Diesel generator sets are often the mainstay for electrical power in remote areas in the developing world. But their associated costs have recently soared, which adds to concerns over CO2 emissions in driving the search for alternative solutions. A hybrid power system incorporating rechargeable battery technology may hold the key.
Joel Brunarie, Saft Industrial Battery Group,UK
Diesel-powered generator sets, thanks to their simplicity, low cost and speedy installation remain the main source of electrical power for many consumers throughout the world, especially in less developed areas such as sub-Saharan Africa, South Asia and South America where grid supplies are generally unreliable and quite often non-existent.
The drawback is that in recent years the costs associated with diesel generation have increased substantially, with fuel prices up by 50 per cent in many locations.
In addition, for more remote sites, especially those operating in war zones or in humanitarian aid projects, the refuelling and periodic maintenance of the generators can be both hazardous and difficult to accomplish, resulting in direct and significant further increases in operating expenditure.
At the same time, environmental concerns are driving operators of diesel generator sites to consider how they might be able to reduce their CO2 emissions.
THE BATTERY BECOMES THE PRIMARY POWER SOURCE
A potential solution to these challenges is now emerging in the form of hybrid power systems incorporating a cycling rechargeable battery. 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 the genset providing the recharging current.
This approach is targeted mainly at installations where grid connectivity is unavailable or unstable, requiring diesel generators to be running for at least 30 per cent of each day. In addition to new-build sites, this system can also be used as a retrofit solution.
The main advantage of this hybrid system is a reduction in operating costs (OPEX) of 50 to 85 per cent combined with a reduction in carbon emissions of 48 to 80 per cent, depending on site load, compared with continuous generator operation. The gensets only need to run for a fraction of the time, reducing fuel consumption, emissions and maintenance requirements.
RELIABLE POWER FOR TELECOM BTS SITES
Early interest in hybrid power systems has been shown by operators of BTS (Base Transceiver Station) sites for mobile communication networks. Typically, off-grid sites are powered by two diesel generators operating alternately to ensure a reliable power source, while sites in regions of unreliable grid power are normally equipped with a single diesel generator. In some rural regions of East Africa or in India the run time of the genset to back up the mobile site can reach up to 12 hours a day.
The use of a battery technology with a proven high level of reliability allows a single diesel generator to be used while still ensuring a minimum power backup time of six hours. This maintains system reliability while reducing capital expenditure, with part of the cost of the battery being covered by the elimination of the second generator.
The hybrid system features a modular, containerized design. This enables it to be sized to meet the immediate requirement and, if necessary, to be expanded with minimal extra cost and equipment. It also has the flexibility to incorporate both solar and wind generation ” the use of a renewable power source can further decrease the generator running time and hence OPEX while both will assist in reducing the carbon footprint. To ensure the lowest total cost of ownership (TCO) of the hybrid solution, the different sub-systems such as battery, generators, alternative power sources, rectifier and controller must be carefully selected and sized to permit the highest system efficiency and the best operating profile, including these key criteria:
- a complete hybrid system of this type will be packaged in an ‘energy container’ to offer quick installation in remote locations;
- the cycling battery must be a temperature-resistant, low-maintenance design, accept fast charging and be able to deliver a large number of charge-discharge cycles;
- the rectifier must offer the highest possible energy efficiency;
- the controller must be equipped with dedicated software enabling the operating profile of the battery and generator to be optimized, thus delivering the lowest possible operating cost;
- the addition of renewable energy sources will permit an increase in cycling time and consequently extend the calendar life of the battery and generator while also increasing the environmental benefits.
The BTS sites where Saft has implemented a hybrid solution are based on a special design of a nickel-cadmium (Ni-Cd) telecom battery (Sunica.plus). The key feature of this battery is its excellent chargeability, which enables highly efficient operation under fluctuating charging conditions. It also offers good cycling capability to withstand daily and seasonal cycling at variable depths of discharge and state of charge.
An additional benefit of this Ni-Cd design is that it provides absolute reliability and the capability to function in extreme temperatures (from -50à‚°C to +70à‚°C) while ensuring continuous operation at any state of charge. Another important advantage is a low maintenance design that not only provides superior behaviour under unstable charging conditions, but also significantly extends the interval for topping up with water. This is especially important in remote installations.
Even in this very demanding application, the Sunica.plus batteries are expected to have a lifetime of five years, with a payback of just two years.
|Fuel consumption by a hybrid power system is a great deal more efficient than that of a diesel-only generator|
The size of the diesel generators varies according to the sizing of equipment but is usually in a range from 10 kVA through to 120 kVA ” for a typical BTS site with a 2 kW load a standard 27 kVA genset is provided.
In addition to the fuel cost of running these generators 24 hours a day, the servicing of the engines can also have a serious impact on the site operating costs with, in some cases, service intervals required every 250 hours.
The controller is the key to the hybrid system since it enables the efficient monitoring of the battery charge levels, and of the optional solar panels and wind turbines along with the generator to ensure that the site load requirements are met while minimizing the generator run time.
