energy and raising the soil temperature to about 149-degrees Fahrenheit (65°C) at its coolest point and 194-degrees (90°C) at its warmest. To extract the thermal energy from the soil, the process is reversed. Cool water is pumped through the condenser coils, heat from the soil transfers into the water, which is then deposited into an insulated water storage tank. Designed for use in rural India, where residents have no ac- cess to a centralized power grid and rely heavily on solar sources to meet their energy needs, the system solves the puzzle of how to provide power during times of insufficient sunlight. The Global Clinic team—comprised of Jennifer Lee ’11,
Hufsa Ahmad ’11 describes her team’s solar thermal energy storage system. Using the Energy in Dirt
Their task was challenging: use inexpensive, natural materials to capture and store solar energy for use during India’s four-month monsoon season.
Their solution was nothing short of brilliant. The six-student Harvey Mudd College Global Clinic team designed a system that collects thermal energy with water and stores it in dirt. “We tested all kinds of soil—loam, sand, rocky soil, com- pacted and not compacted—to determine their effectiveness as a storage medium,” said summer Team Leader Hufsa Ahmad ’11. “The system worked on all soil types and, since you can find dirt almost anywhere, it can be used almost anywhere on the planet.” Powered by a 9-volt lithium battery, the system’s pump cir-
culates water in a tube that runs through a single solar collector. The cold water heats up in the collector, then passes through a series of refurbished refrigerator condenser coils buried in 10.6 cubic meters of soil housed in an insulated, underground cyl- inder.
The water transfers thermal energy into the soil as it passes
through the condenser coils—which act as heat exchangers— and comes out cold again. The cool water then travels back to the collector and the cycle resumes. The process continues for eight months, storing thermal
Andrew Xue ’11, Hufsa Ahmad ’11, Allie McDonnell ’12, Julie Lapidus (Scripps) and Niger Washington (Pomona)—worked on the capture and storage components of the project. Team members coordinated with a second student team at the Birla Institute of Technology (BIT) in Ranchi, India, which focused on how to convert the stored thermal energy into electricity for use in rural Indian homes. “The [HMC] team had to learn what materials were accessi- ble and what the people were willing to use,” said Global Clinic Director L.G. de Pillis. “Working with a team from the area gave them good information about what was acceptable and doable. They also learned about working with people from a different culture and language.”
The BIT team determined there were two potential ways of
converting the thermal energy from the water into electricity to power homes. The first would be to use a Stirling Engine to convert heat to electricity, but it may be too costly, Ahmad said, since few companies manufacture the engines. The second, more feasible option would be to employ an
Organic Rankine Cycle (ORC), which uses low-grade heat to power a turbine and generate electricity. “An ORC would be ideal for our system for two reasons: 1) its operating temperature range is appropriate for our soil sys- tem, and 2) we’re outputting heat into a water tank, which stores water as the working fluid for the ORC,” said spring semester Team Leader Jennifer Lee ’11. The system works theoretically, but will require more work
before a prototype can be installed and tested. “To make the [storage] system feasible, we need to determine the pump requirements, model the heat extraction process, merge the collector system with the water system model and determine the tank insulation thickness,” Ahmad said.
SUMMER 2011 Har vey Mudd College
19
STEVE SCHENCK
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