is maximized at a low ambient temperature the valve 302 closes lowering the effective thermal conductivity of the heat pipe 301 in fig3 a some optional elements are depicted in broken lines in particular fig3 a depicts an optional temperature sensor 310 connected to an optional controller 308 via an optional signal line 312 the controller 308 is connected to the valve 302 by the optional signal line 318 as an alternative to the temperature sensor 310 an optional temperature sensor 314 is connected to the controller via an optional signal line 316 when the controller 308 is present the valve 302 can be an electrically actuated valve that is driven by the signal on the line 318 based upon a temperature t 1 sensed by the sensor 310 or alternatively based upon the temperature t 2 sensed by the temperature sensor 314 the controller 308 causes the valve 302 to open or close when the sensed temperature t 1 or t 2 is greater than or equal to a reference temperature the controller preferably controls the valve 302 to open when the temperature falls below the reference value the controller causes the valve 302 to close by being positioned against the heat sink 106 the sensor 310 can sense the ambient temperature the sensor 314 can sense the temperature of the base plate 104 examples of electrically actuated valves are solenoid valves this will define the system 300 as an active system further in the alternative the valve can be a hydraulically actuated valve in which case the signal line from the controller 308 is a hydraulic rather than electrical signal line the base plate 104 is.