Solar Wind Hybrid Plant : Technology And Policy 

A hybrid power plant in simple terms is a plant which generates power by using two or more sources of energy. These sources could be either conventional or non-conventional or both. Such power plants are preliminarily useful as they reduce dependency on a single source while also enabling the power plant to produce more power output. Now with the world's focus on non-conventional power sources (solar and wind energy pre-dominantly) it is important that such sources are utilized to their maximum potential. While individually, these sources have been performing quiet fair, there are still few concerns which needs to be addressed in order to utilize these sources efficiently.A simple yet innovative solution is hybridization of this technology or a hybrid solar wind plant (shown in Figure 1). Similar to the other hybrid plants, here both solar PV panels and wind turbines could generate electricity and feed it into the grid. With the target of 10GW by 2022 from such hybrid plants and with both the central & various state governments supporting this idea, hybrid plants are poised to increase. This blog thus aims to educate its readers on the basic technical details along with the advantages of such hybrid plants.

Figure 1: A solar wind hybrid plant (Source: Google images)

Before understanding the technicalities of hybrid plant, it is important to understand why such plants are necessary at the first place. Its advantages are mentioned as below:

1. Complementary power generation: The first and foremost advantage of such hybrid plant in the complementary power generation. The solar power plant produce energy during the daytime whereas wind plants are generally known to produce (more) energy during evening and night (compared to daytime). The individual generation curves of solar and wind plant as evident from figure 2 below are intermittent in nature and could not adequately match the overall energy demand. This is where the hybrid power plants could be of utmost importance. Superimposing both these generation curves they could manage the overall demand to some extent (as shown in figure 3). Additionally with the central/state regulators pressing against providing schedule and forecasting of renewable power plants such hybridization (along with storage) could help power generators to closely match the given schedule while helping the SLDC/RLDC/POSOCO in getting power generation values (with certain tolerances) and balancing the against the power demands.

Figure 2: Power output generation curve; solar power (left) & wind power plant (right) (Source: Gujarat SLDC)

Figure 3: Super imposed generation curve of solar and wind energy in comparison to energy demand (Source: Google images)

2. Increased capacity utilization factor (CUF): A capacity utilization factor in simple terms is the ratio of actual energy generation of the power plant to the maximum possible generation in such period (24 hours, 365 days in a year) under operation. A solar power plant (due to the limitation on availability of sun) has a CUF varying from 16-21%. A wind power plant (location wise variation of wind at different heights) may have CUF varying from 20% to 26%. This means that for about 74 to 84% of the time, the plant remains idle without any generation. Studies however suggest that CUF ranging from 35% to as high as 50% (in few cases) have been obtained for hybrid power plants. This means that a higher energy per watt could be obtained which primarily improves the plant statistics while holistically reducing the LCOE of the power plant (compared to LCOE of individual power plant).
3. Increased utilization of transmission capacity: Utility scale renewable energy plants are usually located quiet far from load centers. This means that adequate transmission infrastructure is required to transmit such power to load centers. Individual power generation curve of solar and wind plants clearly suggest that such transmission infrastructure remains un-utilized. With tens of lakhs required to erect a HV transmission substation, it is important to utilize it to maximum. This is where a solar wind hybrid plant comes into play. The hybrid plant due to its complementary nature of generation (as we explained above) could utilize such infrastructure more efficiently when compared to individual plant.
4. Efficient usage of land: The next advantage is that the hybrid power plants efficiently utilize the available land space. The land requirement of a utility scale power plant is in between 4-6 acre/MW. A wind turbine may vary in sizes and power output, however most commonly it would occupy an area ranging from 10 to 50 acre/MW (Source: NREL) (the wind turbine only occupies 5% of the area). The land savings per MW in hybrid plant could be as much from 10-30%. Additionally with same transmission infrastructure used to evacuate power, such costs could be reduced too.
5. Easier renewable energy purchase obligation (RPO) achievements: A RPO as the name suggest is an obligation for an entity (i.e. state distribution companies, open access customers and captive generators) to source a part of their energy need from renewable energy sources. It is divided into 2 components i.e. solar and non-solar RPO. While an enforcement of RPO is in place, (baring mere a few) as almost every obligator has been a defaulter. A hybrid power plant would help where both solar and non-solar RPO of an obligatory entitycould be fulfilled.

As we mentioned above, the solar wind hybrid plant would be of importance given their advantages compared to the individual plant. With such understanding in place, let us understand the technical scheme in practice for a hybrid power plant.There are two categories by which there hybrid plant could be erected. They are:

6. AC coupled hybrid plant: An AC coupled power plant as the name suggest is the one where the individual power plant are interconnected at AC side. A single line diagram (SLD) for an AC integrated hybrid power plant is shown in Figure 4 below. The configuration of both solar and wind power plant here remains almost comparable to the individual standard plant with their output most likely integrated at HT panel of AC yard after individual meter. This ensures that the utility company knows the exact generation from each plant and financial settlements could be done accordingly (if PPA for the solar and wind plants were signed individually at different rates). Such integration may be perfect for plants existing solar/wind power plant are hybridized with new wind/solar power plant. This type of system would be more perfect for fixed speed wind turbine given the fact that they are directly integrated to the grid (using intermediate induction generator).

Figure 4: An AC coupled hybrid power plant

7. DC coupled hybrid plant: A DC coupled hybrid power plant (as shown in Figure 5) on the contrary is the one where the DC power output of individual is connected to a common DC bus. This entire power output is then converted to AC power by using a common inverter which is further connected to the grid. Such plants are termed in the market as "true hybrid" because they use the evacuation infrastructure to the maximum.This configuration is more often suitable for new power plants and/or for specifically with plants having variable speed wind turbine. This is because the capacity of common bus, inverter and the protection on both DC & AC side would require adequate planning which is not possible in existing plant. If optimally erected, such hybrid plant could offer a potential savings from INR 5-10/Wp

Figure 5: A DC coupled hybrid power plant

While the technology is in place, it was necessary that a policy specific to this technology was passed. This was also to ensure that in addition to erecting new plants, the existing solar/wind plants were given fair chance to upgrade their power plant by adding appropriate capacity of wind/solar power plant. The MNRE has released the solar wind hybrid policy where both AC & DC coupling is allowed, however to be termed as hybrid plant, the power capacity of one resource should be at least 25% of the rated power capacity of other resource. The hybrid plants are encouraged to sell the generated energy with all the available business models i.e. captive consumption, sale via open access, sale to Distribution Company via bidding or at APPC. It also allows power plants to install battery storage given its various benefits. Few states (Gujarat, Andhra Pradesh) have realized potential of hybrid plants and have released draft policies (hybrid policy of Gujarat is finalized and notified). With more states realizing its potential, such plants would increase in years to come.