For more than two decades, nearly all renewable energy endeavors have incorporated the latest advances in turbomachinery design and turbomachinery manufacturing technology in an evolving effort to achieve and even surpass the economics of utility scale. Even mature turbomachines continue to be optimized for increments of better performance, efficiency, and energy conservation.

An experimental microturbine is designed to operate on concentrated solar-thermal energy when available, or alternately on biogas

The world’s growing demand for renewable, sustainable, and environmentally friendly electricity is being provided by a broad and fragmented mix of green energy technologies. And virtually all refined, new, and proposed technologies for generating “green” electricity rely on improved designs for turbines, compressors, expanders, pumps, and fans to make them economically and environmentally viable. Solar, wind, hydro, tidal, wave, geothermal, and biomass energy resources require advanced turbomachinery designs to efficiently extract power from low-density flows, and to help justify the value proposition required for project success.

The engineering practices perfected in traditional turbomachinery development are now being applied to the newest generation of turbo green equipment being incorporated into virtually all the latest technologies for renewable and sustainable power. Some of these turbomachines are relatively mature, but continue to be optimized for even better performance, efficiency, and energy conservation, as well as for new applications and special requirements. And the turbomachinery implications for each technology reveal many opportunities for performance improvements as well as potential engineering breakthroughs.

Wind Turbines and Compressors

The United States leads the world in wind energy production with over 30,000 wind turbines, and new installations are rapidly increasing. Utility-scale commercial wind farms are now considered a mature technology that is economically competitive (even with coal-fueled generation) in select coastal locations. Micro wind-capture designs for onsite power generation are now being developed to operate on gentle and less consistent winds.

Other regions with strong and consistent winds include flatlands of the plains states (Iowa already produces 15% of its electrical power by wind turbines), plus various mountain regions and coastal sites including offshore islands. Some of the most lucrative locations, however, are remote and currently restricted by the lack of a high-voltage transmission line infrastructure, or limited to practical offshore installations in depths to only 30 meters.

Several radically different and highly efficient axial and radial impeller designs are now emerging that will effectively accommodate urban and onsite installations. To be viable, these small wind turbines will require much less wind for start-up (as low as 5 mph) and for optimum levels of efficiency.

Wind turbine turbomachinery is also being further refined to reduce the environmental impacts of noise and bird kills. New bird-safe blade shapes are especially needed for offshore regions within migratory flight paths. In another wind-power cycle, a wind turbine is used to drive a compressor. The stored compressed air is available to drive a turbine generator (or some other application) at times when needed.

Solar Turbines

This three-bladed, horizontal-axis turbine is deployed underwater to generate clean, renewable energy from river currents.

As the ultimate renewable and sustainable resource, solar energy will continue to mandate incrementally improved (and less costly) photovoltaic installations, as well as inspire more effective solar-thermal technologies. Theoretically, capturing just 1/1000th of the sunlight falling on the Earth’s land masses could generate today’s total power consumption for all civilizations. Yet Germany, as the world leader, produces less than 1% of its electrical power from solar generation.

Both electrical and thermal-solar-powered energy systems will primarily focus on distributed generation for industrial/ commercial-scale sites. Utility-scale solar plants will require even more sophisticated and efficient turbomachinery to overcome several limiting factors that include only partial predictability for periods of solar generation and the amount of power that will be produced.

However, newer and better solar technologies using concentrated solar heat (at 400 °F and considerably higher) to drive a steam or gas turbine are promising to more effectively, efficiently, and practically utilize solar energy for electrical generation or to power some other thermal application such as an absorption chiller. The overlying turbomachinery challenge in all solar technologies and applications is to further advance turbine and pump efficiency in an effort to reduce their dollars- per-Watt installed cost.

Also, improved technologies for storing solar-powered thermal and electrical energy production are promising to make solar power practical and economically feasible in more regions. Significant breakthroughs in electrical and thermal storage could propel solar energy from a minor to a major contributor of green energy.

