Biological energy for transport

Worldwide road transport is the single most energy consuming sector of our economy. You probably found that the same was true in your country of residence. If it wasn’t top, it was probably second. The way we power our trucks and cars is therefore of prime importance to the world’s energy economy. And as you saw from the Sankey diagrams, almost all of the energy used nowadays to drive vehicles on our roads comes from oil, an ancient photosynthetic source. That won’t always be true. We’ll come to electric and hydrogen powered cars later this week and the biological energy we’ve used for transportation hasn’t always been oil.

The original meaning of the word drive is to force a living thing to move, and for thousands of years, that living thing was a horse. The horse holds a crucial place in the history of industrial biological energy. It was domesticated around 5,000 years ago, probably somewhere Northeast of modern day Turkey, and over the following centuries, was widely adopted for transport and agriculture. Horses can pull up to a third of their body weight. More than any other large domesticated animal and can run twice as fast as cattle can. Their combination of speed and power made horses our most important engines for millennia.

Horses rely on the same myosin and actin system to power their muscles as we humans do, and we can both deliver about one watt of power for every kilogramme of body weight. A horse, though, can weigh 10 times more than a human. As a herbivore, it can access photosynthetic energy more efficiently than we can. A man can work at a rate of about 80 watts, but a big horse weighing 3/4 of a tonne can deliver one horsepower, a unit we still is equal to 745 watts.

The coal powered steam engines we met in week 1 of this course could deliver many kilowatts equivalent to hundreds of horses and railways, steamships, and tractors gradually displaced horses from their traditional role in human society. Horses didn’t disappear, though. The domestic horse population in industrialised nations actually increased through the 19th century. In Britain, for instance, the number of horses grew from just over a million in 1811 to over 3 million in 1901, and they were mostly working animals. Horse numbers peaked during the First World War, during which the British army employed hundreds of thousands of draft horses to hold artillery, ammunition, and supply waggons.

We shipped more horse fodder to the front than human food, because horses were doing most of the physical work. Millions of them died. 21st century Britain is back to around a million horses again, but almost all of them are bred for recreation, not work. The internal combustion engine in both its petrol and diesel variants has replaced it for day-to-day use. Even the smallest modern car engines deliver close to 100 horsepower. Truck engines deliver twice as much power as the biggest Cornish steam engines of the 1850s. These engines are 1,000 times stronger than horse or human muscle, delivering one watt of power for every gramme of metal.