Special Report. Distributed Generation: Application determines DG system configuration

Power, Aug 2007 by Packham, Keith, Generation, Cummins Power

face= Italic; The intended use is the primary consideration (even fuel is secondary) when choosing an on-site energy system. Whether the need is for power alone or for heat as well, distributed generation plants have delivered lower energy costs and improved reliability to industrial and commercial customers worldwide. Following are six case studies of reciprocating engine systems that highlight their versatility.face=-Italic;

By Keith Packham, Cummins Power Generation

Distributed generation (DG) can be configured in many different ways, with many kinds of technologies and equipment. Among the choices are gas turbines, reciprocating engine-generator sets, photovoltaic arrays, and wind turbines. Fossil-fueled DG systems can take the form of cogeneration or combined heat and power (CHP) plants. On-site power systems that partially shed electrical load or remove load during peak demand periods also fit the definition of DG. All of these systems reduce the load that the local utility must serve and benefit the end user with lower energy bills and improved reliability.

Perfectly matching a DG system to its application isn't always easy, but it is usually well worth the effort. The choice of the system's fuel is determined by local availabilities and the fuels' relative Btu content and impact on the environment. Money matters, too; being able to use all of the energy produced by a DG system is a key aspect of its financial viability.

DG's high efficiency, compared with that of utility power (85% to 90% for CHP applications, vs. 33% for central station units), affords tremendous potential for conserving energy and reducing greenhouse gas emissions. Efficient, fossil-fueled DG systems release less COface= Subscript; 2face=-Subscript; per kWh or Btu generated than less-efficient generation.

face= Bold; Recips dominateface=-Bold;

While exotic generating technologies capture the attention of the media, the most widely used on-site generating technology is the workhorse reciprocating engine-generator, fueled by natural gas, landfill methane, or diesel. If the application can make use of the waste heat from the engine, the installed system will also include heat-recovery equipment to produce hot air, hot water, and/or steam.

Economics also plays a big role in the decision to generate your own power. Favored locations have both relatively high electric rates and low prices for natural gas or diesel. Another plus is a government subsidy or incentive to increase efficiency or reduce air pollution. Interconnecting to the local utility still poses a problem in some areas, but the financial viability of most on-site power systems has not hinged on the willingness of utilities to buy surplus electric power produced by end users. The fact is, the power and heat have much more value when they can be fully utilized on-site.

The variability of DG applications is quite broad, and although the hardware may be similar from one application to the next, the ways in which the electric power and/or heat are used vary widely. The following six case studies represent a spectrum of applications that use reciprocating engine technologies to generate on-site power and heat.

face= Bold; Landfill gas helps power Scottish cement plantface=-Bold;

In Dunbar, Scotland, Viridor Waste Management, one of the UK's largest operators of municipal landfills, manages a 193-acre site that uses two gensets fueled by low-Btu landfill gas to produce 3.5 MW of electricity (Figure 1). As paper and other organic materials decompose in landfills, a natural by-product of that decay is methane, one of the major flammable components of natural gas. Though this natural release of methane is dilute, it is still a greenhouse gas with 20 times the global warming potential of COface= Subscript; 2face=-Subscript; . The gensets extract energy from the methane that would otherwise be wasted, while protecting the environment.

The two 1.75-MW gensets operate in parallel. Each is powered by an engine that was modified to run on dilute methane. The engines have an enlarged fuel delivery system, double-safety gas shutoff valves, and special coatings and bearing materials to withstand any corrosive contaminants in the landfill gas.

The two gensets are housed in a power building with room for two more units. Additional units will be installed as the landfill grows and methane production increases, doubling the DG system output to 7 MW. At current "tipping" rates, the landfill is expected to operate for the next 30 years.

The first two gensets were chosen for their high power output relative to capital cost and because Cummins Power Generation and its local distributor helped design the complex connection for exporting their electricity to the nearby Lafarge Cement works, which purchases the power. The cement plant's total electrical demand is about 23 MW, and the gensets supply about 15% of that, at a lower cost than local grid power.

The project's bottom line also benefits from supplemental revenue in the form of renewable obligation certificates (ROCs), part of a UK government scheme to encourage usage of renewable energy like landfill methane. Every megawatt-hour of electricity generated produces one ROC that can be sold. At current market prices of about US$80 to $90 per ROC, the system's net cost of generation is competitive with utility grid power. In other words, Viridor can generate electricity from the landfill gas, sell it to the cement plant below the cost of local power, and make enough money to recoup the system's capital cost over time.