Analysis and Evaluation of Heat Pipe Efficiency to Reduce Low Emission

energies

Article Analysis and Evaluation of Heat Pipe Efﬁciency to Reduce Low Emission with the Use of Working Agents R134A, R404A and R407C, R410A

Łukasz Adrian 1,* , Szymon Szufa 1,* , Piotr Piersa 1, Piotr Kuryło 2 , Filip Mikołajczyk 1, Krystian Kurowski 3, Sławomir Pochwała 4 , Andrzej Obraniak 1, Jacek Stelmach 1, Grzegorz Wielgosi ´nski 1 , Justyna Czerwi ´nska 1 and Przemysław Kubiak 5

1 Faculty of Process and Environmental Engineering, Lodz University of Technology, Wolczanska 213, 90-924 Lodz, Poland; [email protected] (P.P.); fi[email protected] (F.M.); [email protected] (A.O.); [email protected] (J.S.); [email protected] (G.W.); [email protected] (J.C.) 2 Faculty of Mechanical Engineering, University of Zielona Góra, Licealna Street 9, 65-417 Zielona Góra, Poland; [email protected] 3 Faculty of Biology and Environmental Science, Cardinal Stefan, Wyszynski University in Warsaw, Wójcickiego 1/3, 01-938 Warsaw, Poland; [email protected] 4 Faculty of Mechanical Engineering, Opole University of Technology, 45-271 Opole, Poland; [email protected] 5 Institute of Vehicles, Warsaw University of Technology, str. Narbutta 84, 02-524 Warsaw, Poland; [email protected] * Correspondence: [email protected] (Ł.A.); [email protected] (S.S.); Tel.: +48-501-156-231 (Ł.A.); +48-606-134-239 (S.S.) Citation: Adrian, Ł.; Szufa, S.; Piersa, P.; Kuryło, P.; Mikołajczyk, F.; Abstract: This paper presents an analysis of methods to increase the efficiency of heat transfer in heat Kurowski, K.; Pochwała, S.; Obraniak, exchangers. The scope of the research included analysis of efficiency optimization using the example A.; Stelmach, J.; Wielgosiński,G.; et al. Analysis and Evaluation of Heat Pipe of two tubular heat exchanger structures most often used in industry. The obtained efficiency of Efficiency to Reduce Low Emission heat recovery from the ground of the examined exchangers was over 90%, enabling the reduction of with the Use of Working Agents emissions of the heating systems of buildings. The paper presents the results of tests of two types of R134A, R404A and R407C, R410A. heat pipes using R134A, R404A, and R407C working agents. The paper also presents the results of Energies 2021, 14, 1926. https:// experimental tests using the R410A working medium. The results included in the study will also doi.org/10.3390/en14071926 enable the effective use of land as a heat store.

Academic Editor: Manabendra Saha Keywords: heat pipe; heat transfer; heat exchanger; phase change; evaporation; condensation; low emission Received: 18 February 2021 Accepted: 26 March 2021 Published: 31 March 2021 1. Introduction Publisher’s Note: MDPI stays neutral It is widely acknowledged that global demand for energy is constantly increasing. with regard to jurisdictional claims in Industries are finding it increasingly difficult to stay competitive because companies in published maps and institutional affil- every market are looking for new ways to maximize efficiency while lowering energy iations. costs and improving their environmental image. These challenges are complex and multi- faceted [1]. Heat exchangers play one of the most important roles in the pursuit of optimization while also saving energy. Installations of high-efficiency heat exchanger systems provide Copyright: © 2021 by the authors. new opportunities to reduce energy costs and CO2 emissions without compromising the Licensee MDPI, Basel, Switzerland. performance and quality of the final product. This article is an open access article Many manufacturers of thermal equipment design their cooling and heating equip- distributed under the terms and ment to be compact. The compact design of the heat exchanger can significantly reduce conditions of the Creative Commons Attribution (CC BY) license (https:// installation costs in the replacement of older technology and increase plant efficiency. It is creativecommons.org/licenses/by/ easier to install a heat exchanger in an area in which space for facilities is a critical factor. 4.0/).

Energies 2021, 14, 1926. https://doi.org/10.3390/en14071926 https://www.mdpi.com/journal/energies Energies 2021, 14, 1926 2 of 29

The spectacular increase in sales of heat pumps and ground heat exchangers in the last 3 years, and the likely trend for coming years, indicates that the target market is interested in the solutions examined in this study. According to the report of the Polish Organization for the Development of Heat Pump Technology (PORT PC), the heat pump market, and thus that of ground heat exchangers, in Poland in 2020 (one-fifth of heat pumps installed in Poland have a lower source of heat in the form of soil) recorded large sales increases. Since 2017, the heat pump market has grown more than five-fold and, according to the same organization, this trend will continue for the next decade. Renewable energy support programs have contributed to a significant increase in sales of heat pumps in recent years. It should be emphasized that the Polish heat pump market is developing practically without the direct support of investments in this technology from the state. Subsidies and subsidy programs exist that include investments in heat pumps, but there is currently no direct subsidy program for the pumps themselves, as is the case with other renewable energy systems. The results obtained during research into the high efficiency of heat pipe heat exchangers (92%) show that it is possible to use heat pipes as heat exchangers that intensify the heat exchange between lower heat sources from renewable sources (e.g., ground) and heat receivers of central heating devices for buildings (e.g., the heat pump). Contamination downtime is also an important issue in industrial plants. This is not only an issue regarding productivity—it also has an impact on efficiency. As contamination accumulates, thermal efficiency and pressure decrease, requiring more energy. Pumping fluid through a dirty heat exchanger, for example, also requires more power and considerably greater energy consumption. This study clearly indicates that the installation of more efficient heat exchangers is often the best means of removing limitations caused by insufficient thermal or cooling capacity. More effective heat flow means that more energy can be used by the technological process, which allows an increase in the available production capacity while reducing costs [2,3]. Thus, understanding relatively simple means of increasing the efficiency of a heat exchanger can be useful in evaluating new technologies and comparing suppliers. There has been significant development in the use of heat pipes over the last half-century, initiated by the use of heat pipes in space science. Today, they are used in many industries. One of the most interesting applications of heat pipes is their use for drying and cooling air, in addition to heat recovery and passive cooling of rooms. Other important applications are pipes as solar collectors and as passive heat exchangers for regulating ground temperature and heating the surface of bridges and viaducts. Due to its structure, the heat pipe is an economical heat exchanger compared to other commonly used approaches. For this reason, and in an era of striving to save [4] and limit the unnecessary dissipation of energy, interest in this type of heat exchangers is still growing [5]. To facilitate this technology, a series of tests has been found to be necessary to analyze the operation of heat pipes and to identify the most effective heat exchangers of the heat pipe type depending on their operating conditions [6]. The current research included an analysis of the efficiency optimization problem based on the example of two types of tubular heat exchanger structures most commonly used in industry. The research on effective methods of increasing heat transfer in cooling systems was also associated with the need to analyze the most important factors influencing the efficiency of thermal systems using R134A, R404A, R407C, and R410A working agents [7]. These factors were selected due to their thermodynamic properties in the temperature range studied, their availability, and their wide use in civil engineering and air conditioning. As part of this work, the Department of Thermal Technology of the Institute of Fluid-Flow Machinery of the Lodz University of Technology also conducted experimental research on the working factor R410A, the results and conclusions of which are included in the comparative analysis of the current paper. R410A is used in refrigeration equipment, air-cooled chillers, and heat pumps. This factor is also used in sea and land transport, and cold stores [8]. Energies 2021, 14, 1926 3 of 29

Experimental studies were carried out in conditions corresponding to natural work, i.e., in temperature conditions of heat recovery, both in air-conditioning and ventilation installations, and in construction engineering [9,10]. They consisted of testing a heat exchanger of the heat pipe type in the temperature limits of the lower source between 15 and 50 ◦C. The effects of phase transformations for the aforementioned working factors and their quantity were investigated, in addition to their influence on the operation and efficiency of the heat pipe. The main focus of this research was to analyze and evaluate the efficiency of the heat pipe for different working factors for different temperatures of the heat supply to the heat pipe [11].

2. Materials and Methods 2.1. Used Materials and Their Properties In this study, our team conducted research using 4 different refrigerants: R134A, R404A, R407C and R410A. R134A refrigerant is used in medium-temperature, small and medium-sized refrigeration and air-conditioning installations. R134A has a faint ethereal odor and is colorless. Its chemical formula is CH2F-CF3. It is a replacement for the withdrawn R12 refrigerant. R404A refrigerant is a synthetic refrigerant of near azeotropic mixtures. This refrigerant is a replacement for R502, R22, and R507. R404A refrigerant is used in refrigeration and air-conditioning units. R407C is a synthetic azeotropic mixture. It is used as a replacement for R22. Heat pipe tests were carried out using working ﬂuids such as R134A, R404A, R407C, and R410A. These factors were selected due to their thermodynamic properties in the studied temperature range, their availability, and the affordability of their use in construction engineering and air conditioning. R410A refrigerant is a near azeotropic mixture of R32 (50%) and R125 (50%), used in refrigeration and air conditioning devices with an evaporation temperature from −50 to 20 ◦C and, in some applications, from −70 to 40 ◦C, e.g., as a replacement for R13B1.

2.2. Assembly of the Test Bench The experimental tests were conducted on a heat tube exchanger made of copper, with a length of 1769 mm, an 18 mm outside diameter, and a 1 mm wall thickness (Figure1) [12,13]. Heat pipe data: - internal diameter: dw = 0.016 (m) - outer diameter: dz = 0.018 (m) - tube height: h = 1.769 (m) In the Department of Heat and Refrigeration Technology of the Institute of Fluid-Flow Machinery of the Technical University of Lodz, a stand for testing heat exchangers of the heat tube type was designed and built. The setup was equipped with a heat-flow apparatus, which included, among other things, ultrathermostats for supplying pipe- in-pipe exchangers serving the heat pipe, circulation pumps, and liquid coolers. The exchanger was made of copper due to the high heat transfer coefficient and the availability and low price of copper pipes. The length was selected so that the part receiving and giving off heat is in the middle of the 0.5–1 m range, which corresponds to a typical series of heights of ventilation and air conditioning ducts and units. As part of the subject of this work, experimental tests were also carried out on a heat pipe made of brass, with a length of 550 mm, an external diameter of 22 mm, and a wall thickness of 1 mm (Figure2), the results and conclusions of which are included in the comparative analysis in this paper (see work in [14,15]). The first tests were carried out on a tube with vacuum inside and then filled with air to confirm the thesis that heat conduction through the wall of the tube along its length was negligible. The research consisted of placing both heat supply (i.e., the evaporator) and Energies 2021, 14, 1926 4 of 29

Energies 2021, 14, x FOR PEER REVIEW heat removal (i.e., the condenser) in the ﬂow exchanger, which supplies and receives4 of 32 heat from the heat pipe on the principle of a pipe-in-pipe heat exchanger (Figure3).

Figure 1. The tested copper heat pipe, 1769 mm long. Figure 1. The tested copper heat pipe, 1769 mm long.

HeatDuring pipe data: the research, a thermographic analysis of the operation of a heat pipe type - heatinternal exchanger diameter: was dw also = 0.016 carried (m) out [16]. For the purposes of thermographic analysis of - theouter processes diameter: taking dz = place0.018 inside(m) the tested exchanger, a polycarbonate heat pipe was also - builttube (Figureheight: 4h)[ =17 1.769]. (m) In the Department of Heat and Refrigeration Technology of the Institute of Flu- id-Flow Machinery of the Technical University of Lodz, a stand for testing heat exchangers of the heat tube type was designed and built. The setup was equipped with a heat-flow apparatus, which included, among other things, ultrathermostats for supplying pipe-in-pipe exchangers serving the heat pipe, circulation pumps, and liquid coolers.

