- Email: [email protected]
SOLAR POND HEAT REMOVAL USING A SUBMERGED HEAT EXCHANGER
SOLAR POND HEAT REMOVAL USING A SUBMERGED HEAT EXCHANGER
J. R. Hull, A. B. Scranton, and K. E. Kasza Argonne National Laboratory Argonne, IL 60439
ABSTRACT The results of operating a submerged plastic-tube heat exchanger in the 1080 m Research Salt Gradient Solar Pond at Argonne National Laboratory are presented. The heat exchange system simulates grain drying, in which the temperature of the ambient air is raised 8-15°C when it passes through a liquid-to-air heat exchanger tfrft is coupled to the submerged heat exchanger. Approximately 1.0x10 J (100 MMBtu) were extracted from the pond, and the operation had no adverse effects on the stability of the salt gradient. KEYWORDS Solar pond; salt gradient; grain drying; heat exchanger; solar collector; natural convection; stratified fluid. INTRODUCTION 2
A 1080 m Research Salt Gradient Solar Pond (RSGSP) has been operated at Argonne National Laboratory (ANL) since 1980 [1]. In 1984 a heat extraction system was constructed for the ANL RSGSP to simulate grain drying, in which the temperature of the ambient air is raised approximately 8-15°C. Ambient air is blown across a conventional fin-tube liquid-to-air heat exchanger to produce the heated air. The liquid side of this heat exchanger leads to two plastic-tube heat exchanger mats submerged in the solar pond. An ethylene glycol/water solution is pumped through the submerged mats and the liquidto-air heat exchanger. Fig. 1 shows a schematic of the complete system. This paper reports the first use of a plastic-tube heat exchanger in an operating solar pond. A key advantage of the inexpensive and electrically nonconductive plastic tubes is that there is no corrosion due to chemical reaction of the tubes with the salt water, but care must be taken that the plastic chosen can survive the temperatures encountered in the solar pond. In addition to the work reported here, plastic heat exchangers for solar ponds have been studied by Pacetti and Principi [2]· 1505
1506
INTERSOL 85 HEAT EXTRACTION SYSTEM
The submerged heat exchanger consists of two 25 m by 2 m mats. Each mat is headed by CPVC inlet and outlet pipes located above the pond berm, and each contains 23 polypropylene heat exchange tubes. Each tube is 45.7 m long, with inner diameter of 0.94 cm and wall thickness of 0.16 cm. Each tube is connected to the inlet header via a plastic joint, runs about half of its length into the pond, then returns, where its exit end is connected to the outlet header via another plastic joint. The first and the last 6 m of each tube pass through the gradient zone (GZ), leaving 34 m that lie horizontally at the top of the heat storage zone (HSZ), about 1.0 m above the pond bottom. The tubes passing through the gradient zone are insulated in separate inlet and outlet bundles. The headers of a commercial submerged heat exchanger would probably be located in the HSZ. Polyethylene would be preferred to polypropylene for the material of the submerged heat exchanger, due to its higher thermal conductivity. The thermal conductivity of polypropylene ranges from 0.084 to 0.173 Wm~*°C~ , whereas the value for polyethylene is 0.33 to 0.42 Wm C , and that for high density polyethylene is 0.46 to 0.52 Wm °C [3]. Past experience in the U.S. indicated that high density polyethylene would have reliability problems at high solar pond temperatures, however Pacetti and Principi [2] indicate that a high density polyethylene pipe, which appears to be able to withstand high solar pond temperatures, has recently become available in Europe. The air handling system consists of a 3 HP centrifugal blower, a panel of filters, a copper-tube, aluminum-finned liquid-to-air heat exchanger, and associated ductwork. The average air velocity through the 0.51 m wide by 1.24 m high exit duct was measured to be 5.92 m/sec. On the liquid side of the system, a 2 HP pump delivers a flowrate of 2.3 kg/s (36 gpm). Costs for the heat exchange system are shown in Table 1. Shipping, design, and installation costs are not included. The cost of the submerged heat exchanger was estimated at $0.21 per linear m of tube.
n
Surge Tank
i
Air Flow
T denotes thermocouple F denotes flow sensor
Fig. 1.
Schematic of the heat extraction system.
