Concentrating solar power (CSP) plants provide a clean and sustainable form of renewable energy in the southwestern United States, but the costs on local water resources can be high.
“Water requirements for CSP can include using water in wet cooling towers as well as for washing of mirrors,” said director of UNLV's Center for Energy Research and NEXUS scientist Robert Boehm. “The first of these two evaporates water into the ambient air and this can amount to large amounts of water use.”
Most solar thermal power plants use mirrors to concentrate the sun’s energy to heat a fluid that is used to create steam, which then drives turbines that create electricity. Once the steam passes through the turbine it needs to be condensed and cooled before it can be reused to produce more electricity. Typically, solar thermal developers prefer to use “wet cooling” towers, which involve the constant replenishment of water to make up for evaporative losses.
To counteract this water requirement, researchers have developed dry cooling systems that use air instead of water to condense the steam. These systems use about 90 percent less water, as almost none is lost to evaporation. But despite this advantage, dry cooling requires more capital investment and reduces the efficiency of the power plant.
To address these challenges, NEXUS faculty Boehm and Yitung Chen, along with Ph.D. student Kaipo Kekaula, undergraduate student Phillip Vorce, and research engineer Rick Hurt at UNLV are using a combination of experiments and computer modeling to improve dry cooling system performance.
“We are trying to minimize the performance penalty that exists in these kinds of systems so that, between the water savings and the performance penalty reduction, this may be more than enough to overcome the capital cost differential between wet and dry cooling,” Boehm said.
Testing an Optimal Heat Handover
In summer 2015, Boehm and his team finished the construction of a full-sized apparatus to test out modifications to dry cooling systems. Incorporating slanted tubes 20 feet long, the apparatus looks like an upended airplane wing and in it, the air-cooling tubes are nearly the same length as they would be in a fully operational unit.
“Data from experiments on this system allow us to develop computer models of dry cooling that will give us understanding of broad applications for these kinds of systems,” said Boehm.
In particular, the engineers are investigating how different types of cooling tubes might increase the transfer of heat from the steam to the air, allowing the whole concept to become more efficient. They are testing tubes in a variety of configurations and modifications to find out how differences in structure will affect temperature, velocity and pressure for steam, airflow and condensation.
“The experiments we are performing allow us to understand the performance improvements that modifications to the tubes will produce,” Boehm said.
Looking to the Models
In parallel to their experimental efforts the team has developed computer models to simulate how air flows through the system and transfers heat from the tubes. Most previous modeling studies have focused solely on the heat transfer improvements in a single fluid domain, whereas Boehm and his team are investigating the more comprehensive and realistic combined effects of the steam and air.
“We are coupling analyses of the inside phenomena with outside phenomena to show the impact of the tube design,” Boehm said. Using the models, the engineers are investigating the influence on liquid film growth and condensate accumulation on the inside of the tubes. They are also modeling how to improve the fin design to increase the rate of condensation.
Ultimately the team's research has the potential to increase the efficiency of dry cooling systems by 5 to 10 percent. It is research that would have wide scale applicability.
“This will work for any type of steam power plant with dry cooling: geothermal, coal, or nuclear, as well as solar,” Boehm said. “Our research can have far-reaching positive impacts in the power industry.”