Cogeneration

Cogeneration

• Cogeneration (CHP, combined heat and power) is the use of a heat engine to simultaneously generate both electricity and thermal energy (useful heat) from a single source of primary energy.

The useful heat is in the form of high pressure steam (steam process) or hot water (district heating)

Cogeneration is a more efficient use of fuel. In separate production of electricity, some thermal energy must be discharged as rejected) heat, but in cogeneration this residual thermal energy is put to use.

• Cogeneration systems allows

Primary Energy saving compared with the separate heat and electricity production.

Have higher global thermal efficiencies than conventional systems.

CHP installations can achieve global energy efficiency levels of around 90%

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Useful energy output (heat and power)

Fuel input (fuel consumption)

• Natural gas is the source of primary energy most commonly used to fuel cogeneration plants. However, renewable energy sources can also be used (biomass).

• 90% of CHP installations work with natural gas.

• A significant percentage of electricity production from biomass corresponds to cogeneration

Main advantages of cogeneration

• Reduces losses on the electrical network because cogeneration installations are usually closer to the consumption point.

• Increases competition among electricity producers.

• Reduces emissions of CO2 and other substances and contributes to sustainable development.

• Cogeneration reduces energy costs and so, it makes the competitiveness of enterprises increase

Typical cogeneration applications

Industrial- Paper industry- Food industry- Chemical industry- Ceramic industry- Steel industry- Textile industry

Utility sector- Hospitals- Hotels- Universities- Sport Centers

Types of cogeneration systems

• Steam turbine cogeneration system• Gas turbine cogeneration system• Reciprocating engine cogeneration system

Steam turbine cogeneration system

Gas turbine cogeneration systemSteam to process

Evaluation of CHP plants

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Evaluation of CHP plants

The PES is an internationally accepted parameter to assess and compare the “quality” of CHP plants.

• Primary Energy Savings (PES)

Where ηc is the reference efficiency of separate electricity production and ηb is the reference efficiency of separate heat production

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Evaluation of CHP plants

• Heat to power ratio:

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A company uses a constant electric power of 1MW. The heat demand changes from 1 to 6 MW. It has a cogeneration power plant, with a electrical efficiency of 0.3 and a heat to power ratio of 2 and a conventional heater with an efficiency of 0.9. The cogeneration plant is adjusted to the power demand and the conventional heater is switched on when it is necessary. The efficiency reference value of separate electricity production is 0.4 and the efficiency reference value of separate heat production is 0.9.Estimate the possible fuel savings that might be achieved with the proposed cogeneration plant.

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Water is the working fluid in a cogeneration cycle that generates electricity and provides heat for campus buildings. Steam at 20 bar and 320°C (state 1), expands through a two-stage turbine. Some steam is extracted between the two stages at 1.5 bar (state 2) to provide 2000 kW for useful heating, and the remainder expands through the second stage to the condenser pressure of 0.06 bar (state 3). The net power developed by the cycle is 800 kW. Condensate returns from the campus buildings at 1 bar and 50°C (state 4) and passes through a trap into the condenser (state 5), where it is reunited with the main feedwater flow. Saturated liquid at 0.06 bar (state 6). Then the working fluid is compressed through the pump from condenser pressure to boiler pressure (state 7). Each turbine stage has an isentropic efficiency of 80%, and the pumping process can be considered isentropic. Determine:

a) The mass flow rate of steam into the first turbine stage and the extracted fraction for useful heating

b) The heat transfer rate to the working fluid passing through the steam generator

c) The rate of heat transfer to the cooling water passing through the condenser

d) If the efficiency of electric generator is 0.95 calculate the electric power

e) If the fuel consumption is F=4000 kW find the global thermal efficiency of the plant

f) Find the primary energy savings (PES) considering that the efficiency reference values of separate electricity and useful heat production is 0.4 and 0.9 respectively.