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Southwest Energy Efficiency Project Southwest Energy Efficiency Project

Thomas O. Price Service Center

Tucson, Arizona

The City of Tucson has a demonstrated commitment to the promotion of sustainable development practices, and is considered a leader in the area of energy efficiency. This case study will highlight a city effort - the retrofit of an existing municipal building - to practice and promote energy efficiency design principles and technologies in this desert community of nearly 500,000 people. The city's Department of Operations takes the lead in the design of city facilities, and played this same role in bringing this project to fruition. For additional information about the City of Tucson's energy efficiency programs, visit www.ci.tucson.az.us.

The Thomas O. Price Service Center Building One project was a retrofit of an existing 23,400 sq. ft. office building. This one story building houses 80 full-time employees in various City administration roles.

Energy efficiency upgrades on the building began in 1995 with a lighting retrofit. The upgrade cost was $31,300, with an annual avoided electricity cost of $5,700, or a simple payback period of 5.5 years. In May 1999, occupancy sensors were installed throughout the building at a cost of $3,000, with an estimated annual avoided electricity cost of $375, or a simple payback period of 8.0 years. By May 2001, several additional efficiency projects had been completed at a cost of $128,200, including the addition of an energy management and control system and the changeover of a constant volume dual duct air handling system to a variable air volume system. With an estimated avoided electricity cost of $22,400 per year, the simple payback period of these efforts is 5.7 years.

A "cool roof" was added to Building One in June 2001, as part of Tucson's Creating Cool Communities program, an effort to develop mitigation strategies for offsetting the urban heat island effect. Urban heat islands are a phenomenon affecting large urban centers. They occur when urbanized areas accumulate greater amounts of heat during the day than their rural surroundings, sometimes reaching temperatures six to eight degrees hotter. Cities also retain the increased temperatures throughout the night. Dark, heat absorbing surfaces such as building roofs and parking lots contribute to this effect, causing an increase in energy usage, energy costs, air pollution, and greenhouse gas emissions. Building One was chosen as a cool roof demonstration project because of its general setup and ease of monitoring due to its ongoing participation in the U.S. EPA's Energy Star Buildings program.

White Roof in Tucson

Building One's roof is about 28,000 sq. ft. in area, including flashings. The vast majority of it is surfaced with a sturdy aluminum foil over its low-slope, and a copper foil that has turned black with time over the more steeply sloped sections. Although somewhat reflective, bare metallic surfaces have low emissivity and therefore retain high levels of heat. A white elastomeric top coating was applied to the roof in June 2001.

Two methods were used to evaluate the energy savings listed below: First, by looking directly at "raw" data from a week before and a week after the cool roof was applied; and second, by adjusting this raw data to account for weather and occupancy variations.

Table 1 compares pre-installation (May 7 - 13, 2001) to post-installation (July 23 - 29, 2001) peak and average temperatures. These weeks experienced similar weather conditions, and provide a good opportunity for comparison. The cool roof reduces the black portion of the roof's peak temperature by more than 70 degrees Fahrenheit, and the metal portion's temperature is reduced by over 40 degrees Fahrenheit.

Table 1. Hot and Cool Roof Temperature Comparisons

  Hot Roof Surface (Regressed values) Cool Roof Surface (Actual values) Temperature Reduction
Peak Temperatures      
Black Roof 198°F 120°F 78°F
Metal Roof 165°F 119°F 46°F
Avg. Temperatures      
Black Roof 102°F 85°F 17°F
Metal Roof 95°F 85°F 10°F

Table 2 provides estimates of the energy savings due to the cool roof.

Table 2. Cooling Energy Savings Due to Cool Roof

Weekdays Only Energy Use (MBtu)
Hot roof cooling energy use, regressed values (May 7 - 13, 2001) 37.8
Cool roof cooling energy use, actual values (July 23 - 29, 2001) 19.4
Savings 18.4
% savings 48.7%

The reductions in energy consumption in the building due to the cool roof are quite outstanding. Nearly 50% cooling energy usage reduction was seen after installation. This result is even more impressive when taking into consideration that Building One had been retrofitted with modern energy efficient lighting, mechanical systems and building controls prior to the roof retrofit.

Avoided energy usage can be estimated at over 400 million BTUs annually. This translates into an avoided energy cost of nearly $4,000 annually. The cost of the new roof coating was $24,993; therefore, the simple payback period is just over six years, and the rate of return is 16%. The payback is moderately long in part because the building was relatively efficient prior to the roof retrofit (see Figure 1). In addition to the monetary savings, the reflective roof coating eliminated several roof leaks and extended the life of the existing roof.

Figure 1 shows the average monthly electricity use of the building from 1990 through 2001. As a result of the series of efficiency retrofits described above, including the cool roof, total electricity use and cost declined from around 432,000 kWh and $38,600 per year in the early 1990s to about 162,000kWh and $16,100 per year in 2001, a reduction of more than 55 percent. During this time period the building saw a substantial increase in occupants and office equipment, most notably personal computers, making the savings even more impressive. Note that the savings shown in Figure 1 only include the direct building electric usage, as the heating hot water and cooling chilled water come from an adjacent central plant. The utility bills for the central plant have also decreased by over $23,000 annually since 1998.

Total energy efficiency upgrades performed on Building One cost $187,500, and produce over $40,000 in annual energy cost savings, well above the $32,500 estimate. Overall payback on the entire menu of energy efficiency projects is therefore 4.7 years. Pollution prevention is estimated at nearly 350 tons of CO2 equivalent annually.

Figure 1. Thomas O. Price Service Center Building One, Average Monthly Electricity Use (kWh)