Modeling Approach to Design of a Ground-Source Heat Pump Bridge Deck Heating System

The approach taken to design a hydronic snow-melting system for a bridge deck on an Interstate highway in Oklahoma is described. A vertical borehole, closed-loop ground-source heat pump system is to be used as an energy-efficient means to provide the heating requirement. It is proposed to use the bridge deck as a solar collector in the summer months so that the ground can be thermally “recharged” by circulating fluid from the bridge deck to the ground. A design of this type involves the determination of a number of interdependent variables that are difficult or impossible to accurately quantify by conventional design practices. Therefore, mathematical models of a hydronically heated pavement slab, water-to-water heat pump, and a ground loop heat exchanger were developed for use in a detailed system simulation. The simulation allowed design parameters such as minimum heat pump entering fluid temperature, number of heat pumps, and flow rate to be varied, and several potential designs that kept the bridge surface temperature above freezing were selected. The hourly heating loads for candidate configurations were then used to estimate the required size of the ground loop heat exchanger. There is a trade-off between the number of boreholes that make up the ground loop heat exchanger and the number of heat pumps required. Of the configurations examined, a system with 16 heat pumps of nominal 106-kW capacity and 250 boreholes, each 76 m deep, was selected.