Why is there no such thing as a microwave refrigerator?

Answer

Best,

Heat is energy. You therefore cool or heat by transporting energy to (when heating) or from (when cooling) an object from/to elsewhere.

Let’s take a look at a convection oven. A resistance element is brought to a high temperature, and the heat is transferred through the air to the object to be heated. Air is a poor conductor of heat and can store (transport) very little energy. To remedy this problem, a fan is used to circulate the air faster.

If you want to heat something in such an oven, it takes a relatively long time, because the energy can only be transferred in small amounts.

The energy transfer can be done faster by eliminating air as a transport medium. You do that when grilling and baking. When baking, the object to be heated is in direct contact with the hot pan, while the grill uses infrared light (heat radiation) that directly heats the outside of the object.

In the microwave, electromagnetic waves are used (at a high frequency) to indeed bring the water molecules in the object to be heated to a higher temperature. The energy is therefore not only applied to the outside, but also internally in the object to be heated (penetration depth is approximately 2cm). The oven transfers the heat directly into the object.

Cooling an object in the refrigerator is the reverse process of the hot air oven. Again, it is the air in the refrigerator that is responsible for transferring the energy.

Moreover, that cooling will depend on the temperature difference between the object and the walls of the refrigerator.

To make a “fast cooler” there are only 3 possibilities:

-Increase the difference in temperature between the object to be cooled and the walls of the refrigerator. Because that temperature is also the final temperature in the refrigerator, you are limited to about 4 degrees Celsius. Cooling in the freezer is therefore faster than in the refrigerator, but there is a risk of hypothermia (freezing).

-Increase the amount of energy transferred through the air (use a fan in the refrigerator)

-Improve the contact between the object to be cooled and the cold walls. That is practically difficult. An object will cool down faster in a bucket of ice(water). You can hardly fill your fridge with water….

A physicist could make up a whole story about how any form of energy ultimately results in heat. To extract heat from something, energy is needed, which ultimately means that more heat is generated than is extracted. That is the principle of the heat pump (= refrigerator).