Experimental Analysis of Influencing Factors on Heat Exchange Efficiency of Cast-in Place Energy Piles

CHEN Zhi XIAO Yu XIAO HenglinQUE Mengke(闕夢珂)SUN Yang

School of Civil Engineering,Architecture and Environment,Hubei University of Technology,Wuhan 430068,China

Abstract:The energy pile is a kind of building energy-saving technology using shallow geothermal energy,and its heat exchange performance is an important factor affecting its development. In this study,heat exchange tests were carried out on three full-size cast-in place energy piles,and the influence of various factors on the heat exchange amount and the heat exchange rate was analyzed. The following conclusions are drawn:(1) the heat exchange rate of the cast-in place energy pile is greatly affected by the inlet water temperature;(2) increasing the pile length can increase the heat exchange amount,but has little effect on improving the heat exchange rate;(3) the increase in the heat exchange amount by heat exchange pipes in series is not significant,and therefore the parallel-type heat exchange pipes should be considered in practical engineering;(4) the appropriate circulating water flow velocity gives the best heat exchange efficiency.

Key words:energy pile;cast-in place;heat exchange amount;heat exchange rate

Introduction

Energy piles are used in the process of pile foundation construction.The ground heat exchangers (GHE) in the ground source heat pump (GSHP) heat exchange system are embedded in the pile foundation[1-2],and heat exchange between the pile foundation and the shallow ground can be achieved by the circulating heat exchange medium flow in the heat exchanger[3-6].The energy pile not only has the advantages of traditional ground source heat pump technology,but also overcomes the two main disadvantages in the widespread use of GSHP technology:the land area required and the high cost of drilling[7-10].

With increasing attention being given to energy piles,how to achieve the best heat exchange performance for these piles is one of the core competitiveness factors for promoting this technology.At present,researchers have carried out certain experimental and theoretical analyses on the heat exchange rate of energy piles[11-13].Ingersoll[14]proposed that heat exchange models were suitable for vertical buried pipe heat exchangers based on Kelvin linear heat-source theory.Kavanaugh[15]combined the structural characteristics of the energy pile to establish a cylindrical heat-source model to solve the average temperature and the inlet and outlet temperature of the heat exchange medium in the heat exchange pipe.Based on existing models，Diaoetal.[16]proposed a two-dimensional heat-conduction model and a quasi-three-dimensional heat-exchange model for the heat exchange process of a vertical heat exchanger and described practical engineering applications.

Although existing models have been used in engineering design,some deficiencies still remain,such as the treatment of boundary conditions,complex buried pipe types in large pile foundations,and variable formation conditions.The field test of energy pile is the best way to reflect the real heat exchange rate.Many researchers[17-20]carried out the field heat exchange rate test of energy piles in combination with theoretical analysis.Table 1 summarizes the in-situ tests on shallow geothermal energy piles.Among them,cast-in place piles are a common type of pile foundation[17,19-21].Concrete or steel is used as backfill materials for heat exchange pipes because of its high compactness,better contact with earth,low contact thermal resistance,high heat transfer and high exchange rate[22-25].Therefore,the cast-in place energy pile has superior heat exchange performance[26-28].However,due to time/labor consumption of filed test,the different pile types and pile lengths,heat exchange conditions and complex buried pipe types in large pile foundations,the test results differ greatly,and the understanding of the influencing factors of heat exchange performance of energy piles is inconsistent,which needs to be further revealed.

Table 1 In-situ tests on shallow geothermal energy piles

In this paper,the influence of various factors on the heat exchange rate of energy piles is analyzed through heat exchange tests of full-size cast-in place energy piles.

1 Experiments

1.1 Test base

The energy pile test base is located at Hubei University of Technology,Wuhan,China.Table 2 shows the soil type and soil mechanical properties of test the base.The topographical composition of the test site is filled soil,clay,silty clay,and strongly to moderately weathered sandy mudstone from top to bottom,in which the top layer of mudstone is about 24 m,and the area surrounding the test site is all built up with low-rise buildings.This early construction had no obvious influence on the site.The test base contains three energy piles and five GSHP holes;the energy piles are cast-in place piles and five GSHP holes are 80 m in depth on average and 150 mm in diameter.Heat exchange tests were carried out on energy piles Nos.1,2 and 3,with pile diameters of 800 mm and pile lengths of 23,25 and 18 m,respectively.Figure 1 shows the construction process of energy piles,including U-shaped heat exchange pipe preparation,the manufacture of reinforcing cage,steel cage hoisting,pipe binding,pile foundation concrete pouring and pile forming.

