Water Use Reduction at Thermoelectric Plants

Conventional approaches to reducing water consumption in thermoelectric plants typically result in reduced efficiency and increased carbon dioxide emissions. Technological breakthroughs in the areas of dry cooling, non-conventional power conversion, dry carbon- capture methods, and reduced fuel consumption are urgently needed in order to address electricity needs in regions where water is, or will become, scarce. The CERC-WET portfolio includes an array of research projects aimed at developing new approaches to reduce water consumption and carbon dioxide emissions from thermoelectric plants.

Topic Area 1 Projects

 

Project 1.1: Dry CO2 capture based on nanoscale framework materials

Jeffrey Long, UC BerkeleyJeffrey Long, UC Berkeley
CV | Website

Context: Post-combustion CO2-capture process requires large amounts of energy and water. By replacing traditional aqueous amine absorbent capture technology with solid adsorbents, savings in both energy and water resources can be realized.
 
Objective: Identify the diamine-appended metal-organic framework best suited to minimize this energy and water consumption. Develop strategies and partnerships to advance technology readiness of the chosen adsorbent through scale-up, process modeling and slipstream testing. 
 

The Long Group recently discovered a class of solid adsorbents that can capture CO2 with minimal impact on water and energy resources. These materials, diamine-appended metal–organic frameworks of the form (diamine)2M2(dobpdc) (dobdc4– = 2,5-dioxidobenzene-1,4-dicarboxylate), feature inorganic nodes connected by organic dobpdc4– ligands to form one-dimensional, hexagonal channels of 18 Å diameter. One end of each dibasic diamine species binds each metal site lining the channels; this orients the other end of the diamine into the pore to initiate the capture process upon exposure to acidic CO2 gas.

The choice of a solid adsorbent allows the fundamental benefit of capture technology that can function without need for water or other solvents, which can comprise up to 70% of the volume of traditional amine absorbers. Further, adsorbents can afford intrinsically lower regeneration energy requirements due to the elimination of energy inputs in a temperature swing process associated with the high heat capacity of water. Beyond these fundamental gains, we are able to achieve unparalleled working capacities and energy savings with the diamine-appended M2(dobpdc) frameworks by virtue of their recently-elucidated mechanism of operation: CO2 binding takes place in a cooperative manner, resulting in step-like adsorption isotherms with onset pressures that increase with increasing temperature.

While classical adsorbents with Langmuir-type adsorption profiles require either significant heating or vacuum to initiate regeneration, we can regenerate materials with step-shaped adsorption profiles simply by decreasing the pressure or increasing the temperature. This allows access to full working capacity of the material by positioning the operating partial pressure of CO2 below that of the critical step pressure. Importantly, we have found that this threshold adsorption pressure for our materials can be tuned over 6 orders of magnitude by varying the constituent metal and diamine.

Metal–organic frameworks are highly promising new materials for next-generation technology in selective gas adsorption. In this project, we will identify the diamine-appended metal–organic framework best suited to minimize energy and water consumption in a post-combustion CO2 capture process. The greatest challenge in this endeavor will be selecting the specific step position and corresponding material that can minimize the temperature swing required for regeneration while preserving performance in the presence of H2O and other common flue gas components. Based on our preliminary screening of over 100 diamine-appended adsorbents, we are confident in our ability to deliver an optimal solid adsorbent for CO2 capture. 

Project 1.2: Reheat air combined cycles (RACC)

Per Peterson, UC BerkeleyPer Peterson, UC Berkeley
CV | Website

Context: Molten salts can transport and store heat at temperatures useful for combined cycle power conversion, enabling coupling to non-fossil energy sources including nuclear reactors and concentrating solar power towers.
 
Objective: Evaluate RACC system performance with computer simulation and experiments, with specialized models for heat exchange, duct, and thermal storage systems. Collaborate with Chinese partners to develop a roadmap to achieve the major outcomes projected by 2020. 

A key factor affecting the use of fission reactors and concentrating solar power (CSP) in regions with scarce water has been their relatively low operating temperatures, resulting in low thermal efficiency in power conversion and significant water consumption for cooling.  Fluoride salts have emerged as a technologically interesting option for heat transport in these applications because they can transfer heat in the temperature range from 600-850°C using currently available structural materials. The near-term demonstration of a 370-kW molten fluoride salt test loop is expected to occur in China at the Shanghai Institute of Applied Physics before 2020, complementing parallel research efforts in the U.S. and at UC Berkeley. 

