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ISAT 413 - Module III: Building Energy Efficiency

Topic 8:

Thermal Performance Monitoring

Degree Days

Example of Degree days

Thermal Energy Consumption

Example of Thermal Energy Consumption

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Degree Days

One convenient way of comparing fuel heating costs throughout the year and from year to year is by defining the weather conditions by degree days. This is defined as the daily difference in temperature between a base temperature and the 24-hour mean outside temperature when the base temperature, tb is higher than the maximum daily temperature.

The base temperature in the US is 65oF (15.5oC for UK).

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Calculation of Degree Days

The 24-hour mean outside temperature is taken to be the average between the maximum and minimum outside temperatures. Hence the number of degree days in any one month is given by:

On any day for which tmax>tb then the degree days for that day are taken as:

minmaxb tttD

2

1

minbminmaxb

bmaxminbminmaxb

ttDttt

ttttDttt

4

1

2

1 when (ii)

4

1

2

1

2

1 when (i)

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Example of Degree Days

Calculate the degree days for each of the three days shown below for the normal base temperature of 15.5oC: (i) maximum temperature, 10oC; minimum temperature, 2oC; (ii) maximum temperature, 16oC; minimum temperature, 10oC; (iii) maximum temperature, 20oC; minimum temperature, 14oC.

5

3750145154

1

Therefore .2

1 and , case, In this (iii)

8752515164

110515

2

1

Therefore .2

1 and , case, In this (ii)

592102

1515

Therefore . case, In this (i)

..daysDegree

ttttt

...daysDegree

ttttt

..daysDegree

tt

minmaxbbmax

minmaxbbmax

bmax

Example of Degree Days (continued)

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Degree Days for the Heating Season in Various Locations in the United States

Richmond, VA 3,720Anchorage, AK 10,789Barrow, AK 19,994Fairbanks, AK 14,158Tucson, AR 1,776San Francisco, CA3,069Los Angles, CA 2,015Denver, CO 5,673Washington, D.C.4,333Miami, FL 173Boise, ID 5,890Chicago, IL 6,310Des Moines, IA 7,681Seattle, WA4,438

New Orleans, LA 1,175Portland, ME7,681Boston, MA 5,791Detroit, MI 6,404Minneapolis, MN 7,853Kansas City, MR 4,888Great Falls, MT 7,555Reno, NV 6,036Newark, NJ 5,252Rochester, NY 6,863New York, NY 5,050Memphis, TN3,006Dallas, TX 2,272Cheyenne, WY 7,562

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Degree Days for the Midlands and South east England

Month 1986 1988 20-year AverageMid SE Mid SE Mid SE

January 386 368 325 306 379 365February 482 467 316 316 347 331March 335 335 284 267 315 306April 296 277 235 217 244 234May 142 141 134 108 162 151June 77 64 72 63 80 73July 40 30 52 43 44 40August 78 61 54 43 47 44September 150 132 88 72 87 79October 164 130 172 132 173 160November 249 227 332 304 275 267December 301 300 252 259 343 334

Total 2700 2532 2316 2130 2496 2384

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Thermal Energy Consumption

A typical well-run building has a space-heating load which varies with the outside temperature on a controlled basis; in addition the building has a hot water load and perhaps process thermal energy demands all of which remain substantially constant over the year.

Sloping line cuts the base load line at 15.5oC.

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When a complete heating season has been plotted in this way then any subsequent heating can be compared with the graph. Comparison will show whether changes to the building (e.g. new control or improved insulation), have produced a true reduction in fuel consumption.

Fuel Consumption as the function Degree Days

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Example of Thermal Energy ConsumptionThe monthly gas consumption figures for a given year for a building are as shown in the table below with the degree days corresponding to the heating season for the region in which the building is sited.Month Gas Current 20-year

consumption degree daysaverage (therm)degree days

September 6050 79 87October 5740 149 173November 8950 239 275December 8660 298 346January 11340 467 379February 11440 368 347March 10530 346 315April 7490 240 244May 6770 189 162

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Example (continued)

(a) Plot gas consumption against degree days and hence estimate the base thermal load.

(b) For the building described, the fabric heat loss is found to be 75% of the total heat loss with the ventilation loss 25% of the total heat loss. It is proposed to upgrade the insulation of the building with all other factors unchanged and it is estimated that this measure will reduce the fabric heat loss by 10%. Calculate the expected annual cash saving by introducing this measure and plot the new performance line on the graph; take a gas price of 37 p/therm.

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Example (continued)

(a) The base thermal load is 4005 therm.

y = 16.26x + 4004.8

R2 = 1

y = 17.233x + 4004.8

R2 = 0.8769

0

2000

4000

6000

8000

10000

12000

14000

0 200 400 600Degree days

Ga

s c

on

su

mp

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n (

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Example (continued)

)(1136100/373069.4 savingcash annual

i.e.,

therm 3069.4 0.1409250.75 savinggas annual

by given is 10%by loss fabric the reducingby gas in savingthe

hence and total this of 75% is loss fabric the for used gas The

409254005976970

by given is heating- spacefor used gas the i.e.

period, month-nine the for 40059by given is etc. heating

waterhot for used gas The 2375. of seasonheating the in days

degree total for therm 76970 if nconsumptio gas total The (b)

poundL

therm

)(

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Example (conclusion)

#12. slideon Figure in the

shown as plotted becan then line eperformanc new The

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theuse tonecessary isit line eperformanc new thefind To

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slope