7

- i CHAPTER 7 . ENERGY AND WATER USE ANALYSIS

TABLE OF CONTENTS

7.1 INTRODUCTION

................................ ................................ ................................ ....... 7

- 1

7.2 PER

- CYCLE ENERGY AND WAT ER CONSUMPTION BY EF FICIENCY

LEVEL

................................ ................................ ................................ ........................ 7

- 1

7.3 AVERAGE ANNUAL ENERG Y AND WATER CONSUMPT ION BY

EFFICIENCY LEVEL

................................ ................................ ................................ . 7

- 4

7.4 VARIABILITY OF DISHW ASHER USAGE

................................ ............................. 7

- 6

LIST OF TABLES

Table 7.2.1 Standard

- Sized Dishwashers: Annual Energy Use, Per

- Cycle Water Use, and Standby Power Use by Efficiency Level

................................ .................... 7

- 2

Table 7.2.2 Compact Dishwashers: Annual Energy Use, Per

- Cycle Water Use, and Standby Power Use by Efficiency Level

................................ ........................... 7

- 2

Table 7.2.3 Standard

- Sized Dishwashers: Per

- Cycle Energy Use by Efficiency Level

........ 7

- 3

Table 7.2.4 Compact Dishwashers: Per

- Cycle Energy Use by Efficiency Level

.................. 7

- 4

Table 7.3.1 Standard

- Sized Dishwashers: Annual Energy and Water Use by Efficiency

Leve l

................................ ................................ ................................ ................ 7

- 5

Table 7.3.2 Compact Dishwashers: Annual Energy and Water Use by Efficiency Level

..... 7

- 6

Table 7.4.1 RECS 2005 Dishwasher Usage Data

................................ ................................ 7

- 6

LIST OF FIGURES

Figure 7.4.2 Distribution of Dishwasher Annual Usage (cycles per year) based on 2005 RECS Usage Data

................................ ................................ ............................ 7

- 77-1 CHAPTER 7. ENERGY AND WATER USE ANALYSIS

7.1 INTRODUCTION

To carry out the life

- cycle cost (LCC) and payback period (PBP) calculations described

in c hapter 8, DOE needed to determine the savings in operating costs that consumers would

derive from more efficient products

. DOE used consumer energy and water use data, along with energy and water prices, to develop the most significant component of consume r operating costs in the LCC and PBP analyses . (Maintenance and repair costs are the other contributors to operating cost.) This chapter describes how DOE determined the annual energy and water consumption of residential dishwashers.

7.2 PER

- CYCLE ENERGY AND WATER CONSUMPTION BY EFFICIENCY

LEVEL

Dishwasher per

- cycle energy consumption has three components: energy used for heating water, operating the machine, and drying the dishes . The energy used to operate a machine

powers the motor ( to pump water and dispo se of food ) and booster water heater. The DOE test procedure provides the following equations to calculate the total per

- cycle energy consumption

of dishwashers :

D M WH DW CYCLE **+ +**  **=**

D M

e K T V DW CYCLE **+ +**  **× ×**  **=**

W here:

DW CYCLE = d ishwasher per

- cycle energy consumption,

V = w ater consumption in gallons per cycle,

T = n ominal water temperature rise = 70 oF (assuming a nominal inlet water temperature of 120 oF),

K = s pecific heat of water in kWh per gallon per degree Fahrenheit (0.0024), or Btus pe r gallon per degree Fahrenheit (8.2),

e = e fficiency of electric water heater (100 percent ) or gas water heater (75

percent ),

WH = V•T•K/e = w ater

- heating per

- cycle energy consumption,

M = m achine per

- cycle energy consumption, and

D = d rying per

- cy cle energy consumption.

The largest component of dishwasher energy consumption is water

- heating energy use.

