Power-generation systems in high-performance buildings

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

Tom Divine, PE, LEED AP, Smith Seckman Reid, Houston
Power-generation systems in high-performance buildings

Learning objectives

The term "high-performance buildings" has generated a great deal of interest over roughly the past decade. That interest is primarily focused on conservation measures, specifically with regard to energy and water, and their impact on the environment.

Standby generating systems have received little attention as components of high-performance buildings. This general dearth of attention isn't particularly unexpected, as generation systems often support the welfare of human beings under adverse conditions and are, by their nature, high-performance systems. Their unyielding operational and reliability requirements often preclude design decisions that might favor energy conservation and environmental impacts, and the limited run time of standby generators limits opportunities for generator characteristics to have a substantial impact on energy conservation or environmental concerns.

History and definition

The term high-performance buildings entered the legal lexicon in the Energy Policy Act of 2005, commonly called the EPAct. The concept was expanded in the Energy Independence and Security Act of 2007 (EISA), which provides this definition for a high-performance building:

Figure 1: Paralleling switchgear for an 8,000-kW system with four 2,000-kW generators is shown. All graphics courtesy: Smith Seckman Reid Inc. "... a building that integrates and optimizes on a lifecycle basis all major high-performance attributes, including energy conservation, environment, safety, security, durability, accessibility, cost-benefit, productivity, sustainability, functionality, and operational considerations."

This can be called a "soft" definition: It describes the focus in general terms, but it doesn't provide enough information to determine whether a particular building can be classified as high-performance.

EISA also provided for the creation of an Office of Federal High-Performance Buildings, under the General Services Administration, to establish and promulgate more detailed standards for federal buildings. A number of states have followed with high-performance building programs of their own. The aggregate market for facilities that can qualify as high-performance buildings, therefore, is quite large, leading to a great deal of interest and discussion in the building design and construction industries.

Of the 10 characteristics of high-performance buildings listed in EISA, the greatest industry interest is focused on energy conservation, environment, and sustainability.

Standby generation systems

The U.S. Environmental Protection Agency (EPA) rules classify standby generation systems as either emergency systems or as nonemergency systems. The regulations are complex, but they are presented in a simplified form: Emergency systems, as defined by EPA rules, are those that operate only when the electric utility service is either unavailable or unacceptable, and otherwise for certain specific purposes for limited periods of time. Nonemergency systems are those that run under any other conditions. Peak-shaving is an example of an application that would be impermissible for an emergency system, but it is allowed for a nonemergency system.

The EPA promulgates different emissions regulations for emergency systems and nonemergency systems. Because they may run at any time, the rules for nonemergency systems are very restrictive. Rules for emergency systems are, by comparison, relaxed due to the limited conditions under which they are permitted to operate. Most generating systems installed at facilities primarily intended for occupancy by human beings are classed as emergency systems. This article will focus on systems classified as emergency systems under EPA regulations.

Figure 2: A simplified schematic of paralleling switchgear is shown, with a split bus. In operation, the tie breaker is open; generators come to speed and energize high-priority loads. After the system stabilizes, the two buses are synchronized and connected through the tie breaker. Environment

The design decision that might be expected to have the greatest environmental impact is the selection of the fuel source for the generating system. NFPA 110-2016: Standard for Emergency and Standby Power Systems declares that three fuel sources shall be permitted for standby power systems: liquid petroleum products, liquefied petroleum gases, and natural gas. In practice, these fuels are diesel fuel, propane, and methane. Propane units are available only in limited sizes, typically 150 kW and below, and have limited application as standby units for all but the smallest building loads.

Natural gas has a reputation as a clean-burning fuel, and in fact, it does have lower emissions of almost every type at the point of use, with the exception of water vapor. In terms of carbon dioxide, the greenhouse gas that currently gets most of the press, natural gas generates about 30% less than diesel fuel to produce equal amounts of heat. It would seem, then, that natural gas would be the preferred fuel for generator applications from an environmental standpoint.

The overall emissions picture, though, is less clear. A small portion of natural gas produced and transported will escape, appearing as atmospheric methane. Methane is a very effective greenhouse gas, capturing the Earth's radiated heat about 25 times as effectively as carbon dioxide over a 100-year period, as reported by the EPA. So, a small amount of methane released during production, transportation, and delivery can entirely negate the reduced greenhouse effect of reduced carbon dioxide emissions.

On the other hand, atmospheric methane persists for a few decades at most, with the bulk converted to other, more benign substances in the first 10 or so years, while carbon dioxide appears to persist for centuries or longer.

Natural gas engines are somewhat less efficient than diesel engines, though that gap appears to be closing. In terms of carbon dioxide emissions, the advantage of natural gas over diesel is therefore less pronounced when comparing equal amounts of energy delivered at the generator terminals, as opposed to equal heat content.

Trade-offs between the estimated climate effects of these two gases are difficult to estimate, and it appears that general agreement on the equivalence has not been reached among climate scientists. It's not entirely clear which of the two options has a lower impact on climate change, but the balance currently appears to tip slightly in favor of natural gas. Decisions regarding fuel source will, therefore, be based on other considerations.

Diesel generators command roughly 80% of the standby generator market, due primarily to operational advantages and industry familiarity. Diesel generators have a better ability to track sudden large changes in load than similarly sized natural gas units, making them better able to meet the 10-second starting requirements of NFPA 110 for Level 1 installations-generators whose failure could have a serious impact on the safety of human beings.

One of the primary advantages of natural gas as a generator fuel is the fact that it's provided by an offsite supplier and doesn't require onsite storage. For Level 1 installations where the probability of interruption of the offsite fuel supply is high, however, NFPA 110 requires onsite storage of sufficient fuel for the entire required run time of the standby system. This requirement will often negate a significant advantage of natural gas as a generator fuel. The code doesn't provide guidance on the level of likelihood of failure that triggers the onsite storage requirement. For Level 1 installations, the acceptable level of risk could be expected to be quite low, particularly where the risk of interruption of utility power and natural gas service are correlated.

Energy conservation

Emergency standby generators run infrequently and usually for short periods of time. They are permitted by EPA regulations to run for as much as 100 hours/year for testing and maintenance while the utility is available, and for an unlimited period when the utility has failed. In practice, their testing and maintenance run time will be much lower than the allowed maximum, and periods when utility power is unavailable will be limited.

Figure 3: The fuel-rate range plotted against the percentage of rated output, expressed in gallons per megawatt-hour, for typical generators rated from 150 to 2,250 kW. Larger units typically are more efficient than smaller units.  The electric utility industry takes service reliability quite seriously, and will take measures to improve it-sometimes under pressure from regulators and customers-should outage frequencies or durations begin to rise. The limited run time of standby systems makes the efficiency of the engines less interesting from the standpoint of energy conservation.

Standby generators generally operate in a relatively narrow band of roughly 70 to 75 gal/MWh in their most efficient range-usually 75% to 80% of nameplate capacity-and exhibit the familiar bathtub curve over their operating range. Larger units are typically a bit more efficient than smaller units. This narrow range of efficiencies is due to the fact that diesel engine technology is driven largely by the transportation industry, where fuel efficiency is a primary driver of purchasing decisions. Modern designs have wrung out about as much efficiency as the medium can deliver. In general, attempting to select diesel generators for operating efficiency will yield only marginal benefit, if any.



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