The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or transformed. Heat engines, refrigerators, and air conditioners all involve the transfer or transformation of energy.

The efficiency of a Carnot heat engine is given by $\eta = 1 - \frac{T_c}{T_h}$, where $T_h$ is the temperature of the hot reservoir and $T_c$ is the temperature of the cold reservoir.

The coefficient of performance of a Carnot refrigerator is given by $COP = \frac{T_c}{T_h - T_c}$, where $T_h$ is the temperature of the hot reservoir and $T_c$ is the temperature of the cold reservoir.

A heat pump is a device that transfers heat from a cold reservoir to a hot reservoir. This is done by using a compressor to increase the pressure of the refrigerant, which causes it to heat up. The refrigerant is then passed through a condenser, where it releases its heat to the hot reservoir. The refrigerant is then passed through an expansion valve, which causes it to cool down. The refrigerant is then passed through an evaporator, where it absorbs heat from the cold reservoir.

An air conditioner is a device that transfers heat from a hot reservoir to a cold reservoir. This is done by using a compressor to increase the pressure of the refrigerant, which causes it to heat up. The refrigerant is then passed through a condenser, where it releases its heat to the hot reservoir. The refrigerant is then passed through an expansion valve, which causes it to cool down. The refrigerant is then passed through an evaporator, where it absorbs heat from the cold reservoir.

A refrigerator is a device that transfers heat from a hot reservoir to a cold reservoir. This is done by using a compressor to increase the pressure of the refrigerant, which causes it to heat up. The refrigerant is then passed through a condenser, where it releases its heat to the hot reservoir. The refrigerant is then passed through an expansion valve, which causes it to cool down. The refrigerant is then passed through an evaporator, where it absorbs heat from the cold reservoir.

A power plant is a device that converts heat into mechanical energy. This is done by using a boiler to heat water, which turns into steam. The steam is then passed through a turbine, which converts the mechanical energy of the steam into electrical energy.

A solar cell is a device that converts light energy into electrical energy. This is done by using a semiconductor material, such as silicon, to absorb light energy. The light energy is then converted into electrical energy by the semiconductor material.

A fuel cell is a device that converts chemical energy into electrical energy. This is done by using a chemical reaction to produce electricity. The chemical reaction is typically between hydrogen and oxygen, but other fuels can also be used.

A battery is a device that converts chemical energy into electrical energy. This is done by using a chemical reaction to produce electricity. The chemical reaction is typically between two different metals, but other materials can also be used.

A geothermal power plant is a device that converts heat from the Earth's interior into electrical energy. This is done by using a geothermal reservoir to heat water, which turns into steam. The steam is then passed through a turbine, which converts the mechanical energy of the steam into electrical energy.

A hydroelectric power plant is a device that converts the kinetic energy of water into electrical energy. This is done by using a dam to create a reservoir of water. The water is then released from the reservoir through a turbine, which converts the mechanical energy of the water into electrical energy.

A wind turbine is a device that converts the kinetic energy of wind into electrical energy. This is done by using a rotor to capture the wind energy. The rotor is then connected to a generator, which converts the mechanical energy of the rotor into electrical energy.

A solar thermal power plant is a device that converts solar energy into electrical energy. This is done by using a solar collector to collect solar energy. The solar energy is then used to heat a fluid, which turns into steam. The steam is then passed through a turbine, which converts the mechanical energy of the steam into electrical energy.

A biomass power plant is a device that converts the chemical energy of biomass into electrical energy. This is done by using a boiler to burn biomass, which produces heat. The heat is then used to turn water into steam. The steam is then passed through a turbine, which converts the mechanical energy of the steam into electrical energy.