The increasing cost of fuel and U.S. dependency on foreign fuel supplies has renewed interest in conserving energy and in generating electric power using otherwise wasted heat energy from prime mover processes. Such power generation systems are typically based on the thermodynamic Rankine cycle or the reverse Rankine cycles (i.e., a vapor compression, a.k.a. heat pumping). These systems can use water or organic-based (i.e., refrigerant-type) working fluids for Organic Rankine Cycle (ORC) power generation.
The Department of Energy (DOE) continues to be a driving force in the development of such energy systems. When one considers that approximately one-third of the U.S. energy requirements are used in each of the transportation and industrial sectors, it is reasonable that the DOE would support projects that could increase fuel economy of mobile and stationary sources of waste energy. However, the DOE presently needs to provide some impetus to initiate basic research in these areas to make these systems more cost-effective. Whether improving the efficiency of existing energy systems via waste heat energy recovery or generating power from renewable energy resources, continuing the evolution of efficient energy generation through innovation is necessary for industrial nations as well as nations that are continuing their economic rise.
Advanced turbomachinery design and cost-effective manufacturing remain the keys to efficient energy generation from waste heat recovery and renewable energy resources. This article will describe some of the more current energy programs that the DOE is helping to support, and will also explore some additional creative innovations in “green” power generation that can help minimize the United States’ dependency on foreign oil supplies.
A discussion of advanced energy recovery and mechanical and electrical power generation cannot be made without reference to the software advances made in analyzing the thermodynamics of these advanced multi-component systems. Credit must also be given to the extraordinary advances made to CAD and CAM analysis tools and manufacturing techniques to produce efficient turbomachinery components that can meet the challenge of energy conservation, while maximizing the available renewable energy for power generation. Such programs range from the latest efforts in recovery of waste heat from transport vehicles, to the recovery of carbon dioxide and its use in power generation systems.
Waste Heat Recovery for Electric Power Generation
There was considerable interest within the newly formed DOE during the 1970s to develop Organic Rankine Cycle systems for recovering heat from waste heat sources that were generated from prime movers. The only criterion for such prime movers was that they generate large amounts of waste heat for extended periods of time.
Systems developed for mobile vehicles can also take advantage of the cost reductions inherent in systems that are mass-produced in the hundreds of thousands per year. Examples of such systems are long-haul tractor-trailers, locomotive diesels, and barge tow boats. These vehicles have one very important common characteristic that is essential for cost-effective power recovery: continuous and large amounts of waste heat released from the prime mover, typically reciprocating diesel engines.
Some 30-kW waste heat recovery systems for long-haul vehicles were prototyped for the DOE in the 1980s (see Figure 1). This endeavor, along with sim-
ilar research, was in response to the 1970s oil embargo, which caused fuel prices to soar and the lines at gasoline stations to extend over one mile. This interest has been renewed by the DOE, as evidenced by their support of several fuel economy improvement projects, including waste heat recovery from long-haul diesel engines. Such systems can improve the fuel economy by as much as 5%, even while priority is given to maintaining the “best available emissions quality” standards.
The basic ORC technology can be drastically improved upon from systems first prototyped in the 1970s and 1980s due to the availability of advanced computer control systems that are also very compact in size, the advent of advanced materials and coatings, and advancements in CFD modeling of the thermo-fluid analysis required to produce more efficient turbomachines. These improvements would not only increase the overall efficiencies of the cycles, but would also reduce engineering design and analysis costs, and thus improve the simple paybacks for these systems.
Similar applications of ORC power systems were studied for integration with engines that could be fueled by landfill gas, which otherwise would be flared for its disposal. The economics of energy today may now make the compression of landfill gas (L-CNG) economical for use in on-site maintenance vehicles, or even city-wide passenger buses where compressed natural gas (CNG) is in common use today.
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.
CO2 Capture and Power Generation
It is interesting to consider the “advantage” that this fluid has in being a fluid that is almost universally considered to be best kept contained, rather than released into the atmosphere. There is considerable prospect in having carbon dioxide as the working fluid in a waste heat recovery system. We could contemplate that the CO2 containment vessel be the closed piping of a power-producing cycle that generates as much power from the exhaust gas of a fossil-fueled power plant as is needed to drive compressors for later sequestration of the CO2. The DOE has been sponsoring feasibility studies in this field for many years; in particular, cycles that operate in the supercritical region. In the supercritical region, the fluid can sometimes behave as a liquid and then suddenly as a vapor, depending on the small changes in temperature or pressure.
The capture of CO2 from the exhaust products of a conventional fossil-fueled (particularly coal) power generation facility has been given increased interest in the advent of recent “Cap-and-Trade” legislation. Research has been funded by DOE’s National Energy Technology Laboratory (NETL) for methods of capturing and sequestering CO2. Several of these studies have promoted the integration of CO2 capture with power generation using coal as the primary fossil fuel. For example, a project funded by NETL (Advanced CO2 Cycle Power Generation) for Foster Wheeler North America, focuses on a cycle that combines power generation using fluidized bed technology and coal to syn-gas production to generate power while also capturing 100% of the CO2. This cycle has the advantage of not requiring amines to absorb the CO2 from the exhaust products of coal combustion. An additional CO2 sequestration and solid-fueled power generation system, developed by Concepts NREC, reduces CO2 emissions while combusting coal or biomass. Clearly prominent once again is the need for advanced turbomachinery that can improve the power production and thus increase the cost-effectiveness for the user.
A common thread exists through these advanced, but very feasible, waste heat recovery systems, and truly the heart of the engineering challenge is the analysis, design, and fabrication of the necessary turbomachinery. For power generation, this includes turbines, compressors, and pumps. The turbomachinery can be axial or radial, impulse or reactionary type. The choice depends upon the fluid operating pressure ratios and volume flow rates, and these in turn depend on the temperature of the available waste heat, the temperature of the available cooling source, and the power required. The advances in machining technology include CAM software that can transform detailed engineering drawings of the turbine rotor almost immediately into machine code that can guide the 5-axis machining of the entire turbine or compressor impeller. The turbomachinery specialty software, combined with CFD technology, has enabled even the most sweeping or contoured, shrouded turbine or compressor blade to be manufactured without compromising the thermo-fluids structural engineering analysis for the blade shape in order to achieve maximum power recovery efficiency.
The common technical challenge that must be met for each of these turbomachines, in addition to the expert thermo-fluid analysis and design, is the proper selection of compatible materials between the working fluid of choice and the materials of construction for the high-speed turbine or compressor. While it is desirable to increase the operating limits of the rotor tip speeds to reduce the number of stages for a particular application (and thus reduce complexity and cost of the unit), the rotor materials must be chosen for their strength to survive the enormous centrifugal stresses, while also maintaining compatibility with the working fluid. Thus, the proper application of CFD and FEA in the design of the machine is necessary.
This article was written by Frank Di Bella, Large-Product Development Program Manager at Concepts NREC in Woburn, MA. For more information, Click Here