Multi-Energy System Demonstration Pilots on Geographical Islands: An Overview across Europe

Abstract

Smart integration of different energy sectors on islands could serve as a key to achieve their energy independence and carbon neutrality goals and facilitate the large-scale penetration of renewables. Demonstration projects play a vital role in testing innovative sector-coupling solutions and exploring their synergies in the real-life environment. They also help to build the required market confidence and awareness, verify and enhance the societal acceptance, and identify and prevent unnecessary risks and complications. The aim of this paper is to provide the reader with a comprehensive review of 19 research projects, which concern the demonstration of sector-coupling technologies on European islands. The projects are funded by the European Commission and started between 2010 and 2020. The review focuses on the exploration of various technical aspects of 28 demo pilots. In addition, it addresses objectives, contributions, and novelty of the demonstrations and analyses barriers, challenges, risks, and success factors reported by project stakeholders. The paper intends to support the decision-makers on the identification, adoption, and replication of successful technologies. Moreover, it provides researchers and other interested parties with lessons learnt to ensure the successful project execution and technology demonstration in future projects.

1. Introduction

More than 2000 European islands, populated by over 10 million people, are highly dependent on fossil fuels and energy import and experience power supply stability and reliability problems [1,2]. This often makes it challenging to follow ambitious national, European, and global decarbonisation goals, set for 2030 and 2050 [3,4,5]. Although there have been numerous initiatives of different scale, concerning decarbonisation of European islands through renewable energy sources (RES), the potential of the RES penetration into the power system is limited by its stability and energy supply reliability requirements and physical grid constraints. This is exacerbated in cases with a lack of a robust connection of the island’s power system to the external electrical grid. Here, smart integration of the power sector with other energy sectors could serve as a key for unleashing the decarbonisation potential on islands. Indeed, electrification of energy-demanding sectors opens additional sources of flexibility to the power system, allowing to increase the share of renewable generation in the energy balance. At the same time, sector-coupling allows partially or completely replacing fossil fuels with green fuels and electricity. Finally, sector integration leads to the increase in the overall efficiency of the energy system [6,7].

In some countries, for example, in Denmark, the synergies between power sector and heating, cooling, and transportation sectors have been already achieved to some extent, and benefits of these synergies for the power system balance have been proven by different research studies and real-life applications [8,9,10]. However, to reach future sustainable goals, a more complex and close interweaving of different energy sectors is required [11]. This implies, besides replication and scaling up existing solutions, their improvement, as well as development of new sector-linking solutions. Withal, it should be taken into account that, besides bringing advantages, some of the sector-coupling technologies can also pose additional challenges on the grid. For example, the large expansion of the rapidly advancing power-to-hydrogen (P2H2) technologies, allowing to store the excessive renewable production and release it when needed, by this acting as a highly potential power system flexibility provider, will significantly increase the annual power demand and peak loads [6,12]. As another example, the large-scale integration of the electric vehicle (EV) charging infrastructure, in particular, high-power charging stations, into the power system will additionally increase both the overall energy consumption and the peak load demand [13,14]. To prevent these challenges and to reach an overall efficient and robust energy system, a coordinated planning and operation of the entire multi-energy system, comprising, besides energy sectors, the related infrastructure and the consumption sector, is needed. This concept, called the Energy System Integration, formed the basis of the Energy System Integration Strategy [15] released by the European Commission in 2020.

Although the amount of research, dedicated to the multi-carrier energy systems of different scale and aggregation approaches [16], has significantly increased in recent years, there are still many interdisciplinary challenges that have to be addressed. One of them is related to the co-simulation of several energy sectors with different dynamics, while other concerns include:

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To what degree the sector-coupling can be practically achieved,

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How to realize the maximum possible flexibility potential in the most economically feasible way,

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What should be changed in the existing techno-economic, policy, and regulatory barriers to unlock the multi-sector integration.

Research and development projects concerning sector-coupling and energy system integration play an important role in fostering the development and maturity of the required technological solutions. The projects implying demonstration aspect are of a particular value, since they allow to test and validate innovative concepts in real-life conditions, especially when it is rather difficult or impossible to proof their livability without a pilot. In addition, demonstration projects help to build the required market confidence and awareness, verify and enhance the societal acceptance, and identify possible risks, barriers, and challenges of different nature that could potentially be avoided in other projects. At the same time, in the majority of cases results of the technology demonstration activities within research and development projects remain in project reports and do not reach the wide audience. Moreover, synthesis and analysis of specific types of information from these dispersed reports from different projects seems very difficult to conduct. A literature analysis allowed us to identify that, while there are several comprehensive reviews of renewable energy utilization experience on islands worldwide [17,18,19] and research and innovation projects on geographical and energy islands in EU [2], there is no review focusing on the technology demonstration activities related to the multi-energy systems on European islands.

The objective of this paper is to provide a comprehensive review of research and development projects with demonstration activities in sector-coupling and energy system integration on geographical islands across Europe. The analysis took into consideration projects that were funded by the European Commission and started between 2010 and 2020. The analysis is carried out following a systematic review methodology. We highlight the core technological characteristics of the demo pilots and comprising technologies, as well as applied aggregation and control approaches. We also analyse objectives, focus areas, novelty, contributions, and services provided by the demonstrated solutions. In addition, we discuss risks, challenges, and success factors reported by stakeholders of several projects. The paper intends to support the decision-making of local islands administrations, power system operators and other industrial players, and researchers, in particular, on the replication and adoption of specific technologies, as well as lessons learnt to ensure the successful project execution and pilot demonstration.

The rest of the paper is organized as follows: Section 2 describes the methodology of the systematic review applied in the analysis. Section 3 is dedicated to the general overview of the projects and the relevant use cases, as well as islands, hosting the demonstrations. Section 4 is devoted to the qualitative and quantitative analysis of the demonstrations covering different perspectives, mentioned above. Section 5 concludes the manuscript underlying key findings of the analysis.

2. Methodology

This section describes the methodology of the systematic review, followed in the analysis. The methodological approach was adapted from [20] according to the needs of the current investigation. Figure 1 depicts its main steps and factors considered in each step. The analysis consisted of three preparatory steps (Search, Compliance check, and Screening of full project descriptions) and a final project analysis step (Synthesis and analysis). The main and only information source for the initial selection of projects was the Community Research and Development Information Service (CORDIS) database [21], the European Commission’s primary public repository and portal to disseminate information on all EU-funded research projects and their results. Herewith, only sector-coupling related projects that took place on European islands and started between the beginning of 2010 and end of 2020 were taken into consideration.

2.1. Search

The aim of the first screening was to identify a set of projects that relate to energy (in the first approximation) or multi-energy system demonstration on islands.The formation of the initial project pool was carried out using the CORDIS database’s Search and Filter functionality. The following search filters were used:

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Collection: Projects; Results Packs; Results in brief; Report summaries; Project deliverables; Project publications; Exploitable results.

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Domain of application: Industrial technologies; Fundamental research; Transport and mobility; Climate change and environment; Energy;

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Start date (from): 1 January 2010; Start date (to): 31 December 2020.

Several search iterations with the following search strings were further carried out:

1.

‘multi’ AND ‘energy’ AND ‘system’ (427 results);

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‘integrated’ AND ‘energy’ AND ‘system’ (1210 results);

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‘energy’ AND ‘system’ AND ‘demonstration’ (38 results);

4.

‘energy’ AND ‘system’ AND ‘islands’ (49 results);

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‘island’ AND ‘decarbonisation’ (12 results);

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‘island’ AND ‘green’ AND ‘transition’ (15 results).

In each search iteration, the title and the summary of each project shown in the search results was fast-screened. The above mentioned information for each project, that met the search criteria, accompanied by a project abbreviation, a website, and a link to a corresponding project web page in the CORDIS database were gathered into a spreadsheet. The resulting table (Step 1 Tab [22]) contained 129 projects.

2.2. Compliance Check

At this stage, the projects selected in Section 2.1 were screened towards the fulfillment of the following criteria:

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Geographical island involvement;

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Presence of the multi-energy aspect;

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Presence of the demonstration aspect.

