An experimental bridge deck heating system has been constructed and monitored that uses field-assembled heat pipes to transfer energy from 100-ft vertical evaporators in the ground. Measurements indicate that the heated surface was from 2 to 14 C warmer than the unheated portion of the bridge during heating events. This heating was sufficient to prevent the preferential freezing of the bridge deck surface relative to the adjacent road and provided some snow melting. A. computer model that simulates the performance of ground heat pipe systems has been developed and verified by using data from two experimental facilities. This model is being used in conjunction with experimental results to prepare a general design procedure for these ty· pes of systems.

Pavement heating systems to control snow and ice accumulation on bridges and ramps have been incorporated at certain critical locations as an alternative to the more traditional methods of plowing or using deicing chemicals. These systems have typically been either embedded electrical resistive heaters or pipes circulating a fluid. The circulating fluid systems generally use fossil fuel energy sources, although several low-grade renewable thermal energy sources such as geothermal water (1) and the warm ground pelow the frost l ine (2)" have also been tested. The efficiency with whi;;-h low-grade e nergy can be transported to the road surface is an area where improvement can be made. The use of gravity-operated heat pipes to transport thermal energy to a road surface was investigated and developed during the 1970s (].-~). The gravity-operated heat pipe consists of a sealed enclosure that contains a fluid in the liquid-vapor state over its operating temperature range. The lower end of the pipe is the evaporator section, whereas the upper portion is the condenser section. When the evaporator section is warmer than the condenser section, a portion of the liquid vaporizes and travels to the condenser section, where it condenses with the release of its latent heat of vaporization. The evaporation and condensation process creates the driving potential to transport the vapor upward, while gravity returns the condensate to the evaporator. This makes the gravity-operated heat pipe an attractive heat exchanger because it is a completely passive system that does not require any mechanical or electrical parts. Ammonia has been used as the working fluid in these heat pipes, which has the advantage over water-based systems in that it is not susceptible to freezing. Because the thermal energy is transported in the form of latent heat of vaporization, the heat pipe can transport large amounts of energy over relatively long distances (~ 55 m at two installations) with only a small temperature difference. For this reason, low-grade thermal energy sources that miqht otherwise be totally inadequate for the heating of road surfaces may be feasible enerqy sources in some