5.1. Renewable Energy Generation Methods for PEDs
As noted in Section 4
, the energy generation potential of renewable energy technologies varies between different regions within the EU. A renewable energy technology that excels in one region might be impossible to implement in another region. The geographical location and its properties must therefore be taken into account when planning a PED. Solar PV is a good example of an energy technology that is highly dependent on the geographical location. In northern Europe, where there are only a few hours of daylight in the winter season, solar PV generation is significantly lower than in southern Europe. Hence, the capital costs per kWh of generated solar power are significantly higher in the Nordic countries compared to the Mediterranean region. The situation is similar for wind power, which is naturally more remunerative in windy areas, such as the regions close to the northern Atlantic Ocean, the Baltic Sea and parts of the Mediterranean Sea.
Different renewable energy technologies also have different properties when it comes to flexibility, cost and service life [32
]. Intermittent renewable energy generation technologies, such as solar and wind energy, are considered non-flexible energy sources, as they can only generate energy when the wind speed and solar radiation are sufficient. Run-of-river hydropower is more flexible than solar and wind energy, but not as flexible as reservoir hydropower and bioenergy.
The installation costs, costs of electricity and service lives of different renewable energy technologies are presented in Table A1
in the Appendix A
. However, the installation cost is dependent on the size and location of the installation, and the cost of electricity is highly dependent on the geographic location [32
]. Hence, the costs in the table are given as global weighted averages.
The diversification of intermittent renewable energy technologies is a great way to increase the demand coverage and reduce life cycle costs [86
]. Intermittent renewable energy technologies, such as wind and solar energy, are often able to compensate each other, as windy and sunny periods are not synchronized. As the energy export price is often lower than the energy import price for small-scale energy producers [88
], it might be beneficial for a PED to minimize the external grid interaction. By diversifying the intermittent renewable energy generation, it would be possible to achieve a positive annual energy balance with a lower export rate [89
According to a study by Heide et al. (2010), wind power is, in general, more beneficial in Europe from a load-matching perspective since both the wind power generation and the energy demand are higher during the winter than during the summer [86
]. Solar energy generation, on the other hand, is the highest during the summer months. Thus, from a load-matching point of view, a larger share of the PED energy generation mix should be covered by wind energy in most of Europe. This is, however, not that simple, as installing wind turbines in populated areas is complicated and solar energy is, on a global level, a more cost-effective energy generation method [32
In most districts, especially in densely populated areas, space is also an issue. Renewable energy systems must thus be integrated in a smart way, so that energy generation does not conflict with other functions that are essential for the district. Solar power integration in urban districts is convenient since solar PVs, CSPs and solar heat collectors can be installed on rooftops and various available surfaces within districts. Solar PV panels can also be integrated into building façades. So-called building-integrated photovoltaics (BIPV) can be integrated in stable and heavy structural elements as well as in lightweight and transparent structural elements [90
]. According to a study by Fath et al. (2015), building façades provide almost three times the area of roofs in a 2 km2
urban area in Karlsruhe, Germany [91
]. However, due to their angles and positions, they receive only 41% of the total solar irradiation. Hence, solar PV panels on roofs should be prioritized in PEDs, while façade solar PV panels can be considered if the solar radiation on a particular façade is sufficient. Overall, city-integrated solar PVs have a great potential and can satisfy over 60% of the electricity demand in some smaller cities in Europe [92
Wind power integration in urban areas, on the other hand, does have many practicality issues and is thus less suitable for on-site energy generation in PEDs. It would be complicated to install large-scale wind turbines due to their size, aesthetics and noise as well as low and turbulent urban wind-speed and safety issues [94
]. Small-scale wind turbines could be an option, but their cost per installed kWh is about twice as high as large-scale turbines [32
]. Vertical axis wind turbines (VAWTs) are a popular alternative among small-scale wind turbines. These wind turbines are able to handle the higher turbulence and varied wind speeds associated with urban environments [94
]. Another benefit with VAWTs is that the generator can be installed at a lower part of the so-called tower, allowing building-mounted turbines to be more easily serviced [94
]. The hub height of small-scale urban wind turbines is, however, not high enough to access the same wind speeds as large-scale wind turbines [97
Due to the many shortcomings of wind turbine installations in urban areas, wind power is best suited for virtual power plants. The distance between the district and the virtual wind power farm could, however, be relatively short and thereby ease the power transmission to the district. Wind farms could, for instance, be installed in nearby rural areas or even offshore if the district is in a coastal area.
Bioenergy and hydropower can be used to provide PEDs with flexible power when the intermittent energy generation is lower than the electricity demand [98
]. These flexible power generation methods make the district less dependent on electricity supplied by the external grid and thereby foster a positive annual energy balance.
Bioenergy plants can be built almost anywhere in Europe, as biomass is relatively cheap to transport from biomass-producing regions, such as the Nordic countries, the Baltic countries and Austria [50
], to other parts of Europe. Bioenergy generation does, however, produce emissions, which contradicts the PED’s aim to provide a carbon-free energy environment and better life quality in residential areas. Even though bioenergy is carbon neutral from a life-cycle perspective (as the carbon dioxide emissions originate from carbon dioxide captured from the atmosphere by biomass), this does not change the fact that bioenergy plants pollute the air in the district where they operate.
