Introduction

To foster the renewable energy development, the HKSAR Government has introduced the Feed-in Tariff (FiT) Scheme since 2018 as one of the important policy measures to combat climate change. Under the Scheme, the system owners can sell the electricity generated from their solar photovoltaic (PV) or wind systems to the power companies at a rate higher than the normal electricity tariff.

The Scheme has successfully raised public awareness and interest to install solar PV systems at their premises. However, it is difficult for the public to identify suitable locations on their rooftops for solar PV installations and hence estimate the return from the FiT Scheme. As such, a web-based application “Hong Kong Solar Irradiation Map” is developed with the aim of providing users with useful information such as preliminary solar irradiation values on building rooftops, annual electricity generation estimation and the estimated FiT income.

Data set

The Digital Surface Model (DSM) and the Geographic Information Systems (GIS) building data were used in this project to develop the solar irradiation map.

The airborne Light Detection and Ranging (LiDAR) data provided by the Civil Engineering and Development Department (CEDD) were used to generate the DSM.

In addition, the 10-year historical hourly solar radiation data (2009-2018) were obtained from the Hong Kong Observatory (HKO) to carry out the solar irradiation modelling.

Solar irradiation estimation

DSM generation from LiDAR data

At the beginning of the project, only the 2011 LiDAR data were available for DSM derivation and hence for simulation. However, some buildings were built or demolished since then and thus have been some discrepancies between the actual building locations and the DSM. A new set of LiDAR data collected from December 2019 to February 2020 was released by CEDD in 2021. These two sets of LiDAR data were then used to identify the changes and build up a more updated DSM so as to improve the accuracy of simulation.

Solar irradiation estimation

The Solar Analyst tool in ArcGIS was used to simulate the annual horizontal (i.e. 0°) and tilted solar irradiation at 20° and 40° at eight different orientations (i.e. north, northeast, east, southeast, south, southwest, west and northwest) at one metre spatial resolution (Figure 1). The tilted solar irradiation at 5°, 10°, 15°, 25°, 30° and 35° of the eight orientations were estimated by the second-order polynomial fitted from the results of 0°, 20°, 40°. The simulation model considered the terrain, shading effects and the atmospheric condition over the ten-year historical solar irradiation data from the HKO.

Figure 1: Simulation results of annual total solar irradiation on horizontal surface of Kowloon Peninsula and northern part of Hong Kong Island

Field verification of Solar Irradiation Model

Site measurements at five locations were conducted for validation of the data derived from the simulation, including the rooftop of a village house in Kam Tin, the rooftop on the 20th floor of a private housing in Sha Tin, a sky garden at the Hong Kong Polytechnic University (PolyU), the lawn in the HKO King’s Park Station, and the rooftop at a secondary school in Tseung Kwan O.

Figure 2: Field verification conducted in HKO King’s Park Station

Upon comparing the global horizontal solar irradiation simulation results with the field validation measurement using in-situ pyranometers for the specific period, it was found that the accuracy rate of the simulation results reached 95.99% in general.

Area suitable for solar PV system installation

Although the total building rooftop areas could be calculated from the GIS building polygons, the information did not tell whether the selected area was suitable for installing a solar PV system. In view of this, five criteria were set for selecting the optimal area for solar PV installation and details are discussed below.

Ground mask

The GIS building polygons might contain ground pixel since the shape of building polygons were roughly surveyed and some buildings were in irregular shape. Therefore, filtering could be conducted within the building polygon using Object Height Model. The height of object was calculated by subtracting Digital Elevation Model (DEM) from DSM, where DSM was the building’ top level and DEM was the building’s base level. Pixels with object height below 2.5 m (which is a normal height of one floor of a building) would be considered as the ground level.

Slope

Suitably tilted PV panels can receive the maximum amount of solar irradiation. However, Yang and Lu (2007)¹ indicated the solar irradiation received by a PV panel decreased significantly when the slope exceeded 40 degrees. Therefore, the rooftops with slope angle larger than 40 degrees were considered not suitable for installing PV system. Thus, the rooftop area with a steep slope exceeding this threshold value would be excluded from the suitable area for solar PV installation.

Perimeter zone

The shadows of protective barriers casting on the PV panels will significantly reduce the PV system’s energy generation. Thus, the perimeter zone within one metre from the edges of building rooftops were also excluded and considered as the area unsuitable for solar PV system installation (excepting village house). Figure 3 illustrates the solar irradiation at the horizontal level of the EMSD Headquarters where the blue colour represents the areas unsuitable for installing PV panels.

