Ponds and lakes can serve as thermal reservoirs in geothermal heating applications. In this example, fluid circulates underwater through polyethylene piping in a closed system. Inside the pipes, cold water is flowing and picking up low grade heat from the pond. The return water will have a slightly higher temperature than the feed. A heat pump (not included in the model) will upgrade the heat for use in dwellings. Chilled water is then returned back to the pond loop. The pipes are coiled in a slinky shape and mounted onto sleds. The Nonisothermal Pipe Flow interface sets up and solves the equations for the temperature and fluid flow in the pipe system, where the geometry is represented by lines in 3D. A forced convection correlation is used (see Figure 4 above) for the external cooling effect.