The state of Florida is hot. That’s a fact. Businesses tend to run their air-conditioning units year round with the exception of a “cold” day in January. So when the Hernando County School District started to develop plans for a new high school campus in Weeki Wachi, they wanted an efficient, reliable solution for their cooling needs. Insert hybrid cooling. Winning the Florida ASHRAE Chapter Technology Award, Weeki Wachi High School in Hernando County is a success story not to be missed.

Hybrid Cooling 101

Hybrid cooling utilizes a standard packaged chiller to produce and store ice at night, in this case, in IceBank thermal energy storage tanks. The stored ice is then melted the following day to provide cooling in tandem with the building’s chiller system. By using more-efficiently produced nighttime electricity to create the stored cooling, a hybrid cooling system reduces source energy consumption, emissions and cooling costs.

To understand source energy think of an efficiency contest betweem a Hummer and a Prius. If the odometers of each vehicle start at zero and both vehicles travel 100 miles, you could say both cars went 100 miles. However, mileage does not indicate how efficiently the car has traveled the 100 miles. The same is true of the utility meter. A utility meter tells how many kilowatt-hours were used but not how many source Btu were used to make and transport the electricity.

Weeki Wachi High School on Ice

The Weeki Wachi High School campus features a unique “high school as a small town” design concept. Three academic neighborhoods – an administration and media civic center, athletic and dining facilities, and a performing arts town center – are clustered around a central landscaped courtyard. To meet LEED Silver certification requirements, Weeki Wachi High School designers looked into many energy-efficient technologies during the design phase of the campus, including solar domestic water heaters, low-flow water fixtures in all restrooms, and high-efficiency T8 and T5 internal lighting.

To take advantage of cooling load diversity and economies of scale, the high school and adjoining K-8 campus operate with a combined chilled water central energy plant (CEP) design.  The CEP utilizes a hybrid cooling system, which includes CALMAC IceBank energy storage tanks and chillers. The system is a partial energy storage design, which uses chillers to charge the IceBank tanks at night, then the chillers are used the following day along with the ice storage to cool the facility. On a peak design day, the system’s air-cooled chillers cool up to 70 percent of the facility’s load, and the stored ice cools the remaining load. (See Figure 1.) On non-design days, the ice cools a larger portion of the load, and in the winter, the ice contribution can be 100 percent. Energy storage also improves classroom comfort by lowering humidity levels.

A partial storage configuration enables downsized chillers (water or air-cooled) to operate at or near full load continuously, thus eliminating the inefficient cycling and capacity modulation that accompanies part-load operation and normally causes a chiller to operate at peak efficiency only 25 percent of the time. Air-cooled chillers were used at the Weeki Wachi High School, as is commonplace for buildings in Florida. Air-cooled chillers are great ‘icemakers’ because they can take advantage of reduced dry bulb temperatures at night while making ice.

The combined campuses require 2,040 total tons of cooling capacity. By combining two central cooling plants into one and adding ice storage, Matern Professional Engineering Inc. was able to reduce the purchased chiller capacity to 1,200 tons. The hybrid cooling system with ice storage reduced not only the HVAC equipment size and refrigerant required for the project but also the peak energy consumption for the campus. CALMAC’s energy storage tanks are estimated to save 10 percent of the school’s annual electricity costs.

Passing the Test

One of the main challenges at Weeki Wachi High School was the integration of two different load profiles for the common CEP and the ice storage system. During the development process, those involved needed to understand the needs of a high school campus and a K-8 campus as it pertains to occupant schedules and how much off-peak time they had to charge the ice tanks.

The goal of the project was to achieve a 12 percent reduction in energy costs. The team was able to maximize energy savings, and overall the project has far exceeded the goals. The most recent energy calculations showed a 19.8 percent reduction and a cost savings of 20 percent. The calculation includes 43 MBtu of on-site renewable energy from solar panels. The total water savings is 44.8 percent as calculated.

Hybrid cooling is quickly becoming the solution of choice for school administrators, providing payback in as little as one year and improved cash flows for the life of the facility that can add up to millions of dollars. As part of a high-performance sustainable design, energy storage systems provide a demand-responsive cooling system with fuel-source flexibility in an unregulated energy market. 

Weeki Wachi High School, Weeki Wachi, Fla.

School Opens August 2010
Size: 450,000 Square Feet

Chilled Energy Plant savings:

No rebates
First Cost Savings of $400,000
Annual Energy Cost Savings of $48,000

Key Players:

· Mark Johnson, Regional Sales Manager, CALMAC
· Ryan Strandquest, LEED AP, Vice President and Project Manager, Matern Professional Engineering Inc.
· Kevin Bjorgum, PE, Mechanical Engineer of Record
· Marc Sherman, EI, LEED AP, Mechanical Engineer and LEED Representative