Balancing Group of KNESS Energy: experience of the BG participant
15/01/2024
KNESS Energy’s Balancing Group has a new member
17/01/2024

Press center

Design process of rooftop solar power plants for business

Designing rooftop solar stations for facilities is a multi-stage, thorough process that has its own features and requires an individual approach to each project. A wide range of factors and requirements should be taken into account to develop an energy-efficient, integrated solution. The area, type, and roof condition, type of roofing, building’s age, height, and location all have an impact on achieving the goal of a high-quality solar station according to the customer’s needs. This may include the autonomy of the power supply during power outages, saving electricity costs by reducing electricity consumption from the external grid with the possibility of selling excess electricity to the market at free prices, or manufacturing products using environmentally friendly electricity without CO2 emissions. 

Let us consider the details of the design process of rooftop solar power plants for business together with Oleksandr Kucheruk, Chief Engineer of the KNESS Design and Development Department.

How to determine the optimal capacity of a rooftop solar power plant?

The higher the capacity, the shorter the payback period of a solar power plant. This is a rule that does not always work for rooftop solar power plants. There are constraints here: roof area, electrical solutions, and the need that a rooftop solar power plant meets, depending on its type.

Rooftop solar power plants are divided into autonomous, grid, and hybrid. Autonomous ones are intended only for self-consumption without supplying electricity to the grid. Grid-connected ones operate exclusively in the presence of an external grid to be able to sell the unused surplus of generated electricity under market conditions. Hybrid ones involve the installation of energy storage or battery storage units together with the plant so that the surplus generated can be accumulated and used at times when generation is insufficient or missing.

One of the most essential tasks of rooftop solar power plant designers is to determine the optimal capacity of the future plant, which will exactly match the electrical capabilities of the facility. This applies to all types of plants. But there are additional points to consider when designing each type. For example, for a grid-connected solar power plant, an important indicator is the capacity of the power grid. For an autonomous solar power plant, it is vital that the generation covers the company’s consumption schedule as much as possible. If the capacity of the future generation facility is insufficient, the station will not cover the needs of the consumer, which will increase the payback period. If the capacity is significantly higher than required, the consumer may receive penalties from the supplier or transmission system operator, as well as face malfunctions of their electrical equipment, which is definitely not worth the risk.

So, how do you find the optimal solution in terms of capacity?

Based on the data from the Automated Commercial Electricity Metering System (ACEMS), the facility’s consumption is analyzed, and the need to be covered is determined. The level of solar radiation in the area where the facility is located is also investigated. Next, the conditions and possibilities for installing a rooftop solar power plant are checked: the technical condition of the roof, its area, the conditions of electrical connections to the grid (if necessary), etc. The next step is to select the equipment that best meets all the conditions. The forecast generation is calculated using a special software called PVsyst. Taking into account all the data, we determine the optimal capacity of the future facility, proposals for the reconstruction of power grids (if necessary), the cost and potential payback period of the solar project.

How to choose and think through the arrangement of a reliable mounting system for PV modules according to the roof type?

Each roof is unique, even if made according to a standard design. Nevertheless, there are a number of characteristics that are crucial to consider at the pre-project analysis stage to make sure that it is feasible to build a rooftop PV system for commercial use.

Taking roof characteristics into account in design solutions

Roof shape and its effect on the position of solar modules

Photovoltaic modules can be installed on flat and pitched roofs, regardless of the roof angle. However, to do this, it is necessary to correctly calculate the most favorable location of the modules relative to the azimuth, south or east-west, and select the appropriate adaptive solar module mounting system with the optimal angle of inclination relative to the plane. Also, the inclination angle of the modules for more efficient absorption of sunlight can be adjusted with the help of metal structures, regardless of the roof type, but this is not always cost-effective.

It is important to understand what the primary goal of the future rooftop station is: to generate as much power as possible during the day? Or to produce maximum power but only during a certain period of the day or season? Or to generate maximum power throughout the year? Depending on the answers to these questions, we model the options for the inclination and orientation of the module mounting system in the PVSyst program and, if necessary, adjust the inclination angle of metal structures if appropriate/profitable. Nevertheless, for flat roofs, it is not recommended to make the inclination angle of the modules more than 10-15 degrees relative to the plane, as it will increase the windage of the building and the wind load on the entire structure, roof, and building.