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The exchanger was made of copper due to the high heat transfer coefficient and the availability and low price of copper pipes. The length was selected so that the part receiving and giving off heat is in the middle of the 0.5–1 m range, which corresponds to a typical series of heights of ventilation and air conditioning ducts and units. As part of the subject of this work, experimental tests were also carried out on a heat pipe made of Energies 2021, 14, 1926 brass, with a length of 550 mm, an external diameter of 22 mm, and a wall thickness of 1 5 of 29 mm (Figure 2), the results and conclusions of which are included in the comparative analysis in this paper (see work in [14,15]).

Energies 2021, 14, x FOR PEER REVIEWFigure 2 2.. AA brass brass heat heat pipe, pipe, 550 mm 550 l mmong and long 22 and mm 22in mmdiameter in diameter [16]: (a) a [brass16]: (heata) a pipe, brass (b heat6) aof 32 pipe, (b) a

heat pipe pipe with with heat heat exchangers exchangers and andmarked marked inlets inlets and outlets and outlets for hot forand hot cold and water. cold water.

The first tests were carried out on a tube with vacuum inside and then filled with air to confirm the thesis that heat conduction through the wall of the tube along its length was negligible. The research consisted of placing both heat supply (i.e., the evaporator) and heat removal (i.e., the condenser) in the flow exchanger, which supplies and receives heat from the heat pipe on the principle of a pipe-in-pipe heat exchanger (Figure 3).

Figure 3. Tested heat pipe 1769 mm long: (a) complete heat pipe, (b) isothermal part of the heat Figure 3. Tested heat pipe 1769 mm long: (a) complete heat pipe, (b) isothermal part of the heat pipe, pipe, (c) flow heat exchanger, (d) flow heat exchanger, (e) temperature measurement point. (c) ﬂow heat exchanger, (d) ﬂow heat exchanger, (e) temperature measurement point. During the research, a thermographic analysis of the operation of a heat pipe type heat exchanger was also carried out [16]. For the purposes of thermographic analysis of the processes taking place inside the tested exchanger, a polycarbonate heat pipe was also built (Figure 4) [17].

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Figure 4. A 1769 mm long heat pipe made of polycarbonate: (a) complete heat pipe, (b,c) isothermal Figure 4. A 1769 mm long heat pipe made of polycarbonate: (a) complete heat pipe, (b,c) isothermal part of the heat pipe, (d) heat pipe without flow exchangers. part of the heat pipe, (d) heat pipe without ﬂow exchangers.

AA thermographic thermographic analysis analysis was was conducted conducted on an on actual an actual heat pipe heat exchanger pipe exchanger produced pro- fromduced polycarbonate, from polycarbonate, 1769 mm 1769 long, mm 18 mm long, outside 18 mm diameter, outside and diameter 1 mm, walland 1thickness, mm wall i.e.,thickness, with dimensions i.e., with dimensions correlated to correlated the copper to heatthe copper pipe. Bothheat thepipe. heat Both pipe the andheat the pipe ﬂow and exchangersthe flow exchangers were built were from buil polycarbonatet from polycarbonate due to the due transparency to the transparency that allows that observa- allows tionobservation of the processes of the processes taking place taking inside place the inside heat pipe.the heat The pipe. thermographic The thermographic analysis analy- was conductedsis was conducted using a thermovision using a thermovision camera [18,19 camera]. The installation [18,19]. The was installation also equipped was with also basicequipped measuring with equipment,basic measuring which equipment, included thermocouples which include withd thermocouples meters, and digitalwith meters ﬂow , metersand digital with metersflow meters and rotameters with meters [20 ].and A photorotameters of the [20]. constructed A photo testof the stand constructed is shown intest Figurestand5 .is shown in Figure 5.

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FigureFigure 5. 5.Measuring Measuring station station where: where: 1—heat 1—heat pipe pipe with with insulated insulated heat heat exchangers exchangers 2—ultrathermostat– 2—ultrathermostat–cooling water, 3—ultrathermostat–heating water, 4—chilled water aggregate, cooling water, 3—ultrathermostat–heating water, 4—chilled water aggregate, 5—circulation pumps, 5—circulation pumps, 6—rotameter, 7—gauge flow, 8—control valves, 9—pressure gauge, 6—rotameter, 7—gauge flow, 8—control valves, 9—pressure gauge, 10—pressure sensor, 11— 10—pressure sensor, 11—millivoltmeter, 12—heat pipe filling valve, 13—filling station. millivoltmeter, 12—heat pipe filling valve, 13—filling station. The heat pipe (1) was enclosed in the evaporating and condensing parts with heat The heat pipe (1) was enclosed in the evaporating and condensing parts with heat exchangers for its heating and cooling. The evaporator was heated in the temperature exchangers for its heating and cooling. The evaporator was heated in the temperature range of 15–50 °C, and the condenser was cooled with water at 10 °C. To maintain con- range of 15–50 ◦C, and the condenser was cooled with water at 10 ◦C. To maintain constant temperature parameters in the heat exchangers, the station was equipped with two stant temperature parameters in the heat exchangers, the station was equipped with two ultrathermostatsultrathermostats equipped equipped with with a a microprocessor-based microprocessor-based temperature temperature control control system system (2,3) (2,3) cooperatingcooperating with with an an ice ice water water unit unit (4). (4). Photos Photos of of the the heat heat pipe pipe with with exchangers exchangers are are shown shown inin Figure Figure6. 6. To ensure the possibility of regulating and measuring the water flow through the heat exchangers, circulation pumps (5), rotameters (6), electronic flow meters (7), and shut-off and flow regulating valves (8) were installed. The possibility of measuring the pressure in the central part of the tube was provided by a pressure gauge (9) with a sensor (10). Installing thermocouples in conjunction with milli-voltmeters (11) made it possible to measure the thermoelectric force in selected places on the heat pipe, and thus to determine

Energies 2021, 14, 1926 8 of 29

the temperatures in these places. Additionally, this system allowed us to determine the Energies 2021, 14, x FOR PEER REVIEWtemperature of water ﬂowing in and out of the heat exchangers. In the central part of9 of the 32

heat pipe there was a valve (12) for ﬁlling it.

FigureFigure 6. 6.Test Test stand stand before before insulation. insulation.

TheTo ensure main tests the werepossibility carried of out regulating on a real and heat measuring exchanger the made water of copper,flow through 1769 mm the long,heat 18 exchangers, mm diameter, circulation and 1 mm pumps thick. (5), The rotameters heat pipe was (6), filled electronic with the flow refrigerant meters (7), filling and stationshut-off (13), and which flow firstregulating generated valves a vacuum (8) were in installed. the pipe, andThe thenpossibility filled theof measuring pipe with thethe measuredpressure in amount the central of the part working of the medium. tube was provided by a pressure gauge (9) with a sensor (10). Installing thermocouples in conjunction with milli-voltmeters (11) made it pos- 2.3.sible Measuring to measure Points the andthermoelectric Quantities of force Interest in selected places on the heat pipe, and thus to determineThe heat the transfer temperatures tests in in the these heat places. pipe type Additionally, heat exchanger this weresystem carried allowed out us for to vari- de- oustermine temperature the temperature ranges in of terms water of flowing phase changesin and out taking of the place heatinside exchangers. the heat In pipes.the central An analysispart of the and heat evaluation pipe there of the wa efficiencys a valve (12) of the for heat filling pipe it. work was carried out for various workingThe factors main tests and thewere related carried phase out changes,on a real andheat for exchanger different made temperatures of copper, on 1769 the side mm oflong, the heat18 mm supply diameter, and constant and 1 mm temperature thick. The onheat the pipe side was ofthe filled heat with receipt the refrigerant from the heat fill- pipeing station [21]. The (13), analysis which first and generated development a vacuum of the in research the pipe, results and then consisted filled the of assessingpipe with thethe effectiveness measured amount of this of type the of working exchanger. medium. The research methods concerning this heat exchanger consisted of examining a real exchanger and computer modeling of this exchanger to2.3. obtain Measuring a correlation Points betweenand Quantities the simulation of Interest results and experimental studies. Equipment for thermal-flowThe heat transfer measurements tests in and the heatcomputer pipe equipmenttype heat exchanger for numerical were modeling carried ofou thist for heat exchanger was used for the tests, and for the preparation of the test results. Proper various temperature ranges in terms of phase changes taking place inside the heat pipes. functioning of the heat pipe depends highly on the temperature inside it, the pressure An analysis and evaluation of the efficiency of the heat pipe work was carried out for of the working medium, and the temperature of the heating water at the heat exchanger various working factors and the related phase changes, and for different temperatures on inlet. Once the pressure is known, the temperature of evaporation of the working medium the side of the heat supply and constant temperature on the side of the heat receipt from is read from thermophysical tables or with the use of a program, e.g., Solkane. The heat the heat pipe [21]. The analysis and development of the research results consisted of as- pipe can only operate when the evaporating temperature of the working medium inside is sessing the effectiveness of this type of exchanger. The research methods concerning this lower than the temperature of the water washing the evaporator part of the tube. In order heat exchanger consisted of examining a real exchanger and computer modeling of this for the tube to function, the working medium inside the heat tube should be heated by exchanger to obtain a correlation between the simulation results and experimental stud- the thermal energy supplied. Conversely, when the temperature inside the heat tube is ies. Equipment for thermal-flow measurements and computer equipment for numerical higher than the temperature of the water washing the vaporizer part of the tube, it cannot modeling of this heat exchanger was used for the tests, and for the preparation of the test function properly. results.The Proper research functioning plan included of the the heat following pipe depends activities: highly on the temperature inside it, the pressure of the working medium, and the temperature of the heating water at the 1. Emptying the heat pipe (creating a vacuum); heat exchanger inlet. Once the pressure is known, the temperature of evaporation of the 2. Filling the heat pipe with an appropriate amount of the working medium; working medium is read from thermophysical tables or with the use of a program, e.g., 3. Determining the assumed temperatures in the ultrathermostats; Solkane. The heat pipe can only operate when the evaporating temperature of the 4. Pumping heating and cooling factors for the heat pipe; working medium inside is lower than the temperature of the water washing the evaporator part of the tube. In order for the tube to function, the working medium inside the heat tube should be heated by the thermal energy supplied. Conversely, when the temperature inside the heat tube is higher than the temperature of the water washing the vaporizer part of the tube, it cannot function properly.

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The research plan included the following activities: 1. Emptying the heat pipe (creating a vacuum); Energies 2021, 14, 1926 9 of 29 2. Filling the heat pipe with an appropriate amount of the working medium; 3. Determining the assumed temperatures in the ultrathermostats; 4. Pumping heating and cooling factors for the heat pipe; 5. SettingSetting the the uniform uniform flow ﬂow of of heating heating and and cooling cooling fac factorstors in in the the heat heat pipe pipe at at 15 15 L Lh;h; 6. DeterminingDetermining the the resultant resultant temperatures temperatures tz2, tz2, trg, trg, trz, trz, tg2 tg2;; 7. ReadingReading of of measurement measurement data. data. The schematic diagram of the test stand is shown in Figure7 7..

Figure 7. Schematic diagram of the test stand. Figure 7. Schematic diagram of the test stand.

