• Thermal control design of a Bioreactor

The thesis work was carried out with the company Kayser Italia as part of the Biorider project.
It is based on the design of the thermal control system as part of the Biorider incubation system designated for the Space Rider mission, scheduled for 2025 and in which experiments in microgravity will be carried out in what will be the first reusable ESA commercial spacecraft, a real milestone in European space exploration.
After identifying mission characteristics regarding the main requirements from the thermal and space available on board Space Rider, research was conducted regarding the state of the art in the area of thermal control of the main bioreactors on board the ISS and also in unmanned missions.
Hence the widespread tendency to choose a single direction of thermal control, but especially that of keeping the environment insulated and cooling it as needed.
From this also, the chosen path depended on constraints related to stringent thermal requirements (0.5°C range) but especially space requirements, given the small available volume and size of the incubator. So on the basis of what has been said so far, the designated active thermal control system is the Peltier cell, a small thermoelectric device capable of creating a temperature difference between the two faces of which it consists as a result of an input of electrical power.
Starting from a preexisting model in which the incubator consists of 3 experiment containers each housing 4 experiment units, fixed on a support plate whose temperature range adjusts the power demands to meet the predetermined thermal requirements.
The hardware also includes an interface plate between the incubator and the Payload Support Plate of SpaceRider, aimed at facilitating the mounting of the system onto the vehicle. The Experiment Units require a LED lighting system to provide the necessary power likely required for the experiments inside them. Therefore, located between the two pairs of Units is a LED PCB system, which represents the main internal thermal load within the Experiment Container.
The main system is then enclosed and isolated from the external environment by a cover called KIC cover, whose main task is precisely to thermally insulate the units by canceling the radiant contribution through reflective surfaces and also low-emissivity MLI covers.
The planned thermal control system is characterized, in addition to the Peltier cell, by other elements that define this system: a heat sink and two fans. In fact, the intended control is based on forced convection, where heat is removed from the Peltier by the base of the Experiment container (KIC), onto which the heat sink is fixed. At that point, the cold air inside the container is circulated among the Experiment Units thanks to the system consisting of the two fans, one for intake and the other for exhaust.
To evaluate the thermal performance, a thermal model with lumped parameters was built, where the system was divided into a certain number of nodes, neglecting the thermal capacities of the materials and relying solely on steady-state behavior. This resulted in a system whose outputs revealed the temperatures of the main elements of the two systems, with particular attention to the area affected by the Peltier cell and especially the thermal interface resistances. In this regard, further work on the optimal choice of thermal interface devices was carried out, basing the selection on a trade-off between structural integrity, weight, and, above all, thermal resistance.
Based on the thermal interface material used to connect the TEC on one side to the container and on the other side to the cold plate, 3 different designs were born: the first with thermal pads used as the thermal interface between the electrical device and the two components, the second with a thermal pad on the side of the plate and a fastening system for placing a thermal strap on the side of the container, and the last one very similar to the design just described but with the difference that a conductive high-conductive aluminum alloy plate is used instead of the thermal strap, an easier homemade solution.
The results obtained with the Peltier still turned off revealed the necessity, in the majority of cases studied, to actively cool using the thermoelectric device. Its performance have been evaluated for each of these cases using software provided by the chosen device manufacturer. To validate the model, tests were conducted on a pre-existing model in the company, to which some missing components were added, printed in polycarbonate, thus integrating the new materials and geometric characteristics into a new thermal model. Besides these differences between the two thermal models, the contribution of convective motions present in the model under test represented the major point of uncertainty, although the results ultimately obtained were more than satisfactory. Regarding those cases where the steady-state temperature fell below the target, a comprehensive study on how to heat the units was not conducted. However, the thermal power to be provided through heaters was evaluated.