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Storing Power as Ice: Why Boat Cruisers Need Eutectic Refrigeration

Power on a boat is hard to keep steady. Solar and wind power change with the weather.
Most cruisers use batteries to store electricity. But batteries have very limited space.
What happens when your boat batteries are full on a sunny day? Where does the extra solar power go?
You might think you need a bigger, heavy battery bank. But there is a much better way.
You can store that extra power as cold thermal energy.

What is “Eutectic” Storage?

To understand this, we need to look at how we measure heat. We measure heat in watts (just like electricity).
    • Sensible Heat: It takes 1 watt of cooling to drop the temperature of 1 litre of water by 1 degree Celsius. Please see Wikipedia about Sensible Heat.
    • Latent Heat: It takes a massive 93 watts of cooling to turn that same litre of water into solid ice. Please see Wikipedia about Latent Heat.

Turning liquid into ice stores a lot of cold energy! But plain water is not cold enough for a boat fridge. So, we add salt or glycol to the water. As a result, this new mixture is called a eutectic solution.
A eutectic solution  freezing at -5°C is ideal for a fridge. Additionally, it stores about 85 watts of cold energy per litre. This frozen mix acts like a giant battery for coldness.

How It Works: Reducing the Load on Your Batteries

A eutectic fridge looks like a normal fridge, but it has one big difference.
Instead of a thin metal cooling plate, it uses a tank filled with the eutectic liquid. Also inside the tank is the refrigeration cooling coil (technical refrigeration term “Evaporator”).

Here is how the cycle works:
    1. The Freeze: The fridge motor runs and turns the eutectic solution into ice.
    2. The Hold: The motor turns off.
    3. The Melt: The ice slowly melts, keeping your food cold for hours without using any electricity.

For example, a small 5-litre tank needs about 3.5 hours of continuous running to freeze solid.

Imagine it is midday and your solar panels are soaking up the sun. But your boat batteries are already 100% full. Instead of wasting that free solar power, your eutectic fridge turns on. The eutectic system instead uses that extra sun to freeze the eutectic solution. So at night, the fridge can stay off while the frozen eutectic solution keeps your food cold. As added benefit, your batteries can then focus on powering your lights and navigation tools.

COP: The Secret to High Efficiency

Efficiency is measured by COP (Coefficient of Performance). This is the amount of heat a fridge removes for every watt of power it uses. The higher number, the more efficient the fridge is.

Eutectic fridges are far more efficient than standard (cyclic) fridges for two major reasons:

1. Better Temperatures
    • Standard Fridges: They turn on and off quickly due to lack of thermal storage (it is just a thin aluminium plate after all). To compensate for this, they must run at a freezing -23°C much like their version of cooling in advance. However, this gives them a poor COP of 1.01.
    • Eutectic Fridges: They hold their coldness better due to the frozen eutectic mass constantly absorbing heat. They only need to run at -10°C to keep the food at a perfect 2°C. This gives them a great COP of 1.41.

2. Fewer Startups
Pushing a stationary car into motion takes more energy than keeping it moving. Fridge motors are the same. As a result, starting up uses 30% to 100% more current for a few seconds.
    • Standard Fridges: Turn on and off 40 times a day. This wastes power. On top of that, each time a motor/compressor start puts tremendous strain on mechanical and electrical parts. Therefore, life expectancy is reduced. 
    • Eutectic Fridges: Only turn on 1 or 2 times a day. They run for a long time, freeze the block, and shut off. This saves your energy and protects your equipment.

Summary

A eutectic fridge does two jobs at once: It keeps your food fresh, and it acts as a secondary storage bank for your extra electrical power. This is the smartest way to upgrade your boat.
Want to find the perfect setup for your boat? Use our Ozefridge Heat Load Calculator to discover exactly what you need for your next cruise!