Off-road hauling with hot air balloons
Floating a heavy towed load with the engine's waste heat. Could it work?
Last month, my father shared with me a series of books and diaries from the first white settlers that passed through our corner of North Dakota in search of land parcels made available via Abraham Lincoln’s Homestead Act. This land is very flat and dotted with marshes and sloughs; when folks eventually settled down, they would build a small homestead on their land allotment. The primary motive power to haul building materials, plow the earth, and get their grain to market was derived from horse or mule teams in conjunction with a limited rail network.
I’ve tried to imagine how it might be done differently today with the wealth of information and technology we’ve developed. Assuming that we’d want to settle a flat area with no roads and frequent marshes, simply moving heavy tractor-trailers would be very challenging. Your run-of-the-mill 4x4 or pickup might do fine in dry conditions, but anything heavier would face some serious obstacles. The same problem still pops up all the time in places with insufficient or war-damaged infrastructure.
After much daydreaming, I came upon this idea - what if we use the waste heat from an internal combustion engine (ICE)—which is >80% of the original fuel ‘s energy content— to help inflate a tethered balloon, which can then help reduce the ground pressure from a towed load? In short, we let the buoyant hot air reduce the weight on the wheels. To preempt some obvious criticisms, let’s walk through some of the things that this can’t do:
It won’t work for high speeds of travel; we’d need a hot air vessel with an impossibly large cross-sectional area to do any real lifting as the drag would be immense. Working in high winds would also be challenging.
There won’t be an easy on-off switch with instantaneous effect to levitate a load within the span of a minute or two. The heat output of an ICE is large, but so is the thermal mass of the gas that we’re using to levitate.
That said, there’s an obvious incentive to think about this problem - there is free energy (in the form of waste heat) being continuously thrown off by a running engine. It seems promising to pin down whether we could get enough lift to help reduce rolling friction or make a very heavy load suitable for fragile or nonexistent roads.
I’ll be playing rough and fast with the numbers used later. I am aware that semi trucks != gasoline engines. It’s just easier to get quick numbers for gasoline and since the energy density of diesel is ~10% higher, showing viability for the former implies it will work for the latter. I’ll also be ignoring the fact that waste heat is partitioned between an engine’s exhaust system and its cooling system. I’ll be pretending that they’re the same stream and that the heat can be transferred from engine to balloon with nearly 100% efficiency. The ideal system I’m imagining is an engine + exhaust system sitting in a thermal exchange medium which is constantly being cycled through the balloon’s heating elements.
Energetic considerations
What do the back-of-the-envelope calculations show, assuming a round balloon and a fully loaded semi? Drawing from Wikipedia and Wolfram Alpha, we have
Lifting capacity per cubic meter for a balloon heated to 100 degrees (C) is roughly 1 kilogram per 4 cubic meters, assuming ambient room temperature conditions. To float a fully loaded tractor-trailer setup (80,000 lb. or 36,000 kg.), we’d need an inflated balloon with a radius of approximately 30 meters, assuming a spherical shape. Interestingly, this is radius is a little bigger than the semi’s length (21 meters).
Thermal energy for inflation requires a more sophisticated calculation to account for the density of air as a function of temperature. Here’s a very rough number. It takes approximately 700 kJ to heat a kilogram of air from room temperature to 100 C. Scaled over the entire balloon and noting that the density of air is 1 kg / cubic meter, we end up requiring about 8.1 billion joules (2.1 GJ) to inflate our balloon. This sounds like a large number, but we’re forgetting that gasoline is incredibly energy dense. 8.1 GJ is only 400 kg. of gasoline; not that much more than the average semi’s fuel tank size. It is also important to note that much of this energy would otherwise be wasted. This number is also pessimistic because the final density of the air inside the heated balloon should be much lower than the density at room temperature
Balloon inflation time can be hand-waved away here. We know that the ICE is going to throw off all the energy contained in the fuel either as waste heat or mechanical work. Supposing we can capture it efficiently and get heated gases circulated into the balloon, this should only take as long as we need to run down the fuel tank.
At this point, you may want to raise the issue of thermal losses. After all, if it takes huge amounts of energy to stay aloft, then this might not be economical. However, we can take advantage of a few happy coincidences with this setup. First, we can have a sealed balloon (unlike a propane fueled one) because there is no need to let fresh air inside the balloon to oxidize burning fuel. Thus, convective losses should be much, much smaller.
To summarize, we have an appealing set of numbers here. For the waste heat generated from normal engine operations on a relatively modest fuel volume, we can inflate a balloon large enough to lift the entire fully-loaded truck. We would probably want to keep enough load on the ground so that the wheels can adequately drive the load forward (and also so that the wind doesn’t blow us away).
This opens up all sorts of interesting possibilities, though. The main obstacles to driving something heavy through a marsh or snowbank are (1) ground clearance, and (2) weight. Supposing we have enough of #1, why couldn’t we build an off-road truck capable of towing 30 tons across a field and across a pond? The only issue is controlling the amount of lifting power for each phase of travel.
I also think that this use case presents some interesting choices for optimization.
In the case of a stiff cross-wind, perhaps the balloon could be designed to look more like an aircraft wing, and generate large amounts of lift due to velocity differences between the top and bottom.
Using either solar-powered heating or direct insolation to raise the internal air temperature during the daytime could also help improve efficiency.
Comment with any criticisms or thoughts!
Fascinating. Would never have thought about using energy loss to turn every vehicle into an off-road vehicle.