1. 1 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

2. 2  Degree Days z 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, t b is higher than the maximum daily temperature. z The base temperature in the US is 65 o F (15.5 o C for UK).

3. 3 Calculation of Degree Days z 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: z On any day for which t max >t b then the degree days for that day are taken as:

4. 4  Example of Degree Days Calculate the degree days for each of the three days shown below for the normal base temperature of 15.5 o C: (i) maximum temperature, 10 o C; minimum temperature, 2 o C; (ii) maximum temperature, 16 o C; minimum temperature, 10 o C; (iii) maximum temperature, 20 o C; minimum temperature, 14 o C.

5. 5 Example of Degree Days (continued)

6. 6 Degree Days for the Heating Season in Various Locations in the United States Richmond, VA3,720 Anchorage, AK10,789 Barrow, AK19,994 Fairbanks, AK14,158 Tucson, AR1,776 San Francisco, CA3,069 Los Angles, CA2,015 Denver, CO5,673 Washington, D.C.4,333 Miami, FL173 Boise, ID5,890 Chicago, IL6,310 Des Moines, IA7,681 Seattle, WA4,438 New Orleans, LA1,175 Portland, ME7,681 Boston, MA5,791 Detroit, MI6,404 Minneapolis, MN7,853 Kansas City, MR4,888 Great Falls, MT7,555 Reno, NV6,036 Newark, NJ5,252 Rochester, NY6,863 New York, NY5,050 Memphis, TN3,006 Dallas, TX2,272 Cheyenne, WY7,562

7. 7 Degree Days for the Midlands and South east England Month1986198820-year Average MidSEMidSEMidSE January386368325306379365 February482467316316347331 March335335284267315306 April296277235217244234 May142141134108162151 June776472638073 July403052434440 August786154434744 September15013288728779 October164130172132173160 November249227332304275267 December301300252259343334 Total270025322316213024962384

8. 8  Thermal Energy Consumption z 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 15.5 o C.

9. 9 z 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

10. 10  Example of Thermal Energy Consumption The 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. MonthGasCurrent 20-year consumptiondegreedaysaverage (therm)degree days September60507987 October5740149173 November8950239275 December8660298346 January11340467379 February11440368347 March10530346315 April7490240244 May6770189162

11. 11 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.

12. 12 Example (continued) (a) The base thermal load is 4005 therm.

13. 13 Example (continued)

14. 14 Example (conclusion)