RatihChubby

In today’s ‘energy-challenged’ business environments where demand is beginning to outweigh supply and costs are rising ever higher, heads of business are looking for alternative sources of standby back-up power protection to reinforce uninterruptible power supplies (UPS) and provide a continuous source of electricity during mains power failures.

The primary criteria in deciding which alternative to choose are usually: space constraints, location, cost, load types and application. Diesel generators, in particular, require dedicated space away from protected loads and primary work areas, plus room for fuel storage. They are often housed in purpose-built enclosures for which planning permission must be granted and sometimes this can be a challenge in primary urban areas where building and planning restrictions may exist. The issue of cost can often preclude up-coming alternatives like fuel cells, which can overcome some of the difficulties associated with diesel generators but at a price. Load types, too, can be a challenge in terms of how they draw voltage waveform and its effect on standby power generation equipment. However, the following are alternatives that are commercially available:

Diesel Generators: a generator is a mechanical device capable of providing a source of ac electrical power on-demand or as an alternative to mains power supply. In an uninterruptible power supply scenario a generator is used as a standby source of three or single-phase ac power to supplement the runtime provided by the UPS battery set or alternative source of dc power. It may also directly power essential loads such as alarms, air-conditioning or emergency lighting in the event of a power cut.

A generator typically takes up to a minute before it can deliver a suitable supply on start-up, which is why an uninterruptible power supply sits between it and the source of mains power.

Generators burn fuel (diesel being the most common and cost-effective) but they emit heat, noise and exhaust gasses. Generator fuel is usually stored in what is referred to as a ‘day tank’, located on-site, which typically contains eight-hours of supply. This can be supplemented by bulk tanks, also located on-site. Fuel storage on-site introduces health & safety and environmental regulations that must be observed. Some experimentation has been done using bio-fuels but the problem is not so much in usage as in storage. Because they are comprised of organic matter bio-fuels cannot be stored.

Fuel Cells: are increasing in popularity across a number of applications due to their high-efficiency and purported ‘green’ credentials i.e.: zero exhaust emissions. A fuel cell uses a chemical reaction, which occurs when Hydrogen and Oxygen combine to produce water. It harnesses this reaction to produce electrical energy. Fuel cells have two potential UPS applications: an alternative source of ac power to mains or as support to an existing battery set.

UPS Batteries: today’s UPS batteries are typically sealed lead acid and maintenance-free with a design life of five years. As they rely on a chemical process they are prone to deterioration and the speed at which this happens is dramatically affected by room temperature and number of charge/discharge cycles the battery has to perform during its lifetime.

A ‘battery block’ is a self-contained electro-chemical device that can store electrical energy for later use. A UPS battery set comprises a number of battery blocks connected in series into a ‘string’, or number of battery strings.

Flywheels: are energy storage devices based on the principle of Kinetic energy, whereby a motor-generator is used to rotate a mass at high RPM in a vacuum on frictionless bearings inside a floor-mounted cabinet. They provide a source of short-duration dc power and can be used with large application UPS, around 60kVA upwards. As energy is discharged to the load on demand, flywheels can be used in place of a battery set to ride through the time required for generator start-up.

Gas Turbines: a descendent of the Jet engine, like generators, Gas Turbines are a combustion device with similar installation and operational issues. Advances in technology have lead to the development of commercially viable energy-saving applications and the devices are finding useful roles within power protection projects. There are two designs: single shaft (which has a turbine and compressor on the same shaft and output of which is speed controlled by the fuel supply) and two-shaft (which has a higher operating efficiency but poor response to load change). A micro-turbine operates at even higher speed, is compact in design, lighter weight and can run on a variety of fuels.

Super Capacitors: store electrical energy, which can be released on discharge. For small uninterruptible power supplies this can be used in place of a battery set or to reduce the potential for battery discharge during momentary mains power supply failures. The amount of energy that can be stored depends upon the active material used and the capacitor design.

The energy within a capacitor is quickly available. They can extend the working life of a UPS battery set (if used in conjunction) by inhibiting battery cycling for momentary or small interruptions (the majority of power interruptions last for less than five seconds). Super capacitors are also happy to operate in temperatures ranging from -30-45 degrees centigrade but they are pricey in comparison to UPS batteries.

Whilst generators remain the most popular source of standby power in uninterruptible power supply applications, there are alternatives, which may be more relevant and suitable in certain instances. Where planning permission cannot be granted for external generator housing, for example, a fuel cell may be an ideal alternative. Flywheels may also be a viable option as an alternative power protection energy source for larger uninterruptible power supplies.