Balloon Engines for Solar Power

By Victor Bivell

Eco Investor February 2009 Edition

A Queensland 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."

Power output 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 metres."

The author 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.

"This limited operation time is economically possible due to the relatively low infrastructure cost of this type of engine as compared with, for example, tower engines."

Mr Edmonds 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.