Smart Control of a Geothermally Heated Bridge Deck

This manuscript describes the “smart” control system designed for a geothermal bridge deck heating system. The control system integrates concepts of model predictive control with a first-principles bridge deck model and hourly computerized National Weather Service (NWS) forecasts to prevent bridge icing without the use of salt or other chemical de-icing materials. The proactive nature of the control system maximizes motorists’ safety and bridge life while minimizing system operating costs. The basic concepts of model predictive control, first applied in the process industries in the early 1990’s, are presented as applied to control of a geothermally heated bridge deck. An overview is then provided of the detailed bridge model used to predict the thermal response of the bridge deck to constantly changing weather conditions. The proactive element of the control system as provided by the NWS Rapid Update Cycle forecast model is then described. Performance of the integrated control system is illustrated via simulation using actual weather data. The “smart” control system represents a novel approach and can be conceptually applied with any type of bridge deck heating system. INTRODUCTION Bridge icing, especially preferential bridge icing (formation of ice on a bridge deck before ice appears on approaching sections of road), represents a major transportation safety issue. A common response is the application of salt to suppress the freezing point and prevent ice formation. Unfortunately, this creates two problems of major concern. The first is the environmental impact associated with salt runoff in water bodies. The second is a reduction in bridge life due to the corrosive effects of salt on rebar and other structural steel. One alternative to avoid salt is the use of heated bridge technology (HBT). This paper describes the control system for this approach. In 1999, the U.S. Department of Transportation Federal Highway Administration funded the Oklahoma State University (OSU) Geothermal Smart Bridge Project. The mission of this project is to “research, design, and demonstrate technically feasible, economically acceptable, and environmentally compatible Smart Bridge systems to enhance the nation’s highway system safety and to reduce its life cycle cost (1).” The OSU project uses a hydronically-heated deck with geothermal energy as the heat source. The control system described in this paper represents a major step forward from earlier approaches used to control bridge deck heating. The philosophy and capabilities of the control system described in this paper are consistent with the goals for Intelligent Transportation Systems. A hydronically-heated bridge deck has tubes buried in the pavement. Heat is transferred to the bridge deck when a warm fluid is pumped through the tubes. For the application described in this paper, a ground source heat pump, which recovers energy stored in the earth, is used to heat the fluid circulated through the bridge deck. Energy is supplied to the heat pump from a ground loop heat exchanger. The ground loop heat exchanger utilizes a second fluid circulating through tubing buried in the earth. A schematic of the system is presented in Fig. 1. The term “Smart Bridge” associated with the OSU project is a reflection of the sophisticated control system used to operate the bridge deck heating system. Decisions to initiate, continue, or stop heating the bridge deck are based on measurements at the bridge site, information from road weather information systems (RWIS), and weather forecasts for the bridge site. A distinguishing feature of the OSU control system is the ability to leverage state-ofthe-art control, modeling, and real-time weather monitoring and forecasting technology to