Table1 1 shows shows fillings fillings of of the the heat heat exchanger exchanger used used to heat to heat (3) and (3) cooland (2)cool the (2) heat the pipe.heat pipe.The diagram The diagram also shows also shows the installation the installation location location of the pressure of the pressure sensor (4) sensor and the (4) valveand the (5) valvethrough (5) whichthrough the which heat pipethe heat is filled. pipe Theis filled. factor The that factor heats that the heats heat the pipe heat is filled pipe withis filled an withultrathermostat an ultrathermostat (12), in which (12), electric in which heaters electric and heaters coils supplied and coils from supplied the ice water from genera- the ice watertor (13) generator and the water (13) and supply the networkwater supply (15) are network responsible (15) are for responsible maintaining for the maintaining appropriate thetemperature. appropriate The temperature. heating medium The heating from the medium ultrathermostat from the (12) ultrathermostat is forced by the (12) pump is forced (10) bythrough the pump the flow (10) meterthrough (8) the and flow the rotametermeter (8) and (6) to the the rotameter flow heat (6) exchanger to the flow pipe heat in theex- changerpipe (3), pipe where in itthe gives pipe its (3), heat where to the it heatgives pipe its heat (1) and to the returns heat topipe the (1) ultrathermostat and returns to (12). the ultrathermostatValve (8) is responsible (12). Valve for regulating (8) is responsible the amount for ofregulating flowing medium.the amount In theof flowing ultrathermo- me- dium.stat (11) In therethe ultrathermostat is a cooling agent, (11) the there temperature is a cooling of whichagent, isthe kept temperature at a constant of which level byis keptelectric at heaters a constant and levelcoils fed by withelectric chilled heaters water and from coils the fed chilled with water chilled generator water from (13) and the chilledwater from water the generator mains (15). (13) Chilledand water water from flow the is mains regulated (15). byChilled a valve water (14). flow The is coolant regu- latedfrom theby a ultrathermostat valve (14). The (11)coolant is forced from by the the ultrathermostat pump (9) through (11) theis forced flowmeter by the (8) pump and the(9) throughrotameter the (6) flow to the meter flow heat(8) and exchanger the rotameter pipe in (6) the to pipe the (2) flow where heat it exchanger receives heat pipe from in the pipeheat pipe(2) where (1) and it receives then, when heat heated, from the it returnsheat pipe to the(1) and ultrathermostat then, when (11).heated, it returns to the ultrathermostat (11). Table 1. Comparison of the most advantageous fillings of all tested working fluids, vacuum, and air.

R134A 10% R404A 10% R407C 5% Vacuum Air Tg1 ◦ Qg Qz Qg Qz Qg Qz Qg Qz Qg Qz [ C] ï [%] ï [%] ï [%] ï [%] ï [%] [W] [W] [W] [W] [W] [W] [W] [W] [W] [W] 15.00 30.76 27.73 90.00 24.17 20.03 83.00 0.44 0.00 0.00 0.00 0.00 0.00 0.22 0.00 0.00 20.00 39.48 35.65 90.00 37.29 31.70 85.00 0.88 0.00 0.00 0.00 0.00 0.00 0.66 0.00 0.00 25.00 63.48 57.88 91.00 59.11 51.72 88.00 13.13 1.76 13.00 0.00 0.00 0.00 1.09 0.00 0.00 30.00 83.05 78.33 94.00 78.68 71.85 91.00 34.93 11.23 32.00 0.00 0.00 0.00 2.18 0.00 0.00 35.00 104.60 99.04 95.00 109.00 100.70 92.00 54.47 34.12 63.00 1.09 0.00 0.00 4.36 0.44 10.00 40.00 134.90 127.60 95.00 135.10 124.20 92.00 84.84 61.64 73.00 2.18 0.22 10.00 6.53 0.88 13.00 45.00 160.70 151.80 94.00 154.50 140.70 91.00 128.10 97.97 76.00 3.26 0.44 13.00 7.61 1.32 17.00 50.00 199.30 184.80 93.00 176.00 150.80 86.00 162.50 126.60 78.00 4.34 0.66 15.00 8.69 1.98 23.00 Energies 2021, 14, x FOR PEER REVIEW 11 of 32

Table 1. Comparison of the most advantageous fillings of all tested working fluids, vacuum, and air.

Tg1 R134A 10% R404A 10% R407C 5% Vacuum Air [°C] Qg [W] Qz [W] ɳ [%] Qg [W] Qz [W] ɳ [%] Qg [W] Qz [W] ɳ [%] Qg [W] Qz [W] ɳ [%] Qg [W] Qz [W] ɳ [%] 15.00 30.76 27.73 90.00 24.17 20.03 83.00 0.44 0.00 0.00 0.00 0.00 0.00 0.22 0.00 0.00 20.00 39.48 35.65 90.00 37.29 31.70 85.00 0.88 0.00 0.00 0.00 0.00 0.00 0.66 0.00 0.00 25.00 63.48 57.88 91.00 59.11 51.72 88.00 13.13 1.76 13.00 0.00 0.00 0.00 1.09 0.00 0.00 30.00 83.05 78.33 94.00 78.68 71.85 91.00 34.93 11.23 32.00 0.00 0.00 0.00 2.18 0.00 0.00 35.00 104.60 99.04 95.00 109.00 100.70 92.00 54.47 34.12 63.00 1.09 0.00 0.00 4.36 0.44 10.00 Energies 2021, 14, 1926 10 of 29 40.00 134.90 127.60 95.00 135.10 124.20 92.00 84.84 61.64 73.00 2.18 0.22 10.00 6.53 0.88 13.00 45.00 160.70 151.80 94.00 154.50 140.70 91.00 128.10 97.97 76.00 3.26 0.44 13.00 7.61 1.32 17.00 50.00 199.30 184.80 93.00 176.00 150.80 86.00 162.50 126.60 78.00 4.34 0.66 15.00 8.69 1.98 23.00 FigureFigure 8 shows8 shows a diagram a diagram of the of installation the installation of the ofheat the pipe heat on pipe the ontest the stand test with stand with the the temperaturetemperature measurement measurement points. points.

FigureFigure 8. Scheme 8. Scheme of the installation of the installation of the heat ofpipe the at heatthe measuri pipe atng thestand measuring with temperature stand with temperature measurementmeasurement points points,, where where tz1—cold tz1—cold water temperature water temperature at the inlet at to the the inlet cooling to theexchanger, cooling exchanger, tz2— tz2—cold water temperature at the outlet from the cooling exchanger, tg1—hot water temperature cold water temperature at the outlet from the cooling exchanger, tg1—hot water temperature at the at the entrance to the heat exchanger, tg2—hot water temperature at the outlet of the heating ex- entrance to the heat exchanger, tg2—hot water temperature at the outlet of the heating exchanger, trz— heat pipe wall temperature in the center of the condensing section, trg—heat pipe wall temperature in the center of the evaporating section.

2.4. Test Procedure (Used for the Repetitiveness and Archivibility of the Document, Number of Tests Performer) The heat pipe tests consisted of measuring the temperatures at the inlet and outlet of heating and cooling water from pipe-in-pipe heat exchangers, measuring the pressure inside the pipe, determining the water flow in the exchangers, and determining the temperatures at measuring points along the height of the heat pipe. The temperature determined of heating and cooling water in the exchangers was obtained with the use of ultrathermostats. The temperature of the cooling water at the inlet to the exchanger in the condenser part was constant at about 10 ◦C. The value of the heating water temperature at the inlet to the heat exchanger in the evaporator part for subsequent measurements ranged from 15 to 50 ◦C and was changed in increments of 5 ◦C. The flow of heating and cooling water flowing around the heat pipe was about 15 L/h. After stabilizing the heat exchange, the water temperatures at the inlet and outlet of the exchangers and along the height of the heat Energies 2021, 14, 1926 11 of 29

pipe were read. Measurements were made by means of thermocouples installed on the measuring stand, using a millivoltmeter and a temperature recorder. The values obtained in millivolts were converted to temperature in degrees Celsius using the thermocouple constant. The wall temperatures in the middle of the evaporator and condenser were measured in the heat pipe tested. The heat pipe was tested with the use of three refrigerants, namely, R134A, R404A, and R407C. For each of these factors, the heat tube was ﬁlled by volume for subsequent measurements at amounts of 2.5%, 5%, 10%, 20%, 30%, and 40% of the total volume of the heat tube. Knowing the total volume of the heat pipe and the density of a given medium, its amount expressed in grams was determined, which corresponds to its ﬁlling in the volumes indicated above. The density of factors at a given temperature was determined using the tables of thermophysical properties.

2.5. Tests of a Copper (Cu) Heat Pipe 1769 mm Long During the test of a 1769 mm long copper heat pipe, the temperatures at the inlet and outlet of hot and cold water from heat exchangers heating and cooling the heat pipe were measured. The pressures inside the tube and the temperatures of the wall of the heat tube were also measured at measurement points halfway up the condenser and evaporator of the heat tube. The temperature of cold water at the entrance to the heat exchanger in the condenser part was about 10 ◦C. In the evaporator section the hot water temperature at the entrance to the heat exchanger was between 15 and 50 ◦C and it was changed in increments of 5 ◦C for each measurement. The flow rates of cold and hot water flowing around the heat pipe were about 15 L/h. The tests were performed for the heat pipe without any working medium with a vacuum and then with air inside the heat pipe and with the use of refrigerants: R134A, R404A, and R407C. The heat tube was filled by volume for subsequent measurements at amounts of 2.5%, 5%, 10%, 20%, 30%, and 40% of its entire volume.

3. Results 3.1. Heat Copper Tube Exchanger, 1769 mm Long, 18 mm Outside Diameter and 1 mm Wall Thickness 3.1.1. Comparative Analysis of Test Results for Various Charges of the R134A Working Medium Figures9 and 10 show diagrams for different amounts of the R134A working medium Energies 2021, 14, x FOR PEER REVIEW 13 of 32 filling the heat pipe, comparing the heat flux values collected and given off by the heat pipe, and a diagram comparing the heat pipe efficiency.

2.00 2.00 1.80 1.80 1.60 1.60 1.40 1.40 1.20 1.20 1.00 1.00 Qz [W] Qg [W] 0.80 0.80 0.60 0.60 0.40 0.40 0.20 0.20 0.00 0.00 15 25 35 45 15 25 35 45 tg1 [°C] tg1 [°C]

(a) (b)

FigureFigure 9. 9.Comparison Comparison diagram diagram of of heat heat ﬂuxes fluxes (a ()a absorbed) absorbed by by the the heat heat pipe pipe and and (b )(b given) given off off by by the the heat heat pipe pipe for allfor chargesall withcharges the R134Awith the working R134A working medium. medium.

100 95 90 85 80 75

ƞ [%] 70 65 60 55 50 15 20 25 30 35 40 45 50 tg1 [°C]

Figure 10. Comparison diagram of the heat pipe efficiency for all charges with the R134A refrigerant.

The values of the heat flows absorbed by the evaporator are always higher than those taken off by the condenser. This indicates that more heat is taken by the refrigerant than given up. This difference results mainly from heat losses to the environment, such as through the isothermal component, in which the pressure sensor and the valve for filling the heat pipe with the working fluid are located. The heat pipe works with satisfactory efficiency at 5%, 10%, and 20% of its volume filling in the temperature between tg1 15 °C and 50 °C; whereas at 2.5% filling, 30% and 40% of the volume of the heat pipe works with satisfactory efficiency in the temperature range tg1 = 25–50 °C. In all fillings, the pressure in the heat pipe rises linearly with the rise of the temperature at the entrance to the lower heat exchanger. The highest value of the heat flux taken by the evaporator and given off by the condenser occurs when the heat pipe is filled in 10% of its volume with the R134A refrigerant. The highest efficiency was achieved for 10% filling in the heat pipe operating range at 25–50 °C, amounting to approx. 95%. Heat transfer efficiency was also high in the other fillings within the scope of the heat pipe′s operation at temperatures of 25–50 °C. How- ever, in the remaining temperature range, the efficiency of the heat pipe decreased. The

Energies 2021, 14, x FOR PEER REVIEW 13 of 32

2.00 2.00 1.80 1.80 1.60 1.60 1.40 1.40 1.20 1.20 1.00 1.00 Qz [W] Qg [W] 0.80 0.80 0.60 0.60 0.40 0.40 0.20 0.20 0.00 0.00 15 25 35 45 15 25 35 45 tg1 [°C] tg1 [°C]

Energies 2021, 14, 1926 (a) (b) 12 of 29

Figure 9. Comparison diagram of heat fluxes (a) absorbed by the heat pipe and (b) given off by the heat pipe for all charges with the R134A working medium.

