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A comparison of the experimental and predicted temperatures at the inlet and

outlet of the test section as well as 6 mm above the bottom of the tenth capsule are

presented in Figure 37. During the first 2000 sec the difference between the predicted

and experimental results for the temperature in the tenth capsule was less than 3%.

After the tenth capsule starts to melt the simulation begins to predict a higher

temperature than the experimental results. A similar trend is seen with the outlet

temperature of the system. Additionally, the simulation predicts a total melting time of

4612 sec which is 7.6% faster than the 4992 sec melting time indicated by the

experiments. After the capsules are fully melted both the experiments and simulations

show the same rate of temperature increase within the capsules offset by the difference

in melting time. Given the good agreement during the solid sensible heating phase the

likely cause of the faster predicted melting time is the use of an incorrect value for the

latent heat of NaNO3. Bauer et al. [81] reported that the average value is 172 kJ/kg

which is 8% higher than the 162.5 kJ/kg that was used during the numerical analysis.

A total of 22 MJ of energy was lost by the HTF over the course of the charging

process. The EPCM capsules stored a total of 17.5 MJ which accounts for 79.5% of

the energy lost by the HTF. The remaining 20.5% goes into sensibly heating the test

section walls and deflectors. The total energy stored in the capsules is 4.9% less than

the 18.4 MJ reported by Zheng et al. [23]; however it is within their estimated

calculation error. Of the energy stored by the capsules, 3.3 MJ is stored in the stainless

steel capsule shell and 14.2 MJ is stored in the 17.7 kg of NaNO3 which accounts for

MJ reported by Zheng et al. [23] which is likely caused by the way Zheng et al.

calculated the total energy storage as they only measured the temperature of the tenth

capsule and therefore had to estimate the temperature distribution within the test

section based on the inlet and outlet temperatures. Furthermore, of the 142 MJ of

energy stored in the NaNO3, 20.3% is attributed to the latent heat of fusion.

Figure 37. Comparison between experimental and numerical results for the inlet, outlet, and tenth capsule temperature

7.5 Conclusions

A numerical analysis of a pilot scale EPCM-based latent heat TES system was

conducted. In order to improve the heat transfer around the capsules and avoid the

large wake that is present with cross flow around cylindrical capsules, metal deflectors

250 300 350 400 450 500 550 600 650 700 750 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 Te m p e ra tu re (K) Time (s) Sim Outlet Exp Outlet Sim Inlet Exp inlet Sim 10th cap Exp 10th cap

deflectors the presence of the void at the top of the capsule reduces the heat transfer

rate in the upper half of the capsule. This leads to the isotherms being “U”-shaped in

the solid PCM while conduction is the dominant mode of heat transfer. Once the PCM

begins to melt, natural convection in the molten PCM increases the heat transfer rate

accelerating melting along the PCM-void interface. This causes the isotherms, and in

turn the solid-liquid interface, to slant inward in the upper portion of the capsule.

While the capsules initially begin melting along the capsule edge in the center of the

channel, the increased heat transfer from the capsule end to the PCM results in the

solid-liquid interface propagating from the ends towards the center of the capsule. In

addition to the vortices that are present in the air void, two recirculating vortices form

in the molten PCM, one along the capsule edge and the other around the remaining

solid PCM, both in the XY and YZ planes. The 10 EPCM capsules consecutively

experience the same evolution in the solid-liquid interface and therefore show the

same expansion of the PCM and compression of the void from an initial 26% to 19%

when the PCM is fully melted.

Good agreement is seen between the predicted and measured temperature of

the tenth capsule and test section outlet during the sensible heating phase of the

charging process. However, the simulation predicted a total melting time that was 8%

faster than indicated by the experimental results. The good agreement obtained during

the sensible heating phase indicates that an incorrect value of the latent heat of fusion

was used during the simulation. Indeed the value established previously via

calorimetry experiments is 8% lower than the average value reported in literature.

measured temperatures show the same rate of temperature increase just offset in time

by the early completion of melting. It should be noted that the value of latent heat used

only slightly under predicts the total energy stored due to the large fraction of sensible

heat stored by the system.

A total of 17.5 MJ of energy was stored in the 10 EPCM capsules which

accounts for 80% of the energy given up by the HTF. The remaining energy goes into

sensibly heating the test section walls and the metal deflectors. Of the total energy

stored in the EPCM capsules 14.2 MJ is stored in the 17.7 kg of NaNO3 and 3.3 MJ is

stored in the stainless steel capsule shells. Additionally, 20% of the energy stored by

the PCM is attributed to the latent heat of fusion of NaNO3. Therefore the system is

able to store a large fraction of energy supplied by the HTF where a significant portion

is from latent heat. Furthermore, if the system was to consist of either a greater

number of capsules, larger capsules, or a smaller operating range was applied then the

fraction of energy stored via latent heat would be increased. These results demonstrate