Title: Power Sources
Course: 2016FA-MCO-110-401 > Section: Module 3: Power

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Power Sources

Utilities

A utility is a business (or other entity) that supplies a service such as water or power.

Power that comes from a utility must be conditioned and backed up. In some high uptime applications more than one utility will be contracted, with one utility serving as the primary source of power and others serving as back-ups.

Generators

A generator in simple terms is a machine that turns mechanical energy into electrical energy. By definition a generator could be something as simple as a hand crank used to supply a small amount of power or it could be a very large generator capable of powering an entire building. The term genset is sometimes used to refer to a diesel generator or other engine generator and stands for generator set [5].

Generators can be powered by gasoline, diesel, natural gas, and liquid propane. Natural gas has a limitation that prevents it from use in many high uptime environments. The natural gas must remain pressurized for use and to do this electricity is required, so it is not a viable option to use for energy in case of utility loss.

Generator Ratings

Generators have different ratings. Prime, standby, and continuous are the three main generator ratings. A prime rated generator is capable of being used as a primary source of power; it is typically needed for a limited number of hours. A continuous rated generator is what is used in cases where the power grid is unreachable or cost prohibitive; it is typically needed 100% of the time. Standby rated generators are meant to work for several hours at a time to be used in case of a temporary power disruption from the primary supplier (utility). Sometimes the actual equipment has few differences, while the big difference in generator ratings lies with their warranty coverage. A generator rated for standby is covered for fewer hours of use per year (annual hours) than one that is rated for prime [6]. When differences are present they consist of better cooling capabilities and upgraded components in generators rated for continuous use than for those rated standby [6].

Additional considerations that must be planned for and dealt with when using generators include ease of fuel delivery and how it will be delivered, the space required to allow for enough air flow around a generator, fuel storage, fuel treatment, and generator maintenance. Each facility will possess differences in climate, location, available utilities, and other variables that will play a part in the choice of generator.

Generator Components

The parts of a generator will vary in size, appearance, and configuration from one vendor to another and depending on the size, model, and features of the specific generator. Most commonly a genset will consist of an engine, air filter, control panel, radiator, AC generator, and battery charger on a skid or a bed. Generator components can include remote radiator that is designed to dissipate heat taking it away from the generator’s engine for purposes of temperature control, space, or other design requirements.

UPS

A UPS is an uninterruptible power supply or uninterruptible power source. It is a device that when utility power fails, takes over providing back-up power for long enough that a generator can be brought online [7]. “UPS systems can regulate, filter, and otherwise condition the utility supply and seamlessly switch a critical load to a backup power in the event of a subcycle power interruption. They also reduce or eliminate power quality problems caused by voltage sags, surges, or high-speed transients” [3, p. 228].

For safety the UPS system should be equipped with Emergency Power Off (EPO). An EPO is designed to power down equipment, systems, or an entire facility in case of emergencies like fire, flood, or equipment overheating [8].

Types of UPS Configurations

The configuration or topology of a UPS is how the system is set up, how the components interact, and how components are arranged. There are three main types or topologies of UPSs: single conversion systems, double conversion systems, and multi-mode systems [7]. A newer UPS topology the delta conversion system is an updated version of the double conversion system. Some of the most commonly found topologies are explained with more detail below.

A single conversion system senses when power from the utility falls outside of the preset desired parameters, then uses an inverter to draw current from its battery, and it disconnects from the AC supply. (This disconnect is important to prevent back feed from the inverter possibly reaching the utility.) The system has converted usage from AC to DC power. The UPS continues to run on battery power until it senses that AC input has returned to desired levels or until the battery runs out of power. Two popular versions of the single conversion system are line interactive and standby. A line interactive UPS regulates input voltage (up or down as needed) before it lets voltage reach protected equipment. Both line interactive and standby UPSs use their battery to protect against frequency abnormalities.

Standby UPSs monitor utility power, let equipment run off of utility until a problem is detected, and can have the added ability to condition the power [7]. This system is called a standby system since in normal operation the UPS’s battery and inverter are not supplying power to any equipment, instead they are in a standby mode waiting until they are needed. Standby UPSs are sometimes referred to as offline UPSs or standby power supplies or SPSs [8]. The biggest advantage to this topology is the cost, a standby UPS costs less than other configurations. However, the biggest drawback would be that this type of configuration takes longer to bring online when needed. The primary power source in the standby system is the utility and secondary is the battery. The power supplied to equipment is not regulated or conditioned and therefore this topology is not recommended for use in most mission critical environments.

