Barton evaporation engine
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The classical Carnot heat engine
The evaporation engine works by evaporative cooling of dry air at reduced pressure. Key steps are: (1) adiabatic expansion of unsaturated air; (2) evaporative cooling at reduced pressure; and (3) re-compression back to atmospheric pressure with further evaporation. Net work is available in the cycle, so the engine produces power and cooled moist air from water and hot dry air:
- hot dry air + water → power + cooled moist air
The remarkable property of the evaporation engine is that the temperature of an air stream is reduced at the same time that power is produced. This occurs without violation of the 2nd Law of Thermodynamics because the entropy increase as water is evaporated outweighs the entropy decrease as the air cools.
With a modest amount of passive solar pre-heating, the engine theoretically is able to produce power in hot arid climates. As well as being a heat engine, the evaporation engine can also be used as an evaporative cooler.
The evaporation engine has broadly comparable theoretical efficiency to simple Rankine steam turbines, without need for high-pressure boiler or condenser. The evaporation engine can function well on industrial waste heat, particularly the exhaust gas of open-cycle gas turbines.
The thermodynamic cycle can be achieved by at least three separate mechanisms. The most straightforward mechanism is a piston-cylinder device, for which a full thermodynamic analysis was published in 2008. Barton also built an experimental piston-cylinder engine. that provided confirmation of the theory.
As a second option, the evaporation engine can also be configured in continuous-flow form, for which a full analysis was published in 2012.
There is a third possible manifestation based on the Bernoulli effect for compressible gases. As a compressible gas flows through a narrow orifice, the pressure and temperature decrease, thereby allowing the possibility of evaporative cooling at reduced pressure in the high-speed section. On recovery to slow speeds, there will be surplus pressure that can drive a turbine. Barton has also analyzed this mechanism. The analysis has not been published but is available on request to Sunoba Pty Ltd. The Bernoulli turbine would face extreme (perhaps insurmountable) difficulties in construction, much more so than with the other two versions.
In general, the efficiency of the evaporation engine increases with the inlet temperature and the expansion ratio. As an example of the output from a piston-cylinder engine, air at 30°C and 47% relative humidity pre-heated to 85°C can theoretically deliver 4.9 kJ work output per kg of dry air by evaporation of 19 ml of water per kg of air at an expansion ratio of 1.64. If the cycle time is 1 second, the theoretical power output would be 4.9 kW/kg of air.
Barton (at www.sunoba.com.au/previous) gives an example of the evaporation engine as an evaporative cooler, that is operating on ambient air without heating prior to the inlet. The inlet conditions were: temperature 45°C, partial pressures 99.3 kPa (air) and 2 kPa (vapour). The volume expansion ratio was 1.2 and the outlet conditions were: temperature 25.5°C, partial pressures 98.1 kPa (air) and 3.2 kPa (vapour). Under these conditions, the net work available in the cycle is 788 J/kg dry air.
If the inlet air is sourced from an open-cycle gas turbine exhaust at around 500°C, Barton has shown that the evaporation engine can provide about a 20% boost to the power output of the gas turbine. It should be noted, however, that the power boost depends sensitively on the adiabatic efficiency of expansion and compression.
A key issue with this engine is the water consumption, which can be prohibitive for low expansion ratios and low inlet temperatures. The engine works best in hot dry climates, but those are typically the locations where water is most scarce.
Other studies by Barton involving the evaporation engine include:
- Pre-heating of the air prior to the engine inlet using a horizontal double-glazed canopy.
- Pre-heating of the air prior to the engine inlet using a sloping double-glazed canopy.
- Integration of the evaporation engine with thermal storage in a pebble bed.
Abstracts and comments on all cited articles are available at www.sunoba.com.au/references.
- Drinking bird - a toy that works on similar principles.
- Energy tower (downdraft) - another embodiment of the same thermodynamic cycle for multi-MW energy plant
- N.G. Barton, “An Evaporation Heat Engine and Condensation Heat Pump”, ANZIAM J, Vol 49 (2008), 503-524.
- N.G. Barton, “Experimental Results for a Heat Engine Powered by Evaporative Cooling of Hot Air at Reduced Pressure”, Proc ANZSES Conf, Sydney (2008).
- N.G. Barton, “The Expansion-Cycle Evaporation Turbine”, J Eng Gas Turbines and Power, 134 (2012), 051702.1-051702.7.
- N.G. Barton, “A Heat Engine and Heat Pump based on the Bernoulli Effect”, 18 pp, Sunoba Pty Ltd (2006).
- N.G. Barton, “Annual Output of a New Solar Heat Engine”, Proc AuSES Conf, Canberra (2010).
- N.G. Barton, “Output of the Evaporation Engine (Sloping Canopy)”, Proc 2011 Solar World Congress, Kassel (2011).
- N.G. Barton, “Simulations of Air-blown Thermal Storage in a Rock Bed”, Applied Thermal Engineering 55 (2013), 43-50.
- N.G. Barton, “Passive Solar Power Generation with Air-blown Thermal Storage”, Solar2012, Australian Solar Council, Melbourne (2012).