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Power density: balancing greater output with available on-site space

Published: 8 March 2019 - Rachael Morling

Jason Harryman, sales and business development manager – Electric Power-Diesel at Finning UK & Ireland (Finning), explores some of the ways that operators can achieve a balance between greater power output and available space, discussing key design considerations such as noise attenuation, cooling and fuel supply.

Connectivity between human and machine is growing at an exponential rate, meaning that generating sufficient standby power to meet peaks and troughs in demand can be a challenge. With space generally at a premium, either for a new-build site or in an existing plant room, operators are often faced with a compromise – a smaller generator set with less power or a larger machine that will require additional building works to accommodate it.

The latest generators have been re-engineered with higher power densities to meet challenging restrictions on capital, site size, emissions and cost. Increased power density is also advantageous because it reduces the amount of ancillary equipment required for a system. Furthermore, when supplying a new generator set, the size of the unit will naturally play a role in dictating the cost of transporting the generator to site, its installation, maintenance and any ancillary enclosures.

So, when it comes to specifying a ‘power dense’ system, what considerations do operators need to be aware of?

Optimising output

The first area that needs to be considered is the generator’s output. Above all, each generator set should be sized correctly, according to the demand for power. In an attempt to minimise risk, some operators may choose to over-size a system, which can be a costly approach. Not only is equipment specified with a higher rating than necessary under normal operating conditions, it can also result in the installation of larger units that take up more space and will require more regular servicing and maintenance.

As a result, engine suppliers such as Finning are engineering systems that offer a higher maximum expected usage, with varying load for the duration of the outage. This ensures an operator can benefit from a smaller, more compact generator set that is sized closer to the desired load, while still retaining the same power resilience available from a larger unit rated for continuous operation.

Other issues surrounding generator output that must be taken into account include parasitic loads, which is essentially secondary support equipment – such as electrical devices – that are necessary for the operation of the system. It is also important for engine suppliers to consider how emission levels can be lowered at the greatest level of output and the highest level of load acceptance.

Influencing factors

Ever-evolving emission regulations have led to changes in how engines are cooled. Similar to a single jacket water circuit, where the cooling fluid is circulated through internal passages in the engine block to extract and carry away some of the heat produced as a result of the combustion cycle, a separate circuit after-cooled (SCAC) engine also incorporates a separate water circuit to cool the engine’s intake air after it is pressurized by the system’s turbocharger.

In contrast, an air-to-air-aftercooled (ATAAC) engine uses forced air to cool the system’s turbocharged air before it enters the engine’s combustion chamber. Both designs aim to lower the engine’s intake air temperature to help improve emissions and output power efficiency, however the SCAC design requires lower turbocharged air temperatures than the ATAAC model, which needs greater airflows. This, in turn, influences the attenuation required to meet the maximum noise levels.

The noise level of a generator can be an influential factor in a purchasing decision, depending on the customer and their application and requirements. To truly understand the overall sound pressure level in dB(a) from a generator, recordings should be considered from all sides of the unit rather than just a single data point. Furthermore, it is important for manufacturers to calculate the logarithmic mean, rather than simply the average of these values, else there is the danger that the generator will appear quieter on paper than it is in practice.

Air distribution or segregation is another consideration, to manage heat rejection and ambient air intake.

Fuel for thought

Fuel storage can also potentially pose a challenge when it comes to power density. Where space is at a premium and density is a need, structural base tanks with fuel filtration capabilities are an option. These help maintain the condition of the fuel at a suitably high standard, which can potentially be a challenge in applications where fuel is stored for long periods of time, in high volumes and at varying temperatures.

Some low-emission fuels and bio fuels are available, but it is important to bear in mind that not all are compatible with high-pressure fuel rail injection systems, nor capable of being stored for long periods of time. Indeed, some might not even be suitable for engines with traditional combustion cycles.

Other considerations

For large distribution systems with extended resilience features, medium-voltage or high-voltage generators are typically specified. Nevertheless, more local low-voltage back-up systems with simplified infrastructure are also an option, which can be operated more economically.

Generators can also be equipped with a low- or high-voltage frame alternator. A high-voltage frame, for example, allows more machines to sync onto a common bar. This, in turn, can improve load management. This can help avoid over-sizing the genset due to motor starting requirements.

Another area that has greatly improved power density is platform development. For example, the 3516 diesel engine from Caterpillar – the world’s largest manufacturer of generator sets – was originally rated at 1650 kVa but can now achieve 3000 kVa as a LBSFC (low brake-specific fuel consumption) variant, and is expected to achieve an output of 3500 kVa by 2020.

While the unit has a very similar footprint to the earlier model, it now weighs 5 tonnes more. And yet, while the airflows on the unit have increased, it can now deliver approximately double the output.

Space-saving techniques, such as steel structural bases that allow genset containers to be double stacked, are yet another option to help maximise the space available on-site.

To conclude, generator sets with improved power densities will allow operators to fine-tune the selection of power required, through a compact package that can significantly reduce costs in both the short-term and long-term. As manufacturers set out to engineer better-performing solutions, compliant with the latest legislative updates, power density is becoming a more and more prevalent issue.



Industry Connections: Finning (UK) Ltd

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