Hot air balloon with moon on the horizon
Hot air balloons burn propane to heat the air inside the envelope to a lower density than outside air, allowing the craft to lift. Could we replace propane with an electric heat pump to achieve the same effect, producing a electrically-powered, lighter-than-aircraft? Harry Valentine thinks it just might be feasible for a tethered version.

Goodbye Lead Balloon

The possibility of electrically powered airborne hot-air craft.

By Harry Valentine

Hot-air ballooning is a hobby for one a segment of the population and a sightseeing business for a small number of entrepreneurs. They could supplant sightseeing helicopters at several locations as fuel prices rise over the long-term future. To ensure safety they may be tethered to a ground location via a control line or cable. A hot air balloon stays aloft by burning propane or other combustible fuel to heat the air that is enclosed inside the balloon’s envelope. The density of the heated air is lighter than the density of the air outside the balloon.

Some modern designs of hot-air balloons and hot-air airships can actually carry a substantial payload and could be used in place of a crane at some locations. Airborne hot-air craft that operate in a very restricted range could operate using a tether. The tether could include an electrical power cable that may allow the craft to remain aloft without need for propane and related combustion equipment. The nylon material from which the balloon envelope is made may have a melting or thermal breakdown temperature at around 200°-C. That temperature restriction opens a possibility to use an alternative means of heating the air inside the envelope in warm summer temperatures.

One possibility for tethered operation would be to transmit electrical power to the craft and use it directly to heat banks of resistors in or under the balloon. Such a method may make inefficient use of available energy. Advances in air conditioning and heat pumping technology have increased the coefficient of performance (COP) to a level where up to 6-units of heat may be transferred for every 1-unit of energy consumed compared to earlier designs that transferred some 4-units of heat.

On a hot summer day the exterior heat exchanger or hot coil of an atmospherically cooled air conditioner could only reject heat to the hot outside air if the liquid inside that heat exchanger was hotter than the outside air temperature. If that temperature is 50°C or 122°F the temperature of the fluid in the hot coil may exceed 150°F. It may be compared to the blast of heat from an oven or a furnace after the door has been opened.

The hot coil of a heat pump may be located inside air the envelope of an airborne hot air craft. The cold coil could provide air-conditioning inside the passenger gondola and also cool the atmospheric air under the craft as it transfers heat into the air inside the envelope. An airborne craft that maintains buoyancy using heated air may use a compound or cascade heat pump. A liquid-to-liquid heat exchanger would be added between 2-independent closed-loop heat transfer circuits, each with its own refrigerant, compressor and choke valve. A modern cascade heat pump could provide a coefficient of performance of between 3-to-1 and 4-to-1.

A cascade heat pump would reject heat at much higher temperature inside the envelope while using electrical energy efficiently. The craft would initially be prepared in the same manner as hot-air balloons by pumping heated air from an external source into the envelope as it is being inflated. The cascade heat pump would be activated after the envelope is inflated to a predetermined shape and volume. A restricted-range tethered hot-air craft could receive energy from an electrical power cable attached to the tether.

The surface area of the hot-air craft may be sufficiently extensive to allow lightweight, flexible thin-film solar photovoltaic paneling to be attached to the envelope. The solar constant is 1366-watts/square-meter and the best thin-film solar PV material could operate at near 12% efficiency. Some 12 to 15-Hp (9 to 11-Kw) could be available to drive the cascade heat pump and transfer 45 to 60-Hp into the envelope. Modern state-of-the-art ceramic batteries or flow batteries may be carried onboard as a alternative energy source.

The airborne craft may carry a microwave dish that receives microwave power from a nearby ground-based location. That power may be converted to electrical power that drives the heat pump. Advances in magnetically coupled resonance could allow electric power to be transmitted to a receiver on the craft without needing wires. A portion of transmitted and onboard stored power could be used to maneuver the craft via ducted fans. There are several companies and entrepreneurs around the world engaged in developing a new generation of lighter-than-air and other hot-air craft. They may have the option to operate electrically.

Times Article Viewed: 11270
Published: 22-Apr-2008


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