Large Photovoltaic Power Plant Design Guide
Designing a photovoltaic power plant on a megawatt-scale is an endeavor that requires expert technical knowledge and experience. There are many factors that need to be taken into account in order to achieve the best possible balance between performance and cost.
Our team of renewable energy engineers have the technical know-how and the experience necessary to design stellar photovoltaic power plants that strike the perfect balance between cost savings and quality for the greatest possible energy yield.
Plant Design Overview
All decisions regarding the engineering of a large solar PV power system must be carefully considered so that initial decisions made with cost savings in mind do not result in more maintenance costs and decreased performance later in the system’s lifespan. In general, the decisions regarding layout and shading potential, panel tilt angle and orientation, and PV module configuration are the most critical for reaching the optimal balance of cost and yield.
Layout and Shading
Specific site conditions often inform general layout decisions such as row spacing and the overall arrangement of solar energy arrays. The layout should always be designed in such a way to reduce cable run as much as possible, which in turn reduces electrical losses. Space should be reserved for maintenance access as well.
Each location will have its own ideal tilt angle that maximizes annual sun exposure based on the latitude of the site. For fixed-tilt panels, the optimal angle may need to be adjusted due to factors like panel soiling, shading, and seasonal irradiation distribution. The higher the panels are tilted, the more they will be cleaned by rain but also the more they will shade panels in rows behind them. Simulation software can help determine the optimal tilt angle, accounting for these circumstances.
PV Module Configuration
PV module configuration refers to whether individual panels are mounted in landscape or portrait orientation as well as how they are connected to each other within each string. Both orientations have advantages as far as reducing shading in different situations.
The ideal row spacing distance will be a compromise between reducing inter-row shading, reducing cable runs as much as possible, keeping energy losses low, and keeping the overall area of the power plant within a reasonable limit.
The general rule of thumb for determining acceptable inter-row spacing is to arrange the PV modules in a way that allows for no shading at solar noon on the winter solstice. In some cases, detailed energy yield simulations and calculations may be warranted to achieve optimization between yield, shading, and the cost of land.
In the northern hemisphere, the optimal directional orientation for all panels is true south. However, in some markets where producing energy during peak demand times is encouraged, it may be more financially beneficial to orient the panels facing southwest to generate the most power in the afternoon.
Naturally, the technology that is selected for the PV power plant will have an impact on the bottom line due to factors like quality and longevity, initial and maintenance costs, warranty protection, efficiency rating, and so forth.
Modules (Solar panels)
All PV modules (solar panels) should be certified to IEC, CE, and UL standards. Beyond that, potential modules should be assessed against the following metrics: Levelized cost of electricity, quality, performance, power tolerance, flash tests, temperature coefficient, degradation, bypass diodes, warranty terms, maximum system voltage, and any other site-specific concerns or requirements.
There are many different types of inverters, so the local conditions of the site and the nature of the other system components should be analyzed when selecting the best type of inverter for the power plant. Factors to look at include the DC to AC conversion efficiency, DC input voltage and load, average site temperature and altitude, product reliability, serviceability, and total cost.
There are two main types of transformers that are suitable for solar power plants: distribution transformers and grid transformers. Distribution transformers help increase the output voltage for the plant collection system, and if the plant is connected to a distribution network, power can be exported directly to the grid. If the plant is connected to a transmission grid, grid transformers can increase the output voltage even further.
There are several different types of mounting systems that can be used for PV power plants, such as fixed-tilt support structures, single- or double-axis tracking structures, marine-grade support structures that prevent corrosion, and so forth.
The electrical design of a power plant will need to be considered on a case-by-case basis, since each site has unique constraints and parameters. However, we will share some general guidelines and industry best practices below for both DC and AC systems.
A DC (direct current) system is composed of the following elements: PV module arrays, DC cabling, DC connectors, junction or combiner boxes, protection devices, and grounding. All DC components should be rated for thermal and voltage limits based on manufacturers’ data.
PV Array Design
The PV array design will be dependent on the inverter style and the chosen system layout. Safety requirements, inverter voltage limits, federal regulations, and the maximum and a minimum number of modules per string will need to be calculated.
Federal and state regulations dictate the sizing and options available for cabling. Cables that are specifically designed for DC solar power generation should always be used, and the cables must be assessed based on the cable voltage rating, the current carrying capacity of the cable, and the minimization of voltage drop due to the cabling.
Above-ground cables must be routed as efficiently as possible and secured well to the mounting structures. They should also be protected from direct sunlight, standing water, and any sharp edges on surrounding equipment.
The preferred cable type for connecting modules is single-conductor, double-insulated. The number of modules per string will dictate the necessary voltage rating for the cables.
