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4.5 TES Standalone Simulations

The TES was simulated within the Thermosys 4 toolset which was described in detail in chapter 3 of this work. The simulations were performed using two different materials for the PCM: water and PureTemp -5, which is a vegetable oil based solution. Both materials were simulated with and without fins to determine the relative performance gained by interior fin enhancement. All of the simulations were constructed to run through two full charge and discharge cycles of the PCM, and all simulations used the fixed grid method with 10 radial nodes. The relevant properties of each PCM are given in Table 4.1. All simulations used the same geometric and material parameters for the TES unit given in Table 4.2.

Table 4.1: PCM Properties

Property Water PureTemp -5

Latent Heat of Fusion [kJ/kg] 334 180

Melting Temperature [°C] 0 -5

Solid Phase Density [kg/m3] 918 860

Liquid Phase Density [kg/m3] 1000 860

Solid Phase Thermal Conductivity [W/(m*K)] 2.3 0.2

Liquid Phase Thermal Conductivity [W/(m*K)] 0.58 0.2

Solid Phase Specific Heat [kJ/(kg*K)] 2.108 1.66

Table 4.2: TES Unit Geometric and Material Parameters

Parameter Value

Refrigerant R404A

Inner Tube Diameter [m] 0.008

Outer Tube Diameter [m] 0.03175

Refrigerant Length of One Pass [m] 6

Parallel Refrigerant Passes 11

Tube Bank Transverse Pitch Ratio 1.5 Tube Bank Longitudinal Pitch Ratio 1.25

Tube Passes Per Transverse Plane 10

Wall Material Aluminum

Wall Density [kg/m3] 2702

Wall Specific Heat [kJ/(kg*K)] 0.903

Wall Thickness [m] 0.0015875

Air Side Surface Area Enhancement Factor 5

In Table 4.2 the pitch ratios and tube passes per transverse plane parameters are used to calculate the air side heat transfer coefficient through a correlation for a tube bank given in [29]. The air side area enhancement factor accounts for the increase in surface area due to the exterior fins on the tube bank. The model calculates the surface area of the tubes only, and multiplies that by the enhancement factor to simulate the presence of air side fins.

4.5.1 Water Simulation Results

This section shows the results of simulations using water as the PCM with and without interior fin enhancements. Both simulations used the same initial and operating conditions, which are given in Table 4.3.

Table 4.3: TES Unit Initial and Operating Conditions – Water

Initial/Operating Condition Value

Initial Refrigerant Mass Flow Rate [kg/s] 0.06 Initial Refrigerant Pressure [kPa] 550 Operating Refrigerant Inlet Enthalpy [kJ/kg] 259.1

Initial Refrigerant Outlet Temperature [°C] -6 Initial Air Mass Flow Rate [kg/s] 0 Operating Air Inlet Temperature [°C] 10

Initial PCM Temperature [°C] 0.5

4.5.1.1 Without Fin Enhancement

Figure 4.8 shows the TES solid fraction dynamics for the system without the fin enhancement model active. This TES configuration took 9,742 seconds to fully charge and discharge twice without fins. Figure 4.9 shows the enthalpy dynamics of each of the 10 nodes. As mentioned earlier in the modeling section, without fins the PCM nodes freeze sequentially from the innermost node outward and melt sequentially from the outermost node inward allowing only one node’s enthalpy to be between zero and the latent heat of fusion of the PCM at any given time.

Figure 4.10 shows the refrigerant pressure dynamics of the TES. As nodes freeze a larger temperature gradient is required to maintain the heat transfer in the TES. Therefore, the refrigerant pressure drops continuously throughout the charging periods in the simulation causing the saturation temperature to drop to maintain the temperature gradient required to freeze the PCM. One way on a physical system to determine when the TES is fully charged is to monitor the refrigerant pressure. When the pressure reaches a very low value that is PCM dependent, the PCM can be considered fully charged.

Figure 4.8: Unfinned Water Solid Fraction Dynamics

Figure 4.10: Unfinned Water Refrigerant Pressure Dynamics

4.5.1.2 With Fin Enhancement

Figure 4.11 shows the TES solid fraction dynamics for the system with the fin enhancement model active. In this model 8 interior fins are included positioned as shown in Figure 4.6 with a thickness of 0.5 mm. This TES configuration took 5,742 seconds to fully charge and discharge twice without fins. This represents a 4000 second or 41% reduction in charge/discharge time due to the increased system heat transport properties through fin enhancement.

Figure 4.12 shows the enthalpy dynamics of each of the 10 nodes. The interior fins eliminate the earlier model restriction of sequential freeze and melt, allowing heat to transfer in and out of all of the PCM nodes simultaneously. This results in faster charge and discharge of the overall unit. Additionally, since heat transfers through the fins much faster than through the PCM, the PCM freezes and melts from both interior and the exterior walls simultaneously.