For that, summaries of each project, as well as general information available on project web pages and web sites were screened, and the compliance of each project with the listed criteria was marked. As a result, the initial list of projects was reduced to 36 projects approved for further analysis (Step 2 tab [22]).

2.3. Screening of Full Project Descriptions

At this step, the fulfillment of each of the 36 projects for the tree criteria set in the previous step was double-checked. On top of that, a funding program, project start and end dates, and availability of dissemination materials were collected and indicated for each project. The analysis allowed to screen out 17 projects and finally form a resulting list of 19 projects (Step 3 Tab [22]) for the detailed review.

2.4. Synthesis and Analysis

The following projects were finally selected for the review: TWENTIES (2010–2013), EcoGrid EU (2011–2015), NETfficient (2015–2018), TILOS (2015–2019), BIG HIT (2016–2022), inteGRIDy (2017–2020), SMILE (2017–2021), REMOTE (2018–2021), HySeas III (2018–2021), REACT (2019–2022), GIFT (2019–2022), INSULAE (2019–2023), FLEXIGRID (2019–2023), FEVER (2020–2023), IANOS (2020–2024), MAESHA (2020–2024), ROBINSON (2020–2024), VPP4Islands (2020–2024), ISLANDER (2020–2024).

At this stage, a number of qualitative and quantitative indicators for the selected projects were set and addressed, such as:

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General information: Demo island(s); Global target and focus.

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Demonstration-related: Short demo description; Hardware and software demonstrated; Demo size; Sectors coupled; Aggregation concept.

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Parties involved: Coordinator country; Community involvement; Grid operator involvement.

The corresponding information for each of the projects has been collected and could be observed in [22] (Step 4 Tab). This spreadsheet formed the basis for the subsequent analysis, results of which are presented further in the paper.

3. Project Overview

This section summarises the selected projects with a specific focus on the demonstration-related use cases. For usability and visibility purposes, projects consisting of several use cases (UCs) were broken down into the use cases. This resulted in 28 use cases considered in the subsequent analysis. Each UC had a specific number.

Table 1. Characterization of demonstration use cases under investigation. Scale: medium–from 100 kW to 1 MW, large–over 1 MW.

No	Project/Use Case	Island(s)	Scale	Aggregation Concept	Linked Energy Sectors
					Power	Heating	Cooling	Gas *	Transp.
1	TWENTIES	Faroe isl, DK	Large	VPP	x	x
2	EcoGrid EU	Bornholm, DK	Medium	VPP	x	x
3	NETfficient(1)	Borkum, DE	Medium	VPP	x	x	x	x
4	NETfficient(2)	Borkum, DE	Medium	VPP	x	x	x	x
5	TILOS	Tilos, GR	Large	microgrid	x	x
6	inteGRIDy(1)	Isle of Wight, UK	Medium	VPP	x	x
7	inteGRIDy(2)	Isle of Wight, UK	Medium	microgrid	x				x
8	BIG HIT	Orkney isl, UK	Large	Energy hub	x	x		x	x
9	SMILE(1)	Orkney isl, UK	Medium	microgrid	x	x		x	x
10	SMILE(2)	Madeira, PT	Medium	microgrid	x				x
11	SMILE(3)	Samsø, DK	Medium	VPP	x	x			x
12	REMOTE(1)	Stromboli, IT	Medium	microgrid	x			x
13	REMOTE(2)	Froan, NO	Medium	microgrid	x			x
14	HySeas III	Orkney isl, UK	Large	Energy hub	x			x	x
15	REACT(1)	San Pietro, IT	Medium	microgrid	x	x
16	REACT(2)	Aran isl, IE	Medium	microgrid	x	x
17	GIFT(1)	Hinnøya, NO	Medium	VPP	x				x
18	GIFT(2)	Procida, IT	Medium	microgrid	x	x
19	INSULAE(1)	Bornholm, DK	Medium	microgrid	x				x
20	INSULAE(2)	Bornholm, DK	Large	VPP	x	x		x	x
21	INSULAE(3)	Madeira, PT	Medium	microgrid	x				x
22	FLEXIGRID	Thassos, GR	Medium	microgrid	x				x
23	FEVER	Cyprus	Medium	microgrid	x	x	x
24	IANOS	Ameland, NL, Terceira, PT	Medium	VPP	x	x	x	x
25	MAESHA	Mayotte, FR	Medium	VPP	x				x
26	ROBINSON	Eigerøy, NO	Medium	Energy hub	x	x		x
27	VPP4Islands	Formentera, ES	Medium	VPP	x	x		x
28	ISLANDER	Borkum, DE	Large	Energy hub	x	x	x	x	x

* including hydrogen.

provides an overview of the project UCs in terms of location, demonstration scale, aggregation concept, as well as energy vectors coupled, whereas Figure 2 depicts their allocation on the geographical map. Key characteristics of each EU island hosting the project pilots are gathered in Table 2. The rest of the section introduces each project and/or project use case, focusing on technological details, as well as results and experiences of the pilots, if available.

3.1. TWENTIES (2010–2013)

The aim of the project was to remove barriers for wind technologies integration into European energy systems. One of the six project pilots, which took place on Faroe Islands (Denmark), demonstrated the first Virtual Power Plant (VPP) in the world capable of providing Fast Frequency Response (FFR) [23,24]. The so-called Power Hub aggregated wind turbines (WT), a battery energy storage system (BESS), 100 distributed energy resources (DER) from heat and power sectors, and 47 industrial controllable loads (ICL). The aggregation of the VPP components was implemented through an innovative scalable IT platform. The VPP was intended to perform self-balancing of the the local energy system. In addition to the FFR, the VPP was able to provide the ultra-fast balance restoration and blackout prevention in the islanded energy system. The latter was mainly achieved by automatic decoupling of large-scale ICL from the power system [25]. In addition, the VPP provided reserve, dynamic reactive power control, and load shifting services. The successful pilot exploitation over the years has proved the availability of flexibility in DER. It has also demonstrated feasibility of mobilising flexibility of industrial controllable loads on commercial terms with involvement of third party aggregators. Since the beginning of the operation, the VPP has grown with several more wind farms and industrial controllable loads, and is currently an integral part of the Faroe’s energy system. Thanks to the project, the share of wind generation on Faroe islands increased from 5% in 2012 to 25% in 2014.

3.2. EcoGrid EU (2011–2015)

The EcoGrid EU project, which took place on the Danish island of Bornholm, has demonstrated an innovative market concept, enabling owners of smart household appliances to participate in the provision of the close-to-real-time frequency regulation. That was possible through the indirect (price-based) control of residential and industrial heat pumps (HP) and electrical water heaters (EWH) based on a 5 min price signal [26,27].

The four-month operation of the new market, which involved around 1900 households and 18 industrial customers, proved benefits of utilising the five-minute price signal to activate flexibility in RCL and ICL. The automatic price-based control allowed to reduce the peak load by 1.2% while increasing the RES share by 8.6% due to less wind generation curtailment and a lower cost for the activation of spinning reserves. The project contributed to a closer examination of household appliances as full-fledged participants of the frequency regulation market, especially when coupled with distributed generation, which became a focus point in a follow-up project EcoGrid 2.0 [28,29].

3.3. NETfficient (2015–2018)

The NETfficient project, which took place on the German island of Borkum, focused on the increase in energy independence of the island through energy storages and other distributed energy resources, as well as their market integration [30]. The sector-coupling aspect appeared in two project use cases. Both use cases focused on demonstrating energy management approaches for the optimal balancing of renewable-based production and consumption on the demand side, applied to different prosumer types. Among them, 40 residential households (use case NETfficient(1)) and a group of larger non-residential buildings (use case NETfficient(2)) [31]. Both types of buildings were equipped with solar PVs, as well as one or several of the following energy storage technologies: lithium-ion batteries, hybrid energy storage systems (HESS) consisting of lithium-ion batteries and capacitors, second-life EV batteries (2LEV), thermal energy storages (TES), and fuel cells (FC) combined with pressurised (H2SS) or metal hydride (MeH2SS) hydrogen storages. The use cases developed and demonstrated an Energy Management Platform [32] and an Energy Forecasting System (EFS) [33,34,35], as well as several other services, enabling market participation of DER and distributed storages through the VPP aggregation concept. This made possible, for example, the provision of FFR by the aggregated batteries [36].

