Article originally published in The Data Center Journal in July 2011.
In almost any context, the presence of large amounts of waste leads to the question of what could be done with that waste to save money and materials. In power generation, wasted energy in the form of heat seems like such a—well, waste. Although the second law of thermodynamics implies that no process is perfectly efficient, modern power generation facilities leave plenty of leeway for improvement. But if your data center is generating, or planning to generate, its own power, not all the heat that your facilities produce need be wasted: trigeneration can improve efficiency, reduce operating costs and help protect the environment.
Also known as combined cooling, heating and power (CCHP) or “trigen,” trigeneration is a way to exploit the heat energy produced by, for instance, the burning of natural gas or coal to produce electricity by way of turbines. As the more descriptive name of CCHP indicates, trigeneration provides cooling and heating in addition to the typical electrical power produced in a generation facility.
The Data Center Journal has previously discussed the possibility of data centers generating their own power—a strategy that involves greater capital costs but that can also yield tremendous benefits in terms of power quality and freedom from the vagaries of the power grid. For data centers that elect to generate some or all of their own power for operations, trigeneration offers a way to stretch fuel costs even further. Needless to say, implementing trigeneration infrastructure involves initial capital costs, but with energy prices continually rising, a properly planned and implemented trigeneration scheme can quickly provide a return on this investment.
Trigeneration: What Is It?
When a fuel such as natural gas or coal is burned, the result is heat energy and waste materials (gases such as carbon dioxide, for instance). A mechanical apparatus—typically a turbine of some sort—converts some of the heat energy into electrical energy. Generally, the same principle applies to both large-scale power plants that supply the power grid and smaller-scale, dedicated generation facilities for data centers or other industrial applications. In either case, much of the heat energy goes to waste—that is, it is not used for anything, but is simply vented into the atmosphere.
In the context of data centers, however, that unused heat energy need not be entirely wasted. (Again, some will go to waste owing to the inherent inability to create a 100% efficient process.) That’s where trigeneration comes into play. Trigeneration produces power using essentially the same means as discussed above—this is the third point in the CCHP name (power). The second point of trigeneration, heating, involves making use of waste heat to warm office space and to provide hot water. Instead of using the electricity generated by the turbines to heat water, for instance (a wasteful proposition), and even instead of using additional fuel (a somewhat wasteful proposition), why not use the abundant heat produced during generation of electricity? This aspect of trigeneration is particularly helpful when the data center is part of a larger campus that includes plenty of office space and other buildings that company employees use. In a manner similar to how automobiles use waste heat from the engine to warm its interior, the generation facility can provide heat to other areas of the company’s location.
The first point in the CCHP name—cooling—seems a little out of place, but the heat produced when generating electricity can still be used to cool the data center and even provide air conditioning for office space and other employee areas. This heat can be used to drive absorption chillers that can provide cooling capabilities to servers and other IT equipment or to office space.
How Much Better Is Trigeneration?
Trigeneration makes use of heat energy that would otherwise be wasted; thus, it is more efficient. But to justify the capital expense of implementing the necessary infrastructure to support such a system, information is needed on the extent to which it is more efficient than traditional power generation. The fuel input into the generation system is considered the baseline for an efficiency measurement: the fuel is burned to produce a certain amount of heat. How much of that heat is converted to a usable form (electricity, heating, or cooling, for instance) is the efficiency of the generation system.
Electrical generation alone is generally very inefficient. According to the U.S. Environmental Protection Agency (EPA), nearly two-thirds of the energy produced by burning coal or another fossil fuel is wasted: “The average efficiency of fossil-fueled power plants in the United States is 33 percent and has remained virtually unchanged for four decades.” Assuming a data center employs an electrical generation technology similar to that used by a typical large power plant, this number leaves plenty of room for increases in efficiency.
According to several sources, trigeneration can yield efficiencies of over 80 percent (some estimates are as high as 90 percent). Even at 80 percent, however, a trigeneration system is over twice as efficient as an average power plant. In other words, implementing trigeneration could conceivably cut a data center’s fuel costs in half (this, however, depends on how much electricity the data center needs relative to its cooling and heating needs). According to the M + W Group (“M+W Zander at the Hannover Messe: Reducing CO2 Emissions by up to 40 percent using Trigeneration Power Plants”), “A trigeneration power station can be fired using fossil fuels and renewable energy sources. The high level of efficiency of up to 85 percent is achieved by using the heat produced during the generation of electricity to supply a plant with steam, heat and cooling.” The 85% number means that 15% of the energy produced by burning the fuel is lost, and this typically occurs in the form of wasted heat energy and losses in distributing the electrical energy (line losses).
But will you always be able to get 85% efficiency out of a trigeneration plant? Practically speaking, not necessarily. For instance, the electrical load may remain fairly constant throughout the year, thus mandating a certain level of fuel consumption, but heating and cooling needs will vary. In the summer months, demand for cooling will be higher, and in the winter months, demand for heating will be higher. But in spring and fall, the need for cooling and heating may decrease significantly. In such cases, the heat converted into heating and cooling capacity may not be as needed, thereby decreasing the practical efficiency of the trigeneration system.
Nevertheless, the efficiency increases during times of peak usage can translate into large energy savings, even when averaged over the entire year. Properly judging how trigeneration can benefit a particular data center depends on a number of factors, including the need for heating and cooling, the type of cooling infrastructure used in the data center (free cooling would benefit less than a more traditional type of cooling system) and the size of the campus to be served by the generation facility.
What’s the Damage for That?
Greater efficiency, unfortunately, isn’t free. But depending on the particular situation, a company can potentially recover the cost of implementing a trigeneration facility quickly. One of the benefits of a trigeneration system is that it takes the place of separate boilers and chillers—in other words, trigeneration is not simply an additional technology whose cost must be tacked onto that of all the traditional infrastructure. A trigeneration apparatus could also be installed in existing facilities when old boilers or other infrastructure must be replaced.
