The cleaner, greener future of hybrid power for off-grid cellular sites

Joel Brunarie, telecom general manager for Saft’s TTG (Transportation, Telecom and Grid) division, explains how diesel generators and batteries can work together to provide cost-effective and reliable power supplies for remote base stations.
Energy, Sustainability, Society, Future, Battery, Tech, Services, Technology, Lithium, Ion, Environment, Climate change, World, Earth

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Can batteries make cell towers more efficient? Yes, argues Joel Brunarie.

Building out cellular infrastructure to serve the next billion users is a key priority for governments and operators alike. The challenge is that many of these users live and work in areas that are poorly served by electricity grids. Villages and roads in rural areas often have little or no access to a reliable source of electricity, and radio base station sites require their own source of power. In many markets, this makes rolling out cellular coverage financially unattractive.

Historically, remote base stations have been powered by diesel generator sets, usually in pairs for redundancy. This represents a significant ongoing operational cost, especially with the unpredictable, and usually rising, price of fuel. There is also the added cost of transporting fuel and maintenance teams to the sites – which in some cases can actually be greater than the cost of the fuel consumed.

This is why there is growing interest in the use of hybrid power solutions for off-grid cellular sites. Rather than using the diesel generator as the primary power source, the hybrid system relies on the battery as its primary source of power, with a single generator providing the recharging current. To achieve even greater operational efficiency, the power from the diesel generator can be supplemented by a renewable source – typically solar photovoltaic (PV) panels or wind turbines.

In this way, the battery system enables the energy generated and the energy consumed to be efficiently balanced, and ensures continuity of supply. It acts as a buffer to manage situations when the available renewable energy is less than the demand. It also supplies energy when there is no renewable energy available, and stores the excess energy when demand falls below the output from the wind turbines or solar panels.

Lifetime cost savings

Hybrid site power solutions have been shown to reduce both fuel consumption and maintenance needs, leading to significant cost savings for mobile operators over the lifetime of the site. Furthermore, they also provide a significant reduction in CO2 emissions and noise pollution. Experience from the field shows that this approach can reduce operating costs by around two-thirds, and reduce CO2 emissions by about the same amount.

The main reason such savings can be achieved is the ability to run the diesel generator only when it is actually needed, typically for just a few hours a day rather than continuously (as is the normal case). Generator-only sites can require servicing as frequently as every 250 hours. With the addition of a battery and renewable power, this can typically be extended six-fold with the generator running for only four hours a day.

With the right sizing, the energy needed to run the base station site can be provided by a renewable source for most of the day. Any excess power generated (for example, during periods of low mobile traffic) is used to top up the storage battery.

The high reliability of modern batteries enables operators to install just one generator per site, instead of two. The battery supports the system load when the generator is off and there is no power available from the renewable source(s). With a correctly sized renewable power source, battery cycling (the repeated charging and discharging of the battery) can be kept to a minimum, extending the life of both the battery and the generator while further increasing the environmental benefits.

The capital cost of different battery technologies can vary widely. However, this represents only a small fraction of the lifetime system cost. The true Total Cost of Ownership (TCO) of the system is calculated from the initial purchase cost, installation and maintenance needs, service intervals and life expectancy. The installation, servicing and maintenance costs can be significant for remote, off-grid cell sites, where access to qualified engineers is limited. In this environment, a battery technology that provides high performance, reliability, long life and efficient operation represents significantly better value for money, or TCO, than one that requires regular maintenance or which may succumb to ‘sudden death syndrome’ (a characteristic of lead-acid batteries).

Choosing the right batteries

Operating conditions for such installations can be tough. Remote off-grid sites can often experience extremes of temperature, and batteries need to support the demanding load profile over a lifetime of up to 20 years. While lead-acid and nickel-based batteries have traditionally been used for such duties, the latest lithium-ion (Li-ion) technology has clear advantages, including lower weight and higher energy density.

Another advantage of Li-ion batteries is that they are suited to the deep cycling which can be encountered when working in conjunction with an intermittent renewable source. The proven performance and minimal maintenance needs of Li-ion batteries, even in tough conditions, means they are ideally suited to hybrid power site installations.

Battery chemistry is just one selection factor that influences the overall performance of a hybrid system. Other key factors include reliability, operating temperature, energy storage density, energy-efficiency, fast-charging characteristics, smart management capabilities, and battery lifecycle.
It is also important to ensure the right dimensioning and integration of the complete power system. Achieving optimum performance requires careful selection and sizing of the various subsystems involved – including generator, rectifier, controller and battery – to ensure the optimum system efficiency.

The size of the generator can vary widely, but is usually between 10 kVA and 120 kVA for a typical site. Key to the efficient running of hybrid systems is the controller, which monitors renewable generation input and battery charge levels and controls the generator to ensure site load requirements are met, while minimising generator run-time.

When there is insufficient power being generated by the wind turbine or solar panels, the controller brings the batteries online until they reach the specified depth of discharge (which is typically equivalent to six hours of operation, in case the generator should fail to start). At this point, the generator is started up to supply both the site load and the battery recharging current.

Added intelligence

With a suitable controller in place, operators can also set up intelligent remote supervision to monitor performance remotely and reduce the need for site visits.

A complete hybrid power system can be packaged in a compact and light ‘energy container’ to offer a turnkey solution that is quick and easy to install in remote locations, requiring just a plinth and connection to the cell site systems.

Hybrid systems are ready to go right now

Hybrid power systems are able right now to provide mobile operators with dependable power for remote off-grid installations, while minimising the run-time of diesel generators. They can deliver substantial long-term benefits in the form of reduced fuel costs, lower CO2 emissions, reduced maintenance and improved reliability. Furthermore, hybrid power systems lower the barrier to rolling out mobile coverage to the world’s next billion users – with all the socioeconomic benefits that connectivity brings.

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