Wind power is currently one of the most important alternative sources of renewable energy, offering the major advantage of zero emissions of greenhouse gases by reducing and even eliminating the use of fossil fuels. By developing the wind energy sector as the energy of the future, environmental and sustainable development requirements will be met while achieving energy security objectives. Implementation of wind power projects aiming at building wind turbines that produce more energy, thus reducing costs in the context of sustainable development, has been a major concern in recent years.
Over the years, researchers have consistently sought new ways of making wind turbines more and more eco-friendly [1
]. Research in recent years has provided an overview of different solutions to develop a new generation of wind turbines, and feasible and technological trends in wind power generation that could reduce costs. The need to use renewable energy, as well as a review of various wind technologies in connection with their applications and operating devices, is detailed in this paper [2
]. It is demonstrated that the cost of generating electricity, as well as the current economic and environmental policies, supports the installation and development of wind power systems. This paper provides arguments on the usefulness of this research in the development of wind assemblies. In [3
], some technological solutions regarding the power electronics in the wind turbine systems are presented. The paper provides an overview of the state of the technology and discusses the technological trends in the field. There are also some difficulties that arise in the operation of the control part, generators and gearboxes, where the main components are known to be bearings. It is necessary to find solutions to improve the operation and reliability of wind turbines, and the solutions proposed in the present paper follow this direction. Another technical solution, consisting of the implementation of a new concept, “the adaptive-blade concept in wind-power applications”, is presented in [4
]. One of the technological challenges in the wind energy field is the development of a new generation of feasible, improved turbines, which further reduce production costs. As the size of the typical turbine increases, weight reductions, as well as complexity in the design of rotors and their auxiliary mechanisms, are becoming increasingly important. The study proposed for reducing inertial masses by implementing large bearings with hollow rollers is of great interest. After some research on the reliability of wind turbines [5
], analysis of the failure modes, and causes of defects, it was found that the highest failure rate occurs in some key components of the wind turbine assembly, including the generator, gearbox and blades. Transmission and generator faults are mainly caused by bearings. Ref. [6
] discusses the analysis requirements for the design and operation of the main bearings in modern multi-megawatt wind turbines, with a view to finding technical solutions for bearings with high reliability and profitability.
Wearing processes considerably reduce the precision and durability of bearings [7
]. In this paper, an analysis is carried out of bearing failures caused by wear. The presence of different types of wear and the detection of bearing defects are studied. The bearing elements (inner and outer rings, rollers) are put into operation with complex loads, such as: high strains and alternating stretch and compression, rolling friction, abrasive friction and corrosion.
Worldwide, in the field of large-sized bearings, studies only began relatively late (1970–1975), compared with studies in the field of small- and medium-sized bearings. In countries with bearing manufacturing traditions (USA, Japan and Germany), a great deal of research on large bearings has been seen in the last decade. It has been found that wind turbine generator bearings have a surprisingly high failure rate, with failures happening too early due to classical rolling contact fatigue [8
]. Reference [9
] studies the behavior of the bearings in the gearbox of a wind turbine and the impact on its reliability. Inertial forces on small- and medium-sized bearings do not have the negative effects that they have on large-sized bearings.
Today, all the construction solutions for wind turbines use large roller bearings with solid rollers, with expensive logarithmic profiles to reduce contact pressures and increase resistance under variable load conditions [10
]. Based on all of this research, this paper proposes new technical solutions for improving the lifespan of wind turbines, especially the large-sized bearings inside the gearing system, by implementing an automated lubrication system in the hollow rollers. Following previous studies by the authors [12
], a solution to using hollow roller large bearings in the construction of wind power plant assemblies is proposed.
The proposed solution using hollow rollers requires only a cylindrical profile (easy to obtain) with a similar behavior to the solid rollers with a logarithmic profile. Due to the weight of the rollers, inertial masses have been widely investigated, with the use of hybrid or ceramic bearings being proposed [15
]. Their high cost requires new research on the reduction of inertial masses of large-size bearings. Attempts have been made to reduce the negative influence of centrifugal loads by decreasing the density of the rolling elements, in turn achieved by using lower density materials.
There are constructive solutions that have adopted drilled or hollow steel rolling elements [16
], but these are not intended for the construction of large-size bearings for wind turbines. A solution for determining the size of the hollow roller was described in [18
]. The analysis has shown that the durability of hollow cylindrical roller bearings operating at the maximum allowable stress level can be substantially higher than the durability of solid roller bearings.