Water

- heating energy use is directly dependent on water use.7-2 To develop per

- cycle energy use values, DOE used data developed by the engineeri ng analysis. As discussed in c hapter 5, DOE analyzed specific efficiency levels for standard

- sized and compact dishwashers. Table 7.2.1 and Table 7.2.2 provide the annual energy use

, per

- cycle

water use , and standby power for standard

- sized and compact dishwashers , respectively. Also

i ncluded in the tables is the standby power consumption corresponding to each efficiency level

.

Table 7.2.1 Standard

- Sized Dishwashers: Annual Energy Use, Per

- Cycle Water Use, and Standby Power Use by Efficiency Le vel

Level Annual Energy Use Water Use kWh/yr Standby Power gal/cyc W

Baseline 355 6.50 0

1 324 5.80 2.3

2 307 5.00 1.7

3 295 4.25 1.7

4 234 3.80 1.7

5 180 1.60 1.7

Table 7.2.2 Compact Dishwashers: Annual Energy Use, Per

- Cycle Water Use, and Standby Power Use by Efficiency Level

Level Annual Energy Use Water Use kWh/yr Standby Power gal/cyc W

Baseline 260 4.50 2.3

1 222 3.50 2.3

2 154 2.10 1.7

Based on the data in Table 7.2.1 and Table 7.2.2 , DOE derived per

- cycle energy use estimates using equations and assumption in the DOE test procedure.

DOE developed the per

- cycle dishwasher energy use by first subtracting from the total

annual dishwasher energy use the standby power energy use. The result is the annual dishwasher

energy use , i.e., the energy dedicated to dishwashing only

. The per

- cycle dishwasher energy use is simply the annual dishwasher energy use divided by the averag e

- use cycles per year. 1 Arthur D. Little (ADL) conducted a comprehensive analysis of dishwasher usage in 2001 that revealed that dishwashers are used, on average, 215 cycles per year. 2

The following equation from the DOE test procedure demonstrate s how the per

- cycle

dishwa sher energy use is determined. Below is the equation for total annual energy use:7-3 

## ( **)**

1000 _ 

### L

N H S N DW DW

m CYCLE ANNUAL 

### ×
**× +**  **× =** 

W here:

DW ANNUAL = t otal annual dishwasher energy consumption,

DW CYCLE = p er

- cycle dishwasher energy consumption,

N = r epresentative average dishwasher use of 215 cycles per year,

S

m = a verage standby power in Watts,

H = t otal number of hours per year = 8

, 766, and

L = a verage duration of dishwasher cycle.

Because both the total annual dishwasher energy use and stan dby power consumption are known, the per

- cycle dishwasher energy consumption simply is:



## ( **)**

N L N H S DW

DW m ANNUAL

CYCLE 

### 1000 _ 

### ×
**×**

**−**

**=**

Per

- cycle dishwasher energy use is disaggregated into two general categories: (1) water heating; and (2 ) machine (e.g., motor energy for pumpi ng) and dish drying from an electrical heating element. DOE determined the per

- cycle water heating consumption by assuming the use of an electric water heater and multiplying the per

- cycle water consumption by an assumed temperature rise of 70 oF (21 oC) a nd a specific heat of 0.0024 kWh/gal

- oF (4.186 joule/gram

- °C)

. D OE determined t he per

- cycle machine and drying energy by subtracting the per

- cycle water heating energy consumption from the per

- cycle dishwasher energy consumption. Table

7.2.3 and Table 7.2.4 show energy use and component s for standard

- sized and compact dishwashers, respectively

.

Table 7.2.3 Standard

- Sized Dishwashers: Per

- Cycle Energy Use by Efficiency Level

Level Energy Use Water Use kWh/yr Standby

Power gal/cyc W Per

- Cycle Energy Use Components

Total* Water Heating** gal/cyc Machine + Drying gal/cyc

Baseline gal/cyc

355 6.50 0 1.65 1.09 0.56

1 324 5.80 2.3 1.42 0.97 0.44

2 307 5.00 1.7 1.36 0.84 0.52

3 295 4.25 1.7 1.30 0.71 0.59

4 234 3.80 1.7 1.02 0.64 0.38

5 180 1.60 1.7 0.77 0.27 0.50

* Standby annual energy use based on an assum ed dishwasher cycle of one hour and 215 cycles per year; Standby hours = 8

, 766 hours

– 215 * 1 hour = 8

, 551 hours.

