The use of renewable energy sources to produce electricity is one of the key actions of the energy transition for reducing carbon emissions to the atmosphere. On-site generation of electricity from renewables can help local municipalities, cities and communities to have substantial environmental, economic and social benefits. Indeed, generating electricity in the urban environment is possible to reduce transmission and distribution losses, and increase flexibility and energy security. The main technologies used for renewable electricity generation which can be adopted in cities are:
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Wind power: wind represents one of the renewable sources which gained momentum in recent years with a substantial increase in wind power installed worldwide and a decrease in cost [1]. Wind energy is converted into electricity using wind turbines which are a mature technology available in different sizes and energy capacity. Today, small wind turbines are available to provide electricity for small-scale applications [2]. Wind turbines applied to cities can be stand-alone or building-integrated wind turbines. The turbines can be either horizontal-axis or vertical-axis [3].
Urban wind generation
Gil-García IC, García-Cascales MS, Molina-García A. Urban Wind: An Alternative for Sustainable Cities. Energies. 2022; 15(13):4759. https://doi.org/10.3390/en15134759
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Solar photovoltaics (PV): Photovoltaics (PV) represent the most used technique to convert solar radiation into electricity. A PV module is made of cells of semiconductor material which can convert light (photons) to a voltage potential (electricity). The most common installations of PV modules are roof or ground-mounted PV panels. PV modules can be also integrated into the building envelope (building-integrated PV, BIPV) acting as real construction products necessary for the integrity of the building functionality [1]. The efficiency of PV modules can be increased using PV concentrators (CPV) which use optical devices with cheap and suitable technology to concentrate the light on small and highly efficient PV solar cells. Another promising solution is the installation of PV modules in water bodies including oceans, lakes, reservoirs, irrigation ponds, and wastewater treatment plants. A floating PV (FPV) plant consists of a pontoon or separate floats anchored to the bottom of the water, to the shore or to adjacent structures.
Solar photovoltaics (PV)
Source: http://www.hybuild.eu/
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Geothermal power [4]: The thermal energy contained in the deep earth crust can be used both for heating/cooling and to produce electricity. In the latter case, the steam captured in deep ground reservoirs is directly used to drive turbines and generate electricity. Dry steam, flash steam or binary cycle can be employed to generate electricity. Another technology under development is enhanced geothermal systems (EGSs) where e a subsurface fracture is created and a heat transfer fluid is injected and heated by the rocks and pumped to the surface to generate electricity.
Enhanced Geothermal System (EGS)
Sowiżdżał A, Starczewska M, Papiernik B. Future Technology Mix—Enhanced Geothermal System (EGS) and Carbon Capture, Utilization, and Storage (CCUS)—An Overview of Selected Projects as an Example for Future Investments in Poland. Energies. 2022; 15(10):3505. https://doi.org/10.3390/en15103505
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Biomass, waste-to-energy, and biofuels [3,5] where electricity is produced from local biomass waste (e.g., from municipal solid, wood, agricultural wastes, sewage, and plant material). Waste-to-energy can incentivize waste recycling activities in cities promoting circular economy and and minimising the decrease in volume of usable waste, which is necessary to ensure a reliable and constant supply of energy. Biomass feedstocks can be burned producing steam to feed turbines that most commonly generate heat and electricity (combined heat and power - CHP). Biomass can be also converted in combustible oil or biofuels in a gasification process in a low-oxygen environment which can be more efficient compared to the burning of solid biomass (anaerobic digestion). Biogas can be captured from landfills and other facilities such as wastewater and manure treatment plants. The methane produced from the anaerobic digestion can be burned in a combustion process to produce electricity.
Biofuel products from biomass
Clauser NM, González G, Mendieta CM, Kruyeniski J, Area MC, Vallejos ME. Biomass Waste as Sustainable Raw Material for Energy and Fuels. Sustainability. 2021; 13(2):794. https://doi.org/10.3390/su13020794
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Hydropower [6,7]: it is a mature technology which converts the kinetic energy from falling or running water to electricity using turbines. Nowadays, hydropower is the largest source of renewable electricity worldwide and it can be available in different sizes. Small hydropower systems can be also integrated into municipal water facilities or irrigation ditches.
Small-scale hydropower plant in Kempten.
Rotilio M, Marchionni C, De Berardinis P. The Small-Scale Hydropower Plants in Sites of Environmental Value: An Italian Case Study. Sustainability. 2017; 9(12):2211. https://doi.org/10.3390/su9122211
Another technology which can be considered for on-site renewable electricity production are fuel cells [8]. These devices are able to generate electricity through a chemical reaction of oxygen and hydrogen. Since pure hydrogen does not exist in nature it can be generated from other sources including the reformation of natural gas or biogas or by electrolyzing water. The process of electrolysing water, which needs electricity, can be done through renewable sources.
MATURITY:
Renewable electricity generation accounts for 3,146 GW installed in 2021 and most technologies used for on-site renewable generation are nowadays mature and available on the market. Several examples already exist in cities with generation capacity from a few kW to MW. Systems can be installed on or near both residential and non-residential buildings including, schools, hospitals and industries [9].
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