By Zhang Hongguan & Zhang Yufeng

Uninterrupted power supply for remote base stations has been a challenge since the founding of the wireless industry, but alternative sources have a chance of succeeding where traditional solutions have failed.

With users no longer tolerating spotty coverage in the great outdoors, the need for off-the-grid energy solutions is ever growing. However, the costs and hurdles involved with conventional solutions (fuel, labor, construction, regulatory compliance) are ever growing. Diesel generators are now money pits, while lead-acid batteries are becoming environmental pariahs. Neither, individually or in tandem, delivers the reliability that allows a telco's COO to sleep through a storm.

To address this situation, Huawei offers PowerCube, an industry-leading hybrid power supply solution. Built along the lines of a Micro-Grid Energy System (MGES), it comprises four elements – power generation, control, monitoring, and energy storage. Power generation utilizes a variety of sources, including wind, solar, power grid, and diesel, while the control system integrates elements such as ATS, system power supply, solar/wind energy control, and power distribution. The energy storage system can employ a variety of energy storage methods and temperature control modes to maximize energy utilization, while the monitoring system supports Huawei in-band & out-band GPRS/IP transmission through NetEco and M2000 on the back end.

Dual power

Traditionally, when power outages are frequent, onsite power supply combines mains, batteries and generators. Normally, the mains supply power while charging the batteries. When the mains fail, batteries take over; diesel generators are only utilized if the batteries prove insufficient. The typical charging coefficient for an onsite battery is 0.1 to 0.15 and depends on its charging pattern and the charging logic for the system's power supply.

However, charging of onsite batteries is time-intensive. Frequent power outages lead to frequent discharging and incomplete recharging, which dramatically shorten battery service life, while frequent start-up and shut-off of diesel generators wastes fuel, generates noise and leads to more frequent breakdown.

Huawei provides a dual-power solution that alternates power supply duties between the mains and batteries. Batteries are injected with special additives that raise their capacity for received current by up to 0.3C (C: capacity of batteries). The recharging logic of the system's power supply is also improved noticeably, which enables effective control over spikes in battery temperature while satisfying the need for timely recharging..

When the mains resume or diesel generators start, charging current is adapted, as per the capacity of the input source, via slow-start recharging, automatic charging mode adjustment, and current limits; this effectively avoids the instantaneous recharging caused by current surges, which can disrupt or even paralyze the AC-input source.

The operational scenario for this solution is illustrated in Figure 1. When the mains are disrupted, batteries take over. When the mains resume, they power the station and recharge the batteries. Batteries receive the charging current (max. 0.3C) based on the mains' input capacity and their own remaining capacity, which assures system stability and maximizes use of the mains. Diesel generators only start when the mains are unavailable for an extended period and the batteries prove inadequate.

This solution effectively raises battery recharge efficiency, maximizes the operational intervals for the mains, and reduces and even eliminates the use of diesel generators. It helps reduce fuel consumption and maintenance costs, and effectively resolves the onsite power supply conundrum for regions where the mains are unreliable.

Diesel-battery combination

When diesel generators must be used, it is ideal to have at least two onsite that alternate duty, but fuel consumption, carbon emission, and wear & tear remain issues. Diesel generators are configured to supply level current for batteries; in fact, when batteries are undergoing floating-charge, very low currents are needed, which is highly inefficient, in terms of both fuel and electricity.

Huawei has developed a diesel-battery hybrid solution where batteries work as the primary energy source; this is enabled by advances in battery electrode plating composition, so that complete discharge and deep cycling are achieved. When the diesel generators operate, the batteries are charged. When the batteries' charge reaches a threshold, they take over and operate until discharge, when the generators take over and the cycle starts again (Figure 2).

This solution reduces the running time for diesel generators and increases their fuel efficiency, which significantly reduces the O&M costs involved.

Renewable energies

As the environment increases its footprint in the public consciousness and the costs for conventional energy continue to grow, renewable onsite energy solutions (solar & wind) are becoming more than just alternatives.

Huawei provides a variety of green energy solutions, including solar scenarios that feature maximum power point tracking (MPPT) solar energy controllers, and hybrid solutions that combine renewable and conventional energies with specific energy-storage systems. For base stations, there are six power supply combinations-solar-only, solar+diesel, solar+mains, etc.

Solar-only

When there is sufficient sunlight, photovoltaic cells convert solar energy into electric power. Loads are powered by solar energy controllers, which also charge the batteries. When sunlight is not sufficient, the batteries will take over.

Considering that remote base stations must be highly-integrated, inexpensive, and modest, Huawei has developed its all-on-pole EasySite solution, which integrates the base station, antennas, transmission, and tower into one convenient package.

Solar + diesel

This solution introduces diesel generators when loads are heavy or rain is prolonged. Generators operate when neither photovoltaics nor batteries are sufficient.

Solar + mains

Solar or power grid electricity powers the base station and charges the batteries, with solar having priority. Only when neither proves sufficient will the batteries be utilized.

Huawei's PowerCube hybrid power supply solution has been widely recognized for its remote-station viability. Huawei power supply solutions are currently serving more than 80 operators, including Vodafone, MTN, Etisalat, and Zain, etc.