Efficient on-site power generation

The increasing cost of energy has prompted many large energy consumers to invest in on-site power generation. Keith Packham from Cummins Power generation takes us through the many different applications and technologies on offer.

Distributed generation can be accomplished in various 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 and landfill methane. 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. 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 per cent in CHP applications versus 33 per cent), distributed generation has tremendous potential for conserving energy and reducing harmful gas emissions. 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 fuelled 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 favoured 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 utilised 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 electricity (and heat, in the case of CHP) is utilised tends to vary. The following examples provide a spectrum of applications using reciprocating engine technologies to generate 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 which uses two low-Btu gas generator sets to produce 3.5MW of electricity from the methane created by decaying rubbish. 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. 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 1.75MW low-Btu generator sets operating in parallel. The low-Btu generator sets include 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 7MW. At current “tipping” rates, the landfill is expected to operate for the next 30 years.

The conduit coming out of the Viridor power building contains the 11kV transmission line
that powers the nearby Lafarge Dunbar cement plant.

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 23MW, and the Viridor generating system supplies about 15 per cent 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 MWh of electricity that is generated, an ROC is produced, which can then be traded in a market mechanism currently for GBP40-45 (US$80-90), making the cost of generation competitive with the utility grid power. This enables Viridor to invest in 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 1.75mft² 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 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 1750kW lean-burn gas generator sets for a total generating capacity of 5.25MW, enough to power a city of 5000 residents. Since the building is located in a commercial and residential zone, only natural gas-fuelled 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 from Monday to Friday from 18.30h to 21.30h in the summer and from 17.30h to 20.30h in the winter. During that time, the typical load on the generators varies from 3.5 to 4.9MW, 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 per cent 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.

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, California, 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 the feed and food by-products. When Western Milling analysed its need for a more economical source of energy, it chose a combined heat and power system running on natural gas. The particular system it installed includes a lean-burn natural-gas-engine generator that produces 1250kW of electricity, 2200lb 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 which is located outside the facility.

The company’s processing plant runs 24/7 and uses both electrical and thermal energy to process grains into animal feed. To achieve its savings goals, Western Milling needed an efficient system that could operate at better than 95 per cent 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 in this case was installed and commissioned just 12 weeks after it was ordered, which included time spent working with the local utility on paralleling.

On-site power generating systems in California face the most restrictive environmental standards in the world. As a result, the set-up includes a Selective Catalytic Reduction (SCR) arrangement on the generator set’s exhaust that uses a urea injection to reduce the NO_x in the engine’s exhaust. As a result, NO_x in the exhaust stream is reduced to just 5ppm, about half of the legal limit.

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 500bhp. If one of them was down for repair, the remaining steam generator could produce only 85 per cent of the necessary steam for processing.
Now, with the CHP system operating, the plant can run 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. Power is produced which 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 the 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 2.7MW for the electric grid and 3.5MW 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 the 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 2.6MW 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 2.7MW 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 5.3MW 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 maximises the plant’s electrical power output. The system has optimised plant efficiency at about 86 per cent, reduced the customer’s initial investment, and made the system more reliable.

“Distributed generation systems slow the demand for more centralised power plants and delay the need for costly utility transmission and distribution lines.”

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, cut 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 centralised power plants and delay the need for costly utility transmission and distribution lines. The need to reduce fossil fuel consumption, coupled with rising prices at the pump has been generating increased interest in alternative technologies, many of which are better suited to a distributive model of power generation than the standard centralised approach.

Keith Packham is Gas Application Manager for Cummins Power Generation. He has been involved with the design, installation, maintenance and operational aspects of combined heat and power (CHP) plants, boiler plants and power generation and distribution, refrigeration, water and effluent treatment plants and their optimum performance.

For more information on how distributed generation can help your business why not visit: www.cumminspower.com