More about DC Fridges & Freezers 12 and 24 Volt

A) Refrigeration with DC Fridges:

Refrigeration is often the largest electrical load in a home power system. The first option that people often consider is conventional 240V units. Despite improving efficiency in the last few years, standard domestic refrigerators are still rather power hungry. Manufacturers make them with price, appearance and space in mind and efficiency is a minor consideration. We have tested a number of these fridges and in general terms they take 2 - 3 times more power than a 12/24V Danfoss compressor driven fridge. (A 240V type fridge will typically require eight to twelve 80W solar panels at a cost of several thousand dollars). Added to this cost is the cost of a larger inverter to power such a fridge and an ongoing cost for larger batteries. In general terms, we would not recommend using 240V domestic fridges unless you plan on having a back-up diesel generator coupled to a large battery charger or interactive inverter-charger in your home power system.

Another alternative to consider is an LP gas fridge. These are fairly costly to buy and a 220 litre unit will use a 45kg bottle of LP gas every 3 months. They do need a bit of ongoing maintenance and they generate some heat and fumes that need to be vented out of the building. There is also some anecdotal evidence that they can have difficulty coping with extreme tropical conditions such as in North Queensland and Darwin.

Some of these fridges are often known as 2 way or 3 way in that they can also run off 12V or mains power electricity. These use the electricity to first make heat similar to the gas flame. Depending on size, they typically draw 10 - 25A @ 12V and will run non stop for 24 hours per day. This type of fridge is okay to run on gas but is too power hungry to run it off solar.

The last and best option to consider is a 12/24V Danfoss compressor driven fridge. This Danish made compressor is renowned for its efficiency and reliability. These compressors are used in the majority of vaccine fridges around the world for these reasons. With 12/24V fridges, the chest type is more efficient than a front opening one which lets all the cold air fall out. This type of fridge will typically require one to five 80W solar panels (depending on the capacity of the fridge etc).

A further innovation is the use of eutectic fluid in some of these fridges. In the old days this used to be a brine solution. The idea with the eutectic fluid is similar to the esky freeze packs you put in your deep freeze for camping trips.

Eutectic refrigeration is often used in fishing vessels. The basic principle is that when the engine of your boat or vehicle is running (and therefore generating power), you run your fridge so that the eutectic fluid goes below 0°C. This then keeps your fridge cold for a further 24-48 hours (while your engine is off). The advantage of a eutectic fridge in a solar set-up is that you can time the fridge to come on in day light hours only (when your solar panels are generating power). This lowers the system voltage a bit and helps to get a bit more amperage from your panels. More importantly, it helps reduce the battery inefficiency which results from storing power in your batteries during the day to run the fridge at night! And the last bonus is that 1-2 days holding capacity of your fridge is a bit like having some more battery storage. We have monitored a 73 litre eutectic fridge (chest type) for a one year period in our area. If you are not familiar with our location, Nimbin is fairly close to Brisbane. Basically it is a sub tropical climate with typical summer day temperatures around 32°C and winter day temperatures about 20°C. The average daily current consumption @ 12V is Winter 8 - 11 Amp hours Spring & Autumn 12 - 16 Amp hours Summer 17 - 20 Amp hours

We sell a range of DC fridges and freezers ranging in size from 39 to 360 litres, which use between 1 and 5 solar panels (80Watt).

B) Power Consumption 12 and 24 Volt Fridges:

Because of their far superior efficiency we strongly suggest you select a 12/24 Volt compressor driven fridge. If you look at the selection in our price list or web site, you will see an expected daily power consumption expressed as Wh/day. As a very rough guide you will need one 80W solar panel per 250Wh/day of power consumption.

C) Use of DC Fridges & Freezers for Commercial Use:

The daily power consumption of our DC fridges and freezers is tested at 35 C ambient during the day and 28 C at night. It is designed to simulate an average 'normal' domestic usage pattern.

It is important that our customers advise us if the fridge is to be located in an extreme climate or is to be used for a non-domestic purpose - eg vaccine fridges or a commercial usage pattern.

For example, a 220 litre deep freeze is normally rated at 880Whr per day for normal use. However, if it is used to freeze 20 kg of water or fish per day the expected power consumption rises to 1500 Whr/day. Therefore, the number of solar panels and the battery capacity needs to be almost doubled for this application.

