Airwaves Communications manage the TETRA communications network that provides the primary network for the UK emergency services. Where the majority of the cell sites are in locations where grid power is available, many are not and require an alternative source of power supply. Running such a site on a diesel generator comes at a cost in terms of fuel, maintenance, and emissions.
The power requirements of a cell tower are, typically, relatively steady albeit at a low level. In this case, average power consumption was in the region of 6kW. As a result, the diesel generator, rated to cope with the occasional peaks in power demand when A/C starts, spends most of its time running at an inefficient level that means poor fuel economy and, of course, the creation of higher than necessary levels of NOx and CO2e emissions.
In this instance, the site was located close to residential property where the noise was also an issue during nighttime hours attracting complaints from residents. Fuel economy and the cost of maintenance and refueling were issues that the customer (the tower operator) also wanted to address.
To minimise the generator running hours, a pair of 30/60 Powercube’s ( each 30kVA, 60kWhr) were used in a “dual cascade” mode where one unit feeds into a second to provide greater energy storage.
The two units enabled the site to operate from the battery for 16 hours on an average load of 6kW with energy to spare. The diesel generator running hours were then limited to 10 am to 6 pm each day allowing sufficient time for the battery to recharge. During this time, the 45kVA generator was loaded well so that fuel economy was optimised, emissions minimised and, most importantly, there was no noise for long periods.
Diagram. When the generator is running, each POWERCUBE is supplied directly from the generator to charge the battery. The lead unit (B) also supplies the tower. When the generator is turned off, unit A supplies power to unit B until depleted. At that point unit B then uses it’s battery until the generator is then re-started.
SERVICE AND REFUELING
Additional benefits were that the lower generator running hours meant less need for engine service and that servicing could be carried out any time, any day without any interruption to the operation of the tower. Fewer refueling visits were necessary too.
Through managed loading of the generator, maintained at a level over 50%, fuel economy was optimised saving some 40-50% in fuel compared to levels before installing the Powercube. This also meant halving the frequency of fuel deliveries.
Running hours of the generator were reduced by 2/3rds leading to a c comparative reduction in the need for routine maintenance. Fewer breakdown issues were also then encountered because of better utilisation.
It was possible to eliminate generator noise for 16 hours per day which avoided noise complaints from residents.
Lower fuel consumption and better engine utilisation levels resulted in significant reductions in both carbon emissions and improvements in air quality through lower NOx and PM content.