Surface heat exchangers include pavement surfaces with embedded pipes for snow-melting or heat rejection from hybrid ground source heat pump systems. The heat exchanger may be ground coupled or not and when not ground coupled the bottom surface is exposed to the wind but not solar gains.
This type of heat exchanger is intended to be connected to the supply side of a condenser loop, and can be used with any type of plant loop. The surface heat exchanger may be specified as the only heat exchanger on the condenser loop (as shown in the first figure below) or it may be connected in parallel with other condenser loop heat exchangers (such as cooling towers, ground surface heat exchangers) as shown in the second figure below.
Surface Ground Heat Exchanger as only heat exchanger on condenser loop
Surface Ground Heat Exchanger with other heat exchangers on condenser loop
Technical description
The Surface ground heat exchanger model is based on the QTF (Quadratic Transfer Function) formulation of heat transfer through building elements with embedded heat sources/sinks. The model uses a heat exchanger analogy to relate the inlet fluid temperature to the net heat transfer rate and consequently outlet temperature. The model is entirely passive, i.e. it does not set any flow rates or incorporate any controls. In order to deal with the non-linear boundary conditions at the top surface due to the presence of ice/snow fluxes have to be calculated by the QTF model and temperature calculated from the surface heat balance. This requires some iteration. Note, top surface variables correspond to ‘outside’ variables in standard CTF/QTF definition (CTF=Conduction Transfer Functions). Bottom surface variables correspond to ‘inside’ variables.
This alpha field contains the editable identifying name for the outside panel heat exchanger.
You can use this control to load data to the dialog from a pre-defined Ground heat exchanger template as a starting point for your particular component.
Note: After loading a Ground heat exchanger template it is important to make sure to update the Design heat exchanger flow rates as the sum of connected heat pump rated flow rates.
DesignBuilder provides 3 different ground heat exchanger types:
Note: The Type drop down list is not editable. To select a different type of heat exchanger, you must first select an appropriate template (above).
This numeric field contains the hydronic tubing inside diameter (in m or in).
This numeric field contains the number of hydronic tubing circuits.
This numeric field contains the hydronic tube spacing (in m or ft).
This numeric field contains the surface length (in m or ft).
This numeric field contains the surface width (in m or ft).
This alpha field expresses the lower surface exposure. Options are:
The Construction selected here defines the layers of materials used to make the heat exchange surface.
Note: The construction must include an Internal Source to accommodate the heat exchange pipes.
If you select a construction with large thickness to represent the ground you may need to use the 2-Finite differenceSolution algorithm either for this construction only or for the whole model.