Implementation and use of renewable sources have several co-benefits, including: 

• Health 

• Improved air quality – by reducing CO2 emissions and air pollutants related to the combustion of fossil fuels 

• Social: 

• Reduce energy poverty – by reducing the dependence on the grid and thus the cost 

• Resource efficiency 

• Bioenergy and waste-to-energy can improve circularity with a reduction of waste and better management. 

• Economy 

• Growth of local economy and job creation 

• Energy security – by increasing the self-production of energy, by alleviating the peak demand of the grid, and by increasing the energy resilience of cities and municipalities 

The share of renewables can be better exploited in new and renovated buildings with low energy demand, through a variety of building envelope solutions and passive building solutions, as listed below: 

• Envelope insulation 

• Envelope thermal capacity 

• Adaptive facades  

• Kinetic facades 

• Ventilated/double skin facades 

• Biomimetic facades 

• Green roof/green facades 

• Cool roofs/facades 

• Reflective (incl. retroreflective) roof/facades 

• Energy saving glazing  

• Joinery for low-energy houses or passive houses  

• Passive building design strategies 

• Natural ventilation  

• Sunspaces 

The share of renewable energy can be enhanced using: 

• Energy storage solutions 

• Sustainable and energy-efficient active systems 

• Smart solutions  

• Efficient control systems 

• Digital solutions 

• Microgrid 

Bioenergy and waste-to-heat can be integrated with solutions for management of solid waste. 

• Climate and geography: Climate and geography can be determinant factors in the integration of on-site renewable sources. In the case of wind power, suitable locations with enough wind energy are needed in order to make the investment profitable. Hydropower and geothermal are constrained by geological requirements. PV generally does not have particular constraints, however, the cost-benefits are higher in locations with high solar radiation since the solar fraction of thermal energy produced with renewables is higher thus reducing the payback period.  

• Urban form and layout: the built environment can have a strong impact on the space available for the installation of renewable systems. Nowadays, in Europe, in order to promote the integration of on-site generation from renewables, positive energy districts have received attention as they involve the planning of energy efficient and flexible urban areas to produce zero emission and surplus of energy. In the case of roof-mounted PV and BIPV, shading coming from adjacent buildings and vegetation can reduce the system efficiency. For urban wind power, the built environment can have a large influence on the wind flow. Deep geothermal installation may not be always accessible in urban areas. In the case of waste-to-energy conversion, the site can be a challenge both in terms of space availability and of the potential risk of social acceptance related to the impact on air quality and health.  

• Technical aspects/infrastructure: In order to implement renewables in cities the energy infrastructure must be flexible enough. Moreover, the intermittency of renewable sources must be taken into account. In order to mitigate these drawbacks, storage technologies and digital technologies can help to maximise the benefits of renewable sources. The direct integration of renewables in buildings can strongly support the targets of energy efficiency set by the Energy Performance of Buildings Directive (EPBD) which requires all new and renovated buildings to be nearly zero energy buildings (nZEB).  

• Policy and regulatory/legal framework: Generally, solar systems are supported by different dedicated policies [14]. These include targets on the share of renewables, that nowadays in Europe is proposed to be increased to 45% under the Fit for 55 package (including the European Solar Rooftops initiative) [15]. However, cities and governments have also supported renewable integration by setting specific targets and encouraging investment in renewable sources.  In the last decade, as municipal governments increased their awareness of renewable sources and highlighted the importance of distributed energy sources and on-site renewable generation, the dynamics between the energy systems and the infrastructure usually connected to national or regional systems have changed. The installation of on-site renewables can be also related to mandatory building codes. For example, the European Solar Rooftops initiative will demand obligatory PV in all refurbished and new constructions [15]. Moreover, refurbished buildings will benefit from the “Renovation wave” strategy adopted by Europe. In order to increase the share of renewables in cities, local governments can invest their own resources for the development of local solar power projects where citizens and local businesses can be involved [9]. Community solar projects can be suitable for users which have constraints on installing renewable facilities. Other solutions which can incentivize citizens and local businesses to consume energy from renewables are Power Purchase Agreements (PPAs) [16] and virtual net metering which enable users to consume energy generated elsewhere and receive credits on the energy bill according to their share of investment in the project.  Municipalities which own their own utilities have better control on the energy sources and are able to directly provide renewables to the user. The rapid expansion of on-site renewable generation can lead to challenges in the distribution network. In this case, upgrade of the current distribution network and additional storage capacity may be needed. The development of mini and micro-grids can also promote the deployment of on-site renewable generation. On-site renewable energy can be supported by different policy instruments including feed-in-tariffs and tax incentives. 

