1 Introduction

In 2021, the share of renewable energy generation in Germany’s gross electricity consumption was 41.1% (German Government Environment Agency 2022b). Onshore and offshore wind energy has the largest share among the renewable energy resources, followed by bioenergy, solar energy, and hydropower. However, most of the energy is still generated using fossil fuels, such as gas and coal. To mitigate the climate crisis, a transformation of the energy sector towards low-emission energy generation is necessary. The European Union aims to generate 40% of the required energy from renewable energy sources by 2030 and to become climate-neutral by 2050 (European Commission 2022a). Meanwhile, the demand for electricity is expected to increase in the next years: While Germany has consumed 565 TWh of electricity in 2021 (German Government Environment Agency 2022a), its electricity consumption is projected to increase to 658 TWh in 2030 (Kemmler et al. 2022). To ensure that the increasing demand can be met and greenhouse gas emissions are reduced at the same time, a significant increase in the share of renewable energies within the energy mix is necessary. Solar and wind energy is expected to play a key role in future electricity supply; complemented by other technologies such as bioenergy, geothermal energy, hydropower, and energy storage (i.e., Qudaih et al. 2022). Tidal energy, however, plays no role in Germany’s current electricity supply although it has a decisive advantage compared to other forms of renewable energy: Since tides are governed by the constant cycles of Earth, Moon, and Sun, tidal energy is highly predictable and can be calculated well in advance (European Technology 2020). There are specific reasons for neglecting tidal energy in German waters that have not been addressed so far in the pertinent literature; this work hence aims to close this knowledge gap by comprehensively reviewing the available literature and additionally by providing a novel insight into the motives of stakeholders to neglect energy resources in waters with less favorable conditions for current tidal energy technology.

In general, there are two established methods for harnessing tidal energy: tidal barrages and tidal current turbines (Lemonis 2004), also known as tidal range and tidal stream technology, respectively. On the one hand, tidal barrages are dam-like coastal structures typically built across estuaries or bays, which have been constructed since 1966 (e.g., La Rance, France (Gougenheim 1967)), and utilise the potential energy of the tides (see also Ruiz et al. 2022). Tidal current turbines, on the other hand, extract kinetic energy of the tides and operate similar to wind turbines. The power output of tidal current turbines highly depends on the prevailing current velocity regime (Fraenkel 2002). Besides tidal current turbines, which are typically constructed with a horizontal or vertical axis, there are other devices that belong to tidal stream technology (see Sect. 4).

Worldwide, in 2021, a deployed tidal stream capacity of 39.6 MW could be observed (Ocean Energy Europe 2021b). This capacity was steadily building up from the establishment of the first facility for testing and demonstrating tidal stream technology (European Marine Energy Center (EMEC)) in 2003 till today.

1.1 Tidal energy resource assessment

In 2009, a guideline for the assessment of the tidal energy resource was published by EMEC, which aims to provide a uniform methodology for identifying the tidal stream energy resource at a specific location (Legrand 2009). Later, this guideline has also formed the basis for the IEC TS 62600-201 (International Electrotechnical Commission 2015). Based on this guideline, several investigations of the tidal stream energy resource in different countries have been carried out in the past years, for example in Norway (Grabbe et al. 2009), the United Kingdom (Coles et al. 2021), Ireland (O’Rourke et al. 2010), France (El Tawil et al. 2017; Guillou et al. 2018), Spain (Calero Quesada et al. 2014), USA (Work et al. 2013; Gunawan et al. 2014), Mexico (Mejia-Olivares et al. 2018), Chile (Guerra et al. 2017), Iran (Khojasteh et al. 2018; Rashid 2012), India (Murali and Sundar 2017), China (Hw et al. 2011), Indonesia (Orhan et al. 2015), Malaysia (Lim and Koh 2010; Maldar et al. 2022), Australia (Cossu et al. 2021), and Fiji (Goundar and Ahmed 2014).

In previous tidal energy reviews, it is often distinguished between a theoretical, technical, practical, accessible, and viable tidal energy resource, as for example for Ireland by O’Rourke et al. (2010): While the theoretical resource in Ireland was estimated at 230 TWh  y\(^{-1}\), the viable resource was finally determined to 0.915 TWh y\(^{-1}\). The differences are due to the introduction of various boundary conditions in each iteration step (e.g. from theoretical to technical resource), for example specific water depth ranges, d, minimum required current velocities, V, a power coefficient, \(C_p\), the current use of marine space, and commercial limitations. A more detailed description of the differences between the resources is given by Segura et al. (2017). However, Coles et al. (2021) only address the practical tidal energy resource (for the United Kingdom), and reports that UK’s practical tidal energy resource has been re-estimated at 34 TWh y\(^{-1}\) based on the initial Carbon Trust Study (The Carbon Trust 2011). This represents 11% of UK’s current annual power demand and could be achieved by installing tidal current turbines with a total capacity of 11.5  GW by 2050.

Table 1 Resource assessment stages (adapted from Legrand (2009))
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According to EMEC (Legrand 2009), it can further be distinguished between different stages of a tidal energy resource assessment (see Table 1). At the beginning of the investigations it is recommended to conduct a regional assessment (stage 1), in which suitable sites for tidal energy extraction within a region of reference are screened. On this basis, feasibility studies are carried out in site assessments (stage 2), until the final design is developed in the end (stage 3). An example for a stage 1 assessment for Norway is given by Grabbe et al. (2009), where in total 104 sites were identified suitable for tidal energy extraction using a numerical model, resulting in a theoretical resource of 17 TW h y\(^{{-1}}\). Stage 2 assessments are shown in studies for Banks Strait, Tasmania, Australia (Cossu et al. 2021) and in the East River tidal strait, New York, USA (Gunawan et al. 2014), where hydrodynamic data were gained using Acoustic Doppler Current Profiler (ADCP) and Acoustic Doppler Velocimeter (ADV), respectively. Cossu et al. (2021) finally projected that 200 tidal current turbines could generate between 4.4 and 5.34 MW power at the study site. On the other hand, the East River tidal strait, New York, USA has already been licensed for a 1.05 MW array of up to 30 tidal current turbines in January 2012 (Gunawan et al. 2014).

1.2 Objectives

In Germany, no tidal energy resource assessment of any kind can be found in the permanent literature. The present study aims at filling this gap. As an initial step, a regional assessment is carried out in the framework of this study, with the aim of screening suitable sites for energy extraction from the prevalent micro- and mesotidal conditions (Davies 1964) in the German North Sea (stage 1 assessment). In particular, the specific objectives of this work are:

  • To identify and assess the theoretical, technical and practical tidal energy resource along the German North Sea coast.

  • To review and evaluate currently available Tidal Energy Conversion (TEC) devices with regard to their suitability for the German North Sea.

  • To analyse the interest of tidal energy technology developers in projects in German waters, as well as explore barriers for stakeholders to develop projects in waters with less-than-favorable TEC conditions.

  • To provide a framework for future detailed resource analyses of high-potential project sites in the German North Sea

To achieve these objectives, based on the materials and methods detailed in Sect. 2, a resource analysis (Sect. 3), a technology analysis (Sect. 4), and a stakeholder analysis (Sect. 5) are presented. The results of the three main sections of this review are finally jointly discussed (Sect. 6) and conclusions are synthesised (Sect. 7).

2 Materials and methods

2.1 Description of sites and focus area

Germany is bordered in the North by both the North Sea and the Baltic Sea. Since the tides in the Baltic Sea are very small due to its character as an inland sea, the potential for utilising tidal energy there is negligible. In the German North Sea, however, tidal ranges between 1 and 4 m occur, with the highest tidal range in the inner Jade bay, close to Wilhelmshaven.

The tidal portion of the time-history of the sea surface elevation occurring at a particular location is caused by the superposition of various components, also known as tidal constituents (Federal Waterways Engineering and Research Institute 2022). The tidal dynamics in the German Bight are predominantly controlled by the M2 tidal constituent, with an amplitude about 10 times higher than that of the S2 constituent (Hagen et al. 2019a). The tidal waves develop in the Atlantic Ocean and propagate from the North-East Atlantic, as well as through the English Channel, into the North Sea, which results in the tidal waves occurring first in western regions before they propagate eastwards in a counter-clockwise rotation. Since the M2 phase is dominant, there are semi-diurnal tides with a period of 12  h and 25  min. Furthermore, the oscillations of the M2 tide create 3 amphidromic points in the North Sea: the first is located at the eastern tip of Norway, another at 56\(^\circ \)N, at the eastern tip of the Dogger Bank, and the third close to the entry of the Southern Bight (Otto et al. 1990).

