Cooling a cooler

Can a solar panel be used to cool a beer cooler? The lid of a picnic cooler is big enough to hold a typical 20W solar panel ($60).

How much energy does it take to keep a cooler cold?
Heat conduction Q/ Time = (Thermal conductivity) x (Area) x (Thot – Tcold)/Thickness

Equation from here.

I couldn’t find specifications for cooler insulation. I guess picnicers don’t ask. Figures for styrofoam are available, figure 2 in. of styrofoam. Styrofoam has a thermal conductivity of 0.033 W/m°C (or here).

Let’s assume air temp of 90F, cooler temp of 40F, that gives a delta T of 28 °C.

Heat flows in through the walls of the cooler. A cooler has an internal surface area of about a foot on each of six sides, 6 sq ft, about 0.67 m2.

Plug the figures in, the cooler heats up at rate of 12 Watts an hour. So 12W of cooling should keep it cold.

So the 20W solar panel powering a Peltier cooler with heat sinks inside and outside the cooler should be able to keep a picnic cooler cold during the day. The solar panel will provide less power when the sun isn’t overhead. The Peltier cooler is inefficient, figure 50% efficiency, effectively 10W of cooling when it gets 20W of solar power. Peltier coolers also have a peak temperature difference of about 20°C, so poor heat exchangers will cut its effectiveness.

Extra heating by the having the cooler sitting in the sun, or not enough sunlight on the solar panel, or opening the cooler will decrease the system effectiveness. Perhaps it would only cool the contents to 50°F.

This assumes that the cooler starts cold. This system does not have the power to cool down a room temperature cooler full of liquid. Figuring two and a half gallons of liquid (10 l) in the cooler, it would take nearly a day to cool it from 90°F down to 40°F running off a battery or wall current.

The limits of Peltier cooling is a delta T of about 60°C, achieved by sandwiching several layers of coolers together. A small 4 in. square chamber surrounded by 3-4 inches of Styrofoam only needs one Watt of cooling. Sandwiched Peltier coolers are inefficient. At a guess, the same 20W solar panel would work. At night, the chamber would heat up at a rate of about 1°C an hour, reaching say -15°C at dawn. So small scientific samples could be frozen in a portable, off-grid cooler of this type.

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