100 95 90 85 80 75

ƞ [%] 70 65 60 55 50 15 20 25 30 35 40 45 50 tg1 [°C]

FigureFigure 10. ComparisonComparison diagram diagram of the of heat the pipe heat efficiency pipe efﬁciency for all charges for all with charges the R134A with therefrig- R134A refrigerant. erant.

TThehe values values of of the the heat heat flows flows absorbed absorbed by theby the evaporator evaporator are always are always higher higher than than those takenthose taken off by off the by condenser.the condenser. This Thisindicates indicates that that more more heat heat is taken is taken by the by refrigerant the refrigerant than giventhan given up. up. This This difference difference results results mainly mainly from from heat losses heat to losses the environment, to the environment, such as such as throughthrough the the isothermal isothermal component component,, in which in which the pressure the pressure sensor and sensor the valve and for the filling valve for filling thethe heatheat pipe pipe with with the the working working fluid fluidare located. are located. The heat heat pipe pipe works works with with satisfactory satisfactory efficiency efficiency at 5%, 10%, at 5%,and 10%,20% of and its volume 20% of its volume filling in the temperature between tg1 15 °C and◦ 50 °C; whereas◦ at 2.5% filling, 30% and filling40% of inthe the volume temperature of the heat between pipe works tg1 with 15 satisfactoryC and 50 efficienC; whereascy in theat temperature 2.5% filling, 30% and 40%range of tg1 the = 25 volume–50 °C. In of all the fillings, heat the pipe pressure works in with the heat satisfactory pipe rises linearly efficiency with inthe the rise temperature ◦ rangeof the tg1temperature = 25–50 atC. the In entrance all fillings, to the the lower pressure heat exchanger. in the heat The pipe highest rises value linearly of the with the rise ofheat the flux temperature taken by the atevaporator the entrance and given to the off lower by the heatcondenser exchanger. occurs when The highestthe heat value of the heatpipe is flux filled taken in 10% by of the its volume evaporator with the and R134A given refrigerant. off by the condenser occurs when the heat pipeThe is filled highest in efficiency 10% of its was volume achieved with for the10% R134Afilling in refrigerant. the heat pipe operating range at 25–50 °C, amounting to approx. 95%. Heat transfer efficiency was also high in the other The highest efficiency was achieved for 10% filling in the heat pipe operating range fillings within◦ the scope of the heat pipe′s operation at temperatures of 25–50 °C. How- atever, 25–50 in theC, remaining amounting temperature to approx. range, 95%.the efficiency Heat transferof the heat efficiency pipe decreased was. alsoThe high in the other fillings within the scope of the heat pipe0s operation at temperatures of 25–50 ◦C. However, in the remaining temperature range, the efficiency of the heat pipe decreased. The heat transfer efficiency of the heat pipe was found for the R134A refrigerant at a range

of 53–95%. Due to the value of the heat flux exchanged and heat transfer efficiency in the entire temperature range, the highest efficient operation of the heat pipe occurs when it is filled with the R134A working medium at 10%.

3.1.2. Comparative Analysis of Test Results for Various Charges with the R404A Working Medium Figures 11 and 12 show diagrams for different amounts of the R404A working medium filling the heat pipe, comparing the values of heat fluxes collected and given off by the heat pipe, and a graph comparing the efficiency of the heat pipe. The values of the heat flows obtained by the evaporator are in every case higher than those given out by the condenser. This proves that a higher amount of heat is taken by the cooling fluid than given up. These different values can be observed mainly from heat losses to the environment, for example, through the isothermal component, in which the pressure sensor and the valve for filling the heat pipe with the working medium are located. The heat pipe works with a satisfactory efficiency at 10%, 20%, 30%, and 40% of its capacity filling in the temperature limit between tg1 20 and 50 ◦C, whereas, when filling 2.5% and 5% of its volume, the heat pipe works with satisfactory efficiency in the temperature range tg1 from 45 to 50 ◦C. In all fillings, as the temperature at the entrance to the lower heat exchanger rises, the pressure in the heat pipe rises approximately linearly. The highest value of the heat flux taken by the evaporator and given off by the condenser occurs when the heat pipe is filled in 10% of its volume with the R404A refrigerant. EnergiesEnergies 2021 2021, 14,, 14x ,FOR x FOR PEER PEER REVIEW REVIEW 14 of14 32of 32

heatheat transfer transfer efficiency efficiency of ofthe the heat heat pipe pipe was was found found for for the the R134A R134A refrigerant refrigerant at ata rangea range of of 53–5395%.–95%. Due Due to tothe the value value of ofthe the heat heat flux flux exchanged exchanged and and heat heat transfer transfer efficiency efficiency in inthe the entireentire temperature temperature range, range, the the highest highest eff efficienticient operation operation of ofthe the heat heat pipe pipe occurs occurs when when it isit is filledfilled with with the the R134A R134A working working medium medium at at10%. 10%.

3.1.2.3.1.2. Comparative Comparative Analysis Analysis of ofTest Test Results Results for for Various Various Charges Charges with with the the R404A R404A Working Working MediumMedium Energies 2021, 14, 1926 13 of 29 FiguresFigures 11 11and and 12 12show show diagrams diagrams for for different different amounts amounts of ofthe the R404A R404A working working me- me- diumdium filling filling the the heat heat pipe, pipe, comparing comparing the the values values of ofheat heat fluxes fluxes collected collected and and gi vengiven off off by by thethe heat heat pipe, pipe, and and a graph a graph comparing comparing the the efficiency efficiency of ofthe the heat heat pipe. pipe.

2.00 2.00 1.601.60 1.801.80 1.401.40 1.601.60 1.201.20 1.401.40 1.00 1.201.20 1.00 Qg [W] Qg [W] 1.00 0.80 1.00 0.80Qz [W] Qz [W] 0.80 0.80 0.600.60 0.600.60 0.400.40 0.400.40 0.20 0.200.20 0.20 0.000.00 0.000.00 15 25 35 45 1515 2525 3535 4545 15 25 tg1 [°C]35 45 tg1tg1 [°C] [°C] tg1 [°C]

(b) (a)( a) (b)

FigureFigureFigure 11. 11.Comparison 11. Comparison Comparison diagram diagram diagram ofof heatof heat flux ﬂux flux (a (a) ( )absorbeda) absorbed absorbed by bythe the the heat heat heat pipe pipe pipe and and and(b )( bgiven) ( bgiven) given off off by offbythe the byheat heat the pipe heatpipe for pipefor all allfillings for fillings all ﬁllings withwith thewith the R404A the R404A R404A working working working medium. medium medium. .

100100 9090 8080 7070 6060 50 50ƞ [%] ƞ [%] 4040 3030 2020 1010 0 0 1515 2020 2525 3030 3535 4040 4545 5050 tg1tg1 [°C []°C]

Figure 12. Comparison chart of heat pipe efficiency for all charges with the R404A refrigerant. FigureFigure 12. ComparisonComparison chart chart of ofheat heat pipe pipe efficiency efficiency for all for charges all charges with the with R404A the refrigerant R404A refrigerant.. The values of the heat flows obtained by the evaporator are in every case higher than ForThe values 10% filling of the inheat the flows heat obtained pipe operating by the evaporator range at are 25–45 in every◦C, case the higher highest than efficiency those given out by the condenser. This proves that a higher amount of heat is taken by the wasthose achieved, given out by amounting the condenser to approx.. This proves 90%. that For a higher the remainingamount of heat fillings is taken in by the the heat pipe coolingcooling fluid fluid than than given given up. up. Th Theseese different different values values can can be be observed observed mainly mainly from from heat heat operating range at 25–45 ◦C, its efficiency was also the highest. However, in the remaining losseslosses to tothe the environment, environment, for for example example, through, through the the isothermal isothermal component component, in, inwhich which the the temperature range, the efficiency of the heat pipe decreased. The efficiency of the heat pipe pressurepressure sensor sensor and and the the valve valve for for filling filling the the heat heat pipe pipe with with the the wo workingrking medium medium are are lo- lo- wascated.cated. maintained for the R404A refrigerant in the range of 30–92%. Due to the amount of transferred heat flux and efficiency in the entire temperature range, the most effective operation of the heat pipe occurs when it is filled with the R404A working medium. Analysis of measurement error and measurement uncertainty wasnplaced in AppendixA.

3.1.3. Comparative Analysis of the Test Results for Various Charges with the R407 C Working Medium Figures 13 and 14 show diagrams for various amounts of the R407C working medium filling the heat pipe, comparing the values of heat fluxes consumed by the heat pipe and a graph comparing the efficiency of the heat pipe. The values of the heat flows taken by the evaporator are always greater than those given off by the condenser, which proves that more heat is taken by the refrigerant than given up. This difference results mainly from heat losses to the environment, for example, through the isothermal component, in which the pressure sensor and the valve for filling the heat pipe with the working medium are located. Energies 2021, 14, x FOR PEER REVIEW 15 of 32

The heat pipe works with a satisfactory efficiency at 10%, 20%, 30%, and 40% of its capacity filling in the temperature limit between tg1 20 and 50 °C, whereas, when filling 2.5% and 5% of its volume, the heat pipe works with satisfactory efficiency in the temperature range tg1 from 45 to 50 °C. In all fillings, as the temperature at the entrance to the lower heat exchanger rises, the pressure in the heat pipe rises approximately linearly. The highest value of the heat flux taken by the evaporator and given off by the condenser occurs when the heat pipe is filled in 10% of its volume with the R404A refrigerant. For 10% filling in the heat pipe operating range at 25–45 °C, the highest efficiency was achieved, amounting to approx. 90%. For the remaining fillings in the heat pipe operating range at 25–45 °C, its efficiency was also the highest. However, in the remaining temperature range, the efficiency of the heat pipe decreased. The efficiency of the heat pipe was maintained for the R404A refrigerant in the range of 30–92%. Due to the amount of transferred heat flux and efficiency in the entire temperature range, the most effective operation of the heat pipe occurs when it is filled with the R404A working medium. Analysis of measurement error and measurement uncertainty wasnplaced in Appendix A.

3.1.3. Comparative Analysis of the Test Results for Various Charges with the R407 C Working Medium Energies 2021, 14, 1926 14 of 29 Figures 13 and 14 show diagrams for various amounts of the R407C working medium filling the heat pipe, comparing the values of heat fluxes consumed by the heat pipe and a graph comparing the efficiency of the heat pipe.

1.80 1.40

1.60 1.20 1.40 1.00 1.20 1.00 0.80 Qz [W]

Qg [W] 0.80 0.60 0.60 0.40 0.40 0.20 0.20 0.00 0.00 15 25 35 45 15 25 35 45 tg1 [°C] tg1 [°C]

(a) (b)

FigureEnergiesFigure 2021 13. , Comparison13.14, xComparison FOR PEER REVIEW diagram diagram of of heat heat ﬂuxflux (a) absorbed absorbed by by the the heat heat pipe pipe and and (b) given (b) given off by off the by heat the pipe heat for pipe all16 fillings for of 32 all ﬁllings with the R407C working medium. with the R407C working medium.

