Impact of Waste Heat Recovery of Chimney Using U-Shaped Pulsating Heat Pipe to Generate Hot Water- Experimental and Environmental Analysis

Many studies have been done on the Pulsating heat pipes (PHP) using energy applications system. In this study a heat exchanger PHP is analyzed. A heat pipe prototype is manufactured for waste heat recovery. The present study experimentally investigated the effect of pulsating heat pipe on the waste heat recovery of the chimney and produce hot water for household consumption. The evaporator is placed in a smoke exhaust duct and the condenser is located in a water chamber in which the smoke heat is transferred through. The results are presented for different heat pipe angles to the horizon from 0 to 90. The PHP is lled 60% by distilled water as operating uid. The highest hot water temperature in outlet of reservoir was about to 58 o C. Also, The CO 2 mitigation and CPH of the waste heat recovery system was equal to 84.82 tons and 0.1$/m 3 . Moreover, the eciency is changing from 19% for a horizontal PHP to 54% for a vertical one. the thermal performance of the anti-gravity pulsating heat pipe and heat recovery of the system. Their results revealed that the optimum lling ratio of the pulsating heat pipe was about 70%. Also, the system was assisted to use the waste heat recovery to preheat the fuel oil. Li et al. (Li et al. 2020) studied the performance of pulsating heat pipe in low temperature heat recovery using graphene nanouid. They reported that the thermal resistance amount reduces by raising the volume fraction of the graphene nanoparticles and also with raising the thermal load to the evaporator. An end closed one PHP which is used as an air preheater has been studied by Rittidech et al. (Rittidech et al. 2005). This PHP consists of an evaporator, an adiabatic part and a condenser. The 2 mm internal diameter heat pipe is made of copper. Water and R-123 are used as operating uids. It is shown that any increase in temperature, causes the eciency to increase. Also, the system operation will increase if R-123 is used instead of water. A preheater end closed PHP with one-way valve was fabricated by Meena et al. R-134a with a 50% lling ratio was used as an operating uid. Any increase in the hot air velocity and temperature would reduce the eciency. Also this system has the capability to reduce the humidity from 80-100% to 54-72% (Meena et al. 2007). In another research, studied by Nuntaphan et al. (Nuntaphan et al. 2010), a heat exchanger is made of some PHPs. Hot water and air are used in this heat exchanger. The PHP is lled by methanol, Acetone and R-123. The results show that if the PHPs are used as ns, the heat transfer rate will be increased by 10%. Mahajan et al (Mahajan et al. 2020) investigated the performance of the nned and bare tube oscillating heat pipe for waste heat recovery application. The results revealed that the average waste heat recovery of the nned oscillating


Introduction
Heat pipes are effective instruments in the heat transfer of different energy applications (Guo et al. 2022, Jung &Boo 2021. The heat pipe used in solar desalination (Fallahzadeh et al. 2020, Khalilmoghadam et al. 2021, Rastegar et al. 2020) to enhance the drinking water productivity, in thermoelectric generator (Date et al. 2014, Makki et al. 2016) to produce electrical, in photovoltaic thermal system (Jouhara et al. 2021, Zhang et al. 2021) to use from waste heat recovery of the system, and in HVAC system (Sukarno et al. 2021) to use energy recovery of the device. The heat transfer coe cient in the evaporator and condenser is signi cantly high and therefore the heat pipe is widely used in energy systems. As the thermal resistance of heat pipes is too low, it causes heat exchanger to have a smaller area and less mass compared to other traditional heat exchanger types. The use of pulsating heat pipes (PHP) is broadly extended to the vast variety of industrial sciences such as air preheater PHPs in thermal power plants, heat recovery from exhaust smoke and even electronic components cooling. the freshwater generation of solar desalination system with a pulsating heat pipe. The obtained outcomes revealed that the highest productivity of modi ed solar desalination was about 875 ml/m 2 hr. Also, the optimum water height in the solar desalination was equal to 0.01 m. Aref et al. (Aref et al. 2021a) evaluated the effect of closed-loop pulsating heat pipe on water generation of humidi cation/dehumidi cation desalination. The outlet air from the dehumidi er was preheated and prehumidi ed by closed-loop pulsating heat pipe before entering the humidi er of the desalination system. They found that the highest daily freshwater generation of humidi cation/dehumidi cation desalination was about 8.7 L/m 2 . Xu et al. (Xu et al. 2017) evaluated the impact of compound parabolic concentrator (CPC) and PHP on performance of solar collector. The CPC has a signi cantly enhance the intensity of solar radiation to the PHP as an absorber and also decrease the heat loss due to reduce in area of the hot zone. They showed that the highest energy e ciency of solar collector using CPC and PHP was about 50%. Aref et al. (Aref et al. 2021b) investigated the effect of lling ration, inclination angle and solar intensity on performance of the novel closed-loop pulsating heat pipe. A at plate collector was coupled with dual-diameter of pulsating heat pipe. The pulsating heat pipe was made of copper. The obtained results indicated that the highest e ciency of the solar collector with a lling ration of 60% was about 72.4%. Alizadeh et al. (Alizadeh et al. 2020) improved the performance of photovoltaic system with using closed-loop PHP. The PHP was used to cool the photovoltaic module and also increase the electrical e ciency of the PV panel. The outcomes indicated that the improvement performance of the system with PHP was increased about 35%.
Alizadeh et al. (Alizadeh et al. 2018) increased the electrical performance of photovoltaic module using pulsating heat pipe.
The results indicated that the power generation of photovoltaic module using pulsating heat pipe as a cooling system was enhanced about 18% compared with conventional photovoltaic module. The effect of very long PHP on performance of the solar water heater was studied by Arab et al. (Arab et al. 2012). The PHP was made of copper pipes with internal diameters of 2 mm. They reported that the thermal e ciency of the thermosyphon mode of solar water heater using very long PHP was about 36%. Chen et al. (Chen et al. 2020) studied the performance of the solar collector using ower type pulsating heat pipe.
The acetone used as a working uid in pulsating heat pipe. The results revealed that the thermal e ciency of system with lling ration of 50% was equal to 50%.