INTERSOL 85
1507
Table 1 Heat Exchange System Costs (in 1984 U.S. $) Submerged HX (est) Pump Filter Hoses, Valves, & Insulation Liquid-to-Air HX Blower Ductwork Electrical Total
$4500
750 500 500 900
1150 2000
500
$10,800
EXPERIMENTAL RESULTS The heat extraction system was operated for short periods of time during Aug. and Sept. 1984 to evaluate performance factors and to investigate effects of the heat extraction operation on solar pond stability. The effects of this operation are visible in the HSZ temperature history plotted in Fig. 2, where the usual small temperature increase in Aug. became a relatively sharp decrease instead. No detrimental effects on gradient zone stability were observed during these tests. In Oct. 1984, the submerged heat exchange system was operated continuously to simulate grain drying. During this period approximately 1.0 xlO J (100 MMBtu) were extracted from the pond. The heat extraction could easily have continued into Nov. and Dec., but at a slightly reduced rate from the approximately 50 kW of Oct. Fig. 3 shows temperature profiles of the solar pond taken before, during, and after the period of continuous heat extraction. The Sept. profile was taken before heat extraction commenced, the Oct. profile was taken during the middle of the heat extraction period, and the Nov. profile was taken at the very end of the extraction period. As evident from Fig. 4, heat extraction established a strong temperature stratification in the HSZ (lower 1.0 m) and lower portion of the GZ.
80 70
Ü
80 H
Φ 3 50 H CO φ 40
o. E
,Φ 30 20 10 1960 Fig. 2.
1981
1982
1983
1984
HSZ temperature history of ANL RSGSP.
1508
INTERSOL 85
The temperature stratification in the GZ is caused by the rapid decrease in temperature at the level of the heat exchanger. Part of the thermal energy that was stored in the GZ is then conducted downward to the heat exchanger, effectively increasing the size of the HSZ. With both salinity and temperature acting to stabilize the gradient, there is no chance for instability to occur during the submerged heat extraction process. The stratification below the heat exchanger is due to natural convection in the HSZ. As the heat exchange tubes pick up heat, the adjacent pond fluid cools The relatively cold plume of and falls toward the bottom of the pond. descending fluid spreads out over the bottom of the pond, producing the stratification. The resulting colder temperature at the bottom of the pond is beneficial by reducing ground heat loss. During and after the heat extraction period, the temperature of the ground below the pond was warmer than the bottom of the pond, and some of the heat that had been lost to the ground earlier in the year was returned to the pond. This was verified by temperature measurements from thermocouples located under the pond. The influx of heat was responsible for the very small drop in storage zone temperature in Nov. and Dec. As shown is Fig. 4, the effectiveness of the liquid side of the heat exchanger decreased with time. Examination of the heat exchanger after the experiment revealed that minimal fouling had occurred and that the insula tion did not deteriorate. It is believed that the thermal stratification in the HSZ caused a decrease in the natural convection heat transfer coeffi cient of the mats. Based on the scale of an individual tube, one would expect stratification to have little impact [4]. Therefore, the appropriate length scale for this problem appears to be the width of the mats. It was noticed that even when the pump was turned off, some heat was transferred from the HSZ to the surface through the heat exchange tubes. The amount of heat lost from the HSZ in this way was estimated using a correlation for the Nusselt number [5]. The calculated heat transfer was on the order of 10 W. The heat transfer from the HSZ to the surface due to conduction through the GZ, on the other hand, is on the order of 10 kW. Because the heat loss due to the heat exchange tubes is only about 0.1 percent of the total heat loss, it is of no major concern.
■ ■■
«
■
v·
i■■
■■■
• · · ·
·:
,i t
'
* .■
■
" ^- ^· ^ \^ ^ ■
>^
■
\
·
\
.5>1
x©
'/
10
20
■■ i ■ · ■
SEPT. 11 OCT. 22 ; NOV. 4 ■ ■ ■ ■ DEC. 2 8 -
·- 7>
l
Fig. 3.
■■ ' ■ ■
-~-^
30
\
'
·
^ V .
·
^
:
\ ) -
V1 ····
/ I1 · ·
■■ ··
■ ·
40
50
Temperature (C)
60
70
Temperature profiles of the ANL RSGSP.
