Suchen Wu ,Yiwen Ding ,Chengbin Zhang ,*,Dehao Xu

1 Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education,School of Energy and Environment,Southeast University,Nanjing 210096,China

2 Nanjing ACME Electronics Cooling Service Inc.,Nanjing 211101,China

Keywords:Thermoelectric Heat pipe Heat spreader Power generation

ABSTRACT A gravitational flat-plate heat pipe is designed and fabricated in this paper to serve as a heat spreader to diffuse the local heat source to the hot side of the thermoelectric power module.Based on this,an experimental test for the thermoelectric power generation system is conducted to study the in fluences of the heat spreader on the temperature uniformity and power generation performance when exposing to a local heat source.In addition,the effects of the heating power,inclination angle,and local heat source size on the power generation performance of the thermoelectric power module using a flat-plate heat pipe as a heat spreader are examined and compared with thatusing a metalplate.The results indicate thatthe gravitational flat-plate heat pipe has considerable advantages over the metalplate in the temperature uniformity.The superiority of temperature uniformity in the improvement of power generation performance for the thermoelectric power system using a heat pipe is demonstrated.Particularly,the heat pipe shows good adaptability to placement mode and the local heat source size,which is beneficial to the application in the thermoelectric power generation.

1.Introduction

The thermoelectric power generation is an emerging technology based on the Seebeck effect,which is the conversion of heat directly into electricity[1,2].It possesses many unique advantages,including no moving parts,easy installation,and no pollutant emissions,so ithas potentialapplications in waste heatutilization,solarpowergeneration,polar expeditions and deep space missions[3,4].The power generation efficiency increases monotonously with the temperature difference.It is reported that the current power generation efficiency of commercial Bi2Te3-based generator modules is about 2%–5%under the temperature difference from 100 °C to 300 °C[5,6].The heat source used for thermoelectric power generation usually comes from waste heat,which usually exists in the form of local heat sources,such as high-temperature flue gas ejecting from the hole or flowing through the pipeline.Owing to the effectoflocalheatsource,the surface temperature of the hot side for the thermoelectric power module is unevenly distributed,degrading the power generation performance.Therefore,it is urgent to introduce the advanced heat spreader to eliminate the temperature non-uniformity so as to improve the heat utilization ef ficiency of the thermoelectric power system.

As an efficient and reliable thermal device,the heat pipe combines the principles of evaporation–condensation phase-change heat transfer and gas–liquid two-phase flow.It is now widely utilized in electronic cooling[7],micro fluidic systems[8,9],thermal management,etc.At the evaporator section,the liquid absorbs the heat coming from the heat source and then vaporizes into the gas,which travels to the condenser section and then condenses back into liquid accompanied with latentheatrelease.The condensate flows back to the evaporator section by the driving force(gravity or capillary force).The above processes form a complete gas–liquid two-phase cycle[10,11].Due to the gas–liquid phase-change heat transfer,the heat pipe has excellent isothermal performance,which can spread heat from a point to the entire surface or can transport the heat for a long distance[11–13].In this context,the heat pipe is regarded as an attractive thermal device that expands the heat transfer area,reduces the temperature difference during the heat spreading,or maintains the temperature uniformity for long-distance heat transport.

