How to use a fridge in solar house without using extra batteries

Warning:  This post contains ideas and instructions about electrical devices and wiring. I am not an electrician. You should be extremely careful when wiring electrical devices and if you are not sure, please use a qualified electrician. 

Introduction: We have been living off grid for many years and I think we have worked out the best way to have fridges and freezers. Before having electric fridges, we used to have a Butsir gas fridge. Gas fridges work OK for most of the time but they can suddenly develop soot problems, they turn off for no reason or they can become hard to light plus they tend to use a bottle of gas each month which is not ecological. At the moment, a bottle of gas is 15 euros so that is 180 euros per year. In this post, I am going to explain how to have an electric fridge in the cheapest way possible without having to buy extra batteries.

We have 2 fridges and a freezer.  We don’t have a massive solar array but we have enough solar power to run a 600 watt swimming pool pump for 3 hours a day between 11 am and 2 pm.  The maximum power generation we have is only about 1300 watts.  The swimming pool is only in use from June to mid September so that means that without the pool pump we have loads of power during the rest of the year.

If you don’t have a lot of solar panels, by far the best system is to use a chest freezer as a fridge, put it on maximum full power and control it using an external thermostat. The freezer tries to bring the temperature down to minus 20C but as soon as it reaches 1C, it is switched off by our external thermostat and when it reaches 2C, the thermostat switches it on again.

Additional paragraph – digression – skip if you want.
By the way, the difference between the 1C and 2C stated above is called the slewing range.  The default is normally 1C or 2C. The smaller the slewing range, the more times the fridge is switched on and off.  Some people the other day suggested that a fridge could be damaged by switching it on and off. This is just utter nonsense!!!

Why use a chest freezer? The big problem with using a normal, upright fridge is that as soon as you open the door, all the cold air falls out onto the floor. This is especially bad if you have a lot of guests. Rather than quickly opening and closing the fridge door, they will be casually telling you their life story while absent-mindedly looking for the beer and then afterwards not even shut the door properly. (The same sort of person who will  use their 2000 watt electric curling tongs and shut the house electricity down just when there is a penalty shoot out in an international football match) .

Chest freezers are spacious  and they can be filled with lots of liquid such as milk, beer, water, etc. The reason it is a good idea to have lots of liquid in the fridge-freezer is that they can be used to store the cold in thermal mass.  Thermal mass acts as a thermal battery. All the watts of power that you use to cool the liquids down when the sun is shining will be stored and will prevent the fridge or freezer heating up during the night. So you don’t have to spend lots of money on expensive batteries – you just store the energy literally in beer. This has the added advantage that you can have a cool beer any time you like. What is not to like? 

I normally start my fridge at 10am and switch it off at around 5pm.  At 5pm, the temperature is just above freezing and by10am the next day, the fridge has got up to maybe 10C which is an OK fridge temperature.  Most fridges and freezers use about 90 watts.

This is not just some untested bright idea I have thought up: I have been doing this for years and it works perfectly.

Instructions:  This is a very cheap and easy system to set up so even if you are a money-strapped absolute nincompoop you should have no difficulty in doing this.  Having said that, if you have never done any electrical wiring before, make sure that everything is switched off when you actually touch any wires and study the instructions that come with the thermostat.

The first thing to do is put your chest freezer in a suitable place. It could be in your kitchen. In the winter, it will work even more efficiently if you keep it covered outside in the cold.

Near the fridge, you need to have somewhere to put the digital thermostat. I have mine of a shelf above the freezer and I keep it in place with a G clamp I bought from a thrift shop for a couple of euros. You could use gaffer tape.

The only slightly difficult thing is wiring the thermostat. You need to go to a shop and buy some wire and an electrical plug to power the thermostat and also an electrical socket so that you can plug the fridge into it.

This is a plug socket. You can open it up and connect the wires to it.  The wires don’t need to be especially thick because a fridge or a freezer only uses a small number of watts.

 
This image below is the wiring diagram from the thermostat. The NTC are the wires that go to the sensor. The sensor should be put inside the freezer. This is how the above diagram translates into the actual connection.  The black cable goes to the plug socket (two images above) and the white cables go to the wall plug.
Everything should be covered in insulation tape to increase the safety so that nobody could touch any live wires.  We do not use an earth so there must be no exposed metal which could be touched and make sure that you have a good differential switch in your fusebox.  If you don’t know what you are doing, get an electrician. Don’t take any risks!!!!!!!

This is what a chest freezer converted to a fridge looks like.  The only problem with this idea is that people tend to prefer using a fridge with a forward opening door. The things that you use often are kept in the basket at the top and you would have to have good organisation of where things are kept to make it convenient.   In our case we also have a normal fridge but if you want a fridge without also getting bigger batteries then this works very well.

SHOPPING LIST
This is the type of automatic thermostat I use. They are very cheap and normally continue to work for at least a couple of years. I also use these as normal thermometers because I like to see the red numbers in my kitchen.

This is a typical chest freezer. As you can see the price is about the same as 1 year of gas for a gas fridge. I bought mine in 1996 and it is still going strong.

This is how I switch the entire system on and off at 10am and 5pm. It is a Teckin smart plug which works with WiFi. They use an app so I can switch everything on and off as long as I have my smartphone with me. They also have an inbuilt electric meter which tells you how much power they have used.  They would be worth buying just for the electric meter. If it goes cloudy, I can switch everything off from anywhere.

Of course, you also have to have a solar electric system with inverter and solar panels to supply the power which is outside the remit of this post.

Note: In this post, I have talked about converting a freezer into a fridge. You can use a similar idea with a freezer as a freezer.  Just put the freezer on maximum. It should get to -20C during the day and come up to  -5C by the following morning. If you have meat in it, you should be especially careful that is never goes higher than 0.  You could use a min max thermometer just to be sure. Using this technique your batteries never have to power the freezer.

One last tip:
If you grow your own walnuts or almonds, put them in the freezer for a week to kill any moth eggs or larva and then put them in a well-sealed container. If you have ever had your almonds contaminated by moths, you will know what I am talking about.