Rooftop PV Energy Potential Based on Housing Design in Brunei National Housing Planning

: Brunei aims to achieve 600 MW of renewable energy by 2035. Since the country has high solar radiance throughout the year, solar energy is the most feasible. However, the high cost of producing solar farms remains an obstacle and caused the country’s progress towards the goal to fall behind schedule. Thus, this study aims to show the beneficial costs of investing in residential rooftop solar farms instead. This study calculated the optimal PV system sizes based on the residential load profiles (at only 10% of total consumption) and the available detached housing plans. After that, the costs of the optimal PV system sizes are estimated and then projected to the number of detached houses nationwide, which is 49,872 houses. The research found that the optimal rooftop off-grid PV system size for the average resident is 15.3 kW , with an inverter of 12.0 kW and a nominal battery size of 20 kWh . The net present cost of one optimal PV system is BND49,108 for 20 years of lifetime. If all the detached houses in the country are utilized to install solar panels, the total solar capacity is 763 MW , costing BND2.4 billion without involving the homeowners’ funds. Comparing this budget to conventional solar farms such as Tenaga Suria Brunei, the country can save BND10.3 billion. The study shows that Brunei may need to consider the benefits of investing in residential rooftop solar farms more due to savings in potential costs and only using the readily available rooftop space.


Introduction
According to [1], about 59.5% of Bruneian households live in detached houses.The houses in Brunei are typically spacious, and the roofs are typically gable styles, the most common roofs with two sections slanting downwards.The roofs of these houses can be utilized to install solar panels.It may hassle the residents; however, due to the high electricity consumption of the residential sector [2][3][4], the residents may need to adapt to using solar PV systems to meet at least a fraction of the high electricity consumption.Furthermore, the country's electricity is primarily generated by natural gas, negatively affecting the environment [5].However, due to the high investment costs demonstrated in [6], the government may need to step in to catalyze the implementation of residential rooftop PV systems.Since the country is targeting to achieve 200 MW by 2025, 600 MW by 2035, and 1200 MW by 2050 [7], Brunei should investigate financing in residential rooftop solar farms due to the available rooftop area and no land needs to be cleared to install the solar farms.Hence, this study will investigate the potential cost benefits of investing in residential rooftop solar farms.There are a few research questions that need to be answered, which are: • What total solar capacity can be installed by utilizing the residential rooftop area?
• Compared with conventional solar farms, what is the impression of the total costs of utilizing all the detached houses in the country?

Methodology
This study used the free software Sunny Design [8] to calculate the optimal PV system sizing based on the residential load profiles [9]-only 10% of the total load is considered due to the high consumption.Since the building plans from the Rancangan Perumahan Negara (National Housing Scheme) are available [8], this research used the measurements to estimate the maximum number of panels installed on the roofs.This study only considered detached houses.Error!Reference source not found.shows the average residential load profile used on Sunny Design.After keying in the load profiles, the next step was to create a residential building to visualize the placements of the solar panels.Then, the total solar capacity was calculated while placing the optimal number of PV panels on the roofs.Both sides of the roofs were used to support the panels, as shown in Error!Reference source not found.Error!Reference source not found..After calculating the maximum available solar panels that can be placed on the rooftops, the software calculated the optimal inverter and battery sizes.Error!Reference source not found.shows the PV components' specifications used in the calculations done on Sunny Design.The estimated lifetime of the system is 20 years.Since solar panels usually last 20-25 years, the replacement cost will be zero in this study.Meanwhile, the inverter must be replaced every 10 years, so in this study, the inverter will be replaced once.
On the other hand, the lead-acid battery used in this study has an estimated lifetime of 5 years [10].Therefore, the batteries would need to be replaced 3 times during the system's lifetime.An additional cost is added to the software, about BND4500 [6].After the optimal PV system sizes are calculated, the results are projected to the number of detached households in 2021 [11].Tenaga Suria Brunei, with a capacity of 1.2 MW and cost BND20 million [12], will be used as a reference for how much the Brunei can save in comparison with installing residential solar farms.