3.4. TILOS (2015–2019)

The TILOS project aimed at maximizing the use of RES on the Greek island of Tilos. The project demonstrated the first-ever battery-based wind-PV hybrid power plant (HPP) of MW scale in Greece, coordinated using the microgrid approach [37,38]. The core of the large-scale pilot was formed by a 800 kW/2.88 MWh sodium-nickel-chloride battery (NaNiCl2), a 800 kW wind turbine, and a 160 kW PV plant [39,40,41]. In addition, the microgrid also involved distributed thermal energy storages (domestic electric water heaters (EWH)), residential controllable loads (air conditioners and fridges), and community-level controllable loads (water pumps). The centralised remote control of the thermal energy storages allows us to take advantage of the surplus of the RES production, while the controllable loads participated in the grid frequency and voltage regulation through the demand-side management (DSM) mechanism. The microgrid was tested in both stand-alone and grid-connected modes. Its coordinated operation was performed using an Energy Management and Control System (EMCS) with an advanced renewable generation and demand forecasting system (EFS) [42]. The project demonstrated that the grid-scale BESS was able to efficiently balance the microgrid, while providing ancillary services to the upper-level grid through the existing sea cable connection [39,43].

3.5. inteGRIDy (2017–2020)

The project aimed at increasing stability and flexibility of isolated grids by mutual coordination of power, heating, and transportation sectors, primarily through the DR and energy storages [44,45]. Two project use cases took place in the Isle of Wight (UK). The first use case (inteGRIDy(1)) concerned the implication of aggregated household appliances (heat pumps and electric water heaters), as well as residential solar PVs and battery storages for the flexibility provision to the grid, while minimising energy costs for the consumers. The second use case (inteGRIDy(2)) was dedicated to the development and testing of a microgrid solution, integrating a 50 kW fast DC EV charger, a 22 kW three-phase AC EV charger, a 80 kWh lithium nickel manganese cobalt oxide (NMC) BESS, a 10 kW solar PV plant, and a 50 kW battery inverter, and operated using a newly built IT platform for the energy management and control. The microgrid solution was intended to provide, besides EV charging and energy storage, grid services, such as active power control and peak shaving [46]. The successful demonstration of the microgrid solution showed its capability to provide EV charging and participate in the provision of grid services through aggregators.

3.6. BIG HIT (2016–2022)

The six-year project addresses the challenge of the efficient management and reliability of islanded grids with RES through building a fully integrated model of hydrogen production, storage, transportation, and utilisation for heating, power, and mobility sectors. In addition, one of the key aims of the project it to employ a novel hydrogen trading framework. The Orkney Islands, in the north of Scotland, are hosting the demo [47,48]. The demonstration pilot implies a 1.5 MW polymer electrolite membrane (PEM) electrolyser integrated into the RES-dominated energy system of the islands. The electrolyser is intended for balancing the grid and preventing wind power curtailment. It utilises the energy produced by wind turbines and tidal power plants (TPP) on the islands of Eday and Shapinsay and converts it to 50 t of hydrogen per annum. The produced hydrogen is then either transported across the islands and used locally or delivered by sea to Kirkwall. In Kirkwall, it is converted into heat and power in a 75 kW FC, supplying harbour buildings, a marina, three ferries, and a refuelling station for a fleet of up to 10 fuel cell-powered vehicles (FC-EV). The project investigated the replicability of the demonstrated solution for forming the green hydrogen-based integrated energy system model in other isolated/remote regions [49], as well as the environmental impact and the life-cost of hydrogen energy. The latter analysis showed that the cost of hydrogen energy is still higher than of fossil fuels (with the cost of electricity as the main contributor), and its competitiveness in the current energy market conditions cannot be achieved without the policy support [50].

3.7. SMILE (2017–2021)

The project aimed at performing a shift to energy systems relying solely on RES through the better integration and utilisation of local energy generation and storage resources from different energy domains [51,52]. Three medium-scale pilots were deployed and demonstrated within the project: in Orkney (UK), Madeira (Portugal), and Samsø (Denmark) islands. The goal of the pilot on Orkney islands (use case SMILE(1)) was to integrate a new Demand Side Management (DSM) system with the existing smart grid and to apply it to aggregated DER and loads. Specifically, the pilot demonstrated the aggregation of 45 EWHs in residential, public, and commercial buildings, 30 slow smart EV chargers, and hydrogen electrolysers, controlled for the provision of different DSM services. The Madeira’s pilot (SMILE(2)) demonstrated the provision of the DSM services by an aggregation of EVs, equipped with smart EV charging software [53,54,55]. The Danish island of Samsø (SMILE(3)) hosted a multi-energy Virtual Power Plant, integrating a 60 kW solar PV plant, a 50 kW/240 kWh BESS, thermal energy storages, a WT, a HP, several electric transportation types (EVs, e-bikes, boats), and commercial controllable loads situated at the Ballen marina. The VPP was integrated into the market through innovative market mechanisms. The system allowed to integrate more renewables and provide balancing of production and consumption.

3.8. HySeas III (2018–2021)

Another project based on Orkney islands, HySeas III, takes advantage of the excess of hydrogen production over its local demand, achieved, in particular, thanks to the 1.5 MW electrolyser installed on the islands within the BIG HIT project. The objective of HySeas III is threefold: (1) to bring to the market the world’s first zero emission renewable-powered hydrogen ferry; (2) to demonstrate a novel circular economy model for the local production of hydrogen that could transform coastal and island economies around Europe; and (3) to make the proposed concept commercially viable and replicable [56,57]. The ferry is intended to serve on the route between Kirkwall and Shapinsay islands and has a load capacity of 120 passengers and 16 passenger cars or 2 trucks. The core of the ferry, the power-train, is composed of a 600 kW PEM FC, a 600 kg compressed gas hydrogen storage (H2SS), and a 768 kWh Li-ion BESS [58], equipped with an advanced energy management and control system. The combination of both FC and BESS technologies allows capturing the advantages of both systems while reducing the size of the FC system. The hydrogen, which will be used in the fuel cells, is generated by the electrolyser from the electricity produced by local wind, tidal, and wave power plants. This way, the ferry becomes an integral part of the islands’ energy hub. The design of the ferry had been completed in March 2022. The full-size drive train has been successfully preliminarily tested on the mainland in Norway in a long-term testing campaign. A special attention in the project is paid to the development of an innovative business model for overcoming the capital investment barriers to the technology replication, as well as new safety regulations.

3.9. REMOTE (2018–2023)

The project investigated technical and economic feasibility of the fuel cell integration into renewable-based microgrids, by this undertaking an important step to the complete substitution of fossil fuels in off-grid remote areas [59,60]. Two out of four project demo sites are islands of Stromboli (Italy) and Froan (Norway). The pilot in Stromboli (use case REMOTE(1)) represents a microgrid comprised of a 170 kWp PV power plant, an electrolyser and a FC of 50 kW each, a 1.5 MWh (21.6 m${}^3$) hydrogen storage tank, and a 600 kWh Li-ion BESS, forming an integrated Power-to-Power hydrogen-based system [61]. The microgrid was connected with the island’s power system. Different optimization strategies, leading to the reduction in the diesel generator use were developed and tested. The demo pilot on the island of Froan (use case REMOTE(2)) is composed of a 55 kW PEM fuel cell, a 100 kW PEM electrolyser, a 550 kWh Li-ion BESS, and a 3.3 MWh (340 m${}^3$) hydrogen storage tank [61], forming a non-integrated centrally controlled hydrogen-based microgrid. The pilot system allowed to avoid building a new subsea power cable and almost completely substitute fossil fuels with green energy, reaching RES share of over 95%.

