Vive La France Part 2 – The Power of Hot Bodies

In a New York Times Op-Ed (December 29, 2012), Diane Ackerman described an alternative way to get energy that will not pollute the environment – the power of a crowd. The first paragraph is cited below:

As I waited with a throng of Parisians in the Rambuteau Metro station on a blustery day, my frozen toes finally began to thaw.  Alone we may have shivered, but together we brewed so much body heat that people began unbuttoning their coats. We might have been penguins crowding for warmth in Antarctica’s icy torment of winds.  Idly mingling, a human body radiates about 100 watts of excess heat, which can add up fast in confined spaces.

The article reminded me of another non-polluting attractive energy alternative in the form of power-generating exercise bicycles that were used in Lyon-France to “power” outdoor display devices that I described in my December 17, 2012 blog,  “Vive La France.”

I don’t know whether Ms. Ackerman is French, but it seems to me that the French have the capacity to inspire thinking out of the box.

As it happens, the Op-Ed was published before New Year’s Eve, and my French cousin and her son were our guests in New York. Her son was my source of information for the Dec. 17 blog, so, this time, I had her read the Op-Ed and asked for her opinion.

Her first reaction was that her father (my uncle – a psychiatrist – now deceased) thought about a similar scheme while holding an overactive screaming baby. He mentioned that all this energy could be very useful if channeled to, say, light a house. Reading further, she quietly mentioned that a more acceptable solution would be to hold  a crowded sex party in an enclosed space close to some freezing spaces. It sort of seems logical that moving sex parties to spaces that need heat would be more practical than moving Metro stations.

Similar to the Lyon biking experiment, it seems to be a good opportunity to go to the numbers and to some important physics.  (The physics can sometimes sound like a foreign language and, to many people, it is.  Wikipedia is a great help with the physics concepts that I will talk about, so please feel free to consult it if you need to.)

So – why do we emit about 100 watts of excess heat? It turns out that every “visible” object in the universe emits radiation that depends only on the temperature of the object and the surface area of the object. This kind of radiation is called blackbody radiation. “Visible” objects are the regular objects that are all around us and are constituted of atoms and molecules. There are other kinds of objects that don’t emit any radiation – this is dark matter and we can locate it only through its gravitational force. We don’t see this matter around us, but, in the universe, we find five times more dark matter as compared with visible matter and we still have no idea about the structure of dark matter.

Human bodies are obviously a form of visible matter. The amount of radiation that we radiate as excess heat depends only on the temperature and the surface area of our skin. These two parameters are routinely measured because they are indicative of our health conditions. Our skin surface area obviously varies with the individual but they range between 1.5 – 2 square meters (16.1 – 21.5 square feet). Our skin temperature is more complicated – it protects our body temperature so that it’s kept approximately constant at 370C (98.60F), but it strongly depends on the ambient temperature and the length of our exposure to the ambient temperature. Furthermore, different parts of our skin can be at different temperatures depending on the cover that we provide. The amount of excess heat (the blackbody radiation) that we emit is very sensitive to the temperature. It varies as the 4th power of the temperature but the temperature is measured in scales that we are not used to. The scale is the “absolute” temperature (or more often called the Kelvin scale after William Thomson, the 1st Baron Kelvin). We just take the temperature in Celsius scale and add 273 to get the temperature in the Kelvin scale.

Since the skin temperature is not very well known, for argument’s sake, I will take the 100 watts that Ms. Ackerman took and the average skin surface area at 1.75 meters square (19 square feet) and get a skin temperature of 270C (810F). This number is perfectly reasonable. A small electrical heater that I often use when my heating system is not doing its job is rated at 1.5kw (1500 watts) with an adjustment to lower power. This would require 15 people (crammed into my room) to supply the same heat. This is perfectly doable but a somewhat crowded substitute.

I can also try to calculate the relative environmental impact:  assume that the 15 people are making me “comfortable” for a full day, eating normally, consuming on average 2500 food calories for the day. Their emission of carbon dioxide amounts to approximately 11kg (24 lb) (For details on how to do these kind of calculations from first principles, see my book Climate Change: The Fork at the End of Now – Momentum Press – 2011.) My electric heater, at the full power of 1.5 kw and working for the full 12 hours, uses 18 kwh of electrical energy. Electricity is a secondary energy source. It needs a primary source to generate the electrical power. Based on conversion practices, it is limited by physical laws not to exceed an efficiency of around 30% (depends on the conversion temperatures). My electrical company mostly uses natural gas to generate its electricity. Based on this input – the carbon footprint of my electrical heater can be calculated to be 8.1 kg (17.8 lb).

As with the Lyon example, this idea doesn’t look too attractive at the moment.

But here we have an important mitigating element.

The carbon footprint that’s created by burning the natural gas that generates the electricity that’s heating the room with the heater is new carbon dioxide that contributes to the chemical changes in the atmosphere and thus directly contributes to climate change.

However, the carbon dioxide that is being exhaled by our 15 volunteers is generated through the metabolism of the food that they eat. Directly or indirectly (eating meat), the food is produced by processing green products (vegetables, fruit or feedstock for the animals) that grow by sequestering the same amount of carbon dioxide through the photosynthetic process. So, if we calculate the carbon footprints throughout the full cycle, the only net addition of greenhouse gas to the atmosphere should come through externalities such as burning the fuel needed for the cultivation of the agricultural products.  And that number should be considerably smaller than that from the fuel that is needed to generate the needed electricity in the room.

So… if anyone wishes to try using hot bodies as an energy generating alternative, please let me know the results!

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About climatechangefork

Micha Tomkiewicz, Ph.D., is a professor of physics in the Department of Physics, Brooklyn College, the City University of New York. He is also a professor of physics and chemistry in the School for Graduate Studies of the City University of New York. In addition, he is the founding-director of the Environmental Studies Program at Brooklyn College as well as director of the Electrochemistry Institute at that same institution.
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