Ask Pablo: Refrigerator Water Dispenser or Refrigerated Bottles?


Image Source: Alex Muse
Dear Pablo: We drink a great deal of water all day long and wondered which uses more energy: opening the refrigerator door to get a bottle of cold water or using the water dispenser on the exterior of the unit. Also, does it take more energy to get ice from the mechanical dispenser (which also uses energy to make ice) or more by opening and closing the freezer door to get some cubes?

The big picture answer is that it probably doesn't matter. Simply the fact that you are filling your own water bottles rather than buying bottled water is a much more important environmentally-friendly act. Both your refillable bottle and water that is dispensed by your refrigerator are taken from the same starting temperature to the same final temperature, so there is no difference there. Where there is a difference, however, is in the act of opening the refrigerator door and allowing all of that cold air to spill out (hot air rises, cold air falls). By using the door-mounted dispenser this loss of cold air is avoided.

But How Much Energy Is Really Lost By Opening the Refrigerator Door?

The energy loss from briefly opening the refrigerator door is probably quite minimal. This is because air does not have a high specific heat capacity, so cooling the new air in the refrigerator does not use a lot of energy. Let's say that there are 10 ft3 of air space in your refrigerator (or a standard 20 ft3 refrigerator that is half full). If we assume that opening your refrigerator door replaces all of that cold air with ambient room-temperature air, that has to be cooled down to refrigerator temperature we are talking about 6.9 kJ (kilojoules), which is less than 0.002 kWh, or about the amount of energy contained in 1½ Tic-Tacs (1.5 Calories).

What About Ice Cubes?

With ice cubes it's going to be the same story, except that the temperature difference is going to be larger, but since freezers are typically smaller, the energy loss should be about the same. What is different is that the ice maker uses electricity, whereas filling ice cube tray manually doesn't. One could also argue that filling ice cube trays means leaving the refrigerator door open longer, making the two alternatives a bit closer.

So What Can I Do To Improve The Efficiency Of My Refrigerator Use?

First of all make sure that you have an Energy Star rated refrigerator. Even a 10 year-old refrigerator may be costing you money when compared with a newer, more efficient model. Next, minimize the time you spend with the door open. Opening and closing the door has a very minor effect, as we have found, but leaving it open causes heat loss do to radiative heat transfer, meaning that the food in your refrigerator actually begins to warm, requiring more cooling. Next, unless you use a great deal of ice cubes, you may have noticed that the ice cubes in your automatic ice maker shrink over time. This is due to sublimation, where the ice essentially evaporates. If most of your ice cubes are evaporating, why make them in the first place? Simply turn off your ice maker and switch to "ice cubes" made from granite for cooling you beverage of choice (unless your beverage of choice is a blended Margarita of course!). Finally, ask yourself if your water needs to be refrigerated at all.
Pablo Päster is a weekly columnist for TreeHugger.com, an experienced greenhouse gas engineer and the Senior Environmental Program Manager at Hara Software. Send your questions to Pablo(at)TreeHugger.com and connect to his RSS feed.

Additional Resources On Refrigeration:
Could You Unplug Your Refrigerator, For Good?
TreeHugger Tip: Tomm Stanley On Refrigerator Efficiency
Wayback Machine: Solar Powered Refrigerator (in 1935!)

Tags: Ask TreeHugger | Bottled Water | Drinking Water | Electricity | Energy | Energy Efficiency

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