Solar Photovoltaic (PV) technologies convert the energy from sunlight into electricity. The technology has improved significantly in recent years and solar PV costs have reduced by 40-80 per cent since 2010.
We now see solar PV technology more commonly in devices ranging from calculators to streetlights and providing electricity for homes and communities. Solar PV can be deployed at a significant scale either as decentralised rooftop solar PV or as large-scale centralised solar PV farms. In 2018 , rooftop solar PV accounted for a quarter of new additional renewable electricity generation capacity globally.
Being decentralised in nature, rooftop solar PV technology enables families, farmers, businesses and all building owners to directly engage in delivering the solutions to climate change. But at what scale? Do we have enough rooftops to meet a significant portion of global electricity demand?
We at the MaREI research centre in University College Cork, along with our international partners at Columbia University in the US, Imperial College London and Ahmedabad University in India, sought an answer to this question.
Our research centred around devising a novel resource assessment framework that utilises cutting-edge machine learning, large-scale open-source datasets and geographical information systems to deliver the first global assessment of rooftop solar PV potential at a spatial scale ranging from cities to continents.
The motivation for this was to demonstrate how cutting-edge technologies can advance our knowledge of the energy systems in a global context. The results were recently published in Nature Communications in time to make a useful contribution to deliberations at the Cop26 UN climate summit in Glasgow.
We used a machine learning model to learn the relationship between satellite-demarcated human-made built-up area, length of roads, population count, and true rooftop area derived from open-source datasets in squares measuring 10sq km. We then used this trained model to estimate rooftop areas at a global scale. We utilised about 300 million buildings and nearly 16 million kilometres of road to train the model. This research was funded by Science Foundation Ireland and the National Natural Science Foundation of China under an Ireland-China research partnership programme.
The recently published Intergovernmental Panel on Climate Change report highlights how we may reach 1.5-degree increase in mean surface temperature above pre-industrial levels within a decade. This temperature level was the agreed limit in global warming that governments wish to remain within to avoid dangerous levels of climate change.
The report provides even more evidence that we need to rapidly bring renewable sources of energy into our energy mix. Due to its fast deployment time, the availability of “free” real estate (read rooftops) and the benefits of “democratising” energy delivery, rooftop solar PV appears to be a prime candidate to spearhead the move towards a low-carbon future.
Global scale
Our research shows that rooftop solar PV can deliver enough power at a global scale to match our current yearly global electricity demand. This is a very significant finding. The results also show that the total global rooftop area is equivalent to the size of the UK and covering half of the global rooftops at current technology limits will be enough to power the earth.
The resource is higher and cheaper in some areas compared to others, and approximately one-third of the global electricity demand can be met at below one US cent per unit of electricity.
The largest potential along with the cheapest cost of deployment of rooftop solar PV lies in Asia followed by North America with the highest cost of deployment. Next in line in terms of potential, we have Europe where cost of deployment is in the mid-range relative to other areas. The research also revealed variations in solar energy during the year.
The largest intra-year variations in potential exist in Northern latitudes where ±40 per cent variation between summer and winter months can be observed. Countries near the equator fare well, where ±1 per cent variation can be observed.
In addition to the contribution to climate action, the results of this scientific analysis show that rooftop solar PV can also play an important role in meeting other sustainable development goals. Nearly half the potential we computed is dispersed within low-density rural areas globally.
Considering that there are 800 million people without access to electricity power globally, this highlights the potential of rooftop solar PV to increase global access to electricity. Separately, 20 per cent of the potential is concentrated in the high-density areas where rooftop solar PV can contribute to reducing local air pollution.
All this is good news, but what about night-time when the sun does not shine? Our study considers the day/night cycles and provides aggregated values for a month or a year. All the power will of course be generated in the day cycle, and we need system solutions to also deliver electricity in night. Battery and other storage technologies are key here in addition to ensuring we have a smarter grid with more responsive demand-side solutions (ie to enable shifting the time within a day when electricity is used).
What about Ireland?
In our assessment, we find that approximately 220sq km of rooftop area is present in Ireland which can meet more than 50 per cent of current total annual electricity needs.
This is significant as we perceive those perpetual cloudy days may make Ireland less suitable for deployment of solar PV. Nearly half of the aggregated rooftop area exists in lower-density areas (below 200 people per sq km), which is quite different to the situation in the UK (where nearly half of the aggregated rooftop area exists in areas below 1,000 people per sq km).
Rooftop solar PV in Ireland has significant potential outside of the four major cities. Focusing just on households, Ireland currently has about 1.8 million permanently occupied dwellings, of which 0.5 million are being rented in some form. This is an important factor in terms of decision-making power lying with families or landlords, which will need to be considered in measures to further increase the deployment of rooftop solar PV in the country.
Why is this new research important?
While rooftop solar PV has long been seen as one of the important solutions to climate change, this scientific research assesses the global resource for the first time in detail. A key immediate benefit of this is in the identification of rooftop solar PV hotspots globally, which can inform the discussions at Cop26. The analysis also identifies hotspots within each country that local governments can use to formulate inclusive and evidence-based energy policy. This is timely for Ireland given the requirement to develop local authority climate action plans, as called for in the Climate Action and Low Carbon Development (Amendment) Act 2021.
National governments can also benefit from the results and analysis of this research. Effective policy levers and financial instruments that will enable and support the transition to a just and equitable low-carbon energy future need data that captures the spatial detail and diversity of renewable energy resources.
It can also provide new insights on locations where rooftop solar PV might grow at scale, which benefits electricity network operators and market regulators in understanding how future electricity supply might evolve.
This research also provides useful new data for energy modellers and institutions such as the World Bank, Integrated Assessment Modeling Consortium, International Monetary Fund and the United Nations Framework Convention on Climate Change, who will be better able to represent the rooftop solar PV in their global and national models, scenarios and policy frameworks.
This is valuable because of the role that rooftop PV plays as a significant part of the urban and rural energy solutions to climate change, in a just way that also increases electricity access. In this way, the analysis can be used to support broader sustainable development goals and energy justice in addition to climate action. Separately, a global study of this kind adds to the science and engineering contribution that the Irish research system is providing on the solutions to climate change.
Siddharth Joshi is a PhD student at MaREI research centre in UCC. Brian Ó Gallachóir is director of MaREI and led the UCC delegation to Cop26