Heavy load transporter utilizing pressurized hot air balloons.
The transport of heavy loads over difficult or environmentally sensitive terrain is problematic with conventional transport such as trucks. Helicopters are limited in load capacity and are expensive. Helium filled airships are one possibility. However, there are technical and economic problems in using helium as a lifting gas.
(1) Helium is expensive to purchase and to store. Helium also has a strong tendency to leak. When not in use helium airships must be stored in a hangar to avoid wind damage.
(2) Helium airships must shed ballast continually when loading and must continually take on ballast when unloading. This is problematic in difficult terrain and where partial unloading at different sites is required.
Hot air is a much more practical working gas for load transport.
(1) It is inexpensive, being produced by burning a readily available fuel – propane gas.
(2) Hot air lifters do not need to take on or shed ballast. When loading the lift capacity is increased progressively by burning propane gas. When unloading the lift capacity is progressively decreased by venting hot air and replacing with cooler air.
(3) Hot air lifters do not require a hangar as when not in use they can be stored out on the ground in a deflated state.
(4) Hot air lifters can be constructed by adapting commercially available recreational hot air balloons to create heavy lifters.
The Cameron Z-1600.
Fig 1 illustrates the hot air lifter of this invention 1. It comprises one or more large recreational hot air balloons 2. A lifting gantry 3 formed from aluminum beams bolted together is suspended from the balloons 2. Lifting cables 5 connect the load 4 to the lifting gantry 3. The lifting gantry 3 has positioned on its framework air blowers 6 which blow ambient air through a solid lower shroud 16 and a flexible upper shroud 15 into the opening of the balloons 2. Propane burners 7 are supported on top of the lower solid shroud and extend into the opening of balloons 2. The propane burners 7 are supplied from propane cylinders 8. The air blowers 6 have two functions. The first function is to rapidly inflate the balloons 2 with ambient air at the beginning of the lifting operation via the solid lower shroud 16 and the flexible upper shroud 15. During inflation the propane burners are ignited so that the internal air warms and begins to produce lift. When the balloons are fully inflated the propane burning is increased to raise the temperature of the air sufficiently to lift the loading gantry 3. The second function of the air blowers 6 is to produce a slight overpressure in the balloons so that they maintain their shape under the drag forces experienced when the hot air lifter of this invention is traveling through the air.
Each balloon fills into a nylon rope net 11. In Fig 1 only the left hand balloon is shown with a nylon rope net although in operation all balloons would fill into a nylon rope net. The nylon rope net 11 has three functions. (1) To reduce the tensile stress in the fabric of the balloon 2 when it is inflated with high temperature air and is under slight positive pressure generated via the air blowers 6. (2) To transfer a large part of the lifting stress from the balloon fabric to the nylon rope net itself and thence via guy ropes 12 and 13 to the lifting gantry. (3) To bind the balloons together into a stable group via guy ropes 13.
The loading gantry 3 is fitted with two or more electrically driven propellers 10 mounted on swivels so that the hot air lifter can be driven through the air. The propellers 10 can be manually or electrically swiveled about vertical axes so that the direction of the hot air lifter can be steered. All electric motor driven equipment such as the air blowers 6 and the propellers 10 are supplied electrical power from a petrol driven generator 9 fitted to the lifting gantry 3.
Typical operation during a day is as follows. During overnight storage the loading gantry 3 is resting on the ground with the balloons 2 in deflated state resting over the top of the gantry and on the surrounding ground. The whole may be covered by waterproof tarpaulins if stored for a long period. A lifting operation begins with the tarpaulins being rolled back. The generator 9 is started and the air blowers commence to inflate the balloons with cold air. When partially inflated the propane burners are fired up and continue to the heat the air until the balloon is fully inflated and the lifting gantry 3 lifts off the ground. The propellers 10 are started and the air lifter is steered to the loading point. Cables 5 are lowered by winches 17 and the load 4 attached. The propane burning is increased until the air is heated sufficiently to provide buoyancy to lift the load. The lifter rises to transport altitude, perhaps, 100 metres, then is driven and steered by propellers 10 to the unloading point. The balloon is maneuvered over the loading point by propellers 10 and possibly by guide ropes lowered to ground personnel. Parachute vents 14 are opened at the top of the balloons and hot air progressively vented to be replaced by cold air via the blowers 6 so that the buoyancy progressively decreases. When there is negative strain in the lifting cables 5 the load is detached. The cables 5 are wound in, the hot air lifter returns to traveling altitude and returns to the loading point for another load.
Performance of the hot air lifter of this invention depends on
(1) The lifting capacity and the energy (propane) consumption to maintain lift.
(2) The traveling speed and the fuel (petrol ) consumption per km travel.
The performance estimates below are based on a commercially available balloon envelope. The Cameron Z – 1600 has an enclosed volume of 45,300 cubic metres (equivalent shere diameter 44 m), envelope mass 513 kg and cost A $220,000.
