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Efficiency of heat storage in solar energy systems
Energy Convers. Mgmt Vol. 34, No. 8, pp. 677--685, 1993
0196-8904/93 $6.00 + 0.00 Copyright © 1993 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
EFFICIENCY
OF
HEAT
STORAGE
IN SOLAR
ENERGY
SYSTEMS
MOHAMMAD RIAHIt Department of Technology, Northern Illinois University, Dekalb, IL 60115, U.S.A.
(Received 6 April 1992; received for publication 18 December 1992)
Abstract~The purpose of this research was to develop an original method of measurement of the efficiency of heat storage in solar energy systems. The method was tested experimentally by using an electric heater instead of the heat from solar collectors. A rectangular-shaped box was made to hold the material that is capable of holding heat. In this research, pentaerythritol was chosen as being potentially the most effective material for this experiment from all aspects. This report presents a theoretical justification of the methods and the results of the error analysis, Different means and ways of improving the efficiency of a heat storage box, as well as the whole system, are suggested. The report concludes by analyzing the data that the material used is capable of storing heat at 360°F for up to 16 h. Heat storage
Solar
Energy
Efficiency
NOMENCLATURE T* ~ Temperature of heat transition Cs -~ Heat capacity of solid phase CL = Heat capacity of liquid phase 2 = Latent heat of phase transition T, = Temperature of oil entering oil well To = Temperature of oil leaving oil well 0i -- Temperature of oil entering heat generator 0o = Temperature of oil leaving heat generator m = Mass of pentaerythritol in box v -- Volume of hot oil in circuit SAE60 = A heavy duty lubricant oil that is capable of receiving heat up to 570°F without burning [6]
INTRODUCTION
Energy materials such as oil, natural gas and coal are being consumed at an ever increasing rate. By the most generous estimate, the known quantity of oil and natural gas resources will run out only two generations from now [1]. The everlasting source of energy is solar. Employing the radiation of the sun and successfully converting it into some other form of energy will be the answer to the shortage of energy in the future. For centuries, the sun has been viewed only as a light source. It has been only in recent decades that analytical studies of sunlight have come to exist for scientific purposes. One such purpose is to collect the radiated energy and then use it for heating a place. As soon as this hypothesis proved practical, different applications of converted radiation from the sun have emerged. Heating houses, offices, and greenhouses, as well as boiling water for different purposes, are but a few examples of applying the radiated energy from the sun into regular day-to-day usage. As useful and practical as it may seem, solar energy cannot be relied upon as the mere source of energy and possesses its deficiencies. One such deficiency is the night time where there are no rays of radiation, and because of that, all equipment working with the radiated beams from the sun become useless at night or severely diminished on cloudy days. Studies have been conducted on finding materials capable of storing heat for short periods of time [2, 3]. There are a number of materials known to scientists that can hold heat for times long tPresent address: Department of Technology, School of Applied Science, University of Southern Maine, 37 College Avenue, Gotham, ME 04038, U.S.A. 677
678
RIAHI: EFFICIENCYOF HEAT STORAGEIN SOLAR ENERGY SYSTEMS
enough to be useful in this work. However, in this particular case, two factors are involved. One is the high temperature that the material must hold [3]. Second is the length of time involved for storing the heat, which should be no less than 6-10 h for the obvious reason that there is, on average, a 12 h pause in the shining of the sun in every 24 h. It is clear that, for a normal daily use in a household, a temperature up to 500°F is needed for cooking and up to 150°F is needed for hot water and heating of the house [2]. The concept of sensible heat storage represents the current state of thermal energy storage technology. This type of storage system can be designed and engineered, using available technology and materials, with a relatively low cost of time and money. Systems using water as the storage medium are limited in maximum temperature by the high vapor pressure, with costs of these systems being inflated by the need for high pressure containment. A significant decrease in storage cost is projected through the use of underground tanks or aquifers; although the latter is restricted to a relatively low temperature due to the limited artesian pressure head [4]. The vapor pressure problem can be alleviated by the use of heat transfer oils or molten salts [5]. Although reduction in containment cost is offset by higher material costs, these materials offer the possibility of an alternative for higher temperature storage operation [5]. If significant reduction in material costs can be realized, the oil or salt systems will become much more attractive [6]. The use of a solid storage material has the advantages of high temperature and low pressure operation, direct contact heat exchanger, and very low material costs [6]. Hybrid systems consisting of a packed bed of solid particles through which a heat transfer oil or molten salt is flowing have been shown to reduce significantly the cost of an all-liquid system [6]. The high temperature solid storage concept appears quite attractive, however reliable cost estimates are needed [5]. DESIGN
