Comparative Life-Cycle Cost Analysis of Underground Heat Distribution Systems

Abstract

An analytical model was developed to compare the life-cycle costs of two types of underground heat distribution systems: directly buried conduits and concrete trench installations. The costs of initial construction, routine maintenance, operation, repair, and replacement are represented in the model. Comparisons are made to explore the influence of failure rates, heat loss assumptions, the discount rate, and the policy on partial section replacement. Failure rate estimates are obtained from the literature and also from survey results obtained by the U.S. Army. Life-cycle cost computations indicate that, when initial construction costs are the same, concrete trench installations have a cost advantage of $18–$42 per foot over buried steel conduit when other cost components are taken into account over a 25-year lifetime. The cost advantage of concrete trench over fiberglass reinforcement plastic (FRP) buried systems is $32–$133 per foot. The variation in the cost advantage reflects the variation in the estimate of the failure rate for buried steel and FRP systems. Routine maintenance was crucial for the buried systems to avoid dramatic increases in operation and replacement costs that would make the cost advantage of concrete trench installation even greater than stated above. In contrast, the low incidence of failure associated with concrete trench systems made their life-cycle costs relatively insensitive to maintenance policy.