AN ENVIRONMENTALLY FRIENDLY WORKING FLUID FOR A HEAT PIPE

Abstract

The invention provides a gravity heat pipe having a working fluid selected from the group consisting of HFO-1234ze(Z), HFO-1234ze(E), HFO-1336mzz(Z), HFO-1336mzz(E), HFO-1224yd(Z), HFO-1233zd(E), and a mixture thereof. The heat pipes of the invention are environmentally friendly, have good cooling performance and low manufacturing costs, and are suitable for cooling of communication base stations, servers, or data centers.

Claims

1. A gravity heat pipe, comprising a working fluid selected from the group consisting of HFO-1234ze(Z), HFO-1234ze(E), HFO-1234yf, HFO-1336mzz(Z), HFO-1336mzz(E), HFO-1224yd(Z), HFO-1233zd(E), and a mixture thereof.

2. The gravity heat pipe according to claim 1, wherein the working fluid is HFO-1234ze(Z), HFO-1234yf, HFO-1233zd(E), HFO-1336mzz(E), HFO-1224yd(Z), or a mixture thereof.

3. The gravity heat pipe according to claim 1, wherein the working fluid is HFO-1234ze(Z).

4. The gravity heat pipe according to claim 1, wherein the mixture comprises -1234ze(Z), HFO-1233zd(E), or HFO-1336mzz(E) as one component.

5. The gravity heat pipe according to claim 1, wherein the mixture comprises HFO-1234ze(Z)/HFO-1336mzz(E); HFO-1234ze(Z)/HFO-1336mzz(E); HFO-1234ze(Z)/HCFO-1224yd; HFO-1234ze(Z)/HCFO-1224yd; HFO-1234ze(Z)/HCFO-1233zd(E); HFO-1234ze(Z)/HCFO-1233zd(E); HCFO-1233zd(E)/HCFO-1224yd; HCFO-1233zd(E)/HCFO-1224yd; HFO-1336mzz(E)/HCFO-1224yd; and HFO-1336mzz(E)/HCFO-1224yd.

6. The gravity heat pipe according to claim 1, wherein the working fluid is used to directly replace a fluid of a gravity heat pipe originally designed to use HFC-245fa, HFC-134a or R410a.

7. The gravity heat pipe according to claim 1, wherein said gravity heat pipe device is a flat-plate type gravity type heat pipe.

8. The gravity heat pipe according to claim 1, wherein said gravity heat pipe is used for cooling of an electronic equipment, a computer, a communication base station, a server, or a data center.

9. A method for cooling using the gravity heat pipe according to claim 1.

10. The method according to claim 9, wherein the working fluid is fluid selected from the group consisting of HFO-1234ze(Z), HFO-1234ze(E), HFO-1336mzz(Z), HFO-1336mzz(E), HFO-1224yd(Z), HFO-1233zd(E), and a mixture thereof.

11. The method according to claim 9, wherein the working fluid is fluid selected from the group consisting of HFO-1234ze(Z), HFO-1234yf, HFO-1233zd(E), HFO-1224yd(Z), or a mixture thereof.

12. The method according to claim 9, wherein the working fluid is HFO-1234ze(Z).

13. The method according to claim 9, wherein the gravity heat pipe is operated at a temperature from 0° C. to 100° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 shows the saturation vapor pressures of various gravity heat pipe working fluids in a temperature range between 0° C. and 100° C.;

[0024] FIG. 2 shows the Merit Numbers (M′) for various gravity heat pipe working fluids in a temperature range between 0° C. and 100° C.;

[0025] FIG. 3 shows a schematic diagram of a process flow for a gravity heat pipe setup.

DETAILED DESCRIPTION

[0026] Embodiments of the invention will be illustrated with the following examples. However, the invention is not limited to the specific examples. Those skilled in the art will recognize that the invention encompasses other alternatives, modifications, and equivalents that may be included within the scope of the claims.

[0027] Embodiments of the invention relate to working fluids for heat pipes (e.g., gravity type heat pipes), particularly those for cooling electronic equipment, such as computers, communication base stations, servers, or internet data centers (IDC), to name a few. A heat pipe uses phase changes (e.g., between liquid and gas phases) of a working fluid to transfer heat from one location to another. Proper functioning of a heat pipe requires a saturated working fluid. The working fluid absorbs latent heat (heat of evaporation) when evaporating from the liquid phase to the gas phase in the evaporator zone and releases latent heat when the gas is cooled and condensed back to liquid in the condenser zone.

