The EU has set the ambitious target of achieving net-zero greenhouse gas emissions by 2050. Its long-term climate strategy also includes intermediate targets as set in the 2030 climate and energy framework. The key targets for 2030 are: achieving at least 40% reduction in greenhouse gas emissions compared to the 1990 levels, a minimum 32% share of renewable energy in the final energy consumption, and a minimum 32.5% increase in energy efficiency [1
]. The building sector is one of the main energy consumers responsible for about 36% of greenhouse gas emissions (GHG), and therefore it has been the focus of regulatory reform, as part of the strategy towards reducing emissions over the past years [2
]. The main use of energy by households in the EU in 2017 was for heating their homes (64.1% of final energy consumption in the residential sector) with Renewable Energy Sources (RES), accounting for almost a quarter of EU households space heating consumption. Heating and domestic hot water alone accounted for 78.9% of total final energy use of EU households in 2017 [3
]. Provisions in increasing the energy efficiency of buildings are included in Directive 2010/31/EU on the Energy Performance of Buildings (recast) (EPBD). In this context, the EPBD (recast) requires that all new buildings should be nearly-zero energy buildings from 31st December 2020 onwards, while all new public buildings should be nearly-zero energy as from the 31st December 2018 [4
]. Furthermore, Directive 2012/27/EU on Energy Efficiency requires a 3% renovation of the total floor area of public buildings on an annual basis [5
Whilst there is continuing focus on improving the energy efficiency of the construction sector at the building level, there is also emerging interest in improving the energy performance at community scale. The EU’s urban population is ever growing; cities accounted for approximately 75% of the EU population in 2015, while this percentage is expected to reach 80% by 2050 [6
]. It is estimated that urban areas account for 70% of GHG emissions and two thirds of energy consumption in the EU [7
]. Therefore, Net Zero Energy (NZE) settlements where the zero energy principles are considered at the district scale are expected to play a significant part in achieving the emissions reduction targets. Apart from their decarbonisation potential, these are seen as places that will stimulate environmental awareness, innovation, economic growth and social progress; however, it is acknowledged that achieving net zero performance at the community level has its own challenges and opportunities [8
Many studies up to now have been conducted regarding the evaluation of Near Zero Energy Districts (NZED). Robinson et al. (2011) [9
] focused on simulation approaches for sustainability in the urban environment with emphasis to building energy modelling. Pol and Robinson (2011) focused on the impact of the urban morphology on the energy performance of buildings [10
]. Whilst there is a wide range of established tools for the analysis of building energy demand with high levels of accuracy, energy modeling at the district scale is more challenging and computationally intense; district scale modeling considers and integrates at the minimum a group of buildings, a source of energy and an energy distribution network. This often leads to the use of simplified models with somewhat reduced accuracy [11
]. Jebaraj and Inivan (2006) reviewed 252 works focusing on the use of integrated energy models [12
], while Vreenegoor et al. (2003) evaluated various simulation tools and certifications (i.e., LEED etc.) applicable at the district level analysis; the focus of that study was the residential building stock in Germany [13
]. Alegrini et al. (2015) reviewed models and tools for the energy modeling of district systems, renewable energy production and the effect of urban microclimate to the district energy demands [14
]. District heating networks are a core component in many energy communities. Haghighat et al. (2019) provides a review of case studies with district heating networks and the relevant source of thermal energy in [11
]; CHP, geothermal, solar energy, waste to energy technologies and industrial excess heat were considered as well as the simulation tools used in the analysis.
In the work of Synnefa et al. (2017) [8
] the evaluation of four NZE settlements across the EU (Cyprus, France, Italy and the UK) in terms of energy, environmental and cost performance was presented. The analysis conducted at both the building and the settlement level and it was found that that the targets of (i) annual net-regulated energy use less than or equal to 20 kWh/m2
and (ii) annual renewable energy production greater than or equal to 50 kWh/m2
were met in the four settlements investigated. The projects were also considered cost-effective as reduction of at least 16%, compared with current NZEB costs, was also reported. Hachem (2016) investigated the effect of design parameters on the performance of solar community incorporating roof-mounted PV in terms of GHG emissions reduction and the balance between electricity consumption and generation; design parameters included the building insulation levels, neighborhood density and type, as well as the design of streets and distance to commercial center. It was found that the energy upgrade of the community resulted up to 75% reduction in GHG emissions from the buildings, while transport was found to still have a significant environmental impact [15
]. Hachem-Vermette et al. (2019) investigated the performance of a neighborhood in Canada towards achieving net-zero energy performance by means of solar thermal, coupled with borehole energy storage and PV [16
]. Energy production up to 20% higher than the energy consumption of the neighborhood was achieved.