A double conversion system turns the utility’s supplied AC power into DC with an input rectifier. Then it uses an output inverter to change the DC power back to AC before sending it out to power equipment. If the utility power drops out of preferred limits, the input rectifier turns off and then power from the batteries are used to supply the equipment. This double conversion of power is used to isolate or put a barrier between the raw power from the utility and the equipment. This helps to supply clean power to equipment. Double conversion systems are sometime referred to as double conversion online UPSs, true UPSs, or online UPSs [8]. The biggest advantages of these systems are the isolation of protected equipment and the lack of interruption in a utility power failure, there is no wait time to switch over to battery since power always passes from the battery to the equipment with this setup [7].

A multi-mode system takes features from the single conversion and feature from the double conversion systems and blends them together. These systems operate in line interactive mode as long as the preferred parameters are met and normal operation is possible. If AC input power falls outside the predetermined boundaries however, the system changes over to double conversion mode in order to protect the equipment from abnormalities. Then, if AC input is determined to go beyond the double conversion rectifier tolerances (or if power is lost), the UPS uses battery power to sustain until a generator is brought online - at which time double conversion mode is resumed until utility power becomes stable. Once utility power is back to “normal” (predetermined parameters) the line interactive mode can be resumed. The multi-mode systems are effective and reliable [7].

A delta conversion system, created to be more energy efficient, got its name from the term scientific term delta. Delta means the difference or change in something. Identical to the double conversion system, the delta also uses an inverter to supply power to the critical load (equipment). However the delta system has an additional delta converter that can add power to the inverter output [9] and has bidirectional power flow capabilities. The delta conversion system offers the benefit of being able to completely isolate the critical load plus added benefits from the delta converter of being able to balance power flow [10]. Delta conversion systems are approximately 5% more energy efficient to operate than a double conversion system and cost less to operate since less air conditioning is required (less heat is produced) [3].

A rotary UPS system, or flywheel UPS system, uses the inertia of a rotating flywheel to correct short dips in power or short outages (power conditioning). It uses an electric motor and a generator instead of a rectifier or inverter that may be found in many non-rotary systems. (The motor and generator in this setup may be referred to as an M-G set by some manufacturers.) Usually the rotary UPS system does not use batteries and support time can be limited. Multiple instances of rotary UPS systems can be implemented in a series to make up for the limited support time or a rotary UPS system can be used in conjunction with a typical UPS battery topology in order to extend battery life [11]. These systems are heavy and expensive but are not costly to maintain.

A diesel rotary UPS (DRUPS) is the name given to a rotary UPS system that uses a diesel generator backup to be able to provide continuous and uninterruptible power.

Alternative Power Sources

Cogeneration or Co-generation

Cogeneration, also known as CHP (combined heat and power) is the combining of the production of heat and power. Generally in cogeneration one fuel source can be utilized for both heat and power. This process often includes the collection of heat from an engine or other heat source that might otherwise be wasted [18]. In many Mission Critical environments CHP is used an alternative power source. There are many benefits to cogeneration including lower energy bills, lowered emissions (better for the environment), and increased efficiency. Cogeneration can be employed with various systems, the most common in mission critical operations include the fuel cell system, in which heat produced from the chemical reaction involved is used for heating purposes, or boiler or steam turbines and combustion turbines where natural gas or propane is used to produce power while simultaneously creating potentially useful exhaust heat. Heat that would normally be an unused byproduct, can either be used to produce power or to do a number of other things using heat (including heating the facility itself) [13].

Fuel Cell

Like a battery, a fuel cell uses a chemical reaction to produce electricity. A fuel cell needs a constant supply of oxygen (or something that provides oxygen to the reaction) and fuel (usually hydrogen) to keep the chemical reaction going. A fuel cell never needs to be recharged like a battery nor does it run down [14].

Fuel cells come in different types, for example: PEM (polymer electrolyte membrane) fuel cells, Hydrogen-Oxygen fuel cells, AFCs (alkaline fuel cells), and PAFCs (phosphoric acid fuel cells).

Fuel cells can be placed in a series and this is often referred to as a fuel cell stack. Fuel cells are useful in situations where the use of a diesel generator is not feasible, when the battery need would require batteries that were too big or too expensive, or when environmental regulations would favor the fuel cell’s sustainability.

Advantages to the fuel cell include its lack of carbon emissions and the fact that the only by-products from the fuel cell’s chemical reaction are heat and water. There are some drawbacks to fuel cells including cost, durability, and performance. Continued work in the field of fuel cells may serve to remove some concerns in the future [15].

Objectives met:
- C3
- 3.2
- 3.3