100 90 80 70 60

ƞ [%] 50 40 30 20 10

0 15 20 25 30 35 40 45 50 tg1 [°C]

FigureFigure 14. 14. ComparisonComparison diagram diagram of the of heat the pipe heat efficiency pipe efficiency for all charges for all with charges the R407C with refrig- the R407C refrigerant. erant. The heat pipe works with a satisfactory efficiency at 2.5%, 5%, and 10% of its volume The values of the heat flows taken by the evaporator◦ are always greater than those fillinggiven off in theby the tg1 condenser, temperature which range proves from that 35 more to 50heatC, is whereastaken by the at 20%refrigerant filling, than 30% and 40% of thegiven volume up. This of difference the heat piperesults works mainly with from satisfactory heat losses to efficiency the environment, in the temperature for exam- range tg1 fromple, through 45 to 50 the◦C. isothermal In all fillings, component the pressure, in which in the the pressure heat pipe sensor increases and the approximately valve for linearly asfilling the the temperature heat pipe with at the the entranceworking medium to the lower are located. heat exchanger increases. The highest value of theThe heat heat flux, pipe takenworks bywith the a satisfactory evaporator efficiency and given at 2.5% off, 5% by, and the 10% condenser, of its volume occurs when the filling in the tg1 temperature range from 35 to 50 °C, whereas at 20% filling, 30% and 40% heat pipe is filled to 5% of its volume with the R407C refrigerant. For all fillings with the of the volume of the heat pipe works with satisfactory efficiency in the temperature range◦ workingtg1 from 45 medium to 50 °C. R407C, In all fillings, the operation the pressure of thein the heat heat pipe pipestarted increases at approximatelytg = 30–35 C at the inlet tolinearly the heat as the exchanger temperature heating at the theentrance heat to pipe. the lower Up to heat this exchanger temperature, increases. the The tg temperature washighes lowert value than of the the heat R407C flux,evaporation taken by the evaporator temperature and for given a given off by pressure the condenser, and the heat pipe didoccurs not when transfer the heat heat. pipe is filled to 5% of its volume with the R407C refrigerant. For all fillingsFor with the the 5% working filling medium in the heat R407C, pipe’s the operation operating of rangethe heat at pipe 35–50 started◦C, at the tg highest= 30– efficiency was35 °C achieved, at the inlet amountingto the heat exchanger to approx. heating 70%. the heat For pipe. the Up remaining to this temperature, fillings in the the heat pipe tg temperature was lower than the R407C evaporation temperature for a given pressure ◦ workingand the heat range, pipe did its not efficiency transfer heat. was much lower and it increased only at tg = 50 C. The efficiencyFor the of 5% the filling heat inpipe the was heat maintainedpipe′s operating for therange R407C at 35– refrigerant50 °C, the highest in the effi- range of 0–78%. ciencyDue was to achieved, the amount amounting of heat to approx. flux transferred 70%. For the and remaining efficiency fillings in thein the entire heat temperature range,pipe work theing most range, effective its efficiency operation was much of lower the and heat it increased pipe occurs only at when tg = 50 it °C. is The filled with the efficiency of the heat pipe was maintained for the R407C refrigerant in the range of 0– 78%. Due to the amount of heat flux transferred and efficiency in the entire temperature range, the most effective operation of the heat pipe occurs when it is filled with the R407C working medium. Analysis of measurement error and measurement uncertainty wasnplaced in Appendix A.

3.1.4. Analysis of the most Favorable Refrigerants: R134A, R404A, R407C Table 1 shows a comparison of the most favorable fillings for all media tested, for vacuum and air. Figures 15 and 16 show comparative diagrams of heat flux collected and given off by the heat pipe and the efficiency of the heat pipe for the best fillings of all working media tested, and for vacuum and air. Analysis of measurement error and measurement uncertainty wasnplaced in Appendix A.

Energies 2021, 14, 1926 15 of 29

R407C working medium. Analysis of measurement error and measurement uncertainty wasnplaced in AppendixA.

3.1.4. Analysis of the Most Favorable Refrigerants: R134A, R404A, R407C Table1 shows a comparison of the most favorable fillings for all media tested, for vacuum and air. Figures 15 and 16 show comparative diagrams of heat flux collected and given off by the heat pipe and the efficiency of the heat pipe for the best fillings of Energies 2021, 14, x FOR PEER REVIEWall working media tested, and for vacuum and air. Analysis of measurement17 errorof 32 and

measurement uncertainty wasnplaced in AppendixA.

2.00 2.00

1.80 1.80

1.60 1.60 1.40 1.40 1.20 1.20 Qz [W]

Qg [W] 1.00 1.00 0.80 0.80 0.60 0.60 0.40 0.40 0.20 0.20 0.00 0.00 15 25 35 45 15 25 35 45 tg1 [°C] tg1 [°C]

(a) (b)

FigureFigure 15. Comparison 15. Comparison chart chart of heatof heat ﬂux flux (a ()a absorbed) absorbed byby the heat heat pipe pipe and and (b ()b given) given off offby the by theheat heatpipe pipefor the for most the most favorablefavorable ﬁlling filling of all of tested all tested working working media, media, and and for for vacuum vacuum andand air. air.

3.1.5. Thermographic Analysis of the Heat Pipe during Operation 100 A thermographic analysis of the heat pipe in operation was conducted on a heat pipe 90 exchanger made of polycarbonate, 1769 mm long, 18 mm outside diameter, and 1 mm 80 wall thickness, i.e., dimensions related to the copper heat pipe. The heat pipe and the heat 70 ﬂow exchangers were constructed of polycarbonate due to the transparency that allows 60 observation of the processes taking place in the heat pipe. The thermographic analysis 50 was conducted with a thermal imaging camera. Figure 17 represents the thermographic

ɳ [%] 40 analysis done with a connected polycarbonate heat pipe, and the following figures show 30 thermal images of the heat pipe in operation. The heat pipe was analyzed with the R134A refrigerant in the amount of 10% of the total volume. The temperature of the hot water 20 inlet feeding the evaporator section was tg1 = 50 ◦C and the colder water entrance feeding 10 the condenser section was tz1 = 10 ◦C. The heat pipes consist of three distinct areas: the 0 evaporator, which is the heat supply area; the condenser, which is the heat receiving 15 area;20 and the25 adiabatic area30 between35 them. When40 the heat45 pipe is50 in operation, there tg1 [°C] is a closed, two-phase cycle with condensation and vaporization of the working fluid Figure 16. Comparison chartremaining of the heat in pipe saturated efficiency conditions for the most whenfavorable the fillings temperature of all tested is working between media, the and triple for point and vacuum and air. the critical state. The heat applied to the evaporator area causes the working fluid in the evaporator to evaporate. The high temperature and correspondingly high pressure in this area3.1.5. create Thermographic a steam flow Analysis towards of the the Heat opposite, Pipe during cooler Operation end of the vessel, where the steam then condenses,A thermographic giving analysisup its latent of the heat heat of pipe vaporization. in operation They was thenconducted transport on a the heat liquid pipe exchanger made of polycarbonate, 1769 mm long, 18 mm outside diameter, and 1 mm wall thickness, i.e., dimensions related to the copper heat pipe. The heat pipe and the heat flow exchangers were constructed of polycarbonate due to the transparency that allows observation of the processes taking place in the heat pipe. The thermographic analysis was conducted with a thermal imaging camera. Figure 17 represents the thermographic analysis done with a connected polycarbonate heat pipe, and the following

Energies 2021, 14, x FOR PEER REVIEW 17 of 32

2.00 2.00

1.80 1.80

1.60 1.60 1.40 1.40 1.20 1.20 Qz [W]

Qg [W] 1.00 1.00 0.80 0.80 0.60 0.60 0.40 0.40 0.20 0.20 0.00 Energies 2021, 14, 1926 0.00 16 of 29 15 25 35 45 15 25 35 45 tg1 [°C] tg1 [°C]

(a) (b) back to the evaporator by gravity in the gravity heat tubes or by capillary forces in the Figureporous 15. Comparison structure. chart of heat flux (a) absorbed by the heat pipe and (b) given off by the heat pipe for the most favorable filling of all tested working media, and for vacuum and air.

100 90 80 Energies 2021, 14, x FOR PEER REVIEW 18 of 32

70 60 figures50 show thermal images of the heat pipe in operation. The heat pipe was analyzed with the R134A refrigerant in the amount of 10% of the total volume. The temperature of ɳ [%] the hot40 water inlet feeding the evaporator section was tg1 = 50 °C and the colder water entrance30 feeding the condenser section was tz1 = 10 °C. The heat pipes consist of three distinct20 areas: the evaporator, which is the heat supply area; the condenser, which is the heat receiving area; and the adiabatic area between them. When the heat pipe is in operation,10 there is a closed, two-phase cycle with condensation and vaporization of the working0 fluid remaining in saturated conditions when the temperature is between the triple point15 and the20 critical state.25 The heat 30 applied to the35 evaporator40 area causes45 the 50 working fluid in the evaporator to evaporate. Thetg1 high [°C] temperature and correspondingly high pressure in this area create a steam flow towards the opposite, cooler end of the FigureFigure vessel,16. Comparison 16.whereComparison the chart steam of the then chart heat condenses, pipe of the efficiency heat giving pipe for up the efﬁciency its most latent favorable heat for theof fillings vaporization. most of favorableall tested They working ﬁllings m ofedia, all and tested for vacuumthen and transport air. the liquid back to the evaporator by gravity in the gravity heat tubes or by workingcapillary media,forces in and the porous for vacuum structure. and air. 3.1.5. Thermographic Analysis of the Heat Pipe during Operation A thermographic analysis of the heat pipe in operation was conducted on a heat pipe exchanger made of polycarbonate, 1769 mm long, 18 mm outside diameter, and 1 mm wall thickness, i.e., dimensions related to the copper heat pipe. The heat pipe and the heat flow exchangers were constructed of polycarbonate due to the transparency that allows observation of the processes taking place in the heat pipe. The thermographic analysis was conducted with a thermal imaging camera. Figure 17 represents the thermographic analysis done with a connected polycarbonate heat pipe, and the following

Figure 17. Measuring stand with connected polycarbonate heat pipe.

Energies 2021, 14, x EnergiesFOR PEER 2021 REVIEW, 14, x FOR PEER REVIEW 19 of 32 19 of 32

Energies 2021, 14, 1926 17 of 29

Figure 17. MeasuringFigure stand 17. Measuringwith connected stand polycarbonate with connected heat polycarbonate pipe. heat pipe. The mentioned thermographic analysis of the heat pipe during its operation proved The mentioned Thethermographic mentioned thermographicanalysis of the heatanalysis pipe of during the heat its pipeoperation during proved its operation proved the operation of the heat pipe at the analyzed temperature limit (Figure 18). The fact that the operation ofthe the operation heat pipe of at the the heat analyzed pipe attemperature the analyzed limit temperature (Figure 18). limit The ( Figurefact that 18). The fact that both the heat pipe and its supply exchangers were made of polycarbonate made it possible both the heat pipeboth and the itsheat supply pipe exchangersand its supply were exchangers made of polycarbonate were made of madepolycarbonate it pos- made it pos- to compare the moment of starting the process of evaporation of the working ﬂuid and sible to comparesible the tomoment compare of thestarting moment the processof starting of evaporation the process of theevaporation working offluid the working fluid its correlation with other similar research. The high temperature and correspondingly and its correlationand with its correlation other similar with research. other similar The high research. temperature The high and temperature correspond- and correspond- high pressure in this area create a steam ﬂow towards the opposite, cooler end of the ingly high pressureingly in high this pressure area create in thisa steam area flowcreate towards a steam the flow opposite, towards cooler the opposite, end of cooler end of vessel, where the steam then condenses, giving up its latent heat of vaporization. They the vessel, wherethe the vessel, steam where then the condenses, steam then giving condenses, up its latent giving heat up of its vaporization. latent heat of vaporization. then transport the liquid back to the evaporator by gravity in the gravity heat tubes or by They then transportThey thethen liquid transport back theto the liquid evaporator back to bythe gravity evaporator in the by gravity gravity heat in the tubes gravity heat tubes capillary forces in the porous structure. Author did a photos of the evaporation process or by capillary or forces by capillaryin the porous forces structure.in the porous Author structure. did a photos Author of did the a eva photosporation of the evaporation inside the connected polycarbonate heat pipe Figures 19 and 20. process inside theprocess connected inside polycarbonate the connected heatpolycarbonate pipe Figure heats 19 pipeand 20.Figure s 19 and 20.

