Thermal Efficiency of a Human Being

Thermal Efficiency of a Human Being
• A human body is essentially a biological machine. We will analyze its efficiency as compared to machines that we have invented.
• Discussions are included on the specific methods that our bodies use to lose waste heat from the many biological processes necessary to keep us alive, which stabilizes our body temperature.
• Discussions are consider exactly what happens to the food and food energy we ingest each day, which includes some amazing surprises!
• Many other related concepts and subjects are also addressed.



How many BTU’s does the human body generate?


"The human body maintains a basic minimum rate of heat production at about 250 Btu/hr during sleep, the heat equivalent of about 75 watts, and about 400 Btu/hr (120 watts) when awake but sedentary activity increases, the rate of oxidation of food, with its attendant release of energy, must increase. The level of heat production for light work will be about 650 Btu/hr (190 watts), the extreme value for heavy work, about 2400 Btu/hr (700 watts)."

400 BTU's awake and 250 BTU's sleeping


How many watts of energy in BTU's does the human body give off at rest for one hour?

1 met = 1kcal/kg/hr
1kcal = 3.97 Btu
At rest, the human body expends about 0.8 met.
So, a 70 kg person at rest generates 0.8 * (3.97 Btu * 70 kg) every hour.
That would be about 222 btu/hr, which is not a measure of watts, so....
Since 1 btu/hr = 0.293 watts, that same person generates 65.14 watts at rest.

How many watts does the body produce?
Adjusted for male-female mix the approx. total heat generated while seated is 350 btu/h or 110 watts/hr
During heavy work or athletics 1800 btu/h or 550 watts/hr
The total heats listed include latent heat (perspiration and respiration) which is roughly half the total. Men produce about 12% more total heat. (No adjustments for body weight were noted.)
Maximum intensity was not listed in my source, but Lab experiments have shown 800 watts over very short durations for some "normal" people.
An Analyst notation of 1000 watts for brief, intense durations seems reasonable in consideration of the televised international "World Strongest Man" contests.
Source(s):
ASHRAE Pocket Guide

• 1 food Calorie = 1,000 energy (heat) calories.
1 calorie = 4.184 Joules.
1 Watt = 1J/s.
1 kWatt = 1kJ/s.

If a body burns off 1,000 Calories of energy in say,1 hour of jogging, it's equal to 1,000 kilocalories of heat energy. (1,000,000 calories).
1,000kcal x 4.184kJ/kcal = 4,184kJ = 4,184kW in 3,600seconds.

4,184 kiloWatts over 3,600secs (1 hour) = 1.162kW (1,162 W).

This is NOT a 'perhaps' or an 'estimate', it's a correct calculation.

How does the human body generate heat, please explain in detail?
The heat is generated by all of the biochemical reactions carried out by cells. The liver is the main contributor of heat since it carries out hundreds of reactions. Every time that a "fuel food" - protein, carbohydrate or fats - is broken down, heat is released as a by-product. The actual product of this "tissue respiration" is a molecule called Adenosine Tri Phosphate (ATP) which is used by all cells for energy. For example each Glucose molecule will produce 38 molecules of ATP plus carbon dioxide and water as waste products and heat energy. The heat is passed onto blood so that, in the liver, blood entering the liver is cooler than blood leaving it! Shivering is actually just very rapid muscle contraction and each time a muscle contracts it needs ATP which is made by breaking down a fuel food. The equation for tissue respiration involving glucose is:-

Glucose + oxygen + ADP + Pi ----- Carbon dioxide + water + ATP + heat energy

When fats are respired they produce twice as much energy, in the form of ATP, than either carbohydrates or proteins.