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Energy Generation

The Energy Generation area aims to replace carbon-intensive electricity, heating, and cooling with low-carbon energy production. How can cities tackle this issue? by a range of sustainable energy systems, which enable low-carbon energy production, the development of renewable energy solutions, and the efficient use of electricity is necessary to fully decarbonise the energy supply.

RES ELECTRICITY AND THERMAL ENERGY GENERATION

Decarbonised electricity is a powerful solution for reducing energy-related emissions and a key enabler for emissions reductions in other sectors as they electrify over time. This solution includes various demand-side measures that decrease the scope-2 emissions from electricity use, including increased adoption of distributed renewable generation (such as solar PV or geothermal energy) and increased purchasing of certified green electricity from utility-scale wind/solar-PV farms. For some cities, electricity use stands for 50% or more of their total greenhouse gas emissions. The following solutions can be used:

  • 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 Energy Smart 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. These solutions are also mentioned in Stationary Energy area. Besides the ones above-mentioned, other deployments can be done in agriculture (what is also called agrivoltaics), off-shore solar floating structures (like in the Netherlands), or energy communities (Freiburg, Crevillent, or Hunziker in Switzerland). In the future, photovoltaics could also reach space.
  • Distributed wind consists of wind turbines connected to the distribution level of the electric grid, to serve either on-site loads or local loads in the same grid. Use cases are utility (community-owned, like Konkanmäki wind farm, or public-owned like in the USA), industrial (Brande pilot), residential, institutional, governmental, commercial, or agricultural. The most common grid applications are grid-connected microgrids, isolated grids, and remote off-grid. Small wind turbines can also be used, like in Aland.
  • Cities are a framework where the water-energy nexus is becoming critical due to demographic movements, economic growth, climate hazards (draughts, floods, etc.), and the inexorable increase in demand. Urban water networks can be considered a source of renewable energy as they usually hold untapped energy deriving from abundant pressure (water head) or kinetic energy (water flow), Waste heat recovery in district heating networks. The sites with an excess of energy are located in existing storage/service reservoirs, wastewater systems (collection or discharge stages), or in devices already installed to alleviate the excess of energy as pressure reducing values or Break pressure tanks. Micro-hydropower plants can be installed for generating electricity using specially designed in-pipe turbines. Case studies can be found in a river, Grobweil, or in water networks (using pump-as-a turbine), Dwr Uisce.
  • Cities can also use bioenergy, like in Innsbruck where biogas from waste, the accruing sludge of the wastewater plant, solid biomass wood chips, and different types of waste heat is used as a primary energy source to generate heat, electricity, activated carbon, and dried sewage sludge, which can be used as a substitute fuel. Co-generation can be used to simultaneously produce heating and electricity. Biogas can also be produced from the collection of organic waste.

INFRASTRUCTURE

Decarbonisation of heating and cooling can be done through the technologies mentioned in Stationary Energy area (such as solar thermal panels or geothermal), and also, through the integration or renovation of District heating (DHN) and cooling (DCN) networks.  DHN and DCN district heating system transport warm or cold water from an external heat source to several buildings for space heating or cooling and, tap water heating. If the district heating already exists and operates at high temperatures (above 90ºC), which are considered as 1st, 2nd or 3rd generation of DHN [1]. The 4G (from 40 to 70ºC) and 5G (<40ºC) DHCN have reduced temperatures so as to reduce losses and to allow the usage of low-temperature heating sources (e.g. solar thermal) and waste heat (from industries, data centers, showers from buildings, etc.). The heat source can be diverse (water from rivers, sea, or ponds; ground, or even waste heat) and of varying degrees of sustainability, providing the opportunity to reduce greenhouse gas emissions. 5G provides the primary heat source for decentralized heat pumps. 4G and 5G DHCN are usually connected to highly efficient buildings (due to temperature level), unless heat pumps are available at substations (booster heat pumps for DHW like in REWARDHEAT project). Furthermore,  for new buildings, DHCN can also supply the needs of dishwashers and washing machines like in COOLDH project. Cooling can be also provided through DCN, like in Herleen. Renovation can be performed in existing DHCN to upgrade them, but to this first the buildings need to be refurbished, which allows the inclusion of RES in the DHCN (like in Stockholm). New European Bauhaus principles can be considered when deploying DHCN like sustainability, together (where the DHN is community-owned), and beautiful (through multicultural buildings like in Herleen, Copenhill or Hamburg.

Energy communities can be, in fact, a good instrument to uptake most of the above mentioned technologies. For instance, Ollersdorf municipality invests themselves in photovoltaics and e-mobility, so as to set a good example and promote citizen investments. For those roof owners who could not participate economically, they also have the opportunity to offer their roof and still benefit from PV production. In the north, instead of installing PV, the off-shore wind is also promoted and shared among neighbors (see Windcentrale).

ENERGY AND E-FUEL STORAGE

Heat/cold produced at times of peak supply of renewable electricity can be used to meet demand even when the sun is not shining and the wind is not blowing if storage is used. In fact, electricity and heat supply and demand deal with a seasonal mismatch; year-round supply (e.g. from geothermal systems, industrial waste heat) or supply particularly in summertime (e.g. solar energy) does not match the low heat demand in summer and high demand in winter. Underground thermal energy storage (UTES) offers the possibility to store large volumes of excess heat (warm water) in the summer, to be back produced in the winter. Various seasonal storages have been studied and tested in field labs and pilots, such as Aquifer Thermal Energy Storage (ATES), Borehole Thermal Energy Storage (BTES), Pit Thermal Energy Storage (PTES), Mine Thermal Energy Storage (MTES), and Tank Thermal Energy Storage (TTES). These seasonal heat storage technologies allow heat delivery to residential areas by thermal heat networks to make much better use of regional sustainable heat sources, thereby reducing the need for fossil fuels for peak demand (Like in the solar community of Okotoks in Canada, or Vojens in Denmark). Short-term energy storage can also be used when there is no possibility or space to integrate seasonal storage, such as hot or cold-water tanks (like in Lund), or ice storage (DHC in Paris). 

Energy flexibility can also be offered by the city through thermal and electric storage and smart solutions. Electric storage can be deployed at a large scale or community scale. SYMPHONY project uses distributed energy sources (DER) like rooftop solar, batteries, and selected household appliances and it is orchestrated as a Virtual Power Plant (VPP) to decarbonise the power system at the same time lowers electricity bills.

Chemical storage (like hydrogen storage or biogas holders, etc.) can also be deployed such as in the “Climate-neutral urban quarter – New Weststadt Esslingen”, in Germany, which produces and stores green hydrogen on-site.  Low-carbon electricity can be used to produce hydrogen (with electrolysers), store it (H2 storage), and use the hydrogen for power generation again (with fuel cells).

Energy management techniques (smart solutions) such as load balancing (like in Florence), can help to use the storage efficiently.

The combination of solutions can be done through different concepts, such as Positive Energy Districts (PEDs), micro-grids, Nearly Zero Energy Districts (NZED) or green neighborhoods. In PEDs is important to reduce energy needs as much as possible through the improvement of the building façade and building design, so as to then install as much as possible renewable energy technologies to produce more energy than what is needed on-site. Examples of PEDs will be available soon in the EU PED-database.

References:

[1] DHN are classified according to temperature level and technologies integrated. See more in https://5gdhc.eu/different-generations-of-dhc/

LIST OF ENERGY GENERATION SOLUTIONS IN NetZeroCities:

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