1. Introduction
To tackle the problem of global warming, low carbon energy policies have stimulated the widespread installation of commercial solar photovoltaics (PVs) and wind power parks globally and in Europe [
1,
2]. However, the integration of renewable energy sources (RES) to the grid imposes another challenge—grid stability. An appropriate mix of RES distributed generation (DG) chosen for the site of interest can help to overcome this and to support the net load variability with a large share of RES DG [
3]. Ocean and particularly wave energy is considered to be a promising and attractive energy source due to its relatively small short-term fluctuations [
4] and high power density [
5], yet it still remains untapped. Wave energy is also attractive since the wave power potential in some regions follows the seasonality of electrical energy demand [
6,
7]. The estimated wave energy potential has gone from 17 TWh/year in 2007 [
8] to 92 PWh/year in 2016 [
9], available at coastal areas, where about 50 percent of the world’s population lives. The global wave power potential is shown in
Figure 1.
Although the focus of many researchers and wave energy technology developers has been on areas with high wave power potential, even areas with relatively low potential have received attention. For example, the wave power potential has been studied for such regions as the Italian [
11], Lithuanian [
12] and Swedish [
7] coastlines, in the Caspian Sea [
13] and the Red Sea [
14], offshore Mediterranean areas [
15,
16], as well as seas in China [
17] and the Maldives [
18]. The wave power density for these locations is given in
Table 1. In this paper, the Adriatic Sea is of particular interest.
This paper evaluates the possibility of integrating a wave power farm with the power system of an island in the Adriatic Sea, combining the wave power with a battery energy storage system (BESS) and solar photovoltaics (PVs), and its impact on the local weak low voltage grid. The wave power technology is a point-absorbing wave energy converter (WEC) with a direct drive linear permanent-magnet synchronous generator power take-off device, and is presented in
Section 2. Wave power farms (WPFs) consist of two to ten WECs. The main idea of the case studies carried out and presented in
Section 3 was to give a comprehensive analysis of various renewable electricity generation technologies that can be utilized together with a WPF to provide an electricity supply to meet the typical Adriatic island demand (consumption) throughout the entire year. The load profile used represents the typical demand (consumption) of an Adriatic island, in which the demand substantially increases during summer (the tourist season). Because of the large solar energy potential of this region, it is expected that PV systems will be integrated into this power system; therefore, another case involving the simultaneous operation of a WPF and PV system is evaluated. Furthermore, the benefits of a BESS, together with a WPF and PVs, in achieving net zero electricity exchange with the grid is evaluated. Finally,
Section 4 deals with economic, environmental and social aspects of WPFs.
3. Case Study of Possible WEC Integration on an Adriatic Island: Results
This section describes the power flow analysis model of the daily operation of a low voltage distribution network with an integrated WPF applied to conditions typical for a small Adriatic (Mediterranean) island with low wave energy potential and variable consumption throughout the year, namely, low electricity demand (consumption) during the winter period (off tourist season) and high electricity demand (consumption) during the summer period (tourist season).
WPF generation (1-s time resolution) estimated by the model for the two locations (L1 and L4 in
Figure 2) and two seasons (summer and winter in
Table 3) was used to analyze the possibility of deployment of a WPF with and without PV systems and BESS technologies into the low voltage distribution network. The daily electricity generation for various numbers of WECs in a WPF, generated by the model of WPF, for locations L1 and L4 and for both summer and winter season are given in
Figure 9.
The power network (grid) model into which the WPF/PV/BESS were integrated is the Institute of Electrical and Electronics Engineers (EEE) European Low Voltage Test Feeder (LVTF), a 11/0.416 kV AC low voltage unbalanced distribution network. The IEEE European LVTF was developed to represent a 50 Hz test feeder at the low voltage level of 416 V (phase-to-phase), typical in European low voltage distribution systems. The test system consisted of an 11/0.416 kV transformer, 907 buses, 905 lines and 55 loads presented with 55 consumption profiles of household-size consumers with a one-minute time resolution over 24 h. This enabled time-series load flow analysis over a one-day period or static load flow analysis at specific moments in a day. The low voltage 416 V network was connected to an 11 kV upstream network via a three-phase transformer rated at 0.8 MVA and a delta/grounded-wye connection (Dyn) of windings. The resistance and reactance of the transformer impedance were 0.4% and 4%, respectively. The reader can find more details on the IEEE LVTF in [
45].
