Abstract
Design of ground-source heat pump systems requires an estimation of the heat transfer between a buried pipe and the surrounding ground. Since heat pumps normally operate intermittently, the heat transfer with intermittent boundary conditions is important. While a variety of useful tools exist for numerically determining the heat transfer characteristics of any given system, it is unwieldy to use such tools for rough or preliminary estimates of system sizes. This paper describes the development of accurate simple approximations for the heat transfer from a round pipe buried in an infinite medium with the pipe wall intermittently changing between isothermal and adiabatic conditions. It is shown that the steady-boundary case provides an upper bound on the time-averaged heat transfer from the pipe and a lower bound on the instantaneous heat transfer rate. For a low thermal diffusivity medium, the time averaged intermittent heat transfer rate approaches that of the steady boundary case, while for a high value of thermal diffusivity, the time averaged heat transfer decreases faster than it does in the steady boundary case. The overall heat transfer can be well correlated with simple functions over a range of thermal diffusivities, intermittence intervals, and time.
