We've been looking into getting a fridge for when we go camping which made me start to look into how they work and what makes them different from the fridge in our kitchen. A fridge is basically a very well insulated box with a method of extracting heat from the inside and transferring it to the outside. It's the method of extracting the heat that differs.

Here's the science bit.

Before we get into the different methods it's first important to understand the concept of latent heat. For a material to turn from solid to liquid or liquid to gas requires energy. That energy doesn't alter the material's temperature it just changes the material's state. When the material changes back from a gas to a liquid or from a liquid to a solid that energy is released again. So for example when ice at 0°c melts into water at 0°c it absorbs energy from it's surroundings to do so and when it refreezes it releases that energy back to it's surroundings.

A normal household fridge extracts the heat from the inside using a vapour compression cycle. In a vapour compression cycle a gas is compressed and allowed to cool which makes it liquify, releasing the latent heat. It is then allowed to quickly expand which makes it vapourise and absorb latent heat. The gas is then compressed and cooled again and the cycle restarts. In the fridge the cooling and liquifying part of the cycle takes place outside and the expanding and vapourising part takes place inside. This method is about 40-60% efficient but the compressor is heavy, noisy and not very robust. So it's not well suited to camping use.

An electrical camping fridge works differently. It uses thermoelectric cooling also known as the Peltier effect (discovered by Jean-Charles Peltier). Mr Peltier discovered that if you create an element made from 2 different materials and pass a current through it, one side will become cold and the other side will become warm. If you reverse the current the hot and cold sides swap over. In a fridge one side of the element faces the inside of the fridge and the other side faces the outside of the the fridge. When the current flowing in one direction this cools the inside,when the current flows in the opposite direction it warms the inside. Thermoelectric cooling is only about 5-10% efficient but the devices are very robust as they have no moving parts.

Another option, which seems illogical uses heat to cool the fridge by vapour absorption. This works in a similar way to the vapour compression method in that refridgerant flows in a cycle around a closed system of pipes and the refridgerant expands and vapourises inside the fridge and cools and condenses outside. The difference is that heat is used to drive the refidgerant around the system rather than a compressor, so it's quiet and has no moving parts. This method is about 8-12% efficient, does not require an electricity supply to work and is quite robust.

A final method used is evaporation. In this case the fridge has an inner non-porous highly conductive container usually metal, surrounded by a large block of porous material like plaster. The outer layer is kept damp. The heat of the sun evaporates the water from the outer layer. This evaporation requires latent heat energy which is drawn from the inside of the container. This method requires no energy source at all, just a modest supply of water. It was quite popular in the 1970's but has died out now and it's difficult to find a modern fridge that uses this method. These evaporation fridges are better than coolboxes at keeping things cold but can't cool things down very well if they are not cold to start with.

Modern camping fridges usually have a thermoelectric element powered by a 12v supply, some have a built in transformer so they can also be used with a 240v supply (2-way fridges) and others in addition to that have a vapour absorption system powered by camping gas built in too (3-way fridges). They are not as efficient as a household fridge and so can not keep the temperature as low.

Camping fridge temperatures are quoted as degrees below ambient, ambient being the temperature of the air around the fridge. Given the cooling depends on the heat energy being released to the air around the fridge the temperature of that air is important. Where a household fridge can cool to around 40°c below ambient a camping fridge can only manage around 15-20°c below. Also a household fridge has a pretty constant ambient temperature to deal with around 20°c, whereas a camping fridge might have to cope with 30°c or more. If it gets hot you should try to keep your camping fridge in a nice cool shady place with plenty of air flow around it, to give it as much chance as possible. In hot weather camping fridges are unlikely to be able to keep the temperature as low as a household fridge (5°c).

So there we have it, camping fridges use two different methods to keep the inside cool, thermoelectric and vapour absorption, those methods are not as efficient as the vapour compression in a household fridge but are more robust.