1509
INTERSOL 85 0.6 0.5 5
Φ
0.4
e
ai 0.2 0.1 0.0 0
5
10
15
20
25
30
Time (days) Fig. 4.
Effectiveness of submerged heat exchanger.
In the unoptlmized heat extraction system in the ANL RSGSP, approximately 3 kW of electrical energy was needed to run the pumps and blower to obtain 60 kW of thermal energy. A COP of 20 is a reasonable number to assume for solar pond operation to produce heated air. A COP of 60 would be typical in the production of hot water. ACKNOWLEDGEMENTS This work was funded by the 111. Dept. of Energy and Natural Resources. The ANL RSGSP was built by funds obtained from the U.S. DOE under Contract W-31109-ENG-38. Salt for the RSGSP was donated by the Morton Salt Co. The authors are grateful to C. MacCracken of the Calmac Manufacturing Corp., Englewood, New Jersey, for donating the submerged heat exchanger. A. Mele, F. Piotrowski, M. Mattox, J. Mehta, and B. Makkinejad are acknowledged for help in the fabrication, installation, and testing of the heat exchangers. Gratitude is also extended to D. Voss for help in preparing the manuscript. REFERENCES 1.
J. R. Hull, Y. S. Cha, W. T. Sha, and W. W. Schertz, "Construction and First Year's Operational Results of the ANL Research Salt Gradient Solar Pond." Proc. Am. Solar Energy Soc., pp. 197-202, Houston (1982).
2.
M. Pacetti and P. Principi, "Operation and Preliminary Data of a Small Experimental Solar Pond." Report WP/SP-3, Univ. di Ancona, Italy (1983).
3.
C. A. Harper, Handbook of Plastics and Elastomers, McGraw-Hill (1975).
4.
C. C. Chen and R. Eichhorn, "Natural Convection from Spheres and Cylinders Immersed in a Thermally Stratified Fluid." ASME J. Heat Trans. 101, 566-568 (1979).
5.
D. K. Edwards and I. Catton, "Prediction of Heat Transfer by Natural Convection in Closed Cylinders Heated From Below." Int. J. Heat Mass Trans. J_2^, 23-30 (1969).
J. R. Hull, A. B. Scranton, and K. E. Kasza Argonne National Laboratory Argonne, IL 60439
ABSTRACT The results of operating a submerged plastic-tube heat exchanger in the 1080 m Research Salt Gradient Solar Pond at Argonne National Laboratory are presented. The heat exchange system simulates grain drying, in which the temperature of the ambient air is raised 8-15°C when it passes through a liquid-to-air heat exchanger tfrft is coupled to the submerged heat exchanger. Approximately 1.0x10 J (100 MMBtu) were extracted from the pond, and the operation had no adverse effects on the stability of the salt gradient. KEYWORDS Solar pond; salt gradient; grain drying; heat exchanger; solar collector; natural convection; stratified fluid. INTRODUCTION 2
A 1080 m Research Salt Gradient Solar Pond (RSGSP) has been operated at Argonne National Laboratory (ANL) since 1980 [1]. In 1984 a heat extraction system was constructed for the ANL RSGSP to simulate grain drying, in which the temperature of the ambient air is raised approximately 8-15°C. Ambient air is blown across a conventional fin-tube liquid-to-air heat exchanger to produce the heated air. The liquid side of this heat exchanger leads to two plastic-tube heat exchanger mats submerged in the solar pond. An ethylene glycol/water solution is pumped through the submerged mats and the liquidto-air heat exchanger. Fig. 1 shows a schematic of the complete system. This paper reports the first use of a plastic-tube heat exchanger in an operating solar pond. A key advantage of the inexpensive and electrically nonconductive plastic tubes is that there is no corrosion due to chemical reaction of the tubes with the salt water, but care must be taken that the plastic chosen can survive the temperatures encountered in the solar pond. In addition to the work reported here, plastic heat exchangers for solar ponds have been studied by Pacetti and Principi [2]· 1505
1506
INTERSOL 85 HEAT EXTRACTION SYSTEM
