Water Use Reduction at Thermoelectric Plants

Fuel cell power plants can help achieve air quality and climate goals through several high value attributes, including high efficiency, ultra-low pollutant emissions, zero demand for water, and near zero acoustic emissions. When integrated into a Brayton cycle, the resultant “hybrid” technology releases the Carnot Efficiency constraints (due to materials limitations) and combustion pollutant emissions of the heat engine and thereby results in highly efficient generation of electricity with coal, natural gas, and/or biogas from central plant generation to distributed generation. The solid oxide fuel cell (SOFC) is a suitable fuel cell for these applications. This research will develop integration schemes to fully realize the potential of hybrid SOFC/GT systems in both central plant and distributed generation applications. 

Central plant Integrated Gasification Fuel Cell (IGFC)/gas turbine hybrid technology, combining coal and/or biomass gasification, is deployable at large scales (~100 MW). IGFC has the potential to generate electricity at efficiencies approaching 60% on coal higher heating value (HHV) basis, while capturing >90% of the evolved CO2 with substantially reduced water demand. Operated on natural gas in the absence of gasification, the NGFC hybrid has the potential for efficiencies approaching 75%. The higher overall plant thermal efficiency of SOFC/GT hybrid power plants leads to a lower amount of heat rejection, thus requiring less makeup water to wet the cooling towers typically used in current power plants. Further, by keeping the anode and the cathode exhaust gases separate, the H2O vapor entering with the fuel and formed by fuel oxidation may be more easily recovered by cooling the anode exhaust gas, because the H2O vapor will be at a much higher partial pressure than typical flue gases. This further reduces the net water consumption of the plant.

SOFC/GT hybrid technology is ideal for distributed power generation fueled either by natural gas or biogas derived from sources such as wastewater treatment facilities and landfills. Further, because of the increasing demand on electric grids to support high penetrations of intermittent renewables, enhance power quality, increase reliability and resiliency, and provide ancillary services, local power is being generated on both sides of the meter in the U.S. and may also be expected in China in the future. Clusters of SOFC/GT hybrids in the size range of 10 to100 MW, referred to as Transmission Integrated Grid Energy Resource (TIGER) Stations, are ideal to enable electric grid support by providing baseload and various levels of load following services.

Publications:

Rosner, F., Chen, Q., Rao, A., Samuelsen, S., Jayaraman, A., & Alptekin, G. (2019). Thermo-economic analyses of IGCC power plants employing warm gas CO2 separation technology. Energy, 185, 541-553.

Rosner, F., Rao, A., & Samuelsen, S. (2020). Water gas shift reactor modelling and new dimensionless number for thermal management/design of isothermal reactors. Applied Thermal Engineering, 115033

Two important levers to reduce water-intense energy generation needs are: (1) increasing the productive use of waste heat from plant operations, which enhances the efficiency of energy generation while mitigating water cooling needs; and (2) using heat exchangers that enhance dry heat transfer coefficients and make water-free cooling more efficient. Recent advances in novel classes of hybrid thermoelectric and thermal interface materials now enable efficient direct thermal-to-electrical energy conversion (DTEC) with exceptional performance below 400°C and zero net water dissipation. Average energy conversion efficiencies from power plants range from 35-55%, thus plants are typically rejecting hundreds of MW of low temperature waste heat, which we propose to use for energy generation. 

         The Urban group has expertise in design of hybrid inorganic/ organic materials that have best-in-class thermoelectric power conversion efficiencies and are made using scalable room temperature, solution processing methods. Uniquely, these devices also offer customizable engineering – our devices are made in flexible and conformable form-factors that facilitate ease of implementation and widespread utilization/retrofitting. The scientific innovation in this project will be to optimize our materials systems to best match and utilize the specific waste-heat temperatures required for industry, as DTEC efficiency is dependent upon a complex mix of material-specific parameters. Our group has developed a new strategy to overcome these limitations using simple, stable, surface atomic modifications; we will customize these solutions for the specific CERC partner interests.                       

            In addition, we propose to deliver water-free passive cooling solutions with cooling capacities that approach that of water. Here, we take advantage of the fact that the latent-heat of melting in nanocrystalline materials is size-dependent and can be tuned across a wide range by simple chemical growth processes. We will use these characteristics to prepare nanostructured phase-change “tapes” that can be directly applied to hot surfaces where water or spray cooling is used—the nanocrystalline materials will go through phase transition locally on the hot side, absorbing heat, and will then re-radiate this heat on the cool side due to their large surface area and high emissivity. While supplemental air-cooling may be used to enhance this effect if necessary, we have demonstrated water free cooling in preliminary results.

Nanostructured superhydrophilic surfaces enhance impinging water droplet vaporization through spreading and thin film evaporation. This can result in lower steam condensing temperatures as well as higher power cycle efficiency and reduced water consumption. Significant progress in these areas could be made through the development of durable, low-cost superhydrophilic heat exchanger surface coatings that provide highly effective spray cooling heat transfer performance with ultra-low water usage. Applications include reduction of water use for spray cooling of power plant condensers, water desalinization, and manufacturing process such as water quenching of metal forgings and castings. 

Our recent preliminary studies have shown that coating a metal copper substrate with a layer of ZnO nanorods creates a superhydrophilic surface that enhances droplet impingement heat transfer and eliminates loss of water by ejection of splash-created droplets. We fabricated this nanostructured ZnO coating by depositing ZnO nanoseeds on a prepared copper surface using a scalable thermal growth methodology. This coating is durable under conditions typical of spray cooling of power plant condensers.