Figure 4.13 shows the refrigerant pressure dynamics of the TES. In this case the increased system transport properties allow for a smaller temperature gradient to maintain the required heat transfer. Therefore, the pressure does not have to drop as much as in the unfinned case. During discharge mode, the pressure increases more than previously due to the inner wall heating up faster through fin enhancement.

Figure 4.11: Finned Water Solid Fraction Dynamics

Figure 4.13: Finned Water Refrigerant Pressure Dynamics

4.5.2 PureTemp -5 Simulation Results

This section shows the results of simulations using PureTemp -5 as the PCM with and without interior fin enhancements. Both simulations used the same initial and operating conditions, which are given in Table 4.4.

Table 4.4: TES Unit Initial and Operating Conditions – PureTemp -5

Initial/Operating Condition Value

Initial Refrigerant Mass Flow Rate [kg/s] 0.06 Initial Refrigerant Pressure [kPa] 450 Operating Refrigerant Inlet Enthalpy [kJ/kg] 259.1

Initial Refrigerant Outlet Temperature [°C] -8 Initial Air Mass Flow Rate [kg/s] 0 Operating Air Inlet Temperature [°C] 5

4.5.2.1 Without Fin Enhancement

Figure 4.14 shows the TES solid fraction dynamics for the system without the fin enhancement model active. This TES configuration took 11,740 seconds to fully charge and discharge twice without fins which represents a 21% increase from the unfinned water model. The increase is primarily due to the extremely poor transport properties of PureTemp -5.

Figure 4.15 shows the enthalpy dynamics of each of the 10 nodes. Without fins, the modeling restriction of sequential freeze and melt is active. Another interesting point of note is that the enthalpies of the frozen nodes must drop a lot more than the frozen water nodes to maintain the temperature gradient and heat transfer throughout the PCM due to the very poor thermal conductivity of PureTemp -5.

Figure 4.16 shows the refrigerant pressure dynamics of the TES. As nodes freeze a larger temperature gradient is required to maintain the required heat transfer. Therefore, the refrigerant pressure drops continuously throughout the charging periods in the simulation causing the saturation temperature to drop to maintain the heat transfer required to freeze the PCM. The pressure in this simulation drops significantly more by the end of the charge period than in the water simulation due to the lower melting point and poorer transport properties of PureTemp -5.

Figure 4.15: Unfinned PureTemp -5 Nodal Enthalpy Dynamics

4.5.2.2 With Fin Enhancement

Figure 4.17 shows the TES solid fraction dynamics for the system with the fin enhancement model active. This TES configuration took 4,160 seconds to fully charge and discharge twice. This represents a 28% decrease from the finned water model and a 65% decrease from the unfinned PureTemp -5 model. The presence of fins significantly increases the system transport properties and therefore increases the charge and discharge rate of PT -5.

Figure 4.18 shows the enthalpy dynamics of each of the 10 nodes. Fins eliminate the sequential freeze and melt modeling restriction. The phenomena that was mentioned earlier in the finned water section of two way freezing and melting can very easily be seen with PureTemp -5. The nodes freeze and melt in pairs of the innermost and outermost throughout the simulation. Additionally, the enthalpies of the frozen nodes must drop a lot less than the unfinned case due to greatly increased system transport.

Figure 4.19 shows the refrigerant pressure dynamics of the TES. Due to the greatly increased system transport properties, the pressure does not have to drop as much as it did in the unfinned case. Additionally, much like the finned water case, the pressure rises higher than the unfinned case during discharge mode due to the inner wall heating up faster through increased transport properties.

Figure 4.18: Finned PureTemp -5 Nodal Enthalpy Dynamics

4.5.3 Simulation Summary and Conclusions

In general, the above analysis shows that the transport properties of a chosen PCM prove to be the most important factor in judging the overall charge/discharge performance of a TES unit. PCMs with innately favorable transport properties require less augmentation to provide favorable performance. However, even they do see noticeable improvement in performance with transport augmentations such as fins. PCMs with innately poor transport properties nearly necessitate the use of transport augmentation to provide a favorable amount of performance for the system. However, with augmentation their performance can rival the performance of PCMs with innately favorable transport properties.

Another very important property to consider for PCM performance is latent heat capacity. While a lower latent heat value allows for faster charge and discharge, having a lower latent heat value means that the TES will not be able to provide as much overall cooling to a space that might be experiencing highly transient loading periods. This will be explored in the next chapter which studies adding a TES to a VCC cycle to model and simulate a hybrid VCC system.