Off the southern coastline in Lower Saxony are the East Frisian Islands Borkum, Juist, Norderney, Baltrum, Langeoog, Spiekeroog, and Wangerooge and some smaller Islands such as Minsener Oog and Mellum. Off the eastern coastline in Schleswig-Holstein are the North Frisian Islands Sylt, Föhr, Amrum, Pellworm, and the Halligen. The rivers Eider, Elbe, Weser, Jade, and Ems drain into the North Sea. The transitions of the Ems, Weser, and Elbe into the North Sea have characteristics of estuaries, and also represent important shipping lanes. Stratification in the German Bight shows high variability on different time scales (synoptic to inter-annual), leading to intra–tidal variability of the vertical current structure (Kopte et al. 2022).

The German Bight is characterised by the Wadden Sea, which is largely under nature protection and covers an area of 11,500  km\(^{2}\) (Federal Ministry for the Environment 2022). A characteristic of the Wadden Sea is a seabed exposure at low tide but again flooded at high tide. All these geographical features of the German coastal waters make the assessment of tidal energy resources significantly more complex than in places where relatively straight coastlines prevail. Figure 1A provides the location of the study site in the North Sea, Fig. 1B shows a satellite image of the German Bight. The study site is located at 6.367854\(^{\circ }\)/53.37165\(^{\circ }\) as southwestern boundary and 9.010586\(^{\circ }\)/55.102917\(^{\circ }\) as northeastern boundary in WGS84. The reference Coordinate System for the data is EPSG 25832: ETRS89/UTM Zone 32 (Plüß et al. 2020). All data in this work is visualised and further processed using the Open-Source Geographic information system QGIS v3.24.

Fig. 1
figure 1

Overview of the study site, with A the location of the German Bight in the North Sea, B satellite image of the German Bight from NASA Worldview (NASA 2023), as well as C the water depths and D the mean flood current velocities in Germanys 12 NM territorial sea. Administrative boundaries according to EuroGeographics (Geoportal of the European Commission 2023)

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2.2 The EasyGSH-DB North Sea Model

The assessment of the tidal energy resources requires a sound understanding and reliable data, in particular pertaining the the hydrodynamics of the area of interest. A numerical representation of the flow conditions and associated environmental parameters in the German Bight are provided by the EasyGSH-DB North Sea Model (Plüß et al. 2020). The model was built with the model family UnTRIM2 for tidal dynamics, SediMorph for bottom roughness estimation, and UnK for wave computation (Hagen et al. 2021a). Datasets of the tidal dynamics, salinity, and the sea state in the German Bight for the years 1996–2015 were modelled. With a grid resolution of up to 50  m in the focus area, local and regional effects can be reproduced highly detailed. In addition to the significant M2 and S2 tidal constituents, the K1, K2, L2, N2, O1, P1, and Q1 constituents were also considered in the harmonic analysis in EasyGSH-DB, so that the tidal dynamics are reproduced very accurately (for a stage 1 assessment, at least the M2 and S2 constituents are recommended (Legrand 2009)). Detailed information on the validation of the EasyGSH-DB North Sea Model can be found in (Hagen et al. 2019b).

EasyGSH-DB provides a wide range of hydrodynamic model results as GeoTIF-Files, inter alia, the following depth averaged ebb and flood current velocities in a 100  m grid (mean and maximum values) (Hagen et al. 2020):

  • Mean ebb current velocities

  • Mean flood current velocities

  • Maximum ebb current velocities (95% percentile)

  • Maximum flood current velocities (95% percentile)

To evaluate the tidal energy resource, the most recent data for the year 2015 will be used in this study; amplitudes of the inter-annual variations and their contribution to tidal flow velocities are deemed to be negligible (Hagen et al. 20212b). Information on the validation of the 2015 model results are available in (Hagen et al. 2019a). Bathymetry data are obtained from Sievers et al. (2020); however, in a 10  m grid, so that manual resampling is required (see below).

By way of example, Fig. 1C shows the water depths within Germany’s 12 nautical mile (NM) territorial sea. The area between the mainland and the Frisian islands is characterised by shallow waters (\(d < 10\)  m); parts of this area are even exposed at low tide (\(d > 0\)  m). In river mouths, e.g. in the estuaries of the Ems, Weser, and Elbe, as well as between the North and East Frisian Islands the water depths range between 15 and 25  m. The maximum water depth in the German Bight of about 58  m is reached southwest of Heligoland.

Figure 1D shows the mean flood current velocities within the 12 NM territorial sea in the German Bight. In the area exposed at low water, mean current velocities are mostly lower than 0.3 m s\(^{-1}\). Between the East Frisian Islands, mean current velocities range from 0.6 to 0.8 m s\(^{-1}\), in the Outer Jade up to 0.95 m s\(^{-1}\), and north and south of Sylt up to 1.0 m s\(^{-1}\). The highest current velocities are reached in the Elbe estuary near Cuxhaven with 1.0–1.1m s\(^{-1}\). The maximum current velocities in the Wadden Sea are less than 0.5 m s\(^{-1}\) during ebb current, rising to 0.75 m s\(^{-1}\) during flood current. Current velocities above 1.5 m s\(^{-1}\) are reached e.g. between the East Frisian islands Baltrum and Langeoog, north and south of Sylt, as well as in the estuaries of Ems and Weser. In the Outer Jade, current velocities of 1.75 m s\(^{-1}\) are exceeded. The maximum current velocities in the German Bight are reached in the Elbe estuary, where velocities of nearly 2.0 m s\(^{-1}\) are reached during ebb current, and even about 2.2 m s\(^{-1}\) during flood current.

2.2.1 Implementation in QGIS

Given the range of velocities across the ebb-tide cycle over time, it is convenient to define a mean of the ebb and flood velocities provided by the underlying database for subsequent computations; to that end, the available matrixes of the mean ebb and mean flood current velocities were averaged using the matrix computation tool of QGIS. For that, files provided by EasyGSH in GeoTIF format can be imported as raster data into QGIS.

For the computation of the theoretical resource, first, the raster data were cropped to the 12 nautical mile boundary using the polygon function. Then, all bathymetry values with a positive value were filtered out of the bathymetry grid so that they will not be included in the calculation. To filter specific values in QGIS, the command in Eq. (1) was used in the raster calculator:

$$\begin{aligned} \frac{((\text {`X'}>y)\cdot \text {`X'})}{((\text {`X'}>y)\cdot 1+(\text {`X'}\le y)\cdot 0)} \end{aligned}$$
(1)

In Eq. (1), \(\text {`X'}\) denotes the name of the raster whose values are to be filtered, and y denotes the value of the filter limit. For instance, from the bathymetry raster, all values larger than 0 can be filtered out using the command from using:

$$\begin{aligned} \frac{((\text {`Bathy'}<0)\cdot \text {`Bathy'})}{((\text {`Bathy'}<0)\cdot 1+(\text {`Bathy'}\ge 0)\cdot 0)} \end{aligned}$$
(2)

After filtering, the bathymetry was manually re-sampled to a 100  m grid to get the same grid resolution as the current velocities. In case of different raster resolutions, the QGIS raster calculator uses the nearest neighbor algorithm to re-sample the raster data which in many cases is not the optimal solution. With the QGIS operation r.resamp.stats, however, a re-sample with averaged values is possible. Then the theoretical power output can be calculated in the raster calculator twice: once for the mean ebb and once for the mean flood current velocities. The power output can be calculated for each individual grid cell. For the cumulative power output of all grid cells, the raster layer statistics was used, in which the sum of all values is given.

