Backup Power Generators
Block Loading or Load Acceptance:
Diesel generators have been used for decades as a backup power supply for many Australian businesses who need power security; a backup power supply when the mains power is off or out of specification (eg a brownout). Back up generators are often built inexpensively with a short life expectancy, as they do not run for too many hours per year and the diesel engine driving the alternator provides plenty of torque to maintain speed, corresponding to power AC frequency, when large electrical loads are switched on. This solution works well.
The standard, ISO 8528-12 defines that the block load, or Load Acceptance, for diesel generators as a maximum electrical load that can be applied before the speed drops by more than 10%. Diesel generators can typically accept 50% of their maximum power output, but every manufacturer will provide their own block loading values, often when the engine is warm. So, even with a backup generator, there is typically a period of no power between when the power grid fails, the backup power generator starts and then warms up to take the load. A cold engine will perform worse so heaters and glow plugs can be added to speed up engine starting and load acceptance.
Gas engine generators typically have lower % load acceptance than diesel generators, particularly lean burn engines that provide enhanced attractive fuel economy, but at the cost of torque and load acceptance. This is fine when the site is running in parallel with the utility grid as step loads power can be seamlessly imported from the grid for a few seconds while the generator steps up to the higher demand. However, they are not as useful as site back up generators if the site as larger loads. For some biogas generators, they are sold as grid paralleling only and unsuitable for off-grid power as their step load is too poor. In this case, the site may need to have generator CHPs for normal operation running parallel with the grid and a separate diesel generator for back up power supply. We believe that this is a poor outcome for the site owner and operator with more equipment to buy, install and then maintain.
How Inoplex fixes the Load Acceptance problem
Seeing this cost at our client's operations, the engineering team at Inoplex have developed our next generation of inverter generators to address this needless site cost and complexity. We have worked to develop a gas engine with dramatically improved load acceptance so that it can provide both power paralleling with the grid and back up power when the grid is down or out of specification. And yes, it can do this with poor quality biogas too, without just resorting oversizing the generator!
Our inverter CHP engines run lean, or with an excess of air, to achieve excellent fuel economy by burning all the available fuel when running with the grid. When the grid is down or out of specification, the CHP runs in "back up power" or “island" mode. In island mode the engine runs as a “stoichiometric" engine, meaning that the amount of oxygen in the combustion air matched to the fuel provided which yields improved torque and instantaneous response to increased block loads. While this approach provided a significant gain in block loading performance for our customers, we decided we could do better and control the power quality tighter.
The inverters on our CHP always provide the required power frequency and voltage, regardless of engine speed. So, even if the engine starts to slow or head towards a stall because a large step load is applied, the power produced by the CHP will still be precisely 50 Hz or the value required by the site. So, our second approach to producing a biogas engine with leading step loads is using the power electronics to compensate for the momentary slowing of the engine. But, we soon realised we can do better than even this….
Our standard design has banks of capacitors storing electrical energy on the DC bus within the power electronics. Just like a heavy flywheel has mechanical mass, rotating at speed that can be converted from mechanical to electrical energy when a block load is applied, our capacitors have stored energy, ready to top up the supply of power as required for a bock load. This is the third layer of our block load approach to assist clients operations to run smoothly when the grid has failed.
The Inoplex inverter CHP provides a smart solution to embedded power generator running in parallel with the grid and also, providing a substantial back up power supply Load Acceptance. Our ability to de-couple the engine speed and parameters from power production and the use of power electronics provides many benefits to owners
When considering CHP Cogeneration for your site, think about the benefit of having one generator system that is able to provide power security without the need and costs and complexity of an added back up diesel generator too.