Engine engineering

What is cogeneration?

What is cogeneration?

Cogeneration is the process of simultaneous production of mechanical energy (usually transformed into electrical energy) and heat. The heat can be used to heat buildings and / or for industrial production processes.

The spectrum of electrical and thermal performance with respect to cogeneration plants varies from a few to hundreds of kilowatts. Since the year 2000 or so, so-called mini and micro power plants with combined production of single-family homes, small businesses and hotels are available to more and more on the market of the plant the size of a washing machine. In 2009, VW launched a project that envisages the installation of 100,000 small cogeneration plants, with a total output of around 2 GW.

Generally, a cogeneration plant consists of:

  • First engine;
  • Electric generator;
  • Engine system;
  • Heat recovery units;

If they were divided by main engines, we could distinguish:

Cogeneration example

The operation of a car gives an example: the power taken from the crankshaft is used for traction and the production of electricity, the heat that is extracted from the cylinders to heat the cabin and the pressure of the exhaust gases to move the crankshaft. turbocharger turbine. The heat and pressure exploitation does not imply an increase in consumption, since they are remnants of the process of conversion of chemical energy to mechanical energy applied by the engine.

Its exploitation allows transforming 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.

CHP uses

Thermal energy can be used for industrial or environmental conditioning (heating, cooling).

Cogeneration is carried out in private 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, syngas 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 plant) .

A particular field of cogeneration systems is that of trigeneration.

Definition of efficiency

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 relation 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 electrical and 40 MWh thermal has an electrical and thermal efficiency of 40% and an overall efficiency of 80%.

EPA preferably uses another definition of efficiency known as "fuel efficiency", the ratio of net electrical production to net fuel consumption (which does not take into account the fuel used to produce usable thermal energy, calculated assuming a specific efficiency of the boiler of 80%). The reciprocal of this relationship is the net quantity of heat.

There are also other indices for evaluating the performance of a cogeneration plant: the first is the so-called IRE, the energy saving index. This index is defined as the ratio between the difference in power absorbed by the individual plants for the production of electricity and thermal energy separately, minus the power 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 indices are the electrical index defined as the relationship between the electrical energy supplied and the thermal energy produced by the same cogeneration plant, the expected utilization coefficient as the sum of the relationships between the electrical energy and the absorbed energy and the energy thermal and that entered.

However, all these coefficients are related to a specific moment when intervening in their powers, and for this reason these indices are useful to determine the values ​​of the system board, that is, the maximum values ​​of system performance.

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 ratios rather than 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 ratio between the costs that would be obtained when buying energy from abroad minus the costs that you have when buying fuel to power the cogeneration plant that you want to build and that produces an equal amount of energy as 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 such a project entails, of course, a correct and complete economic evaluation implies a calculation of expenses for the maintenance of the plant and the 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 a car engine, it indicates the relationship between the kilometers traveled and the amount of hydrocarbons introduced; In large motors for the production of electricity, 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 efficiencies that range from 20 to 30 percent; cars with diesel engines between 25 and 35 percent, the rest is converted into waste heat.

Large motors have higher efficiency, and although they are widely generalized, it can be said that for thermoelectric motors, the coefficient of efficiency is quite high and can reach 55%. But the same engine when it is produced in cogeneration has coefficients that reach 85%, because the calorific value of the fuel is better used, with an effective optimization of the process.

Of course, the investments to adapt the engines of a thermoelectric plant to cogeneration are considerable, but if it is possible to create a district heating network, the results are always advantageous. In fact, the period of use of these machines must be considered, which reaches even 30-40 years.

Types of cogeneration plants

The Ferrera Erbognone (PV) thermoelectric cogeneration plant

The most common example of a cogeneration plant is one built with a gas turbine / reciprocating engine and recovery boiler. The fumes from the gas turbine or reciprocating engine are transported through a flue pipe to the recovery boiler. Recovery can be simple, if there is no afterburner, or afterburner recovery otherwise. The fumes in the boiler make it possible to produce hot water, saturated steam or superheated steam. Typically 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 dispersion of energy is minimal and consists mainly of the heat introduced into the atmosphere by the fumes that come 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 however has the defect of having too low a convective heat transfer coefficient and therefore much higher heat exchange surfaces are required.

Regarding internal combustion engines, generally only 33% of the total energy available is transformed into mechanical energy, the rest is lost in part due to the irreversibility present in the engine equal to another 33% of the total energy and finally the last 33% is emitted to the external environment in the form of thermal energy that is eventually 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 a low temperature (not higher than 80 ° C), another exchanger to cool the water intended to cool the engine in yes, and finally a last exchanger located in the engine exhaust that allows to considerably raise the temperature of the heat exchange fluid, generally, as has been 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 motors must in any case work with a cooling system,

Finally, the evolutionary fluids particularly used are diathermic oils derived from petroleum, which have the characteristic of remaining liquid at atmospheric pressure up to temperatures of 300 ° C, and have a much lower solidification point than water, which prevents them from freeze. in the pipes

Small cogeneration (and microcogeneration)

Cogeneration with electrical energy 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 reciprocating engines, internal combustion, micro gas turbines or Stirling cycle engines. The main difference between small cogeneration and micro cogeneration consists in the fact that in small cogeneration thermal energy is a secondary product, while micro cogeneration is mainly aimed at the production of heat and secondarily of electricity.

The advantages of small cogeneration

In short, the advantages of small cogeneration are:

  • Use of unused thermal energy, with consequent fuel savings.
  • Less air pollution.
  • Significantly shorter electrical distribution chain, with a net reduction in line losses
  • Infrastructure reduction (power plants and power lines)

The trigeneration

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 in combination ... guaranteeing a significant reduction in use. of fossil fuels and CO2 equivalent emissions ". This is achieved because traditional thermal power plants convert only 1/3 of the fuel's energy into electricity, while the rest is lost as heat. There is still a need to increase the efficiency of electricity production. A method that goes in this direction is the combined production of heat and electricity (also known by the English acronym CHP,

Trigeneration systems

Co-trigeneration systems can be studied and produced to work with any primary heat source. These systems are technically mature and economically suitable today to be widely adopted, among the many possible configurations we mentioned:

  • cogeneration systems with fossil fuels;
  • trigeneration systems with fossil fuels;
  • co-generation with solar thermal systems;
  • cotigenation with biogas;
  • hybrid cogeneration and trigeneration systems.

Heat combined with fuel cells

Currently it is possible to produce hydrogen gas from methane to the public or biogas network (after desulfurization, because the H 2 "poisons" S the proton exchange membranes) with a reforming process that uses steam. Hydrogen is reacted with atmospheric oxygen on a proton exchange membrane to produce direct electrical current. Heat can be recovered for space heating, tap water, steam jet disinfection, etc.

Publication Date: January 10, 2020
Last Revision: January 10, 2020