A Power Menu for Rooftop Telecommunications Applications
A Power Menu for Rooftop Telecommunications Applications A Power Menu for Rooftop Telecommunications Applications Sandra Saathoff, ReliOn As much as 30 percent of total network sites are located on rooftops. This presents a challenge to carriers and their partners to harden sites against grid power outages by selecting solutions that meet the desired amount of power and runtime while accounting for the structural limitations of the building/rooftop, relevant codes and regulations related to fuel storage , logistics and environmental considerations including noise, vibration and emissions. Three viable power solutions: internal combustion generators, batteries and fuel cells, are compared against these parameters providing a matrix from which carriers may choose the best option for each particular rooftop setting. Assumptions For the purposes of this whitepaper, we shall assume a steady state customer equipment DC load of 7kW. Figure 1: Carrier X DC Load Survey Though most carriers routinely specify combustion generators at 25kW or more, the results of several power measurement surveys have indicated that the actual load requirement is much lower. Loads of 5kW and below are common (see Figure 1), with even more power surveys falling under 8kW. Additionally, many rooftop sites are not configured as macro sites due to space and weight limitations. A moderate 7kW assumption allows for nearly 30% overhead at most rooftop locations and allows one to look strategically at each type of power solution. A Power Menu for Rooftop Telecommunications Applications - 2 Generators Industrial internal combustion generators, such as those used for telecommunications equipment sites, can be purchased with one or more of three fueling options: diesel, natural gas (CNG) and propane (LP). A survey of various makes and models of generators resulted in the following comparison chart (Figure 2). On all charts, green highlighting in a matrix cell indicates a satisfactory or good solution, while yellow signifies a moderately acceptable solution and red indicates significant concerns. Figure 2: Generator Comparison Chart Diesel Generators Diesel generators are nearly ubiquitous at ground-level telecommunication sites. They are able to offer runtimes of 67.5 hours at the 7kW load on a small, integrated 54 gallon fuel tank. They become much more difficult in a rooftop situation due to several factors. Safety: If diesel fuel is spilled in a rooftop installation, it will either pool in a spill pan or leak into the building below. Either of these scenarios lends itself to a potential safety issue resulting in siting challenges. Fuel storage: Diesel fuel is susceptible to contamination and breakdown if stored for more than one year. Photo by http://www.enhancedpower.co.uk A Power Menu for Rooftop Telecommunications Applications - 3 Environmental: Generators fueled by diesel, even at Tier 4 levels, give off higher levels of carbon monoxide, nitrogen oxide, particulate matter and carbon dioxide than other power solutions surveyed. This poses substantial issues in dense urban areas which may be considered non-attainments zones, further increasing emission regulation and reporting. Additionally, noise and vibration levels of the solution are high. Weight: At over 3,200 pounds, the diesel generator solution is one of the heavier options available. This could be an issue in some rooftop scenarios. These factors often eliminate diesel generators as a viable solution for placement in rooftop sites. In some cases, diesel generators or their fuel are placed at ground level in order to mitigate the safety challenges. A ground level fueling solution would, of course, require the approval of permitting officials and the building owner for piping from the fuel storage area to the generator. Natural Gas Generators Natural gas generators offer the best runtime and fuel logistics, as long as certain specifications can be met. The building must have natural gas piping available on the rooftop. The flow rate needed for the generator is generally greater than that needed by the tenants, requiring the local utility to install a new feed to the building, which adds cost and time to the project. The building owner must agree to allow the rooftop carrier to use the fuel. If these requirements are met, the natural gas generator offers unlimited runtime for as long as the natural gas supply is not interrupted. The footprint and weight of the generator are moderate and natural gas burns cleaner than diesel fuel. Still, the natural gas generator does have some challenges: Environmental: noise and vibration remain a challenge and lead many building owners to disallow these solutions on their rooftops. Fuel Availability: Natural gas generators may not be allowed or preferable in locations prone to seismic activity. In