How Solar-powered Refrigerators Work

­While developing solar-powered refrigeration systems for the International Space Station (ISS), a couple of NASA scientists decided the technology would do well in the Earth-bound world, too. Scientific-study outposts, eco-resorts, remote hunting cabins, street-vendor food carts and areas of the developing world with no access to electricity could all benefit from solar-powered refrigeration. Not only food but also many medications and vaccines need a cool environment to remain viable. It's a fairly essential technology.

Those NASA photovoltaics engineers created SunDanzer, a line of solar-powered refrigerators and freezers.

The SunDanzer refrigerator works a lot like a traditional one, using a vapor-compression system for cooling. Basically, a refrigerant gas like ammonia is compressed (placed under pressure), causing it to get hot; as it cools back down, it condenses into a liquid. When this liquid travels to a lower-pressure area, it expands and vaporizes. This vaporization absorbs heat, rapidly cooling the coils of the refrigerator.

This is probably how your refrigerator works. The difference between your refrigerator and a solar-powered SunDanzer is that instead of running all the moving parts of the compression-expansion system by plugging into the power grid, you plug into a solar-panel setup. The system simply uses photovoltaic cells instead of coal or nuclear power to generate electricity. It hits the temperature of any other refrigerator (around 38 degrees F, or 3 degrees C, for most people).

To supply enough power to maintain that temperature, the unit's solar panel needs about five hours of sunlight a day [source: NASA]. But it can store excess solar-generated power so it'll stay cold for a week without sunlight. And there's the option of supplemental battery power so food will still stay cold in a pinch. Most solar-powered cooling systems come with this type of backup.

One solar-powered refrigerator model that does not have such a high-tech backup is the so-called "eco-fridge" developed by a 21-year-old student in Britain. That model has no moving parts at all.

­When most of us think of "solar power," we think of panels of photovoltaic cells. A refrigerator introduced in 2009 by British student Emily Cummins used a far more literal version of solar power.

Cummins' refrigerator uses no photovoltaics, no electricity, no chemical refrigerant and no moving parts to produce a cooling effect. Instead, it uses an evaporative-cooling system based entirely on the sun's heat and some water. It's an ideal setup for poverty-stricken Third World villagers with no electricity and no money to spend on a fancy PV system. It is already being used in South Africa and Namibia. (It could also work for a hard-core environmentalist looking for a guilt-free cold one after a long day at work.)

The fridge is brilliantly simple. It's made of two cylinders, one inside the other. The outer cylinder can be made of almost anything on hand -- cardboard, wood or plastic, typically, with holes in it to allow the greatest possible sun access. The inner cylinder is metal. The space in between the inner and outer cylinders is filled with materials like wool, sand or dirt.

The evaporative-cooling system works like this: You place food or medicine inside the inner metal cylinder and close it up. You then pour fresh water into the area between the cylinders, saturating the organic material, and place the whole setup in the sun. As the sun warms the outer cylinder and heats up the wet organic material, the water evaporates. Just like in a traditional fridge, evaporation removes heat (that's why we sweat -- when the sweat evaporates, we cool down). The organic material is touching the inner cylinder, so this heat transfer pulls heat from the inner cylinder holding the food. The result is a very cold inner chamber.

This process is repeated as the water evaporates fully, so water is continually replenished and the cooling continues. The refrigerator can stay at 43 degrees F (6 degrees C) for several days on a single water infusion [source: Flahiff].

This system has no battery backup, so it relies on a good supply of sunlight. Countries like Namibia, with an average of perhaps 10 hours of sunlight a day in some areas, are ideal candidates for this solar-powered setup [source: BBC].

That's always the potential drawback of any solar-powered necessity: Sunlight is seldom guaranteed. In more high-tech PV refrigerators, backups in the form of batteries or generators are common in order to avoid system failure. The low-tech eco-fridge relies entirely on nature to keep things cold.