There were three main parts in the test system circuit that needed to be designed: (a) the storing heat exchanger, (b) the electric pump, and (c) the connecting pipes of the apparatus and the insulation. (a) The design of the heat exchanger was the main task of this experiment. At the beginning of the research, attempts were made in regard to making the shape of the box in a rectangle or square or possibly even a sphere [4]. Since there was no available information pertaining to the practicality of those shapes suggested from previous experiments, rectangular was chosen for its simplicity of making. Also, for fins, aluminum plates of a square shape were drilled at the center so that the l~in. dia copper pipe containing hot fluid would pass through them. Four 18 in. fin tubes were made, and they were connected to the copper pipeline. The box had a size of 18 x 18 x 5 in. The fins had plates of nearly square shape, with dimensions of 5~ x 4¼in. After the copper pipes were connected in the way shown in Fig. 1, the box was filled with pentaerythritol. The direction of the flow of the hot oil was designed to keep a balance for the distribution of heat. Q~ = m[(T* - Ti)Cs + 2 + (To - T*)CL]. In order to create as uniform a temperature as possible inside the heat exchanger, the hot oil was first sent to the first quarter and then to the third quarter. From the third quarter, the hot oil went to the second and then to the fourth quarter so that the decline in the temperature of the oil would be gradual and even. The box was then filled with 21 kg of pentaerythritol, and then the cap of the box was fitted. (b) The electric pump and heater were both purchased according to the needed specifications. The pump used had a continuously variable speed control which required calibration in order to do quantitative work. The pump was tested, and its output was measured at different speeds. Table 1 indicates the output of the pump per unit of time. A submerged electric heater (listed output of 750 W at 120 V) was employed in direct contact with the oil so that the heat generated would only go to the oil and would, therefore, provide a known input power to the oil stream. An aluminum pot in the form of a cylinder was designed to contain the heater. The radius of this pot was 6 in., and the length was 18 in.
RIAHI:
Heater control
EFFICIENCY OF HEAT STORAGE IN SOLAR E N E R G Y SYSTEMS I
Heat e x c h a n g e r with p h a s e c h a n g e material × 0o
Sensor
679
I
×
m
~
1 I
x
×
X
~
X
Oil well
X
Heater
I
I 1
>(
I1 x
Pump
X
Pentaerythritol X
x X
Line losses minimized by shortening and heavy insulation so that T i= 0 oand T o = 0 i Fig. 1. Heat exchanger test system.
(c) Connection and insulation of parts. After the box was ready with an inlet and an outlet, a copper pipe was connected to the outlet of the box and then connected to the pump. This pipe was continued to the pot where it poured from the outlet of the box into the pot. From the inlet of the box, a copper wire was directly connected to the pot or "oil well". Therefore, circulation of the hot oil took place by the suction produced by the pump (Fig. 2). To provide for the insulation of the circuit, four layers of l in. fiberglass were used [7]. However, after running the experiment for the first time, it was noticed that the heat loss was higher than expected [8]. Consequently, for the second run, eight layers of insulating materials (three mineral fiber and five fiberglass [7]), each with a thickness of 1 in. were used. By using wood and long screws, the whole insulation was tied together [9]. This time, the heat loss was minimal, and the result was more satisfactory. Also, up to 8 in. of fiberglass was used around the oil well, and the pipes were tied together with fence wire around them [5]. To place the thermocouple wires, before closing the cap of the heat generator, a 14 in. drill bit was used to make 10 different holes at different locations [4]. There were two wires placed at 6 in. depth inside the box, two at 9 in., two at 12 in. depth and the remaining four at Ti, To, 0i, 0o. One single unit analog thermometer was located in the oil inside the containment vessel [10].
Table 1. Speed of the pump Speed of pump 2.0 2.5 3.0 3.2 3.5 3.8 4.0 4.5 5.0 5.5 6.0 6.5 7.0
Time (s)
Volume (ml)
30 15 10 10 I0 10 5 5 5 5 5 3 3
900 - * 30 ml/s 570 --~ 38 ml/s 550--* 55 ml/s 620 --* 62 ml/s 700 ~ 70 ml/s 850---~ 85 ml/s 500--+ 100 ml/s 600---* 120 ml/s 700--* 140 ml/s 750---, 150 ml/s 850---* 170 ml/s 550 ---* 183 ml/s 600 ---. 200 ml/s
680
RIAHI:
E F F I C I E N C Y OF HEAT STORAGE IN SOLAR E N E R G Y SYSTEMS Solar p o w e r on c o l l e c t o r a p e r t u r e area = m
TO
Qline = 0 0i
Qc
c,ct
Pump
ML n
Ti
0o
Oline = 0 Fig. 2. Overall diagram of the heat exchanger circuit.
