Solar ultraviolet light collector for germicidal irradiation on the moon

Abstract

Prolonged human-crewed missions on the Moon are foreseen as a gateway for Mars and asteroid colonisation in the next decades. Health risks related to long-time permanence in space have been partially investigated. Hazards due to airborne biological contaminants represent a relevant problem in space missions. A possible way to perform pathogens' inactivation is by employing the shortest wavelength range of Solar ultraviolet radiation, the so-called germicidal range. On Earth, it is totally absorbed by the atmosphere and does not reach the surface. In space, such Ultraviolet solar component is present and effective germicidal irradiation for airborne pathogens' inactivation can be achieved inside habitable outposts through a combination of highly reflective internal coating and optimised geometry of the air ducts. The Solar Ultraviolet Light Collector for Germicidal Irradiation on the Moon is a project whose aim is to collect Ultraviolet solar radiation and use it as a source to disinfect the re-circulating air of the human outposts. The most favourable positions where to place these collectors are over the peaks at the Moon's poles, which have the peculiarity of being exposed to solar radiation most of the time. On August 2022, NASA communicated to have identified 13 candidate landing regions near the lunar South Pole for Artemis missions. Another advantage of the Moon is its low inclination to the ecliptic, which maintains the Sun's apparent altitude inside a reduced angular range. For this reason, Ultraviolet solar radiation can be collected through a simplified Sun's tracking collector or even a static collector and used to disinfect the recycled air. Fluid-dynamic and optical simulations have been performed to support the proposed idea. The expected inactivation rates for some airborne pathogens, either common or found on the International Space Station, are reported and compared with the proposed device efficiency. The results show that it is possible to use Ultraviolet solar radiation directly for air disinfection inside the lunar outposts and deliver a healthy living environment to the astronauts.

Figure 1

Spectral solar irradiance in the Ultraviolet band from the SOLar SPECtrometer on board the ISS. The red part of the curve is the reduced UVC bandwidth used for the SAILOR Moon efficiency simulations since ozone formation inside the air duct would occur for light with $\lambda <$ 240 nm.

Figure 2

Proposed design for a possible Sun’s tracking concentrator: Ritchey-Chretien type telescope. A tertiary flat mirror behind the telescope aperture compensates for Zenith angle variations and maintains the focal plane fixed over the quartz window of the air duct.

Figure 3

Sketched designs of the Annular Compound Parabolic Concentrator for solar UVC light concentration: side and top views. The image of Sun’s tracking concentrator at the top-left has the purpose to visually show the two systems’ scale. The two configurations sizes has been chosen to deliver a similar overall Fluence, as shown in Table 2.

Figure 4

The external profile of the annular Compound Parabolic Concentrator. The parameters refer to the upper side. The lower side would have the same parameter values in case of a symmetrical accepting angle between the two sides. Parameter values are listed in Table 1.

Figure 5

Trajectories of some particles inside the cylindrical air duct for the 230 ${\mathrm{m}}^3/\text{h}$ flux. The increased diameter produces a slowing down of the particles’ velocity in the second part of the enlarged section and a turbulent trajectory of some particles. This figure is representative of both the considered air fluxes and the particles’ sizes. The figure size is not in scale for visualisation purposes.

Figure 6

Trajectories of some particles inside the annular air duct for the 230 ${\mathrm{m}}^3/\text{h}$ flux. Particles from the smaller air duct experience some turbulent flow when entering the larger annular duct. At the considered flows, the particle trajectories return to a laminar regime. This figure represents the considered air fluxes and the particles’ sizes.

Figure 7

Cylindrical UVC filter. The image shows how the light rays are reflected ad scattered by the internal surface. The coloured plane is one of the volumetric detectors used to calculate the Fluence Rate inside the filter.

Figure 8

(a) Trajectories of the same particles of Fig. 5, inside the cylindrical air duct for the 230 ${\mathrm{m}}^3/\text{h}$ flux. In the second part of the filter, the reduced particles’ velocity and the higher Fluence Rate in the same region (Fig. 7) produce the local Fluence to increase. (b) Integrated Fluence for the same particles as the upper figure. The two figures’ sizes are not in scale for visualisation purposes.