Your questions answered: Critical power: Power generation systems in high-performance buildings

The Sept. 21, 2017, “Critical power: Power generation systems in high-performance buildings” webcast presenters addressed questions not covered during the live event.


Electrical engineers must consider many factors when designing power generation systems. Safety, maintainability, code compliance, and economics play crucial roles in determining the topology of power generation systems for high-performance buildings. Specific requirements for emergency power vary based on building occupancy type, facility use, and function.

The Sept. 21, 2017, “Critical power: Power generation systems in high-performance buildings” webcast presenters addressed questions not covered during the live event.When designing power generation systems, engineers must ensure that the products and the building electrical systems that they support are appropriate for the specific application. They must make decisions regarding the type of power (alternative, traditional, or combined heat and power) system sizing, load types, whether generators should be paralleled, fuel and/or battery storage, switching scenarios, and many other criteria.

When working on high-performance buildings, designers must consider a building that integrates and optimizes all major high-performance building attributes, including energy efficiency, durability, lifecycle performance, and occupant productivity.

Presenters Randall Ehret, PE, Environmental Systems Design Inc., Chicago; and Cory Duggin, PE, LEED AP BD+C, BEMP TLC Engineering for Architecture, Brentwood, Tenn., respond to questions not answered during the live Critical power webcast: Power generation systems in high-performance buildings on Sept. 21, 2017.

Question: Please discuss interconnecting multiple large units for redundancy.

Randall Ehret: There a several methods that power sources can be combined to build a redundant system. The slides in the presentation gave a high-level overview of several of these. It is always important to consider the project goals when developing the system. Take into consideration factors, such as the reliability of the normal power source, the availability of an alternate power source from an independent grid, and preferably routed to the site along a path disparate from the primary power source. Consider using more than one fuel source and type of emergency power: diesel generators, natural gas generators, uninterruptable power supplies, and fuel cells.

Q: What software is used for energy modeling at TLC?

Cory Duggin: IESVE

Q: Please discuss grounding of separately derived systems and paralleling of generators.

Ehret: Grounding of separately derived systems connected in parallel shall be per the requirements of NFPA 70-2017: National Electrical Code (NEC) Article 250.30.

Q: Please define spark gap.

Duggin: Spark gap, or spark spread, is the difference in the gas utility rate and the electric utility rate. The higher the spark gap, the more economically viable generating electricity onsite via combustion is. This is typically done using a combined heat and power system, which increases the system efficiency by using the waste heat in addition to the electricity. The U.S. Energy Information Administration (EIA) tracks the spark gap variation across the country, so this is a good source.

Q: Does a second utility source qualify as an “alternate source?”

Randall Ehret, PE, Environmental Systems Design Inc.

Ehret: The answer is “it depends.” You need to understand the context and the application of the alternate source as it relates to the applicable code sections. In the context of NEC Article 700, Emergency Systems, a second utility source can be used as the alternate source where approved by the authority having jurisdiction. Proceed carefully and do your due diligence prior to pursuing this as an option.

Q: What spark spread is considered favorable when considering natural gas cogeneration versus utility electric?

Duggin: The higher the better. The minimum spark spread for viability will depend on the electrical generation efficiency of your prime mover.

Q: Where are the generators in Kingdom Tower? What fuel type? Was locating them a challenge?

Ehret: The generators are actually located at the base of the building in two separate vaults. The distribution up through the tower is in separate rated enclosures. The fuel source is diesel. Careful planning was required in the early stages of the project to balance the infrastructure requirements with the programmatic needs of the building. Ultimately, the appropriate answer for locating and configuring the infrastructure always comes down to the best-case compromise.

Cory Duggin, PE, LEED AP BD+C, BEMP TLC Engineering for Architecture, Brentwood, Tenn.Q: Could you share the design methodology that you use to identify the project requirements for high-performance buildings?

Duggin: The high-performance project requirements are often identified during a charrette early in the design process with the project stake holders.

Q: What are the key factors to take into consideration when selecting generator sets for high-performance buildings?

Ehret: Many factors must be considered. Where is the project located? What is the likelihood of a natural disaster impacting power lines to the site? What is the likelihood of a natural disaster impacting the ability to transport a fresh diesel supply to the site? What is the track record of the local utility? Does the owner have any sustainable initiatives he or she is trying to achieve? What is the acceptable downtime? Is a partial outage as impactful to the site as a total outage? Unfortunately, one overriding factor can be budget. I always start every project with a single point failure analysis of my distribution system and then look at options that meet the owner’s requirements and the budgetary constraints.

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