The thermoelectric generator is a device, where electric energy is produced directly from heat energy. They are also called Seebeck generators since they used the Seebeck effect to produce power. In conventional power plants, like thermal power plants, nuclear power plants, fuel is used to heat the water. Generally, coal is burnt in that process. After the water starts boiling, and converted into steam, at high pressure, this steam is used to rotate the turbine and hence mechanical energy is converted into electric energy by Fleming’s law of electromagnetic induction. In this process, there is two-stage energy conversion, first heat energy is converted into mechanical energy and then mechanical energy is converted to electric energy. Because of this two-stage process, the efficiency of the overall system goes down. If we can directly convert heat energy to electrical energy, efficiency will be more. In the thermoelectric generator, heat energy is directly converted to electric energy, which has high economical benefits. Thermoelectric generators fundamentally use the thermoelectric principle of operation, which is based on the temperature gradient.
What is a Thermoelectric Generator?
Definition: It’s an energy conversion device, where heat energy is converted to electrical energy. The fundamental principle of the thermoelectric generator is the thermoelectric effect. In the thermoelectric effect, based on a temperature gradient, the electrons are moved from one place to another place. The temperature gradient is obtained, when there is a difference in temperature levels between two points.
The heat energy which is wasted in thermal power plants can be utilized to create the temperature gradient between two ends. With this, the efficiency of the thermoelectric generator increases as compared to conventional generators. The best part of the thermoelectric generators is, it does not have any moving parts. With that losses reduce exponentially. The thermoelectric generator theory is explained below.
Thermoelectric Generator Construction
Constructional, it is made of up semiconductor materials to create the temperature gradient. The semiconductor materials used to form a thermocouple, which creates the temperature gradient and a potential difference is created. For this semiconductor device, both p-type and n-type materials are used. Metal alloys are formed using this semiconductor device. Metal alloys like Bi2Te3, Sb2Te3 are used to create the semiconductor path for the flow of charges.
Thermoelectric Generator Working
As mentioned, it works on the concept of the Seebeck effect of the thermoelectric effect. In the Seebeck effect, a temperature gradient or temperature difference is created between two ends. When the temperature gradient is created, the electrons flow from one end to the other. The electrons at the high-temperature end would be at high energy levels. So they start moving towards the other ends.
Due to the movement of electrons, a potential difference is created at the two ends. Which forms the output DC voltage. For the flow of electrons, a proper material is used such that the efficiency is more. The material should have high electrical conductivity and low thermal conductivity. For this, metal alloys made up of semiconductor devices are used. These metal alloys, when connected in that manner, form a thermocouple. A thermoelectric generator can be called as the number of thermocouples joined together.
A thermoelectric generator works on the principle of thermoelectric effect. Thermoelectric effects mean, a temperature gradient is created on the two sides of the generator. The temperature gradient means the difference in temperature levels. For example, if we take a metal rod, and heat one end, and cool the other end. A temperature gradient is created between the two ends. The electrons present at the hot end would be at a high energy level as compared to electrons present at the cool end side. This means that the hot electrons will try to tend to move towards the cool end due to the temperature gradient.
Due to the movement of electrons, the hot end will tend to be positively charged, and the cool end is negatively charged. Since the electrons at both ends are at different energy levels, they tend to move towards each side, relatively at different speeds. The hot end side electrons would move faster and create a potential difference between the two ends. The creation of potential difference due to temperature gradient, at the two points, is defined as the thermoelectric effect. The same principle is used in thermoelectric generators.
Thermoelectric Generator Design
For the creation of a temperature gradient, the hot electrons need to flow from one end to another, without any external material used. Like we cant connect two wires between the thermoelectric generators for the current to flow. Hence, now it has been seen that the material used for the thermoelectric generator has to have high electrical conductivity and low thermal conductivity.
With the increase in temperature, the electrons should not move fast. Ideally, to design such thermoelectric generators, semiconductors are used which have high electrical conductivity and low thermal conductivity. Using the semiconductors, the Seebeck effect or thermoelectric effects are used to create the temperature gradient. This device or when the semiconductors are connected together to form the generator is called a thermocouple.
The thermoelectric generator block diagram is shown in the diagram. The input to the generator is waste heat, or also called heat loss. This heat loss may also be obtained from automobiles or other sources of energy generation. This heat loss creates the temperature gradient. When given input to thermoelectric generator, based on the Seebeck effect a DC power is produced. The DC power can be further converted into AC by using the inverter circuit.
Nowadays hybrid vehicles, this DC power is converted to variable DC power and fed to other loads like Air conditioning unit, lighting, etc. For this conversion device, like chopper is used. To dissipate the losses at the generator end, a heat sink is used. For high rated devices, ventilators are also required to remove the heat.
The thermoelectric generator working principle is, it works on the concept of thermoelectric effect or Seebeck effect. As per this effect, when a temperature gradient is produced between two ends, the electrons start flowing from one end to another end and create a potential difference. For the creation of the temperature gradient, thermocouple devices have to be used. Thermocouple devices are fundamentally semiconductor device which has high electrical conductivity and low thermal conductivity. For this, the properties of semiconductor devices are used.
They have generally four valence electrons in the outermost orbit. So they can be either p-type or n-type. In an n-type semiconductor, the majority of charge carriers are the electrons. They are also called donors. In p-type semiconductors, the majority of charge carriers are the holes. Holes are also called the absence of electrons. The p-type material is also called as acceptors. So when a p-type and n-type material are connected in a proper manner, and a temperature gradient is created, then it forms a potential difference across two points. Due to which the electrons start flowing from one end to another. This creates a thermoelectric generator. The thermoelectric generator temperature difference is created by this principle.
Thermoelectric Generator Efficiency Equation
The efficiency of the thermoelectric generator is defined as the ratio of output power to input power. In terms of the device, it can be defined as
Where This the source temperature and Tc is the sink temperature. T is the mean of the temperature at these two ends. Z is defined as
Where ‘s’ is the Seebeck coefficient, sigma is the electrical conductivity and gamma is the thermal conductivity. Based on the above the constant Z is evaluated and efficiency is calculated. It could be noted that the efficiency of the generator is purely based on temperature difference, and does not depends on external factors.
The classification of thermoelectric generators depends on the method by which input heat is produced. Different types of thermoelectric generators are
- Fossil Fuel Generators- In this type, the heat is generated by burning fossil fuels, The fossil fuels used for this purpose are natural gas, propane, butane, etc. Its range is from 10W to 100 W.
- Solar Source Generators – In this type, the heat is produced using solar energy.
- Nuclear Fueled Generators- In this type, nuclear energy is used to input the heat. Its power rating is high as compared to other generators. It may go 1000 W.
Advantages and Disadvantages
The advantages of thermoelectric generators include the following.
- Economically friendly
- Recycles waste heat
- A reliable source of energy
- Lower production cost
The disadvantages of thermoelectric generators include the following.
- Low efficiency
- Limited applications
- Requires a constant source of heat
- Energy cannot be stored
The applications of thermoelectric generators include the following.
- Space applications
- Renewable energy sources
- Gas pipelines
- Radio communication
Thus, this is all about an overview of a thermoelectric generator. Hence we have seen the working principle and design aspects of the thermoelectric generator. One of the major drawbacks it has is limited applications. It must be thought, how by using the output voltage of the thermoelectric generator, it can be further used for more applications. Of course, this requires more converter operations and complex design.