The original consumption profile of the feeder was adjusted to represent the winter period (off tourist season) and summer period (tourist season). Winter-period consumption consisted of 14 households (nearly every fourth is active), describing the consumption of the domestic population on the island, which is multiple times smaller than the consumption during the summer period—influenced by the tourist season. The summer-period consumption profile considered 55 households (original number and consumption profile of the feeder) to encompass the increase in load during the summer period, mainly influenced by the power consumption by air conditioning systems. The power factor of each household was set to 0.95 lagging (inductive). The winter-period and summer-period consumption profiles of the feeder are shown in
Figure 10. Furthermore, the 11-kV upstream network in which the LVTF is connected to a swing bus was represented with a three-phase short-circuit power of 35 MVA, a single-phase short circuit power of 95.26 kVA (isolated 11 kV side) and a reference voltage of 1.05 p.u. These settings were provided by the distribution system operator for the island of Vis, Croatia, and represent realistic island power network input data, located near the location L4.
WECs (a WPF) were connected to the electrically most-distant bus in the LVTF (bus 881), also visible in
Figure 11. Each WEC was connected as a three-phase generator model with a unity power factor.
Operation of the LVTF was carried out for the following case studies:
Case study 1—winter-period (off tourist season) consumption profile of distribution feeder with various numbers of WECs in a WPF installed on location L4 with winter-period generation profile and with or without BESS.
Case study 2—optimal number of WECs (size of WPF) needed to achieve nearly zero electricity exchange with upstream network, as determined in case study 1, were installed on location L4 with the summer-period WPF generation profile, summer-period (tourist season) consumption profile of distribution feeder, integrated 12 PV systems (accounting for about 20% share of households) with the summer-period generation profile and with or without BESS.
Case study 3—optimal number of WECs (size of WPF), as determined in the case study 1, are installed on location L4 with winter-period WPF generation profile, winter-period consumption profile of distribution feeder, integrated 12 PV systems (accounting for about 20% share of households) from case study 2 (winter period generation profile) and with/without BESS.
Case study 1 was carried out to determine the optimal number of WECs needed to achieve nearly zero electricity exchange of the LVTF and the upstream network. Case study 2 was performed to point out the advantages and possible problems in operation that can occur when integrating PV systems with this feeder and to alleviate the increase in consumption during the summer period (tourist season). The same situation was studied with winter-period consumption and generation profiles in case study 3 in order to determine the influence of overall electricity generation from both WPF and PV systems during the winter period (off tourist season) as well.
Time-series (load flow calculations) simulations of LVTF operation were carried out in OpenDSS, an open-source power system analysis software, driven through the Component Object Model (COM) interface with the Python programming language [
46]. Each simulation is carried out for one-day operation with 1-min time resolution, resulting in 1440 load flow calculations.
A one-line diagram of the LVTF used for case studies carried out in this paper with indicated 11/0.416 kV substation, spatial distribution of 55 household-size consumers (1 to 55), 12 PV systems (PV1 to PV12) and the WPF and BESS is presented in
Figure 11.
3.1. Case Study 1–Optimal Number of WECs in WPF for Winter Electricity Demand (Consumption)
The task of case study 1 was to determine the optimal number of WECs in a WPF to achieve nearly zero electricity exchange with the upstream distribution network for winter-period (off tourist season) consumption.
Table 5 shows the observed parameters extracted after the simulations of daily operation. It is visible from the results that two WECs in a WPF results (bolded column in table) in the lowest (closest to zero) net daily import of 15.66 kWh of electricity from the upstream distribution network, which was therefore considered the optimal number of WECs in a WPF integrated into the LVTF. Furthermore, it is visible that minimum voltage, active and reactive energy (bolded) losses occurred in the case of 1 WEC in a WPF (bolded), rather than for the two WECs, which is considered the optimal number of WECs, even though the lost active and reactive energy was close to the minimum. The highest voltage occurred on the bus 881, PCC bus of WPF and LVTF, and the highest active and reactive power losses were observed in the case of 10 WECs (bolded) in a WPF due to its having the largest active power injection, as expected.
Since the output power data of the WECs (WPF) is generated in 1-s time resolution, for better visibility, only the one-hour output power profile of a WPF consisting of two WECs (the optimal number) at the PCC with the LVTF is shown in
Figure 12.
There is large intermittency in the output power profile, which is result of the nature of the WEC operation. To mitigate this behaviour, a smoothing technique was applied in this paper’s case studies, using a BESS installed at the PCC of the WPF and the LVTF, to smooth the output power profile.
