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Use of waste water heat for supply water heating by use of heat pipe diodes
Vol.2, No. 2, pp. 117-124, 1982. Printedin Great Britain
0198-7593/82/020117--0S$03.00/0 PergamonPressLtd
Heat Recovery Systems
USE O F WASTE WATER HEAT FOR SUPPLY WATER H E A T I N G BY USE O F HEAT PIPE DIODES P. BEHRMANN Dornier System GmbH, Friedrichshafen, W. Germany
H. HJ~FNER Daimler-Benz AG, Stuttgart, W. Germany
and L. SPEITKAMP Tobelhalde 2, 7777 Neufrach, W. Germany Abstraet--A hot supply water storage tank, which is heated by waste water heat, has been developed by Dornier System GmbH under a Daimler-Benz AG contract. Copper heat pipes using water as carrier fluid have been developed for this purpose, which are capable to transfer sufficient heat even at temperatures as low as 10°C. Measurements indicate that up to 90% of the usable waste water heat may be recovered depending upon the waste "water flow rate and on the complexity of the system. Stratification within the tank allows that more than 75~ of the stored energy may be drawn without a significant drop in supply water temperature.
INTRODUCTION
ONE MAJOg problem in the recovery of heat from sources like residential waste water is the fact that the energy to be recovered is produced at very irregular intervals and at varying temperature levels. If the energy were to be recovered by means of conventional heat exchangers, sophisticated control of the waste water circuit would be required to avoid heat being transferred from a hot storage tank to cold waste water. Also the effectivity of these systems is rather low since much low grade heat had to be rejected. A concept has been developed at the Daimler-Benz AG to avoid active control of the recovery system and to allow the recovery of both high grade and low grade energy. The concept makes use of heat pipes for heat transfer from waste water to stored supply water since heat pipes operating in the gravity-assisted mode exhibit a Pronounced diode behaviour. Thus it may be achieved to transport heat "one-way-only", i.e. from the waste water to a storage tank, while almost no heat is transferred the opposite way under any operating mode. Another important feature of this concept was to achieve some sort of temperature stratification within the storage tank and to introduce heat pipes at different temperature levels into the tank. Thus low grade waste heat may pass by the heat pipes reaching into areas of hot supply water unaffected, while effectively transferring its energy to other heat pipes connected to cold supply water. A water storage tank and a system of heat pipes for waste water recovery making use of the concept discussed above has been developed at the Dornier System Company under a Daimler-Benz contract. Measurements have been carried out at Dornier System for confirmation of important system parameters and shall be discussed below. STORAGE TANK AND HEAT PIPES SYSTEM
The storage tank and the heat pipe system are depicted in Fig. 1. The storage tank is a stainless steel vessel containing approx. 5001 of supply water. It is divided into six compartments by stainless steel partitions. Each partition has three 30 mm borings to 117
118
P. BEHRMANNet al.
water outlet
Waste water inlet
roll
I
Supply water inlet Waste water outlet
Fig. 1. Storage tank and heat pipe system.
allow for water flow. Each compartment is fitted with an NW 65 flange for heat pipe insertion. The water inlet and water outlet are at the lowermost and uppermost points of the vessel. The vessel is insulated against heat loss. The complete set-up is clad in a stainless steel jacket. Overall dimensions of the complete tank are: height 2000 mm, dia S50 ram. The heat pipes are made of copper, with water being used as working fluid. The condenser section (length approx. 650 mm) is fitted with lateral copper fins of height 4 mm and thickness 0.3 mm. A stainless steel NW 65 flange is brazed to the heat pipe. The evaporator end (length approx. 1000 mm) bears no fins so as to avoid clogging by dirt carried in by the waste water. Small ends of copper wire are soldered to the surface to act as spacers. The waste water conduit is formed of commercially available plastic tubing as can be seen in Fig. 1. The tubes are slipped onto the heat pipe evaporators to form a ring gap heat exchanger. In the system used for the measurements only five out of six compartments of the storage tank are fitted with heat pipes, while the uppermost compartment is left without a heat pipe. DESCRIPTION OF WATER SUPPLY UNIT AND MEASURING SET-UP The purpose of the measurements to be carried out was chiefly to determine the behaviour of the system under various operating modes and to supply information for numerical calculations carried out at the Daimler-Benz AG. Measurements had to include loading the storage tank with various waste water flow rates, unloading it with various supply water flow rates, determine the steady-state temperature distributions for various operating modes and to analyse the behaviour of the system for alternating waste water flow. Also the diode effect exhibited by the heat pipes had to be tested. To conduct
Use of waste water heat for supply water heating
119
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T7
I
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._1 Fig. 2. Waste water and supply water conduits.
