-The following article is co-authored by Dr Ashwani K Sharma, Department of Electrical Engineering, NIT Kurukshetra and Siddharth Singh, Student, Pursuing MTech, NIT Kurukshetra
A comparison of wind and solar in terms of forecasted growth and consumption
This is to highlight the fact that harnessing solar energy is preferred over wind energy due to the difference in the cost of conversion system required i.e. the cost of solar converters are much cheaper than that of wind when compared per unit energy generation. As a result of massive price declines in recent years, solar power is now widely recognised as a cost-competitive and reliable source of energy. The only advantage of installing wind conversion systems is that winds are available during the night.
Figure 1 shows that bioenergy will remain the predominant source of renewable energy. However, its share of total renewable energy declined from 50 per cent (in 2017) to 46 per cent (in 2023) as the expansion of both solar PV and wind accelerates in the electricity sector. The corresponding growth of wind power is from 9 per cent to 12 per cent (33.33 per cent growth) and of solar power is from 4 per cent to 8 per cent (50 per cent growth).
Figure 2 shows a comparison of wind and solar on global cumulative installations and annual new installations. The figures have a close approximation to actual data till the year 2017, beyond that we have predictions till the year 2021.
Over the years, solar has lagged significantly behind wind power in terms of annual installed capacity and cumulative capacity but is rapidly closing the gap in annual installations. Already by 2019, it is predicted that solar will surpass wind in new installed capacity. In 2013, as the wind power industry experienced a big slump in new capacity, solar for the first time surpassed wind in annual installations. Wind retook the lead in new installations in 2014 and 2015 but in 2016, solar opened up a wide gap with 77 GW in new installations or 22 GW ahead of the wind. In 2017 (preliminary figures), new PV installations were almost twice as high as wind (99 GW vs 53 GW). A year of strong growth is predicted for 2018 with new installations expected to surpass the 100 GW mark for the first time with an increase of more than 14 per cent to 113 GW, up from 98.9 GW in 2017.
Why India should focus on improving wind technology?
India has a huge energy potential, as the statistics by Global Wind Atlas shows that the mean wind speed and mean wind power density in India is far better than the world average. In addition, India has not yet realised even the quarter of its on-shore wind potential and has not invested in off-shore wind conversion.
Figure 3 shows the comparison of Wind Energy Installations among the continents. One can clearly observe that Asia stands on the top. The credit goes to China. In 2017 (preliminary figures), China was by far the largest country by both new installed capacity (19.5 GW) and total capacity (188.2 GW). Moreover, India has contributed only a small fraction of wind power to Asia by both new installed capacity (4.148 GW) and total capacity (32.848 GW) despite ranking 2nd all over the continent.
The data matches with various sources. As published by IRENA, total wind installed capacity of the world is 513,547 MW out of which 494,821 MW is on-shore out of which India contributes only 32,848 MW on-shore capacity with no off-shore generation. Similarly, statistics from IEA electricity information shows that total wind electricity generation of the world till 2016 is 957,694 GWhr and that of India is only 44,856 GWh. The examples clearly show that India is lagging behind in this field and it needs to perform better.
Classification of Wind Energy Conversion Technologies
- Wind Turbine (Horizontal axis or Vertical axis)
- High Altitude Wind Energy Technologies
- Air Rotor Systems
- Airborne wind turbines
- Tethered airfoils (kites)
Among these, Tethered airfoils (kites) generator systems are mostly preferred.
The principle of the ‘air rotor systems’ developed by Magenn (MARS) Power Inc, shown in figure 3, is that a helium-filled balloon stationary at an altitude between 200 mëter and 350 meters rotates around a horizontal axis in response to the wind because of the Magnus effect, generating electrical energy via a generator connected to its horizontal axis. The energy produced is then transmitted to the ground by a conductive cable. Magenn tested a 2 kW prototype in 2008, and in 2010 has started manufacturing and commercialising a 100 kW balloon.
The second solution adapted by Sky Wind Power, Joby Energy, and Makani Power is to use airborne wind turbines to harness energy directly in high altitude winds and send it to the ground through conductive cables. Figure 4 shows the airborne wind turbines proposed by Joby energy. This solution has some technical complexities, high cost and heavy structures. Only Makani, acquired by Google.org as part of GoogleX, has passed to the production phase. They have already started producing a 1MW airborne wind turbine named ‘Makani M1.’
The third option is to use power kites as renewable energy generators such as the ‘Kite Wind Generator’ of Politecnico di Torino, and the ‘Laddermill’ of the Delft University of Technology as shown in figure 5. In this case, mechanical power is generated when the kites are pulled by wind, transformed them into an electrical one using an on-ground generator. This allows the flying part of the system to be much lighter and avoid using conducting cables. This technology is expected to produce huge amounts of power using a much simpler and safer structure.
