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

Click here for a downloadable version of this document:(EXTERNAL) Energy Storage

Energy Storage Used in UPSs and Elsewhere

More than Just UPSs

Alternative energy sources (like wind or solar) are sometimes used in data center or other mission critical operations but they have an intermittent nature and therefore fall short as a primary source unless they are used in conjunction with other systems. A big drawback of alternative energies is that because of their nature, they require a lot of energy storage. Since wind is not constant and the sun does not shine 24 hours a day, a method of storing energy for use in these downtimes in necessary. Batteries for this scale of storage are expensive [12]. Just like the UPS systems are not the only reason energy storage is needed, traditional batteries are not the only form of energy storage.

Battery

A battery is a storage medium for power. Power that is to be used at a later time can be stored in batteries to be used when it is needed. A battery commonly contains chemicals that over a period of time undergo a chemical reaction that causes electricity to flow. Batteries have a limited lifespan which varies with battery type, usage, and maintenance.

Battery systems can be made to last just long enough to provide power through a typical short power loss or can be designed to last for hours in applications where that is required [3]. In order to provide enough DC power to supply an inverter in a UPS system, battery cells are connected in series (battery string).

Batteries have maintenance requirements and some have safety concerns that must be addressed (fire hazard or acid leaks for example). The use of batteries is subject to IFC, OSHA, NFP, local fire code, and other codes that affect how they are installed and maintained.

Types of Batteries

Lithium batteries last the longest of the current popular battery forms. One of the properties of lithium is that if a charge is applied to it, it will hold that charge, become positively charged, stay positively charged until something negatively charges is put beside it allowing the charge to flow. Lithium batteries can are made in a number of forms like Li-Ion (Lithium-Ion) and Lithium-Phosphate (LiFePO4). Lithium batteries require very little maintenance. Li-Ion batteries can weigh two to three times less than its lead-acid battery counterpart which makes them an improvement when weight is a consideration. When used with a UPS system, the Li-Ion battery requires a BMS (Battery Management System). A BMS is basically a circuit designed to prevent over current protection and to be sure that the batteries properly recharge. Lithium-Phosphate batteries do not always require a BMS and can often more simply replace a lead acid battery [13].

Lead Acid batteries are commonly found in mission critical facilities and are the most commonly used battery to power UPS inverters.

The two main types of lead acid battery are the flooded wet cell and the valve regulated lead acid (VRLA) battery:

- Flooded Wet Cell batteries are called flooded because they contain electrolyte in a liquid form. These batteries continuously vent hydrogen gas and require special ventilation. Since they contain liquid electrolyte, they require special spill containment systems to be used [14].
- Valve regulated lead acid batteries (VRLA) are designed to emit only limited amounts of hydrogen and require no special ventilation. VRLA batteries contain electrolyte, but not in a completely liquid form like in flooded cell batteries, therefore they do not require spill containment systems. (Other common names for this type of battery include: sealed battery or maintenance free battery [3].)

NiCad batteries require more maintenance than lead acid batteries, but have the ability to survive overcharge situations without damage for much longer than other batteries. NiCad batteries are made up of nickel hydroxide (positive electrode) and cadmium (negative electrode) which, like flooded wet cell batteries, are in an electrolyte solution. Fully discharging a NiCad battery before storing it does not cause harm or damage to the battery like it would to other battery types. The long shelf life, ability to tolerate high temperatures, and general hardiness of NiCad batteries are the best qualities of NiCad batteries. They have some weak points including their need to be watered, the development of battery memory, and difficulty charging and maintain a full charge (which could affect the ease with which a system could come back online after an event) [15].

Super Capacitor

Super capacitors are also known as double layer capacitors or supercaps [20].  A super capacitor is an electrochemical capacitor with a high capacity that can be used in energy storage. A super capacitor consists of two electrodes separated by a porous membrane that allows electron flow and an electrolyte throughout.   A super capacitor can charge and discharge faster than a battery and has the capability for higher energy capacity. (This technology is sometimes used with different types of electrodes and then may be referred to as hybrid capacitors.)

Objectives met:
- C3
- 3.5