Table 6 presents the technical characteristics of the BESS connected at the PCC of the WPF and LVTF. The BESS’s technical characteristics were determined through a trial-and-error method to effectively achieve the effect of smoothing the active power output at the PCC of the WPF and the LVTF.
The dispatch profile of the BESS was generated manually so that the output power of the WPF at the PCC varied ±10% from the daily mean output power. For example, for a daily mean WPF output power of 5.867 kW, the net output power at the PCC would vary from 5.281 kW to 6.454 kW.
Figure 13 gives the one-hour dispatch (1 s time resolution) profile of the BESS for the optimal case (two WECs in a WPF), to achieve the variability of ±10% from the daily mean output power, where negative values represent charging power whereas positive values represent discharging power.
The results of the time-series simulations are presented in
Figure 14, which shows the active power exchange of the LVTF with the upstream network at the swing bus for the following scenarios in case study 1: reference case (load only, no WPF, no BESS); WPF with two WECs only (2 WECs only); and the combination of WPF with two WECs and BESS (2 WECs + BESS).
The results presented in
Figure 14 show that in scenarios where a WPF consisting of two WECs was operating, the exchanged active power was shifted upwards, resulting in alternating upstream and downstream active power flows during the time of the day, depending on the current consumption of loads. In a scenario using BESS installed at the WPF PCC (2 WECs + BESS scenario), the smoothing technique proved to be effective when alleviating the intermittency, and the generation profile trend was preserved.
Figure 15 gives the voltage profiles of each phase (L1, L2 and L3) at the WPF (and BESS) PCC bus (bus 881) for the following scenarios in case study 1: reference case (load only, no WPF, no BESS); WPF with WECs only (2 WECs only); the combination of WPF with two WECs and BESS (2 WECs + BESS).
The results show that there was a continuous voltage rise (in comparison to the levels for the reference case) on the WPF PCC during the day caused by the injection of active power. The smoothing technique using the BESS, as in the case for the active power exchange, also preserved the voltage profile trend while alleviating the intermittency caused by the nature of the active power profile.
3.2. Case Study 2–Operation of WECs (WPF) and PV Systems during Summer Season (Consumption)
In case study 2 we analyzed the operation of the LVTF during the summer period, presented with increased consumption (in comparison to winter-period consumption used in case study 1) due to the tourist season, as well as the additional electricity generation by the household-sized PV systems integrated at various locations in the LVTF. This case study points out the advantages and possible problems in operation that can occur when integrating PV systems, which is a very likely situation to occur due to large solar energy potential of this region, in this feeder and the increase in consumption during the tourist (summer-period) season.
This case study considers that the optimal number of WECs in a WPF (two), as determined in case study 1, are installed on location L4 and presented with a summer-period generation profile. Furthermore, the consumption profile of the distribution feeder is presented with increased consumption during the tourist season, as given in
Figure 10, and integrated PV systems are presented with the summer-period generation profile. Finally, operation with and without BESS was studied. To analyze the worst-case scenario, a scenario with 10 WECs in WPF and with generation of PV systems was also carried out.
There were a total of 12 single-phase (household size) PV systems integrated into the feeder (accounting for about a 20% share of households) with an installed power of 3.6 kVA. PV systems were distributed evenly across all three phases (four PV systems on each phase) and placed randomly in the LVTF. Technical characteristics and locations of PV systems are given in
Table 7, whereas
Figure 16 shows solar irradiance and PV module temperature profiles used for the summer period (used in case study 2) and the winter period (used in case study 3). Solar irradiance and PV module temperatures representing field measurements were acquired from [
47] for the island of Vis, Croatia.
Results of the time-series simulations are presented in
Figure 17, which shows the active power exchange of the LVTF with the upstream network at the swing bus for the following scenarios in case study 2: reference case (load only, no WPF, no PV, no BESS); WPF with two WECs in combination with PV systems (2 WECs + PV); the combination of a WPF with two WECs; PV systems and BESS (2 WECs + BESS + PV); worst case scenario of a WPF with 10 WECs in combination with PV systems (10 WECs + PV).
For the scenario of a WPF with two WECs and PV systems, there was a significant upstream active power flow during the daytime, caused by PV system generation, whereas the entire active power profile was shifted upwards by the WPF generation, and even the consumption of the feeder was increased due to the tourist season. Like in the previous case, the integrated BESS created a smoothed active power profile. In the worst-case scenario of a WPF with ten WECs in combination with PV systems, there was an upstream active power flow for most of the time during the day.
