1. Introduction

Figure 1. An overview of the work undertaken and presented in this paper.

Figure 1. An overview of the work undertaken and presented in this paper.

2. Theoretical Review and Site Conditions

2.3. Baseline Data

The baseline data used throughout this paper applies data from previous work by Borg et al. [26] and Cutajar et al. [27]. The main ESS parameters, which are kept constant throughout this paper, are summarised in Table 1. The ESS applied is the HPES system [17] consisting of an Energy Conversion Unit (ECU) and Pressure Containment System (PCS). The ECU is comprised of two hydraulic machineS, namely a MW-scale centrifugal pump and a Pelton turbine. The HPES system makes use of a liquid piston in order to charge or discharge the system by compressing air in a subsea environment to maintain quasi-isothermal conditions [17].

2.3.1. Renewable Energy Sources

The wind speed data used to estimate the annual energy yield of the wind turbine selected was 3TIER [28] hourly averaged data in a time series format, extending over the 6 year period from years 2005 to 2010. The data were corrected based on wind speed measurements obtained at a height of 80 metres above ground level at a coastal location in the Maltese Islands [29]. The wind speed measurements were applied to a 10 MW NREL wind turbine (WT) [30], with the wind speed being corrected for WT hub height based on a realistic wind shear exponent ($\alpha$) [29], as shown in equation (4) hereunder:

$$v={\left(\frac{H}{H_0}\right)}^{\alpha }\times v_0$$

(4)

where v is the wind speed at the height of interest, $v_0$ is the measured wind speed at a reference height, $H_0$, for the measurement height and H is the proposed WT’s hub height.

The solar PV data were provided from an existing installation at the University of Malta. A multiplication factor of 1.11 was applied in order to estimate the RES generation if the same solar PV installation were to be placed offshore, taking into consideration that the marine environment provides better cooling of PV panels than onshore due to lower ambient temperatures [31]. Table 2 and Table 3 respectively summarise the FOWT farm and solar PV performance metrics assumed and maintained constant throughout this paper.

Once the individual performance parameters for the FOWT pilot farm and for the solar PVs were established, understanding the spatial requirements of the two RES technologies related to the study was the step which followed. Table 4 presents estimates for the spatial requirements for the solar PVs and the FOWT pilot farm. The ‘medium’ value for spatial requirements is simply the average of the optimistic and conservative spatial requirement scenarios.

2.3.2. The Floating Breakwater

The geometric parameters of a single FBW module with the integrated HPES system (presented in Figure 3) described in [27] and being adopted in the present study, are presented in Table 5. Note that, the FBW floater remains unchanged when integrating the HPES system. The PCS in this case is assumed to consist of eight steel pipelines, shown in Figure 3 hereunder. Similarly, the characteristics of the PCS system and the catenary mooring system are listed in Table 5. Further detail may however be found in [27].

Table 6 gives a breakdown of the total hardware-attributable cost for one hybrid FBW module. The total cost adds up to a total of € 29,106,298 (€ 29.1 million) and includes aspects such as shipping, coatings and auxiliary items. It is shown that the moorings are the most expensive component accounting for more than half the total hardware-attributable cost. The anchors are the next, most expensive component of the floating system, followed by the FBW and HPES system. From a detailed study on the logistics of installation of the hybrid FBW, the transportation and installation costs were found to add up to simply 7.2% of the total hardware-attributable costs. Considering also insurance costs during the commission phase, the total CAPEX of one FBW with integrated HPES system has been estimated to be approximately € 31,623,152 (€ 31.6 million).

A breakdown of the described CAPEX is in fact given in Table 7. The latter shows how the hardware, in particular the mooring chains (see Table 6), are the highest expense related to a large-scale FBW. The proposed system needs very heavy chains to remain in place and experience controlled response in deep waters, leading to very expensive mooring solutions.

2.3.3. Case Study Summary

As explained previously in Section 1, the Case Studies considered in this study, and which will be analysed throughout this report are split into two main parts. Table 8 summarises all case studies which have been performed. Studies A1 to A4 considered two onshore solar PV technologies (land-based and rooftop) and two floating offshore solar PV technologies (calm waters and open offshore) to analyse how the energy production, CAPEX, OPEX and DECEX vary from one case to another. The floating FOWT pilot farm was assumed to be the same throughout all studies. Study A5 introduced the FBWs, thus creating a sheltered region. It was assumed that the FBW owners would generate revenue by leasing the sheltered water space to operators of floating solar farms, aquaculture farms and seafaring vessels. For study A5, the FBWs serve the sole purpose of providing a sheltered area, and do not have the ESS incorporated. Studies B1 to B3 introduce ESS which allows the FBWs to serve a dual purpose, i.e., to provide:

• A sheltered area; and
• Energy storage services to the electricity grid.

