Convection

When the temperature of a bee or device at heat equilibrium remains constant, heat production equals heat loss. Input of heat from solar radiation or metabolism or resistances within devices equals the 'leakiness' to heat (heat conductance) times the difference between system temperature and air temperature. In the bee, this conductance measures how much oxygen is consumed to maintain heat balance, expressed as oxygen demand or milliliters of oxygen consumed per weight of bee per unit time.

Small devices or insects may maintain lower temperatures than larger ones. Small systems often produce heat at higher rates, but the higher rates require higher conductances, or put another way, smaller systems require increased circulation of coolant because of their larger relative surface areas. Large bees heat up while flying and elevate their body temperatures, even though the metabolic cost of flight per unit weight drops about two hundred and thirty percent for a tenfold increase in mass.

To increase the heat capacity of a system and decrease variations of temperature it experiences, its coolant must absorb large quantities of heat over a narrow range of temperatures. If a bee or device is small, and its surface to volume ratio is large, it either changes temperature with the surroundings, losing or gaining heat by convection as needed, or the system burns energy to cool or warm itself independently of the environment. In biological systems, enzymes catalyze metabolic processes that are efficient only over small ranges of temperature.

Cooling small computers or miniature systems requires novel cooling systems. Can we learn from bees and insects how they cool themselves, so we then may incorporate insect techniques into our devices?

Consider any small system. Because hemocoels dissipate so much energy from active flight muscles, hemocoels must remain cool. Because distances between interior points in the hemocoel and exterior points on the body's surface are so short, conductive cooling often suffices. On the other hand, if a hemocoel is to operate when a bee is cold, she may have to linger in the sun before taking off.

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