Figure 18. Heat pipe during operation: (a) heat pipe, (b) evaporator section, (c) condenser section. Figure 18. Heat pipeFigure during 18. Heat operation pipe :during (a) heat operation pipe, (b): evaporator(a) heat pipe, section (b) evaporator, (c) condenser section section., (c) condenser section.

Figure 19. Photo of heat pipe during operation showing evaporation process. Figure 19. PhotoFigure of 19.heatPhoto pipe ofduring heat pipe operation during showing operation evaporation showing evaporation process. process.

Energies 2021, 14, 1926 18 of 29 Energies 2021, 14, x FOR PEER REVIEW 20 of 32

Figure 20. Photo of heat pipe during operation showing evaporation process. Figure 20. Photo of heat pipe during operation showing evaporation process.

3.2. Tests of a Brass (CuZn39Pb3) Heat Pipe 550 mm Long The subject of the second part of the research was heat pipes made of brass, 550 mm long, 22 mm outside diameter, and 1 mm wall thickness. During the brass heat pipe test, the temperatures at the inlet and outlet of hot and cold water from heat exchangers were measured. The pressures inside the tube were also measured and the temperatures at the measuring points along the height of the heat tube were determined. The temperature of colder water at the entrance to the heat exchanger in the condenser part was about 10 ◦C, whereas the hot water temperature at the heat exchanger in the evaporator section was within a range of 15–50 ◦C every 5 ◦C for each subsequent measurement. The value of the cold and hot water stream ﬂowing around the tube was about 15 L/h. The heat pipe tests were performed with the use of the refrigerants R407C and R410A. The heat tube was ﬁlled by volume for subsequent measurements at amounts of 10%, 20%, 30%, and 40% of the entire volume. The same heat pipe was tested for R134A and R404A refrigerants. Analysis of measurement error and measurement uncertainty wasnplaced in AppendixA.

Energies 2021, 14, x FOR PEER REVIEW 21 of 32

3.2. Tests of a Brass (CuZn39Pb3) Heat Pipe 550 mm Long 3.2. TestsThe subjectof a Brass of (CuZn39Pb3) the second part Heat of Pipe the 550 research mm Long was heat pipes made of brass, 550 mm long, The22 mm subject outside of the diameter, second part and of 1 the mm research wall thickness. was heat pipesDuring made the ofbrass brass, heat 550 pipe mm test, thelong, temperatures 22 mm outside at thediameter, inlet and and outlet 1 mm of wall hot thickness. and cold Duringwater from the brass heat heatexchangers pipe test, were measured.the temperatures The pressures at the inlet inside and thoutlete tube of hotwere and also cold measured water from and heat the exchangerstemperatures were at the measuringmeasured. Thepoints pressures along theinside height the tubeof the were heat also tube measured were determined. and the temperatures The temperature at the of coldermeasuring water points at the along entrance the height to the of heat the exchangerheat tube were in the determined. condenser The part temperature was about 10of °C, whereascolder water the hotat the water entrance tem peratureto the heat at exchanger the heat exchangerin the condenser in the part evaporator was about section 10 °C, was withinwhereas a rangethe hot of water 15–50 tem °C peratureevery 5 °atC forthe eachheat exchangersubsequent in measurement. the evaporator The section value was of the coldwithin and a rangehot water of 15 –stream50 °C every flowing 5 °C a forround each the subsequent tube was measurement. about 15 L/h. TheThe value heat pipeof the tests werecold andperformed hot water with stream the flowinguse of thearound refrigerants the tube wasR407C about and 15 R410A. L/h. The The heat heat pipe tube tests was filledwere byperformed volume withfor subsequent the use of measurementsthe refrigerants atR407C amount ands ofR410A. 10%, The20%, heat 30%, tube and was 40% of filled by volume for subsequent measurements at amounts of 10%, 20%, 30%, and 40% of the entire volume. The same heat pipe was tested for R134A and R404A refrigerants. Energies 2021, 14, 1926 the entire volume. The same heat pipe was tested for R134A and R404A refrigerants.19 of 29 Analysis of measurement error and measurement uncertainty wasnplaced in Appendix Analysis of measurement error and measurement uncertainty wasnplaced in Appendix A. A.

3.2.1.3.2.1.3.2.1. Comparative ComparativeComparative Analysis AnalysisAnalysis of of Test Test Results Results for for for Various Various Various Charges Charges Charges of of theof the the R134A R134A R134A Working Working WorkingMediumMedium Medium FiguresFFiguresigures 21 21 and andand 22 2222 show showshow diagrams diagrams diagrams for for for various various various amounts amounts amounts of of the ofthe R134Athe R134A R134A working working working medium me- me- fillingdiumdium a fillingfilling heat pipea heat made pipepipe of mademade 550 mmof of 550 550 long mm mm brass, long long lookingbrass, brass, looking at looking the valuesat theat the values of heatvalues of flux heatof collected heat flux flux andcollectedcollected given offand by given the heatoffoff byby pipe, the the heatand heat apipe, pipe, diagram and and a comparing diagrama diagram comparing thecomparing heat transfer the the heat heat efficiency transfer transfer effi- of theeffi- testedciencyciency heat ofof the pipe tested [22]. heatheat pipe pipe [22]. [22].

(a) (b) (a) (b) FigureFigure 21. 21Diagram. Diagram of of heat heat ﬂux flux (a ()a collected) collected by by the the evaporatorevaporator and ( (b)) given given by by the the condenser condenser inside inside heat heat pipe pipe for for various various Figure 21. Diagram of heat flux (a) collected by the evaporator and (b) given by the condenser inside heat pipe for various chargescharges of theof the R134a R134a refrigerant refrigerant depending depending on on the the temperature temperature ofof thethe water washing washing the the evaporator evaporator [1 [5].15 ]. charges of the R134a refrigerant depending on the temperature of the water washing the evaporator [15].

Figure 22. Diagram of the heat pipe efﬁciency for various charges with R134a depending on the temperature of the water washing the evaporator [15].

When filling the heat pipe with R134A refrigerant, the best choice would be to fill it to 10% of its volume due to the highest values of heat fluxes taken by the evaporator and given off by the condenser (max 130 W). The efficiency values of the heat pipe for this filling are also high and reach a level of 92%–98%, whereas the operation of the pipe begins at a temperature of 20 ◦C, and in the remaining fillings the heat pipe operates at a temperature of 25 ◦C.

3.2.2. Comparative Analysis of Test Results for Various Charges with the R404A Working Medium Figures 23 and 24 show diagrams for different amounts of the R404A working medium filling a heat pipe made of 550 mm long brass, showing the values of heat flux collected and returned by the heat pipe, and a graph comparing the efficiency of the tested heat pipe. Energies 2021, 14, x FOR PEER REVIEW 22 of 32

Figure 22. Diagram of the heat pipe efficiency for various charges with R134a depending on the Energies 2021, 14, x FOR PEER REVIEWtemperature of the water washing the evaporator [15]. 22 of 32

When filling the heat pipe with R134A refrigerant, the best choice would be to fill it Figureto 10% 22. Diagramof its volume of the heat due pipe to efficiencythe highest for various values charges of heat with fluxes R134a taken depending by the on the evapo rator and temperaturegiven off of by the the water condenser washing the (max evaporator 130 W). [15]. The efficiency values of the heat pipe for this filling are also high and reach a level of 92%–98%, whereas the operation of the pipe be- When filling the heat pipe with R134A refrigerant, the best choice would be to fill it gins at a temperature of 20 °C, and in the remaining fillings the heat pipe operates at a to 10% of its volume due to the highest values of heat fluxes taken by the evaporator and giventemperature off by the of condenser 25 °C. (max 130 W). The efficiency values of the heat pipe for this filling are also high and reach a level of 92%–98%, whereas the operation of the pipe be- gins3.2.2. at aComparative temperature of Analysis 20 °C, and of Testin the Results remaining for Variousfillings the Charges heat pipe with operates the R404A at a Working temperatureMedium of 25 °C. Figures 23 and 24 show diagrams for different amounts of the R404A working me- 3.2.2. Comparative Analysis of Test Results for Various Charges with the R404A Working dium filling a heat pipe made of 550 mm long brass, showing the values of heat flux col- Energies 2021, 14, 1926 Medium 20 of 29 lected and returned by the heat pipe, and a graph comparing the efficiency of the tested Figures 23 and 24 show diagrams for different amounts of the R404A working me- heat pipe. dium filling a heat pipe made of 550 mm long brass, showing the values of heat flux collected and returned by the heat pipe, and a graph comparing the efficiency of the tested heat pipe.

(a) (b) (a) (b)

FigureFFigureigure 23. 2323Diagram.. Diagram of of of heat heat heat ﬂuxflux flux (a )( acollected collected) collected by by the by the evaporatorthe evaporator evaporator of the of ofheat the the pipe heat heat and pipe pipe (b and) andgiven ( b( b)by) given giventhe heat by by pipe the the heat condenserheat pipe pipe condensercondenser for differentforfor different different charges charges of the of of R404A the the R404A R404A refrigerant refrigerant refrigerant depending depending depending on on the the on temperature temperaturethe temperature of the of water theof the water washing water washing thewashing evaporator the the evaporator evaporator [15]. [15 [15].].

Figure 24. Efﬁciency diagram of the heat pipe for various charges with R404A depending on the

temperature of the water washing the evaporator [15]. A heat pipe ﬁlled with R404A refrigerant works best for charges ranging from 20% to

40% of the tube0s volume, because the values of the transferred heat ﬂuxes are the highest, reaching 135 W. The heat pipe for all charges achieves high efﬁciency values ranging from 90% to 98%.

3.2.3. Comparative Analysis of Test Results for Various Fillings with the R407C Working Medium Figures 25 and 26 show diagrams for various amounts of the R407C working medium filling a heat pipe made of 550 mm long brass, showing the values of heat flux collected and given off by the heat pipe, and a graph showing the efficiency of the designed heat pipe [22]. With the use of the R407C refrigerant, it was found that it is most advantageous to fill the tube to 20%, 30%, and 40% of its volume because the values of heat flux absorbed by the evaporator and given off by the condenser reach the highest values (Q g˙ = ~90 W). By comparison, taking into account economic and efficiency issues, it is more profitable to fill the heat pipe up to 30% of its volume with the R407C factor [22]. Energies 2021, 14, x FOR PEER REVIEW 23 of 32

Energies 2021, 14, x FOR PEER REVIEW 23 of 32

Figure 24. Efficiency diagram of the heat pipe for various charges with R404A depending on the temperature of the water washing the evaporator [15]. Figure 24. Efficiency diagram of the heat pipe for various charges with R404A depending on the temperatureA heat of the pipe water filled washing with the R404A evaporator refrigerant [15]. works best for charges ranging from 20% to 40% of the tube′s volume, because the values of the transferred heat fluxes are the highestA heat, reachpipe filleding 135 with W. R404A The heatrefrigerant pipe forworks all chargesbest for charges achieves ranging high efficiencyfrom 20% values to ranging 40% of thefrom tube 90%′s volume,to 98%. because the values of the transferred heat fluxes are the highest, reaching 135 W. The heat pipe for all charges achieves high efficiency values ranging3.2.3. fromComparative 90% to 98%. Analysis of Test Results for Various Fillings with the R407C Working Medium 3.2.3. Comparative Analysis of Test Results for Various Fillings with the R407C Working MediumFigures 25 and 26 show diagrams for various amounts of the R407C working me- Energies 2021, 14, 1926 diumFigures filling 25 aand heat 26 pipe show made diagrams of 550 for mm various long brass,amounts showing of the theR407C values working of heat me- flux col-21 of 29 diumlected filling and a givenheat pipe off bymade the of heat 550 pipe, mm long and brass,a graph showing showing the the values efficiency of heat offlux the col- designed lectedheat and pipe given [22]. off by the heat pipe, and a graph showing the efficiency of the designed heat pipe [22].