Table 8. A summary of all the Case Studies performed throughout the investigation.

Table 8. A summary of all the Case Studies performed throughout the investigation.

Case Study	Description
Part A - No ESS
A1	Land-based PV and FOWT Pilot Farm
A2	Rooftop PV and FOWT Pilot Farm
A3	Floating PV in Calm Waters and FOWT Pilot Farm
A4	Floating PV in Open Waters and FOWT Pilot Farm
A5	Floating PV in Sheltered Waters and FOWT Pilot Farm (61 FBWs)
Part B - Includes FBWs and ESS
B1	Floating PV in Sheltered Waters and FOWT Pilot Farm (61 FBWs)
B2	Floating PV in Sheltered Waters and FOWT Pilot Farm (41 FBWs)
B3	Floating PV in Sheltered Waters and FOWT Pilot Farm (21 FBWs)

The inclusion of ESS services provides additional revenue streams for the FBW owners. All revenue streams are presented in Table 9. RS1 to RS3 are common revenue streams which were applied to case study A5 and all of Part B, since the said revenue streams are possible with the inclusion of the FBWs, yet do not require the ESS except for the charging services associated with RS1. Revenue streams RS4 to RS6 are introduced in Part B with the inclusion of the ESS incorporated in the FBWs. Figure 4 presents where the RSs are applied in the case studies.

3. Techno-Economic Feasibility Case Studies

3.1. FOWT Pilot Farm Cost Modelling

Since the FOWT Pilot Farm will remain constant throughout all case studies across Parts A and B, this section will separately explain the cost modelling and economic feasibility of the Pilot Farm as a stand-alone project. While Table 2 in Sub-Section 2.3.1 discussed the FOWT pilot farm technical details, Table 10 hereunder presents the main costings assumed. A report carried out by BVG Associates stated that FOWT farm costs range between 4,000 to 5,000 €/kW [40]. While CAPEX costing values were obtained from a variety of sources ([19,20,41]), the highest CAPEX value found from the referenced sources (which exceeds BVG Associates’ cost range [40]) was considered for this particular study due to the following reasons:

• Projects are usually large-scale, meaning that a FOWT farm would consist of a minimum of ten WTs, with some projects reaching Gigawatts of installed capacity (>1000 MW);
• The project in question would be the first of its kind in the Maltese Islands, thus costs will be higher due to lack of previous experience in the sector; and
• Transportation and installation costs would be greater given the lack of local, adequate port facilities which are large enough to assemble the FOWTs at the quayside.

A detailed breakdown of what the CAPEX, OPEX and DECEX costs consist of may be obtained from a cost modelling study performed by Diaz et al. [19] for a FOWT farm consisting of 88 WTs each rated at 10 MW. The cost (€/kW) was calculated and is presented in Table 10. The inputs required for the LCOE, IRR and SPP are listed in Table 11. The interest rate was selected based on the fact that the interest rate for such projects typically varies from 5 to 10% [19,20]. Similarly, the inflation rate is typically assumed to be anywhere between 2 to 2.5% and therefore, an inflation rate of 2.5% was considered [42]. The timing of the project was assumed to be 30 years, with the first 5 years being solely dedicated to project development and consenting. Therefore, the FOWT Pilot Farm would have a lifecycle of 25 years. Staffell et al. [43] concluded that WTs lose approximately 1.6 ± 0.2% in energy production per year. As a result, this energy production loss was applied across the 25-year lifecycle of the FOWT pilot farm. FOWT farms must have an IRR value which is at least 8% [44]. Hence, the feed-in tariff (FIT) required to reach an IRR of 8% was found. Table 12 summarises the FOWT pilot farm’s main economic feasibility results.

Table 10. Main cost parameters of the FOWT Pilot Farm.

Table 10. Main cost parameters of the FOWT Pilot Farm.

Cost Parameters	Value (€/kW)	Reference
CAPEX	5172
OPEX	50.40	[19]
DECEX	138.23

3.2. Part A - No Storage

As explained previously in Section 2.2 and Section 2.3 and as shown in Table 8, Part A involved a number of case studies assuming different Solar PV technologies: Case Studies A1 and A2 analyse two onshore solar PV technologies, comparing the cost difference between land-based and rooftop PVs. Meanwhile, Case Studies A3 and A4 present the cost modelling of the solar PV farm in calm and open waters respectively. Open waters would expose the solar PV structures to large waves, thus demanding robust floating support structures, similar to those used on oil and gas and floating wind turbines. Case Studies A1 to A4 provide information which is required to perform Case Study A5, where the FBW structure is introduced.

