Balloon Engines for Solar Power
February 2009 Edition
author has proposed a significantly cheaper and more efficient variation
of the solar tower concept for generating renewable energy by using a
'balloon engine' instead.
In a paper
Hot Air Balloon Engine published in Renewable Energy journal, Ian Edmonds
of Solartran Pty Ltd outlines a system that can utilize the same thermal
updraft principles of a solar tower by using a large hot air balloon,
a tethering rope, a generator/ motor drive that is coupled to the balloon,
and a glazed solar collector.
As the balloon
ascends - the upstroke - "the buoyancy force delivers mechanical power
via the windlass to the generator/ motor".
At a predetermined
height "a vent in the side of the balloon is opened by pressure sensitive
means and a substantial fraction of the remaining warm air in the balloon
is discharged. The generator/ motor then switches to motor operation and
the partly empty balloon is hauled down to ground level in a down-stroke
that completes the cycle. As the buoyancy force due to the balloon is
larger on the upstroke than on the downstroke a positive mechanical work
output and a positive electrical output are obtained from the engine".
The paper says
at a height of 3,000 metres and with a balloon with an equivalent sphere
diameter of 44 metres, the predicted thermal efficiency of the engine
is about 6 per cent. "With a constant upstroke and downstroke velocity
of 5 m/s the time for one cycle is 1200 s (20 min) and the average power
output over the two stroke cycle is 103 kW. However, recharge time and
energy must be included. Recharge with four 0.55 kW industrial fans each
delivering 7.5 m3/s requires 1256 s. With recharge time and energy included
the average power output over the three phases in each cycle (41 min)
is 51 kW."
increases significantly with balloon diameter. For example an 88 metre
balloon rising to 3,000 metres would have a 37 minute full cycle and an
average power output of 0.47 megawatts.
For a 44 metre
balloon system the estimated overall project cost is $440,000. For the
88 metre balloon it is $1.76 million.
"The very preliminary
cost estimates suggest the installation cost of balloon engines would
be similar in cost to other renewable energy technologies such as wind
power and photovoltaic power," says the paper.
"Both the balloon
engine and the solar tower engines operate in the atmospheric temperature
gradient. However, the thermal efficiency of the balloon is much higher
as the engine can operate over several thousand metres while the solar
tower is restricted by technical constraints to operate over several hundred
envisages the solar engines would be used only at times of low wind, "in
particular during still, hot days in summer to supply the peak air conditioning
loads that occur in Australia at these times. In this way a farm of balloon
solar engines would complement a farm of wind turbines, which would not
be contributing power to the grid in these conditions.
operation time is economically possible due to the relatively low infrastructure
cost of this type of engine as compared with, for example, tower engines."
said he is now extending the concept to a similar engine that inputs the
warm, moist air from the cooling towers of power stations. As the moist
air rises in the engine and expands and cools, the condensation of the
vapor in the moist air maintains the air temperature higher than for dry
air. Thus the engine can operate to much higher altitudes of 10 kilometres
and is called a condenser engine, he said.