The submerged heat exchanger consists of two 25 m by 2 m mats. Each mat is headed by CPVC inlet and outlet pipes located above the pond berm, and each contains 23 polypropylene heat exchange tubes. Each tube is 45.7 m long, with inner diameter of 0.94 cm and wall thickness of 0.16 cm. Each tube is connected to the inlet header via a plastic joint, runs about half of its length into the pond, then returns, where its exit end is connected to the outlet header via another plastic joint. The first and the last 6 m of each tube pass through the gradient zone (GZ), leaving 34 m that lie horizontally at the top of the heat storage zone (HSZ), about 1.0 m above the pond bottom. The tubes passing through the gradient zone are insulated in separate inlet and outlet bundles. The headers of a commercial submerged heat exchanger would probably be located in the HSZ. Polyethylene would be preferred to polypropylene for the material of the submerged heat exchanger, due to its higher thermal conductivity. The thermal conductivity of polypropylene ranges from 0.084 to 0.173 Wm~*°C~ , whereas the value for polyethylene is 0.33 to 0.42 Wm C , and that for high density polyethylene is 0.46 to 0.52 Wm °C [3]. Past experience in the U.S. indicated that high density polyethylene would have reliability problems at high solar pond temperatures, however Pacetti and Principi [2] indicate that a high density polyethylene pipe, which appears to be able to withstand high solar pond temperatures, has recently become available in Europe. The air handling system consists of a 3 HP centrifugal blower, a panel of filters, a copper-tube, aluminum-finned liquid-to-air heat exchanger, and associated ductwork. The average air velocity through the 0.51 m wide by 1.24 m high exit duct was measured to be 5.92 m/sec. On the liquid side of the system, a 2 HP pump delivers a flowrate of 2.3 kg/s (36 gpm). Costs for the heat exchange system are shown in Table 1. Shipping, design, and installation costs are not included. The cost of the submerged heat exchanger was estimated at $0.21 per linear m of tube.
n
Surge Tank
i
Air Flow
T denotes thermocouple F denotes flow sensor
Fig. 1.
Schematic of the heat extraction system.
INTERSOL 85
1507
Table 1 Heat Exchange System Costs (in 1984 U.S. $) Submerged HX (est) Pump Filter Hoses, Valves, & Insulation Liquid-to-Air HX Blower Ductwork Electrical Total
$4500
750 500 500 900
1150 2000
500
$10,800
EXPERIMENTAL RESULTS The heat extraction system was operated for short periods of time during Aug. and Sept. 1984 to evaluate performance factors and to investigate effects of the heat extraction operation on solar pond stability. The effects of this operation are visible in the HSZ temperature history plotted in Fig. 2, where the usual small temperature increase in Aug. became a relatively sharp decrease instead. No detrimental effects on gradient zone stability were observed during these tests. In Oct. 1984, the submerged heat exchange system was operated continuously to simulate grain drying. During this period approximately 1.0 xlO J (100 MMBtu) were extracted from the pond. The heat extraction could easily have continued into Nov. and Dec., but at a slightly reduced rate from the approximately 50 kW of Oct. Fig. 3 shows temperature profiles of the solar pond taken before, during, and after the period of continuous heat extraction. The Sept. profile was taken before heat extraction commenced, the Oct. profile was taken during the middle of the heat extraction period, and the Nov. profile was taken at the very end of the extraction period. As evident from Fig. 4, heat extraction established a strong temperature stratification in the HSZ (lower 1.0 m) and lower portion of the GZ.
80 70
Ü
80 H
Φ 3 50 H CO φ 40
o. E
,Φ 30 20 10 1960 Fig. 2.
1981
1982
1983
1984
HSZ temperature history of ANL RSGSP.
