Study on the structure of offshore wind and solar hybrid power generation device under the background of green energy

There are sufficient solar and wind energy in the sea, which can be used as a good power generation energy and obtain great energy value. Therefore, the development of offshore green energy has become the mainstream trend. This paper mainly focuses on the study of offshore wind solar energy generation structures. This paper first introduces the principle of wind power generation and photovoltaic power generation and the existence of a large amount of energy offshore, and then leads to the basic structure of wind-solar hybrid power generation structure. Then it further studies the power generation, wind load, wave load, initial stability and economic type. Finally, the advantages and future environmental benefits of the structure are summarized, and the structure is reformed reasonably. The results show that the structure has good stability, load bearing capacity and economy, and the construction difficulty of the improved structure has been reduced.


Introduction
Marine green energy refers to renewable energy that can be harvested from the sea, including the unique wave energy, tidal energy, tidal current energy, etc.Among them, tidal energy and tidal current energy are generated by tidal forces, while the rest energy is generated by solar radiation [1].In addition, marine green energy also includes offshore solar, wind energy and other sources.
In order to fully implement the strategy of carbon neutrality and carbon peak, adhere to the harmonious coexistence between human and nature, the development of Marine energy is inevitable.China has unique geographical conditions: 18,000 km mainland coastline and 14,000 km island coastline and more than 10,000 islands [1].In addition, China's offshore wind power energy has developed rapidly and has a large scale.After the 13th Five-Year Plan, the annual installed capacity growth rate exceeds 30% [2].Compared with onshore wind power, the advantages of offshore wind power lie in that there are no buildings at sea, and the wind speed is much higher than that of onshore wind speed and the wind speed is stable.Therefore, the power of offshore wind power is much higher than that of onshore wind power, which is more suitable for the energetic development and utilization.
Offshore wind power generation mainly relies on wind turbines, which mainly include gravity type and floating type [3].Among them gravity type is the most commonly used.However, with the continuous development of offshore wind power, it is an irresistible trend for wind power generation to the deep-sea field, and the structure of wind power generator will evolve to the floating type.Currently, floating wind turbines around the world mainly fall into three types: Spar type, semi-submersible type and tension-leg type, and derivative structures based on these three basic structures are constantly produced [3].At present, the development of offshore wind power in China has been supported by a number of policies.Before 2022, offshore wind power has been subsidized by the central government from 0.85 yuan/kw•h to 0.4 yuan/kw•h, and the cost of offshore wind power is expected to be reduced to within 60% of the original by 2025 [3].However, offshore wind power also faces severe challenges.For example, the offshore environment is more complex than the onshore environment, and the construction is difficult.Furthermore, the manufacturi ng cost, the transportation cost and the maintenance cost are higher.The noise generated by wind turbines will have a certain impact on the Marine ecological environment.The stress situation of offshore fan is complicated, and it has high requirements for structural stress [3].
Offshore photovoltaic power generation converts solar energy into electricity through semiconductors, which is usually converted by solar panels formed by crystal silicon cells in series, so as to supply electricity directly or input it into storage batteries [4].An important factor in the design of photovoltaic power generation is photoelectric conversion efficiency.With the continuous deepening of research, photoelectric conversion rate will continue to improve, and more and more electric energy can be converted by photovoltaic panels per unit area [4].
In order to further promote the development of offshore wind power and promote the development of offshore green energy, this paper will study the structure of offshore wind and solar complementary power generation platform.The usual methods to improve the stability of floating platforms include widening the spacing between the columns, lifting the weight of the platform, and increasing the waterplane area.However, these methods will increase the cost of structural design and affect the platform's own cycle and load [5].The structure selects 10 MW wind turbine and folding photovoltaic panel, and adopts semi-submersible structure [6].The combined structure can not only save the sea area, but also make the energy utilization more efficient, laying a development trend for the comprehensive use of a variety of green energy.

Structure
This paper uses the method of modeling analysis to study the structure.Today offshore wind power is developing towards large size and high efficiency.The research object of this paper is the semisubmersible platform structure with strong wind and wave resistance, and the wind motor with a power of 10 MW and the photovoltaic panel with a power of 2+ MW.The working environment of the research model is set as the sea area with a maximum depth of 100 meters, with a maximum wave height of 9 meters and a corresponding period of 12 seconds, which is a once-in-50-year sea condition.The software tools used for modeling include CAD and Sketch up.By establishing a 1:1 3D model, the stability of the platform is verified by using the theory of ship initial stability simulation, including mass estimation, center of gravity and center of buoyancy calculation.On this basis, considering the further reduction of the center of gravity of the structure and the feasibility of the implementation of the fan hoisting project, this paper studies the 2.0 version of the platform model.Through comparative analysis, evaluate whether the improvement measures are reasonable.

