Fifth-Generation District Heating and Cooling Networks Based on Shallow Geothermal Energy: A review and Possible Solutions for Mediterranean Europe

Abstract

This document presents a comprehensive review of research works, regulatory frameworks, technical solutions, and commercial trends related to the integration of shallow geothermal energy (SGE) technologies in modern 5th-generation district heating and cooling (5GDHC) networks. This literature and market analysis is contextualized by the present geopolitical, environmental, and societal scenario in Europe. In this sense, decarbonization of the heating and cooling sector is a crucial piece in the energy transition puzzle to keep global warming below the critical threshold of 1.5 °C by the next century. Moreover, Ukraine war has added urgency to end up with fossil fuel dependency. The most relevant outcome of this literature review is the synergistic relationship between SGE, 5GDHC networks, and urban environments. SGE is most efficiently deployed in urban environments when it is part of a district heating and cooling network, and the modern concept of 5GDHC is the most suitable scenario for it. Since the potential contribution of SGE to the decarbonization of the heating and cooling supply is mostly untapped across Europe, this synergistic effect represents a possible boost. Hybridization with solar photovoltaics and/or storage makes it even more attractive. Outstanding cases are reviewed, challenges for the future are presented, and tools to overcome social reluctance and/or lack of awareness are described, along with a discussion of the stimuli for the deployment of SGE and 5GDHC networks. A particular focus on Mediterranean countries is presented, where SGE systems and DHC networks of any kind show a particularly low deployment compared to the rest of Europe. To this end, the second part of this work evaluates, justifies, and analyzes the possibilities and potentialities of their application in this zone.

1. Introduction

The progressive increase in temperature due to global warming, together with the energy security of supply, represents a major challenge for the European energy sector. Cooling needs are changing progressively and are expected to increase in the next two decades. This especially affects the countries located in the Mediterranean area that have been suffering more and more frequently from very intense heat waves [1,2]. Besides, since February 2022, Russia’s invasion of Ukraine has added a critical geopolitical factor to the energy security of supply in Europe, which is causing a major concern about the forthcoming winters. The urgency to end dependence on Russian fossil fuels (oil and gas, mainly) has led to the launch of the “affordable, secure and sustainable energy for Europe” plan (REPowerEU) by the European Commission (EC), which is an even more ambitious European roadmap towards decarbonization [3]. Even a controverted energy source, such as nuclear energy, will not be discarded yet by the EU’s decarbonization pathway [4] due to the recent events in the geopolitical field, and also coal-fired power plants have been re-activated in Germany, altering its own roadmap towards phaseout by 2038 [5]. Moreover, an unprecedented cut in energy consumption is being demanded by the EC as an additional measure [6].

With nearly 50% of the final energy consumed for heating and cooling (H&C), and the building sector being responsible for 80% of it [7], the relevance of renewable thermal technologies (RTTs) in Europe is even more prominent under this renewed geopolitical context.

Among the well-established, proven, and reliable RTTs, heat pump (HP) technology is possibly the most versatile. When driven by electrical power and coupled to ground sources/sinks, it is definitely the most efficient one [8,9], since one unit of electrical power is required for the exchange of up to 4–6 units of heat power with the ground. This type of HP is known as ground-source HP (GSHP), and the associated energy resource is named shallow geothermal energy (SGE). Nevertheless, it is noted that the method to produce electricity influences the overall efficiency of these systems. The higher the portion of renewables in the generation mix, the higher will be the efficiency of an electrically-driven GSHP [10,11].

The EU Directorate-General for Energy, in its recent report “Policy Support for Heating and Cooling Decarbonization”, considers electrically-driven HPs as “the most important technology for individual heating” and highlights the leading role of modern, highly efficient district heating and cooling (DHC) networks operating at low temperature for the integration of many renewable and sustainable sources, especially in urban environments [12]. Indeed, the REPowerEU plan proposes to deploy up to 10 million individual HP units in the next five years and to integrate large-scale HPs (along with geothermal and solar thermal energy) in modern DHC networks, especially in densely populated areas and cities [3]. This would add to an existing stock of nearly 42 million operating HPs [13].

Historically, the North and East of Europe have used extensively the concept of district heating (DH) for heating in towns and big cities [14]. However, in Mediterranean countries, this tradition does not exist since space heating of multi-family buildings in the cities is inexistent or is made with individual fossil fuel-fired boilers or electrical heaters along with domestic hot water (DHW), which is still the predominant model [15]. Concerning district cooling (DC) systems, the same trend applies in terms of geographical distribution [16]. The portion of H&C demand met by DH, DC, or DHC networks is remarkably low in Mediterranean and Western countries compared to Northern and Eastern countries. While this percentage is below 1% in Spain, Portugal, or Greece, it exceeds 50% in most Scandinavian and Baltic countries. Besides, there is still a long way to go to reach 100% renewable DHC systems across Europe [17,18,19,20].

Although there are more than 30 GW_th of GSHP-based systems installed across Europe, the distribution is highly uneven, showing a remarkable presence in Scandinavia and just testimonial in Mediterranean Europe [21]. The reasons for this difference are much more related to social aspects and regulatory frameworks than to technical barriers [22,23,24,25,26], so it is a question to find the right stimulus to overcome such difference and unleash the full potential of SGE all across Europe.

This review paper aims to contribute to find this stimulus from a particular perspective: although SGE installations and DHC networks are not yet common in Mediterranean Europe, there is sufficient and consistent evidence in the literature and the market of the synergistic potential of using both concepts in this geographic area under the scheme of 5th-generation of DHC networks (a term coined within the project FLEXYNETS [27]). A massive deployment of both concepts together would contribute very significantly to the decarbonization of H&C systems in this domestic and tertiary sector.

This review paper is structured as follows: Section 2 describes in detail the methodology followed by the authors to guide the bibliographic analysis, the sources employed, and the search and filtering criteria. Section 3 explores the most relevant SGE technologies available and shows how their characteristics are indeed the most suitable to be integrated in modern DHC networks where the use of decentralized HPs is a core feature. Existing stimulus paths for the deployment of these systems are identified and submitted to a critical review. Section 4 identifies the specific challenges that Mediterranean countries are facing with regard to climate change, together with their specific climate conditions, which are presented as an opportunity for SGE to be massively deployed through modern DHC networks. Several hints about the current advances in the H&C sectors are provided, with a focus on district systems.-Section 5 adds a critical analysis of the key findings provided by the literature review A final summary of the most relevant conclusions is given in Section 6.

2. Methods

The bibliographic sources were accessed mainly through the channels listed below:

• Scopus database.
• Web-Of-Science database.
• Proceedings of the European Geothermal Conference.
• Publication Office of the European Union (web portal).
• EUR-Lex Europe (web portal).
• Free web search engines.

The authors followed an iterative method for the literature search. The uneven deployment of SGE installations across Europe [21] was chosen as the “seed fact” that triggered the literature review process. Each iteration step drove to a filtering of topics of interest and set the search criterion for the next iteration step. Besides, the search for contextualizing the literature was guided by a set of “side facts”, as listed below:

• Urgency for decarbonizing the H&C sector in Europe [3,12,28].
• High potential of SGE + GSHPs but mostly untapped [13,21].
• Growing demand for cooling due to climate change, especially in the South [1].

Subsequently, the first search iteration step was to explore the causes of the “seed fact”. The second iteration step was to explore the most relevant advances on SGE. The third iteration step was to explore the relationship between SGE and 5GDHC networks. Finally, the fourth iteration step was to identify the most relevant approaches that could aid a faster deployment of SGE systems in regions where it is still incipient. Contextualizing the literature is employed mostly in Section 1 (Introduction), but also in Section 5 (Discussion). In Figure 1, a detailed flowchart of the iterative literature search process is shown. Notice that the literature search iteration steps do not necessarily correspond to the order established in the storyline of this review article.