The big question is, of course, how much it costs. Needless to say, costs will vary. But the EPA provides an analysis of the costs associated with such a system under certain average conditions. The analysis assumes a turnkey retrofit installation of a trigeneration system, and it doesn’t include any cost affects resulting from removal of existing boilers or similar equipment. The EPA estimates the capital cost for such a system at $1,200 per kilowatt. By way of comparison, manufacturing costs alone for solar panels are currently around $1 per watt, or $1,000 per kilowatt (see the 2009 article “First Solar Reaches US $1 Per Watt Milestone”), so with the usual markup for sales, the cost to a company implementing a solar power solution would likely be more than that of a trigeneration system. (Of course, the “fuel” for solar generation is free, but it is also sporadic—solar power is generally best used as a supplement rather than a full-fledged primary power source.)
Applying a number of other assumptions, such as high efficiency (95% of thermal energy is used), an average interest rate and high fuel costs, the estimated cost per kilowatt hour of electricity using a trigeneration system would be about 6.2 cents ($0.062).
A Working Model: Syracuse University
An example of the use of trigeneration in data center operations is Syracuse University’s “Green Data Center.” This facility employs 12 microturbines running on natural gas, all located in a turbine room. Electricity produced by the turbines is sent to an electrical room for distribution to the data center proper and to a battery room housing the facility’s uninterruptible power supply (UPS) system. Interestingly, the microturbines can “can generate any combination of AC and DC power on site, avoiding the loss of power that typically occurs during transmission and conversion,” according to a Green Data Center fact sheet. Exhaust from combustion (waste heat) is transferred to a chiller room, and chilled water is then supplied to the data center proper for cooling and to an adjacent office building. That office building also receives heated water as well.
The facility employs Thermax absorption chillers, which use a lithium bromide solution to transfer water vapor and promote evaporation (and thus cooling). A cooling tower provides additional cooling capacity. Heat exchangers produce hot water for heating the adjacent office building during cold winter months. Server racks are cooled via liquid piped to rack doors—a system developed for Syracuse University by IBM. The facility is registered with the U.S. Green Building Council (USGBC), and the university is pursuing a LEED Silver rating.
Does Free Cooling Steal Trigeneration’s Thunder?
But what about cases where the data center employs free cooling? Many locations in the U.S. can employ free cooling for a large part of the year, and some can do so nearly year-round. When cooling is supplied without the need for generated power, the practical benefits of trigeneration are greatly reduced. Trigeneration can still be used to provide heating and cooling for office space, for instance, but its need in the data center proper is questionable.
In a DatacenterDynamics blog (“Trigeneration or Free-Cooling?”), Robert Tozer notes that “a simplistic analysis is that a data centre always requires electrical energy and refrigeration, [and] therefore trigeneration is an ideal application. However, when this is examined in more detail some problems are uncovered. Whilst we may need to cool IT equipment, this does not always require refrigeration (chillers), as there are many free cooling (FC) solutions available.” In other words, the recovered heat energy—although it ostensibly means trigeneration is more efficient than simple electricity generation alone—may not have any potential use. Tozer concludes that “free cooling without chillers is the most interesting cost-effective option with lowest CO2 emissions.”
So, which is the best option? That, of course, depends on the needs of the data center. Trigeneration infrastructure may well not be worth the capital cost if the facility is unable to put the recovered heat energy to good use—and this is particularly the case for data centers that employ free cooling. This situation might almost seem to indicate that data centers might as well use traditional cooling infrastructure, since it means that the burning of fossil fuels yields more usable energy. But, of course, traditional cooling costs more than just the price of the energy needed to power it: it includes capital costs, maintenance costs and so on. Free cooling, where possible, seems to be the best option, especially for companies that have a strong focus on reducing costs wherever possible. The savings on capital costs in this case would likely outweigh the long-term benefits of using trigeneration to provide power and traditional cooling capability to the data center.
To some extent, then, free cooling does indeed steal trigeneration’s thunder. The otherwise exciting increase in energy efficiency of trigeneration compared with standard electrical generation is overshadowed by what amounts to the complete (or nearly complete) elimination of cooling as an energy drain. Trigeneration thus becomes more of a novelty that has some limited application but that may never see widespread adoption. In some sense, it is a little behind its time. Again, however, this is not to say that trigeneration has no use, as the Syracuse University Green Data Center illustrates.
Trigeneration promises an increase in efficiency through the use of waste heat left over following the generation of electricity. Perhaps the only unfortunate aspect of this improved technology is that this efficiency isn’t purely in the area of electrical generation. Electricity has more applications than heat—it can be used to power IT equipment and other electronic devices, or it can be used for cooling, heating, light and other purposes. When heating and cooling needs are minimal, the improved efficiency of the trigeneration system lacks the same benefit that it offers during peak times when heating and cooling are in high demand.
Nevertheless, in cases where traditional cooling infrastructure is used, trigeneration can at least help supply the cooling needs of the data center, increasing the efficiency of the generation process significantly. When free cooling is used, however, the benefits become increasingly marginal.
The roughly 33 percent efficiency of typical power generation facilities in the U.S. seems like a horrendous waste. To be sure, some of the limitation is simply physics: you can only get so much mileage out of heat energy using standard turbine technology. Trigeneration offers one way to take advantage of the waste heat and thereby increase efficiency. Although the level to which this improvement applies in a practical sense varies, it is one more option for data centers—particularly those that want to generate their own power—to help stem the effects of rising energy costs and to limit emissions and other negative effects on the environment.
Photo courtesy of EG Focus
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