Innovative Waste Heat Recovery Systems and Improvements with Advanced Turbomachinery
- Saturday, 01 May 2010
Is it too forward-thinking to contemplate that some of the energy of compression for the CNG fuel can be recovered during the necessary pressure reduction of the fuel as it leaves the onboard storage (the gas is initially stored at 3,000 psig and the pressure continually falls as the fuel is drawn from the fixed-volume tank) and is delivered to the engine fuel injection system, which requires that the fuel be only slightly above atmospheric pressure? Shouldn’t the hybrid vehicle engines operating at near constant power and speed have turbogenerators included to be sure none of the waste heat energy from the engine’s exhaust bypasses the engine’s turbocharger (assuming, of course, that a turbocharger is being used with these engines)? Once again, the advent of analyzing, designing, and prototyping such micro-turbomachines has been made possible by the advent of CFD software that can take the engineer’s design from the drawing board to the 5-axis machine, and ultimately into the test cell in a matter of weeks of development time — not the typical months required even 15 years ago.
CFD software isn’t really enough. It must be augmented with specialization software that can provide validated engineering design for specific turbomachines. Such software must include features that make it agile enough to actually provide an interactive experience for the engineering analyst. For example, the Agile Engineering Software Design System® includes single- or multi-dimensional analysis tools to match the CFD with the proper blade contour and boundary conditions, to expedite the final dimensioning of the mechanical system.
The U.S. still leads the world in the use of a plentiful natural resource: geothermal energy that can be tapped to convert the Earth’s thermal energy into electric power. Geothermal heat recovered from the Earth’s mantle, as deep as 10,000 feet, is being utilized for district heating and power generation. These waste heat sources typically have temperatures below 1,000 oF, and often require the use of organic working fluids to effectively recover the waste heat. Such systems can take advantage of the latest design techniques for maximizing the efficiency of the turbomachinery that is a major component of these power systems. For example, the analysis and design of turbine-driven pump units using turbomachinery CFD, integrated with the proper pump software tool, can be quickly performed, and can minimize the design iterations required to match the operating speeds of the turbine and pump.
Marrying Wind Turbines and Water Wave Energy Recovery Systems
Wind turbines have been industrialized to the point where they are among the fastest-growing renewable energy power systems. The industrialization of these systems is so commonplace that the DOE has some difficulty in justifying their further investment in such systems when very successful industrial giants are willing to bear the cost of basic research with the understanding that such research can be paid back with commercial sales. Land or offshore wind turbines that are rated at 1.5 to 2.5 MWe of power are not uncommon. However, the power output of such turbines is affected by the variability of the wind velocities.
A more constant natural energy resource is water waves, and the oscillating water column (OWC)-type of water wave energy recovery system is designed to produce very high-speed air velocities on the order of 100 to 150 ft/sec. It has been estimated that there are 2 tera-watts/yr of power available from ocean waves. There are several well-known ocean wave power prototype plants in operation for research purposes, including LIMPET I and II on the Scottish Island of Islay and the European Pilot Plant on the Pico Island in Portugal. The DOE is also supporting the University of Hawaii as a research center for renewable energy, including ocean water wave research. The Puget Sound Tidal Energy in Washington, the University of Oregon, Maine Maritime Academy, and the PG&E utility in California also have tidal, hydrodynamics, and/or ocean water wave energy facilities and research in progress.
The OWC-type water wave power recovery requires a wind turbine design that can accommodate the constantly changing fluid velocities during each wave period. The vast natural resource of water wave energy warrants the active study of advanced wind turbine designs that can accommodate these continually changing velocities and pressures. For example, Concepts NREC is researching the use of a self-actuating turbine blade that can respond to the changes in air velocity by using the aerodynamic forces that act on the blade. The challenge is to match the torsion stiffness of the blade with its necessary tensile strength. The result of a successful design will be the development of a wind turbine that has the efficiency of a variable-pitch, unidirectional turbine with the lower cost of a fixed-blade turbine design that tends to have lower efficiency. But why not integrate an offshore wind turbine with a water wave energy recovery system, and thus share the costly electrical generator, electrical controls, power cabling, and the structural engineering that is often required to secure the system in the water? There is a cost-savings benefit in sharing the cost of these subsystems when the combined power from the often part-loaded wind or water wave air turbine will always be at close to a 100% rating, even as the wind and/or wave energy power varies due to local weather conditions.
Let’s see how engineering experience and advanced-technology high-speed air turbines can be combined with a current technology-concentrated solar energy system to remediate and environmentally restore a decommissioned open-pit mine to produce “green” energy. Figure 2 outlines the use of a pneumatic chimney tube that utilizes waste heat to induce high-velocity air currents to drive a wind turbine. This concept is not new, but its high structural cost to support a tower over 2,000 feet tall and 300 feet in diameter has made the concept impractical. However, it would be feasible if it was combined with a spherically shaped, open-pit mine that is ready for decommissioning and has also been retrofitted with concentrated collectors whose waste heat can be recovered via the multiple number of smaller-diameter pneumatic tubes that are now structurally supported by the sides of the mine.