Stationary energy is related to energy use and supply in buildings. Buildings account for 40% of energy consumption in cities. Hence, a significant switch to renewable energy in all buildings and facilities is vital. Implement energy-efficient measures to reduce energy demand and improve energy efficiency. For 75% of the EU's existing building stock, energy performance is poor and the buildings were constructed before current energy requirements were in place. Only 11% of the EU’s existing building stock is being renovated each year, and focus more on energy consumption, rather than the renovation of the building envelope. But actually, according to the IEA, energy efficiency and electrification are the two main drivers of decarbonisation of the buildings sector. So, both renovations should be considered: addressing buildings’ energy performance and buildings’ systems. In this sense, there are instruments available in different fields to facilitate the implementation of these kinds of renovations, such as modelling tools (i.e.: urban energy modelling and future scenarios), methodological schemes (i.e.: Sustainable Energy and Climate Action Plans), financial schemes (i.e.: blended finance, soft loans), etc.
Figure: Global direct CO2 emissions reductions by mitigation measure in buildings in the Net‐Zero Emissions by 2050 Scenario 
BUILDING ENVELOPE SOLUTIONS
Specifically, building envelope renovations help deal with the significant emissions of CO2 and air pollution in cities by reducing the large amount of energy needed for heating. With this solution, the heat demand for space heating and domestic hot water is reduced for the existing building stock by addressing heat losses (excluding heat supply) via extensive heating renovations (e.g., windows, insulation, etc.). The aggregated CO2 reduction potential of envelope renovations is great as Europe has many old and poorly insulated buildings. Still, it can also be very costly, with some renovation cases much more expensive than others. Envelope insulation can be done in several ways. In Cityfied project 31 buildings in Torrelago district (close to Valladolid) were renovated using ETICS (External Thermal insulation composite system). To convince the owners was important to involve them in the decision-making process (e.g., they created new clotheslines for them, they were involved in choosing the colours of the façades, etc.). Nevertheless, the users now complain that the external walls (painted in white) are getting dirty due to the exposure to surrounding roads and industries. They would have preferred another coating/final layer. Anyway, the project and savings motivated the owners to continue renovating their buildings (e.g. windows, roof, or ground insulation) to reduce their energy bills. Plus, the company selling the renovation of the windows, offered the district a special discount price to increase the sales. Other projects with retrofitting actions are Ile de Nantes, France, with multi-owner buildings retrofitting (insulating attics, walls, and installing thermostats and new energy systems) or Bergedorf-Süd retrofitting project in Hamburg, Germany, where roofs, façades, and windows were renovated. Regarding the windows, when they are renovated, their design can be improved to avoid solar radiation (with shading devices), or with the right selection of joinery (joinery for low-energy houses or passive houses), considering materials, transmission of heat, low-emissive glaxing, etc., to dress the openings and reduce energy needs. Another possibility is the integration of nature-based solutions such as green roofs, green walls, and green façades. Furthermore, the renovation has multiple co-benefits that are not being considered, such as the creation of jobs and local supply chains, increase of comfort indoors leading to a better quality of life, reduction of GHG emissions and energy poverty (the fewer energy needs, the more affordable it gets to live in that building which can lead to more costs savings to non-energy purposes ), a decrease in future maintenance costs, the energy-efficiency raise awareness among residents, etc.
The truth is, deep renovation is not always achievable in one go. Thus, the Renovation Wave highlights the importance of instruments such as building renovation passports (BRP), which can underpin renovations. The BRP is a document outlining a long-term (up to 15-20 years) step-by-step renovation roadmap to achieve deep renovation for a specific building. In Ireland, a consultation process of BRP was performed to co-design it (which parts should be included and how), as well as to promote it, and decide the roles (who can edit a BRP, who can have access to it, etc.). Another example is the Smart Readiness indicator (SRI), a common EU scheme for rating the smart readiness of buildings, which has been applied in multiple cases, such as in the renovation of Mediterranean buildings. Furthermore, bureaucracy facilitators such as one-stop-shop schemes (one-stop-shop for building renovation) or turnkey retrofitting services (Turnkey retrofit service) are also a good choice to speed up building renovation processes.
PASSIVE BUILDING SOLUTIONS
Energy-efficient new buildings are costlier than their counterpart but offer meaningful energy savings over a long period. New buildings already have to align with the minimum building standards (available in building codes per country). Still, improvements that raise the energy performance to higher standards than required can come with many benefits in cost savings and CO2 emissions reduction over time. With this solution, the heat demand for space heating and domestic hot water is reduced in new buildings, thus reducing associated fossil-fuel or electricity consumption. EPBD has played an important role in promoting NZEB and passive standards, especially forcing the renting market to have a certificate to be able to do their business (e.g., France, Spain ).