UC Berkeley has investigated a range of power conversion technologies optimized to this temperature range, and recently found that conventional gas-turbine combined-cycle thermo-electric plants can be reconfigured to produce base-load electricity using these heat sources, providing highly-flexible, dispatchable peaking power by co-firing with natural or syn-gas. In this configuration, a modified GE 7FB can generate 100 MWe operating with a turbine inlet temperature of 670°C and a single stage of reheat. Co-firing with gas to increase the turbine inlet temperature in the second expansion stage can boost power output to 242 MWe. This enables Reheat Air Combined Cycle (RACC) power conversion to produce rapidly dispatchable peaking power, with 66% efficiency in converting gas to electricity, significantly reducing CO2 emissions to produce peak power.

Furthermore this efficiency is sufficiently high that high-temperature thermal storage using electric-resistance heating with off-peak power should be feasible and attractive, because electric resistance heating can heat firebrick thermal storage media to temperatures >1200°C. This type of thermal storage warrants further investigation because of its potential low cost and potential net efficiency >60%. In the first year of this project, we develop plans for a test facility to demonstrate air heating by molten salts at temperatures and pressures of interest for combine cycle power conversion. In subsequent years we will perform scaled separate-effect test experiments and modeling for air heating and thermal storage for RACC, and will work with Chinese partners to develop the test facility and demonstrate air heating in a closed-loop system. 

This quarter we launched the project, bringing on the first doctoral student Shane Gallagher (who has an undergraduate degree in Chemical Engineering from BYU and is fluent in Chinese).  A candidate facility for a molten salt air heating demonstration was identified in China, a new 370 kW molten salt simulator facility called the TMSR-SF0 being developed at the Chinese Academy of Sciences Shanghai Institute of Applied Physics.  The TMSR-SF0 is currently planned to use a conventional ambient air heat exchanger for heat removal; scoping studies have been initiated at UC Berkeley to identify design options for a pressurized, closed air loop system for demonstration of air heating at pressures and temperatures relevant to combined cycle power conversion and other industrial processes.

Project 1.3: Integrated gasification and natural gas hybrid fuel cell power plants

Scott Samuelsen, UC IrvineScott Samuelsen, UC Irvine
CV | Website

Context: Integrated solid oxide fuel cell (SOFC) gas turbine (GT) hybrid power plants can contribute to achieving air quality and climate goals. Higher efficiency of hybrids reduces heat rejection and makeup water to wet cooling towers typically used.

Objective: Develop integration schemes to fully realize the potential of hybrid SOFC/GT power plants in small to large scale applications with fossil and renewable bio fuels. 


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 Preheated air 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.

  • Subtask 1.3.1 Project planning and coordination with Chinese collaborators

Chinese collaborating researchers have been identified and plans have been established for coordinating SOFC based systems research efforts

  • Subtask 1.3.2 Key equipment design basis and models

The graduate student, Fabian Rosner under the guidance of Professor Scott Samuelsen and Dr. Ashok Rao has been familiarized with SOFC technology and its integration with the GT.  Selection of design constraints and basis of key equipment such as SOFC, reformer, gasifier, dry heat rejection systems have been made.  Development and validation of the models for equipment such as an ejector, reformer, SOFC, GT and flue gas direct contact cooler for water recovery have been completed.  Integration of these models into an overall natural gas fired SOFC/GT hybrid in the Aspen Plus® framework for process simulation has been initiated.

Project 1.4: Dry cooling for steam condensation

Yanbao Ma, UC MercedYanbao Ma, UC Merced
CV | Website

Context: Approximately 1% of thermoelectric power plants utilize air-cooled condensers due to much lower thermal efficiency compared with water-cooled condensers.

Objective: Develop enhanced modular air-cooled condensers (EMACC) to increase the air-side heat transfer coefficient, hair about 3X without net increase in pressure drop. 


To improve the hair of the ACCs by a factor of 3 requires a transformative heat sink design that can introduce flow disturbances to generate significant vorticity without large frictional losses as air flows cross the heat sink surfaces. The innovative design of the EMACC relies on the integration of four effective vortex generation mechanisms for hairenhancement: (a) corrugated fins; (b) a punched delta wing vortex generator array; (c) a V-shape winglet vortex generator array; and (d) dimpled surface design. The major challenge is to optimize the design with a large number of design parameters in conjugate heat transfer. To overcome this challenge, we will implement a new optimization methodology using a volume-averaging theory (VAT) hierarchical physical model and a genetic algorithm (GA) numerical optimizer.