• Funding and financing: Financial incentives including subsidies, green bonds, low-interest loans and tax incentives, and deductions are available to support the initial investment and increase the competitiveness of renewables against other solutions [9]. In Europe the 2021 proposal for the revision of the Energy Taxation Directive continues to allow Member States not to tax electricity of solar origin. Moreover, Europe provides different funding programmes to promote renewables in urban areas. 

Generally, the implementation of renewable sources faces some common barriers including [17]:  

• Lack of public awareness and information 

• Lack of skilled professionals 

• Cost competition with fossil fuels 

• Lack of grant and subsidies 

• High initial capital cost 

• Intangible costs 

• Limited availability of infrastructure and facilities 

• Lack of operation and maintenance skills 

• Lack of research and development capabilities 

• Lack of adequate policies 

• Administrative and bureaucratic complexities: 

• Lack of standards and certifications 

Moreover, technology barriers may be specific for each technology. 

• Wind [18,19]:  

• Size of wind turbine and inability to capture low wind speed and turbulent flows 

• Poor understanding of aerodynamics of wind in cities 

• Visual and acoustic barriers 

• Lack of experimental data 

• Photovoltaic [20,21]:  

• Lack of collaboration between the building and PV industries 

• High installation, maintenance and repair costs compared to other sources of energy 

• Aesthetical constraints of BIPV for some architects 

• Technical constraints on floating PV including durability and environmental footprint on the aquatic ecosystem 

• Hydropower [22]:  

• Technical challenges related to hydrology, dam design, and maintenance. 

• Bioenergy and waste-to-energy [3]: 

• Social acceptance for integration in cities 

• Emissions due to fly ashes and flue gas 

• Geothermal [23]: 

• High initial cost  

• Regulatory issues (i.e. general licensing, permits for ancillary operations) 

• Environmental issues related to drilling and installations 

• Complex project design and technology selection 

 Lack of exploration data 

The main challenges after the implementation can be different for each technology: 

• Wind[19]: 

• Noise and visual aesthetics 

• Impact on the wildlife 

• Photovoltaic (PV) [21]: 

• Efficiency – progresses in research are made in order to increase the conversion efficiency of PV cells 

• Cooling of PV modules – overheating of PV cells reduces efficiency and lifetime. Solutions include employing a PV-T system (PV with solar thermal features) or using water, yet this can come with additional costs and complexity.  

• Soiling can cause power loss. In this case anti-soling solutions and regular module washing are implemented. 

• Shading and vegetation inside and outside the plant for grounded mounted PV 

• Extreme weather events can damage PV installation 

• Bioenergy and waste-to-energy [3]: 

• Local fine dusts 

• Hydropower [24]: 

• Limits navigation 

• Power lines can change the landscape 

• Modifications in aquatic habitats 

• Navigation and public access to some areas may be affected 

• Damming areas rich in biodiverse flora results in carbon emissions 

• Geothermal [25]: 

• Emissions due to the steam power plant cooling towers 

• Subsidence and induced seismicity when extracting fluids from the ground 

• Possible negative impact on wildlife and vegetation  

Literature 

[1] IRENA. Wind Power n.d. https://www.irena.org/costs/Power-Generation-Costs/Wind-Power#:~:text=The global weighted-average cost,%2FkWh%2C without financial support. 

[2] Orrell AC, Sheridan LM, Kazimierczuk K. Distributed Wind Market Report: 2022 Edition. Pacific Northwest National Lab.(PNNL), Richland, WA (United States); 2022. 

