AN INVERTER BACK-UP POWER SYSTEM

A guide to an ideal system sizing that guarantees optimal power supply

Power storage, no doubt has been one of the major problems in the electricity industry. Recent technological developments have significantly reduced this problem as there are some availablepocket-friendly back-up systems that can meet our basic energy requirements.

Generally, a backup power system is used to provide energy when the primary source fails. This system is very important since an uninterruptible power supply is crucial for any operation. The current backup systems include batteries and generators, which operate on diesel, propane, or gasoline. Although these systems are well-established, the cost of operation of generators and its environmental concerns are encouraging the users to seek for sustainable alternative technologies that can provide higher reliability and durability at a rational cost.

So there is no surprise that demand for these system is outstripping equipment supply as well as availability of qualified services providers. The major components of an inverter back-up power system are batteries, inverter/charger and connection accessories. When the supply from the main source is available, inverter/charger supplies power to the home as well as charges the batteries. Upon outage of power, the charged batteries are used for powering the home or office appliances through the inverter.

Functions of different components AC-DC-AC inverter/charger:A typical back up system provides AC power without interruption to keep your critical equipment running during a power outage. Built with an automatic transfer switch, the inverter / charger unit switches to battery power during a blackout. When the main source power is restored it automatically recharges the battery. Inverters come in different KVAs and DC voltages. It is ideal for powering common loads such as refrigerators, computers, printers, lights, small televisions, radios and fans.

In sizing an inverter, the total loads to be connected to the inverter is calculated by noting the individual power consumption of each load. These individual power consumption (in kilowatts, Kw) are summed up and converted to KiloVolts Ampere, KVA by dividing with the applicable power factor value; 0.8 is often used. An extra value is added to this sum to accommodate for losses. Eg if the sum of KVA calculated is 2.9KVA, an inverter size of 3.5KVA is better chosen.

As regards to the inverter DC input voltage, different manufacturers have their own inverter built in different DC input voltage. Generally, most 1KVA-1.2KVA inverters come with a 12V DC input voltage while 1.5KVA to 3.5/5KVA inverters come with a 24V DC input voltage except for some Indian brands. Inverters from 5KVA -10KVA usually come with a 48v dc input voltage except for some Indian makes. It is preferable to utilize higher inverter DC input voltage and have your batteries connected in series to achieve the system voltage. This has a better advantage in the sense that the cross sectional area of cable required will be low, thereby reduces cable cost.

It is also of importance to mention that there is pure sine wave and modified sine wave inverters. The pure sine wave inverter is preferable if one is using sensitive loads like electronics – TV, computers while the modified sine wave can still serve if one is using non-sensitive loads like water pump machine, etc. The major difference between the two is the nature of the output current/voltage. While pure sine wave inverter produces a uniform sinusoidal wave form that are free from noise, the modified sine wave produces a block-type wave form and is accompanied with noise. The pure sine wave is also more expensive than the modified sine wave, but in the long run, it is far better to go for pure sine wave inverter.

Batteries:Reservoirs of charge current that is being produced when there is available supply from the main source, batteries are connected in combination of series or parallel to match the system voltage (i.e Inverter DC input voltage). Batteries come in different Ampere-hour (Ah) from 100Ah to 2000Ah and different voltages ranging from 2volts to 12volts (v). There are different types of batteries and they are classified based on the content of the electrolyte. Generally, we have deep cycle batteries (lead acid batteries) and non-lead acid batteries. The lead acid batteries can be Flooded Lead Acid battery, Sealed Absorbed Glass Mat (AGM) Battery, or Gelled Electrolyte (Gel) Battery.

The flooded Lead Acid battery is also called Wet cells battery. Flooded Lead Acid batteries are very cheap and affordable. The downside is that they are not maintenance free. The user will have to regularly remove the caps and top the acid with distilled water when low.

In Absorbed Glass Mat (AGM) Batteries, the electrolyte is held in glass mats or matted glass fibers which in turn is wrapped around the positive electrode or plate. It is maintenance free type of battery and also has a low internal resistance hence charge faster than other batteries. AGM batteries are more tolerant to temperature variations and can resist vibration unlike others.

Gelled Electrolyte Sealed Lead Acid Batteryis a lot like the Flooded Lead Acid battery except that the liquid Lead-Acid electrolyte solution is ‘thickened’ into gel form with silica. So it makes it a dry cell although it is technically not.

The introduced silica only serves as a thickening agent and doesn’t actively participate in the electrolyte reactions. This kind of battery is Valve-regulated. These valves are the tiny outlets for the escape of gases produced during the use of the battery.

This type of battery actually have a superior deep discharge resiliency and tend to have a greater number of life cycles.

Non-lead acid batteries include Lithium ion (Li-ion) batteries, Nickel-Cadmium batteries, etc. We are not going to be talking about these types of batteries because of their high cost of acquisition for a renewable energy system application.

Each of the battery type described has different lifecycle (ranges from 500 to 1500) which also vary from one manufacturer to the other.

The rule of thumb in sizing your batteries is to first consider and assign some percentage to losses, say 15%. Once that is done, the battery parameters can be convereted to watts-hour by multiplying the voltage with the current into the total number of batteries needed to achieve a system voltage. E.g if 3KVA, 24V Dc Inverter is recommended, the system voltage will be the 24v DC.

If the battery choice is to be 12v, 200Ah battery, The total watt-hour will be 2nos batteries x 12v x 200Ah, ie. 2 x 12 x 200 = 4800Watt-Hour

If we take out 15% of this value to accommodate for losses, we will have 4,080Watt-Hour left. To determine how long this battery will take to completely discharge if fully charged, the value of watt hour calculated above will be divided by the total load the battery powers.

E.g if the total connected load to the battery is to be 500Watts, Back up time = 4,080 Watt-Hour/500Watts = 8.016hours. {Kindly note that this value applies only when there is no power supply to the battery and when it is fully charge before powering the loads)

It is not advisable for battery to discharge completely (to reach 100% Depth Of Discharge, DOD) as many batteries do not have many lifecycle at 100% DOD, because of this, it is advised that batteries are set at a maximum of 90% DOD.