Researchers at MIT have proposed novel approaches to air conditioning that could significantly reduce the amount of energy required to cool and dehumidify indoor spaces. This is becoming an escalating need as populations around the world grow and the climate warms. The air conditioning units of today dehumidify the air by condensing water vapor out of the air. Unfortunately, this is inherently an energy-inefficient process. The researcher’s study focuses on systems that instead use desiccant materials that naturally absorb moisture or membranes that let water vapor molecules, but not air, pass through. Their thermodynamic analyses show that these technologies can yield high efficiencies, but only when they are incorporated into carefully designed systems with integrated heat-recovery devices. The results also demonstrate that a system’s performance can vary significantly at different ambient temperatures and humidity levels. Matching the technology to the climate is critical.
Close to 40% of all energy consumed by buildings worldwide is used for space heating and cooling. With the warming climate, growing populations, and rising standard of living, the demand for cooling and dehumidification is expected to rise precipitously. Those hot, humid regions of the developing world will be demanding cooling to ensure comfort and to protect human health. This will push up global demand.
Today greenhouse gases are frequently used as refrigerants in air conditioners. Unfortunately, most existing systems are very energy inefficient. They need to lower the temperature and remove moisture simultaneously and this takes a lot of extra energy. The standard approach to dehumidification is to run cold water through the pipes inside a building. If that water is colder than the dew point temperature, water vapor in the air will condense on the outer surfaces of the pipes. That water may drop off outside or in a collection pan. The problem arises because running a chiller to get water that cold takes a lot of electricity, and the water is far colder than needed to lower the temperature in the room. By separating these two functions energy is saved on two fronts. Removing moisture from outdoor air brought into the building requires cold water but much less of it than is needed to remove heat from occupied areas. When that job is done, running cool, not cold, water through pipes in the ceiling or floor will maintain a comfortable temperature.
Air conditioning enabled our great modern buildings to rise but it is also fueled today’s energy and environmental crisis. Today environmentally conscious architects are trying to move beyond innovative designs and passive-cooling techniques. Modern buildings cannot survive unless they are supported by an air conditioning system. The glass, steel, and air-conditioned skyscrapers are symbols of status around the world.
In an experiment in Abu Dhabi the energy benefits of maintaining cool water pipes in a room, especially when these indoor spaces are pre-cooled at night when electricity is cheap, and the outside air is cooler. But the dehumidification process remains inefficient because condensing water vapor is energy-intensive.
Researchers also considered membrane technology. Outdoor air enters the membrane unit, and a vacuum pump pulls the water across the membrane. The pump then raises the pressure to ambient levels, so the water vapor becomes liquid water before being ejected from the system. The air, which is no longer humid outdoor air, passes from the membrane unit through a conventional cooling coil and enters the indoor space, providing fresh air for ventilation and pushing some warmer, humid exhaust air outdoors.
Willis Carrier’s invention of artificial refrigeration in Brooklyn in 1902 was a turning point in the history of modern air conditioning. He stumbled upon the technology while trying to create a machine that would dry out printing rooms so the ink would not smear on the presses in humid temperatures. His machine “dried” air by passing it through the water to create fog, which had the by-product of cooling the surrounding space. This engineering marvel had a wide range of uses and Carrier would eventually push for residential applications, the new movie theater market as well as the adoption of residential and office air conditioning.
With the proliferation of air-conditioned space, the types of buildings that have flourished under this recent technology have shown the modern world it creates is far from utopian. Air-conditioned construction quickly changes the urban landscape. The new, windowless wall created fluorescent-lit, dull, and dim office spaces. Unhealthy air quality inside closed-off buildings also was cited as a major health implication. By far the most damaging part of this new cooling system has been the cost, of energy and carbon emissions. We consume more energy for residential air conditioning than all other countries combined! Collectively, we must try to decrease this usage and find other ways to keep ourselves cool in hot weather.