HOW WE’VE MET THE METRICS:

The calculation method for our proposed design yields a peak PUE of 1.049.

The calculation method for our proposed design gives an annual EUE of 0.5488.

The PUE and EUE calculation tables are included.

CONCEPTUAL ENGINEERING APPROACH TO ACHIEVE THE PROPOSED PUE

A. Out of the Box Thinking

Leading edge concepts require a different perspective in order to re-invent the wheel. The requirements set forth in the RFP determined the boundary conditions for determining
the concept design. We analyzed the contributing factors for energy use in the HPCDC’s HVAC system and determined that the biggest contributor to energy use reduction is the reduction of the resistance to flow. In piping systems, the concept of large pipes, small pumps is fairly common. Inair systems, the conduit for flow is at a much larger scale. Reducing the resistance to flow as much as possible meant that the conduits or ducts had to be much bigger than normal. Considering that the building provides the limitation for how big these conduits should be, resulted in the thinking that the building should be part of the conduit and not just a shell that houses the conduit. Therefore, we have determined that the best way to minimize resistance to flow is to let the building be part of the air handling system. The walls that segregate spaces will be used to confine the air from one point to another. As in other facilities, the space below the raised floor will be used as a plenum for the supply air. The plenum space above the DataCenter will be used part of the return air pathway through side wall chases to convey the air from the DataCenter space down to the first floor air conditioning and supply fan space below the DataCenter.

B. Fast Forward to the Beginning

All HVAC concepts must begin with a psychometric analysis. The psychometric analysis provides the process map to resolve the conditioning puzzle. Outside and inside conditionswere plotted on the chart and the resultant process path was determined based on the selected heating and cooling methods. The process was repeated for various locations on the chart that depict the different seasonal variations specific to the NREL site and at the 5,800 feet ASL elevation.

C. Location, Location, Location

The yearly environmental conditions at the NREL site are ideal for the type of process that we have conceptualized. The HVAC concept makes extensive use of evaporative cooling, both direct and indirect. Direct and indirect evaporative cooling can be used for the majority of the year to maintain the HPCDC within the environmental limits provided in the 2008 Thermal Guidelines for Data Processing Environments for ASHRAE Class 1 data centers, without the need for a vapor compression refrigeration system.

D. Keep it Simple

The HPCDC HVAC concept was designed to be simple to understand as well as simple to operate. This not only allowed the energy consumption to be reduced, but it also allows the system to be robust and reliable. The system allows for three modes of operation; once through, recirculating, or a mixture of recirculation and outside air. The HPCDC proposal involves a three floor level concept with a lower level pump systems and thermal storage room, first floor HPCDC air handling and cooling water distribution, and second floor HPCDC computational equipment.

E. Tried and Proven Concept

The concept of using the building as the Air Handling System is not new. We have used this concept in the semiconductor fabrication industry. The industry went through a similar transformation for much of the same reasons. The transformation from re-circulating air handing units to open plenum systems with multiple filter fan units occurred in the mid nineties. The main differences in the system designs are flow direction and environmental parameter tolerances.

F. Challenges

No project is without its challenges. Challenges are the opportunities for improvement and innovation. The challenge of making the building be part of the air handling system requires careful attention to the construction methods for the spaces that convey the air flow. Considering that the systems are negatively or positively pressurized, the walls that form duct will need to be sealed properly to minimize air leakage. Of special concern are the outside walls that will separate areas that are humidified from the outdoor environment. These outside wall partitions will need to be carefully analyzed to determine the extent and location of a vapor retarder/insulating system. On previous projects, we have successfully used methodology described in the ASHRAE fundamentals handbook to determine the composition of the wall members. One such method is the determination of moisture transmission using the dew point method.

Another challenge is the determination of the operational modes to achieve the target PUE or lower. That challenge will be described in the next section.

PROPOSED OPERATIONAL CONCEPTS TO ACHIEVE PUE

A. Understanding the Process

Understanding of the conditioning process is paramount to selecting an effective and efficient operational mode. There will be many tools at our disposal and many methods to achieve the results. Knowing which method is most efficient requires intimate knowledge of the system configurations.

The HVAC system configuration for the HPCDC includes the use of outside air louvers with dampers connected by air chases to the return air / outside air mixing section. The air mixture enters the conditioning unit where it is filtered and conditioned as required to provide the required supply air state to serve the HPCDC. Conditioning of the air involves cooling and humidifying, or dehumidification of the air. The process is determined by the HVAC control system. The heart of the system is the fan array that is mounted on the underside of the concrete floor that supports the equipment on the second level. The fan array consists of multiple fans with N+1 redundancy. The fans will be provided with an isolation damper and an inlet bell. The isolation damper will be used to replace or repair a fan without losing plenum pressurization. The inlet bell will be used to minimize the entrance losses and provide a safety screen from the rotating parts.

