Topic: Microturbines for Distributed Power Generation
Pedro Alvarez Urena
The prefix “micro-,“ is commonly known to imply “small.” Where the word “turbine,” is defined as “a device that converts the flow of a fluid (air, steam, water, or hot gases) into mechanical motion for generating electricity (1).” It should follow then that a MicroTurbine is simply a small device that converts fluid flow into mechanical motion for electric generation processes, but how accurate is this definition? According to one source, MicroTurbines can be defined as follows “MicroTurbines are small electricity generators that burn gaseous and liquid fuels to create high-speed rotation that turns an electrical generator… with size ranges… available from 30 to 400 kilowatts (kW) (2).” But how ‘small’ is small in the case of MicroTurbines? From Figure 1, you can see that a MicroTurbine system appears to be about as large as a household refrigerator unit; keep in mind that this is the entire combined heating and power (CHP) system, and not just the turbine itself. Figure 2 shows the blades found in a steam turbine for industrial power supply applications, the size difference when compared to the MicroTurbine in Figure 1 is simply amazing.
As previously mentioned MicroTurbine Unit’s are commonly used in combined heating and power systems (CHP). For the heat generation aspect “in CHP applications, the waste heat from the MicroTurbine is used to produce hot water, to heat building space, to drive absorption cooling or desiccant dehumidification equipment, and to supply other thermal energy needs in a building or industrial process.” In terms of electrical power generation, MicroTurbines, see Figure 3, operate on the same thermodynamic principles as a Brayton cycle, similar to their gas turbine counter parts2. The fuels used in MicroTurbine system commonly include, natural gas, sour gas, gasoline, kerosene, diesel fuel/distillate heating oil, and in some recovery applications waste gases are used that previously would have been released into the environment2.
Figure 3: MicroTurbine Schematic
“The basic components of a MicroTurbine are the compressor, turbine generator, and recuperator (2).” The compressor increases the pressure of the air which then passes through the recuperator, and onto the combuster where it is superheated, and finally delivered to the turbine. The turbine supplies the compressor with power, and the turbine also turns the generator which produces electricity. The exhaust product exiting the turbine then passes through a recuperator where further heat is extracted to be used in the preheating process of air entering the combuster chamber; this reduces the fuel needed to be added into the combuster. Lastly, the remaining exhaust products can be used in the heat generation aspect of CHP systems, by heating air and/or water for HVAC or hot-water supply uses.
Advantages and Disadvantages
The major advantages of Microturbines are few moving parts, compact systems, good efficiencies in coregeneration, low emissions, can utilize a variety of fuels (including waste fuels), low investment costs, and low maintenance costs.5,10
Disadvantages found in Microturbines are high operating rpms (90,000-120,000), low fuel-to-electricity efficiencies, and reduced power output and efficiency with higher ambient temperatures.5
Microturbines can operate continuously or On-demand and be either grid connected or stand alone. They can run individually or multi-packed and with a variety of fuels: Diesel, Propane, Kerosene, Flare Gas, Biogas, Low or High Pressure Natural Gas, among others. Other applications can be combined heat and power (CHP) and microgrid.6,10
Microturbines are targeted to telecommunication companies, retail services, financial services, financial services, office buildings, restaurants and other commercial services. Microturbines also operate in resource recovery operations like oil and gas production fields.10
“Reliable operation is important since these locations may be remote from the grid, and even when served by the grid, may experience costly downtime when electric service is lost due to weather, fire or animals.” (BioTurbine, 2007)
Figure 4: IR MT70 Microturbine Distributed Generation (11)
Figure 5 and Figure 6 show two different Capston Microturbines. The C30 Microturbine size is 30KW, while the C60/C65 size is 60KW and 65KW.
Figure 7: Record 28.6 in snowfall. Unit operated at full output during the storm. (3)
Figure 8: 30 KW Oil Fired Microturbine at AVEC with Fuel tank and Filter assembly. (3)
Environmental and Cost
The state of current large-scale energy production, relies heavily on environmentally unfriendly industries. Energy supplied to homes and large industrial buildings alike, comes from these sources. Wouldn’t it be great to have households and other users with demand under 1 MW be connected to a small and more efficient power generation system? Microturbines offer this possibility.
From an environmental point of view, this technology produces fewer emissions and allows for the use of waste fuel. The idea of using waste fuel is attractive because it allows to extract energy out of a source that normally goes into the atmosphere as a greenhouse gas.
Figure 9: Landfill operations (ec.europa.eu)
One such waste fuel is in the form of biogas, which is plentiful in landfills. As material is compressed, oxygen content goes down to zero and microorganisms begin to break down the organic compounds in the land fill.4 Such process releases methane and carbon dioxide, called biogas because it is produced naturally. The landfills in the world are of huge proportions, and the methane they produce accounts for 10% of all worldwide methane emissions, so there is no shortage of this fuel for microturbines.7 In terms of emissions, the level is lower than in conventional engines. For example, a microturbine build by Capstone produces NOx and unburned methane emissions each at 3 ppmV at 15% O2.10
Besides taking advantage of biogas, microturbines offer a less intrusive alternative to the surroundings in which the community or office building is located. This small footprint makes it possible to form communities in remote locations and preserve the natural surroundings.
The current cost of energy is on the rise, and microturbines offer an economic advantage. The cost of electricity sold by utilities is based on the demand, making peak times more expensive. A microturbine produces electricity at a lower cost, and its use during peak demand can lower charges from utility industry even further. This results in reduction of overall energy expense.9
There is another cost besides the dollar value, energy has a reliability cost. The supply of uninterrupted electricity is very important in locations such as hospitals and nuclear power plants. Furthermore, interruptions result in loss of productivity and output, which can cost a lot of money. Additionally, loss of electricity disrupts our daily lives, which was evident during the large northeastern blackout in 2003. A community or an office complex that uses microturbines increases its energy security and power reliability. In the event of a blackout, enough power can be supplied to keep essential operations going.9
Figure 10: The 2003 blackout forced thousands of people in New York City to walk home. (Exitmundi.nl/Seinforma)
This technology is developing fast, and for the right reasons. The ability to have a source of power that will reduce the energy bill, provide fewer emissions, and give greater energy security is quite appealing. The growth now depends on greater acceptance of microturbines in this country, and the greener communities of tomorrow will become a reality.