Design and Commissioning of Digester Gas CHP Energy Recovery Projects in a Design/Build
- Robert Bush - City of Atlanta
- Adam Minchey and Richard Porter - Gwinnett County Department of Water Resources
- Michael Reisinger - Crowder Construction Company
- Scott A. Hardy - Hazen and Sawyer
The design/build delivery method for combined heat and power (CHP) systems has the advantage of customizing CHP systems to meet Owner’s budget, a compressing the design and construction schedule, and providing the Owner a unified contactor/engineer team during start up. This paper is a case study for the design and commissioning associated with the 2.1 megawatt (MW) CHP system at Gwinnett County F. Wayne Hill Water Resources Center (FWHWRC) and a 1.6 MW CHP system at the City of Atlanta R. M. Clayton WWTP.
With constrained utility budgets and fixed financing through federal grants, such as the American Reinvestment and Recovery Act (ARRA), the design/build delivery method offers the ability to customize the selection and delivery of equipment associated with CHP systems to meet the Owner’s budget constraints within a constrained delivery schedule. At the FWHWRC, Gwinnett County received a $5.2M ARRA funding for the project and did not have any extra funding for the project and the ARRA funding had stringent constraints on project delivery schedule which would preclude a conventional design-bid-build project delivery system. The design/build team was able to utilize existing equipment that was not originally purposed for the CHP system to decrease cost and maximize the size of the CHP system within a constraint budget and schedule environment. At the RM Clayton WWTP, the design/build team repurposed an abandoned digester gas compression building to install the 1.6 MW engine generator. This saved the City of Atlanta nearly $0.5M on an enclosure for the generator, and allowed the project to meet their budgeted funding levels.
With the design/build method, the design and construction schedule is compressed. On both the FWHWRC and the RM ClaytonWWTP projects, the engine generator, which has the longest lead time, was ordered at the very start of the project delivery phase so that by the time the detailed design is completed, the delivery of the engine is a few months away instead of 8 months away. Timely delivery of the engine-generator set was very critical for the FWHWRC project in order to maintain ARRA grant funding. The RM Clayton project has a 12-month project duration from start of design to completion of startup. The engine is schedule to arrive onsite 2 months after the completion of the design. The design/bid/build delivery method could not meet a 12-month project duration from initiation to commissioning.
The design/build delivery method also has the engineer and contractor on the same team during startup of the CHP systems. These complex CHP systems require multi-discipline coordination between engineers and the general/electrical contractors to work out the start up issues and tune the performance of the system. Examples of Engineer and Contractor working together to resolve commissioning issues much faster than a traditional deign/bid/build will be provided. One example from the startup of the FWHWRC CHP system is that the pressure in the heat recovery system hot water loop increased causing the system pressure relief valve open. With startup in the middle of summer at peak power cost, the Contractor gave the Engineer the freedom to install a second expansion tank at no additional cost to the Owner within a week, instead of finger pointing and debating whether the expansion tank was sized correctly or if Contactor removed all the air from the hot water piping system.
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