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Distributed Generation: A Closer Look
The recent major power blackouts in the United States and elsewhere, as well as the volatility of electricity prices, have increased the awareness among maintenance and engineering managers about the reliability of electricity supplies. In fact, even before August’s major East Coast blackout, many institutional and commercial facilities increased their reliance on alternative-electric sources, primarily in the form of local generation.
Facilities have turned to a variety of technologies — including reciprocating engines, fuel cells, photovoltaic systems, and microturbines — to control costs and ease dependence on traditional sources of electricity. But as with all other systems and technology, managers must consider their overall impact on facilities, including their life-cycle costs and maintenance needs.
The Condition of the Grid
The electrical power industry has served the nation well for close to a century. The system has been designed and operated in such a way that, in the best circumstances, the electrical power grid provides 99.9 percent reliability. This means that on average, customers can experience power outages totaling roughly eight hours annually. Until recently, these assumptions served most power requirements quite adequately.
Today, many critical information technology systems require system reliability of somewhere up to 99.99999 percent. These numbers translate into an average annual power interruption of about 3 seconds per year. In other words, existing electrical service from the power utility is not designed to meet the needs of today’s critical facilities equipment.
The need for back-up power is continually increasing. Ignoring the problem is not option, since the lack of reliable electricity has been costly for many businesses.
A Changing System
A number of factors have led to the increasing number of power interruptions. One main reason for the current situation is the steady drop in the capacity margin of the U.S. power grid over the past decade. For instance, in 1990 the capacity margin in the summer was about 25 percent, meaning the system had the capacity to absorb a load increase of 25 percent. Since the load growth in the past decade has been much faster than the growth in the generation base, this figure has dropped to below 10 percent nationally.
The second main underlying factor is the condition and use of power transmission lines. The number of wholesale power transactions has grown rapidly during the past decade, while the investment in new transmission lines has dropped sharply.
Therein lies the increasing vulnerability to more failures than in the past of electrical systems. Many institutional and commercial facilities recognize they have to find other ways to enhance the reliability of power sources, and distributed generation systems (DGS) can provide that opportunity.
The Emergence of DGS
The advent of new DGS technology offers economically viable options that can help facilities minimize potential electrical interruptions. In fact, demand for DGS has grown rapidly in the last decade. For instance, the number of units ordered the late 1980s was less than 1,000. In the past few years, it has increased to more than 4,000 units per year. It is estimated that today, DGS technology accounts for more than 20 percent of newly installed generation.
About 10 percent of installed generation capacity in the United States is DGS and other standby-power systems. According to some projections, this number might increase to more than 30 percent within the next decade.
Traditionally, DGS ranges from a few kilowatts to 5 megawatts. Reciprocating engines traditionally have been the workhorse of DGS, due to their relatively low installed cost of roughly $400 per kW. Although manufacturers have made improvements in reciprocating-engine technology, high maintenance costs and emission rates are the main disadvantages of this technology. For this reason, new installations of reciprocating units generally are restricted to standby applications.
Major types of DGS include renewable sources, such as photovoltaic systems, and non-renewable sources, such as fuel cells and microturbines.
Fuel cells have a number of desirable characteristics. They use oxygen and hydrogen to produce electricity, as well as some water and heat. More importantly, they have no moving parts and few emissions, and they require minimal maintenance.
Although fuel cells have been used for space applications for decades, based on current technology, it still is not a commercially viable DGS, due to its relatively high installation costs.
Photovoltaic (PV) applications are most prevalent in the Sunbelt states. They tend to be desirable from an environmental standpoint. There are basically two types of PV devices: fixed and movable. As the name states with the fixed units, the orientation of the cells cannot be changed after installation. Movable units can track the sun’s direction through out the day, and they have a higher efficiency. But their initial cost is higher, and they tend to require more maintenance.
The main impediment to these DGS technologies is installation cost, which can be $4,000-$6,000 per kilowatt. Without some type of rebate or other incentive from a local utility or other source, facilities have little motivation to specify these technologies, so they do not tend to be economically viable alternatives yet.
Microturbines are the only DGS technology that offers practical and economically viable alternative for most applications. Microturbines are high-speed gas turbines that generate 15-1,000 kW per unit.
The heart of the equipment is a fast compressor turbine that operates at 1,600 rotations per second. A permanent magnet generator operates on the same shaft.
The use of air bearings keeps the unit cool and makes it virtually maintenance-free. Due to the high rotation speed, the units generate electricity at much higher than 60Hz frequency, which is reduced electronically to 60Hz.
The fundamental advantage of microturbines is the inherent simplicity of the technology. Other advantages include high reliability, low maintenance, low emissions, and their smaller and smaller size compared to reciprocating units with the same rating.
Although microturbine costs more than reciprocating engines on a dollar-per-kW basis, they cost less than photovoltaics, fuel cells and wind turbines. Microturbines cost $600-800 per kW, while the other technologies cost $4,000-$6,000 per kilowatt.
Microturbine efficiency can range from 20-40 percent, while the NOx emission level is about seven parts per million. In comparison, a reciprocating engine operating for one hour generates the same amount of NOx that a microturbine of similar size generates in almost nine days.
The units can operate for 11,000 hours between major overhauls, and their average service life is more than 45,000 hours. Microturbines also are highly reliable and have close to 99 percent availability without significant maintenance. These units do not require any cooling, which makes them very desirable for remote applications.
The low-emission characteristics of microturbines make them good candidates for small cogeneration applications, where exhaust heat is used to generate hot water. As cogeneration units, the overall efficiency of these turbines can be 70-80 percent. Finally, microturbines can operate on a variety of primary fuels including natural gas, propane, diesel and kerosene.
When specifying microturbines, managers must consider a number of factors:
- Does the facility need units only to produce electricity? Or is there a continuous thermal need where a cogeneration unit might be more appropriate?
- What is the type of primary fuel the unit will use?
- Is noise a concern? The noise level of microturbines might be an issue in some applications. In such instances, managers can make sure the unit is enclosed to dampen the sound level.
- Is a maintenance program in place?
Although microturbines require little maintenance, managers must develop a maintenance program for them based on manufacturer recommendations.
A series of factors, including the deregulation of electricity and the continual drop in power reserve margin, have lowered the reliability of the nation's electrical power grid, and price volatility has become the common characteristic of the system that is likely to be present years to come.
But the factors that affect the reliability and availability of electricity, in turn, make DGS a more attractive option for facilities seeking more reliable power supplies. DGS technology can offer cost-effective electricity choices that can reduce the congestion in transmission lines and provide a non-interrupted power source.
DGS: Making the Case