In April 2021, a research team from the Department of Energy Science and Engineering at the Indian Institute of Bombay (IIT Bombay) installed a 50 kWp solar microgrid facility for an army base camp in Sikkim. The Sustainable Energy Storage Suitable for Microgrid (SENSUM) project comes under the Mission Innovation India plan. It is a step towards India’s ambitious goal to tap into renewable energy sources, and the Department of Science and Technology, Government of India, funded the 95- lakh venture.
The project, spearheaded by Prof Prakash C Ghosh, is one of a kind to be installed at an altitude of 17000 feet. The solar microgrid is integrated with a hybrid energy storage system comprising Vanadium Redox Flow Batteries (VRFB) and Hydrogen fuel cells. Presently, the microgrid reduces the site’s dependency on a 30 kVA diesel generator, setting precedence for clean, eco-friendly energy sources.
The team braved rough terrains, low-oxygen and unpredictable weather conditions to install the microgrid.
“Hauling our equipment was a huge challenge,” says Prof Ghosh, recalling how they transferred heavy battery equipment without the help of cranes. “Moreover, we had just two months to install the facility before the monsoons sealed off access to the site,” he adds.
The solar microgrid
Microgrids operate autonomously as power backups for the primary power supply grids. Whenever the main power supply is disrupted due to a power shortage, an outage, or repairs, the microgrids take over the supply to the load. Unlike a diesel gen-set, microgrids generate power from renewable sources such as solar or wind energy. In remote and inaccessible regions (such as the army camp in Sikkim), where there is no central power supply, microgrids offer an attractive and sustainable option to meet the region’s power demands.
A solar microgrid consists of solar panels, photovoltaic cells, and suitable storage batteries. The photovoltaic cells convert solar to electrical energy to feed the load and charge the batteries. The batteries store the energy in an electrochemical form and drive the load during the night or on bad weather days.
Capitalising on the ample sunshine that the camp gets for nearly 300 days a year, the team proposed 50 kWp (p-peak) photovoltaic cells with a hybrid storage system of Vanadium redox flow batteries and hydrogen storage cells. The integrated system can cater to the camp’s power requirements through all weather conditions. Prof Ghosh says, “The microgrid provides a 20kW/200KWh backup for the camp’s 30 kVA generator set.”
Why Vanadium flow batteries?
Although several types of batteries are available, the emerging technology of Vanadium flow batteries has multiple long-term advantages. “Lead-acid batteries have a short lifespan and the capacity is substantially reduced at low temperature, while supercapacitors are not suitable for large-scale energy storage. Hence, we used VRFBs in conjunction with the Photovoltaic solar cell for their reliability, lower cost, long term stability and battery life,” says Prof Ghosh.
At the heart of the VRFB are two energy-storing electrolyte tanks that contain chemically active Vanadium compounds (in ionic states of V4+ and V3+) mixed with sulphuric acid. The electrolytes are pumped through a stack of two-chamber electrochemical cells separated by an ionic membrane. The chambers have current-carrying electrodes.
During the charging cycle, the photovoltaic cells pass a current through the electrodes. V4+ containing electrolyte enters the positive side, get oxidised to V5+ by giving up an electron. This electron passes through the external circuit to reach the negative side, absorbed by the V3+ ions, reducing to V2+. The separating membrane ensures the electrons do not pass through and neutralise.
“Together, one single cell generates an electromotive potential of 1.26 V,” says Prof Ghosh. The charging continues, and the state of charge of the battery starts to build up in the tanks. A pair of tanks is capable of generating 50 kWh of energy, he says.
When the battery discharges to the load, the reverse process occurs. The V2+ releases the electrons convert back to V3+. The reverse current now drives the load connected to the battery and reaches the positive terminal of the cell where V5+ absorbs it and converts it to V4+. Thus, VRFBs have enormous energy storing capacity, are non-inflammable, and have long battery life.
“Another advantage of a redox battery is that energy and power capacity (kW and kWh) can be segregated. Therefore, by increasing the tank capacity (more electrolyte), the energy capacity component increases. In contrast, increasing the number of stacks will provide higher output power,” says Prof Ghosh.
The project deployment
The project’s first phase took place in November 2019. First, the team erected the trusses to mount eight solar arrays on the mountainous terrain. The optimally aligned structure could generate 50 kWp (kiloWatt-peak) power. “It was the toughest job for us as we had minimal manual support to complete the structure before the rains started,” says Prof Ghosh. Although the next accessible time slot was in April 2020, the raging pandemic disrupted their schedule. As a result, the next phase was completed recently.
The team will set up a 5 kW capacity fuel cell with a hydrogen storage system to integrate the microgrid in the subsequent phase. A fuel cell is another low-cost electrochemical storage system that uses hydrogen to charge the battery. The fuel cell will be used when the flow battery cannot provide sufficient output due to bad weather or seasonal variations. The combined storage of flow battery and fuel cell will improve the overall power reliability of the microgrid.
“A useful by-product of fuel cells is thermal energy. The fuel cell is expected to operate with an electrical efficiency of ~45-50%, implying an equal amount of heat is dissipated by the cell. The heat can be used to maintain the flow battery’s operating temperature and for heating the premises,” informs Prof Ghosh. The team envisages providing similar microgrids for Sikkim tourism.
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