these measurements a test rig has been constructed at Dornier System. This rig is sketched in Fig. 2 and is described below. Two thermostats (1, 2) are used to keep two reservoirs (3, 4) at a desired temperature level. One of these is kept at approx. 80°C, while the other is kept at 20°C. The reservoirs are mounted to the top of a test rack on order to provide the driving pressure for both waste water and supply water flow. Thus the pressure may be kept perfectly constant by simply using an overflow. To simulate the waste water circuit both reservoirs are connected to a three-way valve (5). The line continues via two regulator valves (6,7) and two flowmeters (8, 9) to the waste water inlet. The waste water outlet is connected to another three-way valve (10) which in turn is connected to the thermostats from where the water is again pumped to the reservoirs. The three-way valves are operated to lead either hot or cold water to the waste water heat exchanger. The supply water circuit is made up in a similar way, except that it is connected to the cold reservoir only. Thus both three-way valves are omitted in this circuit. The supply water outlet is connected to a cooler (11) of 25 kW cooling capacity to assure that the cold thermostat will not be overheated by the stored hot water. The flowmeters have been calibrated individually. Additionally the effect of water temperature on the flowmeter reading has been calculated. The relevant temperatures within the system are measured by thirteen NiCr-Ni thermocouples numbered T1 to T13 in Fig. 2. The thermocouple readings are plotted on a 12-channel point printer. Since only twelve thermocouples may be registered on this unit, the thermocouple (13) is omitted in most cases since this temperature is associated to the cold water inlet and is very effectively kept at 20°C. MEASUREMENTS
Heatino of cold storaoe tank In Fig. 3 the temperature distribution within the system is plotted vs time. Starting conditions are: The hot reservoir is kept at its predetermined temperature. No water flow is allowed yet. The storage tank is completely cold.
120
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~
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~,II 500
30 I I000
I I~00 Time
I 2000
I 2500
20 3000
in rains
Fig. 3. Heating of storage tank. Waste water flow rate 36 l/h.
A t = 0 a mass flow of 361/h of hot water is introduced into the waste water conduit. No supply water mass flow is allowed. The temperatures within the waste water conduit are drawn by solid lines. The left temperature scale has to be applied for these lines. It is seen that thermocouple T1 (waste water inlet) reaches a constant value after a fairly short time. Minor fluctuations are due to fluctuations in mass flow, which in turn result in varying heat losses in the waste water conduit. The other waste water temperatures quickly rise to medium temperatures with the differences between those temperatures being proportional to the heat transferred by each heat pipe. Note that the temperature difference between T1 and T2 is extremely high. This is attributed mainly to the fact that the uppermost heat pipe has to supply heat to two compartments. The heat exchange between these two chambers is apparently very effective (heating from below), as temperatures T8 and T9 are basically equal. Thus we have improved convection in the upper chambers resulting in better heat transfer from the uppermost heat pipe. An important result is that for a cold storage tank (start of measurements) the waste water outlet temperature is below 30°C. Thus more than 80% of the energy available has been recovered. As time advances the temperatures T2, T3, T4, T5 and T6 asymptotically approach values slightly lower than T1. This may well be expected, since as the temperature inside the storage tank rises the driving force for heat transfer diminishes. The temperature distribution inside the storage tank is depicted by dashed lines. These lines have been shifted with respect to the solid lines for better reading of the diagram. Thus the temperature scale on the right hand side applies for the dashed lines. The behaviour of the temperature distribution inside the tank is quite as expected, exhibiting a gradually diminishing increase as the waste water temperatures are approached. Thermocouple T7 shows a nonconformative behaviour. This thermocouple is placed at the supply water output and is poorly insulated. Thus it reaches equilibrium with the environment as no supply water flow is given. The fluctuations of temperature are mainly due to the formation of air bubbles at the output as the water within the tank expands. Measurements at higher waste water flow rates exhibit basically the same behaviour, although the starting waste water outlet temperature is increased significantly. Thus the recovery of heat becomes less effective with increasing waste water flow. Therefore
Use of waste water heat for supply water heating
121
Table 1. Results of steady state measurements
Flow rate
mA ffi mB = 721/h m A = mB ffi 180 l/h
mA ffi 1801/h mB ffi 90 l/h
Waste water input temperature
79°C
80°C
71.5°C
Waste water output temperature
40°C
47°C
51°C
Efficiency of energy recovery
75%
59%
51%
Supply water output temperature
56.5°C
47°C
55°C
m A : waste water flow; roB: supply water flow.
further thought should be given to improvement of the ring gap heat exchanger. This may be achieved quite simply by introduction of finned exchanger tubes. However, this solution may necessitate the use of filters to avoid clogging of the heat exchanger.