3. Review of Synergies between SGE and DHC Networks

According to the EU Directive 2018/2001, “geothermal energy” means strictly “energy stored in the form of heat beneath the surface of solid earth” [29]. SGE is a specific case where the temperature of the resources is generally under 30 °C (also known as “very-low enthalpy geothermal energy”). In contrast, the heat exchange with surface-water bodies (sea, lakes, or rivers), ambient air, and sewage water is considered as “ambient energy” in the same document. However, unlike in deep geothermal energy (where the heat source is mostly due to the decay of radioactive elements uranium, thorium, and potassium [30]), the ultimate origin of SGE is actually the energy irradiated by the sun, as in the case of ambient energy. Ambient air, surface water, and the ground all store energy from the sun, but only the ground shows a thermal inertia high enough to keep a constant value throughout the year from a depth of 10–15 m downwards. This value corresponds to the yearly ambient temperature of the location close to the surface, and grows with depth according to the geothermal gradient, which is also location-sensitive and ranges typically from 0.02 °C/m to 0.03 °C/m, unless a thermal anomaly is present [30]. For the same reason, SGE can be exploited either in the form of heat exchange or heat storage. Since the ground temperature is higher than the ambient temperature during winter and lower during summer, heat can be extracted from or injected to the ground for heating + DHW and cooling purposes, respectively, more efficiently than in the case of ambient air or surface water, and with a single machine covering all operation modes.

SGE is a resource available virtually anywhere in Europe, at any time and any size. Typical values of installed thermal capacity (corresponding to the nominal capacity of GSHPs) are 3–10 kW_th for individual households [31]; 10–100 kW_th for mid-size applications, such as central heating of multifamily buildings and tertiary buildings (private and public offices, commercial, and educational or health centers) [32]; or 0.1–10 MW_th (and even higher) for very large facilities, such as big hospitals [33] or airports [34].

The exploitation of SGE resource requires a ground heat exchanger (GHE) connected to a HP. Horizontal GHEs (also known as “agrothermal collectors”), vertical borehole heat exchangers (BHEs), and thermo-activated foundations (TAFs) are the most common “closed-loop” systems. The term “closed-loop” refers to the fact that the heat exchanging fluid (usually water or a brine composed of a water–glycol mixture) circulates through a plastic probe embedded in the ground medium. Probe materials are typically high-density or cross-linked polyethylene (HDPE and PEX, respectively). “Open-loop” systems use groundwater directly as a heat exchanging fluid (GWHEs), although it is usual to separate the ground loop from the building loop via an intermediate HE to minimize corrosion or scaling problems [35]. Underground thermal energy storage (UTES) is mainly carried out through BHEs (BTES), GWHEs in aquifers (ATES), and in artificial ground cavities (CTES), which mostly correspond to flooded old oil-storage caves or mines (indeed, CTES is also known as MTES). An important difference to stress between heat storage and heat exchange is that the heat carrier flow is bidirectional in UTES systems and unidirectional in exchange systems. This important distinction influences the requirements concerning ground and groundwater properties when it comes to design, sizing, and operation of GHEs [36]. Another important difference is that UTES does not imply necessarily the use of GSHPs [37] and/or the involvement of renewable energy technologies [38].

3.1. Specific SGE Solutions Applied to Urban Environments

Highly urbanized environments may be seen as a major barrier for SGE projects, given a limited availability of land to drill boreholes and usually intricated underground infrastructure networks. Nevertheless, most European cities possess outstanding examples of successful implementation of SGE projects, from a technical, economic, or environmental perspective.

BHEs are undoubtedly the most versatile, feasible, and environmental-friendly technology to exploit SGE everywhere. At the other extreme, application of GWHEs is firstly limited by the presence of aquifers with specific characteristics. Generally, more stringent conditions must be met to exploit groundwater for thermal uses because of possible conflicts of use, effects on water quality, microbiology, etc. On the bright side, investment cost is usually lower compared to BHEs. In the following paragraphs, specific solutions for urban environments are presented.

In urban environments, available land imposes a clear limit to drilling possibilities. Maybe the most obvious solution is to drill as deep as possible to reduce the number of wells, especially when GSHPs are used mainly for heating purposes. This is the case of Sweden, where the average borehole depth has doubled since 1995 [39]. Sweden shows a large portion of its territory with Precambrian shield rocks (crystalline and metamorphic rocks), and therefore the vast majority of Swedish SGE systems are based on vertical BHEs implemented in hard rock [40]. As an alternative for non-rocky grounds, the lack of space for drilling can be overcome by shifting from a set of parallel wells to a set of a tilted wells, resembling a pyramid-like structure (Figure 2a). This way, the required surface footprint of the BHE field can be minimized. While the wells’ heads are packed together at the surface, their bottom ends are more distanced, so thermal interferences between BHEs are claimed to show an effective magnitude similar to that of traditional parallel well patterns [41]. Since 2020, a facility has been in operation at the Schlumberger Riboud Product Center in Clamart, France. The GHE is composed of 10 BHEs, which required less than 20 m² for the drilling process.

Concerning GWHEs, horizontal wells are a good solution when there are limitations related to maximum drilling depth of wells and a low hydraulic conductivity takes place or a high extraction rate is wanted, [42] (Figure 2b). A good example of their application can be found in the Wirtschaftsuniversität Campus (Wien, Austria), where a set of 10 horizontal extraction wells provide a pumped flow rate of 150 L/s with a maximum drilled depth of 12 m (>3 MW_th installed) [43]. In urban environments with a shallow groundwater table (near a river, lake, or sea), many dewatering stations are used permanently to keep the structural stability of building foundations or to keep water away from underground infrastructures, such as basements or metro stations. This groundwater abstraction usually represents an external cost due to its pumping towards sewers, but depending on the yearly flow profile, it can be turned into a cost-effective energy source [44,45].

A novel concept of GHE is the one called “Dynamic Closed-Loop” (DCL^®) [46]. It can be imagined as a U-tube probe immersed into an abstraction well, where a forced convection flow is created around the closed loop to enhance its heat exchange rate with the surrounding ground. This is a commercial hybrid solution between GWHEs and BHEs which gets the best of both worlds [47,48]. A similar concept has been applied in Japan but taking advantage of an artesian well for the forced convection flow [49].

Another solution especially relevant in urban environments is that of TAFs [50]. The use of buildings’ foundations as GHEs (Figure 2c) is something that can only be taken into consideration in an early stage of the construction project, and it generally applies to large buildings with enough deep foundations, including piles, pile walls, and diaphragm walls, or underground infrastructures, such as metro tunnels [51,52] (Figure 2d). In Barcelona (Catalonia, Spain), the refurbishment of the old Sant Antoni modernist market required a perimeter concrete diaphragm wall, with a total surface of 16,500 m² (40 m deep). This massive foundation was employed as a TAF (the largest in Spain up to date) by embedding nearly 44,000 m of PEX probes (O.D. 25 mm SDR11) in the reinforced concrete structure, which yielded a specific heat exchanging rate of 40 W/m², allowing an installed power of 750 kW_th that suffices to meet the entire heating demand and 65% of the cooling demand [53].

UTES systems are not so common as heat exchange systems, although the technology is mature enough [54]. Its relevance in urban environments is clear since UTES systems are linked mostly to large tertiary buildings or DHC networks [55,56]. BTES is postulated as the natural partner for solar thermal energy in small DH networks [57], allowing the seasonal storage of surplus solar energy during summer for its later use in winter, achieving yearly solar fractions up to 100% [37] for space heating. For small/individual installations, hybrid concepts, such as solar-assisted GSHPs (SAGSHPs), exploit a similar principle. Especially in heating-dominated installations, the surplus energy from solar thermal collectors during summer can be used to regenerate the ground heat depletion caused by the GSHP during winter. This way, GSHP performance is enhanced, and shorter BHEs are required [58]. Notice that these systems operate with unidirectional flow and cannot be strictly considered as UTES. However, a heat storage process certainly takes place. Regarding ATES, a low groundwater flow speed (v_gw ≲ 25 m/year) [59] and a moderate hydraulic conductivity (K ≳ 10⁻⁵ m/s) [54] should converge to guarantee low thermal losses. Additionally, injection and extraction wells should be located across the perpendicular direction of the groundwater flow to minimize thermal interferences between them. In contrast, heat exchange systems (unidirectional flow) should install extraction and injection wells upstream and downstream, respectively. CTES, although is the least common of all UTES systems, represents the best chance to reconvert old mines or other artificial underground cavities [60,61,62,63]. Since groundwater is present as a free-running fluid (not mixed with surroundings rocks), heat injection and extraction show the highest rates among all UTES types, which is attractive for any heat-storage time scale, from daily to seasonal [64].

