Mechanical Technology Archive

MTN, through energy solutions partner Clean Energy Investments is exploring the use of hydrogen fuel cells and dc-powered energy efficient cooling solutions for use in its base station transceiver towers. MechTech talks to Gavin Coetzer, director of Clean Energy Investments.

Altergy hydrogen fuel cell MTN base station transceiver equipmentCellular coverage depends on the distribution of a network of fixed-location transceivers, which are known as cell-phone towers or, more formally, base station transceivers (BSTs). Each base station is used for local wireless transmission and reception of voice and data communication to and from all nearby cell phones.

Together, the network of towers enables a phone near one tower to be connected to another phone near any other tower in the world.

The interconnection between base stations can be through traditional telecoms cables; wireless via a relatively powerful line-of-site parabolic antenna; or, increasingly with the role-out of LTE, via an interconnecting high-speed fibre line.

“MTN is currently adopting a holistic approach to improving reliability, increasing energy efficiency, reducing its carbon footprint and driving down operating costs of its BST network,” Coetzer tells MechTech. From a reliability point of view in the South African context, this involves developing sustainable back-up power solutions that mitigate against theft.

“Because of increasing design efficiency, the size of BST equipment has reduced substantially. The equipment cabinets, which used to be the size of large refrigerators, are now down to the size of bar fridges, so there is much more space in shelters than there was before,” says Coetzer, adding that MTN sees this as an ideal opportunity for base station sharing.

“Instead of each cell phone service provider erecting its own tower and its own interconnecting infrastructure, the additional space allows for three different sets of BST equipment to be incorporated into the same shelter. This approach could drive down operating costs dramatically, for all the cellular networks,” he notes.

Describing the resources required inside a typical shelter, he says that each BST typically draws 3.0 kW of power. “The shelter has to be cooled to prevent the equipment from shutting down due to heat overload, so every shelter has to have its own air conditioning system. Each tower also needs a mains electricity supply along with a backup power system to cover outages,” he notes.

“As well as the physical space and power connections, cooling and the backup energy systems can now all be shared. This reduces investment costs and helps service providers to sweat their assets. It also reduces their physical and carbon footprints,” Coetzer says.

MTN’s new BST vision

According to Coetzer, MTN is specifically targeting three areas of change for its base stations. “The first is a move to a dc-only telecoms environment. The second is to reduce the theft value of the equipment in the shelter, by reducing onsite battery holding, for example, so that it becomes unlucrative for thieves. And on the energy side, MTN want to use passive cooling more effectively to reduce the runtime of its chiller systems,” he says.

On cooling alone, MTN estimates that it can save some 6 600 kWh per base station per year. Across its 8 000 base-station network, this currently amounts to between R50-million and R60-million per year in electricity cost savings.

Three BST cooling system pilots are currently being run in Johannesburg, Gauteng, at the base stations near Pirates and Old Parks and at 206 Long Road in Albertville, where both the novel cooling system and the use of hydrogen fuel cells for backup power are being trialled.

Transceiver equipment running at 3.0 kW causes the temperature to rise to about 70 °C within an hour. Without cooling, this would shut down that power station, even if running on backup power. “So backup power without cooling is not a solution. And while a dc to ac inverter solution can be added in conjunction with battery backup or a hydrogen fuel cell, this drops the efficiency, raises investment costs and increases the heat load,” Coetzer explains, adding: “hence the drive to find a dc solution.”

Clean Energy Investments, therefore, was asked to find a dc-based cooling solution that would be compatible with both battery and fuel cell-based backup power systems. “We found that ideal technology was available from CoolSure, which has now become a partner on this project,” he reveals.

Passive cooling using ambient air is first being used to increase the airflow through the shelter and to improve energy efficiency. “We use a ΔT of 5.0 °C from ambient as the threshold, that is, if the outside air temperature is more than 5.0 °C cooler than the air inside the shelter, then the cooling system uses only the ventilation fans.” These fans, which operate though the air-handling units, are also under VSD control, so that when possible, their speed and power draw can be optimised to maintain the indoor temperature required.

“The temperatures of both the outside and inside air are continuously being monitored and, as soon as the 5.0 °C threshold is breached, the chillers kick in to reduce the inside temperature,” Coetzer explains. “These chillers run on dc-power, via a dc to dc converter that raises the supply voltage from the 48 V on the dc busbars to the 300 V dc required by the compressors. And a sophisticated control strategy ensures that the energy use is optimally matched to the cooling requirements, significantly reducing electricity consumption,” he assures.

The use of hydrogen fuel cells, however, is the main reason for Clean Energy Investments’ involvement in the project. “At 206 Long Road, we have installed a 10 kW Altergy hydrogen fuel cell directly into MTN’s rectifier and transceiver equipment cabinet, a system that has now been under test for nearly a year,” Coetzer reveals.

This 48 V fuel cell system, along with the mains-connected rectifier and a battery bank, are all connected, via switchgear, to the common 48 V busbars. “Normally, the busbars of BSTs are energised by rectifiers. If there is a power outage, the battery banks are switched in to carry the load, so the equipment shouldn’t know if the mains power is on or not.

“The idea with this project is to replace the high-theft value battery bank with a fuel cell, which has the added benefit of being refuelable. A typical battery bank – three 48 V strings of four batteries per string – can only provide about eight hours of backup power before it needs to be recharged. When a power outage lasts for several days, however, a fuel cell is a better option, because when the hydrogen becomes depleted, it is easily replaced,” Coetzer suggests.

Explaining how the system works, he says: “We continuously monitor the voltage level on the busbars. If the rectifier is supplying at 54 V, then we set our fuel cell to trigger if the voltage drops below, say, 52 V. But we still need a battery connection for a transition period of up to a minute. The fuel cell has a start-up procedure that involves some self-checks, for hydrogen leaks, for example, which delay start up by between 30 and 60 seconds. So for continuous BST operation we need to cover this delay with batteries, he explains, adding, “but we can use very small and virtually worthless batteries to cover this period.”

On detection of power outage, the small 48 V battery bank is immediately switched to supply the busbars, but 30 to 60 s later, the hydrogen fuel cell starts to supply the power and the batteries switch off again.

Why a 10 kW fuel cell for a 3,0 kW BST? “We are testing the feasibility of site sharing,” he responds. “MTN’s current thinking is that competing on an operational level is counterproductive for the whole industry. In the same way as roaming has become an integral part of improving network access for all service providers, site sharing benefits everyone equally, reducing costs and improving reliability,” Coetzer suggests.

The hydrogen fuel cell solution meets a number of objectives: it offers a refuelable standby system that is far more reliable with almost no theft value as compared to either battery backup or generator based solutions. It is also a zero carbon solution, so it ticks the green box, and it removes the need for an expensive battery bank, again reducing theft potential.

Also, as well as being part and parcel of MTN’s offering and expertise, connectivity, remote monitoring and the Internet of things capabilities are incorporated into all Altergy fuel cell designs. “We have a modem that links back to a monitoring station at our Auckland Park premises. We continuously monitor all three of MTN’s trial sites to ensure that we are meeting our mandate and that MTN’s objectives are all being met,” Coetzer concludes.

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