Mixing Sodium and Lead Battery Technologies in Telecom Applications

Transcription

1 Mixing Sodium and Lead Battery Technologies in Telecom Applications Paul Smith Shanon Kolasienski Technical Marketing Manager Application Engineer GE Critical Power GE Energy Storage Plano, TX Schenectady, NY Abstract Customers are seeing a mix of battery technologies available for use in telecom applications (Flooded cells, Sealed Lead Acid, Sodium, Lithium, etc.) This paper examines how Sodium Nickel Chloride batteries can be used to reduce fuel consumption in telecom applications primarily powered by on-site generators. The high charge acceptance rate and low state of charge operational characteristics of Sodium Nickel Chloride batteries are harnessed to maximize the generator s efficiency in turning fuel into electricity. By optimizing the generator efficiency, significant operating expense reductions are achieved. Accessing the low state of charge operational capability of the sodium battery reduces the reserve capacity available, should the generator fail to restart when required. A novel architecture is presented that combines the Sodium battery and traditional VRLA batteries in a seamless, reliable package that delivers fuel savings and reduced site outage. The architecture plays to the strengths of each technology as the sodium battery is designed to cycle repeatedly without significant reduction in performance and the VRLA battery is well suited for float applications where extended duration discharges occur infrequently. Test results and performance verification are discussed, and additional applications are examined. In addition to providing performance benefits, this architecture solves several deployment and reliability problems that have been experienced with hybrid system retrofits at existing sites. Introduction Providing reliable and cost-effective power for telecom applications is a challenge in many regions of the world. Mobile telecom operators face rising fuel costs, increasing emission pressures and unreliable or unavailable electric grids. Add to this the high risk of theft of fuel, lead and copper used in base stations, many mobile and tower operators are turning to alternative battery and hybrid solutions to improve the fuel efficiency of the Diesel Generator (DG) without sacrificing operational reliability or back-up power supply duration. Generator Performance (efficiency) When a DG is the only source of power people make the often-easy association of reducing run hours will reduce fuel consumption. Really, the goal should be viewed as maximize the generator s efficiency of turning fuel into electrical power. When efficiency is maximized the fuel usage will, by necessity, be minimized. Take for example a 2kW telecom site load that is being supported by a 25kVA (20kW) DG. A typical DG efficiency and fuel consumption curve is shown in Figure 1. It can be seen that, at an output power of 2kW, the efficiency will be only 11.8% and will require 1.7L of fuel per hour. For the generator to achieve its optimum efficiency (32%), the output power must be increased. 2-1

2 Figure 1. Generator Performance Curve Example (20KW). The easiest way to increase efficiency is to place more load on the DG and use the extra energy to charge batteries. The reduction in DG hours is just a result of the DG operating more efficiently and storing excess energy in the batteries, which can be used for discharge at a later stage. This cycling of the battery is referred to as charge-discharge cycling (CDC). This operation will increase the fuel consumed per hour of DG operation (liters per hour), but since the DG runs less hours, there is a reduction in daily consumption (liters per day). Charge-Discharge Cycling a path to fuel savings Charge-discharge cycling (CDC) or hybrid operation is the combination of a generator power source with a battery energy storage system with a goal of improving the efficiency of the generator operation. Improved generator efficiency results in a reduction in the amount of fuel required to power a given load over an extended period of time. Sodium Nickel Chloride Battery technology is one of the first batteries in the world to be well suited for CDC telecom applications. With high charge acceptance and excellent cycling performance, Sodium Nickel Chloride Batteries can act as the primary energy source for telecom installations that have historically relied on DGs as the primary power source with batteries as the backup. Sodium Nickel Chloride batteries are not affected by ambient temperature, so operators save money and space by removing unnecessary HVAC and complex cooling systems, making it an excellent solution for installations in extreme environments. 2-2