Steady-state results Measurements have been conducted with the waste water flow being equal to or double the supply water flow. These tests have been continued till steady-state conditions were obtained. Three different flow combinations have been tested. The important results are given in Table 1. The efficiency has been calculated by taking the ratio of the waste water temperature drop to the difference of waste water input temperature and temperature of lowest partition [(T1-T6)/(T1-T12)]. It is found that even for moderate mass flows supply water temperatures are at an acceptably high level. Nevertheless some improvement on the waste water heat exchange seems to be desirable. Alternating waste water flow measurements The reason for these measurements was to test the response of the storage system to changes in waste water temperature. A series of measurements was conducted with a constant supply water flow of 901/h and a constant waste water flow of 1801/h. The waste water temperature, however, has been alternated between 80 and 20°C at regular intervals by operation of the three-way valves. Intervals were taken from 2 rain to 1 h. The results for 1 h intervals are depicted in Fig. 5. The measurement has been started from steady-state conditions with above mentioned flow rates. Please note that the diagram has been transformed from a printer output to a computer-made plot. Since the computer programme uses linear interpolation between supporting points the originally smooth curves are approximated by straight line segments. It has been attempted, however, to bring out the characteristic features of the curves. Cooling of loaded storage tank The results of this test are plotted in Fig. 4. Start-up conditions are: The temperatures inside the tank are above 60°C. No water flow. Only the cold reservoir is operated. At t = 0 a water flow of 360 l/h is introduced into the supply water conduit. The diagram shows that the stratification inside the tank is quite effective as it is clearly seen that a cold front moves slowly through the tank. Each compartment exhibits a sharp onset of cooling as the cold front approaches. The outlet temperature is kept almost constant for approx. 1 h. By this time the storage tank has given off approx. 75% of its energy at the maximum storage temperature. Even when the tank has given off about 95% of its energy the outlet temperature is still at 40°C.
122
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al.
8O 70
,L) 60 .c_ 5O 40
T~
TI2
N so
I00
o!5
I Time in hours
1!5
i 2
Fig. 4. Temperature distribution for unloading of storage tank. Flow rate 360 l/h.
The irregularities at the beginning of the test (thermocouples T12 and T13) are due to start-up problems as the supply water path was blocked by air bubbles which had to be removed by introducing an external water current resulting in overheating of the cold reservoir. Similar results have been obtained for lower supply water flow rates. Heat pipe diode test 'This test was conducted with a loaded storage tank, no supply water flow and a cold waste water flow of 36 l/la. The temperature difference between waste water inlet and outlet indicates a heat loss through the heat pipes of as little as 50 W. As the waste water is switched to cold, the lower portion of the storage tank (lines T11 and T12) loses most of its energy, while the outlet temperature remains almost unaffected. After two cycles the lower portion of the tank stabilizes to an oscillating temperature function which may well be considered steady-state condition. The uppermost partition temperature (T8) starts to drop sharply after approx. 3 h of testing, indicating that the initial cold front has moved through the tank. Note that the temperature gap between T8 and T7 (outlet temperature) behaves quite anticyclicly compared to the waste water cycle, as T8 drops while the tank is heated. Also note that the gap between T8 and T7 closes at regular intervals. Although the outlet temperature did not yet reach steady-state conditions by the end of the test, it may be assumed that an oscillating steady state will be reached eventually, with the outlet temperature oscillating around approx. 40°C. The peak in the T1 curve at the beginning of the heating periods is due to the fact that at these times cold water is brought to the hot thermostat from the cold waste water system, and as the heating power of this thermostat is limited, the input temperature T1 drops. It may be taken from Fig. 5 that the thermal capacity of the storage tank smooths out the sharp changes in waste water temperature quite effectively. By shortening the intervals the supply water outlet temperature may be brought to an almost constant level. For example, if the intervals are set to 30 min, the supply water outlet temperature stabilizes at approx. 48°C, with still some oscillations in the lower part of the tank. If the intervals are further reduced to 15 rain, the temperature distribution within the entire storage tank becomes practically time independent. Going to the shortest intervals, it was found that even for 2 min periods energy was still transferred to the cold storage tank. The equilibrium supply water outlet temperatures, however, become quite low as the energy losses within the thermal capacity of the outer waste water circuit become paramount. However, these losses are a quality of the entire apparatus rather than of the heat pipe system. Therefore a true critical frequency could not be obtained with the test rig.
