# Case Study of an Integrated Design and Technical Concept for a Scalable Hyperloop System
## 4 Functional Definition
### Cooling  
Cooling presents a major engineering task for the hyperloop concept. Due to the low pressure, air cooling is only partially effective. Additionally, heat radiation is difficult to manage because creating a sufficient area with a large temperature difference is challenging. Heat storage is impractical for longer trips, as the required mass becomes unmanageable, and discharging the accumulated heat at the station can interfere with high-frequency passenger operations. Therefore, sublimation or evaporation cooling, which is used in space applications with similar requirements [28], is proposed. The large enthalpy of the phase change offers a significant advantage in terms of mass. Additionally, the speed of sound in water vapor is higher than in air, improving the aerodynamic behavior. Nevertheless, several factors must be considered when designing the cooling system in detail. The vapor released must be removed from the tube to prevent pressure increases. The volumetric flow resulting from the evaporation is substantial, making it inconvenient to handle with common vacuum systems. Instead, a condenser can be placed in front of the vacuum system. This arrangement allows the condenser to capture most of the water vapor, while the vacuum system handles the air leakages and the remaining moisture, thus reducing the load on the vacuum system considerably. This method effectively transfers the required cooling power from the pod to the infrastructure, specifically to the chillers required for the condensers. It operates without issues as long as the outside temperature remains above a certain threshold. For water, the boiling temperature at 10 mbar is around 7 \({}^{\circ}\)C. Below this value, depending on the amount of air in the mixture, condensation can occur in the tube. Below 0 \({}^{\circ}\)C, ice will form on cold surfaces. This is not a problem for underground tracks, where temperatures do not reach such low values, but it can be problematic for above-ground tracks. Insulation of the tube can help, and coatings to prevent ice formation in certain areas of the tube are possible. One way to completely resolve this issue is to lower the pressure to 1 mbar. At this pressure, only vapor exists down to a temperature of \(-20\,^{\circ}\)C, where it deposits in the form of ice. Sublimation, instead of evaporation, would be used on the pod, and cyclic ice condensers would be required instead of normal vapor condensers in front of the vacuum system. Nevertheless, due to the large surface area of the tube, any forming ice or water will distribute well. For an average load of 8 \(\frac{\text{Mipax}}{\text{yr}}\), a layer of less than 1 mm of ice would build inside the tube if temperatures remain consistently below 0 \({}^{\circ}\)C for an entire month. This is seen as acceptable and not worth the considerably higher effort for keeping a vacuum of 1 mbar and featuring ice condensers instead of vapor condensers. Consequently, a pressure of 10 mbar and evaporation cooling, possibly enhanced by some air cooling, was chosen. Inside the pod, heat is transferred to the evaporator via a liquid cooling circuit, enabling EMS levitation even at a standstill. As shown in Table 8, even for a four-hour trip, the required mass of distilled water is reasonable, with approximately 371 kg. The amount of necessary water can be reduced \begin{table}
\begin{tabular}{l|c} \hline \hline
**Description** & **Value** \\ \hline Pod energy consumption per seat per km (nom speed) & 14.20 \(\frac{\text{Wh}}{\text{seat km}}\) \\ Electrical power for cooling (condenser) per seat per km & 1.27 \(\frac{\text{Wh}}{\text{Neat km}}\) \\ Losses of IPS per seat per km & 5.95 \(\frac{\text{Wh}}{\text{seat km}}\) \\ Occupation/seat usage & 80\% \\ Vacuum energy consumption per parx per km & 16.93 \(\frac{\text{Wh}}{\text{pax km}}\) \\ Total power per pax per km & 43.71 \(\frac{\text{Wh}}{\text{pax km}}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Total energy characteristics.