Refrigeration on a Solar System

LP Gas Fridges:

One 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.

Small 12/24 volt Fridges:

If you want a small chest fridge or bar size fridge up to 110 litres we suggest you use a 12/ 24 Volt compressor driven model.  The 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 three 87W 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

Larger 240 Volt Fridges:

Fortunately these have become a lot more efficient in the past couple years.  You can see a list of all fridges & freezers sold in Australia and how much power they use at this web site: 

http://www.energyrating.gov.au/appsearch/default.asp

The power consumption is measured at 32C for a 24 hour period and a yearly figure is given (kWh/ year)

Ray our Service Technician wrote:

"The new 240V fridge models are amazing re their efficiency. Considering they are frost-free, have the heat exchanger coils on the outside side-skins of the fridge, and about 25mm of insulation between there and your cold food at 4 degrees, it's a wonder they can do it. I have a 390 litre 1997 model F&P fridge/freezer (not frost-free) which has a 780kWh/yr sticker on the door. I've watched over the years how the new models have come down in power requirements and even in around 2000 it was down to about 550 kWh. I looked at them a year ago and my 390 litre model was discontinued, but a larger 430 litre fridge (frost-free only) was 51% power requirements of my model, as I remember working it out at the time - approx 398kWh/yr. I haven't looked at fridge/freezers of today but if ever I have to get another, I'm going to be in front powerwise as it will save me about 450kWh per year......fantastic, about $77. Last week one of our customers bought a Fischer & Paykel 373 litre fridge-only model which is rated at a very respectable 296 kWh/year (only 811Wh/day).

A new fridge has recently been released-  it is a 416 litre Stainless Steel model with an ice cube maker-  Electrolux ETM4200SC-R 

367 kWh/ year  =   1005 Whh/ day

Start up current 7.46 A (1790 W)

Continuous current  0.67 A (161 W)

There are a couple disadvantages of having a 240V fridge.  You will probably need to operate your inverter in Run mode rather than on the more effecient Standyby setting.  This is because 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.

 Another downside of running 240V fridges on solar is that you need a larger inverter than you would otherwise need because of their high start up current.  These 3 current consumptions were measured in the USA:

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)

However, to conclude, new 240V fridges are getting more efficient and their pricing is more economical than an equivalent sized DC fridge.