by Ryan McGillicuddy
figures by Sean Wilson

When I think of the challenges associated with exploring space, I usually think of explosive rockets, speeding meteorites, deadly radiation, and the empty vacuum of space.

Admittedly, my first worry about space is not the freezing temperatures. But in reality, temperature control in space is a challenge that NASA constantly faces. For example, the sun-facing side of the International Space Station can reach temperatures over 120 °C, while the dark facing side can fall below -150 °C. Besides being uncomfortable, these temperature differences can strain the spacecraft and destroy electronics and biological samples. To battle the cold environment of space, scientists at NASA have gotten creative, and one solution was to store heat in thermal batteries. Just last year, NASA scientist Michael Choi used such a battery on the Neutron Star Interior Composition Explorer (NICER), an instrument for studying the remnants of stars that had exploded as supernovas. This thermal battery stored heat and kept NICER warm, preserving the machine’s functionality during its installation at the International Space Station.

Managing Thermal Energy Using Thermal Energy Storage

Thermal batteries are materials that store thermal energy. Though thermal batteries might sound unfamiliar, you may have one in your basement! The most common type uses sensible heat, where heat is stored by changing a material’s temperature, and larger temperature changes store more heat. For example, consider hot water tanks, where water is held at a high temperature, storing thermal energy. This thermal energy, stored in the hot water, can then be delivered in pipes to give a hot shower.

Another type of thermal battery can store thermal energy in phase changes, often solid-liquid, at a single temperature, the melting temperature. To get an idea of how much energy is stored in a solid-liquid phase change, consider ice, the most common phase change material for thermal energy storage. It takes the same amount of thermal energy to melt an ice cube at 0 °C, a solid-liquid phase change, as it does to heat water from 0 °C to 80 °C. The large amount of thermal energy stored in the phase change allows more energy to be stored over a given temperature range versus a sensible heat battery, allowing for smaller, more energy-dense batteries.

So how can this phase change be used? Solids absorb heat when they melt and liquids release heat when they freeze (figure 1). The heat released or absorbed during a solid-liquid phase change, called the heat of fusion, can regulate the temperature of an object. To understand this temperature regulation, consider a hot object, like electronics, in contact with a phase change material. The object, when overheating, transfers its heat into and melts the phase change material. Later, when the electronics are not in use, the phase change material freezes and releases heat, warming the electronics back up. The result is temperature regulation without external energy input using the thermal energy stored and released during solid-liquid phase changes.

Figure 1: Temperature vs. Energy plot of a phase change thermal battery. During charging, heat is transferred into the solid material, increasing its temperature until the melting point is reached. Then, the material melts by absorbing a large amount of heat at a constant temperature. Finally, fully in its liquid phase, the material absorbs heat and increases temperature again. During discharging, the material freezes and cools down, releasing the absorbed heat.

Phase Change Materials as Thermal Batteries in Space

NASA became interested in phase change materials during the 1960s and ‘70s to help manage thermal energy in space, publishing reports on thermal energy challenges and phase change material solutions for space. One promising class of materials were paraffins, chains of carbon and hydrogen commonly known as a type of wax.

Paraffins have several attractive properties. They can melt and freeze many times without losing their energy storage capacity. In contrast to other phase change materials, paraffins don’t corrode the containers they are stored in. Lastly, paraffins of many different lengths and melting points can be custom made, allowing specific paraffins to be chosen for specific applications (figure 2).

Figure 2: Paraffins. Paraffins with otherwise similar properties are available in many sizes and melting temperatures.

NASA first used paraffins during the 60’s and 70’s, most notably to keep electronics from overheating on the Apollo lunar rovers. So, when Michael Choi needed a thermal battery to keep NICER’s electronics warm, he returned to paraffins.

Keeping NICER warm

Choi’s thermal battery needed to heat NICER for 6 hours during its extraction from SpaceX’s Dragon capsule, which delivered NICER to the space station. Choi planned to charge the battery before the move by melting the paraffin with the electric heaters, making the paraffin absorb thermal energy. Then, during NICER’s extraction when the electronics risked cold-induced damage, the paraffin battery would discharge, freezing and releasing heat to warm NICER.

Choi considered several factors for his thermal battery design. First, the thermal battery needed to be charged, or melted. This required a paraffin with a melting temperature well below 13 °C, the electric heater’s upper temperature limit. Second, the freezing temperature, which for paraffins is equal to the melting temperature, had to be above ‑30 °C. Otherwise, the heat released during the phase change, which would keep the electronics warm during most of the extraction, would not arrive before the electronics suffered damage at -30 °C. Third, the battery had to work for 6 hours.

Choi found that a paraffin called dodecane satisfied these requirements (figure 3). Dodecane, with a freezing/melting point of -10 °C, could be melted by the heater below 13 °C and freeze above -30 °C. Choi also favored dodecane over other suitable paraffins because of its relatively high heat of fusion, which meant that the solid-liquid phase change could store more energy than other paraffins and thus the battery size could be reduced.

Figure 3: The dodecane-based thermal battery. When Choi’s thermal battery is charged solid, dodecane is melted into a liquid. Upon freezing, energy is released, heating up NICER’s electronics. The solid-liquid phase change allows a large amount of energy to be stored over a small temperature range.

Using dodecane’s thermal properties, Choi calculated NICER’s temperature during the extraction, assuming no extra heat from sunlight and accounting for 3 processes that would occur. First, the liquid paraffin would cool from 10 °C to -10 °C over about 1 hour, releasing a small amount of sensible heat. Then, the paraffin would freeze over many hours, releasing the heat of fusion and holding the temperature of the electronics at -10 °C. Lastly, the solid paraffin would cool, again releasing sensible heat. The calculations showed that the final temperature would be -13 °C, well above the limit of -30 °C.

Choi’s thermal battery was implemented in June 2017, when NICER was sent to the International Space Station, and the battery successfully kept NICER’s electronics warm in the cold environment of space.

Looking Forward – Future Uses of Phase Change Material Thermal Batteries

One challenge facing phase change materials in space is the poor understanding of how low gravity affects these systems. For instance, volume changes often accompany solid-liquid phase changes, which necessitates built-in empty space to accommodate the volume change. It is not known, however, how this empty space, called ullage, redistributes over many melt-freeze cycles under low gravity and whether the system performance could be affected. Last year NASA sent a test paraffin module to the International Space Station, which has returned to Earth and will be cut open to examine the distribution of paraffin inside the module.

Thermal energy management systems using phase change materials are also being explored for terrestrial use in building walls and data centers to regulate temperatures and for thermal energy storage in electric cars. Phase change thermal batteries can help keep electronics warm (or cold) in space, but they may also be part of a more energy efficient future on Earth.

Ryan McGillicuddy is a fourth-year Ph.D. student in the Department of Chemistry and Chemical Biology at Harvard University 

 Sean Wilson is a fifth-year graduate student in the Department of Molecular and Cellular Biology at Harvard University

For more information:

  • To learn more about Michael Choi’s work on the thermal battery for NICER, see the original paper.
  • To read about thermal management and phase change materials in space during the 1970’s, read NASA’s phase change materials handbook.
  • For more information about the Neutron Star Interior Composition Explorer, read here or watch this video.

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