Types of Cogeneration System – Topping Cycle & Bottoming Cycle | New Topic [2024]

In this note, we are going to learn about the Types of Cogeneration System on basis of sequence of energy use, like Topping Cycle and Bottoming Cycle. Welcome to Poly Notes Hub, a leading destination for engineering notes for diploma and degree engineering students.

Author Name: Arun Paul.

What is Cogeneration?

Co-generation, commonly known as Combined Heat and Power (CHP), is a process that produces both electricity and useable thermal energy from a single energy source. It is a highly effective method of energy conversion that lowers greenhouse gas emissions while increasing overall energy efficiency. Co-generation systems are categorized into two types based on the order in which they use energy: topping and bottoming cycles.

Types of Cogeneration System

There are basically two types of cogeneration system on basis of sequence of energy use –

1. Topping Cycle
2. Bottoming Cycle

1. Topping Cycle in Cogeneration

In a topping cycle, electricity is generated first, and the waste heat created throughout the process is used for heating or other thermal uses. This is the most prevalent type of cogeneration, which typically includes the following steps:

• Step1 – Fuel Combustion: Fuel (natural gas, diesel, biomass, or coal) is burned to generate high-temperature, high-pressure steam or hot gas.
• Step 2 – Electricity Generation: Steam or hot gasses are utilized to power a turbine, which is coupled to an electric generator.
• Step 3 – Heat Recovery: After passing through the turbine, the low-pressure steam or hot gases are used in a heat recovery steam generator (HRSG) or for industrial activities, space heating, or water heating.

Topping Cycle Diagram

Advantages of Topping Cycle

1. Efficiency around 60% to 80%.
2. Reduces energy and operational cost.
3. Lower greenhouse gas emission and reduces pollutants like NO_x and SO_x.
4. Ensures a stable energy supply system.
5. Improved heat recovery technology.

Applications of Topping Cycle Cogeneration System

Commonly used in industrial facilities, commercial buildings, hospitals, universities, and district heating systems.

2. Bottoming Cycle in Cogeneration

A bottoming cycle’s primary goal is to provide thermal energy for industrial processes, and the waste heat from these activities is then used to generate electricity. This sort of cogeneration is less prevalent and typically found in energy-intensive sectors that create a lot of waste heat. The steps are as follows:

• Step 1 – Industrial Process: High-temperature thermal energy is employed directly in an industrial process (for example, cement making, glass production, or metal smelting).
• Step 2 – Heat Recovery: Waste heat from the industrial process is absorbed, rather than being wasted to the environment.
• Step 3 – Electricity Generation: The captured waste heat is utilized to generate steam, which powers a turbine attached to an energy generator.

Bottoming Cycle Diagram

Advantages of Bottoming Cycle

1. Converts excess thermal energy into electricity.
2. It provides a reliable source of electricity.
3. It improves the overall efficiency in the industrial system.
4. Reduces green house gas emission.
5. Ideal for high temperature operations.

Applications of Bottoming Cycle Cogeneration System

It is mostly utilized in businesses with high-temperature operations, including cement plants, steel mills, glass factories, and petrochemical facilities.

Differences (Topping Cycle Vs Bottoming Cycle)

Here we listed some differences between Topping Cycle and Bottoming Cycle Cogeneration System –

• Efficiency: Both topping and bottoming cycles increase the overall energy efficiency of the system. The topping cycle, on the other hand, is generally more efficient in terms of electricity generation because it is designed for this purpose.
• Flexibility: Topping cycle systems are more adaptable and can be employed in a wide range of settings, including small-scale installations. Bottoming cycle systems are typically large-scale and tailored to certain industrial operations.
• Implementation: Because of their flexibility and efficiency, topping cycle cogeneration systems are more easily implemented and extensively used. Bottoming cycle systems rely on high-temperature waste heat and are consequently confined to specific sectors.