Cogeneration Plant: How it works and what benefits it offers

A cogeneration plant simultaneously produces electricity and heat. Applications, advantages, and limitations

Cogeneration is an efficient and sustainable system for producing both electricity and heat simultaneously. This process is made possible through the use of a cogeneration plant, designed to maximize the energy contained in the fuel, minimizing waste.

The choice of a cogeneration plant to be technically and financially feasible must be carefully evaluated, with a thorough analysis of the utilities (trends over time of electrical and thermal energy loads) and available prime movers (each utility may fit better with one technology over another).

Cogeneration can significantly increase efficiency in the use of fossil fuels, allowing, on one hand, cost reduction in the energy bill, and on the other hand, determining lower emissions of pollutants and greenhouse gases.

To identify the suitable cogenerator for the building’s loads and increase its energy efficiency, you might find useful a cutting-edge thermotechnical software updated in terms of regulatory and technological advancements.

Cogeneration: Meaning

Before delving into cogeneration plants, we must start with the meaning of cogeneration. Cogeneration is a process that allows for the simultaneous production of thermal energy (heat) and electricity using a single energy source.

This approach maximizes energy efficiency by making the best use of residual heat that would otherwise be lost in other forms of energy production.

What Is a Cogeneration Plant

A cogeneration plant, also known as CHP from the English acronym Combined Heat and Power, is a system that maximizes the energy potential of a single source, reducing waste and contributing to cost optimization. This plant originates from the attempt to utilize the waste heat from a power generation plant.

In a power plant, heat, through a thermodynamic cycle, is first converted into mechanical energy and then subsequently into electrical energy. During this process, a significant portion of heat (30-45%) is lost to the environment.

With a cogeneration plant, however, the residual heat produced is recovered and utilized for heating or power generation purposes.

It is worth noting that not all waste heat can be effectively recovered. Some of it remains inevitably lost even within the context of the cogeneration cycle.

How Does a Cogeneration Plant Work

Understanding the operation of a cogeneration plant requires knowing first how conventional electricity production works. The operation of a conventional thermoelectric plant involves 4 phases:

  • fuel supply;
  • use within the plant;
  • electricity generation;
  • disposal of waste heat.

The dissipated heat is released into the atmosphere, causing considerable environmental damage when fossil fuels such as oil or gas are used, contributing to the greenhouse effect. In cogeneration, however, the dissipated heat is recovered.

The operation of a cogeneration plant is based on the following elements:

  • prime mover which can be fueled in various ways;
  • electric generator which, driven by the prime mover, produces electricity;
  • heat exchangers which allow the recovery of the produced heat.

Regarding prime movers, the main variants of cogeneration plants are distinguished precisely by the type of primary engine used. The basic technologies most used today are:

  • gas turbines (used in simple cycle with heat recovery for cogeneration directly from exhaust gases, or in combined cycle, heat recovery for cogeneration after using exhaust gases also for the production of feed steam for a steam turbine);
  • steam turbines (they can be back-pressure, if the heat is recovered from the steam discharged from the turbine, or condensing, if the heat is obtained from steam extracted at an intermediate stage of the turbine);
  • internal combustion engines (Diesel cycle or Otto cycle; in both cases, the heat comes mainly from exhaust gases and cooling liquid of the engine body);
  • combined cycle gas turbine/steam turbine plants.

Steam turbines and combined cycle gas turbine/steam turbine plants are mainly used for high-power industrial applications. While internal combustion engines and gas turbines are used both in high-power plants and in mini and micro-cogeneration systems.

Additionally, some innovative technologies, or still not fully established commercially today, can be added:

  • microturbines;
  • Stirling engines;
  • fuel cells.

Therefore, the cogeneration process is divided into 4 main phases:

  • energy generation: a turbine produces mechanical force using a fuel;
  • electricity production: mechanical energy is transformed into electrical energy through a generator. The final product will be ready to be used within domestic networks;
  • waste heat recovery: the turbine produces a significant amount of residual heat. This can be recovered using a heat exchanger, which transfers heat from the generation system to the desired use;
  • use of recovered thermal energy: the recovered heat is used to meet the thermal needs of the structure. For example, it can be used to heat domestic water.

Cogeneration Plant Diagram

Here is the diagram of a cogeneration plant.

Cogeneration Plant Diagram

Cogeneration Plant Diagram Cogeneration Plant Diagram

Cogeneration Plant: How It Is Powered

In cogeneration systems, various types of fuel, known as primary energy, can be used. These plants can be powered by both traditional fossil fuels, such as coal, natural gas, diesel, and fuel oil, and by renewable sources such as biomass:

  • natural gas cogeneration: it is the most common type of cogeneration plant. It uses natural gas as the primary energy source to power an internal combustion engine or a gas turbine that generates electricity. The produced residual heat is captured and used for building heating, hot water production, or other thermal purposes;
  • biomass cogeneration: in this case, biomass, such as wood, agricultural residues, or organic waste, is burned to generate heat, which is then used to power a steam turbine or an internal combustion engine for electricity generation;
  • high-efficiency cogeneration: this type of cogeneration plant uses advanced technologies, such as fuel cells or combined cycle engines, to achieve high efficiency levels. These plants can efficiently use primary energy and generate both heat and electricity with limited waste.

