Turbine Technology: Everything Old is New Again

Advancements continue to take place asinnovative applications are being discovered

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loonger/E+/Getty Images

Since the invention of the first ever electrical generator (the hand-cranked Faraday disc generator in 1832), mankind has been able to convert mechanical energy directly into electrical energy. A turbine’s function has not changed since then, though its applications and power sources have greatly diversified.

The mechanical energy that a turbine uses to make electricity can find its motive power from gravity (the elevation head of a water reservoir behind a hydroelectric dam), high-pressure steam (created by burning coal that boils water), hot, expanding gases (the combustion of natural gas in a combined cycle gas turbine), blowing wind (at wind farms, aerostats, and offshore wind power facilities), combustion of biogas, rising and lowering tides, flowing ocean currents, wave action at the ocean surface, and rushing waters in a stream. Though definitely a mature technology, turbine design and operation continue to improve. Technical advancements continue to take place, innovative applications continue to be discovered, and new methods of installation developed.

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How Turbines Work
A turbine is a machine that performs a multi-stage transformation of latent energy into electrical power. Latent energy is available either in the form of chemical bonds, nuclear forces, or weight of gravity. The first uses heat to release stored chemical energy in solid fuel (peat, coal, and bulk solid waste), gaseous fuel (natural gas, propane, or biogas from anaerobic digesters), and liquid fuel (oil, gasoline, diesel, or biodiesel)—anything that can burn and has a significant latent heat energy content. The other source of latent energy involves the fission of unstable nuclei in atomic reactions. The last utilizes the weight of water stored in a reservoir to a height the provides enough pressure to directly move the turbine.

The first two methods include an intermediary step: the conversion of latent energy into heat energy. This is typically expressed in the form of water boiled to high temperature and high-pressure steam. Natural gas turbines use the expanding gases from the ignited gaseous fuel directly to spin the turbine in a manner similar to that of a jet engine. The components of this modified design include a compressor module, fuel injection ports, and a combustor unit. As with a jet engine, air intake sucks air into a compressor unit where it is compressed to much higher pressures and densities and mixed with the gaseous fuel. This compressed volume of mixed fuel and air is ignited, creating a rapidly expanding mass of high-pressure hot air (with temperatures as high as 2,900°F). The expanding mass of hot gas passes through the turbine like steam, turning the turbine’s drive shaft.

Most modern, highly efficient turbine designs of any type utilize combined heat and power systems (CHP) to capture the heat that was imparted to the steam or created by the expanding gaseous fuel. Heat exchangers use this radiated heat (which would have been wasted to the surrounding air) to provide heat for buildings and other facilities. Turbines that utilize CHP methods are usually referred to as “combined cycle turbines” and achieve very high levels of overall operating efficiency.

The heat energy available in the fuel that is burned to provide energy to run the turbine is measured in British Thermal Units (BTUs) per pound (the amount of energy needed to raise one pound of water from 60°F to 61°F at sea level). The fuel is ignited to release its latent heat energy. The heat is then directed via burner/flare units to a tank of stored water.

The amount of energy required is easy to calculate based on the weight of water, 8.33 pounds per gallon. Therefore, 8.33 BTUs are required to raise the temperature of a gallon of water by 1°F. Starting at room temperature, 60°F, water has to have its internal temperature raised to 212°F to become steam. This is a temperature increase of 152°F and requires at least 1,266 BTUs (at an assumed 100 percent efficiency translating chemical energy into heat energy) for each gallon being turned into steam. Older boilers have efficiencies in the range of 55–70 percent, while modern boilers can achieve efficiencies of 90–95 percent. So, a modern steam boiler would require 1,300 BTUs to 1,400 BTUs per gallon.

Once created, the steam becomes a super-heated working fluid that generates the force needed to operate the turbine. The force is a result of the pressure created by the expanding steam. Under this pressure, the steam flows through the interior of the turbine until it enters the chamber containing the turbine. The turbine consists of a rotating shaft whose axis of rotation is aligned with the flow of the steam. The shaft is equipped with vanes that are attached at an angle relative to the alignment of the turbine shaft so that, when impacted by the force of the expanding steam, the force is translated into rotation. The vanes are formed into impeller blades whose surfaces are curved to maximize mechanical efficiency.

