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Photocatalytic operating principle. Polluted air flows through TiO filter which has been activated by UV-A and UV-C lights. System traps particulates, kills viruses, bacteria. (Marelli)

Marelli’s COVID-killing cabin purifier

Marelli has detailed a photocatalytic HVAC cabin air purifier that will kill viruses, including COVID-19.

Marelli North America is offering a solution to the problem of interior air quality in shared-use/autonomous vehicles. The upgraded HVAC setup purifies passenger car cabin air, including the destruction of COVID-19 virus, the system’s developers claim. The photocatalytic HVAC shows rapid, >99% performance using a titanium-dioxide-impregnated filter and dual UV lighting to clear cabin air of COVID-19 within a 1.0 m³ (35 ft³) chamber in 15 minutes. The system also kills other viruses and eliminates HVAC odor, according to the company.

Offered for taxis, ambulances and other shared-use vehicles, including autonomous-driving models, the system also has retrofit potential. The design originated at Calsonic-Kansei NA, whose parent company merged with Magneti-Marelli in 2019 and now goes by Marelli. It is a photocatalytic system that combines a titanium dioxide-impregnated filter with short wavelength ultraviolet light shining on the filter. Titanium dioxide (TiO₂), a naturally-occuring oxide of titanium, is a common ingredient in toothpaste and sunscreen. It forms a semi-conductive film on the filter surface.

Radical formation
When UV light shines on the filter, electrons on the TiO₂ surface are released. They interact with water molecules in the air, and the combination forms hydroxyl radicals, a highly reactive pairing of a single hydrogen atom and single oxygen atom, often referred to as a natural air purifier. These small, aggressive hydroxyl radicals then attack and kill bigger organic (carbon-based, including virus) pollutant molecules, breaking apart their chemical bonds and turning them into harmless substances, such as CO₂ and water.

The Marelli system incorporates two fluorescent ultraviolet lights, one (UV-C) with a wavelength of 100-280 nm that results in zapping (instantly killing) viruses and bacteria. A second (UV-A) has a wavelength of 315-400 nm and it attacks complex VOCs (volatile organic compounds), to eliminate odors commonly formed from fungus grown on airborne particles, including cigarette smoke, in the evaporator case. To demonstrate concept feasibility, UV-C and UV-A lights were supplied with 120V AC, stepped down to 29 V AC, with power outputs of 5-12W. The two-UV-light design is key to Marelli patents.

Sensors capable of detecting viruses are very expensive and primarily in hospital use, explained Marelli’s Dr. Gursaran D. Mathur, senior manager for Climate Control Design & Development. To moderate cost (currently 40-80% above the cost of the HVAC itself), the Marelli system has a simple on-off control, with timing based on empirical data derived from test chamber results. Other parameters that contribute to overall cost are the number of UV bulbs and attendant circuitry, and TiO₂ filter loading (gm/m2). The filter presently is made only in China, so although the materials are commodities, sourcing costs could become an issue.

All laboratory tests necessary to prove the system concept’s efficacy are complete. The Marelli evaluation procedure used MS2, the virus commonly employed as a safe substitute for COVID-19, because it is structurally similar, does not cause disease in humans, and is a suitable size for testing (0.027 microns), slightly smaller than the 0.125 of COVID-19.The TiO₂ filter eventually will become restricted by particulate debris, at which time a pressure differential sensor on the filter is triggered, producing a “change filter” signal. A variant design may incorporate a particulate filter ahead of the TiO₂ to increase operating life. A removable panel provides service access to the TiO₂ filter and UV lights.

Ineffective alternatives
The photocatalytic system’s price premium led Mathur’s research group to consider alternative methods to remove or kill viruses and other pollutants (airborne dust particles, pollen, bacteria, mold, spores, dust mites and fungus). HEPA filters can remove most of these pollutants and also some larger viruses. But it cannot remove the small COVID-19 (among others), leading to its rejection.

The most efficient HEPA filters only trap particles 0.3 micron with greater than 99.97% efficiency, which is over twice the particle size of the COVID-19 virus. And in any case, any trapped virus (and bacteria) remain live on the filter (although eventually both die). Also rejected was an ionizer, which produces negative ions that collide with dust and some virus particles, causing them to adhere to surfaces within the HVAC ductwork or even fall on passengers. The ionizer’s primary flaw, Mathur said, was its inability to eliminate airborne virus.

Photocatalysis produces ozone, which can cause throat irritation and other respiratory-related issues. In particular, ozone is created by UV wavelengths in the 100-240 nm range. So the Marelli system was tested for this, Mathur said, but only trace amounts were found. Testing was done in a chamber of 35 ft³ (1.0 m³), which is one-third the volume of a typical passenger car cabin, Mathur noted. The >99.97% virus destruction rate measured for 15 minutes would be slightly longer for a full-volume cabin. Marelli calculated the COVID19 virus kill rate for 106 ft³ (3.0 m³) volume would still be >95% for 15 minutes.

A charcoal-coated conventional filter also would be far less expensive than the dual UV design, and would trap pollen and some other allergens. However, Mathur noted, such filters are ineffective against COVID-19 and other viruses. For AV operation, Marelli recommends running the system for 15~20 minutes prior to entry of passengers. There are vehicle-use conditions in which the system could be turned off after about 15 minutes, such as when carrying one passenger, or two passengers side-by-side in a single seat. However, Marelli advises continuous use if occupants are facing one another.

Marelli is continuing research to reduce weight, improve durability and cut power consumption, to be reflected in a third-generation HVAC unit now in development with LED UV bulbs operating at 5-V and 12-V DC. The system also is being proposed for ambulances, among many possible retrofit applications.

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