Pepco Energy Services (PES) provided an energy services performance contract project to the State of North Carolina, and selected DN Tanks to build a 2.7 MG stratified chilled water Thermal Energy Storage (TES) tank on the state capital campus in downtown Raleigh. The precast, prestressed, wire-wound concrete TES tank is at the heart of this significant energy savings project. Coupled with a new high efficiency packaged chilled water plant, this TES tank is capable of delivering 27,260 ton-hrs of cooling capacity. The system is the primary chilled water source for 3.5 million square feet in 20 buildings. In addition, it is projected that this chiller plant and TES tanks will save nearly two million square feet in tax payer money.
Per Richard Beversdorf, PES Project Develoment Engineer, DN Tanks was selected to build the TES tank,
” … because of their willingness to assist in the design and partner with us to ensure all design objectives were met. For these reasons, combined with a strong reputation, ability to meet a demanding design and construction schedule and overall value.”
In general, DN TES tanks provide these other valuable features to owners:
- Watertight Integrity- All DN TES tanks are built to a zero measurable loss tolerance.
- Maintenance-Free – Will never be out of service to repaint the interior surface.
- Flexibility of Construction – Concrete tanks can be partially or fully buried.
- Speed of Construction – There are no long lead materials.
- Aesthetics – The tank adapts easily to architectural enhancements.
- Turnkey Services – DN Tanks has decades of experience designing and building TES tanks.
In addition to building the new tank, Pepco Energy Services contracted Natgun to repair an existing 0.75 MG underground TES tank. This rectangular, cast-in-place tank was built in 1988 by others, but was never completely put into service because it leaked and did not perform properly.
DN Tanks repaired the leaks, and replaced the diffuser system within the TES tank with a combination of welded steel-PVC piping specifically designed to meet the cooling capacity of 7,100 ton-hrs. The two TES tanks are configured differently, (one is rectangular and the other circular).
The performance of the new circular tank has been independently verified and meets the PES efficiency requirements.
DN Tanks worked very closely on the design and construction with both PES and TAS, a manufacturer of modular chilled water cooling and on-site energy systems. This close working relationship was vitally important to maximize the benefits of the system while building a new central plant within a relatively “tight” urban jobsite. PES’s selection of DN Tanks and TAS proved extremely advantageous for the overall project because of the synergistic effects and the high performance nature of the products. TAS Sales Manager Jim Riesenberger said,
“By installing the new central plant and storage system serving the administration complex, Pepco Energy Services significantly reduced the investment capital to support the retrofit/expansion project. By introducing the TES tank into the technical solution, thousands of tons of centrifugal chiller were displaced.”
In addition, the overall central cooling plant construction schedule was optimized because both major suppliers (DN Tanks and TAS) were never on-site at the same time. Since DN Tanks has no long lead items and builds the tank on site, the tank was constructed first. While the tank was being erected, TAS was designing and building their modular chiller plant at their Houston manufacturing facility. By the time the chiller plant was shipped to the jobsite, the TES tank was near completion. Per John Andrepont, consultant for this project and President of The Cool Solutions Company,
“DN Tanks was very attentive to meeting the specific needs of the performance contracting team and the state throughout all the phases of the project.”
The high quality and high performance synergies of these two systems, as conceived by PES and delivered by DN Tanks and TAS, will provide the majority of the project’s overall annual energy savings.
The central plant retrofit and expansion will reduce the state’s energy consumption by improving operating efficiency from an average of 0.9 kW/ton to less than 0.75 kW/ton. Additional monetary savings were achieved by installing the DN Thermal Energy Storage system, which enabled Pepco Energy Services to shift electric load to lower off-peak rates. As a result of the maintenance-free aspects of the concrete TES tanks, the State of North Carolina will be assured of decades of continuous service and energy savings.