CAMPING - GEAR
Portable fridges and coolers look much the same from the outside – they’re all insulated boxes with lids – but there are significant differences in the way they work and the type of energy they use. It’s necessary to know something about the ebbs and flows of the cooling business if you want to be sure of buying the right unit for your needs.
Ice Box Cooler
The simplest and by far the cheapest type of food and drink cooler is the faithful ‘Esky’: an insulated box with an insulated lid, into which you put ice that someone else has created. When you buy bagged ice, you’re actually buying a ‘slug’ of pre-packaged refrigeration.
We’ve been checking these units for their insulating quality over many years and Coleman units are relatively expensive, but consistently do better than most others, keeping contents cool for several days, even in high ambient temperatures.
Plusses: Low price; relatively portable on built-in wheels; doubles as a picnic ‘seat’; can be used for general storage when not cooling; zero maintenance.
Minuses: Bagged ice is not available in remote areas.
There is an increasing number of thermoelectric cooler boxes on the market, filling the price gap between an ‘esky’ and a true fridge.
Thermoelectric types employ an effect that occurs between two closely-coupled, dissimilar materials. When an electrical current is passed through the materials a temperature variation occurs and can be switched to create heat inside a container or cooling.
Unlike true fridges that can achieve a target internal temperature, thermoelectric coolers are rated to achieve a cooling effect in relationship to the ambient.
Claims for their performance usually quote an internal temperature ’22 degrees below ambient’ or similar. In that respect they’re not reliable for storing temperature-critical contents.
However, the thermoelectric effect works both ways, so these units can be set to heat their contents, up to around 65ºC.
At this stage of development, thermoelectric coolers work as compact means of cooling or heating non-critical consumables. They’re not very efficient, with current draws around 4-5 amps, or in the same province as compressor-type ‘real’ fridges, but they’re cheap, have no moving parts and some are designed specifically for car-seat stowage, with integrated seat belt mouldings. Some can work as a centre-seat armrest.
Plusses: low cost; noiseless; compact; able to heat or cool contents.
Minuses: relatively inefficient; no guaranteed cold temperature.
Compressor-type Portable Fridge
The most popular form of portable refrigeration is the portable, compressor-type fridge. The best-known brands are probably Waeco and Engel, and both employ the same refrigeration principle, but with different types of compressor.
The principle is simple enough: The fridge piping contains a gas that liquefies at low temperatures and this gas is pressurised through fan-cooled condenser coils that are located outside the food storage area.
As the gas sheds its heat through the coils it condenses into liquid, still under pressure and is forced through an expansion valve (tiny hole), into a low-pressure area created by the suction effect of the compressor. This action causes the liquid to vapourise and drop its temperature to around -25oC.
Compressor suction pulls this cold gas through the evaporator coils that are inside the food storage area, lowering the temperature of the contents and warming the gas, which then passes through the compressor and the cycle continues.
Portable compressor-type fridges are 12V/240V compatible, so can run off a car power outlet, inverter or mains plug. Waeco and most compressor-type fridges use a reciprocating compressor, while Engel has a ‘swing motor’ type.
A compressor-type fridge can run off car alternator power while the vehicle is being driven, but it needs auxiliary electrical power when the engine isn’t working.
A deep-cycle auxiliary battery is a common source of 12-volt power and can normally power a fridge for between one and three days, without re-charging, depending on ambient temperature and battery capacity. A solar panel can run a fridge when sunlight is available, or it can be powered at any time by a generator.
Plusses: can operate as a fridge or a freezer and some models can do both; median pricing; relatively portable with handles on each end.
Minuses: needs to cycle on and off periodically, so electrical power needs to be available 24/7 while cooling; fridge noise can be a problem at night.
Compressor-type Eutectic Portable Fridge
Eutectic or ‘ice-bank’ fridges employ the compressor refrigeration cycle, but add a significant additional component.
Instead of evaporator coils (the coils that get cold) being inside the food compartment, they’re housed in a tank that surrounds the food compartment.
Inside the tank is a liquid that freezes as a result of the refrigeration cycle and this external ice-bank is what cools the contents of the food storage area.
A eutectic fridge can operate off the car’s alternator power when the engine is working and, in the case of an overnight stop, won’t need to operate again until the car starts up next morning, so there’s no compressor cutting in and out during the night. The ice-bank keeps the contents cold in the meantime. As with a straight compressor-type fridge a eutectic unit can function as a freezer, or a combination fridge/freezer.
Our testing in real-world conditions has shown that a eutectic fridge uses less electrical power when working on the same cooling duty-cycle as a straight compressor-type fridge. However, you get what you pay for and a eutectic fridge costs a lot more than a conventional portable fridge.
Plusses: efficient cooling; ‘overnight’ power-free potential.
Minuses: high initial price; bulkier and heavier than a straight-compressor-type fridge for its internal capacity.
Absorption-type Portable Fridge
An absorption-type fridge uses heat energy, rather than an electrically-operated compressor to pump refrigerant around the system. As the name suggests, the refrigerant is released from solution and absorbed again during the process.