The shaft is connected to a rotating disc assembly called a rotor. The rotor is the moving part of the assembly that is set inside the stationary part referred to as the stator. The stator encases and surrounds the rotor, which spins inside of the stator. In its simplest configuration, the rotor is wrapped with coils of wire, while the stator is equipped with stationary magnets creating a strong magnetic field. Movement of a conduction wire through a magnetic field creates current within the wire. The rapid spinning of the coil wires on the rotor does exactly this, creating electrical current and electrical energy. Electricity then flows from the turbines output terminals to the electrical wire carrying the current to its users. In principal, every turbine will function in this manner whether they are massive hydroelectric dam turbines generating over 2,000 MW of energy for entire regions or portable microturbines producing 25 kW to 50 kW for specific applications.

In a manner similar to that of CHP, the excess heat generated by a gas turbine can be utilized to power a secondary turbine. This configuration is referred to as a combined cycle system. Once the hot, expanding gas resulting from combusting the gaseous fuel passes through the primary turbine, the super-heated exhaust can be reused instead of letting all that heat go to waste. In doing so, the combined cycle system can provide a 50 percent increase in overall efficiency. The exhaust heat passes through a heat exchanger where the heat is transferred to another working fluid—water—which it heats into steam. This steam in turn drives a traditional generator, creating more electricity. After passing through the secondary turbine, the steam is cooled back into a liquid state and sent back to the heat exchanger to be heated again and reused.

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Micro- and Small-Scale Turbines—Not Just a Difference in Size
A mentioned above, microturbines are at the small end of the power scale. Microturbines are divided into three broad categories based on their rated power output: 12 kW to 50 kW, 51 kW to 250 kW, and 250 kW to 500 kW or higher. Most are capable of integration with CHP systems, but the majority are used either for provided power to microgrids or as standby systems. Their ability to provide power after a quick start gives them the ability to serve as emergency backup power systems. Similarly, they can start up quickly when peak power demand exceeds supply, providing power shaving capabilities.

A well-designed microturbine really shines when it can provide outsized power performance when compared to its weight and size. They also provide high efficiency, which results in low emissions, with NOx emissions less than 9 ppm, by converting a much higher percentage of their fuel into actual power output. Their sizes are similar to that of a small refrigerator with research aimed at producing handheld models. The small size in turn allows for flexibility in mounting and installation, allowing a microturbine to fit into areas of limited floor space and reduced height. When outfitted with a sheet metal heat exchanger (commonly referred to as a recuperator), they can provide cogeneration and CHP capabilities, adding to their already high operational efficiency.

Small size does not limit design options. Different operational functions can be met with different internal configurations: single-shaft with a bearing configuration using either oil or air, or more complicated and sophisticated designs such as double-shaft with inter-cooled and reheat systems, a split-shaft for machine-drive applications. Inherent CHP capabilities can be provided with a recuperated CHP system or a simple unrecuperated single-cycle design.

Microturbines also operate at a wide variety of speeds, with shaft rotation speed of 40,000 rpm to 120,000 rpm. Microturbines operating at lower speeds are cheaper to own and easier to operate. The higher rotating microturbines, with higher operating temperatures or sophisticated CHP systems, are much more expensive. One reason is the need for construction from heat-resistant ceramic parts. But, depending on the application, the cost may be worth it since these systems can achieve overall operating efficiencies of 85 percent.

In addition to the more obvious applications of microturbines in emergency situations, they can provide steady, baseline power output to associated microgrids. Several power sources are available to provide power to microgrids, including distributed energy and renewable power sources (solar and wind) and electrical storage batteries (often used in conjunction with the variable output from renewables). They all provide appropriate scale power output to microgrids, but microturbines can have a significant advantage in production efficiency and operational lifetime. Right-sizing the power output to the receiving microgrid avoids several problems such as consistency of power quality, safety issues (especially during “islanding” situations), and integration of the microgrid with the main regional power grid.

Microturbines have other characteristics that make them preferable for microgrid applications. Many microgrids are serviced by renewable energy sources, but these can be highly variable and reliant on battery energy storage systems to provide a baseline and for when the renewables are not producing energy (like at night). Microturbines, on the other hand, provide a steady power load and start up quickly no matter what time of day or what the weather conditions are. They can also adjust their output to meet variations in daily power demand. Microturbines are also flexible enough to instantaneously cover load changes whenever a microgrid is connected to or disconnected from a main power grid.

