Under funding from the U.S. Department of Energy (DOE), Grant DE-FE0031886, a collaboration between Lehigh University, the University of North Carolina at Charlotte (UNCC) and Worley have been working to develop a solution to enhance the performance of air-cooled condensers using thermal energy storage. Conventional and concentrated solar power plants, as well as industrial processes, require large quantities of clean water for cooling. Due to the scarcity of water west of the Mississippi in the U.S., and many parts of the world, air-cooled systems would be preferable to conventional once-through and cooling tower options which consume water due to evaporation and plume drifting. However, the heat transfer characteristics of air are much less effective than that of water. The concept researched in this project consists of a low-temperature thermal energy storage (LT-TES) module, to be integrated into dry cooling systems, which integrates a Phase Change Material (PCM) into pervious concrete. In this concept, charging of this supplemental cooling module will happen at night time, with cold ambient air used to extract heat from the supplemental LT-TES module resulting in freezing of the PCM (called "Cold Storage"). Conversely, charging of the LT-TES module happens during the hottest period of the day, with the supplemental LT-TES module acting as an integrated direct contact heat exchanger; the internal heat transfer processes caused by the stored latent heat of the frozen PCM and the sensible heat of the pervious concrete effectively cools the air resulting in improved operation of air-cooled condensers, and hence increased plant power output at times of peak loads.
The team from Lehigh University includes Drs. John Fox, Clay Naito, Sudhakar Neti, Carlos Romero, and Muhannad Suleiman, as well as graduate students Lida Yan and Emad Yaghmour. Dr. Nenad Sarunac was the lead from UNCC, and Jagatheesan Senthilvel and Qinghua Xie participated from Worley. The project is completing a 3-yr schedule, which included PCM and pervious concrete development, engineering of integrated PCM-infused modules, laboratory testing and techno-economic evaluation of the technology. The PCM chosen was calcium chloride-hexahydrate (CaCl2·6H2O), due to its availability, low cost, suitable melting temperature and relatively high latent heat of phase change. The PCM material was engineered to achieve melting congruency at 25°C and less than ±5% decay in thermal performance after 1,000 cycles, as demonstrated in a cycling machine designed for the specifics of the application. A vacuum method was employed to infuse the PCM into the pores of lightweight aggregate used to make high-porosity pervious concrete.
An epoxy coating technique was developed to seal the PCM into the aggregate and subsequent covering with Type I cement. Optimized pervious concrete mixtures resulted in water/cement ratios of 0.30 and measured porosity close to 35%. This feature is important to enable the flow of cooling/heating air through the LT-TES module, while minimizing pressure drop across the module and maintaining its structural strength when cast into the size of cooling modules needed in an air-cooled power plant. Different designs of PCM micro-encapsulated and macro-encapsulated pervious concrete modules were built for thermal performance testing in flow rigs at 0.25 and 150 kWh scale. Measurements from the LT-TES modules, under flow conditions, showed the expected combination of sensible and latent heat transfer behavior and no supercooling on the part of the PCM during the entire freezing-thawing cycle. The LT-TES modules exhibited a considerable thermal capacity, with energy densities as high as 45 kWh/m3 and thus could make a positive impact on the operation of air-cooled condensers and its subsequent impact on the power output of the power plant.
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