2.3 Implementation of the survey

A survey aimed at tidal energy technology developers is conducted and was performed as a means to specifically obtain data for our third objective. The survey was created using Limesurvey.Footnote 1 An invitation to the questionnaire was sent to tidal energy technology developers worldwide. To identify tidal energy technology developers, EMEC’s listing (EuropeanMarine Energy Centre 2022), as well as reports regarding the tidal energy market (e.g. Ocean Energy Technology Market Report 2018 (Magagna 2019)) were used to compile a representative list of stakeholders. A total of 91 tidal energy developers were identified, from which 36 companies could not be contacted due to a lack of information. A reminder was sent at two week intervals in case no response had been received. After two reminders, no further attempt was made. This resulted in 14 developers participating in the questionnaire. This work considers surveys that were originally collected from 31.05.2022 to 19.07.2022. However, the participation in the survey has remained possible since and can be accessed via (Korte et al. 2022).

The content of the survey can be divided into three parts. First, it is queried whether there is interest in the German North Sea as a project location, and which reasons there are for neglecting tidal energy projects in Germany. Second, the deployment conditions of the developers’ devices (TRL rating, required water depth, required mean annual current velocity) are identified to assess if deployment in the German North Sea is feasible. To the end, it is investigated, which aspects represent potential barriers in tidal energy projects, and how tidal energy technology might be further developed. For more information on the questionnaire, see (Korte et al. 2022).

3 Resource analysis

As introduced in Sect. 1.1, the tidal energy resource can be distinguished in a theoretical, technical, practical, accessible, and viable resource. In this study, however, only the theoretical, technical, and practical resource is considered in detail, since the assessment of Germany’s tidal energy resource is at an early stage and an accurate determination of the accessible and viable resource is outside the scope of this study (see Sect. 7).

The average power density (APD) in an oceanic cross-section is usually calculated according to Eq. (3) (Legrand 2009):

$$\begin{aligned} \hbox {APD} = \frac{1}{2} \cdot \rho _{\textrm{SW}} \cdot V_{\textrm{rmc}}^3 \end{aligned}$$
(3)

In Eq. (3), \(\rho _{\textrm{SW}}\) is the density of seawater (\(\rho _{\textrm{SW}}\) = 1025  kg m \(^{-3}\)) and \(V_{\textrm{rmc}}\) is the root of the mean cubed current velocity at a specific site, following

$$\begin{aligned} V_{\textrm{rmc}}=\root 3 \of {\sum _{i=1}^{N_B}(U_i^3 \cdot f(U_i))}, \end{aligned}$$
(4)

where the index i refers to the bin number (current velocity range), \(N_B\) is the total number of bins, \(U_i\) is the central velocity value of the ith bin, and \(f(U_i)\) is the percentage of time, that the velocity falls within each bin (Legrand 2009).

Since the root mean cubed velocity \(V_{\textrm{rmc}}\) is not provided in the hydrodynamic data of the EasyGSH-DB North Sea Model, the assessment in this study is performed using the simple mean current velocities, \(V_{\textrm{mean}}\) (see Sect. 6.1 for a discussion of the expected implications). Hence, Eq. (5) is used to determine the APD:

$$\begin{aligned} \hbox {APD} = \frac{1}{2} \cdot \rho _{\textrm{SW}} \cdot V_{\textrm{mean}}^3 \end{aligned}$$
(5)

3.1 Theoretical resource

In the calculation of the theoretical resource, the area under consideration is defined first. Although EasyGSH-DB provides data for the whole German Bight, the seaward boundary is set to the 12 NM territorial sea (see Fig. 1), thereby following O’Rourke et al. (2010). Furthermore, the potential for tidal energy generation further seaward is estimated to be comparatively low due to decreasing current velocities at increasing water depth (El Tawil et al. 2017). Apart from this, no further boundary conditions are introduced. The theoretical resource can therefore be considered as the amount of kinetic energy in the tidal wave that would be converted into electrical energy without losses. Due to the grid cell size of 100  m in the EasyGSH-DB North Sea Model, it is assumed that the energy is extracted at intervals of 100  m. Under these assumptions, the theoretical power output, \(P_{\textrm{mean},T}\), can be calculated using Eq. (6):

$$\begin{aligned} P_{\textrm{mean},T} = \frac{1}{2} \cdot \rho _{\textrm{SW}} \cdot \sum _{i=1}^n(A_{\textrm{cross}_i} \cdot V_{\textrm{mean},i}^3) \end{aligned}$$
(6)

In Eq. (6), \(A_{\textrm{cross},i}\) is the cross-section of the grid cell i (\(A_{\textrm{cross},i} = 100 \cdot h_i\), with \(h_i\) = water depth in grid cell i), \(V_{\textrm{mean},i}\) is the ebb and flood averaged mean current velocity in grid cell i (see Sect. 2.2), and n is the total number of grid cells.

The theoretical resource is then the theoretical power output over the duration of 1 year (8760  h). The accumulated power output is 61.36 GW. The theoretical resource resulting from the accumulated power is 537.51 TWh y\(^{-1}\).

Fig. 2
figure 2

Sites of interest for technical tidal energy resource in the 12 NM territorial sea

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3.2 Technical resource

The technical resource is the amount of energy that can be extracted from an area under technical constraints. The following constraints are considered in this study:

  • Tidal stream technology usually requires current velocities above 2.5 m s\(^{-1}\) (Lewis et al. 2015). However, the maximum current velocities occurring in the German Bight are up to 2.2 m s\(^{-1}\) (see Sect. 2.2). Based on the limitations imposed by O’Rourke et al. (2010) and Carballo et al. (2009), in this study, sites with maximum current velocities \(V_{\textrm{max}}>\) 1.5 m s\(^{-1}\) are considered suitable for tidal current turbine installation.

  • It is further assumed that a generic Horizontal Axis Turbine (HAT) with a rotor diameter of D = 3  m is deployed due to the relatively shallow waters in the Wadden Sea (see Sect. 2.2). A realistic device of such size is the Tocardo T1 turbine (HydroWing, Tocardo 2022).

  • In order for turbines with D = 3  m to safely operate, the minimum required water depth is defined as d = 4.5  m, thereby following recommendations by EMEC (Legrand 2009) and O’Rourke et al. (2010).

  • This study uses a device spacing of 10 D in flow-direction and 2.5 D in lateral direction (Legrand 2009).

  • For the MeyGen Phase 1A turbines, a power coefficient of only \(C_p = 0.41\) is reported (Black & Veatch 2020) and previous assessments assumed power coefficients ranging from 0.39 to 0.45 (El Tawil et al. 2017; O’Rourke et al. 2010; Cossu et al. 2021; Orhan et al. 2015). Hence, in this study, the power coefficient of a generic HAT is set to \(C_p = 0.4\).

Employing the above stated assumptions, the power output of the maximum available number of turbines in the German Bight is computed following

$$\begin{aligned} P_{\textrm{mean}} = C_p \cdot \frac{1}{2} \cdot \rho _{\textrm{SW}} \cdot n \cdot A_{\textrm{rotor}} \cdot V_{\textrm{mean}}^3, \end{aligned}$$
(7)

where \(C_p\) is the power coefficient, \(A_{\textrm{rotor}}\) is the cross-section of the turbine, and n is the number of deployed tidal current turbines following

$$\begin{aligned} n = \frac{A_{\textrm{Site}}}{10\,D \cdot 2.5\,D}, \end{aligned}$$
(8)

where \(A_{\textrm{Site}}\) is the area size of the considered site, and \(10\,D \cdot 2.5\,D\) is the device spacing, which is simplified as the area occupied by one turbine.

Ten sites along the German coast are considered suitable for tidal energy extraction, which are depicted in Fig. 2. Table 2 shows the characteristics of these sites.

The technical resource for the whole German Bight is estimated at 2468 GWh y\(^{-1}\), resulting from a total power output of 281 MW. The prevailing mean current velocities at the individual sites range from 0.77 to 0.92 m s\(^{-1}\). The largest potential can be found in the Outer Elbe (Site 7) with a potential of about 176.84 MW (1550 GWh y\(^{-1}\)), followed by the Outer Jade (Site 4) with a potential of 52.74 MW (462 GWh y\(^{-1}\)).

