Cogeneration System: Working Principles, Types & Application

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What is Cogeneration System? – A cogeneration system uses one primary energy source to simultaneously generate heat and electricity in a single facility, resulting in a higher energy output than would be achievable with two independent production sources. This prevents almost all of the thermal energy generated by combustion processes from being lost to the environment, as is the case with conventional plants, and instead allows it to be recovered and used again. The burning of fuels like natural gas, GPL, diesel, biogas, bio-methane, vegetable oil, or biomass is a common feature of the most extensively used cogeneration technology.

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What is Cogeneration System?

Cogeneration, often known as combined heat and power (CHP), is the simultaneous production of electricity and usable heat using a heat engine or power plant.

Because cogeneration uses otherwise wasted heat from producing electricity for some useful use, it is a more efficient use of fuel or heat. CHP plants recover thermal energy for heating that would otherwise be lost. District heating using combined heat and power is another name for this. One example of decentralized energy is small CHP plants. Absorption refrigerators may also utilize by-product heat at moderate temperatures (100-180 °C, 212-356 °F) for chilling.

First, a generator powered by a gas or steam turbine is driven by the delivery of high-temperature heat. After that, the low-temperature waste heat is put to use heating water or a room. It is possible to employ a gas engine or diesel engine at lesser sizes (usually below 1 MW). Due to their propensity to generate relatively low-grade heat, geothermal power facilities frequently engage in cogeneration. To generate energy at all, adequate thermal efficiency may be required, requiring binary cycles. Nuclear power stations utilize cogeneration less frequently than equivalent chemical power plants because of NIMBY and safety concerns, while district heating is less effective in locations with lower population densities due to transmission losses.

Some of the early electrical generation plants used cogeneration. Industries that generated their own power utilized exhaust steam for process heating before central stations provided electricity. Large office and apartment complexes, hotels, and retail establishments frequently produced their own electricity and heated their structures with waste steam. These CHP activities persisted for many years after grid energy became available because the early bought power was so expensive.

What is Cogeneration System? — Schematic view of a cogeneration system (Reference: **sciencedirect.com**)

The Story Behind CHP

Combining heat with power is not a novel idea. CHP was first utilized in Europe and the US between 1880 and 1890. Many industries at that time produced the electricity needed to run their mills, factories, or mines at their own coal-fired power plants.

The byproduct steam was utilized to heat the area or provide thermal energy for other industrial activities.

The first power plant in the United States was developed and built by Thomas Edison in 1882, and it just so happened to be a cogeneration facility. Edison’s Pearl Street Station in New York supplied steam, a thermal waste, to surrounding factories and heated buildings close by.

Working Principle of a Cogeneration System

In conventional power plants, electricity is produced by boiling water, which generates steam to turn a turbine and produce the kinetic energy required to produce electricity. Typically, fossil fuels like coal, oil, or natural gas are used to heat the water. Every step of this process wastes energy, especially when the heat produced to make steam is only discharged into the sky. Approximately 60% of the energy used to produce conventional power can be lost. Because some energy is lost during transmission, energy efficiency is thus only approximately 30%. Instead, a cogeneration facility utilizes this heat by, for example, delivering the hot water to a customer (be it a factory or a group of buildings). Because of the advantages of cogeneration, only 10% to 30% of energy is lost, increasing energy efficiency to 70% to 90%.

Advantages of Cogeneration System

Because it generates heat and electricity simultaneously, a cogeneration system, also known as a combined heat and power system or CHP, may benefit commercial and industrial (C&I) clients significantly. By producing heat and electricity from the same fuel, energy efficiency is increased, the environment is protected, and cost savings are guaranteed. Cogeneration power plants often run at efficiencies that are 50–70% greater than those of conventional power plants. Cogeneration is a component of the European Union’s energy strategy, which aims to cut greenhouse gas emissions and achieve carbon neutrality by 2050. According to data from Eurostat, cogeneration produced 14% of the heat and 12% of the energy in Europe in 2019. COGEN Europe predicts that percentage will rise to 20% of heat and 25% of electricity by 2030.

Cogeneration technologies can:

By integrating the generation of heat and electricity into a single generator, increase the overall effectiveness of how much energy you use.
Lower the cost of energy
Reduce emissions
Lessen the likelihood of power outages caused by grid issues
Be eligible for financial incentives for energy-efficiency initiatives.
Utilize sustainable energy sources, such as biomass
Be modified to meet the requirements of all customers, including residential
Reduce dependency on the electricity grid since a CHP is often present or close by.
Encourage energy independence and cut back on energy imports.

