I was wondering the same. All units where I live and work are installed on a concrete pad outside the building. Air is from outside, then is cooled. Then goes through whatever filter is between the unit and the ductwork.
We do not have one, we often wish we did.
So...the filter would have to go on the outside, where the air is first brought in, zap it at a high temp (but very briefly) and then work the A/C system hard to cool it off. When it's hot here, the A/C can't usually get the temps down below 78-80° inside (at work). It's a school, where things are antiquated (although most of the buildings I work in are only about 20 years old).
There was an HVAC expert here on this thread, weeks ago - he gave some reasons why this type of thing wouldn't work without upgrading the motor or whatever it is that sucks the air and pushes it through the ducts.
By the time it's been approved for schools, it'll be 2030.
It sounds quite promising to me (no expertise in this area, though). The story comes from a University of Houston news release. BBM
“This filter could be useful in airports and in airplanes, in office buildings, schools and cruise ships to stop the spread of COVID-19,” said Ren, MD Anderson Chair Professor of Physics at UH and co-corresponding author for the paper. “Its ability to help control the spread of the virus could be very useful for society.” Medistar executives are is also proposing a desk-top model, capable of purifying the air in an office worker’s immediate surroundings, he said."
...
"The researchers knew the virus can remain in the air for about three hours, meaning a filter that could remove it quickly was a viable plan. With businesses reopening, controlling the spread in air conditioned spaces was urgent.
And Medistar knew the virus can’t survive temperatures above 70 degrees Centigrade, about 158 degrees Fahrenheit, so the researchers decided to use a heated filter. By making the filter temperature far hotter – about 200 C – they were able to kill the virus almost instantly.
Ren suggested using nickel foam, saying it met several key requirements: It is porous, allowing the flow of air, and electrically conductive, which allowed it to be heated. It is also flexible.
But nickel foam has low resistivity, making it difficult to raise the temperature high enough to quickly kill the virus. The researchers solved that problem by folding the foam, connecting multiple compartments with electrical wires to increase the resistance high enough to raise the temperature as high as 250 degrees C.
By making the filter electrically heated, rather than heating it from an external source, the researchers said they minimized the amount of heat that escaped from the filter, allowing air conditioning to function with minimal strain.
A prototype was built by a local workshop and first tested at Ren’s lab for the relationship between voltage/current and temperature; it then went to the Galveston lab to be tested for its ability to kill the virus.
Ren said it satisfies the requirements for conventional heating, ventilation and air conditioning (HVAC) systems." ...
Researchers Create Air Filter that Can Kill the Coronavirus - University of Houston
And here is the journal citation from
ScienceDaily article:
- Luo Yu, Garrett K. Peel, Faisal H. Cheema, William S. Lawrence, Natalya Bukreyeva, Christopher W. Jinks, Jennifer E. Peel, Johnny W. Peterson, Slobodan Paessler, Monzer Hourani, Zhifeng Ren. Catching and killing of airborne SARS-CoV-2 to control spread of COVID-19 by a heated air disinfection system. Materials Today Physics, 2020; 100249 DOI: 10.1016/j.mtphys.2020.100249