3.10. REACT (2019–2022)

The project aims at demonstrating solutions enhancing the overall energy security of geographical islands through the large-scale deployment of RES and storages and optimal energy management and control of energy assets at the community level for the provision of flexibility to the grid [62,63]. Multi-energy solutions are deployed and demonstrated on two out of three demo islands—San Pietro (Italy) and Aran islands (Ireland) [64,65]. The pilot in San Pietro (REACT(1)) comprises: (i) new remotely controlled heat pumps, installed in residential and public buildings; (ii) 4–8 kWh Li-ion BESS (in two cases—combined with small solar PV sites) in 11 residential buildings; and (iii) 8 hybrid PV-battery solutions, including 16 kWh BESS and up to 20 kWp solar PV sites and inverters, in several public buildings and hotels. The pilot on Aran islands (REACT(2)) implies upgrading of existing heat pumps of 8–11 kW heating capacity in residential buildings with new Wi-Fi adapters to allow remote monitoring and control. Apart from that, a new air-to-water HP system.

Table 2. Characterization of EU islands hosting MES pilots.

Island(s)	Use Case	Size, km${}^2$	Population	RES Share by Year, %	Mainland Grid Connection	Demand, GWh/year
Ameland, NL	IANOS	54	3683	2035—100%	Yes	n/a
Aran isl, IE	REACT(2)	46	1300	2030—100% [66]	Yes	3 [67]
Borkum, DE	NETfficient, ISLANDER	30.74	5266 *	2020—39%, 2030—100%	Yes	17 [68]
Bornholm, DK	EcoGrid EU, INSULAE(1,2)	588.3	39,439	2018—56% [69], 2035—100% [70]	Yes	250 [69]
Cyprus	FEVER	9251	864,236	2018—13.8%, 2030—23% [71]	No	4900 [72]
Eigerøy, NO	ROBINSON	20	2394	n/a	Yes	n/a
Faroe isl, DK	TWENTIES	1400	52,337	2018—49% [73], 2030—100% [74]	No	350 [75]
Froan, NO	REMOTE(2)	n/a	38	n/a	Yes **	n/a
Formentera, ES	VPP4Islands	83.24	12,216	2050—100%	Yes	n/a
Hinnøya, NO	GIFT(1)	2204	32	n/a	Yes	n/a
Isle of Wight, UK	inteGRIDy(1,2)	380	141,500	n/a	Yes	575 [76]
Madeira, PT	SMILE(2), INSULAE(3)	741	262,000	2019—24% [77]	No	883.226 [77]
Mayotte, FR	MAESHA	374	279,000	2020—10%	No	0.34 [78]
Orkney isl, UK	HySeas III, BIG HIT, SMILE(1)	990	22,100	2016—120% [79]	Yes	220 [80]
Procida, IT	GIFT(2)	4.1	10,500	n/a	Yes	n/a
Samsø, DK	SMILE(2)	112	3724	2030—100% [81]	Yes	31 [82]
San Pietro, IT	REACT(1)	51	200-400	n/a	n/a	n/a
Stromboli, IT	REMOTE(1)	12.5	300–600	n/a	No	0.17 [83]
Terceira, PT	IANOS	402.2	55,300	n/a	No	n/a
Thassos, GR	FLEXIGRID	380.1	14,000	n/a	n/a	n/a
Tilos, GR	TILOS	64	500–800	n/a	No	3 [41,81]

* 3–5 times more in the summer; ** outdated; n/a–not available.

Table 2. Characterization of EU islands hosting MES pilots.

Island(s)	Use Case	Size, km${}^2$	Population	RES Share by Year, %	Mainland Grid Connection	Demand, GWh/year
Ameland, NL	IANOS	54	3683	2035—100%	Yes	n/a
Aran isl, IE	REACT(2)	46	1300	2030—100% [66]	Yes	3 [67]
Borkum, DE	NETfficient, ISLANDER	30.74	5266 *	2020—39%, 2030—100%	Yes	17 [68]
Bornholm, DK	EcoGrid EU, INSULAE(1,2)	588.3	39,439	2018—56% [69], 2035—100% [70]	Yes	250 [69]
Cyprus	FEVER	9251	864,236	2018—13.8%, 2030—23% [71]	No	4900 [72]
Eigerøy, NO	ROBINSON	20	2394	n/a	Yes	n/a
Faroe isl, DK	TWENTIES	1400	52,337	2018—49% [73], 2030—100% [74]	No	350 [75]
Froan, NO	REMOTE(2)	n/a	38	n/a	Yes **	n/a
Formentera, ES	VPP4Islands	83.24	12,216	2050—100%	Yes	n/a
Hinnøya, NO	GIFT(1)	2204	32	n/a	Yes	n/a
Isle of Wight, UK	inteGRIDy(1,2)	380	141,500	n/a	Yes	575 [76]
Madeira, PT	SMILE(2), INSULAE(3)	741	262,000	2019—24% [77]	No	883.226 [77]
Mayotte, FR	MAESHA	374	279,000	2020—10%	No	0.34 [78]
Orkney isl, UK	HySeas III, BIG HIT, SMILE(1)	990	22,100	2016—120% [79]	Yes	220 [80]
Procida, IT	GIFT(2)	4.1	10,500	n/a	Yes	n/a
Samsø, DK	SMILE(2)	112	3724	2030—100% [81]	Yes	31 [82]
San Pietro, IT	REACT(1)	51	200-400	n/a	n/a	n/a
Stromboli, IT	REMOTE(1)	12.5	300–600	n/a	No	0.17 [83]
Terceira, PT	IANOS	402.2	55,300	n/a	No	n/a
Thassos, GR	FLEXIGRID	380.1	14,000	n/a	n/a	n/a
Tilos, GR	TILOS	64	500–800	n/a	No	3 [41,81]

* 3–5 times more in the summer; ** outdated; n/a–not available.

For the Community Development Offices building, accompanied by a solar PV plant, is installed and used for the space heating and domestic hot water supply [84]. All energy assets and flexible loads in both pilots are integrated in microgrids through a cloud-based ICT solution allowing to perform automated and manual energy dispatch control, demand response, and flexibility management at the community level based on real-time generation and load forecasts.

3.11. GIFT (2019–2022)

The GIFT project aims at decarbonising the energy mix of European islands through the development of a group of solutions allowing the synergy between power, heating, and transportation sectors [85,86]. Two pilots that differ by type of flexible assets for the load shifting and grid balancing services are foreseen in the project [87]. The pilot on Hinnøya island (Norway) (GIFT(1)) aimed at the congestion prevention at the connection point between Hinnøya and Grytøya islands through the integration of power and transportation domains. This is completed by using flexibility of aggregated prosumers of the Harstad municipality, including 30 EVs, a local energy community (LEC) formed by 12 local fish farms, and a hydrogen-bromine flow battery, all integrated in a VPP [88].

The Italian demo on Procida island (GIFT(2)) aims at demonstrating, how the operation of the whole island as a local energy community could lead to the reduction in its energy dependency from the mainland, avoiding summer congestion and blackouts, and increase the citizen implication in the energy network. The overall system is operated in the same way as the Norwegian pilot. A few public buildings—a city hall, a public school, and a hospital—equipped with heat pumps and more than 200 kW of solar PVs would be eligible for providing flexibility to the grid. The local energy community is driven by Procida municipality and run by an external actor also acting as a balance-responsible party [87].

3.12. INSULAE (2019–2023)

The INSULAE project aims at decarbonising the energy mix of European islands through the development, testing, and replication of several interventions enhancing RES integration [89,90]. Three project use cases out of seven, taking place on the island of Bornholm (Denmark) and Madeira (Portugal), touch upon sector-coupling. Bornholm hosts two pilots: a battery-buffered PV-powered fast EV charging station (INSULAE(1)) and a renewable-based virtual power plant, coupling assets from power, heating, gas, and transportation domains (INSULAE(2)) [91]. The first solution represents a DC microgrid comprising a 61 kW solar PV plant, a 312 kWh lithium-ion BESS, two 33 kW bi-directional inverters (limited to 43 kW), and two high-power EV chargers with a total operational limit of 350 kW. The microgrid is coordinated through a cloud-based energy management and control system. The core of the microgrid is an innovative BESS with adaptive cell switching, which allows to improve the EV charging efficiency, reduce the required grid connection capacity and associated costs, and provide grid services [92,93]. The pilot has been in operation since June 2021, providing EV charging and aggregating valuable data for the subsequent evaluation of the effectiveness of the solution and its impact. The second demo, the VPP, currently comprises a 3 MW CHP biogas plant, two large-scale wind farms of 12.5 MW in total, a 10 MW PV park, and a 2.4 MW electric boiler. The boiler, together with the biogas plant, tie the electrical grid with the local district heating network. Withal, it is expected that in the nearest future the VPP will also aggregate a 1 MW/1 MWh stationary BESS, a MW-scale electrolyser, a fleet of electric vehicles, as well as RCL and ICL [94,95] The focus of the use case it given to the analytical and experimental investigation of flexibility potential of the biogas plant, power capping capabilities of wind turbines and the solar PV plant, energy management and control of the VPP, and holistic optimisation of the island’s energy system to reduce curtailment of renewables and balance the grid [96,97]. The demo pilot in Madeira (INSULAE(3)) aims at expanding the existing EV charging infrastructure on the island with four 10 kW vehicle-to-grid (V2G) chargers and three 50 kW fast chargers, one of which is based on silicon carbide semiconductors. All newly deployed equipment will be aggregated through an intelligent control system and be able to participate in frequency and voltage support [90], by this enabling a higher penetration of renewables into the grid.

