HVAC for Data Centers: Q&A Session

Bill Kosik and Tom SquilloWebcast Q&A session with presenters Bill Kosik, PE, CEM, BEMP, LEED AP BD+C, HP Enterprise Business, Technology Services; and Tom R. Squillo, PE, LEED AP, Environmental Systems Design Inc.

  1. Q: Describe how the efficiency of batteries gets affected with increased temperatures. What is ideal temperature for life and efficiency of batteries?
    • A: Tom Squillo: Based on information provided by several UPS battery manufacturers, UPS batteries are rated at 25 C (77 F) and it is recommended that they be operated at that temperature for optimum life. The rule of thumb is that the battery life will be reduced by 2.5% per degree Celsius (vented) and 5% per degree Celsius (valve-regulated lead-acid battery, or VRLA) rise in temperature. Decreased temperature reduces battery capacity by about 1% of rated capacity per each degree Celsius. 

  2. Q: As servers become more compact, although they may be more efficient, isn't the possibility of the idle power usage not changing or even increasing above the industry believed 30%?
  3. Q: Generally, what vintage of server equipment can tolerate the higher temperatures that we're shooting for? Purchased in past 5 years? 10 years? 15?
    • A: Bill Kosik: Given the wide range of available hardware, it is not possible to given a definitive answer. The first place to look is the manufacturer’s specifications. Then contact the manufacturer to discuss the implications of operating at higher temperatures.

  4. Q: Does the latest edition of ASHRAE Standard 90.1 prevent the use of heat wheel technology is high humidity regions? If so, why?
    • A: Tom Squillo: In ASHRAE Standard 90.1-2013, Section 6.5.1.6 states that systems with hydronic cooling and humidification systems designed to maintain dewpoint temperatures ≤ 35 F shall use a water economizer. This used to be section 6.5.2.4 in the 2010 version. So, strictly by the letter of the code, if your system has hydronic cooling, you could not use airside heat wheels or even indirect evaporative cooling using the prescriptive compliance approach. Most of these types of systems use direct expansion (DX) for supplemental cooling so this may not apply. In my opinion, this requirement is unnecessarily restrictive because it does not distinguish between direct and indirect airside, and the actual energy used by efficient humidification systems is quite low compared to other components.

  5. Q: If higher inlet temperatures are allowed into a data center, will this affect longevity of the IT equipment?
    • A: Tom Squillo: It’s very possible, though I think the jury is still out. Intel has performed some proof-of-concept testing and saw no substantial increase in server failures with large temperature and humidity variations, but much more testing is warranted before this is widely accepted in the industry and recommended by server manufacturers.

  6. Q: What would be the most recommended HVAC software for design purposes in the U.S. for industrial facilities (e.g., CHVAC, HAP, etc.)?
    • A: Bill Kosik: There are several tools. For example, eQUEST is a free hourly energy analysis package that is widely used in the industry. E+ is another free package. Although not as intuitive as eQUEST and others, it is very powerful. There are many other commercially available programs.

  7. Q: How do you see say ice thermal storage as a PUE management strategy?
    • A: Tom Squillo: In most cases, using ice storage will increase your PUE! Depending on your on-peak/off-peak electricity rates, ice storage may reduce your electricity costs, but will most often increase overall energy use. This is because it takes more energy to create ice than chilled water and additional energy to melt that ice when you need it. PUE calculations do not take into account utility costs or time of day usage.

  8. Q: Is filtration a big problem with direct air systems?
    • A: Tom Squillo: I wouldn’t say it’s a big problem, but I would definitely recommend higher efficiency filtration with direct airside economizers. The level of extra filtration depends on the local conditions (dust storms, smoke, salty air, other contaminants). Potential chemicals in the outdoor air may also be a problem that would be increased with airside economizers. Extra filtration increases pressure drop, energy use and maintenance/filter costs.

  9. Q: How do you plan on cooling the IT equipment with a discharge air temperature of greater than 60 F? The IT equipment does not like temperature above 78 F typically and if the servers reach 85 to 90 F, they start automatically shutting down. So in order to provide a 75 F SA temperature, you are essentially doubling the airflow requirements to the space in order to maintain the load the IT equipment produces. This would be a huge wind tunnel.
    • A: Tom Squillo: The quantity of air required would not change, assuming there is appropriate control of recirculation and bypass air within the data center space (containment). Unless server inlet temperatures rise above 75 to 80 F, the amount of air needed by a typical server does not change substantially due to inlet temperature. If a server heats up the cooling air from 60 to 80 F, for example, that same server will heat 75 F cooling air to 95 F. The air-handling unit (AHU) then cools the air from 95 F back down to 75 F. Both hot aisle and cold aisle temperatures increase, but air quantity remains essentially the same. If a data center requires 55 F cooling air to ensure 80 F server inlet temperatures, then it means that there is massive recirculation from hot aisle to cold aisle. Re-balancing and containment strategies should be considered. It is true that more attention needs to be paid to proper air management as temperatures increase. Reduction in energy use is not without some added risk and operational effort.