GSHPs combined with UTES represent a key element in decentralized and renewable energy generation, especially for intermittent electrical power sources, such as solar photovoltaic (PV) and wind energy. The tandem GSHP + UTES is an efficient and cost-effective alternative to electrical storage (hydroelectric storage or large battery stations). GSHPs can use the surplus electricity from solar PV and wind energy to produce thermal energy, which can be stored in UTES systems for its later use. Although this process downgrades electricity to hot water as the energy vector, the fact is that buildings consume far more thermal energy than electricity as final energy use, so UTES can be seen as a realistic and cheap alternative to electrical storage [65,66]. Indeed, this operation strategy is already available as a commercial solution in individual GSHP installations hybridized with solar PV panels [13,67,68,69,70]. Instead of storing surplus self-produced power in Li-ion batteries, this electricity is used to produce DHW, which is stored in a tank. By doing so, no matter how less efficient a GSHP can be in producing DHW, the renewable fraction in this case is 100% [71]. Moreover, the stability of the power grid is favored by reducing the amount of surplus electricity from the solar PV injected into it [72,73].

In addition, it is worthy to mention a particular phenomenon which is intrinsic to highly anthropized areas, such as dense urban centers: the subsurface urban heat island (SUHI) effect. This effect is observed through differences in shallow ground temperatures between dense urban centers and the surrounding less-populated areas. The surplus temperature can range between 2 and 6 °C [74], and supposes a potential energy source that several authors have identified and quantified [75,76]. However, the major concern about SUHI might not be its potential as an energy source, but the potential hazards to chemical quality of groundwater, and its microbial ecosystems [77]. In this sense, SGE can definitely contribute to mitigate thermal anomalies by balancing the heat exchange with the subsurface thanks to the dual H&C operation modes of GSHPs [74].

3.2. Modern DHC Installations: 5GDHC Networks

An important aspect for an efficient generation and distribution of energy is the economies of scale. While the power generation model is still based on centralized large plants with long distance transmission and distribution, this concept would hardly apply to thermal energy transported in the form of hot/pressurized steam or water as the energy vector. This is due to thermal and pressure head losses in water pipes, which makes heat transport economically feasible over much shorter distances than electricity, in general no longer than 20–30 km from the production plant to the cluster of consumers [78]. However, production and distribution of thermal energy at a district level is still the most versatile, energy-, and cost-efficient option over individual installations even if the same primary energy source is considered [79,80,81,82].

The current geopolitical context, along with climate change mitigation strategies, demands an even faster energy transition towards a 100% renewable-based model. Society must, therefore, switch from centralized, high-power, fossil fuel-based to decentralized, low-power, and renewable energy sources. At the same time, a progressive electrification of final energy consumption [83] and a progressive concentration of the population in cities [84] are expected. From the H&C perspective, to confront these trends requires adapting the demand to available low-grade, renewable, and local energy sources. The key to this challenge is offered by 5th-generation DHC (5GDHC) networks.

5GDHC networks are those where the heat exchanging fluid is water or brine at ambient temperature (close to that of the medium through which it is transported, between <0 °C and 20 °C, typically [85,86]), and small-to-medium-sized HPs are installed at each building of the network to provide heating, cooling, and DHW. Several review papers have contributed decisively to define the concept and to identify examples around the world [16,85,87,88,89,90,91,92,93,94]. From these works, the most representative changes with respect to traditional DH schemes are summarized below:

• Change from centralized generation to distributed generation. This feature favors an easier extension of the network, although at a higher investment cost per connection point.
• Almost null heat losses due to transport in cost-effective pipe circuits. Since the water/brine temperature is that of the surrounding ground, thermal losses become a minor concern. Therefore, the use of pre-insulated pipes can be avoided, contributing to an important cost reduction. As a counterpart, heat transport by water/brine at ambient temperature implies generally much lower gap between supply and return temperature, thus requiring higher water/brine flows in larger-diameter pipes if compared to high-temperature networks.
• Buildings conceived either as providers or consumers (prosumers) of heat. Apart from purchasing heat from a network for heating purposes, the same building can now be a supplier by selling the heat rejected when a cooling demand takes place. Monitoring and smart metering are indispensable, analogously as in the case of prosumer electricity, since heat flow requires bidirectional meters in 5GDHC networks, so customers can benefit from purchasing heat or selling it at the right time during the day.
• There is no longer a centralized source of energy, but a new figure known as the “Balancing Unit” (BU) [88,95] which acts as a subsidiary heat provider/absorber of a bidirectional network since, in 5GDHC networks, the priority heat sources and sinks correspond to the buildings themselves. Four generic scenarios can be defined for the BU:
  • Ideal Scenario: H&C loads are compensated between buildings simultaneously, so the heat supplied and rejected in the network cancel each other out.
  • Quasi-ideal Scenario A: H&C loads are compensated, although they do not take place simultaneously.
  • Quasi-ideal Scenario B: H&C loads are not compensated, although they take place simultaneously.
  • Realistic scenario: H&C loads are not compensated between buildings and do not take place simultaneously.

In case 1, the BU would be a dispensable component of the network. In case 2, the BU must be understood as a heat storage unit. In case 3, the BU acts as an additional heat source or sink. Finally, in case 4, which might be the most common scenario, the BU acts as an effective heat source or sink, and eventually as a heat storage unit. A good example is the project Mijnwater 2.0 in Herleen (The Netherlands) [64] where multiple types of buildings in the network exchange heat between them, and the groundwater in a flooded old mine is used as a heat source and sink but also as a heat storage system.

• Two pipes are enough. 5GDHC networks are bidirectional, which means that heat can flow into and out from buildings. An important distinction is made between “directed” and “non-directed” medium flow. In directed networks, a central pump station is used to always circulate water/brine in the same direction. The supply line comes from the BU at an ambient temperature, either for heating or cooling purposes, and the return line goes towards the BU. In directed networks, even 1-pipe scheme is possible (and simpler) if all buildings are topologically connected in series [96]. Under this configuration, the return temperature of a building becomes the supply temperature of the next one. However, for this configuration to be advantageous, the H&C demands need to be well balanced between adjacent buildings, and the extension of the grid is not as simple as in the previous case, where building would be connected in parallel. In non-directed networks, each station has its own, decentralized pump station. A warm pipe (15–25 °C) and a cool (5–15 °C) pipe act as the supply and return channels of heating demand substations, while for cooling demand it is just the opposite (see Figure 3). The direction of the heat exchanging fluid with respect to the BU is determined by the balance between H&C demand at each moment.
• No restriction in building typology. Although highly efficient buildings are preferable nowadays under any circumstance, 5GDHC networks do not impose constrains on the type of building for its connection to them. Different degrees of thermal insulation or different emitter technologies (low or high temperature radiators, radiant floor, fan coils, etc.) can be addressed with tailored HP solutions. Therefore, in the same network, most modern and efficient buildings can coexist along with historical buildings or tertiary buildings of any type or age, with all of them requiring water supply temperatures ranging from 40 °C to 90 °C [97].
• In 5GDHC networks, electricity is inextricably linked to heat. Unlike traditional DH networks, where steam or water is the true energy vector by itself, in 5GDHC networks, ambient-temperature water/brine can be an energy vector only if combined with electrically driven HPs. Therefore, parallel concerns about renewable and decentralized electricity generation are indispensable in designing and planning efficient 5GDHC networks.
• Increased operation complexity. For the operation of bidirectional networks, supply temperature or hydraulic pressure setpoints are not useful anymore. The temperatures of the warm and cool ring can oscillate according to the admissible range of HP performance and the BU (e.g., a BHE field). The actual temperature must be determined by an optimization function. This function considers the balance between H&C demands and a chosen optimization parameter (minimum operational cost, minimum emissions, or maximum HP efficiency, for instance). On the side of hydraulic pressure control, directed networks can be operated as traditional networks. In non-directed networks, the volumetric flow and its direction are mostly determined by the set of decentralized circulation pumps.
• Flexibility for different business models. In 5GDHC networks, old DH business models cannot apply anymore since electricity and heat are interlinked with prosumers, so new retail tariffs, investment costs, ownership, and operation schemes are necessary. As mentioned above, bidirectional metering of both electricity and heat is the most important technical prerequisite to establish any possible business model for 5GDHC networks. On one extreme, an external investor could own the network, the BU, and the set of decentralized HPs. Then, a one-time connection fee would be applied, a €/kWh price would be charged for consumption, and a different one would be charged for production. At the other extreme, a community of prosumers can associate to invest in a 5GDHC network, letting new prosumers connect to the grid by sharing its costs and ownership, and bearing the investment cost of their own HP. In between these two extremes, there are plentiful appealing choices. The following are some examples:

  ○

  When the BU is based on a UTES system, a private company that owns the whole infrastructure could charge only for the heating while offering cooling for free as a stimulus to recharge heat depletion.

