Energy solutions for low-carbon cities
IRENA has published a report which analyses the ways that cities can scale up their use of locally available renewables as they move to decarbonise their energy systems. Today, more than half the world’s population lives in cities and accounts for 80% of the global GDP. The Intergovernmental Panel on Climate Change has found that cities contribute 71–76% of global energy-related carbon dioxide emissions. Fossil fuel and other emissions are associated with serious air pollution problems in over 80% of the world’s cities, generating approximately 7 million premature deaths each year1. Urban settlements are expected to grow by another 2.5 billion people over the next thirty years2. In the coming decade this growing urbanisation will coincide with the pressing need to decarbonise the global energy system.
As a result of their numerous roles, including urban planning and the provision of services, cities have great potential to increase the use of renewable energy while achieving local goals such as reducing air pollution to improve public health, mitigating climate change, supporting the local economy and building resilient infrastructure. In addition, it is important to include renewable energy technologies in the infrastructure development of urban areas as soon as possible to decrease the need for costly retro fitting in the future.
In many urban areas, trends in renewable technology are already developing, such as energy storage, smart charging for electric vehicles, renewable power-to-heat and renewable power-to-hydrogen, digital technologies and intelligent energy management. The trend has accelerated due to the rapid cost reductions in solar PV panels and battery storage systems. This development is expected to continue, along with the arrival of innovative business models such as energy-as-a-service, aggregators, peer-to-peer electricity trading, community-ownership models, pay-as-you-go and urban energy planning.
One of the most distinct characteristics of an urban energy system is that it serves as the sociotechnical interface that links the physical energy system with its users. Hence, consumer behaviour matters more than just the energy system alone. Innovations in energy systems have contributed to the view and practice of the prosumer, an actor that is both a consumer and a producer of energy. Examples are rooftop solar PV systems with battery storage and smart energy management. New actors are also appearing, such as aggregators, which bundle several distributed energy resources into a single entity (a virtual power plant) to interact and trade in power or markets.
The dynamics between system operators and consumers are changing and the boundary between energy production and consumption is becoming blurred. It is important for cities to see and use the potential of this development and form appropriate institutional support. Over the past decade, several cities have sought to gain greater control over their energy systems. By 2019, some 671 cities had set at least one target favouring the use of renewables in their jurisdictions. More than 60% of these cities had set a target to achieve 100% renewable energy, and 45% of them are in Europe. City governments can be trendsetters, leading by example to push change. They act as laboratories of innovation for new policies and business models, testing concepts and approaches. As such, the actions taken by cities can provide important lessons and influence change at the state and national levels, while at the same time providing valuable case studies for other cities around the world.
The technologies that are of special interest in urban environments are as follows:
Solar photovoltaics (PV): When it comes to solar irradiance the analysis shows that 95% of the cities that have the highest solar potential (i.e., cities in the top 10% for global horizontal irradiance, or GHI) do not have a set target for supporting renewable energy development. Even among the cities in the top 30% for solar potential, only 6% (39 cities) have a renewable energy target and only 2% (14 cities) have a target for 100% renewables. Urban-based solar PV systems are generally smaller in scale than ground-mounted systems located on the outskirts of cities. These systems are usually installed on, or integrated with, the roofs and facades of buildings.
Solar thermal: Solar thermal systems, which rely on different types of solar collectors, are usually used for water and space heating and in some cases for industrial process heat. Increasingly, cities and countries have adopted building codes mandating the use of solar water heaters for all new buildings. The solar system can be installed on the ground or on a building roof to supply heat for the building, community, district or city. However, in countries where natural gas is cheap and is the dominant heating source, solar thermal systems are less competitive in the absence of incentives or promotional schemes to support their social and environmental benefits.
Solar thermal cooling: With the growth in global cooling demand tripling from 600 terawatt-hours (TWh) in 1990 to 2,000 TWh in 2016, and projected to at least triple again by 2050, solar thermal energy has gradually extended into the cooling sector. For cooling purposes, solar thermal is typically coupled with absorption chillers to lower peak demand on the grid during hot summers, reducing blackouts and the costs for grid enhancement.
Bioenergy and waste-to-energy: These biomass- based energy sources can provide a relatively reliable and consistent supply of energy in comparison with solar PV. For cities, waste-to-energy offers a promising way to create a circular economy. However, the uncertainties of obtaining a sustainable supply of feedstock need to be addressed.
Urban wind power: Wind power has been used only marginally in cities and faces huge challenges to scale up. While examples exist of urban wind turbines generating electricity, their performance needs to be improved substantially, and large-scale implementation is scarce. The use of wind turbines in urban environments is mainly in the research and development phase. The lack of experimental data is a big drawback in the development of urban wind turbines.
Geothermal energy for direct use: With the need to decarbonise the heating sector, and recognising the potential and advantages of direct use of geothermal energy, applications in cities have been growing. Globally, the installed capacity of geothermal direct use has more than doubled since 2010, reaching 107,727 megawatts-thermal deployed across 88 countries in 2019. Geothermal technology is used mainly for space heating and cooling as well as for hot water in cities, through both stand-alone and district heating systems. For new cities or for the expansion of existing cities, installing geothermal energy systems would be much more cost efficient than integrating the systems into established infrastructure.
For most cities, integrating the renewable energy technologies described above would require upgrading the urban infrastructure to accommodate them, without compromising on operational reliability and stability. This highlights the importance of developing “smart” grids through innovation and the adoption of enabling technologies such as electric vehicles, energy storage systems and intelligent energy management systems. Smart grids present opportunities for using higher shares of variable renewables and for improvements in system efficiency.
In summary, renewable energy technologies that are integrated with local energy systems will be a vital foundation for creating the transformation needed in the cities of the future. When it comes to climate change “Cities are on the frontline of impact, but also of the solutions,” said Inger Andersen, Executive Director of UNEP3. How we decide to build our urban energy systems today will shape our collective future.
1. WHO Global Ambient Air Quality Database (database), World Health Organization, Geneva, www.who.int/airpollution/data/cities/en.
2. UN DESA (2018), The world’s cities in 2018 – data booklet, United Nations Department of Economic and Social Affairs, Population Division, New York, www.un.org/en/events/citiesday/assets/ pdf/the_worlds_cities_in_2018_data_booklet.pdf