Wednesday, December 31, 2008

The 'Passive House' and the Seal's Flipper

It's wintertime in the Northern Hemisphere and word of something snug, warm, and money-saving will certainly receive a lot of attention or a lot of envy. Therefore it is no surprise that the Passive House concept (Passivhaus in German) is being widely reported in the American news (the New York Times's most-emailed article for the past day or two) as a great European invention that could potentially help Americans cut their considerable expenditure on heating. One happy German family reports that they get "all the heat and hot water they need from the amount of energy that would be needed to run a hair dryer." How does it work? Wikipedia helpfully explains the following points of design that contribute to a warm house (though the diagram is still in German...):

  • Very efficient insulation (including super airtight windows)

  • Orientation to pick up solar energy

  • Using passive heat sources like body heat and appliances

  • Shunting ventilation through a heat exchanger

The last element is also known as a heat recovery ventilation system (description) and works by having incoming fresh cold air be in thermal contact with outgoing stale warm air, so as to transfer some of the heat from the outgoing air to the incoming air, and reducing the heat loss through the ventilation system. The air is not being mixed, rather they are fed through closely interlaced tubing that maximize heat transfer between the two streams. How does such a simple system achieve such high efficiency? The New York Times article describes an exchanger that retains up to 90% of the heat. This would seem incredible if not for the fact that animals of cold environments have been using the same passivhaus design for a long time.

Like a passivhaus, the seal has a superb insulation system of blubber and fur. On top of that, it has its own heat exchanger system to minimize heat loss from the body's interior to the external environment. This heat exchanger is the counter-current exchange system, found in the seal's flipper and in the limbs of many other animals. The problem that it solves is how to keep the extremities supplied with blood while losing as little heat as possible (because arterial blood coming from the heart is essentially at core body temperature - as anyone who has been disconcerted by the warmth of his own blood in the tubing at a blood donation drive can attest). In a counter-current system, the arteries (carrying warm, oxygenated blood from the heart) run parallel to and in contact with the veins (carrying cool, deoxygenated blood back to the heart), but with the blood flow in opposite directions, i.e. in a loop. At the steady-state, this will result in a temperature gradient in both vessels will be such that the innermost end is warmest, and the outermost end is the coolest.

This serves two purposes: (1) the blood returning in the vein at the base of the limb is warmer than the venal blood at the tip of the limb, hence the core body warmth of the animal is protected, and (2) the average temperature of the limb is reduced, which is desirable. After all, a high average temperature in the limb means that there is a higher temperature gradient between the limb and the environment, hence a higher rate of heat loss, by Newton's law of cooling.

So a simple arrangement of tubing can lead to dramatic improvement in function. Countercurrent exchangers are also found in other organs and body parts. For example, tuna use a countercurrent system in its body wall to keep its active swimming muscles warmer than the surrounding water. Fish also use a countercurrent system to maximize yield of oxygenation in their gills. A countercurrent arrangement can also be used actively (rather than passively), e.g. in the loops of Henle in the kidney, to actively concentrate the urine, and in the rete mirabile of fishes, to actively concentrate gas in the swim bladder. Doubtlessly there are many points of similarity between German houses and seals (or tuna, or kidneys) but the use of a countercurrent exchanger is probably one of the more ingenious among them!

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