Gravity-operated heat pipes are remarkably simple devices that can passively transport thermal energy over relatively long distances. These attributes permit some thermal control of structures with renewable, low-grade, thermal-energy sources. This paper provides a brief historical synopsis of the development of ground-coupled heat pipe systems, and construction and performance details of field tests on two water-coupled and two ground-coupled heat-pipe systems for bridge deck heating. The results from the 77 F (25°C) geothermal-water and the 46 F (8°C) well-water systems indicate that heat pipes can effectively heat a deck with low temperature water without exposing it to the dangers of freezing water. Field tests of the ground-coupled systems used 80 ft (24.4 m) long heat pipes, and 60 field-constructed heat pipes with 100 ft (31 m) long evaporators. This latter system, which heated an entire bridge, increased the weekly averaged surface temperature by as much as 18 F (10°C).

Units: Dual