The term cogeneration refers to the combined production of electricity and heat.
These two forms of energy are produced simultaneously through a single plant.
The ultimate aim of the cogeneration plants is to exploit the heat dissipated by power plants.
Generally electricity and heat are produced separately. To produce electricity, thermoelectric power plants dissipate thermal energy in the environment. The thermal energy produced comes from fuel conversion.
It therefore seems clear that if a user needs at the same time electricity and thermal energy, one can expect to realize a system that produces both electrical energy and thermal energy. The energy savings generated are obviously due to lower fuel consumption.
Cogeneration is carried out in particular thermoelectric power plants where hot water or process’ steam and / or fumes are recovered, produced by engines fueled by fossil fuel (natural gas, fuel oil, etc.) or organic fuels fossil fuels (biomass, biogas, synthesis gas, or other): it is then possible to get a significant energy saving compared to the separate production of electricity (through generation in power plant) and thermal energy (via traditional thermal power plant).
The process can be exemplified as follows:
Obviously not all the heat dissipated in the environment can be recovered, a portion is always dispersed in the combined production cycle.
A plant can be defined “cogeneration” only if it satisfies a set of performance indixes defined by the Electricity Authority (AEEG) introduced to ensure that there is effective fuel economy and that energy production is not too much unbalanced towards electricity alone.
A cogeneration plant works thanks to:
- A “first engine” (which can be powered in several ways)
- An electric generator that, moved by the engine, converts the mechanical energy into electricity
- Heat exchangers, which perform the heat recovery function.
All the key types of cogeneration plants differ for the type of “first engine” adopted. The most used and consolidated are:
- Internal combustion engines
- Gas turbines
- Steam turbines
- Combined cycle gas turbine / steam turbine plants
Cogeneration plants arise from the need to increase the efficiency of power generation systems by exploiting the heat dissipated by the plant for other purposes. The advantage comes from reducing fuel consumption. The improvement obtainable is measurable in a reduction in the consumption of 35-40%. There is another important advantage associated with reducing pollutant emissions. Another feature, resulting from logistical factors, is the need to locate the cogeneration plant close to the end user plant, a factor that reduces transmission energy losses coming from distribution and transportation.
Often, these plants work in stand-alone mode, which is useful to avoid the risk of power disruption due to network problems.
Finally, another noteworthy element is the economic incentives that cogenerative systems enjoy.
Obviously, the energy savings generated by the various cogeneration systems are not all equals. For this reason, the regulator introduced the definition of High Performance Cogeneration, in case the saving exceeds a predetermined minimum.
According to the European Directive 2004/8 / EC and s.m.i. the values to consider a cogeneration system a high performance one are the followings:
- For small cogeneration (less than 1MWe installed capacity) and micro-cogeneration (maximum capacity below 50 kWe) it is sufficient to generate primary energy savings compared to separate production.
- In other cases, the cogeneration system should ensure primary energy savings of at least 10% with respect to the reference values for separate electricity and heat production.
High-efficiency cogeneration enjoys further incentives and tax benefits.
Since the 1970s, cogeneration has been used to improve the efficiency of production systems, being highly use, both in the industrial and civil field.
Utilities that benefit from the use of cogeneration systems are generally those that are characterized by:
- High electrical and thermal power
- Constant electrical and thermal consumption over time
- High electricity costs
- Necessity to prevent any blackouts from the public network
Apart from the purely economic benefits generated by the cogeneration plants, incentives and tax advantages for plants that show energy savings indexes in line with the legislation can be summarized as follows:
- Priority in dispatching power to the network
- Exemption from the obligation to purchase Green Certificates
- A 20-year comprehensive overhauled tariff for cogeneration plants using renewable sources (biogas, vegetable oil …)
- Tax exemption of the fuel used, with the application of a reduced tax
- Obtainment of “White Certificates” or “Energy Efficiency Certificates” (TEEs) without the need to purchase them
- Exemption from payment of general system charges if the requirements of Legislative Decree 115/2008 are met
As reported in the Annual Generation Report submitted to the European Commission on April 30 of 2014, in Italy, 13,986 MW of power were generated through cogeneration in 2012, of which over 85% was attributable to large size combined cycle gas systems.
To confirm this, the primary energy used for production is attributable to gas for 88.7%.
In 2013, the national power park had a capacity of 128 GW. This capacity is combined with an output of 290 TWh. A subset of the national power park is made up of thermoelectric park. It has a weight of 79 GW to which are associated 192 TWh of power production.
The Italian thermal power park consist partially in cogeneration plants: this group has a power of 23 GW and produces 91 TWh of electricity and 59 TWh of useful heat. Indeed, the actual energy produced in the cogeneration system is equal to 37 TWh. Of these 26 TWhs come from High Performance Cogeneration.
Summary chart of the national thermal power station
Focusing on the cogeneration applied to the industrial sector, it is good to note that according to the ENEA scenarios, final consumption in the industrial sector is expected to recover in the next 10 years. In particular, The sectors defined as having an incremental economic potential if they decide to invest in cogeneration plants are chemical and petrochemicals, food and building materials (ceramics).
Scenario of the evolution of final consumption of electricity in the Industrial sector
Italian receptivity with respect to the diffusion of cogeneration can in general be considered satisfactory, especially when compared with the percentages of other European countries.
It seems that in the future the potential of “efficient plants” will continue to be a valuable tool for supporting decision-making process in formulating strategies and policies aimed at developing more efficient heating and cooling systems in terms of the use of resources and costs.
Maria Mura | Energy Consultant