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TES system modification results and analysis

7. Conclusions

7.1. TES system modification results and analysis

The present work has taken into account several possible changes of the TES system in order to find way to enhance the thermal efficiency of the system. The reactions involved in the chemical loop are relatively simple as they involve at most two reactants or two products and from the physical point of views the only parameters that can be changed in order to have a different system behaviour are temperature and pressure.

The first modification taken into account has been the reduction of the dehydration temperature in order to reduce the energy needed to the preheating of the inlet stream of the charging step to see if the efficiency could be enhanced. As the reaction temperature is directly linked to the steam partial pressure and the reduction of the reaction temperature is possible only if also the pressure is lowered the only way to reduce the temperature is working on the dehydration steam partial pressure. As reference we can remind that at ambient pressure the dehydration process happens at temperature higher than 792 K.

The method used to reduce the dehydration steam partial pressure has been discussed in chapter 5.2 and involves the introduction of a neutral gas, instead of steam, as fluidizing agent in order to dilute the steam and so have a reduction of its partial pressure. The chosen neutral gas is the nitrogen as in literature are present some works where this element is used in some experiment on the same topic without any change in the reaction behaviour. Moreover, nitrogen is also a

157 commonly used substance. The introduction of the nitrogen rises the needs of the separation between it and the steam generated in the reaction. The proposed solution is the condensation of the steam, process that is not anymore at constant temperature due to the mixture of gaseous species.

The results of this first test is that the dehydration temperature can be reasonably lowered only of some tens of degrees. Moreover, the efficiency of the system is not improved as the lower heat needs for the preheating of the streams is lower than the increment of heat needed to run the dehydration reaction due to the reduction of temperature.

The second try is to introduce a series of dehydration reactors in order to be able to have always the same final dehydration conversion but with lower steam production in each reactor and so a lower steam partial pressure that can be exploited to reduce more the temperature. This study has been made using a number of reactors between 2 and 5. The result is that the temperature reduction is also in this case not so high and in any case the thermal efficiency shows a further reduction.

The present work has taken into account only physical expedients in order to reduce the dehydration temperature but, it could be interesting, to make some studies about the possibilities of employ catalysts in the dehydration process to reach lower temperatures.

Also the hydration reaction conditions have been modified but in order to increase the temperature at which the TES can release the stored heat. The physical way to do that is the increment of the hydration pressure. The study on the hydration pressure increment ranges between 1 to atmospheres and in conjunction with the temperature increment it brings also a thermal efficiency decrement due to the higher stream preheating needs.

After all the system modifications have been also made a pinch analysis for a specific case in order to have a more real idea of the potential thermal efficiency of the TES.

158 Figure 103 shows a comparison between the efficiency obtained before and after the pinch analysis for all the for all the TES modification cases for which have been made the pinch analysis.

Figure 10325: TES thermal efficeicny comparison before and after pinch analysis.

As can be seen from figure 103 the efficiency for the TES systems before the pinch analysis show lower and mainly very similar values. Between the base case and the case with the addition of the nitrogen the difference before the pinch analysis are very low.

The efficiency of the cases with three reactors in series and the one with higher hydration pressure before the pinch analysis have similar efficiencies that show both a percentage gap of about 2 or 3 points in relation to the base case and the case with the addition of nitrogen.

After the pinch analysis the thermal efficiency of all the cases are very similar and are between 56.5 to 59 %; the only case in which the efficiency is remarkably lower is related to the turbine direct integration. It can be seen that from the TES thermal efficiency point of view any kind of physical modifications made to have some kind of improvements brings to a lower thermal efficiency but, in most of the cases, a well made network of HEX is able to reduce these differences.

The pinch analysis seems to have a higher influence in the TES layouts with higher hydration pressure and the one with three dehydration reactors in series as they have a lower initial thermal efficiency and after the pinch analysis they reach an efficiency more similar to the one of the other cases that have as starting point already a higher efficiency.

36,86 58,95

36,78 57,37

36,01 57,75

33,69 56,47

33,87 57,3

32,42 26,3

0 10 20 30 40 50 60 70

TES thermal efficeicny [%]

Before and after pinch analysis TES efficiency comparison

Reference case before Reference case After Base case before Base Case after N2 before N2 after 3R+N2 Before 3R+N2 After HP 5 bars Before HP 5 bars After HP+Turbine before HP+Turbine After

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