Fonte: Consulting – Specifying Engineers
Por: SEPTEMBER 28, 2018
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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.
- Understand the limited overall impact of generating system design decisions on the environment, sustainability, and energy conservation, due largely to limited run time.
- Learn about characteristics of high-performance buildings that are affected by generator system designs.
- Become familiar with elementary noise-management concepts.
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:
“… 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.
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.
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.
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.
Paralleling generators can yield meaningful increases in overall fuel efficiency, particularly for systems whose total load shows a high degree of variability. Generating systems must be sized to serve the largest loads that they will be required to serve, and they are often sized to accommodate expansion that may be delayed, or may never occur. In practice, though, they will normally see a load considerably below their projected peak demand, resulting in them operating well below their optimal efficiency.
Most modern paralleling systems are capable of adjusting the number of generators online in response to changing loads. This feature is sometimes called “load demand.” In a load-demand system, all available generators will start in response to a power outage. After the system stabilizes, the system compares the load to the online capacity, and if adequate headroom exists, it will de-energize generators until the load and capacity are well-matched, maintaining an adequate online reserve capacity of typically 20%. The benefit of this feature, in terms of system fuel efficiency, is that the control system can keep the generators running as close to their maximum efficiency as the system load and generating-unit sizes will allow.
From the viewpoint of fuel efficiency, the benefit of paralleling is reduced due to the limited run time of emergency standby systems. As a simple example, a 2-MW generator running at 40% would burn about 7 gal more of diesel fuel per megawatt-hour than two paralleled 500-kW units at 80%. For 100 hours of run time, the difference amounts to 700 gal-about what a single good-sized diesel pickup truck might burn in a single year. The environmental impact of improved efficiency by paralleling is limited.
Parallel systems provide a number of operational advantages in addition to fuel efficiency. An N+1 system can tolerate the failure of a single generator, improving reliability and maintainability. A system can be designed to be expandable, allowing the postponement of expenditures for additional units until they are actually needed. Full-load testing can be simplified by testing one unit at a time, requiring a load bank the size of a single unit rather than the entire system.
Those advantages come at a considerable cost, in terms of the cost of the paralleling system itself and the additional complexity of the system. There are many good reasons to parallel, but energy efficiency and environmental concerns normally will not drive that decision.
Productivity is influenced by the quality of the indoor environment. A variety of studies have concluded that environments that don’t intrude on the perceptions of building occupants lead to higher productivity. Generating systems affect indoor environmental quality in terms of acoustical and visual comfort: noise and views. If a generator is visible at all from the occupied space, it will have a negative impact on view quality. Such aesthetic concerns are the province of the project architect. The engineer, though, can have a substantial impact on the system’s noise level.
Generator noise will be an important consideration for facilities that are intended to maintain a level of normal operation during a power outage. Even in facilities that don’t continue operating through a power outage, some level of noise management may still be necessary to ensure that emergency instructions and communications among emergency responders can be understood. Many municipalities have noise ordinances that limit the sound-pressure level at the property line from all sources.
The impact of generator noise on occupant productivity will have a limited impact on overall economic performance, due again to the limited run time of emergency standby generators.
For outdoor installations, noise-management strategies are based primarily on barriers and distance. An outdoor generator will require some form of enclosure. The manufacturer’s standard offering will typically provide minimal sound attenuation. Where there’s adequate distance from the generator to the occupied space, or to the property line, no further sound reduction may be required. Otherwise, a sound-attenuating enclosure will be necessary.
Sound-attenuating enclosures are normally rated for a specific generator, with a specific sound-pressure level at approximately 23 ft from the enclosure. The resulting sound-pressure levels are usually specified in decibels, a logarithmic measure of sound energy per unit area, and are usually frequency-weighted.
Generator sound pressure levels are typically described in “dBa.” The nomenclature dBa means that measurements are in decibels, and that the frequency components of the sound have been weighted using an industry standard scale, arbitrarily named “A,” giving the greatest weight to frequencies between 1 and 6.5 kHz. Sound-attenuating enclosures are typically rated to limit generator noise at 23 ft to 85 dBa, 75 dBa, or 65 dBa, and will depend on the municipality or jurisdiction. A 65-dBa enclosure is quieter, larger, and more expensive than a 75-dBa enclosure.
Distance from the source provides sound attenuation. As sound radiates from its source, its power is spread of the surface sphere of increasing radius, and the sound power per unit area decreases with the square of distance from the source. For an uncomplicated arrangement, without large reflective surfaces near the generators, the sound-pressure level will decrease to a quarter of its initial intensity when the distance to the source is doubled.
That decrease corresponds to an attenuation of approximately 6 dB. Looking at a 75-dBa enclosure, with a sound-pressure level of 75 dBa at a distance of 23 ft, the sound-pressure level would be decreased to 69 dB at a distance of about 46 ft. Distance provides effective sound attenuation on multibuilding campuses, where generators can be placed far from principal occupied spaces.
Generators installed inside the structure they serve, as may be the case in tight urban sites, present a much more complex set of conditions for noise management. In these installations, generators are surrounded by close reflective surfaces, complicating the analysis, and the building structure itself will participate in transmitting sound through the building. In these cases, the project team is well-advised to engage an acoustical consultant to analyze the installation and recommend attenuation measures.
Codes and standards