After analyzing the specialty literature, it was found that there is no standard method for calculating the optimal hollowness of hollow roller bearings. This depends mainly on the applied load, and the size and the material of the roller. The use of hollow rollers in large bearings has some advantages over the solid rollers. These include reducing the material used in roller making, reducing the weight of the roller bearing, and attaining the preloading capability of the hollow cylindrical element, thus generating more stability, and less noise and vibrations. Hollow roller bearings are single- or double-row radial bearings with an inner ring, outer ring, and cylindrical or thin wall rollers. The thin wall of the rollers allows preloading, as opposed to cylindrical bearings with solid rollers. Preloading rollers on large bearings with hollow rollers increases radial stiffness and reduces vibrations [19
On the other hand, major problems in the operation of a wind turbine occur in the lubrication of the bearings. Appropriate lubrication is essential for the correct operation and lifespan of the bearing. Existing lubrication system automation involves expensive and complex solutions [20
], or solutions that require highly qualified personnel and maintenance stops for wind farms. Operation and maintenance costs are estimated to reach up to 30% of the total lifecycle expenditure [21
]. The proposed lubrication system is inexpensive, and easy to carry out and maintain.
Reducing subsidies in this area transforms this topic into a mainstream one, as it leads to important savings by lowering the cost of maintenance of wind farms by increasing the durability of large bearings, as well as by increasing the energy efficiency and yield of the whole system. The expected impact of the implementation of hollow roller bearings and the automated lubrication system is important in times of economic crisis, mainly in the area of sustainable growth.
1.1. Wind Energy, Strategies and Directions of Development
The wind power industry has grown greatly around the world, and is among the green energy resources. The use of wind energy has increased nearly four times between 2004 and 2015 and currently accounts for about one third of renewable electricity. The use of terrestrial wind energy is quite close to the anticipated trajectory over the years [22
]. The Global Wind Energy estimation [23
] explores the future of the wind energy industry by 2020, 2030 and by 2050. Several scenarios have been developed by the International Energy Agency [24
]. Based on the advanced scenario, GWEC estimates that by 2050, global wind power will reach 5806 GW (Table 1
At the end of 2016, in the Global Wind Energy Council Report [23
], 341.320 wind turbines were catalogued as being in operation, of which 104.934 were in China, 52.343 were in the USA, and 3589 were offshore wind turbines in Europe at the end of 2016. The evolution of the total capacity of all wind turbines installed worldwide during the period 2013–2017 is presented in Figure 1
, according to preliminary World Energy Association (WWEA) statistical data [25
]. Thus, in 2017, a capacity of 6145 MW was installed, with a growth rate of 12.30% compared to the previous year (Figure 2
), representing a revival.
Many countries have based their strategies of phasing out fossil and nuclear energies on the development of wind power energy sources. A recent study in the specialty literature [26
] presents the current state of lifespan extension of offshore wind turbines in Germany, Spain, Denmark and UK. According to [23
], among the top 20 countries in total wind power capacity in 2017, the fastest growth of new facilities was found in the United Kingdom (+4.3 GW/+29.2%), Brazil (+2.0 GW/+18.8%), Ireland (+0.4 GW/15.8%), India (+4.1 GW/+14.5%) and France (+1.7 GW/+14.0%). Year 2017 was a spectacular year in the field of wind energy generation, with Denmark setting a new world record, with 43% of its power coming from wind. Countries like Germany, Ireland, Portugal, Spain, Sweden or Uruguay have reached a double-digit electricity share, as shown in the WWEA Report [25
The Global Wind Energy Council (GWEC) considers, based on market statistics for February 2018, that wind energy has a major role to play in the sustainable development, “wind is the most competitively priced technology in many if not most markets, and the emergence of wind/solar hybrids, the more sophisticated grid management and the increasingly affordable storage, begin to paint a picture of what a fully commercial fossil-free power sector will look like [23
].” Cumulative total installations are expected to reach 840 GW by the end of 2022 (Figure 3
For the European Union, combating climate change is an important objective to ensure sustainable development. The measures taken by the EU to meet this main target, as well as the necessary activities in this area, both for the next period up to 2020 and for the period after 2020, are detailed in the GWEC Report [23
]. Thus, it is considered that “Europe is expected to align with its 2020 targets, and current discussions within the EU indicate that overall renewable targets could be raised to 35% by 2030, putting the industry in a position stronger for the post-2020 market. In Europe, it is expected to install 76 GW of new wind power energy by the end of 2022, reaching a cumulative total of 254 GW”.
