Cogeneration systems: efficiency and applications

Cogeneration, often referred to as CHP (Combined Heat and Power), is an engineering solution that enables the simultaneous production of electrical energy and heat from a single fuel source. This approach offers significantly higher overall efficiency compared to separate electricity and heat production, as it harnesses thermal energy that would otherwise be wasted. In the current context, characterized by decarbonization goals and rising energy costs, cogeneration plays a crucial role, as it helps reduce emissions and optimize consumption across numerous sectors.

Over the years, the regulatory framework has recognized the importance of this technology, with incentives for installations meeting "high-efficiency cogeneration" requirements under the U.S. Environmental Protection Agency (EPA) and Department of Energy (DOE) guidelines. As a result, many companies and manufacturing facilities have undertaken a modernization process of their thermoelectric plants to integrate cogeneration systems. This article aims to provide a technical overview of cogeneration plants, focusing on operating principles, efficiency, applications, and concluding with ORC solutions offered by Turboden.

Definition and operation of cogeneration

A cogeneration plant is based on waste heat recovery generated during electricity production. In a traditional power plant, much of the thermal content is dispersed into the environment without any further use. In contrast, in a CHP system, this residual heat is captured through heat exchangers or recovery boilers, then reused for building heating, supporting industrial processes, or generating domestic hot water. Depending on the prime mover machine (internal combustion engine, gas turbine, steam turbine, or fuel cell), the temperature and pressure conditions of the recovered heat vary, as does the power output in terms of electricity and heat.

The thermodynamic logic behind cogeneration leverages the second law of thermodynamics, which always involves energy losses in the form of heat. Reducing such dispersions by converting them into useful resources significantly increases the overall efficiency of the entire process. Therefore, the plant layout of a cogeneration system must adapt to the thermal needs of its users, choosing the configuration that optimizes the balance between electrical production and heat.

Efficiency

Cogeneration plants can achieve overall efficiencies (sum of electrical efficiency and thermal efficiency) exceeding 80-90%, while traditional power plants often operate between 30% and 40%. This fuel saving translates into lower CO₂ emissions and reduced operating costs. The performance index of a cogeneration system is a function of the heat-to-power ratio and the quality of heat required by the user.

In the United States, the concept of High-Efficiency CHP is regulated and incentivized by the EPA's CHP Partnership Program and qualifies for various federal and state incentives when meeting specific efficiency parameters. At the design level, the ability to correctly size a plant to the actual thermal and electrical demands of the user is crucial to avoid waste or underutilization of the system.

Types of cogeneration plants

Cogeneration types vary depending on the fuel used, power ranges, and conversion systems:

1. Natural gas plants Widely used due to high availability of methane and lower environmental impact compared to more polluting fossil fuels. Often utilize internal combustion engines (Otto cycle) or gas turbines.
2. Biomass plants Utilize agricultural or forestry residues, promoting a circular economy. However, they require an adequate supply chain and suitable combustion systems.
3. Biogas plants Convert produced biogas from anaerobic digestion of organic waste or sewage sludge into energy. Ideal for farms or waste treatment consortia.
4. Heat recovery plants Recover waste heat from high-temperature industrial processes (e.g., melting furnaces, cement plants, paper mills), transforming waste into a valuable resource.

Applications

The applications of cogeneration are numerous:

1. Industrial: large metallurgical complexes, chemical industries, food processing, or refining facilities benefit from a constant heat demand, thus optimizing the efficiency of their CHP plant.

2. Commercial and Tertiary: hospitals, hotels, shopping centers, or sports facilities require large quantities of hot water, heating/cooling, and electricity. Self-produced cogeneration reduces costs and ensures supply stability.

3. Residential and Micro-cogeneration: compact, small-scale units can power condominiums, homes, or small commercial structures. This enables cost containment and reduces emissions, especially in areas not covered by district heating networks.

Advanced technologies

Technological development has led to increasingly efficient and versatile cogeneration solutions.

Here are some relevant innovations:

1. Latest Generation Gas Turbines: designed for high temperatures and increased pressurization, they guarantee high electrical efficiencies, especially in combined cycle configuration (paired with a steam turbine).
2. Internal Combustion Engines: offer operational flexibility and good performance, with the ability to modulate production. Often powered by natural gas or biogas, they achieve total efficiencies above 80%.
3. Fuel Cells: directly convert the chemical energy of fuel into electricity and heat, avoiding conventional combustion processes. Emissions are extremely low, and electrical efficiencies potentially very high, especially with low-carbon hydrogen.

Advantages of cogeneration

Cogeneration ensures numerous advantages in terms of performance and eco-sustainability:

1. Reduction of Energy Costs: internal production of electrical energy and heat limits external supply, decreasing expenses related to traditional energy vectors.
2. Emission Reduction and Environmental Protection: the ability to recover heat reduces the need for additional combustion, lowering CO₂ emissions and harmful pollutants.
3. Security and Supply Stability: a well-designed cogeneration system improves plant resilience, minimizing service interruption risks.
4. Integration with Renewable Systems: cogeneration can cooperate with photovoltaic, wind, or geothermal systems and further reducing fossil fuel consumption.

Integration of Turboden solutions

An example of advanced technology in CHP is represented by Organic Rankine Cycle (ORC) systems offered by Turboden, part of the Mitsubishi Heavy Industries group. Unlike conventional steam cycles, ORC solutions use organic fluids with a lower boiling point, enabling the conversion of heat at medium or low temperatures into electrical energy and useful heat. This feature is particularly advantageous when it comes to harnessing waste heat from industrial processes or renewable sources such as biomass and geothermal energy.

Turboden's ORC solutions offer significant advantages:

• High adaptability to customer needs, thanks to a wide range of sizes and configurations
• Reliability and low maintenance costs, as organic fluids work at lower pressures and temperatures compared to traditional steam
• Emission reduction and efficient exploitation of heat sources that would otherwise be wasted

In this way, ORC-based cogeneration guarantees high efficiency and is suitable for various industrial processes, producing both electrical energy and heat in a highly performant manner.

Conclusions

Cogeneration represents one of the most effective methods for optimizing energy use, both from an economic and environmental perspective. Thanks to its ability to recover heat that would otherwise be dispersed, a CHP plant can achieve overall efficiencies far superior to traditional installations. These advantages manifest across various sectors, from heavy industry to commercial and residential structures, with positive impacts on cost reduction and emission cutting. Furthermore, technological evolution now allows for integration of cogeneration with renewable sources, storage systems, and advanced solutions such as ORC proposed by Turboden.

In a future increasingly oriented toward energy transition, cogeneration confirms its role as a fundamental component in reconciling sustainability and efficiency. From proper design and sizing of plants, through load analysis of thermal and electrical requirements, to the selection of technologies most suitable for the specific context, mechanical and energy engineers play a primary role in designing customized solutions capable of maximizing benefits from both environmental and economic perspectives.