A new type of cooling system invented in India replaces a water pump with biodegradable material to lower costs, save energy and, perhaps most importantly, limit the transmission of waterborne and airborne diseases, including dengue fever and COVID-19.

The team's technology is built around an easy-to-acquire and sustainable material: cow manure-based vermicompost. It could potentially save families 600 rupees per unit every year, thanks to the removal of an energy-intensive pump from the system. Overall, the new model is 21.7% more energy efficient than conventional models. The study describing the new system, partially funded by the government of India, was published May 5 in Sustainable Cities and Society.

The system utilizes the natural process of evaporation to remove heat from the air inside homes or businesses. Inside an evaporative air cooler, a pump sprays water into a stream of air to lower the temperature of the surrounding environment. But in hot, dry climates such as Jaipur, the increased use of evaporative cooling systems could increase the risk of infectious disease.

While water is a relatively cheap and abundant natural resource, water-based cooling systems have two major downfalls when it comes to disease. First, a dry environment indoors can lead to respiratory airborne droplets of COVID-19, which is spreading at alarming rates in India: On June 4, the country reported an average of 145,556 cases and 3,115 deaths over the previous week, according to the New York Times.

The next concern is the water itself.

"In stagnant water, breeding mosquitoes leads to transmittable diseases like dengue," lead author Sujatha Abaranji told The Academic Times. 

Dengue is endemic in over 100 countries around the world, and case counts are notably high in India. One recent study showed that rates have been steadily rising over the past two decades, and it projected that outbreaks will only continue to become more common as the global climate warms.

The team's novel system provides a safer alternative to other air-cooling systems in the midst of a global pandemic. It can supply humid air to a house or office building at lower velocity. 

"Low humidity in the indoor environment plays a significant role in reducing the size of respiratory droplets that causes the viruses to become air-borne particles," the authors note in the paper. Additionally, by maintaining a low air velocity of no more than 3 meters per second, the system can prevent aerosols from being created, the authors noted.

A unique feature of the novel system is the increased control over moisture in the air. This means the system can also also be used to generate cool, humid air. 

"We can also control the humidity in a residential setting, like the floral shop or a greenhouse, where humid conditions are called for," explained Abaranji. 

The novel system takes advantage of the high water retention of a natural material called vermicompost — which, in the current study, was made from solar-dried cattle manure — to eliminate the need for a water pump, therefore preventing waterborne diseases that arise from stagnant water. Because vermicompost is highly porous, it can absorb large quantities of water sprayed to cool the air.

"Vermicompost already has a wide application in agriculture, in water-storing capacity and plant growth," Abaranji explained. Though people often use it for the benefit of plants, the current application recycles animal waste for the benefit of humans in a sustainable cycle.

When cow manure is directly applied to a garden or burned over a fire, it has the potential to spread pathogens that are dangerous to humans, such as E. coli, salmonella and listeria. However, composting cow manure correctly — that is, above 60 degrees Celsius, or 140 degrees Fahrenheit — kills those hazardous microorganisms. 

Abaranji and her colleagues tested the new system in the city of Vellore, in Tamil Nadu, which is known for having a hot, arid climate during much of the year. In February 2020, when the team tried out their innovation, the ambient temperature during the peak heat of the day ranged from 30 degrees Celsius to 34 degrees Celsius (86 degrees Fahrenheit to 93.2 degrees Fahrenheit), and relative humidity was anywhere from 30% to 60%. The team notes that they also tested the technology in May, a month when the weather is typically drier.

"The temperature drop produced by the vermicompost based cooling system is quite similar to the performance of the conventional air cooler system for velocities of 2.7 meters per second," the authors noted in the paper. The study showed a temperature drop of 5 degrees Celsius to 8 degrees Celsius, roughly equivalent to a 5.4 degree Fahrenheit change, during the month of February, which was sustained for over five hours. 

The team hopes to make their system widely available to businesses and families across India. A previous study reported that 52% of the country's energy consumption in commercial buildings comes from heating, ventilation, and air-conditioning systems. As the vermicompost system omits the pump and reduces the energy needed for air cooling, it has great potential to lower the total energy consumption if widely distributed.

Abaranji created the project under the guidance of Velraj Ramalingam, who is a Professor at the Institute for Energy Studies at Anna University. Abaranji notes that Ramalingam is particularly interested in thermal engineering and energy-efficient materials such as vermicompost. He also deeply cares about the coronavirus pandemic.

"Nowadays, we know the issues with COVID-19," Abaranji said. "Droplets will become airborne when the humidity is low and velocity of the air is high. [Our system] has the possibility to reduce the transmission of these particles and may also help families save money."

The study, "Direct evaporative air-cooling system with vermicompost material as the water storage medium," published May 5 in Sustainable Cities and Society, was authored by Sujatha Abaranji, Thanthai Periyar Government Institute of Technology; Karthik Panchabikesan, Concordia University; and Velraj Ramalingam, Anna University.