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 lowhead, 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 tidalcurrent 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.
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.