Copyright © Karl Dahlke, 2023
If you understand the conservation of energy, then you understand the riddle:
Q: How do you warm up your kitchen?
A: Leave the refrigerator door open.
The heat in yourfridge cannot be destroyed; it can only be moved about. It is mechanically pumped out of your fridge and into your kitchen. As the fridge runs, under normal operations, it warms your kitchen, commensurate with the heat that was pushed out of the fridge.
Furthermore, refrigerators aren't terribly efficient. It varies by model, but let's say your fridge is 50% efficient. As it pumps one watt of heat out of the fridge or freezer, and into your home, it burns an additional watt running the compressor. If it has to push 100 watts out, on average, to compensate for the heat that leaks in, and the heat that pours in when you open the door, it consumes that 100 watts of course, pushing the heat out, and another 100 watts through inefficiency. So that's 200 watts.
Now leave the fridge door open - and no I don't recommend this, it will burn out the motor eventually. 200 watts flows into the kitchen, but 100 watts flows around and back into the fridge, where it heats the space that was just cooled. Since the space is warm again, this continues, until you close the damn door. The result is a net heating of your kitchen, in the amount of 100 watts. It's the same as plugging in an old style 100 watt light bulb.
An air conditioner has to pump it's heat somewhere outside of your house. That's the only way it can function. Central air always includes a unit in the back yard, that sheds its heat to the outside. If you don't have central air, you have to have a window unit, that can vent its heat outside. In a hot summer in Arizona, the temperature can be 120 degrees F, or 45 degrees C. The tubes of your air conditioner must be substantially hotter than this, to shed their heat to the summer air. The coolant in the air conditioner must exist in a range of temperatures: well below room temperature in the inside, to cool your home, and well above the hotest summer day on the outside, to shed the heat that was extracted from your home. We'll see how this presents an incredible engineering challeng on Venus, in a later chapter.
With this in mind, let's ask a question that comes up in cooking, a thought experiment if you will. You have a frozen roast that you want to eat tomorrow or next day; no hurry. Which is more energy efficient?
A) Put roast on table and let thaw by the heat of the house.
B) Put roast in fridge and let thaw mostly in the fridge, then put on the table a couple hours before cooking, just to make sure it is thawed all the way through.
A is one step, B is a 2 step process. What do you think?
Say the roast is 500 grams, about one pound. For a larger roase, multiply all the following numbers by the number of pounds.
I'm also going to assume the roast starts at the freezing point of water, 0 degrees C, your freezer is actually a bit colder but these are just estimates.
We can view the roast as water; close enough. It takes 80 calories to thaw one gram of ice to water, thus we need 80*500 or 40 thousand calories to thaw the roast, leaving it still at 0 degrees C. Then, to bring it up to room temperature, it must rise 20 degrees C, times 500 is 10 thousand, so total 50 thousand calories. Under option A, all this comes from the heat of your house.
Now there are three more cases to consider.
1) Winter. You pay for that heat by your furnace. It's a gas furnace, incredibly efficient, so I'll say the cost is straight up 50 thousand calories.
2) Spring. Windows are open, you are sharing your climate with the earth. The earth has thawed your roast and it costs you nothing.
3) Summer, and the air is on. That's 50 thousand calories of cool you didn't have to pay for. AC is 50% efficient at best, so if the roast was not cooling off your house, you would pump out another 50 thousand calories, 50 thousand calories pushed out to the back yard, which costs 100 thousand calories of electricity. You have saved 100,000 calories. Cool!
Now let's consider option B, roast in the fridge. That's 40 thousand calories to thaw the roast, and then 2500 calories to raise it up to 5 degrees C, which is fridge temperature. That's 42,500 calories keeping your fridge cool, keeping the food in your fridge cool. The fridge is much like an AC unit, 50% efficient, so if it had to pump that heat out of the fridge and into your kitchen, instead of just letting it flow into the roast, if it had to pump it out instead it would be twice 42,500, or 85,000. In other words, you've saved 85 thousand calories of electricity, no matter the season. Also, 85 thousand calories did not enter your kitchen. Does that matter? You take the roast out of the fridge and it rises from 5 degrees C to 20 degrees C. That's 15 times 500 or 7500 calories. So your kitchen loses 85,000 + 7500 = 92,500 calories under plan B. In the winter, your furnace has to make this up. It burns natural gas, which you pay for, to put 92,500 calories back into your house. Save 85,000 calories, spend 92,500, net cost 7500. In the spring with windows open it doesn't matter what happens to your kitchen, earth takes care of it. So you save 85,000 via the fridge. In the summer, the 92,500 calories of heat difference in your kitchen translates into twice as much through the AC, 185,000 calories you didn't have to spend cooling your house. That adds to the money you saved via the fridge. That's 270,000 total.
Here is A and B by season. Negative number is cost, energy you had to purchase from the gas or electric company, positive is energy you saved. The last column is the difference between plans B and A.
| Season | A | B | difference |
|---|---|---|---|
| Winter | -50,000 | -7500 | +42,500 |
| Spring | 0 | +85,000 | +85,000 |
| Summer | +100,000 | +270,000 | +170,000 |
In all seasons, but especially summer, it is more efficient to let food thaw, or mostly thaw, in the fridge, if you have the time; then take it out to cook.
Disclaimer: health experts recommend plan B at all times. Thaw food in the fridge, or quickly in the pan or oven as you cook it, so that it never rests at room temperature, where bacteria can grow. The most efficient method is also the safest. Nice.
My parents remember the ice box, prior to electric refrigeration. A tray of ice melted, and kept the food cool, hence the name. Of course the ice had to be replenished on a regular basis. Out of habit, my parents sometimes referred to the refrigerator as an ice box, and so did we. “Can you get the milk out of the ice box?” my mother might ask, referring to the fridge.
As I grew up I eventually dropped the word "ice box" from my lexicon, and started calling it a fridge. This is the only word my kids ever heard, and they would be baffled if I ever called it an ice box.
The first freezers were very much like the first refrigerators, but colder. They kept the food frozen. But there was a problem. Moisture would condense on the walls and freeze, building up a layer of ice. This is particularly true around the tubes of coolant that pull the heat out of the freezer. Inches of ice can build up, which makes the freezer less efficient. This ice had to be chipped away every couple of weeks. This was one of the tassks I performed for my grandfather, when I spent summers with him in Utah. He had an old-style fridge + freezer in his workshop.
Newer models became "frost free", and that was a wonderful technology. How does it work?
Several times a day, the freezer will warm up to a few degrees above freezing, just for a few minutes. This is sufficient to thaw the frost off the walls. The water drips down onto the bottom of the freezer, and through a hole, and into a drip pan. This pan rests on top of the compressor. When the motor starts up again, it generates heat, which evaporates the water into your kitchen, which is where the water vapor came from in the first place. There is no need to empty or drain the pan.
This is a clever idea. Most people don't realize there is a heating element inside your freezer. There is, and I have put my hand on it during the defrost cycle, out of curiosity. It is a tall thin rod near the back wall. It is warm to the touch, but certainly not hot enough to burn your hand. Slightly warm air rises up the walls and thaws the frost away. The warm air generally does not circulate around the food, as you don't want that to thaw.
If you have an automatic ice maker in your freezer, you probably noticed that the older cubes, at the back of the tray, are smaller than the newer cubes in front. They are all the same shape, but some are smaller than others. The defrost cycle gradually melts the older ice cubes, until they shrink down to almost nothing. It is more efficient to use the older ice cubes first, or, lift the bar and empty out the ice tray from time to time.