# Investigation on an Improved Household Refrigerator for Energy Saving of Residential Buildings
## 5 Results and Discussion  
### Performances at Different Ambient Temperatures  
Ambient temperature seriously affects the operating state of household refrigerators. When the temperature gear of the BCD-111 refrigerator is fixed to C, the temperature boundary conditions at ambient temperatures of 17.0 \({}^{\circ}\)C to 31.0 \({}^{\circ}\)C are adopted for analyzing the performances of the improved refrigerator. The common ratio of wall thickness inside of heat pipes to the whole wall thickness, \(X_{in}\) (namely \(X_{in}=X_{t,in}=X_{s,in}\)), is preliminarily set to 16%. The heat transfer area of the evaporating section of heat pipes (\(A_{eva}\)) is given as its maximum value of 0.146 m\({}^{2}\). In the side and the topside freezer walls of the improved refrigerator, considerable heat is transported to the freezer by heat pipes, which changes the thermal characteristics of the freezer walls. Since the outside surface temperature of side freezer walls is greatly affected by the ambient temperature, the side freezer walls of the original and the modified refrigerators are firstly compared in Figure 5. The convective heat transfer coefficients inside of the side freezer wall (\(\alpha^{\prime}{}_{s,in}\) and \(\alpha_{s,in}\)) both increase steadily with ambient temperature, but their difference gradually drops from 0.37 W-m\({}^{-2}\)-K\({}^{-1}\) to 0.22 W-m\({}^{-2}\)-K\({}^{-1}\) (the radiative heat flux is relatively low and thus is neglected to simplify the calculation). Correspondingly, the outside surface temperature of the original side freezer wall decreases from \(-\)11.5 \({}^{\circ}\)C to \(-\)13.8 \({}^{\circ}\)C, while that of the modified one reduces from \(-\)9.0 \({}^{\circ}\)C to \(-\)12.0 \({}^{\circ}\)C, and the gap between them gradually narrows from 2.5 \({}^{\circ}\)C to 1.8 \({}^{\circ}\)C. The above phenomena show that inside heat transfer of the modified side freezer wall is enhanced due to the existence of heat pipes, and the influence of heat pipes steadily decreases as the heat leakage increases with the ambient temperature. This section is not mandatory, but may be added if there are patents resulting from the work reported in this manuscript. Figure 6 further compares the cold loss recovery behaviors of the side and the topside freezer walls. When the ambient temperature is 20 \({}^{\circ}\)C, the cold loss at the inside surfaces of the two side modified freezer walls (\(C_{s,in}\)) is 5.0 W, and the corresponding outside one (\(C_{s,out}\)) is reduced to 2.4 W. Meanwhile, the cold loss at the inside surface of the topside modified freezer wall (\(C_{t,in}\)) is 2.2 W,  
Figure 4: Computation algorithm for the improved refrigerator. and the corresponding outside one (\(C_{t,out}\)) drops to 1.0 W. In comparison to the original freezer walls, about 24.7% of the cold loss of side freezer walls and 21.7% of the cold loss of topside freezer wall are recovered by the heat pipes. With a raising ambient temperature, \(C_{s,out}\) increases from 2.1 W to 3.8 W, and \(C_{t,out}\) enlarges from 0.9 W to 1.7 W. The cold loss reduction ratios of side and topside freezer walls (\(\eta_{b,re}\) and \(\eta_{t,re}\), respectively) significantly drops from 28.3% to 15.0% and from 25.3% to 12.8%, respectively. The cold loss recovery behavior of modified side freezer wall remains better than the topside one, and their cold loss recovery capabilities are both considerably restricted by a high ambient temperature. The overall cold loss recovery performances of the modified freezer at various ambient temperatures are presented in Figure 7. The cold loss of original freezer increases from 6.4 W to 10.0 W, while that of the modified freezer raises from 5.3 W to 9.2 W with a raising ambient temperature. Correspondingly, the cold loss reduction ratio of the freezer (\(\eta_{re}\)) decreases from 16.5% to 8.3%. The results indicate that more cold loss could be recycled by the heat pipes when the refrigerator runs with a lower ambient temperature.