<p>A heat pipe comprises an evaporator section 5 having a radiation absorbing plate 6 a portion of an elongate tube 7, and a condenser section 10 at a distal end of the elongate tube 7 remote from the evaporator section 5. A flow control valve 20 is provided between the evaporator section 5 and the condenser section 10 to selectively interrupt communication between said sections when the temperature within the condenser section 10 exceeds a predetermined maximum. A temperature sensitive member, ( coil of memory metal 34', or discs 34") acts between a support plate 26 and a seat 36 located on a valve pintle 30 to urge the valve head 22 towards the valve seat 24 when the temperature of the temperature sensitive member 34 exceeds a predetermined limit. A return spring 38 is provided to bias the valve head 22 away from the valve seat 24.</p>
