The increasing occurrence of extreme weather events over the past few years has underscored the impact of climate change is having on our planet. As a developed city that has set itself a target of carbon neutrality by the year 2050, Hong Kong must do everything within its power to reduce its carbon emissions in order to contain the global temperature rise to below 1.5°C. Despite Hong Kong's potential to expand the use of renewable energy, the scarcity of the territory's land resources means that on top of fierce competition on the usage of every piece of land, owners are already maximising the utility of every square inch on their plot.

In order to improve the visual appeal of urban environments and mitigate urban heat island effects, building owners and property managers are finding ways to carry out greening, such as by transforming conventional roofs into green ones. At the same time, there are also people choosing to install solar photovoltaic systems on their rooftops so as to obtain extra electricity and earn income generated from the Feed-in Tariff (FiT). In addition, some property developers in Hong Kong are considering installing solar panels on unused farmland in a drive to optimise land use. It may seem counterintuitive to combine solar power systems with the cultivation of plants or crops as plants need water, sunlight and soil for growth, and covering them with solar photovoltaic systems goes against the fundamental requirements for their growth. However, with careful planning, these two approaches can be mutually beneficial as well as maximise the utility of our valuable space. This article will explore the advantages, challenges and precautions so required of combining solar photovoltaic systems with greening projects or crop cultivation on rooftops or farmland in Hong Kong.

Solar power in Hong Kong

To truly transition towards a low-carbon economy, it is crucial that Hong Kong increases the proportion of renewable energy in its total energy generation. Hong Kong's Climate Action Plan 2050 sets out the long-term goal of achieving carbon net zero in its power generation by 2050, with the proportion of its energy coming from renewable sources rising from less than 1% today to 7.5-10% by 2035, and 15% by 2050. As part of this strategy, installing solar power systems on rooftops, hence, is becoming more prevalent. For example, the Action Plan states that new government buildings must dedicate 25% of their available rooftop area to the installation of renewable energy systems, which is significantly higher than the 10% previously stipulated. The Housing Authority similarly must install solar power systems on all new buildings in public housing estates, with a view to using these to generate at least 1.5% of the public electricity supply.

In truth, these targets are relatively low and far less ambitious than those set by other east Asian cities, which has led to criticism from environmental and climate change concerned groups. According to a study by The Hong Kong Polytechnic University, among the 309,000 buildings in Hong Kong, 233,000 are suitable for installing solar photovoltaic systems on their roof or rooftop, which would potentially be able to generate 4,674 GWh of solar energy every year — equivalent to 10.7% of Hong Kong's energy needs [1]. Hong Kong has had a FiT system in place since 2018, to encourage non-government organisations and private individuals to install renewable energy systems on their property. However, despite the fact that rooftop solar photovoltaic systems can provide an enormous source of renewable energy, the proportion of renewables in Hong Kong's energy mix has barely risen in the past five years. In fact, the government even reduced the FiT in April 2022, leading to fears that it will discourage others from signing up as they would be unable to recoup the high set-up cost before the scheme's end in 2033.

Green roofs in Hong Kong

Green roofs are incredibly popular in Hong Kong. Green roofs are visually appealing, provide residents with space for recreational activities. Moreover, they also reduce the amount of heat a building absorbs, help insulate buildings and improve their energy efficiency, thereby reducing the need for air conditioning while still cooling the building and reducing heat island effects.

Given these benefits, it is not surprising that the Hong Kong government has been actively promoting the installation of green roofs. It has encouraged the construction of green roofs on newly completed buildings, both public and private, as well as schools since 2000. In 2007, the Architectural Services Department commissioned the Study on Green Roof Application in Hong Kong, which cited a research finding that adding green roofs to buildings can lower their temperature by at least 2°C, and room temperatures by 4-5°C. Other local report also found that green roofs can reduce the surface temperature on building rooftops by 18-26°C in the month of August. This can play a huge role in energy conservation and carbon reduction, leading the Study to call on the government to do more to promote roof greening.

The benefits and challenges of combining green roofs with solar power systems

Green roofs are aesthetically pleasing, but their cooling effect most likely benefits the top few floors of the building only. Solar power systems, on the other hand, provide renewable energy and reduce carbon emissions, but visually leave a lot to be desired. However, together they can make up for each other's shortcomings and improve the efficiency of solar power generation. The efficiency (η) of photovoltaic cells depends on the amount of solar radiation and the cell's output power, with thermal power being one of the key factors affecting the efficiency of rooftop PV panels. High rooftop temperatures increase conductivity of the system's crystalline semiconductors, which in turn suppresses charge separation and decreases the cell's voltage. As a result, high temperatures can actually reduce the productivity of solar power systems by as much as 25%. However, greening on rooftops helps reduce surface temperature, and therefore improves the system's efficiency. Another factor that affects the efficiency of solar power systems is dust and grime that can accumulate on solar panel surfaces over time, which blocks sunlight and reduces the system's output. If a green roof is coupled with an irrigation system, this can also be used to clean the panels and maintain their efficiency.

