APPLICATIONS OF AN HEAT PIPE HEAT EXCHANGER IN THE RECOVERY OF WASTE HEAT FROM URBAN WASTEWATERS

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1 17 th Building Services, Mechanical and Building Industry Days Urban Energy Conference, October 2011, Debrecen, Hungary APPLICATIONS OF AN HEAT PIPE HEAT EXCHANGER IN THE RECOVERY OF WASTE HEAT FROM URBAN WASTEWATERS GABOR Timea, PhD. Student Eng., RUSU Tiberiu, PhD. Eng. Professor, DAN Viorel PhD. Eng. Reader, CENAN Dumitru, Eng. Technical University of Cluj-Napoca, Faculty of Materials and Environmental Engineering, Departament: Environmental Engineering S.C. EnergiQ Ind.C.O. S.R.L Cluj-Napoca, str. Trăian Vuia, nr. 7A KEYWORDS: heat pipe, heat exchanger, wastewater, waste heat Abstract: The reduction of energy consumption has become a priority for all countries in the world. This fact is due to the limitation of Earth's natural resources, global warming and the increasingly high costs of energy consumption. Wastewater, from drains, represents a waste heat source which is untapped so far. The recovery of heat from wastewaters would reduce the dependencies on fossil fuels, the greenhouse gas emissions and would increase the energy independence. This paper presents a research on the use of heat pipe heat exchangers in the recovery of waste heat from urban wastewaters. 1. Introduction The energy recovery from secondary resources has become an important issue at European Union level. The European Commission proposed the promotion and development of "intelligent city" initiative, by investments in "clean" energy. In this way the investments are aimed at improving energy efficiency and climate change mitigation through the use of sustainable energy, thus supporting the environment. This progress could be achieved by exploiting the potential offered by urban sewage networks (Gabor, 2011) and building drainage pipes (Gabor, 2010). 101

2 17th Building Services, Mechanical and Building Industry Days Urban Energy Conference October 2011, Debrecen, Hungary This paper proposes a waste heat recovery technology for municipal wastewater. The utilization of thermal energy contained in municipal and industrial wastewater, could open new markets in the energy sector and residual energy resources. The residual energy recovery from different wastes has become an important goal in energy conservation programs, a significant progress being made for heat exchangers development. The heat pipe heat exchanger is a type of heat exchangers, which are increasingly used in different technological processes due to its advantages (high thermal conductivity, low cost, low volume, easy to manufacture, etc.). Heat pipe consists of three parts namely: evaporation section, adiabatic section and condensation section, its operation is given in Figure 1. When the evaporation part is in contact with a hot fluid (wastewater), the heat pipe working fluid absorbs heat through the wall, it evaporates and, due to the difference of pressure in the tube, it moves to the opposite end to the condensation zone where it loses its heat to cold fluid. The resulted condensate returns to the evaporation section and the cycle repeats (Te-En, 2010), (Ahmet, 2009). Qassignation CONDENSER section QPipe ADIABATIC section Qabsorbed EVAPORATOR section Fig. 1: Schematic of a heat pipe. The efficiency of heat pipe heat exchanger depends on the temperature of warm source (Patankar, 1980), (Proskiw, 2003.). In the present study the heat source is of low temperature, so heat transfer is carried out only by free convection. Although there are many applications where waste heat is recovered from municipal wastewater (Kalberer, 2007), there is no report on the effectiveness of heat pipe in the process. The purpose of this research is to determine the efficiency of Q-Pipe heat pipe heat exchanger for waste heat recovery from municipal wastewater. 102

3 T. Gabor et al. Applications of a Heat Pipe Exchanger in the Recovery of the Waste Heat from Urban Wastewaters 2. Materials and method Heat pipe heat exchangers (Brochures EnergieQ) are devices that made the exchange of energy (waste heat) from a waste heat source to a colder source. Figure 1 shows the schematic diagram of the experimental apparatus. The system is composed of three major parts: water heater (for wastewater preparation), Q-Pipe heat pipe heat exchanger and devices for measurement and control of parameters. In the installation there are two circulating fluids: the hot agent (wastewater) in the lower chamber of the heat exchanger and the cold agent (cold water) in the upper chamber of the heat exchanger. Fig. 2: Schematic diagram of the experimental apparatus. The heat pipe heat exchanger was equipped with 40 heat pipes arranged vertically at an angle of 90 (Figure 3). A heat pipe has a length of 1390 mm, 16 mm external diameter and wall thickness of 1 mm. The working fluid used in heat pipe has a filling ratio of 20%. 103