Figure 6 shows the basic elements of a Kite Generator System (KGS). On the top is the kite in the shape of a parachute. The natural path followed by this kite is upward with the wind in an eight shaped orbit. The tether is a cheap rope made of fibre having good mechanical strength. The one end of the tether is connected to the kite and the other end is wounded on a drum. The drum rotates to unroll the tether and the kite goes upwards. An electromechanical energy conversion (EMEC) device is connected on the same shaft as the drum through a gearbox. Hence, the linear kinetic motion of the kite is converted into rotational motion of the drum and is used to generate electricity using an EMEC device.
The Kite Generator System (KGS) is a Relaxation Cycle System; it is composed of a Traction phase. In Traction phase, the kite goes up following an eight shaped orbit, hence, drum unrolls and EMEC device acts as a generator. In the recovery phase, the electrical energy is consumed to bring the kite back down, the EMEC device acts as a motor to deliver the power to the shaft and the drum rolls back the tether.
The need to operate kite in a specific range of height (generally 350 to 500m) is explained in figure 9. Figure 8 shows the different kite power region which is in the shape of a quarter sphere, considering the direction of wind going into the page. There is a lower limit under which the kite has a risk of falling. Slightly above is the maximum power region where one gets a peak in the power curve and then comes the medium power region where power output is almost constant with an increase in height. The efficiency above a certain height is low. Hence as soon as kite reaches minimum power region, a relaxation cycle is initiated in order to bring the kite back in the maximum power region of operation.
Comparison of Wind Energy Conversion Technologies
From the grid connection point of view, wind turbines are not able to produce their rated power continuously due to wind irregularity at their working altitudes, a problem that is less significant in the case of HAWE systems which are supposed to be working at an altitude higher than 400m where the winds are more regular.
Concerning the quality of generated power, it depends whether the system returns power to the energy source or not, meaning whether it has a recovery phase or not. In general, a classic turbine has only one phase of functioning that is a generation, which means that while generating, the resulted power is continuous as long as the turbine is in the power region limited by its cut-in speed and cut-out-speed. This is the case of stationary air rotor systems also.
Meanwhile, kite-based systems and airborne wind turbines have a recovery phase whose goal is to maximize the average generated power and the respect of the constraints of the system, but reflects negatively on the generated power which becomes intermittent. This, however, may be balanced out by the high reversibility of these systems that allows using two or more systems with a suitable choice of the kite’s orbits to filter the resulted generated power.
Furthermore, HAWE systems offer mobility and can be invested hugely as it works at a high altitude where the strong wind could be present with little or no wind at low altitudes. Besides, they offer very high adaptability, as their rated power, as well as, generation or consumption phases can be modified by changing the orbit the kite is following e.g. size, rotation and inclination, or changing the altitude. Notably, a kite-based system rated power can be adjusted by changing the kite surface. These adjustments are important to optimise the system’s generated power for changing conditions and constraints on it, e.g. Wind speed and direction.
Cost-wise, HAWE systems economise the manufacturing, transportation and construction cost as compared to a wind turbine, e.g. they eliminate the turbine mast cost. Finally, a kite-based system backs down when it comes to the real-time control issue. That is due to the complexity of the system’s behaviour, a matter that will not be a problem thanks to the rapid development in computer and information technology, allowing having fast and reliable real-time data processing.
Advantages of High-Altitude Wind Energy (HWAE) systems over Turbines
The four major reasons why people are interested in airborne wind energy for electricity production are the following:
- First, like solar, wind power is one of the few renewable energy resources that is in principle large enough to satisfy all of humanity’s energy needs, and are available at night.
- Second, in contrast to ground-based wind turbines, airborne wind energy devices might be able to reach higher altitudes, tapping into a large and so far unused wind power resource. The winds in higher altitudes are typically stronger and more consistent than those close to the ground, both on and off-shore.
- Third, since the equipment is portable and lightweight, the same equipment could be adopted for both on-shore and off-shore.
- Fourth, airborne wind energy systems might need less material investment per unit of usable power than most other renewable energy sources. This high power-to-mass ratio promises to make large scale deployment of the technology possible at comparably low-costs. In fact, the initial investment is ten times lower and the cost of maintenance is negligible when compared to the conventional wind turbine.
The reasons are strong enough to force a developing country like India to invest in HWAE technologies.
Challenges in adopting Tethered airfoils (Kites) generator system
The power electronic circuitry to act as an intermediate between the grid and the generator is easily available. But it is difficult to design a power electronic circuitry which will act as an intermediate between the generator and the Tethered airfoil (kite) because of the variations in speed. Further, it is difficult to design a control mechanism for the orientation, the curvature of the kite and the gearbox. Finally, the most difficult task is to control and optimise the trajectory of the kite. But the solution to the problem exists and is adopted by various organisations like Makani, Kitenge etc all over the world.
KGS is being adopted worldwide; it is the future of the power industry. Since India is having huge wind energy potential, it should start focusing on technology. As India’s on-shore wind capacity remains underutilised and off-shore wind capacity is un-utilised, KGS provides an economical solution to the problem. This technology has the potential to pace up the growth of wind energy contribution to the whole world and it may outperform existing solar PV modules.