Discussing various residential and commercial PV + BESS systems, I observed a common gap in understanding among users, and even designers of these systems. This gap pertains to the basics of PV energy production, the importance of defining a load profile, and the design process of a PV system tailored to meet this profile within certain constraints. This lack of understanding often resulted in extended discussions about the reasons behind my component sizing choices, expected system performance, and optimization strategies.
In this post I will present an overall approach to designing a PV + BESS system in a way that hopefully would clearly lay out the why and the how.
Photovoltaic (PV) Energy
A PV system is designed to harness solar energy hitting a specific surface area and transform it into electricity, which is then tailored to match the energy needs of a given load as closely as possible.
Figure 1 depicts the typical hourly solar irradiation profile for a day in January and a day in June in Lubumbashi, DR Congo, data sourced from the Global Solar Atlas. The graph presents both hourly values and the cumulative irradiation throughout the day.
It’s crucial to understand that solar energy is available only during certain hours and its intensity fluctuates over the day, generally forming a bell-shaped curve. Moreover, the total daily energy in Wh/m2 changes significantly across the year, ranging from about 3.3 kWh/m2 in January to almost double that in June, at around 7.3 kWh/m2.
Solar panels convert the incident irradiation on their surface area (m2) into power (W), following a similar bell-shaped production pattern.
The LOAD Profile
Energy demand from residential or commercial facilities fluctuates over the course of a day. The following graphic offers four distinct daily load profile examples, each consuming the same total energy amount but in different patterns. The first profile shows a constant energy demand throughout the day. The second illustrates a two-tier demand, where energy use significantly increases during working hours from 8 AM to 6 PM. The third profile mirrors the bell shape of the PV energy production, aligning with periods of solar irradiation. The fourth profile represents a typical household, with peak energy usage occurring in the morning and evening.
Sizing up the PV System
Designing a PV system involves generating and managing energy to align with a specific load profile as shown in Fugure 3 abive. Here’s my step-by-step approach:
- Define the daily load profile to derive the total energy required per day, the shape of the load and the maximum power required.
- Quantify PV Panels to ensure the panels produce enough energy to cover the daily load, or more.
- Size the Inverter to transfer power from the PV panels and meet the load’s power requirements.
- Size up the batteries to store the pv energy such that it can later deliver energy to the load as required.
Consider a residential load of 100 kWh per day, as shown in Figure 4. Using January’s PV irradiation (Figure 1), we map and scale it on Figure 4 so the curve’s area is at least 100 kWh, matching the load.
Figure 4 displays the overlay of load and PV energy profiles, revealing three overlapping areas. Area 1 is where the PV energy equals or is less than the load, representing the energy that can de transferred directly to the load. Area 2 is the excess PV energy that can be stored in batteries. Area 3 is when the load exceeds the PV output, when the energy will be drawn from storage. In this case, Areas 1, 2, and 3 are 46, 54, and 53 kWh respectively.
We have assumed an operating mode whereby the power produced is first sent to the load and the excess stored to the batteries. One could also consider a mode whereby priority is first given to charging the batteries.
In the outlined design, the operating mode prioritizes directing the power produced by the PV panels to the load first, with any surplus energy then being stored in the batteries. Alternatively, a different mode can be considered where the priority is to first charge the batteries. In this alternative approach, the batteries act as a buffer, managing the entirety of the energy supplied to the load. In this strategy the batteries regulate and distribute power as needed, regardless of the variability in solar production.
Sizing the panelsTo produce 100 kWh daily, considering January’s lowest irradiance (3.3 kWh/m2), a 25% system loss, and 2.58m2 per panel, we need:
- Total kW of Panels = 100 kWh / 3.3 kWh/m2 / 0.75 = 40.2kW
- Total Area of Panels = 40.2 / 0.55 × 2.58 m2 = 189 m2
Sizing the inverter(s). Choose an inverter capacity that can handle the highest power output, either from the PV panels or the load. As per figure 4, the power from the pv panels reaches the highest value at 14kW. A single 16kW inverter would be appropriate.
Sizing the battery storage. Area (2) in figure 4 represents the amount of energy produced by the PV panels that will be stored and then at a later stage transferred from the batteries to the load. Area (2) should be equal or more than area (3). Area (1) = 46 kWh and Area (2) = 54 kWh, whose sum should be equal or more than the load kWh 100 kWh + system losses. Assuming a maximum depth of discharge of 80%, the BESS capacity should be sized at 60 kWh. This is the minimum size to meet the load requirements of one day. It could be increased further.
In summary the system components are:
- PV Panels : 40.2 kW
- Inverter : 14kW
- BESS : 60kWh
Sensitivity Analysis
Expanding the analysis, we’ll conduct a sensitivity analysis on the cost of the system and the electricity produced for the four types of load profiles shown in Figure 2.
Figure 5 overlays these load profiles onto the PV energy profile.
In these scenarios, the PV + inverter setup remains constant across all cases. However, the energy storage requirement, indicated in yellow as area (2) in Figure 5, changes based on the load profile shape. The constant load profile necessitates the largest storage (largest area 2), while a load profile matching the PV energy profile requires no storage (no area 2).
For cost analysis, I sourced average prices for panels, inverters, and lithium batteries from a South African supplier:
- Panels : 289 usd/kW
- Inverter : 152 usd/kW
- BESS : 311 usd/kWh
Assuming a solar irradiance value of 3.3 kWh/m2 per day, the cost of Panels + Inverter gives a total average cost of 128 usd/kWh.
The following table details the sizing and costs for each component across the four profiles, along with the overall electricity cost over 5 and 10 years, factoring in a 10% interest rate and 1% annual battery replacement.
The table clearly shows that the price of the inverter and PV panels are the same accross of cases. The four cases have different BESS sizes as shown in Figure 5 as area (2) in yellow. As can been seen in the table, the value of the BESS system is zero when the load profile matches that of the PV energy generation. The case with the load at a constant value results in the largest requirements for storage at 54kWh. The difference in tariff is thus driven by the size of the BESS system. As per table 1, the system with no BESS has a cost of electricity of 0.12 usd/kWh over 10 years. Case (4) with a residential profile has the largest BESS resulting in the highest cost of electricity of 0.30 usd/kWh over 10 years.
The table reveals consistent inverter and PV panel costs across all scenarios. The BESS size, represented by area (2) in Figure 5, varies, being zero when the load profile aligns with PV generation. The highest storage requirement, 54 kWh, is for the constant load profile. The variation in the cost of electricity is mainly influenced by the BESS size. For example, the system without BESS has a 10-year electricity cost of 0.12 USD/kWh, while Case 4 (residential profile) with the largest BESS results in the highest electricity cost of 0.30 USD/kWh over 10 years.
Conclusions
In this post, I’ve outlined the fundamental principles in designing PV systems. While the actual design process involves more intricate details, such as accounting for system losses and accommodating specific user requirements (like sizing the BESS for a 2-day supply), the core concepts remain the same.
Importantly, I’ve demonstrated how the load profile significantly influences both the required storage size and the overall system cost, as well as the effective cost of electricity. Understanding and carefully considering these elements is crucial in designing an efficient and cost-effective PV system.