Offshore Wind Energy and the Romanian Energy Future

. The aim of the present work is to assess the electricity production coming from an offshore wind farm that may operate in the northern part of the Romanian coastal area. In the first part, a complete description of the Romanian energy sector is presented considering the time interval from January 2008 to December 2018. In general, the electricity sold is negative (exports exceed imports), with the mention that a significant contribution comes from hydroelectric and coal generation. It is important to mention that, if one of these two sectors will no longer perform on full capacity, the electricity balance will be shifted to the electricity imports. As for the wind energy, the average values from the vicinity of Sulina site may vary between 5.6 m/s and 8 m/s depending on the season, these results being reported at a wind turbine level (80 m). By using an offshore wind farm which replicates the Greater Gabbard project (504 MW), England, was possible to estimate the annual energy production and to indicate the expected impact on the energy sector. For example, a single wind farm may cover 1.7% of the total production, which may be further associated with 9.6% from nuclear, 7.6% from hydroelectric or 6.4% from coal, respectively.


Introduction
The renewable energy sources represent an important part of any energy market, being possible at this mo ment to implement smart energy systems in order to develop a sustainable future [1]. The countries located near the coastal areas may expand the energy portfolio, by adding some particular natural source, such as the offshore wind or waves. By looking on Europe map, we can see that most of the countries are defined by marine areas capable to support renewable marine projects [2,3].
Ro mania is located in such region, being defined by 245 km of coastline facing the north-western part of the Black Sea. Co mpared to some other renewab le sources, the evolution of onshore wind is visible, starting fro m 1.32 MW in 2005 and being expected a 4000 MW for the year 2020. The hydropower sector may increase fro m 6289 MW (2005) to 7729 MW (2020), wh ile in the case of photovoltaic and biomass a cumulated value of 860 MW is predicted for 2020 [4]. Regarding the electricity coming fro m fossil fuels, more precisely fro m coal production, we can see that as in any other countries, the Ro manian electricity market is heavily supported by this sector. Ro mania is a major coal producer, being included in a Top 6 producers from the EU countries and in a Top 25 reported on a global scale [5]. The impact of the restructuring measurements performed in the min ing sector in 2012, w more v isible in 2016 when the production was reduced with almost 39% and 49% in the case of the lignite and bituminous coal, respectively.
Nevertheless, during the last years, this market was more stable being possible to report some exports, which was estimated to be around 2% from the entire production [5].
Another important source of electricity comes fro m the hydropower p lants, which are defined by total capacity of 6443 MW reported fo r the year 2015. As expected, this sector is significantly in fluenced by the presence of dry season, being reported events when the mu ltiannual average flow rate may fall fro m 1226 m 3 /s to almost 679 m 3 /s (the dry year 1990) [6].
During the recent years, a significant amount of work was dedicated to the assessment of the wind conditions fro m the Black Sea basin. A comp lete description of these resources is presented in Ganea et al [7], which also cover the interval fro m 2021 to 2050. Th is is also the case of Rusu et al [8] where a hindcast databas e was considered for investigation. In Onea and Rusu [9], the performances of some state-of-the-art wind turbines that may operate near the Black Sea coasts were estimated. A full spectrum of systems was considered, starting fro m a 3 MW device and ending with a 9.5 MW system. The efficiency of some wind turbines was discussed in Onea and Rusu [10], where a special attention was given to the diurnal and nocturnal fluctuations of the wind resources. Rusu et al [11] carried out a joint evaluation of the wind and wave resources, the emphasis being put on the main shipping routes that cross this region. From these results, it is clear that the western part of the Black Sea reveals more impo rtant wind resources, this area being also defined by a continental shelf.
As for the Romanian coastal environment, previous studies suggest that the northern part of this region show more significant wind resources [12,13]. The operational onshore wind farm Fantanele/Cogealac is located near to this region, being considered to be one of the largest project from Europe with 600 MW [14].

Methods and materials
For the present work, a reference site located in the northern part of the Ro manian coast will be considered for evaluation, as can be noticed from Figure 1. Th is is located at appro ximately 23 km offshore fo r which correspond a water depth of 32 m, all these characteristics replicat ing the conditions reported by the Greater Gabbard wind farm fro m England. A nu mber of 140 units of SWT-3.6-107 defines this project, which is operational since 2013 [15].  Table 1 [16], and based on these values was proposed a case study. The wind data processed in this work are related to the ERA-Interim dataset, that is assembled at the European Centre for Mediu m-Range Weather Forecasts (ECMWF) [17]. These values cover the time interval fro m January 1999 to December 2017 (19 years), being defined by a spatial resolution of 0.125° × 0.125° and a temporal resolution of 6 hours (4 values per day corresponding to 00-06-12-18 UTC, respectively). The wind conditions fro m this dataset are co mputed at a height of 10 m (U10) above sea level.
The main focus of the present work is to assess the expected electricity production of the SWT-3.6-107 wind turbine wh ich operates at a minimu m hub height of 80 m, an therefore the initial wind values (U10) will be adjusted to this height (U80) by using the following logarithmic law [18,19]: where, U80 -wind speed at 80 m, U10 -init ial wind speed (at 10 m), 0 z -roughness of the sea surface (0.0002 m), 10 z and 80 z -reference heights.
The Annual Electricity Production (AEP) of a particular wind turbine can be obtained as [15]: where, AEP is in MWh, T -average hours per year (8760), f(u) -Weibull probability density function, P(u)turbine power curve. The cut-in and cut-out values define the operational limits of a wind turbine, being mentioned in Table 1.
The literature review shows that the project Greater Gabbard generated in 2013 an annual net output of 1800 GWh [20]. We have estimated the performances of this wind farm (140 x SWT-3.6-107) for the Greater Gabbard site, by using the ERA-Interim values (U80) reported for the year 2013. When comparing our result (1792.85 GWh ) with the reported one, we may notice that the differences are very small (0.004%), wh ich indicate that the results expected for the Sulina sites are solid enough.

