(463b) Wet Vs. Dry Cooling for the Operation of Concentrated Solar Plants

Authors: 
Martín, M., University of Salamanca

Energy consumption has increased over the last decades and more sustainable sources such as biomass, solar and wind are being evaluated. Together with it, a recent concern is water consumption since, according to recent reports, two thirds of the plant will suffer water stress in the next decade (Rosengrant et al., 2002). There is a strong connection between energy production and water consumption that is well established for fossil based sources of energy (Water in the West, 2013). While thermal power plants and biomass based facilities have the luxury of being allocated in areas with access to freshwater, solar based thermal facilities present another challenge. Typically regions where the solar incidence is high present low water availability. Therefore, the production of power from concentrated solar power, in spite of the plentiful source of energy, is limited by the cooling capabilities. There are two main systems, wet cooling using a cooling tower and dry cooling, using typically an A-frame. The first one has the drawback of the water consumed. The second one does not require water for its operation, but it consumes a fraction of the produced power to operate the fans.

In this work we present the year-round optimization of the operation of a concentrated solar power facility. Such plants comprise a solar field, a molten salts storage system, the power block consisting of a regenerative Rankine cycle and a cooling system, using either an A-frame or a cooling tower. The salts are heated up using solar energy and stored so that day operation of the facility is secured. A system of three heat exchangers is used to heat up water to saturation, evaporate and superheat it. Only a fraction of the salts are directly used to superheat the steam, that is sent to the high pressure section of the turbine. The rest is used to reheat up the steam leaving the first body of the turbine before it is fed to the second section. The entire flow of salts is then used to heat up the recycled water and evaporate it. In the second body of the turbine, part of the steam is extracted at a medium pressure and it is used to heat up the condensate. The rest of the steam is finally expanded to an exhaust pressure, condensed and recycled. The condensation of the low pressure steam is performed either using a heat exchanger, cooled with water that is next cool down using a cooling tower, or directly using an A-frame system. The aim is to analyze the tradeoffs between the savings in water using dry cooling versus the power consumed due to the operation of the fans. We locate the plant in the south of Europe, Almería (Spain), where high solar radiation is available. The operation of the plant is a function of the solar incidence as well as the climate and atmospheric conditions. The optimization of the system is formulated as a multiperiod Non-linear Programming problem (NLP) that is solved for the optimal production of electricity over a year defining the main operating variables of the thermal and cooling cycles, the operating pressures and temperatures at the different bodies of the turbine, the fraction of salts used for reheating up the steam before feeding it to the second body of the turbine, the fraction of steam extracted to reheat up the liquid, the wet cooling tower operation (air flow and water losses by evaporation) or the A-frame air cooler performance (number of fans and units needed, the fans power consumption and the air inlet and outlet temperatures). Finally, both systems are compared based on the mitigated CO2 emissions due to the production of power from renewable sources

The operation of the wet cooled facility produces a maximum of 25 MW in summer and a minimum of 9.5 MW in winter with an annual production cost of electricity of 0.15 €/kWh consuming an average of 2.1 Lwarter/kWh. The investment for the plant is 260 M€, close to that of a similar plant already in operation. For the dry cooled facility, the cooling system consumes an average of 5% of the energy produced, increasing the production cost of energy up to 0.16 €/kWh and the investment to 265 M€. The advantage is the negligible water consumption. In terms of CO2 mitigation, it turns out that, for the particular allocation, using the wet system is 5% better. Finally, scale up studies reveal that the production cost can decrease by half while the investment per unit of power should become competitive with current coal based power plants if solar and coal facilities present similar production capacities.

References

Rosegrant, M. W.; Cai, X.; Cline, S. A. (2002) Food Policy Report; Global Water Outlook to 2025 (AVerting an Impending Crisis); International Food Policy Research Institute: Washington, DC, 2002.

 Water in the West (2013) Water and Energy Nexus: A literature Review. http://waterinthewest.stanford.edu/sites/default/files/Water-Energy_Lit_Review.pdf