Table 2 Physical and mechanical indices of soils

(a)

(b)

(c)

(d)

(e)

(f)

Fig.1 Construction process of energy pile:(a) U-shaped heat exchange pipe preparation;(b) the manufacture of reinforcing cage;(c) steel cage hoisting;(d) pipe binding;(e) pile foundation concrete pouring;(f) pile forming

The buried heat exchange pipes in all three piles is five pairs of single U-shaped pipes,and the heat exchange pipes are PE pipes with an outer diameter of 32 mm and a wall thickness of 3 mm.Figure 2 shows the cross section of the buried pipes in the energy pile heat exchange pipe,in which 1 and 2 are respectively the inlet and outlet of a single U-shaped (referred to U(L)-shaped) pipe with a pitch of 700 mm,3 and 4 are respectively the inlet and outlet of a single U-shaped (referred to U(S)-shaped) pipe with a pitch of 220 mm,and the W-shaped buried pipe is a series connection of a single U(L)-shaped pipe and a single U(S)-shaped pipe.To test the influence of different buried pipe types on heat exchange in the pile foundation,an energy pile heat exchange pipe set of a water collector and distributor was produced,as shown in Fig.3.The device can adjust the series and parallel connection of single or multiple U-shaped pipes according to the switch setting of the valve.

1.2 Test plan

This experiment considers the influence of inlet circulating water temperature,buried pipe type and pitch,pile length,circulating water flow velocity and other factors on the heat exchange rate of energy piles.Table 3 shows the heat exchange test scheme for energy piles.The different inlet water temperature conditions are carried out by the ①-② tests.The different pile length conditions are carried out by the ③-⑤ tests.The different flow velocity conditions are tested by the ①,⑥-⑧ tests.The different buried pipe types and pitch conditions are carried out by the ④,⑨-⑩ tests.The running time of each set is 24 h,and the interval between each operating condition is 48 h.

Fig.2 Schematic diagram of cross section of the buried pipes in the energy pile heat exchange pipe

Fig.3 Energy pile heat exchange pipe water collector and distributor

The heat exchange test was carried out by a HGNY-03 geotechnical thermal response meter (Ground Source Heat Pump Research Institute,Huazhong University of Science and Technology,Wuhan,China).The structure is shown in Fig.4.The inlet and outlet water pipes of the tester are connected with the inlet and outlet water pipes of the energy pile.Driven by the water pump,the circulating water exchanges heat with the underground rock and soil,and the circulating water flow velocity can be controlled by adjusting the valve.The tester can use the constant heating power to heat the circulating water,and the temperature sensor automatically reads the inlet and the outlet water temperatures according to the different heating powers to achieve the corresponding inlet temperature.

Table 3 Heat exchange test scheme of energy piles

Note:Test ① U(L)+2.0 kW+0.9 m3/h represents U (L)-shaped pipe,a heating power of 2.0 kW and a flow velocity of 0.9 m3/h.

Fig.4 Structure of the geothermal response instrument

1.3 Geotechnical thermal response test

The constant heat-flow method is recommended for the geotechnical thermal response test used in this research.The circulating water is heated by a constant heating power in the internal heater of the instrument,and the water temperature and the circulating water flow velocity in the inlet and outlet of the heat exchange pipe are recorded.The heat of the circulating water in the heat exchange pipe is exchanged with that of rock and soil,and the temperature of the circulating water in the heat exchange pipe changes over time until the temperature is basically balanced.Using the inlet temperature of circulating water,the outlet temperature of circulating water and the circulating water flow velocity,the heat exchange amount and the borehole’s heat exchange rate can be calculated as[29]

Q=ρwνCw(Tin-Tout),

(1)

q=Q/h,

(2)

whereQis the heat exchange amount,W;νis the circulating water flow rate,m3/h;ρwis the fluid density,assumed as 1 000 kg/m3;Cwis the heat capacity of the circulating water,assumed as 4.2×103J/(kg·℃);Tin(Tout) is inlet (outlet) temperature of the circulating water,℃;qis the average heat exchange rate,W/m;andhis the pile length,m.

2 Results and Discussiom

2.1 Inlet temperature

Adjusting the various heating powers can change the inlet water temperature of the heat exchange pipes.Figure 5 shows a comparison of inlet and outlet water temperatures for the tests ① and ② in No.1 pile as a function of time.

