Modified Designs

Hybrid gensets emerge as a preferred choice for backup power

Diesel generator set (genset) suppliers and manufacturers are constantly in the process of developing innovative technology and modifications to refine traditional diesel gensets. Over the past few decades, diesel gensets have become the primary choice for reliable mobile, stationary or temporary on-site power. The working of diesel engines is such that air is introduced into a combustion chamber, which is then raised to about 1,000 °F temperature by super-compression. Fuel is then injected into the combustion chamber via a pre-chamber, where it vaporises and ignites. In recent modifications, the chambers of the engine are modified for maximum combustion and acoustical attenuation. To gain maximum power output and performance, the engine is turbocharged. Further, analogue controls have been replaced with digital controls. Fumes released are subjected to emission treatments. Also, hybrid gensets have come into existence for cost efficiency and compliance with environmental norms.

Technology trends

Hybrid technologies

Hybrid diesel generators combine diesel gensets with a renewable energy source such as solar, wind or biomass. With this, the dependence on diesel decreases and helps in complying with the environmental norms. The most common hybrid technology is diesel gensets, combined with solar photovoltaic (PV) systems. Using two sources of energy makes it cost effective (as diesel is expensive) and also resolves intermittency issues, purely using solar power. In fact, renewable energy generator hybrid systems can double or triple the amount of continuous service time compared to a stand-alone diesel generator with an equivalent fuel tank size.

Solar energy is produced during sunlight hours, which reduces the load on diesel gensets. An automatic digital system controller maximises the load on solar panels during peak sunlight hours and produces energy from PV panels. At the same time, power from the diesel engine provides the required frequency and voltage, which firms up the direct current produced by solar panels and stabilises the load. But the use of diesel is minimised to save fuel.

Diesel gensets with solar hybrid systems mostly include a battery or energy storage. This helps to further reduce the consumption of diesel as the battery provides frequency to the current produced by solar panels for consumption during daytime or if there is a load failure from the grid at on-grid locations. In off-grid systems in remote locations, battery storage provides stability for the current flow, while also minimising the use of diesel generators. Although adding battery storage to this configuration of a diesel-solar hybrid system involves high capex costs initially, there are high operational cost efficiencies in the long run.

Engine modifications

Changes in configurations and sizing of the combustion chamber are also being taken up so as to achieve maximum combustion rates. The higher the combustion rate, the higher the fuel efficiency and thus the power output. At the same time, exhaust emissions are lower. Another modification done to the engine is sound attenuation. The noise in diesel engines is due to the fuel injected into the pre-chamber being forced to vaporise at a high temperature. In recent developments, the pre-chamber has been fitted with sound attenuation pockets filled with porous or metallic ceramic pellets coated with platinum or rhodium. The technology is such that the pellets diminish noise levels, with platinum and rhodium acting as catalysts to achieve complete combustion. Other strategies applied are placing noise-diminishing materials such as porous ceramic or sintered metal near the mouth of the passage, which connects the pre-chamber to the combustion chamber.

WHR systems

Waste heat recovery (WHR) is the use of thermal energy that would otherwise be transferred to the environment, to accomplish a useful function. A WHR system uses devices such as regenerators, recuperators and economisers for energy conversion. About 40 per cent of the input energy gets expelled from the exhaust of a diesel engine and gets wasted. The WHR system recovers heat from steam and transforms it into electrical energy for utilisation. Energy consumption can be reduced by 5-30 per cent with the help of this technology.

Bi-fuel generators

Bi-fuel technology can be significantly helpful in providing environmental benefits and fuel efficiency. Diesel generators that use bi-fuel technology can use 70 per cent natural gas during operation and the system controller manages the fuel mixture that helps the equipment perform optimally under load. It would be a cheaper and environment-friendly option. A retrofitted diesel genset can function with 60 per cent gas and 40 per cent diesel. The initial cost of retrofitting of about half a million can be recovered in about six months. Indraprastha Gas Limited is planning to replace diesel gensets with gas gensets in housing complexes and factories.

Digital controls

Originally, the functioning of diesel engines was monitored through analogue controls. But digital controls are more suited, especially in situations requiring complex power backup and distribution systems.

Digital controls can report the real-time status of all aspects, that is, fuel, engine oil, coolant levels, engine temperatures, battery status and transfer switch status. This is useful to get an overview of the entire system. They also display the status and errors of a power generator on a computer and accurately indicate the nature and place of malfunction. All this helps in the quick detection of any problem and finding a solution to it.

Built-in redundancy in digital controls allows for continual system function even if a component or a part of a circuit fails. Its ability to make the required adjustments to the fuel input and injection timing under varying load conditions increases the efficiency of the engine and reduces exhaust emissions. This also helps in stabilising the voltage and output frequency.

Emission treatments

Diesel combustion in generators at extremely high temperatures results in the emission of nitrogen oxides (NOx) as a by-product. Such emissions are extremely harmful to people and hazardous to the environment. Technologies to tackle this problem include the exhaust gas recirculation (EGR) and selective catalytic reduction (SCR).

In EGR treatment, a part of the exhaust gases emitted are mixed with air in the combustion chamber as it is recirculated into the engine. NOx formation is maximised at high temperatures and in the presence of sufficient oxygen. However, the adiabatic flame temperature is lowered due to this process and there is an increase in the heat capacity of the mix. This allows for combustion at a much lower temperature, hence, reducing NOx formation.

The SCR method has been in use for waste management systems and commercial boilers, but its integration with diesel gensets is a recent phenomenon. In the SCR method, a reducing catalyst such as anhydrous or aqueous ammonia, or urea is used to convert NOx into diatomic nitrogen and water, that is, N2 and H2O. It is known to have reduced NOx emissions in the range of 75-95 per cent.

Fuel systems and treatments

Modifications have been done not just  in engines but also fuel systems and composition. The advanced common-rail fuel (CRF) injection system is a form of the direct injection system. The technology followed is that the fuel is directly injected from a fuel tank, which stores the fuel at high pressure, into individual solenoid or piezoelectric valves. This system is electronically monitored and permits greater control time and quantity of fuel injection. Maintaining a high pressure allows homogenisation of the fuel and air mixture by allowing for finer vaporisation of fuel. This enables a complete combustion. CRF systems can bring about a 20 per cent decrease in fuel emissions and 5 per cent increase in fuel economy.

As for the fuel composition, diesel is high in sulphur content, which produces high particulate matter emissions on combustion. This, in turn, hampers engine performance. However, the use of a new version of diesel with low sulphur content of 15 parts per million (ppm) called ultra-low-sulphur diesel or ULSD can help in lowering exhaust gas emissions.

Conclusion

Although alternative backup solutions have emerged, diesel gensets are still the primary choice for industrial customers with their characteristics of durability, reliability and performance. Use of hybrid gensets and introducing modifications in designs to help lower the carbon footprint is expected to ensure that they remain a preferred choice for backup power.

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