Enthalpy as it relates to free cooling in HVAC

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Enthalpy is defined as the amount of internal energy within a system combined with the product of its pressure and volume. When dealing with the term in the HVAC industry, we usually assume that the process is at a constant pressure and, as such, the change in enthalpy is equal to the heat absorbed or released. At its core, the main function of an HVAC system is to transfer heat, which is a form of energy. The first law of thermodynamics, the Law of Conservation of Energy, tells us that energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another. From that, we can infer that the only way to cool a space down is to remove the heat energy and transfer it somewhere else, typically outdoors. This is generally accomplished by absorbing heat from an airstream and then distributing this cool air to the area we wish to control; it helps to think of conditioned air like a sponge with the capacity to ‘soak up’ heat. The air, now laden with unwanted heat, is routed back via a Return Air pathway to start the process over again. The newly absorbed heat is ‘wrung out’ and expelled, and the cool air is distributed again in a constant cycle. This ‘wringing out’ process is mainly accomplished by mechanical cooling, such as a compressor, or free cooling provided by an air-side economizer.

Mechanical cooling uses electricity to transfer the heat from the air stream to another medium such as water or refrigerant. Free cooling takes advantage of favorable outdoor conditions to introduce air with a greater capacity to absorb heat than the air being returned to the system from the space we wish to control. Most economizers simply look at Dry Bulb temperature, the reading shown on a thermometer, to determine which option to use. The reason that Enthalpy is so important when it comes to making that determination is that outdoor air is a mixture of water vapor and dry air. Therefore, the enthalpy of moist air includes both the heat absorption capacity of the dry air, called sensible heat, as well as the capacity of the evaporated water in the air, called latent heat. We use the total enthalpy, the combination of both sensible and latent values when performing calculations related to heating and cooling processes. This is due to the fact that the Relative Humidity, or amount of moisture of a particular parcel of air expressed as the percentage of full moisture capacity, changes the amount of energy the air can transfer.

The entire point of free cooling is to save energy and so it is helpful to know that the energy used by a cooling coil is proportional to the difference in enthalpy between the air before it enters the coil and the air after it leaves the coil. To determine whether free cooling would be the most energy efficient choice we need to look at the Dew Point of the Return Air, or air entering the coil. The Dew Point is the temperature at which the Relative Humidity reaches 100% and begins to condense into water droplets, often referred to as Dehumidification. If the Dew Point of the entering air is higher than the desired Supply Air (air leaving the coil) temperature then dehumidification will occur, leading to what is referred to as a Wet Coil. In Wet Coil situations, the most energy efficient option would be to cool the airstream with the lowest enthalpy. This may or may not be the airstream with the lowest Dry Bulb temperature since the moisture levels play a role in determining Enthalpy. If the Return Air has a higher Enthalpy than the Outside Air it would take more energy to cool down, even if the Outside Air Dry Bulb temperature was higher than the Return Air Dry Bulb temperature.

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If the Return Air Dew Point is less than the Supply Air setpoint, then no dehumidification occurs, and we end up with what is referred to as a Dry Coil situation. This situation differs because the humidity ratios are the same for both entering and leaving airstreams, and so the most efficient tactic for Dry Coils is to cool the airstream that has the lowest Dry Bulb temperature, regardless of what the comparative humidity levels are.

All of these calculations are done with Dry Bulb Temperature and Relative Humidity levels, but different applications of these two variables result in a myriad of factors to consider when determining which option is the most energy efficient cooling method at the time. Ideally, both the Return Air and Outside Air should be evaluated consistently in order to ensure the most effective method is being leveraged.

Many people are hesitant to implement Enthalpy evaluations into their Air-Side Economizer functions, either because they don’t fully understand the process or due to the high cost of enthalpy sensors.

Computrols has worked hard to remove these hurdles by including a slew of native calculations in our flagship CBAS software. This powerful program can determine many variables, such as Enthalpy, Dew Point, or Wet Bulb, simply by looking at the input from a Dry Bulb sensor and a Relative Humidity sensor. As such, there is no need to purchase expensive specialty sensors that perform those same calculations at the sensor level because the software can use standard sensors to achieve the same thing. By simplifying the required steps, we have also removed the need for the user to understand the minutiae of the processes in order to implement these highly valuable energy-saving measures. In humid climates, proper application of these techniques can result in roughly 10% energy savings. In more favorable climates that permit free cooling to be enabled a greater majority of the time potential savings could reach between 20-30%, showing just how important Enthalpy can be.