Application determines distributed generation system

Distributed generation can be accomplished in many different ways, with many kinds of generating technology. These technologies include on-site gas turbine and reciprocating engine generators, photovoltaic systems, small wind turbines, wellhead gas, livestock confinement and landfill methane—just to name a few. On-site power systems that partially shed electrical load or remove load during peak demand periods also fit the definition of distributed generation. All of these systems reduce the load the local utility must serve and can offer improved energy efficiency, cost savings and reduced production of greenhouse gases.

While the energy efficiency benefits of distributed generation are well understood and utility interconnection standards are moving toward consensus, the manner in which an individual user implements on-site generation is still a matter of finding the right generating technology for the application. Fuel and energy sources differ not only in availability but also in Btu content and impact on the environment. And, having a local application that can use all the energy produced by an on-site power system is a key element in making the system financially viable.

Due to distributed generation’s high efficiency compared with normal utility power (85–90 percent in CHP applications vs.33 percent), distributed generation has a tremendous potential for conserving energy and reducing greenhouse gases. In addition to reducing the amount of CO₂ per kWh or Btu generated, it can also convert another more powerful greenhouse gas—methane from landfills—to something less damaging to the environment.

Systems with reciprocating engines dominate
While exotic generating technologies catch the attention of the media, the most widely used on-site generating technology usually involves a reciprocating engine—fueled by natural gas, methane or diesel—and an alternator and control system. If the application can also make use of the waste heat from the engine, the system will also include associated heat recovery equipment to produce hot air, hot water or steam, depending on the need.

The viability of an on-site power system for distributed generation largely depends on economics.  Locations are favored that have relatively high electric rates and low prices for natural gas or free methane (and in a few cases, diesel fuel). Additionally, in those areas of the world where government subsidies and incentives exist, the installation of these energy-saving systems has been more popular. Interconnection issues with local utilities still present a problem in some areas, but the financial viability of an on-site power system has not been very dependent on the willingness of utilities to buy back extra electric power for use on the national grid. The fact is, the power and heat have much more value when they can be fully utilized on site.

The variability of distributed generation applications is quite broad, and while the hardware may be similar from one application to the next, the way in which the electric power (and heat, in the case of CHP) is utilized tends to vary widely. The following examples represent a spectrum of applications using reciprocating engine technologies to generate on-site power and heat.

Landfill methane project powers large cement plant
In Dunbar, Scotland, Viridor Waste Management, one of the UK’s largest operators of municipal landfills, manages a 193-acre site that uses two low-Btu gas generator sets to produce 3.5 MW of electricity from the methane created by decaying rubbish. As paper and other organic materials decompose in landfills, a natural byproduct of that decay is methane—one of the major flammable components of natural gas. While this natural release of methane is dilute, it is a powerful greenhouse gas that can contribute to global warming. Harnessing this gas to produce electricity protects the environment while generating valuable energy.

The installation features two 1.75 MW low-Btu generator sets operating in parallel. The low-Btu generator sets feature a gas engine that is specifically modified to run on dilute methane. The engine has an enlarged fuel delivery system, double-safety gas shutoff valves, and special coatings and bearing materials to withstand any corrosive contaminants that can occur in landfill or resource-recovery gas.

The generator sets are housed in a power building that has room for two additional generators. As the landfill grows and methane production increases, two additional generator sets will be installed to produce a total of 7 MW. At current “tipping” rates, the landfill is expected to operate for the next 30 years. The company selected the equipment for the Dunbar site because of the high power output of the generators per pound sterling of investment, and because the manufacturer and its local distributor were able to help solve the complex connection issues for exporting the electricity to the nearby Lafarge Cement works, which purchases the power. The plant’s total electrical demand is approximately 23 MW, and the Viridor generating system supplies about 15 percent of its needs at a lower cost than the utility.

The project was eligible for increased revenue in the form of Renewable Obligation Certificates (ROCs), a government scheme to encourage the development of renewable energy projects and make the cost of power competitive. For every megawatt-hour of electricity that is generated, an ROC is produced. The ROC may be traded in a market mechanism currently for £40 to £45 GBP ($80 to $90 USD), making the cost of generation competitive with the utility grid power. This enables Viridor to invest in environmentally friendly waste-to-energy projects, generate electricity from landfill gas, sell it to the cement plant below the cost of power from the grid and still make money. 

Urban peaking project in Brazil
Far across the Atlantic, the World Trade Center (WTC) in São Paulo, Brazil, is also making use of distributed generation to cut costs and provide more reliable electric power for its tenants. The 1.75-million-square-foot complex includes the state-of-the-art WTC Business Tower, the elegant Hotel Gran Meliá São Paulo WTC, and one of Latin America’s most upscale malls, the D&D shopping center. The facility has installed three gas-powered generator sets to reduce the cost of energy during the peak demand period and to guarantee power availability to the complex in the event of a utility outage or power crisis. Since 2003, the power system has reduced electricity costs and improved reliability to such an extent that the WTC promotes the power system in its advertising for tenants. 

