Choosing a Portable Fridge

By Tom Olthoff

At a recent caravan show, I was amazed by the number of portable fridge/freezers being sold.

For optimum performance of these units, an adequate supply of 12V electrical energy, good insulation used in the cabinet and an efficient cooling system all play a big part.

To get a better understanding of the principles of refrigeration we spoke to Tony Boyd, managing director of Refrigeration Research. The company, based in Mount Barker, SA, specialises in portable refrigeration equipment for the leisure and marine industries.

A few relevant factors

When it comes to portable refrigeration, there are several important criteria. These include maximum internal storage space for a given outside dimension, coping with extremes of temperature, low power consumption and a capable cooling system.

High quality insulation can reduce the external dimensions or increase internal space. It also helps to cope with high temperatures. But ultimately the cooling system still has to be powerful enough to remove the heat from the cabinet.

Low power consumption is related to both of these. If the heat is prevented from entering the fridge and the compressor only has to run for short periods, less power will be used.

Tony Boyd also highlighted another factor: the amount of power needed to operate the unit compared to the amount of cooling achieved. This is referred to as efficiency – for an analogy we can use a car engine.

If fuel is burnt efficiently, less is used to produce a given amount of power. If the engine is badly designed or in a poor state of tune, additional fuel will be used thus reducing efficiency.

When operating a refrigerator and/or freezer from a 12V source, efficiency becomes very important as there is often only a limited amount of electrical energy available.

Contrary to what might seem obvious, a refrigerator does not cool its contents, it removes heat from the cabinet which in turn lowers the internal temperature.

The workings of a refrigerator are known as a heat pump – it transfers heat from one area to another. The three major components of a refrigerator system are the compressor, the evaporator and the condenser.

A compressor is responsible for circulating the refrigerant around the system, while inside the cabinet the evaporator collects the heat. The refrigerant gas transports the heat from the evaporator to the condenser which in turn dissipates the heat to the surrounding atmosphere.

Basically the job of the refrigerator system is to remove heat from the cabinet and its contents. Heat can enter through the walls or lid and sneak in past the seals, however it varies with the quality of insulation and the difference between inside and outside temperatures.

Thicker is not always better: the quality of the insulation is most important. Of course, anything added to the cabinet also introduces heat.

Why the confusion?

According to Tony, most of us take refrigeration for granted. We expect the downsized version of our kitchen fridge to perform equally as well with very little power, even in hot, dusty conditions bouncing about in the back of a 4WD.

He believes there would be more consistency if all products were tested to Australian Standards AS/NZS 3350.2.24-2001. This standard relates to the “Particular requirements for refrigerating appliances (including for use in camping, touring caravans and boats for leisure purposes)”.

Tony also believes that the only way to correctly compare and evaluate the power consumption of refrigerators is calculate each unit’s coefficient factor.

Let’s take two examples: Fridge A is able to deliver 150W/hr (Watt hour) of cooling energy while consuming 100W/hr of electrical energy.

This gives fridge A a coefficient factor of 1.5 (150 divided by 100). Fridge B on the other hand delivers 90W/hr of cooling energy while consuming 80W/hr of electrical energy. Therefore the coefficient factor for fridge B is 1.125. Although fridge B uses power at a much lower rate, fridge A is far more efficient.

Before doing some more sums (apologies to the non-technically minded) we need to appreciate that it takes exactly the same amount of power to cool a given amount of produce, for example a dozen cans of soft drink, irrespective which refrigerator is used.

Each unit will need to remove the same amount of heat (assuming that both lots of cans were at the same temperature) to cool the cans to the required temperature.

Let’s assume each lot of cans requires 450W of cooling energy to chill them. Fridge A will achieve this in three hours (450 divided by 150) while consuming 300W of electrical energy (100 multiplied by three). Fridge B takes five hours to cool the contents (450 divided by 90) and consumes 400W of electrical energy in the process (80 multiplied by five).

Alternatively we can compare the number of amps required. For fridge A this is 25A (300W divided by 12V) and fridge B, 33.3A (400W divided by 12V). Whichever way you look at it, fridge A is more efficient.

However, a buyer might look at the relevant specifications sheets and judge fridge B to be the better buy as it would show a lower current draw.