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A pulsating heat pipe was used in waste heat recovery system. Deng et al. (Deng et al. 2017) experimentally investigated the thermal performance of the anti-gravity pulsating heat pipe and heat recovery of the system. Their results revealed that the optimum lling ratio of the pulsating heat pipe was about 70%. Also, the system was assisted to use the waste heat recovery to preheat the fuel oil. Li et al. (Li et al. 2020) studied the performance of pulsating heat pipe in low temperature heat recovery using graphene nano uid. They reported that the thermal resistance amount reduces by raising the volume fraction of the graphene nanoparticles and also with raising the thermal load to the evaporator. An end closed one PHP which is used as an air preheater has been studied by Rittidech et al. (Rittidech et al. 2005). This PHP consists of an evaporator, an adiabatic part and a condenser. The 2 mm internal diameter heat pipe is made of copper. Water and R-123 are used as operating uids. It is shown that any increase in temperature, causes the e ciency to increase. Also, the system operation will increase if R-123 is used instead of water. A preheater end closed PHP with one-way valve was fabricated by Meena et al. R-134a with a 50% lling ratio was used as an operating uid. Any increase in the hot air velocity and temperature would reduce the e ciency. Also this system has the capability to reduce the humidity from 80-100% to 54-72% (Meena et al. 2007). In another research, studied by tube oscillating heat pipe for waste heat recovery application. The results revealed that the average waste heat recovery of the nned oscillating heat pipe was about 80% higher than bare tube oscillating heat pipe at the same lling ration. Khodami et al. (Khodami et al. 2016) evaluated the effect of PHP used to waste heat recovery of the chimney. They showed that the exergy of the system using silver nano uid is much better that ethanol as working uid and also increased the exergy e ciency about 3%. Dhone and Pise (Dhone &Pise 2021) investigated the effect of waste heat recovery of the diesel generator using pulsating heat pipe. The different lling ratio of working uid was used in pulsation heat pipe. The results showed that the optimum lling ratio of pulsating heat pipe on waste heat recovery was equal to 50%.
The couple of PHP by energy applications can have high impact on the performance of these systems. The pulsating heat pipe used to reduce the heat loss of the system and also recovery of the waste heat. To our best knowledge, the PHP is not used for waste heat recovery of the chimney and use this thermal energy for increase water temperature. In this present study, the effect of pulsating heat pipe on the waste heat recovery of the chimney and produce hot water for household consumption is experimentally investigated. Also, the CO 2 mitigation, economic analysis and environmental parameters was studied to assess the performance of the waste heat recovery system with PHP.