Use of waste water heat for supply water heating
123
"',
60~--TX__."
~'m+;7
I
][",,
%N.
' - . J J ii
", Joo
150
200 250 Time in rains
300
350
" 400
450
Fig. 5. Temperature distribution for 1 h alternating waste water flow.
ENERGY TRANSFER
The energy transferred to the storage tank from the waste water conduit may be calculated from the temperature differences within the waste water conduit and the waste water flow rates. The valfies obtained for zero supply water flow and various waste water flow rates have been tabulated in Table 2. The efficiency has been calculated taking the ratio of the waste water temperature drop to the difference of waste water input temperature and temperature of lowermost partition [{T1-T6)/(T1-T12)]. We have the interesting result that the efficiency of heat transfer is not dependent upon the storage state. That means that for a given waste water flow a set percentage of the heat theoretically transferable is transferred at each moment of the heating period. It is found that the energy transfer rates are quite impressive, especially for high flow rates. However, as the flow rate increases the effectiveness decreases. It has been stated above that the effectiveness may be increased by improving the waste water heat exchanger. However, it must be anticipated that for very high flow rates we are already close to the maximum heat pipe transfer capacity. The energy transfer rates for steady-state measurements are tabulated in Table 3. We find that between 88 and 93~ of the energy lost by the waste water is recovered within the supply water, depending upon the flow rates. It seems that losses increase with waste water flow. This may well be possible for high flow rates the average temperature of the waste water conduit is increased. The results may be compared to those in Tables 1 and 2. It is found that the energy transfer rates compare quite well, while the efficiency appears to be slightly reduced for steady-state calculations. Table 2. Energy transfer to storage tank with zero supply water flow Waste water flow 36 I/h* 72 l/h* 180 l/h* 360 l/h* 36 l/ht 180 l/h? 360 l/ht
Energy transferred to one heat pipe Total 0.88 kW 1.97 kW 2.93 kW 3.77 kW 0.17 kW 1.58 kW 1.46 kW
1.9 kW 4.0 kW 6.9 kW 11.3 kW 0.54 kW 4.07 kW 3.77 kW
Efficiency 90% 81~ 60~ 44~ 90~ 60°/0 45~
* Storage tank completely cold. t Storage tank hot. Temperature of lowermost partition _>60°C.
124
P.
BEHRMANNet
a[.
Table 3. Energy balance of storage tank for steady-state conditions Flow rates
Energy loss of waste water
Energyoutput of supply water
mA = mB = 72 l/h mA =mB = 180 l/h mA = 1801/h mB = 9 0 1 / h
3.26 kW 6.9 kW 4.3 kW
3.05 kW 6.07 kW 3.85 kW
GENERAL DISCUSSION AND OUTLOOK The results of the series of measurements have shown that the concept proposed for waste water recovery leads to quite favourable results. The only major drawback was that the waste water outlet temperature becomes unfavourably high for high waste water flow rates. However, this problem may be solved relatively easily. Anyway, up to 11 kW of waste water energy have been recovered already with the present apparatus. The integrating ability of the storage tank is quite impressive as changes in waste water temperature were smoothened very effectively. If the period even of violent waste water oscillations is less than 30 min, no perceivable oscillation of supply water outlet tempera-
ture occurs. The supply water outlet temperature was kept at an almost constant level even when 75~o of the tank's energy had boon drawn out. Thus the stratification within the tank may be considered very satisfactory. The results of the measurements are now turned over to the Daimler-Benz AG to be compared to results of computer calculations on the storage system. Prelimhlary calculations have indicated general agreement between calculations and measurements. The comparison is to lead to further confirmation of important system parameters, so that simulation of very complex modes of operation such as typical resiclontial operation or operation in commercial laundries will become possible. Future presentation of those results is intended.
