I recently reviewed an energy colonoscopy report aimed at saving energy for a small hospital’s data center – an embedded data center. Many building types can have substantial embedded data center loads including schools, office buildings, banks, and government. Connected data center loads vary from a few kilowatts to perhaps 50 kW.
You can do your own research on energy saving opportunities in data centers. Here is one from Lawrence Berkeley National Lab that covers fourteen energy-saving opportunities, but it’s missing one: the giant canacorn custom measure that exists for nearly every embedded data center.
My point in this post is not to pick one colonoscopy report to shreds. My point is to show what a robotic, plug and chug study misses, and where big impacts are often missed.
Ring Em Up!
About that colonoscopy report: the data center has a connected IT load of about 10 kW and the server room is cooled by any of three separate systems: (1) two split systems not unlike central AC in most homes, (2) some portable units and (3) large air handling system serving a lot of occupied space in the hospital.
The report includes some fancy charts of metered electrical consumption from the first and second cooling systems noted above, and the space temperature over time. Great.
This report fails on several fronts.
First, this report does not include energy impacts associated with the large air handling system. Strike one, two, and three; you’re out. That’s like assessing and working on batting only, while completely ignoring the fielding.
Second, it makes recommendations that will be problematic.
Third, it leaves a mess and plenty of opportunities for fighting systems and waste.
Central System Sins
Since the deployment of computer networks over the last thirty years, embedded data centers have been plopped into any open space available in existing buildings. Space conditioning was an afterthought of ham-and-egg solutions that offer great opportunities today.
However, I should add that we see new building HVAC systems that serve a bunch of adjacent, “normal” spaces and an embedded data center. The person who designs such a data center cooling solution should be fined till it hurts. This is the tail wagging the dog in a most wasteful way.
Cooling a data center with a large central system hemorrhages energy year round. The data center sets the temperature (low) for the entire system and may call for the supply fan to run much faster than necessary, all year. Embedded data centers are in the core of buildings and have a virtually constant cooling load all year. Whether the air handler is using free cooling in winter, or chilled water or direct expansion cooling in summer, the result is a lot of simultaneous heating and cooling.
Therefore, step one, take the data center out as a zone on the central air handling system. Note, this does not mean the data center cannot be served by the large central system. More below.
A Recommendation that Won’t Work
The colonoscopy recommends installing a packaged HVAC unit capable of heating, cooling, and free cooling with outside air. These are the things that populate the tops of Walmarts, Targets, Lowes, and every box store on earth.
The recommendation is to use outside air free cooling between 20F and 65F outdoor conditions. Twenty degrees is well outside ASHRAE’s guide for space conditioning in data centers. Engineering nerds may appreciate this shown on a psychrometric chart nearby. The ASHRAE guide says though shalt stay within the magenta square. The recommended operation will result in the space being out of bounds as shown, during cold weather.
Moreover, a lot, if not most, direct expansion cooling systems like the recommended unit, will not operate in cooling when it is cold outside. Residential units, for example, are designed for operation above 65F outdoor temperature only. This lockout is to protect the compressor.
Here are some options for big HVAC savings with embedded data centers.
- Circulate air from the adjacent space through the data center with appropriate containment and barriers – once through and out. This is what we do for our little data center. It even helps heat our space in winter, but it doesn’t wag the dog as its own temperature control zone. The air flow and heat gain for the small-hospital application may be excessive. Calculations are required.
- Use outdoor air as described with ultrasonic humidification. The outdoor air and humidification both cool the space and obviously keep humidity levels above the “out of bounds” zone. This is a bit risky in my mind because it’s an engineered solution, and the humidification water must be pure.
- Use outdoor air as described with a water/antifreeze (water-side) economizer. This avoids introducing excessive cold, bone-dry outdoor air to the space, but again, there is risk and cost of an engineered solution.
- Select a computer-room-air-conditioning unit that has a pre-engineered economizer cycle for very cold weather that is suitable for California Title 24. Like many things, if a reputable company has already engineered the solution for a specific application (data center cooling in cold climates) it makes sense to me to go with this option.
 This is a red flag all day and an indication that anything could be happening with the HVAC serving this space.
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