Several studies also focused on the cost aspects of NZE settlements. Isaac et al. (2020) studied the cost of Net Zero Energy communities, considering various scales and densities. A model was developed for identifying the optimum configuration of RES technologies for an NZE settlement in a cost-effective manner [17
]. It was found that increasing the scale of the community up to a certain point reduced the associated energy cost, while urban density was found to have a more complex impact on costs.
Paduos and Corrado (2017) investigated the effectiveness of retrofit packages of measures for buildings to achieve nearly zero energy performance [18
]. The analysis considered thirty reference buildings and several different packages of retrofit measures. Following a cost-optimal evaluation approach, the measure packages that met the nearly zero-energy target and were cost-effective were defined. It was shown that in most cases NZEB retrofit was technically feasible, with a high reduction of the non-renewable primary energy consumption. Nevertheless, it was found that the costs of retrofitting to such degree were still too high to be considered as attractive investments.
Planning district level energy systems often requires an iterative calculation process and the use of optimization techniques. Evins (2013) highlighted the need for tools that are able to conduct parametric analysis at the district scale and integrate them to optimization processes [19
]. Allegrini et al. (2015) considered the provision of simple tools that can support decision makers at the early stages of project design of significant importance to district energy modeling [14
]. Ala-Juusela et al. (2016) used a decision support tool, called AtLas, designed to inform the long term planning of neighborhood energy solutions, in order to evaluate the energy positivity level of a Finnish residential neighborhood, and part of a French university campus [20
]. Positive energy neighborhoods were defined as those with annual energy consumption lower than the annual locally produced renewable energy. The energy positivity level of an area was estimated with calculating energy matching indicators: on-site energy ratio, annual mismatch ratio and other mismatch indicators. Rehman et al. (2015) investigated the development of positive energy community for the Nordic climate, considering the use of a district heating system combined with wind turbines and PV for electricity production, as well as electrical storage and electric vehicles [21
]. Multi-objective optimization was performed to minimize the lifecycle cost and the imported electricity.
Despite the increased challenges of the districts, considering the near zero performance at the wider scale offers the flexibility of utilizing different levels of energy performance and energy production capacity and aggregating resources, costs and requirements [22
]. In the 2050 Vision of the European Technology and Innovation Platform on Renewable Heating and Cooling, energy communities are considered to have the potential to shift to a new Renewable Heating and Cooling business model where citizens, rather than the operators, own the assets [23
Cities and communities, therefore, will have an important role in EU’s transition towards a carbon-neutral economy as they are ‘a locus for innovation, they provide great opportunities for learning and networks, and they offer the possibility of achieving whole system change at local scales’ [24
] (pp. 81–82). The European Strategic Energy Technology Plan highlighted the importance of smart cities, and supported the roll-out of positive energy blocks and districts within cities that take advantage of the synergies and energy flows between buildings to deliver energy efficient heating, cooling and lighting [25
Within this context, the European project ‘Integrated and Replicable Solutions for Co-Creation in Sustainable Cities’ (IRIS), funded under the Horizon 2020 Program, aims to support the development, demonstration and replication of near zero and positive energy districts and neighborhoods. This is done as part of the transition of three Lighthouse (LH) and four Follower cities (FC) towards becoming smart cities [26
]. Lighthouse are the cities with the technical capacity to implement district-wide projects integrating and demonstrating novel technologies; LHs act as exemplars for the FCs. Follower Cities do not have the full competency to implement such wide scale projects; they aim at replicating the most appropriate solutions demonstrated by the LHs adapted to the local conditions. The IRIS Lighthouse Cities are Gothenburg (Sweden), Nice Cote d’Azur (France) and Utrecht (Netherlands); the Follower Cities include Alexandroupolis (Greece), Focsani (Romania), Santa Cruz de Tenerife (Spain) and Vaasa (Finland). The IRIS smart city transition takes place through increasing the share of renewable energy and energy management, e-mobility services and citizen engagement, while beneficiating from available multi-type available storage systems and is organized around the following five Transition Tracks (TT):
TT#1—Smart renewables and closed-loop energy positive districts;
TT#2—Smart Energy Management and Storage for Grid Flexibility;
TT#3—Smart e-mobility sector;
TT#4—City Innovation Platform;
TT#5—Citizen engagement and co-creation.
