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The performance of a TES system cannot be determined simply by looking at

the charging or discharging process alone, therefore the overall cycle efficiency needs

to be determined. The overall energy and exergy efficiencies are defined as:

These efficiencies depend on the temperature change of the HTF and thus a smaller

difference would lead to an increase in efficiency as well as increase the percent of

energy stored via latent heat. However, it should be noted that for the three materials

considered here the temperature range can only be minimally decreased due to the

melting points of the materials. A key example of the importance of the overall cyclic

performance is seen by looking at the NaNO2 system. When you look at the charging

process alone, the system stores the most energy at the highest rate. During the

discharging process however while it releases the second highest amount of energy, its

efficiency was the lowest and the system does not completely solidify. This leads to

the NaNO2 system only recovering 88% of the energy and exergy that was stored. The

energy stored, exergy stored, energy released, exergy released, charging and

discharging Ste, overall charging and discharging efficiencies, and the percent energy

stored via latent heat are listed in Table 9. Although the KNO3 system was the worst

performer during the charging process, its good performance during discharging leads

to it having the highest overall energy and exergy efficiencies. It is closely followed

by the 2- and 3-PCM systems. The multi-PCM systems not only store more energy stored released overall Q Q Q    ₍₅₉₎ stored released overall Ex Ex Ex    (60)

than the KNO3 and NaNO3 single PCM systems but they do so in a more efficient

manner. Furthermore, while it is possible to recover all of the energy stored in the

sensible heat system, it can only store 40% of the energy the NaNO3 system stores.

This highlights the improvement latent heat-based systems offer over pure sensible

heat TES systems.

Table 9. Results of the charging and discharging process for the NaNO3, NaNO2,

KNO3, 2-PCM, 3-PCM, and sensible heat systems

NaNO3 NaNO2 KNO3 2-PCM 3-PCM

Sensible Heat Energy Stored (MJ) 116 142 84.4 130 118 47.1 Exergy Stored (MJ) 55.9 66.4 42.0 61.4 56.5 22.8 Stechar 0.30 0.79 0.013 0.55 0.37 --- Energy Released (MJ) 107 125 83.8 128 116 46.5 Exergy Released (MJ) 51.9 58.5 41.7 61.6 55.8 22.5 Stedis 0.029 0.038 0.73 0.034 0.27 --- ηQ-overall (%) 92 88 99 98 98 98 ηEx-overall (%) 93 88 99 100 99 99 %LH 69 60 57 64 60 --- 8.4 Conclusions

A two-dimensional analysis of an example EPCM-based TES system was

conducted to evaluate the improvement of its performance by employing a multi-PCM

system. Six systems were considered: three single PCM systems (NaNO3, NaNO2, and

KNO3), a 2-PCM system (NaNO3 and NaNO2), a 3-PCM system, and a sensible heat

only system as a comparison. The performance of the charging and discharging

processes are investigated as well as the overall cyclic performance of the systems.

increase in the efficiency of the system as well as an increase in the fraction of energy

stored via latent heat. As expected the latent heat systems are able to store more

energy than the sensible heat system. Due to the thermal properties of NaNO2 and the

large temperature difference experienced during the charging process, the NaNO2 has

the highest efficiency, stores the most energy, and has the highest exergy content at

the end of the charging process. The small temperature difference between the melting

point of the PCM and the inlet temperature for the KNO3 system coupled with the low

values of latent heat leads to the system having the worst performance of the latent

heat systems. With the exception of the NaNO2 system, the multi-PCM systems

outperform the single PCM systems by storing more energy at a higher rate and

having a higher final exergy content.

Despite being the best system during the charging process, the NaNO2 system

has the worst performance during the discharging process as the HTF fails to retrieve

all of the stored energy. The prolonged solidification time of the NaNO3 system leads

to it having the highest discharging efficiency. The performance of the NaNO2 system

during the discharging process highlights the importance of looking at the overall

cyclic performance of the system. Since the system fails to solidify fully, only 88% of

the energy and exergy stored was able to be recovered and turned into useful work.

The NaNO3 system releases 92% of the stored energy while the HTF in the remaining

systems are able to extract all of the stored energy. Furthermore, the system cannot be

judged on efficiency alone and the total energy and exergy stored is an important

factor in determining the best-suited system. For the systems considered, the 2-PCM

This was partly due to the extremely small temperature difference between the melting

point of KNO3 and the 611 K inlet temperature. It was also due to the higher energy

storage density of NaNO2 compared to KNO3. The 2-PCM system also has the second

highest fraction of energy stored via latent heat behind the NaNO3 system.

These results show that a cascaded multi-PCM has a better energy and exergy

performance over that of a single PCM system. While these results indicate a 2-PCM

system to be best, as the length of the system is increased the number of PCMs used

will have a greater impact. Furthermore, care needs to be taken as to the inlet and

melting temperatures of the PCMs as they impact the performance. While the results

of this investigation lend insight into key aspects of the performance of EPCM-based

latent heat TES systems, additional research is required to determine the optimal