(a)( a) (b) (b)

FigureFigureFigure 25. Diagram 25. 25. Diagram Diagram of of heat ofheat heat ﬂux flux flux ((aa) ( collecteda) collected by by theby the evaporatorthe evaporator evaporator and (and andb) given (b (b) )given by given the by condenser by the the condenser condenser for various for various for charges various charges with charges with with R407CR407CR407C depending depending depending on on the on the waterthe water water temperaturetemperature temperature at at the at the inletthe inlet inlet to the toto heat thethe heatexchanger exchanger exchanger in the in evaporator in the the evaporator evaporator section sectio [22]. section n [22]. [22].

100100

95 95

90 90

85 [%][%] 85

80 80

75 10% 20% 30% 40% 75 10% 20% 30% 40% 70 70 35 40 t [oC] 45 50 35 40 g1 t [oC] 45 50 g1

FigureFigure 26. HeatHeat pipe pipe efficiency efficiency chart chart for various for various R407C R407Ccharges chargesdepending depending on the water on tempera- the water temperature tureFigure at the 26.inlet Heat to the pipe heat efficiency exchanger chart in the for evaporator various R407C section charges [22]. depending on the water tempera- at theture inlet at the to inlet the heatto the exchanger heat exchanger in the in evaporatorthe evaporator section section [22 [22].]. 3.2.4. Comparative Analysis of Test Results for Various Charges with the R410A Working Medium Figures 27 and 28 show diagrams for different amounts of the R410A working medium filling a heat pipe made of 550 mm long brass, showing the values of heat flux collected and given off by the heat pipe, and a diagram showing the efficiency of the tested heat pipe [22]. In that case of filling the heat pipe with R410A refrigerant, the values of heat flux collected by the evaporator and given off in the condensation section occurring at 20% and 30% fillings were the highest (Q cmax˙ = ~128 W), and much lower values were achieved with 10% and 40% fillings of the tube. The highest value of the heat pipe efficiency (~95%) was obtained during 40% filling, within a temperature range of 25–50 ◦C, whereas the lowest value in the same temperature range was obtained for 10% filling. On the basis of the above graphs, it was noticed that the heat pipe was found to be the most efficient for 20% and 30% R410A refrigerant. For efficiency reasons, it is best to choose 20% of the total volume of the heat tube with R410A. The least effective solution is to fill the tube to 40% of its volume.

3.3. Comparative Analysis of the Most Advantageous Fillings of the Heat Pipe 550 mm Long Compared to the Heat Pipe 1769 mm Long Of the four refrigerants tested (R134A, R404A, R407C, and R410A), R404A is the best refrigerant to ﬁll the brass heat pipe with a length of 550 mm and a diameter of 22 mm. Energies 2021, 14, x FOR PEER REVIEW 24 of 32

Energies 2021, 14, x FOR PEER REVIEW 24 of 32

With the use of the R407C refrigerant, it was found that it is most advantageous to fill the tube to 20%, 30%, and 40% of its volume because the values of heat flux absorbed byWith the evaporatorthe use of the and R407C given refrigerant, off by the itcondenser was found reach that itthe is highestmost advantageous values (Q ġ to= ~90 W). fill Bythe comparisontube to 20%,, 30%,taking and into 40% account of its volume economic because and theefficiency values ofissues, heat fluxit is absorbed more profitable Energies 2021, 14, 1926 22 of 29 by tothe fill evaporator the heat pipeand given up to off 30% by of the its condenser volume with reach the the R407C highest factor values [22]. (Q ġ = ~90 W). By comparison, taking into account economic and efficiency issues, it is more profitable to fill3.2.4. the Comparative heat pipe up to Analysis 30% of its of volumeTest Results with thefor R407CVarious factor Charges [22]. with the R410A Working Medium The3.2.4. value Comparative of the heatAnalysis ﬂux of taken Test Results in the for evaporation Various Charges section with and the givenR410A offWorkin by theg condenser, Figures 27 and 28 show diagrams for different amounts of the R410A working me- inMedium this case for 20% of the volume ﬁlled, reaches a value of ~135 W (similar to the R410A dium filling a heat pipe made of 550 mm long brass, showing the values of heat flux col- charge),Figures but 27 the and operating 28 show diagrams range of for the different heat pipe amounts is greater of the R410A for the working R404A me- refrigerant and lected and given off by the heat pipe, and a diagram showing the efficiency of the tested dium filling a heat pipe◦ made of 550 mm long brass, showing the values of heat flux◦ col- amountsheat pipe to 20–50[22]. C compared to R410A, where it is in a range of 25–50 C (Table2). lected and given off by the heat pipe, and a diagram showing the efficiency of the tested heat pipe [22].

(a) (b) (a) (b) FigureFigure 27. Diagram 27. Diagram of heat of heat ﬂux flux (a) ( collecteda) collected by by the the evaporator evaporator and ( (bb) )given given by by the the condenser condenser for forvarious various refrigerant refrigerant Figure 27. Diagram of heat flux (a) collected by the evaporator and (b) given by the condenser for various refrigerant charges R410A depending on the inlet water temperature of the heat exchanger in the evaporator part [22]. chargescharges R410A R410A depending depending on onthe the inlet water water temperature temperature of the of heat the exchanger heat exchanger in the evaporator in the evaporator part [22]. part [22].

100100

90 90

[%][%]

80 80

10% 10% 20% 20% 30% 30% 40% 40%

70 70 25 30 35 40 45 50 25 30 35o 40 45 50 t [ C] o g1 t [ C] g1 FigureFigure 28. 28. HeatHeat pipe pipe efficiency efﬁciency diagram diagram for various for R410A various refrigerant R410A charges refrigerant depending charges on the depending on the waterFigure temperature 28. Heat at pipe the heatefficiency exchanger diagram inlet forin the various evaporator R410A section refrigerant [22]. charges depending on the waterwater temperature temperature at at the the heat heat exchangerexchanger inlet inlet in in the the evaporator evaporator section section [22]. [ 22]. In that case of filling the heat pipe with R410A refrigerant, the values of heat flux Table 2. Comparison of the most advantageous ﬁllings of a heat pipe 550 mm long and 22 mm collectedIn by that the case evaporator of filling and the given heat off pipe in thewith condensation R410A refrigerant, section occurring the values at of20% heat flux in diametercollected madeby the of evaporator brass, compared and given to a off heat in pipe the 1769condensation mm long section and 18 occurring mm in diameter at 20% made of copper.

Copper Tube. 1769 mm Long. 18 mm in A Brass Heat Pipe. 550 mm Long. 22 mm in Diameter. Filled with 10% of the Total Diameter. Filled with 20% of the Total ◦ Tg1 [ C] Volume of R134A Volume of R404A

Qg [W] Qz [W] ï [%] Qg [W] Qz [W] ï [%] 15.00 30.76 27.73 90.15 7.00 6.48 0.00 20.00 39.48 35.65 90.30 21.00 20.23 0.00 25.00 63.48 57.88 91.19 39.28 38.73 98.59 30.00 83.05 78.33 94.32 58.26 55.03 94.23 35.00 104.64 99.04 94.64 75.25 75.02 99.70 40.00 134.92 127.65 94.61 91.84 88.30 96.68 45.00 160.70 151.86 94.50 117.90 106.57 90.39 50.00 199.38 184.87 92.72 134.89 125.73 93.21 Energies 2021, 14, 1926 23 of 29

Based on the results of this analysis of the of experimental studies, it can be concluded that the most effective working medium of a heat pipe made of copper with a length of 1790 mm and a diameter of 18 mm in the tested temperature range is the working medium R134A with a 10% filling of the heat pipe. The maximum heat flux absorbed by the heat pipe reaches a value of 199.3 W, whereas the maximum heat flux emitted by the heat pipe reaches a value of 184.8 W. In addition to the largest heat fluxes transferred through the heat pipe, this factor in the said filling achieved the best heat exchange efficiency reaching 95%.

4. Discussion Both numerical and experimental studies of heat pipes have been carried out by numerous scientists. Heat pipes with similar geometry and similar diameters were tested by Chen and co-authors [23]. In their research, they built a theoretical model of a heat pipe, the results of which are similar to the experimental results presented in this publication. Gao et al. [24] investigated the use of heat pipes for the decomposition of sulfuric acid based on heat transfer surfaces and catalytic volume, and obtained a near 7% acceleration of the decomposition process due to the intense heat exchange taking place in the heat pipes. Ji-Qiang Li and others [25] investigated the power and efficiency of heat exchange in a Stirling engine where there are high technological requirements in terms of heat exchangers. Taking into account the range of temperatures of the applicability of heat pipes and the results of the efficiency of heat transfer through them, it is possible to suggest the use of heat pipes for their tests. By comparison, Song et al. [26] investigated heat pipes in the form of concentric annular tubular heat sinks, obtaining even higher efficiency and values of the transmitted heat flux than those tested in this study. The heat exchangers analyzed, however, have many limitations, which do not allow them to be used in such a wide range. In turn, Xiang [27] and co-authors experimentally tested geometrically similar heat pipes, acting in an anti-gravity manner and thus equipped with a conical chamber. The heat pipes tested were characterized by high efficiency, as for anti-gravity heat pipes, but are still less efficient than gravity tubes and have greater limitations resulting from the need to use a wick.

4.1. Discussion on the Choice of the Working Medium and the Selection of Its Quantity Four refrigerants were used in the current research: R134A, R404A, R407C, and R410A. Based on the analysis of experimental studies, it can be concluded that the most effective working medium of a heat pipe made of copper with a length of 1769 mm in the tested temperature range is the R134A refrigerant with a 10% filling of the heat pipe. With an assumed water flow of 15 L/h for the measured temperature differences, the heat flux collected by the heat pipe reaches a value of 199.3 W, and the heat flux emitted by the heat pipe reaches a value of 184.8 W. In the case mentioned, the evaporator washes water at 50 ◦C. When operating in the remaining temperature range, the R134A refrigerant, filled with 10%, also performed best. In addition to the largest heat fluxes transmitted through the heat pipe, this factor in the said filling achieved the best heat exchange efficiency, reaching 95%. From the remaining tested working fluids, the R404A refrigerant, also with 10% filling, proved to be effective for the efficiency of the heat pipe tested, although it achieved worse results than the R134A refrigerant. The R407C factor was found to be the worst of the factors tested, both in terms of the transferred heat fluxes and the efficiency of heat transfer through the heat pipe.

4.2. Discussion on the Efficiency of Heat Pipes and the Value of the Heat Flux Transferred by Them Comparing the heat pipes analyzed on the basis of the analysis of experimental and thermographic studies [28–30], it can be concluded that when comparing the most favorable fillings of a heat pipe with a length of 550 mm compared to a heat pipe with a length of 1769 mm, a 1769 mm long tube is more favorable when made of copper. A copper heat tube transfers higher heat fluxes and has comparable efficiency, which was also used in different branch scale installations for thermo-chemical conversion of biomass [31,32]. The Energies 2021, 14, 1926 24 of 29

advantage of the copper tube is primarily due to the larger heat transfer surface area of both the evaporating and condensing heat tube sections which can also be used in different applications for heat recovery [33,34]. Of the four tested refrigerants (R134A, R404A, R407C, and R410A), R404A is the best refrigerant to fill the brass heat pipe with a length of 550 mm and a diameter of 22 mm. The value of the heat flux absorbed by the evaporator and given off by the condenser, in this case for 20% of the volume filled, reaches a value of ~135 W (similar to the R410A charge), but the operating range of the heat pipe is greater for the R404A refrigerant and amounts to 20–50 ◦C compared to R410A where it lies in a range of 25–50 ◦C. The heat tube was filled to amounts of 2.5%, 5%, 10%, 20%, 30%, and 40% of its total volume with the specified refrigerants. Each of these factors has different physicochemical properties that affect the operation of the heat pipe. It was not the authors’ intention to calculate the heat transfer coefficient in this paper. The aim of the research was to balance the heat pipe heat exchanger in terms of the lower–upper heat transfer efficiency and to determine the efficiency of the entire apparatus, because this is important for practical air conditioning and ventilation applications. The heat transfer coefficient between the liquid flowing around the tube and the boiling medium for industrial applications is not a key parameter in experimental research according to the authors. In the next publication on the numerical model built by the authors and the simulation of heat transfer processes in a heat pipe, a large section will be devoted to the issue of the heat transfer coefficient.