Hydropower, on the other hand, is extremely dependent on the location of the district since hydropower can only be generated in regions that satisfy the requirements described in Section 4.1.3
. Most of the potential hydropower sites in Europe are already in use or unattainable due to regulations and environmental protection [100
]. Hence, hydropower is best suited for a virtual power plant for virtual PEDs, where the district boundaries are virtual instead of geographic. According to Graabak et al. (2019), a 2050 Central-West European grid with large shares of intermittent renewable energy could benefit from using Norwegian hydropower as flexible energy for grid balancing [98
Heat pumps are expected to provide a significant share of future heating [101
]. Due to the flexibility and high coefficient of performance (COP) of modern heat pumps [101
], they could be a highly valued source for heating in future PEDs. Due to the relatively large operating temperature interval, heat pumps can be used to recover low temperature heat from the ground and the ambient air as well as low temperature waste heat from sewage systems, ventilation air and other waste heat flows. Heat pumps are thus able to increase the total energy efficiency of PEDs and minimize the import of externally generated thermal energy. Moreover, heat pumps provide additional flexibility to PEDs, as they can be used to transform electrical energy into heat that can be stored in TESs [88
]. It is thereby possible to reach a higher utilization rate for electricity generated by on-site intermittent renewable energy technologies.
5.2. Energy Storage Methods for PEDs
Energy storage enables PEDs to store excess energy instead of exporting it. Hence, energy storage can be used to increase the on-site utilization of intermittent energy sources, such as solar and wind. This is particularly important for self-sufficient PEDs, so-called autonomous PEDs, as they are not allowed to import energy from the external grid. For dynamic PEDs, energy storage is not as crucial since they allow bidirectional interaction between the district and its surroundings, and can thereby use the external grid to balance the energy demand during periods of low on-site energy generation.
in the Appendix A
presents the installation costs, energy densities, lifetimes and round-trip efficiencies of different energy storage technologies that can be utilized in PEDs. Based on this table, the most cost-effective energy storage methods are pumped hydro and compressed air energy storage. As explained earlier in the paper, these energy storage methods are extremely dependent on the geographical characteristics of the site, and hence, they are not possible to implement anywhere [61
]. Another issue with these storage methods is their low energy density, which makes it difficult to install them in densely populated districts [61
Pumped hydro and compressed air energy storage do, however, have great potential as virtual energy storage. A virtual PED with a periodical intermittent energy surplus and shortage could, for example, interact with virtual storage located far from the geographical location of the district itself. Similar energy management strategies have, for instance, been implemented between Denmark and Norway, where excess Danish wind power is stored in pumped hydro storage in Norway [102
]. This collaboration between nations is possible due to the high level of wind power generation in Denmark (>20% of the annual electricity generation) and the enormous pumped hydro storage potential in the mountains of Norway.
Batteries, on the other hand, are not so reliant on the geographical site of the PED, but they are considerably more expensive than pumped hydro storage and compressed air storage [60
]. It is therefore often more cost-effective for dynamic PEDs to interact with the electricity grid than to use batteries [26
]. The combination of decreasing battery prices and an increasing share of intermittent energy in the electricity grid could, however, open up more opportunities for batteries in the future.
Even if pumped hydro and compressed air energy storage would be an available option for autonomous PEDs, it could be beneficial to also install a battery for short-term energy storage. Batteries have a significantly shorter reaction time and can thereby add more flexibility to the energy system of the district and increase the utilization of on-site intermittent renewable energy [103
Compared to electricity storage systems, TES systems are relatively cheap to install [72
]. Sensible heat storage in the form of hot/warm water tanks is, by far, the most common TES method for heating and domestic hot water applications [104
]. Short-term energy storage can be implemented at the building level without causing significant heat losses. The storage temperatures of these forms of storage are usually kept at 55–60 °C in order to avoid bacterial growth [104
When heat is stored for longer periods, heat losses become an issue. As heat losses can be minimized by increasing the water volume and lowering the storage temperature, it might be beneficial to implement centralized low-temperature systems for long-term or seasonal TES [71
]. The temperature of these TESs can be increased by utilizing heat pumps.
5.4. District Heating/Cooling and Electricity Networks
Due to the surge in heat pump installations during the last decade, electricity grids and district heating and cooling networks are becoming more and more interconnected [101
]. Thanks to heat pumps, energy systems can reach a higher degree of flexibility, as energy can be converted from electricity to heat with high COPs.
