Tubes on a Hot Tin Roof
Solar thermal systems are helping reduce MU’s coal use and energy costs.
On the roof of MU’s power plant, 480 black rods installed this spring stand at a rakish angle. Unlike solar panels, one side of the cylindrical tubes always faces the sun, even during the early morning and late evening. Nearby, an identical rod is mounted horizontally atop what looks like a long, narrow bathtub coated in mirrors. The parabolic mirrors track the sun and focus light on all sides of the solar tube for optimal energy conversion. Together, these arrays are helping keep carbon dioxide out of the air and keep money in MU’s ledger book.
The power plant’s boilers all operate on the same principle: boil water to create steam, which turns a turbine and creates electricity. However, a significant portion of the steam is diverted to campus buildings for heat, equipment sterilization and humidification. About 80 percent of the steam returns as liquid condensate, but 20 percent doesn’t come back and must be replaced with make-up water.
But before being added to the boiler, explains plant superintendent Gregg Coffin, make-up water must be warmed, which has been done using coal-heated steam. The new solar thermal arrays allow the sun to share that burden, which Coffin estimates will reduce the energy used to heat the water by about one-third.
Each solar thermal rod is vacuum sealed and contains a copper heat pipe filled with a water/alcohol solution. When sunlight hits the rod, it boils the solution, pushing steam to the top of the tube, which is mounted in a manifold and surrounded by another fluid. The heat from the rod is transferred to the fluid, which is circulated through a heat exchanger where its heat is passed to the make-up water.
The arrays are expected to last 30 years.
Coffin says the project adds yet another on-campus opportunity for MU students to learn about sustainable energy technologies.