3.13. FLEXIGRID (2019–2023)

The FLEXIGRID project aims at making the distribution grid operation more flexible, reliable, and cost-efficient through the development of several hardware and software solutions [98,99]. One of the project use cases, taking place on the Greek Island of Thassos, concerns the holistic optimization of the energy supply system of a vacation resort, consisting of a hotel and a number of bungalows. The pilot will comprise a solar PV facility, EV chargers, a stationary BESS, several distributed BESS, and a WT [100]. The microgrid will operate in medium- and low voltage levels, not only supplying the hotel but also feeding into the grid. At the demo site, Elin Verd will develop and test different grid management algorithms in medium- and low-voltage grids. Under normal grid operation conditions the local energy system will be optimised towards minimising energy costs, whereas under abnormal conditions (i.e., blackout) the local energy system will be optimised towards covering energy demands of critical loads. Under normal grid operation conditions the local energy system will be optimised towards minimising energy costs, whereas under abnormal conditions (i.e. blackout)–towards covering energy demands of critical loads. On top of that, the use of DER by multiple consumers and prosumers with or without low carbon assets in a shared manner will be investigated to derive the business case for a community energy system and provide recommendations for streamlined approaches to operation of similar systems. For that, novel generation and load forecasting and control tools will be developed for commercial and residential customers with RES and BESS to maximise the benefit for the owner while ensuring optimal operation for the local grid.

3.14. FEVER (2020–2023)

The FEVER project demonstrates solutions and services enhancing the security and resilience of the distribution grid operation [101,102]. One of the solutions is a platform for the flexibility aggregation and management of energy storage assets and peer-to-peer trading. One of the pilots, situated on campus of the University of Cyprus, aims at demonstrating and assessing the impact of the aggregation of local flexible assets with different availability and dynamics, such as battery storages and power-to-cold technologies. The microgrid intends to provide the demand response, congestion management, and reactive power ancillary services for the distribution grid balancing and handling of islanded operation of the local grid. The flexibility will be offered to the day-ahead, balancing, and reserves market via aggregators. The demonstration is expected to start in summer 2022.

3.15. IANOS (2020–2024)

The IANOS project aims at delivering technological, economic, and social innovations allowing to further reduce dependency on fossil fuels on the islands of Terceira (one of the Azores islands, Portugal) and Ameland (Netherlands). Through nine use cases the energy systems of the islands will be optimised via the energy efficiency measures, electrification of fossil fuel-dependent areas, introduction and adoption of carbon-neutral fuels, and empowerment of local energy communities using the energy community-centric approach [103,104]. The demos will involve at least 900 prosumers and consumers. The use cases will demonstrate different technologies—hybrid transformers, a flywheel storage, bio-based batteries, thermal storages, a tidal kite, and an auto-generative digester—integrated and operated in a VPP architecture. It is expected that, besides the decrease in fossil fuel consumption and increase in RES utilization, the project interventions will lead to the reduction in energy bills of end-users by at least 15%. The replicability of the solutions proposed in the project will be investigated in the follower islands of Lambedusa (Italy), Bora Bora (France), and Nisyros (Greece). Since at the moment there is no information regarding the exact technological scope of both pilots available, we consider them as one whole.

3.16. MAESHA (2020–2024)

The MAESHA project aims at developing and demonstrating flexibility, storage, and energy management solutions enabling large penetration of renewables into the energy systems of geographical islands. The pilots will be hosted by the French overseas island of Mayotte, whereas their replicability potential will be studied for other five follower islands with more than 1.2 million inhabitants [105,106]. The following technological solutions will be tested: (i) Virtual Power Plant, which will comprise renewable, battery storage, and Power-to-Hydrogen technologies, and provide frequency and voltage control services. (ii) Demand response-based frequency and voltage control and peak shaving, performed by industrial and residential controllable loads. (iii) Smart Vehicle-to-Grid (V2G) EV charging utilising solar PV generation and providing frequency control and peak shaving. The aggregated flexibility providers will be optimised together through a Flexibility Management and Trading Platform [107]. The innovative technological solutions will be supported by efficient modelling tools for real-time and long-term energy economy observations, and adapted local business, market, and regulatory frameworks following the community-centric approach. The expected impact of the project implies reaching at least 70% of RES penetration into the island’s energy system.

3.17. ROBINSON (2020–2024)

The ROBINSON project aims at decarbonising islands through the development of an integrated, smart, and cost-efficient energy system, coupling local electrical and heating networks, energy resources, and storage technologies using hydrogen as an energy carrier [108,109]. The demonstration pilot will take place on the island of Eigerøy in Norway. The island’s energy system will integrate: (i) commercially available technologies and storages; (ii) a set of novel technologies, such as a micro gas turbine-based CHP unit running on a mixture of local green fuels, an anaerobic digestion system accompanied by a bioelectrochemical process producing bio-methane from wastewater, and an innovative wind turbine; and (iii) industrial symbiosis concepts, such as waste, heat, and oxygen valorization. The whole system will be managed and optimised through an intelligent Energy Management System with advanced control algorithms considering weather forecasts and market price fluctuations. The project will create new business opportunities for local communities and open up markets for the developed technologies. The project interventions will contribute to the reduction in energy costs and energy dependence on the mainland, while ensuring the grid stability, power balance, and security of supply, and decrease fossil fuel consumption, particularly by the local industry.

3.18. VPP4Islands (2020–2024)

The VPP4Islands project aims to accelerate the transition towards smart and green energy systems on islands. This is to be achieved through the grid integration of aggregated distributed energy resources (DERs) from different energy vectors and controllable loads. One of the islands, hosting the interventions, is a Spanish island of Formentera. The interventions in Formentera will focus on the (i) increase in self-consumption, (ii) implementation of peer-to-peer energy trading, and (iii) energy community management. The interventions comprise renewables, energy storage systems, and controllable loads on the hardware level, and peer-to-peer trading platform and VPP energy management and forecasting tools on the software level. The solutions will upgrade the existing virtual power plant. The peer-to-peer trading will be carried out between residential, commercial, and industrial consumers/prosumers via smart contracts. In addition, the energy community will trade energy with and provide auxiliary services to the DSO, using solar PV installations, flexible loads, and a hybrid lithium-ion and hydrogen-based energy storage system. One of the project interventions will be built to supply the local football camp. The multi-vector energy system will comprise an energy mix of 90 kW solar PV, a 75 kWh Li-ion battery, and a regenerative hydrogen fuel cell system, consisting of a 20 kW PEM electrolyser, a 1 MWh hydrogen storage, and a 5 kW PEM fuel cell [110]. In addition, the system will connect rooftop solar PV systems on private and public buildings of 40 kW in total. The energy consumption of the prosumers, participating in the project, will be optimised to increase self-consumption. The project solutions will be replicated in three follower islands.

