Four-step method to evaluate rooftop PV potential based on system layout, economic parameters

A Chinese research group has created a novel method to calculate city-scale rooftop PV potential not only based on roof areas but also including installation parameters and economic feasibility. The novel methodology was demonstrated in Nanjing, east China, and was found to be superior to conventional methods when it comes to estimating planned capacity.

Researchers from China’s Nanjing University of Information Science and Technology have proposed a novel method for estimating rooftop solar energy potential on an urban scale. While traditional methods focus their calculation extensively on the available roof area, the proposed technique is based on installation parameters as well as economic feasibility.

“The method assesses the economic feasibility of roof areas suitable for solar PV panel installation through required energy demand and payback times and calculates the solar PV power generation potential of urban rooftops based on the placement of solar PV panels,” the team explained. “Few studies have considered the actual installation layout of solar PV panels in rooftop solar PV potential assessments at the city-scale.”

The proposed method consists of four steps. First, an empirical model based on sunshine duration uses 3D data to calculate solar irradiance on building rooftops. The second step considers the ideal PV panel arrangement, calculating the minimum bounding rectangle of each roof. Then, a greedy algorithm from previous scientific literature is used to fit as many panels as possible, avoiding obstacles. Safety margins near roof edges are also considered.

In the third step, the method calculates a threshold for the minimum solar irradiance required for solar panels to be economically viable. The parameters of the calculation include installation costs, payback time, the proportion of electricity sold to the grid, feed-in tariff, electricity price, panel area, and panel efficiency. The fourth and final step compares whether the solar irradiance received by each solar PV panel is greater than the solar irradiance threshold.

“Solar PV panels with average solar irradiance smaller than the calculated threshold are excluded,” the group said. “This process yields the final solar PV panel arrangement and the corresponding solar irradiance. Subsequently, the module efficiency of the solar PV panels is applied to calculate the potential distribution of rooftop solar PV power generation.”

A case study to demonstrate the novel technique was conducted in the city of Nanjing, in east China. Various remote sensing data, vector data, and meteorological parameters were taken from sources such as the AutoNavi Map, the Digital Elevation Model (DEM), NASA, and the Nanjing Meteorological Bureau. Longi’s LR5-72HPH-550 M module was used as a reference for data on price, size, and efficiency.

“We calculate the solar irradiance threshold by setting the payback time to 10 years to ensure that most solar PV panel locations are economically viable,” the scientists added. “The electricity scenario parameter is set to be 1, which means that all solar PV power generation is sold. The fully on-grid distributed electricity price is CNY 0.49/kWh ($0.07/kWh) in Nanjing. The cost of one solar PV panel is CNY 672 ($94.8). The module efficiency is 21.3%.”

Per the results of the new method, the available roof area for PV in Nanjing is 122 km2, with 28.87 TWh electricity potential each year. That results could meet 39.85% of the city’s electricity demand in 2022, which was 72.45 TWh. Compared to the traditional solar PV potential estimation method, the novel technique avoids overestimating generation potential per unit area by 13.08%.

The new tool was introduced in “Evaluating solar photovoltaic potential of buildings based on the installation parameters of photovoltaic modules,” published in Solar Energy.