Each Transition Track comprises several Integrated Solutions (IS). Planning and development of near-zero energy and positive energy blocks and districts is the main focus of TT #1; more specifically, IS1.1-Positive Energy Buildings and IS1.2-Near Zero Energy Districts of TT#1 examine the use of various energy efficient and renewable technologies in buildings, as well as the integration of smart-grids and thermal networks.
The work presented in this paper focuses on the replication activities to develop near and net zero communities in the Follower City of Alexandroupolis, Greece. The aim of the work is to evaluate the feasibility of such energy communities in Greece by selecting and replicating technologies and activities that are currently being demonstrated in the Lighthouse Cities, making them fit within the local context of Alexandroupolis. The most suitable technologies and combinations of these technologies that meet the technical requirements for nearly-zero or positive energy performance in a financially viable manner are identified. Various integrations of these technologies are evaluated with the use of suitable technical and financial Key Performance Indicators (KPIs), thus ensuring that proposed integrated solutions are also financially viable investments. KPIs are indicators designed to measure the degree that specific objectives of a project have been achieved and their selection is critical for measuring and communicating the level of the project’s success [27
It is envisaged that the study may act as a roadmap for the uptake and development of positive energy and near-zero energy communities in Greek Cities, similar to Alexandroupolis. In addition, it can be used as a decision support tool for policy makers when assessing alternative options towards the path to the decarbonization of the EU economy by 2050. This is in line with the recommendation made in [14
5. Results and Discussion
Results of the analysis for the two energy communities investigated are presented in the following paragraphs.
5.1. New-Built Positive Energy Neighbourhood
Each configuration presented in Table 3
is assessed against the set technical and financial criteria. It is noted that in order for a scenario to be considered as positive-energy, the degree of electrical and thermal self-supply should be at least 100%. Results of the analysis are presented in Table 8
It can be seen that achieving at least 100% of thermal and electrical energy self-supply can be met by various combinations of the technologies considered; however, most configurations do not meet the financial criteria. Financially viable positive energy performance can be achieved in only three scenarios (Configuration 10 of Case A and Configurations 6 and 10 of Case B). Moreover, electrical storage providing one day of autonomy is identified as the optimum solution; smaller capacities are not adequate for achieving 100% self-supply, and sizing the battery for increased autonomy is not considered to be cost-effective, since the relevant scenarios did not meet the financial criteria (NPV > 0, IRR > 5%). The three configurations calculate a simple payback between 15.6 and 16.3 years and a dynamic payback period (i.e., considering an annual increase in energy prices and discount rate 5%) ranging from 12.5 to 12.9 years. The achievable emissions reduction is between 1325 and 1370 tonnes of CO2 per year.
With regard to the insulation levels, it can be seen that current relevant Greek building regulations (Case A) appear to be adequate for attaining a positive energy performance in just one system configuration (Configuration 10); maximum PV capacity (550 kWp) is required to achieve the electrical self-supply target. Increasing the levels of insulation results in Case B achieving slightly higher technical performance. Due to the reduced energy consumption, the self-supply component imposes a greater share in the energy balance, resulting in a slightly higher degree of self-supply for all configurations of Case B compared to Case A. This results in an additional configuration, during which the technical (DET and DEE > 100%) and financial (NPV > 0, IRR > 5%) set targets are met with a smaller PV capacity (Configuration 6 with 525 kWp PV capacity). Increasing the energy efficiency of the building fabric therefore provided the flexibility to reduce the size of the PV system and still meet the positive energy requirements.
On the other hand, Case C scenarios present the best technical performance, due to the reduced energy consumption of the buildings. However, they are not cost-effective, mainly due to the increased cost of triple glazing compared to the standard double glazing used in Cases A and B. Increased insulation levels result in reduction of the peak load and, consequently, the size of the heat pump. Nevertheless, these savings are not adequate to justify the additional investment cost for replacing the glazing and increasing the insulation. The use of a larger PV system that will lead to increased revenues from the electricity exports can be considered; however, the potential to increase the PV capacity is limited due to the roof space restrictions.