3.19. ISLANDER (2020–2024)

The ISLANDER project aims at accelerating the transition of European islands to the 100% energy independence, starting from the island of Borkum [111,112]. The energy hub in Borkum will comprise distributed renewable energy assets and small-scale storage systems, large-scale battery energy storages, a seawater-based district heating network for 100 residential buildings with a thermal storage, providing heating and cooling, as well as the EV charging infrastructure. The energy assets will be aggregated through a smart IT platform, forming a renewable energy community. The platform, equipped with an energy system optimisation tool, will allow the energy community to provide demand response services. The project aims at replicating the Borkum’s interventions on the follower islands of Lefkada and Skopelos in Greece, Orkney in the UK and Cres in Croatia.

4. Results and Discussion

This section analyses the demo pilots from the following perspectives: (i) global targets; (ii) technologies involved; (iii) technology aggregation approaches; (iv) coupled energy sectors; (v) provided services; (vi) focus areas and contributions; and (vii) challenges, risks, and success factors. It is worth mentioning here, that although a comparative analysis of investment and operation costs and the global efficiency of the pilots would have a great importance, the analysis is not possible due to the lack or limited access to relevant publicly available data.

4.1. Global Targets

Despite the wide spacial distribution of the concerned European islands, the differences in available energy sources and grid connection to the mainland, regulations, and market conditions, they have common energy-related goals. This is reflected in the goals and objectives of the demonstration projects as well. The analysis allowed to distinguish six main project global targets. Through the development and demonstration of multi-energy system and sector-coupling technological solutions the projects intend to contribute to the achievement in the islands:

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Energy independence—in NETfficient, SMILE, REMOTE, and ISLANDER;

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Carbon neutrality—in GIFT, INSULAE, IANOS, ROBINSON, and VPP4Islands;

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Enhancing RES integration—in TWENTIES, TILOS, INSULAE, MAESHA, HySeas III;

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Improving stability, reliability, and security of power systems—in inteGRIDy, BIG HIT, and REACT;

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Improving flexibility, security, and resilience of distribution grids—in FLEXIGRID and FEVER;

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Enabling market participation of loads and DER—in EcoGrid EU and NETfficient.

4.2. Technologies Participating in the Pilots

Figure 3 provides the reader with a detailed overview of the energy technologies and controllable loads, participating in each demonstration. It also indicates main software tools and technology aggregation concepts, utilised in each pilot, and shows a temporal distribution of the demos over the 14-year timeline. The figure makes it possible to conclude on the most and least frequently appearing technologies throughout the whole set of projects. The following of this subsection is focused on the discussion of the sector-linking technologies in the demos. In addition, other technologies, appearing in the pilots, are also touched upon.

4.2.1. Intermittent Renewables

As the analysis of the projects’ global targets showed, one of the main reasons for building sector-interlinked energy systems on islands is the need for facilitating the large-scale penetration of intermittent renewable energy technologies into the power energy system, necessary for achieving their decarbonisation goals.

There are three types of highly variable renewable energy sources appearing in the demos: solar photovoltaics, wind, and wave energy plants. Solar PVs are inevitable in two-thirds of the pilots. In its turn, the wind technology is present in nearly 40% of the multi-energy demos. It is worth noting that in some pilots the renewable energy facilities, particularly solar PV and wind have already existed on the pilot islands before the project started and are integrated with the pilots’ solutions, as in cases of INSULAE(2) and TWENTIES projects. As to the wave energy technologies, they have a limited consideration in the analysed projects, appearing in only one demo of the HySeas III project. The project contributes to the overall ocean technology development effort of the UK [113,114]. The current application of wave energy technologies in general is limited due to several factors [115]. One of them, which creates implications for the power energy system and causes grid integration concerns [116], is the high spatio-temporal variation in the wave resource.

4.2.2. Other Renewables

However, some of the drawbacks of the technology could potentially be offset if it is part of a highly flexible integrated energy system, as in the case of the HySeas III pilot. Due to the natural availability of the wave resource on islands, the wave energy technologies could potentially become a valuable asset in the islands integrated energy systems.

Unlike the wave power, the tidal power is a fully predictable renewable energy source. Similarly to the wave energy technologies, tidal technologies have a limited consideration in the studied projects, appearing in only three pilots: two of them on the Orkney islands in BIG HIT and HySeas III projects, and one on the Dutch island of Ameland in the IANOS project. This is aligned with the overall pace of the technology development through the last several years. The technology still has a limited attractiveness for the investors due to its immaturity and cost-ineffectiveness. In this case, research and development projects with real-life demonstrations of tidal energy plants can bring it closer to competitiveness. The integration of tidal technologies, characterised by the availability of the water resource and its predictability, into multi-energy systems could potentially bring additional operational and cost-related benefits to the system. Herewith, islands and remote coastal areas are considered as the technology’s niche markets [115].

4.2.3. Sector-Linking Technologies

The list of the sector-linking technologies utilised in the investigated pilots contain seven energy production, storage, and conversion technologies, and two categories of controllable loads. The most frequently used sector-coupling technology, appearing in eight pilots, is the thermal energy storage. It implies both medium and large scale hot water tanks and domestic electrical water heaters. The cost and operational benefits of coupling power and energy systems through thermal energy storages and electric water heaters have been known. However, their optimal usage in energy systems with multiple energy domains is a subject of more and more studies [117] and demonstration projects.

The second most used sector-linking technology is the heat pump. Small-scale heat pump systems appear in six pilots, whereas industrial scale HPs–in five pilots. Although heat pumps are expected to constitute a significant electricity demand in future energy systems and, thus, may potentially load the grid, their participation in DR and DSM programs and complementing them with thermal storages make them a valuable source of flexibility for the grid [118].

Relatively new for the large-scale use, but very promising and fast-growing are the hydrogen production, storage, and conversion technologies. The polymer electrolite membrane electrolysers are part of four pilots, utilising the excessive production of renewables and by this contributing to the grid balancing and the increase in the overall energy utilisation efficiency. In contrast, the PEM fuel cells are deployed in one-fifth of the analysed sector-coupling pilots. In most cases (except the REMOTE(1) demo) FCs are accompanied with either pressurised or metal hydride hydrogen storages. In addition, the pilots on Orkney islands deploy not only the above mentioned technologies, but also a complete infrastructure allowing for the full cycle from the production to the storage, transportation, and versatile utilisation of the hydrogen. Due to the extreme flexibility of hydrogen as an energy carrier that can be produced from various primary energy sources and used in different energy domains, the replacement of fossil fuels with hydrogen is becoming an emerging way to address the uncertainties of the renewable generation in energy systems. Thanks to the study initiated in 2020 by the European commission “Hydrogen strategy for a climate-neutral Europe” [119], numerous national initiatives for the hydrogen development are currently beginning in different European countries (in Denmark [120], in Finland [121], etc.). Thus, it can be assumed that the range of R&D and R&I projects dedicated to the green hydrogen application in multi-vector energy systems in general and particularly on islands will be subject to further growth.

Another technology being considered as part of multi-energy systems in three pilots (INSULAE(2), IANOS, and ROBINSON) is the biogas technology. Despite the limited presence in the studied projects, a biogas power plant potentially entails a large flexibility potential for the renewable-driven energy systems. In addition to being generally versatile from both input and power and heat output sides, with the latter depending on the biogas storage characteristics, the biogas plant can be upgraded to produce bio fuels (particularly, biomethane). This unlocks the interconnection of four energy sectors at once—power, heat, gas, and transportation. Therefore, the biogas plant can be considered as a central element of the islands’ future sustainable economies [96].

4.2.4. Sources of Flexibility and Providers of Grid Services

The electrochemical battery energy storages are part of two-thirds of the demo pilots running throughout the considered timeline. Possessing inherently different characteristics, the BESS can significantly affect the grid at the distribution or transmission level. In addition, they are generally capable of offering operational flexibility to the power system, participating in the grid islanding and restoration, providing frequency- and voltage-based grid balancing services, or locally compensating for wind or solar production fluctuations [122]. It is also worth noting that investigating benefits of combining several types of BESS with other energy storages into the hybrid storage systems is a subject of the pilots of NETfficient, REMOTE, and VPP4Islands projects.