3.2.1. Onshore Solar PV (Case Studies A1 and A2)

Case Study A1 considers the rental of unused, onshore land for developing a 40 MWp solar farm together with a 30 MW FOWT pilot farm off the West Coast of the Maltese Islands. Table 13 presents the main costings related to the land-based and rooftop PV plants. While the CAPEX costs related to the solar PV hardware are not high, the total initial project investment required becomes excessively high due to lease of space costs to rent out the land for a project lifetime of 25 years. Since the CAPEX cost did not include project development and consenting in this case, a study by Alsheghri et al. [45] was used, where it was assumed that such costs made up 0.56% of the total CAPEX costs of the solar farm. Meanwhile, as shown in Table 13, The Energy and Water Agency’s National Renewable Energy Action Plan [46] showed that the CAPEX for rooftop PV systems is higher than for land-based PV systems. Table 14 presents the inputs applied for the calculations of the LCOE, IRR and SPP. The cost parameters presented in Table 13 were applied to the cost analysis.

Since Case Studies A1 and A2 consist of onshore PV systems, the interest rate related to project investment was assumed to be 5%. This value is based on the average interest rate of European Solar PV projects [20]. Additionally, a 0.2% decrease in solar panel production per annum was applied based on a study by Fraunhofer [49]. Table 15 presents the economic analysis outputs of the land-based and rooftop PV systems, indicating that the rooftop PV plant requires a FIT which is 46.70% less than that of the land-based PV plant in order to achieve an IRR of 8%. Once the individual economic outputs of the different onshore solar PV plant setups and the FOWT pilot farm were obtained (presented in Table 12), an overall economic output could be obtained for the two case studies (A1 and A2). Table 16 summarises the main economic parameters of Case Studies A1 and A2.

3.2.2. Offshore Solar PV (Case Studies A3 and A4)

Case Study A3 considered a floating offshore solar PV farm assumed to be in operation in calm waters (i.e., on reservoirs and lakes) together with the 30 MW FOWT pilot farm. Case Study A4 considered a floating offshore solar PV farm, assumed to be in operation in open waters, thus being exposed to harsher wind and wave conditions and demanding a robust support structure. As explained in Table 8 in Sub-Section 2.3.3, the 30 MW FOWT pilot farm is included throughout all the case studies. Table 17 presents the main costings related to the floating offshore solar PV farm assumed to be operating in calm waters and open waters respectively. While a number of research papers were considered to understand the typical CAPEX and OPEX costs of a floating PV (FPV) system [20,50], the most appropriate cost breakdowns were provided by Islam et al. [51] and by Ghigo et al. respectively [52]. While the CAPEX cost difference between floating PVs in calm waters and in open waters is significant due to the reasons explained above, the OPEX cost difference is negligible. Additionally, the replacement of hardware (for example PV panels, inverters and mooring chains) and decommissioning costs were assumed to be equal due to a lack of literature related to the said costs.

In the case of Case Studies A3 and A4, the interest rate related to project investment was assumed to be higher (set at 8%) than the onshore case studies to reflect the higher risk levels associated with offshore-based projects [45,47]. On the other hand, however, the energy produced per year for the offshore solar PV case studies was assumed to be 11% more due to the reasons previously explained in Sub-Section 2.3.1 [31]. Table 18 presents the applied parameters to carry out the economic evaluation for the solar PV systems in calm waters and open seas. Table 19 compares the economic outputs of the solar PV system in calm waters and open seas for an IRR of 8%. Table 20 summarises the main economic parameters of Case Studies A3 and A4.

3.2.3. The Influence of Ground Rent Cost

A key finding from Case Studies A1 to A4 is that although land-based PV systems are far cheaper than those offshore in open sea, the cost advantage is easily absorbed by the high costs for renting land. In fact, Case Study A1 ended up being the most expensive project in terms of CAPEX, thus resulting in requiring the highest FIT to obtain the desired 8% IRR for investors. In order to understand if this was a case-specific issue, different coastal regions around the Mediterranean were analysed to understand the typical land rental pricing.

Table 21 presents the average rental price per square metre per year for the different places analysed. One may observe that the rental price rises based on the regional population density. As a result, Malta resulted in having the greatest rental prices per year, thus heavily impacting the overall economic viability of a solar PV farm on unused, onshore territory. Figure 5 presents a graph indicating the relationship between the IRR and land rental price per year for different FITs. The results show that a FIT as low as 5 €c/kWh would not be enough to make the onshore-based PV system profitable, even if no land rental price was incurred.

