Desiccants that remove moisture from the air may play an important role in cooling our homes and businesses in the future. Warm air holds more moisture than cool air. As hot, humid air passes over a chilled surface, excess water precipitates out, which is why air conditioners and heat pumps have drains.
The human body senses heat in two ways. The first is the actual temperature of the air and the second is the relative humidity of the air. We feel more comfortable in 90ºF heat if the relative humidity is 40% than if it is 90%. When humidity is high, we tend to lower the temperature of our air conditioners to compensate, but that just means we are using more electricity than necessary.
As the world gets hotter, the electricity needed to meet the demand for cooling is expected to triple. The International Energy Agency predicts air conditioning to consume 4000 TWh (that is not a misprint) of electricity by 2050. That is equivalent to all the electricity consumed by the United States in 2022.
Air conditioning equipment today does two things. First, it dries the incoming air. Then, it cools that drier air. What if there was a way to separate the two functions so the air was dehumidified first before cooling took place? Several new companies propose doing exactly that by using materials known as desiccants. They estimate doing so could cut the need for electricity to cool our homes and businesses by 50 to 80 percent. These systems could cool more efficiently, even in extreme heat and humidity, reducing stress on the grid.
Desiccants Permit More Cooling With Less Electricity
A conventional air conditioner works by using a compressor to pressurize a gas, which later cools as it expands. That approach, called vapor compression, is over 100 years old, and the basic design hasn’t changed much since its invention, Ankit Kalanki, a manager in the carbon-free buildings program at the Rocky Mountain Institute, tells MIT Technology Review. Pumping refrigerant around and compressing it enough to shuttle heat outside requires a lot of energy, especially when temperatures are very high.
Keeping a building comfortable has a lot to do with maintaining a low-humidity environment, Kalanki says, but air conditioners must cool down air to pull moisture out of it. Without a designated system to tackle humidity, he says, buildings are often “over-cooled,” which can add a huge energy burden. Desiccants could help lower energy demand by separating the dehumidifying process from the cooling process.
You know those little white packets that come in the box with new electronic devices, the ones that say “Do Not Eat This?” Those are desiccants that keep the items inside the box dry during shipping, but there are other types of desiccants.
Two Approaches To Desiccants
Transaera, an MIT spinoff founded in 2018, is developing a hybrid system that uses a type of material called metal organic frameworks. Adding it to conventional air conditioners could allow them to use 35% less energy than average models, according to Transaera CEO Sorin Grama.
On its website, the company says, “We use a novel sponge-like material that grabs moisture from the atmosphere enabling our air conditioner to cool the air more efficiently. Then we use the heat generated by the air conditioner — instead of wasting it — to dry the material for the next cycle.”
Blue Frontier, based in South Florida, has an entirely different approach. Its cooling technology relies on a salt solution that is so concentrated that it can pull moisture from the air. First, a stream of air passes through a channel and over a thin layer of desiccant, which pulls moisture out of the air. Next, the dried air goes through an evaporative cooling step, which lowers the temperature of the air.
It basically works the same way sweat cools your skin, using what scientists call the heat of evaporation. In arid climates such as Arizona and New Mexico, evaporative coolers are used instead of air conditioners because the ambient humidity is so low. Sometimes known as swamp coolers, they can use up to 80% less electricity than conventional air conditioners.
By pairing evaporative cooling with desiccants, Blue Frontier’s system can work in virtually any climate, CTO Matt Tilghman says. Its operations can be tweaked to handle changes in the weather or in the thermostat set point, altering the balance between cooling and dehumidifying, which could help unlock further efficiency gains. The company’s approach should be able to cut annual electricity use by a total of between 50% and 80% compared with a conventional air conditioning system, depending on the environment, Tilghman claims.
Drying Out The Drying Agent
One of the biggest roadblocks to the widespread use of desiccant cooling is the need for a method to recharge the materials efficiently. Desiccants are like sponges. They can only absorb a limited amount of water before they need to be wrung out or regenerated. That means in addition to the parts that dry and cool air, a desiccant cooling system needs a section that can regenerate the desiccant, releasing the water into another stream of air that in turn is released outside.
Most desiccants can be regenerated through heating, which releases water from the material, but that step can be energy-intensive and often involves fossil fuel-powered boiler systems.
Blue Frontier instead uses a heat pump to regenerate its desiccant. The heat pump adds energy demand, but while the cooling system can run continuously during a hot summer day, the regeneration system can run in the evening or overnight, when there’s less stress on the grid and electricity prices are lower, Tilghman says. Offsetting the regeneration will mean that Blue Frontier’s system could help reduce peak power demand by between 80% and 90%.
Cooling & Climate Equity
Blue Frontier has two demonstration cooling systems running, one in Florida and one in Canada. The company plans to install several dozen more in late 2023 and 2024, Tilghman says. It will focus first on systems for larger commercial buildings before designing systems that can be used for homes, condos, and apartments. While the systems cost more than traditional air conditioning systems, lower utility bills should offset the higher upfront cost within 3 to 5 years.
Access to efficient cooling technology could be crucial to helping more people live and work in safe environments without overloading grids. “If you look at the warming world, the way temperatures are rising, you need people to have access to cooling,” Kalanki says. “It’s not just an issue of thermal comfort or feeling productive, it’s also an issue of equity now.”
If there is a way to keep people comfortable without tripling the world’s demand for electricity, that would be good for all humans. Just because we can make electricity from renewable sources doesn’t mean we shouldn’t use it as efficiently as possible.
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