Cold Comfort
Part 2: Harnessing Vermont's frigid winter to keep food cool and electricity usage down.

BY BEN HEWITT
Photography by Ben Stechschulte
Published in the March, 2006 issue.

The author built this cold box to replace his refrigerator during the winter--saving about 1 kilowatt-hour every day.

Since the fall of 2005, my family has been on a quest to reduce our dependence on external energy sources in our northern Vermont home. We've been documenting our journey both in the magazine ("Living Unplugged," Dec. 2005) and on my blog at PopularMechanics.com. One of the biggest drains on our electricity supply--which comes from solar power supplemented by a gas-powered generator--is refrigeration. When we first moved onto our land in 1998, all we had was a Coleman cooler fed with blocks of ice from a local store. After a year we were given a small propane fridge ripped from an RV. (The tiny freezer just fit a couple of pints of Ben & Jerry's Chocolate Chip Cookie Dough.) Five years later, we finally bought a real, 14.9-cu.-ft. Hotpoint refrigerator. It was chosen because a) it boasted an Energy Star rating and b) it cost only $400. The fridge uses about 380 kilowatt-hours annually.

Our 1.1-kilowatt photovoltaic system has proved robust enough to run the fridge about eight months of the year. But it falls short come late autumn, when the days become briefer and the cloud cover increases.

There's an irony to the situation: Up here near the Canadian border, the short days are accompanied by a deep drop in temperature. My wife, Penny, deserves credit for connecting the dots: Why not use the cold weather to cool our food? We could design and construct an insulated cold box, and attach it to the outside casing of an exterior door that's ideally located next to our conventional fridge. The door is rarely used, as it's intended to provide access to a summer porch that doesn't yet exist. In other words, it's pretty much useless for anything else, and even when we do get around to building that porch (I'm thinking this summer, although I thought the same last summer), we won't need to access it during the winter. Smart, huh?

 



 

 

Chest Fridge



I came across the idea of converting a chest freezer into a hyperefficient fridge while trolling the Internet. All you need to do is bypass the freezer's internal thermostat with one set at a higher temp (around 40 degrees). It's efficient because cold air is heavier than warm air, and it settles naturally into the freezer cavity. Very little is lost when you open the lid. I bought a secondhand, 8-cu.-ft. Vestfrost freezer for $200 and installed an external thermostat (at left) purchased from a home-brewing supply Web site. Turns out, home brewers have been pulling this trick for years because chest fridges allow them to ferment their goods in kegs at the ideal temp. The upshot? Our Vestfrost fridge draws a mere 1/8 kilowatt-hour per day--a pittance compared to our conventional fridge.
There's a growing online community of people trading ideas on alternative energy, and I was curious to see if anyone else had built such a box. When I Googled "building insulated cold box," I found information on the corrosive properties of zinc, plans for constructing a cat shelter and a treatise on building a "kegerator." Interesting stuff, all, but not what I was looking for. It's surprising, considering the amount of ink--or just as often, kilobytes--spilled by energy misers discussing the intricacies of solar panels and efficient light bulbs. I even joined a Yahoo group called Refrigerator Alternatives and heard about another interesting idea. (See "Chest Fridge," below.) But when it came to cold boxes, we seemed to be pioneers.

Penny and I designed a simple box, sized to fit against the exterior casing of the door. I prebuilt the three walls using 2 x 4s and sheathed them with 1/2-in. plywood. (The dimensions are 3 ft. wide x 16 in. deep x 7 ft. high.) I cut 2-in. rigid foam insulation, aka "pinkboard," to fit snugly between the 2 x 4s, then used expanding foam insulation to fill in any gaps between the pinkboard and the frame. Over the plywood, I installed vertical shiplap siding, mostly because it's what I had lying around and because it matched the natural wood clapboards on our house.

The interior of the cold box was sized to fit conventional fridge racks (25 in. wide x 16 in. deep), which I scored for free from a local repair shop. I lined the interior with 5/16-in. cedar paneling, attaching it to surface-mounted nailers on the inside of the walls to increase the dead air space. I coated the cedar with a water-based polyurethane for moisture resistance and easy cleanup.

I also fitted the box with two 24-volt DC fans that are thermostatically controlled. The fans are mounted on the outside of the box; PVC pipe extends from each fan through the plywood, insulation and cedar to the inside. When the temperature in the "fridge" gets too high, the fans kick on, one sucking in cold air near the bottom of the box, the other exhausting warm air at the top. (Eventually, I'll install an override thermostat so that the fans won't start when the outside air temp is above 40.) We went with DC fans and wired them directly to our battery circuit (via a small hole drilled in our box sill) so that we could cut our inverter and its associated efficiency loss out of the loop.

I stapled a layer of sill seal to the frame of the cold box to keep out the drafts. I attached the unit to the house with 3-in. PrimeGuard screws--to remove or install it, all I need is a screw gun. Once the porch is in place, I'll mount casters on the bottom so that I can roll it in and out of position. I'll take off the temporary roof I built over the cold box after the porch roof is built.

I figure the whole box has an R-value of about 12: The pinkboard has an R-value of 10; the other materials and 1-1/2 in. of air space supply the rest. The box hasn't been in operation long enough to give a truly educated appraisal (stay tuned to my blog on PopularMechanics.com), although one real downside has reared its head: There's no freezer and, thus, no storage space for cookie dough ice cream. But the primary goal has been met. Since installing our cold box, we've kept our perishables cool. And our generator silent.

      


(1) The author's insulated cold box rests on the sill of a doorway leading to the kitchen.
 


(2) After building the walls in his (warm) basement, the author screws them in place.
 



(3) Two fans exchange warm air for cool when the interior of the box climbs above 40 degrees.
 


(4) The interior is lined with cedar planks. The entire structure can be removed easily come spring.
 

Read PM's Energy Family Blog written by Ben Hewitt.

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