1.2. Wind Energy Potential in Romania
To make the EU a truly smart and sustainable low-carbon economy, the European Commission and the member states need to work together to use renewable energies. The strategy of the European Union and its member states on wind energy development consists of “stepping up urgent efforts to use wind energy as part of a global strategy for renewable energy and to develop a roadmap for a future 100% renewable energy”, as highlighted in the (WWEA Report, 2018) [25
]. The adoption of the “Clean Energy for All Europeans” package of measures aims to maintain the EU’s competitiveness as the transition to clean energy changes the world’s energy markets [28
Currently, in Romania, wind energy has priority development through national strategies for sustainable development by 2030 and 2050 [30
]. Recent Eurostat statistics for 2017 [32
] highlight the efficiency of wind energy implementation in the EU Member States. Thus, at the end of 2016, EU wind turbines generated 315.00 GWh of electric energy (equivalent to more than 10% of the 3.1 million GWh produced in the EU). Denmark ranks first (43%), and is followed by other countries: Ireland (21%), Portugal (20%), Spain (18%) and the United Kingdom (14%). Romania, with a rate of about 10%, ranked tenth (Figure 4
Regarding Romania’s wind energy, five wind zones were identified, depending on the environmental, topographical and geographical conditions, taking into account the level of energy potential of such resources at an average height of 50 m and over. The results of recorded measurements show that Romania is in a temperate continental climate with a high energy potential, especially in the coast and coastal areas (mild climate), as well as in alpine areas with mountain plateaus and peaks (severe climate). Based on the evaluation and interpretation of recorded data, it is possible to conclude that in Romania, the wind energy potential is most favorable on the Black Sea coast, in the mountain areas and plateaus in Moldova or Dobrogea.
Measures performed in our country demonstrate a high wind capacity, confirming that Dobrogea is, along with northern Scotland, the most promising wind exploiting region in Europe, according to the Romanian Wind Energy Association (RWEA) specialists. In the RWEA Report [33
], “Wind Energy and Other Renewable Energy Sources in Romania”, there are areas (Scotland, Iceland, Denmark, Costa Rica, Tasmania, Australia) that have planned different deadlines for full reduction of carbon emissions and the development of programs to ensure green energy independence for the period 2020–2030. On 1 January 2017, Romania recorded 3025 MW in investments of over 5 billion EUR. Today, our country numbers 20 large wind farms, ranging from 70 MW up to 600 MW of installed power. Romania sources 12.3% of its electricity consumption from wind power [34
Analyzing the widespread use of wind energy in Romania, the proposal to implement a completely new automated lubrication system positioned in the hollow rollers of large bearings is, from an economic and technical point of view, one of major interest with respect to the sustainable development of the green energy field.
2. Materials and Methods
Renewable energy sources are playing a major role in turning the EU into a world leader in innovation, with the EU owning 30% of all world-wide patents on renewable energy [22
]. In the field of wind energy generation, a significant part of the cost reductions can be achieved through technological improvements and the implementation of innovative solutions that lead to sustainable development.
This research was conducted in two stages. In the first stage, the design of the hollow rollers was carried out using the finite element analysis method applied by established software, Nastran and Catia, and validation was done through direct measurement. The results consisted of studying the distribution of stresses and deformations in the rollers and bearings. The second stage was the designing, developing and testing of a prototype lubrication system implemented in the hollow rollers of large bearings.
Large-size bearing modeling has become a mandatory issue, precisely because of its size. Reducing inertial masses on these bearings would be a leap forward across the whole bearing industry. Referring to the product catalogues of the leading bearings companies (SKF, TIMKEN [35
], INA, FAG [37
]), cylindrical, conical or barrel roller bearings were identified in sizes up to 7 m and weighting several tons. All of these bearings have massive rollers—solid, large masses, with high inertial moments, which are disadvantages. The implementation of hollow roller bearings in wind power plants responds to the two major problems encountered in the operation of wind farms: increasing lifespan and increasing energy yield.
At present, the lifespan of a turbine is approximately 20 years, with 1/3 of this time being scheduled for maintenance. Maintenance costs increase with the age of the wind farm. According to WMI—Wind Measurement International [39
]—a first-generation turbine has a maintenance cost of about 3% of its initial value; currently, this cost is falling to about 1.5%. Reduction of maintenance time can be done by increasing turbine durability to external factors: wind turbulence, variable air density, humidity, salinity, temperature. It has been found that the resulting defects lead to a 10% reduction in produced energy [40
], and half of these are due to defects in bearings. All wind turbines have their own wind measuring devices and, based on the information gathered by them, the computer itself makes the adjustments necessary for optimal operation without the need for a human operator to be present at all times. The wind turbines include remote means of verification and remote control, thus making management of production units easier. Since this is an installation with moving parts, wear and maintenance costs will occur [35
2.1. Operating Principle of a Wind Turbine
The system describing the operation of a turbine is based on a simple principle. The wind moves the blades that, in turn, actuate the electric generator. The mechanical system [41
] includes a speed multiplier that directly operates the central shaft of the electric generator.
shows the main rotation movements of a horizontal wind turbine; the pivot system can be seen to feature a large bearing.
