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With PFC enabled, the drive uses its internal PI regulator to control the speed of the first of the three motors to match the demand for pressure, temperature or flow. When the first motor is running at full speed, the drive issues a command for the second motor to start. Similarly, when demand exceeds capacity with two motors running, the drive starts the third.

As shown through theory and case study, speed control of refrigeration components provides maximum flexibility, control and energy efficiency

There are several incentives for using speed control on screw compressors: - Drive control will reduce the power penalty associated with slide valve, poppet valve, or throttling capacity control. On compressors with no capacity control, speed control will eliminate other poor control strategies. - Drive control will reduce wear and tear associated with slide valve action. - Drive control allows a precise suction pressure to be maintained. With slide valve, a broad dead band is often maintained to avoid excessive wear. - Drive speed control provides compressor size reduction with the same system capacity demand.

One of the major benefits of VSDs is the flexibility they allow to dryer users, helping them both control costs and manage the drying process. Many dryer users specify an oversized dryer, as they may be uncertain of the initial market size for their finished products. A VSD allows them to start at 50 per cent drying capacity and then increase this as demand grows, without having to buy another machine.

It recognises that the potential for pump energy savings is substantial. Europump (a pan European association of pump manufacturers) estimate that systems could be 30 to 50% more energy efficient by careful consideration of components, design and installation. Indeed our research has shown that a potential energy saving of 82% is achievable between the worst operating constant flow systems and the best designed variable flow systems.

This paper examines both the potential of and barriers to grid-scale energy storage playing a substantive role in transitioning to an efficient, reliable and cost-effective power system with a high penetration of renewable energy sources. Grid-scale storage is a term that describes a number of different technologies with a wide range of characteristics. This versatility leads to the use of storage to perform a number of grid-services. We first enumerate these services, with an emphasize on those that are best suited to mitigate the effects of uncertainty and variability associated with intermittent, non-dispatchable renewable energy sources. We then provide an overview of the current methods to evaluate grid-integrated storage, summarize key findings, and highlight ongoing challenges to large-scale adoption of grid-scale energy storage. We focus on one particular area that is critical to both the efficient use of energy storage in the power grid and its long-term economic viability: the conflict between the technical benefits of this resource, which can provide both power and energy related grid-services (in some cases simultaneously), and the economic challenges of compensating these services within the current market structures. We then examine recent progress in addressing these issues through regulatory changes and other initiatives designed to mitigate previous market failures. This discussion is followed by some remarks about ongoing regulatory and market design challenges. The paper closes with a summary of the ideas presented and a discussion of critical research needs.