5.2. Near Zero Energy Retrofit District
The scenarios for the NZE retrofit district presented in Table 6
are evaluated according to the same technical and financial criteria. Results of the analysis are presented in Table 9
In order to evaluate the various configurations examined, a minimum level of self-supply for considering ‘near-zero’ performance needs to be determined. A degree of 75% is set as a reasonable threshold. The results indicate that a certain degree of autonomy is required as configurations without electrical storage do not meet the near-zero target, despite the fact that they are the most attractive options in economic terms. Furthermore, it is also apparent that none of the configurations in-between 11–15 that have a 250 kWp total PV capacity (corresponding to approximately 2.6 kWp available per house delivering on average about 4100 kWh annually) are able to meet the 75% self-supply target. This suggests that the A+ rating that considers 2.2 kW PVp capacity is not sufficient for achieving the near-zero energy neighborhood target, as defined here. This highlights the need to update the definition and provide specific guidelines for near zero-energy buildings, as well as to establish a definition for near zero energy districts, if a truly near-zero energy performance is to be achieved.
Oversizing the battery for achieving a degree of autonomy greater than 1 day has limited value in terms of technical performance. For example, considering 1.5 days of autonomy instead of 1 day delivers only an additional 0.6% at maximum for all PV capacities considered, with a significant reduction in economic performance. Considering 2 days’ autonomy was not financially viable in any case. Therefore, the optimum battery size is that providing day autonomy between 0.5 and 1. The size of the PV system is required to be 300 kWp at minimum. The self-supply requirement is met with both 0.5 and 1 day of autonomy at the higher capacity of the PV system (350 kWp), by Configurations 2 and 3, respectively. When the 300 kWp PV system is considered, the battery size is required to be 1100 kWh, providing 1 day of autonomy, in order for the 75% electrical self-supply threshold to be exceeded (Configuration 8).
Therefore, Configurations 2, 3 and 8 meet the required technical and economic criteria. These configurations present a simple payback period ranging from 13.3 to 15.6 years or a dynamic payback period (i.e., considering an annual increase in energy prices and discount rate 5%) from 9.7 to 12.2 years. The resulting emissions reduction from these solutions varied between approximately 795 tonnes to 893 tonnes of CO2.
5.3. Building Level Analysis
The analysis of the specific measures is also considered at the building level. Financial performance of the retrofit measures to a single dwelling is investigated when (a) these are applied individually (which is predominately the case for retrofit projects of privately owned houses), and (b) when they are applied to a house as part of a larger community energy upgrade project. The purpose of this study is to highlight potential advantages that each household can benefit from, due to economies of scale achieved, when the retrofit measures are applied as part of a community scale project rather than individually. It is also envisaged that the findings will stimulate the interest of local communities in investing in energy upgrade measures.
The analysis was conducted for the two typical types of houses representing the blocks of the neighborhood, i.e., mid-terrace and end-terrace. Configuration 8 (Table 8
) that meets the near-zero energy criteria at the community level is used as case study: In this configuration, a 300 kWp PV system with 1100 kWh battery capacity providing one day’s autonomy to the district is used. This corresponds to approximately 3.15 kWp
PV and 11.5 kWh battery capacity on average, per house. Three following scenarios were then investigated:
(a) Conducting the energy upgrade of the house as part of a district energy upgrade scheme as discussed in the previous paragraphs, i.e., increased insulation of the building envelope (walls, roof and windows) and roof-mounted PV with battery storage. Heating and cooling is provided through the district heating and cooling network. The cost of the PV system is set to €1000/kWp, and the cost of the battery equal to €400/kWh. The capital costs of the GSHP and the DHN for each house are approximately €4200 and €6850, respectively.
(b) Conducting the energy upgrade individually using the same technologies and considering the same levels of PV utilisation, i.e., insulation of building envelope, ground-source heat pump for heating and cooling and roof mounted PV with battery storage. The same PV and storage capacity are considered; 3.15 kWp PV and 11.5 kWh storage capacity. The cost of the PV system is again assumed to be €1000/kWp. The cost of the battery and the GSHP for an individual house are €6000 and €15000, respectively.
(c) Business-as-usual retrofit, i.e., insulating the envelope to current building regulations and considering conventional equipment for heating (heating oil boilers) and cooling (A/C units). No PV system is considered. The cost of boiler and A/C units is set to €1500 each.
