Energy is neither created nor destroyed, but transformed: this is one of the fundamental laws of physics. The generation of electricity consists of transforming different forms of energy into electrical energy. In this process, efforts are made to optimize efficiency, that is, to maximize the amount of energy that is transformed for use, while reducing the amount that is wasted. It is in this regard that cogeneration presents itself as a highly effective solution. What cogeneration means Cogeneration, also known as combined heat and power (CHP), is the process by which electricity and thermal energy (i.e., heat) are produced at the same time and used for practical purposes. In a thermoelectric plant, the chemical energy of a fuel is converted into heat by means of combustion. This heat is used to heat water and produce steam, which drives a turbine. Thermal energy is thus transformed into kinetic energy – that is, motion – which in turn, through a generator, produces electricity. However, only part of the heat produced by combustion is converted into electricity, while another part is dispersed into the environment: on average, only 30% to 55% of thermal energy is actually converted into electricity. Cogeneration recovers this heat and uses it to heat rooms or water: in this way, the overall energy yield reaches 65% to 90%. In which sectors can cogeneration be applied? CHP systems can be applied to thermal power plants, but they are often installed directly in facilities with a stand-alone electricity generator – that is, they fall under distributed generation, which is the consumer's ability to obtain energy without drawing it from the power grid. Facilities may be residential or, more often, commercial or industrial, such as hotels, factories and shopping centers. Cogeneration is most cost-effective in settings with high electricity or heat requirements: for example, in the food, paper and ceramics industries. In such cases, one can turn to specialized energy service companies (such as Enel X, the Enel Group company that deals with high value-added technology solutions), which are able to offer clients specialized advice both from a technical point of view and in terms of the authorization process, proposing advantageous contracts and accompanying clients through all the steps, from the initial assessment to the commissioning and maintenance of the plant. The advantages of cogeneration Cogeneration has many advantages: greater energy efficiency: thanks to the use of heat, the combustion process wastes less energy and is therefore more efficient; cost savings: thanks to increased efficiency, the amount of fuel needed for heating is reduced. Cost savings can be up to 30%; lower environmental impact: using less fuel also reduces emissions of greenhouse gases and other pollutants; distributed generation: a cogeneration plant located close to the point of consumption of electricity and heat, with a view to distributed generation, allows the consumer to free himself (in whole or in part) from the power grid. This also avoids the slight losses of electricity that occur along the transmission and distribution grids, with additional benefits in terms of efficiency; economic incentives: in some countries, cogeneration plants are eligible for incentives because of their contribution to energy efficiency and environmental sustainability. How cogeneration systems work A CHP system is a thermoelectric system in which heat from the turbine is recovered by means of a heat exchanger, which is a device used to produce hot water or steam. In some cogeneration systems, the turbine may be replaced by different machines that have the same purpose: for example, ORC (Organic Rankine Cycle) turbogenerators, which use a denser fluid instead of steam, or internal combustion engines, similar to those in automobiles, used especially in small CHP systems installed directly at consumers’ locations. However, the general principle of operation remains the same. Plants are categorized according to their performance, in other words, the energy savings they make possible: this is the benchmark against which regulatory aspects are defined and, in particular, by which incentives are evaluated. There is no single threshold for defining high-efficiency cogeneration: the value depends on the rules in individual countries. In the European Union, Directive 2004/8/EC of Feb. 11, 2004, states that a cogeneration unit of more than 1 MW is defined as high-efficiency if the primary energy savings is at least 10% compared to the references for separate production of electricity and heating; for smaller plants, it is only necessary for energy savings to be present. Energy sources of cogeneration systems A CHP system can be powered by different fuels, which can be solid, liquid or gaseous, from renewable or nonrenewable sources. The most common are: fossil fuels: natural gas (methane), LPG (liquefied petroleum gas), fuel oil, coal; biomass: agricultural and forestry waste, wood derivatives; vegetable oil (or vegoil), which is fuel oil made from plant residues; synthesis gas (or syngas), which is an artificial mixture of gases, mainly carbon monoxide and hydrogen, in various ratios; biogas (gas mixtures made from organic plant or animal residues). A new frontier of innovation – especially suitable for very small systems – is the use of fuel cells: apparatuses that generate electricity and heat from chemical reactions in special containers (the "cells"). The fuel used is mainly hydrogen, which is obtained from water by electrolysis. In the future, fuel cells are expected to play an increasingly prominent role in combined heat and power systems. Alternatively, hydrogen can be combined with other gases and used to directly power a normal cogeneration system, and pure hydrogen systems have also been developed. To ensure true sustainability, it is necessary for hydrogen production to take place with instruments powered by electricity generated from renewable sources (so-called green hydrogen). In addition to these fuels, a cogeneration system can be combined with a solar power system. In this case, it is not photovoltaic energy (which generates electricity directly from sunlight), but rather solar thermal energy, which uses solar heat to generate steam – like thermoelectric plants do – which is then used to produce electricity by means of a generator. A very particular area of cogeneration is geothermal district heating: in this case, the system is installed not at a private customer's premises but at a geothermal power plant. Heat from the depths of the Earth is used to generate electricity and the residual portion is used to heat water, which is then distributed via pipelines to buildings in neighboring areas to supply heating. Clearly, when the power source of a cogeneration system is renewable, the overall environmental impact is reduced even further, but regardless of the fuel used, cogeneration is still a useful tool for energy efficiency and thus for sustainability. Trigeneration A variant of cogeneration is trigeneration. In this case, energy is produced in three different forms: in addition to electricity and heat, cooling energy is also produced and can be used to chill water or cool rooms. Trigeneration, which can use the same fuels as normal cogeneration, is suitable for private consumers and especially for large facilities like factories. Again, specialized energy service companies, such as Enel X, provide subsidized contracts with packages that include all the necessary technical and administrative steps (including procedures for obtaining incentives, where they are available). Technically, the way it works is similar to cogeneration, with one addition: the system also includes an absorption refrigerator, which is an apparatus that uses part of the heat recovered with heat exchangers to generate cooling energy. Actually, when we refer to cooling energy, we’re misusing that term for the sake of convenience: energy is a physical quantity equivalent to heat, and if it is supplied to a body, it cannot – in and of itself – cool it, but only heat it. The technique that is used here is that of the refrigeration cycle: a special cooling liquid is kept at low pressure so that it can evaporate even at low temperatures, absorbing heat from the water to be cooled and thus lowering its temperature. The cold water thus obtained is then used, directly or indirectly, to operate cooling units. In order to operate (as is also the case in ordinary refrigerators) a refrigeration cycle needs energy: in the case of trigeneration, this energy comes from the waste heat of combustion. The great potential of trigeneration lies in using the laws of physics to channel energy transformations as profitably and efficiently as possible.