Optimal Residential Rooftop PV System Size and Costs
After all the calculations were completed on Sunny Design, the costs of the optimal sizes were then estimated using the costs from [2][3][4].Error!Reference source not found.shows the optimal PV system sizes.Based on the average residential load profile and at a 10% consumption level, the optimal PV panels is 15.3 kW.According to the simulation design, all these panels can fit on the roofs of the average detached houses in Brunei.The costs of the solar panels are estimated to be BND5508, including the additional installation cost [6].For the inverter, the calculated optimal size is 12 kW which costs BND13800.Since the replacement is only once, the replacement cost is also BND13800, including the installation cost estimated in [6].The optimal battery size is calculated to be 20 kWh , which has an initial cost of BND4000.Since the replacement needs to be done thrice during the system's lifetime, the total replacement cost is BDN12000, as shown in Error!Reference source not found..The software also calculated the output of the optimal PV system, and the results are shown in Error!Reference source not found..The maximum output energy of the system is 18,702 kWh per year.However, only 3711 kWh of the generated electricity can be used by a household.In a year, the household may consume about 1239 kWh directly from the system, and about 2472 kWh can be stored.The total usable energy by the system is calculated to exceed the annual consumption of the household, 2971 kWh.Hence, it can be assumed that the calculated optimal PV system size is feasible for the average household.

Projection of Residential Solar Capacity Nationwide
After calculating the optimal PV system size for the average Bruneian household that lives in detached houses, the capacity and the costs were then projected to the total number of detached occupied houses in the country.Brunei has four districts: Brunei-Maura, Tutong, Belait, and Temburong.The optimal PV system's capacity and costs were projected using the number of households in each district.Error!Reference source not found.shows the costs and the projection of solar capacity for each district and the total costs and capacity nationwide.The total solar capacity that can be generated for Brunei-Muara, Belait, Tutong, and Temburong are 527 MW, 121 MW, 94 MW, and 20 MW, respectively, which cost BND1.7 billion, BND0.4 billion, BND0.3 billion and BND0.06 billion for each district.The total solar capacity nationwide is 763 MW which cost in BND2.4 billion.If Brunei wants to invest in the same solar capacity but in conventional solar farms, the cost would be around BND12.7 billion.Investing in residential rooftop solar farms would save Brunei around BND10.3 billion.

Conclusions
This study investigated the costs and benefits of investing in residential solar farms.The optimal residential rooftop PV system size for one household was 15.3 kW, with an inverter of 12.0 kW and batter size of 20 kWh, calculated using Sunny Design.As found in the study, the calculated optimal PV system size is feasible for the household as the usable energy from the PV system, 3711 kWh, exceeds the annual consumption of the average household, 2971 kWh .The net present cost of the PV system is found to be BND49,108 for 20 years.By projecting the capacity and costs of one optimal PV system to the total detached houses in the country, 49,872, the study calculated that the total capacity of the residential rooftop solar farms would be 763 MW, achieving 163 MW more than the national target of 600 MW by 2035.The cost to achieve this capacity is BND2.4 billion.However, suppose the country invests in rooftop solar farms instead of conventional ones.The nation can save a budget of BND10.3 billion.This study concluded that investing in residential rooftop solar farms is worth considering.

Figure 1 .
Figure 1.Average Bruneian residential load in a week.

Figure 2 .
Figure 2. Top-view of the solar panels' placements.

Figure 3 .
Figure 3. Placements of the solar panels on the roofs in 3D view.

Figure 4 .
Figure 4. Energy distribution estimated by Sunny Design.

Table 1 .
Summary of the PV components' specifications and estimated cost.

Table 2 .
Optimal PV system sizes and costs.

Table 3 .
Projection of total residential PV systems and net present costs nationwide.