0198-7593/82/020117--0S$03.00/0 PergamonPressLtd
Heat Recovery Systems
USE O F WASTE WATER HEAT FOR SUPPLY WATER H E A T I N G BY USE O F HEAT PIPE DIODES P. BEHRMANN Dornier System GmbH, Friedrichshafen, W. Germany
H. HJ~FNER Daimler-Benz AG, Stuttgart, W. Germany
and L. SPEITKAMP Tobelhalde 2, 7777 Neufrach, W. Germany Abstraet--A hot supply water storage tank, which is heated by waste water heat, has been developed by Dornier System GmbH under a Daimler-Benz AG contract. Copper heat pipes using water as carrier fluid have been developed for this purpose, which are capable to transfer sufficient heat even at temperatures as low as 10°C. Measurements indicate that up to 90% of the usable waste water heat may be recovered depending upon the waste "water flow rate and on the complexity of the system. Stratification within the tank allows that more than 75~ of the stored energy may be drawn without a significant drop in supply water temperature.
INTRODUCTION
ONE MAJOg problem in the recovery of heat from sources like residential waste water is the fact that the energy to be recovered is produced at very irregular intervals and at varying temperature levels. If the energy were to be recovered by means of conventional heat exchangers, sophisticated control of the waste water circuit would be required to avoid heat being transferred from a hot storage tank to cold waste water. Also the effectivity of these systems is rather low since much low grade heat had to be rejected. A concept has been developed at the Daimler-Benz AG to avoid active control of the recovery system and to allow the recovery of both high grade and low grade energy. The concept makes use of heat pipes for heat transfer from waste water to stored supply water since heat pipes operating in the gravity-assisted mode exhibit a Pronounced diode behaviour. Thus it may be achieved to transport heat "one-way-only", i.e. from the waste water to a storage tank, while almost no heat is transferred the opposite way under any operating mode. Another important feature of this concept was to achieve some sort of temperature stratification within the storage tank and to introduce heat pipes at different temperature levels into the tank. Thus low grade waste heat may pass by the heat pipes reaching into areas of hot supply water unaffected, while effectively transferring its energy to other heat pipes connected to cold supply water. A water storage tank and a system of heat pipes for waste water recovery making use of the concept discussed above has been developed at the Dornier System Company under a Daimler-Benz contract. Measurements have been carried out at Dornier System for confirmation of important system parameters and shall be discussed below. STORAGE TANK AND HEAT PIPES SYSTEM
The storage tank and the heat pipe system are depicted in Fig. 1. The storage tank is a stainless steel vessel containing approx. 5001 of supply water. It is divided into six compartments by stainless steel partitions. Each partition has three 30 mm borings to 117
118
P. BEHRMANNet al.
water outlet
Waste water inlet
roll
I
Supply water inlet Waste water outlet
Fig. 1. Storage tank and heat pipe system.
allow for water flow. Each compartment is fitted with an NW 65 flange for heat pipe insertion. The water inlet and water outlet are at the lowermost and uppermost points of the vessel. The vessel is insulated against heat loss. The complete set-up is clad in a stainless steel jacket. Overall dimensions of the complete tank are: height 2000 mm, dia S50 ram. The heat pipes are made of copper, with water being used as working fluid. The condenser section (length approx. 650 mm) is fitted with lateral copper fins of height 4 mm and thickness 0.3 mm. A stainless steel NW 65 flange is brazed to the heat pipe. The evaporator end (length approx. 1000 mm) bears no fins so as to avoid clogging by dirt carried in by the waste water. Small ends of copper wire are soldered to the surface to act as spacers. The waste water conduit is formed of commercially available plastic tubing as can be seen in Fig. 1. The tubes are slipped onto the heat pipe evaporators to form a ring gap heat exchanger. In the system used for the measurements only five out of six compartments of the storage tank are fitted with heat pipes, while the uppermost compartment is left without a heat pipe. DESCRIPTION OF WATER SUPPLY UNIT AND MEASURING SET-UP The purpose of the measurements to be carried out was chiefly to determine the behaviour of the system under various operating modes and to supply information for numerical calculations carried out at the Daimler-Benz AG. Measurements had to include loading the storage tank with various waste water flow rates, unloading it with various supply water flow rates, determine the steady-state temperature distributions for various operating modes and to analyse the behaviour of the system for alternating waste water flow. Also the diode effect exhibited by the heat pipes had to be tested. To conduct
Use of waste water heat for supply water heating
119
I,r
II
Cooling waft"
.I
T7
I
$.19.08
I )i IO
Y
m
) I I
I ) I
I J I I I t I I I
._1 Fig. 2. Waste water and supply water conduits.