The buoyancy force, Fb, experienced by a balloon of volume V is given by
Fb = Vg(r –ri) – Mbg (1)
Mb is the mass of the balloon envelope, g the gravitational acceleration and r and ri the density of air outside and inside the balloon respectively. The outside air density varies with pressure, p, and temperature, T, and may be calculated from r = pM/(RT). The molecular weight of dry air is M = 0.02896 kg/mol, the gas constant, R = 8.3145 J/(mol.K) and the air temperature T = T0+L.h where T0 is the sea level air temperature and L is the temperature lapse rate, -0.065 K/m. The pressure at altitude h is given by p = p0(1+Lh/T0)-gM/RL where p0 is the sea level air pressure. Assuming adiabatic expansion and compression of the air in the balloon the temperature of the air in the balloon, Ti, is given by Ti = Tc(p/p0)(1-1/g) where Tc is the temperature of the charge air and g, the specific heat ratio for dry air, is 1.4. With these relations the buoyancy force on the balloon, Fb, as a function of altitude, h, may be found.
For the air lifter of this invention a reasonable height of operation is 100 metres.
The graph shows that when the ambient temperature is 15 C and the internal air temperature is 60 C the lift generated by one Z-1600 is 7 tonnes. The lift decreases to 4 tonnes when the ambient temperature is 30 C. To restore the lift to 7 tonnes the internal air temperature needs to be increased to 80 C by burning propane . This range of temperature is easily accommodated by the Z-1600 or any conventional hot air balloon.
The lift capacity can be increased by increasing the number of balloons coupled to the air lifter of this invention or it can be increased by increasing the effective diameter of the balloons. The graph below shows the lift generated when the effective balloon diameter is increased from 44 metres (Z-1600) to 88 metres.
When the ambient air temperature is 15 C and the internal air temperature is 60 C the lift generated per balloon is 57 tonnes. The lift falls to 35 tonnes when the ambient air temperature increases to 30 C. To maintain lift at 57 tonnes the internal air temperature would need to be increased to 80 C by propane burning . A two balloon lifter would lift 114 tonnes.
Consider now the propane energy requirements of the air lifter in Fig 1 with two Z-1600 balloons. Assume the ambient air temperature is 15 C and a total lift of 14 tonnes is required. The input air for each balloon is 45,000 cubic metres and must be heated from 15 C to 60 C. The mass of the air is 52,000 kg so the heat supplied is 52,000 x 1007 x 15 = 784 MJ. The energy content of propane gas is 50 MJ/kg so 784/50 = 15.7 kg of propane gas is required. For the two balloons 31.4 kg is required.
Calculations show that the combined radiative and convective heat loss from the Z-1600 is 1.65 MW for a 40 C temperature difference between ambient air and internal air. This is largely compensated by radiant gain from sunlight during direct sunlight conditions in the middle of the day. However, ignoring sunlight gain, the rate of heat replacement required is about 1.65 x 3600 = 6000 MJ per hour. Thus the burning rate of propane during flight is about 6000/50 = 120 kg/hr for each balloon or 240 kg/hr for two balloons. The rate is about the same whether the ambient temperature is low (15 C) or high (30 C). As radiant gain from the sun is useful in compensating for heat loss it would probably be useful to operate on sunny days in preference to overcast days.
When traveling the hot air lifter has to overcome air friction. A conservative approach is to assume the air lifter behaves as a sphere (drag coefficient Cd = 0.1) of diameter equal to the effective diameter of the balloon – 44 metres in the case of the Z 1600. Then the drag force and power may be found from F = (1/2) Cd.r.A.v2. Power versus speed is shown for the Z-1600 air lifter below:
If the travel speed is 30 km/Hr the power to overcome the air drag force is 50 kW. Assuming the propeller efficiency is 60% the electrical power to the propellers is 83 kW. Assuming the petrol generator is 20% efficient the power of fuel burning is 415 kW and fuel consumption is 1500 MJ/Hr. The energy content of petrol is 44 MJ/kg so the fuel consumption when operating at 30 km/Hr is 34 kg/hr. The fuel consumption increases rapidly with operating speed so speeds of about 30 km/hr would be appropriate. The fuel consumption also increases as the square of the balloon diameter. Thus for the 88 m diameter balloon lifter using two balloons and lifting 114 tonnes the fuel consumption when operating at 30 km/hr would be 4 x 34 = 136 kg/Hr.
The combined fuel consumption for the two balloon Z-1600 air lifter adds to about 300 kg/Hr. Thus a 3.3 hour transport at 30 km/hr over a distance of 100 km would require 1 tonne of fuel. The lift capacity is 14 tonnes. Assuming the loading gantry plus equipment and personnel has a mass of 3 tonnes the air lifter could transport a 10 tonne load. The return journey without load would require much less fuel to maintain lift of only the loading gantry and personnel.
It is expected that with double the balloon diameter (88 m) the load able to be transported would be about 100 tonnes.
The capital cost is primarily the cost of the balloon envelopes. $440,000 for a two balloon lifter. Gantry, equipment, nylon netting and assembly of the order $300,00. Suggesting a capital cost less than $ 1 million.
Solartran Pty Ltd