An indoor thermal storage system can re duce winter heating bills by trapping sunlight as it enters your house and storing the heat from the sunlight until it’s needed. The principle behind thermal storage is simple: A solid substance or a container of liquid placed beside a window in direct sunlight absorbs heat from the sun’s rays. When the air surrounding the heat-storing mass becomes cooler than the surface of the substance or of its container, the material begins to release the stored heat.
Putting the principle of thermal storage into practice is also simple, provided that several conditions are met. Thermal storage systems work best in rooms that have their windows facing south. The more windows the better; as a general rule, the glazed surface area should equal at least 10 per cent of the floor space in the room. Such rooms are prone to overheating as the sharply angled rays of winter sunlight pour through the windows during the day; it’s this tendency to over heat that makes the room a good candidate for a thermal storage system. The added mass near the windows absorbs the extra heat before the room has a chance to become too hot; the stored heat is not released until it’s needed — after the sun goes down.
You can perform an easy test to deter mine whether added thermal mass in a room or section of your house will store and radiate enough BTUs of heat to help cut heating bills. On a sunny winter day, pull back drapes and blinds to uncover all the windows in the room, then place a thermometer in the center of the area. If, between 10a.m. and 2 p.m., the tempera ture in the room reaches 90° or higher, thermal storage will work efficiently and cut heating costs.
Masonry, concrete, water and a liquefied chemical compound called phase-change material (PCM) are all used to provide the mass required for thermal storage. Some of these materials are more practical for new construction than for a solar retrofit.
Masonry and concrete, for example, are generally installed as part of the floor area adjacent to windows or as columns partially blocking windows; these materials are most efficiently utilized when they are incorporated in the design of a room. Certain water and PCM containers are made to be built into the structure of a wall; these are also easier to install during rather than after construction.
The water and PCM containers illustrated are designed for use as part of a solar retrofit. The cylindrical tanks are made of fiberglass and are available in 12- or 18- inch diameters, in heights of up to 10 feet. They hold up to 132 gallons of water. You can add dye to the water to
===
Determining Storage Needs
Calculating quantities of thermal mass. To use the chart at right, first total the square feet of floor space in the room or area to be heated; then total the square feet of south-facing glass in the same room or area. In the left-hand column of the chart find the description that best de fines the type of insulation in your house. Read over to the next column and choose the tempera ture closest to the average January-February temperature in your region. Find the conversion factor directly to the right of the temperature and multiply the floor-space figure by this number.
If the result of your multiplication is greater than the south-facing-glass figure, thermal storage won’t work efficiently in the planned area. If the result of the multiplication is less than the south-facing-glass figure, multiply the difference by the thermal-mass base quantity for the storage material you plan to use. The final figure will tell you how much thermal mass to add for optimum heat storage in your house.
House insulation | Average January— February temperature | Conversion factor
Standard: 3½” fiberglass batts or equivalent in 4” walls; 6” fiberglass batts or equivalent in ceilings; double-pane glass; weather- stripped windows and doors.
20°F. .115
30°F. .105
40°F. .90
Heavy: 6” fiberglass batts or equivalent in 6” walls; 9” fiberglass batts or equivalent in ceilings; 3½” batts in floors or 1” styrene or urethane insulation in basement; triple-pane glass or double-pane glass with night insulation; weather-stripped windows and doors.
20°F .92
30° F. .84
40°F. .72
Super: 6” fiberglass batts or equivalent in 6” walls; 1” styrene or urethane insulative wall sheathing (sub-siding); 12” fiberglass batts or equivalent in ceilings; 6” batts in floors or 2” styrene or urethane perimeter insulation in basement; triple-pane glass or double-pane glass with night insulation; weather-stripped windows and doors.
20°F. .83
30°F. .75
40°F. .64
Thermal-mass base quantities darken it and thus increase its absorptive capacity, and algicide to retard the growth of organic matter.
Water 7 gal. (56 lb.)
Phase-change material (PCM): 8 gal. (10 lb)
Bock: 5 sq ft. (217 lb., 38 bricks)
Concrete slab: 4 sq. ft., 4 in. thick (200 lb.)
3 sq. ft, 6 in. thick (200 lb.)
The tanks are deceptively light when empty—the fiberglass in each cylinder weighs only about 10 pounds; however, an 8-foot-tall, 12-inch-diameter tank, For example, weighs 380 pounds when filled with water. Unless you can set the tanks on a floor that covers a concrete slab, local building codes, which you should check before beginning the installation, will generally require that you set each tank directly over a floor joist that is rein forced by the method shown, Steps 1 and 2.
The rest of the installation is simple: 1- inch-thick Manila or cotton rope looped through 1¼-inch eye screws secures the tanks at the top (do not use nylon rope because it stretches). The kickplate tray underneath the tanks is optional; if you want one to provide a flat surface and to protect the bottoms of the cylinders, you can fabricate it with 30-gauge, galvanized sheet steel.