these measurements a test rig has been constructed at Dornier System. This rig is sketched in Fig. 2 and is described below. Two thermostats (1, 2) are used to keep two reservoirs (3, 4) at a desired temperature level. One of these is kept at approx. 80°C, while the other is kept at 20°C. The reservoirs are mounted to the top of a test rack on order to provide the driving pressure for both waste water and supply water flow. Thus the pressure may be kept perfectly constant by simply using an overflow. To simulate the waste water circuit both reservoirs are connected to a three-way valve (5). The line continues via two regulator valves (6,7) and two flowmeters (8, 9) to the waste water inlet. The waste water outlet is connected to another three-way valve (10) which in turn is connected to the thermostats from where the water is again pumped to the reservoirs. The three-way valves are operated to lead either hot or cold water to the waste water heat exchanger. The supply water circuit is made up in a similar way, except that it is connected to the cold reservoir only. Thus both three-way valves are omitted in this circuit. The supply water outlet is connected to a cooler (11) of 25 kW cooling capacity to assure that the cold thermostat will not be overheated by the stored hot water. The flowmeters have been calibrated individually. Additionally the effect of water temperature on the flowmeter reading has been calculated. The relevant temperatures within the system are measured by thirteen NiCr-Ni thermocouples numbered T1 to T13 in Fig. 2. The thermocouple readings are plotted on a 12-channel point printer. Since only twelve thermocouples may be registered on this unit, the thermocouple (13) is omitted in most cases since this temperature is associated to the cold water inlet and is very effectively kept at 20°C. MEASUREMENTS
Heatino of cold storaoe tank In Fig. 3 the temperature distribution within the system is plotted vs time. Starting conditions are: The hot reservoir is kept at its predetermined temperature. No water flow is allowed yet. The storage tank is completely cold.
120
P. BEHRMANNet al.
TI 70 6O
"I"6
oO
80
,o 401~
~
30
,,
~ I
'
,,~-
7 ....Y"
• ~
/
.... x
W'TI2
---.
6o
,..,
,50
t,l I
i //~'
loll -
I
0
" 0
~,II 500
30 I I000
I I~00 Time
I 2000
I 2500
20 3000
in rains
Fig. 3. Heating of storage tank. Waste water flow rate 36 l/h.
A t = 0 a mass flow of 361/h of hot water is introduced into the waste water conduit. No supply water mass flow is allowed. The temperatures within the waste water conduit are drawn by solid lines. The left temperature scale has to be applied for these lines. It is seen that thermocouple T1 (waste water inlet) reaches a constant value after a fairly short time. Minor fluctuations are due to fluctuations in mass flow, which in turn result in varying heat losses in the waste water conduit. The other waste water temperatures quickly rise to medium temperatures with the differences between those temperatures being proportional to the heat transferred by each heat pipe. Note that the temperature difference between T1 and T2 is extremely high. This is attributed mainly to the fact that the uppermost heat pipe has to supply heat to two compartments. The heat exchange between these two chambers is apparently very effective (heating from below), as temperatures T8 and T9 are basically equal. Thus we have improved convection in the upper chambers resulting in better heat transfer from the uppermost heat pipe. An important result is that for a cold storage tank (start of measurements) the waste water outlet temperature is below 30°C. Thus more than 80% of the energy available has been recovered. As time advances the temperatures T2, T3, T4, T5 and T6 asymptotically approach values slightly lower than T1. This may well be expected, since as the temperature inside the storage tank rises the driving force for heat transfer diminishes. The temperature distribution inside the storage tank is depicted by dashed lines. These lines have been shifted with respect to the solid lines for better reading of the diagram. Thus the temperature scale on the right hand side applies for the dashed lines. The behaviour of the temperature distribution inside the tank is quite as expected, exhibiting a gradually diminishing increase as the waste water temperatures are approached. Thermocouple T7 shows a nonconformative behaviour. This thermocouple is placed at the supply water output and is poorly insulated. Thus it reaches equilibrium with the environment as no supply water flow is given. The fluctuations of temperature are mainly due to the formation of air bubbles at the output as the water within the tank expands. Measurements at higher waste water flow rates exhibit basically the same behaviour, although the starting waste water outlet temperature is increased significantly. Thus the recovery of heat becomes less effective with increasing waste water flow. Therefore
Use of waste water heat for supply water heating
121
Table 1. Results of steady state measurements
Flow rate
mA ffi mB = 721/h m A = mB ffi 180 l/h
mA ffi 1801/h mB ffi 90 l/h
Waste water input temperature
79°C
80°C
71.5°C
Waste water output temperature
40°C
47°C
51°C
Efficiency of energy recovery
75%
59%
51%
Supply water output temperature
56.5°C
47°C
55°C
m A : waste water flow; roB: supply water flow.
