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Experimental Investigation of an Inverted Brayton Cycle for Exhaust Gas Energy Recovery

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Experimental Investigation of an Inverted Brayton Cycle for Exhaust Gas Energy Recovery Proceedings of ASME Turbo Expo 2018 Turbomachinery Technical Conference and Exposition GT2018 June 11-15, 2018, Oslo, Norway GT2018-75386 Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT2018/51173/V008T26A003/3768225/v008t26a003-gt2018-75386.pdf by guest on 28 September 2021 EXPERIMENTAL INVESTIGATION OF AN INVERTED BRAYTON CYCLE FOR EXHAUST GAS ENERGY RECOVERY Ian Kennedy Zhihang Chen University of Bath University of Bath Bath, UK Bath, UK Bob Ceen Simon Jones Colin D Copeland Axes Design Ltd HIETA Technologies Ltd University of Bath Malvern, UK Bristol, UK Bath, UK ABSTRACT the vehicle. A window of opportunity still remains for this until Exhaust gases from an internal combustion engine (ICE) alternatives such as electric vehicles become more readily contain approximately 30% of the total energy released from available and cost effective. Recovering energy from exhaust combustion of the fuel. In order to improve fuel economy and gases is one key means of improving the efficiency of ICE reduce emissions, there are a number of technologies available powered vehicles. Turbomachinery can be used to recover this to recover some of the otherwise wasted energy. The inverted energy using technologies such as the IBC, which is shown Brayton cycle (IBC) is one such technology. schematically on Fig. 1. The purpose of the study is to conduct a parametric experimental investigation of the IBC. Hot air from a Coolant Heat turbocharger test facility is used. The system is sized to operate exchanger using the exhaust gases produced by a 2 litre turbocharged engine at motorway cruise conditions. A number of parameters Electric are investigated that impact the performance of the system such C T as turbine inlet temperature, system pressure drop and machine compressor inlet temperature. The results confirm that the output power is strongly affected by the turbine inlet temperature and system pressure Exhaust Engine gas drop. The study also highlights the packaging and performance advantages of using a 3D printed heat exchanger to reject the FIGURE 1: INVERTED BRAYTON CYCLE excess heat. Due to rotordynamic issues, the speed of the system was limited to 80,000 rpm rather than the target 120,000 rpm. The IBC can operate using hot gases at pressures as low as However, the results show that the system can generate a specific atmospheric pressure or lower. This makes it applicable to any work of up to 17 kJ/kg at 80,000 rpm. At full speed it is estimated industry where such gases are available, not just the automotive that the system can develop approximately 47 kJ/kg, which industry. It also represents one of the few types of heat recovery represents a thermal efficiency of approximately 5%. that does not need to impose any additional backpressure on the internal combustion engine. Such backpressure is often viewed INTRODUCTION as detrimental to its performance due to the increase in pumping In the automotive industry, government legislation requires loss and combustion instability. manufacturers of ICE powered vehicles to meet stringent targets The cycle has been studied for a number of years by for exhaust gas emissions. Emissions such as NOx and different groups for various applications. For example, in 1919 hydrocarbons can be reduced through techniques such as exhaust Kohler [1] invented a process for operating combustion engines gas recirculation or catalysis, but production of CO2 is inevitable with expansion to sub atmospheric pressure and compression when hydrocarbon fuels are combusted. CO2 emissions of ICE back to ambient. A steam generator and condenser were powered vehicles can be reduced through improved efficiency of proposed to cool the gases after expansion. Hingst [2] proposed 1 Copyright © 2018 ASME a number of variants of the IBC with cooling after the turbine IBC could increase electrical efficiency by about 5%, at the using surface or spray type coolers. Hodge [3] evaluated the expense of thermal efficiency. thermal efficiency and specific work output of the cycle over a A number of papers have compared IBC performance with range of temperatures and pressure ratios. The thermal efficiency that of other heat recovery techniques. Copeland and Chen [16] was shown to increase with turbine inlet temperature. The compared the performance of an IBC with turbocompounding optimum pressure ratio was also demonstrated to increase with and the pressurized Brayton cycle, when applied as a bottoming turbine inlet temperature. The disadvantage that the cycle cycle to a turbocharged Otto cycle engine. They found that the requires larger components due to the operation at reduced IBC had superior performance, provided that the turbomachinery Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT2018/51173/V008T26A003/3768225/v008t26a003-gt2018-75386.pdf by guest on 28 September 2021 pressure was also noted. efficiencies are high enough. They