PCM Average Temperature Versus Discharging Time
At the end of the charging process, the PCM is in the liquid phase at nearly 600C. The system was kept un-touched for 4 hours to store the energy. The temperature was observed to get reduced to 540C at the starting of discharge process; this is due to the heat loss through insulation, as shown in Fig.4.3.\nDuring the discharge process, the cold HTF have passed through the heat exchanger to extract the energy from the system. The temperature profile during the discharging process is shown in figure. The mass flow rate is kept constant at 0.3 Lpm and temperature of cold HTF is 30°c, which is known that temperature difference due to driving force for heat transfer therefore initially temperature decreases drastically from large temperature difference between cold HTF and PCM.
Discharge using water. Efficiency
As it name refers that how the system is working efficiently. This is the most important portion of any study that will provide an information about working condition of the system after refining all thermal properties with the help of various alternatives such as extended fins, encapsulated PCM etc. It can be measured in terms of charging, exergy and overall energy efficiency. The efficiency results are based on the comparison of the outcome of water and nanofluid. On the basis of these analyses, the effective change is noticed and shown by the
It can be observed that in the case of enhancing the HTF temperature the charging efficiency is continuously improving. The reason behind this the thermal conductivity of Nano fluid is enhancing due to its energetic na-noparticle make the Brownian motion. The result of water is being compared with the consequences of Nano fluid.
Charging efficiency at different HTF temperature.
When water used as an HTF, the charging efficiency is enhanced by 27.51% at variation in inlet HTF tempera-ture. By applying the nanofluid as an HTF by varying HTF temperature and at constant concentration, the enhancement is noticed as following. At 0.25% concentration, the charging efficiency is enhanced by 29.65% on the variation in HTF temperature from 60 to 850C with respect to water. Similarly, at 0.5, 0.75, 1.25 and 1.5% concentration, charging efficiency is enhanced by 32.56, 35.49, 39.12, 40.14 and 40.65%, respectively. The overall improvement of charging efficiency, approximately 17% is recorded at varying HTF temperature, as illustrated in Discharging Efficiency.
As the charging process gives the information about capability of the system, in the same manner, the discharg-ing process also has a significant role for determining the storing capacity of the system. The storing of energy for a longtime eliminates mismatching of demand and supply. The energy is stored up to end of the charging from the beginning of charging by disregarding all the losses during charging process. This stored energy is accumulated for several times after discharging phenomenon takes place. The discharging efficiency depends upon recovered as well as available energy. It has been observed that, there is no effective impact of nanofluid for improving the storing or discharging efficiency because of it is depended on insulation, overall heat transfer coefficient of heat exchanger and surface of cylinder. In this stage the energy stored in the PCM is extracted to use in the load. The efficiency for the discharging period can be expressed as follows Energy recovered from TES during discharging = Amount of energy available to discharge
The enhancement in HTF temperature at the constant concentration, the change in overall efficiency of nanofluid is obtained as compared to that of base fluid. As in case of water, the enhanced efficiency 16.41% is to be observed but as in nanofluid at 600C HTF temperature, the change in efficiency is obtained 20.72 %. Similarly, at 65 to 85OC, the change in efficiency is obtained 23.92, 25.76, 27.56, 29.73 and 30.26%, respectively. The overall change in energy efficiency is recorded approx. 12% that is more drastic change compared than that of water. But enhancing in concentration at constant inlet HTF temperature, the change in overall efficiency of nanofluid is lower than as enhancing the HTF temperature. [13-14]The optimum result of overall efficiency is obtained at 800C HTF temperature and 1% concentration.
The experimental study is carried out for improving the thermal performance of thermal energy storage system using nanofluid as an HTF under varying concentration from 0.25% to 1.5%, as a recess of 0.25% and inlet HTF temperature from 600 to 850 0C, as a recess of 50 0C. The thermo-physical properties of nanofluid are determined from the laboratory. Based on performance factors, the outcomes of the water and nanofluid are compared to each other under varying concentration and inlet HTF temperature.
- The reduction in charging time is found up to19.81% while using water as an HTF whereas the reduc-tion in charging time is found up to 26.98% when using Graphene as an HTF under varying inlet HTF temperature. Under varying concentration, the charging time has been reduced but that is significantly not as much of as variation in HTF temperature. However, the discharging time is not considerable af-fected as using nanofluid as an HTF.
- Enhancement in charging efficiency is found up to27.51% when using water as an HTF but the devia-tion in HTF temperature at a constant concentration, the enhancement in charging efficiency is found up to40.65% while using nanofluid as an HTF.
- The enhancement in overall efficiency is found up to30.26% as variation in inlet HTF temperature of nanofluid at constant concentration. The enhancement in available energy (exergy efficiency) is rec-orded up to 17.02% under varying inlet HTF temperature and constant concentration.
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