Combiner boxes are needed to combine the individual strings together while marshaling the power output. Adequate signage should be placed around combiner boxes since they are live during the daytime.
AC Solar System
AC (alternating current) systems come with many similar design concerns as DC systems, with some unique considerations as well.
Cabling systems must be designed to provide a safe and cost-effective way to transmit the AC power from the inverters to the transformers and beyond. Cables must be rated for the appropriate voltage of the system, taking into account the operating currents and potential short-circuit currents.
Switchgear and protections systems are critical to providing proper disconnection ability, isolation, grounding, and protection. Metal-clad, air-insulated, or gas-insulated switchgear may be necessary based on the exact system specifications.
Sizing and Selecting Transformers
Grid connection for commercial solar power plants is often 11 kV or higher, so it’s usually necessary to step up the voltage using one or more transformers. The type of transformer should be selected based on the required capacity, its position within the electrical system, and the physical location and environmental conditions of the site.
The plant substation will likely contain equipment such as LV/MV transformers, MV switchgear, Supervisory Control and Data Acquisition (SCADA) systems, protection, and metering systems.
Grounding and Surge Protection
As with any electrical system, it’s critical to ground the system to protect against electric shock, fire hazards, and lightning strike damage. The array frames, system, and inverters need to be grounded and lightning and surge protection devices installed.
Grid connection codes will vary based on the exact location of the power plant, but some items of consideration may include limits on harmonic emission, limits on voltage flicker, limits on frequency variation, reactive power export requirements, voltage regulation, fault level requirements, and system protection.
Designing a large-scale PV power plant requires infrastructure that can handle such an installation. For instance, the location must be selected carefully to avoid shading from buildings, trees, or other obstructions. However, some on-site buildings may be necessary to house things like an office and bathroom, an LV/MV station or MV/HV station, and communication and security equipment.
The equipment necessary for a utility-scale power plant represents a significant investment, so security measures should always be taken to protect that investment. A combination of several security measures is often warranted, including things like a security fence, CCTV cameras, video analysis software, sensors, warning devices, security staff, and remote alarms.
Monitoring devices are also an essential part of any utility-scale PV power plant. These devices can help calculate liquidated damages, automatically acquire data, help maintain performance levels, ensure quick detection of problems, and reduce downtime for repairs. Without a monitoring system in place, faults can go undetected for months and lead to significant amounts of lost revenue.
Optimizing System Design
There are several factors to take into account when optimizing the system design to minimize loss, and many ways to mitigate these factors. For example, to avoid loss from shading, choose a location with few or no existing shade obstacles, ensure that there is sufficient space between modules to minimize inter-row shading, and manage the growth of vegetation that may cause shading.
Design Documentation Requirements
At a minimum, design documentation for a large-scale PV power plant should include the datasheets of all system components, comprehensive wiring diagrams, layout drawings that include the row spacing measurements and location of the site infrastructure buildings, mounting structure drawings with structural calculations that have been certified by a licensed engineer, a detailed resource assessment and energy yield prediction report, and a design report that includes general information such as the site location and characteristics as well as the results of a geotechnical site survey.
Design Documentation Requirements
At minimum, design documentation for a large-scale PV power plant should include the datasheets of all system components, comprehensive wiring diagrams, layout drawings that include the row spacing measurements and location of the site infrastructure buildings, mounting structure drawings with structural calculations that have been certified by a licensed engineer, a detailed resource assessment and energy yield prediction report, and a design report that includes general information such as the site location and characteristics as well as the results of a geotechnical site survey.
Permits and Licensing
Permitting and licensing requirements vary based on the exact location of the solar power plant, but they generally include some or all of the following requirements.
Land Lease Agreement
If the site is not privately owned, a lease agreement signed by the owner will be necessary in order to install a power plant. These lease agreements typically last 25 years with the potential for extension, and they should include restrictions regarding the development of structures on neighboring land that might shade the modules. Additionally, the need for access roads should be accounted for in the lease agreement.
Planning and Land Use Consents
Some areas require a separate building permit to be acquired before construction begins, which may be issued by a different Authority Having Jurisdiction than the planning and land use permit authority.
Environmental permits are required in some places, such as environmental impact assessment permits, endangered or protected species licenses, agricultural protection permits, historic preservation permits, and forestry permits.
Grid Connection Application
In order to export the produced power to the grid, the power plant needs to be interconnected, which generally requires another permit. The utility company that maintains the grid will perform their own inspection of the interconnection before allowing power to flow.
Electricity Generation License
The operator of the power plant is required to hold an electricity generation license. This may be required before or after the construction of the power plant depending on the location.
Consultation and Disclosure
We recommend that in the early stages of developing a large PV power plant, the project owners consult with key authorities, statutory bodies, nearby communities, and any other stakeholders in the process.