Types of Cogeneration Systems

Steam turbines are the main source of energy production in topping cycle facilities. The partially expanded steam is then condensed in a heating condenser at an appropriate temperature, such as for district heating or water desalination.

A waste heat recovery boiler then supplies an electricity plant with the high-temperature heat produced by bottoming cycle facilities for industrial activities. Bottoming cycle plants are less prevalent since they are only utilized in industrial processes that demand extremely high temperatures, such as furnaces for producing glass and metal.

Large cogeneration systems supply electricity and warmth for an industrial site or a full town. Typical forms of CHP plants include:

Gas turbine CHP facilities that use the waste heat from the gas turbines’ exhaust gas. Natural gas is frequently utilized as fuel.
Gas motor Reciprocating gas engines are used in CHP plants because they are, up to a capacity of 5 MW, more cost-effective than gas turbines. Natural gas is often utilized as a gaseous fuel. These plants are often produced as completely packed units that can be easily connected to the site’s gas supply, electrical distribution network, and heating systems to be put within a plantroom or outside a plant complex.
A biofuel engine CHP plants are extremely similar in design to gas engine CHP plants and employ a modified reciprocating gas engine or diesel engine, depending on which biofuel is being used. Utilizing biofuel has the benefit of reducing fossil fuel usage and, consequently, carbon emissions. These plants are often produced as completely packed units that can be easily connected to the site’s electrical distribution and heating systems and put inside a plantroom or outside the plant complex. Another option is a wood gasifier CHP plant, which gasifies wood pellets or wood chips as biofuel at high temperatures and with no oxygen before using the generated gas to power a gas engine.
Combined cycle power plants with CHP conversions
Both solid oxide fuel cells and molten-carbonate fuel cells have a hot exhaust that is excellent for heating.
Steam generator CHP systems where the steam condenser for the steam turbine is the heating system
Nuclear power stations can be equipped with extractions in the turbines to bleed partly expanded steam to a heating system, just as normal steam turbine power plants. For every MW of power loss, it is feasible to extract around 10 MW of heat at a heating system temperature of 95 °C. At 130 °C, the increase is a little less significant—roughly 7 MW are gained for each MWe lost.

In what follows, more detailed explanations of various types of cogeneration systems based on the topping and bottoming cycle are presented.

A Topping Cycle

In this kind of power plant, if the given fuel is utilized first to produce electricity, it will then produce heat energy later on throughout the process. The primary purpose of this energy is to provide process heat instead of other thermal supplies. The most well-liked and often utilized form of cogeneration is this one. Power plants with a topping cycle can be broadly categorized into four categories.

Plant with Combined Cycle CHP

A mixed cycle CHP plant is primarily made up of a diesel engine or a gas turbine that produces electrical or mechanical power and is tracked by a heat improvement system that helps produce steam and powers an associated steam turbine.

CHP Plant With A Steam Turbine

The goal of a CHP plant is to burn coal to produce high-pressure vapor that is then utilized by a steam turbine to provide the necessary electricity. Finally, the exhaust vapor is used as low-pressure procedure steam to heat water for a variety of uses.

Engine with Internal Combustion

A heat recovery system for creating vapor in the CHP plant’s cooling system uses hot water that would otherwise be used for gap heating.

Gas Turbine

In this gas turbine CHP plant, a standard gas turbine drives a generator to produce power. Utilizing a heat recovery boiler to produce process heat and steam, the turbine exhaust is provided.

Bottoming Cycle System

The primary fuel in a bottoming cycle CHP plant is used to produce thermal energy at a high temperature. A recovery boiler and a turbine generator are used to generate electricity from the heat lost in this process. These days, this kind of plant is widely employed in manufacturing processes that require heat at high temperatures in boilers as well as heat rejection at extremely high temperatures, even though they are utilized by the cement, steel, ceramic, petrochemical, gas, and other sectors. Plants in the bottoming cycle are uncommon and are not appropriate for plants in the topping cycle.

Applications of Cogeneration Systems

Many variables influence the adoption of cogeneration across sectors. Two of these criteria are the price of energy procurement and the profile of the facility’s thermal requirements.

Different firms use cogeneration in different ways, some more so than others.

Healthcare facilities: To enhance the standard of care, nursing homes and hospitals use high-tech air management systems. Cogeneration is a perfect choice for these facilities since they frequently require continuous heating or cooling.

Greenhouses: A large number of greenhouse facilities employ cogeneration applications because of the constant demand for heat and carbon dioxide to grow the product.

Universities and colleges: These establishments sometimes have sizable buildings that require a lot of heating and energy because they are utilized all year round.

Cogeneration is used in several different industries, from chemical plants and industrial facilities to hotels, to boost their financial performance and lessen their environmental impact.