Harvesting offshore energy through the combination of different generation technologies: Hybrid Offshore Farms

  ⁽¹⁾Senior Researcher, INESC TEC & Assistant Professor, Faculty of Engineering of the University of Porto

When thinking about offshore energy, wind power comes directly to our minds. In fact, this technology is the major responsible for offshore energy generation, being already on a commercial state. From the 2000s decade onwards, wind power industry had a massive and sustained growth, with great emphasis on Europe. Portugal is one of the good practice examples regarding wind power integration - which, allied with favourable hydro power, allowed achieving considerable share of renewable integration in its electricity mix. Many other countries are pursuing wind power generation and, more recently, photovoltaic (PV) generation to attain the so-called sustainable electricity by reducing the fossil-based fuels need and consequently CO2 emissions.

Notwithstanding, the major spots for wind power generation are already in use across Europe. In addition, the plans for decommissioning nuclear power plants, carbon markets and more recently, the natural gas price increase, are putting an additional burden on electricity generation, where wind power plays an important role. Aware of the challenges, the European Commission (EU) has anticipated the need of going offshore with an expected amount of 60 GW of offshore wind and 1 GW of ocean energy, to be installed by 2030 (European Commission, 2020). According to WindEurope association (WindEurope, 2020), by the end of 2020, 25 GW of offshore wind power were installed in Europe. These farms are typically installed close to shore in monopiles or through foundation jackets. However, the floating technology, already deployed in Viana do Castelo offshore farm, unlocks the possibility for going further offshore, exploring deeper water depths.

Offshore energy is not strictly associated with wind power. As a matter of fact, several developments have been made on wave energy harvesting. Some interesting concepts have been explored, but failed on real-word tests due to the inability of the wave energy converters’ (WEC) on withstanding the wave forces. Recently, some companies are presenting successful designs to generate electric energy through the wave motion at offshore level. Moreover, PV technology has also been utilised over water through floating devices. Even though most of the projects are implemented on rivers or dams, there are some pilot projects at offshore level, presenting very satisfactory results, namely in terms of withstanding harsh sea conditions (e.g., sea storms). Despite being reduced in size and on maturity, wave and floating PV are seen as an interesting alternative for offshore energy harvesting.

All the offshore renewable technologies share the common challenge of the interconnection to mainland. The connections must be performed through the adoption of underwater cable circuits, which brings high costs. The connection costs together with the offshore farm costs leads offshore projects to having a high levelised cost of electricity (LCOE), meaning that the €/MWh generated tend to be higher than onshore similar projects. In this sense, the need of having more renewable-based electricity at lower costs, motivates exploring the possibility of combining different generation resources on hybrid offshore energy farms.

Hybrid offshore energy farms will make it possible to harvest more renewable-based electricity from the sea and, by sharing the same interconnection infrastructure, will allow reducing the connection costs, contributing for lower LCOE. Although more installed capacity commonly means the need of more cable capacity, more energy generation may not have that impact, if the offshore hybrid systems are designed to exploit the correlation between the renewable sources, increasing the usage of the given connector cable. This means that for a given offshore wind farm, a certain amount of hybridisation can be installed (i.e.,: wind + wave energy, wind + floating PV, wind + wave + floating PV) making a better use of the interconnection infrastructure, harvesting more renewable-based energy, not impacting on using a cable with more capacity - thus not increasing interconnection costs and consequently, helping on reducing the LCOE.

What is the ideal offshore technology mix?

Offshore hybrid farms, as previously stated, are composed by a combination of different offshore generation technologies that share a common interconnection infrastructure. There are two ways of pursuing a hybrid offshore farm. The first consists of hybridising existing offshore wind farms with other technologies to increase the generated electricity without exceeding the interconnection cable capacity, by exploiting the correlation of involved energy sources. The second solution consists of building a hybrid farm from the scratch. On both cases, it is of upmost importance determining how much of each technology will constitute the hybrid farm. To achieve such fulfilment, it is necessary to have renewable resource data and computational models responsible for simulating different combination of technologies to determine what is the best set to be implemented. There is also additional data related with specificities of the geographic location, such as water depth, sea soil composition and environmental restrictions, that must be incorporated to the problem, to shape the solution for the case that is being analysed. Thus, there is not a common solution that can be extrapolated for everywhere, but there will be important guidelines that could be considered for different geographic areas.

What is the amount of hybrid offshore renewables that can be connected to mainland power system?

The renewable integration, especially from offshore, will depend on the mainland onshore grid connection capability, on the interconnection technology (high voltage direct current – HVDC or high voltage alternate current- HVAC) and on the dynamic stability aspects of mainland grid. So, it is important to perform an thorough assessment to determine the maximum interconnection volume and locations.

From a regional scale point of view, it is necessary to determine the connection capability of each onshore substation, defining a hypothetical amount of connection capacity. Moreover, at the system level, it is important to understand the impact of transposing the regional interconnection capacity to the power system operation. To do so, a set of analysis on N-1 operation and dynamic security assessment must be performed to determine the effective capacity for secure offshore farm interconnection. In this sense, the adoption of offshore grids (HVAC or HVDC) may also pave the way for increasing the interconnection between countries and globally the share of renewable energy integration.

How to maintain offshore hybrid farms?

Maintenance is one critical aspect related to offshore operations, since it is hard to be performed, especially on harsh sea conditions, and it is quite expensive (at least in comparison to mainland maintenance operations). It is consensual that the hybrid offshore farms will need maintenance, with different requisites that vary according to the selected technology mix; it is crucial to assess how to deliver it at the most affordable way. Determining the maintenance needs and the best timeframe to perform such actions, requires the incorporation of computational tools combined with accurate monitoring of the hybrid farms and involved renewable resources and sea conditions forecast. Digitalisation will play a key role on monitoring, together with proper models, and they will help decision makers on determining the most cost-effective maintenance schedules.