Our major challenge is to develop a superhydrophilic thin film coating for an aluminum substrate that simultaneously enhances heat transfer, maximizes the reduction of water use, and is robust, durable, and inexpensive to produce. We will achieve this goal through three research tasks: (1) Adapt to aluminum the ZnO nanostructured surface fabrication methodology we have used successfully to create superhydrophilic surface coatings on copper substrates. [goal: Create a durable nanostructured coating on aluminum with water contact angles less than 3˚.] (2) Fabricate coatings with at least three different nanoscale surface geometries on aluminum substrates to facilitate the experimental heat transfer testing of the surfaces. [goal: Fabricate nanostructured surperhydrophilic surfaces with three different nansocale geometries on aluminum and document them using the electron microscope imaging and wetting experiments.] (3) Conduct experiments to determine how nanoscale geometry variations affect impinging droplet spreading and the resulting evaporation heat transfer for the different nanosurface geometries on the aluminum substrates fabricated in Task 2, and compare our experimental results to Volume of Fluid (VOF) CFD predictions of droplet impingement fluid dynamics and heat transfer. [goal: Complete droplet spreading and heat transfer testing on three different nanoscale coating geometries on aluminum and document them using the electron microscope imaging and wetting experiments.] (3) Conduct experiments to determine how nanoscale geometry variations affect impinging droplet spreading and the resulting evaporation heat transfer for the different nanosurface geometries on the aluminum substrates fabricated in Task 2, and compare our experimental results to Volume of Fluid (VOF) CFD predictions of droplet impingement fluid dynamics and heat transfer. [goal: Complete droplet spreading and heat transfer testing on three different nanoscale coating geometries on aluminum substrates, and define the best nanoscale superhydrophilic coating design for enhancement of spray cooling of aluminum air cooled condensers.]

Figure 1 displays a time lapse of a droplet being deposited on one of the coated surfaces.

Figure 1: Water droplet deposited on zinc oxide coated Aluminum surface. Superhydrophilic spreading observed.

The rate at which water wicks out on the surface is one of 2 main metrics used to quantify the wettability of the surface. The second is total spreading area. The larger the droplet spreads, the thinner the resulting water droplet is. In a heat transfer situation, this can lead to thin film evaporation, which is a highly effective method of cooling. Figure 2 shows the maximum spreading for a 2μL droplet on the coated surface.

Figure 2: Water droplet spreading on a zinc oxide nano-coated surface.

Publications:

Mizerak, J.P. and Carey, V.P., Modeling Of Transport During Droplet Deposition And Spreading On Smooth And Microstructured Superhydrophilic Surfaces , Proceedings of the ASME 2016 International Mechanical Engineering Congress & Exposition IMECE 2016, November 11-17, 2016, Phoenix, USA.

Kunkle, C.M. and Carey, V.P., Metrics For Quantifying Surface Wetting Effects On Boiling And Evaporation At Nanostructured Hydrophilic Surfaces, paper PRTEC-15163, Proceedings of the First Pacific Rim Thermal Engineering Conference, PRTEC, March 13-17, 2016, Hawaii's Big Island, USA. 

Kunkle, C.M. and Carey, V.P., Metrics for Quantifying Surface Wetting Effects on Vaporization Processes at Nanostructured Hydrophilic Surfaces, paper HT2016-7203, Proceedings of the ASME 2016 Summer Heat Transfer Conference HT2016, July 10-14, 2016, Washington, DC, USA.

LaBrie, Russell J., Jorge Padilla Jr, and Van P. Carey. "Experimental Study of Aqueous Binary Mixture Droplet Vaporization on Nanostructured Surfaces." Heat Transfer Engineering 38.14-15 (2017): 1260-1273.

Kunkle, Claire M., Jordan P. Mizerak, and Van P. Carey. "The Effects of Wettability and Surface Morphology on Heat Transfer for Zinc Oxide Nanostructured Aluminum Surfaces." ASME 2017 Heat Transfer Summer Conference. American Society of Mechanical Engineers, 2017.

Wemp, C.K. and Carey, V.P., Tuning Superhydrophilic Nanostructured Surfaces To Maximize Water Droplet Evaporation Heat Transfer Performance, paper IMECE2017-72679, presented and included in the Proceedings of the ASME 2017 International Mechanical Engineering Congress and Exposition, November 3-9, 2017, Tampa, Florida, USA.

Wemp, Claire K., and Van P. Carey. "Water Wicking and Droplet Spreading on Randomly Structured Thin Nanoporous Layers." Langmuir 33.50 (2017): 14513-14525.

Wemp, Claire K., and Van P. Carey. "Tuning Superhydrophilic Nanostructured Surfaces to Maximize Water Droplet Evaporation Heat Transfer Performance." Journal of Heat Transfer140.10 (2018): 102401.

Carey, V. P., Wemp, C. K., McClure, E. R., & Cabrera, S. (2018, November). Mechanism Interaction During Droplet Evaporation on Nanostructured Hydrophilic Surfaces. In ASME 2018 International Mechanical Engineering Congress and Exposition (pp. V08AT10A029-V08AT10A029). American Society of Mechanical Engineers.

Wemp, C. K., & Carey, V. P. (2019). Heat transport for evaporating droplets on superhydrophilic, thin, nanoporous layers. International Journal of Heat and Mass Transfer, 132, 34-51.