Table 2 Characteristics of individual sites of interest as depicted in Fig. 2
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3.3 Practical resource

For the assessment of the theoretical and technical resource, no boundary conditions regarding the current use practise of the available marine space in the German 12NM territorial sea were introduced. To assess the practical resource, it is next screened whether the sites of interest, identified in the technical resource assessment, are located in military zones, shipping lanes, nature reserves, or if there is already existing infrastructure like pipelines. As a data base for existing infrastructure and ship traffic in the German Bight, the GeoSeaPortal from the Federal Maritime and Hydrographic Agency is used exclusively (Federal Maritime and Hydrographic Agency 2022). No further data were considered, however, the chosen database is federally maintained and thus this work assumes that all relevant use purposes are accurately noted.

  • Military zones No military zones are located in the 12NM territorial sea.

  • Nature reserves As the German coastal sea consists to a large extent of mud flats, a large part of the coastal areas have been declared nature reserve “Wadden Sea National Park”, and are under nature protection. Coles et al. (2021), however, discuss the environmental impact of tidal current turbines, which were found to be an order of magnitude less than the environmental impacts of climate change. For the present stage 1 assessment, the practical resource is thus computed under the assumption that a permit to deploy tidal current turbines in the Wadden Sea National Park may eventually be obtained.

  • Shipping lanes The extent to which tidal turbines can be installed in locations with ship traffic is not assessed within the scope of this study. As long as a sufficient clearance above the turbine is maintained, a deployment of tidal current turbines might be possible. However, the installation of turbines, as well as its maintenance, will likely interfere with ship traffic. Important shipping lanes are the Ems estuary (Site 1), the Outer Weser (Site 5 and 6) and Elbe estuary (Site 7), the ship traffic density exceeds partly 150 vessels per day and   km\(^{2}\) (Federal Maritime and Hydrographic Agency 2022). Thus, for the estimation of the practical resource these sites are excluded. Another site with lower ship traffic is the Outer Jade (Site 4). Ferry routes are located at Site 9 (Sylt South) and Site 10 (Sylt North).

  • Pipelines A deployment of turbines near pipelines is also theoretically possible, e.g., using a floating support structure (Orbital Marine Power 2022). However, the extent to which maintenance work on the pipelines will be affected by a nearby turbine deployment is not investigated within the scope of this study. The pipeline Europipe runs through Site 2 (Borkum).

The above outlined use purposes may eventually limit or omit the installation or operation of tidal turbines in the identified regions. However, uncertainties exist as to how severe a use purpose may eventually affect a tidal turbine to operate. To that end, and to estimate the effect of these uncertainties regarding the feasibility to deploy and operate tidal current turbines in areas with existing marine infrastructure, three different scenarios are considered in this work:

  • Scenario 1 “weak restrictions”: The practical resource is calculated assuming that it is possible to deploy tidal current turbines in the area of the pipeline east of Baltrum (Site 2), in the Outer Jade (Site 4) and in the fairways between Sylt - Amrum (Site 9) and Sylt - Rømø (Site 10).

  • Scenario 2 “moderate restrictions”: The practical resource is calculated assuming that it is possible to deploy tidal current turbines in the area of the pipeline east of Baltrum (Site 2), and in the fairway between Sylt - Amrum (Site 9), but not in the Outer Jade (Site 4) and in the fairway between Sylt - Rømø (Site 10).

  • Scenario 3 “strong restrictions”: The practical resource is calculated assuming that it is not possible to deploy tidal current turbines in the area of the pipeline east of Baltrum (Site 2), in the Outer Jade (Site 4) and in the fairways between Sylt - Amrum (Site 9) and Sylt - Rømø (Site 10).

Fig. 3
figure 3

Installed tidal stream capacity from 2010 to 2021 (adapted from Ocean Energy Europe (2021b))

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In each scenario, the economically important shipping lanes of Ems, Weser and Elbe (Site 1, 5, 6, 7) are excluded entirely. Table 3 provides an overview of the resulting mean power for the three different scenarios described above.

The practical tidal energy resource in Germany is estimated at 565.8 GWh y\(^{-1}\) for Scenario 1, 98.6 GWh y\(^{-1}\) for Scenario 2, and 66.6 GWh y\(^{-1}\) for Scenario 3. This highlights that the practical tidal energy resource in Germany is highly dependent on the criteria by which individual sites are considered as suitable. Especially the feasibility to deploy tidal current turbines in the Outer Jade (Site 4) has a significant impact on the practical resource (about 462 GWh y\(^{-1}\)). Therefore, it is worthwhile to examine more closely the extent to which the deployment of tidal current turbines is feasible at each location.

Table 3 Sites included depending on the scenario and the resulting total power output
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4 Technology analysis

Figure 3 shows the annual and cumulative capacity of installed tidal stream capacity for Europe and the rest of the world. Up until 2021, a worldwide installed tidal stream capacity of 39.6 MW can be observed, which results from various small plants, many of which are deployed at the EMEC (Coles et al. 2021). The installation of MeyGen Phase 1 in 2016, with a capacity of 6 MW consisting of HAT by Andritz Hydro Hammerfest and SIMEC Atlantis, played a particularly large role. However, compared to other energy technologies, such as wind or solar energy, tidal energy technology is still rather in its infancy: In 2021, the global cumulative installed capacity of wind energy reached 837  GW, and that of solar energy 940  GW (Global Wind Energy Council 2022; SolarPower Europe 2022). The expansion of tidal stream energy thus amounts to just about 0.00005% of the expansion of wind energy in 2021.

EMEC distinguishes between six main types of tidal current turbines (European Marine Energy Centre 2022). Descriptions of the operating principles are based on (European Marine Energy Centre 2022; TETHYS 2022).

  1. a)

    Horizontal Axis Turbine. The tidal stream causes the rotation of the rotor around the horizontal axis, similar to the principle of wind turbines.

  2. b)

    Vertical Axis Turbine. The principle of power generation is similar to that of the HAT; however, in this case, the turbine is mounted on a vertical axis.

  3. c)

    Venturi. The principle of power generation is similar to that of a HAT or VAT. In this design, the turbine is placed in a duct, in which the tidal current velocity increases through the Venturi effect. The turbine can be mounted in axial (horizontal or vertical) or cross-flow direction.

  4. d)

    Tidal Kite. The kite construction, which consists of a hydrokinetic wing with a turbine attached to it, is tethered to the sea bed. As soon as the wing is lifted through the flow, the kite flies eight-shape loops through the water, which increases the current velocity at the turbine.

  5. e)

    Oscillating Hydrofoil. The Hydrofoil is attached to an oscillating arm. The tidal stream flowing on both sides of the hydrofoil causes the motion of the hydrofoil, which in turn can be used for power generation.

  6. f)

    Archimedes Screw. The device consists of a helical surface, which surrounds a cylindrical shaft. The tidal stream flowing through the spiral causes a rotation of the shaft, which is driving a generator.

Fig. 4
figure 4

Examples for the main types of tidal current turbines. A AR 1500 by SIMEC Atlantis; B OceanQuest turbine by HydroQuest; C Open Centre Turbine by OpenHydro; D DG100 by Minesto; E EEL tidal turbine by EEL Energy; F Bay of Fundy 2 MW tidal turbine by Jupiter Hydro

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4.1 Technology maturity and trends

The Technology Readiness Level (TRL) is a common measure to assess technology maturity. While TRLs 1–3 correspond to the early stages of technology (basic principles, proof of concept), TRL 4–5 correspond to technology validation in laboratory and relevant environment. TRL 6–9 reach from prototype demonstration to system proving in operational environment (Day et al. 2015).

According to the 2018 Ocean Energy Technology Market Report (Magagna 2019), 59% of tidal energy developers are based in Europe, for the most part in the UK, the Netherlands, and France. The largest developers outside Europe are located in Canada and the USA. There are 43 developers with a TRL > 5 and 15 developers with a TRL > 7, of which 13 developers are located in Europe. Only 4 developers have a TRL rating > 8 (Andritz Hydro Hammerfest, Orbital, SIMEC Atlantis, Nova Innovation) (Magagna 2019). A wide range of mature tidal energy technology is therefore not yet available on the world market at the present time.

Figure 4 provides practical examples for each device type. The technology maturity can be clearly seen from the conditions under which the different devices have been tested to date. Information on each tidal energy project presented in Fig. 4 is provided below. Unless otherwise indicated, TRL ratings are based on data from the Ocean Energy Technology market report from 2019 (Magagna 2019).