The electric vehicles are being part of two-fifths of the pilots (12 out of 28) since 2016 (including one, BIG HIT, with fuel-cell-powered electric vehicles). The role of EVs varies across the pilots. In some cases, EVs act as full-fledged electrical energy storage system, capable of performing the respective functions, such as frequency and voltage control [123]. In other cases, electric vehicles are treated as flexible electrical loads that can be aggregated and controlled through DR and DSM mechanisms along with other flexible loads. Despite the relative novelty of the technology and the relatively little consideration in the demos, the existing real examples of their successful utilisation as flexibility and grid services providers, supported by numerous analytical studies, prove its important role in facilitating large-scale integration of renewables in future integrated energy systems [124].

Controllable loads are present in two-thirds of the multi-energy pilots. Herewith, along with residential and commercial loads, the industrial and community-level loads participate in half of the cases. The role of controllable loads in multi-energy pilots vary. For example, in case of the TWENTIES demo, the aggregation of large-scale industrial loads is one of the main enablers of the ultra-fast balance restoration and blackout prevention of the local energy system. Smaller controllable loads, usually aggregated through the aggregation platforms, are usually controlled for the provision of DR and DSM services to the grid. In other cases the loads, together with different distributed energy technologies, become a part of local energy communities. The energy communities, in turn, could pursue the local goals of self-balancing (optimization of the local energy consumption) and reduction in the impact on the grid (BIG HIT, SMILE, REACT, GIFT) or peer-to-peer energy trading (FEVER, VPP4Islands), or participate in the energy and flexibility markets. The energy community scale in the studied pilots varies from several buildings (FLEXIGRID) to the whole island (GIFT(2)).

4.2.5. Software Tools

Every multi-energy pilot participating in the analysis has as an inevitable part of a Energy Management and Control System (EMCS). Depending on the complexity of the multi-energy system architecture, number of different intermittent components, and required functionality, EMCSs are usually accompanied by other additional tools, which, together, form multi-functional ICT platforms. For example, advanced renewable energy production and energy consumption forecasting tools are inevitable for ensuring optimal multi-energy system operation. Thus, Energy Forecasting Systems (EFS) are being developed and demonstrated in nearly a third of the pilots. Other tools may enable (i) market participation and energy trading for aggregated and individual players of multi-energy systems, (ii) demand response forecasting, (iii) peer-to-peer energy trading within energy communities, and (iv) energy cost management and optimization for consumers and prosumers, etc. In addition, four pilots concerning optimisation of energy supply systems of single buildings develop and test building management systems.

4.3. Aggregation Approaches

The discussion in this subsection refers to the Table 1, which indicates technology aggregation approaches, or concepts, used in the multi-energy demos. It is worth noting that while for some of the pilots their belonging to one approach or another was explicitly stated in the dissemination materials, for other pilots it was defined by authors based on the classification and definitions proposed in [16]. According to that, all multi-energy system aggregation concepts could be categorised by three types: the Virtual Power Plant, the energy hub, and the microgrid. In this analysis, the microgrid is interpreted as a low- or medium-voltage distribution system with various DERs (generation, storage, loads), controlled in a coordinated way and able to operate in islanded mode while guaranteeing a certain level of power quality and reliability in the case of grid failure. The VPP is considered as a flexible aggregation of DERs coordinated in an optimal way, that is capable to participate in the energy market and provide the same services as conventional large-scale power plants. Herewith, there are commercial (with only an economic focus) and technical VPPs (aggregated resources are situated in one geographic area, and their coordinated control also considers power grid constraints). Finally, the energy hub implies several input and several output energy carriers and may include objects that realize the transmission, storage, conversion, and transformation of different kinds of energy.

The analysis revealed that 13 out of 28 pilots are organised as microgrids: 11 as VPPs and 4 as energy hubs. In case of the latter, the four energy hub pilots (BIG HIT, HySeas III, ROBINSON, and ISLANDER) couple at least three energy sectors and exploit hydrogen as an energy carrier.

It is worth noting that no correlation has been found between the aggregation approach and the scale of the pilot. The same applies to the level of spatial distribution of the pilots, despite the fact that the single-consumer pilots (as in the FLEXIGRID) tend to be organised as a microgrid.

4.4. Sectors Coupled

Both finished and ongoing research projects are strongly focused on building synergies between several energy domains. Table 1 gives an overview of project use cases and corresponding combinations of energy sectors coupled. Herewith, 57% of the pilots integrate the power sector with only one additional energy sector, in which the combinations of power and heating and power and transportation sectors equally share 88% of the pilots. The remaining 12%, represented by two pilots of the REMOTE project (Section 3.9), couple the power and gas sectors by integrating hydrogen technologies into the isolated electrical grids of the islands. Around 42% (or five pilots) of the remaining share of demos couple the power sector with two other energy sectors, 50% (six pilots)—with three, and the remaining 8%, represented by one pilot of the ISLANDER project, forms the most advanced multi-energy system that brings together all five energy sectors.

In regards to the presence of each energy domain in the combinations of coupled sectors, as expected, the heating sector is the most frequently appearing component of multi-energy systems, taking place in 64% of pilots.

Along with the heating sector, the transportation sector is considered as a full-fledged player of multi-energy systems, appearing in nearly half of the pilots started earliest in 2017. Not far behind, the transportation sector is followed by the gas sector, taking part in two-fifths of the pilots. Finally, the cooling sector is integrated into only 18% of pilots.

It is also interesting to note that there is no clear tendency on the increase in the complexity of the sector mix since 2015, whereas the two projects taking place earlier couple only power and heating sectors.

4.5. Services Provided by Multi-Energy Pilots

To achieve islands’ and projects’ targets, different technologies of multi-energy systems or their combinations perform different control actions and play specific roles within the islands’ energy systems. Figure 4 identifies seven services performed by the technologies located on the consumer side and six services—by medium- and large-scale generation facilities, constituting the multi-energy systems. The connecting lines indicated in which projects such services have been demonstrated. Distributed energy production and storage technologies, installed on the consumer side, and controllable loads participate in the provision of a number of DSM services, as well as distribution grid congestion management, frequency, and voltage control services. Herewith, load shifting, peak shaving, and frequency control are foreseen to be covered more often, being part of 11, 9, and 10 pilots, respectively. In addition, the increase in self-consumption, congestion management, and voltage control are still part part of substantial shares of pilots. On the contrary, the power limitation by the consumer-side DER and loads is demonstrated in only two pilots. The technologies integrated and exploited on the grid side demonstrate the provision of frequency and voltage compensation measures, as well as services related to the system management (congestion management, feed-in management) and reconstruction of supply (black start, islanding). Herewith, the most frequently covered services are frequency and voltage control, congestion management, and islanding and restoration (each covered by 4–5 pilots), while the black start performed by cross-sectoral energy systems is part of only two pilots.

4.6. Focus Areas and Contributions of Multi-Energy Pilots

A detailed analysis of the demo pilots allowed to identify seven main focus areas, appearing in the multi-energy demos, which also represent the main contributions of the demos to either reaching the global targets of the projects, or to specific areas of knowledge in the sector-coupling field. Figure 5 provides an overview of how many and which projects exactly contribute to each of the focus areas. It could be seen that 26 out of 28 multi-energy pilots concern the energy management and control in multi-energy systems.

The market integration of distributed energy resources, energy storages, and controllable loads is the focal point in 17 pilots, whereas the holistic optimisation of multi-energy systems is the focal point in 10 pilots. It is worth noting that while there is no clear indication of the novelty aspect in the names of the three above described categories, it is usually present by default. For example, the demos with the focus area of energy management and control could comprise either innovative combinations of technologies, or new algorithms of energy management and control, or both. Eight pilots contribute with the development and demonstration of innovative market concepts: 6 pilots—new regulatory frameworks, 6 pilots—new business models, and 4 pilots—innovative energy technologies. It is worth noting here that this specific analysis did not focus in detail into the software side of the pilots, since the implementations tend to differentiate from vendor to vendor.

4.7. Barriers, Challenges, Risks, and Success Factors of Demonstrations

This section identifies, classifies, and analyses barriers and challenges faced by the project partners in any stage of the whole life cycle of the project, as well as risks and success factors, reported in the available project dissemination materials. Despite the fact that this information is available only in the minor share of projects, particularly due to the ongoing status of nearly half of them, the accessible data possess important outcomes.

