Question 10.4: Consider an open office space with 150 m2 located in Denver,......
Consider an open office space with 150 m2 located in Denver, CO, with a total window area of 20 m², with double-pane glazing, which has a visible transmittance of 0.55. The office space has 10 fluorescent fixtures, each rated at 52 \ W. Determine the energy use and cost savings due to daylighting harvesting strategies. Assume that without daylighting harvesting, each fixture is operated 8 \ h/day when daylighting can be harvested, 5 \ days/week, 250 \ weeks/year. The cost of electricity is 0.10/kWh.
TABLE 10.5
Coefficients a and b of Equation 10.4 for Various Locations throughout the World
| b | a | Location | b | a | Location |
| 72.7 | 19.2 | Casper | 74.3 | 19.6 | Atlanta |
| 70.9 | 17.8 | Portland | 71.7 | 18.4 | Chicago |
| 69.8 | 18.8 | Montreal | 72.9 | 19.4 | Denver |
| 70.6 | 19.1 | Quebec | 74.8 | 22.3 | Phoenix |
| 68.7 | 16.9 | Vancouver | 67.0 | 18.7 | New York City |
| 70.5 | 20.0 | Regina | 70.8 | 18.7 | Washington, DC |
| 70.5 | 19.3 | Toronto | 67.1 | 18.7 | Boston |
| 70.9 | 19.6 | Winnipeg | 74.8 | 25.1 | Miami |
| 67.3 | 19.4 | Shanghai | 74.0 | 20.6 | San Francisco |
| 72.4 | 20.2 | K-Lumpur | 69.2 | 16.6 | Seattle |
| 73.7 | 23.3 | Singapore | 74.2 | 22 | Los Angeles |
| 74.2 | 27.0 | Cairo | 70.0 | 18.8 | Madison |
| 74.7 | 36.9 | Alexandria | 74.7 | 21.6 | Houston |
| 74.1 | 25.2 | Tunis | 72.9 | 19.7 | Fort Worth |
| 71.2 | 29.4 | Sao Paulo | 70.7 | 17.9 | Bangor |
| 73.6 | 28.6 | Mexico | 72.6 | 18.8 | Dodge City |
| 67.7 | 20 | Melbourne | 70.4 | 20 | Nashville |
| 72.4 | 16.0 | Roma | 74.4 | 20.2 | Oklahoma City |
| 69.7 | 15.2 | Frankfurt | 72.3 | 18.6 | Columbus |
| 65.3 | 22.0 | Kuwait | 71.5 | 17.9 | Bismarck |
| 72.7 | 21.2 | Riyadh | 72.0 | 18.2 | Minneapolis |
| 72.30 | 18.9 | Omaha |
Based on Equation 10.4, using the coefficients a = 19.4 and b = 72.9, the percent reduction in electrical lighting associated with daylighting harvesting is
f_d=b\left[1-\exp \left\lgroup\frac{-a\tau _wA_w}{A_f} \right\rgroup \right] (10.4)
f_d=72.9*\left[1-\exp \left\lgroup-19.4*0.55*\frac{20}{150} \right\rgroup \right]=55.3\%Therefore, the savings in annual electrical energy use for the office space are computed as follows:
\Delta kWh=55.3\%*10*52*\frac{1}{1000}*8*250=2876 \ kWh/yearThus, the annual energy use cost saving is $287.6/year that can justify the installation of one or two photosensors for implementing daylighting control in the open office space since one sensor costs about $250.