[0028] Because a heat pipe functions under saturated conditions (i.e., co-existence of liquid and gas phases in the heat pipe), the first factor to consider in selecting a working fluid is the operating temperature range, which should be between the triple (freezing) point and the critical point of the working fluid. In reality, the operating temperature range for any given fluid is smaller, because the power that the heat pipe can carry drops off sharply near the freezing and critical temperatures. If the operating temperatures are too high, the fluid may not be able to condense. However, if the operating temperatures are too low the fluid will not be able to evaporate. For applications in cooling electronic equipment, such as those in IDC, the operating temperature rage is typically between 0 to 150° C., more commonly between 0 to 100° C.

[0029] Many potential working fluids can function in this temperature range. Proper choices for working fluids would depend on many factors related to the properties of the working fluids.

Environmental Performance

[0030] Climate change is an urgent problem. Any working fluids should have no or little environmental impact, i.e., no ozone depletion potential (ODP) and low global warming potential (GWP). Based on the ODP and GWP values, several candidate working fluids listed in Table 1 are selected as candidate working fluids. It can be seen from Table 1 that all these working fluids all have zero or very low ODP. Some of these working fluids have high GWP, and some have very low GWP.

[0031] Based on these environmental performance parameters, promising working fluids include, for example, HFO-1234yf, HFO-1234ze(Z), HFO-1234ze(E), HFO-1233zd(E), HCFO-1224yd(Z), HFO-1336mzz(Z), HFO-1336mzz(E), and a combination of these working fluids.

TABLE-US-00001 TABLE 1 Working Fluid ODP GWP HFC-134a 0 1430 R410a 0 2100 HCFC-22 0.05 1810 HFC-245fa 0 1050 HFO-1234yf 0 <1 HFO-1234ze(Z) 0 <1 HFO-1234ze(E) 0 <1 HFO-1233zd(E) 0.00034 1 HCFO-1224yd(Z) 0.00012 <1 HFO-1336mzz(Z) 0 2 HFO-1336mzz(E) 0 7

Saturation Vapor Pressure

[0032] Because a heat pipe uses phase changes (e.g., between liquid and gas phases) of a working fluid to transfer heat from one location to another, the working fluid should have sufficient saturation vapor pressures within the operating temperature range to conduct sufficient heat. On the other hand, the saturation vapor pressures of the working fluids in the operating temperature range should not be too high. Otherwise, it may exert too much pressure on the heat pipe envelope. Therefore, a good working fluid should have sufficient vapor pressures within the operating temperature range. In addition, the pressure change as a function of temperature should not be too steep.

[0033] FIG. 1 shows saturation vapor pressures as a function of temperatures for various candidate working fluids. As shown in FIG. 1, while most working fluids shown have acceptable vapor pressures within the operating temperature range of 0-100° C., some have better vapor pressure-temperature profiles. For example, HFO-1336mzz(E), HFO-1234ze(Z), HFO-1233zd(E), HCFO-1224yd(Z), and HFO-1336mzz(E) have similar saturation vapor pressures and temperature dependency as those of HFC-245fa. These properties suggest that these working fluids can be “drop-in” replacements for HFC-245fa—i.e., it is probably unnecessary to modify the pipes when replacing HFC-245fa with any of these working fluids.

[0034] On the other hands, R410a, HCFC-22, HFC-134a, HFO-1234yf, and HFO-1234ze(E) have significantly higher saturation vapor pressures and also have higher temperature dependencies, as compared to those of HFC-245fa. These properties suggest that these working fluids should not be used as drop-in replacements for HFC-245fa. Among these working fluids, R134a and HFO-1234yf have very similar properties, making HFO-1234yf a suitable drop-in replacement for R134a. Interestingly, the trans isomer, HFO-1234ze(E), has much higher saturation vapor pressures and temperature dependency, as compared to its cis isomer, HFO-1234ze(Z).