further thought should be given to improvement of the ring gap heat exchanger. This may be achieved quite simply by introduction of finned exchanger tubes. However, this solution may necessitate the use of filters to avoid clogging of the heat exchanger.
Steady-state results Measurements have been conducted with the waste water flow being equal to or double the supply water flow. These tests have been continued till steady-state conditions were obtained. Three different flow combinations have been tested. The important results are given in Table 1. The efficiency has been calculated by taking the ratio of the waste water temperature drop to the difference of waste water input temperature and temperature of lowest partition [(T1-T6)/(T1-T12)]. It is found that even for moderate mass flows supply water temperatures are at an acceptably high level. Nevertheless some improvement on the waste water heat exchange seems to be desirable. Alternating waste water flow measurements The reason for these measurements was to test the response of the storage system to changes in waste water temperature. A series of measurements was conducted with a constant supply water flow of 901/h and a constant waste water flow of 1801/h. The waste water temperature, however, has been alternated between 80 and 20°C at regular intervals by operation of the three-way valves. Intervals were taken from 2 rain to 1 h. The results for 1 h intervals are depicted in Fig. 5. The measurement has been started from steady-state conditions with above mentioned flow rates. Please note that the diagram has been transformed from a printer output to a computer-made plot. Since the computer programme uses linear interpolation between supporting points the originally smooth curves are approximated by straight line segments. It has been attempted, however, to bring out the characteristic features of the curves. Cooling of loaded storage tank The results of this test are plotted in Fig. 4. Start-up conditions are: The temperatures inside the tank are above 60°C. No water flow. Only the cold reservoir is operated. At t = 0 a water flow of 360 l/h is introduced into the supply water conduit. The diagram shows that the stratification inside the tank is quite effective as it is clearly seen that a cold front moves slowly through the tank. Each compartment exhibits a sharp onset of cooling as the cold front approaches. The outlet temperature is kept almost constant for approx. 1 h. By this time the storage tank has given off approx. 75% of its energy at the maximum storage temperature. Even when the tank has given off about 95% of its energy the outlet temperature is still at 40°C.
122
P.
BEHRMANN et
al.
8O 70
,L) 60 .c_ 5O 40
T~
TI2
N so
I00
o!5
I Time in hours
1!5
i 2
Fig. 4. Temperature distribution for unloading of storage tank. Flow rate 360 l/h.
The irregularities at the beginning of the test (thermocouples T12 and T13) are due to start-up problems as the supply water path was blocked by air bubbles which had to be removed by introducing an external water current resulting in overheating of the cold reservoir. Similar results have been obtained for lower supply water flow rates. Heat pipe diode test 'This test was conducted with a loaded storage tank, no supply water flow and a cold waste water flow of 36 l/la. The temperature difference between waste water inlet and outlet indicates a heat loss through the heat pipes of as little as 50 W. As the waste water is switched to cold, the lower portion of the storage tank (lines T11 and T12) loses most of its energy, while the outlet temperature remains almost unaffected. After two cycles the lower portion of the tank stabilizes to an oscillating temperature function which may well be considered steady-state condition. The uppermost partition temperature (T8) starts to drop sharply after approx. 3 h of testing, indicating that the initial cold front has moved through the tank. Note that the temperature gap between T8 and T7 (outlet temperature) behaves quite anticyclicly compared to the waste water cycle, as T8 drops while the tank is heated. Also note that the gap between T8 and T7 closes at regular intervals. Although the outlet temperature did not yet reach steady-state conditions by the end of the test, it may be assumed that an oscillating steady state will be reached eventually, with the outlet temperature oscillating around approx. 40°C. The peak in the T1 curve at the beginning of the heating periods is due to the fact that at these times cold water is brought to the hot thermostat from the cold waste water system, and as the heating power of this thermostat is limited, the input temperature T1 drops. It may be taken from Fig. 5 that the thermal capacity of the storage tank smooths out the sharp changes in waste water temperature quite effectively. By shortening the intervals the supply water outlet temperature may be brought to an almost constant level. For example, if the intervals are set to 30 min, the supply water outlet temperature stabilizes at approx. 48°C, with still some oscillations in the lower part of the tank. If the intervals are further reduced to 15 rain, the temperature distribution within the entire storage tank becomes practically time independent. Going to the shortest intervals, it was found that even for 2 min periods energy was still transferred to the cold storage tank. The equilibrium supply water outlet temperatures, however, become quite low as the energy losses within the thermal capacity of the outer waste water circuit become paramount. However, these losses are a quality of the entire apparatus rather than of the heat pipe system. Therefore a true critical frequency could not be obtained with the test rig.
