Introduction Backup Power Systems for Mobile Telecommunications Telecommunication networks are a major user of backup powering systems also termed uninterruptible power systems UPS and t
Trang 1Uninterruptible and Back-up Power: Fuel Cells
U Benz, D Busche, and D Lutterbeck,P21 GmbH, Brunnthal, Germany
& 2009 Elsevier B.V All rights reserved.
Introduction
Backup Power Systems for Mobile
Telecommunications
Telecommunication networks are a major user of backup
powering systems (also termed uninterruptible power
systems (UPS)) and they serve as the primary example of
a fuel cell (FC) application in this article Mobile or
wireless networks are often the primary source of
tele-communications especially in emerging markets and
developing countries where fixed line networks have not
been extensively built Customers expect to continue to
use their mobile phones, even when the power grid fails
In extreme cases, need to be reached and need to
con-tinue with their communication
The mobile network is based on a matrix of base
transceiver stations (BTSs) In general, the electrical
energy is provided by the electric power grid In a BTS
site, the 230 VAC (volts alternating current) power is
generally inverted into 48 VDC (volts direct current)
power, which is used to power the radio equipment In
the case of a blackout, batteries and/or diesel generators
traditionally step in to ensure continuous operation
Today, the mobile network operators (MNOs) rely
mainly on lead–acid batteries as the emergency power
source These are accumulators that are maintained at
full capacity by applying an adequate charge current
during normal grid operation In the case of a
(tempor-ary) loss of the electrical power grid, the voltage level in
the BTS falls below the open-circuit voltage of the
bat-tery and the batteries take over the power supply as the
lead–acid batteries are discharged As soon as the
elec-trical grid is reestablished, the batteries are recharged
until they reach their full capacity, or until the grid fails
once more
Although batteries are the most established provider
of backup power, they suffer from a number of
drawbacks:
• Long and unpredictable outage periods (e.g., several
hours of grid loss) require large battery banks The
size of the banks has to be designed for the most
probable worst case The weight of the batteries can
be a problem especially on rooftop sites and the costs
increase with the size of the battery bank
• Continuous monitoring of the condition of lead–acid
batteries, and therefore their available energy capacity,
is impractical The available capacity falls over time
and with the number of discharge cycles, especially at
high depths of discharge (DoDs) Therefore, the op-erator does not know how much backup power is available at the sites
• Lead waste is highly toxic and it is not guaranteed that the recycling is up to the required standard
• Batteries suffer if they are operated at temperatures above 25 1C Consequently, the BTSs are actively cooled, especially in hot regions This is a major contributor to energy consumption and operation costs
For very long backup times and for remote off-grid op-eration, diesel generators are generally used These diesel generators represent a very significant investment and operational cost for both fuel and maintenance and, in addition, there are problems with reliability Diesel generators contribute to air pollution and have a large carbon footprint
Backup Power – Early Market for Proton-Exchange Membrane Fuel Cells?
Fuel cells could offer an alternative power backup to both batteries and diesel generators They are compact and lightweight and can be refueled, thereby offering extended backup power for long periods of time
An FC is a highly efficient, environmentally friendly, relatively simple, and high-potential technology that generates electricity by electrochemically combining hydrogen and oxygen It is commercially available for backup power applications It has no moving parts such as
an internal combustion engine, and therefore does not vibrate and produces only minimal noise The only emissions are water It does not require fossil fuels and has greater fuel-to-electrical energy efficiency than any other technology
The basic principle of a proton-exchange membrane fuel cell (PEMFC) is shown inFigure 1
Despite their high potential with respect to efficiency and environmental friendliness, until recently and for most of the broad field of potential applications, FCs showed significant disadvantages compared to established competitive technologies, such as
• higher investment costs,
• limited lifetime, and
• poor fuel availability and a high fuel price
Owing to the most recent development efforts spent on
FC systems for backup power applications, the costs of FCs and FC systems have reached a commercially viable
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