[3] IRENA. Rise of Renewables in Cities: Energy Solutions for the Urban Future 2020. 

[4] Center for Sustainable Systems University of Michigan. Geothermal Energy Factsheet 2021:Pub. No. CSS10-10. 

[5] Local Government Climate and Energy Strategy Series. On-Site Renewable Energy Generation 2014. 

[6] IEA. Hydroelectricity. Paris: 2022. 

[7] Office of Energy Efficiency & Renewable Energy. Hydropower Basics n.d. https://www.energy.gov/eere/water/hydropower-basics. 

[8] Laporte A, Shook R. Fuel Cells. 2015. 

[9] Ranalder L, Busch H, Hansen T, Brommer M, Couture T, Gibb D, et al. Renewables in cities 2021 global status report. REN21 Secr Paris, Fr 2021. 

[10] Office of Energy Efficiency & Renewable Energy. How Wind Energy Can Help Us Breathe Easier 2022. https://www.energy.gov/eere/wind/articles/how-wind-energy-can-help-us-breathe-easier. 

[11] Sullivan JL, Clark CE, Yuan L, Han J, Wang M. Life-cycle analysis results for geothermal systems in comparison to other power systems: Part II. Argonne National Lab.(ANL), Argonne, IL (United States); 2012. 

[12] Chaliki P, Psomopoulos CS, Themelis NJ. WTE plants installed in European cities: A review of success stories. Manag Environ Qual An Int J 2016. 

[13] Las-Heras-Casas J, López-Ochoa LM, Paredes-Sánchez JP, López-González LM. Implementation of biomass boilers for heating and domestic hot water in multi-family buildings in Spain: Energy, environmental, and economic assessment. J Clean Prod 2018;176:590–603. 

[14] IRENA, IEA and REN21. Renewable Energy Policies in a Time of Transition: Heating and Cooling. 2020. 

[15] European Commission. REPowerEU: A plan to rapidly reduce dependence on Russian fossil fuels and fast forward the green transition 2022. 

[16] European Commission. EU Solar Energy Strategy 2022. 

[17] Moorthy K, Patwa N, Gupta Y. Breaking barriers in deployment of renewable energy. Heliyon 2019;5:e01166. 

[18] Abohela I, Hamza N, Dudek S. Urban Wind Turbines: Potentials and Constraints. CISBAT 2013, Clean Technol Smart Cities Build 2013. 

[19] Office of Energy Efficiency & Renewable Energy. Advantages and Challenges of Wind Energy n.d. https://www.energy.gov/eere/wind/advantages-and-challenges-wind-energy. 

[20] Karakaya E, Sriwannawit P. Barriers to the adoption of photovoltaic systems: The state of the art. Renew Sustain Energy Rev 2015;49:60–6. 

[21] IRENA. Future of solar photovoltaic - Deployment, investment, technology, grid integration and socio-economic aspects. Abu Dhabi: 2019. 

[22] Sovacool BK, Dhakal S, Gippner O, Bambawale MJ. Halting hydro: A review of the socio-technical barriers to hydroelectric power plants in Nepal. Energy 2011;36:3468–76. 

[23] Pan S-Y, Gao M, Shah KJ, Zheng J, Pei S-L, Chiang P-C. Establishment of enhanced geothermal energy utilization plans: Barriers and strategies. Renew Energy 2019;132:19–32. 

[24] Okot DK. Review of small hydropower technology. Renew Sustain Energy Rev 2013;26:515–20. 

[25] Kabeyi MJB. Geothermal electricity generation, challenges, opportunities and recommendations. Int J Adv Sci Res Eng 2019;5:53–95. 

On-site renewable electricity generation can lead to substantial benefits in cities. Each of the available technologies has proved its own related benefits and expected impact [9]. In Dublin, the council issued a contract to install PV in different sport centres which led to a saving of 129,000 EUR and 321 tonnes of carbon emission annually. In 2019, in UK, 300 communities were involved in energy generation with more than 250 MW installed. The Community Energy London network, which connects 700 people in 30 projects resulted in 450 tonnes of CO2 savings since 2018.  

