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Evaporation
engine introduction |
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Introduction to Evaporation
Engines* * Patents pending |
Sunoba Renewable Energy Systems |
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Evaporation
engines were invented by the founder of Sunoba Pty Ltd in 2004 and have been
extensively investigated since then, as described in the References web page. You might think
that the thermodynamic cycle hot air + water →
power + cooled moist air violates the second law of
thermodynamics. How is it possible to
convert the heat energy in the vibrations of air molecules into power and
thereby cool the air, and yet not decrease the entropy in the overall system,
which would of course violate 2LT? The
answer lies in the fact that entropy increases as water evaporates into
vapour; the entropy gain in evaporation offsets the entropy loss in the air
as it is cooled. There is no violation
of the second law. The
evaporation engine uses a low-pressure gas cycle in which hot dry air is
expanded and then evaporatively cooled at reduced pressure. Evaporative cooling continues as the air is
compressed back to atmospheric pressure.
The moistened air is released to the atmosphere. Surplus work is available during the
thermodynamic cycle, which can be implemented in continuous-flow and
piston-in-cylinder configurations.
P-V plot of the piston-in-cylinder
evaporation engine as an evaporative cooler, with 10 stages during
re-compression. Inlet partial
pressures are 99.3 kPa (air) and 2 kPa (vapour); inlet temperature is
45°C. Outlet partial pressures are
98.1 kPa (air) and 3.2 kPa (vapour); outlet temperature is 25.5°C. The nett work available in the cycle is 788
J/kg dry air. The dotted line shows
the dry adiabat – i.e. re-compression without evaporation. As an
example of the output from a piston-in-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 average power output would be 4.9 kW/kg of
air. (At 30°C, 1 kg of air occupies a
little less than 1 cubic metre.) Full
thermodynamic analyses of the evaporation engine have been developed for
piston-in-cylinder (Article [4]) and
continuous-flow (Article [8]) versions. In these analyses, air and water vapour
were considered as ideal gases with constant specific heat capacities. The jump in internal energy or enthalpy on
evaporation (i.e. the latent heat)
was given as a function of temperature by interpolation from tabulated
values. In common
with all gas-cycle engines, the evaporation engine has a high back-work
ratio. For
piston-in-cylinder engines, the back-work ratio is the work required to
expand the gas divided by the work received during re-compression. Piston-in-cylinder engines will be
preferred when the inlet temperatures are not particularly high, say 150°C or
less. In its piston-in-cylinder form,
the engine will be quiet, large but unobtrusive, lightly stressed,
slow-revving and well suited for power
generation from passive solar heat collection. For
continuous-flow engines, the back-work ratio is the work done by a compressor
in taking the air from the low-pressure evaporation chamber to the outlet
divided by the work received by an expansion turbine as the air passes from
the inlet to the evaporation chamber.
The continuous-flow version involves greater expansion/compression
losses than the piston-in-cylinder version and will be similarly bulky;
however it is mechanically simple and therefore cheap to build. The continuous-flow version will be
applicable for utility-scale power generation at high
inlet temperatures, for example from the exhaust of Open-Cycle Gas Turbines. Evaporative cooling
is preferably achieved by spraying a fog of microscopic water droplets into
the expanded air, and evaporation takes place in a fraction of a second
provided the water droplets are sufficiently small. Various aspects of evaporation have been
studied within Sunoba Pty Ltd as follows: Technical Report 2007-1 was an experimental
investigation of the pressure drop caused by droplet evaporation at fixed
volume. Technical Report 2009-2 describes numerical
simulations for the rate of droplet evaporation. Technical Report 2010-1 looks at droplet
evaporation during the re-compression stroke in a piston-in-cylinder engine. The piston-in-cylinder engine
has been tested experimentally. The
photograph below is of the experimental engine under construction, and some modifications were
subsequently made before the engine first ran in August 2008. Features to note are the transparent cylinder,
the piston mounted on the chrome-plated steel rod, valve assemblies (inlet
and outlet) at each end, the air heater and the hot air ducts to the inlet
valve “bellows” (grey material at each end).
The water pump to drive the spray injectors is mounted at the front of
the engine and the injection nozzles are embedded in the inlet valves. Valves were activated by compressed
air. The whole device is controlled by
a computer (out of shot) connected via the black cable at the front. Article [5]
has full details.
This video clip (MPG, 2.81 MB) shows
four strokes of the engine in operation.
You can hear three separate noises – the “click” of valves as the
water injection is turned on/off, the “clunk” of the brakes that are applied
to hold the rod at the end of each stroke, and a “buzzsaw” sound of the air
compressor (that powers the valve actuators and the water pump). Since these experiments were carried out, a
superior piston-in-cylinder mechanism to that for the experimental engine has
been developed; details are only available under Non-Disclosure Agreement. To conclude this web page, it is stated
with confidence that the evaporation engine is based on a mature and well
documented theoretical analysis and has been experimentally verified. |
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© Sunoba Pty Ltd 12 May 2011 |
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