In the emerging world markets such as the Sub-Saharan regions, South Asia, and South America, the reliability, and sometimes availability, of power from a central source or grid is virtually non-existent. Since alternative energy sources such as solar and wind power have proved both costly and difficult to deploy, telecom network operators have come to rely on diesel generators for their Base Transceiver Station (BTS) sites.

It is usual for off-grid sites to be powered by 2 diesel generators operating alternately to ensure a reliable power source, while sites in regions of unreliable grid power are normally equipped with a single diesel generator. In some rural regions of West Africa or in India the run time of the gen-set to backup the mobile site can reach up to 12 hours a day.

Conventional fossil-diesel-fuelled generators are a simple, low CapEx power solution for off-grid sites with well known architecture, multiple vendors for the sub-systems, and easy and quick installation. But such sites have seen their operating costs substantially increase in the last 2 years, with diesel prices increasing by up to 50% in many locations. For the more remote sites, the refuelling and periodic maintenance of the generators create logistical problems, and result in direct and significant further increases in operating expenditure.

At the same time, the global warming issue has persuaded operators to implement programs to reduce the environmental footprint of their telecom sites.

The Might Behind Right
In response to this new environment, telecom vendors have developed new generations of mobile equipment with a lower power load, typically in a range of 0.6 to 1.0 kW. This reduction in power has been made possible by the development of new software to optimize the operating of the radio modules, while the introduction of remote radio heads and equipment with a higher tolerance to extreme temperatures reduced, or even eliminated, the need for air conditioning.

In parallel, the major operators in the developing world have also been researching new power solutions for their mobile sites to combine both significant savings in OpEx and a drastic reduction in carbon emissions. The typical requirements for these new solutions are (compared with existing sites powered purely by fossil diesel):

• Reduction in OpEx by more than 50% Payback in less than 24 months
• Reduction in carbon emissions by more than 50%
• Lead time to implement a new site less than 3 weeks

Saft is currently working with telecom power providers to implement turnkey hybrid power systems incorporating a cycling nickel-cadmium (Ni-Cd) battery. Rather than using the generator as the primary power source, the hybrid power system uses the battery as its primary source of power, with the diesel generator providing the recharge current. It is aimed at mobile operators in countries where grid connectivity is not available or where centralized power is unstable, with diesel generators running for at least 30% of each day. In addition to new-build sites, this system can also be used as a retrofit solution for existing BTS sites. (See Figure 1.)

Figure 1. A schematic of a typical hybrid installation with diesel and battery working together.

The advantage of this approach is a reduction in operating costs (OpEx) of between 50 to 85% combined with a reduction in carbon emissions between 48 and 80%, depending on site load, compared with continuous generator operation. The diesel generators need to run only for a fraction of the time, thus reducing fuel consumption, emissions, and maintenance requirements.

The hybrid system features a modular, containerized design. This enables it to be sized to meet the immediate requirement and, if necessary, to be expandable with minimal extra cost and material. It can also be expanded to include both solar and wind options; the use of a renewable power source will further decrease the generator running time and hence OpEx while both will assist in reducing the carbon footprint. The addition of either or both still gives operators a minimum energy backup time of 6 hours.

The use of battery technology with a proven high level of reliability can allow a single diesel generator to be used; this maintains system reliability while reducing capital expenditure, with part of the cost of the battery being covered by the elimination of the second generator.

To ensure the lowest Total Cost of Ownership (TCO) of the hybrid solution, the different sub-systems such as battery, generators, alternative source, rectifier, and controller must be carefully selected and sized to permit the highest system efficiency and the best operating profile.

When choosing a system, the following criteria should be evaluated:

• A complete hybrid system of this type should be packaged in an “energy container” to offer a turnkey solution for easy and quick installation in more remote locations.
• The cycling battery should be a temperature-resistant, low-maintenance design, accept fast charging, and able to deliver a large number of charge-discharge cycles.
• The rectifier should offer the highest possible energy efficiency.
• The controller should be equipped with dedicated software enabling the operating profile of the battery and generator to be optimized, thus delivering the lowest possible operating cost.
• The addition of renewable energy sources such as solar panels or a wind turbine permits an increase in cycling time and consequently extends the calendar life of the battery and generator while also increasing the environmental benefits.

