1. Introduction

Because of the development of solar energy technology, there has been an increase in solar power system installation in urban areas. Consistently lower solar panel prices have made it possible to install solar energy systems in complex areas. Many solar power systems are installed not only on roofs but also on the ground and building façades. Mapping of the accurate spatial distribution of solar energy potential and analysis of shadowing should be considered to achieve maximized efficiency. However, as solar power generation increases, there are factors to consider besides simply increasing efficiency. The difference pattern of solar power supply and power demand requires additional energy or energy storage. The most representative case is ‘duck curve’, which shows net demand, the difference between the electric load and the supply of solar power, in California [ 1 ]. It represents the steep ramping needs of energy demand in the evening, as solar power increases. As this sudden ramping causes difficulties in providing a stable energy supply, it is necessary to find methods to stabilize it. One of the methods to alleviate this volatility is to change the orientation of the solar panel.

Power systems have traditionally consisted of a macrogrid at the national level, but in recent years, many small microgrid forms have been studied. Microgrid, which is an independent and distributed group of power systems, has many potential advantages such as low transmission losses, low grid congestion, utilization of local renewable resources, etc. [ 2 ]. The word microgrid has various synonyms, such as minigrid, depending on capacity or characteristics; however, it is used to refer to all terms that match the definition in the preceding sentence in this study. The detailed renewable energy potential of the site and its variability should be analyzed to construct the microgrid. At microgrid level, the main factors for solar energy are shading, shadowing, and other system variables rather than solar irradiance of the region. Analysis of the spatial distribution and temporal variance of the potential should be conducted to maintain the stability of the grid.

The university campus is one of the most suitable sites for running such an independent microgrid. In general, although each university has a different area and number of buildings, it has many regular energy users and various facilities in the university. The electricity consumption of the university is in the range of a few to hundreds GWh/year, but varies widely depending on the university. Therefore, universities around the world already started to construct a microgrid in universities such as Wesleyan University in Connecticut and the University of California at San Diego (UCSD) [ 3 ]. In addition, there are over 45 global activities for sustainable campus and green university [ 4 ]. Korea is also making efforts to construct microgrids to various regions, including universities based on the Renewable Energy 2030 plan by the Ministry of Commerce, Industry and Energy (MOTIE). According to the Korean Association for Green Campus Initiative (KAGCI), electricity usage of university and college in Korea was approximately 4000 GWh/year in 2018.

6,7,8,9,10,6, Many previous studies have analyzed the optimal orientations in various regions [ 5 11 ]. They focused on calculating methods [ 5 7 ], the spatial distribution of optimal orientation [ 8 9 ], seasonal optimal tilt [ 5 10 ], and special application such as panels on large ship [ 11 ], however, they estimate optimal orientation based on the total value of power generation. The optimal orientation considering the temporal volatility should be studied to maintain the stability of the grid. If the change in orientation of the solar panel reduces volatility with a loss of little power, it can be an optimal choice considering the high price of batteries. In this respect, it is necessary to calculate the optimal orientation considering the generation pattern and energy demand according to various time distributions. There are still many areas using fixed solar panels because of cost and maintenance, and the volatility optimization can help stabilization of these solar power systems.

13,14,15,16,17,18,13,14,21,24,25,26,27,31, There is considerable previous literature regarding estimating solar potential in urban areas such as a university. Some studies have focused on building roofs [ 12 19 ]. These mainly studied the extraction of high potential roofs of buildings considering roof orientation and shadowing. Some have focused on the detection of building roofs [ 12 15 ] and various scales of this method were applied from specific detailed building roofs [ 12 16 ] to the national scale [ 17 ]. This method is appropriate for urban areas with low buildings. Other studies have focused on both building roofs and façades using simple box-shaped buildings [ 20 22 ]. They calculated solar potential in each plane of the building containing a vertical plane. These methodologies are appropriate to highly urbanized areas and simple box data are easy to acquire and use; however, in most cases, these data contain only buildings without other objects. Other studies have focused on all the objects in an area using detailed surface models such as Light Detection And Ranging (LiDAR) data [ 23 28 ], UAV-derived data [ 29 ] and designed three dimensional (3D) models [ 30 32 ]. These detailed models contain all objects containing trees, but usually, have complex shapes and are difficult to analyze. Using these data enables accurate calculation of potential distribution in urban areas, but they require many resources to create.

Estimating the solar potential of specific urban areas or optimal solar direction for high potential has been done. However, as the solar power supply ratio increases, new perspectives are required to address future temporal volatility. Therefore, the objectives of this study are the estimation of photovoltaic potential and analysis of optimal orientation considering temporal volatility to increase the stability of the grid. A new tool and methodology have been proposed to achieve these objectives.