[0035] From this group, working fluids with preferable profiles include HFO-1336mzz(E), HFO-1234ze(Z), HFC-245fa, HCFO-1224yd(Z), HFO-1233zd(E), and HFO-1336mzz(Z). The saturated vapor pressures of these working fluids are below or only slightly over 1.0 MPa at 100° C. The lower vapor pressures for HFO-1336mzz(E), HFO-1234ze(Z), HFC-245fa, HCFO-1224yd(Z), HFO-1233zd(E), and HFO-1336mzz(Z) would not demand a strong heat pipe envelope, thereby reducing the system manufacturing costs. More importantly, the vapor pressures of these working fluids all have very similar temperature dependence to that of HFC-245fa, which is the working fluids in the current generation heat pipes. The similar pressure-temperature profiles of these working fluids suggest that these working fluids can be “drop-in-replacement” for HFC-245fa in the current heat pipe systems with no or minimal modification.

The Merit Number

[0036] In addition to the above considerations, several properties inherent in the working fluids would impact their performance as heat pipe working fluids. For example, high liquid density and high latent heat reduce the fluid flow required (i.e., the amounts of working fluids required) to transport a given power. A low liquid viscosity reduces the liquid pressure drop for a given power.

[0037] Taking into accounts these properties of working fluids, a merit number (or figure of merit, M′) may be used to assess the relative performance of a range of prospective working fluids. For a uavity type heat pipe (Wickless heat pipe or thermosyphon), the merit number (M′) is defined as follows:

[00003]$M^{\prime }={\left(\frac{L{k}_l^3{\rho }_l^2}{{\mu }_l}\right)}^{\frac{1}{4}}$

wherein the dimension of the merit number M′ is

[00004]$\frac{kg}{K^{3/4}{S}^{5/2}},$

wherein L is the latent heat of vaporization with a dimension of (kJ/kg); ρ.sub.1 is the density of the saturated liquid, with a dimension of kg/m.sup.3; k.sub.1 is the liquid thermal conductivity; and μ.sub.1 is the hydrodynamic viscosity in dimension of Pa.Math.s.

[0038] FIG. 2 shows the merit numbers (M′) for several candidate working fluids within the temperature range of 15-95° C. Among these working fluids, HFO-1234ze(Z) has the highest merit numbers within this operating temperature range, even higher than those of HFC-245fa. Thus, HFO-1234ze(Z) not only is good for drop-in-replacement for HFC-245fa in current heat pipe system, but also will have high performance characteristics compared to HFC-245fa. Other working fluids having high merit numbers include HFO-1233ze€, HFO-1224yd(Z), HFO-1336mzz(Z) and HFO-1234ze(E). In contrast, the merit numbers of HFO-1234yf, which is the coolant of choice in modern automobiles, are significantly lower than others in this temperature range. Interestingly, the merit number of the trans isomer, HFO-1234ze(E), is significantly lower than those of its cis isomer, HFO-1234ze(Z), within this temperature range. Thus, HFO-1234ze(Z) would be a significantly better working fluid than HFO-1234ze(E) due to a larger merit number and lower saturation vapor pressures with a lower pressure-temperature dependency.

Latent Heat of Evaporation

[0039] Latent heat of evaporation relates to the amount of heat transfer in phase transition (e.g., from liquid phase to gas phase in the evaporation zone) per unit weight of a working fluid. As shown in the above merit number equation, the higher the latent heat of a working fluid is, the higher its merit number will be. Therefore, everything else being equal, working fluids with higher latent heats are preferred because they would require less amounts of the working fluids to transfer the same amounts of heat, as compare with a working fluid with a lower latent heat of evaporation. The following Table 2 shows the latent heats of evaporation for various working fluids and mixtures.

TABLE-US-00002 TABLE 2 Working Fluid Latent Heat at 23° C. (KJ/Kg) HFC-245fa 192.4 HFO-1234ze (Z) 207.4 HFO-1336mzz (E) 160.00 HCFO-1233zd (E) 192.2 HFO-1234yf 147.0 HFO-1234ze (E) 168.4 HCFO-1224yd 164.9 HFO-1336mzz (Z) 169.4 R1234ze (Z)/R1336mzz (E) = 80/20 199.0 R1234ze (Z)/R1336mzz (E) = 20/80 169.6 R1234ze (Z)/R1224yd = 90/10 202.3 R1234ze (Z)/R1224yd = 10/90 168.1 R1234ze (Z)/R1233zd (E) = 90/10 204.9 R1234ze (Z)/R1233zd (E) = 10/90 192.7 R1233zd (E)/R1224yd = 90/10 188.9 R1233zd (E)/R1224yd = 10/90 167.0 R1336mzz (E)/R1224yd = 90/10 188.9 R1336mzz (E)/R1224yd = 10/90 165.3

[0040] As shown in Table 2, several working fluids (e.g., HFO-1234ze(Z) and mixtures thereof) have high latent heats of evaporation. With these working fluids, less amounts (mass) would be needed to achieve the same amount of heat transfer.

