Indirect Dry Cooling of Power Plants using Spray-Freezing of Phase Change Materials Hamidreza Shabgard, Han Hu, Md Mahamudur Rahman, Philipp Boettcher, Matthew McCarthy, Young Cho and Ying Sun

Department of Mechanical Engineering and Mechanics, Drexel University

Complex Fluids and

Multiphase Transport

& Multiscale

Thermofluidics Labs

Background

• Cooling of power plants account for 40% of total fresh water withdrawals in the US • Dependency of power plants to increasingly scarce water resources is not affordable • Novel cooling systems are to be developed for power plants

Closed-cycle cooling (Fig. b): Partial evaporation of recirculating water removes heat from the power plant. Water usage may not be sustainable at some locations

Motivation

Once-through cooling (Fig. a): intake structures withdraw water, which is run through power plant for cooling. Thermal discharges face increasing regulatory challenges

(b) Commonwealth Edison’s Byron Nuclear Plant, IL (http://commons.wikimedia.org/wiki/File:Byron_IL_Byron_Nuclear_Generating_Station_2.jpg)

(a) Encina Power Plant, CA (http://www.kpbs.org/news/2012/apr/19/power-plant-replace-encina-needed-future-reliabili/)

Array of Air Cooled Condensers (http://www.hudsonproducts.com/products/stacflo/tech.html)

Dry-air cooling • Uses essentially no water • Steam runs through large number of finned-tubes • Large fans are used to circulate air Up to 10% power production penalty Costly

Water-Based Cooling

Source: U.S. Energy Information Administration, Form EIA-860, Annual Electric Generator Report

Cost effective technology needed for reducing water usage for power plant cooling

Funding for this work was provided by the National Science Foundation (CBET-1357918) and The Electric Power Research Institute (EPRI).

Innovative Solution

Focus Areas of On-Going Research Slurry Side Thermal-Fluid Analysis

Experimental Work

Test rig: (a) Test section, (b) PCM reservoir, (c) control box

(a)

(b)

(c)

Outer Dimension:

2 m x 1.5 m x 0.6 m

Major components of the control system; (a) and (b) DAQ and control

hardware, (c) control software

(b)

(a)

(c)

t = 0.5 s t = 1.5 s t = 2.0 s t = 2.5 s

Parameter Real

System

Scaled-down sub

system

Power load, Ptotal 700 MW 5 kW

Heat flux, Jtotal 2,000 W/m2 2500 W/m2

Number of tubes, Ntube 345,000 36

Reynolds number, ReD 450-1,100 300-1,000

Test section dimensions (mm) - 199.31191244

Heat transfer coefficient, h 200 W/m2K 250 W/m2K

Total heat transfer area, At 350,000 m2 0.7 m2

Solid PCM volume fraction 0.1-0.4 0.1-0.4

PCM slurry flow rate - 0.168-0.569 L/s

A 5 kW test setup designed and manufactured Melting of PCM particles in slurry flow through heated tube bundle

Key design parameters of the large-scale and pilot-scale systems

dparticle = 6 mm, particle loading 2000/sec, Re = 1000

Theoretical Analysis

• Obtain insight on heat transfer between solid and liquid phases • Complementary tool for designs of slurry-side • Establish Nu correlations for PCM slurry flow with melting and settling

0

5

10

15

20

25

0.01 0.1 1 10 100

Re

t* = t/(d/Umax)

Gan et al. (2003)

Current simulation

temperature field for 50 particles with sedimentation

(6% solid fraction)

T

Wall Nusselt number and solid volume fraction for

28 particles with sedimentation (solid Vf = 6%)

Time variations of settling velocity of a

single particle with simultaneous melting

Combustion, Flammability, and Safety

Eicosane

280 μm particles

c = 0.1262 kg/m3

Experimental apparatus to study flammability

• Experimental and theoretical assessment of flammability risk

• Guarantee the safety and minimal environmental effects

5 mm PCM particles

• Millions of spherical particles required for the experiments

• A particle manufacturing unit is built for timely production of uniform spherical particles

PCM and Phase Change Characterization

Thermal Conductivity Measurement

Novel air-cooled power plant cooling tower/condenser based on

spray-freezing of recirculating phase change materials (PCMs)

20 30 40 50 600.0

0.2

0.4

0.6

0.8

1.0

Eff

ecti

ve

ther

mal

con

du

ctiv

ity, k ef

f (W

/mK

)

Temperature, TPCM

(oC)

Solid Liquid

0 1.5wt% 3.0wt%

Solid

Liquid

• Hot wire method; well established and accurate for low k material

1μm

Graphite nanoplatelets (GNP) from XG Science (25 μm dia., 15 nm thick)

Thermal conductivity enhancement of eicosane with various GNP loadings

Hot wire test rig

About 80% enhancement in keff is obtained with 3 wt% GNP loading

Wax Reservoir

Nozzle

Blower System

Micro pump

Vibration Damper

• Design and construction of spray-freezing PCM sub-system

• Spray characteristics of liquefied PCM

• Freezing characteristics of PCM spheres in air

• Design and construction of 5kW PCM slurry heat exchanger

• CFD analysis

Material Characterization Air Side

Slurry Side

• Thermal conductivity • Melting/solidification

Safety • Combustion and

flammability Solid-liquid PCM bath Steam/water tubes

Freezing PCM droplets

Settling solid PCM particles

Air inlet

Air outlet

Small-Scale Cylindrical Melting and

Freezing

Validation Solidification of eicosane in cylinders with

inner diameters of 14 mm and 6 mm

• Millimeter-scale melting and solidification

• Constant wall temperature • Center temperature

monitored • Pressure transducer to

track phase change fraction during process

Experimental Setup

Air Side Spray Freezing PCM Spray Characteristics

• Fluid delivery system for PCM spray nozzle

• Controlled PCM flow rate and temperature

• Controlled air flow rate

Experimental apparatus to study PCM spray characteristics

Freezing of PCM spheres in air

dsphere = 38 mm, freezing in wind tunnel, Thermocouples at the center and inner wall

About 25% reduction in solidification time for 1.5 wt% GNP

Tair = 23 °C

likelihood of ignition as a function of particle concentration

minimum concentration vs. particle size causing ignition

• Significant reduction in steam condensation temperature using environmentally benign PCM for > 8% production gain

• Improved air-side heat transfer coefficient by up to 4 times due to the use of sprayed droplets • Reduced primary steam tubing and pressure drop • Reduced system cost by 50% and size by 20%

Potential Advantages

(http://www.rubitherm.de/english/)

0

0.02

0.04

0.06

0.08

0

4

8

12

16

20

24

28

0 10 20 30 40

Solid

vo

lum

e f

ract

ion

Nu

wa

ll

t (s)

Slurry flow

Single phase

Solid VfVf

Modeling Approach: • Arbitrary Eulerian-Lagrangian method with deforming mesh • Simultaneous melting/settling of PCM particles