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Solar Panels – Everything You Need to Know
Designing a solar system is a complex task that involves various challenges, such as accurately determining the energy demand, selecting appropriate components, choosing the right solar panels, and optimising the system for efficiency and affordability. Getting the design right is crucial for the system's performance, safety, and economic viability.
If you’ve been wondering where to start with solar panels for your home or business, This article will guide you through the essential steps and considerations of how to design a solar PV system.
What is a solar PV system?
A solar photovoltaic system (PV system), or solar power system, is a renewable energy system that uses PV modules to convert sunlight into electricity. The electricity generated can be either stored or used directly, fed back into the grid line, or combined with one or more other electricity generators or renewable energy sources.
It’s a reliable and clean source of electricity that can suit a wide range of applications such as residence, industry, agriculture, livestock, etc.
What are system components?
The first thing to know when answering “What solar panel system do I need?” is learning about the individual components that constitute it. A solar PV system consists of several components, each serving a specific function to harness solar energy, convert it into usable electrical power, and deliver it for use or storage.
Understanding the different solar system components and their roles is essential when designing one for your unique needs. It helps in selecting the appropriate equipment, optimising the solar system for maximum efficiency, and ensuring its smooth operation.
Solar panels
Solar panels, or photovoltaic modules, are the primary components that convert sunlight into direct current (DC) electricity. Each panel consists of multiple solar cells made of semiconductor materials, usually silicon, that generate electric charges when exposed to sunlight.
Critical considerations for solar panels include their efficiency, capacity (measured in watts peak, Wp), and the panel generation factor (actual electrical output vs. potential output), which varies by location. New design solar panels can reach efficiencies of 23%.
Solar charge controller
The solar charge controller regulates the voltage and current coming from the solar panels to the battery and prevents overcharging, reverse current flow, and battery drainage, thus prolonging the battery life and ensuring the safety and efficiency of the solar system.
Critical considerations for the solar charge controller include its capacity (measured in amperes), type (PWM or MPPT), and compatibility with the PV array and battery voltage.
Inverter
The inverter converts the DC output of the PV panels or battery into alternating current (AC) suitable for AC appliances or feeding back into the grid. It’s a vital component as most household appliances and the grid operate on AC power.
Critical considerations for the inverter include its capacity (measured in watts), efficiency, and compatibility with the system voltage and grid requirements.
Battery
The battery stores energy generated by the solar panels for use when there is a demand, such as at night or during cloudy periods. Deep cycle batteries are recommended for solar PV systems as they are designed for repeated charge and discharge cycles. Depending on your exact location, batteries could be a vital component of your solar system in countries like the UK with inclement weather.
Critical considerations for the battery include its capacity (measured in ampere-hours, Ah), voltage, depth of discharge, efficiency, cycle life and maximal charging discharging power, and days of autonomy (the number of days the system can operate without solar power).
Auxiliary energy sources
Auxiliary energy sources, such as diesel generators or other renewable energy sources, provide backup power when the solar PV system cannot meet the demand. They ensure continuous power supply during extended cloudy periods or peak demand times.
Critical considerations for auxiliary energy sources include their capacity, fuel efficiency, and integration with the solar PV system.
Determine the energy demand
To design an efficient and effective solar PV system, it’s essential to accurately determine the energy demand that will be required.
Simply, this involves calculating the total power and energy consumption of all the loads that need to be supplied by the solar PV system. However, it may be more complex than you expect at first. The following will provide you with a complete breakdown of what to consider:
1. Calculate Total Watt-hours per Day for Each Appliance Used:
List all the appliances and devices that will be powered by the solar PV system and determine their total power consumption per day in watts. To be safe, you should design your system with the worst-case scenario, i.e., when demand will be at its highest.
This will ensure you’ll never be stranded without enough power for your needs.
Multiply the power consumption rating of each appliance (power rating in W) by its daily usage hours to get the watt-hours per day for each appliance. Add the watt-hours needed for all appliances together to get the total watt-hours per day, which must be delivered to the appliances.
Watt-hours per appliance (Wh) = power rating (W) × hours (h)
Total Energy Demand Watt-hour = Sum Wh for all appliances
2. Calculate the total watt-hours per day needed from the PV modules:
To be safe, you need to compensate for the potential energy loss in the system due to minor inefficiencies or environmental conditions. Typically, a factor of 1.3 is used as a rule of thumb. So, multiply the total appliance watt-hours by this amount to get the total energy required from the PV system.
In reality, this factor is determined by the efficiency of your panels, temperature, shading, dust & debris, conversion/transmission losses, and battery charging/discharging losses.
For example, if a household has the following appliances:
The total energy demand watt-hour would be calculated as follows:
Total Energy Demand Watt-hour = (10W * 5h) + (150W * 24h) + (100W * 4h) + (50W * 6h) = 50Wh + 3600Wh + 400Wh + 300Wh = 4350Wh or 4.35kWh
Therefore, the daily energy demand for this household is 4.35kWh. To account for energy loss in the system, multiply by 1.3:
Total Watt-hours per Day Needed from PV Modules = 4.35kWh * 1.3 = 5.655kWh
Inverter sizing
It's essential to select an inverter with a power rating that suits the system's needs, typically being 25-30% larger than the total wattage of appliances and three times the capacity for motors or compressors.
