Optimizing Conventional Electricity Generation

The process can however be optimized. Richard Campbell (2013) analyzed the performance of coal power plants in the United States. According to him, the yield could be improved by an average of four points. While each plant is unique, the sources of inefficiencies are the same everywhere. They come from two main factors. First, mechanical imperfections in the various ventilation systems and pumps which circulate air and vapor through the system can deteriorate over the course of the power plant’s lifetime. This deterioration leads to decreased performance of the mechanical conversion of the plant and, therefore, of the overall production of electricity achieved for a given quantity of fossil fuel. Leakages and lack of isolation lead as well to a lower efficiency of the heat transformation and to issues with maintaining the gas pressure at desired levels. Other than mechanical imperfections, the overall management of the power plant operation also contributes to inefficiency. A power plant is designed to operate optimally at a certain level. Operating a power plant at other than this level, whether below or above it, leads naturally to lower yield because of the resulting thermal inefficiency. Production variations can also lead to decreased thermal efficiency. A brutal drop of the electric power output followed by an increase requires raising the boiler temperature back up and therefore to increased consumption of fossil fuel; stable production makes better use of fossil fuels to maintain temperature and pressure levels. Finally, the combination of different variables (oxygen, temperature and pressure), if not optimized, decreases the overall efficiency of the process. Optimizing this combination requires the synchronized operation of different elements of the system: pumps, ventilation systems, burner, etc. Real-time process optimization is thus one of the primary sources of efficiency (or inefficiency) of thermal power plants.

Pilot projects have been run in the United States that use renewable energy such as thermal solar or biomass to complement the traditional heating process. This allows plant operators to minimize the amount of fossil fuel required by the heating process.

All these optimizations grant a few points of additional efficiency. Beyond this, the process itself needs to be revisited. Supercritical plants improve the yield of the Carnot cycle by increasing the pressure and the temperature of the vapor. This can result in yields of 46%, more than half more of the 30% yield for traditional plants. In such power plants, the pressure is up to 285 bars and the temperature around 620 degrees (Hansen and Percebois 2015). Circulating Fluidized Bed technologies operate at a lower burning temperature and less greenhouse gas is emitted while operating the boiler. Coal gasification creates a synthetic gas which does not generate CO2 emissions. These technologies are mature but require heavy investments—they generally cost double what a traditional coal power plant does (Barre and Merenne-Schoumaker 2011).

Gas power plants have a yield around 40%, slightly above that of coal power plants. Combined cycle power plants provide a much better yield, around 60%, as they use part of the heated gas that goes through the first turbine to heat up vapor in a second cycle which then will expand through a second turbine. This doubling of the production cycle leads to a higher power output for a given quantity of gas initially used.

Cogeneration plants (L’Expansion 2012) operate with two circuits. The first fluid is heated to operate through a turbine and cooled down while in contact with a second fluid. The second fluid, heated up by the thermal exchange, is then used in a district heating network. The total yield of a cogeneration plant can reach 80-90% in the best cases. This is a significant improvement over the yield of conventional plants. Now, for cogeneration to be efficient, the calibration of electricity production and heat must be well done. Any deviation from nominal operation leads to a sharp decrease of the power plant yield.

To sum up, there are a number of solutions to optimize the yield of a power plant. However, their yield is bound to remain extremely low, except when using combined cycle gas plants or cogeneration plants. This means that, whatever improvements are made (and they must be made), the level of losses remains colossal. Even though CO2 emissions can be tamed to a certain extent with the use of carbon capture systems (© OECD/IEA, Coal 2014), such waste cannot continue to exist in a world where the demand for fossil fuels keeps increasing and where greenhouse gas emissions are skyrocketing. This is why engineers have long started to look for alternative energy sources, such as nuclear or renewable energies.

 
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