Another aspect to consider is that, while plants require sunlight to grow, if it is too strong, it will damage the plant's chlorophyll and tissue, and thus impede its growth. Rooftop solar power systems that are installed on green roofs can provide much-needed shade for the plants, improving their growth and reducing the amount of water they lose through evaporation. These systems also provide shelter from storms, protecting smaller plants and shoots.

Scholars have been recommending the installation of solar panels on green roofs since 2011. Dr C. M. Hui of the Department of Mechanical Engineering of The University of Hong Kong conducted a study at the University's main library, and found that solar panels generated 4.3% more energy when installed on green roofs. Furthermore, he observed that drought-resistant plants growing near the library's ventilation systems grew better in the shade of the panels [2]. An earlier Australian study also found that solar power systems were 3.63% more efficient when installed on green roofs compared to conventional roofs [3].

There are many benefits of installing solar power systems on green roofs, but it is important to take several precautions into consideration, of which the most important concerns safety. Legislation currently requires open roofs to have a minimum load of 2 kilopascals (kPa). With ordinary green roofs, the plants, soil, and water absorbed by the soil after rain or irrigation have already constituted a considerable load on the roof, and that is before adding on the weight of a solar power system (admittedly, the systems themselves are usually lightweight, but their base and mounts need to be heavy and strong enough to support them in the event of strong winds or storms). As such, when considering installing solar power systems on green roofs, it is crucial to ascertain whether the roof can support the weight and take necessary steps to ensure this, such as by selecting thin, lightweight soil that requires little to no irrigation (known as an Extensive Green Roof), or by developers pre-installing supporting poles on the roofs of new buildings for the potential addition of solar power systems at a later date.

Another consideration is the angle and orientation of the solar power system: although sunlight changes over time, solar panels need to be angled and oriented in such a way as to ensure the plants can still grow, and it would be best to select shade-tolerant plants that do not require much sunlight.

The high potential for installing solar panels on farmland

Hong Kong's agricultural industry is small, with the territory's 2,500 farms occupying a mere 0.68% of its land mass and predominantly growing vegetables or ornamental plants. However, replacing imported food with locally grown and supplied crops will help stabilise food supplies, which could be at risk of disruption due to climate change, as well as reduce carbon emissions. In order to meet ever-growing food and energy needs, and mitigate the impact of climate change, regions such as Japan, Taiwan and the UK are expanding the adoption of agrivoltaic systems, whereby crops are grown and electricity is generated on the same plot of land.

The main factor to consider here is whether the installation of solar power systems on arable land will affect crop harvests. A study in the US investigated the effect of agrivoltaic system on tomato fields found that such system reduced water requirement of certain crop types and increased the efficiency of irrigation on crop growth. For this study, photovoltaic panels were installed facing south at an 18° inclination for optimum irradiation, with the mount brackets on the higher side set at 2.2 metres and the lower side 0.8 metres above the ground respectively. Tomatoes were planted underneath and on either side of the panels, and the surrounding microclimate was monitored, including air temperature, relative humidity, soil temperature, soil moisture, sunlight and wind speed. Testing was carried out when the soil moisture was at 75% and 40%. The results showed that the shade from the solar panels significantly increased the amount of water in the soil, and that planting crops beneath solar panels and only irrigating when soil moisture is low can help maintain the soil moisture level in arid regions, and improve the efficiency of irrigation on crop growth. However, it is important to note that tomatoes require direct sunlight to grow, and that increased solar panel coverage therefore leads to lower yields. As a result, this strategy should not be deployed for f crops that require high light intensity [4].