The Conditioning Units condition a variable amount of air before it enters the cooled air plenum in the first floor of the HPCDC. The supply air plenum provides an equalizing chamber to evenly distribute the cooled airflow throughout the area. The circulating fans will draw the cooled air and supply it to the pressurized raised floor plenum above. A bypass damper around the conditioning units will be used to reduce the energy requirements during certain conditions. These conditions are explained below.

At the HPCDC floor, grated panels will be placed at strategic locations in order to provide the cooled air where it is needed. Depending on the selected equipment, the air will be supplied entirely into the cold aisles or a combination of cold aisles and underneath the cabinet frames. Heated air from the HPCDC equipment will be discharged into the contained hot air aisles and drawn into the ceiling return air plenum. The return air plenum will be connected to the mixing section by return air chases next to the outside walls of the room. The amount of air flow will be determined by the leaving air temperature above the corresponding hot aisles.

Primary cooling water will be distributed throughout the facility to serve the AHU and the HPCDC cooling systems. The configuration of the cooling system for the HPCDC will be a reverse return system with valved branches for each lateral that serves a portion of the HPCDC. The cooling systems will have a high enough temperature rise so that the return water can be used for heat recovery to other parts of the facility.

B. System Redundancy

The HVAC N+1 redundancy will be provided for critical components that would affect HPCDC operation if these components would fail. N+1 redundancy will be incorporated into the systems that will directly affect the HPCDC. These systems include but may not be limited to; cooling towers, heat exchangers, cooling pumps, and fans.

C. 2008 ASHRAE Recommended Environmental Envelope Area for ASHRAE Class 1 Data Centers

The 2008 expanded Environmental Envelope limits are as follows:

Low End Temperature: / 18° C (64.4° F) Dry Bulb
High End Temperature: / 27° C (80.6° F) Dry Bulb
Low End Moisture: / 5.5° C (41.9° F) Dew Point
High End Moisture: / 60% RH and 15° C (59° F) Dew Point

These limits define the operational characteristics of the Air systems that supply the HPCDC.

The limits are as follows:

  • Above an Outdoor Air dew point of 15° C (59° F),dehumidification is required.
  • Below an Outdoor Air dew point of 5.5° C (41.9° F),humidification is required.

Between an Outdoor Air Wet Bulb of 5.5° C (41.9° F) and 15° C (59° F) and below 5.5° C (41.9° F) Dew Point, direct evaporative cooling is effective.

Between the High End moisture and Low End moisture limits and to the left of the Low End Temperature, Mixing of a portion of the Return Air and a portion of the Outside Air is required.

Between the High End moisture and Low End moisture limits and to the right of the High End Temperature, Sensible Cooling of the Return Air is required.

Within the Environmental Envelope, conditioning of the air is not required.

This is just a sample of some of the operational states that the system will operate at based on outside air conditions. However, the conditional parameters do not end there. The amount of air flow to be supplied to the HPCDC has to be controlled for optimum performance. We propose that temperature control zones be created to assure that the maximum temperatures are not exceeded while the lowest amount of air is used for air based cooling. The control of air flow for optimum performance will be described in the following section.

D. Air System Operation to Obtain 1.06 PUE or Lower

The Air system will be controlled by the building automation system (BAS) who will determine its operational state. In the determination of the operational state, the BAS system will use such inputs as:

  1. Outside air Dry Bulb Temperature
  2. Outside air Dew Point
  3. Barometric Pressure
  4. Primary Cooling Temperature(PCWS)
  5. Primary Cooling Water flow rate
  6. Primary Heating Water Temperature (PHWS)
  7. Primary Heating Water Flow rate
  8. DataCenter Water Temperature (DCWS)
  9. Data Center Water flow rate

The primary goal to achieve the lowest PUE is to reduce the electrical requirements for conveying the heat generated at the HPCDC to a location where it can be beneficially used or at worse case, rejected to the atmosphere. Knowing that heat transfer with water is more economical and effective than with air, the primary cooling system will be used to perform the majority of the equipment cooling. The cooling towers will be used to produce the lowest possible temperature to cool the HPCDC but not so low that condensation could occur. Preventing condensation means that the supply water temperature of the DataCenter cooling systems will be controlled to be higher than the HPCDC dew point. Priority will be given to water based cooling in lieu of air based cooling.

