Adding Cheap Solar Power to Your Home

If you're looking for ways to cut your fuel bill, here are two cheap solar power projects you can take up this weekend.
By the MOTHER EARTH NEWS editors
June/July 1994
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Even an older home can be retrofitted with a thermal mass or batch heater system.
MOTHER EARTH NEWS STAFF
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Maybe in your mind the term "solar power" conjures up images of blue photovoltaic panels lining a broad expanse of south-facing roof on a residential home, or perhaps vast arrays of mirrors aimed to concentrate the sun's light in one spot. And maybe you also think about the cost of these big ticket implementations, and know they are well beyond your means. Well don't despair. Cheap solar power isn't an oxymoron. Tapping the sun's energy for heating purposes is affordable and relatively simple. Here are two ways to do it.

Thermal Mass

When I was around ten, my mother decided to build herself a modest 3' x 7' step-in greenhouse as a small addition to the family room. She and my father sat around the kitchen table for weeks, poring over plans and deciding how they could piece it together without going broke. And each day I came from school to find something different; a large hole in the south-facing wall, sheets of plastic as temporary windows, piles of bricks, concrete. For a while it was better than television. In a month, construction was finished, and we had a handsome little topiary bubble sticking out of the house like a turret. The first hot weekend in May, however, brought a shock for my Mom. That bubble, while we were away for just one warm, sunny weekend, had managed to kill all but the hardiest of Mom's plants and turn the family room into a convection oven. It was my first lesson in just how much heat a few windows can muster. Mom and Dad restocked the plants, invested in some reflective shades and monitored the weather a little more closely after that.

Nearly all homes have some southern facing glass, but most homes, especially older ones, were not built to take advantage of the heat energy those windows can transmit. One low-cost and simple way of taking advantage of southern exposure is through the addition of thermal mass material, which absorbs the heat energy and stores it, radiating the stored heat during the cool fall and winter evenings. You can do this by placing a substantial amount of heat-holding material such as masonry, concrete, or containers of water where the sun can shine on them. This thermal mass will help to prevent the sunny room from being overheated and will reduce the need for opening windows or shading out the sun's free energy just to maintain comfort. At night, this mass will radiate its stored solar heat and reduce the need for conventional heat.

There are many ways to add thermal mass to a room. The challenge is to add enough. Try to provide 2 to 4 gallons (1/4 to 1/2 cubic feet) of water per square foot of south window, or 1/2 to 1 cubic foot. (75 to 150 lbs.) of concrete or masonry per square foot. If the mass is not in direct sunlight, double or triple these amounts. If you have a concrete slab, you already have plenty of thermal mass — just allow the sun to reach it. The mass should be dark-colored to absorb radiation and should be placed as close to the window as possible so that the sunshine hits it all day. It's important, though, that the windows should normally overheat the space where the thermal mass material is placed. If the windows don't overheat the space, the mass won't attain the higher temperatures needed to help with heating, except by moderating uncomfortable temperature change.

Variations  

  • If you have a strong floor, you can pour a thin 2" - 4" concrete slab over it, or lay down brick or heavy tile.
  • Steel drums or racks of water-filled containers could act as partitions or furniture.
  • An interior wall that receives sunlight could be lined with stone or brick. 

Advantages 

  • The addition of thermal mass does not increase the amount of solar energy available to a space, but it lengthens the time you can use the heat.
  • Thermal mass moderates temperature swings between day and night, keeping the space more comfortable.
  • Excess heat that might otherwise be thrown away is saved and used later.
  • Thermal mass has no moving or complex processes. It simply sits there and does its job without your time or effort.
  • Thermal mass materials are common and inexpensive. Concrete runs $40.00 to $60.00 per cubic yard (27 cubic feet). Water is almost free. Containers, such as steel drums or plastic jugs, can be scrounged or bought secondhand. Figure $1 to $2 per square foot of window area. 

Disadvantages 

  • Thermal mass requires bulky materials and often takes up a lot of space. It's often difficult to add more in a graceful way that's acceptable to your existing decor.
  • The structure of your house may need strengthening before it can support the considerable weight of thermal mass. To be sure, check out the foundation and floor before you load in the mass. 