Testing the Performance of the Working Fluids

[0041] The cooling performance of working fluids of the invention are tested with a gravity type heat pipe setup. In this setup, two independent sets of gravity heat pipe systems (shown schematically in FIG. 3) are arranged closely together. One set of the gravity heat pipe systems uses HFC-245fa for comparison, and the other set of the gravity heat pipe systems uses a test working fluid of the invention (e.g., HFO-1234ze(Z), etc.). This side-by-side setup provides a better comparison between two working fluids. However, other setup (e.g., a single heat pipe system) may also be used for the tests. In addition, the performance of these working fluids may also be investigated with computer modeling/calculations.

[0042] Referring to FIG. 3, to run the test, the inner and outer back plates were opened at the same time for heat pipe circulation experiments. The experimental conditions were as follows: the indoor air intake dry bulb temperature was 35° C., and the indoor air intake wet bulb temperature was 22° C. The air circulation volumes are the same for the test working fluid (e.g., HFO-1234ze (Z)) and the comparison (e.g., HFC-245fa) system. The water temperatures for water circulating to the outside condensers are also the same for both systems (in: 15° C. and out: 20° C.). [0043] The thermocycle performance parameters for the working fluids, as compared to that of HFC-245fa, are shown in Table 4. Under the same inlet and outlet water temperatures, the heat exchange capacities and coefficients of performance for several candidate working fluids are larger than those for HFC-245fa. The temperature of the inlet area of the cold passage can be maintained in compliance with the national standards, 18-27° C.

TABLE-US-00003 TABLE 4 Gravity Heat Pipe Cycle Test Results Indoor Air side Air Temp. water Optimal heat Coefficient of outlet System inlet and outlet amount Exchange Performance temperature Pressure Working Fluid (° C.) (kg) (KW) (COP, W/W) (° C.) (MPa) HFC-245fa 15° C./ 1.05 5.98 56.42 23.50 0.143 HFO-1234ze(Z) 20° C. 0.80 6.09 58.94 22.30 0.154 HFO-1336mzz(E) 1.00 5.54 63.93 23.95 0.196 HCFO-1233zd(E) 0.90 5.59 65.39 23.92 0.130 HFO-1234yf 0.85 6.29 72.26 21.50 0.615 HFO-1234ze(E) 0.88 5.36 77.70 24.35 0.455 HCFO-1224yd(Z) 1.03 5.49 65.23 24.06 0.116 HFO-1336mzz(Z) 1.05 5.29 59.63 24.43 0.065 HFO-1234ze(Z)/ 1.0 6.43 61.41 22.08 0.162 HFO-1336mzz(E) = 80/20 HFO-1234ze(Z)/ 0.95 5.31 62.35 24.26 0.177 HFO-1336mzz(E) = 20/80 HFO-1234ze(Z)/ 0.85 5.94 59.89 23.55 0.166 HCFO-1224yd = 90/10 HFO-1234ze(Z)/ 1.03 5.55 64.29 23.95 0.146 HCFO-1224yd = 10/90 HFO-1234ze(Z)/ 0.97 5.68 62.15 23.85 0.132 HCFO-1233zd(E) = 10/90 HFO-1234ze(Z)/ 0.94 5.99 60.12 23.50 0.165 HCFO-1233zd(E) = 90/10 HCFO-1233zd(E)/ 1.02 5.54 65.28 23.92 0.139 HCFO-1224yd = 10/90 HCFO-1233zd(E)/ 0.98 5.55 65.35 23.90 0.125 HCFO-1224yd = 90/10 HFO-1336mzz(E)/ 1.00 5.52 64.12 23.98 0.173 HCFO-1224yd = 90/10 HFO-1336mzz(E)/ 1.03 5.50 65.02 24.00 0.144 HCFO-1224yd = 10/90

[0044] As shown in Table 4, working fluids of the invention in general have better performance parameters than those of the currently used working fluids, such as HFC-245fa. For example, these working fluids need less amounts to achieve the same heat transfer (i.e., lower optimal amount). The coefficients of performance of these working fluids are better than that of HFC-245fa, suggesting that all these fluids would have better performance than HFC-245fa. In addition, most of these working fluids have relative low system pressures, except for HFO-1234yf and HFO-1234ze(E), suggesting that these working fluids can be used to safely replace HFC-245fa without substantial modification to the heat pipes.

