Integrated systems for UPS: New solutions in the power quality chain

Power quality solutions for critical loads have to satisfy some specific needs such as reliability, cost, environmental impact and power conditioning capability. A correct trade-off between these depends on each specific application. The most typical solution for the supply of critical loads is by means of a double conversion UPS that takes the back up energy from batteries. The main purpose of this paper is to show how new technologies already successful in other applications (such as flywheel energy storage systems and fuel cells) can become a useful alternative to existing power quality solutions for critical loads. In particular, the focus is on flywheels as a partial or complete replacement for batteries as a backup power in UPS. The flywheel itself is a very old technology that allows energy to be stored mechanically in the form of a rotating mass. Indeed, the flywheel has been used in the power quality industry for the last few decades in rotary UPS and as a replacement for batteries, with heavy and slow rotating masses supported by ceramic bearings. New technologically advanced materials and magnetic bearings are making a wider Held of application possible thanks to the higher rotational speeds which can be achieved, and hence, lower weight, increased efficiency and higher energy density. This paper will focus on the empirical results derived from the use of a new fully levitated flywheel, using magnetic bearings, with a core rotating mass that uses aerospace grade high strength steel providing 140 kW of power for 15 seconds. The aim of this work is to give an indication of the field of application for a power quality solution with flywheels. This result will be contrasted with other rotating or other completely static solutions used to supply a power solution to the critical load. In particular, two main aspects of the UPS and flywheel solution will be covered. In the first case we will look at the use of the flywheel in parallel to a battery string to solve the problem of the frequent short mains power problems. In the second case we will discuss a flywheel solution used as a standalone DC back up energy source for systems that use a diesel generator as a source of energy for longer periods of time. The latter case will include a review of the precautions to be considered when a generator set has to be relied on for longer periods of time. In both cases, there will be a review of the overall system in terms of reliability, availability of the load, back-up time, footprint, cost and environmental impact. Also presented is a study on the use of fuel cells in a UPS application including a review of the steps that still are needed for a complete integration of the fuel cells in a UPS application.