Microturbines are operationally inexpensive, being relatively cheap and easy to maintain. Their ability to use flexible fuels gives them another cost advantage, utilizing fuels that are currently the least expensive on the open market. However, their overall efficiency may be less than standard-sized turbines due to their inability to take advantage of scale efficiencies. Capital costs (the startup costs associated with purchase, installation, instrumentation, controls, communications, data storage, staffing, and training) can range from $700 to $1,100 per kW. Standard labor (after properly trained) can run between $200 and $500 per kW. Specialized add-ons such as CHP systems cost between $75 and $350 per kW. Altogether, a microturbine system costs between $1,000 and $2,000 per kW, with annual operations and maintenance costs between $50 and $150 per kW.

Microturbines also have an edge in operational lifetimes. Fewer moving parts result in higher reliability, less maintenance, and fewer replacement parts. Typically, a once per year maintenance schedule is sufficient for the rugged units, with maintenance intervals of 5,000 to 8,000 hours. On a per kWh basis, these maintenance costs run between $0.005 per kWh and $0.016 per kWh, similar to small reciprocating engine systems with comparable power outputs.

Technical Advances and Continuing Research
Microturbine research and development continue on multiple fronts, but size and weight are primary concerns. Designers seek to reduce both the overall size and the weight of turbines relative to their power generating levels. By making them more compact, they make them more portable and can install them in new locations, replacing even portable batteries currently in use. Micro gas turbines—and even ultra-micro gas turbines—are in development for the 50-wattto 100-watt power range. Efficiencies are being improved (in terms of pressure ratios, inlet temperatures, recapturing waste heat via CHP, compressor performance, and turbine efficiencies). Further development is leading to turbines with power-to-weight ratios of 300 watt to 600 watt hours per overall kilogram of weight (the turbine and its housing, fuel tanks, electronic controls, etc.).

This work involves the reduction in compressor diameter, smoother radial inflow turbine blades, gas bearings, reducing the combustion chamber size and allowing it to use flexible fuels (natural gas, biogas, propane), increasing turbine rotational speeds, and using improved materials that are more durable and heat resistant. The goal is a tiny turbine that can replace and outperform batteries of equivalent size while producing more useful alternating current than the direct current generated by a battery. Technically, microturbines offer several additional advantages over traditional turbines: flexible fuel sources, low levels of emissions, and minimal maintenance requirements. By using gaseous fuels, microturbines can utilize natural gas, landfill methane, biogas from natural sources, gas from anaerobic digesters, propane, and even diesel.

Why the demand for microturbines? And where is the need for this research coming from? First, there is a need for a continuous and efficient power source in emergency situations, particularly during natural disasters. Second, grid electricity costs are increasing while natural gas costs are falling as a result of an abundance of gas produced by fracking operations. Third, flexible microgrids have a need for steady power supply as well as electricity from renewable energy sources. Fourth, low emissions from cleaner burning natural gas help meet mandated environmental standards. Fifth, small-scale power sources (particularly those that outperform batteries on a power to weight/size basis) provide much needed operational flexibility.

As research continues, however, it has several roadblocks to overcome. The first is the relatively high capital cost of a microturbine (relative to its power output). These are often specialized machines and as such would have limited production runs. Economies of scale are often not applicable to their production. Challenges limiting the adoption of microturbines include high capital cost, cheap grid power from major power grids, competition from reciprocating engines, and access to natural gas pipelines and supplies. Other areas of research include increasing efficiency, reducing operating costs, simplification of operations and controls, and utilizing flexible fuels from multiple sources (including hydrogen, syngas, and synthetic fuels).

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New Wind- and Solar-PoweredTurbine Applications
One increasingly obvious and popular example of an electric power turbine is one powered by the wind. Indeed, next to solar panel arrays, the electrical turbine windmill has become the symbol of the distributed energy industry. It continues to be the focus of ongoing and intense research and development to improve efficiency, reduce weight, and minimize costs.