many of these regions, the natural gas supply is shut down as a safety precaution due to public safety concerns, during the very time period when continued operation of telecommunications equipment is most important. Propane Generators Propane generators run cleaner and are lighter than diesel generators. The addition of a 150 gallon propane tank and fuel remains lighter than the 54 gallon diesel solution specified above. It gives the propane generator four additional hours of runtime over the diesel genset at 71 run-hours between refueling visits, or nearly 3 days within a specified temperature range. Propane-fueled generators, however, have many of the same issues as diesel generators. Safety: Most notably, the fuel vapor is heavier than air and much more hazardous than diesel. In a leak scenario propane fuel forms a vapor cloud that pools at grade-level on the rooftop. Depending on the local jurisdiction, propane can be either challenging or impossible to site on a rooftop. Environmental: As with diesel and natural gas generators, propane generators have issues with noise and vibration that many building owners will not allow. Footprint: Additionally, the need for a separate propane tank leads to a larger footprint increasing rent/lease expense for the carrier. Temperature: As the temperature drops, the availability of propane vapor decreases significantly, impacting the runtime of the generator. A Power Menu for Rooftop Telecommunications Applications - 4 Weight: At over 3,000 pounds, the propane generator solution is one of the heavier options available. This could be an issue in some rooftop scenarios. Common Generator Challenges Reliability: Anecdotal evidence suggests that generator reliability is somewhat suspect. A survey of third-party measurements of generator reliability shows numbers in the 20% to 88% range, dependent on manufacturer, customer up-keep with maintenance routines, and fuel availability. Maintenance: All three generator options strongly suggest quarterly maintenance visits in order to keep the equipment in good working order, so that the equipment can perform during an outage. This adds operations budget expense to the solution. Overall, the natural gas generator appears to be the best generator option for a 7kW load needing long runtime as long as its required operating conditions can be met. Batteries For short term outages, batteries have many positive characteristics. Both VRLA and Lithium Ion batteries have no requirement for an outside fuel source, so the siting issues experienced by generators are mitigated. Their environmental specifications are generally quite good, with few to no air emissions, no vibration and, in cases where HVAC cabinets are not used, no noise. Additionally, installation logistics are quite manageable due to their compact packaging. For a runtime of 4 hours, both solutions offer a very small footprint, at 6.25 square feet for the battery cabinet. For this scenario, though, we are assuming that the batteries are the only backup power solution for the rooftop application with a 7kW load. That requirement poses some overwhelming challenges for both types of batteries. Figure 3: Battery Comparison Chart Photo by http://www.extremetech.com A Power Menu for Rooftop Telecommunications Applications - 5 Common Battery Challenges Runtime: The largest challenge for both battery options outlined here is runtime. Because they are energy storage devices, batteries require power to recharge between outages. A 4 hour outage is a very short time, with most customers specifying at least 24 hours in order to consider the site “hardened”. Weight: In order to bump the runtime up to 24 hours, the battery solution – including its footprint and weight – would need to be increased by a factor of six. For both solutions, this would quickly lead to excessive weight on the rooftop. Even at a 4 hour runtime, battery weight measured in pounds per square foot, is greater than the weights of any of the other options. Lifetime: Battery manufacturers generally recommend that batteries be climate controlled in order to extend their useful life. They recommend that the batteries be housed in cabinets with air conditioners. These cabinets have a noise level of about 65 decibels. This is lower than the generator solutions typically are, but still not quiet. Most customers, however, find that it is less expensive to simply replace the batteries every 3-5 years rather than purchase an HVAC cabinet for them. Thus, in most cases, noise becomes a non-issue. Differences between VRLA and Lithium Ion batteries Emissions: The VRLA batteries emit a small amount of hydrogen. As hydrogen is lighter than air, venting of the battery cabinet is important, so that the hydrogen gas can dissipate. The lithium-ion batteries have no emissions. Weight: The VRLA batteries and their cabinet are approximately