Publication: Comparación de ciclos sub, super y ultracríticos para centrales termosolares
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2015-06
Defense date
2015-07-09
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Abstract
El presente proyecto fin de grado tiene como objetivo la comparación de ciclos subcríticos, supercríticos y ultracríticos en centrales termosolares. Para ello se ha realizado un programa que permite calcular las variables termodinámicas de interés (temperatura, presión, entalpía y gasto másico) en los puntos singulares del ciclo de producción de potencia, una vez introducidos los valores que permiten calcular el mismo. El programa calcula a su vez el rendimiento de la central y la potencia neta generada. Además del balance energético de cada ciclo de producción de potencia, el programa permite calcular los parámetros característicos de transferencia de calor de los intercambiadores cerrados que poseen los ciclos, así como sus dimensiones.
Se comparan en total diez ciclos: ciclo subcrítico condensado por agua o aire, supercrítico condensado por agua o aire y con un solo o doble recalentamiento y ultracrítico condensado por agua o aire y con un solo o doble recalentamiento, introduciendo en el programa las condiciones propias de cada uno de ellos. Los ciclos subcríticos trabajan con ~125bar en el foco caliente y el vapor alcanza los 540°C. Estos ciclos son los utilizados hasta hoy para centrales termosolares. En los ciclos supercríticos la presión se eleva hasta los 300bar y la temperatura del vapor alcanza los 590°C, mientras que en los ciclos ultracríticos estos parámetros aumentan hasta los 330bar y 630°C. Por tanto, en estos dos últimos casos, se produce una transición continua desde el estado líquido al estado vapor ya que el cambio de fase no se produce a la temperatura de saturación.
La utilización de mayores temperaturas permite mejorar la eficiencia que hoy día se tiene con los ciclos subcríticos: ~48% en ultracríticos frente a ~38% en subcríticos. Sin embargo la mayor parte de la mejora se obtiene al aumentar la temperatura a solo 590°C, propia de los supercríticos, con una rendimiento de ~46,5%. Al condensar por aire los ciclos muestran peores resultados, disminuyendo sus rendimientos del orden de un 2%, mientras que los ciclos con doble recalentamiento aumentan su eficiencia en un 0,2%. Los ciclos super y ultracríticos constan de un mayor número de intercambiadores cerrados que los subcríticos. De esta forma, aunque las condiciones sean más extremas en super y ultracríticos, los intercambiadores no se diferencian apenas de los de los ciclos subcríticos ya que la carga se reparte entre un mayor número de ellos.
The present project aims for making a comparison between subcritical, supercritical and ultracritical cycles for power towers. To do so, it has been created a program that allows the calculation of some variables (temperature, pressure, enthalpy and flow rate) in the singular points of the power plant cycles, once the values that make possible the calculation have been introduced. This program calculates also the efficiency and the power generated by the plant. Apart from the cycles’ heat balance, the program allows the calculation of the characteristic parameters of the cycles’ feedwater heaters, and also the dimensions of them. In total ten cycles are compared: subcritical with wet and dry condenser cooling, supercritical with wet and dry condenser cooling and with only one and with two reheaters and ultracritical with wet and dry condenser cooling and with one and with two reheaters, introducing to make the comparison the specific thermal conditions of each cycle. Subcritial cycles work with 125bar in the hot focus and the steam reachs 540°C. Nowadays these are the cycles used in power towers. In supercritical cycles the pressure increases to 300bar and the steam reachs 590°C, whereas in ultracritical cycles these parameters increase to 330bar and 630°C. Therefore, in these two last cases, there is a continuous transition from liquid water to steam because of the phase change is not at the saturation temperature. The use of higher temperatures lets improve the efficiency that nowadays subcritical cycles have: ~48% in ultracritical cycles versus ~38% in subcritical. However, most of the improvement can be expected by raising the temperature to only 590°C, typical of supercritical cycles, with an efficiency ~46,5%. With dry condenser cooling, the cycles get worse results, decreasing their efficiency ~2%, whereas cycles with two reheaters increase their efficiency ~0,2%. Supercritical and ultracritical cycles have more feedwater heaters than subcritical cycles. By this way, although the conditions are more extreme in super and ultracritical cycles, there isn´t hardly any differences between super and ultracritical heat exchangers and subcritical heat exchangers because the load is divided in a higher number of them.
The present project aims for making a comparison between subcritical, supercritical and ultracritical cycles for power towers. To do so, it has been created a program that allows the calculation of some variables (temperature, pressure, enthalpy and flow rate) in the singular points of the power plant cycles, once the values that make possible the calculation have been introduced. This program calculates also the efficiency and the power generated by the plant. Apart from the cycles’ heat balance, the program allows the calculation of the characteristic parameters of the cycles’ feedwater heaters, and also the dimensions of them. In total ten cycles are compared: subcritical with wet and dry condenser cooling, supercritical with wet and dry condenser cooling and with only one and with two reheaters and ultracritical with wet and dry condenser cooling and with one and with two reheaters, introducing to make the comparison the specific thermal conditions of each cycle. Subcritial cycles work with 125bar in the hot focus and the steam reachs 540°C. Nowadays these are the cycles used in power towers. In supercritical cycles the pressure increases to 300bar and the steam reachs 590°C, whereas in ultracritical cycles these parameters increase to 330bar and 630°C. Therefore, in these two last cases, there is a continuous transition from liquid water to steam because of the phase change is not at the saturation temperature. The use of higher temperatures lets improve the efficiency that nowadays subcritical cycles have: ~48% in ultracritical cycles versus ~38% in subcritical. However, most of the improvement can be expected by raising the temperature to only 590°C, typical of supercritical cycles, with an efficiency ~46,5%. With dry condenser cooling, the cycles get worse results, decreasing their efficiency ~2%, whereas cycles with two reheaters increase their efficiency ~0,2%. Supercritical and ultracritical cycles have more feedwater heaters than subcritical cycles. By this way, although the conditions are more extreme in super and ultracritical cycles, there isn´t hardly any differences between super and ultracritical heat exchangers and subcritical heat exchangers because the load is divided in a higher number of them.
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Keywords
Centrales termosolares, Ciclos de producción de potencia