The energy prices considered are the same as previously (Table 7
). The capital and operational costs for each scenario and house type are presented in Table 10
Results of the financial evaluation are presented in Table 11
. In both cases, savings are greater for the end-terrace house type. The increase in capital cost for the near-zero energy retrofit is almost 2.5 times that of the BaU retrofit when the measures are applied as part of a larger community scale scheme; when measures are applied individually the increase in capital cost is almost three times that of the BaU retrofit. Results suggest that investing in a NZEB as part of a wider community scheme is a more attractive option than doing so individually, since it is financially viable. On the other hand, it appears that when these measures are considered only at the individual building level the increased capital cost is hardly compensated for by the reduction in operational costs; the investment is not considered viable for the mid-terrace house, while it barely meets the financial criteria in the end-terrace house.
6. Conclusions and Lessons Learnt
The feasibility of the development of a newly-built positive energy district and the retrofit of an existing district to near zero energy performance levels is examined in this paper. The analysis is conducted within the framework of the IRIS project that supports European cities to deliver, among others, upgraded energy services to their citizens in an effective and sustainable manner. The main objective of the work is to identify the technologies and the combinations of these technologies that can achieve the stringent requirements of zero or nearly-zero emissions, while also being financially viable investments. It is envisaged that the work presented here will act as a roadmap for planning and developing additional energy communities in Greece.
The design approach to the highly efficient energy communities considers high levels of building fabric efficiency, renewable energy production and storage (PV, battery and GSHP), as well as the energy efficient delivery of thermal energy (district heating and cooling). The measures considered are able to meet the technical and financial criteria set. This is the case when:
Increased insulation levels are applied. In the case of the near-zero retrofit district, increasing insulation to a higher standard than required by building regulations is a pre-requisite for complying with the current definition of NZEB. With regard to the new-built district, again, technical performance is better when insulation is slightly higher than required. The degree of electricity self-supply is 3% higher for the same configurations of technologies when increased insulation levels are considered (from 104.6% in Case A—Configuration 10, to 107.6% in Case B—Configuration 10). Furthermore, due to the reduced energy consumption, there is an additional configuration in Case B meeting the technical and financial criteria with reduced PV capacity (Configuration 6). However, care should be taken when considering increasing the levels of fabric efficiency. Replacing the double-glazed windows with triple glazed ones is not considered a viable option based on the results of the analysis.
Electrical storage providing approximately one day of autonomy is used. This is the optimum configuration in order to deliver the necessary levels of self-supply and be financially viable. In the case of the new-built neighborhood, the positive energy criteria are met only when considering 1500 kWh of electrical storage. In the existing retrofit neighborhood, the respective criteria are met mostly when 1100 kWh of electrical storage are used, corresponding to one day of autonomy. Requirements are also met in a scenario with smaller battery size, 550 kWh, however technical performance is significantly improved when considering the higher capacity (DEE 95.9%, compared to 82.7% for Configurations 3 and 2, respectively).
Increased PV capacity is considered. In both case studies, results suggest that the district performance is optimum when the highest size of PV system considers the limitations imposed by the available roof areas. Higher PV capacities result in shifting the energy balance positive in primary energy terms and increased revenues.
Additionally, the benefits for the occupants of participating in an energy community scheme, rather than performing the energy upgrade on an individual house basis are demonstrated, in an attempt to stimulate the interest of stakeholders for investing in energy community schemes. The need to update the definition of near zero buildings for the uptake of truly near-zero energy buildings and communities is also highlighted. Finally, it should be noted that the technologies considered here are selected from a pool of technologies based on their suitability for the Greek context. Additional research investigating the feasibility of other renewable energy and energy efficient technologies will facilitate the development of energy communities and support the transition towards carbon neutrality.
Based on the experience gained from this study and the limitations identified, it is suggested that similar research studies in the future can benefit greatly from monitoring existing buildings, in order to feed robust data in the simulation software and increase the reliability of the results. If this is not possible due to limited resources or the degree of intrusiveness to the residents’ homes, other means of data collection may be used: interviews with the occupants, questionnaires and analysis of energy bills (electricity, fuel) can be used for a better representation of the occupancy profile and the energy consumption in the simulation software. Furthermore, dynamic building simulation software and software specialized in specific technologies have the potential to increase results accuracy, should the resources become available and the project maturity allow for their use. It should be noted, however, that the accuracy of the results relies again on the quality of input data. This highlights the value that monitoring or data collection can add on the simulation study.