Technical Outline of a Co-generation System

1 - The Concept
2 - Features of the System
3 - Benefits
4 - The Co-generation Scheme

1 - The Concept.        

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  • Fuel - Fuel contains energy. When it is burned, the heat generated in this combustion process is utilized to the most possible extent. This corresponds to the point when the temperature of the products of combustion is now at ambient conditions of temperature and pressure. One immediately sees that there are better chances of utilizing the energy in the fuel more efficiently when the ambient conditions are colder, than when they are warmer.

  • Power in Thermal Power Plants - In conventional thermal power technology, fuel is burned in a boiler. The hot products of combustion (gases) is made to boil water under pressure and the resulting steam is sent to a steam turbine to turn it and convert heat into electrical power. Manufacturers have attempted many ways of increasing the efficiency of conversion from the energy of the fuel into electrical energy, but their efforts brought efficiencies to the 40% range. The most efficient thermal power plant in the Philippines has an efficiency of 30%.

  • Power in Internal Combustion Engines - Fuel is also burned inside the combustion chamber of reciprocating engines and gas turbines. The hot gases are also made to do work, not through steam, as described above, but directly to the head of the piston in an internal combustion engine, or to its blades in the case of a gas turbine. The best efficiencies in reciprocating engines and gas turbines have now breached the 50% mark.

  • Wasted Energy - Despite the high efficiencies developed in newer power plants, there is still the fact that only about half (mostly less than half) of the energy content of the fuel is used productively. A shameful amount is wasted into the atmosphere to pollute our earth.

  • NEW ENERGY - If one were to recover this wasted energy by some affordable means, then we will have, indeed, been able to mine new energy, since we will have recovered that which has already been lost and brought it back to do work for us.

  • Co-generation Technologies - Co-generation technologies using this "lower grade" recovered energy (which we will refer to as "new energy" from now on). Some co-generation technologies add a gas turbine before the boiler in order to draw power at that point, while still allowing the boiler to generate steam for the steam turbine. (Not all boilers work efficiently with this scheme.) I will give a very brief listing, below, of the popular co-generation technologies in common use today.

    1. District Heating - In cold countries, new energy is used to produce steam or hot water and circulated around town to provide heating to houses in the wintertime.

    2. Process Heating - This is a common application in the manufacturing sector, when new energy is used for pre-heating or drying applications.

    3. Air-conditioning - Japanese engineers have taken the lead in the utilization of low temperature steam and low temperature water, derived from new energy to produce chilled water at about 5 degrees Celsius. This is done in an energy-exchange process using a solution of lithium bromide.

    4. Energy Estate - I beg the reader's indulgence but I coined this term "energy estate" to refer to an energy source that provides power, steam for heating, and chilled water for airconditioning. I will accept "co-generation facility" with a heavy heart because it does not carry the richness and the fullness of the message that fuel as an energy source is a resource we must use with extreme care because it is not renewable, and that it can be depleted.

2 - Features of the System.        

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An "energy estate", which I personally nurtured until the negotiation stage has the following general features

  • Fuel - Fuel is conceived to be initially diesel fuel, but in due time would convert to natural gas from the Philippines' Malampaya source.

  • Gas Primemover - Fuel would be burned in two gas turbines, whose hot exhaust gases would be sent to two heat recovery steam generators (HRSG) to generate steam.

  • Steam Primemover - Steam from the two HRSGs would combine to drive a steam turbine operating under the following conditions

    1. Extraction Port - An extraction port is to be provided at the 10 bar gage pressure to deliver about 90% of the steam to the chiller plant and to the drying plants.

    2. Condensers - The remaining 10% of the steam would be sent to the condenser.

    3. Back Pressure - At the second stage when the demand for electricity and steam increases, the steam turbine to be installed would no longer have a condenser. It would be a "back pressure" turbine, where the exhaust is steam at a pressure of about 10 bar gage. This steam would be sent directly to the chiller and the driers.

  • Lithium Bromide Chillers - For this energy estate, the 10 bar gage steam would act as a heat pump, similar to the compressor of a refrigerator. The output is chilled water at a temperature of 5 degrees Celsius. This would be transmitted by way of a piping system to end-users.

  • Overall Thermal Efficiency - At the start of the project, when the load is still low, the overall thermal efficiency is estimated to be between 60% and 65%. As the demand for electricity, steam and chilled water increases, the overall thermal efficiency is expected to increase to about 75% to 85% with the use of the "back pressure" steam turbine.

3 - Benefits.        

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The recovery of new energy provides the following minimum benefits

  • Foreign Exchange - The amount of utilization of the fuel is now doubled. The importation would of fuel would be reduced by the amount corresponding to the new energy.

  • Cleaner Effluents - Effluents would have a much lower sulfur content, since gas turbines are designed to burn low-sulfur fuels. This effectively cuts the formation of SO3 to zero. SO3 is a primary ingredient to the formation of sulfuric acid at the low temperature products of combustion. This allows the exhaust to be brought as close as possible to ambient.

  • Foster a Healthy Technological Outlook - Consciousness of new energy, efficiency and a concern for the environment, within the context of an economically sound project is a healthy environment in and of itself.

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