Another example might be using a fridge to cool drinks for a village shop. Using a chest fridge at 32 C ambient to cool a carton of drinks (9 litres) from 32 C to 2 C would require about 20Ah @ 12V (assuming the fridge is equipped with the efficient BD35F Danfoss compressor).

D) Save Power with a DC Fridge

As you know we still recommend 12/24V fridges for solar systems as being considerably more efficient than the 240V ones. Despite their higher cost, we still think it is a cheaper option for residents outside of Queensland. The situation in Queensland might be more 'line ball' in that most qualify for a 50% subsidy on the batteries as well! However, in several years time, this larger battery bank will need replacing. An extra several hundred-amp hours of battery storage to be purchased every several years is not cheap these days!

We have come across problems with two 240V fridge installations (which we did not recommend). Both have to do with the autostart function on some inverters. One customer had a fridge that tripped an internal switch when the inverter went into standby mode. This is apparently a 'brown out' protection device for the grid. Another one of our customers has a fridge that won't start up when the inverter is in standby mode (with nothing else connected). It would seem that this fridge has some sort of soft start relay that the inverter does not see as a load. So this customer now needs to keep their inverter on run mode all the time which further adds to the inefficiency of the system.

Peter McCloy encountered a problem with a customer who bought a fridge with automatic defrost. It would not restart after the defrost cycle, despite the fact that the light still went on, and if you left it off for a while it would start again.

These refrigerators use a small heater to defrost, but after the defrost cycle do nothing for a while, except for a timer which controls the cycle. This timer does not pull enough current to start the inverter, and so the cycle never finishes. However, if you open and close the door enough, each time the door is opened the light trips the inverter and the cycle will eventually finish. Similarly if you leave it switched off, temperature rises and the machine will start when you turn it back on.

Remedy: have someone disconnect the auto defrost cycle, or leave the inverter on, but preferably don't buy an auto-defrost model. However, we have also had customers who experienced problems with this 'remedy' as the fridge then iced up excessively (more than an ordinary non auto defrost fridge).

Neil McIntosh pointed that all chest type fridges use less power than an upright. In the Queensland summer his 160 litre Fischer & Paykel 240V chest freezer (run as a fridge) used 768Wh. The same unit uses 389Wh on 12/24 Volts.

I'd also suggest that if you are using a unit designed as a freezer, you may, in some cases, need to change the thermostat. Consult your refrigeration mechanic about this.

The other point you may wish to consider is that any modifications (like disconnecting the auto defrost etc) may void the manufacturer's warranty. This is particularly true if it is not carried out by a qualified person.

While I appreciate that some 240V fridges are getting more efficient, we suggest you still think twice before going down this track.

E) Start up Currents for an AC fridge

Another disadvantage of running a 240V fridge on an inverter is that you will need a larger good quality sine wave inverter. The inverter will need to run your other AC loads as well as the fridge.

Vaughn in the USA measured the following start up currents;

150W Run, 1,900W Peak (fridge over, non-frost-free)
200W Run, 2,400W+ Peak (Side by Side, icemaker, FF)
200W Run, 2,300W Peak (Side by Side, Icemaker, FF)

These start up currents are a very significant load to consider when selecting an inverter to run an AC fridge!

Insulating a Chest Type Freezer
by Ian Morrison

1. Most models of chest freezer in Australia have the condenser coils wrapped just under the metal outer skin of the appliance. If you insulate the outer walls you will stop the proper radiation of the heat. This will result in high condensing pressures and temperatures and a dramatic drop in efficiency. The compressor will get very hot and maybe damaged. Also the compressor will run a very long time as the temperature of the inside of the chest freezer will not drop to minus 18 or minus 20 degrees celcius and the thermostat will just keep the compressor running and running.

2. The correspondent who mentioned not drilling in to the outside panels of the appliance is quite correct as I had to service a brand-new chest freezer at a High School in Hobart, Tasmania where the handyman had tried to fit a hasp and staple lock to keep the kids from stealing the food. The first rivet hole he drilled grazed a condenser tube and all the R12 gas was lost.

3. Refrigeration compressors in chest type freezers usually run quite hot, and some models have a fan to assist will some ventilation air. Most compressors are painted black to radiate the heat.

4. If you are able to get a chest freezer model with a static condenser (serpentine coils and fins and no fan) fitted to the back of the chest freezer or maybe a type of chest freezer with a condensing unit under the freezer cabinet which will have the compressor and small finned condenser WITH a condenser fan then you could INSULATE the outside of the freezer chest. The effect will be that you will slow down the heat infiltration through the insulation put there by the manufacturer and thus REDUCE electrical energy usage.

