The solar industry in India set up as early as 1977 sustained for over three decades purely on the government supported remote home lighting programmes like lanterns, home lighting and street lighting and remote industry applications like rural telephony. Battery was a key component in all these off-grid solutions. However for the battery industry, the solar market was too insignificant to be taken seriously and for the solar industry module prices were of far greater concern than all else and batteries did not receive the attention it deserved. In fact, with many failures in the off-grid systems, traced to battery, arising out of improper use and poor maintenance, it was considered “a weak link”.
The solar landscape however witnessed dramatic transformation since the Gujarat Solar policy in 2009 and the National Solar Policy in 2010 were announced. From a mere 12 MW in 2010, the grid connected solar installed capacity rose to 12 GW in March 2017. By March 2018, the installed capacity is projected to be approaching the 25 GW mark and the capacity addition will be ramped up further in the following years to reach the 100 GW target by 2022.
With continued large scale integration of the intermittent solar power into the grid, the manageable threshold is fast approaching and the time for introducing grid scale energy storage system has arrived. Battery Energy Storage Systems (BESS) turns out to be the most preferred among the storage systems. It would therefore be helpful, to know the various issues arising out of large scale solar injection into grid and the solution BESS provides in managing them. Described below, in brief, are some of the key issues and how BESS help in smoothening them.
1. Harvesting Clipped Energy
With module prices falling, it has become attractive to increase the Inverter Load Ratio (ILR) to generate more energy. ILR exceeding 1.4 are being currently considered. This higher ILR also increases the occurrence of clipping of power by the inverter during high levels of sunshine. (Inverter clips the power by dragging the array operating point to higher voltage).
In large power plants even a 2 to 3% clipping could result in MW/hrs of lost energy and lost revenue. With BESS, the energy which would otherwise have got clipped could be saved.
2. Energy Time Shifting
Energy prices depend on the time of the day and also on the congestion in the distribution network. BESS helps store excess energy generated by solar during the day and shifting its use or sell it during the peak evening tariff period. It is also possible to use the same storage system to draw and store energy from the grid during low tariff period in the night and sell it during the high tariff evening or morning period (buy-low/sell-high transactions). Energy shifting to peak evening load period also helps in “peak load shaving” which helps utilities reduce the cost of generating power during peak periods, reducing transmission and distribution congestion and extend the life of existing infrastructure and delay investment on them.
3. Meeting Local Load Needs and Saving Energy During Grid Disruptions
In grid connected systems, any grid outage, be it necessitated by scheduled maintenance or due to natural disasters, results in the inverter disconnecting itself from the grid resulting in wasted solar energy generation. With EBSS the generated energy could be used to power local loads or saved for later use.
4. Capacity Firming
Maintaining committed levels of power generation is major issue with large renewable power integrated to the grid. A sudden cloud cover or dispersal can sharply bring down or raise generation. Under such conditions, batteries which can rapidly ramp up charging and discharging at mandated rates (MW/min) can smoothen the rapid swings in power generation.
5. Capturing Power During Curtailment and Supplying Power During Ramp Up
As the number of solar systems tied to the grid increase continuously, the net connected load to the grid falls sharply during peak sunshine in mid-day. The California Independent System Operator (CAISO), working on current and future scenarios brought out the net load profile that looked like a duck with a neck, belly and a tail and appropriately named the duck curve. (Prior to the integration of large scale solar, the profile looked more like camel humps with morning and evening peaks).
With steady fall in the net load during mid-day, it would become necessary to shut down peaking, intermediate and in extreme cases even base load generation (as warranted in Hawaii in 2013). As the day advances, the situation reverses totally with solar radiation steadily falling and evening loads rapidly rising. In the CISO load profile, it can be seen that on February 2016, the generation was required to be ramped up by nearly 11,000 MW in three hours. This would require all generation units to be started up and ramped up rapidly to match the demand of the rising load and to maintain grid stability. To get over the problem, the peaking solar generation during the day time could be clipped to keep the net load well above the base load and stored. The stored energy can then be used along with other generators to match the rising evening load demand.
6. Improving Power Quality
Intermittent power generation from renewables source and the changing net load connected to grid causes voltage sags and spikes and also frequency deviations. Energy storage systems can improve power quality and protect the loads.
7. Harvesting at Low Voltages
During early morning and late evening and during heavy cloud cover, the system voltage will be lower than the minimum “wake up” voltage of the inverter and in a large multi megawatt system, a substantial energy can be lost over a period of time. Batteries along with DC-DC converters can help capture this energy which would otherwise have been lost.
Battery Energy Storage System thus helps provide multiple, stacked services and creates additional value for all electricity system stakeholders: end users, utilities, independent system operators and transmission companies. Putting BSES to multiple services helps in utilizing the full potential of the battery resources over its life and enhances the return on investment.
Despite the several benefits the high cost of BESS has remained a deterrent for their integration in grid scale projects. In the past few years, there has been growing convergence of clean energy and clean transportation technologies with strong linkage provided by BESS. There had been concerted efforts by companies like Tesla who have interest both in solar and EVs to bring down costs. With global efforts and with electric vehicles driving scale, costs are rapidly falling. BNEF reported early this year that lithium-ion battery prices in 2016 fell to half the 2014 price.
Simultaneously several battery technologies are emerging each competing and offering unique value propositions in key parameters of cost, energy density, cycle life, reliability and safety. Table below gives the relative merits and demerits of some of the batteries available in the market.
Renewable energy capacity in India, which stood at 57 GW as of end April 2017, constitutes 17% of the total electricity generation. Keeping in view the unfolding grid energy scenario government has already announced projects totaling more than 300 MW with energy storage. Private initiative is also picking up. In January AES Corporation and Mitsubishi Corporation announced a partnership to deliver India’s first grid scale (10 MW) energy storage solution to Tata Power Delhi distribution solution.
Late in March, government of India announced its aim to make India a 100% electric vehicle nation by 2030. With the demand from the renewable and the transportation industry expected to rise steadily in the coming years, India would soon emerge as one of the largest global markets for BESS.
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