Chiller Loop
When you're in space, temperature control is a major concern. Mike Lotto, LifeLAB Graduate Research Assistant, explains:
"If you're in space, and you’re in the shadow of the earth, it can get extremely cold (around -250 C). If you’re in the sun, it can get pretty hot ( around 250C). You either have a lot of heat going in, or a lot of heat going out. If you stick something up there (i.e. a spacecraft cabin full of astronauts), you can’t expect the temperature to be constant – you have to either actively or passively control it."
LifeLAB's chiller loop is aimed at finding improved methods of actively controlling temperature aboard spacecraft such as the ISS.
How the old system works:
Currently, the ISS uses a water and ammonia loop system to regulate cabin temperature:
Originally, heat is produced by the crew members working in the spacecraft cabin and by the on-board avionics, including the ECLSS (Environmental Control and Life Support System). This heat has to go somewhere - otherwise, it will build up in the cabin to uncomfortable, and ultimately lethal, levels.
On the interior of the ISS cabin is a water loop. Heat from the cabin is then transferred to the relatively cooler water. Water was selected as the medium of this primary loop because:
- It has a high specific heat.
- It is non-toxic.
The water is circulated so that it contacts a metal exchanger, causing the heat previously stored in the water to be transferred to this exchanger. The water, now cooled, returns to the cabin to restart its loop.
The metal exchanger also contacts an ammonia loop that runs on the outside of the spacecraft. Ammonia was selected as the medium of this loop because it has a low freezing point. If the water loop were to run on the outside of the spacecraft, it would freeze due to low temperatures, causing pipe and infrastructure damage. Unfortunately, ammonia is highly toxic; thus, it must be kept on the outside of the spacecraft.
The heat from the metal exchanger is transferred to the relatively cooler ammonia loop, where it contacts a "radiator." This radiator absorbs the heat from the ammonia and radiates it out into deep space. The ammonia, now cooled, continues on its loop.
The radiator must be designed to accomdate the maximum thermal load (the bigger the radiator, the more heat it radiates). However, the astronauts and ECLSS system will not always be producing a maximum amount of heat; thus, the radiator, if unregulated, could cause the cabin temperature to dig to dangerous levels.
To avoid this problem, the system incorporates a bypass valve that diverts some of the ammonia away from the radiator; that way, some of the heat is transferred back to the spacecraft cabin and not into deep space.
How the improved system works:
The goal of LifeLAB is to improve this temperature regulation system by eliminating the toxic ammonia loop. Professor Nabity explains:
"Our goal is to eliminate hazardous coolants, such as ammonia. At the end of January [of 2015], there was a scare with ammonia being leaked aboard the ISS. Though there ended up not being a leak, because of ammonia's high toxicity, the astronauts had to respond to the threat."
LifeLAB's design uses controlled freezing to regulate the amount of heat that is transferred to the regulator.
Lotto explains:
"Instead of using a valve to alter the flow [to the radiator], you can instead allow the water in the heat exchanger to freeze in a certain controlled fashion. Since ice has a lower thermal conductivity than water, it will act as a passive insulator as it forms, reducing heat flow to radiator."
Suppose, for example, that the spacecraft cabin is producing max thermal load (the astronauts are exercising and producing lots of heat, the ECLSS system is working at full capacity). In this case, maximum heat from the cabin would be transferred to the water loop, heating it up to its max temperature. In this scenario, no ice would form within the water loop piping, full heat would be transferred to the radiator, and the cabin would be adequately cooled.
On the other extreme, suppose that the spacecraft cabin is producing minimual thermal load (the astronauts are sleeping, the ECLSS system is working at low capacity). In this case, minimal heat from the cabin would be transferred to the water loop. In this scenario, the water in the loop would mostly freeze, minimum heat would be transferred to the radiator, and the cabin would be kept comfortably warm.
Not only would this system eliminate the need for the hazardous ammonia loop; it would also eliminate the need for a bypass valve (the more moving parts in a system, the more possible points of failure).
-Written By: Ari Sandberg, Intern