  ○

  The network and the BU can be owned by a private company, while an energy service company (ESCO) would own the set of building substations. The final clients would pay only for H&C at an agreed price which, in theory, should be lower than traditional alternatives (gas boilers + air conditioning), but still high enough for the ESCO to obtain a payback for its initial investment plus an additional profit.

  ○

  The network, the BU, and the substations could be all owned by separate entities. The network would pay to the BU owner, and the final prosumers would pay to the network owner. Under this scheme, the owner of the BU would pursue to maximize its use with compensated H&C loads on a seasonal basis. The network owner would seek, as its main business driver, to achieve a balance set of H&C demands on an hourly basis (reducing the need to use the BU). From the side of final prosumers, their main driver would be to compensate their own H&C demands instantly (reducing the need to use the network).

Figure 3. Illustrative scheme of a generic 2-pipe 5GDHC network, where H&C demands could take place simultaneously and the BU is a BHE field. This particular scheme corresponds to a “bidirectional energy flow with non-directional medium flow”, as defined in [16]. An additional third ring (black-colored) corresponding to electricity has been added to emphasize the coupling of heat and electricity that takes place within 5GDHC networks.

Figure 3. Illustrative scheme of a generic 2-pipe 5GDHC network, where H&C demands could take place simultaneously and the BU is a BHE field. This particular scheme corresponds to a “bidirectional energy flow with non-directional medium flow”, as defined in [16]. An additional third ring (black-colored) corresponding to electricity has been added to emphasize the coupling of heat and electricity that takes place within 5GDHC networks.

The energy flows of a 5GDHC network are analogous to the heat-recovery processes in large and complex modern buildings, where simultaneous H&C can take place just by pumping heat from the cooling-dominated zone to the heating-dominated one [98,99]. From a theoretical perspective, four key performance indicators are identified and defined in [100] to establish the proper metrics when characterizing and comparing 5GDHC networks:

• Specific supply costs for H&C. It is defined as a yearly indicator, expressed in €/MWh_th/year.
• Exergy efficiency. It is defined as the ratio between total useful exergy and the total exergy expenditures.
• System COP. This indicator is analogous to that of an individual HP, but it includes the influence of self-produced electricity by means of solar PV panels, denoting the expected close relationship between 5GDHC and renewable electricity generation:

$$CO{P}_{system}=\frac{Q_{h,dem}^{tot}+Q_{c,dem}^{tot}+W_{feed in}^{tot}}{W_{grid}^{tot}+W_{PV}^{tot}}$$

(1)

where $Q_{h,dem}^{tot}$ and $Q_{c,dem}^{tot}$ are the total H&C demand, respectively; $W_{feed in}^{tot}$ is the total self-produced electricity injected to the grid; $W_{grid}^{tot}$ is the total electricity consumed from the grid; and $W_{PV}^{tot}$ is the total self-produced electricity.

• Specific CO₂ emission. It is expressed as tones of CO₂ equivalent divided by MWh of thermal energy (heating + cooling). Here, the emission corresponds to the primary energy used, so it depends on the CO₂ factor of the power system mix.

Additionally, a new indicator was proposed in [100], named Demand Overlap Coefficient (DOC) in order to quantify the proportion of H&C that can be compensated between buildings within a 5GDHC network. DOC shows values from 0 to 1, being 0 for the case where the H&C demands never take place simultaneously, and 1 for the ideal case scenario defined above, when the BU is unnecessary (case 1). This indicator can be either defined for a network or a single building.

3.2.1. Overview of Concept Definition

Among the engineering community, the categorization of DH systems introduced in 2014 by Henrik Lund and co-authors [14] is nowadays widely accepted. It classifies DH systems into four different generations characterized by a set of disruptive technological and conceptual jumps in chronological order, since their conception in the nineteenth century until present. These jumps follow a clear trend in the following aspects:

• Progressive decrease in the supply temperature of the heat carrying fluid (steam/water) in the network.
• Progressive increase in the efficiency of the system due to a decrease in thermal losses, both in the generation and distribution of the thermal energy.
• Progressive transition from fossil fuels to renewable energy sources as the primary energy consumed by the network.

The term “5th generation DHC” (5GDHC) emerged just one year later, in 2015 [27]. This name was cleverly coined in the wake of Lund’s previous work, with the aim to attract attention on a specific concept of DHC networks. On purpose or not, by naming it with the term “5th generation”, it conveyed a sense of being the natural next step following the abovementioned trends that DHC networks had been showing throughout time (lower temperature, higher efficiency, and higher share of renewables). Furthermore, it added extra features, such as decentralized production and intrinsic coupling between electrical and thermal networks [101]. Nevertheless, the concept of 5GDHC is not newer than 4th-generation DH (4GDH) in terms of existing operative examples [16]. However, although its acceptance within Lund’s categorization is controversial [17,102,103], the fact is that 5GDHC has become the most widely and consistently used term for this kind of modern DHC projects [104]. Particularly, the research European D2Grids project has decidedly contributed to the prevalence of the term “5GDHC networks” over the rest of nomenclatures. In its webpage, the concept is clearly and concisely stated through a list of five principles [105].

Simone Buffa and co-authors were the first to provide an exhaustive list of nomenclatures strictly fitting the concept of 5GDHC networks, but also of those corresponding to closely-related types of networks, for the sake of clarification and harmonization [16,104]. In the present work, additional comments add to the still open discussion.

The concept of “low temperature DH” (LTDH) was before Lund’s categorization, since it was a natural consequence of trying to lower the supply temperature of traditional DH networks as much as possible (T_sup = 60–70 °C) in order to integrate a larger portion of RTTs, and to lower transmission losses.

The term “ultra-low temperature DH” (ULTDH) has become unfortunately ambiguous. Initially, ULTDH was defined as an evolution from LTDH systems, where the limit of direct supply temperature (still with no booster HPs) was pushed to the lowest possible value (T_sup = 40–45 °C) [106,107]. More recently, other authors assimilate ULTDH as 5GDHC networks, where decentralized HPs are an intrinsic part of such concepts [108,109]. Although it is accurate to equate the terms LTDH and 4GDH, it is not so with ULTDH and 5GDHC. In [90], a clear distinction is made between ULTDH and 5GDHC networks, which are defined as ambient-temperature DH (ATDH) networks. In fact, LTDH and ULTDH can be considered as two subcategories of 4GDH [87], so any reference to “low“- or “ultra-low”-temperature networks should be avoided when referring to 5GDHC networks.

3.2.2. The Synergistic Effect of SGE+5GDHC Networks

Looking at the existing examples of 5GDHC networks in Europe, Simone Buffa and co-authors firstly identified 40 installations in Europe [16], mostly in Germany and Switzerland, but also in The Netherlands, Belgium, Norway, Italy, and UK. From this list, 25 out of the 40 installations used SGE in their BU. More recently, Marco Wirtz and co-authors updated the list of installations only in Germany, where 53 cases were identified [86], with some of them being a hybrid system between 4GDH and 5GDHC. In 77% of the surveyed installations, the BU was based on SGE, mainly in the form of horizontal GHEs and BHEs, but also GWHEs. From these examples, it is undeniable that SGE and its associated myriad of exploitation schemes is the natural partner of 5GDHC networks. Interestingly, the first 5GDHC network that could be identified as such by Marco Pellegrini and Augusto Bianchini [87] consists of a small DHC operative since 1991 that re-uses the groundwater inflow from the Furka Tunnel (Switzerland) as the energy source, in combination with decentralized GSHPs [110].

This is not in vain as community schemes for the exploitation of SGE are attracting attention in recent years in parallel to 5GDHC networks. In Denmark, the non-profit association Termonet [111] is named after a concept which consists of a cluster of customers sharing the same GHEs (BHEs, GWHEs, or TAFs) within a neighborhood. This kind of 5GDHC network is a simplistic version of 5GDHC networks since buildings connected to the grid are alike and usually not showing compensated thermal loads between them (Figure 4).

