Cogeneration

A 250 MW cogeneration plant in Cambridge, Massachusetts.

Cogeneration is the procedure by which electrical energy and useful thermal energy (steam, sanitary hot water) are simultaneously obtained. If cold is also produced (ice, cold water, cold air, for example) is called trigeneration. If carbon dioxide emissions are captured in addition to this, it is called quadrigeneration (not to be confused with tetrageneration, where the fourth type of energy produced is mechanical energy).

The advantage of cogeneration is its greater energy efficiency since both heat and mechanical or electrical energy are used from a single process, instead of using a conventional power plant and a conventional boiler for heat needs.

Another advantage, and not a small one, is that by producing electricity close to the point of consumption, voltage changes and long-distance transport are avoided, which represent a significant loss of energy due to the Joule effect (it is estimated that in large networks this loss is between 25 and 30%).

According to Hertler, it is the process by which electrical energy and useful thermal energy are simultaneously obtained. It is energy efficient and prevents voltage changes. It differs from parallel generation since two processes are used, one to generate electrical energy and the other to generate thermal energy. This electrical energy is produced by power plants of limited capacity, belonging to private companies or cooperatives that can be integrated into the national electrical system.

Operation

When generating electricity by means of a dynamo or alternator, moved by a thermal engine or a turbine, the use of the chemical energy of the fuel is from 25 to 46% (over the lower calorific value), and the rest must be dissipated in the form of heat. With cogeneration, an important part of the thermal energy that would normally be dissipated into the atmosphere or a body of water is used and avoids generating it again with a boiler. It also avoids possible problems generated by unused heat.

The efficiency of the plant can be measured by means of some coefficients: the FUE, energy use factor, which is the quotient of the electrical energy generated, plus the useful heat, between the energy supplied to the internal combustion engine. And the RCE, heat/electricity ratio, which is the ratio between the useful or usable heat, and the electrical power generated by the motor-generator set. The first is the most important since it gives an idea of the global performance of the installation.

This procedure has both industrial applications and in certain large buildings in which the heat can be used for heating, cooling (through absorption systems) and domestic hot water preparation, such as large sales areas, university cities, hospitals, etc...

Although it is difficult to couple it to private homes (the truth is that there are more and more installations, specifically called microcogeneration), it is much more favorable to carry out large installations, such as district heating.

Regulation

The cogeneration process has a more or less fixed distribution between electrical/mechanical production and heat. As the needs of both energies can vary in a different way, it is frequent that there is a surplus of one of them.

Cogeneration energy is included in the Special Energy Regime (Royal Decree 413/2014, of June 6, which regulates the activity of electrical energy production from renewable energy sources, cogeneration and waste) that allows you to use cogeneration to provide yourself with all the heat you need and inject into the electricity grid the electricity you do not need at a fixed rate. In this way, another plant is prevented from producing that energy in a less efficient way.

Trigeneration and others

There is an extension of the concept of cogeneration that allows to have apart from the usual heat and mechanical/electrical energy, cold from residual heat.

It is possible to obtain cold from a heat source through absorption systems. As a result, greater efficiency is obtained: in most climates, heating is not necessary for more than a few months a year, while with trigeneration the system is also used in the hot seasons, which improves (decreases) the time amortization of the system, by increasing the service hours of the installation.

An absorption refrigeration system needs a minimum temperature of about 80°C to work, from which discharge water is obtained at about 40 or 50 °C and cooling water at about 0 to 4 °C.

There are other ways to maximize the concept of cogeneration.

In greenhouses, exhaust gases from the cogeneration system can be reused, previously treated with a catalyst such as carbonic fertilization.

Another modality is the so-called Tetrageneration, in which in addition to the three previous forms of energy, usable mechanical energy is simultaneously generated, for example, to generate compressed air. In Spain there are few examples of tetrageneration, except for cases such as the FORD factory in Almusafes, which is a good example.

Machinery used for cogeneration facilities

The principle of cogeneration can be materialized with the help of countless technologies. Fundamentally, a first differentiation takes place between the technologies by which the fuel can be burned. Among the possibilities, we find Stirling engines, steam engines, steam turbines and technologies such as ORC (Organic Rankine Cycle) or Kalina Cycle.

• Steam turbine
  • Steam turbines to counterpressure: when the steam volume required for auxiliary services is equal to that of the turbine.
  • Steam turbines with intermediate socket, when only one part of the turbine steam is necessary for auxiliary services.

• Gas turbine
• Internal combustion engine
• Fuel pipe
• Microturbines
• Stirling motors
• Rankine Cycles with Organic Fluid
• Kalina Cycle

Turbines and steam engines typically have efficiencies in the 10-20% range. Otto engines have an electrical efficiency of 1 to 2 points below the efficiency percentage of combustion engines. For their part, gas turbines have a performance of up to 20 MW and, at the same time, most of them have an efficiency percentage below 40%. The electrical efficiency of Stirling engines, which is in a low power range (micro-cogeneration), varies between 10 and 15%. The most powerful technology is fuel cells, which can reach a degree of electrical efficiency of 60%.