double the weight of the lithium-ion batteries and their cabinet. Hazards: While VRLA batteries can be prone to battery acid spills, which must be cleaned up at some point, some lithium-ion technologies have been vulnerable to thermal runaway. Newer chemistries have been successful in alleviating this hazard in lithium-ion batteries. Overall, lithium-ion batteries appear to provide the best solution in the battery category for a 7kW rooftop load needing a short runtime. A Power Menu for Rooftop Telecommunications Applications - 6 Fuel Cells For short to long term outages, fuel cells offer several positive attributes. Reliable: Third-party reliability data substantiates that fuel cells provide availability greater than 96%. Quiet & Clean: All four fuel cell options have lower noise levels than any of the generator solutions and have no vibration issues. Hydrogen fuel cells have no emissions, while the methanol fuel cell emissions are much lower than the generator options. Low Maintenance: Fuel cells lower operations expenses due to their minimal maintenance requirements. Aside from refueling as needed, routine on-site maintenance is limited to an annual air filter inspection and replacement as needed. Safe: Hydrogen is lighter than air and hydrogen storage cabinets are vented, so fuel siting is not generally a problem on rooftops. Hydrogen fuel cells have several options for fueling. Each of the fueling methodologies has advantages and disadvantages. Hydrogen Cylinder Swap model The hydrogen cylinder swap model is a packaged gas solution which utilizes standard industrial gas cylinders. This model requires that empty 140 pound hydrogen cylinders are exchanged for full ones as needed. A practical number of cylinders that keeps the total weight of the solution around 2,100 pounds is six. This model provides 8 hours of runtime at a 7kW load or 140A @ 50 VDC. For locations not susceptible to longer term outages, this may be a feasible option. This solution can also be used to extend the battery reserve time. The footprint for this solution is moderate. What’s a fuel cell? A fuel cell is a power generator that converts the chemical energy of a fuel (hydrogen, natural gas, methane, methanol, etc.) and an oxidant (air or oxygen) directly into electricity. While there are a number of fuel cell technologies available, the most common and practical technology for small to medium-sized standby power is the proton exchange membrane, or PEM, fuel cell. The PEM fuel cell generates electricity through an electrochemical reaction using hydrogen and oxygen. This process happens without combustion. A fuel cell operates electrochemically through the use of an electrolyte and connects to the DC bus, just like a battery, but it does not run down or require recharging. It is similar to a combustion generator in that it operates for as long as the fuel is supplied; but unlike a generator, it is simple, quiet and clean with few moving parts. One of the most important features of fuel cells is their scalability. Customers can purchase robust, efficient power the size needed by the equipment at the site and scale up if their needs change. Most fuel cells being used for backup power today range from hundreds of Watts to approximately 20 kilowatts. For sites with these relatively low power loads and outages lasting from hours to days, fuel cells can be the backup power source of choice. In a disaster recovery scenario, provisioning the site with enough fuel to accommodate a sensible refueling window keeps the site running as long as is necessary to complete repairs to grid power. Photo by ReliOn Photo by ReliOn A Power Menu for Rooftop Telecommunications Applications - 7 Challenges Fuel logistics: The weight of the hydrogen cylinders can make it challenging in some rooftop architectures to replace fuel. Runtime: At 8 hours, this solution may not offer enough runtime for critical sites to be considered hardened. Figure 4: Fuel Cell Comparison Chart Hydrogen Bulk Refueling model The hydrogen bulk refueling model offers multiple refueling options. In one option, a bank of larger hydrogen cylinders is housed on the rooftop with a pipe residing on the outside of the building leading from ground to rooftop. Refueling is accomplished with a bulk refueling truck, similar to a ground-level diesel or propane refueling model. In the second option, the entire fuel storage cabinet is housed at the base of the building, with a fuel pipe running up the side of the building and connecting to the fuel cell housed on the rooftop. In this model, the fuel cell may be delivered for installation via the service elevator, which lowers installation costs by eliminating the need for a crane. Permission from the building owner is required for either piping installation, just as it is for the natural gas fueled generator option. This solution offers 48 hours of