Fig.5 Curves of inlet and outlet temperatures with time under different heating powers

Figure 5 shows that in the heat exchange process,the inlet and outlet water temperatures are continuously increased,and the temperature difference between the inlet and outlet tends to be stable after about 4 h.The inlet temperatures were 36.68 ℃ and 42.60 ℃ for tests ① and ② respectively,and the outlet temperatures were 34.67 ℃ and 39.56 ℃ for tests ① and ② respectively.

According to Eqs.(1) and (2),the heat exchange amount and the heat exchange rate under different heating powers can be calculated as shown in Fig.6.When the heating power increased from 2.0 kW to 3.5 kW,the inlet temperature increased by 5.92 ℃,the inlet and the outlet water-temperature difference increased from 2.01 ℃ to 3.04 ℃ (a total increase of 1.03 ℃),and the heat exchange rate increased by 51.2%.Clearly,as the inlet temperature increases,the heat exchange rate of heat exchange surface increases correspondingly,and the heat exchange efficiency of the energy pile also increases.

Fig.6 Comparison of heat exchange amount and heat exchange rate under different heating powers

2.2 Pile length

Heat exchange tests under three of the operating conditions in Table 3 (tests ③,④ and ⑤) were carried out,and the influence of different pile lengths on heat exchange rate was analyzed.Figure 7 shows the curves of inlet and outlet temperatures with time.

Fig.7 Curves of inlet and outlet temperatures with time under different pile lengths

Figure 7 clearly shows that in the heat exchange process.The variation law of inlet and outlet water temperatures for the three operating conditions is similar to that of the operating conditions described earlier.Under the same heating power and flow rate,the length of the pile body affects the total amount of fluid in the heat exchange pipe and at the same time has a certain influence on the inlet temperature.

The length of pile in the test ⑤ was 18 m.The inlet temperature of test ⑤ was higher than that of tests ③ and ④ in the whole heat exchange process.The temperature differences between the inlet and the outlet for tests ③,④ and ⑤ were 1.99,2.20,and 1.64 ℃,respectively.The longer the pile,the larger difference between the inlet temperature and the outlet one was.

According to Eqs.(1) and (2),the heat exchange amount and the heat exchange rate under three different pile lengths were calculated.The results are shown in Fig.8.When the pile length increased from 18 m to 25 m,the difference between the inlet temperature and the outlet temperature gradually increased,and the heat exchange amount of the energy pile also gradually increased.This indicates that the longer the pile,the more sufficient the heat exchange amount with the fluid medium will be,and the greater the total heat exchange amunt will be.There was little difference in the heat exchange rate for the three pile-length conditions,with the maximum occurring with a pile length of 18 m,and the minimum with a pile length of 23 m.The heat exchange medium was more stable in the deep part of the pile foundation.As pile length increased,the heat exchange amount was theoretically higher.The heat exchange results for No.1 pile with a pile length of 23 m and No.2 pile with a pile length of 25 m can be verified.However,the influence of the pile length on the heat exchange rate was limited.Although the pile length of No.3 pile was the shortest,the inlet temperature was the highest,and the heat exchange rate was also the highest,indicating that the influence of inlet temperature on heat exchange efficiency is more significant than that of pile length.

Fig.8 Comparison of heat exchange amount and heat exchange rate under different pile lengths

2.3 Buried pipe type and pitch

Heat exchange tests of three operating conditions in Table 3 (tests ④,⑨ and ⑩) were carried out,and the influence of different buried pipe types and pitches on the heat exchange rate was analyzed.Figure 9 shows the curves of inlet and outlet temperatures under different buried pipe types and pitches.

Fig.9 Curves of inlet and outlet temperatures under different buried pipe types and pitches

Figure 9 showed that in the heat exchange process,the inlet and the outlet temperature trends under the three tests were the same,and that the difference between the inlet temperature and the outlet temperature became stable after about 4 h.The change laws of tests ④ and ⑨ were almost the same,and the difference between the inlet temperature and the outlet temperature was only 0.20 ℃.After the pipe in the test ⑩ was heated for 12 h,the inlet temperature was 33.98 ℃,which was about 3.40 ℃ lower than that in tests ④ and ⑨,but the difference between the inlet temperature and the outlet temperature in test ⑩ was 2.60 ℃,which was about 0.40 ℃ higher than that in tests ④ and ⑨.