Reality Hits With Site Trials
Recently, field testing of one hybrid solution was done at a site in Nigeria. The location was selected primarily due to the interest being expressed by the various operators in this region of Africa. (See Figure 2.)

Figure 2. A hybrid test site in Africa with the batteries installed externally and subjected to extreme temperatures.

To test the system at its fullest capability it was assumed that the grid was not available and a dummy load of a constant 2 kW was used to simulate the site working at 100% capacity for a period of 2 months.

The site layout was a containerized solution with the generator housed within the container, and the rectifier cabinet and batteries placed on a rack on the open platform. This was done to demonstrate that the batteries would operate effectively at the ambient daytime temperature of +35°C, as well as under varying temperatures, without any degradation.

The batteries were set as the primary source of power, with the diesel generator as the secondary source for provisioning of power to the site and for recharging the batteries. The batteries’ charge was also monitored so that when they had discharged to 27% DoD, the generator was started to provide the recharge current. Once the batteries were fully charged, the system switched the power source back to the batteries.

During the trial, the generator ran for 6 hours per day, with the batteries as the primary power source for 18 hours per day. This resulted in a fuel saving of 75%, compared with a similar site using generators only, at a 100% site load per day.

As the actual site load varies according to call usage patterns, and generally averages around 60 to 70% of the maximum load, it can be assumed that the real-life savings could be substantially greater.

The use of Ni-Cd battery technology enables a hybrid installation to be based on a single diesel generator, maintaining system reliability while reducing capital expenditure. This approach can potentially reduce the annual fuel cost of each site by approximately 75%. It also allows a significant reduction in the generator service costs by increasing the engine life and reducing maintenance requirements.

The addition of a wind turbine and/or solar panels can also enable an increase in cycling time. This will permit a subsequent reduction in the hybrid system OpEx through a combination of reduced fuel consumption and the extension of the battery’s calendar life.

Powerful Environmental Benefits
As shown in Figure 3, a hybrid system can reduce CO2 emissions drastically when compared with a pure diesel site.

                                           2.5 kW        6 kW           12 kW         24 kW
Operational Details           System        System       System       System
Daily Generator Operation    8.25 Hrs       9 Hrs           9 Hrs           11.75 Hrs
Daily Battery Operation        15.75 Hrs     15 Hrs         15 Hrs         12.25 Hrs
CO2 Savings                       58 tons/yr     56 tons/yr   58 tons/yr    42 tons/yr
Figure 3. CO₂ savings for the hybrid system compared with pure diesel site.

Over a 5-year operating period, corresponding approximately to the life duration of the Ni-Cd battery and of the gen-set in such a hybrid application, the reduction in the CO2 emission for a 2 kW mobile site will be more than 200 tons. This should be compared with about only 2.7 tons of equivalent CO2 for the cradle-to-grave global warming potential of a string of 48 V, 555 Ah Ni-Cd telecom batteries -- from the extraction of metals at the mine to the production of materials to be used in the manufacturing of batteries and recycling -- representing just over 1% of the total savings in CO2 emission.

The addition of a renewable energy source further emphasizes this reduction in CO2 with an increase in cycling time.

A suitable hybrid power system, based on advanced Ni-Cd cycling battery technology, can meet the site load requirements of telecom installations while minimizing the generator run time. This approach offers substantial benefits in terms of reduced fuel costs and CO2 emissions, lower maintenance, and increased reliability.

About the Author
Joel Brunarie is Telecom Business Development Manager for Saft Industrial Battery Group. He has more than 25 years of experience in the advanced batteries and telecommunications industries. Saft is a designer, developer, and manufacturer of advanced technology batteries for industrial and defense applications. The Group has put in place the key building blocks to deliver their strategy for high technology lithium-ion batteries for clean vehicles and renewable energy storage. For more information, email joël.brunarie@saftbatteries.com or visit www.saftbatteries.com.

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