Batch Heaters

Another way to take advantage of the sun's considerable heat energy is through a batch heater (sometimes called a breadbox). A batch heater, nothing more than a greenhouse for heating water, is a water tank painted black, mounted in a well-insulated box, and covered by a sheet of glazing. The batch heater preheats the cold water from your well or municipal water supply, reducing the amount of energy your water heater uses. The best location for a batch heater is an unshaded area in the yard on the south side of your home or as close as possible to your existing tank.

The sun's energy passes through the glazing and is absorbed by the black surface of the tank. This heat is immediately transferred to the colder water in the tank, supplied from your well or municipal water service. On a warm sunny summer day, the water in the tank may reach 140°F. This preheated water then flows to a well insulated outlet pipe to your home's water heater. As the warm water is delivered to your home, it is replaced in the tank by cold water from the inlet pipe and the cycle continues.

To get the most hot water from this solar system, use only a single layer of glass. Including a reflector in the design will increase the amount of solar energy the tank receives. The water lines going to and from the solar tank should be well insulated and kept as short as possible.

The batch heater needs to be drained at the first sign of a severe frost and refilled in the spring when all danger of frost has passed. This heater needs little maintenance — every two years add a fresh coat of paint and check the sacrificial rod that is placed in the tank to reduce corrosion.

A batch heater is an excellent choice for a water-heating system. It's inexpensive and takes a moderate amount of experience with carpentry and plumbing to build. It will pay for itself very quickly, especially if it is combined with hot water conservation measures. The batch heater is flexible — if you have a sunny wall, you can build a freestanding version.

Some commercial solar water heaters are based on this design. By using advanced materials, they do not freeze in the winter and can be used year round.

Construction of a batch heater, including $50 for energy conservation measures on your existing hot water system, will cost between $500 and $ 1,000. A batch heater can cut your hot water bill by half when it operates, or up to 1200 kWh per year.

Variations  

  • The batch heater can be oriented in a north-south direction or it can also stretch in an east-west direction.
  • Purchased versions of this design use several layers of glazing and operate year round

Advantages  

  • This hot-water system is the cheapest and simplest to build. For the cost it puts out a good amount of hot water.  

Disadvantages  

  • The total hot water production is lower than some other systems because the batch heater is not used all year.

Adapted from The Fuel Savers (Morning Sun Press, 1992) by Bruce N. Anderson. Copyright © 1992 by Bruce N. Anderson  


What It Costs to Run Electrical Appliances

Unless indicated, costs are measured per hour. Estimated electricity rate: 14.7¢ per kwh. Figures courtesy of Consolidated Edison

Coffee maker              1.2¢
Television, color         2.1
Television, B&W           0.7
VCR                       0.5
Computer monitor          0.8
Computer and disk drive   1.2
Computer printer         
 (standby)                1.2
 (printing)               1.8
Light bulb
 (100 W)                  1.5
 (60 W)                   0.9
 (25 W)                   0.4
Electric blanket          1.1
Blow dryer (per use)      2.0
Clock (per month)        29.4
Vacuum cleaner            9.7
Washing machine (load)    3.8
Clothes dryer (load)     44.1
Iron                      8.4
Fan                   1.3-2.9

Room air conditioner (10,000 BTUs/500 hrs for one summer)

 New unit (10.0 EER):   $77
 Old unit (7.0 EER): $110

Refrigerator (dollars per year)

 Single door
  12 cu ft, manual defrost:          $72
 w/ freezer
  14 cu ft, manual defrost:         $113
  14 cu ft, frost free:             $108
  18 cu ft, frost free:             $115
 
Freezer (stand alone)
  15 cu ft chest, manual defrost:    $70
  16 cu ft upright, manual defrost:  $97
  16 cu ft upright, frost-free:     $129

Sizing a Room Air Conditioner

Floor area   Type of Outside Wall
             Heavy       Light
100          4,500       5,300
125          5,150       6,100
150          5,700       6,800
175          6,200       7,500
200          6,650       8,100
250          7,550       9,300
300          8,300      10,400
400          9,700      12,400
500         11,000      14,250

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