Due to the above inherent advantages,the heat pipe has been used for thermoelectric power generation in waste heat recovery.For example,when collecting the waste heatofcar exhaust,itis difficultto place a large number of thermoelectric generators in the car exhaust pipe owing to the small,confined space.To solve this problem,a heat pipe is installed in the exhaustpipe to transportthe heatto the externallarge space where the thermoelectric generators can be laid out[14].The thermoelectric generators receive the heat from the heat pipe and are cooled down by air cooling,where the thermoelectric generator produces the electricity from the waste heat.Nowadays the mature commercial thermoelectric material(such as Bi2Te3)cannot work under high temperatures over 250°C,making it unable to be in contact with high-temperature flue gas carrying waste heat.For this reason,the variable-conductance heat pipe is applied to adjust the operating temperature for the thermoelectric generators in an acceptable range[14,15].There is also a novel hybrid solar system for heat utilization,photovoltaic and thermoelectric by the use of heat pipes to collect solar energy,which increases the efficiency by 30%compared to the traditional photovoltaic power generation technique[16,17].In these above thermoelectric power generation applications,the axial heat pipe is used for the long-distance heat transport and then the fins or metal plates are used to expand the heat transfer area of the hot and cold sides for the generator.However,the axial heat pipes are difficult to spread the local heat source from a point to the whole surface in the confined space.To meet the demand of a local heat source,a gravitational flat-plate heat pipe[18–20],which efficiently spreads the heat from a point to an area in a confined space,is introduced in the present study to improve the surface temperature uniformity ofthe thermoelectric power module.

In general,the performance of thermoelectric power generation is dependent on the material figure of merit(ZT)and the difference between the cold and hot side temperatures.The thermoelectric material has a suitable working temperature range,in which the higher temperature difference tends to achieve better performance.The available literature in terms of thermoelectric power generation focuses on either the performance or efficiency of thermoelectric materials as affected by a single temperature of the cold and hot sides,i.e.,the relationship between ZT(efficiency)and the temperature difference[21].Some studies also compare the ZT for different materials within a certain temperature range to select suitable materials for power generation[22].In these studies,the constant temperature boundary condition for the hot side of thermoelectric generators is considered,i.e.,the temperature of the whole hot surface is uniform.However,in the actual application,the thermoelectric power module(consisting of a number of thermoelectric generators)is in direct contact with the large-area hot surface where there exists a temperature gradient owing to the imposition ofa localheatsource.The presence ofthe temperature gradient reduces the capability of the thermoelectric power module,resulting in an inefficient use of waste heat.Until now,there have been relatively few studies on the in fluence of temperature uniformity on the performance of a thermoelectric power system.

Therefore,in this paper,a gravitational flat-plate heat pipe is designed and fabricated,which is used as the heat spreader to diffuse the heat from a local heat source to the hot side surface of the thermoelectric power module.Based on this,the temperature uniformity of flat-plate heat pipe is experimentally identified and discussed as compared with the metal plate.In addition,the performance of a thermoelectric power generation system using a heat pipe is experimentally evaluated and compared with that using a metal plate,in an effort to demonstrate the advantage of the heat pipe for a thermoelectric power generation system.Particularly,the in fluence of inclination angle,heat load,and heat source size on the power generation performance is also analyzed.

2.Experimental

2.1.Heat pipe

The heat pipe is a kind of high-efficiency heat spreader via the principle of evaporation and condensation heat transfer.In the current study,the gravitational flat-plate heat pipe is used to effectively ensure the temperature uniformity and greatly extends the contact area for the hot side of a thermoelectric power module.The geometric structure of the gravitational flat-plate heat pipe is shown in Fig.1(a).The length,width,and thickness of the heat pipe are L=300 mm,W=300 mm,and H=14 mm,respectively.The heat pipe is made from a piece of brass plate.To ensure the flatness of the heat pipe surface with the high-pressure of the internal working fluid and the superior communication among the channels,the metal plate is machined with crisscrossed cylindrical channels,comprised of 26 channels,5 mm in diameter and a spacing of 11 mm in both the transverse and longitudinal directions,as illustrated in Fig.1(b).The machined brass plate is vacuumed firstly,then filled with a certain amount of water and finally sealed to fabricate a heat pipe.For heat pipe imposed by a local heat source,the amount of liquid filling affects the degree of temperature uniformity.Too many or too few liquid filling weakens the thermal performance of the heat pipe,and the appropriate liquid filling is beneficial to enhance the temperature uniformity.The available investigation indicates that the liquid filling rate of about 50%is a good candidate for the gravitational flat-plate heat pipe.Thus,the filling ratio of working fluid is chosen as 50%(i.e.163 g)to ensure the high-performance operation of the heat pipe.

