investment is nil or very low. Storing useful amounts of heat for more than three or four days is hard.”
“We did this about 25 years ago,” writes Tom of American Solartechnics. “It works good for carrying a superinsulated structure for a week or so in the winter here in Maine. Seasonal storage is not realistic, but weekly storage is. This can even out sunny and cloudy days and is a decent tool with that realistic expectation in mind.”
(American Solartechnics, as it turns out, is a company based in Searsport, Maine, that specializes in heat storage tanks and heat exchangers.)
Rock or water storage more effective
Mike writes that seasonal heat storage has been practiced in Europe for some time, although designers there are more likely to use super-insulated tanks filled with water, or rock.
Mark Klein says he’s built a number of homes that use high-mass solar thermal system, which can provide about half the space heating load and up to 95% of domestic hot water needs.
But, he adds, these systems aren’t as responsive as large-tank based systems.
Instead, Klein points to a project in Canada called the Drake Landing Solar Community. Eight hundred solar thermal collectors mounted on garage roof tops capture heat in a glycol solution, which is then piped to a number of bore holes in rock. Over the summer, the rock heats up. In winter, the heat is reclaimed and used to heat the 52 homes in the community.
But this is no backyard project. The 144 bore holes are more than 120 feet deep and cover an area about 114 feet in diameter.
This is a “district” system, as opposed to one designed for a single home.
Store your energy on the grid
“Long term thermal storage has never yet panned out in practice,” writes Kevin Dickson, but what does work is banking electrical energy on the utility’s grid with the help of grid-tied photovoltaic panels and net-metering.
“You want to save all that extra solar energy from the summer and use it in the winter?” he says. “Get a PV system and put it on the grid. Then pull it off the grid. The grid stores it for you, not in BTUs or even kWhs, but in Dollars. Problem solved.”
Holladay agrees. Grid-tied PV solves the two major drawbacks of solar thermal storage: first, that solar thermal generates the most heat precisely when people don’t need it; and second, there’s no effective way of storing it.
“Grid-connected PV systems, on the other hand, give homeowners credit for 100% of the energy produced,” he says, “so they come out ahead.”