  10. Q: You mention 1.5% to 2.5% increase in chiller efficiency for every 1 degree increase in chilled water supply temperature. Do you have estimates for air-cooled DX computer room air conditioning (CRAC) savings when raising their supply air temperatures?
    • A: Tom Squillo: I haven’t received this info from DX CRAC manufacturers, but will investigate and share this info when received. The ability to control a typical DX CRAC unit by supply air temperature is limited and applies more for those with digital scrolls or variable speed compressors. The new units with integral pumped refrigerant economizers would obviously also benefit from extra hours of economization.

  11. Q: Where does the Kyoto Cooling configuration fit into these requirements?
    • A: Tom Squillo: Though the code language is not written with indirect airside economizers in mind, Kyoto cooling is an indirect air-cooled technology with DX supplemental cooling. I believe this meets the intent of the airside economizer sections of the codes. The clause regarding waterside economizers for humidified spaces described above would not apply since the system doesn’t include hydronic cooling.

  12. Q: In those countries not adopting standards as mandatory codes like here in Qatar, which is more applicable: ASHRAE 90.1 or IECC 2012?
    • A: Tom Squillo: ASHRAE 90.1 does not require climate zones 1A ,1B, 2A, 3A, and 4A to have economizers. Miami is zone 1A. Qatar is hotter and more humid than Miami, so it would be a challenge to make economizers actually be economical, and it certainly wouldn’t be required if it fell under the U.S. energy codes. IECC-2012 has no exceptions for complex systems based on climate.

  13. Q: Have you completed any studies about trigeneration effectiveness?
    • A: Bill Kosik: We have done a number of studies on combined heat and power. At a regional level, this type of system reduces the transmission losses, but depending on the efficiency of the turbine, there may be no net energy use reduction. This strategy is primarily used for utility cost reduction and running autonomous from the electric grid, not necessarily energy efficiency.

  14. Q: What is the value of energy monitoring of the data center?
    • A: Bill Kosik: Monitoring is critical to gain an understanding of the energy efficiency of the discreet pieces of equipment (if that level of granularity is envisioned). Also, the data can be rolled up to generate metrics such as PUE and water usage effectiveness (WUE). Certainly the monitoring has to include the IT systems and should also be extended to lighting and other miscellaneous power systems in order to get the most accurate energy use data. The Green Grid has a number of guidelines on this topic.

  15. Q: What type of energy intensity (kBtu/SF) is typical for a data center?
    • A: Bill Kosik: For a data center that has 1 MW of IT load, is 10,000 sq ft in area, has a PUE of 1.30, and is running at 100% load, the energy intensity is 3,515 kBtu/ sq ft. A rule of thumb is that a data center will consume approximately 100 times the energy of an equally sized commercial office building.

  16. Q: What type of conservation measures are applicable to data centers?
    • A: Bill Kosik: There are too many to list, but the primary strategies revolve around running elevated air temperatures so mechanical cooling is not needed, or needed only a small portion of the year. Along these lines, using indirect energy transfer, using both dry and evaporative processes will eliminate the need for mechanical cooling a large part of the year (in many climates). We are actually “cooling” the return air using the outside air using a heat exchanger, with no mixing of air streams. And this is not limited to cold weather. This cooling goes on into the hot summer months—the elevated temperatures in the data center produce return air temperatures that are greater than the outdoor temperature. So as long as the temperature outside is less than the return air temperature, the mechanical cooling load will be reduced. And since the energy exchange process is based on sensible heat transfer, the outdoor humidity does not impact the effectiveness of the process.

  17. Q: Please provide information about commissioning techniques, retrofitting existing HVAC systems, future alternative systems.
    • A: Tom Squillo: Not sure of the question, but the energy codes require some level of commissioning to ensure proper operation, though stop short of specifying the level or complete scope because project requirements vary. HVAC retrofits and renovations would generally not need comply unless there was a major change to the system or type of occupancy in the space. Future alternative systems that do not fall into specific categories would either require official interpretation or could use the Energy Cost Budget method of compliance.

  18. Q: What are typical W/sq ft seen at actual operating data center?
    • A: Bill Kosik: Typically we are designing for 100 to 150 W/sq ft, but are seeing criteria for more than 200 W/sq ft, especially in smaller data centers that house very high density server cabinets.

  19. Q: Compare DX cooling to CHW cooling, and which is preferable?
    • A: Bill Kosik: Focusing only on the energy efficiency aspects, the climate and size of data center will influence the choice. A straight DX system with CRACs will have the lowest energy efficiency, but an indirect evaporative air handling system with DX assist will generally have a better efficiency than a chilled water system. Advances in HVAC technology such as refrigerant free cooling also have to be considered. Even though it is DX, the operating strategy and equipment potentially make this type of system very energy efficient. Parametric (comparing systems and parameters) energy analysis is necessary to make an informed decision on the long-term operating costs of each option.