  • SIMEC Atlantis AR1500 - TRL 9 One AR1500 turbine by SIMEC Atlantis, rated at 1.5 MW, is installed in the MeyGen Phase 1A in Scotland’s Pentland Firth since March 2018. Besides the AR1500, three AH1000 MK1 by Andritz Hydro Hammerfest are deployed in MeyGen Phase 1A, resulting in a 6 MW array. The four turbines have a rotor diameter of 18  m, operate in water depths of 34  m to 36  m, and the power coefficient was found to be \(C_p\) = 0.41. The lifetime energy yield from the four turbines is expected to amount to 450  GWh over a duration of 25 years (Black & Veatch 2020).

  • HydroQuest OceanQuest - TRL 7 From 2019 to 2021, the 1 MW OceanQuest turbine has been deployed as a demonstration project at the test site of Paimpol-Brehat (Brittany). As a next step, Hydroquest plans to deploy seven next generation turbines with a total capacity of 17.5 MW in the FloWatt project (HydroQuest 2022). The new HydroQuest turbine rated at 2.5 MW has a width of 26  m and a height of 21  m, and is designed for water depths of 35  m (HydroQuest 2022). No TRL rating is provided by Magagna (2019), but the technology has potentially already reached TRL 7, due to the tests under real life conditions.

    Table 4 Characteristics of selected tidal stream devices, sorted from large-scale to small-scale devices
    Full size table

  • Minesto DG100 - TRL 7 Two units of the DG100 by Minesto, rated at 100  kW each, have been installed in the Vestmannasund project in the Faroe Islands, representing the first tidal kites delivering power to grid in December 2020. The eight-shape loop of a DG100 has a dimension of 50  m width and 17  m height. The tether connecting the kite with the seabed is 35  m long and the devices are operating in a water depth of about 50  m (Minesto 2022).

  • EEL tidal turbineFootnote 2- TRL 6 After testing several prototypes, a commercial-scale 50  kW EEL tidal turbine has been tested in the Port of Brest, France in May 2022. In the range of products, the smallest EEL tidal turbine has a membrane size of 3\(\times \)3   m, requires a minimum water depth of 2  m, and is rated at 5  kW; the largest device has a membrane size of 15\(\times \)22   m, requires a minimum water depth of 15  m, and is rated at 1 MW (EEL Energy 2022).

  • OpenHydro - TRL 7 OpenHydro was the first developer deploying a tidal turbine at an EMEC test site in 2006. The first test rig had a generating capacity of 250  kW and was the first tidal turbine generating electricity to UK’s national grid. This was followed by larger demonstration projects, e.g. in Canada and France. After the 7th generation of OpenHydro with a rotor diameter of 6  m was installed in 2014, Naval Energies, parent company of OpenHydro, decided to liquidate OpenHydro.

  • Jupiter Hydro 2 MW tidal turbine - TRL 7 Jupiter Hydro started working on its Archimedes Screw in 2011 and went through several open water tests in 2013 and 2014. Jupiter has a 2 MW Power Purchase Agreement issued by the Nova Scotia Department of Energy and Mines, and aims to deploy the TEC device in the Minas Channel, Bay of Fundy in early 2024. The unit consists of four screws with a diameter of 7.26  m which are placed at a 30\(^{\circ }\) angle to the flow (Jupiter Hydro Inc. 2022).

Table 4 provides an overview of currently available tidal stream technology, for which relevant data is publicly accessible. It can be seen that there is a wide range within the technology in terms of rated power. While large scale devices in the 1  MW to 2 MW range are equipped with large rotors (> 10   m) and, therefore, require relatively large water depths, Tocardo and Nova Innovation also offer turbines in the medium scale range with 100  kW to 250 kW, which have smaller rotor diameters, theoretically resulting in smaller required water depths, and also provide power at lower flow rates. The T-1 and T-2 from Tocardo offer the special feature that the rotor diameter and the rated power can be varied, which allows a versatile use of the turbine depending on site-specific conditions. In the small scale range < 100 kW, the P66 and the EEL Tidal Turbine are the smallest devices available, which can even operate in particularly shallow waters. Since the average water depth at the sites of interest in the German Bight is between 13.7  m to 21.6  m (see Table 2), small-scale or medium-scale turbines would be more suitable for locations in the German Bight in order to provide enough top and bottom clearances at the sites of interest.

The 2018 Ocean Energy Technology Market Report by JRC (Magagna 2019) stated that a massive expansion of tidal energy technology is expected in the next years. For 2023, an additional capacity of more than 700 MW was planned (Magagna 2019). However, due to the COVID pandemic, tidal energy expansion stalled in 2020, but recovered to 2019 levels in 2021 (see Fig. 3) (Ocean Energy Europe 2021a, b). Especially MeyGen Phase 2, where the installation of an additional 80 MW tidal stream capacity is planned, can be considered a significant step for tidal energy technology. The expected increase in the coming years might also have a significant impact on the cost of tidal energy technologies, which would make tidal energy technology more competitive with other energy technologies.

4.2 Levelised cost of energy

The LCOE is a common measure for the techno-economic assessment of energy technologies. It compares the costs incurred over the entire life cycle of the project with the energy yield over the entire life cycle. According to Vanegas-Cantarero et al. (2022), the LCOE can be calculated as follows:

$$\begin{aligned} \text {LCOE} = \frac{\sum _{t=1}^m \frac{I_t+A_t+\hbox {DC}_{m+1}}{(1+r)^t}}{\sum _{t=1}^{m} \frac{\text {AEP}_t}{(1+r)^t}}, \end{aligned}$$
(9)

where \(I_t\) denotes the capital expenses (CapEx) in year t, \(A_t\) denotes the operational expenses (OpEx) in year t, DC are the decomissioning costs at the end of the project’s lifetime (assumed as one year after the end of the lifetime), r represents the discounts rate, \(\text {AEP}_t\) is the generated power in year t, and m is the total economic lifetime of the project (Vanegas-Cantarero et al. 2022).

In MeyGen Phase 1A, for example, CapEx include turbine development, onshore balance of plant, offshore works, substructures, cabling, project initiation & management, as well as insurance. In total, the CapEx are quantified at £ 51.3 million. OpEx include Lease & insurance, unplanned & planned maintenance, spare parts, onshore inspection & maintenance, cost for operating teams, decommissioning, offshore inspection & maintenance, corporate operations, and equipment purchase. In total, the OpEx are quantified at £ 1.4 million per year (Black & Veatch 2020).

Vazquez and Iglesias (2015), in their study on LCOE mapping for tidal stream energy, provide a new geospatial tool for tidal stream energy. Their results highlight the potentially more significant role of CapEx and OpEx on the LCOE than the power coefficient. The same authors (Vazquez and Iglesias 2016) highlight drawbacks of existing models for the estimation of the CapEx. To overcome this, the authors develop a new formation of the estimation of these costs, considering specifically tidal stream resource, water depth (thus rotor size), or distance to the shore (thereby cable length). A further in-depth discussion of the LCOE of tidal energy can be found in (Greaves et al. 2018, Chap. 11).

4.2.1 Current LCOE of energy technologies

No tidal energy projects have been implemented in Germany so far so that no LCOE data is available for German sites; therefore, data from the UK is referred to. Currently, the LCOE for tidal stream technology is approximately 240  £/MWh (Coles et al. 2021), which corresponds to 28.36  € cent/kW h.Footnote 3 With that, the current LCOE is not in line with the European Union’s targets for reducing the LCOE for tidal energy projects: The EU’s Strategic Energy Technology Plan (SET-Plan), adopted in 2008 to establish an energy technology policy for Europe, aims to maintain EU leadership in low-carbon energy technologies and to realise a low-carbon economy by 2050 (European Commission 2022b). With respect to ocean energy, the objective is to deploy 100 MW of ocean energy capacity (wave and tidal) by 2025 and to reduce the LCOE for tidal energy to 15  € cent/kW h by 2025 (and for wave energy to 15  € cent/kW h by 2030, respectively) (Implementation Working Group Ocean Energy 2021).

The Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) regularly publishes data on the LCOE of energy technologies in Germany. Figure 5 shows the LCOE for renewable energies and conventional power plants in Germany for the year 2021 (Kost et al. 2021).

