 # Solar Energy # Thermodynamics Thermodynamics is the branch of physics that studies the effects of changes in temperature, pressure and volume of a physical system (a material, a liquid, a set of bodies, etc.), at a macroscopic level. The root "thermo" means heat and dynamics refers to movement, so thermodynamics studies the movement of heat in a body. Matter is composed of different particles that move in a disorderly manner. Thermodynamics studies this messy movement.

The practical importance lies primarily in the diversity of physical phenomena it describes. Consequently, knowledge of this diversity has resulted in enormous technological productivity.

## Study of thermodynamics

The main elements we have to study thermodynamics are:

The laws of thermodynamics. These laws define the way in which energy can be exchanged between physical systems in the form of heat or work.

Entropy Entropy is a magnitude that can be defined for any system. Specifically, entropy defines the disorder in which the internal particles that make up matter move.

Enthalpy Enthalpy is a state function of the physical system considered. Actually, the first law of thermodynamics, depending on the enthalpy, takes the form dQ = dH - Vdp, that is, the amount of heat supplied to a system is used to increase enthalpy and do external work - Vdp.

In thermodynamics, the interactions between various thermodynamic systems are studied and classified, which leads to the definition of concepts such as thermodynamic systems and their environment. A thermodynamic system is characterized by its thermodynamic properties, related to each other by the state equations. These can be combined to express internal energy and thermodynamic potentials, useful for determining the equilibrium conditions between systems and spontaneous processes.

With these tools, thermodynamics describes how systems respond to changes in their environment.

## Laws of thermodynamics

The principles of thermodynamics were enunciated during the nineteenth century, which regulate thermodynamic transformations, their progress, their limits. Actually, they are real axioms based on the experience on which the whole theory of thermodynamics is based.

In particular, three basic principles can be distinguished, plus a "zero" principle that defines the temperature and is implicit in the other three.

### Zero principle of thermodynamics

The zero law of thermodynamics states that when two interacting systems are in thermal equilibrium, they share some properties, which can be measured by giving them a precise numerical value. Consequently, when two systems are in thermal equilibrium with a third, they are in equilibrium with each other and the shared property is temperature.

### First thermodynamic principle

The first law of thermodynamics states that when a body comes into contact with another relatively colder body, a transformation occurs that leads to a state of equilibrium in which the temperatures of the two bodies are equal.

The first principle is, therefore, a principle of conservation of energy. In each thermal machine, a certain amount of energy is transformed into work: there cannot be a machine that produces work without consuming energy.

In short, the first thermodynamic principle is traditionally affirmed as: The variation of the internal energy of a closed thermodynamic system is equal to the difference between the heat supplied to the system and the work done by the system in the environment.

### Second principle

There are several statements of the second law of thermodynamics, all equivalent, and each of the formulations emphasizes a particular aspect. First, he states that "it is impossible to make a cyclic machine that has the sole result of transferring heat from a cold body to a warm body" (Clausius statement).

On the other hand, it can also be affirmed, equivalently, that "it is impossible to carry out a transformation whose result is only that of converting heat taken from a single source into mechanical work" (Kelvin's statement).

### Third principle of thermodynamics

The third principle of the laws of thermodynamics is closely related to the latter, and in some cases it is considered as a consequence of the latter. In this sense, it can be said that "it is impossible to reach absolute zero with a finite number of transformations" and provides a precise definition of the magnitude called entropy.

Additionally, the third law of thermodynamics also states that the entropy for a perfectly crystalline solid, at the temperature of 0 kelvin is equal to 0.

In the study of thermodynamics different concepts appear that should be known:

### Thermodynamic system

A thermodynamic system refers to a limited area used for thermodynamic research, and is the subject of thermodynamic research. The outer space of the thermodynamic system is called the environment of this system.

The limits of a system separate the system from its exterior. This limit may be real or imaginary, but the system must be limited to a limited space. The system and its environment can transfer matter, work, heat or other forms of energy at the limit.

### Thermodynamic cycle

In thermodynamics, a thermodynamic cycle is a circuit of thermodynamic transformations carried out in one or more devices destined to obtain work from two heat sources at different temperatures, or conversely, to produce through the contribution of work the step of source heat from lower temperature to higher temperature.

The objective of a thermodynamic cycle is to obtain work from two thermal sources at different temperatures, for example, in a solar thermal energy installation. The work obtained is generally used to produce movement or to generate electricity.

Performance is the main parameter that characterizes a thermodynamic cycle. The thermal efficiency of a thermodynamic cycle is defined as the work obtained divided by the heat expended in the process.

### Thermodynamic properties

Thermodynamic properties are the properties that define and intervene in the thermodynamic state of a system. Thermodynamics is characterized by having a state of equilibrium in which pressure, volume, temperature and composition are present.

Thermodynamic properties can be classified as extensive or intensive. Among these properties are internal energy, entropy, enthalpy, heat, temperature, pressure, volume, etc.

### Thermal performance

The thermal efficiency or efficiency of a thermal machine is a coefficient or dimensionless ratio calculated as the ratio of the energy produced (in an operating cycle) and the energy supplied to the machine (so that it manages to complete the thermodynamic cycle). It is designated with the Greek letter η

Depending on the type of thermal machine, the transfer of these energies will be done in the form of heat, Q, or work, W.

In 1824, the French physicist Sadi Carnot derived thermal efficiency for an ideal thermal machine as a function of the temperature of its hot and cold reservoirs: where:

Th is the temperature of the hot reservoir;
Tc is the temperature of the cold reservoir.

In conclusion, the thermal efficiency equation suggests that higher levels of efficiency are obtained with a higher temperature gradient between hot and cold fluids. In practice, the hotter the fluid, the greater the efficiency of the engine.

## Applications of thermodynamics

Thermodynamics can be applied to a wide variety of science and engineering topics, such as motors, phase transitions, chemical reactions, transport phenomena, and even black holes.

The study of thermodynamics is of great importance in the case of solar thermal energy because these types of solar installations are based on heat exchange.

In short, thermodynamic results are essential for other fields of physics and chemistry, chemical engineering, aerospace engineering, mechanical engineering, cell biology, biomedical engineering, and materials science to name a few.

### Thermodynamic solar energy

What is thermodynamic solar energy? This application of solar energy is a technological system that takes advantage of the difference between the temperature of the liquid in the solar panels (in this case thermodynamic panels) and the ambient temperature.

In thermodynamic solar energy, solar panels carry a coolant at a very low temperature. The coolant, in contact with the ambient temperature, undergoes a thermodynamic heat exchange process as long as the outside temperature is not lower than that of the coolant.

The advantage of this system is that thermal energy can also be generated at night, in adverse weather conditions, rain, wind, etc.

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## References

Last review: November 27, 2019