A new type of heat recovery and thermal conversion process is analyzed that produces electricity by recovering thermal energy over a wide range of temperature differences, including relatively small ones. The process induces fluid motion and drives a piston assembly connected to an electric generator. Temperature differences as low as 15oC produce useful power from the process. The performance can be enhanced through solar energy input, which would provide temperatures upward of 90oC at the collector side and increase the Carnot efficiency of the process significantly.
As a consequence, the device has the potential to reduce greenhouse gas emissions significantly, by allowing renewable energy resources and waste heat to be utilized and fossil fuel use to be correspondingly decreased. Hence, the device may have a role as a technology for mitigating global warming and the impact it is having via climate change.
The unit consists of two main parts: (i) one that absorbs heat from a higher temperature source and (ii) one that converts thermal to mechanical and finally electrical energy, before discharging the remaining heat to a lower temperature sink (see Fig. 14.1). The device uses pairs of vessels that are pre-pressurized with gas (dry air, hydrogen, or nitrogen) to absorb and transfer heat. The heated gas exits the pressure vessel and flows into an enclosed cylinder, which drives a piston to produce shaft power. Thermal energy is converted to kinetic energy via the working fluid, which is subsequently converted to mechanical energy in the high-pressure pneumatic cylinders, and a transmission system with gears designed to optimize the energy potential of the fluid.
I. Dincer et al. (eds.), Global Warming, Green Energy and Technology,
DOI 10.1007/978-1-4419-1017-2_14, © Springer Science+Business Media, LLC 2010
Fig. 14.1 Schematic of Marnoch power device. (a) schematic of overall system and (b) piston assembly.
Finally, the mechanical energy is transferred to an electric generator sized appropriately to the system. The device uses pairs of highly pressurized tanks in a closed loop, with operating principles similar to a Stirling engine, except the manner of heat inflows/outflows; valve assembly and other unique changes are aimed at overcoming the deficiencies of a Stirling engine.
The process operates similar to a conventional heat engine, which absorbs heat from a high-temperature source, produces power, and rejects waste heat to a lower temperature medium. In contrast to conventional heat engines that combust fossil fuels, however, the proposed system does not emit greenhouse gases and can operate effectively over relatively small temperature differences. Such heat source temperatures are available in many applications.
The Marnoch unit (Fig. 14.1a) consists of four cylindrical tanks, each fitted with helical copper tube coil heat exchangers. Each tank is connected to both a hot source and a cold source, via tubes and valves that allow a controlled mass flow at any step of the process.
In this chapter, which extends previous work (Armstrong et al., 2007), the operating performance and economic viability of the Marnoch unit for various operating circumstances and applications are examined. In particular, the performance and operation are studied of a prototype Marnoch engine developed at UOIT, for a range of operating temperatures and tank configurations. The environmental performance of the device, particularly with respect to global warming, is also assessed.
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