The reduction of fossil fuel CHP plants in the energy generation mix would require a more sophisticated district heating network that is better suited for decentralized heating. This field has recently received increased attention from researchers, and hence, the properties of the next generation, i.e., the fourth generation, of district heating and cooling networks have been investigated and discussed in several research papers [110
The fourth generation district heating (4GDH) network will be an integrated part of smart energy systems and thus able to interact with other components, such as heat pumps, solar heat collectors and TESs [110
]. Hence, the 4GDH networks rely on the optimized distribution, consumption and interaction between renewable energy sources [112
]. Another key objective of the 4GDH network is to enable heat recovery from low-temperature sources and to decrease the temperature of both the supply and return district heating water [110
]. The low temperature of the district heating network district also benefits heat pumps, as their efficiency is higher for lower output temperatures [101
District cooling solutions are also a relatively new technology, and they are not as widely used as traditional district heating [101
], but they can be implemented with the same operating principles as the 4GDH networks [110
]. District cooling is usually supplied by natural cold resources, absorption chillers, mechanical chillers and cold storage [114
]. During periods when heating and cooling demands are occurring simultaneously, synergies between the district cooling and heating networks can be utilized by using heat pumps to produce cold and warm water at the same time [114
Both 4GDH and district cooling can be implemented as local networks (to which all energy consumers and producers are connected) in the PED with connections to the external district heating and cooling networks. This way, PEDs can balance their internal heating and cooling demands before exporting or importing energy from the external network. The same principles can also be applied to the electricity grid in the district. In order to streamline the utilization of such local energy networks, centralized control systems can be implemented. A centralized control system can optimize the energy flows between energy consumers, producers and storage in the PED so that the economic benefit of the PED is maximized.
Connections to the district heating/cooling network and electricity grid are an essential part of the PED concept, as one of the main targets of a PED is to interact with other PEDs and provide renewable energy to other parts of the metropolitan area. Hence, the energy transfer connections in and out of the district must be carefully planned and designed based on the purpose and capacity of the PED energy system.
5.5. Construction of PED Networks
Cities can be very different when it comes to size, population, population density, economic situation, public transportation, etc., and consequently, there are also significant differences in energy consumption. Cities in cold and hot climates consume a large amount of energy for heating and cooling, respectively [94
]. Industrial cities also also consume more energy; however, they usually have a greater potential for district heating [94
]. Even within the same city, there can be considerable variations in energy consumption between different districts [94
]. According to a study by Jones and Kammen (2011), there is a clear correlation between income and household energy consumption [115
]. Additionally, the energy consumption per household of big American metropolitan areas is usually higher in the suburbs than in the urban cores, due to longer driving distances and bigger homes [116
]. All in all, there are numerous factors that affect the energy usage of cities and districts within cities, and therefore, it is impossible to develop specific PED construction guidelines that can be applied to every district in every city.
The high population density of urban cores complicates the installation of renewable energy systems. The population density does, however, usually decrease as the distance to the city centre grows, and therefore, it is easier to install renewable energy systems in the suburbs, where there is more space in relation to the number of residents. Hence, we propose an onion model for PED networks, where most of the PEDs are constructed in the outer-most layers, i.e., the districts furthest away from the city centre. These outer-layer PEDs produce more renewable energy than they consume and can thereby export excess renewable energy to the inner layers of the city. This way, networks of PEDs can increase the renewable energy share of the city centre and the self-sufficiency of the whole metropolitan area. A visual explanation of the onion model is depicted in Figure 11
There is a strong correlation between the share of a country’s population that lives in urban areas and CO2
]. Air quality might thus become an increasing problem as global urbanization continues and metropolitan areas around the world grow [117
]. By ensuring that the cities are surrounded by PEDs, the amount of polluting fossil fuel power plants can be reduced in the region. This way, PEDs can improve the air quality of densely populated areas and contribute to decelerating climate change.
5.6. Regulative Aspects
The EU has, in several ways, highlighted the importance of preventing climate change and global warming. This is also noticeable from a legislative point of view. The European Green Deal, initiated by the European Commission in December 2019, aims to tackle climate- and environment-related challenges [118
]. One of the main goals of this deal is for the EU to become climate neutral (no net greenhouse gas emissions) by 2050 [118
]. The President of the European Commission Ursula von der Leyen has stated the importance of this deal, by calling it the EU’s “new growth strategy” [118
Since the goal of the PED concept is in line with the aim of the Green Deal, the enormous focus on the deal might benefit the development and construction of PEDs in the future. Some of the EU’s Green Deal key actions, such as the “‘Renovation wave’ initiative for the building sector”, the “Assessment of the final National Energy and Climate Plans” and the “Zero pollution action plan for water, air and soil”, are directly enhancing the preconditions for the application of PEDs [119
The Clean Energy Package proposed by the European Commission in 2016 is also a ground-breaking act for PEDs and other small-scale energy producers since it recognizes, for the first time under EU law, the rights of communities and citizens to engage directly in the energy sector [120
]. As a result of this, renewable energy and energy storage could be shared within communities, using internal electricity grids [120
]. The energy community and its shareholders cannot, however, be engaged in large-scale commercial activity in the energy sector.
The legislative features of energy communities might benefit the PEDs since they reduce the economic friction between renewable energy producers and consumers within the community. Regulations might, however, prohibit PEDs defined as energy communities from exporting energy to the external electricity grid and district heating network, as energy communities are not allowed to engage in commercial energy trading.