Challenges and potential of offshore solar

In the past few years, there has been an increasing interest in the development of offshore floating solar power plants.

The technology has a very large power generation potential, in particular in the areas used by offshore wind farms.

In some scenarios, 45GWp solar power in the Dutch part of the North Sea has been projected in2050. The Dutch Roadmap Solar on Water concludes that in ten to twelve years, large scale application of offshore solar on the North Sea could offer possibilities, but it also expresses major concerns about the technical and economic challenges of offshore floating solar systems.

Perspective

Significant innovation in this field by commercial enterprises will only be undertaken if there is enough perspective for a profitable business case in a large market. This is only possible if are likely to be solved and costs can be decreased considerably. Nevertheless, several companies (especially in the Netherlands and in Norway) started investing in system developments that may lead to feasible and affordable solutions for offshore solar. Based on this, some smaller scale pilots have been established or are planned.

CO2 free

There are substantial technological challenges to build and operate floating solar systems in a harsh maritime environment, with wave heights up to 15m. At the same time, there are major challenges on system design, O&M schemes, grid connection and cost. The Dutch top sectors strengthen the economy through innovations that target to help solving important societal challenges such as the energy transition. In this approach the societal and economic goals go hand in hand. Recently, this has substantiated in so called missiondriven innovation. For the energy transition thirteen multiannual mission-oriented innovation programs (MMIP’s) have been formulated, and MMIP 1, titled ‘Renewable electricity at sea’, has the goal to contribute to the mission ‘A CO2 free electricity system in 2050’.

For the development of the MMIP 1 program, a further analysis of the challenges related to offshore floating solar was needed. For this, RVO (Netherlands Enterprise Agency) commissioned a report, requested by TKI Offshore Wind, based on an analysis conducted by TNO. Recently, the results of this report, called ‘Challenges and Potential for Offshore Solar’, was presented during a webinar.

According to Wiep Folkerts, Program Manager Solar at TNO, the Dutch energy system would need an additional 1,000km2 of space to develop solar power plants. “On shore, the Netherlands lack sufficient space to realise the ambitious solar energy goals”, he says. “At sea however, there still is ample space available in the Dutch Economic Zone. Offshore solar and offshore wind offer important synergies, for example because of the infrastructure that could be shared, such as cables and substations. Therefore, offshore wind farms seem to be ideal location for solar power plants. Another benefit of developing solar power plants at sea is that, because of the higher radiation of the sun at sea, the output of offshore of solar panels may be slightly higher compared to that of onshore panels.”

Various concepts

He continues, “Currently, in Europe, such as in the Netherlands and in Norway, there are already various concepts under development. The first concept is the so-called elevated truss concept. A truss structure is equipped with floaters and some kind of mooring system. A platform with solar panels is placed on the truss structure, supported by floating elements. The platform is located on a certain height above the waterline. Waves can move underneath the truss structure. The concept avoids the direct contact between (slamming) waves and the solar panels. This way, the mechanical load on the solar panels caused by waves will be less, and the fouling of the solar panels due to sea water residues on the panel surfaces may be smaller.

Companies like SolarDuck and Tractebel are working with this concept. The second concept is a pontoon where the solar system is much closer to the water level. The pontoons are substructures that are mechanically coupled into a massively modular structure. The complete floating structure is moored. Solar panels are mounted on the pontoons, relatively close to the water level. The advantage is that the wind load on the panels and the structure is relatively low. Oceans of Energy, and Heliorec are some of the pioneers in developing this concept. Then you have the mattress-like soft and flex approach that fully moves with the waves. The basic idea of this concept is to let the floating structure move with the waves as much as possible. The transfer of mechanical energy from the waves to the structure will therefore be smaller, compared to for instance the pontoon concept, which results in reduced mooring forces. Because of this the structure needs less strength and may achieve lower costs. Solar panels are mounted on the floating body. During high waves, part of the structure may be overflown. Being near the water surface, it is less sensitive to wind loads. Bluewater/ Genap have embraced this concept. The final concept is based on the well-known technology of fish farms. Especially in Norway, with companies such as OceanSun and Inseanergy, there is expertise on this approach. A membrane is stabilised with a moored ring and on top of the membrane a solar power installation is installed.”

“Offshore solar and offshore wind offer important synergies, for example because of the infrastructure that could be shared, such as cables and substations.”

Oceans of Energy’s offshore solar energy pontoon installed at the North Sea. The pontoons are substructures that are mechanically coupled into a massively modular structure. Image courtesy of Oceans of Energy.

Challenges

The TNO report registered four main challenges for the development of offshore solar power plants. “When looking at constructing offshore solar power plants,” Mr Folkerts states, “the first challenge is how to design and construct a system that will survive the circumstances at sea, and also how to prove this survival. Relevant subjects in this are the connectors between the subsystems, the choice of materials used, the structural integrity and mooring, the dynamic power cable connection, and the calculation methodologies for prediction of hydrodynamic response. In fact, it is a matter of interaction between development of calculational methodologies and making the designs, do scale model testing for those designs, build pilots, see and measure how these models behave, give feedback to the methodologies, and thereby building up the knowledge on how to construct such floating structures in a cost- effective way.”

Operations & maintenance has its challenges too, and therefore there is work to be done and more knowledge to be built up over the coming years for this as well. Think for example of offshore resistant PV modules and electrical components. Not only is it important to find out for them how to survive at sea, but also how to make an effective maintenance schedule, for example for fouling prevention and fouling removal. For optimising O&M one could also look at digital twinning approaches.” The third challenge lies in the power production and LCOE, and according to Mr Folkerts this actually is the key challenge. “All offshore solar projects are aimed at producing substantial volumes of power. It is important, for example for judging investment proposals, to make validated power yield models and reliable models for cost calculations and to show, how scale effects and other effects such as the combination of offshore wind and solar will affect the LCOE.” Societal acceptance is the fourth challenge described in the report. Relevant aspects in this are recycling and other elements of circularity, the ecological effects of offshore solar power plants, and the interaction with other uses of the sea. “All societal acceptance elements come together in permitting and permitting frames”, Mr Folkerts explains.

Bluewater/Genap have embraced the soft and flex approach. Image courtesy of TNO.

Solid solution

“Offshore solar is promising, so much is true”, Mr Folkerts concludes. “It can be the solution for the large-scale solar Ocean Sun Floater. The fish farm concept is based on the well-known technology of fish farms. Especially in Norway, there is expertise on this approach. A membrane is stabilised with a moored ring and on top of the membrane a solar power installation is installed. power production that we need. Still, it is a challenging development that needs innovation investments from the sector, which should work together with governments and knowledge institutes. There are different concepts in development and all those concepts have their specific advantages, values, and promises. Several pilots and demonstrators for accelerating the learning curve are underway, however more of them are needed towards scaling up and for winning the confidence of all stakeholders that offshore solar indeed is a solid solution.”

Source: Yellow & Finch Publishers