Comparing the current LCOE of tidal stream energy in the UK to those of the energy technologies included in Fig. 5, it can be seen that the LCOE of tidal stream energy is significantly higher than any other renewable energies and conventional power plants in Germany. Offshore wind energy, for example, falls in the range of about 8  € cent/kW h to 12  € cent/kWh, rendering it much more favourable at this point. However, tidal energy technology is not yet as advanced as the other energy technologies. Nevertheless, progress has already been made: Installation of the first 8 MW of tidal stream energy in the UK has already reduced the LCOE by 25%, which is comparable to the reduction in LCOE for onshore wind between 1985 and 1990 (IRENA 2018).

Fig. 5
figure 5

Current LCOE of other energy technologies according to Fraunhofer ISE (Kost et al. 2021) and the current LCOE of tidal stream (dashed line) according to Coles et al. (2021)

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4.2.2 LCOE projections

Future LCOE projections are usually based on a technology learning rate, which expresses the percentage by which costs are reduced when the total installed capacity doubles (Coles et al. 2021). According to Offshore Renewable Energy Catapult (OREC) (ORE Catapult 2018), the near term cost reduction potential for tidal stream energy depends upon three aspects: Initial accelerated reductions, Learning by doing & Innovation, and Costs of capital:

  • Initial accelerated reductions These include volume effects, e.g., lower production costs per unit for mass production. Economies of scale, which represent the size of the installed array, are also taken into account here. In the case of a tidal current turbine, this could be achieved, for example, by increasing the rotor diameter (Orbital O2) or optimising the hub height of the turbine, as reported by Coles et al. (2021).

  • Learning by doing & Innovation Over time, processes are better understood and procedures are optimised as a result, which has historically been observed in other industries, but also at EMEC tidal test sites. This includes, for example, developing a strategy for O &M measures to maximise turbine uptime or optimising processes in the supply chain. Also, innovation, e.g. improved structures and moorings, as well as reduced offshore operational costs, can reduce the costs (ORE Catapult 2018).

  • Cost of Capital Funding for tidal energy projects in the UK is currently provided through grant support and private finance. However, due to increasing technology maturity and the resulting lower financial risks, part of the project could be financed through commercial loans, as in the case of offshore wind energy (ORE Catapult 2018).

LCOE projections for tidal stream energy are provided by ORE Catapult (2018) and Coles et al. (2021), among others. The LCOE projections are based on different technology learning rates ranging from 9 to 26%. For example, OREC’s LCOE projection (ORE Catapult 2018) is based on a technology learning rate of about 26% in the early stages of development; however, drops to about 15% above a cumulative installed capacity of 100 MW (Coles et al. 2021). It is estimated that the LCOE drop to 17.7  € cent/kWh at 100 MW, 15.4  € cent/kWh at 200 MW and 10.6  € cent/kWh at 1  GW. According to OREC, a capacity of 1  GW in the UK could be achieved with the right policy support by 2030, based on an annual expansion of 100 MW (ORE Catapult 2018). Coles et al. (2021), however, stated that OREC’s projection is unattainable if tidal energy projects depend on subsidies that will only allow expansion on this scale from 2026 onwards. Rather, a cumulative capacity of 160 MW in the UK could be expected by 2031; with expected LCOE of 21.5  € cent/kWh, 17.7  € cent/kWh, and 11.3  € cent/kWh for technology learning rates of 9%, 17% and 25%, respectively (Coles et al. 2021).

LCOE projections for other energy technologies in Germany are provided by Fraunhofer ISE (Kost et al. 2021). The LCOE for biogas and solid biomass are expected to remain constant (referred to Fig. 5). In the near future, tidal stream energy in the UK is not expected to come into a similar LCOE range as wind and solar energy in Germany since a LCOE reduction of these technologies is also expected. However, by 2031, the costs for tidal stream energy in the UK could fall into a similar range as for biogas and solid biomass in Germany, where the LCOE is expected to remain constant (referred to Fig. 5) (Kost et al. 2021). Tidal stream energy could also achieve lower LCOE than gas turbines or combined cycle gas turbine (CCGT) in Germany, where the LCOE are expected to increase, when the tidal stream energy capacity increases up to 1  GW.

Fig. 6
figure 6

Status of planned tidal energy projects in Germany

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In this context, the LCOE data from tidal stream energy in the UK may not be transferable to Germany. For more information on the extent to which these data can be transferred to Germany, see Sect. 6.2.

5 Stakeholder analysis

On the basis of the resource analysis in Sect. 3 and the present knowledge gained in the technology analysis in Sect. 4, a stakeholder analysis was conducted in the framework of this work. The current status of planned tidal energy projects in Germany is addressed, as well as reasons for and against tidal energy projects in Germany. An overview of the characteristics of the developers’ TEC devices serves to assess which TEC devices would be suitable for the conditions in the German Bight, or low-potential sites in general. Furthermore, the analysis indicates which aspects are of concern in planning tidal energy projects, and in which way the developers would like to further evolve tidal energy technology.

5.1 Germany as site for tidal energy projects?

Project sites of the participating developers are the UK (10x), Canada (5x), Netherlands (3x), France (2x), USA (2x), Indonesia (2x), Philippines (1x), Nepal (1x), Chile (1x), Channel Islands (1x), Sweden (1x), Norway (1x), Belgium (1x), and potentially Germany (1x). When asked whether a tidal energy project in Germany had ever been considered (see Fig. 6), two developers stated that there are currently considerations and one developer has discontinued the efforts. Most developers have not yet considered projects in Germany. The reasons for neglecting tidal energy projects in Germany are mainly site-specific conditions, i.e. the range of occurring current velocities, and the lack of political support, i.e. missing public funding schemes. Among other reasons, developers named, for instance, the prioritised focus on other projects, no knowledge about the tidal currents in Germany, or the assumption of a low potential in Germany. Also, it was mentioned that until now no appropriate partners were met with whom the further development of the technology is possible, and that there is no great attraction due to the geographical location, the potential of harnessing tidal energy in Germany, and the research that has been carried out.

5.2 What conditions are needed for deployment?

Table 5 lists the device type of the developers, how they would rate their current TRL, which minimum water depths are required to install their technology, and at which mean annual current velocity they would classify a site as viable. In summary, the developers’ technology fall, with one exception, in a TRL range of 6–9. Among the participants of the survey, there are five developers offering technology for shallow water depths (< 3  m). However, especially those devices that can operate in shallow water are not yet fully developed, but are in a TRL range from 6 to 7. Especially VATs and types other than the HATs are considered suitable for shallower water depths, which could be explained by the fact that the rotor diameter is not decisive for the minimum water depth. Since the average water depths at the sites of interest in the German Bight are between 13.7 and 21.6 m (see Table 2), larger devices that require water depths of 7–18  m could also be considered. However, the extent to which such larger devices can be deployed at the sites would need to be investigated in a subsequent assessment. Furthermore, four developers would classify the mean annual current velocities occurring at Germany’s sites of interest as viable (0.77–0.89 m s\(^{-1}\), see Table 2). The missing values for the required mean current velocity in Table 5 are due to the fact that for Developer #13 it is not usual to classify a site as viable using the mean annual current velocity, but to differentiate between low, medium, and high energy sites. Medium (current velocities between 1.5 and 2.1 m s\(^{-1}\)) and high (between 2.1 and 2.5 m s\(^{-1}\)) energy sites are targeted. Developer #8, on the other hand, mentions that the slack period duration between ebb and flood is of big importance, which is the reason why they classify sites as viable with consistent current velocities between 1.7 and 5.0 m s\(^{-1}\).

Table 5 Information on technologies provided by stakeholder questionnaire
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5.3 What are potential barriers and future trends for tidal energy technology?

Figure 7 provides an overview of the importance of economic concerns, the impact to the environment, political support, social support, and site-specific conditions when planning tidal energy projects, as judged by the developers. As the distribution of votes for importance of political support and importance of site-specific conditions turned out equal, they were combined in one chart (see Fig. 7c). In most cases, economic concerns are important to very important, which is not unexpected when considering that a lot of effort has gone into R &D of the technology over several years, and companies are fundamentally dependent on economic returns. The impact to the environment is also found to be a significant aspect when planning tidal energy projects. Political support, i.e. public funding schemes, is, besides site-specific conditions, the aspect most often rated as very important by developers. Site-specific conditions, such as soil conditions, sediment transport, interference with other marine infrastructure, water depth, and current velocities are particularly relevant in project development at any site. Social acceptance and support of the planned projects is also considered relevant, though it is not rated as important as the previously mentioned aspects.