Japan similarly needs to massively expand its renewable energy production to meet its carbon reduction targets, and agrivoltaic systems offer a promising solution. However, to minimise impact on agricultural production and to ensure that the sale of electricity would not become farmers' main source of income, Japan's Ministry of Agriculture, Forestry and Fisheries has mandated that yields must not fall below 80% of normal levels, alongside a host of other conditions. These include the requirement that solar panel mounts be lightweight and easily removable, that there is enough space for the use of agricultural machinery, and that installed panels have no impact on other plots. With these restrictions in place, Japanese researchers studied four fields used to cultivate rice, a staple in Asia, each with its own size, density and angle of installed solar panels. Factors known to affect rice yields, including fertiliser application, temperature and solar radiation, were then observed to evaluate the changes caused by shade from the photovoltaic systems installed above the rice field. The study found that, in order to meet the 80% target yield, the maximum permissible shade range from agrivoltaic systems was 27-39%. Supposing the density of agrivoltaic systems in Japanese rice paddies was 28%, this would still produce 284 GWh of electricity every year, equivalent to almost 29% of Japan's total energy demand in 2018. However, the study also found that the lower levels of sunlight resulted in lower rice yields. Further research would be needed to study possible solutions to this, such as installing solar panels on the north side of the paddies (and thereby reducing the amount of sunlight reaching the panels instead), choosing to plant crops based on how much sunlight they need, or considering different solar panel materials [5].

The need for more research and community engagement in Hong Kong

Climate change, heat island effects, insufficient greenery and unstable food supplies are all problems that are faced by cities around the world. Most of them are taking multi-pronged approaches to tackling these problems. Whilst developing renewable energy sources, they are also rethinking ways in which their land resources are used to best serve their populations. With its low rate of renewable energy generation and reluctance to consider dual land use, Hong Kong has got off to a slow start. The examples cited above show that Hong Kong has a number of avenues it can deploy to increase the proportion of its energy generated from renewables; agrivoltaic systems alone could generate up to 190 GWh if panels were installed on just 10% of the territory's arable land [6]. Whether solar energy systems are installed in fields or on rooftops, there needs to be constant scientific testing and data collection to identify which solar panel angles, orientations, materials, surface areas, shapes, arrangements, neighbouring crops or plants, irrigation methods and soil drainage measures work best for Hong Kong, in order to use its limited space effectively and bring other benefits besides renewable energy generation. CarbonCare InnoLab's Solar Care programme has partnered with NGOs to install solar panels and connect these to utility companies' FiT schemes, as well as organise a number of community events aiming to raise public awareness about the dangers of climate change and the importance of renewable energy. The solar power systems we are currently installing at 73 selected sites are expected to generate around 4.8 MW of electricity every year.

CarbonCare InnoLab also hosted the Asia Solar Energy for Climate Change Conference (ASECCC) webinar on 23-25 August, in which over 40 speakers across the Asia Pacific Region shared their efforts in utilising solar energy to combat climate change. Various speakers acknowledged that beyond technological advancements, it is paramount to assess the societal impacts of energy transition. They also shared inspiring examples of solar projects catering to communities' needs. While there is rapid growth in global solar energy investments, the global investments in renewable energy to achieve the net zero goal fall significantly short of the target. To bridge this gap, funds with more grants and concessional loans will be crucial. We would like to see more collaboration among various stakeholders, including green groups, NGOs, government departments, electricity companies, renewable energy experts, building owners/managers, agricultural land users and the general public, to expand the use of renewable energy sources in power generation and together build a zero-carbon Hong Kong.

References:

1. Polytechnic University of Hong Kong. "Potentials of generating clean solar energy in Hong Kong," https://www.polyu.edu.hk/cpa/excel/en/201703/viewpoint/v1/index.html
2. Dr. Sam C. M. Hui and Miss S. C. Chan. "Integration of green roof and solar photovoltaic systems"
3. University of Technology Sydney. "Green Roof & Solar Array – Comparative Research Project, Final Report July 2021," https://opus.lib.uts.edu.au/bitstream/10453/150142/2/City%20of%20Sydney%20Final%20Report%20EPI%20R3%20201920005.pdf
4. H. A. AL-agele, K. Proctor, G. Murthy and C. Higgins. A Case Study of Tomato (Solanum lycopersicon var. Legend) Production and Water Productivity in Agrivoltaic Systems. https://doi.org/10.3390/su13052850
5. R. A. Gonocruz, R. Nakamura, K. Yoshino, M. Homma, T. Doi, Y. Yoshida and A. Tani. Analysis of the Rice Yield under an Agrivoltaic System: A Case Study in Japan. https://doi.org/10.3390/environments8070065
6. According to the Agriculture, Fisheries and Conservation Department, roughly 7.6 km² of land in Hong Kong is used for growing crops. Calculated on the basis of 10% of this figure, 340,000 solar panels covering an area of 2.2 m² each could be installed on Hong Kong farmland. Assuming that each panel is capable of generating 550 W of electricity, this would give a total output of 190 GWh.

Authors:
Mr Kwok Hiu Chung, Senior Programme Officer, CarbonCare InnoLab
Mr Ken Tai, Project Manager, CarbonCare InnoLab

October 2023