Hydroelectric Turbines

In many regions, the damming of rivers and streams to manage flows for hydroelectric generation has been an environmental disaster. As a result, it is highly unlikely that new hydro-electric dams will be permitted in the U.S., with the exclusion of dams for flood control, irrigation, and navigation.

There is still potential for improving the “water to wire” efficiency of most existing hydropower turbines, and with no further negative consequence to the environment. And there is also enormous potential for new hydroelectric designs that can provide continuous and predictable power from various low-head, low-power water flows — without the need for dams — that may also vary dramatically by location or season. With an energy density 850 times greater than wind, even slowly flowing waters can be an effective energy resource for a highly efficient hydrokinetic turbine.

Current and planned hydrokinetic technologies are attempting to extract the maximum amount of energy from stream and river flows (as low as four to five knots) without the need for dams. The higher efficiencies progressively achieved by these systems will ultimately determine the extent of their feasible applications. These same turbo-green technologies are also planned to operate effectively in the low flows of underground streams and falling water. The underwater streams of ocean estuaries are an especially reliable energy resource for driving a hydrokinetic turbine.

Tidal-Current and Wave-Compressor Turbines

Within the mix of natural (and national) energy resources, the enormous potential in repetitive ocean waves and tidal currents is now being addressed by several turbo-green technologies that propose to most efficiently extract energy without affecting the flow or the environment. Because all waterflow and wave-powered systems operate in a relatively harsh environment, turbomachinery must be designed to operate at peak effectiveness over a wide range of conditions, as well as be optimized for reliability and durability.

One promising technology is a tidal current turbine designed to harness energy from various marine currents. The unique Golay vertical-axis tidal-current turbine uses hydrofoil blades placed helically around an axis to operate bidirectionally as tides go in and out. An underwater tidal turbine farm would operate much like an offshore wind farm except for the added complexities involved in a relatively harsher underwater environment.

Another turbo-green solution uses an oscillating water column (OWC) to capture and convert ocean-wave energy into compressed air to drive an air-turbine generator. Ascending and descending waves alternately compress and evacuate air inside a chamber to drive the air-turbine- powered generator. The core technology incorporates a patented high-efficiency, variable-pitch turbine in which an electromechanical blade-pitch control enables rotation in the same direction irrespective of the bidirectional airflow of the OWC system.

Geothermal Pumps and Turbines

Newer geothermal electric-power plants using binary-cycle systems are capable of operating efficiently at relatively low temperatures of about 225 to 360 °F (compared to dry-steam or flashsteam plants). However, to further reduce the high capital cost per kilowatt of power generated, there is a great incentive to modify the turbomachinery used in this Organic Rankin Cycle (ORC) in an attempt to achieve maximum potential effectiveness that would reduce the high capital cost per kilowatt of power generated.

In a typical geothermal ORC cycle, a pump transfers hot geothermal brine from the production well to a vaporizer where the fluid’s heat is transferred to an organic refrigerant. The cooled brine is then pumped into a reinjection well. In a second loop, a pump circulates the vaporized refrigerant to power a turbine generator. The refrigerant is then condensed and pumped back to the preheater to again be vaporized in the unfired boiler heated by the hot geothermal fluid.

To obtain meaningful improvements, these turbomachinery systems require more efficient pumps and turbines. One such gravity head energy system (GHES) uses a compact and highly efficient turbo-expander pump installed deep within the wellbore. This highly advanced turbo-green design significantly increases the overall cycle efficiency by at least 20% and up to 30%.

Biomass Steam Turbines

A wood-chip-fired boiler can generate pressurized steam at 950 °F to drive a steam turbine, and biomass power plants can operate 24/7 or on call as needed. And although steam-turbine technology in a biomass cycle is considered quite mature, advanced capabilities in turbomachinery design will now allow even higher efficiencies, cleaner emissions, and reduced expenses.

Steam turbines have long been a natural green energy choice for onsite CHP plants with access to a nearby supply of low-cost biomass fuel. On a utility scale, a biomass-fueled steam-turbine plant is limited only by the size of the renewable “wood basket” available within a 30- or 40-mile radius. In heavily wooded regions, several operators of coal-fueled power plants are now converting to burn much cleaner biomass wood chips — and this trend is increasing.