We will achieve this objective through the following four tasks:

Subtask 1.4.1 Conduct CFD simulations for parametric studies of wavy fin with longitudinal vortex generators (LVGs) on both sides of the fin surface [Goal: demonstrate a 2X improvement in the hair compared with that of flat fins];

Subtask 1.4.2 Conduct targeted CFD simulations for design and assessment of dimpled surface [Goal: identify two most promising dimpled surface designs];

Subtask 1.4.3  Develop a design and optimization tool based on CFD-VAT [Goal: demonstrate the validity of the CFD-VAT design tool];

Subtask 1.4.4  Find the optimal design of a lab-scale EMACC with 3X improvement in the hair[Goal: demonstrate the 3X improvement goal is achieved]. 

Project 1.5: Synthesis of flexible, low-temp thermoelectrics and heat exchangers

Jeffrey Urban, LBNLJeffrey Urban, LBNL
CV | Website

Context: Our custom hybrid materials provide water-free cooling and also enable waste heat recovery for co-generation. We aim to eliminate high water consumption on spray cooled condensers. 

Objective: Prepare nanostructured phase-change "tapes" (PCT) that do water-free cooling, providing passive thermal management. Additionally, our materials perform energy co-generation via direct thermal to electrical energy conversion (DTEC), thus reducing water use and enhancing energy generations.


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.

We did not receive funding until late October 2016, so project work has been delayed due to lack of funding.  In the specified period, we have recruited students and postdocs for the CERC-WET project (vide supra). We have begun to list and identify chemicals and supplies necessary for the project and are ready to order when funding is approved. We are also in the process of gathering information and identifying potential Chinese partners from the list provided by our Chinese organizer. 

Project 1.6: Nanostructured surface enhancement of spray cooling water vaporization processes

Van Carey, UC BerkeleyVan Carey, UC Berkeley
CV | Website

Context: Inadequate heat transfer and high water consumption for water spray cooling of power plant air cooled condensers.

Objective: Develop scalable methods to create nanostructured superhydrophilic surface coatings on aluminum, and experimentally assess coating durability and effectiveness to enhance heat transfer and reduce water consumption for water spray cooling of power plant air cooled condensers.

 

 

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.]

The month of June was full of fabrication and testing of the hydrothermal synthesis of zinc oxide nanoparticles on aluminum substrates. The methodology for depositing and growing nano-structure was perfected by running consecutive lab tests to test out different growth times for the surface coating. The length of growth was hypothesized to change the morphology and wettability of the surface based on previous studies using the same hydrothermal synthesis method on copper surfaces. Lab tests confirmed the success of nanoparticle deposition on an aluminum substrate using a variety of growth lengths, annealing temperatures, and growth temperatures. This significant step is coupled with a five-fold increase in the inventory of the coated aluminum surfaces. The advantage of having a range of surfaces with different growth times is that specific analysis can be done comparing the wetting and wickability of the different surfaces in order to optimize the fabrication of the surfaces to maximize wettability.

Wettability tests were run using water on each of the hydrophilic coated aluminum surfaces. A high speed camera with up to 2000 frames per second was acquired by the lab to use to analyze the wicking and wettability of the coated surfaces. 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.

These tests were repeated throughout the months of June-September to confirm the consistency and durability of the surfaces. In cases where wettability decreased, a reheating of the surface worked to desorb the surface and the wetting of the surface was restored. By testing over a multiple month period, with continued extremely wetting characteristics, we are able to draw conclusions about the performance of these surfaces over time.

Concurrently with the water droplet testing, the surface was analyzed on a nano-scale. The UC Berkeley Marvell Nanofabrication Laboratory’s Zeiss SEM was used for visual analysis of surface morphology. By comparing the changes in morphology from the different growth times and relating it back to the observed wettability, we can identify key factors in the increased wetting of certain surfaces and certain fabrication techniques.

The high speed videos that were taken of water droplet spreading were analyzed using a Matlab image analysis code developed during the month of August in combination with video enhancement using Adobe Premier Pro. By identifying different regimes of wetting, we can better understand the mechanisms that lead to advanced wetting on the surfaces and exploit that knowledge to optimize further surface production.

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