Microturbines
Abstract
Microturbines are very small gas turbines with sizes as small as 1 kW although the main commercial machines are in the range 30 to 500 kW. These small turbines usually have a single stage compressor and a single stage turbine, with a generator mounted on the same shaft. Both compressor and turbine are normally radial rather than axial in design, as would be the case for their larger relatives. Rotational speed is extremely high, usually greater than 40,000 rev/minutes and power electronics are used to match the output frequency to the grid. Efficiency is low compared to a large gas turbine but many of the more efficient microturbines use recuperators to improve performance. Others use waste heat to produce hot water in a cogeneration system.
Keywords
Microturbine; radial compressor; radial turbine; recuperator; cogeneration; distributed generation; ultra-microturbine
Historically most of the gas turbines that have been used for power generation have been relatively large, with electrical outputs of over 1 MW. Many have outputs of several hundred megawatts. However there is a group of much smaller turbines, called microturbines. These turbines are intended for use in distributed generation applications where they supply electrical energy, and often heating or cooling too, to group of local energy users. For the smallest of these microturbines, the users might be in a single household. Even smaller units, called ultra-microturbines are being developed for portable use, as a replacement for batteries.
There is no standard definition of a microturbine. Some commentators include all turbines with an electrical output of less than 1 MW. This covers a broad field since the smallest of them can be of the order of tens of millimeters in diameter and have outputs of 10 to 100 W. For stationary applications the minimum size is likely to be 1 to 5 KW, sufficient to provide power for a small domestic dwelling. However, most of the commercial microturbines available are much larger with sizes ranging from 30 to 500 kW. These larger machines can also be deployed in parallel to provide even larger microturbine installations. The market for these devices is still evolving and the may take several more years to establish a firm position. Many commercial microturbine systems are still under development.
8.1 Microturbine Technology
While the main gas turbine technology can be traced back to the development of aero engines during the middle years of the 20th century, microturbines are generally considered to have evolved from the auxiliary turbines that are fitted to aircraft to supply electricity and heat.1 Similar devices were also tried as automotive power units in the 1970s but the technology did not thrive.
Microturbines are small gas turbines and being small they are generally also much simpler than their larger relatives. A typical microturbine will use a radial compressor and turbine rather than the axial components that are common in larger gas turbines. There will probably be a single stage to the compressor and a single stage turbine too, both mounted on the same shaft, with a combustion chamber between the two. A high speed generator will share the shaft, making for a compact design. Air bearings are often used to minimize friction although conventional oil-lubricated bearings are also common.
These small turbines have extremely high rotational speeds, usually between 40,000 and 120,000 rev/minutes, and the output frequency of the generator is similarly high, perhaps as high as 1000 Hz. In order to match grid frequency, these systems are therefore fitted with power electronic frequency conversion hardware. This has the advantage of conditioning the supply and controlling both voltage and frequency and the unit should be able to operate independently of the grid if necessary.
The compression ratio that can be achieved using a single stage radial compressor in a microturbine is low compared to a conventional multistage compressor of larger gas turbines. Typical compression ratios are 3:1 to 4:1. In order to attain high efficiency these machines usually need to incorporate a recuperator. A microturbine with a recuperator is shown schematically in Fig. 8.1 while Fig. 8.2 shows a cross-section of a commercial microturbine. As discussed in Chapter 6 the recuperator captures energy from the exhaust gases of the turbine and uses it to preheat the compressed air from the compressor before it enters the combustion chamber. The typical efficiency of a microturbine with a recuperator is 25% to 30%. There are also microturbines that do not use recuperators. These devices tend to be more sturdy than those with recuperators but their efficiency is low at around 15%.
All the components of the microturbine will usually be integrated into a simple package that is ready to link to the mains supply. This ease of installation is one attraction of this type of device. Nevertheless, the efficiency that can be attained is low compared to many other forms of generation. This can be offset if further waste heat, in the exhaust gases exiting the recuperator, is used to heat water in a combined heat and power system. This will raise the overall efficiency to as high as 85% although it may still fall short of alternatives, such as a reciprocating gas engine. The waste heat can also be used to drive a chiller for cooling. Where cogeneration is important, a simple microturbine without a recuperator can be more effective since there is more waste heat energy available.