** Based on the use of an electric water heater at 100% efficiency.7-4 Table 7.2.4 Compact Dishwashers: Per

- Cycle Energy Use by Efficiency Level

Level Energy Use Water Use kWh/yr Standby

Power gal/cyc W Per

- Cycle Energy Use Components

Total* Water Heating** gal/cyc Machine + Drying gal/cyc

Baseline gal/cyc

260 4.50 0.0 1.12 0.76 0.36

1 222 3.50 2.3 0.94 0.59 0.35

2 154 2.10 1.7 0.65 0.35 0.30

* Standby annual energy use based on an assum ed dishwasher cycle of one hour and 215 cycles per year; Standby hours = 8

, 766 hours

– 215 * 1 hour = 8

, 551 hours.

* * Based on the use of an electric water heater at 100% efficiency.

7.3 AVERAGE ANNUAL ENERG Y AND WATER CONSUMPT ION BY

EFFICIENCY LEVEL

DOE determined the average annual energy and water consumption by multiplying the

per

- cycle energy and water consumption by the number of cycles per year.

In 2003, DOE revised its test procedure for dishwashers to more accurately establish their

effici ency and energy and water use. The 2003 test procedure amendments included a reduction

in th e average

- use cycles per year. As discussed above, ADL conducted a comprehe nsive analysis of dishwasher usage in 2001 in 26,000 households that revealed that dishwashers are used, on average, 215 cycles per year. 2 The 200 5 RECS provides data indicating the annual usage of households with dishwashers. Of the almost 4400 households in RECS, 248 0 have

dishwashers. The average

- use value for dishwashers in the 2005 RECS households is 174 cycles

per year. But b ecause the ADL survey is a much more comprehensive and larger survey than the survey performed for RECS, DOE chose an average usage of 215 cycles per year as the most representative value for average dishwasher use.

DOE calculated dishwasher annual ene rgy consumption from the per

- cycle values reported in Table 7.2.3 and Table 7.2.4 by multiplying them by the average

- use cycles as shown

below with the following equation s :



### N WH DW ANN WH 

### × **=**

**−**



### N M DW ANN MACH 

### × **=**

**−**



### N D DW ANN DRY 

### × **=**

**−**7-5 W here:

DW WH

- A NN = t otal annual dishwasher energy consumption,

DW MACH

- ANN = t otal per

- cycle dishwasher energy consumption, and

DW DRY

- ANN = t otal per

- cycl e dishwasher energy consumption.

DOE calculated dishwasher annual water consumption with the following equation:



### N DW DW CYC WATER ANN WATER 

### × **=**

**− −**  W here:

DW W

ATER

- ANN = t otal annual dishwasher water consumption, and

DW WATER

- CYC = t otal per

- cycle dishwasher water con sumption.

The annual energy and water consumption shown in Table 7.3.1 for standard dishwashers

and in Table 7.3.2 for compact dishwashers reflect an annual usage of 215 cycles per year. T he

annual water

- heating energy consumption reflects the use of an electric, gas, or oil water heater.

