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Combined cycles / Cogeneration
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Combined cycles
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In Gas turbine, an important part of energy is loss with exhausted combustion gases. This energy can possibly used as hot source for a vapor turbine. We then talk about **combined cycle**.

Cogeneration
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Finally, whatever the heat engine is, thermal energy is provided to a cold sink. **Cogeneration** consists in using these residual energy for a heating system. By the way, the installation produces mechanical work and heat. The global efficiency of a cogenerative installation may be evaluated by:

.. math::

  \eta_{cogen} = \frac{-\dot{W}_t + |\dot{Q}_{recup}|}{\dot{Q}_C}

With :math:`|\dot{Q}_{recup}|\leq \dot{Q}_C`.

These installation may reach 70 to 80% or efficiency.