Distributed Generation Technology

Traditionally, electricity is generated at large central power plants using fossil fuel (coal, oil, natural gas), nuclear, or hydraulic means. Size does matter to these generation plants due to economies of scale. Smaller scale power generation using fossil fuel combustion, electrochemical processes (fuel cells), and renewable processes (wind, solar, hydraulic, geothermal, biomass or landfill gas combustion) are alternatives to centralized power plants.

Combined Heat and Power Production

By capturing the otherwise “non-utilized heat” or “waste heat” released from the generation process on-site, the users of distributed generation are able to utilize the heat and achieve substantial savings.

Types of Natural Gas Distributed Generation Equipment

Distributed power generation can be accomplished via reciprocating engines (internal combustion engines), microturbines, gas turbines, steam turbines and fuel cells, generally fueled by natural gas. Other distributed generation by solar panels (photo-voltaic cells), wind turbines, and, hydraulic turbines are also available but usually at a higher cost. 

Reciprocating Engines (Internal Combustion – IC)
Reciprocating engines fueled by natural gas for power generation is a well proven technology that has been commercially available for many years. This technology which has evolved considerably over the years is economically viable for supplying power continuously to a site when power generation capacity is required. Natural gas reciprocating engine power generation systems are commercially available in sizes from 25 kw and up.

Reciprocating engines include spark-ignited gas engines, and diesel engines. Spark-ignited gas engines require an external source of energy to ignite the air-fuel mixture in the combustion chamber of the engine, while diesel engines rely on the heat generated by the compression of the air-fuel to cause combustion to occur. IC engines are available from small residential generators up to large 5-10 MW utility generators.

Spark-ignited engines typically use natural gas as the fuel although some engine manufacturers allow the use of landfill gas, bio-gas, and other gases that contain methane. There are various options for spark-ignited gas engines depending on the requirements for efficiency, emissions, and heat recovery.

Spark-ignited gas engines, and diesel engines, when used for power generation, typically operate at speeds from 514 to 3600 rpm. Large diesel engines are available that operate at speeds less than 514 rpm, however, these are typically used in areas where other fuels are not practical for generating power, such as remote communities. Generally a higher engine speed allows a smaller engine requirement, with a lower capital cost, but potentially higher maintenance costs. Selection of the engine operating speed is based on the evaluation of annual hours of operation, electrical output, capital cost, maintenance costs over the expected equipment service life, and space required for the equipment.

Engine manufacturers can offer complete design services to the end user that can include the necessary controls, monitoring systems, and safety interlocks required by the engine generators, as well as equipment to recover heat from the engine exhaust, and engine cooling systems. Cooling of engines generate significant quantities of relatively low temperature hot water, and the economic viability is usually determined by how much of the heat from the cooling water can be recovered. The amount of heat recoverable is typically determined by available uses of the heat.

Spark-ignited natural gas engines are typically more economically viable than diesel engines unless there are other design criteria that cannot be met with a natural gas engine. Generally engine generators are suited for combined heat and power installations where there is a greater need for power than heat, and the required heat is of a lower temperature. Generally, the size of the engine generator will be determined by the heat load of the installation, and the ability to recover the waste heat from the engine.

Reciprocating engines operate efficiently at part load compared to similar generating technologies. Consequently, a reciprocating engine-generator can be oversized for the minimum electrical load without suffering a significant increase in fuel consumption per kWh produced. By comparison a gas turbine usually is sized for the minimum electrical load to avoid prolonged operation at part load where the fuel consumption per kWh can increase significantly.

Microturbines
Microturbines are small, single-stage gas turbine generator sets, that burn gaseous or liquid fuels to create a high-energy gas stream that is expanded through a turbine to drive an electrical generator. Microturbines which are commercially available, or in development, range from 30 to 500 kW.

Most microturbine designs are single-shaft systems, producing variable voltage and frequency AC power. Digital power controllers convert the generated high-frequency AC power into commercially usable electricity. Because single-shaft turbines have only one major moving part, they have the potential for low maintenance & high reliability. Two-shaft models use one turbine to drive the compressor and a second (power) turbine to drive the generator, with exhaust from the compressor-drive turbine supplied to the power turbine. The two-shaft design has more moving parts but does not require power electronics to convert high frequency AC power output to a common 60 Hz frequency. Electronic components control all of the engine/generator operating and start-up functions.

Microturbines can be equipped with controls that allow the unit to be operated either in parallel with or independent of the grid, and they incorporate many of the grid-protection and system-protection features required for acceptable electrical system interconnections. The controls usually also allow for remote monitoring and operation.

A recuperator is incorporated in most microturbine designs, to increase efficiency to competitive levels for continuous-duty service. A recuperator is an air-to-gas heat exchanger that utilizes the hot turbine exhaust gas (typically around 649ºC or 1200ºF) to preheat the compressed air (typically around 149 to 205ºC or 300 to 400ºF) before it goes into the combustor section. This reduces the amount of fuel needed to heat the compressed air to the design turbine inlet temperature. In CHP operation, a second heat-recovery heat exchanger – the exhaust-gas heat exchanger – can be used to transfer remaining energy from the microturbine exhaust to a hot water or similar system. Some microturbine-based CHP applications do not use recuperators, and some have the ability to bypass their recuperator to adjust their thermal to electric ratio. The temperature of the exhaust from these microturbines is much higher (up to 649ºC or 1200ºF) and thus more heat is available for energy recovery.

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