4.7.1. Barriers and Challenges

The reported technical barriers and challenges include: (i) lack of existing hardware and software solutions for the automatic provision of auxiliary services (EcoGrid EU); (ii) lack of energy, communication, and measurement infrastructure, needed for facilitating the demonstration (TILOS); and (iii) inability or difficulty of executing load demand forecasting in tourism-oriented small-scale islands (TILOS).

Among the reported legal and regulatory barriers and challenges are: (i) inadaptability of regulatory conditions for the implementation of specific technological concepts (TWENTIES); (ii) lack of standards for the aggregation and control of household appliances (EcoGrid EU); (iii) difficulty of obtaining building permits for the long-term deployment of equipment (INSULAE(1)); (iv) difficulty of finding a suitable for the demo pilot location and receiving an approval (INSULAE(1)); and (v) lack of safety regulations for the hydrogen market development (HySeas III).

The market-related barriers and challenges comprise: (i) inadaptability or lack of specific market mechanisms (EcoGrid EU); (ii) lack of access to the real market due to not meeting the power capacity requirements (NETfficient); and (iii) lack of market required for the successful demonstration (HySeas III).

Among challenges and barriers on the societal front projects report: (i) attracting potential users of the demonstrated solutions (INSULAE(1)) and (ii) receiving a long-term commitment of consumers and prosumers, required for the successful demonstration (Ecogrid EU).

The organisational barriers and challenges indicate: (i) a lack of representatives from the local administration in the project consortium and (ii) lack of representatives of power system operators in the project consortium. The latter led to serious consequences that affected the demonstration part of another project, SINGULAR (2012–2015) [125,126], investigating the effects of large-scale integration of renewables and demand-side management on the planning and operation of insular electrical grids. Due to inability to integrate the solutions into the island power system, the real demonstration did not take place. Therefore, the project, despite the relevant scope and involvement of geographical islands and multi-energy aspects, but lacking the demonstration aspect, was excluded from the systematic review in Step 3.

4.7.2. Success Factors

As to the success factors, while most of them could be retrieved from the reported barriers and challenges, several projects emphasized some additional ones that have to be taken into account at the initiation and planning stages of the demonstration project life cycle. Among them: (i) wide public awareness, (ii) dedicated involvement of end users (NETfficient, TWENTIES, EcoGrid EU, and other projects), (iii) involvement of DSO or TSO, (iv) carefully planned versatile composition of the project consortium (NETfficient), and (v) meticulous planning of the project.

4.7.3. Risks

The following risks were reported by the project stakeholders:

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Technical: (i) non-compliance of the technologies with the Grid Code (INSULAE(1)) and (ii) incomplete project implementation due to the complexity of the demonstrated solutions;

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Economic: (i) unfavourable long-term price development (NETfficient), (ii) inability to replicate the developed solutions due inability to mitigate capital investment barriers (HySeas III);

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Legal and regulatory: (i) legal and regulatory changes influencing the value of the demonstrated technologies, (ii) inability to realise the real-life demonstration due to the lack of market or too advanced new market concepts (EcoGrid EU);

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Societal: low acceptance of the interventions by the society;

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Temporary: (i) delays in the equipment development and supply by contractors, especially when it comes to the innovative technologies (INSULAE(3)) and (ii) delays related to obtaining grid integration permits from the TSO/ DSO (INSULAE(1));

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Organisational: (v) inability to realise the demonstration due to the lack of DSO or TSO involvement in the project.

5. Conclusions

This paper provided a comprehensive review of 28 demonstration use cases, deploying and testing sector-coupling technologies and multi-energy systems on 21 European islands. The analysed pilots are part of 19 research and development projects, funded by the European Commission and running between 2010 and 2024. The initial selection of projects for the analysis was carried out using the CORDIS database, the EU’s main research project repository and channel for disseminating their results. The projects were screened out based on their fulfilment of the following three main criteria: (i) involvement of an European island as a pilot host, (ii) presence of the sector-coupling aspect, and (iii) presence of the technology deployment and demonstration aspect. As a base for the analysis, a spreadsheet with a number or quantitative and qualitative indicators characterising each project and demo pilot was created. The spreadsheet was filled out utilising all available project dissemination materials, including the dedicated project pages in the CORDIS database, official websites, as well as project reports and scientific papers.

The paper first presented the summaries of the selected demonstration projects and corresponding pilots, focusing on the technological aspects and demonstration results. This was followed by results of the synthesis and analysis of specific demo-related information. In particular, the demo pilots were analysed from the following perspectives: (i) global targets; (ii) technologies involved; (iii) technology aggregation approaches; (iv) coupled energy sectors; (v) provided services; (vi) focus areas and contributions; and (vii) challenges, risks, and success factors.

Despite the wide spatial distribution of the European islands hosting the demonstrations, differences in the availability of energy sources and grid connection to the mainland, regulations, and market conditions, they have common energy-related goals. This is reflected in the goals and objectives of the demonstration projects as well. Through the development and demonstration of sector-coupling technologies, substantial shares of projects contribute to reaching the energy independence and carbon neutrality goals, as well as facilitating the large-scale penetration of renewables. In addition, several projects aim at improving stability, reliability, and security of islands’ power systems, flexibility and resilience of distribution grids, and enabling market participation of loads and DER.

The detailed analysis of the technologies comprising the demonstrations allowed to identify the most and least frequently appearing solutions and their combinations throughout the investigated timeline. In particular, besides the more mature intermittent renewable energy technologies, such as solar PV and wind, appearing in the major share of projects, there are also wave and tidal energy technologies participating in the demos. Despite the current limited consideration and low attractiveness of both technologies due to remaining technological, market, and economical challenges, the research and development projects with real-life demonstrations of the technologies as part of an integrated energy system could potentially offset some of their drawbacks. The consideration of the biogas technology in the investigated pilots is also very limited despite a significant flexibility potential it entails for the renewable-driven energy systems and future sustainable economies.

As to the trends in the sector-linking technologies, besides the conventional thermal energy storage and heat pumps, more and more sector-coupling projects involve hydrogen production, storage, and conversion technologies in the demonstration. One can assume that the range of research and development projects dedicated to the green hydrogen application in multi-energy systems, particularly on islands, will be subject to further growth in the coming years.

Among other flexibility and grid service providers in multi-energy systems there is an increasing consideration of distributed energy storages and versatile controllable loads, often combined into local energy communities, as well as electric vehicles.

The analysis of the multi-energy system aggregation approaches allowed to identify the trend of the more frequent application of microgrid and VPP concepts in comparison to the energy hub. At the same time, though requiring more complex control and optimisation approaches, the energy hub concept generally enables more flexibility opportunities. The increasing complexity of multi-energy systems requires advancing the energy management and control and forecasting tools, ensuring their optimal operation. As to the sector combinations, it was revealed that, though more than half of the demonstrations couple the power sector with only one additional sector (either heat or transportation), there is also a considerable number of projects exploiting synergies of three and four energy domains, and one demo, bringing together five energy domains. Herewith, the potential of the cooling sector in the multi-energy systems on islands seems to be underexplored.

The analysis also identified a variety of services and control actions, essential for achieving the projects’ and islands’ energy-related targets, and main contributions of the demonstrations. As to the latter, while energy management and control, market integration of distributed energy resources, and holistic optimisation receive the most wide representation, substantial shares of projects create a value through the development and demonstration of innovative market concepts, new regulatory frameworks, new business models, and innovative energy technologies.

The paper also identified, classified, and analysed barriers and challenges faced by the project partners during the project life cycle, as well as risks and success factors, reported in the available project dissemination materials. It was particularly observed that the demonstration of new technologies or their combinations is often associated with high risks of facing regulatory and market barriers, which may influence the ability to carry out the demonstration or its success. A carefully planned versatile composition of the project consortium, implying the involvement of the grid operators and other critical to executing the demonstration partners, is named as one of the main success factors. In addition, the importance of the dedicated involvement of the end-users and local authorities, as well as the support of the local community should not be underestimated. These findings allow a better preparation for the future demonstration projects, prevent the appearance of similar issues, and eliminate unnecessary risks.

The results of the systematic review provide a solid basis for the decision making for different parties—researchers, industry players, and islands’ administrations—concerning future sector-coupling and multi-energy system-related research and industrial projects.