The area where microturbines may offer their greatest advantage is in their emissions performance. The main pollutant generated in a gas turbine is NOx and the level of these emissions from a microturbine are usually very low. This will usually allow microturbines to be installed without the need for additional emission control systems, even in urban areas.
8.2 Microturbine Enhancements
Current microturbines have limited appeal as electricity generators because of their low energy conversion efficiency. Other limitations include a drop off in power as the ambient temperature rises and a fall in power at higher altitudes.
There are a number of enhancements that can improve the performance of a microturbine although not all will be cost-effective.2 One that may be cost-effective is inlet air cooling. This has two advantages. The first is to insulate the turbine from changes in ambient air temperature. The second is to increase power output under normal operating conditions by reducing the inlet air temperature, so increasing the temperature difference between inlet and outlet. Air cooling can be carried out either by refrigeration of the air before it is delivered to the input or by inlet air fogging, as discussed in Chapter 6. The latter also adds an element of mass injection.
A second possibility is to create a micro-combined cycle plant by using a closed cycle organic Rankine turbine to exploit the heat from the microturbine exhaust. This is a complex and potentially costly solution to the efficiency problem but has the potential to raise the efficiency significantly. Another cycle modification is to apply mass injection of steam into the combustor of the microturbine. Again this adds complexity and may not be cost-effective.
Other ways of increasing the performance of microturbines involve the same strategy used in larger turbines, by increasing the inlet temperature at the turbine. The small size of a microturbine means that the production of high temperature ceramic components may be much more cost-effective than for larger machines. Thermal barrier coatings, not normally used in microturbines, might also be introduced. As with other possible enhancements, cost is likely to be the determining factor.
8.3 Applications for Microturbines
There are, in principle, a wide range of applications for microturbines. One major use is likely to be for standby or backup power. This might be in a hospital or a data center where power availability is critical but it could also be in less sensitive commercial settings if the grid supply is insufficiently reliable. In addition, companies could use microturbines for peak shaving, using the unit to supply electricity when the cost from the grid is high but returning to the grid when costs fall. In regions where demand management is common grid practice, microturbines might offer small companies a cheap way of joining a demand management scheme which pays a company to reduce its consumption during peak demand periods.
Alternative fuels are another area where microturbines are beginning to find a niche market. Methane gas from small landfill sites offers one possibility but so does biogas generated from a variety of sources. The simplicity of a packaged microturbine may make it more attractive than a reciprocating gas engine, the type of engine commonly used to burn these gases, in situations where a relatively small amount of gas is available.
Where there is a need for heat as well as electricity, microturbines can offer a cost-effective source of energy. If the site is in an urban area where emissions regulations are tight, the microturbine could be the easiest solution to install. In the United States there are examples of microturbine installations in restaurants and on a university campus.
8.4 Ultra-Micro Gas Turbines
There is a type of gas turbine called an ultra-microturbine that is even smaller than the micro gas turbine. These ultra-micro gas turbines are still in the research stage of their development but they offer intriguing possibilities. The machines offer outputs of tens to hundreds of watts and are typically only a few centimetres in diameter. With machines this tiny, fabrication of parts such as the turbine and compressor present major problems and techniques such as those used to manufacture microchips are being exploited. These tiny turbines can rotate at more than 100,000 rev/minutes. The small size means that a variety of effects that are not significant in large machines, such as air flow friction, will play a major role in their operation. Novel designs for combustion chambers are also required and new materials may be needed to build all the components.
Efficiency of these tiny gas turbines is expected to be lower than for the microturbines discussed above. Their advantage is power density, particularly when compared to batteries which are their likely competitors. Applications for such devices include use by military personnel and in drones, the latter offering perhaps the most attractive market. Civilian uses may be limited because of the high operating temperature of the devices.