By Mike Carr and Michael Kirchner
Published in the September 2007 issue of Today’s Facility Manager
Diesel and gaseous fueled generators have been common solutions for industrial and commercial standby power requirements for decades. Diesel gensets have traditionally dominated the market for larger kilowatt applications (greater than 125 kilowatts). For applications below 125 kW, the market has historically been a mix of diesel and gaseous (natural gas or liquid propane vapor) powered generators.
Currently, the market trend in new and replacement installations is toward an expansion of gaseous and a contraction of diesel powered solutions. Meanwhile, several key issues remain, which is why technology has shifted beyond the status quo.
Compression Ignited Generators
Compression ignited (diesel fueled) generators are the market norm for larger applications due to a significant capital cost advantage when compared to traditional large natural gas generators. Though they are in wide use, diesel generators are challenged by major, extended outages that cripple infrastructure and make refueling problematic or impossible.
Hurricanes and storms can close roads. Grid failures may make it impossible for suppliers to pump fuel into delivery trucks or replenish all customers in a timely manner when the simultaneous demand becomes overwhelming.
These realities often place system designers in a difficult position. How much diesel fuel storage is enough and how much is too much? Large fuel tanks bring their own challenges with regard to fuel contamination and breakdown.
When storing a large amount of diesel on-site, facility professionals need to pay close attention to various aspects of fuel handling and maintenance. Large quantities of diesel fuel are typically kept in a main storage tank and then transferred to a smaller tank (day tank) at the generator for usage.
Two significant competing concerns arise with these systems. The first problem is designing the system to be fail-safe so no spillage occurs. No single point of failure in the fuel delivery system should result in fuel outflow.
The environmental concerns associated with the spillage of on-site fuel are considerable. Many locations have various local code requirements covering fuel containment and delivery. Some include concrete walled secondary containment, double walled piping, fire rated tanks, specific fill and spill requirements, special permitting, and other requirements.
In order for a diesel tank system to be well designed, it not only must prevent fuel spills, but it must also deliver the fuel reliably. The second aspect becomes more difficult as system complexity increases.
Various components become mission critical and may require redundant elements or increased monitoring and control to maintain high generator reliability. These include pump motors, the pump power circuit, float switches, shut-off valves, isolation solenoids, and other elements.
In addition to the handling issues, diesel fuel contamination and breakdown are real concerns, because the fuel is typically stored for long periods of time. For this reason, biodiesel fuel is not well suited for standby power applications because it is best used when fresh.
The two most common contaminants are water and biomass. Water enters the tank as humidity through the tank’s normal vent and condenses during the daily thermal cycle. Initially, moisture binders in the fuel capture and contain the moisture. As these binders become overloaded, the water drops to the bottom of the tank and begins accumulating. At some point, the moisture may be sucked into the diesel engine, potentially resulting in loss of power, loss of lubrication, and corrosion.
Water also creates an environment that could support biomass that can grow at the water/fuel interface. When these microbes are pulled into the generator, they cause the fuel filter to clog, resulting in the engine losing power and shutting down.
Contamination is a significant problem that can be treated with additives and a watchful preventive maintenance program. Additives are also helpful in treating common fuel breakdown issues such as varnishing and the formation of gums. Though additives are helpful, at some point the fuel may need aggressive filtering and treatment requiring the fuel to be cycled out of the storage tank.
In an effort to minimize the negative aspects of on-site diesel fuel, systems that use simple sub-base (generator mounted) fuel tanks and limit on-site fuel to a more easily maintained quantity are generally preferred. But doesn’t this leave the system at a greater risk of running out of fuel? How can system designers offset this risk?
One solution is to develop comprehensive and coordinated refueling contingency plans. Another solution is bi-fuel technology (discussed later in this article).
Spark Ignited Generators (Gaseous Fueled)
Spark ignited generators offer advantages over diesel. The most noteworthy is the extended run time offered by an endless supply of natural gas. Other advantages include the omission of fuel permitting requirements, reduced preventive maintenance costs, less risk of environmental contamination, and cleaner engine emissions.
Spark ignited (natural gas or LP vapor fueled) generators for standby power are cost-effective solutions that fall into the automotive engine classification. Historically, this meant a maximum power output of 100 kW. Through turbocharging and optimized rpm technologies, various manufacturers have increased this power range to 125 kW and 150 kW within the last few years.
Implementing spark ignited solutions for applications requiring more than 150 kW has been traditionally limited by capital cost. That is because large, higher output gaseous engines are produced in much smaller numbers and are considerably more expensive. Typical capital cost premiums for these higher output gaseous systems have been 1.75 to 2.5 times the comparable diesel generator price point.
Though natural gas fueled products offer run time advantages, they do rely on natural gas distribution systems to deliver the fuel. Thus, the gas utility is providing backup for the electric utility.
Is this acceptable? For non-earthquake environments, the natural gas infrastructure has shown itself to be extremely reliable and not interdependent with the electric utility. Through four Florida hurricanes in 2004 and the Northeast grid failure of 2003, the natural gas infrastructure was unaffected.
Though proven reliable and often used for optional standby (NEC 702) applications, natural gas may not meet the requirements for certain emergency system (NEC 700) applications. Those types of emergency system loads may require the generator’s fuel to be on-site (if there is any question regarding this matter, facility professionals should consult with their local authorities).