5. If you puncture the outer wall of the freezer you will destroy the VAPOUR BARRIER and allow the insulation the breathe. A hole will allow moisture from the air to be pulled into the insulation and this will freeze and eventually make the freezer useless and while you discover this you will find the compressor will run for long periods of time due to the insulation being ineffective.

6 If you were able to make the condenser a water-cooled type then the freezer unit would run more efficiently than with the usual "refrigerant to air" condensers. To be technical, the water cooled condenser will allow the high side of the refrigeration system to operate at a lower "head" or "discharge" pressure than air-cooled. This allows the compression ratio to be less and the electricity consumption less. Tired old compressors with worn out pistons and cylinders where the internal slippage or leakage of high pressure refrigerant gas is greater will run a very long time on hot summer days with air cooled condnsers.. In my work I did several air conditioning jobs substituting air-cooled condensers with water-cooled condensers. Several I did had marine condensers with seawater.

I hope these thoughts help the discussion floating around the world..

Ian Morrison

Power Consumption of a 73 Litre Eutectic Fridge or Freezer

Eutectic refrigeration is often used in fishing vessels. A eutectic fridge has a `cold sink´ of very cold fluid to help it to maintain its cold temperature once power to run the fridge compressor is switched off or unavailable. The basic principle is that when the engine of your boat or vehicle is running (and therefore generating power), you run your fridge so that the eutectic fluid goes below 0°C. This then keeps your fridge cold for a further 24-48 hours (while your engine is off). The 73 litre eutectic fridge- freezer that we sell has this fluid inside the walls where one would expect to find normal fridge insulation.

Another application is for camping and touring. If you drive your vehicle for at least 4-5 hours every 2 days, you can use your fridge without any solar or other charging source.

The advantage of a eutectic fridge in a solar set-up is that you can time the fridge to come on in day light hours only (when your solar panels are generating power). This lowers the system voltage a bit and helps to get a bit more amperage from your panels. More importantly, it helps reduce the battery inefficiency that results from storing power in your batteries during the day to run the fridge at night! And the last bonus is that because the fridge does not need power to stay cold for 1-2 days you may be able to use a smaller battery bank!

One of our staff ran and monitored his eutectic fridge power consumption for a whole year in the Nimbin area. This is a sub- tropical area of Australia with summer day temperatures around 32C and winter around 20C.

The monthly averages were:

73 Litre Eutectic Auto Fridge Power Consumption - A/hr @ 12V

AUG... 11.1
SEP... 13.7
OCT... 14.8
NOV... 15.5
DEC... 18.4
JAN... 19.3
FEB... 19.9
MAR... 18.2
APR... 12.5
MAY... 12.2
JUN... 8.29
JULY... 9.82

In our shop in Nimbin, we ran 4 tests to establish how much power the eutectic fridge uses in different situations.

1) The first test was conducted by bringing 4 litres of water at room temperature 23 C down to 3 C. The cold water was replaced with another room temperature 4-litre bottle of water daily. The power consumption was then measured with a data logger, and the battery voltage was regulated at 12.8 volts throughout the test. The average number of amp hours required daily by the eutectic fridge was only 13.2 Ah.

2) Considered by many to be a very small load, the 4 litres of water was replaced with a carton of beer (24 bottles each 375 ml). The amount of energy required to bring the beer to 3 C from an ambient temperature of 23 C was then logged. The beer was then left in the eutectic fridge for the remaining 3 days of the test to establish how much energy was required to keep it at this temperature. The average daily power consumption was only 12 Ah.

3) As the eutectic fridge can be used as a freezer, the third test required the freezing of 10 litres of water. Ten 1-litre bottles of water were added with a starting temperature of 23 degrees. The eutectic fridge took around 12 hours to bring the water to freezing point. A total of 96 Ah was used over the first day to bring the ten bottles down to -9C. To hold the ice at this temperature used less power over the next 3 days; 80, 65 and finally 43 Ah were required over the next three days.

4) The final test was to measure the eutectic fridge´s ability to maintain a safe cold temperature after being disconnected from the power supply and allowed to act as an "esky". The gradual rise in temperature was monitored for a further 3 days. Due to the eutectic layer, the eutectic fridge took 3 days to rise above 10C.

This performance makes the eutectic fridge appropriate for freezing and transporting goods such as fish in areas where power supply is limited or intermittent from a vehicle engine or other power supply.