The human body as a radiator - Air conditioning

[eemaestro]

After reading the thread "how many watts does the human body use?"

I calculated that my body requires 86 W of energy on average throughout the day.

However, I am wondering how much excess energy my body radiates as waste heat (and blackbody radiation) ? Obviously this depends on a lot of variables--body mass, pulse rate, rate of expenditure of physical energy, how much caffeine is in my body (causes blood vessels to dilate), energy of the Kcalories that have been eaten (as opposed to the
Kcalories my body actually needs), state of consciousness, etc.

I am just interested in a rule-of-thumb that people who design air conditioning systems use for waste heat dumped by people, awake and sleeping.

 

[russ_watters]

You've accounted for all of it. All of the energy we produce by burning food gets radiated (or convected) away as heat.

And your number sounds about right.

 

[ravenprp]

It is fascinating to think of the human body as a radiator and how much energy it radiates as waste heat. As you mentioned, there are many variables that can affect this, making it difficult to come up with a specific rule-of-thumb for air conditioning systems. However, there are some general estimates that can give us a better understanding of this concept.

According to a study published in the Journal of Applied Physiology, the average human body radiates about 100 watts of heat per square meter of body surface area. This means that for an average adult with a body surface area of about 1.8 square meters, the waste heat radiated would be around 180 watts. This number can vary based on factors such as body composition, clothing, and physical activity.

Another factor to consider is the body's metabolic rate, which is the rate at which the body consumes energy. The higher the metabolic rate, the more waste heat is produced. On average, a person's metabolic rate is about 1 watt per kilogram of body weight. So, for a 70 kg person, the waste heat produced would be around 70 watts.

It's also important to note that the body's waste heat production is not constant throughout the day. It tends to be higher during physical activity and lower during rest. Additionally, as you mentioned, factors like caffeine intake and state of consciousness can also affect the body's heat production.

In terms of designing air conditioning systems, engineers typically take into account the average metabolic rate and body surface area of occupants to determine the amount of waste heat that needs to be removed from a space. They also consider factors such as insulation, ventilation, and outdoor temperature to ensure the system can effectively maintain a comfortable indoor temperature.

In conclusion, while it's difficult to determine an exact rule-of-thumb for the waste heat produced by the human body, understanding our body's role as a radiator can give us a better appreciation for the complexity of designing and maintaining comfortable indoor environments.