Wind load.
The wind load on the floating fan can be expressed by F and calculated as follow.
In this expression,  ℎ represents height coefficient and   represents shape coefficient.The values of height and shape coefficient can be obtained based on different situations.A is the projected area of the structure in the wind direction, P is the wind pressure.P can be expressed as follow.
In this expression, v represents wind speed.

Wave load.
According to Ranges of suitability of various wave theories, this paper classifies the wave types of assumed sea states as linear waves.The wave load is then calculated using the linear wave theory along with wheeler's stretching method, which has less error and is more realistic than the more conservative constant stretching method.The formula for calculating Morrison force of wave load per unit length can be seen in following equation.
The constants C M and C D are determined by assuming that the structural material is a new type of uncoated steel.Finally, the calculation of Morrison force is assisted with Matlab.

Cost analysis
The cost of the structure was evaluated considering the initial construction cost, including the cost of the fan generator set, tower, photovoltaic panel, semi-submersible floating platform and construction installation.The detailed costs of each facility come from the past studies or projects.Through basic calculations, the cost of the whole structure can be derived, while the return cycle can be estimated.

Load
3.1.1.Wind load.According to the above formula and data, the wind load of this structure is estimated.Below 15.3m above the sea level, the height coefficient is taken as C h =1.0 for semi-submersible floating platform, photovoltaic panel, connecting truss, beam and other structures.In the check Table, the shape coefficient is taken as C s =0.5 for floating platform, and 1.5 for photovoltaic panel and 1.3 for truss, respectively.According to the estimation of the wind area, the wind area of the buoy of the semisubmersible floating platform is 840.94 m 2 , and the wind area of the photovoltaic panel is 10324.376m 2 with inclination of 35°.The wind area of the truss is about 1000 m 2 .
Through the basic additions of the product of area, height coefficient and shape coefficient, a value of 18,783.87m 2 can be derived.The actual wind speed at sea is 12 m/s.In extreme circumstances, such as once-in-50-years sea conditions, the extreme wind speed at sea is 40m/s.It is calculated that the actual wind load at sea is about 1658.09kN, and the wind load under extreme conditions is about 18423.22 kN.
Renderings of the two wind energy structures are shown in the Figure 1 and 2, espectively.We assume that the structural material is a new type of uncoated steel, and the roughness k is 5*10 -5 .According to the above formula and data, it can be seen in Table 1.
Table 1.Parameters of wave load.

Checking the initial stability of the structure
According to the ship initial stability theory [7], when the ship tilts at a small Angle (10° ~ 15°), it is assumed that the equal volume tilt, and the intersection line of the waterplane passes through the centroid (also known as the drift center) of the original water line [8].The initial stability radius BM is only related to the drainage volume and the moment of inertia of the water line.GM can be used as a basic indicator to measure the initial stability of a ship.To make the ship stable, GM>0 is necessary.
The above conclusions are also applicable to the semi-submersible floating wind turbine platform.The structural diagram of the semi-submersible platform can be seen in Figure 4 and Figure 5, respectively.The photovoltaic panel area is divided into two layers, the upper layer area is 3600 m 2 , the lower layer area is 14400 m 2 , the total power of 2.16 MW.The body of the fan is a tower with variable section, and the rated power is 10 MW.The total mass of photovoltaic panel and fan is 844,400 t.
The data for platform 1.0 and 2.0 are summarized in Table 4 and 5, respectively.1.47 M Platform 2.0 has a lower center of gravity than platform 1.0 and requires less ballast, but the ballast is better.The initial stability of both is greater than 0.

Cost analysis
With reference to the wind turbine equipment of Zone A project of Changle Offshore Wind Power Plant, which is the target of Dongfang Wind Power in 2020, the unit cost of the fan generator set of this structure is about RMB 6,590 /KW, and the cost of 10 MW fan is about RMB 65.9 million [9].The total weight of the fan tower is 628.4 tons, and the material of the tower tower is Q345E.The cost of each ton is about 4350 yuan, so the cost of the tower tower is about 2,733,354 yuan.The cost of each WP of polysilicon photovoltaic panel is 0.00040 million yuan, and the cost of 2.16 MW is about 8.64 million yuan [10].60 sets of 60 KW charging piles are selected for photovoltaic panels, each of which costs about 295,000 yuan, a total of 17.7 million yuan.Photovoltaic panels support the truss structure of about 180 tons, and the cost of the truss structure is about 3,800 yuan/ton, a total of about 684,000 yuan.For the floating platform, 35,425.28tons of concrete were poured, and the unit price of C20 concrete per ton was about 300 yuan, totaling 10,627,580 yuan.Q235 steel was used for the buoy, and the unit price was about 4,000 yuan/ton.The total mass was 17,241.98tons, the cost was about 68.9679 million yuan.The mass of the platform truss was 3,947 tons, the unit price is 3500 yuan/ton, about 13.8145 million yuan in total.Construction, installation, cable and sea use costs account for about 15% of the total cost.To sum up, the total cost of this project is about 222.43 million yuan, and the cost per KW is about 18,292 yuan.Estimate that the profit of the electricity per KWh is 0.85 yuan, the annual working hour is about 4000 hours and the annual maintenance cost is about 10 million yuan, so the profit per year is about 31.344 million yuan.So the return period is about 7 years.However, the structure could be combined with fishery to construct a fish farm right behind the structure, which could greatly reduce the return period.

