## 4 Results and Discussion
### Evacuated Tube Modification
The minimum temperature that 10 mol% of ethylene-glycol in water at 1 atm completely vaporizes is 140 \({}^{\circ}\)C. However, the pressure drop and temperature drop could occur due to effect of evaporation and condensation of the heat transfer fluid leading to vapor flow and the difference of temperature inside the heat pipe. Therefore, the experiment may be designed to evaluate this factor in the future. Overall heat transfer coefficient as shown in Table 2 was calculated for investigating the performance of heat transfer from the inner glass wall to the inner heat pipe wall. The overall heat transfer coefficient was determined from the thermal resistance of symmetry cylindrical layer. The total thermal resistance was a series network of the individual thermal resistances, consisting of thermal resistance of copper heat pipe layer, thermal resistance of inner aluminum fin layer, thermal resistance of air layer (convection), and thermal resistance of outer aluminum fin layer. In case of modified tube, the thermal resistance of stainless-steel scrubber was added into the total thermal resistance in parallel  
Figure 6: Comparison of the storage temperatures of heating medium between the experiment and the model with different storage capacities. with the thermal resistance of air layer. The two assumptions for this calculation are: (1) the heat transfer surface area and the difference of temperature between inner glass wall and inner heat pipe wall are assumed constant, and (2) the thermal conductivity and heat transfer coefficient are constant. The results show that the stainless-steel scrubbers could reduce thermal resistance of the air led to shorter heating time. The mathematical model was used to evaluate the effect of modifying inner structure of evacuated tube on the performance of ETSC. The 20-standard and modified evacuated tubes were observed. The climatic and operating conditions were based on the experimental data of 120 L palm oil case. The initial temperature of palm oil and the flow rate were 38.7 \({}^{\circ}\)C and 0.032 kg/s, respectively. The solar intensity and the ambient temperature were fixed at the average values of 848.86 W/m\({}^{2}\) and 30 \({}^{\circ}\)C, respectively. Figure 9 shows that when a set of modified evacuated tube is operated, the palm oil temperature leaving the manifold at 9.00 a.m. increases from 48.63 \({}^{\circ}\)C to 52.67 \({}^{\circ}\)C. The heating medium energy production \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Standard Tube** & **Modified Tube** \\ \hline Heating time 1 & 60 min & 60 min \\ Steady temperature of tip & 138.71 \({}^{\circ}\)C & 160.32 \({}^{\circ}\)C \\ Vapor fraction at tip & 0.986 & 1 \\ Overall heat transfer coefficient per unit area, _UA_ & 30.30 W/K & 306.74 W/K \\ \hline \hline \end{tabular}  
* Heating time is time taking to heat the tip from ambient temperature to steady temperature.  
\end{table} 
Table 2: Summary of standatd evacuated tube and modifided evacuated tube performance. Figure 8: (a) T-x-y diagram of 10 mol% of ethylene-glycol in water at 1 atm; (b) Relation between vapor fraction of 10 mol% of ethylene-glycol in water and temperature at 1 atm. Figure 7: Comparison of the heat pipe tip temperature of the standard tube and modified tube. rate increased from 382.19 W to 515.82 W. The efficiency of 20-standard and 20-modified evacuated tubes solar collector were 30.02% and 40.51%, respectively. The efficiency of evacuated tube was improved by 34.96%.