When a local heat source is imposed to heat the flat-plate heat pipe,the liquid at the bottom of the cylindrical channel is evaporated into vapor.The vapor diffuses to the condenser section and condenses into the liquid at the top of the channel with latent heat release.With the gravity effect,the condensate falls to the bottom of the channel.The above gas–liquid two-phase circulation spreads the heat from a local heat source to a large heating surface.The working principle of the gravitational flat-plate heat pipe is shown in Fig.1(c).The latent heat of water is far higher than that of the sensible heat,and the fluid temperature in the phase-change process remains almost constant.Due to that,the heat pipe exhibits superior temperature uniformity.The thermal conductivity of the heat pipe is dozens of times,or even hundreds of times,that of the brass(λbra=108.9 W·(m·k)-1).Therefore,under a local heat source with the same heating power,the temperature distribution of the heat pipe is more uniform than the metal plate and,hence,improves the performance of thermoelectric power generation system.

2.2.Experimental apparatus

As shown in Fig.2(a),the experimental apparatus of thermoelectric generation system mainly includes the local heat source,heat spreader,thermoelectric power module,cooling unit,adjustable holder and data measuring device.In the experiment,an electrical heating unit with a small area on one side is used to simulate the local heat source,and it is the energy input for the thermoelectric power module that is fixed to the adjustable holder.As an efficiency heat spreader,the flat-plate heat pipe diffuses the local hot point to the entire surface of the condenser section,so the thermoelectric generators configured on the condenser section can absorb heat efficiently and uniformly.The cold side of the power generation module is directly in contact with the cooling unit,which is the aluminum serpentine channel.The cooling water flows in the serpentine channel and takes away the heat that is not utilized by the thermoelectric power module.The temperature distribution of the heat pipe and the power generation of thermoelectric generators are measured and recorded by the data measuring device.

The electrical heating unit is composed of a copper rod with some electric heating rods embedded inside,and its external surface is wrapped by the thermal insulation material.There are 10 electric heating rods embedded inside the copper rod with a maximum heating power of 100 W per electrical heating rod,indicating that the local heat source is able to provide 1 kW of heating power at a maximum.The arrangement of the electric heating rods for the local heat source is shown in Fig.2(b).The electric heating rods input heat to the copper rod and arefinally transferred to the heat pipe.In this experiment,four copper rods with diameters of d=20 mm,40 mm,60 mm,and 80 mm,respectively,are used to simulate the local heat source with a different area.Note that,without specification,the following experimental data are obtained under the case with diameters d=80 mm.The total input heating power can be adjusted by changing the input voltage of the electrical heating rods,by which the input heat load of the thermoelectric generator is manipulated.

Fig.1.Geometric structure and working principle of a gravitational flat-plate heat pipe.

The flat-plate heat pipe utilizes the gravity as the driving force for the recirculation of working fluid,indicating that the heat pipe is able to work at different inclination angles.The adjustable holder(see Fig.2(c))is applied to regulate the inclination angle of a flat-plate heat pipe.In this paper,three placement modes(i.e.the inclination angle α =0°,45°,and 90°)are considered to analyze the effect of inclination angle on the performance of power generation.When α =0°,the electrical heating unit is configured at the center of the heat pipe,while the electrical heating unit is located at the bottom of the heat pipe when α =45°and 90°,as shown in Fig.2(d).Note that,without specification,the following experimental data are obtained under the case with inclination angle α =0°.

The thermoelectric power module consists of 36 commercial thermoelectric generators,which produce electricity when there is a temperature difference between the cold and hot sides of the thermoelectric generators.The length,width,and thickness of a thermoelectric generator are 40×40×3 mm3,respectively.The thermoelectric material is Bi2Te3and each thermoelectric generator has 120 pairs of PN junctions connected in series.The relationship between the output power of a single thermoelectric generator and the temperature difference between the cold and hot sides is shown in Fig.3(a).As shown in the figure,the output power of thermoelectric generator increases with the increase of the temperature difference between the cold and hot sides.In this experiment,36 thermoelectric generators are divided into three groups;these three groups are connected in parallel with each other while 12 thermoelectric generators in each group are connected in series,as shown in Fig.3(b).

