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Diesel Gensets: Economics and Efficiency

October 31, 2011
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Saurabh Diddi, Energy Economist, Bureau of Energy Efficiency

Power and energy consumption for network operations in the telecom sector has been very high. Given the lack of continuous grid power, the telecom infrastructure industry has to rely heavily on diesel generator (DG) sets to bridge the energy gap.

A DG set is a comprehensive system, comprising several parts such as a diesel engine, an alternating current (AC) generator and controls. DG-based captive power plants are frequently used in small power (captive non-utility) systems. The power capacity ranges from 4 MW to 15 MW. The specific fuel consumption for diesel engines is currently 160 g per kWh.

Captive power plants running on DG sets have various advantages over other types of captive plants. They offer higher efficiency (of 43-45 per cent) as compared to small steam and gas turbines. Other merits include low installation costs, short delivery and installation periods, minimum cooling water requirements and short start-up time.

Depending on the speed, diesel engines can be broadly classified as slow speed and high speed engines. In terms of fuel consumption, a slow speed diesel engine, with its flat fuel consumption curve over a wide load range, is favourable over a high speed diesel engine. While low speed diesel engines are best suited for continuous use, high speed diesel engines are more suitable for intermittent use. Direct operating costs including lubricating oils, filters, etc. are much lower for slow speed engines. However, the duration between the overhauls is less for high speed engines. Further, the weight-to-power ratio, representing the sturdiness and life of the engine, is greater in slow speed engines.

These DG sets can be categorised as water-cooled and air-cooled sets, depending on the requirement of the cooling system. Although both sets are comparable in terms of performance and maintenance, it is necessary to use a water-cooled model during larger capacity generation to prevent overheating of engines during the summer months.

Safety features

It is advisable to have short circuit, overload and earth fault protection on all DG sets. According to the electricity rules, neutral earthing should be employed in all electrical equipment to ensure safe working. The equipment should be provided with two independent earth wires, one connected to the body of the equipment and other to the neutral circuit conductor. This gives adequate protection to the equipment by providing a return path for earth faults and current outflow. However, such a system becomes uneconomical in the case of small er-capacity DG sets. Other safety measures like high temperature and low lube oil pressure cut-outs should be provided so that  the engine automatically stops functioning in case of any abnormality, thereby preventing major damage. It is also essential to provide reverse power relay in the case of parallel running of DG sets in order to avoid backfeeding from one alternator to another.

Energy saving measures for DG sets

DG sets can be made more efficient in several ways. These include ensuring steady load conditions on the DG set by avoiding fluctuations and imbalance in phases, providing cold and dust-free air intake, improving air filtration and frequent calibration of fuel injection pumps.

It is recommended to have the load balanced since unbalanced loads can cause heating of the alternator, resulting in unbalanced output voltages. The maximum unbalanced load between the phases should not exceed 10 per cent of the generator sets’ capacity.

Overloads are also detrimental to a DG set and hence must be carefully analysed. The DG set selection should be such that overloads remain within the specified permissible limits of 10 per cent for one hour in every 10 hours of operation in the case of diesel engines, and a 50 per cent overload for 15 seconds in the case of AC generators.

Further, there are cases where the load is not constant throughout the day. Under such variable load conditions, parallel operations of DG sets can be considered for improved loading. Various capacity combinations are now being utilised to improve the flexibility of the system, as it is viable to stop one DG set while there is another generating at least 50 per cent of the power requirement. Also, during lean and low power cut periods, one DG set can act as a 100 per cent standby. Multiple DG sets can run at optimum operating points for optimum fuel consumption. The scheme can also be applied when there are loads that can be segregated as critical and non-critical to provide standby power to critical load in the captive power system. Moreover, these combinations are more economical in terms of space, operation, maintenance and the initial capital investment, when compared to one large DG set.

Compliance with manufacturers’ guidelines and oil company stipulations for fuel storage and system handling is also mandatory to improve DG set efficiency. Regular field trials must be carried out to monitor its performance and maintenance.

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