[0045] In addition, the optimal masses (amounts) needed are mostly lower than that for HFC-245fa, making these working fluids more economical to use than HFC-245fa.

[0046] These data together indicate that the HFOs and HCFOs of the invention are excellent working fluids for heat pipes, particularly, gravity type heat pipes. Examples of working fluids of the invention include HFO-1234ze(Z), HFO-1234ze(E), HFO-1234yf, HFO-1336mzz(Z), HFO-1336mzz(E), HFO-1224yd(Z), and HFO-1233zd(E), or a mixture thereof. Preferred working fluids of the invention include HFO-1234ze(Z), HFO-1233zd(E), HFO-1234yf, HFO-1336mzz(E), and HFO-1224yd(Z), or a mixture thereof.

[0047] A mixture of these working fluids may comprise two components with a ratio of 1:99, preferably 10:90, or 20:80, or 30:70, or 40:60, or 50:50, and any number therebetween. A preferred mixture comprises HFO-1234ze(Z), HFO-1233zd(E), or HFO-1336mzz(E) as one component. Non-limiting examples of a mixture of working fluids of the invention may include: HFO-1234ze(Z)/HFO-1336mzz(E) in a suitable ratio (e.g., 80/20); HFO-1234ze(Z)/HFO-1336mzz(E) in a suitable ratio (e.g., 20/80); HFO-1234ze(Z)/HCFO-1224yd in a suitable ratio (e.g., 90/10); HFO-1234ze(Z)/HCFO-1224yd in a suitable ratio (e.g., 10/90); HFO-1234ze(Z)/HCFO-1233zd(E) in a suitable ratio (e.g., 10/90); HFO-1234ze(Z)/HCFO-1233zd(E) in a suitable ratio (e.g., 90/10); HCFO-1233zd(E)/HCFO-1224yd in a suitable ratio (e.g., 10/90); HCFO-1233zd(E)/HCFO-1224yd in a suitable ratio (e.g., 90/10); HFO-1336mzz(E)/HCFO-1224yd in a suitable ratio (e.g., 90/10); and HFO-1336mzz(E)/HCFO-1224yd in a suitable ratio (e.g., 10/90).

[0048] That the HFOs of the invention have better performance coefficients than HFO-1234ze(E) and HFC-245fa is unexpected. HFC-245fa is considered a good alternative working fluid for a gravity type heat pipe. HFC-245fa lends itself to a number of heat transfer and working fluid applications based on its thermophysical properties. The thermophysical properties of HFC245fa make it suitable in various applications such as centrifugal chillers, Organic Rankine Cycle for energy recovery, sensible heat transfer in low-temperature refrigeration and passive cooling devices. (G. J. Zyhowski et al., “An Overview of the Properties and Application of HFC-245fa,” International Refrigeration and Air Conditioning Conference, 2002).

[0049] In addition, the trans isomer, HFO-1234ze(E), was developed as a fourth-generation refrigerant to replace fluids such as HFC-134a. HFO-1234ze(E) has zero ozone-depletion potential (ODP=0), a very low global-warming, potential (GWP<1), even lower than CO.sub.2. HFO-1234ze(E) has been adopted as working fluid in chillers, heat pumps, and supermarket refrigeration systems. (Wikipedia: https://en.wikipedia.org/wiki/1,3,3,3-Tetrafluoropropene). However, here we found that the cis isomer, HFO-1234ze(Z), would actually be between than the trans isomer, HFO-1234ze(E), for use in heat pipes (particularly, gravity type heat pipes) for cooling electronic equipment, internet data centers, etc. because the cis isomer, HFO-1234ze(Z), has a lower system pressure and would require less amount to transfer the same amount of heat.

[0050] Embodiments of the invention have been illustrated with a limited number of examples. One skilled in the art would appreciate that these examples are for illustration only and are not meant to limit the scope of the invention and that other modifications and variations are possible without departing from the scope of the invention. Therefore, the scope of the invention should only be limited by the accompanying claims.