Deep Thoughts by Deepinder Singh, CEO at 75F
An economizer is a device designed to make a package rooftop unit (RTU) more energy efficient. The economizer controls the outsideair damper of a rooftop unit (RTU) and brings in fresh outside air which can provide free cooling when conditions are right. It also helps meet indoor air quality (IAQ) requirements. While few RTU economizers today are utilizing the Internet of Things (IoT), its inception enables intelligent companies to make smarter, more efficient decisions that increase savings and decrease our impact on the planet.
Based on the maximum occupancy of a building, IAQ standards require a certain amount of air exchanges every hour with fresh air from the RTU’s outside air damper. There are two typesof air exchanges. The first is when an RTU simply moves the air in a room by recirculating the air in the building and the second type freshens a room with outside air. When fresh air comes in, the same amount of air must also be exhausted out. Without smart controls, an outside air damper will typically be set to open a minimum of 20-30% to provide enough fresh air to meet the IAQ standards. The precise percentage opening of the damper is determined by factors such as how many Cubic Feet per Minute (CFM) is blowing, the size of the outside air damper relative to the ductwork and the size of the spaces being served.
Demand control ventilation (DCV) is a controlstrategy in which we only open the outside air damper when IAQ demands it, based on CO2 levels measured in the return air system. The outside air damper is kept nearly closed (well below the minimum of 20-30% outlined above) most of the time. Only if the CO2 levels rise is the outside air damper opened to reduce them. If CO2 levels are low (say because the building is only partially occupied), DCV can save a lot of energy when it’s freezing outside and we don’t have to heat that outside air up to 72°F. The same principle enables DCV to also save energy in hot, humid weather.
That same fresh outside air can provide free cooling when conditions are right. For example,if it were 72°F inside and 55°F outside, fresh air coming in at 55°F would help cool down the building. Free cooling through the economizer frequently works out nicely in the spring and fall and the outside air damper can be opened up well past the minimum. It’s just like opening up a window on a spring day instead of turning on the A/C, which helps save energy.
There are different strategies to determine when to bring in outside air and IoT is changing the game. One could just look at the outside airtemperature alone and make the decision based on that, which is called “dry bulb.” But the total amount of energy in the air (and therefore the amount of cooling power needed to remove it) is determined by both temperature and humidity. This is where it gets interesting and enthalpy comes into play. Typical economizer controls will have either a dry bulb or comparative enthalpy control. Comparative enthalpy controls measure the outside air enthalpy based on the temperature and humidity and compare it with the indoor enthalpy, to determine how much outside air to bring in.
HOW IS THE ENERGY OF AIR RELATED TO HUMIDITY?
If you take water and let it evaporate, the water will convert into water vapor. During this time,the water left behind is actually cooled. If you’ve ever walked through a greenhouse that had water misters stationed within, you may have noticed you felt cooler inside. The reason behind this is mist will eventually evaporate and as it does, it cools the air around it. The water vapor in the air stores a huge amount of energy.
Enthalpy determines the total amount of energy in the air and is based on both humidity and temperature. While there is no simple mathto determine enthalpy, we use a psychometric chart to discover how much energy the air has. This graphical representation calculates thermodynamic properties like dry bulb temperature, wet bulb temperature, humidity, enthalpy, and air density. Let’s take an example of a building that has an inside temperature of 72°F with 40% humidity, while the air outside is cooler at 60°F with 65% humidity. In the chart below we plot the enthalpy as downward sloping diagonal lines from left to right. We can see that 60°F with 65% humidity has less enthalpy than 72°F air with 40% humidity. So according to the graph, the outdoor enthalpy is less than the indoor enthalpy, so bringing in fresh air to cool the building (instead of mechanical cooling) makes sense.
However, enthalpy alone is not a good metric to determine when to use free cooling, since it does not account for what bringing in the outside air would do to the indoor building humidity. Evenwhen the enthalpy is favorable, the humidity might still increase beyond what is comfortable for humans and good for the building (mold growth due to high humidity is a big concern). Using the psychometric chart, we can see that 60°F air with 65% humidity will end up at 43% humidity when it heats up to 72°F.
75F measures the indoor humidity and also gets a ‘desired target humidity’ for each building it automates. Now if that building’s desired target humidity was 40%, we’d see that bringing in the outside air would make the building more humid, so we’d pass on getting that outside air even though it’s tempting just looking at the enthalpy.
The comparative enthalpy method works most of the time, except when there are high humidity levels inside the building (restaurants with steam cooking lines and high occupancy offices are notorious for this), which cause the enthalpy comparison method to break down. At 75F wetake into account not just enthalpy, but also what bringing in the outside air will do to the building’s indoor humidity.