The hybrid system is configured so that the controller monitors the available power sources and then brings them into operation in a defined order as follows:
- if present, the wind turbine or solar panels are set as the primary source of energy and are monitored to ensure availability to provide for the site load;
- in the absence of sufficient energy from the primary source of supply, the batteries are used as the primary source of power and are monitored to ensure that they only reach a 27 per cent depth of discharge (DoD) before recharging;
- when the batteries reach 27 per cent DoD the generator is started and provides both the recharging current and the site load;
- in the event that the generator fails to start, the site load reverts to the batteries and an alarm is raised. The remaining 73 per cent charge is calculated to last for a minimum of six hours of backup;
- if power from a local grid is available at any time it is monitored and utilized as the supply to the site load. This prolongs the battery discharge time and hence reduces the daily fuel consumption.
Saft has worked with Eltek Valere, a specialist developer of hybrid telecom power systems, to carry out field tests on a BTS site in Lagos, Nigeria. This location was selected since Nigeria is the most important market in the African continent with over 77 million mobile subscribers.
To test the system at its fullest capability it was assumed that the grid was not available and a dummy load of a constant 2 kW was used to simulate the site working at 100 per cent capacity for a period of two months.
The site layout was a containerized solution with the generator housed within the container, and the rectifier cabinet and batteries placed on a rack on the open platform. This was done to demonstrate that the batteries would operate effectively at the ambient daytime temperature of +35 à‚°C, and also under varying temperatures, without any degradation.
The batteries were set as the primary source of power, with the diesel generator as the secondary source for provision of power to the site and for recharging the batteries.
The level of charge of the batteries was monitored so that when they had discharged to 27 per cent DoD, the generator was started to take over and provide the recharging current. Once the batteries were back to full charge the system switched the power source back to the batteries.
During the trial, the generator was running for six hours per day, with the batteries as the primary power source for 18 hours per day. This resulted in a fuel saving of 75 per cent, compared with a similar site using generators only, at a 100 per cent site load per day.
As the actual site load varies according to call usage patterns, and generally averages at around 60 to 70 per cent of the maximum load, it can be assumed that the real-life savings would be substantially greater.
|Table 1: A comparison of operating expenses of hybrid systems compared with those for a diesel site|
As shown in Table 2, a hybrid system can reduce CO2 emissions drastically when compared with a pure diesel site.
Over a five-year operating period, corresponding approximately to the life duration of the Ni-Cd battery and of the genset in such a hybrid application, the reduction in the CO2 emission for a 2 kW mobile site will be more than 180 tonnes. This should be compared with about only 2.5 tonnes of equivalent CO2 for the ‘cradle-to-grave’ global warming potential of a string of 48 V, 555 Ah Ni-Cd telecom batteries, representing just over 1 per cent of the total savings in CO2 emission.
Following the success of the trials, Eltek Valere is now rolling out hybrid telecom power systems to 80 mobile telecom sites across Nigeria.
TERRACON ENERGY CONTAINER
Saft Ni-Cd batteries also provide the vital energy buffer for a self-contained transportable energy container developed by Terracon Energy GmbH to provide an almost instant source of up to 10 kW of renewable energy in remote locations.
The Terracon Energy Container integrates a complete hybrid renewable power system ” comprising wind generator, photovoltaic (PV) modules, diesel generator, batteries, associated control and monitoring systems and inverters ” within a standard 6.1-metre ISO shipping container. On arrival at site the container simply has to be craned into the required position. There is no need for a base plate and the container also acts as the foundation for the 15-metre-high wind generator.
|Table 2: Example of CO2 savings achieved by using a hybrid system in place of a pure diesel installation|
The battery system provides the vital balance between the energy generated and the energy consumed to ensure continuity of supply. So it acts as a buffer to make up the difference 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 generator and PV modules. The energy container guarantees a peak output of 10 kW and a continuous output of 5 kW. Efficient use enables it to produce up to 15 000 kWh per year, and as much as 37 000 kWh with the diesel generator.
The exact specification of the battery installed in each energy container will vary according to specific customer needs. Typically, it will be a nominal 48 V and 800 Ah capacity.
Titan Energy’s renewable energy mobile utility system (REMUS) trailer concept can provide a utility-scale electricity supply ” up to 307 V and 138 kW. It is powered by a combination of a wind turbine, PV solar panels and a 40 kW diesel generator that also charges the Li-ion battery pack. It is an important development in the emergency energy industry as it offers a solution that is less dependent on traditional fuels, particularly valuable in remote locations and during humanitarian missions where supplies may be limited. REMUS requires a small, lightweight battery that can control many operations while maximizing fuel efficiency. Saft’s advanced Li-ion technology, part of a standard line of products for hybrid electric military vehicles (HEMV), offers an ideal solution.
Hybrid power systems are able to provide dependable power for remote off-grid installations while minimizing the run time of diesel generators. This approach can offer substantial benefits in terms of reduced fuel costs and lower CO2 emissions as well as reduced maintenance and improved reliability.