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I just saw the following block quote and table (Table 4.4-1) in this answer about temperature limits of spacecraft electronic systems. A source for the document appears to be http://vibrationdata.com/tutorials2/TR_2004(8583)_1_REV_A.pdf although I don't know it's the ideal source or not.

Question: What is a "diode heat-pipe" and why is it considered passive? If it is a thermoelectric device such as a Peltier cooler, then you can vary the current or just turn it on and off to control the rate of heat flow, so I wouldn't necessarily classify that as passive, but there may be more to the situation than meets the eye.

4.4.2 Thermal Uncertainty Margins

For the purpose of thermal uncertainty margin specification, thermal control hardware is categorized as either passive or active. Passive hardware uses a thermal uncertainty margin, whereas active hardware uses excess power as a thermal uncertainty margin. Examples of passive and active thermal control hardware for purposes of uncertainty margin are identified in Table 4.4-1.

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From a company that builds these:

Standard heat pipes will transfer heat equally in both directions. If the nominal condenser is hotter than the evaporator, then heat will flow in reverse, from the “condenser” to the “evaporator”. A diode heat pipe is used when it is necessary to prevent heat flow in the reverse direction. There are two basic types of diode heat pipes, Liquid Trap Diodes, and Vapor Trap Diodes. Note that a thermosyphon will also act as a diode heat pipe (the thermosyphon condenser is typically wickless, so liquid is not supplied to the nominal condenser).

A Liquid Trap Diode has a wicked reservoir located at the evaporator end of the diode heat pipe. The wicks in the heat pipe and reservoir are designed so that they can’t communicate with each other. During normal operation, the heat pipe behaves like a standard heat pipe. Heat applied to the evaporator and reservoir causes liquid to evaporate. The vapor travels to the condenser, and capillary action in the heat pipe wick returns the condensate to the evaporator. Since the reservoir wick is not connected to the main wick, the reservoir quickly dries out, and becomes inactive.

When the condenser becomes hotter than the evaporator/reservoir, the role of the evaporator and condenser are switched. Vapor evaporates from the hotter nominal condenser, and travels to the nominal evaporator and the reservoir, where it condenses. Since the reservoir wick doesn’t communicate with the heat pipe wick, any liquid that condenses in the reservoir can’t return to the nominal condenser. In a short time, all of the liquid is trapped in the reservoir. The main part of the pipe contains only vapor, so the only heat transfer from the condenser to the evaporator is by conduction through the heat pipe wall and wick, which has a much, much higher thermal resistance than the resistance during normal operation.

As soon as the evaporator and reservoir become hotter than the condenser, the liquid evaporates from the reservoir, and the heat pipe resumes normal operation.

So they don't contain any mechanical or electronic parts, which qualifies them as passive systems.

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  • $\begingroup$ Thanks! There may be a better way to determine if something is considered passive or active than whether it "contain(s) any mechanical or electronic parts." $\endgroup$
    – uhoh
    Commented Jul 10, 2018 at 9:37
  • $\begingroup$ Isn't a passive system one that has no moving parts per say? $\endgroup$
    – ironduke97
    Commented Jul 11, 2018 at 8:06
  • $\begingroup$ depends on context. In electronics, components like transistors are considered active, diodes and resistors are passive. $\endgroup$
    – Hobbes
    Commented Jul 11, 2018 at 9:06
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    $\begingroup$ In this area, "passive" means "there are no control parameters to the operation": It does what it does, without on/off or other settings that have to be considered. Once you have a pump that can be on or off, or a controller with a variable set point, etc, then it's "active". $\endgroup$
    – Bob Jacobsen
    Commented Jul 11, 2018 at 17:19

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