RogerBW's Blog

Food pills and energy density 22 September 2017

In Marty Jopson's new book The Science of Food, he demolishes the idea of food pills by looking at the energy density of fat and working out the mass of fat-pills one would need to eat. But why would one restrict oneself to the energy that an unmodified human body can get from food?

One can look at energy per mass, and hydrogen tends to win there, but in practice the volume of a pill is more important than its weight. Let's assume a 1cm-wide cylinder, 3cm long, as the largest practicable pill; that's a volume of about 2.3ml.

Figures for human energy consumption vary, but let's assume about ten megajoules to be on the safe side. So to live on one pill per day we need about 8,500MJ/litre.

Chemical energy tends to run about 10-40MJ/litre, with kerosene at the top end. So unless we're taking a hundred pills a day, mere chemistry isn't going to cut it. No, we need to go nuclear, or at least RTG. We only need around 115W sustained, and a lot of that doesn't even need to be electrical - waste heat can heat the body, after all.

Unfortunately RTGs tend to be large – all that pesky shielding – and some people have a strange aversion to having powerful radioactive sources embedded in their bodies. So how about using an external power source? A Tesla Model 3 battery holds around 750kJ/kg, so that's only around 13kg for a day's power. And much of that weight can be saved by removing the respiratory and much of the digestive systems! (While vitamins and amino acids are still needed as humanity creeps incrementally towards perfection, they can be provided by food pills.)

So never mind food pills - what we need is rechargeable humans, as an intermediate stage on the way to nuclear-powered ones. You shall be like us.


  1. Posted by John Dallman at 12:41pm on 23 September 2017

    It gets better with fuel cells!

  2. Posted by RogerBW at 12:51pm on 23 September 2017

    Hydrogen's still horrible for energy per volume, though. Unless you can compress it down to the metallic phase, I suppose.

  3. Posted by Dr Bob at 03:35pm on 24 September 2017

    Surely quite a lot of the energy demand is pesky things like insisting on having a body temperature of 37C, or humans wanting to DO stuff like walk the dog, type entries into the accounts spreadsheet, play cricket or have sex? I'm sure you could streamline that down...

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