Considering purchasing solar PV equipment? Use these guidelines.
Are you considering purchasing solar PV equipment? Follow the guidelines below to ensure you derive maximum benefits from your system.
1.Determine power consumption demands
The first step in designing a solar PV system is to calculate the total power and energy consumption of all loads that need to be supplied by the solar PV system as follows:
1.1 Calculate total Watt-hours per day for each appliance used.
Add the Watt-hours needed for all appliances together to get the total Watt-hours per day, which must be delivered, to the appliances.
1.2 Calculate total Watt-hours per day needed from the solar PV modules.
Multiply the total appliances Watt-hours per day times 1.3 (the energy lost in the system) to get the total Watt-hours per day which must be provided by the panels.
Total appliance use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 24 x 0.5 hours)
= 1,092 Wh/day
Total PV panels energy needed = 1,092 x 1.3
= 1,419.6 Wh/day.
2.Size the solar PV modules
Different sizes of solar PV modules will produce different amount of power, therefore there is need to determine peak watt (Wp) that can be produced. Given that peak watt (Wp) produced depends on the size of the PV module and climate of site location, there is need to determine peak sun hours (the average daily amount of solar energy received on a site). Note that in Zimbabwe, the worst-case peak sun hours in winter are usually 4.5 on average. To determine the sizing of PV modules, calculate as follows:
2.1 Calculate the total Watt-peak rating needed for solar PV modules
Divide the total Watt-hours per day needed from the PV modules (from item 1.2) by 4.5 to get the total Watt-peak rating needed for the PV panels required to operate the appliances.
2.2 Calculate the number of solar PV panels for the system
Divide the answer obtained in item 2.1 by the rated output Watt-peak of the PV modules available to you. Round up any fractional part of the result to the next highest full number and that will be the number of PV modules required.
The result of the calculation is the minimum number of PV panels. If more PV modules are installed, the system will perform better and battery life will be improved. If fewer PV modules are used, the system may not work at all during cloudy periods and battery life will be shortened.
2.1 Total Wp of PV panel capacity needed = 1,419.6 / 4.5
= 315.47 Wp
2.2 Number of PV panels needed = 315.47 / 110
= 2.87 modules
Actual requirement = 3 modules
So this system should be powered by at least 3 modules of 110 Wp PV module.
An inverter is used in the system where AC power output is needed. The input rating of the inverter should never be lower than the total wattage of appliances. The inverter must have the same nominal voltage as your battery.
For stand-alone systems, the inverter must be large enough to handle the total amount of Watts you will be using at one time. In addition, the inverter size should be 25-30% bigger than total Watts of appliances. Note that if the appliance type has a motor or compressor, the then inverter size should be minimum 3 times the capacity of those appliances and must be added to the inverter capacity to handle surge current during starting.
For grid tie systems or grid connected systems, the input rating of the inverter should be same as PV array rating to allow for safe and efficient operation.
Total wattage of all appliances = 18 + 60 + 75 = 153 W
For safety, the inverter should be considered 25-30% bigger size.
The inverter size should be about 190 W or greater.
The battery type recommended for using in solar PV system is deep cycle battery. Deep cycle battery is specifically designed to be discharged to low energy level and rapidly recharged or cycle charged and discharged day after day for years. The battery should be large enough to store sufficient energy to operate the appliances at night and cloudy days. To find out the size of battery, calculate as follows:
4.1 Calculate total Watt-hours per day used by appliances.
4.2 Divide the total Watt-hours per day used by 0.85 for battery loss.
4.3 Divide the answer obtained in item 4.2 by 0.5 for depth of discharge.
4.4 Divide the answer obtained in item 4.3 by the nominal battery voltage.
4.5 Multiply the answer obtained in item 4.4 with days of autonomy (the number of days that you
need the system to operate when there is no power produced by PV panels) to get the required
Ampere-hour capacity of deep-cycle battery. In our case, 3 days are recommended.
Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of autonomy
(0.85 x 0.5 x nominal battery voltage)
Total appliances use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)
Nominal battery voltage = 12 V
Days of autonomy = 3 days
Battery capacity = [(18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)] x 3
(0.85 x 0.5 x 12)
Total Ampere-hours required 642.35 Ah
Therefore, the battery should be rated 12V, 650 Ah for 3-day autonomy.
5.Solar charge controller sizing
The solar charge controller is typically rated against Amperage and Voltage capacities. Select the solar charge controller to match the voltage of PV array and batteries and then identify which type of solar charge controller is right for your application. Furthermore, ensure that the solar charge controller has enough capacity to handle the current from the PV array.
According to standard practice, the sizing of solar charge controller is to take the short circuit current (Isc) of the PV array, and multiply it by 1.3
Solar charge controller rating = Total short circuit current of PV array x 1.3
Example: A house has the following electrical appliance usage:
One 18 Watt fluorescent lamp with electronic ballast used 4 hours per day.
One 60 Watt fan used for 2 hours per day.
One 75 Watt refrigerator that runs 24 hours per day with compressor run 12 hours and off 12 hours.
PV module specification
Pm = 110 Wp
Vm = 16.7 Vdc
Im = 6.6 A
Voc = 20.7 V
Isc = 7.5 A
Solar charge controller rating = (3 strings x 7.5 A) x 1.3 = 29.3 A
So the solar charge controller should be rated 30 A at 12 V or greater.
The solar PV system will be powered by 12 Vdc, 110 Wp PV module.