However, this type of turbine has its vanes on the outside of the turbine body structure, those huge blades that spin with the slightest breeze. While experiencing both improvements in efficiency and decreases in costs over the past few years, there is an upper theoretical limit to how much energy a windmill can produce. In 1919, physicist Albert Betz discover what is now referred to as the Betz Law Limit, which states that the fundamental laws of conservation of mass and energy limit the amount of wind energy that can be converted into mechanical power (and therefore electrical energy) to only 16/27 (59.3 percent). Improvements in turbine design in recent years have approached 80 percent of this theoretical limit, or 47.4 percent of the energy in the wind.

Also, the amount of energy available in the wind is a function of the cube of the wind speed. So, doubling the wind speed increases the available energy by a factor of 8. For a wind turbine the energy is equivalent to that of braking the wind. By doubling the wind speed, the blades perform twice as many slices of wind moving through the rotor every second, and each of those slices contains four times as much energy.

Given these physical characteristics, the process of designing a wind turbine involves extracting the maximum amount of available wind energy. Doing so involves pointing the turbine axis of rotation so that it is aligned with the wind direction, curving the rotor blades so as to efficiently convert as much of the directional wind energy into rotational spin, efficiently controlling the operation of the wind turbine (starting, stopping, etc.), and optimizing the physical support structure of the wind turbine (hub, generator, support pylons, foundation support, etc.).

Although mechanical design, such as the aerodynamic shaping of the rotor blades to maximize efficiency, is important, what is critical is proper integration of the wind turbine into the local power grid or microgrid. This is done in coordination with dozens or hundreds of wind turbines arrayed together in a wind farm, each feeding a separate stream of electrical energy into the grid. Intense research continues in all of these areas, yielding incremental but cumulative improvements in efficiency and performance with each new cycle of design and innovation.

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Less obvious or common is a new niche category of advanced solar-powered turbines. A solar turbine generator is defined as “a device that uses steam from a solar power system to convert the sun’s heat into usable electric energy.” It represents a whole new area of turbine research and development. Heat is absorbed to turn the water into steam using either a concentrated solar panel system or a thermal solar panel.

A solar turbine has one distinct advantage over a photovoltaic solar cell array: The solar turbine generates alternating current, while a solar cell produces direct current. It also has advantages over traditional wind-powered turbines, such as a steady and consistent power output that is not so highly variable. This direct current has to be passed through the inverter, which transforms it into alternating current, but at a high cost in overall operational efficiency. The solar turbine avoids this step completely while being just as “green” as the solar cell.

Furthermore, it is all done with mirrors. Concave mirrored panels concentrate reflected sunlight on a focal point containing a reservoir of water. These panels can be attached to a mechanical system that allows them to track the sun across the sky while keeping the focal point heated. This provides maximum operational efficiency throughout the daylight hours. The intense heat of the concentrated sunlight at the focal point flashes the water into high-temperature and high-pressure steam. The steam is pumped through tubes outside of the mirror array to a heat exchanger. There the steam heats up the turbine’s working fluid, also steam, and then condenses back to water that is recirculated back the focal point for reheating. The heated working fluid also becomes steam at the correct pressure and temperature needed to run the turbine. After leaving the turbine, the steam is also condensed into water for return to the heat exchanger. This exchange avoids the problem of variable solar heat caused by time of day or weather conditions and ensures a consistent temperature and pressure for the turbine’s working fluid. The overall process involves recirculation of the steam and working fluid in a continuous operating cycle.

Major Suppliers
Elliott Group manufactures steam turbine generators (STGs) that offer reliable, efficient, and cost-effective on-site power generation. Its customized STG sets support commercial energy requirements for continuous or standby power up to 50 MW. Elliot’s turbines are suitable for both renewable energy and green energy applications. Meeting industry standards such as API 611 and API 612, and National Electrical Manufacturers Association (NEMA) specifications SM-23 & SM-24, their installed systems include: steam turbine, speed-reducing gear, generator, integrated control system, lubrication system, and baseplate.

Sulzer’s core strengths are flow control and applicators. They specialize in pumping solutions and services for rotating equipment, as well as separation, mixing and application technology. Sulzer provides cutting-edge maintenance and repair solutions for turbines, compressors, pumps, motors, and generators dedicated to increasing customers’ life-cycle cost effectiveness. Sulzer is a service specialist that is renowned for its technology-based solutions, fast execution, and expertise in complex maintenance projects with a network of over 100 service sites around the world. Sulzer has been headquartered in Winterthur, Switzerland, since 1834.

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