3 Figure 2. Sodium Nickel Chloride Charge Curve. It can be seen from the battery charge curves in Figure 2 that, as charging progresses, the rate of charge acceptance decreases. This is true for most battery technologies, and in the Sodium Nickel Chloride battery is a result of increased internal resistance at higher State of Charge (SoC) levels. Decreasing charge acceptance results in decreasing load on the generator as charging progresses, and as seen in Figure 1. Lower load translates to lower efficiency. To keep the generator efficiency up we must limit charging to the region of highest charge acceptance, and operate the battery in partial state of charge. If charging is limited to 2.5 hours as shown in Figure 2, the average charging power to the battery is 2.08KW and the approximate cycling range for SoC is 20% to 60%. Example 1 Looking at the generator efficiency data in Figure 1, with a 2KW load, a simple energy equation shows that: Daily energy required by the load is 2KW x 24 hours = 48KWH and that all this energy must come either directly from the generator, or from the generator through the battery. If the generator is run 24 x 7 with a 2KW load, it operates at only 11.8% efficiency and consumes 1.7LpH for 24 hours = 40.8L per day. If a single battery is used the average load on the generator increases to = 4.08KW and the fuel consumption is 2.25LpH. But at an output of 4.08KW it operates at 17.8% efficiency and only needs to run for 48/4.08 = 11.8 hours to generate the required 48KWH. The fuel used in 11.8 hours at 2.25LpH is 26.4L per day. The generator efficiency improves from 11.8% to 17.8% and the daily fuel consumption decreases from 40.8L to 26.4L. Daily Fuel consumption is reduced by 35.3%. 2-3

4 Example 2 Clearly the generator is capable of greater than 17.8% efficiency, but to get to the higher efficiency we must add more load. Each battery is limited in its ability to load the generator by Figure 2. Adding batteries would increase the generator load, improve the efficiency further, and more fuel savings would result. If the charge acceptance of each battery averages 2.1KW, adding a total of 5 batteries increases the average generator load to = 12.5KW. At this level the efficiency increases to 30.5% and the fuel consumption increases to 4.1LpH. To generate the requisite 48KWH at 12.5KW generator output only takes 3.9 hours per day, resulting in a daily fuel usage of 3.9 x 4.1 = 16L. Daily Fuel consumption is reduced by 60.7%. These examples only consider the primary effect of improving generator efficiencies. We have not considered the secondary effect of battery round trip efficiency, which will offset a small amount of the savings. Energy Storage Criteria Not all batteries are suitable for CDC applications. The key parameters highlighted by the above include: Deep discharge the Sodium Nickel Chloride battery, unlike some other technologies, does not have any side chemical reactions when it is deep discharged, which can result in permanent capacity loss. For this reason, the Sodium Nickel Chloride battery is considered a deep discharge battery and can operate at low SoC s that permit high charge acceptance. More charge current means more load on the DG (higher DG efficiency) and more opportunity to reduce fuel consumption. High charge acceptance rate in order to maximize the load on the generator and achieve optimum generator efficiency the battery must be able to recharge at a high rate. It is important that any Battery Management System (BMS) electronics not limit the recharge acceptance rate. High Cycle life at partial SoC operation since charge acceptance rate is typically higher at lower SoC's it is essential that the battery maintain cycle life when operated in a partial SoC mode. Careful choice of operating conditions ensures that Sodium Nickel Chloride chemistry batteries fulfill these needs and extend the life of the battery. Additional benefits include: Temperature independence the Sodium Nickel Chloride battery is a warm battery, which means it does not need any active cooling. Typically, the temperature specs listed in the battery s technical specification sheet apply to the Battery Management System (BMS) electronics and not the battery itself. This leads to: - The reduction or elimination of air conditioners for battery enclosures. - No need for temperature compensation while charging. High Energy Density - the high energy density of Sodium Nickel Chloride Batteries makes them ideal for back-up applications in applications where space or floor loading is at a premium 2-4

