Welcome to the first post in our concise series on enthalpy and its significance in HVAC and AHU systems. Our team of experts has delved deep into this critical concept, uncovering its essential role in optimizing efficiency and performance. We aim to bring a clearer understanding of enthalpy to professionals in the field, making this complex topic more accessible and relevant.
“Free cooling” is a highly efficient and cost-effective cooling regime that leverages the natural outdoor air to assist in the cooling process of indoor environments.
Instead of relying solely on energy-intensive mechanical refrigeration, free cooling harnesses favorable outdoor conditions to reduce the thermal load within a building.
This approach is particularly beneficial:
- during cooler seasons (or cooler parts of the day),
- during spring and fall (transitional seasons),
- or in climates where the ambient air temperature is consistently lower than the desired indoor temperature.
The principle behind free cooling is straightforward yet ingenious. When outdoor air is cooler and less humid than indoor air, it can be drawn into the AHU and used to absorb and dissipate indoor heat. This process significantly reduces the reliance on traditional cooling systems, thereby saving energy and lowering operational costs.
The common mistake when configuring AHU systems for free cooling
In practice, we utilize free cooling when the outdoor air temperature is at least one degree Celsius (1°C) lower than the indoor air temperature.
In many instances, the application of free cooling in an AHU is done without evaluating the energy content of the indoor and outdoor air, which is a common oversight.
The precise method to determine the suitability of free cooling in AHUs involves comparing the enthalpy of indoor and outdoor air. Enthalpy, as a measure of total energy content, provides a significantly more accurate assessment than temperature alone. Read more about Enthalpy here.
Free cooling should only be applied when the enthalpy of the outdoor air is lower than that of the indoor air. This ensures that the energy content is genuinely reduced, leading to effective cooling.
In modern cooling processes, particularly those where AHUs regulate both relative and absolute humidity, it is common for indoor air to have lower humidity levels compared to outdoor air. Despite a higher temperature, the lower moisture content means the indoor air has a lower energy content.
Activating free cooling without first comparing enthalpy can inadvertently introduce excess humidity into indoor spaces.
This increased humidity, due to higher partial pressure, can permeate materials that have been previously dried to maintain low indoor humidity levels.
Consequently, this can compromise indoor air quality and negate efforts to control humidity effectively.
Sensors measuring enthalpy for optimal free cooling
High quality industrial grade sensors play a crucial role here, meticulously measuring enthalpy, dew point, and the density of moist air to ensure optimal conditions for implementing free cooling.
Andivi offers Enthalpy Sensors with Modbus and / or BACnet designed to precisely calculate enthalpy. Housed in practical, easy-to-install casings, these sensors seamlessly integrate into Air Handling Units (AHUs) and other HVAC systems, providing a reliable solution for optimized performance. For example: Modbus Enthalpy Sensor ANDRFFT/R-MD/S.
Implementing free cooling in AHUs involves sophisticated control systems that continually monitor and compare indoor and outdoor air conditions.
These systems adjust dampers and fans to maximize the intake of outdoor air with lower enthalpy while maintaining indoor air quality and comfort.
By intelligently managing the mixing of outdoor and recirculated air, AHUs can maintain a consistent and comfortable indoor environment while extensively reducing the energy footprint.
Sustainability and free cooling
Moreover, free cooling is an environmentally friendly approach, aligning with sustainable building practices. By reducing the dependency on mechanical refrigeration, it lowers greenhouse gas emissions and promotes energy conservation.
This aligns well with modern energy policies and the growing emphasis on sustainable and eco-friendly building operations.
Free cooling by night
To optimize the application of free cooling in AHUs, it is crucial to compare enthalpy not just during the day but also at night.
Diurnal temperature and humidity fluctuations can significantly impact the energy content of the air. By continuously monitoring and comparing the enthalpy of indoor and outdoor air throughout the 24-hour cycle, HVAC systems can more accurately and effectively implement free cooling.
This vigilant approach ensures that cooling processes are consistently efficient, leveraging the most favorable outdoor conditions at any given time, and thereby enhancing overall energy savings while maintaining optimal indoor air quality.
Does free cooling make sense?
In summary, free cooling in AHUs is a smart and sustainable method that optimizes the use of natural outdoor air to maintain indoor thermal comfort. It requires precise control and monitoring of air properties to be effective, highlighting the importance of advanced sensors monitoring enthalpy (e.g. Modbus Enthalpy Sensor ANDRFFT/R-MD/S) in achieving this balance.
For professionals in the HVAC industry, understanding and leveraging free cooling can lead to significant energy savings, reduced operational costs, and enhanced sustainability in building management.
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This article was prepared by Danijel Mursic, a mechanical engineer and thermodynamics expert with over 30 years of experience in the AHU and HVAC sector.