Experimental setup
The heat exchanger consists of three main parts includes the pulsating heat pipe, exhaust air channel from chimney and water reservoir for produce hot water. The PHP is fabricated by using copper pipes with suitable internal diameter of 0.002 m. The rolling around a 0.02 m shaft, the 0.002 m copper pipe turns to 21 U shape ones in only 1 m distance. The pipe rstly is evacuated to the 175 Pa and then lled with distilled water up to 60% lling ratio. The test setup was installed in which the exhaust air goes through the channel for heat exchanging (PHPs) in a constant temperature 180℃, and then it leaves to the atmosphere in a temperature near 108℃. In this experiment exhaust air is the air produced by household devices. Fig. 4 shows the test setup of waste heat recovery system using pulsating heat pipe. The water reservoir has an inlet at the bottom and an outlet at the top. Household water is used to be heated. Actually in this setup a heat exchanger for indoor use is described. Temperatures are measured by 5 sensors, a thermometer and a key selector to switch between sensors. The sensors are installed at reservoirs inlet and outlet, channel inlet and outlet, and heat pipes. The water ow is measured by a 10-100 L/h rotameter. The water ow rate of water reservoir was considered equal to 30 L/h and air exhaust velocity from chimney was equal to 0.7 m/s. An anemometer is used for air velocity measurement. Fig. 5 illustrates the schematic of pulsating heat pipe.

Uncertainty of results
Uncertainty evaluating of test measuring data was investigated. The value of uncertainty is achieved by data sheet of the devices and some by the builder and is calculated by (Shoeibi et al. 2020): {u}=\frac{{a}}{\sqrt{3}} (1) Where a and u represent the standard uncertainty and accuracy of equipment. Table 1 illustrates the uncertainty of the measuring device.

Thermal e ciency
The energy e ciency of the system is de ned as thermal energy produce for raising the water temperature to the heat energy inlet from exhaust air of chimney and is calculated as follow: If there was no heat loss in the system, the e ciency would be equal to one, but the heat loss affects directly on the e ciency reduction.

Price of hot water generation
The price per liter of hot water generation was assessed to cost effective of the fabrication of the waste heat recovery application using pulsating heat pipe. The capital recovery coe cient is used for economic evaluation, which de nes the achievement of the investment and is de ned by the ratio of a xed annuity to the value of obtaining that annuity for a speci c where i and n depict the interest rate of a bank loan (20% in this paper) and the lifespan of waste heat recovery system, which is assumed to be twenty years. The rst annual cost is shown as follow (Shoeibi et al. 2021c): \mathbf{F}\mathbf{A}\mathbf{C}=\mathbf{P}\times \mathbf{C}\mathbf{R}\mathbf{F} (7) where P shows the capital price of waste heat recovery application. The annual salvage value is determined as the value of the goods of waste heat recovery system and is calculated by (Shoeibi et al. 2021d): \mathbf{A}\mathbf{S}\mathbf{V}=\mathbf{S}\times \mathbf{S}\mathbf{S}\mathbf{F} (8) where S is the salvage amount of waste heat recovery system and is about 20% of the used all goods value of the system. The sinking fund factor is obtained by (Shoeibi et al. 2021b): \mathbf{S}\mathbf{F}\mathbf{F}=\frac{\mathbf{i}}{{\left(1+\mathbf{i}\right)}^{\mathbf{n}}-1} (9) The annual maintenance costs such as the annual cost of maintenance of the system, which is assumed to value for ten percentage of the FAC and obtained by (Shoeibi et al. 2021a): \mathbf{A}\mathbf{M}\mathbf{C}=0.10\times \mathbf{F}\mathbf{A}\mathbf{C} (10) The uniform annual price of the application is obtained as follow: \mathbf{U}\mathbf{A}\mathbf{P}=\mathbf{F}\mathbf{A}\mathbf{C}+\mathbf{A}\mathbf{M}\mathbf{C}-\mathbf{A}\mathbf{S}\mathbf{V} (11) The cost per cubic meter of hot water generation is determined as the ratio of the UAP to the annual hot water generation (HW) of the system and is achieved by (Shoeibi et al. 2021d): \mathbf{C}\mathbf{P}\mathbf{H}=\frac{\mathbf{U}\mathbf{A}\mathbf{C}}{\mathbf{H}\mathbf{W}} (12) 3.

CO 2 mitigation
The annual CO 2 mitigation in the waste heat recovery system is calculated as {{(\text{E}}_{\text{e}\text{n}})}_{\text{o}\text{u}\text{t}}× 2 and the CO 2 removal in the period of lifespan is determined as

Enviroeconomic parameter
The enviroeconomic parameter is speci ed as the price achieved by the CO 2 reduction in the period of lifespan of the waste heat recovery system and is achieved by: The cost of CO 2 is about 14.5$ per ton (Parsa et al. 2020).

