## 4 Results and Discussion
### Evacuated Tube Modification  
The heat transfer fluid is a key component used to drive the ETSC cycle. One simple way to improve the ETSC performance is by increasing the rate of heat transfer from inner glass wall to heat transfer fluid inside the heat pipe. Achieving higher heat transfer fluid temperature leads to the increase in heat storing rate of heating medium. As shown in Figure 1, there is a gap between the aluminum fin folded around the heat pipe. The gap of circular aluminum fin contained air which the thermal conductivity is 0.03 W/m\(\cdot\)K. It indicates that the ability of heat conduction of air was lower than metals. Therefore, in this work, stainless-steel scrubbers, a common and low-cost material, were used to promote heat transfer rate by inserting them in the gap of the circular fin to increase heat conduction surface area. The experimental results corresponding to the observation of the temperature of heat pipe tips of standard and modified evacuated tubes during the day is illustrated in Figure 7. The Figure 7 show that the solar intensity during the day is fluctuated because the sky was sometimes unclear. The temperatures of heat pipe tips of the standard tube and modified tube sharply increase between 9:15 a.m. to 10:15 a.m. During 11:24 a.m. to 12:19a.m., the temperature of both tubes dramatically drops which is consistent with the solar intensity trend and then the temperature turns to increase until reaches steady temperature at 12:44 a.m. to 4:06 p.m. The heating time of standard and modified tubes for heating the heat pipe tip from ambient temperature to the steady temperature level is approximately 60 min. The average temperature of modified tube (160.32 \({}^{\circ}\)C) was approximately 20 \({}^{\circ}\)C more than a standard tube (138.71 \({}^{\circ}\)C). These temperatures were used to investigate phase change behavior of the heat transfer fluid by the VLE theory by assuming that: (1) the temperature of the heat pipe tip is approximately equivalent to the temperature of the heat transfer fluid inside the heat pipe, (2) the temperature drop between the bottom and the tip of the heat pipe is neglected, (3) pressure drop from vapor flow is neglected. The T-x,y diagram of 10 mol% of ethylene-glycol in water at 1 atm calculated by using the binary VLE relation is represented in Figure 8a. With this T-x,y diagram mole fractions of ethylene-glycol at any particular temperature in equilibrium can be identified. At 138.71 \({}^{\circ}\)C, the 0.1 mol of ethylene-glycol consists of 0.73 of liquid mole fraction (\(x_{1}\)) and 0.09 of vapor mole fraction (\(y_{1}\)) of ethylene-glycol; the 0.9 mol of water consists of 0.27 of liquid mole fraction (\(x_{2}\)) and 0.91 of vapor mole fraction (\(y_{2}\)) of water. Therefore, the vapor fraction and liquid fraction of the mixture are 0.986 and 0.002, respectively. On the other hand, at 160.32 \({}^{\circ}\)C, the 10 mol% of ethylene-glycol in water consist of 0.87 of \(x_{1}\), 0.26 of \(y_{1}\), 0.13 of \(x_{2}\) and 0.74 of \(y_{2}\). The vapor fraction of mixture increases to 1 which shows that the stainless-steel scrubbers could promote the heat transfer from inner glass wall through aluminum fin to the copper heat pipe excellently. Figure 8b shows the summary of vapor fraction of 10 mol% of ethylene-glycol in water from calculation at different temperature. At the beginning, the heat transfer fluid absorbs the thermal energy in sensible heat form until 100 \({}^{\circ}\)C, boiling point of water. Then, the heat transfer fluid absorbs large amounts of the thermal energy from vaporization during 100 \({}^{\circ}\)C to 140 \({}^{\circ}\)C which the T-x,y diagram explains that the water will start vaporizing before ethylene-glycol.