## 4 Results and Discussion  
### Experimental Results  
The mathematical model was developed to facilitate the study of operating and climatic condition effects on the performance of ETSC. Furthermore, the validated model could be applied to study the feasibility of adapting the ETSC with thermally application in a small-scale industry. In this work, the palm oil was used as a heating medium in the 20-ETSC system. Figure 5a-f show the manifold and storage temperatures with different storage volumes of palm oil experimentally and theoretically and the solar intensity. The storage volume was one of the parameters affecting the performance of the ETSC.  
With 50 L and 80 L storage volumes as shown in Figure 5a,b, the storage temperature increases rapidly between 9:00 a.m. to 1:00 p.m. since the rise of solar radiation in this period directly affected the temperature of heat transfer fluid inside the copper heat pipe. The heat transfer fluid took 1 h for heating up to steady state temperature at approximately 140 \({}^{\circ}\)C. In addition, as above storage volumes were small, they needed small amount  
Figure 5: The storage and manifold temperatures of heating medium from the experiment (dots) and the mathematical model (lines) at different times of the operation. (**a**) 50 L; (**b**) 80 L; (**c**) 100 L; (**d**) 120 L; (**e**) 140 L; (**f**) 160 L.  
of thermal energy to heat the whole palm oil up and, therefore, the temperature of the palm oil inside the storage tank increased rapidly. The maximum storage temperatures of 50 L and 80 L reached at 4:00 p.m. are 97 \({}^{\circ}\)C and 90.4 \({}^{\circ}\)C, respectively. Although the temperature increase rate of the 50 L and 80 L storage volume was very fast, the heat storing capacity was inferior. The heat loss quantity of the 50 L and 80 L was 83.84 W and 43.50 W, respectively.  
On the other hand, with 100 L, 120 L, 140 L, and 160 L storage volumes, as shown in Figure 5c-f, the results show that the storage temperature steadily increases throughout the measured period because the heat storing capacity of such storage volumes were large and, thus, needed more thermal energy to heat more volume up. The maximum storage temperatures of 100 L, 120 L, 140 L, and 160 L are 81.6 \({}^{\circ}\)C, 84.6 \({}^{\circ}\)C, 79.9 \({}^{\circ}\)C, and 74.8 \({}^{\circ}\)C, respectively.  
It can be seen that the storage temperature of 50 L at 4:00 p.m. in Figure 5a drops and the maximum storage temperature of 100 L in Figure 5c is less than that of 120 L which is against the fact that less storage volume should provide higher maximum temperature. These are because the limitation on controlling climatic condition to be equivalent for every experiment leading to the lower solar intensity during 1:00 p.m. to 4:00 p.m. at the day of the 50 L and 100 L palm oil experiments were conducted comparing with the other days. Moreover, the trend of storage and manifold temperatures of 140 L and 160 L, as shown in Figure 5e,f could be increased further. However, the quantity of solar radiation during the day was not enough to drive the ETSC for both storage volumes.  
The selection of optimal storage volume with size of the ETSC could lead to the increase in ETSC performance. The 120 L storage volume was a suitable volume for 20-ETSC system which provided 11.06 M) of heat storage with about 30% of efficiency.