The low-grade wood typically harvested for biomass fuels is mostly scrap from forestry management over a 40-year growth cycle. However, biomass resources are expected to dramatically increase as second-generation biomass fuels are produced from annual corn and switchgrass harvests. And a new type of tree with high fuel value and a 3-year growth cycle is now being genetically engineered specifically for biomass.

Biogas Turbines

Methane gas and other biogas wastes that are environmentally harmful byproducts of aging landfills, municipal wastewater treatment processing, and farm digesters can economically fuel clean-burning gas turbines that provide continuous electricity at efficiencies and rates approaching or comparable to a utility. Combined efficiencies are especially favorable (approaching 90%) when both the electricity and the heat produced by the turbine can be utilized in a CHP application.

Today’s second- and third-generation microturbines are now employing advanced and refined turbomachinery designs that permit operation at higher temperatures to achieve ever better efficiencies and cleaner emissions. And new turbine designs are in development for multifuel hybrid cycles that might, as in one case, operate alternatively on biogas fuel when solar-thermal energy is not available.

Many sites currently operating reciprocating- engine generators on biogas are retrofitting microturbines for onsite power because the latest turbomachinery technology provides significantly lower emissions, better reliability, longer service intervals, and reduced maintenance. Microturbine development is now focused on design advancements and technology breakthroughs that could eventually nearly double current efficiencies.

Other Turbo-Green Machinery

Nuclear power plants. Considering that nuclear fuel releases a million times (projected up to 50 million times) more energy per unit than fossil fuels, with zero harmful emissions, this alternative energy technology is gaining renewed interest for refinement — but not for application in the U.S. If nuclear generating capabilities are to be expanded in the U.S., what you can expect for future plants (and their turbomachinery systems) is one standardized design to dramatically confine costs. Variations to accommodate some variables will be developed and supplied by capable vendors.

Fuel cells. Although extraordinary electrical efficiencies have resulted from fuel cell and turbine-compressor combinedcycle, closed-loop systems, turbomachinery development is currently focused on future hydrogen supply-line compressors for fuel cells. Reciprocating compressors currently in use rely on lubricants that can seep into cylinders and contaminate the hydrogen. A centrifugal compressor avoids such contamination and allows higher compression efficiency for an equivalent duty.

A large-scale centrifugal compressor now being designed for hydrogen pipeline transmission is expected to service a future demand to be created by the adoption of practical fuel cell power plants and cars. The 8,000-hp, six-stage compressor will move 240,000 kg of hydrogen a day at pressures up to 1,200 psi, and be considerably smaller (1/4 the size) and more efficient than compressors currently in use on hydrogen pipelines.

Energy storage. The rapidly advancing development and acceptance of green energy technologies for both distributed generation and to supply the smart grid, demand that renewable energy be paired with energy storage. With the exception of battery systems, all other storage methods depend on the efficiency of various turbomachines for pumping fluids and compressing gases.

Of recent interest is a new generation of flywheels using multi-ton carbonfiber rotors spinning on magnetic bearings in a vacuum. Each flywheel is expected to store up to several megawatts of backup power for buffering power fluctuations from intermittent energy sources (wind and solar) or to stabilize power for changing loads. In one system, a motor powered by green energy accelerates a flywheel to terminal speed, then the rotor maintains its inertial energy “indefinitely” until released by reversing the process and using the motor as a generator.

The success of a highly effective and sustainable turbo-green machine operating over a wide range of conditions can often depend on advances and breakthroughs in turbomachinery design and manufacturing facilitated by highly specialized CAE and CAM software systems. To meet optimum performance goals, many turbo-green impeller designs will utilize thin, highly sculpted, and closely spaced blade shapes. But to be cost-competitive with fossil-fueled power generation, these elegant solutions must be complemented with sophisticated toolpath programs for accurate and economical 5-axis milling.

This article was contributed by Concepts NREC, White River Junction, VT. For more information, Click Here .

NASA Tech Briefs Magazine

This article first appeared in the May, 2013 issue of NASA Tech Briefs Magazine.

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