Fig. 7
figure 7

Importance of a economic concerns, b impact on the environment, c political support and site-specific conditions, and d social support in tidal energy projects

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Fig. 8
figure 8

Importance for developers to enable the use of tidal energy technology in a shallower water depths, b deeper water depths, c in areas with lower current velocities, and d in areas with larger current velocities

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When asked to name other moderately important to very important aspects, the developers mentioned the seabed conditions (relevant for the anchoring of the devices), the wave characteristics, the currently existing grid infrastructure, the proximity of the site to the shore, a fair financial risk management for funding new innovative technologies, the availability of personnel and suitable vessels (crane vessels as a huge cost driver), governmental regulations, as well as support to find private investors, and the allocation of test sites for companies. Further suggestions include an early certification for tidal energy applications, the implementation of feed-in tariffs, the combination of the power output from offshore wind, TEC devices and wave energy converter to reduce transmission costs, and the research of production and storage of hydrogen at the site for power balancing and energy expansion. Criticism was expressed with regard to the lack of governmental will to support the tidal energy industry with grants and the reluctance to try new principles based devices.

Finally, it was examined in which way the developers want to drive the technology in the future. With regard to the German Bight as a potential location for tidal energy extraction, it is particularly interesting to see whether further development is planned towards enabling the deployment in shallower water depths and at locations with lower tidal current velocities than currently optimal. Figure 8 shows the results of the question, how important it is to the developers to enable the use of their technology in shallower and deeper water depths, as well as in areas with lower and larger current velocities. A trend can be identified in which the developers want to enable the use of their technology in shallow water, taking into account that, among those who disagree with the statement, there are two developers whose technology can already be used in shallow water. With regard to enabling the technology to be used in deeper waters, the votes are relatively evenly distributed and no trend is apparent. Regarding current velocities, a trend towards enabling the use of tidal energy technology in areas with lower current velocities is clearly visible. Vice versa, there is also a positive tendency to a use in areas with higher current velocities. Overall, it can be seen that, in most cases, the developers want to enable the most versatile use of their technology possible.

6 Discussion

6.1 Germany’s tidal energy resource

The tidal energy resource in the German Bight was iteratively determined using data from the EasyGSH-DB North Sea model. While the technical tidal energy resource in Germany is estimated at 2468 GWh y\(^{-1}\), the practical resource falls in a range between 66.6 and 565.8 GWh y\(^{-1}\), depending on the scenario. The three scenarios highlight particularly the dependence on the feasibility to deploy tidal current turbines in shipping lanes. With respect to the total energy consumption of 595 TWh in Germany in 2018 addressed in Sect. 1, tidal stream energy could only make a minor contribution, even under Scenario 1: weak restrictions (about 0.01% of the total power consumption). Given the projected increase in total power consumption to 658 TWh in 2030, the share of tidal energy would become even less. Nevertheless, with the deployment of tidal current turbines in the German North Sea, power could be generated for more than 21,200 households annually, even under Scenario 3: strong restrictions (assuming a household power consumption of annual 3100  kWh, which represents the mean power consumption of households in Germany in 2019 (Federal Statistical Office of Germany 2021)). However, in international comparison, Germany’s practical tidal energy resource is found to be rather small. For example, for the Alas Strait, Indonesia, the practical resource is estimated at up to 640 GWh y\(^{-1}\) (Blunden et al. 2013), for Ireland at 2.6 TWh y\(^{-1}\) (O’Rourke et al. 2010), and for the UK at 34 TWh y\(^{-1}\) (Coles et al. 2021).

In the present stage 1 assessment, some important aspects regarding turbine deployment could not be considered. Also, the EasyGSH-DB North Sea Model was not primarily developed to conduct a tidal energy resource assessment. Simplifications were made which result in uncertainties. Thus, potential sources of error in this study are listed below, which should be addressed to improve the accuracy of a subsequent assessment.

  • Current Velocities The root mean cubed velocity \(V_{\textrm{rmc}}\) is not provided by the EasyGSH-DB North Sea Model and could therefore not be for used the calculation of the APD. Instead, the resource is calculated using the mean current velocity \(V_{\textrm{mean}}\), representing a slightly less accurate way in calculating the APD. Using \(V_{\textrm{mean}}\) results in an underestimation of the resource, since in particular the cubic influence of high current velocities in \(V_{\textrm{rmc}}\) is neglected in \(V_{\textrm{mean}}\). For instance, Polagye and Thomson (2013) show the correlation of the cumulative probability density functions of power (therefore, power generation) and velocity, highlighting that in their example at the Admiralty Inlet, Puget Sound, WA, 50% of the power is generated for current velocities >2 m s\(^{-1}\), which occur 10% of the time, in turn, underscoring the importance of considering the velocity distribution, rather than simply the mean value. A subsequent site-specific assessment should take \(V_{\textrm{rmc}}\) into account, which could be derived from the time series of the hydrodynamic model.

  • Averaging the current velocities The current velocities were averaged over the entire grid at each site. This procedure disregards the distribution of velocities at the site and underestimates the effects of higher current velocities on the power output. A more precise approach would be to determine the power output for each grid cell and then sum it up for the entire site. However, this approach would also be more complex, which is why this simplification was employed in this initial site screening assessment.

  • Number of turbines The number of turbines envisaged is relatively high (see Table 2), which stems from the fact that the prevailing mean current velocities in the German North Sea are comparatively low, the assumed rotor diameter of D = 3  m is relatively small, and the turbine spacing potentially non-optimal. However, the mean water depths at the individual sites range from 11.1 to 21.6  m, and the minimum required water depth of 4.5  m is only approximately reached at Site 10 (see Table 2). Thus, the deployment of turbines with a larger rotor diameter might be feasible. It is therefore recommended to investigate the water depths at each individual location more accurately using rotor diameters that suit the specific conditions at each site. Similarly, the device spacing shall be investigated in more detail. While the considered assumption by Legrand (2009) are established for generic turbines, the required spacing for, e.g., optimal turbulence diffusion, flow restoration, and angle of attack on the blades may be influenced by the turbine characteristics and the local flow conditions. Sub-optimal spacing may lead to significant drops in turbine efficiency, thus, power output of the turbine array (Thiébot et al. 2020). It is likely that larger spacing compared to the herein considered 10 D and 2.5 D are required. Thus, considering the above stated, the number of turbines can be minimised while the power output of single turbines is increased.

  • Further restrictions It is assumed that the entire site can be equipped with tidal current turbines as long as the conditions of both minimum current velocity and minimum water depth are met. However, there are practically other constraints, such as the availability of the turbines and the required vessels, the O &M costs, the connection to the existing power grid, etc. As a result of this simplification, the number of turbines that can in fact be installed might be overestimated.

  • Impact of turbines on flow In the EasyGSH-DB North Sea Model, the influence of tidal current turbines on the flow was not considered. Information on numerical modelling of tidal current turbines can be found in (Day et al. 2015). However, most typically, follow-up studies with a more specific economic interest in specific sites will lead to detailed investigations of fluid–structure-interaction and these will then reveal a richer set of information for the estimated power.

Aspects that were neglected in the resource analysis but need to be taken into account in a future assessment are the current direction, which is necessary to assess whether a HAT with a yaw system is needed or a bidirectional fixed axis is sufficient. Also, the hub height of the turbine has an impact on the power output since the current velocity is not constant along the water column but can rather be estimated using a power law (Legrand 2009). Furthermore, considerations regarding the support structure, as well as regarding the power transmission, are necessary. Finally, the status of the Wadden Sea as a nature reserve needs to be included in more detail. In the framework of the present resource analysis it was assumed that a permission to deploy turbines in the Wadden Sea is obtained. The Wadden Sea National Parks in Germany are designated as biosphere reserves, which in turn are divided into three different zones: Core areas, Buffer zones and Transition areas (Nationalpark Wattenmeer 2022). While the deployment of tidal current turbines in core zones could be complicated, it could be conceivable in buffer zones (if the project is designed in an environmentally responsible way), as well as in transition zones (human activities are not restricted). Hence, for subsequent studies, it is recommended to investigate the sites in terms of which zone they fall into.

