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".
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.
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.
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 metres."
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.