(601d) Cooling Limitations in Power Plants: Optimal Multiperiod Design of Natural Draft Cooling Towers

Thermal plants, either regular ones based on fossil fuels, geothermal or Concentrated Solar Plants (CSP) [1], require cooling units to reject the heat from the exhaust. The use air results in the fact that the operation becomes complex due to the variability of its conditions over time, temperature and humidity. On top of the variability of the cooling agent, it is even more challenging to consider the operation of CSP plants, where, there is a large rage in the power produced over time due to seasonality. Martín and Martín [2] and Martín [3] evaluated the operation of such plants for wet and dry cooling systems. However, no detailed structural design for the wet cooling tower was carried out.

In this work we develop a multiperiod optimization formulation for the geometric design and monthly operation of cooling towers for power plants aiming at minimum water consumption. Coal based and CSP plants are considered. While the operation of both depends on the weather, CSP plants operation is also characterized by a non steady production of energy that also depends on atmospheric conditions. CSP and regular power plants show that extreme conditions do not allow fully operation of the plant and worst case design results in large unused capacity. Cooling towers are modelled using Mickley method to compute the mass transfer operation [4] together with geometric considerations to account ofr the pressure drop across the structure [5,6]. A multiperiod formulation is developed in GAMS for the optimal design and operation of such units over time. Furthermore, a rigorous CHEMCAD simulation of the power block based on data from a real power plant is also developed to evaluate the limits in power production as a result of the cooling capabilities in different climates based on an actual Power plant in Spain [7]. We use a hot climate as a reference for the location of CSP plants [2] and the same location and another one in a colder location to compare the design of the tower under different weather conditions for plants whose fuel or energy source is biogas, natural gas or coal.

The formulation shows that the driving force is limited in winter. Furthermore, the extreme temperatures of summer reduce the production capacity of the plant due to the heat transfer capacity. Colder climates require larger towers and show lower water consumption. Hotter climates need additional heat transfer area until the point when production must decrease by increasing the exhaust pressure of the low pressure turbine. Furthermore, a correlation for the power produced as a function of the air temperature and exhaust pressure of the turbine has been developed as a reference to evaluate the efficiency losses depending on the weather.

Acknowledgement

M. Vicente. La Robla power plant.La Robla, León, Spain.

References

[1] Hooman, K (2010) Dry cooling towers as condensers for geothermal power plants. Int. Comm. Heat Mass transf. 37, 1215, 1220

[2] Martín, L, Martín M. Optimal year-round operation of a Concentrated Solar Energy Plant in the South of Europe Applied Thermal Engineering 2013;. 59: 627-633.

[3] Martín, M (2015) Optimal annual operation of the dry cooling system of a Concentrated Solar Energy Plant in the South of Spain. Energy 84, 774-782

[4] Geankoplis, C.J. (1993) Transport Processes and Unit Operations (3rd Edition) (1993) Prentice Hall Upper Saddle River, New Jersey. U.S.A.

[5] EPA (1970) A method for predicting the performance of natural draft cooling towers Pacifict Northwestern Water Laboratory

[6] Kreith, F., Goswami, D.Y (2005) The CRC Handbook of Mechanical Engineering. 2^nd Edition CRC Press Boca Ratón

[7] Martín, M. (2013) Design of a power plant with carbon capture (In Spanish). MEng Thesis University of Salamanca