The cooling unit is a serpentine channel heat exchanger made of aluminum with the same length and width as the flat-plate heat pipe,which is 300 mm in length and 300 mm in width,respectively.The serpentine channel is connected to the constant-temperature water bath.A flowmeter is installed at the entrance of the serpentine channel,and two thermocouples are put in the outlet and inlet of the serpentine channel.The heat transfer quantity of the serpentine channel can be determined by Qf=mCpΔtf,where Cpis the heat capacity of water,m is the mass flow rate,and Δtfis the temperature difference between the inlet and outlet.The heat load of the thermoelectric power module(Q)is the sum of the output power(P)and heat transfer quantity(Qf),Q=P+Qf.

The data measuring device,including the thermocouples and electric meter,is used to measure the wall temperature of the heat pipe and the capacity of power generation module.The electric meter composed ofan ammeter and a voltmeteris used to measure the output current and voltage of the power generation module so as to determine the output power.The T-type thermocouples are connected to the acquisition board of the data acquisition instrument(Agilent 34970a).The data acquisition instrument is controlled by the computer to carry out signal conversion and recording,realizing the real-time monitoring of the temperature during the experiment.

Fig.2.Experimental apparatus of a thermoelectric generation system.

In order to verify the superior isothermal performance and the improvement of power generation capacity owing to the use of a heat pipe,a comparative experiment of the thermoelectric power system is carried out by using the metal plate as a heat spreader.In the comparative experiment,the metal plate is of the same external dimensions(width W=300 mm,length L=300 mm,height H=9 mm),and weight(G=4.06 kg)as the flat-plate heat pipe.

3.Results and Discussion

3.1.Temperature uniformity

Temperature uniformity is the key factor for the evaluation of the thermal performance of the heat spreader.If the heat is diffused more efficiently from the local hot point to the whole surface,the whole surface temperature distribution is more uniform for a heat spreader.In order to intuitively display the performance of the heat spreader,it is necessary to evaluate the temperature distribution of the heat pipe and metal plate under a local heat source.To record the unsteady-state wall temperature distribution of the heat spreader,13 temperature measuring points are set on the solid surfaces of the heat pipe and metal plate,as shown in Fig.4(a).Four typical temperature points(marked as①,②,③,④)are selected from these 13 temperature measurement points to exhibit the dynamic temperature variations of the heatspreader.In the experiment,the heatload supplied by the electrical heating unit(d=80 mm,see Fig.2(b))is imposed on the center of the lower surface for the heat spreader,and the upper surface of the heat spreader is convected into the air(velocity:3 m·s-1,temperature:24°C)supplied by the fan,as shown in Fig.4(b).Note that the heat load starts from 100 W and increases gradually as every point temperature reaches the steady state.

Fig.4.Dynamic temperature distributions of the heat spreader under a local heat source.

In order to clearly understand the temperature distribution and thermal response behavior of heat spreader under a local heat source,Fig.4(c)and(d)compares the temperature evolutions oftypicalsurface positions for the heat pipe and metal plate under the step-wise increasing heat flux.As shown,irrespective of the type of heat spreader,the temperature of the center point(i.e.,Point①)that is located at the center of the local heat source is the highest,and the temperature of the corner point(i.e.,Point④)which is located farthest from the heat source is the lowest.There is a very small difference among the temperatures of four typical positions for the heat pipe,attributing to the superior performance of the evaporation and condensation heat transfer.In addition,under the step-wise increasing heat flux,the thermal response time of the heat pipe is shorter than that of the metal plate.This can be explained by the fact that the high thermal conductivity of the heat pipe improves the thermal diffusion property,and hence the heat is diffused efficiently and shows a rapid dynamic response.

