Delta T optimisation – the next frontier in data-centre cooling efficiency?
14 June 2016
As traditionally the largest energy expenditure in data centres once the IT equipment is powered, data centre cooling is closely scrutinised for any potential savings that can be achieved.
In the last decade, both the IT-equipment OEMs (original equipment manufacturers) and the building services/HVAC (heating, ventilation and air conditioning) vendors have risen to the challenge, building upon Trojan work by organisations such as the American Society of Heating, Refrigerating, and Air-Conditioning Engineers’ (ASHRAE’s) Technical Committee 9.9, which has defined a number of different permissible temperature and humidity ranges for the data-centre cold aisle (as measured at the inlet to the IT equipment).
Each ASHRAE defined range (envelope) is wider than the last, permitting lower and lower air-cooling costs, beyond the tightly controlled conditions that evolved into the norm in years past.
Both the IT equipment OEMs and the HVAC industry now build their equipment with these ASHRAE envelopes in mind, with IT equipment being rated for certain ASHRAE envelopes (e.g. the Dell Fresh Air Server, or the HP ProLiant ASHRAE A3A range), and data centre HVAC systems coming on stream to capitalise on these envelopes with cold aisle/hot aisle containment, direct free air cooling and adiabatic cooling (as offered in CommScope’s DCoD with SmartAir).
This convergence of two distinct skillsets to enable wider temperature ranges means that in many cases, tremendous savings on the cost of conditioning the cold-aisle air can be realised, up to and including solely external direct free air-cooling without any chiller.
Of course, to remove heat from IT equipment, there are two ingredients required: suitable air in the cold aisle, and a means to move that air through the IT equipment, absorbing waste heat as it passes. While ASHRAE along with the IT OEMs and HVAC vendors are operating in concert to define the parameters for the cold-aisle conditions, there is less consensus in the data-centre industry about what is a suitable delta T (ΔT) or temperature rise across the IT equipment.
How the IT equipment transfers its heat to the air stream flowing from cold aisle to hot aisle is quite similar to how a traditional hair dryer works: the temperature at the outlet depends upon the speed of the fan blowing air across the heating element.
In contemporary data centres, with correct separation of cold aisle and hot aisle, it is possible to control the delta T (hot-aisle temperature – cold-aisle temperature) by varying the speed of the air circulation fans (same principle as speeding up and down the hair dryer fan, and consider the IT equipment to be equivalent to the heating element in the hair dryer).
Presuming that there are no operational costs associated with conditioning the cold-aisle supply air, the energy associated with moving the air through the IT equipment now becomes the principal cooling energy cost. And note that the larger the delta T allowed, the less energy you will consume (and vice versa).
Two main methods have been identified to control the data centre circulation fans, the most common of which is known as fixed delta T (with variable hot aisle temperature). In this mode, the fan speeds are modulated to maintain a particular heat rise across the IT equipment, such that if your cold-aisle temperature is 20C, and your delta T is set at 10C, your hot-aisle temperature will be 30C. And if your cold-aisle temperature is 25C, your resultant hot-aisle temperature is 35C.
This fixed delta T approach is perfect if your cold-aisle temperature is assured in a tight band, and your IT equipment required a particular delta T (e.g. as tight as 5C or as wide as 15C). Its drawbacks include the fact that regardless of what the inlet and exhaust temperatures are, to achieve a fixed delta T, a fixed volume of air must be moved. This means the energy consumption is fixed – so even if your cold aisle is a brisk 15C, your delta T of 10C is fixed, and you will spend energy to result in a hot aisle of 25C (which in other circumstances is quite acceptable as a cold-aisle temperature).
Enter the variable delta T control method with a fixed hot-aisle temperature. In this mode, you specify what your hot-aisle temperature will be (e.g. 30C), and allow the fans to speed up and down to ensure that regardless of what the cold aisle temperature is, the air volume (and consequently the energy consumption) is modulated to ensure the hot-aisle temperature setpoint is achieved.
So, if your hot-aisle temperature is set at 30C, and your inlet air is 20C, you will have a delta T of 10C. However, as the contemporary ASHRAE cold-aisle ranges denote, a wide range is permitted in the cold aisle, so your cold-aisle temperature could just as easily be 15C, in which case your delta T is 15C (along with a reduced energy consumption).
This is a boon for energy savings. However, it takes patience to set-up correctly, considering degree of positive pressurisation to be maintained in the cold aisle, and modulation range of the fans.
I feel that in future, more and more data-centre managers will switch to a fixed hot aisle/variable delta T strategy once they expand their cold-aisle range, and a new frontier in energy savings will be identified. In advance of that, any interested parties are more than welcome to visit CommScope’s DCoD demonstrator modular data centres with SmartAir cooling to experiment with variable delta T strategies and more, in our Executive Briefing Centre in Richardson, Texas, USA.
Author: Darragh Rogan is technical services manager for CommScope’s Data Centre on Demand (DCoD) product in the Europe, Middle East and Africa regions. He supports the DCoD commercial team in the EMEA regions through his technical expertise, participates in developing a world class Partner Pro network for DCoD, and provides input to the product development team in shaping the future direction of the DCoD product. He holds a Bachelor’s Degree in Electrical and Electronic Engineering, and is a Chartered Engineer.http://www.engineersjournal.ie/2016/06/14/delta-t-optimisation-the-next-frontier-in-data-centre-cooling-efficiency/http://www.engineersjournal.ie/wp-content/uploads/2016/06/TelecityGroup-data-centre-1024x717.jpghttp://www.engineersjournal.ie/wp-content/uploads/2016/06/TelecityGroup-data-centre-300x300.jpgMechdata,energy,technology