Table 7.3.1 Standard

- Sized Dishwashers: Annual Energy and Water Use by Efficiency

Level

Efficiency

Level Annual Energy Use Annual

Water

Use Total gal/y r Water Heating*

kWh/y r Machine + Drying + Standby kWh/y r Electric kWh/y r Gas kWh/y r Oil MMBtu/y r

Baseline MMBtu/y r

355.0 234.8 1.068 1.068 120.2 0.0 1

, 398

1 324.0 209.5 0.953 0.953 94.8 19.7 1

, 247

2 307.0 180.6 0.822 0.822 111.9 14.5 1

, 075

3 295.0 153.5 0.698 0.698 127.0 14.5 914

4 234.0 137.3 0.624 0.624 82.2 14.5 817

5 180.0 57.8 0.263 0.263 107.7 14.5 344

* Electric, gas, and oil water heating based on water heater efficiencies: 100% for electric, 75 % for gas, 75 % for

oil.7-6 Table 7.3.2 Compact Dishwashers: Annual Energy and Water Use by Efficiency Level

Efficiency

Level Annual Energy Use Annual

Water

Use Total gal/yr Water Heating*

kWh/yr Machine + Drying + Standby kWh/yr Electric kWh/yr Gas kWh/yr Oil MMBtu/yr

Baseline MMBtu/yr

260.0 162.5 0.739 0.739 77.8 19.7 968

1 222.0 126.4 0.575 0.575 75.9 19.7 753

2 154.0 75.9 0.345 0.345 63.6 14.5 452

* Electric, gas, and oil water heating based on water heater efficiencies: 100% for electric, 75% for gas, 75 % for

oil.

7.4 VARIABILITY OF DISHWASHER USAGE

The 2005 RECS indicates that 2,480 of the 4, 382 households in the survey use

dishwashers. For each household using a dishwasher, RECS provides data on the number of dishwasher cycles in the following bins: (1) less than once per week, (2) once per week, (3) 2

– 3 times per week, (4) 4

- 6 times per week, (5) at least once per day. DOE converted the above to annual values and created a triangular or uniform distribution for each bin. Table 7.4.1 shows the

share of households in each bin and the distribution us ed. DOE randomly assigned a specific numerical value from within the appropriate bin to each household in the dishwasher sample. The average number of cycles per year derived from the RECS 2005 data is 174.

Table 7.4.1 RECS 2005 Dishwasher Usage Data

Bin Share of RE CS Households Distribution Used Percentage * % Count
Less than once per week 17 402 Triangular, 1 to 52 Once per week 14 325 Uniform, 26 to 78

2

– 3 times per week 34 834 Uniform, 78 to 182

4

– 6 times per week 18 464 Uniform, 182 to 338

At least once per day 17 455 Triangular, 300/400/500
*Percentages represent weighted values.

To determine the variability of dishwasher usage, DOE normalized the household usage values from RECS so that the average

- use value equaled 215 cycles per year rather than the weighted

- average value of 1 74 cycles per year as indicated by RECS. DOE determined the usage for each RECS household with a dishwasher based on the following equation:

AVG RECS AVG DOE

HH DW DW 

### N N N N

_ _

_ 

### × **=**7-7 W here:

N

DW = m odified dishwasher usage for specific RECS household,

N

DW_HH = d ishwasher usage for specific RECS household as specified by RECS,

N

DOE_AVG = a verage dishwasher usage of 215 cycles per year as established in the DOE test procedure, and

N

RECS_AVG = a verage dishwasher usage of 174 cycles per year as established by RECS.

With the normalized dishwasher usage known for each RECS household, DOE determined the corresponding annual energy and water consumption. For all RECS households, dishwasher use frequency varies from 1 to over 6 00 cycles per year. Figure 7.4.2 shows the probability distribution of the modified dishwasher usage that DOE determined for each RECS

household.

Figure 7.4.2 Distribution of Dishwasher Annual Usage (cycles per year) B ased

on 2005 RECS Usage Data7-8 REFERENCES

1 U.S. Department of Energy

– Office of Energy Efficiency and Renewable Energy. Energy Conservation Program for Consumer Products: Test Procedure for Dishwashers

, Final Rule. Federal Register . August 29, 2003. Vol. 68, no. 168: pp. 51887

– 51903.

2 Arthur D. Little. Review of Survey Data to Support Revisions to DOE’s Dishwasher Test

Procedure , December 18, 2001. Prepared for the U.S. Department of E nergy by Arthur D. Little: Cambridge, MA.