5 Materials - Constructed from abundantly available materials, Sodium Nickel Chloride Battery modules attract fewer thieves looking to recoup expensive materials. The single 48V block is also much less appealing to thieves than the ubiquitous 12V monobloc to operate consumer electronics. Potential Issues Operation of the Sodium Nickel Chloride battery at low SoC maximizes its charge acceptance, and allows maximizing the generator efficiency, but also means that the point at which the generator is started to resume charging is very low on the SoC curve around 20%. This means that, should the generator fail to start, there is very little battery capacity left to support the load until a repair / maintenance operation can be completed. Low residual reserve capacity is a problem in remote locations where support personnel may be hours away. Lead to the Rescue The traditional lead acid battery, in the form of the ubiquitous Valve Regulated Lead Acid (VRLA) battery, has been used for many years to provide extended duration, infrequent use, back-up energy for Telecom applications. Unfortunately this chemistry is not well suited for frequent, or deep, cycling applications. The typical float voltage for a VRLA battery is also not directly compatible with the higher charge voltage preferred for the CDC application of Sodium Nickel Chloride chemistry. The remainder of this paper will propose a novel architecture that allows utilization of both Sodium Nickel Chloride and VRLA batteries in a CDC application to advantageously exploit the unique properties of both batteries. Since we called the Generator / Battery combination in CDC operation a Hybrid system, we will call the Hybrid / VRLA combination a Hybrid Hybrid system (H 2 ). Voltage considerations In Sodium Nickel Chloride batteries, achieving high recharge rates requires that the BMS does not limit the recharge current. This means that the optimum charging voltage is determined by the battery chemistry alone. For the GE Durathon battery, the optimum charging voltage has been determined to be 55.5V. This is a full 1 volt higher than that typically recommended for float charging VRLA batteries. The architecture shown in Figure 3 allows for this voltage difference. 2-5

6 Figure 3. Mixed Battery Technology (H 2 ) Architecture. Operation During normal CDC operation the Sodium Nickel Chloride batteries are cycled as previously detailed, turning the generator on and off, optimizing generator efficiency. The output from the Sodium Nickel Chloride batteries is fed to the load through a DC/DC converter system that is set up with a constant output voltage fractionally (ΔV) higher than the float voltage for the VRLA batteries (V1). This ensures that the rectifiers DC1 supply little or no current in normal operation, yet remains within the acceptable float voltage range of the VRLA batteries. The converters are selected to be able to regulate their output with an input voltage swing corresponding to the voltage range of the Sodium Nickel Chloride batteries during cycling operation (typically V). When the generator is running the DC1 rectifiers can also top off the VRLA batteries, supplementing charge current available from the converters. If the DG fails to start for any reason, the Sodium Nickel Chloride battery will continue to discharge until it reaches its end of discharge voltage usually around 46.1V. Once the end of discharge voltage is reached, the Sodium Nickel Chloride BMS opens its contactors and power ceases to be provided through the DC-DC converters. At that point, the VRLA battery begins to discharge to support the load. This will continue until the VRLA battery reaches its own end of discharge voltage. When the generator restarts, it will provide power to all rectifiers, bringing the system back to a normal operating/cycling condition. One benefit of the topology described is that the DC2 rectifiers provide redundant rectification in the event that the top set of rectifiers fails to charge the VRLA battery. Since a generator non-start is an alarm event, maintenance personnel will be signaled to service the site. Typically, the Sodium Nickel Chloride battery will discharge for up to an hour before it reaches its end of discharge voltage. The VRLA battery bank will discharge for another 4-8 hours, depending upon installed capacity. This gives maintenance personnel significant time to respond ensuring site downtime is minimized. 2-6