EXPERIMENTAL APPARATUS
The equipment used in this research was as follows: (1) Electric variable-speed pump. An electric pump designed to perform at high temperature (up to 500°F) for the circulation of the liquid. (2) Electric multireader thermocouple. A 10 channel thermocouple that could detect temperatures at 10 different locations at all times and record them with selected repetition of time interval. Also, two single thermocouples for separate readings. (3) Copper pipes. One-half inch copper pipes that were used to connect the whole circuit. (4) Insulating materials. Fiberglass and some asbestos that were used for insulating the heat generator and the pipes. (5) Electric floating heater. A 1500-W floating heater that was place inside the boiling pot to heat the fluid. (6) Boiling pot. A cylinder used for boiling the fluid to be circulated in the circuit. EXPERIMENT
The main principle of this research was to build an oven for cooking purposes that would be capable of holding the heat it received for at least 6-10 h. There were three elements involved in this design. First, the heat storage (box), which was made of galvanized steel. The shape of the heat storage could be in any geometrical form. However, since there was no information available in regard to the performance of one form compared to the other, a rectangular shape was selected for the purpose of practicality. The second element in the design was the material used for filling the heat generator, known as the box. This material had to have the capability of storing heat in it for the length of time desired and up to 500°F. From presently available information on materials, the most practical one for this research appeared to be pentaerythritol [11]. Pentaerythritol has physical properties, such as solid to solid phase change and large latent heat, that are very suitable for this experiment [12]. In addition to that, pentaerythritol with 98% purity would be economically attractive [6, 11]. The third element involved was the insulation of the box. Since no heat loss of any amount should take place, the insulation had to be as effective as possible. For this reason, different layers of fiberglass were used. Also, for the purpose of heat exchanger performance measurement, instead of using heat collected from the sun, an electric heater was used in the pumping circuit. This was done to minimize the control problems involved. An electric heater with specific output was used so that the heat generated by it would be equivalent to that being received through the solar collectors [13]. There were two different runs involved in this research. From the 10 thermocouples involved, two of them were malfunctioning; and after calibration, they were found to be out of order. The
RIAHI: 500
--
400
--
E F F I C I E N C Y OF H E A T S T O R A G E IN S O L A R E N E R G Y S Y S T E M S Power off Cooling starts • •*•
L.
• •41 300
--
200
--
100
--
Latent cooling
•
•
O•
•
••
••
•
•
•
•
Sensible cooling •
i.
[-
681
0
1
I
I
I
I
I
I
I
I
I
I
I
I
I
•
•
I
•
•
•
I
Time ( h ) Fig. 3. Performance of heat generator l, thermocouple
I
I
I
I
I
I
2.
remaining eight were recorded on the same print the thermocouple printed. Also, the other two singie-wire thermocouples had functioned properly. Since the reading of those two wires was not crucial to the outcome of this research, the main focus became concentrated on the main thermocouples with eight wires. When the run started, all the probes involved were reading the room temperature which was at about 75°F. Then the heater and the pump were turned on. The thermocouples recorded the temperatures every 5 min as programmed and showed its reading every 5 s digitally. This procedure went on for a period of nearly 12 h. At that time (after 12 h), the pump and the heater were both turned off. The thermocouples then continued printing at every 10 min. The graphical data shown in Fig. 3 was collected during a 6-h heating and 24-h cooling process. The second run took place about 2 months later. The same procedure took place. This time, 3 thermocouples out of 10 were not functioning correctly. Therefore, the data collected were based on the readings from 7 thermocouples. This time, the pump and the heater were turned off after 8 h. The cooling position of the data shown on Figs 4 and 5 were collected for a period of 39 h after the pump was turned off. ANALYSIS
Results of the first experiment, shown in Fig. 3, indicated that there were two rather undesirable curves in both achieving the desired temperature of over 360°F and maintaining it at that level for a considerably long period of time, e.g. 20 h. Items to be considered for analyses of the curves in Fig. 3 were the following: lack of proper insulation, too much resistance between the heater and the fluid [5], the use of a small powered heater [13], and the mass of pentaerythritol that filled the heat exchanger box [14]. However, despite the deficiencies mentioned above, the overall result was satisfactory. One result that could be drawn from the graph is that, after turning the heater off, the temperature of the heat exchanger remained above 350°F for a period of 12 h and, thus, achieved the original purpose of this research. Therefore, although in later runs, better results were 500
Power off Cooling starts Latent cooling
400 00a
QO
Q 00
00
00
01
•
0
e
•
Sensible cooling
300
0 0 0 0 0 0 0
Q
neoo
QO go
OOQ
0
200
E [-
100
0
I
I
I
I
L
I
I
I
I
I
L
I
I
I
I
I
Time (h) Fig. 4. Performance of heat generator 2, thermocouple 2.