Passive building design strategies such as building orientation, passive heating, and cooling or natural ventilation should be considered, when possible, to be able to introduce nearly zero energy buildings in cities. Singapore’s university designed a high-comfort net-zero energy building with the integration of green inside and outside the building, over-sailing roofs, shades, and renewable energy production with a high-tech control system with adaptive comfort techniques. Passive standards can be used to promote building efficiency improvements, such as BREEAM, LEED, WELL, or Passive House.
In addition, energy performance certificates (EPCs) describe how efficient a building is, giving it a rate, which allows increasing transparency of the performance of the building stock, and helps municipalities to tailor policies and plans towards those non-efficient areas. In Greece and Cyprus, to raise people’s awareness of EPCs, financial incentives have been linked to the issuing of EPCs; examples include the Greek ‘saving at home’ and the Cypriot ‘save & upgrade’ programme. In Milan, an Environmental Decision Support System Tool (EDSS) that fees with information from the EPCs Italian database (Urban-scale environmental decision support system based on EPC databases), allowing to propose solutions for building retrofitting and development of district heating networks. The tool can potentially be calibrated with real-time monitor data, becoming this way a digital twin. Other tools are available, like in Portugal, where the energy certificate has become an "aggregator of information" interconnecting the energy certificate with supply and demand platforms (of the 'one-stop shop' type ), such as, for example, the casA + Portal.
There are other integrated solutions as innovative buildings adapted to climate change conditions, such as Climate-smart greenhouses, using nature-based solutions to do so.
LOW-CARBON AND SUSTAINABLE BUILDING MATERIALS
Furthermore, materials either for renovation or new buildings, have embedded emissions that should be considered. Circularity in the built environment and the use of bio-based materials (bricks from compressed earth, woods, hemp, straw, cellulose wadding, or recycled textiles) can play a significant role in reducing cities’ carbon emissions.
RES AND ENERGY-HARVESTING SOLUTIONS
Fossil-free electricity, heating, and cooling are, in many cases, one of the most critical steps to reduce a city’s emissions since the aggregated energy demand and the associated emissions can be substantial. Heating in cities is often produced through a mix of fossil fuels, electricity (e.g. heat pumps), and bio-based fuels. The heating can either be produced locally or centralised as district heating. Eliminating fossil fuels from the heating system is a solution with high CO2 abatement potential, which also positively impacts air quality if generation shifts to, e.g. heat pumps with low-CO2 electricity. The following examples show different options for decarbonising the building sector:
- Photovoltaics (PV) is the main technology for decarbonising the electricity supply. PV can be installed in many ways: solar canopies, solar ponds, solar parking lots (like in Florida or Zaragoza), and also, as a way to provide shading in cities (through organic photovoltaics in Tel Aviv, structures like in Madrid, solar trees in Germany). To promote its installation different incentives can be used, such as reduction of municipal taxes for collective self-consumption installations (like in Sant Cugat), through funding schemes (e.g. Finland), public-private partnership (like in Smart Energy Aland), etc. When there is no space, solar parks outside city boundaries can be installed (e.g., the community-funded solar park in Oxfordshire or the brownfields solar fields in New York City, Philadelphia, or Chicago), but even heritage cities like Evora are making strong efforts to integrate photovoltaic systems in their protected buildings, using transparent or ceramic innovative photovoltaics to produce electricity inside the city.
- Solar thermal panels can also be used for producing heating, either at a building scale or large scale (integrated with district heating networks like Silkeborg or Okotoks). The EPB in each country has been the most supportive scheme to the uptake of solar thermal panels in buildings (especially new buildings), although now solar thermal competes with heat pumps. But a combination of both can be deployed, like in Madrid where the public building company (EMVS) renovated a social housing building and then, through an EU project, installed hybrid photovoltaic-thermal (PVT) panels coupled with a water-water heat pump in a to produce heating, domestic hot water, electricity, and cooling. Germany has been supporting 45% of the cost of replacing boilers with solar-assisted heating. Additionally, solar thermal systems can be Evacuated Tube Collectors (ETC), in which the absorber plate and heat pipe are located in vacuum-sealed glass tubes to improve solar radiation absorption, thus reducing heat transfer losses and achieving a greater performance ratio.
