Pumps & Systems, May 2008

Under an Energy Conservation Performance Program, Miami-Dade County commissioned a study to evaluate potential capital improvement projects fully funded from the projected savings in energy consumption and/or operating and maintenance costs. Through this program, the cost of new equipment is financed through energy and/or other realized operational cost savings. The capital funding otherwise required to replace existing mechanical systems is avoided, and those funds can then become available for other County needs.

The proposed efficiency improvements to the County's existing chilled water loop were roughly placed in two groupings:

  1. Measures which reduce the energy consumption at the building level associated with chilled water distribution (pumping energy)
  2. Measures that address energy consumed at the Central Plant, associated with the production of the primary cooling medium (chilled water) 

Schemes considered to reduce building water-side energy consumption included adding variable frequency drives to existing building pumps, and replacing three-way valves with two-way valves, where applicable. Finally, measures were explored to reduce Central Plant energy and conversion of the chilled water distribution system to a variable volume primary distribution system.

FPL Services, LLC has an energy performance contract with the County to assist in reducing energy costs. As part of the overall energy saving strategy, FPL Services examined the application of variable speed pumping for the Downtown Miami District Cooling Plant. The Central Cooling Plant provides chilled water to approximately nine office buildings that encompass more than 1.8 million square feet of conditioned space in downtown Miami. The plant has a pumping capacity of 11,200-gpm, which provides 4,700 tons of chilled water to the buildings. Because of Miami's subtropical climate, the cooling plant operates year round.

Pumping, a large consumer of electrical consumption, provided great potential for energy savings. Reducing energy costs with this application would finance the construction of these energy conservation improvements and allow for infrastructure improvements. An extension of the chilled water piping to serve a new County office building was funded through the energy savings.

To evaluate the energy conservation potential through variable speed pumping, an engineering evaluation was preformed on the entire pumping and piping system. There were three steps preformed during the engineering evaluation. The first step was to develop building load profiles for each of the building on the district loop. Each building had a baseline model developed. This model was calibrated to the actual building consumption. Once the baselines were within 5 percent of the actual energy bills, the energy conservation measure of variable speed pumping was analyzed. Each building was modeled with variable speed pumping and compared to the baseline for energy savings.

Once the energy analysis was completed, the second step was to determine a baseline flow analysis for all of the buildings supplied by the central chilled water plant. The data developed by the load analysis model in the first step was used to develop the baseline flows and pressures. In the flow analysis, the entire piping loop and all buildings were modeled. Once the baseline was developed, a retrofit baseline was modeled applying variable speed pumping to the various buildings. These models were developed to ensure the District Cooling Plant could provide chilled water to the buildings based on the proposed operating scenarios.

After the load analysis models, flow analysis models and diagrams were completed, engineering drawings and scopes of work were developed for implementation. The design was developed by applying variable speed drives to existing pumps on the primary, secondary and tertiary systems. There were a total of 15 variable speed drives installed, ranging from 15-hp to 150-hp. A control sequence was developed to vary the flow of the pumps based on building load. The variable speed drives are controlled by differential pressures sensors in the building. As the pressure increased, the pumps would slow down. Conversely, as the pressure dropped, the pumps would speed up. By allowing the pumps to match the chilled water requirements of the building, energy savings could be achieved.

The projected energy savings for the variable pumping energy conservation measure are in the table below.

Bldg.

Baseline energy use

Post-Installation energy use

Calculated Annual Energy Savings (kWh/Yr)

Calculated Annual Electrical Savings ($/Yr)

SPC

11,225,339

10,625,164

600,175

$42,913

Cult Cntr

4,792,217

4,391,923

400,294

$28,621

Flagler

3,120,589

2,979,480

141,109

$10,089

MD Crthse

3,154,263

2,850,636

303,627

$21,709

Central Support

2,670,537

2,288,863

381,674

$27,290

Total

24,962,945

23,136,066

1,826,879

$130,622

                     

The electrical savings achieved by the application of variable speed pumping was approximately $130,622. The electrical savings compensated for construction costs for 15 years. Through the project savings, the County was able to connect an additional building to the District Chilled Water Loop.

After construction, the long-term measurement and verification process begins. Each year, for 15 years, FPL Services LLC will be verifying the savings achieved through the variable speed pumping application. A report of the savings will be issued each year. FPL Services is in the first year of the 15-year guarantee.