6.2 Tidal energy technology

The analysis of technologies has shown that tidal energy technology is still in it’s infancy when compared to other energy technologies, but has nevertheless further matured in recent years and there are now proven systems suitable for larger-scale deployment, as planned for MeyGen Phase 2. A significant increase in installed capacity is expected in the coming years, which is expected to further reduce the LCOE. From an installed capacity of 1  GW, the LCOE of tidal energy in the UK is estimated to be between 9.5 € cent/kWh to 16.5  € cent/kWh, depending on the projection, which would put the LCOE of tidal energy in a similar range as biogas in Germany. It is debatable to what extent the LCOE data from the UK can be used for Germany. In the UK, many turbines have been tested in recent years and learning effects have been achieved. In Germany, on the other hand, tidal current turbines have not yet been deployed. Nevertheless, it could be expected that especially through learning by doing & innovation in other projects, processes will be better understood and procedures optimised by the developers, which could reduce costs independent from the project site. Learning effects could also be drawn from the economies of scale (Li et al. 2017), and since the deployment of tidal current turbines is expected to increase worldwide, the production costs per turbine probably decrease. However, there are uncertainties regarding project financing in Germany. Furthermore, as it is not expected that a large-scale array will be deployed in the German Bight immediately, but that several tests will presumably be carried out first, the LCOE could initially be higher than in the UK. Over time, however, cost-reducing factors would also set in. Tidal energy is not expected to compete with the LCOE of key technologies, such as offshore wind and solar energy in the future. However, diversification of energy technologies is essential to enable a stable power supply. In this context, tidal stream energy represents a reliable energy technology due to the predictability of the tides.

6.3 Tidal energy projects in Germany

The Stakeholder Analysis has shown that, surprisingly, there are several tidal energy developers who also design devices for use in shallow water. The participants of the conducted survey classify sites as economically viable where mean annual current velocities of 0.5–2.0 m s\(^{-1}\) are exceeded. Mean current velocities at Germany’s sites of interest fall in a range between 0.77 and 0.89 m s\(^{-1}\) and would therefore be suitable for tidal energy projects. As Table 2 has shown, the mean water depths at Germany’s sites of interest range from 14  m to more than 21  m, and might therefore be also suitable for devices with larger rotor diameters. Furthermore, there are two developers currently considering tidal energy projects in Germany, and that one developer had considerations in the past. One of the main reason for neglecting tidal energy projects in Germany are site-specific conditions (more precisely the current velocities). However, since developers aim to enable the use of their technology in areas with lower current velocities than currently, the German North Sea could become a suitable project site in the future. Another main reason for neglecting tidal energy projects in Germany is the lack of political support, which was also indicated in the Ocean Energy Technology Market Report (Magagna 2019). Since the development of technologies is associated with high costs (Magagna 2019), policy support for tidal energy in the EU and Germany is therefore particularly relevant when it comes to the realisation of tidal energy projects. The Ocean Energy Technology Market Report 2018 (Magagna 2019) noted that fixed feed-in tariffs for ocean energy systems are established in Germany, which range from 12.4  € cent/kWh for systems below 500  kW to 3.47  € cent/kWh for arrays above 50 MW (Magagna 2019). However, the SET-Plan indicates that there are no available public funding schemes and no test site facilities for ocean energy in Germany (Implementation Working Group Ocean Energy 2021). To enable the deployment of tidal energy in the German North Sea, there is therefore still room for improvement.

7 Conclusions and outlook

7.1 Conclusions

Based on the objectives established in Sect. 1.2, the following conclusions can be drawn:

  • In international comparison, the potential for harnessing tidal energy in Germany is found to be rather low. The practical tidal energy resource is estimated in a range between 66.6 and 565.8 GWh y\(^{-1}\). The magnitude of the resource is highly dependent on the feasibility to deploy tidal current turbines in shipping lanes. Since the most energetic sites (the estuaries of Ems, Weser and Elbe) were excluded in the estimation of the practical tidal energy resource due to their status as shipping lanes, it is recommended to conduct further feasibility studies towards the installation of tidal current turbines in such shipping lanes without interfering with ship traffic is feasible.

  • Tidal energy technology has matured in recent years and a significant expansion can be expected globally. In recent years, various devices have been developed which are also suitable for deployment in shallow waters and can viably generate power even at low tidal current velocities (mainly VATs). However, these devices are often not fully developed (TRL 6–7). There are several small-scale and medium-scale devices, which could be suitable for deployment in the German North Sea.

  • Only a few developers consider the German North Sea as a viable site for tidal energy projects. For an expansion of tidal energy in Germany, further political support, such as public funding schemes, as well as the availability of test sites are necessary. It is considered important to develop the technology in such a way that it can be used at low current velocities, so that Germany (and further low-potential sites) could become more interesting as project sites for future tidal energy projects.

7.2 Outlook

This outlook provides a framework for a potential subsequent resource assessment in the German North Sea. One approach would be to adjust the current velocity limit. Setting the limit to \(V_{\textrm{mean}}>{0.5}\) m s\(^{-1}\) (as rated viable by three developers, see Sect. 5) in the resource assessment would enable the deployment of tidal current turbines at many more sites, which are shown in Fig. 9. However, these sites were not investigated in the framework of this work, yet it might be worthwile to conduct subsequent research regarding their suitability for harnessing tidal energy.

Fig. 9
figure 9

Sites of interest with adjusted current velocity to \(V_{\textrm{mean}} > {0.5}\) m s\(^{-1}\) in the 12 NM territorial sea

Full size image

Alternatively, one of the sites considered in this study could be investigated in more detail. Depending on the scenarios established in Sect. 3.3, four to six sites of interest were identified in the German North Sea. In this context, the Outer Jade (Site 4) has a particularly strong influence on the actual size of the practical resource: The deployment of tidal current turbines in this area could generate up to 460  GWh power per year. It is therefore recommended to conduct more detailed research at this site. The minimum water depth in the Outer Jade is about 6.3  m (see Table 2), which is higher than the established minimum value of 4.5  m, and the average water depth is approximately 21.6  m. This would allow the deployment of tidal current turbines with larger rotor diameters than 3  m, as assumed in the present study. Tocardo’s HATs were rated TRL 7 in 2018 (Magagna 2019), and can vary the rated velocity and rotor diameter, allowing them to be optimally adapted to site-specific conditions (see Table 4). Tocardo’s T-1, for example, has a rated grid power of 42  kW at current velocities of 2 m s\(^{-1}\) when set to a 6.3  m rotor diameter, and the cut-in speed is 0.4 m s\(^{-1}\) (HydroWing, Tocardo 2022). In the Outer Jade, maximum current velocities larger than 1.5 m s\(^{-1}\) occur, the highest values exceed 1.7 m s\(^{-1}\). The mean current velocities range between 0.8 and 0.96 m s\(^{-1}\). Due to the low cut-in speed of the Tocardo T-1, power might be generated over a large period of the flow duration. In large parts of the site, water depths of 15  m are exceeded. Setting the minimum water depth in a subsequent assessment to 15  m would result in additional space for both bottom clearance (to avoid shear stress) and top clearance (to reduce wave loading on the turbine and to provide space for vessels). Due to the variable rotor diameter of the Tocardo T-1 turbine, shallower water depths could also be included, although with decreasing rotor diameter the cut-in speed and rated speed of the turbine increases.

To further progress in the resource assessment stages (see Table 1), also more advanced modelling approaches should be considered. The use of advanced numerical models should first be restricted to depth-average approaches (e.g., Delft3D (Sánchez et al. 2014)) to model hydrodynamic effects on a tidal energy farm scale. Such models may be extended to include morphodynamic effects (e.g., in and around tidal inlets), which may be of particular importance in the Wadden Sea. An example can be found in (Chatzirodou et al. 2019). If such a study then reveals the requirement for a hydrodynamic analysis on a turbine scale (e.g., Malki et al. (2014)) high-fidelity fully resolved CFD models should be considered. These models enable the consideration of, e.g., wake effects. However, the use of such models should be well justified due to the associated high computational cost.