To provide peaking power for the enormous structure, the WTC São Paulo relies on three 1750 kW lean-burn gas generator sets for a total generating capacity of 5.25 MW, enough to power a city of 5,000 residents. Since the building is located in a commercial and residential zone, only natural gas–fueled power plants are approved by the environmental agency. In addition to the three gas-powered generators, the facility also installed a diesel generator with “black start” capability to ensure that the system would be able to start during a total power outage. 

The main purpose of installing the generators was to reduce costs during peak times when electricity rates are at a premium. The generator sets in the WTC São Paulo run Monday through Friday from 6:30 to 9:30 p.m. in the summer and from 5:30 to 8:30 p.m. in the winter. During that time, the typical load on the generators varies from 3.5 to 4.9 MW, depending on which facilities are in use. By being able to produce its own power, the WTC São Paulo is able to save as much as 30 percent on energy costs during peak hours.

When the generators are running during the peak hours, they are paralleled with the local utility, but do not export power. If there is a utility failure, then the generators are automatically isolated from the grid and provide power independently to the WTC São Paulo. In the event of a major utility power outage, the installed generators are capable of powering the entire building; however, some load shedding would be required at certain periods of the day. 

Colombian pasta manufacturer saves with CHP
Near Bogotá, Colombia, a large Colombian manufacturer of pasta products was experiencing lost production time due to frequent utility voltage instability and power failures. The food processing giant also had high energy costs for electricity and fuel oil. In order to keep the plant’s production line up and running and at the same time save money on energy expenditures, Pastas Doria installed a combined heat and power (CHP) system to generate more reliable electricity and produce heat for pasta drying.

Pastas Doria has been producing a wide variety of pasta products in Colombia for over 53 years and makes more than 50,000 metric tons annually—about 40 percent of all the pasta consumed in the country. By getting both electricity and heat from the natural gas the company purchases, the company benefits from savings on both forms of energy. As a result, Pastas Doria purchases less fossil fuel and electricity than before it installed the CHP system, while solving its voltage stability problems and reducing total emissions. The company estimates that it has reduced its electricity purchases by 60 percent and its fossil fuel purchases by 70 percent, resulting in a savings of approximately $50,000 USD per month on its energy bills.

The CHP system consists of a natural-gas-powered reciprocating engine generator set, an exhaust gas heat exchanger, switchgear and controls. The key component of the CHP system is a lean-burn 1750 kW natural gas generator set. The generator set operates 24 hours a day in parallel with the local utility in order to stabilize the voltage of the utility power coming into the facility and to replace a significant portion of the power the company purchases every day. Should the utility fail for any reason, the CHP system would continue to operate, providing up to 1750 kW of electricity to run various plant operations. Waste heat from the generator set’s exhaust is used to provide 3.4 million Btu/hr of heat energy to the plant’s boilers, pasta-drying operations and space heating, which offsets fuel oil purchases. 

Diesel CHP in India
Distributed generation systems occasionally run on diesel fuel when natural gas supplies are not available or when local electric pricing conditions favor diesel. In India, Gujarat Fluorochemical Limited (GFL), a leading manufacturer of refrigerants, installed a 3 MVA diesel cogeneration system.  The company was experiencing up to four power outages per month from the local utility that continually interrupted its 45-day long refrigerant gas production cycle, resulting in lost materials and delayed deliveries. Additional, the worldwide phase-out of CFC refrigerants had reduced production and sales from the plant, forcing GFL to seek creative ways to cut its operating expenses for power and process steam. In addition to solving these major problems, GFL had a list of needs, all aimed at reducing the cost of kilowatt-hours. 

GFL’s first priority was to be able to purchase or generate cheaper kilowatt-hours than it was currently getting from the local utility. Second, it needed operational reliability in excess of 8,000 hours annually, plus uninterrupted power in at least 1,000-hour blocks to accommodate its 45-day refrigerant production cycle. In addition to those needs, GFL wanted the power system to be as flexible as possible to handle a manufacturing process that required electricity, steam and chilled water.

The on-site power system installed included two 1500 kVA diesel generating sets with remote radiators, and a heat-recovery boiler and associated control equipment for utility paralleling. The generator sets were rated to produce a combined 2.1 MW on a continuous basis, and they were capable of operating on diesel fuel, light diesel oil or superior kerosene oil, depending on local supplies and prices.

When the engine-generators are operating, an exhaust gas heat recovery boiler produces 2,000 pounds of steam per hour at 12 bar for manufacturing processes. The generator sets are also equipped for the option of running a chilled water generator with energy from the engine cooling jacket if further reductions in operating costs are deemed necessary. 