Compared to individualized installations, a shared set of GHEs is advantageous in many ways:

• Smart and synchronized operation of decentralized HPs permits a lower aggregated peak load in the network, so the total size of the GHEs can be minimized, allowing much lower installation costs, reduced land utilization, lower impact to the ground, and minimized thermal interferences [112].
• When integrated in new urbanizing plans, its implementation is the most efficient and cost-effective. However, in existing urban areas, this option might be the only one possible to exploit the SGE resource.
• As in the general case of 5GDHC defined previously, shared GHEs permit the uptake of new business and operation models.

A representative and outstanding example of a Danish “Termonet” is in Silkeborg, which is operative since 2017. It consists of 15 individual houses sharing 6 × 120 m deep BHEs. The total installed HP power is 90 kW_th (15 units of 6 kW_th each), with no back-up equipment of any kind [113]. In Denmark, there are 11 Termonet projects (operative or under construction) [114].

In the UK, the same concept is commonly known as “Shared Ground Loop Arrays” [115,116,117,118]. In North America, the commercial term GeoGrid^TM [119] coexists with “GeoMicroDistrict” or “Networked Geothermal Energy”, which were coined by the “Home Energy Efficiency Team” (HEET) located in Massachusetts (US) [116]. This non-profit organization advocates for a progressive substitution of old and leaky natural gas grids by BHEs [120]. This is a very interesting approach where many issues converge, such as the mitigation/suppression of greenhouse gas (GHG) emissions from leaks in an extensive old gas pipe grid, the transition from fossil fuels to renewable sources, building electrification, and the shift/renewal of business and jobs. However, the implementation of such a big-scale transformation in energy generation and distribution is not evident, especially in densely packed urban areas. In locations with a markedly vertical distribution of the population (tall and packed buildings), the H&C demand density might be too large to be met by SGE systems, despite the lower demand of multi-family buildings (typically between 20% and 50% less in terms kWh_th/dwelling/year with respect to single-family detached buildings, depending on year of construction and climate conditions [121,122]). Even if all the buildings have been previously enhanced in efficiency, the implementation of large enough GHEs would still be challenging, but not impossible.

In densely populated areas, the exploitation of the SGE resource is more efficient in the form of a DHC network rather than individual installations. Alina Walch and co-authors demonstrated that the exploitation of SGE under a DHC scheme would increase the potential of meeting H&C demands in Western Geneva by 2050, from 63% to 87% for cooling and from 55% to 85% for heating, thanks to direct heat exchange between buildings [123]. Most importantly, these numbers are independent from the Representative Concentration Pathway (RCP) scenarios defined by the International Panel on Climate Change (IPCC) [124]. Additionally, it is demonstrated that balanced seasonal ground heat exchange is highly beneficial in order to minimize thermal interference between BHEs, and to maximize the total amount of exchanged heat (comprising both H&C operation modes) along the year.

3.3. Specific Stimulus Paths for SGE and 5GDHC

According to the EU’s Eurostat on Energy from Renewable Sources [125], the contribution of HPs to H&C in the EU-27 showed a linear growth from 0.3% in 2004 to 2.9% in 2020, which include all kinds of HPs. In the same period, the total share of renewables for the same final energy use went from 11.7% to 23.1%, where the predominant fuel continues to be biomass. Moreover, only 4.4% of the operating HPs are GSHPs [13]. These data demonstrate that there is still a long way for an effective electrification of the H&C sector, especially when it comes to GSHPs.

On the side of DHC networks, the current contribution of DHC to the H&C sector is around 12% [18] in Europe, although this could reach up to 50% by 2050, even considering a conservative scenario [126]. Currently, the share of renewable sources employed just for DH is about 30% [17], although a 100% renewable is envisioned for 2040 in [18], where HPs would represent the second largest contribution (~23%), being only surpassed by biomass (~35%).

The EU has outlined its major guidelines towards the decarbonization of the H&C sector with targeted quantitative goals in the 2030 horizon [17]. These can be found throughout the most recent revisions of the Renewable Energy Directive (RED) [127], the Energy Efficiency Directive (EED) [29], and the Energy Performance of Buildings Directive (EPBD) [7]. Concretely, a minimum of 1.1% of yearly increase in renewable H&C is required for all member states (1.3% in case that waste heat and cold is used). Additionally, a yearly 2.1% penetration of renewable energy source is expected in DHC networks [127].

For SGE and 5GDHC to contribute to these objectives, specific stimulus paths are identified in the following paragraphs.

3.3.1. Economic Incentives

The EU have many active economic incentives for research and innovation in H&C through grants, loans, guarantees, equity funds, and subsidies, which are addressed mainly to private and public entities, such as small companies, research institutes, universities, or municipalities. Additionally, there are many other national and regional subsidies and funding programs. An inventory of these schemes can be found in [128]. Within these actions, HPs are benefited along with other RTTs, such as biomass or solar thermal collectors. However, GSHPs might deserve a focus on specific actions.

Compared to other alternatives, GSHP is the most expensive RTT in terms of investment costs (CAPEX), and it is still relatively little known among the population. At the same time, it is the one showing the highest efficiency and lowest maintenance cost (OPEX) [129]. Therefore, the yearly savings in operating and maintenance costs more than compensate for the extra investment attributed to GSHP systems, and payback periods in the range of 5 to 10 years are generally feasible when compared to traditional solutions, such as gas boilers + air conditioning systems, even less when considering current subsidies. In fact, as upfront costs can constitute a real barrier for a vast majority of citizens and small private/public entities, economic incentives (such as direct and indirect subsidies or soft loans) are the true main pillars for an effective massive deployment of SGE systems. Since GSHPs constitute one of the building blocks of modern 5GDHC networks, the same reasoning applies to them.

Further side actions should be carried out in order to provide a more accurate (or actual) cost of efficient and low-emission RTTs, such as the GSHP. On the one hand, the application of taxes to CO₂ emissions, which is currently limited to large installations, should be extended to all sizes of H&C producers. On the other hand, installation of new individual gas boilers should be restricted or directly prohibited, which can contribute also to reducing the exposure of customers to the high volatility of gas prices [130]. In this sense, several countries have already announced their own regulations to ban new installations of such systems [131].

3.3.2. Local (Thermal) Energy Communities

Although popularly known as “Local Energy Communities” (LECs) or simply “Energy Communities”, “Citizen Energy Communities” (CECs) was the term used by the European Union in the first directive (EU 2019/944) addressing the regulation of production of electricity by cooperating groups of citizens at a local scale [132]. Previously, the Renewable Energy Directive 2018/2001 included a specific article on “Renewable Energy Communities” (RECs) as multiple citizen entities with a common purpose of self-producing renewable energy [133]. Both directives aim to protect and promote the participation of small groups of aggregated producers, generally non-profit associations, in the energy market. CECs concern electricity generation, not necessarily from renewable sources, while RECs concern renewable energy generation, not necessarily electricity. The underlying philosophy of energy communities addresses all three components of the Energy Trilemma (Security-Equity-Sustainability) [134], and despite their local nature, they have a great potential to be implemented across Europe. Concerning H&C, according to the report on Policy Support for Heating and Cooling Decarbonization from the European Commission [12], “local communities have an important role to play in awareness creation, exchange of experience, and creating demand for solution providers, planners and system operators”. However, as stated by Adamantios G. Papatsounis and co-authors [135], the vast majority of research, regulation, and implementation of LECs carried out so far are electricity-related, while thermal energy communities (TECs) are still at an early stage of research, lacking specific clear regulation and with very few examples of implementation. Given the differences between generation, distribution, and storage of thermal and electrical energy, Javanshir Fouladvand and co-authors suggest that an independent research and regulation agenda for TECs should be attained [136]. In fact, both review works, [135] and [136], highlight the close relationship between TECs and DHC networks. Interestingly, the e-Neuron project introduces the term “Integrated Local Energy Communities” (ILECs) [137]: “A set of energy users deciding to make common choices in terms of satisfying their energy needs, in order to maximize the benefits deriving from this collegial approach, thanks to the implementation of a variety of electricity and heat technologies and energy storages and the optimized management of energy flows”. This is probably the definition that best applies to 5GDHC networks incorporating renewable electricity generation. Energy communities prone to be engaged in such kind of projects would have to address multi-vector self-production distribution and storage. With approximately three-quarters of households’ end-use energy corresponding to heating, cooling, and DHW [138], TECs should receive at least as much attention as electricity-related LECs at an institutional level.

