Solar, Storage

Balancing Solar Gain and Thermal Storage

Two homeowners — Tom of Stillwater, Minnesota, and Jan J of New York City — both asked me related questions about the effects of passive solar gain and the possible diminishing returns of solar thermal mass in their homes.

Tom and Jan, I went right to Ron Judkoff, who’s Director of the Buildings & Thermal Systems Center at the National Renewable Energy Laboratory. Ron says, “You are right about diminishing returns. For direct gain passive solar systems the “rules of thumb” are as follows: * Place mass inside the insulated envelope of the building (you had that part right). * Place mass where it will receive as much direct gain radiation as possible and make sure it is at least in the same room/space as where the sun enters. * Mass for direct gain systems is only effective for the first 2 to 3 inches, so 8″ concrete block is overkill (diminishing returns) unless you are doing a trombe wall system in which case the trombe wall should be 12 to 16″ thick. * Surface area is much more important than thickness. So the same weight of mass widely distributed in a thin layer is much more effective than a thick layer less widely distributed. * The mass (except for trombe walls) does not have to be in exterior walls. It can be in interior walls, or floors. In fact a 4″ brick partition wall separating two rooms with each room having South facing windows is a very effective design because you utilize the first two inches of depth from each surface of the mass wall. * If the floor is used for mass, do not cover in carpet or some other non-conductive material. Use tile or masonry interior floor finishes. Also insulate under the entire floor area (not just perimeter insulation) being used for thermal mass. * The mass surfaces do not have to be dark in color (except for trombe walls), but it would be bad to have the non-mass surfaces dark and the mass surfaces light. If both the mass and non-mass surfaces are light in color, then there will be enough reflections to get the energy to the mass. * Be careful with Low E windows. Work with your window dealer to specify high solar transmissivity low e on the south facing windows (not easy to get) (one product is Heat Mirror 88). If you can’t get a high solar transmissivity low-E window then specify triple clear with a krypton fill for the south windows. Windows on all other orientations can be the usual low-E products.” Ron goes on to say that the overall approach should be to super-insulate the exterior walls, roof, and floor, and then to widely spread a thin surface area of mass all around the rooms that receive south sunlight. We also suggest you obtain passive solar design guides for your climatic region from the Sustainable Buildings Industry Council (SBIC) and you can purchase from SBIC a computer program called Energy-10, which will allow you or your architect to run different design scenarios and determine which performs best from an energy standpoint. Good luck! You’re smart to consider good design and optimization of thermal mass for new home construction. — Scott Sklar