Steam turbines convert steam pressure energy into power, being widely used in power plants throughout industry and electric utilities. They use high pressure and high temperature superheated steam that flows through an expander, forcing the turbine to rotate, moving any equipment attached to the same shaft. After expansion, steam exits at lower pressure and temperature. The major difference of the steam turbine, relative to other primary movers like internal combustion engines and gas turbines, is that initial heat source for the process, combustion, occurs externally in a boiler, a separate device. The exit steam, depending of course on pressure and temperature, can be used for heating or can be sent to move another lower pressure steam turbine.
There are two basic steam turbines types, backpressure and condensing. The backpressure turbine produces power through expansion of high pressure steam to a lower pressure and the exiting steam is sent to an industrial process in a cogeneration system. A condensing turbine is similar to the backpressure turbine, but the low pressure side is usually below atmospheric pressure. This yields a greater heat-to-power efficiency, but the rejected steam has much lower energy content, hence reducing its value for heat recovery, so this steam leaving the turbine has to be condensed by a heat exchanger. In utility electrical plants, where electricity generation is the unique purpose, all turbines used are commonly condensing.
For cogeneration objectives, the ability to control the heat and power balance using the two types of steam turbine is helpful. This is accomplished by the extraction turbine, which simultaneously permits partial removal of steam from the turbine during expansion before condensation. This turbine can be designed to allow extraction at various pressures and flows, increasing the flexibility to satisfy both variables heat and power loads by a great deal. They are frequently used in cogeneration applications, but their average efficiency is lower than other turbine types.
The choice between backpressure turbine and extraction condensing turbine relies mainly on the balance between power and heat demands, hot temperature demand and economic factors. For same power level application, like electrical generation, compared with condensing, backpressure turbines present the simplest configuration, fewer components and lower capital cost. And, if properly projected to match process heat demand, they signify no need of cooling utility and less environmental impact. On the other hand, a backpressure turbine tends to be larger than the condensing type for same power output, because it works under a smaller enthalpy difference. Also, steam flow through the turbine depends exclusively on heat demand, which results in little or no flexibility to match its own power demand.
Therefore, to balance these amounts, there is the need either to purchase external electricity when the heat load doesn't allow enough power generation, or venting steam directly to the atmosphere to comply with power needs, a very inefficient option. In the inverse conditions, when the heat load produces more power than necessary, electricity exportation might eventually be an opportunity, but intermittent power production has almost no commercial value. All these cases present poor economical and energy performance.
Energy efficiency opportunities in steam turbines begin on selection, by choosing the appropriate turbine type that allows best control of the heat and power balance. Guaranteeing steam temperature and pressure design conditions at the turbine inlet is essential. Variations from optimal conditions can impair turbine ability to operate at maximum efficiency, regardless of its type. For condensing turbines, maintaining back pressure or vacuum is the most important factor, once deviations from optimum can reduce significantly efficiency. This can be obtained by ensuring appropriate cooling of the utility inlet temperature and the flow rate, to control fouling and scaling inside the condenser. Ensuring vacuum condition is another important aspect, because if the pressure rises, the enthalpy difference available for power generation diminishes drastically, spoiling turbine performance. Good sealing to avoid air infiltration into the condenser is mandatory.
Incorporating extracting and condensing steam turbines into industrial cogen-eration systems increases the ability to control the heat and power balance. If heat demand diminishes simultaneously to an increase in power demand, steam can be passed all the way to the condensing turbine part to produce additional power, maintaining a uniform and efficient load without extra fuel consumption. As a matter of fact, industrial cogeneration produces power by steam condensation less efficiently than a large utility plant. Better overall energy efficiency can be reached if more possibilities are in hand to manage diverse optimum operation points that match process needs, availability requirements and external power purchasing costs.
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