By Anirudh Sharma, Assistant Manager, WRA, Industrial Services, TÜV Rheinland India
India has set an ambitious target of reaching 175 GW of installed capacity from Renewable Energy sources by the year 2022, which includes 100 GW of Solar and 60 GW of Wind power capacity. Various policy initiatives have been taken to achieve this target. As of December 2019, the total renewable power installed capacity in the country was almost 86 GW.
To meet the day to day increasing load demand, conventional energy sources are no longer a viable solution as they are depleting rapidly. Solar, due to its dependence on sunlight, can produce power only during the day mainly between 8 am and 5 pm. Wind, on the other hand, usually is more during late evenings and reaches its peak at night. Due to this complementary intermittent nature of wind and solar, power production can be leveled out throughout the day with a Solar-Wind Hybrid. With a hybrid, reliability of the grid is improved by ensuring peak power requirements are met.
The introduction of the National Wind-Solar Hybrid Policy has been instrumental in encouraging the hybridization of existing wind and solar plants as well as promoting Hybrid plants in India. Further, the objective of the policy is to optimize and improve the efficacy of the usage of transmission infrastructure and land, which in turn will mitigate inconsistencies associated with the generation of renewable power and help in attaining better grid stability.
The primary aspect of the hybrid plant is the configuration - AC or DC integration and the use of technology. The second important aspect would be related to sizing - which would depend on resource characteristics. In order to achieve the benefits of a hybrid model in terms of optimal and efficient utilization of transmission infrastructure and better grid stability by reducing variability in renewable power generation, in the locations where wind power density is quite good, the size of the Solar PVs capacity to be added as the Solar-Hybrid component could be relatively smaller. On the other hand, in the case of sites where wind power density is relatively low or moderate, the component of the Solar PV capacity could be on the higher side.
Advantage of Wind Solar Hybrid Plants
- Complementary resource characteristics: Wind and Solar energy resources are complementary on a diurnal basis, with peak wind times after sunset and before sunrise and peak solar times aligning with periods of lower wind resource.
- Efficient use of land: To make efficient use of land between wind turbines which are duly spaced apart to avoid row effects.
- Analogous technical processes: Both Wind and Solar rely on natural sources and can be integrated into common AC or DC output to feed into the local utility grid.
- Cost efficiency from shared infrastructure: Shared data collection systems, O&M service facilities, asset management and common point of interconnection are beneficial for cost efficiency, especially for projects with higher capacities.
According to MNRE, the implementation of a Wind-Solar Hybrid system will depend on different configurations and use of technology.
a. Wind-Solar Hybrid - AC integration: In this configuration, the AC output of both the Wind and Solar systems is integrated either at the LT side or at HT side. In the latter case, both systems use separate step-up transformers and the HT output of both systems are connected to the common AC Bus-bar. Suitable control equipment is deployed for controlling the power output of the hybrid system
Figure 1. Diagram of AC-coupling topology for integrating the wind/ solar/ battery
The only advantage of this system is that it is most compatible with the existing transmission framework in India.
b. Wind-Solar Hybrid - DC integration: DC integration is possible in case of variable speed drive wind turbines using converter - inverter. In this configuration, the DC output of both the Wind and Solar PV plant is connected to a common DC bus and a common inverter suitable for combined output AC capacity is used to convert this DC power into AC power.
Figure 2. Diagram of DC-coupling topology for integrating the wind/ solar/ battery
The DC-coupling topology has the advantages of simplified hardware, lower cost and higher energy efficiency.
Aim to make maximum use of the Land area. Solar and Wind resources could be explored separately in different regions or the same region but Solar and Wind resources should exhibit complementary peaks in production on an annual and daily level.
Sizing of Hybrid Power Plant
Optimization is the main objective achieved by minimizing the system cost while maintaining reliability.
Sizing is done by mathematical models to get the best output from the combination. The number of PV panels, wind turbines, battery cells, load profiles, and available renewable resources play a significant role in the sizing of PV-Wind Hybrid power systems. In order to utilize renewable energy cost-effectively, many researchers have studied the algorithm to calculate the capacity of applicable generator units that can constitute a reliable power system at a low cost. Many different sizing methods, such as the iterative method and artificial intelligence method, have been reported to design a techno-economically optimum hybrid renewable energy system.