Figure 18 presents voltage profiles of each phase (L1, L2 and L3) at the WPF (and BESS) PCC bus (bus 881) for the following scenarios in case study 2: reference case (load only, no WPF, no PV, no BESS); WPF with two WECs in combination with PV systems (2 WECs + PV); the combination of WPF with two WECs; PV systems and BESS (2 WECs + BESS + PV); worst case scenario of WPF with 10 WECs in combination with PV systems (10 WECs + PV). Like in a case study 1, there was a voltage profile rise that was present continuously during the day, caused by the generation, even though there was no distinguishable rise in voltage during the daytime caused by PV system generation. Furthermore, in the worst-case scenario, voltage levels approached the upper voltage margin of 1.1 p.u., which can be exceeded in case of further integration of RES-based generation.
Results given in
Table 8 present the parameters extracted after the daily operation simulation for case study 2 scenarios: reference case (load only, no WPF, no PV, no BESS); WPF with two WECs in combination with PV systems (2 WECs + PV); the combination of WPF with two WECs, PV systems and BESS (2 WECs + BESS + PV); worst case scenario of WPF with 10 WECs in combination with PV systems (10 WECs + PV). Results with two WECs in combination with PV systems with or without BESS, considered as optimal cases based on achieving net zero electricity exchange with the grid, have been bolded in the table. The highest voltage occurred on bus 881, the PCC bus of WPF and LVTF for the case of 10 WECs (bolded) in a WPF due to the largest active power injection, as expected.
The results show that integration of BESS has minimal influence on the observed parameters while the effect of intermittency is mitigated, as visible in
Table 8.
3.3. Case Study 3–Operation of WECs (WPF) and PV System during Winter Season (Consumption)
In case study 3, we studied the operation of the LVTF with the same generation units as in case study 2 but with the seasonal conditions of case study 1 (the winter period). Therefore, the following situation was presented—two integrated WECs (the optimal number from case study 1) in a WPF installed on location L4 with the winter-period generation profile (
Figure 12), winter-period consumption profile (
Figure 10) of LVTF, 12 integrated PV systems (accounting for about a 20% share of households) from case study 2, with the winter-period consumption profile (input profiles presented in
Figure 16) and with/without BESS. In comparison to case study 1, the task of this case study was to analyze the influence of additional generation from PV systems during the winter period.
The results shown in
Figure 19 indicate the active power exchange of the LVTF with the upstream network at the swing bus for the following scenarios in case study 3—reference case (load only, no WPF, no PV, no BESS); WPF with two WECs in combination with PV systems (2 WECs + PV); the combination of WPF with two WECs, PV systems and BESS (2 WECs + BESS + PV); worst case scenario of WPF with 10 WECs in combination with PV systems (10 WECs + PV).
For the scenario of a WPF with two WECs and PV systems, there was an upstream active power flow throughout the day and the generation from the PV systems did not significantly influence the upstream power flow. Furthermore, the entire active power profile was shifted upwards by the WPF generation in comparison to the reference case. As in the previous case study, the integrated BESS smoothened the active power profile. In the worst-case scenario of a WPF with 10 WECs in combination with PV systems, there was a significant upstream active power flow during the entire day.
Figure 20 presents the voltage profiles of each phase (L1, L2 and L3) at the WPF (and BESS) PCC bus (bus 881) for the following scenarios in case study 3—reference case (load only, no WPF, no PV, no BESS); WPF with two WECs in combination with PV systems (2 WECs + PV); the combination of a WPF with two WECs; PV systems and BESS (2 WECs + BESS + PV); worst case scenario of WPF with 10 WECs in combination with PV systems (10 WECs + PV).
As in case studies 1 and 2, there was a voltage profile rise that was present continuously during the day caused by the active power generation, even though there was no distinguishable voltage rise during the daytime when PV systems were generating power.
The results given in
Table 9 present the parameters extracted after the daily operation simulation for case study 2 scenarios—reference case (load only, no WPF, no PV, no BESS); WPF with two WECs in combination with PV systems (2 WECs + PV); combination of a WPF with two WECs; PV systems and BESS (2 WECs + BESS + PV); worst case scenario of a WPF with 10 WECs in combination with PV systems (10 WECs + PV). The results of scenarios with two WECs in combination with PV systems with or without BESS—considered to be optimal cases based on achieving net zero electricity exchange with the grid—have been bolded in the table. The highest voltage occurred on bus 881, PCC bus of WPF and LVTF for the case of 10 WECs (bolded) in a WPF due to the largest active power injection, as expected.