Figure 3: Barriers which are determined as unsuitable for installation

Shadow identification

Hong Kong is a densely urbanised city with high-rise buildings, the amount of solar irradiation received by a rooftop will be significantly reduced by the shadowing effect caused by the nearby buildings. When the global horizontal irradiation of a pixel was lower than a threshold value of 800 kWh/m² (Wong et al., (2016)²), it could be considered as the shadow cast area where the electricity generation by a solar PV system was not efficient. These areas would be excluded from the suitable area for solar PV installation as well.

Minimum area for installing solar PV system

The size of the usable rooftop area is also a critical factor to determine whether solar PV system can be installed. Considering the size of a standard crystalline silicon panel is about 1.7 m² and a minimum of two panels were required for solar PV employment, the extracted locations with area less than 3 m² were filtered out.

Estimated electricity generation

Even the locations suitable for solar PV installation could be identified, users might be confused about the quantity of PV panels to be installed and how much electricity would be generated. Utilisation factor and the performance ratio are crucial to estimate the installed capacity and the annual electricity generation of a solar PV system.

Utilisation factor

In the actual application of solar PV systems, certain space should be allowed between the front and back rows so as to minimise the self-shading effect as well as reserve sufficient space for maintenance. The distances between the front and back rows for different orientations and tilting angles are determined by ensuring that there is no partial shading on the direct irradiance component caused by the front row of PV panels between 8.30 am and 3.30 pm on the winter solstice in Hong Kong.

Unlike direct irradiance, diffused irradiance comes from all directions yet the effect is minimal if the obstructions are far away from the PV panels.

A utilisation factor (UF) is then introduced to account for the inter-row spacing during the calculation of installed capacity.

Performance ratio

Performance ratio (PR) is one of the indicators for system performance evaluation. It is defined as final yield (actual energy output of the solar PV system) compared to the reference yield (nominal energy output of the solar PV system).

The performance ratio is affected by several factors, e.g. temperature of the solar PV panel, conduction losses, efficiency of the inverter and isolation transformer, soiling, etc. The performance ratio of 0.75 was adopted in this project.

Responsive web-based application

After setting out the criteria for solar irradiation simulation and defining the calculation parameters, a responsive web-based application was developed to display the map and estimate the annual electricity generation as well as the FiT income.

Base map

Digital Topographic Map Application Programming Interfaces (API) and the Digital Image Map API developed by Lands Department were used as the base map of this application. However, the building images shown on the Digital Image Map API were not orthorectified (looking straight down) and was not appropriate for this application. A set of DSM orthorectified photos was obtained from Lands Department to generate the orthorectified base map (Figure 4) so that users could have a preliminary check on the rooftop conditions and draw a preferred location for solar PV installation easily.

Figure 4: Orthorectified photo vs DSM orthorectified photo (map from LandsD)

Electricity generation

In this application, users can draw a polygon on the planned location for installing the solar PV panels and select the installation options such as tilting angle and direction. The application would automatically display the results including the average annual solar irradiation, estimated installation capacity of solar PV system, estimated annual electricity generation and estimated annual income by joining the FiT Scheme (Figure 5).

A payback calculator built on simple payback is in place to facilitate preliminary check on the payback period. Users just need to input the installation cost and the installed capacity, the application will then calculate the payback period automatically.

All in all, this application provides users with a one-stop and user-friendly interface to identify the optimal location for solar PV installation.

Figure 5: The responsive web-based application

Conclusion

Solar irradiation is critical for the estimation of electricity generation. This project generated the pioneering detailed annual solar irradiation map of the entire Hong Kong territory for over 300,000 buildings at one-metre fine spatial resolution with the consideration of topographic and shading effects of surrounding environment, on hourly basis with the 10-year historical solar irradiation data from HKO.

To find appropriate locations for solar PV system installation on building rooftops in a compact city is challenging. With the aid of this web-application, users could identify the suitable rooftop area for installing solar PV system conveniently, and hence estimate the electricity generation and the FiT income automatically. This could greatly arouse public interest on solar PV system installation, whereby facilitating a wider penetration of low-carbon renewable energy in Hong Kong.

This web-application garnered a silver medal at the Special Edition 2022 Inventions Geneva Evaluation Days, which was the virtual edition of the International Exhibition of Inventions of Geneva held in March 2022.

For details of the Hong Kong Solar Irradiation Map, please visit EMSD’s website: https://solarmap.emsd.gov.hk

References

1. Yang H X and Lu L (2007). The Optimum Tilt Angles and Orientations of PV Claddings for Building-Integrated Photovoltaic (BIPV) Applications. Journal of Solar Energy Engineering, 129(2), pp. 253-255.
2. Wong M S, Zhu R, Liu Z, Lu L, Peng J, Tang Z and Chan W K (2016). Estimation of Hong Kong’s Solar Energy Potential Using GIS and Remote Sensing Technologies. Renewable Energy, 99, pp. 325-335.