Designing modular data centers

When planning for modular data center design, the engineer should focus on attributes such as system efficiency and operational characteristics.

12/21/2016


This article is peer-reviewed.Learning objectives

  • Provide a high-level understanding of topics related to modularization in data centers and other critical facility types that have sophisticated and complex power and cooling infrastructure.
  • Present arguments on the aspects of financial and operating outcomes for a range of modular design approaches, ranging from a minimal application to a highly modular design with multiple levels of redundancy. 

In the design of power and cooling systems for data centers, there must be a known base load that becomes the starting point from which to work. This is the minimum capacity that is required. From there, decisions will have to be made on the additional capacity that must be built in. This capacity could be used for future growth or could be held in reserve in case of a failure. (Oftentimes, this reserve capacity is already built into the base load). The strategy to create modularity becomes a little more complex when engineers build in redundancy into each module.

In this article, we will take a closer look at different parameters that assist in establishing the base load, additional capacity, and redundancy in the power and cooling systems. While the focus of this article is on data center modularity with respect to cooling systems, the same basic concepts apply to electrical equipment and distribution systems. Analyzing modularity of both cooling and power systems together-the recommended approach-will often result in a synergistic outcome. 

What is modular planning?

When planning a modular facility, such as a data center, there are three main questions that need to be answered:

  • What is the base load that is used to size the power and cooling central plant equipment (expressed in kilovolt-amp, or kVa, and tons, respectively)? In the initial phase of the building, if one power and cooling module is used, this is considered an "N" system where the capacity of the module is equal to the base load.
  • In the base load scenario, what is the N that the central plant used as a building block? For example, if the base cooling load is 500 tons and two chillers are used with no redundancy, the N is 250 tons. If a level of concurrent maintainability is required, an "N+1" configuration can be used. In this case, the N is still 250 tons but now there are three chillers. In terms of cooling in this scenario, there would be 250 tons over the design capacity.
  • How do we plan for future modules? If the growth of the power and cooling load is determined to be linear and predictable (which is a rare scenario), the day one module will be replicated and used for future growth. However, when the growth is not predictable or the module design has to be changed due to changing loads or reserve-capacity requirements, there has to be a strategy in place to address these issues. This is where the module-in-a-module approach can be used. 

Module-in-a-module

Each module will have multiple pieces of power and cooling gear that are sized in various configurations to, at a minimum, serve the day one load. This could be done without reserve capacity, all the way to systems that are fault-tolerant, like 2N, 2(N+1), 2(N+2), etc. So the growth of the system has a direct impact on the overall module. For example, if each module will serve a discreet area within the facility without any interconnection to the other modules, the modular approach will stay pure and the facility will be designed and constructed with equal-size building blocks. While this approach is very clean and understandable, it doesn't take advantage of an opportunity that exists: sharing reserve capacity while maintaining the required level of reliability.

If a long-range strategy includes interconnecting the modules as the facility grows, there will undoubtedly be opportunities to reduce expenditures, both capital expense and ongoing operating costs related to energy use and maintenance costs. The interconnection strategy results in a design that looks more like a traditional central plant and less like a modular approach. While this is true, the modules can be designed to accommodate the load if there were some type of catastrophic failure (like a fire) in one of the modules. This is where the modular approach can become an integral part in achieving high levels of uptime. Having the modules physically separated will allow for shutting down a module that is in a failure mode; the other module(s) will take on the capacity that was shed by the failed module.

Figure 1: This graph shows the three scenarios (one to three chillers) running at loads of 10% to 100% (x- axis) and the corresponding chiller power multiplier (y-axis). The chiller power multiplier for the one-chiller scenario tracks the overall system l

Using the interconnected approach can reduce the quantity of power and cooling equipment as more modules are built, simply because there are more modules of N size installed (see Figure 1 through 4). Installing the modules with a common capacity and reserve capacity will result in a greater power and cooling capacity for the facility.

If uncertainty exists as to the future cooling load in the facility, the power and cooling equipment can be installed on day one, but this approach deviates from the basic design tenets of modular data centers. And while this approach certainly provides a large "cushion," the financial outlay is considerable and the equipment will likely operate at extremely low loads for quite some time. 

Equipment capacity, maintenance, and physical size

When analyzing the viability of implementing a modular solution, one of the parameters to understand is the size of the N and how it will impact long-range costs and flexibility. To demonstrate this point, consider a facility with a base load of 1,000 tons. The module could be designed with the N being 1,000 tons. This approach leaves little reserve capacity or the ability to maintain the equipment in a way that minimizes out of range temperature and humidity risks to the IT systems. In this N configuration, taking out a major piece of HVAC equipment will render the entire cooling system inoperable (unless temporary chillers, pumps, etc., are activated during testing or maintenance).

Figure 2: This graph demonstrates the same concept as in Figure 1, but the facility cooling load is at 75%. As the cooling load decreases, the energy use of the different scenarios begins to equalize.

Going to the other end of the spectrum yields an equipment layout that consists of many smaller pieces of equipment. Using this approach will certainly result in a highly modular design, but it comes with a price: All of that equipment must be installed, with each piece requiring electrical hookups (plus the power distribution, disconnects, starters, etc.), testing, commissioning, and long-term operations and maintenance. This is where finding a middle ground is important; the key is to build in the required level of reliability, optimize energy efficiency, and minimize maintenance and operation costs. 


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