runtime between refueling visits. Challenges Weight: The challenge for the roof-mount bulk refueling option is weight. At 7,400 pounds, this solution is the heaviest of the hydrogen fueling solutions. This challenge can be mitigated by installing the fuel storage at the base of the building. Photo by ReliOn A Power Menu for Rooftop Telecommunications Applications - 8 Hydrogen Carbon Fiber Tank model The hydrogen carbon fiber tank model uses lighter hydrogen cylinders housed in a wheeled frame for ease of installation and exchange. The tanks are designed to be installed via a service elevator in order to circumvent the need for the expense of a crane for installation. In this model, the fuel cell may also be delivered via the service elevator. Once installed, a bank of eight cylinders allows for 37 hours runtime between refueling visits. Refueling may be accomplished either via the service elevator or a ground fill port using a fuel pipe installed on the side of the building. As with the hydrogen bulk refueling model, this would require permission from the building owner. The weight of this solution is moderate at just over 2,100 pounds. Challenges Footprint: The carbon fiber tank model footprint is the largest of all the solutions, thus increasing rent/lease expense for the carrier in some cases. Methanol Reformer model The methanol reformer model combines a fuel cell with a fuel processor, or reformer. The reformer is fueled with a methanol-water mixture, which it “processes” to extract the hydrogen molecules from that mixture to feed to the fuel cell. On the positive side, the methanol reformer model offers the greatest number of run-hours of any of the fuel cell solutions at 57 hours on a 120 gallon tank at 7kW. Challenges Reliability: The reformer adds complexity to the overall fuel cell solution, which may impact its reliability. Permitting: This solution adds the presence of a liquid fuel to the rooftop, which makes the siting process similar to that of diesel or propane generators. In certain areas, methanol fuel will be difficult to attain permission to use. Methanol is heavier than air, thus it will pool if a leak occurs and can pose a safety issue. Overall, the carbon fiber fueling solution appears to be the best in the fuel cell category, offering acceptable runtime, installation and fueling logistics and ease of permitting. Conclusions The housing of telecommunications equipment on rooftops offers myriad challenges carriers must navigate in order to find the best solution for each individual site. Regional and local regulations as well as individual building owner requirements create a maze of parameters for consideration. Figure 5 shows the top-ranked solutions from each category for final consideration. Photo by ReliOn Photo by ReliOn A Power Menu for Rooftop Telecommunications Applications - 9 Figure 5: Best in Class The natural gas generator offers the benefit of continuous operation as long as the carrier is able to attain permission from local permitting authorities and the building owner for use of natural gas on the rooftop and as long as gas is not interrupted during an outage event. Its weight is moderate and its emissions are better than those of the diesel generator. Its challenges include potential cost of a larger-flow pipeline to the rooftop, installation cost of a crane to carry the generator to the rooftop, noise and vibration. The lithium-ion battery solution offers the benefits of no fuel logistics and the ability to use the service elevator for installation. The battery solution has none of the noise or vibration issues of the generator, as long as the HVAC cabinet is not used. For short term outages or as a bridge solution to a longer term power solution, this option appears to be the best of the group. Challenges for the battery solution are short runtime within acceptable weight limits and, for some technologies, risk of thermal runaway. The fuel cell with carbon fiber hydrogen storage tanks offers the benefits of acceptable runtime between refueling visits, installation logistics and fuel siting. The carbon fiber tanks offer multiple fueling options, allowing the carrier to choose the best solution for their individual locations. The fuel cell’s low noise and lack of either emissions or vibration are superior to the generator option. The challenge for this option is footprint on the rooftop when fuel is stored there rather than at ground level. For many rooftops footprint is not an issue, and the pounds per square foot measurement indicates that the fuel cell offers a very attractive load option. In the end, no one solution is applicable across all rooftop sites. It is helpful to be familiar with the variety of solutions on the market as carrier hardening plans become ever more important. ReliOn, a Plug Power company 15913 E. Euclid Avenue Spokane, WA 99216 www.relion-inc.com