Cogeneration is the process of simultaneous production of mechanical energy (usually transformed into electrical energy) and heat. Heat can be used to heat buildings and / or for industrial production processes.
The spectrum of electrical and thermal efficiency with respect to cogeneration plants varies from a few to hundreds of kilowatts. Since the year 2000 or so, the so-called mini and micro power plants with combined production of single-family homes, small businesses and hotels are available for more and more in the market for the size of a washing machine. In 2009, VW launched a project that provides for the installation of 100,000 small cogeneration plants, with a total yield of around 2 GW.
Generally, a cogeneration plant is composed of:
- First engine;
- Electric generator;
- Motor system;
- Heat recovery units;
If they were divided by main engines, we could distinguish:
- Turbogas plants;
- Turbo steam plants;
- Alternative engines to internal combustion.
The operation of a car gives an example: the power taken from the crankshaft is used for traction and electricity production, the heat that is subtracted from the cylinders to heat the passenger compartment and the pressure of the exhaust gases to move the crankshaft. supercharger turbine The heat and pressure exploitation does not imply an increase in consumption, since they are remnants of the process of converting chemical energy to mechanical energy applied by the engine.
Its exploitation allows the transformation of the primary energy introduced (the fuel supplies chemical energy) into different forms of secondary energy produced (movement, heat). A system that operates from cogeneration is called a co-generator.
L ' thermal energy can be used for industrial or environmental conditioning use (heating, cooling).
Cogeneration is carried out in particular thermoelectric plants, where hot water or steam and / or process fumes are recovered, produced by a primary engine powered by fossil fuel (natural gas, fuel oil, etc.) or non-organic fuels. fossils (biomass, biogas, synthesis gas or other): this generates significant energy savings compared to the separate production of electricity (through generation in the power plant) and thermal energy (through the traditional thermal power plant) .
A particular field of cogeneration systems is that of trigeneration.
Efficiency can be expressed in different ways, which do not always lead to a correct comparison between the various plants. The definitions adopted by the Environmental Protection Agency (EPA) are illustrated below.
The efficiency of a simple process is the relationship between the conserved energy, at the end of the process, and the input energy.
Since cogeneration systems produce electricity and heat, their total efficiency is given by the sum of electrical efficiency and thermal efficiency. For example, a plant that uses 100 MWh of methane to produce 40 MWh of electricity and 40 MWh of heat has an electrical and thermal efficiency of 40% and an overall efficiency of 80%.
The EPA preferably uses another definition of efficiency known as "fuel efficiency", the relationship between net electrical production and net fuel consumption (which does not take into account the fuel used to produce usable thermal energy, calculated assuming a specific efficiency of 80% boiler) The reciprocal of this relationship is the net amount of heat.
There are also other indices of performance evaluation of a cogeneration plant: the first is the so-called IRE, energy saving index. This index is defined as the ratio between the difference in powers absorbed by individual plants for the production of electricity and thermal energy separately, less that absorbed by the cogeneration plant, given the power absorbed by the separate plants, this power being evaluated in terms of fuel in equal electrical and thermal power produced by the respective plants. This index gives the idea of how much energy can be saved with these systems; It is possible, through simple analytical calculations,
Other important indexes are the electrical index defined as the relationship between the electric power supplied and the thermal energy produced by the same cogeneration plant, the expected utilization coefficient as the sum of the relationships between electric energy and absorbed energy and energy thermal and that introduced.
However, all these coefficients are related to a specific moment when intervening in their powers, and for this reason these indices are useful for determining the system board values, that is, the maximum system performance values.
Very often it is convenient to refer to a finite period of time and evaluate the indices in that period: this is equivalent to evaluating the indices in terms of energy relations instead of powers, these evaluations are important because they allow you to establish where it is most convenient carry out a specific project Cogeneration plant, according to the energy consumption obtained in these areas.
Finally, the economic savings index that is defined as the relationship between the costs that would be obtained when buying energy from abroad minus the costs it has when buying fuel to feed the cogeneration plant that you want to build and that produces an equal amount of energy that you want to buy, fraction of the cost of the energy you want to buy. This index makes it possible to evaluate the economic convenience that this project entails, of course, a correct and complete economic evaluation implies a calculation of expenses for the maintenance of the plant and related investments.
The energy efficiency of cogeneration
Cogeneration is a technology that allows you to increase the overall energy efficiency of an energy conversion system. But to explain why, we need to analyze the returns.
The efficiency coefficient is characteristic for each type of engine and represents the relationship between the resulting energy efficiency and the fuel introduced. In the engine of a car, it indicates the relationship between the kilometers traveled and the quantity of hydrocarbons introduced; In large engines for electricity production, the coefficient indicates the relationship between kilowatt hours produced and the fuel consumed.
These relationships are characteristic for each type of engine. For example, gasoline car engines have yields that range between 20 and 30 percent; cars with diesel engines between 25 and 35 percent, the rest is converted into waste heat.