According to Eqs.(1) and (2),the heat exchange amount and the heat exchange rate under three buried pipe types and pitches were calculated.The calculation results are shown in Fig.10.When the buried pipe type was U(L)-shaped or U(S)-shaped,the calculated heat exchange amounts and the heat exchange rates were almost the same,but the values for the U(L)-shaped pipe were slightly higher,indicating that when the pipe pitch was greater than 220 mm,the values rose.Afterwards,pipe pitch had less influence on the heat exchange rate.When the buried pipe type was W-shaped,the heat exchange amount increased by 14.7% and 15.4% compared with U(L)-shaped and U(S)-shaped pipes,respectively.This showed that when the heat exchange path of the circulating medium in the pipe became longer,the heat exchange was more sufficient,and a higher heat exchange amount could be obtained.The W-shaped buried pipe consisted of a single U(L)-shaped and a single U(S)-shaped pipes connected in series,meaning that its heat exchange path was double that of each pipe type.Although the inlet water temperature under this operating condition was lower than that under tests ④ and ⑨,its heat exchange rate did not increase in the same proportion,and its percentage growth was less than 20%.This test suggested that a multi-U buried pipe in a large-diameter cast-in place energy pile was an acceptable way to obtain higher heat exchange rate.

Fig.10 Comparison of heat exchange amount and heat exchange rate under different buried pipe types and pitches

2.4 Circulating water flow velocity

Heat exchange testing of four operating conditions in Table 3 (tests ①,⑥,⑦ and ⑧) was carried out,and the influence of different circulating water flow velocities on heat exchange efficiency was analyzed.Figure 11 shows the inlet and the outlet temperature curves over time.The flow rates of the heat exchange pipes were set at 0.5,0.7,0.9 and 1.1 m3/h,respectively,which translated into flow velocities of 0.26,0.37,0.47 and 0.58 m/s,respectively.

Fig.11 Inlet and outlet temperatures over time under different circulating water flow velocities

Figure 11 showed that during the heat-exchange process,the water-temperature trends of the inlet and the outlet under the four operating conditions were the same,the temperature difference between inlet and outlet was stable after about 4 h,and the inlet temperatures were 37.17,37.90,36.68 and 37.80 ℃,respectively.The outlet temperatures were 33.46,34.96,34.67 and 36.26 ℃,respectively,and differences between the inlet and outlet temperature were 3.71,2.83,2.01 and 1.54 ℃,respectively.As the flow velocity increased,the difference between inlet and outlet water temperatures became smaller.

According to Eqs.(1) and (2),the heat exchange amount and the heat exchange rate at four different circulating water flow velocities were calculated.The calculation results are shown in Fig.12.As the flow velocity of the circulating medium in the heat exchange pipe increased,the change in heat exchange amount and the change in heat exchange rate of the energy pile behaved differently from the temperature difference between inlet and outlet water temperatures.The heat exchange amount and the heat exchange rate generally showed a trend that first increased and then decreased.When the circulating medium flow velocity was 0.7 m3/h,the heat exchange amount and the heat exchange rate of the energy pile were the largest.Therefore,increasing the flow velocity of the circulating medium brings about a certain improvement in the heat exchange rate of the energy pile,which is the most obvious when the efficiency rises to a certain value and then gradually decreases.

Fig.12 Comparison of heat exchange amount and heat exchange rate at different circulating water flow velocities

It is possible that the heat exchange between the circulating medium and the surrounding soil is insufficient due to excessive flow velocity under experimental conditions.Therefore,it is suggested that the flow velocity of the circulating medium in the heat exchange pipe should not be too large,and it might be more suitable to set it to about 0.7 m3/h.

3 Conclusions

In this paper,the influence of various factors on heat exchange efficiency has been analyzed through heat exchange tests of three full-size cast-in place energy piles.The conclusions are as follows.

(1) As the inlet temperature increases,the heat exchange rate also increases,and the heat exchange efficiency of the energy pile also increases.

(2) Increasing the pile length can bring about a certain increase in heat exchange rate,but this effect is limited.The influence of inlet temperature on heat exchange rate is more significant than that of pile length.

(3) When the pipe pitch is larger than 220 mm,it has less influence on heat exchange rate.The heat exchange efficiency of a W-shaped buried pipe is better than that of a single U-shaped buried pipe,but the heat exchange rate is improved by less than 20%.It has been suggested that a multi-U buried pipe in a large-diameter cast-in place energy pile is an acceptable way to obtain higher heat exchange efficiency.

(4) There is an optimal range for the influence of the circulating medium flow velocity on the heat exchange rate of the energy pile.If the flow velocity is too high or too low,the heat exchange efficiency will be reduced.It is suggested that the circulating medium flow velocity of the cast-in place energy pile should be set to about 0.7 m3/h.