Energy communities represent, from a social perspective, a unique platform for stimulating renewable energy self-production among groups of population showing diverse profiles concerning education, income, and awareness on sustainability issues, but facing common challenges. Both SGE and 5GDHC are characterized by high investment costs compared to their alternatives. Although the savings in operational costs usually may pay off the difference in the long run, an initial scenario with limited monetary resources combined with a low concern about energy transition and climate change can be a major barrier. Even with attractive financial incentives from public and private sectors accompanied by facilitating regulation frameworks, a lack of awareness can be still a remarkable showstopper. Therefore, empathic initiatives at the community level [139] are essential to reach all possible audiences, addressing their respective idiosyncrasies, generating trust, and promoting involvement.

3.3.3. Progress in the Learning Curve through Design and Modeling Tools

5GDHC is still an incipient concept in the energy field and it is practically unknown among most of the population. Given its relevance, it is necessary to create solid evidence of its benefits through experience and knowledge to boost awareness. Accurate sizing and optimal operation of DHC networks are key issues, but they will become critical in order to start new projects in the near future. In the case of 5GDHC networks, which are complex systems because of their decentralized and bidirectional nature, modeling and simulation software becomes an indispensable tool.

Traditional DHC systems can be divided into three independent modeling blocks [140]:

• Heat sources or heat plants. A main distinction is made between permanent heat plants (heat production continuously exceeds the network demand) and non-permanent heat plants (heat production fluctuates over time).
• Distribution networks. Usually, it is composed of two pipes (supply and return) for DH networks, or four pipes for DHC networks. Modeling can be based upon hydraulic or thermal balance. A primary circuit is established between the heat plants and the buildings, and a secondary circuit is established between the buildings and the heat transfer elements (radiators, radiant floor, fan coils, etc.). The topology of the network is mostly radial (or tree-type), where the heat plant is at the topological center or root from which the pipes branch out and connect to the buildings [141].
• End-users (consumers). End-users can also be understood as the thermal loads or demand profiles. For their modeling, basic deterministic methodologies are used, such as the degree-day method [142] and the bin method [143]. Specific building physical behavior modeling requires specific software tools, such as Energy Plus [144] or TRNSYS [145]. Alternatively, stochastic methodologies include regression models [146] and artificial neural networks [147].

However, under the scheme of modern 5GDHC networks, these three blocks cannot be addressed independently, and the modeling of such systems would require a more holistic approach (see Figure 5). Heat sources (and sinks) can be centralized installations, such as solar thermal plants; however, in the case of SGE systems, a central heat source/sink (BHE fields, GWHE in aquifers) is coupled to a distributed set of water-to-water/brine-to-water HPs installed at each building. Solar PV panels, although not being a thermal energy technology, should be considered as a new “must” for modern DHC networks, since most efficient HPs are electrically driven. Not only this, circulation pumps, booster pumps, and monitoring systems are all electrically-driven machinery.

Concerning the concept of end-user, the buildings themselves can alternate the role as end-user and heat/cold producer, so traditional DHC networks turn into grids of multiple nodes where heat can flow in and out (“ring” or “meshed” grid typologies [141]). Despite the general increase in complexity, the neutral transport temperature (between <0 °C and 20 °C, typically) of 5GDHCs simplifies somehow the modeling of the network. In principle, a 2-pipe scheme, or even a 1-pipe scheme, is enough for coexisting H&C demands, as previously mentioned in Section 3.1.

Marwan Abugabbara and co-authors created a bibliographic analysis to identify the current challenges in modeling and simulating DHC networks [93]. They found that most of the current research is performed in MODELICA, and the co-simulation between building and network models is performed through the Functional Mock-up Interface (FMI) tool. The control strategy when integrating building performance in a decentralized and bidirectional network was identified as the main challenge in modeling and simulation of 5GDHC networks.

In

Table 1. Review of modeling tools suitable for 5GDHC design.

Ref.	Name	Language	IDE	GUI	5GDHC	SGE	Sources	Network	End-Users
[148]	DiGriPy	Phyton		X	X	X	X	OK	X
[149]	DHNx	Phyton		X	X	X	X	OK	X
[150]		Modelica	Dymola	X	OK	X	OK	OK	~
[141]		Modelica		X	OK	X	~	OK	OK
[151]		Matlab	Simulink	X	OK	~	X	OK	X
[109]	TEGSim	Matlab		X	OK	OK	~	OK	X
[152]		Octave		X	OK	~	OK	OK	X
[153]	SIM-VICUS	C++		X	OK	X	X	OK	OK
[154]	FLEXYNETS tool	-	Excel	~	OK	~	~	OK	~
[155]	Fluidit HeatTM	Phyton		OK	?	X	OK	OK	X
[156]	THERMOS tool	Clojure		OK	X	X	X	OK	OK
[157]	nPro	Phyton		OK	OK	X	OK	OK	OK

, a list of representative software tools for DHC design is presented. It is observed that most of the tools are valid for 5GDHC systems, but they mostly lack a Graphical User Interface (GUI), and very few include a single package to design and model GHEs.

4. Opportunities for SGE and 5GDHC Networks in Mediterranean Europe

Within the continental EU-27 territory, the Mediterranean region is the most vulnerable to the impacts related to climate change [1,158]. Most of the effects will be related to a decrease in precipitation and an increase in temperature, especially during summer, with more frequent and prolonged heat waves and droughts. With almost 71% of European population [159], urban areas will be facing the largest share of the most critical challenges related to climate change mitigation, and they have been called to become cleaner, more energy self-sufficient, and more climate resilient [160]. Particularly in densely populated areas (39.3% of Europeans live in cities [159]), heat waves will cause the most severe effects on human health due to the more intense urban metabolism, which results in higher concentration of air pollutants and higher ambient and ground temperatures (heat island effect) [161]. In this sense, Mediterranean countries will be increasingly more concerned about addressing their cooling demand, which is currently met mainly by air-to-air HPs (air-conditioning systems). Despite the threats, new opportunities arise. Mediterranean countries show lower thermal energy demand than other regions in general, with a more balanced H&C. This scenario is particularly favorable for an enhanced performance of HPs [162]. Besides, Southern Europe receives more peak sun hours compared to the North, which is an obvious stimulus for solar energy exploitation and its hybridization with SGE systems. These issues will be analyzed in more detail in the following paragraphs.

4.1. Heating and Cooling Demands and Future Trends

Concerning the H&C demand, a first approximation of the present and future scenarios can be outlined by looking strictly at the heating-degree-day (HDD) and cooling-degree-day (CDD) indicators. Mediterranean countries show values in the range of 1000–2500 HDD and >165 CDD, while Scandinavian and Baltic countries are mostly in the range of 4000–5500 HDD and <15 CDD (NUTS 3 scale). On one extreme, Sweden and Finland show 5325 HDD/0 CDD and 5664 HDD/1 CDD, respectively. On the other extreme, Malta and Cyprus show 534 HDD/574 CDD and 780 HDD/577 CDD, respectively (values averaged through the period 1979–2021) [163] (Figure 6). HDD and CDD cannot be considered as actual indicators of the total share of H&C demands, since other important end uses come into play, such as DHW production or food conservation, which are less weather and climate dependent. However, it is possible to obtain an idea of how climate change will influence H&C demands in households (see correlated values of HDD/CDD with actual space heating/cooling demands in Figure 7).

Future prospects for HDD and CDD values are calculated in relation to the RCP4.5 and RCP8.5 scenarios, which are defined by the IPCC in its 5thassessment report [124]. RCP4.5 corresponds to an intermediate scenario (still optimistic) with a radiative forcing increase of 4.5 W/m² by 2100, while RCP8.5 corresponds to a (pessimistic) scenario showing a radiative forcing increase of 8.5 W/m² by 2100. According to the European Environment Agency [1], most of the Mediterranean region would experience a reduction of 60–120 HDD and an increase of 60–90 CDD by 2050 with respect to the 2020 levels under a RCP4.5 scenario. Under a RCP8.5 scenario, a decrease of 120–180 HDD and an increase of >120 CDD could be expected. Translated into H&C demands, the same work reports an expected overall increase of total H&C demand in Mediterranean countries. This will be caused by an increase in cooling demand that will not be compensated for the decrease in heating demand. In the rest of Europe, it is expected an overall decrease in H&C demands, which is associated with lower values of HDD, but with no actual influence from an increase in CDD. This draws a significant contrast between the North and the South of Europe regarding the impacts that climate change will impose on energy consumption.

