02/13/2015
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| For small power producers such as those operating mini-grids, a new generation of generator enclosures is increasing choice and reducing or eliminating some of the challenges with diesel. Photo courtesy: HIPOWER Systems |
Do the Old Rules Still Apply?
By Rafael R. Acosta, HIPOWER SYSTEMS
Only a few years ago, traditional logic for generator sets (gensets) held that diesel meant reliable and inexpensive but also noisy, loud and messy. Natural gas meant expensive and temperamental but also quieter and cleaner. Today, thanks to the development of new technologies in engines, enclosures and other components, the differences between diesel and natural gas generators is no longer so clearly defined.
Additionally, many firms follow conventional logic in their comparisons of these two fuel types and fail to give sufficient weight to the operating realities of their particular applications. In this article, we’ll take a look at both new developments and long-standing considerations that impact generator selection by fuel type in the power generation industry.
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The Engine Equation
Traditionally, the viewpoint has been that diesel engines provide response, power and longevity, while natural gas engines are more environmentally friendly. Although the environmental argument for natural gas still holds true, diesel is no longer the clear winner in terms of power and response. Spark-ignited (natural gas) industrial engine manufacturers can now optimize the RPM of these engines to make their transient response similar to that of diesel. Manufacturers are also producing natural gas units that can meet the 10-second startup requirement for backup systems that is traditionally associated with diesel engines, alone.
Use of gear-on-gear powertrains or two-pole alternators (as opposed to traditional four-pole alternators) has increased the overall performance and power of natural gas engines, as well. Manufacturers have also incorporated stronger, more resilient engine parts, such as hardened valves and seats, to boost performance and increase reliability.
In the area of energy density, there is no doubt that diesel has greater peak energy density than natural gas-by a factor of more than of three (generally 129btu versus 37btu). Even here, there are mitigating factors that may tip the scales in favor of natural gas.
Density is impacted by both engine and fuel conditions. A poorly maintained diesel engine, or one running sludgy fuel from a fuel tank filled with particulates, likely will outperform a natural gas engine, but it will not perform at its peak.
Even more significantly, diesel engines have a sweet spot of 50-70% of load, with 80% being the recommended maximum for long-term prime operation. Running them under a lighter load for long periods of time results in wet stacking, a condition that sends unburned fuel and soot into the exhaust system.
Operators that run engines under light loads often employ load banks to consume the excess energy. This approach reduces wet stacking but can waste a considerable amount of fuel. As an alternative, those operators could instead choose natural-gas-powered generators, which burn hotter than diesel engines. These engines are less likely to experience problems with unburned fuel, even if they are run at a lighter-than-optimal load.
Built to Last
Regarding longevity, diesel engines still tend to have longer lives, on average, than natural gas engines. However, many of the new technologies mentioned above not only enhance performance in natural gas engines; they also increase engine resilience and longevity.
In short, companies that plan to keep their engines operating at peak condition will still likely enjoy the greatest longevity from a diesel engine. If they allow it to fall prey to the operating challenges discussed in the previous section, longevity will suffer-in extreme cases, potentially cutting engine life in half.
For firms that use generators only for backup power, a natural gas engine may provide nearly the same effective life, given how infrequently it is used. Other considerations such as continuity of fuel source also come into play, perhaps outweighing diesel’s benefits of performance and engine life.
Cost Comparison
It’s a common misconception that industrial diesel engines are considerably less expensive than comparable natural gas models. Below 150kW, natural gas engines are actually more cost effective, even without factoring in the fuel differential.
For applications where more kW are required, power producers can create parallel configurations of smaller engines to provide them with the cumulative kW needed for the operation. Parallel systems have the advantage of supporting load sharing and management, making them one of the most cost-effective and environmentally friendly options, in terms of fuel use, for variable load applications such as mini-grids. Add to this savings the reliability and scalability of parallel systems where they replace a single, larger diesel generator (such as for backup power) and the benefits of such a solution are considerable.
For the past two years, natural gas genset suppliers have also been touting fuel prices as cost benefit. While the fall in natural gas prices is certainly making them a more attractive option, plummeting oil prices are having a similar effect on diesel generator fuel. We won’t make a prediction here, because the outcome of fuel prices is anyone’s guess.
Saving the Planet
One of the big advantages of natural gas, of course, is that it burns more cleanly than diesel. This comparison is exacerbated for any of the operating conditions mentioned in the previous section, where wasted fuel increases soot and dangerous emissions.
In addition, shortened engine life from wet stacking, light loads, inadequate maintenance and other common diesel generator issues can negatively impact emissions even more. This can be an problem, not only for the environment, but also for regulatory compliance with the EPA and other agencies that protect it.
The new rules require MACT (maximum achievable control technology) emissions controls and GACT (generally achievable control technology) management practices for both major sources and area sources of HAPs (hazardous air pollutants).
Engine maintenance and condition monitoring must be verified through reporting, and if operators allow engines to fall outside acceptable ranges with poor maintenance or fuel conditioning, steep fines can result. The fines for not keeping an engine in optimal running condition can quickly eclipse the added cost of a natural gas model. Consequently, firms should consider all of these factors before they make an engine choice.