To display the improvementofthermalperformance by the heatpipe,Fig.5 presents the steady-state surface temperature distributions for the heat pipe and metal plate induced by a local heat source with the same heatload(Q=604 W).In the plot,the whole surface temperature distribution is calculated through the above 13 temperature measurement points using cubic polynomial interpolation;the green plane represents the average value of the whole surface temperature.For the heat pipe and metal plate,the temperature in the center is higher than that at the periphery,forming a peak area.Due to the superior radial heat diffusion performance,the surface temperature variation of the heat pipe is relatively gentle,and the difference between the highest and lowest temperatures is small.The contourofthe surface temperature distribution is only a slight bulge.Considering that the thermal conductance capability of the metal plate is far lower than the heat pipe,the temperature uniformity of the metal plate is much weaker than the heat pipe.In this case,the temperature difference between the local heat source zone and the non-heating zone is very large,and the central zone of the surface temperature contour for the metal plate forms a steep mountain.

Fig.5.Surface temperature contours of heat spreader under a local heat source.

Fig.6.Surface temperature uniformity of heat pipe and metal plate under a local heat source.

Fig.6 presents the effect of the heat load on temperature uniformity of the heat pipe and metalplate.In the plot,Δt represents the difference between the highestand lowesttemperatures ofthe heatspreader;σtis defined as the mean square error ofthe surface temperature;and t‾is the surface mean temperature.These two parameters,Δt andσt,are applied to describe the temperature uniformity of heat spreader.As shown,for the same heat load and cooling capability,the average temperature of the heat pipe is close to the metal plate.However,Δt and σtof the heat pipe are far smaller than those of the metal plate,indicating the superior temperature uniformity of the heat pipe.For the case with heat load of 600 W,Δt of the heat pipe is only 13 °C,but Δt is up to 117 °C for the metal plate;σtof the heat pipe is 2.5 °C while it is 49.1°C for the metal plate.

Itis also seen from the figure thatΔt(σt)increases in a linear fashion with the heat load for the metal plate because the heat transfer mechanism is solely the heat conduction of the metal.However,Δt(σt)is almostunvaried with increasing heatload forthe heatpipe.Thisimplies that the temperature uniformity capability of the heat pipe becomes stronger as the heat load increases.The heat transfer mechanism for the heat pipe is the evaporation and condensation heat transfer.The higher the heat load,the stronger the gas–liquid two-phase phasechange heat transfer,which leads to more efficient heat diffusion from the local heat source.Hence,the thermal diffusion capability of the heat pipe is superior to the metal plate.In this situation,the difference of Δt(σt)between the metal plate and heat pipe increases as the heat load increases.

Itis notable thatthe metalplate has a highermean temperature than the heat pipe with the increase of input power.For the heat pipe,the surface temperature is very uniform,so the heat exchange between the heat pipe surface and the air is highly efficient for every part of the heat pipe surface.For the metal plate,the surface temperature of central region is very high,so there is a large heat exchange between surface and air for this small area.However,the surface temperature of its neighboring regions for the metal plate is very low,which causes small heat exchange for this large area.In this case,the surface utilization rate of the heat pipe for heat transfer is better than that of the metal plate.Therefore,for the identical ambient air temperature,the metal plate which only has a higher surface temperature can transport the same heat transfer capacity as compared with the heat pipe.In other words,the metal plate shows a higher mean temperature than the heat pipe when the local heat source power was large,as shown in Fig.6.

3.2.Power generation performance

Forthe thermoelectric powermodule,itsoutputpowerisaffected by the temperature uniformity of the heat spreader under a local heat source.Fig.7 compares the output power and the generation efficiency of the thermoelectric power module as a function of the heat load in both the horizontal and vertical states.The power generation efficiency ηPis defined as ηP=P/Q.As seen in Fig.7,no matter whether the heat spreader is placed horizontally or vertically,the output power of thermoelectric power module with the flat-plate heat pipe is higher than that with the metal plate.In addition,the output power is larger for a higher heat load for the thermoelectric power module using both the metal plate and heat pipe.Take the horizontal placed(α =0°)mode for example,it is indicated that the generation efficiency of the TE power module shows an improvement by 33.3%under the input of about 400 W when comparing the use of heat pipe with the brass plate.With the input increased up to 640 W,the efficiency is even improved beyond 50%.

