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Cogeneration in a small scale

Cogeneration is a simultaneous production of electric energy and heat, which leads to a more efficient use of primary energy. Sample quantitative gains from cogeneration are displayed in the figure below. As seen from the picture, in order to produce 21 units of electric energy and 33 units of heat in cogeneration (assuming the theoretical total cogeneration efficiency of 90%) there are 60 units of primary energy required, whereas 97 units of primary energy are needed to produce the same amount of final energies in separate generation.

Thus, cogeneration brings considerable increase of energy efficiency and contributes to decrease the level of harmful gases emissions into the environment. The opportunities for cogeneration are however usually determined by the demand on heat, which can vary seasonally and with the daytime.

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Production of electric energy and heat in a separate mode and in cogeneration.

Cogeneration as a simultaneous production of electric energy and heat can especially be applied in small power units of distributed generation systems. In small cogeneration systems, the produced energy goes first to local communities. One can mention here the energy generation for households, residence buildings, large farms, public buildings or small and medium enterprises. The surplus of electric energy goes to the power network, heat surplus goes to local district heating networks, whereas the surplus of fuel can be used for transportation or compressed into a gas network.

The main components of distributed cogeneration systems (also integrated with the production of fuels from biomass in biogas stations) are combustion piston engines (spark and diesel engines) topped with a recovery heat node. Equipped with appropriate feeding and ignition systems combustion piston engines can burn both gas and liquid fuels, also less caloric fuels such as biogas from fermentation biogas stations, gas obtained from pyrolytic gasification, liquid products of fermentation and pyrolysis or products of estrification of animal fat. A basic power range for ignition engines is from a dozen kWe to a few MWe.

A cogeneration cycle for ignition engines is illustrated below. The piston ignition engine drives a generator of electric energy. Heat from the cooling and lubrication cycle can be used for heating net water. Heat recovered from exhaust gases can be used for the production of technological steam and also for district heating.

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A cogeneration cycle for the piston ignition engine.

The main advantages of small power stations based on piston ignition engines are:

  • high efficiency of electric energy production, also during low load operation,
  • possibility of quick start-up to the nominal operation conditions,
  • possibility of operation in places distant from entry to the distribution network and as an emergency supply,
  • variable fuel supply,
  • relatively low investment costs.

In small or micro distributed cogeneration systems gas turbines or microturbines can also be applied. A sample cogeneration cycle with a gas turbine operating in an open cycle is illustrated below. Compressed air is fed to the combustion chamber where the fuel is burned under constant pressure. Heat is passed to the flue gases which expand in the turbine driving a generator. The exhaust gases from the turbine of temperature still in the region of 400-600oC first go to the recuperator, then to the heat exchanger where the district net water is heated. Gas turbines are characterised by a significantly longer exploitation time as compared to piston engines and require less frequent maintenance services. The efficiency of electric energy production however is usually by a few percent lower than that of the ignition piston engines in the considered range of power. Initial investment costs are also higher.

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A cogeneration cycle with a gas turbine.

Cogeneration devices dedicated for biomass stations are small steam turbines or microturbines that operate in an organic Rankine cycle (ORC). Main components of this cogeneration cycle are ecological boiler fit to combust different kinds of biomass or biofuels, intermediate heat cycle to extract heat from flue gases to thermal oil as a heat carrier, evaporator, turbine with a low boiling liquid as a working medium, generator, condenser and circulating pumps for the working medium and thermal oil. In the presented heat cycle, electric energy is a by-product and forms only about 10-20% of the total heat. The remaining superheat and condensation heat of the working medium is used for heating net water. The solution offers a possibility to apply low temperature heat sources, allows utilisation of different types of fuels and modular construction, which facilitates adaptation of the CHP unit to the required power range. It is anticipated that many ORC turbines will be installed in the near future in the distributed cogeneration systems. They are:

  • micro heat and power stations of heat capacity up to 20kWt and electric power up to 4kWe dedicated for individual households,
  • mini heat and power stations of heat capacity up to 20kWt and electric power up to 4kWe dedicated for gminas and poviats as local energy centres.

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A cogeneration unit with an ORC system;
E – evaporator, TV – steam turbine, C – condenser, G – generator.

Cogeneration technologies integrated with systems of production of fuels from biomass (biogas stations and biorafineries) need to be characterised by a high efficiency of electric energy production (40-50%), as the electric energy is usually considered the most precious output from the cogeneration unit. Therefore works on combined steam/gas cycles illustrated schematically in the figure below are in progress at IMP PAN. The main heat cycle and generator is driven by the ignition engine or gas turbine. An additional heat cycle is a steam ORC cycle working based on heat recovered from piston engine/gas turbine exhaust gases or cooling systems. The steam turbine drives another generator, which produces additional amount of electric power. The remaining superheat and condensation heat of the working medium in the ORC cycle is then used for heating net water or for own needs of biogas station (eg. heat supplied to the fermentor). It seems that cogeneration units of electric power 0.5-1MWe will most often be used.

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Schemes of cogeneration units in a combined cycle:
A – diesel engine + ORC cycle;
B – gas turbine + ORC cycle; D – diesel engine, TP – steam turbine, TG – gas turbine, G1, G2 – generators, C – compressor, BC – gas turbine combustion chamber, HE – heat exchangers.

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