4.3. Discussions on the Advisability of Using Heat Pipe Heat Exchangers for the Needs of Civil Engineering and Air Conditioning to Reduce Low Emissions During the past half-century, there has been a significant development in the use of heat pipes, pioneered by their use in space science. Today they are used in many industries. One of the most interesting applications of heat pipes is their use for drying and cooling air, and for heat recovery and passive cooling of rooms. Other important applications are uses in solar pipes of solar collectors, and as passive heat exchangers for regulating ground temperature and heating the surfaces of bridges and viaducts. Due to its structure, the heat pipe is an economical heat exchanger compared to those commonly used. For this reason, in an era of striving to save and reduce the unnecessary dissipation of energy, interest in the use of this type of heat exchanger is still growing. A series of tests was thus necessary to analyze the operation of heat pipes and to determine the most effective heat exchangers of the heat pipe type depending on their operating conditions. The results of experimental tests on the heat pipes and thermographic tests fully confirmed the legitimacy of using heat exchangers of the heat pipe type to reduce emissions. The research results prove the possibility of a wide application of heat pipes to reduce the consumption of primary energy by reducing energy dissipation in heat transfer processes and by their intensification. The results of the experimental studies obtained prove the high efficiency of heat transport by means of heat pipes, which can significantly affect the efficiency of thermal devices.

5. Conclusions The obtained efficiency of heat recovery from the ground of the examined exchangers obtained values over 90%, which will enable the reduction of low emissions in heating systems of buildings. The results included in the study will also enable the effective use of land as a heat store. As part of this work, issues related to the analysis of the efficiency of heat pipes were analyzed to reduce emissions based on experimental studies. In relation to the above, it was proved that the analysis of the operation of the heat pipe type heat exchanger enables the selection of the best working medium among those tested, and its optimal amount used to fill the heat pipe. This allows the maximum efficiency and the maximum value of the transferred heat fluxes in the temperature range of the lower source of 15–50 ◦C to be obtained. The experimental studies conducted permitted a number of conclusions to be drawn regarding the solutions used in heat pipes. These conclusions mainly concern: Energies 2021, 14, 1926 25 of 29

• selection of the best working medium and determination of its optimal quantity; • the efficiency of the heat pipes and the value of the heat flux transmitted by them; • the desirability of using heat pipe heat exchangers for structural engineering and air conditioning to reduce low emissions. Based on the results obtained from this research regarding the high efficiency of heat pipe heat exchangers (92–98%), it is possible to use heat pipes as heat exchangers that intensify the heat exchange between lower heat sources from renewable sources (e.g., ground) and heat receivers of central heating devices for buildings (e.g., the heat pump). The conclusions from the research conducted prove that the use of heat pipes in construction engineering and air-conditioning is consistent with activities limiting primary energy consumption and increasing energy efficiency by limiting emissions [33]. Conclusions resulting from this study also confirm the possibility of heat pipe operation in a wide temperature range and the ability to work at the smallest temperature differences, which contributes to more efficient use of renewable sources and allows their use in places which, thus far, are limited by profitability constraints. In the next stage of work, the authors plan to carry out research on a ready-made prototype of a ground heat exchanger of the heat tube type in real conditions.

Author Contributions: Conceptualization, Ł.A., S.S., P.P., P.K. (Przemysław Kubiak), P.K. (Piotr Kuryło); methodology, Ł.A., S.S., P.P., P.K. (Piotr Kuryło), P.K. (Przemysław Kubiak), G.W., J.C.; software, Ł.A., S.S., P.P.; validation, Ł.A., S.S., P.P., P.K. (Piotr Kuryło), P.K. (Przemysław Kubiak); formal analysis Ł.A., S.S., P.P., P.K., G.W., J.C.; investigation, Ł.A., S.S., P.P., P.K., F.M., K.K., S.P., A.O., J.S., P.K. (Piotr Kuryło), P.K. (Przemysław Kubiak); resources Ł.A., S.S., P.P., G.W., J.C.; data curation, Ł.A., S.S., P.P.; writing—original draft preparation, Ł.A., S.S., P.P., P.K. (Piotr Kuryło), P.K. (Przemysław Kubiak), K., F.M., K.K., S.P., A.O., J.S., P.K.; writing—review and editing, Ł.A., S.S., P.P., P.K. (Piotr Kuryło), P.K. (Przemysław Kubiak), F.M., K.K., S.P., A.O., J.S., P.K., G.W., J.C.; visualization, Ł.A., S.S., P.P.; supervision, Ł.A., S.S., P.P., P.K. (Piotr Kuryło), P.K. (Przemysław Kubiak), G.W., J.C., F.M., K.K., S.P., A.O., J.S., P.K. (Piotr Kuryło), P.K. (Przemysław Kubiak), G.W., J.C.; project administration, Ł.A., S.S., P.P., P.K. (Piotr Kuryło), P.K. (Przemysław Kubiak); funding acquisition, Ł.A., S.S., P.P., G.W., J.C.; All authors have read and agreed to the published version of the manuscript. Please turn to the CRediT taxonomy for the term explanation. Authorship must be limited to those who have contributed substantially to the work reported. Funding: Statutory resources of the Lodz University of Technology. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: NCN GRANT: Preludium II no: DEC-2011/03/N/ST8/05912 pt. “Research on heat exchange processes and distribution of velocity fields by the PIV method and the effects of phase changes and capillary transport occurring in heat exchangers of the heat tube type”. The grant was financed by the National Science Center in Krakow. Conflicts of Interest: The authors declare no conflict of interest.

Appendix A The tests that were carried out are affected by errors resulting from the measurements, the equipment used, and calculations. Below is an example of calculating errors for 10% of the heat pipe volume with R134A refrigerant and the hot water temperature at the heat exchanger inlet of ~35 ◦C. Limit error of hot and cold water ﬂow through the rotameter:

. l ∆ V = 0.5 , (A1) lim c h

∆lim–limit error. Energies 2021, 14, 1926 26 of 29

Total uncertainty of hot and cold water ﬂow through the rotameter: . . ∆lim Vc l 0.5 h l uc Vc = √ = √ = 0.3 (A2) 3 3 h

Limit error of the millivoltmeter for hot water ﬂowing into the exchanger is E9 = 1.34 mV: ∆lim(E9) = 0.015%·E9 + 0.004%·Z, (A3)

∆lim(1.340 mV) = 0.015%·1.340 mV + 0.004%·200 mV = 0.008 mV, (A4) where 0.015% and 0.004%–the values stated by the manufacturer of the millivoltmeter; Z—measuring range of a millivoltmeter. Total uncertainty of a millivoltmeter for hot water ﬂowing into the exchanger: E9 = 1.34 mV. ∆lim(E9) 0.0082 mV uc(E9) = √ = √ = 0.005 mV, (A5) 3 3

Millivoltmeter limit error for hot water ﬂowing from the exchanger: E10 = 1.10 mV.

∆gr(E10) = 0.015%·E10 + 0.004%·Z, (A6)

∆gr(1.1 mV) = 0.015%·1.1 mV + 0.004%·200 mV = 0.008 mV, (A7)

The total uncertainty of a millivoltmeter for the warm ﬂowing water: E10 = 1.10 mV.

∆lim(E10) 0.008 mV uc(E10) = √ = √ = 0.005 mV, (A8) 3 3

Thermoelectric force limit error:

∆gr(∆Ec) = ∆gr(E10) + ∆gr(E9) = 0.008 mV + 0.008 mV = 0.016 mV, (A9)

Total uncertainty of measuring the difference in hot water temperature:

∆gr(∆E) °C uc(∆tc) = √ ·25 , (A10) 3 mV

The relative error of the hot water heat ﬂux: v . u . 2 u Q u u V 2 c g u c c uc(∆tc) . = t .  + (A11) ∆tc Qg Vc

. v u !2 uc Qg l 2 u 0, 3 h 0.24 °C . = t + = 0.044 (A12) l 6.05 °C Qg 15 h Uncertainty of the hot water heat ﬂux: . uc Qg . . u = . ·Qg = 0.044·104.64 W = 5 W (A13) Qg Qg

The value of the hot water heat flux, taking into account the measurement error, is: . Qg = 104.64(5) W. The error of the value of the hot water stream is therefore 5 W. The limit error of the cold water flow and the total uncertainty of its flow are specified above. Energies 2021, 14, 1926 27 of 29

Error calculation example for the cold water temperature at the inlet to the heat exchanger of ~10 ◦C: Millivoltmeter limit error for cold water ﬂowing into the exchanger for E11 = 0.390 mV:

∆gr(E11) = 0.015%·E11 + 0.004%·Z, (A14)

∆gr(0.390 mV) = 0.015%·0.390 mV + 0.004%·200 mV = 0.008 mV, (A15) Total uncertainty of a millivoltmeter for cold water ﬂowing into the exchanger for E11 = 0.390 mV: ∆gr(E11) 0.008 mV uc(E11) = √ = √ = 0.005 mV, (A16) 3 3 Millivoltmeter limit error for cold water ﬂowing from the exchanger for E12 = 0.568 mV: ∆gr(E12) = 0.015%·E12 + 0.004%·Z (A17)

∆gr(0.68 mV) = 0.015%·0.568 mV + 0.004%·200 mV = 0.008 mV, (A18) Total uncertainty of a millivoltmeter for cold water ﬂowing from the exchanger E12 = 0.568 mV: ∆gr(E12) 0.008 mV uc(E12) = √ = √ = 0.005 mV, (A19) 3 3 Thermoelectric force limit error:

∆gr(∆Ez) = ∆gr(E11) + ∆gr(E12) = 0.008 mV + 0.008 mV = 0.016 mV (A20)

Total uncertainty of measuring the cold water temperature difference:

∆gr(∆E) °C uc(∆tz) = √ ·25 , (A21) 3 mV

0.016 mV °C uc(∆tz) = √ ·25 = 0.23 °C , (A22) 3 mV the relative error of the cold water heat ﬂux: v . u . 2 u Q u u V 2 C z u c z uc(∆tz) . = t .  + (A23) ∆tz Qz Vz

. v u !2 uC Qz l 2 u 0.3 h 0.23 °C . = t + =0.045 (A24) l 5.67 °C Qz 15 h cold water heat flux uncertainty: . uC Qz . . u = . ·Qz = 0.045·99 W = 4 W, (A25) Qz Qz The value of the cold water stream, taking into account the measurement error, is: . Qz = 99(4) W. The error of the cold water flow value is, therefore 4 W. Heat pipe efficiency relative error: v u u . !2 . !2 Q u uC(η) u g Qz = t . + . , (A26) η Qg Qz Energies 2021, 14, 1926 28 of 29

s u (η) 5 W 2 4 W 2 C = + = 0.06, (A27) η 104.64 W 99 W uncertainty of heat pipe efﬁciency:

u (η) u = C ·η = 0.06·0.95 = 0.06 (A28) η η

The efficiency value of the heat pipe, taking into account the measurement error, is written as: η = 0.95(6). The error of the heat pipe efficiency value is 6%. The errors in all other cases were determined in a similar way. As a result of the performed error calculations, their values are within the range 3–8%. With the increase in hot water temperature at the entrance to the heat exchanger heating the heat pipe, the errors in the values of heat flux absorbed and returned by the heat pipe increase, and the errors in the efficiency values reach smaller and smaller values.

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