**1. Introduction**

Recently, the Positive Energy District (PED) concept has been discussed substantially as it could become the key solution to energy systems in transition towards carbon neutrality. According to European Strategic Energy Technology (SET) Plan Action 3.2 [1], PED could be defined as an energy-efficient and energy-flexible urban area with surplus renewable energy production and net-zero greenhouse gas emission in a certain time frame. Some PED initiatives aim to create a knowledge base and a roadmap to achieve the energy transition of cities according to established time horizons [2].

Most of the studies and practical experiences about PEDs are based on newly built districts or planning of future districts. Monti et al. [3] described the process of adaption and the challenges/barriers faced by the PED decision makers. They also proposed how simulation, optimization, ICT approaches and business models are combined in a holistic and pragmatic way. Lindholm et al. [4] defined three types of PEDs (i.e., PED

**Citation:** Zhang, X.; Penaka, S.R.; Giriraj, S.; Sánchez, M.N.; Civiero, P.; Vandevyvere, H. Characterizing Positive Energy District (PED) through a Preliminary Review of 60 Existing Projects in Europe. *Buildings* **2021**, *11*, 318. https:// doi.org/10.3390/buildings11080318

Academic Editor: Francesco Nocera

Received: 24 May 2021 Accepted: 20 July 2021 Published: 24 July 2021

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autonomous, PED dynamic, and PED virtual), depending on the system boundary and energy import/export conditions. They also pointed out that PED is highly dependent on local context with many impacting factors, such as the available renewable energy sources, energy storage potential, population, energy consumption behavior, costs and regulations, which affect the design and operation of PEDs in different regions. A series of technical solutions, such as the integration of batteries, electric vehicles (EV), and grid-responsive control, were discussed to promote the development of PEDs [5]. Samadzadegan et al. [6] developed a framework to design energy systems for PED or zero-carbon districts, by focusing on estimating heating and cooling demand and sizing related renewable energy systems, e.g., solar photovoltaic (PV) and heat pumps. Shnapp et al. [7] proposed handling the energy performance targets by transferring to the district level the minimum energy requirements imposed by the energy performance of buildings directives to individual buildings. Gabaldón Moreno et al. [8] proposed a methodology for calculating the energy balance at the district level and energy performance of those districts with the potentials to become PEDs. A "double density" simulation scenario was studied further by Bambara et al. [9] to test residential densification potential for PED, where each existing detached house in a community is replaced with two energy-efficient houses of equal living area on the same land lot. From economical and technical points of view, Laitinen et al. [10] concluded that it is more feasible to achieve PED or net-zero energy district, rather than full energy self-sufficiency after they studied a series of technologies (e.g., local centralized wind power, solar PV, battery, heat storage and heat pump), using Helsinki as a case study. Moreover, Soutullo et al. [11] suggested that urban living labs could be a driver to achieve PED. Fatima et al. [12] studied PED's implementation potential from a citizen engagement aspect. Uspenskaia et al. [13] recommended planning and modeling the replication of PED at the very early stage because it is important to find tailor-made solutions to fit spatial, legislative, socio-economic conditions and historical growth of the cities.

Apart from the newly built districts, an explanatory study was carried out as the first step to support the complex planning urban refurbishment, in order to achieve PED [14]. In their study, the key information on the different district types (e.g., energy consumption) was simulated to identify the districts with the highest potential for energy refurbishment. Civiero et al. [15] provide a view of a district simulation model able to analyze a reliable prediction of potential business scenarios on large scale retrofitting actions and to evaluate a set of parameters and co-benefits resulting from the renovation process of a cluster of buildings. Gouveia et al. [16] also argued that the transformation of the existing districts is essential, including historic districts, which present common challenges across EU cities, such as degraded dwellings, low-income families, and gentrification processes due to massive tourism flows. In their report, they discussed how the PED model can be an opportunity for historic districts to reduce their emissions and mitigate energy poverty. Moreover, a methodology for the evaluation of positive energy buildings and neighbourhoods is proposed in the report [17], where a set of Key Performance Indicators (KPIs) are defined with details on the calculation procedure for categories of Energy and Environmental, Economic, Indoor Environmental Quality (IEQ), Social, Smartness and Energy flexibility.

A research gap is thus observed that there are many studies starting to address technical, economic, social aspects of PED, but very limited studies are found in characterizing PED. The Joint Programme Initiative Urban Europe (JPI UE) [18] plays an important role in coordinating PED projects across Europe, it actively engages the interests of different stakeholders, particularly, cities in PEDs. To accomplish its objectives, only Bossi et al. [19] summarized part of PED's characteristics in aspects of geographic distribution, implementation status, building structure, land use, energy typology, success factors/challenges, and barriers. While Brozovsky et al. [20] identified different terminologies of PED, and related focused aspects (i.e., energy, social, climate). JPI UE needs more comprehensive scientific advice on the knowledge and methods for guiding the design, monitoring the operation and evaluating the performance of PED projects. Therefore, many other PED

characteristics need to be abstracted and categorized for further development of PED, such as district size, finance source, energy concepts, building archetypes, spatial/temporal scale and keywords. Moreover, as PED projects are expanding all the time, it is necessary to use a common tool/database to increase the semantic interoperability among different stakeholders, for an updated summary of PED's main characteristics.

In the framework of both International Energy Agency—Energy in Buildings and Communities (IEA EBC) Programme Annex 83 [21] and EU Cost action CA19126 [22], the working groups are now collecting data of PEDs and characterizing them for potential proposal of reference and replication of PEDs in different contexts. This paper, therefore, reviews the existing 60 projects within the European area from the JPI Urban Europe PED booklet, establishes the database, and further analyze/visualizes them for the main characteristics. The paper aims to illustrate the basic characteristics of existing PED projects in the EU, and then deliver the information to the targeted stakeholders, such as municipality, urban planner, real estate developer, utility company, policy/regulation maker, renewable energy provider, energy engineer etc., for them to further define, design, promote and implement potential PED projects. As the PED concept is new to most of the stakeholders, this paper intends to transfer the knowledge to the targeted groups through the review/analysis and the development of a database. The result will be also used for the iterative definition of PED in the two initiatives of IEA and EU Cost action.

#### **2. Data Source and Research Methods**

#### *2.1. Data Source*

The data of PED related projects is collected from the PED booklet [23] by JPI UE updated latest on 2019. JPI Urban Europe is conducting a programme on 'Positive Energy Districts and Neighbourhoods [24] for Sustainable Urban Development' with an implementation plan, SET (Strategic Energy Technology) Plan Action 3.2 [1], participated by about 20 European member states, in the context of Europe commitment towards clean energy transition and carbon neutrality. The total databank consists of 60 projects' data that have similar goals to PED projects in Europe. These projects have been identified and updated by the participated cities of workshops conducted by JPI Urban Europe. The database is divided into several key parameters shown in Table 1.

**Table 1.** Table parameters for data collection.


However, it has been challenging to understand the energy typology and detailed strategies due to unclear/insufficient information for many projects from the JPI Urban Europe booklet. The data for the temporal scale of the projects are only available for very few projects. Due to this insufficient information, external sources, such as the website/publication of the specific project, have been studied and reviewed in order to collect more detailed information [25–42].