The electrical current obtained is either transmitted for storage in batteries and then used by an inverter for low-capacity turbines or delivered directly to the AC network for distribution. The turbine that drives the electric generator is also driven by the wind pressure. The amount of electricity produced by a wind farm depends on the type and size of the turbine and the location of the farm. At low speeds, it does not generate electricity. From Beaufort 2 (about 3 m/s) onward, the turbine delivers its maximum power. At a wind speed of over 25 m/s, the turbines are designed to lock and brake in a controlled way to avoid overloading and damaging the turbine installation or construction. The latest developments are equipped with a tilt angle control device that changes the angle of the rotor blade under unfavorable weather conditions [42
Wind turbines are equipped with a robust safety system including an aerodynamic locking system. In case of danger, or for shutdowns needed in maintenance, a locking disc is used. The rotation systems that actuate the electric generator have roller bearings provided with logarithmic profile rollers on which carburetor thermal treatment is made at high depths of 7–10 mm [12
Reducing roller masses directly reduces inertial moments, increasing start-up speed, offering easier handling of the plant, reducing static load in the bearing, and reducing vibrations. The influence of inertial masses in the wind turbine construction, implicitly in the construction of large-size bearings, is studied by Song, Dhinakaran and Bao [43
2.2. Logical Scheme for Choosing the Optimal Construction Solution for Large Bearings
illustrates the logical scheme of the interaction of influence factors in the operation of large-size bearings. Each category of factors interacts and influences the end point of the logic scheme, namely the duration of operation. Both the static load created by the bearing construction and the dynamic load due to the external wind conditions directly determine the contact stresses, hence the deformations. The roller construction, material conditions and thermal treatment are designed to counteract the negative effects of variable loads, and the temperature generated by additional frictional forces.
An appropriate lubrication system, reducing loads by reducing inertial masses, as well as faster and clearer system response to external disturbances, is a result of using hollow roller bearings in the construction of large-size bearings. Generally, when studying a bearing, the lifespan of the bearing and the system in which it is mounted are studied. In the analysis of a bearing, dynamic loading is an essential element in its definition and mathematical modeling. Dynamic loading occurs between rolling elements and rolling treads due to the movement of the rolling elements, both the movement around the bearing axis and the movement around its own axis. These movements at higher or lower speeds, depending on the use of the bearing, generate forces that interact and produce stresses between rolling elements and rolling treads.
Controlling the stresses that appear on the surface of the rolling elements leads to increased lifespan and improved bearing efficiency. In the case of large cylindrical roller bearings, due to their mass, the stresses are higher and generate significant defects and high maintenance costs.
The purpose of this applied research was to design and test a functional hollow roller model provided with an internal automated lubrication system for the large bearing as a sustainable solution in the field of wind energy. The optimal solution of the automated roller lubrication system was identified and the dynamic behavior of such a roller was studied. The center of gravity of the roller changes with the removal of the lubricant from the roller to the lubrication pathways. The prototype was made, and the test stand will be made as the next research step. The stand will validate or invalidate, based on actual measurements, the behavior of the lubrication system using different geometries as a result of the lubricant discharge from the rollers.
Several hollow roller patterns with different hollowness were modeled and developed, starting from the actual situation of large bearings with solid rollers. CAD/CAM programs were used, the hollow rollers being the main element in their design. To encapsulate a lubricant that improves the lubrication of the bearings, the covers were also designed and assembled. For the study of stresses by finite element analysis, the contact stresses and deformations encountered during contact with the roller types proposed in this research were compared.
Based on the dynamic analysis of the rotor assembly, a comparison was made between a solid roller bearing assembly and a hollow roller bearing assembly. By performing Von Mises analysis, the results demonstrate that with the decrease of inertial masses, the loading forces decrease and, at the same time, the effort across the system decreases.
The solution of using hollow rollers in large bearings has important effects in reducing material consumption, increasing energy yield (in the field of wind farms), increasing the durability of large bearing systems, reducing operating noise, increasing resistance to vibrations, decreasing losses through friction and lowering the working temperature. Performing studies on the behavior of these bearings under extreme vibrational conditions and variable forces would be one of the future research directions. The benefits obtained by introducing large bearings with hollow rollers into wind power assemblies are demonstrated by presenting the estimated economic calculation of product implementation. The economic impact is achieved by reducing the cost price of the whole energy package, which could be the starting point for the revitalization of the wind energy industry.