4.2. Why SGE and 5GDHC Are Especially Attractive in Mediterranean Europe?

In connection with the above, there are two main basic technical factors why SGE should be more present in Mediterranean countries compared to the rest of Europe. The first is a lower demand (as inferred from Figure 6), and the second is more balanced H&C (heating demand still predominates, although climate change will cause a reduction in the gap between H&C demands in the forthcoming years). These two factors contribute to smaller GHEs; hence, the implementation of SGE becomes technically and economically more feasible. In addition to this, there is no significant difference in average ground properties across the European territory that could justify the uneven deployment of SGE between the North and the South [165].

Notice that, when it comes to H&C balance in SGE systems, the focus must be put on the heat extracted from and injected into the ground, which depends on the seasonal performance factor (SPF) of the GSHP:

$${\mathrm{E}}_{\mathrm{ground}}^{\mathrm{out}}={\mathrm{E}}_{\mathrm{building}}^{\mathrm{in}}·\left({\mathrm{SPF}}_{\mathrm{h}}-1\right)$$

(2)

$${\mathrm{E}}_{\mathrm{ground}}^{\mathrm{in}}={\mathrm{E}}_{\mathrm{building}}^{\mathrm{out}}·\left({\mathrm{SPF}}_{\mathrm{c}}+1\right)$$

(3)

Therefore, for a real balance to take place with the ground (${\mathrm{E}}_{\mathrm{ground}}^{\mathrm{out}}={\mathrm{E}}_{\mathrm{ground}}^{\mathrm{in}}$), a proportion between heating (${\mathrm{E}}_{\mathrm{building}}^{\mathrm{in}}$) and cooling (${\mathrm{E}}_{\mathrm{building}}^{\mathrm{out}}$) demands should comply with the following expression:

$${\mathrm{E}}_{\mathrm{building}}^{\mathrm{in}}·\left({\mathrm{SPF}}_{\mathrm{h}}-1\right)={\mathrm{E}}_{\mathrm{building}}^{\mathrm{out}}·\left({\mathrm{SPF}}_{\mathrm{c}}+1\right)$$

(4)

For instance, if a GSHP shows a SPF of four for both H&C operation modes (SPF_h = SPF_c = 4), this means that, during winter, for every four units of heat delivered to the building, three comes from the ground and one comes from the GSHP itself. During summer, for every four units of heat removed from the building, five units are injected into the ground. Under this scenario, the cooling demand should be 60% of the heating demand to guarantee a real heat balance in the ground.

Table 2. Ratio of cooling demand with respect to heating demand (${\mathrm{E}}_{\mathrm{building}}^{\mathrm{out}}/{\mathrm{E}}_{\mathrm{building}}^{\mathrm{in}}$) to guarantee a balanced heat exchange with the ground (${\mathrm{E}}_{\mathrm{ground}}^{\mathrm{out}}={\mathrm{E}}_{\mathrm{ground}}^{\mathrm{in}}$), depending on different values of SPF_c and SPF_h. Cells corresponding to more pronounced heating-dominated demands show a more intense red color.

$\frac{E_{building}^{out}}{E_{building}^{in}} \left[\mathit{\%}\right]$		$SP{F}_c$
		3	3.5	4	4.5	5	5.5	6
${\mathrm{SPF}}_{\mathrm{h}}$	3	50.0	44.4	40.0
	3.5	62.5	55.6	50.0	45.5
	4	75.0	66.7	60.0	54.5	50.0
	4.5		77.8	70.0	63.6	58.3	53.8
	5			80.0	72.7	66.7	61.5	57.1
	5.5				81.8	75.0	69.2	64.3
	6					83.3	76.9	71.4

shows a wider comparison of H&C scenarios depending on SPF_h and SPF_c values. Heating-dominated demands are usually favorable for a balanced heat exchange with the ground, but a cooling demand in the range of 40% to 70% of heating demand is required to that end. Although these scenarios can take place at specific locations across Europe, they are only possible in a generalized way in the southernmost regions of Mediterranean Europe (e.g., Malta or Cyprus in Figure 7).

Another important aspect of climate influence is the SPF values obtained by GSHPs, which depend on the location. In [162], a simplistic model was built assuming same building and subsoil characteristics across Europe, in order to assess strictly the influence of climate on the performance of HPs and comparing air-source and ground-source types (where the BHE is assumed as the GHE). As a result, SPF_h in Mediterranean region would be 3–3.4 for ASHPs and 4–4.6 for GSHPs. Comparing between the South and the North, it should be expected a reduction of 1–1.5 points in SPF_h in Scandinavian and Baltic countries with respect to Mediterranean Europe, either for ASHPs and GSHPs.

From an economic point of view, in [15], it is concluded that GSHP is the most cost-optimal technology for H&C in Mediterranean Europe under RCP4.5 scenario. Under a RCP8.5 scenario, this conclusion is even more robust for the Mediterranean region, Ireland, and the UK, but also to most of central Europe. Nevertheless, there is an open controversy about ASHPs and GSHPs as the most cost-optimal solution. It is generally assumed that GSHPs can only compete with ASHPs when they are sufficiently subsidized [166]. Despite the fact that ASHPs and GSHPs should be seen as complementary rather than competing technologies, ASHPs show several limitations at low ambient temperatures and high sink temperatures that can only be effectively overcome by GSHPs [167]. Besides, the recent changes in gas and electricity markets require a deep redefinition of the long-term economic and environmental analysis usually attained when ASHP and GSHP are compared.

Apart from the improved techno-economic feasibility of SGE installations, the benefits of having well-balanced H&C demands extend also to the long-term ground dynamics, and eventually to the thermal interferences with adjacent installations. Very heating-dominated demands cause a long-term decrease in the undisturbed ground temperature $T_{ground}^0$, while cooling-dominated demands cause the opposite effect. When sizing an installation, this issue is usually accounted for, resulting in larger GHEs, and consequently higher CAPEX. 5GDHC networks offer the possibility to establish a balanced heat exchange with the ground by connecting heating-dominated and cooling-dominated demands from several buildings, contributing to reducing even more the required size of the GHEs when they are smartly shared [123]. Therefore, 5GDHC networks represent optimal platforms to deploy SGE installations in areas with limited land-use and coexistence of diverse demand profiles, such as urban areas.

Finally, another important aspect of the Mediterranean area is its capacity for electricity generation with solar PV. Solar PV energy is roughly 50% more productive in Spain than in Germany, and 70% more than in UK (in terms of kWh/kW_p) [168]. However, Germany still leads in Europe in installed capacity in terms of global amount, and The Netherlands leads in terms of per capita, which is currently more than twice of other countries such as Spain or Italy. Hybridization with solar PV panels enhances the renewable fraction of GSHPs, which can be easily higher than 75%, depending on the efficiency of the GSHP itself (SPF) and the average efficiency of the power grid (η_e = 46.7% according to the EU directive 2018/2002 [29]). Assuming a SPF of four (one unit of electricity to provide or remove four units of heat), just 50% of self-consumption by PV panels can increase the renewable fraction of the GSHP from 87% to 93%. Several authors also emphasize the benefits of hybridizing GSHPs with PV panels from the operational, economic, and environmental perspectives [71,72,73,169,170,171].

4.3. Contextual Issues Concerning the Heating and Cooling Sector in Mediterranean Europe

The eventual deployment of any RTT in the European territory, in this case SGE, must be framed within the objectives set by current European policies. Progress in decarbonization of the H&C sector is provided by country in [12], according to their respective binding targets as defined by the RED (maximum 1.3% yearly increase of renewables from 2020 to 2030) [133]. From Mediterranean Europe, only Spain and Greece are apparently on track to meet their targets based on their trends in the last years, while Italy would be partially achieving it, and Portugal not yet. However, Italy is the country with the highest share of H&C in final energy consumption, showing also the most prominent share of HPs (mostly ASHPs). Interestingly, Greece is the Mediterranean country with the highest share of GSHPs per capita (~0.7 GHSPs per 1000 inhabitants.) [13].

DHC networks have a residual contribution to H&C in Mediterranean Europe. Italy has a significant 4% of its H&C demand covered by DHC networks [18] if compared to its Mediterranean neighbors. Although its systems are mostly run on fossil fuels, Italy is showing an active research on modern 5GDHC networks [16,24,87,104,151,152,172], with already four operative networks in the North, being “Complesso della Torre” district (Savona) the oldest one, which is operative since 2007 [16]. Greece and Spain show about 1% share of DHC in their respective H&C sectors, while Portugal shows a 2%.

