AN EMPIRICAL STUDY OF ENERGY EFFICIENCY OF CLOTHES DRYERS.

AN EMPIRICAL STUDY OF ENERGY EFFICIENCY OF

CLOTHES DRYERS

Goals of the Research

• Assess the overall environmental impact of a clothes dryer

• Evaluate a “real” efficiency value for clothes dryers• Find the most significant factors contributing to the

efficiency• Note the effect of different variables• Suggest a means for consumers to reach a “best

efficiency” setting• Discover ways in which to improve the efficiency of a

dryer• Suggest ways to improve the safety of the dryer

Impact of a Dryer

• On average, a dryer uses 1079kWh per year• Total consumption of approximately 66B kWh

each year in the U.S. alone– or ~7 x 1GW power plants– Emitting ~ 70M metric tons of CO2

– Equivalent to 1M cars driven 12,000 miles

• In 1998, dryers caused:– 15,600 fires– 20 deaths, 370 injuries– $75.5M in direct property damages

Experimental Setup

• Monitoring and datalogging of: input/output temperature, input/output humidity, air velocity, power consumption

Experimental Procedure

• dry weight of clothes was measured. • clothes were soaked in water then wrung out to

simulate the spin cycle of a conventional clothes washer.

• damp clothes were weighed, and the amount of water in the clothes was calculated.

• damp clothes were placed in the dryer. • The data logging equipment was initialized and the

dryer was started. • The dryer was stopped when the absolute humidity of

the exhaust air reached ambient absolute humidity. • The clothes were weighed again to determine if any

noticeable moisture was left.

Calculations

• From raw data, we can calculate useful values– Estimated dryer

efficiency (based on latent heat)

– Mass flow rate of evaporated water at any single time

7.5 2048.625

35.851 2

( )10

x

x

ToTx

atm xxi

VAk P k H

T

dryer

Heat of vapourization

Electrical energy

Sample Data ResultsAnalysis Unit 1,125 kg 2,22kg 2,97kg 4.34kg

Drying time min 37.5 55.5 68.5 84

Total electrical power used kJ 4080.9 6005.8 7343.2 8569.2

Actual mass of water evaporated kg 0.56 1.15 1.55 2.175

Energy needed to evaporate water kJ 1347.78 2780.56 3751.94 5281.86

Actual dryer efficiency 33% 46% 51% 62%

Cumulative mass air flow water evaporated kg 0.33 0.84 1.24 2.18

Energy needed to evaporate water kJ 792.13 2046.02 3014.13 5296.55

Computed dryer efficiency 19% 34% 41% 62%

Error % 41% 26% 20% 0.3%

Observations

• For loads with 0.56, 1.15, 1.55, and 2.18 kg of water in the clothes

• there is a correlation between the weight of the dryer load and:– drying time (37.5, 55.5, 68.5, 84.0 min) – maximum rate of water evaporated (0.21, 0.35, 0.43,

0.59 kg/sec) – Efficiency (19, 33, 39, 62%)

• ...and an inverse correlation with:– average in/out temp increase (16.7, 13.6, 12.0, 10.1

C) – max in/out temp increase (21.3, 19.9, 19.2, 19.0 C)– Computed error (41, 26, 20, 0.3%)

Plot of Experimental Data

Plot Explained

Sources of Error? 1

• To explain higher error rates in low mass tests, calculations redone via psychrometric charts– 15C @ 30% RH: chart=0.174 g/s, formula=0.179– 20C @ 30% RH: chart=0.232 g/s, formula=0.241– 20C @ 90% RH: chart=0.717 g/s, formula=0.722– 30C @ 50% RH: chart=0.704 g/s, formula=0.704– 35C @ 30% RH: chart=0.551 g/s, formula=0.550 – 40C @ 10% RH: chart=0.232 g/s, formula=0.237– 45C @ 10% RH: chart=0.305 g/s, formula=0.303

• Difference less than 4%, therefore formula not high source of error

Sources of Error? 2

• Formula uses density of “dry air” instead of density of the moist air exiting dryer– Density of air changes as moisture content

changes– Taking highest value of moisture content from

data, density of moist air was calculated– Moist air was 0.9% less dense than dry air– Using density of dry air in calculations not a

significant source of error

Conclusions

• Good preliminary results, but some notable error

• More tests to be conducted to determine impact of more variables

• More testing to determine sources of high error

• Wish to find a better approximation for the absolute energy needed to evaporate water form a medium

Thank you for your attention!

Questions?