A group of researchers from India's National Institute of Technology and the National Institute of Solar Energy (NIT) Kurukshetra and the National Institute of Solar Energy (NISE), Gurugram, have developed a PV-powered thermic solar geyser (TSG) that enables daytime charging of thermal oil by converting PV electricity into heat via a nichrome wire heating rod.
“Our TSG ensures consistent performance and reliable hot water even during periods of low sunlight due to its thermal storage capability and solar photovoltaic integration,” the research's lead author, Dinesh Kumar Saini, told pv magazine. “It also provides long operational life, while requiring less maintenance and allowing effective heat retention and reliable hot water delivery during low solar irradiance and nighttime hours.”
The TSG system features a nichrome wire heating rod designed with aluminum fins attached to its radial heat transfer and a circular heat exchanger made from flexible stainless-steel pipes and aluminum fins. It uses 450 L of a thermal oil known as Therminol VP as a thermal energy storage medium, due to its high-temperature stability and efficiency in solar thermal applications.
In the proposed system configuration, the circular heat exchanger facilitates efficient heat transfer from the thermic oil to the incoming cold water, delivering hot water at the outlet. The cylindrical heating rod includes a ceramic strip, nichrome wire, aluminum fins, and fine sand. The nichrome wire consists of a resistance heating element made from a nickel-chromium alloy.
The experimental setup included six 300 W polycrystalline solar panels provided by Indian manufacturer PV Power Tech, a heating rod, a fin, a pipe heat exchanger, the TSG, a pyranometer, thermocouples, a data logger, and a power analyzer.
“The wire is wrapped around the ceramic strip in a helical configuration and centrally positioned within the rod,” the scientists explained. “Fine sand fills the rod, effectively preventing direct contact between the nichrome wire and thermic oil, thus eliminating the risk of oil evaporation. The nichrome wire converts electrical energy from the PV panels into thermal energy, which is stored and transferred by the fine sand to the Therminol VP1 through aluminum fins attached to the heating rod’s surface.”
Image: National Institute of Technology
The scientists evaluated the performance for 240 L of daily hot water demand over two consecutive days at a constant flow rate of 1 liter per minute (LPM), under three realistic domestic usage scenarios: 80L (morning, afternoon, evening), 120L (morning and evening), and 240L (morning).
The analysis showed that the system's charging behavior varies with solar radiation intensity. As radiation levels increase, the thermal energy input also rises. In contrast, under low radiation, heat generation is slower, and the charging duration is longer. It also demonstrated that, over a four-day charging operation, the average thermic oil temperature increased from 32.67 C to 101.83 C, with the average charging efficiencies of the PV panels, TSG, and the overall system being 12.32%, 89.69%, and 11.05%, respectively.
“During a 12-hour discharging operation, the thermic oil temperature decreased from 103.63 C to 48.39 C at 1 LPM, successfully heating 720 L of water with an average temperature rise of 16.4 C between the inlet and outlet,” Kumar Saini explained, noting that the system was also able to heat 240 L of water daily for two consecutive days, ensuring reliable hot water delivery for domestic applications. “It also exhibited effective overnight heat retention under varying operational conditions, with recorded heat retention efficiencies of 94.15 %, 93.84 %, and 86.68 % for discharging 80 L, 120 L, and 240 L of hot water, respectively.”
Further techno-economic analysis showed the system is reportedly viable with an initial capital cost of $1373.39, an expected lifespan of 25 years, and a calculated levelized cost of energy (LCOE) of $0.051 per kWh. “The estimated payback period is 5.24 years, confirming its cost-effectiveness for domestic hot water applications,” Kumar Saini stated. “Over its lifetime, the TSG system achieves a total CO₂ mitigation of 104 tons and a net mitigation of 77.92 tons, emphasizing its environmental benefits and contribution to carbon emission reduction.”
The system was described in the study “Development and performance evaluation of a photovoltaic-integrated thermic solar geyser for domestic water heating,” published in Thermal Science and Engineering Progress.
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