The carbon emissions generated through the life-cycle of technologies using renewables is much lower compared to fossil fuel plants. For example, using wind power, the life-cycle greenhouse emissions are around 11 grams of CO2 per KWh of electricity generated which is 90 times and 40 times less than the electricity generated using natural gas or coal [10].  Similar considerations can be proposed for photovoltaic, hydropower, and geothermal power [11]. 

The use of biomass and the implementation of waste-to-energy installations has also a huge potential in cities to achieve sustantial environmental savings. Through combustion, it is possible to reduce the volume of urban solid waste by up to 90% [12,13]. Moreover, the methane generated from landfills has a higher global warming potential than CO2, therefore the recovery of landfill gases and manures can effectively reduce the amount of air pollutants [5]. 

Implementations should follow the technical requirements and standards included in national and local legislation.  

In this context, education and training to promote and install renewables are needed and the availability of skilled workers is critical. Further, training programmes and certification schemes are necessary to promote local employment.  

In order to promote these solutions, the following instruments might be considered: 

• energy audits 

• certification and labelling 

• infodays 

• workshops 

• awareness campaigns 

• promotional campaigns 

• grants/subsidies 

• loans/soft loans  

• incentives/disincentives 

• taxation  

• integrated action plans 

• decarbonisation plans for industry 

• building codes/standards 

• energy supplier obligations 

• smart readiness indicator 

• provision of energy efficient products and services 

• GHG scenario modelling 

• LCA/LCC 

Citizen participation can also play an important role in the deployment of renewable energy systems as citizens can be active individual prosumers by choosing to purchase energy in a way that increases the share of renewables or by participating in community energy projects [17]. Moreover, community energy projects can help to promote on-site renewables. 

WINDUR (EU-funded FP7 project – Grant Agreement: 605067). WINDUR proposes a small vertical-axis wind turbine (VAWT) optimised for use in urban environments as a roof-top mounted system. Proposed novel developments include (1) a variable speed control system developed to maximise VAWT´s energy yield under rapidly changing wind speeds, (2) an aerodynamic design based on a helical rotor, refined for reducing rotor weight and loads on the roof, to lower WINDUR installation complexity and cost, and (3) an assessment of wind resources in urban areas, for characterising those locations with better wind resources. 

DESTRESS (EU-funded H2020 project – Grant Agreement: 691728). Demonstrating enhanced geothermal systems (EGSs) for renewable heat and electricity production from deep geothermal resources in a wide range of geological sites in France, Germany, Lithuania, The Netherlands, South Korea, and Switzerland 

DEEPEGS (EU-funded H2020 project – Grant Agreement: 690771)  Demonstrating the feasibility of enhanced geothermal systems (EGsS) for delivering energy from renewable resources in Europe by testing stimulating technologies for EGS in different geologies: superhot conditions (550 °C) or in very deep wells (5 km) 

Hydro4U (EU-funded H2020 project – Grant Agreement: 101022905). The EU-funded Hydro4U project will adapt European small-scale hydropower (SHP) technologies to meet Central Asia’s needs. Specifically, it will install and assess two demo plants with reduced planning and construction costs that do not compromise efficiency. The overall aim is to find innovative solutions that are fit-for-purpose and to demonstrate EU quality standards and create entry points in developing markets for the entire European small-scale hydropower industry. 

FORBIO (EU-funded H2020 project – Grant Agreement: 691846). Assessing bioenergy production potential in underutilised lands in Europe to determine suitable types of advanced biomass and to analyse the barriers to their deployment in three target countries. 

PV-Prosumers4Grid (EU-funded H2020 project (Grant Agreement ID 764786) with the main objective to increase the market share and market value of PV by enabling consumers to become PV prosumers in a system-friendly manner. PVP4Grid aims at a better power system integration of PV with a focus on market integration. New management and business models to combine PV, storage, flexible demand and other technologies into a commercially viable product, will be assessed, improved, implemented and evaluated.