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.