# Experimental and Numerical Study of Thermal Performance of an Innovative Waste Heat Recovery System
## 4 Conclusions  
A heat pipe heat recovery system was designed, manufactured and tested. The thermal performance of the equipment was investigated through experimental tests and confirmed by numerical simulations. Also, an efficiency calculation was made based on the experimental data obtained.  
Compared with classic heat recovery system, this equipment produces hot water at lower costs, given the fact that the heat pipes are a passive technology and do not require any moving parts. The results showed that it needs about 10-15 min until it is able to produce hot water at almost constant temperature. The efficiency of the heat pipes is increasing directly proportional to the temperature of the primary agent. An efficiency of 76.7% was obtained with a temperature of 70 \({}^{\circ}\)C of the primary heat source, but the results showed that this percentage can be further improved.  
It can be concluded that the heat pipe heat recovery system can be a feasible solution for recovering thermal energy from used waters. As further researches, the equipment can be tested at even higher volume flow rates for the secondary agents and the number of heat pipes could be varied. Also, industrial tests may be carried out to check the efficiency of the heat pipe heat recovery system in factories.  
Figure 10: The efficiency of the heat recovery system.  
**Author Contributions:** Conceptualization, R.S.V., A.B. and M.C.B.; methodology, A.B.; software, R.S.V.; validation, R.S.V., A.B., C.A. and M.V.; formal analysis, R.S.V., C.A. and M.B.; investigation, R.S.V., A.B., C.A. and M.B.; resources, M.V.; data curation, R.S.V. and M.B.; writing--original draft preparation, R.S.V. and M.B.; writing--review and editing, A.B., C.A., M.V. and M.C.B.; visualization, R.S.V. and A.B.; supervision, A.B., C.A. and M.C.B.; project administration, A.B., C.A., M.V. and M.C.B.; funding acquisition, M.V. and M.C.B. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was supported by a publications grant of the TUIASI, project number GI/P28/2021.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** This work was supported by a publications grant of the TUIASI, project number GI/P28/2021.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**Nomenclatures**  
HPHE Heat pipe heat recovery system  
T\({}_{\text{Ev,1}}\) Temperature of the primary agent at the inlet of the evaporator [\({}^{\circ}\)C]  
T\({}_{\text{Co,1}}\) Temperature of the secondary agent at the inlet of the condenser [\({}^{\circ}\)C]  
q\({}_{1}\) Volume flow rate of the primary agent [L/min]  
q\({}_{2}\) Volume flow rate of the secondary agent [L/min]  
CH1 Thermal sensor at the inlet of the evaporator  
CH2 Thermal sensor at the inlet of the condenser  
CH3 Thermal sensor at the outlet of the condenser  
T Time [min]  
\(\lambda\) Thermal conductivity [W/m\({}^{\circ}\)K]  
\(\varepsilon\) Thermal effectiveness of the HPHE [%]  
Q Heat transfer rate of the HPHE [W]  
Q\({}_{\text{max}}\) Maximum theoretical heat transfer rate of the HPHE [W]  
C\({}_{\text{ev}}\) Heat capacity rate of the evaporator [W/K]  
T\({}_{\text{Ev,2}}\) Temperature of the primary agent at the outlet of the evaporator [\({}^{\circ}\)C]  
C\({}_{\text{co}}\) Heat capacity rate of the condenser [W/K]  
T\({}_{\text{Co,2}}\) Temperature of the secondary agent at the outlet of the condenser [\({}^{\circ}\)C]  
r\({}_{\text{Ev}}\) Mass flow rate of the primary agent [kg/m\({}^{3}\)]  
m\({}_{\text{Co}}\) Mass flow rate of the secondary agent [kg/m\({}^{3}\)]  
C\({}_{\text{p,Ev}}\) Specific heat of the primary agent [J/kg\({}^{\circ}\)K]  
C\({}_{\text{p,Co}}\) Specific heat of the secondary agent [J/kg\({}^{\circ}\)K]  
C\({}_{\text{min}}\) Minimum heat capacity rate between the two fluids [W/K]