E. Outdoor Operational State – Cold and Dry

This condition occurs whenever the Outside Air Wet Bulb Temperature is lower than 5.5° C (41.9° F). During this condition, the system will be in full recirculation mode with the only outside air required for ventilation and pressurization of the space. During steady state conditions, the amount of moisture addition will be minimal and the direct evaporation will be called on to increase the moisture level as necessary. Being that cooling of the HPCDC will be performed using the indirect (sensible only) cooling coils. During this condition, the majority of the heat transfer will be water based. Therefore, a portion of the Return Air flow can be bypassed around the sensible cooling coil in order to maintain the supply air near the upper end of the Dry Bulb Temperature. Heat recovered from the DCW system will be used to preheat outside air at Make Up Air handling units serving laboratories.

F. Outdoor Operational State – Cold and Wet

This condition occurs whenever the Outside Air Dry Bulb Temperature is lower than 18° C (64.4° F) and between the Low End and High End moisture limits. During steady state conditions, a portion of the Outside Air will be mixed with a portion of the Return Air in order that the air mixture condition falls within the EnvironmentalEnvelope. The mixed air is then supplied to the HPCDC through the supply air chamber. Since Outside Air is introduced into the facility, a portion of the Return Air must be exhausted from the facility. Exhaust (exfiltration) from the facility will be accomplished by the use of exhaust air louvers and dampers located in the return plenum space. The position of the exhaust air dampers will be controlled to assure that the building is maintained at a slight positive pressure.

G. Outdoor Operational State – Hot and Humid

This condition occurs whenever the Outside Dew Point Temperature is above 15° C (59° F). During this condition, the system will be in full recirculation mode with the only Outside Air being that required for ventilation and pressurization of the space. During steady state conditions, the amount of moisture removal in the HPCDC will be minimal. Moisture removal (dehumidification) from the Outdoor Air will be performed using fan coil units with low temperature coils to remove moisture. Cooling of the HPCDC will be performed using the indirect (sensible only) cooling coils.

H. Outdoor Operational State – Hot and Dry

This condition occurs whenever the Outdoor Air condition is between an Outdoor Air Wet Bulb Temperature of 5.5° C (41.9° F) and 15° C (59° F) and below 5.5° C (41.9° F) Dew Point. During this condition, the system may operate in one of two modes:

  1. 100% Outside Air flow – Direct Evaporative Cooling
  2. Return Air/Outside Air Mixing controlled by mixed air temperature sensors

Determination of which Operational mode to use will require knowledge of the availability of reheat (summer conditions) and the direct evaporating water temperature. Direct evaporative water temperature could have a negative effect if the water temperature is below the dew point of the air. If this is the case, cooling and dehumidifying of the air could occur.

Operating Mode 1; During steady state conditions, the system will bring in 100% outside air and humidify it so that the resultant state is above the Low End Moisture limit. From that point, the air can be mixed with Return Air in order to enter the Environmental Envelope before being discharged into the supply air chamber. Since 100% Outside Air is introduced into the facility, most of the Return Air must be exhausted from the facility. Exhaust (exfiltration) from the facility will be accomplished by the use of exhaust air louvers and dampers located in the return plenum space. The position of the exhaust air dampers will be controlled to assure that the building is maintained at a slight positive pressure.

Operating Mode 2; During steady state conditions, the return and outside air dampers will be positioned so that the condition of the air mixture, falls within the Environmental Envelope. If the outdoor air conditions change and the mixed air condition reaches a pre-determined limit, the BAS will add or remove heat from the mixture to return the air to the Environmental Envelope before discharging into the supply air chamber (plenum.) Since Outside Air is introduced into the facility, a portion of the Return Air must be exhausted from the facility. Exhaust (exfiltration) from the facility will be accomplished by the use of exhaust air louvers and dampers located in the return air plenum space. The position of the exhaust air dampers will be controlled to assure that the building is maintained at a slight positive pressure.

I. Outdoor Operational State – Within the Environmental Envelope

This condition occurs whenever the Outside Air condition is inside the environmental window. During steady state conditions, the conditioning system does not need to condition the air supply air in any manner. During this condition, the 100% of the Outside Air will be used to cool the HPCDC. In order to achieve the lowest energy usage, the dampers in the conditioning unit andthe bypass dampers shall be opened to 100% position. This will allow the air to flow through the path of least resistance and promote uniformity. Since 100% Outside Air is introduced into the facility, most of the Return Air must be exhausted from the facility. Exhaust (exfiltration) from the facility will be accomplished by the use of exhaust air louvers and dampers located in the return air plenum space. The position of the exhaust air dampers will be controlled to assure that the building is maintained at a slight positive pressure.