Step-by-step design of a Solar PV System

The recently concluded Micro Grid Academy (MGA) training saw over seventy energy experts across the African continent trained on decentralised renewables against Covid-Operation and Maintenance. One of the most impactful take-homes for me was:

 

“ Design is not about the deliverables; design is a way of thinking”

 

In this post, I will break down the steps required in designing a solar photovoltaic (PV) system. Solar photovoltaic system or Solar power system is one of renewable energy system which uses PV modules to convert sunlight into electricity. The electricity generated can be either stored or used directly, fed back into grid line or combined with one or more other electricity generators or more renewable energy source. Solar PV system is very reliable and clean source of electricity that can suit a wide range of applications such as residence, industry, agriculture, livestock, etc.

If you are still reading at this point, I just want to reiterate just how much I miss creative writing, technical writing can also be done in a creative way, but boy don’t I just miss the fiction! I may do a post soon, on the other blog, yes, the other one, remember it? End of rant, as you were…

 

Step 1: Select a location; which can be a village, community, or residence…

 

Step 2: Perform a Solar Resource Assessment, don’t worry, there are plenty of online applications that can help. For example; 

(https://www.energyaccessexplorer.org/), (https://globalsolaratlas.info/map) amongst others. 

This information is necessary to determine whether solar is a viable resource for the project.

 

Step 3: Collect the results of the Energy Access Explorer (or other Atlases). The value you’ll be looking for are as shown below; the specific PV output, direct normal irradiation etc. The example below is for Kandiege village in Homabay county. 

 

Step 4: Identify the type of Installation (Rooftop or On-Ground)

This step should not be avoided because the choice of installation determines whether you require a roof survey to determine whether the roof can withstand the weight of the solar PV modules. Site survey is also necessary to ensure there is available space at the location for ground mounted PV installations.

 

Step 5: Identify Load (per day) and plot a Load Profile and calculate the capacity of PV to be installed (PV sizing). This is usually a determination of the power consumption demands. In designing a solar PV, find out the total power and energy consumption of all loads that need to be supplied by the solar PV system as follows:

 

·      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.

·      Calculate total Watt-hours per day needed from the 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.

 

Step 6: Size the PV using the PV Module Data Sheet, this you can easily obtain online from different PV vendors. Different size of PV modules will produce different amount of power. To find out the sizing of PV module, the total peak watt produced needs. The peak watt (Wp) produced depends on size of the PV module and climate of site location. We have to consider, the panel generation factor, which is different in each site location. We’ll use 3.43for this post. To determine the sizing of PV modules, calculate as follows: 

·    Calculate the total Watt-peak rating needed for PV modules. Divide the total Watt-hours per day needed from the PV modules by 3.43 to get the total Watt-peak rating needed for the PV panels needed to operate the appliances.

·      Calculate the number of PV panels for the system. Divide the answer obtained above by the rated output Watt-peak of the PV module available to you. Increase any fractional part of result to the next highest full number and that will be the number of PV modules required.

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.

 

Step 7: Size the Inverter data using Inverter Data Sheet also available online.

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 watt 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. The inverter size should be 25-30% bigger than total Watts of appliances. In case of appliance type is motor or compressor 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.

 

Step 8: Sizing of Battery (Autonomy = 2 No Sun Days)

The battery type recommended for using in solar PV system is deep cycle battery. Deep cycle battery is specifically designed for to be discharged to low energy level and rapid 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:

·       Calculate total Watt-hours per day used by appliances.

·       Divide the total Watt-hours per day used by 0.85 for battery loss.

·       Divide the answer obtained in item above by 0.6 for depth of discharge.

·       Divide the answer obtained in item above by the nominal battery voltage.

·       Multiply the answer obtained in item above 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.

 

Battery Capacity (Ah) = (Total Watt-hours per day used by appliances x Days of autonomy)

                                                             (0.85 x 0.6 x nominal battery voltage)

Step 9: 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. Make sure that solar charge controller has enough capacity to handle the current from PV array. For the series charge controller type, the sizing of controller depends on the total PV input current which is delivered to the controller and also depends on PV panel configuration (series or parallel configuration). 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

 

Point to note: For MPPT charge controller sizing will be different. Please research more on this.

 

Step 10: System Line Diagram

This drawing is a helpful way to oversee the design and all the components fitting in together. If coupled with a proper wiring diagram, it is sufficient to guide installation works for a well-designed system.

 

Step 11: Economic Analysis CAPEX and O&M

A bill of quantities is a breakdown of all the costs required to actualize the project. Don’t forget to factor in labor costs for specialists with the necessary technical expertise. Operations and maintenance is also an important factor that includes the cost of equipment and labor to sufficiently maintain the system. It is an annual cost.

1st Photo courtesy of : UNESCO

Now go forth and renewABLEs! (Queen of bad puns 😂)