Study and Design of a Hybrid-Diesel-Compressed Air Energy Storage System for Remote Areas

In Canada it is estimated that throughout the country, over 200,000 people live off of the grid, and require electricity from other sources. Most of this electricity is supplied through small, localized power plants containing diesel generators. This situation is not ideal for several reasons. Primarily, diesel itself is not the most efficient or inexpensive way of generating energy. However local power plants in Canada suffer other economic stressors too; it is far more expensive to run a small power plant than it is to run a large one, and the costs and risks associated with the transport of the diesel to the plant add further financial burdens. Some locations are more remote than others, and this is reflected in the cost of fuel transport, however as Canada has a uniform price for energy, some plants lose much more money than others. For all of these reasons these small power plants expend more money than they receive, and need to be subsidized by the Canadian government. On an additional note, diesel also releases a high amount of greenhouse gas. Diesel electricity production emits 1.2 million tons of greenhouse gas per year in Canada alone. Ibrahim et al. (2009) conducted a study in which they attempted to mitigate many of these issues through hybridizing the current diesel generators with a combination of wind turbines and a compressed air storage system.  While it was already known that the addition of wind turbines to these generators could cut fuel consumption modestly, the authors found that with the added addition of a well-designed supercharger fueled by the compressed air storage unit, they could reduce fuel consumption at a much higher rate.¾Emily Grace Cole

Ibrahim, H., Younés, R., Ilinca, A., Dimitrova, M., Perron, J., 2010. Study and design of a hybrid-diesel-compressed air energy storage system for remote areas. Applied Energy 87, 17491762.

 While wind-turbine, diesel-hybrid generators did exist before Ibrahim and his colleagues conducted their study, they have many known disadvantages. Low penetration systems, in which the overall energy derived from wind is no more than 1015% are viable, however a larger amount of wind energy usage would lead to even more reduction in fuel consumption. The issue lies in the unpredictability of wind energy. In higher penetration systems, where the overall percentage of wind energy contribution is higher, the generator must often be left on so that in the event of a loss of wind it can respond quickly and deliver energy. This is a major fuel drain, as the generator can remain on for long periods of time, serving no actual purpose. Energy storage is a solution to this, as it allows the wind turbines to harvest and store excess energy when they are producing more than their load demands. In this scenario, the generator can then be powered down, and energy reserves can be drawn on from storage in the event of fluctuation. However the current storage systems have their flaws as well. Some systems use hot water to store energy quickly but hot water is difficult to convert back to electricity, and the reserves must be quickly accessible. Batteries are used as well, but they are expensive, need to be maintained, and are difficult to dispose of. Fuel cells are a possibility too, but they are not commonly used, as they are expensive and technically challenging. Compressed air does not have these disadvantages. It is clean and it converts back to electricity efficiently and quickly.
The addition of compressed air storage to wind-diesel-hybrid systems alone would increase wind penetration and therefore cut down on fuel use, however compressed air has another advantage in that it lends itself to supercharging. Supercharging is a process which facilitates the injection of air into the combustion chamber of the diesel generator, giving it greater power for the amount of fuel it uses, leading to an overall decrease in the amount of fuel it needs. Ibrahim and his colleagues designed several possible models for implementation of the supercharger into the existing Canadian diesel generators. The models varied in their use and placement of the compressed air, as well as their number of turbines. They were all thought to bring different advantages and disadvantages. In order to select which model to proceed with in the study, the researchers came up with a weighted rating system based on the following criterion, in order of descending importance: efficiency, simplicity, adaptability with the diesel engine, cost, control system, and reliability, system 1, which relied upon one turbine directly connected to the supercharger shaft, was judged to have the highest score.
When Ibrahim and colleagues experimented System 1 in practice, they found that it led to a significant fuel saving, as the generator could be made to work at 25-30% of it’s original fuel consumption. 

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