Meanwhile, the only FITs which are feasible at a land rental price of 16 €/kWh fall between 20 and 25 €c/kWh. In most cases, the maximum land rental price which made the FITs ranging from 10 to 25 €c/kWh able to achieve 8% IRR or more, was 4 €/m²/year. It is important to note that the graph analysis does not apply to the onshore rooftop PV system, since a rooftop rental cost was established through a study by Rebe et al. [47]. The rooftop rental cost is also based on the assumption that the required area to house a 40 MWp solar farm would be found.

Due to the wind turbine farm already being situated offshore (in waters with a sea depth of 200 metres), and considering the high land prices for onshore PVs in Malta, co-locating floating wind and solar farms offshore seems to be a promising alternative. Case Study A2 provided competitive results with those in A3, however based on co-location and the assumption that a 40 MWp solar PV farm would need to be placed on rooftops, shifting offshore seems to be most sensible. As a result, Sub-Section 3.2.5 will explain Case Study A5, which is titled ‘Floating PV in Sheltered Waters and FOWT Pilot Farm including FBWs’, as previously presented in Table 8 in Section 2.3.3. Prior to Sub-Section 3.2.5, Sub-Section 3.2.4 will discuss Part A of the revenue streams created as a result of the introduction of the FBWs.

3.2.4. Revenue Streams for the FBWs (Part A, no ESS integrated)

The FBWs will create a large, sheltered area in deep water to accommodate different marine activities. The FBW technology assumed in the FORTRESS project is explained in detail in [27]. When assuming that the FBWs are purely structures to provide an area of sheltered waters (i.e., the storage aspect presented in Figure 3 in Section 2.3.2 within the FBWs is omitted), the following revenue streams may be generated:

• Berthing of seafaring vessels (e.g., yachts),
• Renting of space for aquaculture cages,
• Renting of space for floating solar PVs.

Figure 6, Figure 7 and Figure 8 present an example of how the sheltered waters could be utilised, and what Project FORTRESS would look like overall. The FBW array provides a sheltered area spanning 8.30 km², which provides space for two 40 MWp offshore floating solar PV farms (spanning 0.57 km² excluding spacing) , aquaculture through fish farms (peach section spanning 0.14 km²) and a berthing section (orange and purple sections spanning 0.23 km²). An additional 27 MWp of solar PVs are assumed to be placed on the FBW array surfaces, spanning an area of 0.12 km².

Berthing Facilities

The first revenue stream considered is the offering of berthing facilities (RS1, Figure 4) to seafaring vessels within a sheltered area created by the FBWs. The assumed business model is to have this area owned by the operators of the FBWs, but leased to a third party operating the facilities. In order to obtain realistic pricing for the berthing services being offered, prices from different berthing locations across the Maltese Islands were reviewed, such as for example those being quoted by the Excelsior [69] and Marina di Valletta [70] marinas. Table 22 presents the assumed pricing being assumed in the presented study. These are presented at the price per day for yachts as a function of vessel size. The pricing was set to one half of that found in [69,70], given that Project FORTRESS will offer the berthing in offshore environment rather than at port. Additionally, the number of berth spaces and area required are also presented. The approximate number of berth spaces was based on analyses for Universal Berth Parameters [71] and based on a report related to the development of yachting facilities in Malta [72].

The berthing section was assumed to be operational for five months per year, spanning from mid-May to mid-October (153 days). The estimated revenue being generated was estimated to be based on a half-half mix: 284 yachts sized between 16 to 18 metres and 284 yachts sized between 18 to 20 metres. Since assuming that the berthing area would be fully utilised consistently for all 153 days is an unrealistic scenario, three different usage percentages were considered. Lastly, Project FORTRESS would charge the berthing owner a percentage of the revenue generated for making use of the sheltered area created by the FBWs. The percentage charges are shown in Table 23 (10%, 17%, 25%). Through this charge, a cost in euro per square metre was established, which would then be utilised for the other revenue streams. The obtained cost per square metre was required to be less than the lease of land and rooftop rent. Table 23 also presents the results of the yacht berthing revenue stream financial analysis.

