Cold Comfort

Adding a personal fridge unit to your van or truck means more than cool drinks on a hot day. Delivery reviews what’s cool in the market

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. Here we help you find the right unit for your needs.

Thermoelectric coolers

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 electric 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 65o C.

At this stage of development, thermoelectric coolers work as compact means of cooling or heating non-critical consumables. They’re not very efficient, 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 in the back of a crew-cab ute or people mover.

Pluses: low cost; noiseless; compact; able to heat or cool contents.

Minuses: relatively inefficient; no guaranteed cold temperature.

Waeco_P1Compressor-type portable fridge

The most popular form of portable refrigeration is the portable, compressor-type fridge. 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 12 V/240 V compatible, so can run off a car power outlet, inverter or mains plug.

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 driving the alternator.

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 charge a fridge battery when sunlight is available, or the fridge can be powered at any time by a generator.

Pluses: 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; cannot run off a starting battery without risk of flattening it.

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.

Pluses: efficient cooling; ‘overnight’ power-free potential.

Minuses: high initial price; bulkier and heavier than a straight-compressor-type fridge for its internal capacity.

Waeco_P2Absorption-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 one advantage that makes them popular with caravanners – 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 240-volt 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 12 V power delivery around 10 amps – a compressor-type fridge draws about half that. An alternative is to run it via a 240 V inverter, but you’ll still need plenty of electrical grunt.

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. Also, absorption-type portable fridges need to operate on a level, or almost level, surface.

An absorption-type portable fridge comes into its own when it’s required to operate for long periods outside a vehicle, without generator or solar panel power supply. We’ve run an absorption-type portable fridge non-stop for around two weeks on one 9 kg LPG gas bottle.

Pluses: silent operation, three types of power input.

Minuses: slower ‘pull-down’ time; ambient and level critical; greater power draw than compressor fridges.

Most fridges last for many years – my own compressor-type fridge has worked faithfully for 15 years – so select the one you need with a long-term investment in mind and don’t choose purely on initial price. Fridges are not identical, so be sure to buy the one that best fits your needs.

Frozen Assets

How to convert your van into a refrigerated delivery vehicle

Waeco and Thermo King both produce a range of portable fridge/freezer modules designed to fit inside delivery vans and light trucks, converting conventional bodywork into refrigerated ones.

There’s a choice of capacities, with Thermo King offering its ColdCube as 150-litre, 350-litre and 950-litre units, and Waeco producing the 850-litre capacity CoolFreeze. Weights vary from around 40 kg to 160 kg.

The CF-850 is so compact and so convenient that it straps into location in a light van such as the Renault Kangoo. Operators with longer and wider vans can carry refrigerated or chilled products inside the van fridge and have all the rest of the cargo area available for conventional loads.

The Waeco CoolFreeze CF-850 compressor cool container provides 850 litres of available cold space in a maintained environment of temperature regulation from 2-12 degrees Celsius, all within a compact module with dimensions of 1100 mm width x 1000 mm height x 1400 mm depth. Its powerful cooling capacity is achieved through a Danfoss compressor and air circulation system.

The CF-850 CoolFreeze has its own integrated reloadable battery, enabling the unit to be operated for eight hours without any power supply (at 25 degrees C ambient temperature with pre-cooled load). It is powered by either 240 Volts AC supply or 12 Volts DC supply.

For Thermo King the 350-litre and 950-litre cooling versions have a claimed minimum temperature capability of 0o C, and the 150-litre, -10o C, in a 30o C ambient. The same units specified as freezers are said to achieve -21o C.

The ColdCube takes its power from 240 V AC or 12/24 V DC, and 12 V peak current ranges from 9 A to 25 A, with averages from 3 A to 9.5 A. Pre-wiring delivery vehicles for the ColdCube units would need to be professionally done, and alternator capacity to power vehicle aircon and accessories, as well as powering the ColdCube, may be an issue with some vehicles. An auxiliary power supply might be necessary to maintain internal temperature when the vehicle alternator isn’t charging the starting battery.

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