High-Efficiency Cogeneration Plant

High-efficiency cogeneration plants (HECPs) are designed to optimize the use of primary energy resources, ensuring a significantly more efficient conversion into useful energy compared to traditional systems. Usually, the efficiency of such plants exceeds 90%, in stark contrast with the 30-40% of conventional systems.

High-efficiency plants often integrate advanced technologies, such as high-efficiency gas turbines, state-of-the-art internal combustion engines, or fuel cells. These technological solutions maximize the conversion of primary energy into electricity and heat.

This type of plant is gaining popularity not only in the industrial sector but also in other sectors such as commercial, hospitality, healthcare, and in general, in all facilities requiring both electricity and thermal energy.

When Is Cogeneration Cost-Effective: The Advantages

Cogeneration plants, as mentioned earlier, are developed to optimize the efficiency of electricity generation processes.

Improved energy efficiency translates into significant savings on energy costs for businesses. By using the thermal energy produced by cogeneration for building heating or to power industrial processes, companies can substantially reduce their energy bills. These companies may even become partially or fully self-sufficient compared to the public power grid.

The approach to cogeneration can be customized to meet the specific needs of each individual company, thus ensuring a usage flexibility that makes it suitable for a wide range of applications.

From an environmental sustainability perspective, cogeneration is considered a clean energy source, as it utilizes the primary energy source more efficiently. Consequently, there is a reduction in greenhouse gas emissions and the overall carbon footprint of the company.

Finally, lower primary energy consumption represents a significant economic advantage. This advantage can be further strengthened by the incentives that can be obtained by investing in a cogeneration system.

In conclusion, the advantages are of 3 types: energy-related, environmental, and economic.

Limitations of a Cogeneration Plant

However, it is also important to highlight the main limitations to consider when evaluating a cogeneration plant. Although the principle of cogeneration is generally valid, sometimes it cannot be applied in an energetically and economically feasible manner if the following conditions are not met:

  • presence and proximity of thermal users: for a cogeneration plant to be feasible, there must be thermal users, industrial or civil, nearby. This necessity clashes with the tendency to locate thermoelectric plants far from urban or work centers in order to limit the population’s exposure to atmospheric emissions;
  • simultaneity of users: another condition for a cogeneration plant to be used properly is that the demand for thermal and electrical energy be simultaneous. A cogeneration plant typically can provide heat and electricity simultaneously, so it is necessary that the users simultaneously absorb this energy. For this reason, cogeneration plants are often connected to the national power grid, providing excess electricity to it and operating the plant according to the thermal energy demands of the users;
  • temperature compatibility: not all cogeneration plants make heat available at the same temperature. It may happen, therefore, that a cogeneration system is not suitable for serving a thermal user because it requires heat at too high temperatures;
  • plant flexibility: although there is simultaneous demand for heat and electrical energy from a user, sometimes the ratio between the energy required in the two forms may vary. It may happen, therefore, that at certain times the demand for electrical energy is proportionally higher than that for thermal energy or vice versa.

Industries Where Cogeneration Plants Can Be Applied

Cogeneration plants are suitable for both residential and industrial applications. Cogeneration is very advantageous especially in contexts characterized by a high and constant need for electricity or heat, such as:

  • hospitals and clinics;
  • swimming pools and sports centers;
  • shopping malls;
  • paper mills;
  • food industries;
  • petroleum refining industries;
  • chemical and pharmaceutical industries;
  • ceramics industries;
  • textile industry;
  • plastics manufacturing industry.

Cogeneration, Micro-Cogeneration, Domestic Cogeneration

Cogeneration plants are distinguished both by size (power) and by the characteristics of the primary combustion engine.

Regarding size, we talk about:

  • micro-cogeneration: electrical power < 50 kW;
  • small-scale cogeneration: electrical power < 1 MW;
  • cogeneration: electrical power > 1 MW.

The concept of micro-cogeneration (or small-scale cogeneration) refers to systems where electrical powers range from kilowatts to megawatts, with plants designed for domestic/residential use (also known as domestic cogeneration) and for small/medium-sized businesses.

The main distinction, therefore, between cogeneration and micro-cogeneration lies in the intended use: in domestic cogeneration, the main goal is heat production, while the electrical component tends to be in excess of immediate needs and can be sold to the electrical company.

Difference Between Cogeneration and Trigeneration

The distinction between trigeneration and cogeneration is based on the functions performed by the respective systems. Trigeneration can be defined as an extension of cogeneration and allows its use even during warmer periods.

While cogeneration focuses on the combined production of heat and electricity from a single energy source (such as natural gas), trigeneration goes further and allows the generation not only of heat and electricity, but also of cool air or water for air conditioning systems, maximizing the use of available thermal energy.

It is sufficient to connect an absorption refrigeration unit to the cogeneration system, which can add cold water production to electricity and heat generation. This cold water can be used both for process activities and for cooling indoor environments.

Cost of a Cogeneration Plant

The cost of a cogeneration plant can vary based on a series of variables, such as:

  • plant size;
  • technology used;
  • installed power;
  • plant configuration;
  • fuel availability;
  • market prices.

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