12V fan does not work when in parallel but does in series

Question

I am building a food cooler for camping this summer. My plan is to build a thermoelectric cooler that is solar powered. My cooler is looking much like this:

including a CPU fan on the hot side and heat sink on the cool side. I bought a 5W, 12V solar panel to run it (and make it so I can also plug it into my car). Both the fan and the TEC will be connected in parallel to the source. When testing with a power adapter, the fan runs only if in series, which would have it going at 6V. In parallel the fan does not run. This is a CPU fan that runs at 12V.

What is happening? Any suggestions for a fix?

Answer 1

I hate to avoid the stated question, but you need to rethink your basic cooler idea.

Let's start with your solar cells. A 5W, 12V panel implies a short circuit current of ~0.4 amps. That is 0.4 x 12 = 4.8. Solar cells typically have an essentially constant current output, rather than a constant voltage output.

A standard 50W, 40mm x 40mm Peltier device will have a resistance of ~ 5 ohms.

So, when driving your cooler, the solar panel will produce a voltage of 0.4 amps x 5 ohms, or 2 volts at the cooler. Another way to look at it is the power delivered to the cooler will be 0.4 x 0.4 x 5, or .8 watts.

TECs typically run at about 10% efficiency, so your 5W solar panel will only produce about .8 watts at the cooler, and the cooler will only produce .08 watts of cooling.

I think you have much more pressing issues than your fans.

Answer 2

The TEC has a very low resistance, comparable to or lower than the internal resistance of the solar panel. This means that when it is connected across the solar panel, the voltage across it drops too low to run the fan. On the other hand, when you connect it in series with the relatively high-resistance fan there is plenty of voltage across the fan to run it since there is very little voltage drop across the TEC. Unfortunately this means that more power is being used by the fan than the TEC, so not much cooling will happen.

Answer 3

There's nowhere near enough power from your solar panel to run an inefficient TEC and produce sensible amounts of cooling.

Something like this for the solar panel is getting close: (20x, in line with the 10x+ bigger solar panel as Russell suggests). You might need more than one.

TECs are kind of a thermal short when they're not conducting current, so TEC will create a lot of heat loss when the sun is switched off (unless you're camping in the high Arctic in summer and have sunlight (or at least some light) 24/7). Of course the fan will be off, but it would be better if you could replace the TEC with a block of good insulation when it is not producing a net cooling effect. Or add a battery and charger and sufficient solar panel capacity to run everything.

Maybe get used to pemmican, which has high energy content per unit mass and per unit volume. Or opportunistically pick up ice and consume perishables before it melts. A 100-quart Coleman cooler packed with dry ice and water ice will reportedly stay cold for up to two weeks, but it's 100lb before adding food. A phase-change material in the cooler would act as storage, like the battery suggested above.

If the region is desert with clear skies at night, it might be practical to use the cold from the sky at night rather than the light from the sun during the day, but that's straying way off topic.

Answer 4

Simple answer, the Peltier is stealing all of the electricity. Electricity flows in the path of least resistance. And if the Amp draw of the peltier is higher than the Power supply can deliver the circuit will not work. The peltier will leach all the power and the fans will not work. You best bet it to power the device off a Motor cycle battery and use the Solar Panel to trickle charge the battery with a PWM charger.