Cause analysis of structural deviation
Firstly, according to the stability analysis and load calculation of the structure, the calculation results show that the safety and rationality of the structure meet the requirements.In the process of calculating the wave load, some results do not accord with the actual situation, and a large wave load is obtained.This is because Morrison force is used in the calculation of wave load in this paper, but the actual wave is not completely composed of linear waves, which does not fully conform to the assumptions of the model.The Kirilov force can be approximated by multiplying the estimated wave load by 2 to achieve accurate wave load estimation as far as possible.Unlike most installations, this structure considers the placement and installation difficulty of photovoltaic panels while maximizing the contact area with light.In the design of the foundation column, the stability and economic effect are calculated respectively in the form of the column and the round table.The advantages and disadvantages of each structure are obtained through the comparison of the two designs, which has a certain reference function in the actual construction.This comparative research method can also promote the improvement of the structure and better find its shortcomings.

Suggestions
This structure will install the fan in the geometric center, in the installation stage is very difficult, operability is not very high.Therefore, the fan can be installed in the corner is more reasonable.In addition, the economic cost of the device can be further optimized.Currently, wind and photovoltaic power generation installations around the world are still relatively expensive, and further optimization of materials and specific construction operations needs to be implemented.In addition, the folding setting of photovoltaic panel can be considered.Through the three-dimensional setting, it can not only increase the light area, but also reduce the footprint space and improve the utilization rate of space.Due to the lack of fan models on the market, the adoption and adjustment of parameters can be further adjusted.

Directions for prospective research
This structure is slightly insufficient in innovation, and further innovation can be carried out based on the setting and photovoltaic panel.For example, the basic form can be improved to the round table and so on.In addition, the size of the structure platform is too large, which may bring problems to the installation.In addition, in terms of load estimation, the model adopted by the structure may be different from the actual situation, but not completely consistent with it.In order to ensure safety, we have adopted a larger estimate in a prudent way, which may lead to the waste of some building area, which is not fully utilized.

Conclusion
By installing 10 MW wind turbine and 2.16 MW photovoltaic panel, the offshore wind power generation device realizes the comprehensive utilization of solar energy and wind energy, and the wind load and wave load data are reasonable.Under normal conditions, the wind load it receives is about 1658.09kN and the wave load is about 17.96×10 6 KN.And the sea wind generator has good stability.The center of gravity height is 11.62 meters and the center of buoyancy height is 13.41 meters.The center of gravity is lower than the center of buoyancy.It shows good initial stability, workability and serviceability when being exposed to various environmental loads.What's more, marine wind power generation device can bring good environmental benefits.The use of clean energy can reduce carbon dioxide and other greenhouse gas emissions, reduce the loss of coal and other natural energy and eliminate noise bringing to the environment which can impact less on the marine lives.For constructability and material stiffness, the offshore wind turbine has been improved by moving the wind turbine from the center to the corner, and changing the shape of pontoon from a cylinder to a platform.All these facts show good sustainability.

Figure 2 .
Figure 2. Wind power structure 2.0.3.1.2.Wave load.We assume that the structural material is a new type of uncoated steel, and the roughness k is 5*10 -5 .According to the above formula and data, it can be seen in Table1.Table1.Parameters of wave load.
force, according to the code procedure of calculating k by Newton method above, k=0.0282 of this structure is calculated.Stretching was calculated by Wheeler's stretching method.Using Matlab's function to generate image function, we get the results as shown in the Figure3.

Figure 5 .
Figure 5. Schematic diagram of Platform 2.0 structure.Data of photovoltaic panel and fan are summarized in Table2 and Table 3, respectively.Table2.Photovoltaic panel research parameters.

Table 2 .
Photovoltaic panel research parameters.