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Celebrating the independent kiwi spirit of invention.


Solar PV: Camping Fridge Types

By Ian Mander, 2 & 5 November 2019, 5 & 9 January 2020

 

Keeping stuff cool is now one of the objectives of having a good camping solar PV system. This is because my previously favoured option, the Dometic ACX40G mini fridge, which uses commonly available 220 gram gas cartridges, is not available in New Zealand. The New Zealand Dometic supplier does not import them any more because there wasn't enough demand. This is a pity, because using a gas cartridge instead of a gas bottle means it's completely self contained (and thus much more convenient), and because importing one will be rather expensive (5% duty on the fridge but not on the freight; GST on the fridge, duty, and freight; import entry transaction fee; and MPI levy, all coming to $760 or more).

There are three main types of camping fridges.

Thermoelectric cooler/warmer

These use a Peltier device running on 12 V, and sometimes come with an AC mains adapter.

The Peltier device is a small flat square of semiconducting material (eg, 40 mm x 40 mm x 3.6 mm). When a current is applied, one side of the device becomes cold and the other side gets hot. Basically, heat energy gets pushed across it. They use a lot of power to do this (eg, 5 A * 12 V = 60 W) and thus produce a lot of excess heat (60 W) in addition to the heat pushed across the device.

They have the advantage that if the current is reversed, the heat is pushed across the Peltier device in the opposite direction, so a single unit can both cool and warm its interior (but not at the same time).

Unless you have a good solar panel, the power requirements of a thermoelectric cooler mean it's not practical to use when a car is not running – it'll run the battery flat too quickly. Also, they're not particularly effective. The one I have draws about 4 A but only produces a cooling of 15 °C below ambient, which is not enough to keep milk adequately cold even in the spring, let along summer.

Aside: A homemade solar powered Peltier cooler could be a useful way of cooling drink cans very quickly. The hot side of the Peltier devices would need fan cooling.

 

Three way fridges, aka absorption fridges

DC & AC electricity, and gas (LPG bottles or butane cartridges depending on model).

Use an ammonia absorption cycle and have no moving parts. Because of this they must be kept very level to work well. Related to this, if a three way fridge is not used for a while it may not start up. The fix is to unplug it from gas and electricity, turn it upside down and leave it for five minutes, turn it right way up again, reconnect it to power or gas, and start it. Older models may need more time upsidedown – refer to the user manual.

Three way fridges are very versatile for power source and the quietest – almost completely silent – but less efficient than two-way fridges (eg 85 W for a 40 L absorption fridge when running on electricity). On gas they run at a fixed setting (some models have three modes – L, M, H) and can be a maximum of about 30 °C under ambient.

The Dometic ACX40 and ACX40G have a small ice tray, so making ice cubes is possible.

 

Two way fridges aka compressor fridges

Run on 12/24 V DC and 100-240 V AC mains with a mains adapter/power supply (normally included).

Use an electric compressor – basically just like a normal kitchen fridge except running on low voltage DC. They are quite efficient (eg, 60 W for a 50 L fridge) and the compressor cycles on and off as needed so they can hold a fixed, settable temperature without drawing power the whole time (eg, total energy used in 24 hours is about 250 Wh for a 50 L fridge).

For most efficient operation the fridges should be at an angle of 5° or less, although it can be operated at an angle of up to 45° for short periods. I believe this is because of heat buildup because the compressor needs to run for longer when it's not level.

Most cooling needs to be done during the day when the ambient temperature is hottest, which is convenient because 12 V power can be supplemented with solar power.

Compressor fridges can be 50 °C under ambient, which makes a freezer compartment an option! This opens up whole new possibilities.

 

Leading option

Jaycar now sells the Brass Monkey range of portable two-way fridges. The dual zone fridge/freezers in particular seem quite versatile. They sport two compartments which can be set to run at different temperatures, or one compartment can be turned off for economy. The compartment divider can also be removed to make a single large compartment. The fridge automatically senses the divider has been removed.

All models have battery protection, with a selection of three different voltage levels at which it will turn off. Note that running one of these fridges on a battery overnight or during the day without solar power reinforcement requires a battery which is not suffering from aging (with a lot of voltage drop under load due to increased internal resistance).

A couple of models were on display in the shop running on mains. They were turned off at night but were easily able to get frost on the inside of the freezer compartments by midday when I was there. (It actually takes only about ten minutes.) Each zone has a digital display of its temperature, which varied by about 3 °C as the compressor cycled on and off. The fridge gives a small shake as the compressor starts up and is pretty quiet. The small amount of noise from the compressor is not too intrusive in timbre and I had to try hard to hear it at all over the radio station the shop was playing. (While camping, with a bit of wind noise in trees, it's hard to tell if the fridge is working.)

Some of the Brass Monkey fridges have an LG compressor (for example, the 80 L fridge model which supposedly only draws 2.9 A). This appears to only be the fridges with a stainless steel box, and the 15 L model.

Which model? The 36 L dual compartment is too short to take a 1.25 L soft drink bottle vertically. Defeats the point, really. The 50 L dual zone fridge is tall enough for a 1.25 L soft drink bottle, and wide enough for two 3 L juice bottles side by side. Being big enough to put in it the largest things I'm likely to put in it, the Brass Monkey 50 L dual zone fridge seems the obvious choice to meet my needs. My review here.

How does it compare to alternatives? The equivalent from Waeco including an insulating cover is $1464. The Brass Monkey 50 L dual zone fridge is $588 including a cover (or $499 if you can get the insulated cover for free), which seems like an amazingly good deal even without the wireless app the Waeco has.

The Brass Monkey fridges have a three year warranty, the same as Waeco DC compressor driven refrigerator/freezers. This is impressive, and reassuring, since Brass Monkey seems to be a very new brand with hardly any reviews anywhere. They have been sold in Australia since at least July and New Zealand since at least September. (I've seen implications that they've been available since April 2019. The box for the 50 L fridge I bought has a date sticker 24/06/2019.)

Other than the lack of history, the only other downside I've found is the confusion in specs. The Waeco datasheet looks rock solid. The Brass Monkey specs say several different things, depending on where you look – on the website, in the downloadable PDF manual, on the box, in the printed manual. See Specifications in my review for more details.

 



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