For applications that require on-site fuel, spark ignited generators can use stored liquid propane (consumed in vapor form) to meet code requirements. These systems can also be designed to run in a dual fuel configuration in which the primary fuel is natural gas and the secondary fuel is liquid propane. Such configurations are very common for small emergency systems but are not typically used on larger kW applications due to capital cost.
Because of advances in technology and the increasing market pressure to minimize diesel related issues, generators that use gaseous fuel are growing in popularity. The technologies that are expanding the use of natural gas address the historical cost disadvantage of natural gas powered generators. This is accomplished by improved power outputs and/or extending the use of automotive based engines. The key technology shifts are the optimization of engine rpm, integrated approaches to generator paralleling, and bi-fuel (combined diesel and natural gas operation).
Optimizing engine rpm. The AC frequency of the genset’s electrical output is a function of engine speed and alternator design. To achieve 60 Hertz, the alternator rotor must turn at a specific rpm for a given alternator pole configuration.
Fifty years ago, most generators operated at speeds below 900 rpm. Within the last 30 years, the diesel standby generator market has moved from 1,200 to 1,800 rpm as engine outputs have increased.
The prime power, natural gas driven generator market has migrated from 900 to 1,200 rpm with some recent offerings at 1,800 rpm. These trends have also extended into the automotive style, spark ignited engines serving applications up to 150 kW. Historically operating at 1,800 rpm, current technology is optimizing these engines for operation at 2,300, 3,000, and 3,600 rpm.
For operating speeds between 1,800 and 3,600 rpm, some manufacturers use a simple gear reduction device between the engine and a four pole alternator to operate the engine in its peak power band, thus achieving the optimal amount of mechanical power while maintaining a 60 Hertz electrical output. The trend toward increasing the operating speed of automotive derived engines provides multiple advantages, including improved transient performance, less stress on engine bearings, increased power densities, and reduced capital cost.
As manufacturers optimize operating speeds on automotive and truck derivative engines, these units become more powerful and cost-effective. The automotive engines (<150 kW) provide the greatest value at a significant discount to diesel engines, but optimized truck derivative engines (<300 kW) also become financially feasible.
Integrated generator paralleling. As automotive and truck derivative spark ignited engines become more cost-effective, they can be used as building blocks of power to meet the needs of larger applications, providing a cost effective alternative to traditional diesel or larger (>200 kW) natural gas engines. To achieve this goal, manufacturers are using an integrated approach to generator paralleling which connects the generators and combines their output without using external equipment.
Parallel power solutions have always offered the standby generation marketplace advantages; however, the implementation has been limited to mission critical applications and large kilowatt projects.
This is due to the constraints in implementing traditional paralleling solutions using switchgear. These constraints include costs, space, issues of single source responsibility, and a significant level of complexity.
To access the benefits of parallel generation while removing the cost and complexity limitations, generator manufacturers have developed integrated generator paralleling. For example, three 150 kW gaseous fueled gensets operating in parallel offer the same output as a single large 450 kW unit—but with the advantage of built-in redundancy. If for some reason the 450 kW unit does not operate, the facility is without backup power; but if one of the 150 kW units doesn’t run, the other gensets will supply two-thirds of the system’s normal output.
Bi-fuel generators. Bi-fuel gensets combine the power density and capital cost benefits of diesel engines with the extended run time of natural gas. Using mass produced diesel engines as prime movers, bi-fuel generators start up on diesel fuel in a normal manner. As the generator picks up load, bi-fuel delivery systems introduce natural gas to the combustion air while reducing the amount of diesel fuel. Under full load conditions, bi-fuel generators will operate on a ratio of 25% diesel and 75% natural gas, with no reduction in power.
At just a slight cost premium to diesel-only designs, bi-fuel gensets offer several powerful advantages:
- The lower capital cost of the diesel engine is retained;
- Run times per tank of diesel fuel are greatly extended;
- On-site fuel storage and maintenance issues are minimized; and
- The exhaust emissions profile is improved.
The reduced consumption of diesel fuel by the engine under bi-fuel operation means that run times per tank of fuel are significantly extended.
Because natural gas is the predominant fuel, smaller diesel tanks are a viable option. With smaller fuel tanks, the risk of fuel going bad and the cost of fuel maintenance are significantly reduced. If the natural gas supply is interrupted for any reason, or if there is a fault in the bi-fuel system, the controls will automatically direct the unit back to 100% diesel without interruption of operation.
A Change For The Better
In the wake of multiple grid failures and severe weather events, facility managers have discovered the challenges of maintaining operation during area wide outages of long duration. Many building professionals are attracted to the idea that a diesel standby generator with a 24 hour fuel tank may take up to 10 years to consume one turn of the fuel.
Natural gas can be extremely reliable with no interdependency on the electrical infrastructure. It also significantly reduces the burden of fuel maintenance and refueling contingency planning, which are absolute requirements for steady diesel generator operation.
The standby generator industry continues to adopt new technologies that address the fundamental problems with diesel fuel. Bi-fuel solutions, integrated paralleling, and optimized rpm generators may make natural gas solutions feasible and minimize or remove the negative issues associated with on-site diesel fuel storage.