Cogeneration system: what is and how does it work

The cogeneration system has expanded to become a key trend at a time when energy efficiency and cost savings are becoming priorities.

Cogeneration processes are known for being able to generate both electricity and heat using the same energy source. In fact, the operation of a cogen power plant aligns with current efforts to avoid energy losses and wasted byproducts, as they rely on capturing and utilizing waste heat from electricity generation. 

As such, official authorities such as the European Commission or the US Department of Energy are promoting the cogen power plant, which can achieve efficiencies of up to 90%.

Well-designed cogeneration systems bring in cost savings, a decentralized approach to energy and the possibility to incorporate renewable energies. A bold step forward for the energy transition and global decarbonization efforts, that is pushed even further when paired with advanced technologies such as TIAC systems or district heating infrastructure.

What is a cogeneration system

A cogeneration system is a technology that simultaneously produces heat and electricity by employing one single primary energy source.

Also known as combined heat and power, the CHP engine departs from conventional energy systems because it captures and employs the heat produced during electricity generation, which is otherwise lost in traditional systems.

As such, thermal energy produced by combustion processes to generate electricity is recovered and reused. An outstanding achievement towards efficiency which can be pushed even further when incorporating innovations such as TIAC technologies.

Key characteristics of cogeneration systems

• Combined output, generating both electricity and thermal energy in a single process.
• Based repurposing energy that would otherwise be wasted. The cogeneration system recovers and utilizes this heat.
• Offers efficiencies of up to 90%, a significant improvement when compared to the average for conventional power generation systems, which typically offer efficiencies of up to 50% and below.
• Can employ multiple fuel types, including natural gas, coal and oil but also renewable sources such as waste materials (municipal solid waste or industrial by-products), biogas or biomass.
• Provides decentralized power production, so that power generation can be located near the point of consumption.
• Can be scaled to fit a wide range of applications, from small residential projects to large-scale industrial processes.
• The heat produced presents a variety of thermal applications, from space heating in buildings, to process heating for industrial operations and hot water production. Cogeneration systems are also successfully incorporated into district heating, thus providing heating to entire districts and improving the efficiency of urban heating systems.
• An advanced model of the cogeneration system involves trigeneration. In this case, the heat is repurposed not only for heating but also for cooling. In order to do so, absorption chillers are involved to produce electricity, heat, and cooling simultaneously, in an effort to further enhance energy efficiency.

4 types of cogeneration systems

The types of cogeneration systems are roughly divided depending on the type of energy source and technologies that are in charge of producing power. 

While some other categories (like biomass cogeneration or fuel cell-based cogeneration) are worth mentioning as they open the door to new opportunities, below we list the 4 main categories of cogeneration system:

1. Engine-based cogeneration

This technology is based on internal combustion engines (which may use fuels such as diesel, gas or biofuel), to generate electricity. The waste heat (in the form of flue gasses from the engine, or deriving from the engine’s cooling system) is captured and repurposed for use. It’s a suitable configuration for small to medium-scale facilities, including industrial plants.

2. Gas turbine cogeneration

Gas turbines are employed to produce electricity, with natural gas being the most common fuel. The resulting waste heat from the process is captured to generate energy. This model is suitable for facilities with high power and heat demand, including large-scale industries or district heating.

3. Steam turbine cogeneration system

In this case, high-pressure steam produced by a boiler is employed to drive a steam turbine that generates electricity. As low-pressure steam exits the turbine, it is captured and used for heating or industrial processes.

4. Combined cycle cogeneration

The combined cycle cogeneration system combines gas turbines and steam turbines and is at the heart of some of the most efficient large-scale power plants. The combined cycle process starts with a gas turbine producing both power and exhaust gases. The latter are recovered using a Heat Recovery Steam Generator (HRSG) in order to produce steam, which runs a steam turbine to produce additional electricity and heat.

How do cogeneration systems work?

While the exact mechanism for cogeneration depends on the choice of technology, the basic underlying working principle for the most common cogeneration technologies begins with a combustion process. 

Available fuels range from natural gas and diesel to biogas, bio-methane, biomass or hydrogen. 

The combustion is at the heart of engine-based cogeneration. When it comes to turbine-based cogeneration, it is also the first step to produce the steam that drives turbines, which then generates electricity via kinetic energy.

It’s after this step that cogeneration comes in. While conventional power plants discard the heat generated in the process to generate electricity (which can account to 60% of total energy generated during the process), the cogeneration system is in charge of capturing it and processing it for reuse. In this case, energy losses can be reduced to just 10%.

In order to do so, Heat Recovery Units (such as a Heat Recovery Steam Generator, or HRSG) are in charge of capturing the heat and converting it into thermal energy in the form of steam or hot water. This waste heat is thus repurposed so that it can be used to heat buildings, for industrial processes or as part of combined cycle processes for additional electricity generation. 

At the same time, the different types of cogeneration systems require diverse technologies in order to harness waste heat. For instance, in the case of waste heat recovered from cooling systems in engines, heat from the coolant is recovered using coolant-to-water heat exchangers.

In turbine-based cogeneration, the incorporation of TIAC technologies for further efficiencies involves modifying the first step in the power generation process. As such, TIAC systems are installed at the air inlet of the gas turbine, cooling ambient air before it enters the turbine and increasing power production for turbines by up to 30%.

What are the benefits of cogeneration?

• By employing energy that would otherwise be wasted, cogeneration allows for lower costs and operational savings, making better use of available resources.
• The efficiency of a well-designed cogeneration system can climb up to 90%. Thanks to improved energy efficiency, the cogeneration system is also more environmentally-friendly, reducing emissions.
• Cogeneration allows for on-site power generation and promotes energy security.
• It can incorporate cleaner and renewable fuel options, including green hydrogen, biomass and biogas. In the case of biogas or biomethane, cogeneration systems are further involved in circular models, as they repurpose waste materials including agro-industrial waste and organic urban waste, among others.
• Authorities at a global scale are promoting subsidies and incentives to support the adoption of cogeneration systems

As such, cogeneration stands out as a key option for energy-intensive operations (including service and industrial processes), as well as operations where waste resources can be exploited.

Because of their capacity for on-site power generation, the cogen power plant also represents an extraordinarily resilient option suitable for recovery systems or locations that are isolated, remote or often affected by blackouts. In fact, cogeneration has been a recurrent choice for providing backup power during grid outages, and also as backup systems for typically intermittent renewable energy sources, such as solar or wind. 

As operators worldwide look for efficient alternatives to energy generation, combined heat and power emerges as a versatile and efficient solution. This is particularly true in the case of combining cogeneration systems with TIAC technologies.

This quest for efficiency must nonetheless be informed by each project’s needs and requirements. This is precisely where nuances and discussions can emerge such as the comparison between combined heat and power and heat pumps.

When applied to initiatives such as district heating, heat pumps can offer distinct benefits that may align with certain project’s needs. Such is the case of their capacity to upgrade low-grade heat, which is at the heart of this equipment’s proficiency in incorporating renewable energy sources.

In this context, the role of thermal engineering professionals is a key step for picking the right formula for each project, matching both available resources and the project’s desired goals.

At Araner, we’re committed to help companies achieve their projects’ fullest potential. Putting our thermal engineering expertise to work, we offer a wide range of solutions to cater to the diverse needs and requirements of each initiative. From advanced TIAC technologies to cutting-edge, tailor-made district heating, our systems aim at pushing thermal energy capacities to new levels, all while maximizing efficiency and sustainability. Want to learn more? Get in touch with us and discover how we can help you.