A FORTRAN computer program called GROCS (Ground Coupled Systems) has been developed to study three-dimensional underground heat flow. Features include the use of up to 30 finite elements or “blocks” of earth which interact via finite difference heat flow equations and a subprogram which sets realistic time and depth-dependent boundary conditions. No explicit consideration of moisture movement or freezing is given. GROCS has been used to model the thermal behavior of buried solar heat storage tanks (with and without insulation) and serpentine pipe fields for solar heat pump space conditioning systems. The program is available independently or in a form compatible with specially written TRNSYS component TYPE subroutines. This paper first describes the approach taken in the design of GROCS, the mathematics contained and the program architecture. Then, the operation of the stand-alone version is explained. Finally, the validity of GROCS is discussed. A companion paper serves as a user’s guide to the TRNSYS-compatible subroutine version.

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