California animal feed producer saves money
In Southern California, soaring rates for both electricity and natural gas prompted a large animal feed supplier in the San Joaquin Valley to do something to reduce its energy costs. Western Milling, located in Goshen, Calif., produces a full range of liquid, bagged and bulk animal feed products ranging from organic feeds to food by-products. It uses large amounts of electricity to run conveyors, mixers, grinders, blenders and pellet mills. In addition, it uses steam and hot water for processing feed and food by-products.

When Western Milling analyzed its need for more economical energy, it chose a combined heat and power system running on natural gas. The system it installed includes a lean-burn natural-gas-engine generator that produces 1250 kW of electricity, 2,200 pounds of steam at 115 psi and 30 gallons per minute of hot water at 190° F.  The entire unit is enclosed in an ISO-style container and located outside the processing facility. 

The company’s processing plant runs 24/7, and uses both electrical and thermal energy to process grains into animal feed. In order to achieve its savings goals, Western Milling needed an efficient system that could operate at better than 95 percent availability. So far, it has been successful.

Another advantage of systems based on reciprocating engine technology is that they can be ordered and installed quickly. The CHP system at Western Milling was installed and commissioned just 12 weeks after it was ordered, which included working with the local utility on paralleling.

On-site power generating systems in California face the most restrictive environmental standards anywhere in the world. As a result, the system includes a Selective Catalytic Reduction (SCR) system on the generator set’s exhaust that uses a urea injection to reduce the NOx in the engine’s exhaust. After treatment, NOx in the exhaust stream is reduced to just 5 ppm, about half of the amount allowed in the standard.

In addition to energy savings, the CHP system is helping to improve reliability of the electrical service and supply of steam and hot water at Western Milling. The original system for producing steam and hot water consisted of two steam generators, each rated at 500 brake horsepower. If one of them was down for repair, the remaining steam generator could produce only 85 percent of the necessary steam for processing. Now, with the CHP system operating, the plant can operate on one steam generator, leaving the other one for backup.

Gas supplier in Belgium employs unique CHP system
In a novel installation of a CHP system, the Electrabel Gas Distribution Centre in Brussels employs gas reciprocating generator sets to generate electricity and thermal energy for its gas distribution system. The system generates power that is directly supplied to the national power grid, while waste heat from the engines is used to pre-heat high-pressure pipeline natural gas prior to pressure reduction and distribution. Electrabel is Belgium's main electric generating company and that country's largest distributor of gas and electricity. 

The Electrabel facility is designed to reduce natural gas pressure from as much as 14 bar to about 1.7 bar in preparation for delivery into the final gas distribution network. This network supplies approximately 300,000 domestic and industrial natural gas customers in north Brussels and seven surrounding municipalities in the Flanders region.

The CHP system is powered by two lean-burn gas engine generating sets that produce a total of 2.7 MW for the electric grid and 3.5 MW of thermal energy. Most of that thermal energy is used to pre-heat pipeline gas entering a pressure-reducing turboexpander.

In traditional gas transmission and distribution systems, the reduction or regulation of pressure in gas transmission and distribution systems is usually controlled with a special valve. However, the energy lost in this process can be a substantial proportion of all the energy put into the original compression of the gas system. An alternative approach uses a turboexpander in place of a valve to provide a controlled reduction of gas pressure, and at the same time produce useful work by turning a turbine. The output from the turbine can be used to generate electricity and thus contribute to overall energy efficiency and reduce impact on the environment.

As the gas passes through the turboexpander, its pressure is reduced and energy is extracted by the turbine blades to turn a separate 2.6 MW alternator. However, as the gas is allowed to expand across the turbine, its temperature drops, causing any moisture in it to freeze. To prevent this from happening, the incoming gas is heated with 2.7 MW of waste heat from the lean-burn gas engine generator sets. Most of that heat is produced by the exhaust system, and the remainder is recovered from the engine cooling water jacket and the oil sump. 

Excess waste heat from the engines not used for heating the pipeline is used for building space heating during the winter. During the summer, this excess heat is dispersed using roof-mounted radiators.

The combined output of the CHP plant’s lean-burn gas engine generator sets and turboexpander-driven alternator is 5.3 MW of electric power that is fed directly into the grid. The customer chose the cogeneration option for pre-heating the gas rather than using a conventional boiler because it maximizes the plant's electrical power output. The system has optimized plant efficiency at about 86 percent, reduced the customer’s initial investment, and made the system more reliable.

Conclusion
While on-site power and CHP systems based on reciprocating engine technology dominate the distributed generation landscape, the specific user application is the most important factor in determining the design and operation of the power system. Fuels used range from natural gas to landfill methane and even diesel, depending on local energy pricing conditions and environmental regulations. Whether on-site power systems are used to produce prime power and heat, or just peaking power, they all help to improve energy efficiency, reduce energy costs to the user and reduce the amount of greenhouse gases released to the atmosphere. Additionally, distributed generation systems slow the demand for more centralized power plants and delay the need for utility transmission and distribution lines.

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