After reading this complete thread on ballasting or storing heat absorbing energy it seems the basis thermodynamic facts are in some cases distorted. Ice has been used to store heat absorbing energy for thousands of years but water
must first be frozen by a separate energy source. For one pound of water
at standard day temperatures to go through a phase change to ice a number of BTUs of heat must be removed from it and disposed of. It requires one Btu per pound of water per degree to lower water temperature to 33 degrees F and an additional 144 Btu to complete water phase change to ice. After the change from water to ice only ½ of a Btu per pound per degree can be stored. To test these facts of water as a cold holdover system yourself place one pound of water in a plastic container with a thermometer inside. If this one pound container is placed in a 4 to 6 degree F freezer
it will freeze in 12 hours because of its small size.
Your test containers ice temperature after 12 hours should be down to less than 10 degrees. Place container somewhere at room temperature and watch thermometer rise within a short time to temp of 32 degrees consuming the very small amount of energy stores after ice was solid. Unlike other liquid eutectics water/ice temperature will remain at 32 degrees as long as there is any amount of ice in the container.
Any container whether holding plate or a gallon plastic container ice made from water alone will never provide a box temperature colder than 40 degrees.
To reach box temperatures lower than frozen water, mixtures of chemicals called eutectics can be mixed to form a colder desired freeze point. The blue ice packs sold in supermarkets have a gel eutectic solution that freezes at a temperature just below 32 degrees. There are companies on the web like Tundra that sell low temperature cold packs. Ice and cold packs work well in coolers where 40 to 60 degree temperatures are adequate but not recommended for a cruising boat life style. The biggest problem with ice packs is energy and time required to freeze them.
There are those who would like to convince you that holding plate refrigeration
is overall a better performer than conventional evaporator system refrigeration
, this is not true. Holding plates serve but one purpose and that is to store surplus energy if it is available from an alternative energy sources, wind
, sun, water, shore power
or an extra large refrigeration compressor
. Small 12/24 volt compressors like the Danfoss BD models without alternative energy producing any surplus energy unless box and plates are very small holding plates are a poor investment.