Using a Geothermal System to Cool Your Data Center

Thursday, 11 December 2008 12:30

Written by John Peterson, PE

For data centers, this means cutting back on the staggering amount of energy required to keep them running.  In 2006, data centers consumed about 60 billion kilowatt-hours (k WH) of electricity, equaling roughly 1.5 percent of total U.S. electricity consumption.  Estimates project that by 2011, this figure will double, if current trends in design and operation continue.

The good news is that there are plenty of energy efficient improvements that do not compromise data center availability, performance or security.  When creating energy efficient environments, companies should focus upon implementing effective, efficient cooling methods for the infrastructure workload.  Ensuring that equipment is within the optimal temperature range while balancing power needs is critical. 

Looking for an efficient, cost effective, and environmentally-friendly heating/cooling system? A geothermal system is the greenest way to go.

Geothermal Introduction
A geothermal system consists of a series of underground pipes that take advantage of the constant temperature of the earth to cool data centers which results in higher efficiency and lower energy use.

 
Figure 1: Typical Geothermal system

Energy and cost savings of geothermal heat pumps will vary by region and type of conventional system to which they're connected. While the initial cost of installing a geothermal system is higher than traditional heating and cooling methods, the long term benefits and cost savings, not to mention the efficiency of a geothermal system, can be quickly realized in a short timeframe.

Another key benefit to using the ground is that the systems performance is not affected to outdoor weather conditions such as rain or heat waves that can limit or overburden the equipment. The result is less time and money spent on maintenance and operation of the system.

The energy costs of geothermal versus conventional heating, ventilating, and air conditioning (HVAC) systems will always be lower over the life of the system and the geothermal system will always be greener.
 
Geothermal Types
The most common geothermal system used for cooling larger, energy-intense buildings is a closed loop ground-coupled system. This is primary geothermal design which we will discuss in this article.  In most cases, a ground-coupled system is composed of an array of vertical wells beneath the ground, spaced anywhere from 15 to 30 feet apart with a vertical depth of up to 800 feet.  Together the wells form what is known as a well field, with all of the piping buried safely below the frost line to escape freezing conditions in the winter. 

Geothermal First-Cost and Operation Comparisons
As mentioned earlier, the upfront capital cost of a geothermal system is greater than conventional HVAC systems.  Where a conventional system might cost about $1000 to $1400 per ton of cooling, a geothermal system may cost up to three times as much.  This initial cost might appear unattractive, but a geothermal system can pay for itself in energy savings in as little as four years.

The primary reason behind the larger upfront cost is largely due to the labor and materials to install the geothermal well field.  Long lengths of piping and heat transferring fill need to be buried in the wells, and all of the piping must be below the frost line to prevent freezing. A geothermal system requires a relatively large amount of land to install the wells, and disapate the heat. Fortunately, the well field can be located beneath a parking lot or other similar areas.  Once installed, the geothermal system requires almost no regular maintenance.  Should a rare leak develop in a well, it can be isolated to prevent water and system losses. 

While a conventional design using cooling towers might be cheaper at the beginning, the ongoing maintenance and operating costs are much higher than with a geothermal system. 

• Cooling tower fans must operate to provide even minimal cooling, and their efficiency is directly dependant on the outside air temperature. 
• Chemicals are necessary to prevent growth of contaminants in the cooling tower basins. 
• Both the cooling towers and the associated chemical treatment system need to be monitored and maintained by the building staff.
• Costs must also include replacement parts such as belts, motors, and basin heaters.
• Also of concern is cooling tower down-time, as during this time the facility has temporarily lost either capacity or redundancy. 

A geothermal system begins to save energy as soon as it is operational, significantly reducing the amount of energy consumed since there are fewer motors required for operation.  Since the system is a closed loop, there is little contamination of the water as it circulates.  There is also little need for replenishing water. This can save over 30 gallons per minute of water when compared to a cooling tower. 

Integration with Data Center Facilities Equipment
From a design perspective, a geothermal system rejects heat from the same condenser water system that a cooling tower typically serves.  The geothermal system can be designed to completely replace the cooling towers, but often is coupled with a type of cooling tower called an evaporative cooler.  The evaporative cooler, also called a closed-circuit cooler, can provide extra capacity during high load conditions, and is able to recharge the geothermal system during off-peak hours.  A geothermal system incorporating an evaporative cooler in this way is known as a hybrid system. 

A hybrid system design can shrink the well field size, thereby reducing the high cost of installation from 20 percent to 45 percent as compared to a ground coupled system.  The geothermal and cooling plant designer should closely consider planned and future cooling needs, perhaps utilizing the hybrid solution at a later time when the demand for cooling has increased.

The geothermal system will operate in a wider temperature range, using ground temperatures as low as 45 degrees F to 70 degrees F, dependant upon the geographic location.  The equipment coupled with this, most often chillers, should be considered when operating over the wider temperature range.  A heat exchanger may be used to keep the geothermal system water separate from other cooling equipment to eliminate the small risk of sharing ground water with equipment cooling coils.  A heat exchanger can also be introduced to separate the water systems should glycol (anti-freeze) be necessary. However the introduction of a heat exchanger reduces overall efficiency of the geothermal system. 

Geothermal systems will have an operational life span of up to 200 years and are often given warrantees for 50 years. This is much, longer than the life of a typical 30-year cooling tower.  During its operational lifespan, a geothermal system may see an increase in water temperature, especially over a prolonged period with a constant load almost equal to the geothermal field capacity. At this point, the well field temperature is gradually rising and losing its ability to discharge heat into the ground.  This causes the equipment to be less efficient, in turn adding more heat to the field – unless checked, this downward spiral will continue. 

Fortunately, there is an easy solution. Like a battery, the well field can be recharged.  As mentioned above, this can be done by operating an evaporative cooler or cooling tower to assist with cooling; the equipment can recharge the well field during the winter to take advantage of the lower ambient temperatures.  The cooling equipment can also operate at night while electricity costs are lower and the tower efficiency is higher. Operating the equipment during normal hours can also assist a discharged geothermal system with high peak loads.  The evaporative cooler can completely recharge the geothermal system during the winter and off-peak hours, resetting the well fields to reduce the high energy costs during the summer months. 

Taking full advantage of a geothermal system requires planning and foresight to properly couple with the rest of the cooling system, but geothermal should always be considered as a possible energy saving solution for data center heat rejection.   

About the Author: John Peterson, PE, LEED AP, HP Mission Critical Facilities Services

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