ECBM CASE STUDY

The good way to understand the ECBM calculation as it relates to a LEED Credit EA-1 building is provided in this example.

A new two story office building located in St. Louis with 16,900 sq ft per floor (total of 33,800 sq ft) was modeled using a commercially available energy simulation program.¹ The occupancy is 100 sq ft per person. A common variable-air-volume rooftop air-handling unit with zone reheat was the selected HVAC system.

The budget model inputs:

• Glazing: U = 0.57 Btu/hr ft² F; solar heat gain coefficient (SHGC) = 0.39.

• Walls: U = 0.077 Btu/hr ft² F (R-13).

• Roof: U = 0.053 Btu/hr ft² F (R-19).

• Lighting: 1.3 w per square foot.

• Miscellaneous power: 0.75 w per square foot.

• Air-cooled rooftop unit (RTU) with gas heating and 30 percent minimum zone airflow.

• Gas-fired heating: 80-percent efficient.

When the utility costs were calculated, the annual energy costs were found to be $23,112 for electricity and $1,172 for natural gas (for a total of $24,284). This then became the base case (BBD), against which the annual energy cost savings were compared for improved energy performance (PDM).

One of the first conclusions evident from the BBD was that the heating cost was only about 7 percent of the total energy cost. Therefore, the heating was not going to achieve target 15 percent total annual energy cost savings goal for one point in Credit EA-1, even reduced to zero cost. Therefore, the key to this endeavor was to reduce the cooling costs.

It is a good idea to start with the most straightforward energy reduction opportunities that will maximize energy savings. This includes changes to lighting, which have a doubling effect because electrical energy is used to both power the lights and to run the air conditioning, which cools the heat that is shed by that same light.

It is also important to focus on the building envelope. For this example, the lighting was revised to 1.2 w per square foot, and the insulation values of the walls, roof, and glazing, were improved to R-19, R-30, and U = 0.34 Btu/hr ft² F, respectively. The SHGC was also reduced to a 0.32 value with an actual glazing selection.

These modifications resulted in an 8 percent reduction in the annual energy cost for the building.

HVAC

The first option to consider when optimizing the HVAC system in this case study should be a water-cooled chiller and the associated air-handling unit. Water-cooled chillers use significantly less kilowatts per ton when compared to air-cooled chillers. However, this turned out not to be the optimum approach, as our model indicated that this system needed 6-percent more energy dollars than the base, which totals 14 percent in the wrong direction. So there is a good reason why this building type (mid-size office building) in this climate (St. Louis) has more air-cooled equipment than water-cooled chillers. (Note that LEED EMP does allow the use of a water-cooled system in lieu of an air-cooled system in the budget model for systems less than 150 tons.)

Additional HVAC system options were then considered. These included improving the motor efficiencies by going to premium efficiency electric motors in lieu of the high efficiency motors. Next, friction losses in the ductwork were reduced to require only 3-in. static pressure for the supply-air fan in lieu of 4 in. in the base model. A water-cooled RTU selection was considered. This actually reduced the compressor size because the needed performance could be obtained with the next size smaller, also reducing the first cost impact to less than a 2-year payback. These modifications resulted in a 22.4 percent annual energy savings (as compared to the base) and two points in Credit EA-1.

The goal of the project team was to earn three points for Credit EA-1 so additional options were then considered. Standard 90.1 does not require economizer in the St. Louis climate, but it was worthwhile to determine if economizers would help in this instance. The energy simulation model was run again, making this revision and the result indicated a total annual energy cost of $18,341 (26.3 percent savings). The three points for Credit EA-1 were now possible by using economizers.

The proposed model inputs (final):

• Glazing: U = 0.34 Btu/hr ft² F; SHGC = 0.32.

• Walls: U = 0.053 Btu/hr ft² F (R-19).

• Roof: U = 0.033 Btu/hr ft² F (R-30).

• Lighting: 1.2 w per square foot.

• Miscellaneous power: 0.75 w per square foot.

• Water-cooled RTU with gas heating and 30 percent minimum zone airflow.

• Gas-fired heating: 80 percent efficient.

• Air-side economizer.

SUMMARY

The conclusion that is evident from this example is that to effectively improve the energy performance of a building, the engineer should always begin with the basic parameters of the energy-consuming systems utilized. The first costs involved do not have to be major. Look for the low-hanging fruit obtained by the progressive use of basic engineering principles, rather than searching first for more exotic solutions that will likely not be cost beneficial to the owner. This approach will also be helpful in identifying the specific impact of a particular component and clearly define the cost-benefit relationship of each.

ACKNOWLEDGEMENTS

The author wishes to thank Scott Hunke of Trane in St. Louis for his assistance in the modeling and equipment selections for this case study. He also wishes to thank Jordan Heiman, Al Black, both former members of the Standard 90.1 committee, and Jerry Williams of McClure Engineering for their time and input.

REFERENCES

1. Trane Trace 700

For HPAC Engineering feature articles dating back to January 1992, visit www.hpac.com.

Peter W. McDonnell, PE, is a senior mechanical engineer for McClure Engineering Associates in St. Louis. He is a LEED Accredited Professional and a past adjunct professor of construction engineering at the University of Central Florida. He has spoken several times on ANSI/ASHRAE/IESNA Standard 90.1, including at HPAC Engineering's Engineering Green Buildings Conference in July 2004. He can be reached at pmcdonnell@mcclureeng.com.

U.S. Commercial Energy Codes

On July 15, 2002, the U.S .Department of Energy (DOE), under the directive of the Energy Conservation and Production Act (ECPA), issued a determination that ANSI/ASHRAE/IESNA Standard 90.1-1999 would save more energy in commercial buildings than the 1989 edition of the standard. “As a result of this positive determination regarding Standard 90.1-1999, each state is required to certify that it has reviewed and updated the provisions of its commercial building code regarding energy efficiency to meet or exceed Standard 90.1-1999.”

The DOE gave the states two years (until July 15, 2004) to submit certification of compliance. The graphic below shows the status of each state's commercial energy code adoption process as of mid-November 2004. These mandatory statewide codes include all state-owned buildings.

This graphic is courtesy of the Building Codes Assistance Project (BCAP). BCAP is a not-for-profit organization working to save energy in homes and buildings by promoting the adoption and implementation of building energy codes. BCAP provides free assistance to states and municipalities in their efforts to adopt and implement both commercial and residential energy codes. BCAP also functions as a clearinghouse of information about building energy codes and other measures for saving energy in homes.

Notes: 1) Code implementation depends upon voluntary adoption by local jurisdictions. 2) 90.1 is mandatory for state-owned buildings.

This map should be used in conjunction with BCAP's Status of the State newsletter. Go to www.bcap-energy.org to view the current issue.

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