Use of waste water heat for supply water heating
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150
200 250 Time in rains
300
350
" 400
450
Fig. 5. Temperature distribution for 1 h alternating waste water flow.
ENERGY TRANSFER
The energy transferred to the storage tank from the waste water conduit may be calculated from the temperature differences within the waste water conduit and the waste water flow rates. The valfies obtained for zero supply water flow and various waste water flow rates have been tabulated in Table 2. The efficiency has been calculated taking the ratio of the waste water temperature drop to the difference of waste water input temperature and temperature of lowermost partition [{T1-T6)/(T1-T12)]. We have the interesting result that the efficiency of heat transfer is not dependent upon the storage state. That means that for a given waste water flow a set percentage of the heat theoretically transferable is transferred at each moment of the heating period. It is found that the energy transfer rates are quite impressive, especially for high flow rates. However, as the flow rate increases the effectiveness decreases. It has been stated above that the effectiveness may be increased by improving the waste water heat exchanger. However, it must be anticipated that for very high flow rates we are already close to the maximum heat pipe transfer capacity. The energy transfer rates for steady-state measurements are tabulated in Table 3. We find that between 88 and 93~ of the energy lost by the waste water is recovered within the supply water, depending upon the flow rates. It seems that losses increase with waste water flow. This may well be possible for high flow rates the average temperature of the waste water conduit is increased. The results may be compared to those in Tables 1 and 2. It is found that the energy transfer rates compare quite well, while the efficiency appears to be slightly reduced for steady-state calculations. Table 2. Energy transfer to storage tank with zero supply water flow Waste water flow 36 I/h* 72 l/h* 180 l/h* 360 l/h* 36 l/ht 180 l/h? 360 l/ht
Energy transferred to one heat pipe Total 0.88 kW 1.97 kW 2.93 kW 3.77 kW 0.17 kW 1.58 kW 1.46 kW
1.9 kW 4.0 kW 6.9 kW 11.3 kW 0.54 kW 4.07 kW 3.77 kW
Efficiency 90% 81~ 60~ 44~ 90~ 60°/0 45~
* Storage tank completely cold. t Storage tank hot. Temperature of lowermost partition _>60°C.
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Table 3. Energy balance of storage tank for steady-state conditions Flow rates
Energy loss of waste water
Energyoutput of supply water
mA = mB = 72 l/h mA =mB = 180 l/h mA = 1801/h mB = 9 0 1 / h
3.26 kW 6.9 kW 4.3 kW
3.05 kW 6.07 kW 3.85 kW
GENERAL DISCUSSION AND OUTLOOK The results of the series of measurements have shown that the concept proposed for waste water recovery leads to quite favourable results. The only major drawback was that the waste water outlet temperature becomes unfavourably high for high waste water flow rates. However, this problem may be solved relatively easily. Anyway, up to 11 kW of waste water energy have been recovered already with the present apparatus. The integrating ability of the storage tank is quite impressive as changes in waste water temperature were smoothened very effectively. If the period even of violent waste water oscillations is less than 30 min, no perceivable oscillation of supply water outlet tempera-
ture occurs. The supply water outlet temperature was kept at an almost constant level even when 75~o of the tank's energy had boon drawn out. Thus the stratification within the tank may be considered very satisfactory. The results of the measurements are now turned over to the Daimler-Benz AG to be compared to results of computer calculations on the storage system. Prelimhlary calculations have indicated general agreement between calculations and measurements. The comparison is to lead to further confirmation of important system parameters, so that simulation of very complex modes of operation such as typical resiclontial operation or operation in commercial laundries will become possible. Future presentation of those results is intended.