Heat sinks, which sharing names with real world passive radiators found in computers, are actually complete heat pumps, not "true" heat sinks in the engineering sense . They serve as a BattleMech's means of protecting itself from internal damage caused by heat, as most 'Mechs' weapons easily cause enough heat to fry the machine's own electronics or melt its synthetic muscles, the myomers that make 'Mechs possible.
Heat sinks operate by collecting heat with coolant distributed to heat sources (weapons, engines, myomers, electronics, etc.) and delivering that to a radiator. Because a BattleMech may operate in environments considerably hotter than the interior of the 'Mech, the system includes a heat pump to "force" the heat out of the 'Mech by elevating the temperature of the coolant in some reversible fashion. (The vapor-compression heat pump of home air conditioners is a typical example, but 31st Century BattleMechs may make use of more exotic heat pumps.)
Note that if the heat sinks were true heat sinks or simple combustion engine radiators, a BattleMech operating in a hot environment would find the exterior heat being driven into the BattleMech rather than having internal heat rejected.
BattleMechs generally have two types of heat sinks: those mounted in the fusion engine and those mounted elsewhere on the chassis. The chassis-mounted heat sinks perform as described above, while the engine-mounted heat sinks constitute a "regenerative cooling" system that scavenges excess heat for power.
Heat sinks are made of a number of materials. Traditional heat sinks, the so-called single strength heat sinks, typically use radiators made of very thermally conductive oriented graphite (which may have up to five times the thermal conductivity of copper). Double heat sinks have largely replaced normal heat sinks. They dissipate twice as much heat for the same tonnage, but take up more space to operate effectively because of their use of a crystalline polymer. This polymer, similar to the engine shielding of XL fusion engines, is not as thermally conductive as graphite but significantly lighter and more durable, allowing it to be formed into a larger radiator for the same mass.
The actual coolant circulating in the heat sink varies from manufacturer to manufacturer and military to military. Heavy oils (hydrocarbon and silicone) are favored for their high boiling points and thus may be contained by low pressure tubing. Water-based coolants, typically modified with glycols, have nearly unbeatable heat capacities and are readily available for resupply. Freons work very effectively with the heat pumps in heat sinks. Some gases, like helium, also find use in heat sinks. Contrary to what might be thought, liquid nitrogen is a very poor coolant - it has very little heat capacity and boils far too easily.
Despite the heat pump, heat sinks are affected by the surrounding environment as a matter of basic thermodynamics - it takes less work to dump heat into a cold environment than a hot one, and water is a better coolant than air. Many 'Mechs on desert worlds often have to be refitted for the different conditions, or risk overheating from the compounded effects of weapons-fire and the environment. Conversely, heat sinks operating in cold regions and worlds dissipate heat much more effectively, as the environment's natively-cold temperature helps cool the BattleMech by default. This often leads to many garrisons on ice-worlds using a larger number of energy weapons, as they don't have to worry about heat as much. A 'Mech submerged in water, however, is often the one with the most efficient heat sinks as they can pump dense water (with its enormous heat capacity) over the heat sink radiators instead of low density air.
Heat sinks were introduced by the fledgling Western Alliance in 2022AD to deal with the heat loads imposed by the earliest battlefield energy weapons and to protect the first battlefield fusion engines from their own heat. Modern heat sinks retain the same features as their ancient predecessors: heat collection systems (typically flowing liquid coolant delivered to hot spots), a heat pump (which may or may not be the same as the coolant circulation pumps), and a radiator to drive heat into the environment. They have shrunk in size and are now offered in forms with greatly improved efficiency, but the basic principles of heat sinks have not changed.
- TechManual, p. 38, "Heat Pumpers"
- Classic BattleTech Companion, pp. 245-246, "Heat Network"
- MechWarrior, First Edition, p. 63, "Heat Sinks"
- Tech Manual, p. 220, "Heat Sinks"