Different types of inverters include on-grid, off-grid, and microinverters, each suitable for different applications. For grid-tied systems, the inverter's input rating should match the PV array rating. Advanced 'smart inverters' allow two-way communication between the inverter and the electrical utility, which can help balance supply and demand, reduce costs, and ensure grid stability.
Lastly, it's essential to include proper protection mechanisms, such as fuses, RCDs (Residual Current Devices), and MCBs (Miniature Circuit Breakers), to ensure the system's safety.
Controller sizing
To select an appropriate charge controller, you must consider the total wattage of your solar power system and the battery bank voltage. For this, we can use the formula Amps x Volts = Watts, derived from Ohm's law.
For instance, a 4,000-watt solar panel system combined with a 24V battery bank results in 166.67 amps. This is the minimum amperage that your charge controller should be able to handle, but it’s always a good idea to choose a higher-amp model to have a margin of safety.
Typically, charge controllers are rated based on their amperage and voltage capacities. Ensure that the chosen charge controller matches the PV array and battery voltages and can handle the PV array's current. For series charge controllers, the size depends on the total PV input current delivered to the controller and the PV panel configuration (series or parallel).
As a rule of thumb, the solar charge controller rating can be calculated by taking the PV array's total short circuit current (Isc) and multiplying it by 1.3:
Solar charge controller rating = Total short circuit current of PV array x 1.3
For example, if the total short circuit current of your PV installation array is 20A, the solar charge controller rating should be 20A x 1.3 = 26A. Therefore, a charge controller rated at least 26A would be suitable for this application.
You can get the total short circuit of your PV array by checking the individual Isc of each panel and multiplying it by the total number of strings panels.
Battery sizing
Sizing the battery for an off-grid solar PV system is crucial to ensure there is enough power to run the required load for 24 hours, as well as fully recharge the battery each day. It’s recommended to use deep cycle batteries, which are designed for repeated discharging and recharging.
To size the battery, follow these steps:
- Calculate the total Watt-hours used by appliances daily.
- Adjust for battery loss by dividing the total Watt-hours by 0.85.
- Adjust for depth of discharge by dividing the result from step 2 by 0.6.
- Divide the result from Step 3 by the nominal battery voltage.
- Multiply the result from step 4 by the days of autonomy (the number of days the system needs to operate without power from the PV panels) to get the required Ampere-hour (Ah) capacity.
Formula:
Battery Capacity (Ah) = (Total Watt-hours per day x Days of autonomy) / (0.85 x 0.6 x nominal battery voltage)
Sizing of the PV array
As the crux of your solar PV system, correctly sizing the PV array is perhaps the single most important factor. The PV array size can be determined using the lowest mean daily insolation in peak sun hours and the energy demand per day.
The formula for calculating the total size of the PV array (W) is:
Total size of PV array (W) = (Energy demand per day (Wh) / TPH) × 1.25
Where TPH is the lowest daily average peak sun hours of a month per year, and 1.25 is the scaling factor. The number of PV modules required can be determined by dividing the total size of the PV array (W) by the rating of the selected panels in peak watts. Typical solar panels today have a peak wattage of between 300W and 450W per panel.
The number of modules = Total size of the PV array (W) / Rating of selected panels in peak-watts
Additional losses due to factors like sunlight angle, dirt on panels, temperature above 25°C, and panel aging should be considered to find the exact panel generation factor (PGF).
For example, if your daily energy need is 3000 Wh, then:
Total WPeak of PV panel capacity = Energy demand per day / PGF = 3000 Wh / 2.4 = 1250 WPeak
Where 2.4 is a typical PGF value for the UK.
If you consider using 400W panels, then:
Number of panels = 1250 WPeak / 400W = 4 panels (rounded up).
Sizing of the cables
Optimal cable sizing is crucial for maintaining minimum voltage drop and resistive losses while ensuring safety and economic affordability.
The cable cross-sectional area (A) can be calculated using the formula: A = (ρIML / VD) × 2
Where:
ρ is the wire material resistivity
I is the maximum current
M is the maximum current carried by the cable
L is the cable length
VD is the maximum permissible voltage drop.
It's also essential to select the proper size circuit breaker, rated plugs, and switches to avoid energy loss and accidents. You’ll find online calculators that may help you make precise calculations.
Summary
While many think designing a solar system is as simple as finding the best solar panel, it’s a process that actually requires meticulous planning and a comprehensive understanding of various components and factors. A well-designed system ensures efficiency, safety, and cost-effectiveness. Following the guidelines and considerations discussed in this article, you can design a solar panel system that meets your unique needs and contributes to a more sustainable future.