Cogeneration is the simultaneous production and use of two different forms of energy from a single energy source in a single process. The goal is to use the second form of energy, that is a by-product of the production of the first form of energy, rather than wasting it. For example, a generator is used to produce electricity, but also produces a significant amount of heat that is often discarded. With heat recovery systems, it is possible to use this energy to heat water, to heat the supply air of a building or for any other purpose.This is a principle that is increasingly used throughout the world.
The advantage of this approach is that it produces fewer greenhouse gases (per unit of energy produced) than using separate power and heat generation systems. Another positive aspect of cogeneration is that the energy produced can come from a wide range of different sources that may be available locally such as geothermal, municipal waste incineration, etc. It can also be used with less polluting or recycled energy sources such as biodiesel and biomass. A typical generator has an energy efficiency of about 35%, 65% of the energy being lost as heat. Cogeneration allows to recover up to 50% of this waste heat.
Description of combined heat and power systems
There are four general categories of combined heat and power application :
- Small-scale systems. These systems are designed to heat and/or produce hot water for buildings. Heat is produced by spark-ignited piston engines (combustion engine).
- Large-scale systems. These are most often used for steam production in large industrial plants or buildings. These systems make use of compression ignition engines, steam turbines or gas turbines.
- Large-scale systems for district heating. A power plant or waste incinerator supplies heat to a local heating network.
- Systems powered by renewable energy sources. These systems can be of any size.
A cogeneration station consists of a main engine, a generator, a heat recovery system and a control system. The engine (or turbine, depending on the application) drives the generator. A heat exchanger recovers heat from the engine or its exhaust gases to produce hot water or steam. The process is managed by the control system. The production of electricity and heat by this principle requires 10% to 30% less fuel than using two separate systems.
Factors to take into account
Although cogeneration has many positive effects on the environment by using waste energy, air pollution is a problem that should not be overlooked. Any form of combustion produces pollutants. New cogeneration plants are often subject to government regulations to ensure that they meet air quality standards. When developing the project, it should be kept in mind that meeting these standards may increase the initial cost of some cogeneration systems. On the other hand, if pollution control equipment is already required by the main process, cogeneration may be an interesting option.
The type of fuel must also be taken into account. Some systems, such as those using diesel engines, will not recover as much heat as other systems. Others will not be able to efficiently use all the heat they produce. As a result, they are less efficient and the environmental benefits are less.
It is important to know the precise heat and power requirements of the facility during the design phase. The goal is to balance the demand and variability of heat production with the demand and variability of electricity production. The ideal case, from an energy efficiency point of view, is that the electricity production exactly matches the heat demand. This can be difficult to achieve if the demand variations of the two types of energy are not directly related to each other.
Finally, due to their complex and interdisciplinary nature, cogeneration systems require qualified personnel to operate and maintain. This risk can be mitigated by contracting a specialized maintenance company and setting up training sessions.
Cogeneration is a concept that is being used more and more around the world. Its advantages are appreciated as environmental issues are taken more and more seriously. Advantages such as:
- Reduction of electrical peak costs
- Reduction of energy costs through heat recovery
- Potential reduction of fuel consumption by 10 to 30%.
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