Fig.7.Power generation(P)and efficiency(ηP)of the thermoelectric power module.(a)Horizontal placement(α =0°);(b)vertical placement(α =90°).

It is interesting that,in the case of the same heat load and cooling capability,the mean hot side temperature of the thermoelectric power module with the heat pipe is nearly the same as that with the metal plate,but there exists a large difference of output power.When the heat pipe is used as a heat spreader,the surface temperature of the hot side for the thermoelectric power module is almost the same,so all of the thermoelectric generators operate at the same working state.This is beneficial to the coordination among the thermoelectric generators and,hence,results in a superior capability of the thermoelectric power module.However,using the metal plate as a heat spreader,the surface temperature at the local heat source zone is much higher than that at the non-heating zone,leading to various working conditions for the thermoelectric power module.A few thermoelectric generators at the local heat source zone produce a large amount of power,but most ofthe thermoelectric generators in the non-heating zone generate a small amount of power.Consequently,under the same heat load,the overall output power of the thermoelectric power module using the heat pipe is far larger than that by using the metal plate.In summary,the superior temperature uniformity of heat pipe ensures the improvement of the power generation capability for the thermoelectric power module.

In order to provide further insight into the above phenomenon,the power generation capability of the thermoelectric power module induced by the temperature uniformity of the hot side is analyzed in terms of the in fluence factors of the thermoelectric materials.The thermoelectric efficiency is dependent on the temperatures of the cold and hot sides,as well as the thermoelectric figure of merit(ZT),and is defined as[15].

where tCand tHare the temperatures of the cold and hot sides,respectively.It is demonstrated by the previous investigation that the effect of hot side temperature on the thermoelectric generator efficiency is shown in Fig.8.As shown,for the same cold-side temperature,the thermoelectric efficiency does not increase in a linear fashion with the hot-side temperature,and the increasing trend is gradually slower as the hot-side temperature increases.Therefore,for the same average hot-side temperature,if the hot-side temperature uniformity is better,the average efficiency of the thermoelectric power module is higher.

Fig.8.Schematic of thermoelectric efficiency affected by the temperature of heat side.

3.3.In fluencing factors

In reality,the thermoelectric power module cannot always operate in the horizontal state.For example,the thermoelectric power generator may be positioned inclined owing to the constraint of the actual working environment.In the inclined condition,the flat-plate heat pipe can still operate normally,in which the liquid evaporation/boiling heattransfer occurs atthe evaporator section and the condensation heat transfer occurs at the condensation section.The gas–liquid two-phase circulation inside the heat pipe is still completed via the gravity effect.

In the experiment,three placement modes,including the horizontal placement(α =0°),inclination(α =45°),and vertical placement(α =90°),are considered.Based on these three conditions,the effect of the inclination angle on the power generation performance of the thermoelectric power module using a heat pipe is analyzed.When the heat pipe is inclined,the liquid working fluid is accumulated in the lower-part of the heat pipe.In order to ensure the normal work of the heat pipe in the inclined condition(α =45°,90°),the heating unit is located at the center of the lower part of the heat pipe.For the horizontal placement,the heating unit is located at the center of the lower surface of the flat-plate heat pipe.In the study,we take all the 13 points(see Fig.4(a))into consideration to determine the steady-state temperature and then evaluate the surface temperature uniformity of the flat heat pipe.The temperature uniformity of the heat spreader with three placement modes(α =0°,45°,90°)is provided in Fig.9.Owing to the boiling/evaporation and condensation phase change heattransferin these crisscrossed channels,the gravitational heat pipe utilized in the experiment has an efficient heat spread capability even it is inclinedly placed.The experimental results confirmed its superior performance of temperature uniformity,as shown in Fig.9.

