How it works
Your air conditioner compressor’s job is to raise the temperature of a refrigerant fluid, from 50°F or so up to about 170°F. We’ll explain why this is necessary to produce cold air in just a minute.
The important idea, though, is that solar energy can be used to provide a portion of this temperature increase, reducing the compressor’s workload and allowing the compressor to consume less electricity.
The idea that a very hot solar collector can help an air conditioner cool more efficiently can be confusing. In order to understand how solar air conditioning can save so much energy, it helps to first have a basic understanding of how conventional air conditioners work.
Picture a sponge.
Refrigerators and air conditioners use a special kind of fluid called refrigerant to produce cold air. A refrigerant works like a sponge, soaking up heat from the indoor air and then transporting it outdoors, where the heat is squeezed out of the sponge into the air.
The refrigerant fluid flowing through a coil—a series of metal tubes—inside an air conditioner’s air handler is very cold—typically 20–40°F. Heat always flows from a hotter place to a colder place. So if, say, 75°F indoor air is blowing through the air handler, heat will be removed from the air and absorbed by the much colder refrigerant fluid. The temperature of the air falls (the air gets colder) as heat energy is removed.
In a correctly sized and properly functioning system, air leaving the air handler should be about 15 to 20°F cooler than the room temperature. If the inlet air temperature is 75°F, then cooled air leaving the air handler should be 55–60°F.
What the compressor does
The metal coil inside the air handler is called the evaporator coil because heat absorbed by the refrigerant causes the refrigerant to evaporate, or turn into a gas. The temperature of refrigerant gas leaving the evaporator coil is about 50°F.
This presents a problem.
Remember that heat always flows from a hotter place to a colder place. We want to send the heat absorbed by the refrigerant into the outside air, but if the refrigerant gas is 50°F and the outside air is 80°F to 95°F, this won’t work. We need to find some way to get the temperature of the refrigerant gas higher than the outside air temperature, so the heat energy stored in the refrigerant gas can be dumped into the outside air.
It just so happens that compressing a gas—forcing it into a smaller space-increases its pressure, which also increases its temperature. An air conditioner compressor’s job is to compress the refrigerant gas coming from the evaporator coil enough so that its temperature is increased from 50°F to 170°F or more. With the temperature of the refrigerant gas now significantly higher than the outside air temperature, heat energy stored in the refrigerant gas can now be easily transferred into the outside air.
How solar energy helps
We can send the refrigerant through a solar energy collector before the fluid reaches the compressor, and solar energy can do a portion of the work—heating up the refrigerant—that would normally be accomplished by the compressor. This allows the compressor to run less, or to operate at a lower speed, and thus use less electrical energy. You can click on the image at right to see a larger diagram and explanation of this process.
Dumping the heat outdoors
The very hot refrigerant gas leaving the compressor flows into another coil called the condenser coil. This coil looks and behaves very much like a car radiator, radiating heat from the hot refrigerant into the outside air as a fan pulls outside air across the coil. The condenser coil gets its name from the fact that the refrigerant gas condenses back into a liquid as it gives up heat to the outside air. You can easily verify that heat is being dumped into the outside air by placing your hand into the exhaust air flow of the condenser fan. This exhaust air should be hotter than the surrounding air—even on a 95°F summer day.
Completing the cycle
The refrigerant cools as it moves through the condenser coil, down to perhaps 85–110°F. We employ another physics trick to get the refrigerant back into the 20–40°F range necessary for cooling. Just as compressing the refrigerant increases its temperature, allowing the refrigerant to expand will cause its temperature to fall. This is accomplished by forcing the refrigerant through a tiny opening—aptly named an expansion valve—that allows the fluid’s pressure and temperature to fall substantially as it emerges from the tiny passage.
With the refrigerant cooled back down to 20–40°F, the cycle repeats.
Air conditioners also control humidity.
Humidity is water vapor in the air. Humidity makes people uncomfortable because as the moisture content of air increases, it becomes increasingly difficult for sweat—the body’s built-in cooling mechanism—to evaporate from the skin. The clammy or sticky feeling you experience during hot, humid weather is caused by sweat on your skin that cannot evaporate.
Central air conditioning systems dehumidify indoor air by the same principle that causes moisture to form on the surface of a glass of iced tea on a hot summer day. As air in contact with the glass cools, the air falls below the minimum temperature necessary to hold water as a vapor. As a result, water vapor in the air condenses on the glass surface.
Similarly, warmer house air cools as it passes over the very cold evaporator coil in the air handler, losing the ability to hold water as a vapor. As a result, water vapor in the air condenses on the outer surfaces of the evaporator coil. This condensation drains off the coil and collects in a pan below the evaporator coil, which empties through a drain line to a discharge point outdoors.