compared the results in terms Various studies have investigated the performance of the of specific work and thermal efficiency. Bianchi and De Pascale IBC as a bottoming cycle for devices such as gas turbines. [17] compared the IBC (with and without condensation of water) Wilson et al. [4] investigated the business case of adding a fan with the Stirling engine and the organic Rankine cycle. They downstream of a gas turbine fitted with a downstream boiler. The compared the cycle performance using a number of different intent was to increase net power out through a greater expansion efficiency metrics based on heat used, heat available and ratio across the existing turbine by expanding to atmospheric efficiency compared with an ideal reversible cycle. They found pressure and below. A standalone IBC device operating off of hot that the organic Rankine cycle gave the highest efficiency using gases was also studied. The findings were up to 10 percent return all efficiency metrics, particularly at low heat source on investment for the standalone device and up to 30 percent for temperatures. The Stirling engine and IBC both had efficiencies the addition of a fan. Holmes [5] investigated the addition of an that increased with heat source temperature, with the Stirling induced draft fan to a marine gas turbine and found an increase engine having superior efficiency compared with the IBC. in power and efficiency of approximately 9%. Condensation improved the IBC efficiency to close to the Frost [6] proposed a combined Brayton and Ericsson cycle Stirling efficiency for high IBC turbomachinery polytropic where the Ericsson cycle is a bottoming cycle with expansion to efficiency. Lu et al [18] compared the performance of a very low pressure (0.04 bar absolute). The bottoming cycle is turbocharged engine with IBC versus a turbocompounding effectively an inverted cycle with isothermal compression as in system with decoupled turbine and continuously variable the Ericsson cycle. They predicted an efficiency similar to that transmission driven compressor. They found that both systems of a combined Brayton and Rankine cycle. Tsujikawa et al. [7] offered a significant improvement in fuel efficiency and power investigated intercooling in an IBC, finding that three stages can output at full load. Bhargava et al [19] compared the IBC with improve thermal efficiency by approximately ten percent. They the pressurized Brayton and the organic Rankine cycle in a found that a Brayton cycle with an IBC as a bottoming cycle had cogenerative application as a gas turbine bottoming cycle. a thermal efficiency of over 60% for a turbine inlet temperature Performance was evaluated using an energy saving index, with of 1500°C. Fuji et al. [8] showed that the IBC could either the Rankine cycle giving the best performance for most gas replace a Rankine cycle as a bottoming cycle or could be used as turbines followed by the IBC and then the pressurized Brayton an alternative to recuperation. Agnew et al. [9] showed that cycle. overall efficiency could be improved with higher inverted cycle Another means to evaluate and compare cycle performance inlet pressures when used as a bottoming cycle. is based on exergy efficiency. Zheng et al [20] investigated the Alabdoadaim et al. [10] investigated a combined Rankine, exergy efficiency and losses in a Braysson cycle. They found that Brayton and two parallel inverted Brayton cycles, and found that the exergy efficiency was higher than for a Brayton cycle and the system had a maximum thermal efficiency of 54%, achieved that the largest exergy loss was for combustion. Zhang et al [21] by altering the expansion ratio of the second inverted Brayton performed an exergy analysis on the system proposed in [12], cycle. Alabdoadaim et al. [11] studied a combined Rankine, finding the largest exergy losses to be in the combustor followed Brayton and inverse Brayton cycle and attained a maximum by the heat exchanger. Chen et al [22] used exergy analysis to thermal efficiency of 57.7%. Alabdoadaim et al. [12] also studied optimize the performance of a combined intercooled a combined Brayton and inverse Brayton cycle. They found that regenerative Brayton and inverse Brayton cycle. They found that a system with regeneration could achieve an efficiency of at low pressure ratios regeneration could significantly improve 49.36%. efficiency and at high pressure ratios intercooling had a marked Bianchi et al [13] investigated the use of an IBC for impact on efficiency. repowering existing gas turbines. They found that the cycle could Some investigations have been conducted into the increase the electrical efficiency by up to 30%. application of the IBC to microturbines. Henke et al [23] Tsujikawa et al [14] proposed an atmospheric pressure simulated the use of an IBC in a micro gas turbine combined heat turbine to recovery energy from a solid oxide fuel cell. The and power (CHP) plant. They noted that for small systems, small system was effectively an inverted Brayton cycle with two stage turbocharger components are needed and these suffer from high intercooled compression.
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