- When the sun is not available, sustainable biogas or biomass technologies can be used. In Lund’s nearly zero energy district, a biomass-fired combined heat and power plant was used to produce both district heating and electricity. The biomass resource is mostly from pulp and forest industries and to reduce transport emissions, the source comes from a 200 km radius.
- Geothermal is also common for heating and cooling. When the temperature is high (200 ºC), like in Montieri, the geothermal source can be used directly for electricity production and heating the houses. When the temperature is not enough, heat pumps can be used, like in Bolzano.
- Lastly, but the least common, fuel cells can also be used. In Japan 120000 fuel cells have been installed in buildings, saving around 50 to 57% of CO2 emissions. A public-private partnership was used to speed up the process.
The RES sources applied to a building should be linked with digital or smart solutions, such as demand management to be able to adapt demand to prices (implicit demand response) or to the necessity of the grid (explicit demand response controlling heat pumps in large markets to avoid grid congestion), by varying set points of the buildings, running or stopping controllable loads (heat pumps, appliances, etc.) or providing feedback to users through apps to undertake a specific action in a period of time. Demand response has been widely applied in EU projects, such as the Helsinki lighthouse demonstrators. Lastly, Building Automation and Control Systems (BACS) & Building Energy Management Systems (BEMS). They enable the energy-efficient and cost-effective integration of local renewable energy supply technologies, as well as the management of energy storage and flexibilities to optimally balance energy demand and supply in the operation of building energy systems. Examples can be found in Seestadt Aspern Vienna, Barcelona, or Cologne. Additionally, aspects of building maintenance can be integrated into building automation systems, which allows for maintenance demand predictions that enable the cost-effective operation of buildings.
HEAT RECOVERY SOLUTIONS
For improving the efficiency of heating systems, heat recovery from showers can be used to recover up to 80% of heat, although only few Member States (e.g. Portugal, France, Germany and The Netherlands) are considering the benefit of waste water heat recovery within their energy performance of buildings calculation method . As well as air-to-air heat recovery systems (in passive house, 75% needs to be recovered ).
It is also worth mentioning the sewage heat recovery via pump system as a way to withdraw heat from warm wastewater.
SUSTAINABLE AND ENERGY-EFFICIENT ACTIVE SOLUTIONS
Efficient lighting and appliances are both important as old units use much more energy on average than the latest efficiency standards, are relatively easy to upgrade, and even have negative abatement costs in many cases. Instead, the challenge for cities lies in identifying how the costs and benefits can be appropriately redistributed, as citizens now bear the cost but stand little to gain from spending extra money on lighting and appliance efficiency. Additionally, efficient appliances can underpin connections to grids: for example, to power grids through the use of controllable loads and smart IoT, or to district heating networks, through heating water circuits appliances that consume from the network instead of producing heat through electric resistance. Cities can for sure tackle efficiency in their own administrations and city infrastructure. For example, efficient street lighting plays a huge role and it has been tackled in many smart city EU projects. For example, in Hamburg Smart street lighting or Humble Lamppost was installed and it included not only modern LED lighting, but also adaptive lighting (e.g., according to people detection, light levels variation, and optimization of the communication between light poles), a bicycle counter and Wi-Fi at selected places, allowing the device to become smart. Furthermore, other possible additional services for public lighting can be included in the lamppost, such as traffic sensors (Bikes/Pedestrians/Cars), e-charging stations, and environmental Sensors to measure air pollutants, noise pollution, or weather information, among others. In New York, Smart traffic signal systems are connected with buses (that are geolocated) to reduce commute times and improve public transport.
 International Energy Agency (2021), Net Zero by 2050, IEA, Paris: Net Zero by 2050 Scenario - Data product - IEA (https://www.iea.org/reports/net-zero-by-2050)
LIST OF STATIONARY ENERGY SOLUTIONS IN NetZeroCities:
- Building envelope solutions
- Passive building solutions
- Integrated solutions
- Climate-smart greenhouses https://netzerocities.app/resource-276
- Low-carbon and sustainable building materials
- Reducing embedded emissions of buildings https://netzerocities.app/resource-286
- RES and energy-harvesting solutions
- Photovoltaics https://netzerocities.app/resource-388
- Solar thermal panels https://netzerocities.app/resource-438
- Solar thermal systems with Evacuated Tube Collectors (ETC) https://netzerocities.app/resource-458
- Hybrid systems (PVT, PV+HP, ...) https://netzerocities.app/resource-648
- Geothermal energy for H&C https://netzerocities.app/resource-668
- Sustainable biomass and biogas technologies https://netzerocities.app/resource-678
- Smart solutions
- Heat recovery solutions
- Sustainable and energy-efficient active solutions