Large motors have greater efficiency and, although they are widespread, it can be said that for thermoelectric motors, the efficiency coefficient is quite high and can reach 55%. But the same engine when produced in cogeneration has coefficients that reach 85%, because the calorific value of the fuel is better used, with an effective process optimization.
Of course, the investments to adapt the motors of a thermoelectric power plant to cogeneration are considerable, but if it is possible to create an urban heating network, the results are always advantageous. In fact, the period of use of these machines, which even reaches 30-40 years, must be considered.
Types of cogeneration plants
Ferrera Erbognone thermoelectric power plant (PV)
The most common example of a cogeneration plant is the one built with an alternative gas / engine turbine and recovery boiler. Fumes from the gas turbine or the alternative engine are transported through a smoke duct to the recovery boiler. Recovery can be simple, if there is no afterburner, or otherwise post-combustion recovery. The fumes in the boiler allow to produce hot water, saturated steam or superheated steam. Generally, hot water is used for heating, saturated steam for industrial users and superheated steam for steam turbines and users.
Finally, electricity is obtained through the alternator coupled to the gas turbine and possibly through the alternator coupled to the turbo steam, and the production of thermal energy in the form of steam, which is then exploited by the connected users.
In the presence of turbo steam, a combined cycle is obtained in which the energy dispersion is minimal and consists mainly of the heat introduced into the atmosphere by the fumes coming out of the recovery boiler.
As for the evolving fluid, this is usually the water that, in many cases, reaches the state of superheated steam, but in others it can reach temperatures that are not high enough. For this reason, you will need intermediate heat exchangers to increase the temperature.
More rarely, the evolving fluid is air, which nevertheless has the defect of having a convective heat transfer coefficient that is too low and, therefore, much higher heat exchange surfaces are required.
As for internal combustion engines, generally only 33% of the total available energy is transformed into mechanical energy, the rest is partly lost due to the irreversibility present in the engine equal to another 33% of total energy and finally the last 33% is emitted to the external environment in the form of thermal energy that is finally lost.
To recover this lost heat, different heat exchangers are used: a first exchanger that allows the cooling of the lubricating oil, is available at low temperature (not exceeding 80 ° C), another exchanger to cool the water destined to cool the engine in yes, and finally a last exchanger located in the engine exhaust that allows to raise the temperature of the heat exchange fluid considerably, generally, as said, water, which for this additional heat exchange can reach the state of superheated steam. Through these plants it is possible to produce electricity and heat. Except for the cost of the exchangers. this does not constitute an excessive complication of the system because such engines must work in any case with a cooling system,
Finally, the evolutionary fluids particularly used are petroleum-derived diathermic oils, which have the characteristic of remaining liquid at atmospheric pressure up to temperatures of 300 ° C, and have a solidification point much lower than water, which prevents freeze in the pipes
Small cogeneration (and microcogeneration)
Cogeneration with electric power of less than 1 MW is defined as small-scale cogeneration, one with a power of less than 50 kW micro-cogeneration, and is carried out by alternative, internal combustion engines, gas micro turbines or Stirling cycle engines. The main difference between small cogeneration and microcogeneration consists in the fact that in small cogeneration thermal energy is a secondary product, while microcogeneration is mainly directed to heat production and secondarily electricity.
The advantages of small cogeneration
Simply put, the advantages of small cogeneration are:
- Use of unused thermal energy, with consequent fuel savings.
- Less air pollution.
- Significantly shorter power distribution chain, with a net reduction in line losses
- Infrastructure reduction (power plants and power lines)
Trigeneration involves the simultaneous production of mechanical energy (electricity), heat and cold using a single fuel, in fact, a trigeneration plant is "capable of producing electricity, heat and cooling combined ... guaranteeing a significant reduction in use of fossil fuels and equivalent CO2 emissions. " This is achieved because traditional thermal power plants convert only 1/3 of the fuel energy into electricity, while the rest is lost in the form of heat. The need to increase the efficiency of electricity production continues. One method that goes in this direction is the combined production of heat and electricity (also known with the English acronym CHP,
The trigeneration systems
Co-trigeneration systems can be studied and produced to work with any primary source of heat. These systems are technically mature and economically convenient today to be widely adopted, among the many possible configurations mentioned:
- fossil fuel cogeneration systems;
- trigeneration systems with fossil fuels;
- co-allocation with solar thermal systems;
- cogeneration with biogas;
- hybrid cogeneration and trigeneration systems.
Heat combined with fuel cells
It is currently possible to produce gaseous hydrogen from methane to the public or biogas network (after desulfurization, because the H 2 "poisons" S proton exchange membranes) with a reforming process that uses water vapor. Hydrogen is reacted with atmospheric oxygen in a proton exchange membrane to produce direct electrical current. Heat can be recovered for space heating, running water, steam jet disinfection, etc.