The battery bank with the total nominal capacity is permitted to discharge up to a limit defined by the minimum state of charge. For a good knowledge of the real State of Charge (SOC) of a battery, it is necessary to know the initial SOC, the charge or discharge time, and the current.
Factors to Consider during Energy Simulations
- Turbine Shading: Shading impacts the generation of Solar PV panels. This is required to be quantified. Shading will cause mismatch losses in PV arrays
- Roughness because of solar arrays needs to be quantified
- Effective Hub Height of turbines will change because of PV arrays. This can be taken care of by using the concept of Displacement Height
Wind Farm Loss
- Wake Effects
- Availability (Machine & Electrical System)
- Electrical (Component efficiencies, wiring)
- Turbine Performance (operation, power curve, high-wind hysteresis, inclined flow)
- Environmental (blade degradation, temperature shutdown, site access, lightning)
- Curtailments (directional, environmental, PPA, operational)
Solar PV Farm Loss
- Effective Irradiation (Shading, Soiling, Reflection)
- Photovoltaic Conversion (Mismatch, Quality Degradation, Non-STC Temperature & Irradiance)
- Electrical (Component Efficiencies, Wiring)
- Operational (Availability, Curtailment)
Hybrid Energy Uncertainty (Mutual Interactions)
Apart from individual uncertainties, there are mutual interactions between the two resources that are needed to be taken into account:
- Wind Uncertainty (Affected by PV arrays)
- a) Wake loss uncertainty (due to solar arrays)
- b) Availability loss uncertainty (due to more difficult crane access around solar arrays)
- Solar Uncertainty (Affected by wind turbines)
- a) Turbine shading loss uncertainty
- b) Shading-induced mismatch uncertainty
- c) Inverter availability uncertainty due to shadow flicker
- Reliability constraint
- Fluctuations of power injected into the grid
- Battery constraints - As the batteries are the components with the least lifespan in the system, they need to be given the most attention. In this method, the battery State of Charge (SOC) is maintained within the minimum (SOCmin) and maximum (SOCmax) limits to ensure that the battery is neither completely discharged nor fully charged at any point in time
- Physical accessibility of turbines because of PV arrays
The Ministry of New and Renewable Energy (MNRE) released a solar-wind hybrid policy in 2018 which provides a framework to promote grid-connected hybrid energy through set-ups that would use land and transmission infrastructure optimally and also manage the variability of renewable resources to some extent. India is not the only country planning hybrid projects; over 50 hybrid projects of MW-scale have already been announced or are under construction globally, with Australia and US being the leaders. Hybrid Energy Systems can be potential solutions for the electricity problems especially in rural India. However, more exhaustive research is needed on this subject to make it technically feasible and deployable.
Renewable hybrids can play a key role in helping India accelerate the decarbonisation of power generation and lower the cost of electricity generation in the medium term. However, there is a need to ponder on the pros and cons of building new coal-fired plants and renewable hybrids. There is also a need for investors, who are evaluating existing stressed coal-fired assets, to understand the future value and potential of such assets 5-7 years from now. Most importantly, the policy and regulatory environment in India needs to evolve more in order to fully capture the potential of Hybrid Energy.
Source and further readings
National Wind-Solar Hybrid Policy — MNRE (2018)
M. Engin, “Sizing and Simulation of PV-Wind Hybrid Power System,” Hindawi Publishing Corporation International Journal of Photoenergy, March 2018
R. A. Badwawi, M. Abusara & T. Mallick (2015) “A Review of Hybrid Solar PV and Wind Energy System, Smart Science, 3:3, 127-138, DOI: 10.1080/23080477.2015.11665647
K. S. Sandhu and A. Mahesh, “Optimal sizing of PV/wind/battery Hybrid Renewable Energy System Considering Demand Side Management,” from International Journal on Electrical Engineering and Informatics - Volume 10, Number 1, March 2018
C. D. Ziesler, P. N. Johnson, and J. R. Doty, “Wind-Solar Hybrid Design and Modeling,” webinar presentation from Underwriters Laboratories, September 2019.