Heat, sourced from 12-volt, 240-volt or LPG supply, is used to raise the temperature of a strong ammonia/water solution, so that most of the ammonia evaporates. From there the ammonia passes through a fan-cooled condenser, where it becomes liquid, before passing through a restrictor that causes it to evaporate. This action cools the gas as it passes through tubes in the food storage area, drawing heat from the contents.
The warmed gases then flow into an absorber, filled with the weak ammonia solution, where they’re absorbed, forming a strong solution once more and the cycle repeats.
Absorption-type fridges aren’t as energy efficient as compressor-type fridges, but they have advantages that make them popular as the main fridges in caravans. The most obvious one is silence! With no moving parts, an absorption-type fridge won’t disturb your slumber.
Another major advantage is the ability to operate on 12-volt, 240-volt and LPG power. However, our experience over many years of travel is that 12-volt operation is much less efficient than 240V or LPG.
The reason for this is simple: electrical energy is used least efficiently in a radiant-heat application – the heating element for an absorption fridge – but is 90+ percent efficient when powering an electric motor, such as a fridge compressor. To supply current for an absorption-type portable fridge in a vehicle it’s necessary to have a powerful alternator and guaranteed 12V power delivery around 10 amps – a compressor-type fridge draws about half that. An alternative is to run it via a 240V inverter, but you’ll still need plenty of electrical grunt.
Those who go off-roading over steep or uneven ground may find that absorption-type portable fridges don’t work very well en route, because these units operate optimally on a level, or almost level, surface.
As with a compressor-type fridge an absorption-type can function as a freezer, or a combination fridge/freezer. However, very high ambient temperature can upset the absorption-cycle efficiency.
An absorption-type portable fridge comes into its own when it’s required to operate for long periods, without generator or solar panel power supply. We’ve run an absorption-type portable fridge non-stop for around two weeks on one 9kg LPG gas bottle – and we could still tee-off gas supply to our LPG stove to grill some fish!.
Plusses: silent operation, three types of power input.
Minuses: slower ‘pull-down’ time; ambient and level critical; greater power draw.
Stowing your cooler
No matter what your choice of camping cooler, you need to make sure it’s safely stowed, so that it can’t move around or become a missile in the event of an accident. It’s also important that you can access the contents and allow sufficient ariflow around the condenser plate for correct heat-exchange operation.
A fridge slide is a popular way of improving access, because the fridge can be slid out when entry is required. Another solution is a swing-and-slide frame that allows the fridge to be pulled out and then swung out of the way. Often, a camping fridge is stowed high-up in the vehicle – usually on top of a set of storage drawers – making access to the lid impossible unless you pack a small step-ladder.
One alternative is a tilting slide, with a drop-down action that allows the fridge to tilt forward in the slide frame, improving access.
The Esky Turns 60
‘Esky’ is as much a household name as the ‘Hills Hoist’, ‘Victa’ and ‘Vegemite’ and has turned 60 years of age, but the brand goes back much further than most people think. The first ice box sold under the trade mark Eskimo (Esky) was manufactured by Francis Malley’s household products company in1884, but it wasn’t a portable unit.
Back then, houses had ice chests for refrigeration and the ‘ice man cameth’ with block-ice every day in a horse-drawn cart. (I can remember as a kid growing up in Sydney’s eastern suburbs, in the 1950s, jumping onto the back of the ice truck and sucking ice splinters while old mate did his house deliveries, carrying the ice blocks with scissor-shaped grapples.)
In 1952 Malleys launched a portable cooler, branded ‘Esky Auto Ice Box’. It was formed from sheet metal and had a wire handle that doubled as a lid clamp.
The first portable Esky used cork sheeting for insulation, had room for six pint bottles and included a removable three-tray food rack. (I can remember my old man questioning the salesman closely about how many bottles of Resch’s Dinner Ale it could hold.)
Aimed at motorists, the Esky was advertised in motoring publications with the claim that the product was: ‘just as essential in the boot as the jack’. The smiling Eskimo who became the familiar Esky brand character for the next 30 years made his first appearance near the end of 1959, with the catch-cry: “Cool, man, cool”.
Over the years the Esky cooler took on a new shape and new colours. By 1960, it was claimed that ‘500,000 happy picnickers’ were relying on their Eskys, but it took Malleys until 1961 to register the trade name ‘Esky’ for the portable food and drink container.
Although Australia views the portable Esky as a local invention this is a contentious claim, because of a 1951 American patent application that was formally granted in 1953. American leisure goods company Coleman produced its first US-market, portable cooler box in 1954.
Plastics manufacturer Nylex took over production of the Esky product range from Malleys in 1984 and in 1989 signed a licensing agreement with Coleman. Under the terms of this agreement Nylex manufactured Coleman coolers and Coleman sold the Esky brand to camping stores.
Unsurprisingly, after Nylex went into receivership in 2009, the Esky brand was acquired by Coleman.