Results
In the first part of this section is evaluated the Romanian electricity system, by considering the values reported by the national energy portal [21]. This info cover the interval fro m January 2008 -present, being reported for the entire energy sector, and the temporal resolution of this data being close to 10 minutes.
In order to reveal the main trends, in Figure 2 was represented the evolution of the annual electricity production (average values) for each production sector, by considering the values reported between January 2008 and December 2018. In general, the electricity production is located in the range of 57960 GWh and 64550 GWh, more consistent values being reported for the interval 2014-2018. This energetic sector is main ly supported by the contribution coming fro m the coal, hydroelectric and nuclear sectors. The evolution of the electricity sold is presented in Figure 3 and Table 2 (average values), being also included a short time interval fro m December 2018 to January 2019. However, according to the values reported during the latest two months, December 2018 and January 2019, we may notice that this pattern is severely changed, the balance being shifted to imports (390 MW). The main reason for this change is that during this interval, a significant percentage of the Ro man ian miners working at the Oltenia Energy Co mplex were on strike during an entire week. It is important to mention, that this min ing sector cover almost 30% of the brown coal supply used to generate electricity [22]. In order to see what happen if the coal sector will no longer generate electricity one day, we proposed some scenarios in Table 2. According to these results, Ro mania will beco me a net importer of electricity, wh ich on a long term will be reflected by a rising electricity price.
Go ing to the offshore wind conditions, in Table 3 are presented some statistical values reported for the total distribution and for the representative seasons, where: Spring -March, April, May; Su mmer -June, July, August ; Autumn -September, October, November; Winter -December, January, February. As expected, the wind speed (average value) is more consistent during winter, when a maximu m of 8.02 m/s may be reported. In summer, we may expect a minimu m of 5.56 m/s, which is below the values reported during the spring and autumn, respectively.
The downtime index represents the time percentage during which the turbine will not generate electricity, being reported to the cut-in value of the SWT-3.6-107 wind turbine. The rated capacity is defined as the time percentage during which a wind turbine will operate on full capacity, being taken into account only the values located between the nominal wind speed and the cut-out value of the SWT-3.6-107 generator. A maximu m downtime of 23.67% may be expected in summer, co mpared to winter where only in 8.7% percentage of the time the turbine will not operate. In winter, such turbine will obtain better performances in almost 6.77% of the time, being followed by autumn (3.16%), spring (1.5%) and summer (0.76%).
A more detailed investigation of the downtime windows is presented in Figure 4, by considering the consecutive period during which the wind speed does not exceed the cut-in value. Fo r examp le, sequence 1 means that during two consecutive hours (ex: 00-06) the wind speed was below cut-in, while for a sequence 2 this number was increased to three consecutive hours (ex: 00-06-12). In this way, it is possible to estimate the number of suitable time windows of inactivity during which the turbine will not operate. One limitation of this evaluation is that only 4 data per day are available for evaluation. For sequence 1, all the events putted together sum almost 56 days, which is followed by sequence 2 with 32 days and by sequence 3 with 20 days.
In Figure 5 is presented the annual energy production reported by the SWT-3.   By co mbin ing the results provided in Figure 2 and in Figure 5, was possible to identify the electricity percentage covered by an offshore wind farm that may operate in the northern part of the Ro manian coastal area, these results being provided in Figure 6. As we can see, a single pro ject may cover almost 1.7% fro m the entire p roduction, being possible to replace almost 10% fro m the energy generated from nuclear sources and close to 15% in the case of the onshore wind. For the hydroelectric and coal productio n, a ma ximu m of 7.6% may be expected. The electricity coming fro m the photovoltaic pro ject can be easily replaced by a single offshore wind farm, wh ile the current biomass output is already covered.
In an ideal scenario, the electricity demand per each sector can be covered fro m: 10.4 pro jects -Nuclear; 6.8 projects -onshore wind; 13.2 projects -hydroelectric; 9.6 projects -hydrocarbons; 15.6 projects -coal or 1.3 projects -photovoltaic. The investment required for the Greater Gabbard project was close to £1.6bn, being possible to supply renewable energy to almost 530000 homes per year and creating in this way around 100 permanent jobs [23].

Conclusions
In this work, the expected benefits that may occur from the imp lementation of an offshore wind project near the Sulina site (Ro mania -north) was evaluated. Based on the electricity values reported on the national level, the fact that at this moment there is no backup plan in terms of the energy production was highlighted. If one of the main sectors (nuclear, coal or hydroelectric) will reduce their contributions, this will mean that the electricity need will be covered from imports.
Since Ro man ia has the possibility to develop offshore wind pro jects, it was considered interesting to assess the performances of a wind farm that may operate close to the Sulina site at appro ximately 23 km fro m the shore. Based on these results, it was noticed that a significant percentage of electricity demand can be covered throughout a such project. Many scenarios can be developed, only if we take into account that at this mo ment in European waters are being imp lemented generators defined by 8 MW rated capacity.