Often batteries are blamed for poor portable refrigerator operation. More likely it is the ability of the charging system to keep the battery fully charged or replace what was taken out.

This is assuming that the fridge is an efficient unit. Let’s examine what happens during the charging process.

After the vehicle’s engine has started, the alternator will produce a high output to quickly replace the electrical energy that was taken out to operate the starter motor.

Once the regulator senses that the battery is fully charged, the alternator’s output will be automatically reduced.

This can be referred to as a ‘float’ mode. But what happens if there is a second battery?

Usually a second battery is connected into the system using an ‘isolator’. This isolator allows the second battery to supply electrical power to accessories like a portable fridge but not to the starter motor.

At the same time both batteries can be recharged by the alternator. But once the main or starting battery is fully charged, only a reduced alternator output is available to the second battery.

In a typical scenario the shortfall increases each night and will result in the fridge not being able to keep its contents cold.

The Solution

So how can efficient portable refrigerator operation be maintained?

Tony Boyd says it is easy as long as certain steps are followed.

These steps include: purchasing the correct fridge for the application, making sure that the characteristics of the second battery match the first, always using the right sized conductors, fuses and joiners, and finally installing a battery monitoring system that ensures proper charging and protection.

Waeco

Waeco offers a huge array of fridges, freezers and compressors for the RV market. The Coolmatic CF-35 runs on both 12V and 24V powered by the company’s kompressor motor.

The 32.5L fridge features a soft-touch operating panel with hard-wearing membrane keypad, secure locking mechanism, ergonomically shaped recessed grips and sturdy, detachable carrying grips.

An interior light is standard for fishing out a beer at night and the detachable lift-up lid provides flexibility.

It is 360mm wide, 385mm high and 580mm long, and weighs just 15kg. Temperature range is 50°C below ambient. Warranty is five years on the kompressor motor, three years on everything else.

<a href="http://www.waeco.com.au/"><u><font face="Arial" color="#0000ff" size="2">www.waeco.com.au</font></u></a>

Engel

Engel’s range of durable portable fridge/freezers is an appropriate choice for an auxiliary unit in your motorhome. There’s everything from a basic 13L chest fridge, a 40L fridge to one of Engel’s latest, the MT80FC model which has a 75L capacity (42L for fridge and 33L for freezer).

Like all Engel fridges, the MT80FC (pictured) uses the Sawafuji swing motor, a reciprocating compressor unit with one moving part. Weighing 40kg, the MT80FC has an electrical current draw of 0.5 to 4.5A and will operate from both 240V AC and 12/24V DC.

A digital cabinet temperature readout gives a constant monitoring of the internal temperature. Engel offers a three-year warranty on its fridges, and a low power consumption.

<a href="http://www.engelaustralia.com.au/"><u><font face="Arial" color="#0000ff" size="2">www.engelaustralia.com.au</font></u></a>

Reefer Traveller

The portable refrigerators marketed by Refrigeration Research are sold under the Reefer brand, with the slogan ‘Serious Refrigeration’. The fridges range from 50L to 200L capacity, all with dual compartments.

The smaller 50L and 70L Reefer Travellers have a cooling capacity of 50°C below ambient and use high-tech Danfoss compressors.

The larger Reefer Professionals, some with dual temperature control, are fitted with efficient Mitsubishi Rotary compressors.

Refrigeration Research also produces marine refrigeration units that utilise the Eutectic principle, and custom builds fridge/freezer compartments and refrigeration units.

<a href="http://www.reefer.com.au/"><u><font face="Arial" color="#0000ff" size="2">www.reefer.com.au</font></u></a>

Dometic

Dometic three-way fridges, from small units to domestic sized models, are found in the great majority of caravans and motorhomes both here and in Europe. Three popular models in the portable range are the Finch B533, and the two Chescold models, the F400 (pictured) and the RC1180.

The Chescold fridges are versatile because of their ability to run solely on LPG when out camping, and can also run on a 12V battery and a 240V base supply. They are CFC and HCFC-free, meaning they won’t pollute the environment, are fast cooling, and exceptionally quiet.

The F400 is a three-way fridge/freezer with a 39L capacity and a weight of 23kg. Dometic, PO Box 1140, Clayton South, Vic 3169, (03) 9545 5655, fax: (03) 9545 5966, email: dometic@dometic.com.au