4 17th Building Services, Mechanical and Building Industry Days Urban Energy Conference October 2011, Debrecen, Hungary Fig. 3: EnergieQ Heat pipe heat exchanger. The input and output temperature, the flow of thermal agents and running time were measured in this experimental research. Determination of efficiency of heat recovery was achieved under the following conditions: wastewater temperature equal to 30 C, different flow rates: 0.5 m 3 /h, 0.4 m 3 /h and 0.3 m 3 /h, the flow and temperature of cold water from network was constant (0.15 m 3 /h and 17 C). Measurements were made of 5 in 5 minutes, the operation of the heat pipe heat exchanger was 80 minutes. The heat current transferred by the wastewater ( Q 1 ) and heat flux received by the cold water ( Q 2 ) were determined based on measurements results (Shah, 2000), (Chiriac, 1992): m c T W (1) Q1 1 w 1 T1 T1 T 1 C (2) m c T W (3) Q2 2 w 2 T T C (4) T2 2 2 Q % Q (5) 1 where: Q 1 is the heat current transferred by the wastewater [W]; Q 2 is the thermal current received by the cold water [W]; m 1 weight rate of the wastewater [kg/s]; m 2 weight rate of the cold water [kg/s]; c w water`s heat capacity [kj/(kg C)]; ΔT difference between initial and final water temperatures [ C]. 104

5 T. Gabor et al. Applications of a Heat Pipe Exchanger in the Recovery of the Waste Heat from Urban Wastewaters In formulas 1 and 2 subscriptions indicate the hot agent (wastewater) and the cold agent (cold water), and ' and '' indicate the entry and the exit of the two agents. 3. Results and discussion In Figure 4 is presented the variation of recovered temperature depending on the operation time of the heat pipe heat exchanger, for the three flow rates of wastewater. It can be noted that the highest value of the recovered temperature (5.16 C) was obtained at a rate of 0.5 m 3 /h. Fig. 4: The variation of the temperature of cold water depending on the time. In Figure 5 is shown the heat current variation transferred and received for each wastewater flow rates over an operation period of 80 minutes of heat pipe heat exchanger. This graph shows that in the case of the wastewater flow rate of 0.3 m 3 /h is recovered the most amount of the current transferred by the heat exchanger. Fig. 5: The variation of the transferred and received thermal current of agents depending on the wastewater flow rate. 105

6 17th Building Services, Mechanical and Building Industry Days Urban Energy Conference October 2011, Debrecen, Hungary In Figure 6 are shown values of the efficiencies determined for the three wastewater flow rates. The graph shows that the highest efficiency of %, was obtained from a wastewater flow rate of 0.3 m 3 /h. Fig.6: The variation of the effciency determined for Q-Pipe heat pipe heat exchanger depending on the three fow rates of wastewater. 4. Conclusions From this study some conclusions can be made: The recovery of residual heat from municipal wastewater depends on their flow rates: the higher the flow rate is the greater the recovered temperature gets; The efficiency of the heat exchanger depends very much on the preheated cold water flow rate. The cold water flow rate must be closed to the flow rate of wastewater; The operating time influences the entire recovery process. From measurements made it can be observed that after minutes the system has started to enter a steady state regimen, the temperature difference being slightly increased. By connecting these heat pipes heat exchanger to the water-water types heat pumps their efficiency is further increased, thus contributing substantially to reduction of pollution in cities. The waste heat recovery system is very practical, cost effective and economically efficient for the environmental protection compared to other heating systems (based on oil, electricity or coal). 106

7 T. Gabor et al. Applications of a Heat Pipe Exchanger in the Recovery of the Waste Heat from Urban Wastewaters Acknowledgement This paper was suported by the project POSDRU 6/1.5/S/5, project co founded from European Social Found through Sectorial Operational Program Human Resources The experimental heat exchanger was suported by S.C. EnergiQ Ind.C.O. S.R.L Cluj-Napoca. References Ahmet, K., Hakan, D., Zakir, T., (2009). Experimental study of heat transfer of buried finned pipe for ground source heat pump applications, International Communications in Heat and Mass Transfer Vol. 36, p Chiriac, F., Leca, L., Pop, M., Badea, A., Luca, L., (1992). Procese de transfer de căldură şi masă în instalaţii industriale, Editura Tehnică, Bucureşti. Gabor, Timea, Rusu, T., Dan, V. (2011). Heat recovery from wastewater using heat exchangers and heat pumps, The 7 th Edition of the Carpathian Basin Conference on Environmental Science, Vol.1, Eds. Abel, Cluj-Napoca, ISSN , p Gabor, Timea, Rusu, T., Dan, V., (2010). Technological variations for domestic wastewater heat recovery, ProEnvironment, Vol.3, Nr. 6, TODESCO Publishing House pissn: ; BIOFLUX Publishing House eissn: , p Patankar, S. V., (1980). Numerical Heat Transfer and Fluid Flow, Taylor & Francis, New York, pp Proskiw, G., (2003) Design and Analysis of a Residential Greywater Heat Recovery System, Report Prepared for CANMET Energy Technology Centre, Canada, p. 4, 5, 52. Shah, R. K., Thonon, B., Benforado, D. M., (2000). Opportunities for heat exchanger applications in enviromental systems, Applied Thermal Engineering, Vol. 20, Issue 7, p Te-En, T., Guan-Wei, W., Chih-Chung, C., Wen-Pin, S., Sih-Li, C. (2010). Dynamic test method for determinining the thermal performances of heat pipes, International Journal of Heat and Mass Transfer, Vol. 53, p Kalberer, D. (2007). Energie aus abwasser aktueller denn je, Journal Spektrum der Gebaudetechnik, Nr. 3, p ***Brochure EnegieQ 107