Originally Posted by jheldatksuedu
It takes no more energy to keep something frozen or cold for that matter once it is frozen or cold as long as it stays in the refrigerator, the only energy is what it takes to keep the air cold around it. Adding mass to the inside of the refrigerator or freezer only changes the length of time before the compressor comes on again, if you add mass it will come on for a longer time, basically using the same total amount of power in the long run. Mass (or a eutectic solution) will help keep things cold longer but then it will take longer to return them to the cold state. The only thing that determines the amount of power required is the insulation and the average interior and exterior temperature, that's what controls the heat transfer through the insulation. If you bring cold from home that reduces power on the boat but increases power use at home.
The above quote says it well regarding steady state temperature and energy balances.
On page 35 of Nigel Caulders Refrigeration For Pleasure Boats he illustrates a number of graphs for eutectic fluids. Y axis is temperature, X is % of solute. The solutions are all water based. The graph for glycol mixes does not have a eutectic point but has a notation that below a certain temperature the fluid "sets to a viscous mass" indicating separation. This appears to happen at about minus 44 degrees F. He also explains that antifreeze
solutions are not suited to eutectic refrigeration because the eutectic point tends to change over a range of temperatures, the extremely low temperatures required by the antifreeze
solutions and, the separation of glycol and water as the water freezes tends to remain in the tank as glycol is heavier than water. He does not mention alcohol water solutions anywhere as a eutectic fluid.
Page 18 provides a graph indicating the effectiveness of insulation. The BTU loss per square foot of icebox
per 24 hours versus inches of insulation curve is not linear and shows that after 4" the law of diminishing returns tends to kick in fairly savagely after you achieve 4", still helps to have more but not as effective after 4".
Eutectics were used with large, engine driven compressors in order to exploit the high refrigeration capacity of these compressors over a short period to minimise the cycling time.
Be mindful that solar panels as a source of electricity on boats is a relatively recent phenomenon and when I built my boat 28 years ago I made no provision for panels as they were horrendously expensive and did not produce much power. I now have 520 watts of the damned things and am becoming concerned about their effects on stability.
In the absence of the large ice block the eutectic systems exploit the low capacity DC powered compressor systems we use on boats need to cycle constantly through the day and night to maintain the required temperature in the box.
Not a problem during a nice sunny day when the solar panels are providing lots of electricity but not so good during the night when consumption
is high anyway because we are running lights and entertainment systems and sometimes navigation
I am in the process of rebuilding my constant cycling 12V refrigeration into a 12V eutectic system in order to transfer electrical
power usage during the night when the panels don't produce power to the day time when I have an excess of power from the panels.
To answer the original question - you have the right idea in wanting to "store cold" in your ice box you just need to improve your methods. In the short term the ice in a container solution one may purchase
at the supermarket is a good way to go as you are emulating a eutectic system with them but in the long term building the fridge or freezer is a better solution.
You can now buy off the shelf 12V eutectic systems which tends to indicate that the manufacturers have recognized the value of electrical
It will be interesting to see when this starts to enter the domestic refrigeration and air conditioning
market with the proliferation of rooftop solar.