How to use small variances of temperatures

[exponent137]

It is known, that perpetum mobile of the second kind does not exist. This means, ff temperature is everywhere the same, the work cannon be obtained from heat. But temperature is not uniform anywhere.

Let us assume that we have very small cheap heat engines. How much energy can they get from, for instance, one usual room. Are T variations in such a room useful or not, for such theoretical small cheap heat engines?

Is this topic analysed in physics?

 

[mfb]

Small temperature differences lead to very small maximal efficiencies, and even worse actual efficiencies.
Within the air in a room, I doubt that you can get a single Joule of useful energy until the temperature is equal everywhere.

As comparison, one kWh is equal to 3.6 million J, and costs some cents.

 

[exponent137]

One Joule is almost nothing. But how do you estimate this Joule? Are anywhere any calculations on this topics?

But time is also important. When we use this Joule, temperature differences arise again very fast.

 

[mfb]

Consider two volumes of air with 1m^3 with a temperature difference of 1K. If we cool down one side and heat the other, we get ~600J flowing. At room temperature (~300K), we can extract 1/300 out of this - in an ideal world, we get 2J. This is completely unrealistic, however. First, your volumes of air are separated, you have to transport those 500J of thermal energy without too much loss (=the heat bridge should not be in contact with the environment). Second, mechanical devices won't work with those small temperature differences. You can use the Seebeck effect, but with a difference of 1K you get at most 1mV per contact - good luck transforming this to a useful voltage.
I think an efficiency of a percent is still a optimistic value for the conversion efficiency (relative to the theoretical maximum), so a room with ~50m^3 might give something like 1J.

Using temperature differences between solid materials might be a better approach (especially between the room and the outside), but then you are kind of ruining the purpose of insulations... it might be interesting in spring/fall or (depending on the temperatures) in summer, if you can use the daily temperature cycle.

 

[sardar]

The concept of small variances of temperatures and their potential use in energy production is a topic that has been extensively studied and analyzed in the field of thermodynamics. The laws of thermodynamics state that it is not possible to create a perpetual motion machine of the second kind, meaning that it is not possible to continuously generate work from a uniform temperature. However, this does not mean that variations in temperature cannot be utilized in some way.

In fact, the concept of utilizing small temperature differences to generate energy is a well-established field in physics known as thermoelectricity. This involves using the Seebeck effect, which is the conversion of temperature differences into electrical energy, to create small heat engines that can generate power from temperature variations. These small heat engines are often referred to as thermoelectric generators, and they have been used in various applications such as powering satellites and remote sensors.

The amount of energy that can be obtained from temperature variations in a room depends on the size and efficiency of the thermoelectric generator. Generally, the smaller the temperature difference, the less energy can be produced. However, with advancements in technology and materials, researchers are continuously working on improving the efficiency of these generators and finding ways to utilize even smaller temperature differences.

Therefore, while it is true that a uniform temperature cannot generate work, small variations in temperature can still be useful for generating energy through thermoelectricity. This topic has been extensively studied in physics and continues to be an area of research and development in the field of energy production.