Results And Discussion
In this study, the inlet and outlet air exhaust temperatures from chimney, the inlet and outlet water temperature from water reservoir, and thermal e ciency of the waste heat recovery application. The experimental study was used to assess the performance of the waste heat recovery application using pulsating heat pipe to produce hot water. Furthermore, cost of one cubic meter of hot water generation (CPH), and environmental analysis of waste heat recovery system are presented and discussed. The different angles of the pulsating heat pipe was tested to optimized best angle of heat pipe. Each experiment is done four times to reduce the errors. With respect to the annual measuring of energy produce and hot water, the data of the one-day experiment (December 2, 2020) were applied to all days of the year. Fig. 6 shows the inlet and outlet temperature of the air exhaust from chimney. The outcomes revealed that the lowest temperature of outlet air temperature from channel was obtained at angle of 90 degrees, which was equal to 105.1 o C. Also, the amount of inlet air temperature in channel was the same at different and was equal to 180 o C. Moreover, the angle of the pulsating heat pipe has an inverse effect to air outlet temperature of channel due to increasing the heat transfer rate between air exhaust and water reservoir. Figure 7 depicts the temperature of inlet and outlet water of the hot water reservoir. The results showed that the maximum hot water temperature in outlet of reservoir was achieved at angle of 90 degrees, which was about to 58 o C. Also, the amount of inlet water temperature in reservoir was about 33 o C. Moreover, the angle of the pulsating heat pipe has a direct impact to hot water outlet temperature of reservoir due to enhance in heat transfer rate between an air exhaust and water reservoir. Figure 8 indicates the energy e ciency and pulsating heat pipe temperature of the waste heat recovery system. As observed, the highest energy e ciency and pulsating heat pipe temperature was achieved in angle of 90 degrees, which were equal to 54% and 80 o C, respectively. By increasing the evaporator surface temperature of PHP, the thermal e ciency of the application increased due to increasing the heat transfer between water and heat pipes. Also, the lowest energy e ciency was about 19% which was occurred in angle of 0 degrees. Figure 9 illustrates the energies of hot water, air exhaust and waste heat energy from the system. The obtained outcomes showed that as the energy of hot water raised, the energy of air exhaust decreasing due to high heat transfer between heat pipes and water. The highest energy of hot water was equal to 872 W which was occurred in angle of 90 degrees. Also, the waste heat energy of the system was increased by increasing the angle of the PHP. The highest waste heat energy of the system was occurred in angle of 0 degree, which was about 1303 W. Table 2 provides the price of fabrication of the waste heat recovery system. The results revealed that the price of fabrication and salvage value of the waste heat recovery system are about 117$ and 23.4$, respectively. Table 3 depicts the price of one Page 7/18 cubic meter of hot water production with using waste heat recovery system. The results indicated that the annual hot water generation and CPH of the system were equal to 262.8 m 3 /year and 0.1 $/m 3 , respectively.  Table 4 presents the embodied energy to generate various goods and material used in the application using PHP for generate hot water. The embodied energy of the waste heat recovery system was about 439.7 kWh.  Table 5 shows the environmental, enviroeconomic parameters in the waste heat recovery system for a lifespan of 20 years.
The results indicated that the CO 2 reduction and enviroeconomic parameter of waste heat recovery application were equal to 84.82 tons and 1217.2 $, respectively, during life time. Also, the CO 2 emission of the waste heat recovery system during life time based on embodied energy was equal to 879.4 kg.

Conclusion
In this research a novel application of PHPs in heat exchangers are reviewed and experimentally tested. The results show that this prototype of heat exchanger that is working by PHPs can reach an e ciency up to 54% which is noticeable. This type of heat exchanger is applicable in any place that hot air and cold water do exist. In many industries, hot exhaust air produced by industrial instruments is easily left to atmosphere which can simply affects unwanted global warming. If such a heat exchanger were used on the exhaust air channel before leaving to the atmosphere, not only the heat could be easily be recovered, but also it would prevent the unwanted global warming. This type of heat exchanger can even be used as an appliance for such houses which use fossil fuel (such as oil or gas fuel) as the main resource for household heating. Exhaust air of heating devices can easily be used to get the household water hot. The main conclusion presented as follow: The CO 2 mitigation and enviroeconomic parameter of waste heat recovery system were equal to 84.82 tons and 1217.2 $, respectively.
The annual hot water generation and CPH of the system were equal to 262.8 m 3 /year and 0.1 $/m 3 , respectively.
The price of fabrication of the waste heat recovery system was about 117$.
The highest energy of hot water was equal to 872 W which was occurred in angle of 90 degree.
The highest energy e ciency and pulsating heat pipe temperature was achieved in angle of 90 degree, which were equal to 54% and 80 o C.
The angle of the pulsating heat pipe has a direct impact to hot water outlet temperature of reservoir.
The highest hot water temperature in outlet of reservoir was about to 58 o C. The fabricated pulsating heat pipe.

Figure 2
Exhaust air channel which the evaporators are placed inside.

Figure 3
The water reservoir coupled with PHP.

Figure 4
The experimental setup of heat recovery using PHP.