Aquaculture

The second possible revenue stream is to exploit the sheltered waters created by the array of FBWs for aquaculture through the operation of fish farms (RS2, Figure 4). Wang et al. [73] explained that while moving aquaculture further offshore increases expenses, advances in technology and the opportunity for co-location can lead to important cost reductions. Furthermore, Holmer [74] discussed how environmental detriment is predicted to decline with aquaculture moving further offshore due to greater sea depths and harsher weather conditions, leading to more efficient waste dispersion. Due to being set up further offshore, the carbon footprint of fish farms may increase due to increased transportation, unless green transportation is introduced. One small fish farm in the case of this project was assumed to consist of four 50 metre diameter cages and two 60 metre diameter cages (shown in Table 24). It was assumed that ten of such small fish farms would be placed in the sheltered water area. Similar to the berthing revenue stream, Project FORTRESS’ responsibility as the owner of the FBWs is to rent offshore sheltered space to be utilised by the fish farm operators. The cost in euro per square metre calculated previously in Table 23 was applied in this case in order to calculate the annual revenue for FORTRESS for the area utilised. Table 25 presents the financial results obtained.

Renting of Solar PV area

The third revenue stream is the renting of sheltered water areas created by the FBWs to operators of floating solar PV farms. With the introduction of the FBW array, the offshore floating PVs have a level of protection from open sea conditions. As a result, the solar farm cannot be considered to be in calm waters but is protected from the high waves of open seas. Thus, a separate analysis assuming a compromise between Case Studies A3 and A4 was carried out to estimate the LCOE and required FIT for a minimum of 8% IRR in the case of a solar PV farm in sheltered waters. The CAPEX cost value (1,370 €/kWp) was assumed since it is the average value of the CAPEX costs for the PV system in calm waters (693 €/kWp) and for the PV system in open seas (2,047 €/kWp). All economic analysis input parameters were also maintained. Due to the CAPEX savings in designing a Floating PV system for sheltered seas, a rental price (in terms of €/m²) is assumed to be charged by Project FORTRESS covering two different methods:

• Locating floating PV systems in the sheltered water areas created by the FBWs, with a pricing arrangement that is similar to yacht berthing and aquaculture.
• Locating additional solar PVs on the deck area available on the FBWs themselves. Such panels would be integrated in a similar way as roof-top systems, with no floaters and moorings required.

Table 26 presents the main economic results for a PV system in sheltered waters.

In the present feasibility analysis, it is assumed that the rental price for space for the second option is twice that of the first. Table 27 presents the power ratings and areas occupied in the sheltered waters together with the area occupied on top of the FBWs. Once again, Project FORTRESS is solely responsible for renting out the sheltered water area being utilised by the solar PV farm operators. The revenue resulting from the energy generated from the solar PV farm will be earned by its operators and not by the owners of the FBWs. Table 28 presents the yearly revenues for Project FORTRESS resulting from the two different areas enlisted in Table 27.

3.2.5. Sheltered Solar PV (Case Study A5)

Case Study A5 analyses an offshore floating solar PV system in sheltered waters and the FOWT Pilot Farm. The solar PV system is said to be sheltered due to the introduction of a FBW array, consisting of 61 FBWs. The FBW design and costings have been explained in detail in Sub-Section 2.3.2. The number of FBWs was derived based on previous research [75]. Table 29 presents the costings related to the FBW structure, with the FBW CAPEX amounting to € 27,586,954. A full economic analysis could be performed amalgamating the costings and revenue generated via the applicable revenue streams. Since the revenue streams will be applicable across the project lifetime (25 years), the annual revenue generated considered an inflation rate of 2.5% from the start to the end of the project [20]. Table 30 presents the results of Case Study A5, highlighting that the project would not be financially feasible with the discussed setup.

Through the economic evaluation of Case Study A5, a number of conclusions may be drawn:

• The financial modelling shows that revenue streams RS1 (yacht berthing), RS2 (aquaculture farms) and RS3 (solar PV farms) do not provide sufficient revenue in total to outweigh the life cycle costs of the overall project, primarily due to the high cost of the FBW array. Figure 9 presents the breakdown of the different costs associated with the FBW array and the portion of revenue generated.
• The combined savings incurred by locating the offshore solar PV farms in sheltered waters created by the FBW array together with the additional revenue streams instead of open seas, do not justify the high capital investment required for the FBWs in deep waters.
• Case Study A5 did not consider the integration of energy storage within the FBW array to provide additional revenue streams, which would lead to further FBW investment.

As a result, the following section (Section 3.3) will analyse new case studies whereby the FBW array will not only be utilised to create a sheltered area, but to also store energy and thus, offer the opportunity for new revenue streams.

Figure 9. A pie chart showing the breakdown of the FBW array costs and revenue generated.

Figure 9. A pie chart showing the breakdown of the FBW array costs and revenue generated.

4. Conclusions

5. Outlook

Abbreviations

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