The current target set by the EU’s RED is a yearly increase of 2.1% in the share of renewables for the DHC sector [127], although there is not yet a specific target of DHC deployment.

The Spanish DHC networks deserve special attention for their high share of renewables. The catalogue of DHC networks regularly updated by the Spanish non-profit company association ADHAC (Asociación de Empresas de Redes de Calor y Frío) accounts for 497 operating district networks [173]. From these, 366 run exclusively on biomass, 83 run exclusively on natural gas, and 11 run on biomass and natural gas. More than 90% are just 3rd-generation DHs. Interestingly, nine DHC networks run partially or totally on geothermal energy, although these cases are testimonial. Unfortunately, this trend is not limited to Spain. Biomass, biofuels, and renewable waste represent by far the largest share of renewable energy feeding DHC networks all across Europe [17].

As far as barriers for DHC deployment is concerned, technical barriers deal with the existing building stock and their heat distribution systems, which imposes a clear limitation on the minimum supply temperature for certain old, high-temperature radiators [18]. Financial barriers are due to high investment cost, especially from the side of the commissioners and operators. However, notice that 5GDHC may allow a more varied range of business models, as mentioned in Section 3.2, where the investment cost could be shared by network and household owners. Social and knowledge barriers have to do with the tradition of DHC being present in certain territories compared to individual solutions and the multivariable perception of the customer. The latter comprises complexity of DHC solutions, operation costs, connection cost, and non-monetary benefits, such as CO₂ emission savings, primary energy savings, or climate change mitigation (if CO₂ taxes do not apply) and comfort.

At the European level, many research projects and support activities have been funded by the European Union so far in the fields of H&C and DHC networks [174], and many more are ongoing [175,176]. Focusing mostly on the Mediterranean region, a list of relevant research projects is presented:

• Ground-Med (Advanced ground source heat pump systems for H&C in Mediterranean climate) EU project [177] demonstrated that a yearly SPF over five was feasible in Mediterranean climates. In this case, SPF would comprise global heat and cold production, taking into account the electrical power consumption of external and internal circulation pumps, i.e., SPF₃.
• LEGEND (Low Enthalpy Geothermal Energy Demonstration cases for Energy Efficient Buildings in Adriatic area) EU project [24] was devoted to promoting the deployment of SGE-based solutions, contributing to enhancing building energy efficiency in the Adriatic sea area.
• FLEXYNETS (Fifth generation, Low temperature, high ExergY district heating and cooling NETworkS) EU project [27], mostly participated by Italy and Spain, is the first European project where decentralized HP networks for H&C were introduced as the future of DHC networks. A Microsoft Excel -based tool was generated for simplified design and sizing of such networks.
• RELaTED (REnewable Low TEmperature District) EU project [178] pursues a demonstration of the feasibility of using ultra-low-temperature (<45 °C) DHC networks for building complexes at a district scale in four European locations with “complementary operation environments”: Denmark, Estonia, Serbia, and Spain. The specific pilot project in Spain consisted of substituting an existing network powered by gas boilers (accounting for 650 kW_th) with new RTTs based on renewable and waste heat sources [179].

5. Discussion

Despite the ambitious and brave positioning of the EU against new geopolitical challenges, the rush to become energetically independent from Russia, while accelerating and intensifying the energy transition, raises serious questions about the security of supply. In the very short term, Europe is being inexorably doomed towards contingency plans on the side of demand. A fast response from the energy generation perspective necessarily implies a focus on consolidated, reliable, and already available renewable technologies.

The H&C sector represents the largest front in decarbonizing Europe by 2050. To this end, HPs are recognized by the EU as the most relevant RTTs, and modern (4th- and 5th-generation) DHC networks as key enabling infrastructures. Together with TES, HPs and modern DHC networks can contribute decisively to a full electrification of final energy consumption. Thanks to these RTTs, a large portion of final energy use corresponding to H&C would be covered in a truly local manner. As a result, this would contribute to a less constrained, more flexible, and more reliable supply of electricity by renewable (but intermittent) energy sources. Particularly in densely populated areas, any chance of achieving energy self-sufficiency would be possible only if massive retrofitting of buildings is attained as well, focusing on energy efficiency measures, optimal urban planning strategies, and smart operation of the networks. It must be noted that population growth rate in Europe is close to stagnation and tending to negative values around 2030 [180] (unlike in the case of global prospects), but it is expected that big cities will still experience a population increase [181]. In contrast, alternative views foresee a collapse of the urban model which would favor a population redistribution towards rural areas [182]. Indeed, the recent experiences of lockdowns in cities around the world during the COVID19 pandemic, as well as the flourishment of the digitalized era, have fostered people to rethink of their settlement patterns [183,184,185]. From a residential perspective, it would be even more favorable to achieve energy self-sufficiency in anthropized areas through SGE-based solutions if these areas are more sparsely populated (higher portion of available land use per inhabitant).

A special emphasis is placed on the Mediterranean countries, given their lower H&C demands with respect to other regions and their favorable climate conditions for an efficient operation of GSHPs. Although the SGE market is incipient, the industry is generally mature and backed by recent incentives. DHC networks are even more poorly deployed, and these two facts (low deployment of SGE and DHC networks) precisely represent a great opportunity for decarbonizing the H&C sector in these countries.

The lack of a specific target for DHC deployment poses a clear barrier from a policy perspective. The example of Spain is illustrative of a DHC model based on biofuels to achieve renewable share targets, but without a clear commitment to innovation beyond traditional boilers. With regard to DH systems, biomass is the easiest way to “change everything for everything to remain the same”, and, paradoxically, burning wood is being perceived nowadays as a modern technology compared to fossil fuels. Moreover, most of the Spanish DH and DHC networks are in small towns or are serving a few tertiary buildings, such as libraries, schools, city halls, hotels, and sport centers. By no means is this a scalable model for entire towns, and even less so for cities. Indeed, massive burning of wood as an alternative to fossil fuels for heat and even power generation is highly controversial, as stated in an open letter signed by more than 500 scientists in 2021 and addressed to the presidents of the US, the EU, Japan, and South Korea [186].

Unlike fossil fuels, SGE deployment is far from being affected by the physical limits of extraction. Unlike biofuels, SGE is a truly local and low-emission resource. Unlike solar thermal energy, SGE is time and weather independent, and, unlike H₂, SGE is readily available and scalable, exergy efficient, and economically accessible. Finally (and paradoxically), unlike electricity, thermal energy has the key to fully electrify the end uses of energy and to meet the target of a 100% renewable and carbon-neutral energy system. This key is the HP.

6. Conclusions

Among the available options for decarbonizing the European H&C sector, SGE and 5GDHC are presented as the most exergy-efficient combination to decarbonize residential and tertiary buildings, allowing an optimal match between low-grade demands and low-grade energy sources in the most versatile range of configurations.

The synergistic effect of combining SGE and 5GDHC networks is corroborated through most of the existing 5GDHC cases, but also through the flourishing market of homeowner communities sharing a set of GHEs (mainly in the form of vertical BHE fields). A massive adoption of this model implies an effective coupling between thermal energy and electricity, which is not only unavoidable but desirable.

The question about why SGE deployment is still so low in most of Europe has been analyzed. Two main non-technical barriers can be identified. One is related mostly to the high CAPEX of GSHPs, which is dragged down by the cost of GHEs. The second relates to the lack of awareness that permeates through public institutions, academic bodies, investors, and the general public. This review paper has identified three stimulus paths to overcome such barriers. Firstly, economic incentives are the simplest but effective paths to address the CAPEX barrier. Secondly, considering the synergistic alliance of SGE and 5GDHC networks, ILEC emerges as a powerful concept to empower and engage the population in energy transition, accessing to all possible audience regardless of knowledge or income levels and addressing both electricity and thermal energy. Finally, knowledge-related stimuli are especially necessary to make the potential benefits of 5GDHC networks more widely known. The design, optimization, operation, and even the business models of these networks require new approaches to address higher levels of complexity compared to previous DHC schemes.

Mediterranean Europe shows a striking contrast between the low deployment of SGE systems and its favorable conditions to achieve the opposite, which applies also to DHC networks. The course of the current decade will provide clearer proofs of SGE’s real potential and how 5GDHC networks can contribute to it. This is not in vain as this is supposed to be the “Geothermal decade” [187].