Roof type and reliability of solar module mounting

There are solutions on the market for mounting photovoltaic modules for different types of roofs, but there are certain limitations. From our own experience, we have seen the reliability of fixing metal structures on roofing felt, membrane, and metal coatings. For them, in cooperation with our RnD Center, we have developed and are manufacturing our own support structures with ballast, bracket, and adhesive fastening systems. Metal structures by KNESS make it possible to mount solar modules securely while maintaining the thermal insulation and waterproofing characteristics of the roof. Thanks to the modeling and calculation of the optimal distributed load, which designers for each individual project carry out, we manage to maintain the reliability of metal, membrane, or roofing felt in each case.

We do not recommend installing solar power plants on other types of roofing, such as slate, ceramic tiles, or wood, as there is a high risk of damage to the integrity of the roof and leaks.

Roof’s age, technical condition and load-bearing capacity

At the pre-design analysis stage, the technical condition of the roof and the building itself must be inspected for the ability to bear additional loads. We calculate the load-bearing capacity of the building (roof) on which the solar power plant is to be installed using field surveys, comparing the design documentation for the roof and its actual condition. Then, mathematical analysis and engineering calculations are performed, taking into account data on wind and snow loads typical for a particular region and a particular location where the building is located, as well as data on the load the future solar station will carry. Interestingly, even buildings that are very close to each other can have different wind loads, for example, due to height or neighboring buildings that stop or change wind flows. A report is generated based on all the data obtained. In 95% of cases, the roofs are suitable for installing SPPs. However, if the building was built decades ago, we often recommend reconstructing the roof or using additional reinforcements before installing a PV system.

Specifics of selecting and installing electrical equipment for rooftop solar power plants

Photovoltaic modules (PV modules)

Models of solar modules are universal for both ground- and rooftop solar power plants. The only difference in the recommendations is that for rooftop solar power plants it is inappropriate to use bifacial PV modules, which allow for additional generation due to the absorption of sunlight from both the front and back sides. In ground-based plants, this type of module can increase generation by 6-9%, but in rooftop PV plants, the distance for sunlight to “return” to the back of the module is too small. That is why this type of module will not work out.

Inverters

Two types of inverters can be used for rooftop solar power plants: centralized inverters, the same as for ground-based solar power plants, or string inverters. What is the difference between them? String inverters are smaller in size. Moreover, string modules are connected to such inverters directly, without power collection cabinets (junction boxes), which are required for the operation of centralized inverters. Although string inverters are smaller in capacity than centralized inverters, in almost all cases for rooftop PV plants, this solution is more practical in terms of saving roof space. On a flat roof, inverters are installed on a ballast system or by mounting on the frame of the building itself. It is also possible to install them in a separate enclosure under the roof on a separate rack or a standard bracket.

Cable lines

According to fire regulations, cables should not be lying on the roof. Therefore, for rooftop solar power plants, we place all cables in special mesh trays. This solution is safe because it prevents mechanical damage to the wires, provides easy access to the cables for visual or thermal inspection or maintenance, and protects them from overheating.

Feasibility study and project documentation for a rooftop solar power plant

The process of performing a feasibility study and preparing project documentation for a rooftop and ground-based solar power plant is quite similar. In both cases, careful calculations are required. We described these design stages in detail in a previous article from our designers. The differences for rooftop solar power plants include the absence of geodetic surveys and the mandatory report on the load-bearing characteristics of the roof if the building is not new. At the same time, autonomous rooftop stations require approval exclusively from the customer, without involving other authorities, unlike ground-based solar power plants. And for hybrid and grid-based plants, if necessary, an update of ACEMS is added.

Over 14 years of operation, KNESS project portfolio includes 22 rooftop solar power plants with a total peak capacity of almost 14 MW. Therefore, taking into account the diverse features of each facility in the design is a challenge that the team of highly qualified KNESS project engineers has learned to accept with a high sense of responsibility and interest. We have certified specialists with experience in designing CC1, CC2 and CC3 consequence class facilities, who ensure the highest quality of projects of any complexity. You can learn more about the design of electricity facilities by KNESS here:

https://kness.energy/en/proiektuvannya-ob-iektiv-elektroenergetiki/

Приєднатися до командиЗакупівліJoin our teamProcurement