7 When generator operation is restored DC1 rectifiers will recharge the VRLA batteries and DC2 rectifiers charge the Sodium Nickel Chloride batteries. Some recharge of the VRLA batteries will occur from the converter output also, depending on the provisioned capacities. Applications This architecture has applications in both new and retrofit scenarios. In a new installation the system could be configured as shown in Figure 3 or alternatively as shown in Figure 4. Figure 4. Mixed Battery Technology (H 2 ) Architecture Alternate topology for new installations. The advantage of Figure 4 topology is that is only has one set of rectifiers. Care must be taken however that the rectifiers and converters are provisioned in sufficient quantity to support recharging BOTH batteries. Retrofits In many retrofit applications, there may be an existing installation that includes generator, rectifiers, VRLA batteries for long term backup. In these cases, the module labeled DG Cycling Module is intended to mount alongside the existing equipment and only require the minimal connections of ac and dc cables, as shown in Figure 5. When applying this solution in a retrofit scenario, we also see operational benefits from the performance gains in the new equipment (particularly rectifier efficiency) as compared to the legacy equipment, although some of this will be offset by the addition of losses in the converters. The new equipment may also add feature enhancements, such as remote monitoring and control. In retrofit scenarios where there are existing, serviceable VRLA batteries already installed, this capacity is re-purposed to the benefit of final operation. In retrofit applications with existing VRLA batteries, the battery enclosure is probably already environmentally conditioned. If adding VRLA batteries, it may be necessary to add HVAC to cool the VRLA batteries. 2-7

8 Figure 5. Mixed Battery Technology (H 2 ) Architecture Retrofit applications. The main benefit of this implementation is that of improving generator efficiencies. The primary power flow is now through DC2 and the converters instead of DC1. We expect to see a better efficiency in DC2 rectifiers than in the older DC1 rectifiers, but we have also added losses in the converters. 2-8

9 Test Results Figure 6 shows the results of initial tests performed on a lab setup of the configuration shown in Figure 3. Figure 6. Mixed Battery Technology (H 2 ) Architecture Test Results. The first segment of results shows the generator cycling, with the Sodium Nickel Chloride battery first discharging, then charging and discharging again. At the end of the second discharge the generator does not restart and the VRLA battery supplies current to the load. Ac power was supplied from the grid rather than a generator. It will be noted also that at the generator fail point the Sodium Nickel Chloride battery disconnects since it is at its end of discharge point. The switching thresholds in a real system would be set somewhat higher. The shape of the discharge current curve portions of the result reveal that the simulated load was resistive rather than constant power, this is evident in the VRLA discharge portion, where both voltage and current from the VRLA battery slowly decay. The Sodium Nickel Chloride battery sees the dc/dc converters as a constant power load, so the current is seen to increase as the voltage decreases during the discharge parts of the cycle. The test performed was to demonstrate the ability of the converter assisted architecture to transition from battery to battery. In a real scenario, when the generator restarts the system would resume the Generator Cycling mode once the Sodium Nickel Chloride battery reaches the upper SoC set-point. For the sake of test time the VRLA was not discharged fully, as reflected by its rapid recharge. 2-9

10 Power Plant DC1 Power Plant DC2 Batteries Rectifier capacity 165A Rectifier capacity 150A Sodium Nickel Chloride Battery GE Durathon 4815 Converter capacity 92A VRLA Battery Deka Unigy HR7500ET Load ~75A Figure 7. Mixed Battery Technology (H 2 ) Architecture Test Configuration. Summary The use of an energy storage system to allow generators to more efficiently convert fuel in to electrical energy yields significant operational savings, primarily in fuel costs. The high charge acceptance rate and low SoC operation characteristics of the Sodium Nickel Chloride Battery make it well suited to this application. Operation at low battery SoC levels maximises generator efficiency gains through increased charge acceptance, but leaves little battery capacity should the generator fail to start. Combining CDC operation with float charged VRLA batteries can mitigate this risk significantly, resulting in a system with excellent operational efficiency, and extended back-up capacity. The system described can be easily adapted to retrofit applications, with minimal disruption to the legacy system. This systems architecture is not limited to the combination of Sodium Nickel Chloride and VRLA chemistries; it could be applied to many combinations of battery technologies having different voltage characteristics. References 1. Job Rijssenbeek, Herman Wiegman, David Hall, Christopher Chuah, Ganesh Balasubramanian and Conor Brady, "Sodium-Metal Halide Batteries in Diesel-Battery Hybrid Telecom Applications", 2011GRC699, August