3.5 Heat Pump Models
3.6.6 Comparisons
Four models with different geometry were simulated in COMSOL Multiphysics and compared. This was to find out which parameters play the biggest roles, and to see how the tank is best built. All simulations with changes were compared to the base case. The base case was built as explained in the previous section. All other simulations had only one changed parameter from this base case.
The second simulation had larger pipes. The larger pipes in the simulation were chosen to align with the next size up from PipeLife product line. This is the pipe with diameter of 32mm and wall thickness of 3mm.
In the third simulation, the pipes in the bottom of the concrete tank were re-moved, with exception of a few pipes close to the wall. This was to see whether or not the heating from the bottom changed the water flow within the tank, and thereby ruined the natural convection. Figure 3.17 shows the six pipes that were kept in the bottom concrete for the simulation with fewer pipes.
In the final simulation the concrete tank was replaced by steel. This was to see the major change happening with a tank material with higher thermal conductivity.
These cases were compared in three different areas. The first was the water tem-perature of the gray water. The water temp was measured at 4 different locations and plotted in a graph. Figure 3.18 shows the locations of the four points. The comparisons between simulations were done at the top middle point 2 as this rep-resented the temperature in the body of gray water furthest away from any warmer solid wall, as well as being closest to the gray water outlet.
Figure 3.17: The model gray water tank for the third simulation run with fewer pipes in the bottom concrete in COMSOL Multiphysics
Figure 3.18: The four points (blue dots) where temperatures were compared in the gray water tank in COMSOL Multiphysics
The normal total heat flux was also derived from the simulations and compared.
Normal total heat flux, Φ [W], is evaluated by COMSOL Multiphysics as the heat transfer rate, ϕ [W/m2], over the total area of a boundary, perpendicular to the plane of the boundary. Normal total heat flux from the pipes to the wall, from the wall to the gray water, and from the wall to gravel were acquired and plotted in a graph. In addition, the difference between the first and the two latter was calculated and plotted in the graph as well.
3.19 shows the boundaries that were used for the normal total heat flux values in the simulations.
The third comparison was made on the water flows happening in the tank.
Snapshots from three different time periods, respectively, 200 minutes, 360 min-utes and 720 minmin-utes were taken and compared.
Different Coil Temperatures
Different temperatures of the water in the pipes representing the coil was also sim-ulated in COMSOL Multiphysics. The temperatures tested were respectively 10 K, 20 K, and 30 K higher than the temperature for the base case. No other adjustments
(a) Heat Flux Pipes (b) Heat Flux Gray Water (c) Heat Flux Gravel
Figure 3.19: The three different boundaries (blue) examined for Normal total heat flux from the COMSOL Multiphysics simulations
were made. This comparison was to see how much impact it would have to design a heat pump with a higher condensation temperature. Temperatures at the top mid-dle point (Point 2) was once again used to compare the temperature development over time in the base case to the three other simulations. Normal total heat flux between the three boundaries, pipes, concrete wall and outside gravel as shown in 3.19 were compared for the three simulations.
Table 3.5: Variable parameters in the COMSOL Multiphysics simulations that were different from Base Case
Simulation Variable Base Case Simulation
Value Value
Larger Pipes Case Pipe Diameter, Dpipe [m] 0.025 0.032
Fewer Pipes Case Number of bottom pipes 33 6
Steel Tank Case Tank material Concrete Steel
50◦C Case Tpipe [◦C] 40 50
60◦C Case Tpipe [◦C] 40 60
70◦C Case Tpipe [◦C] 40 70
Table 3.5 gives an overview of the variables that were changed between the various simulations.
3.6.7 Pressure Drop Calculations
It is of importance to determine the pressure drop within the pipes in the tank walls.
This is needed to regulate the correct circulation pump pressure, and also to avoid too high pressures.
The heating capacity of the condenser for the space cooling circuit was found with SIMIEN. Further, an assumption was made that the water circuit gets a 10 K temperature rise in the condenser, regardless of the condensing temperature. With
this information, and given the pipe parameters, the mass flow and velocity of the water was calculated.
The dynamic viscosity was found using a temperature in accordance to the base case introduced earlier in this section. The rest of the calculations were performed based on the equations from section 2.4.
It was further tested if more parallel coils in the tank would provide a lower pressure drop. The numbers of parallel coils tested were one, two, three, and four.
Finally, the pressure drop in different bends were evaluated, and pressure drop were calculated for different geometric forms. The forms tested were circular, quadratic, hexagon, and octagon.
The pressure drop was then used to find out how much circulation pump power was needed in each case.