Refrigeration....Ice as a battery (change of state) & eutectics

[owly]

My home refrigeration is a machine I absolutely hate. At 65, I've owned numerous refrigerators, and hated every one of them. The worst aspect is that every single refrigerator I've ever owned will occasionally go on a tear and decide to freeze my produce. The thermostats in fridges suck! The problem can be allevated by replacing the thermostat with something like an STC 1000 and a installing a small circulating fan, but this is a pain. In most households the fridge is opened and closed often enough that the crappy thermostats don't cause a lot of trouble apparently.

I have an old time spring house, where I keep produce to prevent this problem. I live in a remote area, and like to buy things in quantity to avoid a lot of trips to civilization. Another and very effective solution is an ice box. An insulated space cooled with blocks of ice does not get down to freezing temps and maintains fairly good temp control.
Many boats use ice boxes obviously. Marine refrigeration is expensive and tends to be maintenance intensive. Ice boxes are simple and reliable so long as you have ice.



Ice has about the same embodied energy as a charged lead acid battery per pound, except that it can be "discharged" 100% forever and never degrades. I had the brainstorm of looking at very small freezers with the idea of freezing blocks of ice on a boat. The idea is that rather than relying on a stupid thermostat to control temp, you simply run the freezer when you have plenty of solar or wind energy as a way to store that energy. You freeze water to use in your ice box. This would eliminate the need to have a big battery bank to run a fridge that mindlessly turns on and off just based on internal temp. You transfer the containers of ice into the ice box, the temp stays uniform. It takes human intervention, but on a boat a chore like that is trivial... you have a lot of spare time.

What I found was a cheap 1 cubic foot freezer at Walmart for about $100 To run it you would need an inverter..... but the draw is going to be fairly small, and most people have an inverter anyway. The freezer is essentially a "throw away" at that price, and about a quarter of the cost of the cheapest marine freezer. It also is mechanically simple with a single sealed compressor / motor assembly, not multiple voltage systems. It should be extremely reliable based on it's simplicity.


The beauty of this is that with an insulated space, you can store as much energy as you want.... you just need more fresh water and containers. Batteries are a fixed value. Granted you cannot light your boat or cook with ice or do anything but refrigerate or cool drinks, but in my experience refrigeration is one of the largest energy loads.
You can of course store heat as well, but it's not quite as simple. Hot water does not have the energy from change of state that ice does. The transition from liquid to solid and back takes up far more energy than simply heating a liquid or solid.
With this in mind there are eutectic materials, generally salts that transition between solid and molten at specific temp ranges. They are well known, but to my knowledge nobody has built marine appliances that for example store heat by transitioning at say 350F so that you can store energy and use it for baking and such. There is absolutely no reason why this cannot be done.
There is a good article on Wikipedia on eutectic materials that explains the concept, and has charts of both naturally occurring eutectic materials and commercially available ones. It is a potentially useful technology, and at least interesting. Water is the ultimate as far as being cheap and having a tremendous capacity for energy storage and release.

Enjoy the read H.W.

Phase-change material

[ttex]

Doesn't sound very efficient to me at all but what do I know.

[Bill O]

Will agree the amount of power needed for refrigeration/freezer on a boat is way up there w/power needed to create hot water.
Did see someone else freeze water in jugs then transfer it to a well insulated cooler for refrigeration. Interesting idea, but not really certain you will be saving any energy in the long run by doing this vs. straight refrigeration. Anyone one have the calculations for this strategy? (Richard?)

[dougweibel]

Storing thermal energy relies on insulation and the rate of energy transfer through insulation (or any material) is dependent on the temperature difference. We keep our freezer at 11 degrees F and ambient temperatures in the boat are usually below 95 F, so there is a 84 degree or less temperature differential for the freezer. The refrigerator is set at 44F, so only a 51 degree or less difference there.

If you want to store something at 350F+ as a heat source and the ambient temperature is 70F then that is a 280 degree temperature difference. With the same insulation value as I have in my freezer the rate of heat loss will be 3.3+ times higher than in the case of my freezer, so even with a proper phase change material storing heat for cooking is going to be inefficient due to heat loss.

Using an old fashion ice box is great if you can freeze ice at home and use that to get by for some number of days. Notwithstanding the potential efficiency benefit or lack thereof, for long term cruising our freezer is fully utilized for its intended purpose - keeping frozen food frozen. There is no room to be freezing large blocks of ice.