Despite recent briskness in installations, rooftop solar capacity in India is only around 6 GW, against a 2022 target of 40 GW.

The reason is that state-owned electricity distribution companies (discoms) have typically baulked at letting their paying customers put up rooftop solar plants and reduce their dependence on the discoms — primarily by refusing to buy any surplus energy from the rooftop plants or paying a pittance for it.

However, technology is making it possible for rooftop solar plant owners to go it alone.

Battery costs

Energy storage costs are falling, making it affordable for rooftop solar plant owners. According to Bloomberg New Energy Finance, the average price of batteries was $137 a kWhr in 2020, compared with $1,100 in 2010 — with clear indication that it will fall below $100 by 2023. But even that is a conservative estimate, not accounting for technology breakthroughs.

Much is happening in battery research. The article ‘Looking beyond lithium’ ( Quantum dated September 27, 2021) mentions a Canada-based company, Zinc8 Energy, offering zinc-air batteries at $45 a kWhr. Sceptics say metal-air batteries are still a distance away. Even if that’s true, there are other lithium chemistries that can aid a fall in storage costs.

Lithium sulphur, for instance, boasts an energy density of 1,000 Whr per kg, compared with 290-300 Whr per kg of lithium-ion. Lithium polysulphide has a problem — the polysulphides migrate from the cathode to the anode through the electrolyte, hampering operations. However, such challenges are being overcome. For instance, Prof Kothandaraman Ramanujam of the Department of Chemistry, IIT-Madras, has developed a ceramic-polymer insulator to prevent such migration.

Rooftop hydrogen

Even keeping batteries aside, there are other options for delinking a rooftop solar plant from the grid. For instance, there are indications that in the future there would be electrolysers so small that they can be treated as a household appliance. Rooftop solar plant owners can use surplus energy to generate hydrogen for sale or self-use.

Pashupathy Gopalan, one of the principal investors in the US-based electrolyser manufacturer Ohmium, says this will “definitely happen”. He told Quantum that electrolyers, “being a stackable technology, can easily go down to residential scale”.

Keep cool

Other uses for energy from rooftop solar includes running an ice-cream freezer. There are ‘phase changing materials’ (PCM) that can remain cold for 2-3 days. Thermal pads made with PCMs take about five hours to freeze in an ice-cream freezer, but you can keep stuff in the box cold for 2-3 days.

This is an ideal solution for farmers, floriculturists and fishermen who need to transport perishable goods across distances. A Chennai-based start-up, Tan 90, manufactures PCMs. “Starting from 4 degrees Centigrade, Tan90 provides cooling solutions till minus-25 degrees Centigrade,” it says.