5. Discussion
Case study 1 was carried out to determine the optimal number of WECs needed to achieve nearly zero electricity exchange of the LVTF and the upstream network. Case study 2 was performed to point out the advantages and possible problems in operation that can occur when integrating PV systems with this feeder and to alleviate the increase in consumption during the summer period (tourist season). The same situation was studied with winter-period consumption and generation profiles in case study 3, in order to determine the influence of overall electricity generation from both WPF and PV systems during the winter period (off tourist season) as well.
The aim of this study was to achieve net zero exchange of electricity, which was easier to accomplish using PV systems to cover the increased demand during the summer season (tourist season). Electricity generation from wind power plants is very low during the summer in regional climatic conditions. Additionally, wind turbines can be too large for small communities, and additional benefit analysis could be done in the future research to identify the most economical and technically viable energy mix for small island communities.
The presented case studies have a number of limitations. Firstly, an average seasonal sea state was utilized to create a wave time series using a modified JONSWAP spectral function. It was assumed that the sea state remained constant during the whole day. However, the sea state may vary during the season, including completely calm days. More detailed analysis based on the wave power potential available in the Adriatic Sea, correlated with solar irradiation, would help to analyze the energy flow and dimensions of an energy storage more precisely with the goal of maintaining zero power exchange with the grid. Secondly, the power potential was assessed using the third generation wave model Wave Modelling (WAM) for deep sea waves and calibrated with satellite measurements. For the precise assessment of wave potential nearshore, a different prediction model such as Sea Waves Nearshore (SWAN) should be utilized. Thirdly, WECs are usually installed at relatively shallow waters with a depth of up to 100 m. The UU’s WEC can safely be installed and operated at 50 m depth. Location L4 has a larger depth, and therefore the WPF should be installed closer to the island shoreline, but then the wave climate would be milder that at L4. Fourthly, offshore installations (e.g., WPF) can face resistance from local communities. Although the UU’s WPF has minimal visual impact if located several kilometers offshore, its installation inevitably leads to creating protected areas where fishing and sailing are forbidden. Finally, all presented calculations are influenced by the network element ratings and upstream grid strength (voltage robustness). The calculations were performed for a realistic weak island grid strength; however, in the case of even weaker upstream grid strength, expected particulary on smaller islands, more voltage and power fluctuations as well as exceeding of limits could be expected.
6. Conclusions
The study demonstrated the technical possibility of the integration of a WPF into the low voltage weak grid of an Adriatic island with a typical demand profile which is higher for the summer season. The study showed that a WPF consisting of two WECs is optimal for such a small power grid, and it is effectively complemented by the installation of domestic PVs. The combination of RES with a BESS contributed to the reduction of the intermittency of the power flow, which has a positive effect on the grid and reduces dependency on the grid connection to the mainland. Integration of a larger number of WECs to the same grid leads to the overproduction of electrical power. Finally, the possible deployment of a WPF in combination with PV systems and BESS into a distribution network or microgrids on the coastline of the Adriatic Sea (also applicable to other Mediterranean areas) and particularly on islands, indicated that there are clear benefits to enabling a reliable supply of rapidly increasing electricity demand (consumption) during the tourist season, reaching a net zero grid exchange standard that could play a significant role in faster commercialization of WECs in low energy potential seas.
The optimal number of WECs determined was mainly influenced by the net consumption of the feeder. In the case of the very likely scenario in which the additional generation of electricity is provided by the residential-scale PV systems, concerning problems with voltage regulation and loadability can occur. These possible problems can be mitigated with grid hosting capacity enhancment methods, such as grid reinforcements, network reconfiguration, energy storage devices, OLTCs, reactive power provision, etc. Hovever, as presented in the study, the concept of self-sufficient electricity supply from RES generation is possible, and very likely to be utilized in small-scale islands with wave and solar energy potential.
A BESS used for WPF output power profile smoothing proved to be very efficient with intermittency mitigation. This kind of power-electronic-based device can be utilized in a different manner, such as a community-scale BESS connected at the beginning of the feeder to regulate active and reactive power flows with a reactive power provision to regulate loading and voltages.
Future research should focus on this, as well as on its influence on power quality indices, particulary flickers and harmonic distorsion. Finally, electricity generation by WECs is generally environmentally friendly. It creates new jobs and is economically effective, since operation and maintenance costs are lower for the resource price than when fossil fuels are utilized for energy production. However, economical and life cycle analysis of WPF installation needs to be carried out to see the financial benefits and to estimate the CO2 reductions for the region.