15/9/2017 0 Comments
The use of off-grid energy supply is becoming increasingly popular for domestic, industrial and municipal applications. Due to the variable nature of renewable energy sources, many of the installations include an energy storage system to enable supply for peak demands and when energy generation is limited. There are alternative storage technologies but the method of calculating the size of the battery required is common to all chemistries. To ensure a system that satisfies the usage requirements it is necessary to obtain a reasonably detailed picture of the loading and run time autonomy of the battery. Allowances must be made for the efficiency of the components in the system in converting energy from the input source to the demand on the battery. For this, the size of individual load, the total load and the individual run times are crucial factors in calculating an accurate battery capacity for the system requirement.
Whether as the sole source of electricity or as a hybrid fuel supply, the characteristics of the equipment and the application need to be thoroughly understood to design and specify an effective and trouble-free installation. To provide electricity during the night, either all or in part, from a PV array requires storage of electrical energy. A meticulous approach to calculating the autonomy load will also ensure that the battery selection will be accurate. A correct battery specification will ensure not only a satisfactory autonomy but also a long and cost-effective battery life. The following is a guide to obtaining the detailed and correct information necessary to calculate the size of battery required to maximise its performance, energy efficiency and cost effectiveness.
Summary of method
This section provides an understanding of the overall method to provide an explanation of the methodology used to obtain the data. The detailed calculation and methods of obtaining loads and efficiencies are given in the operations section.
This is designated as H
Generally, there is more than one load from various devices and these loads may not be operating continuously. For these individual loads there will be individual autonomies. These will be listed separately as load 1, 2, 3 etc., with corresponding times in operation, i.e. corresponding autonomies.
These individual autonomies are designated as h1, h2. h3 etc.
If the load is obtained by measurement then this step is unnecessary and the measured value can be used directly.
The load and average load can be calculated by taking the sum of the individual loads or the maximum load measured (Lt) then dividing by the number of hours for the battery operation (H) to give the average load (La).
A more accurate method is to look at the individual loads and their time of operation. To calculate the total watt hours required, the loads are multiplied by their operating time.
PV array ------------> DC:DC -------------> Inverter --------------> Load
Output from the solar panels x DC converter efficiency (EDC) x Inverter Efficiency (EI)= total available output.
With energy storage, the efficiency of the battery charger, the efficiency of the battery chemistry on discharge and charge must also be considered. The voltage loss through the cables is another factor to be added in for calculating the battery output requirement.
The peak loads and their occurrence during the discharge period are important as this will cause a voltage drop. The battery should be sized to prevent this drop, including the voltage losses in the system, from falling below the required operating voltage for the loads or inverter.
The battery capacity will vary with temperature. The lower the temperature then the lower the capacity. The life of the battery will also depend upon the battery operating temperature: generally the higher the temperature the shorter the battery life. This information regarding capacity and life will be provided by the Microtex technical team.
Total watts including inefficiencies taken out of the battery = total watts including inefficiencies put into the battery.
Two more factors are the ambient temperature and the depth of discharge and recharge to provide the required cycle life and recharge time for the operation of the battery. The amount of battery capacity used can be expressed as a fraction e.g. Minimum SOC = 20% and maximum SOC = 95% the capacity fraction is 75% or 0.75. The operating temperature will provide the compensation for capacity and the DOD and %SOC will determine the battery size so that:
Battery size = (total watts out/capacity fraction) x temperature compensation
This will give the correct battery size with no margin for error. It is recommended that there is a contingency of +5% added to this final value to ensure a trouble-free operation.
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