Fridge/Freezer Optimizer Smart thermostat replacement

Usually fridges and freezers are controlled by a dumb thermostat. It just regulates the set temperature within a rather wide margin. We can improve on that, and since cooling compressors are usually the most power hungry devices on a ship, the potential gain is quite significant.

If you are on shore power, you just set a temperature and that's it. However, if you are at anchor, and have an irregular power availability (a generator which only runs once a day, solar panels, wind generator, or the alternator of the engine) the fridge could use energy more wisely. Why not use energy when it becomes available to cool the fridge a little bit more, so we can relax a little bit on energy consumption when power is scarce? This is what the Fridge/Freezer optimizer is about. It monitors the battery voltage and uses it to dynamically change the target temperature within the limits set by the user.

While we were at it, we added a few more features too:


  • Uses the fridge to store extra cooling energy when there is an excess of available power
  • Switches beverage-only fridges off during the night or when the electricity is in short supply
  • Instantly switches off temporarily when a heavy consumer is switched on (winches, heaters, etc)
  • Smart control of cooling fans
  • Minimizes ice-buildup on the evaporator
  • Automatic de-icing mode
  • Battery protection (shut off below a certain voltage)
  • Reliable voltage measurement with optional voltage reference inputs
  • LCD display with boiler temperature, trend, voltage and heater status
  • Fully configurable by an on screen menu
  • Switchable between degrees Celsius and Fahrenheit


Let's talk a bit more about these features and how they work.

Automatic mode

There are a few parameters that need to be configured first, let's get them into the right order of priority.

  1. Minimum voltage. If the batteries are almost empty, you probably want to use the remaining energy to keep some light and the GPS going and the fridge should shut down.
  2. The next important parameter is the highest temperature you find acceptable for the fridge's or freezers content. Above that temperature, the fridge/freezer will start cooling as long as it can stay above the minimum voltage.
  3. The third most important parameter is the lowest temperature you will find acceptable. You don't want your beer or vegetables to actually freeze. This parameter is important because it will limit the amount of "cooling energy" you will be able to store in your fridge/freezer.
  4. Hysteresis. This is the "gap" between the temperature the compressor switches off and the temperature the compressor switches on again. If the hysteresis is small, the temperature variations inside the fridge are small, but the compressor will switch on and off very frequently. It is better to have a larger hysteresis; one advantage is that the inevitable ice on the evaporator has a bit more time to melt before the next cycle sets in.

Now we can start optimizing. When the batteries are fully charged, the fridge/freezer can use the excess energy to cool down to the minimum temperature you have configured. You will actually use the fridge/freezer as an energy storage. If the voltage goes down, the fridge/freezer can relax for a while, utilize the stored energy and allow the temperature to slowly rise to the upper limit you have set. It will then maintain this temperature, until the voltage goes up and energy can be used to cool down the fridge to the lower limit again.

  • For a beverage fridge you can set the maximum and minimum temperatures far apart (unless you want to have very cold drinks early morning), resulting in the fridge to shut off around sundown and to start up again somewhere in the late morning.
  • For a standard fridge the energy saving is less dramatic but still a lot of energy can be saved, especially if a wide margin between allowable temperatures is set.
  • For a freezer there is no lower limit, so they are ideal to store lots of cooling energy when energy is available, and then to switch off for quite a long time until the temperature has climbed up to the upper allowable limit.

Cooling fan control

Usually the condenser/compressor unit has a cooling fan. Sometimes, the evaporator (inside the fridge) has a fan too. The fans are normally wired in parallel with the compressor, so they turn on and off at the same moment the compressor turns on and off.

There is a smarter way to control the fans: When the compressor starts, it takes some time before the condenser and compressor warm up, until that time there is no need for the fans and any energy used to run them is wasted. The fan inside the fridge should not switch off together with the compressor, but should run a bit longer. This makes sure that the "cold" which is still inside the evaporator is not used to form an ice-layer but is distributed inside the fridge to cool its content. Again the energy is used more wisely and it reduces the ice buildup on the evaporator.


This program maintains the temperature of the fridge at the upper limit, regardless of battery voltage.


This program maintains the temperature of the fridge at the lower limit, regardless of battery voltage.


This just switches the fridge completely off


With this program the compressor is stopped, but the fans keep running to speed up the melting of the ice. After a configurable amount of time, the de-icing program is terminated and the automatic mode is enabled again.

ZwerfCat Example

On ZwerfCat, one of our fridges is dedicated to beverages. In the morning we usually don't drink any cooled beverages so it is useless to use energy to cool them. Actually, we only start to use cooled beverages in the afternoon. So, before we had the fridge optimizer, we turned off the "beverage fridge" at sundown, and turned it on again somewhere late morning when the solar panels were already generating a lot of power. This was giving the fridge enough time to cool our drinks to be ready at sundown. Another observation was that our demand for cool beverages not surprisingly correlates with the amount of sunshine. And this of course correlates with the amount of electricity generated by the solar panels.

What we were missing with our fridge thermostat was an option to link it somehow with the availability of electrical power. Fridge and freezer temperatures are not really set in stone but somewhat flexible. To be able to link the cooling process with the power availability we designed the Fridge Optimizer.

Cooling fan control

On ZwerfCat, we actually had two sets of cooling fans: one for the condenser/compressor unit and one for the evaporator inside the fridge. They were wired in parallel with the compressor, but this is less than optimal. When the compressor starts, it takes some time before the condenser and compressor warm up, until that time there is no need for the fans and any energy used to run them is wasted.


We often have ice accumulation on the evaporator inside the fridge, even though we have the temperature of the fridge set above freezing temperature. What's going on?

  • The interval (hysteresis) of the fridge is too low. The thermostat starts the fridge again before all the ice has melted.
  • If you have fans inside the fridge, they stop together with the compressor, while they could be running for much more time to make sure that all the energy stored inside the ice is used to cool down the fridge and the ice is melted before the compressor fires up again.


Characters left: