Keeping Water Cold

One of my sisters is about to move to a part of the world where the weather is hot and electrical energy has limited availability. That tropical paradise is actually cooler than Iowa right now, but they don’t have winters. She asked me the following question:

One of our co-workers in SI puts ice and water in a big water jug (like a little one of Mom’s orange jugs, with a lid on top and spout at the bottom). I think they put refrigerated water in it but I’m not sure. They drink out of it during the day so they don’t have to open the fridge 100 times a day. Seems like a good idea to me. Is that a good idea energy-wise? Downside is they have to make more ice.

Since she’s my sister, I can clarify that Mom’s orange jug is similar to the one pictured. I thought this would be a good topic for the week, what with record heat waves around this country.

This question got my physics-teacher mind going. I thought of how to figure this out by collecting all the information and using a bunch of equations. The information necessary would include:

• How much water the family drinks.
• The size of the family’s refrigerator.
• How many times the family would open the refrigerator.
• How long the refrigerator would be open each time.

About this time, I realized there is no way I could answer these questions about my own family, much less another family on another continent in a different climate. So, I switched to a back-of-the envelope calculation. What’s a back-of-the-envelope calculation? The kind that doesn’t necessarily include measured quantities, but is more a bunch of estimates scratched out on the back of an envelope because that’s what is sitting next to the sofa where you’re discussing the question.

I am going to go formal physics enough to treat this problem as a heat-transfer question rather than referring to “losing coldness,” because it’ll work better this way. I boiled the issue down to the specific question:

Is the total heat let in the refrigerator by opening the door multiple times during the day greater than the amount of heat gained by the insulated jug during the day?

While I can’t answer that question for everyone’s family, I would say that the answer is “yes” for a family with children living in the tropics drinking a lot of water. A good insulated jug doesn’t let in much heat. My careful calculation was thinking about all the times I have moved an insulated jug and remembering they generally don’t feel cold. If the jug is absorbing significant heat, it will absorb heat from my hand when I touch it. This loss of heat is interpreted as feeling cold. Regular water pitchers feel cold because they absorb heat from my (and your) hand, but insulated jugs feel, at most, slightly cool.

I don’t know about opening the refrigerator in every house, but I know in my house it has always been difficult to keep the fridge closed. If the water drinker were to open the fridge, quickly grab a jug of water, close the fridge, pour the water for as many people wanted some cold water, and quickly replace the jug, this might be a tough calculation. However, I just don’t see that happening every time, especially in a house with small children. If the fridge gets opened a couple times an hour for ten seconds, I think the heat let into the fridge might surpass the heat lost by the insulated jug in that hour. Then, if one four-year-old kid doesn’t get the fridge shut while pouring the water, that would make the insulated jug a winner.

Some tips to keeping the water cool with the least amount of refrigeration possible:

• Use the coldest water available for whichever method you choose. You probably already thought of this. To clarify, it doesn’t matter whether refrigerator water or simply cool water is used in the insulated jug, as long as whatever goes into the refrigerator, freezer, or insulated jug is as cool as readily available.
• It will take less ice in the insulated jug if you use refrigerated water, but that doesn’t matter energy-wise. It might matter if you don’t have freezer capacity to make enough ice every day.
• Don’t use more ice or water in an insulated jug than is necessary for the day. If there is cold water or ice left by evening, put it in the refrigerator or freezer to use the next day. It might take a few weeks to figure out the appropriate amount of ice and water for your family.
• The fuller the refrigerator/freezer, the less heat it gets every time you open the door–heat enters as warm air, and limiting the airspace limits the warm air. It might be worth putting some empty boxes (or some jugs of water) if the refrigerator is generally mostly empty. There is a limit to this, though, as very full refrigerators mean you leave the door open longer while you search for items. Also, if the air cannot flow freely inside the fridge, the cooling system looses effectiveness.
• If you trust your family to quickly close the door each time, it might be a good idea to fill small water bottles and keep them in the refrigerator. Then it’s a quick open, grab, close sequence for some refreshing water. It might even make sense to only fill the bottles partially full if it’s important for the water to be cold until the end of the bottle.
• Keep the insulated jug in a cool place, out of direct sunlight.

What energy-efficient tips do you have for staying cool on hot days?