Solar geyser turns sand into thermal energy storage system for hot water

Researchers from India’s National Institute of Technology (NIT) Kurukshetra and the National Institute of Solar Energy (NISE) Gurugram have created a prototype solar geyser system that uses ordinary sand to store heat, driving down energy costs to an impressive $0.0233 per kilowatt-hour.

“This experimental study evaluates the thermal performance of a photovoltaic-integrated fine sand solar geyser (PFSG) system,” said the researchers in a new study.

This innovative “Fine Sand Solar Geyser” (FSG) offers a robust solution to the age-old problem of solar energy’s intermittency. 

“The FSG system offers a sustainable, environmentally friendly, and cost-effective solution for water heating by storing thermal energy in fine sand, overcoming the intermittency of solar radiation,” Dinesh Kumar Saini, a researcher involved in the project, told PV Magazine.

System’s design is straightforward

“The system comprises a fine sand-based solar geyser (FSG), incorporating a nichrome wire heating rod (NHR) and a circular heat exchanger (CHX) to enhance thermal energy conversion, storage, and extraction,” explained the study.

Electricity generated by three 300W polycrystalline solar panels powers a nichrome wire heating rod. This rod converts electrical energy into heat, which is then efficiently stored within 120 kg of fine sand.

The sand was specifically chosen for its impressive thermal mass, excellent heat retention capabilities, uniform heat distribution, and its low cost and abundance.

When hot water is required, a circular heat exchanger with aluminum fins springs into action, transferring the stored heat from the sand to water flowing through embedded pipes.

Impressive results

The prototype has already demonstrated remarkable performance. During initial testing, the sand temperature inside the FSG soared from 25.2°C (77.36°F) to 164.2°C (327.56°F) on the first day of operation and from an already elevated 94.7°C (202.46°F) to a scorching 211.9°C (413.42°F) on the second day. 

“Correspondingly, the FSG stored 14.02 MJ and 11.81 MJ of thermal energy with charging efficiencies of 97.41% and 90.76%, respectively,” noted the study.

In practical terms, the system delivered a substantial 140 liters and 152 liters of hot water over the two days, maintaining a flow rate of 0.67 liters per minute.  

Users experienced a significant rise in water temperature, with an average difference of 11.7°C (53.06°F) and 13.75°C (56.75°F) between the FSG inlet and outlet on these days. The system also boasted a discharging efficiency of around 90% on both days.

“The system achieved an average water temperature increase of 11.7°C for 140 L and 13.75°C for 152 L, with discharging efficiencies of 90.0% and 90.05%,” highlighted the researchers.

Highly cost-effective

The economic implications of this development are particularly compelling. A techno-economic analysis indicates the FSG is a highly viable option for domestic water heating. 

“The PFSG system demonstrated an energy cost of $0.0233/kWh, payback period of 4.45 years, and an expected operational lifespan of 25 years, offering a sustainable and cost-effective solution for domestic water heating, even during low solar radiation or nighttime conditions,” concluded the study.

This positions the sand-based solar geyser as a potentially transformative technology for affordable and sustainable hot water access.