Fig.9.Effect of placement modes on the surface temperature uniformity of heat pipe.

The effect of the placement mode on the output power of the thermoelectric power module is shown in Fig.10.Under the same heat load,the mean temperatures and temperature uniformity of heat pipe are almost the same in the different placement modes,so the output power of thermoelectric power module does not change with the inclined angle.In other words,the output power is insensitive to the placement mode when the gravitational heat pipe is used as a heat spreader for the thermoelectric power module.The good adaptability of the placement mode is beneficial to expand the application of the heat-pipe heat spreader in the thermoelectric power system.

Fig.10.Effect of placement mode on the thermoelectric power generation using a heat pipe.

Note that the heat pipe with inclination angle 90°provides higher out-put power when the heat power was between 550 W and 650 W in Fig.10.It can be interpreted that the nucleation boiling dominates the gas–liquid phase change heat transfer at the evaporator section when the heat power was between 550 W and 650 W.As inclination angle increases,the more importance of gravity effect for gas–liquid phase change is induced,which is beneficial to the bubble motion during boiling heat transfer and hence improves the thermal performance ofthe heatpipe.This indicates thatsurface temperature is more uniform as inclination angle increases,which contributes to the higher surface utilization rate of the heat pipe for power generation.

The local heat source size is another important factor to affect the power generation performance of the thermoelectric power module.To analyze the effect of local heat source size,the electrical heating rods with different diameters(d=80,60,40 and 20 mm)are considered.Fig.11 presents the effect of the local heat source size on the output power of the thermoelectric power module using a heat pipe.As shown,when the diameter of the electrical heating rods for the heat pipe is larger than 40 mm,the output power of the thermoelectric power module is insensitive to the local heat source size.Note that once the input power increased beyond 600 W,the temperature of the rod even reached 500°C with the diameter being 20 mm,which resulted in the failure of the thermal grease and then the subsequentincrease of the temperature.Thus,for the sake of security,the experiment with higher inputpower was suspended.By comparison,itis concluded thatthe heatpipe is more adaptive to the localheatsource size due to its superior thermaldiffusion capability,which improves the power generation performance of the thermoelectric power system.

Fig.11.Effectoflocalheatsource size on the thermoelectric powergeneration using a heat pipe.

4.Conclusions

A gravitational flat-plate heat pipe is designed and fabricated in this paper to serve as a heat spreader to diffuse the local heat source to the hot-side surface of the thermoelectric power module.The temperature uniformity of the flat-plate heat pipe is experimentally identified and discussed as compared with the metal plate.In addition,the performance of the thermoelectric power generation system using the flatplate heat pipe is experimentally evaluated and compared with that using the metal plate.The in fluence of the heat load,inclination angle,and heat source size on the power generation performance is also analyzed.The main conclusions are drawn as follows:

(1)The gravitational flat-plate heat pipe has considerable advantages over the metal plate in the temperature uniformity when exposing to a local heat source.The thermal response time of the heat pipe is shorter than the metal plate.The temperature uniformity capability of the heat pipe becomes stronger as the heat load increases.

(2)The role of the heat spreader on the power generation performance of the thermoelectric power system is demonstrated.Although the mean temperatures of the hot side are almost the same,the output power of thermoelectric power module using the flat-plate heat pipe is far greater than when using the metal plate.

(3)The gravitational flat-plate heat pipe shows good adaptability for the placement mode and local heat source size.The output power of thermoelectric power module does not change with the inclined angle of the heat pipe.When the diameter of electricalheating rodsforthe heatpipe islargerthan 40 mm,the output power of the thermoelectric power module is insensitive to the local heat source size.These inherent advantages are beneficial to the application of the gravitational flat-plate heat pipe in thermoelectric power generation.