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BEE
cycle |
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Illustration of the
thermodynamic cycle for the Barton Evaporation Engine * *
patent pending |
Sunoba Renewable Energy Systems |
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The BEE
can use industrial waste heat in the form of hot air presented directly to
the BEE inlet. As an example, consider
the exhaust from an open-cycle gas turbine (OCGT), which is at atmospheric
pressure and typically very hot. The
exhaust is also somewhat humid, especially if inlet air to the OCGT is
evaporatively cooled before compression and if the fuel for the OCGT is
natural gas. Assume the
inlet air for the BEE is taken from the exhaust of a large OCGT with
compression ratio of 17:1 and heating rate of 840 kJ/kg air. Typical values are temperature of 500°C and
partial pressures for air and vapour 92,500 Pa and 8,800 Pa
respectively. The density of the air
is 0.442 kg/cubic meter. The inlet
data corresponds to Station 1 in the P-V diagram below. The air is
expanded to 4 times its initial volume (Station 2). Work is expended. The pressure, temperature and density
decrease, but not to the point that the air becomes saturated. Water at 20°C is sprayed into the chamber,
and evaporative cooling at constant volume occurs rapidly. The temperature and pressure decrease
further (Station 3). As the gas is
allowed to re-compress back to atmospheric pressure (Station 4), evaporation
continues to occur and work is received.
The theoretical nett work output of the cycle is 148 kJ/kg dry
air and the BEE exhaust is at 65°C, that is 435°C cooler than the inlet. Corresponding
to the above data, the actual work output of the OCGT is approximately
290 kJ/kg air, so the BEE can potentially make a significant improvement to
efficiency, without need for high pressure boiler or condenser, as with
Rankine cycle steam turbines. |
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The work
output of the BEE increases with the temperature and dryness of the inlet
air, and with the expansion ratio (up to the point where the air becomes
saturated upon expansion). The BEE
will theoretically deliver small amounts of power when used as an evaporative
cooler when the inlet air is, say, 35-40°C and with low relative humidity. None of
the above says anything about the engineering aspects of the BEE,
particularly how big it would be, how much it would cost to build, what are
the losses, and so on. |
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How quickly does evaporation occur? Provided the
droplets are sufficiently small, evaporation takes place in a fraction of a
second. Condensation is similarly fast,
as can be seen from this photograph of condensation clouds forming and then
disappearing above the wings of the space shuttle in flight. 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. |
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© Sunoba Pty Ltd 22 April 2010 |
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