The PCM pod strips, although considerably more expensive than water tanks, take up one quarter the amount of space and store more than five times the BTUs of heat per pound of material. (The enhanced heat-storing capability of the pod strips is due to the phase-change material—a saline com pound—sealed within the fiberglass pods of each strip; this chemical changes from a solid to a liquid at 81°.) Because they weigh no more than 29 pounds apiece and slip readily in and out of their aluminum support channels, you can take them out in the summer, when heat storage is undesirable.
Both the solar water tanks and the PCM pod strips with the necessary hard ware for installation are obtainable from solar-equipment dealers; if you have difficulty finding them in your area, check with the U.S. Department of Energy for a list of manufacturers.
Before you install any type of thermal storage, use a simple mathematical formula in conjunction with the chart shown below, opposite, to determine how much mass you must add to the area that will be heated. Your calculations will be based on several factors. First, it’s essential for you to know how well your house is insulated—use the descriptions in the chart for guidance. If your house does not meet the standards listed as average in the chart, you should add the required insulation before installing thermal storage materials.
The amount of thermal mass you add also depends on the ratio between the number of square feet of glazed surface in the room and the number of square feet of floor space to be heated. You must know the average January-February temperature in your region; check this by contacting the National Climatic Center in Asheville, North Carolina.
Finally, the amount of mass depends on the type of storage material you are planning to add. The table underneath the chart translates your calculations into an appropriate volume of water, phase- change material, brick or solid concrete.
Cylindrical tanks and sealed pod strips. Anchored 4 inches from the stationary section of a sliding glass door, three water-filled translucent fiberglass cylinders trap and store the solar radiation transmitted through the glass of the door. At night, when the room temperature drops below the temperature of the water in the tanks, the thermal energy stored in the water radiates through the fiberglass walls and heats the air in the room.
The considerable weight of the cylinders is sup ported by reinforced joists beneath the floor (Steps 1 and 2); the tank bottoms rest in a sheet-metal tray, which provides a smooth, level surface as well as a 2-inch-high protective kickplate. The top of each cylinder is secured to the wall with rope looped through eye screws. Sealed fiberglass pod strips containing phase- change material cover large windows along a south-facing wall (above, right). Resting in aluminum slip channels supported by wall brackets at each end, the pod strips can be re moved easily during summer months.
Fiberglass Water Tanks for Thermal Storage
1 Preparing joists for reinforcements. For each joist that needs reinforcement, cut enough 18-inch spacer blocks from matching lumber to run, spaced at 2½-foot intervals, along both faces of each joist. Nail each block to a joist face with three 12-penny nails, staggered in a zigzag pattern as shown.
Cut two reinforcement joists for each joist; use matching lumber and make each reinforcement the same length as the existing joist. Miter the ends of each new joist at opposing 60 degree angles; this makes them easier to slip into place.
2. Installing the reinforcement joists. With its longer edge facing down, tilt a reinforcement joist into place beside an existing joist: rest the ends of the new joist on the sill plate and push the face of the joist against the spacer blocks. Nail the reinforcement to each spacer block with three 16-penny nails, reversing the zigzag pattern used for the blocks in Step 1, above. Nail a second reinforcement to the blocks on the opposite face of the joist; repeat the same procedure for every joist that needs reinforcement.
3. Anchoring the water tanks. For each water tank, drive two 1¼-inch eye screws, spaced at a distance equal to the diameter of the cylinder, into the header beam above the window. Leave at least 2 inches of space between screws for adjacent tanks. Then set one cylinder in place, 4 inches from the window with its bottom end in a kickplate tray. While a helper checks with a level to be sure that the tank re mains plumb, loop a 1-inch-thick rope snugly around the tank and through the pair of eye screws, securing the ends of the rope with double square knots. Anchor the remaining tanks in the same manner.
Use a garden hose to fill the tanks with water to within 3 inches of their tops; add any desired dyes or algicides and then push the cap onto the top of the cylinder.
Heat-storing Pods Installed in Strips
1. Installing support brackets. Set the predrilled flange of a channel-support bracket against the wall beside the window, the lower edge resting on the window stool and the corner of the bracket flush with the casing. Make marks on the wall at the screw-hole locations and along the top edge of the bracket. Measure 15% inches up from the top-edge mark and set the top edge of a second bracket at this point; with the corner of the bracket against the window casing, make the same marks as before.
Continue up the wall making similar pairs of bracket marks—one lower and one upper—for each pod strip you will install. Then mark the other side of the window in the same way. At the marks, screw the brackets to the jack studs on either side of the window, using 2-inch wood screws. For drywall with no framing member underneath, use Molly bolts; for masonry, use lead anchors and screws.
2. Setting the pod strips in place. Rest each aluminum channel strip in the bracket slots at opposite sides of the window. To install each pod strip, slip the pod’s upper edge into a channel groove, pushing up until its lower edge will clear the bottom lip of the channel be low, then drop the lower edge of the pod into the bottom lip.
Measure the horizontal distance between the protruding flanges of two brackets; use a hacksaw to cut Strips of aluminum channel 1 1/2 inches longer than this measurement; cut one strip for each horizontal pair of brackets.