1508
INTERSOL 85
The temperature stratification in the GZ is caused by the rapid decrease in temperature at the level of the heat exchanger. Part of the thermal energy that was stored in the GZ is then conducted downward to the heat exchanger, effectively increasing the size of the HSZ. With both salinity and temperature acting to stabilize the gradient, there is no chance for instability to occur during the submerged heat extraction process. The stratification below the heat exchanger is due to natural convection in the HSZ. As the heat exchange tubes pick up heat, the adjacent pond fluid cools The relatively cold plume of and falls toward the bottom of the pond. descending fluid spreads out over the bottom of the pond, producing the stratification. The resulting colder temperature at the bottom of the pond is beneficial by reducing ground heat loss. During and after the heat extraction period, the temperature of the ground below the pond was warmer than the bottom of the pond, and some of the heat that had been lost to the ground earlier in the year was returned to the pond. This was verified by temperature measurements from thermocouples located under the pond. The influx of heat was responsible for the very small drop in storage zone temperature in Nov. and Dec. As shown is Fig. 4, the effectiveness of the liquid side of the heat exchanger decreased with time. Examination of the heat exchanger after the experiment revealed that minimal fouling had occurred and that the insula tion did not deteriorate. It is believed that the thermal stratification in the HSZ caused a decrease in the natural convection heat transfer coeffi cient of the mats. Based on the scale of an individual tube, one would expect stratification to have little impact [4]. Therefore, the appropriate length scale for this problem appears to be the width of the mats. It was noticed that even when the pump was turned off, some heat was transferred from the HSZ to the surface through the heat exchange tubes. The amount of heat lost from the HSZ in this way was estimated using a correlation for the Nusselt number [5]. The calculated heat transfer was on the order of 10 W. The heat transfer from the HSZ to the surface due to conduction through the GZ, on the other hand, is on the order of 10 kW. Because the heat loss due to the heat exchange tubes is only about 0.1 percent of the total heat loss, it is of no major concern.
■ ■■
«
■
v·
i■■
■■■
• · · ·
·:
,i t
'
* .■
■
" ^- ^· ^ \^ ^ ■
>^
■
\
·
\
.5>1
x©
'/
10
20
■■ i ■ · ■
SEPT. 11 OCT. 22 ; NOV. 4 ■ ■ ■ ■ DEC. 2 8 -
·- 7>
l
Fig. 3.
■■ ' ■ ■
-~-^
30
\
'
·
^ V .
·
^
:
\ ) -
V1 ····
/ I1 · ·
■■ ··
■ ·
40
50
Temperature (C)
60
70
Temperature profiles of the ANL RSGSP.
1509
INTERSOL 85 0.6 0.5 5
Φ
0.4
e
ai 0.2 0.1 0.0 0
5
10
15
20
25
30
Time (days) Fig. 4.
Effectiveness of submerged heat exchanger.
In the unoptlmized heat extraction system in the ANL RSGSP, approximately 3 kW of electrical energy was needed to run the pumps and blower to obtain 60 kW of thermal energy. A COP of 20 is a reasonable number to assume for solar pond operation to produce heated air. A COP of 60 would be typical in the production of hot water. ACKNOWLEDGEMENTS This work was funded by the 111. Dept. of Energy and Natural Resources. The ANL RSGSP was built by funds obtained from the U.S. DOE under Contract W-31109-ENG-38. Salt for the RSGSP was donated by the Morton Salt Co. The authors are grateful to C. MacCracken of the Calmac Manufacturing Corp., Englewood, New Jersey, for donating the submerged heat exchanger. A. Mele, F. Piotrowski, M. Mattox, J. Mehta, and B. Makkinejad are acknowledged for help in the fabrication, installation, and testing of the heat exchangers. Gratitude is also extended to D. Voss for help in preparing the manuscript. REFERENCES 1.
J. R. Hull, Y. S. Cha, W. T. Sha, and W. W. Schertz, "Construction and First Year's Operational Results of the ANL Research Salt Gradient Solar Pond." Proc. Am. Solar Energy Soc., pp. 197-202, Houston (1982).
2.
M. Pacetti and P. Principi, "Operation and Preliminary Data of a Small Experimental Solar Pond." Report WP/SP-3, Univ. di Ancona, Italy (1983).
3.
C. A. Harper, Handbook of Plastics and Elastomers, McGraw-Hill (1975).
4.
C. C. Chen and R. Eichhorn, "Natural Convection from Spheres and Cylinders Immersed in a Thermally Stratified Fluid." ASME J. Heat Trans. 101, 566-568 (1979).
5.
D. K. Edwards and I. Catton, "Prediction of Heat Transfer by Natural Convection in Closed Cylinders Heated From Below." Int. J. Heat Mass Trans. J_2^, 23-30 (1969).