Wednesday, August 12, 2015

New Energy: Cogeneration

       Cogeneration: using the waste heat of power production for useful purposes, for example, for industrial processes requiring steam or for commercial and domestic space heating. Capturing and recycling the waste heat from power plants (thermal or photovoltaic solar, fossil fuel, nuclear..) boosts their overall effective (thermodynamic) efficiency from about 50% to over 80% in the best conditions. This means that to get 1 watt of energy out a typical power plant you need to put in 2 watts. To calculate this you simply invert the efficiency (which tells you that you get .50 watts out for every watt of fossil fuel burned). For a cogeneration plant you need only 1.25 watt in fuel for every watt delivered as power or useful work: 1.0 watt out / 1.25 watts in = .80 (or 80% efficiency). If we compare the energy requirements, the cogeneration unit delivers burns 1.25 watt for every watt delivered, compared to 2 watts for the conventional plant. This gives us 1.25 / 2.00 = .625 or 62.5%. Thus the cogeneration plant requires only 62.5% ( or 5 / 8) of the input energy of a conventional fossil fuel burning electrical production unit, a saving of 37.5% (or  3 / 8). It also means that cogeneration is about 40% less polluting per unit of power produced (or useful work delivered)! 

          The Scandinavian countries are way ahead of us North Americans in terms of extracting the environmental benefits of cogeneration. About two thirds of commercial and domestic heating in Sweden is from some form of renewable energy (hydro, solar..) including burning organic wastes (wood chips, household waste). Cogeneration schemes are often employed. In these, a conventional electrical production unit burning, say, natural gas provides heat to the local community. Hot steam is piped from the plant through a system of pipes into homes and local commerces. The heat can also be used to drive air conditioning devices thus providing the option of heating or cooling in addition to power production.

          When cogeneration is integrated into a distributed energy production / consumption network, it can reduce the cost of distribution infrastructure greatly. In this scheme, institutions and factories produce their own power internally (for example through solar energy). Excess power would be sold back to the local utility. When power was needed it could be extracted from the common grid. Because most power is generated and / or consumed close to the point of production, distribution losses are cut to near zero. In addition, distribution infrastructure need be simpler and less costly.

           Cogeneration need not be dependent upon fossil fuels. The main source of energy could be solar, for example, with back up production kicking in when solar production dropped off or at night. A solar power unit might also store excess energy in the form of stored heat, using solar energy to melt chemical mixtures of salts. At night the heat stored in molten salt could power steam turbines to produce electricity and hot steam for domestic heating.

           We have failed to exploit the options opened by cogeneration as we should have because the fossil fuel lobby has its fat arm up the back side of federal politicians in all western countries. In addition they are a major client of the mass media, providing much of their advertising revenue..



  1. Nice explanation. I understand the process now. How can we simulate a a combined heat and power? Thanks in advance for answering my question.

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