Thermal Control Systems in Satellites: Loop Heat Pipes Technology

Space technology has advanced significantly in recent decades, and satellites play a crucial role in this evolution. The thermal control of the constituent elements and structures of satellites is vital to the proper operation of satellites in space. Heat management in space is a fundamental challenge, and one of the most effective technologies in this field is Loop Heat Pipes (LHPs).

Importance of Thermal Management in Satellites and all types of space structures

In space, satellites are primarily exposed to extreme temperature and thermal radiation conditions. The environment in space is quite different from terrestrial conditions when it comes to heat dissipation. In space, heat can only be eliminated or received by thermal radiation, which is the least effective form of transferring heat compared to convection (natural or forced) or thermal conduction, which can be used on earth. This can cause electronic components to overheat or, conversely, to cool down too much, affecting their performance and useful life. Therefore, an efficient thermal control system is essential to maintain temperatures within the appropriate operating ranges.

These systems are used in all types of satellites, whether for telecommunications, observation, navigation, science or exploration.

What are Loop Heat Pipes (LHPs)?

Loop Heat Pipes are heat transfer devices that use evaporation and condensation of a working fluid to transport heat from a hot source to a heat sink with minimal temperature difference. Its unique design enables heat to be transported efficiently and without the need for additional energy, making optimal use of the thermodynamic properties of the fluid and the geometry of the device.

The choice of ammonia, propylene and ethane as working fluid

The most common temperature range for satellites is from -70 °C to approximately 90 °C. In that range, ammonia is the most efficient working fluid used in LHPs due to its excellent thermodynamic properties. For certain cryogenic applications, it is necessary to drop down to temperatures as low as -160 °C, where propylene and ethane replace ammonia as the most efficient working fluids. All these fluids have a high latent heat of vaporisation, high surface tension and high density of their vapour phase, with low viscosity, making them ideal candidates for thermal systems in satellites in these temperature ranges.

The LHPs designed and manufactured at ARQUIMEA cover a wide operating temperature range, from -160 °C to +120 °C, and can transport from 1 W to 1200 W, making them well suited for extreme space conditions.

Operation of Loop Heat Pipes

The operating process of an LHP begins in the evaporator, where the heat generated by the satellite components causes the liquid to evaporate. This steam travels through a mission-specific steam pipe to the condenser, which is in contact with a radiator that emits and dissipates the heat into space. In the condenser, the working fluid is condensed back to liquid and returns to the evaporator through a liquid pipe. The evaporator is fitted with a capillary wick which is the element that provides the necessary pumping pressure to circulate the working fluid, overcoming the pressure losses in the different components of the LHP (evaporator, vapour line, liquid line and condenser line). This continuous cycle enables effective heat transfer, with a minimal temperature gradient between the heat source (heat sink component) and the cold sink (radiator), and with no moving parts.

The integration and testing of Loop Heat Pipes in satellites entails meticulous design and rigorous trials. LHPs must be carefully assembled and subjected to thermal trials and environmental tests to ensure that they survive the mechanical environment of the satellite launch and their operation in the extreme conditions of space, thus ensuring the reliability and efficiency of the thermal control system.

Advantages of Loop Heat Pipes (LHPs) in satellites

1. High heat transfer efficiency: LHPs are highly efficient in heat transfer providing excellent thermal conductivity, which is crucial for maintaining the temperature of the satellite components at optimal levels with minimal radiator size. Poor conductivity implies a radiator operating at a lower temperature, and therefore the need for a larger radiating surface, as the radiated power varies with the fourth power of the temperature.
2. Passive operation: They operate without the need for external energy, using only the capillary properties of the wick. This avoids energy consumption and reduces system complexity.
3. Reliability and durability: Since there are no moving parts, LHPs are extremely reliable and have a long useful life, which is essential for long duration space missions. This efficiency is maintained even when heat transfer is required over long distances of up to >5 m and for evaporator lengths from 25 to 400 millimetres.
4. Lightweight: LHPs are very lightweight devices.
5. Anti-gravity operation: LHPs can operate in an adverse orientation, with the condenser up to several metres below the evaporator.
6. Mechanical flexibility: The connection between the condenser and the evaporator is made with very light conveyor lines, which introduces little mechanical stiffness between the two components. In addition, flexibility can be increased to enable wide relative movements by using flexible hoses or ‘pigtail’ lines. This possibility is required, for example, for deployable radiators or to facilitate integration into the satellite.
7. Variable thermal conductivity: The thermal conductivity can be modulated so that the evaporator is thermally decoupled from the condenser in the event that the radiator becomes too cold. This can be done passively by introducing a pressure regulation valve in the steam line, or actively with a heater in the evaporator tank.
8. Adaptability to temperature variations: They can adapt to a wide range of temperatures, which is beneficial in the variable space environment.

Applications in Space Missions

LHPs on satellites are used in a variety of space missions, from communications satellites to interplanetary probes. For example, they have been successfully implemented on NASA and ESA satellites, where their ability to handle high thermal loads and their reliability have been repeatedly demonstrated.

ARQUIMEA has 52 units already in orbit on various missions and 38 units awaiting launch. Among the most recent Loop Heat Pipes manufactured are those for the state-of-the-art SpainSat NG active satellite antenna, a genuine technological challenge.

In addition, we have worked on major projects with leading space agencies and satellite operators on different international missions, such as ExoMars, Intelsat 19, Intelsat 20, Star One C4, Astro-H, Sentinel-1/2, EDRS-A/C, Alphasat, etc.

Challenges and Future of LHP

In the future, advances in materials and manufacturing technologies, such as additive manufacturing, promise to make LHPs even more effective and accessible (cost-effective), broadening their use in a wider range of space applications.

Conclusion

Loop Heat Pipes represent an advanced and efficient solution for thermal management in satellites. Their ability to transfer heat efficiently, lightly and without the need for additional power means that they are a crucial component of modern space exploration. As we continue to advance in our understanding and use of space, LHPs will play an increasingly important role in ensuring the success and sustainability of our space missions.