Sustainability Associate Director at Cundall

What is Hybrid Cooling?

If you imagine relaxing in a tropical paradise, chances are you might picture slowly revolving fans above a daybed with an outlook on lush greenery. It’s an image straight out a glossy travel brochure – but it is also simply a very practical application of building physics strategies.

The tropics pose particular challenges for providing cooling that is both effective and energy-efficient. In other climate zones, seasonal temperature variations and diurnal variation allow for strategies such as operable windows, or night-purge ventilation for extremely low-energy cooling and fresh air.

What does work in the topics is that slowly turning fan we visualised. A low velocity fan delivers extra degrees of cooling that mean the air conditioning can have a higher set point such as 26 or 27 degrees instead of the usual 22-23 degrees and people will still generally feel as comfortable. This is hybrid cooling; the combination of a traditional air-conditioning system with ceiling/desk fans.

Benefits of this System/Design Approach

Hybrid systems save significant energy and Opex, as air conditioning uses more energy the lower the set point goes. Fans being a simple small motor, they use very little energy. A common reference is the SDE4 building at the National University of Singapore, that designed a hybrid cooling system as part of overall energy efficiency and has reported a EUI (energy use intensity) for the entire building of 48 kWh/m2/year is substantially below industry average for comparable spaces.

“While it is relatively straight forward to retrofit fans – it is incredibly complex, expensive and time-consuming to replace or upgrade a chiller.”

Another benefit of the hybrid cooling approach is having more than one way to cool the human body. The standard air conditioning system relies on a low ambient temperature and the human body transferring heat into the cooler air around it. A fan moves air over the body increasing heat dissipation through both convection and evaporation – cooling the person faster.

The impact is also on the occupant’s perception of comfort. Research from Berkley’s Centre for the Built Environment (CBE) found that it can take up to 15 minutes for an occupant to register a change in the room conditions with a standard HVAC system. Whereas a fan aided strategy instantly provides a cooling sensation (and effect).

Additionally, introducing fans also means occupants can have greater personal control over their immediate environment.  Because of conventions around office attire, for example, the traditional air conditioning set point of 22 degrees is comfortable for people wearing suits – but often too cold for others in the office. So, if the temperature set point is higher on the main system – the HVAC – those who feel the need for extra cooling can turn on a fan in their personal space, while those dressed for slightly higher ambient temperatures will be comfortable too.

Hybrid Approach—Why is it not a Standard Practice?

One of the obstacles might be perception. Tenants could have concerns about how guests might respond to a slightly higher set point for the general air conditioning and fans to supplement the cooling. Leasing agents may be worried that it won’t be attractive, or owners may believe that the presence of fans somehow makes their asset seem less luxurious.

Another barrier could be the structure of commercial leases. In buildings with centralised HVAC systems, landlords/property managers often pass on cooling costs in the form of a service charge, typically as a flat $/m2. The result is that tenants are not individually charged for their cooling consumption and are therefore not incentivised to use a hybrid system when doing the fit-out. (Note, for areas with high cooling, such as server rooms, it is typical for cooling to be sub-metered and charged. But this is not the majority of the building’s GFA or cooling consumption)

A factor could also be industry awareness and gaps in technical knowledge. The successful design/installation of any system requires close collaboration between the client, design team and the contractor; hence all parties need to be aware of the Opex, environmental and comfort benefits from this approach.

And lastly, hybrid cooling seems new and therefore risky. The BCA’s Braddell campus in Singapore is probably the closest regional example of an office building that has successfully adopted the approach, but we are yet to see it deployed at scale by a commercial developer. The concept needs to be derisked – many developers are comfortable with being the second adopter, but not the first!

Additional and Less Obvious Benefits

As discussed earlier, there is the immediate energy saving on operating cooling. However, there is a deeper cost saving also which is by operating traditional HVAC systems at a higher set point, it is likely the service life of the chiller could be extended. While it is relatively straight forward to retrofit fans – it is incredibly complex, expensive and time-consuming to replace or upgrade a chiller. The strategy might also unlock capacity in the system, facilitating changes in tenancy mix/need, increasing GFA, or providing resilience.

There are a body of studies that demonstrate occupants’ satisfaction improves when they have control over their space; improving staff retention and lowering absence days. Giving occupants greater control over their comfort through being able to switch on, speed up or switch off fans in their immediate personal zone also improves their personal sense of being comfortable.  

In addition, as fans are extremely economical with energy consumption, when a building is being retrofitted for improved resilience (including power outages) while HVAC is in many cases too demanding energy-wise to operate from backup power, it may be possible to keep the fans on and keep people more comfortable. This is a strong selling point, particularly as next year SGX and HKEX listed REITs and Asset owners will be required to report climate change risks (and eventually, mitigations) under the ISSB standard.

How can we make hybrid cooling common place?

Metered cooling and green leases:

The current flat cooling cost ($/m²) charge needs to be replaced with direct cooling cost ($/kWh). To aid in standardisation, green lease toolkits should set this as a minimum requirement. It will incentivise energy saving generally and facilitate tenants considering hybrid cooling for their fit-out.

Technical guides and training:

Engineers and contractors will need to upskill on how to design and install a hybrid system in new and existing buildings. Currently, designers can access the CBE’s guide on fan design and The ASHRAE 55-2023 guide is an excellent resource for establishing thermal comfort criteria for hybrid systems. In Singapore the BCA has started offering training courses, other regional authorities or green building councils should do the same. Additionally, a localised design guide would help with adoption and eventually the development of a National Standard.

A High profile, Commercial Case Study:

An example multi-tenant project that has deployed hybrid cooling as a core strategy. Preferably it would be in the CBD or a major business park, where the perception shift can take place.