I
I
I
I
I
I
682
RIAHI:
E F F I C I E N C Y O F H E A T S T O R A G E IN S O L A R E N E R G Y SYSTEMS
Power off Cooling starts
5O0
•
o~" 400 • "~ 300 ~D
E cD ["
200
•
•
Latent cooling •
o
o
•
o
•
•
•
*
•
o
o
°
•
Sensible cooling
• •
•
•
•
•
•
*
•
•
•
•
•O
•
•
•
100
0
I
I
I
I
[
I
P
I
I
I
I
P
I
I
P I
I
I
P
I
I
J
T i m e (h) Fig. 5. Performance o f heat generator 2, thermocouple 6.
achieved. Nevertheless, considering the overall circumstances, the first run did accomplish precisely what the whole experiment was designed to accomplish. In the second run, some changes took place that could be looked upon as the main reasons for the better results. New insulation was installed that, in addition to having two more layers of fiberglass, covered the whole circuit, including the pipes. Also, the amount of pentaerythritol was increased [14]. It could be deduced from Figs 4 and 5 that the latent cooling has increased from 12 to nearly 19 h, which is a 53% improvement over the previous run. Also, the sensible cooling, extends to approx. 38 h after the heater was turned off, which is considerably longer than the sensible cooling of the first run. Figure 3 represents the data collected by a thermocouple that was placed 4 in. deep in the middle of the heat exchanger box in the first run. Figure 4 shows the temperatures of a thermocouple placed 8 in. deep inside the box and next to the front cover. Figure 5 is the temperature of the outlet of the fluid in the second run. CONCLUSION The main goal in performing this research was to conclude from the collected data whether the idea of storing heat was feasible or not. Therefore, the outcome was dependent upon the numbers collected from the thermocouples placed in the circuit. To make assurances that the readings of the thermocouples were sufficiently accurate, two calibrations were conducted. In the first run, by observing the digital numbers that appeared on the thermocouple, it was noticed that some of the reported temperatures did not agree with the theoretical physics laws. As a result, all the wires were calibrated. In order to calibrate, all 10 wires were wrapped together with a band. All of their thermojunctions were separated from each other by bending them from one end, and the wires were placed in the oil inside the oil containment vessel. The wires were connected to the thermocouple on the other end. Also, another single-wired thermocouple was connected to the inside of the oil well, where the other wires were placed. The heater was turned on, and after it reached 300°F, the attached thermocouple was stopped. It was noticed that the temperature read from the smaller thermocouple, and seven of the other thermocouples agreed at any given time. Therefore, the three defective ones were discarded. Also, in the second run, the same procedure with the calibration of the thermocouples took place. This time as well, the inaccurate thermocouples were removed. As mentioned earlier and the results are indicating, the second run was much more efficient than the first, in that, for the first run, an effective storage time of 14 h was recorded for the temperature over 360°F, whereas in the second run, the reported temperature stayed above 369°F for 23 h constantly. Another important point needs to be mentioned, which is to notice that, in the first run, the circuit was operating for nearly 14 h before it was turned off. However, in the second run, the time that the circuit was in operation and the heater and the pump were running did not exceed 8h. Since the ultimate goal was to build an oven for household or commercial use that would stay hot long enough to cook or bake for at least 6-10 h; the required temperature had to be at least
Reference
[10, 1I] [13, 14] [14] [4] [131 [13, 8, 9] [7] [7] [6] [12] [13, 5] [4] [4] [5, 10]
PCM for thermal storage
NaCLO 3 *LiNO3 MgCI2 • 6H20 • Pentaerythritoi {2-2-Bis hydroxymethyl 1,3-propanediol} CsHt204 2,2-Bis(hydroxymethyl) (proprionic acid) 2-Amino-2-(hydroxymethyl) 1,3-propanediol Adipic acid Thermkeep {91.8"/, NaOH, 8°/0 NaNOj, 0.2% MnO2} • Draw salt {50°/. NaNO3, 50% KNO3} • K-salt (50% NaNOs, 35% KNO3, 15% NaNO2) NaOH-KOH (50%, 50*/,) Solder (Pb-Sn, 40%, 60%) Anthracene Urea (NH2CONH2) Phthalimide (C6I-I4 (CO)2NH) Hydroquinone ~453
363 306-311 268-273
Tss (°F) 491 482 242 498 381-387 381-387 306 559 440 390 338 361 424 271 453 342
TSL (°F)
1.328
0.9
1.87 1.89
2.38
Specific gravity
Table 2. Phase-change storage materials
68.946
Molecular wt (g)
64 ~ 1! 38.5 57.6 47.9 58.8
49.7 87.8 41.2 8.8 6.4 6.0 59.6
Lsg (cal/g)
98
72.5 68.6 67.6
Lss (eal/g)
78.1
209
L
"'Quizor' C6H4(OH)2
Safe on all counts CHjC(CH2OH)2CO2H
Hazards
,<
,<
Z erl
O 3-
l-n
> ,-4 -4 O
0,.q
,.<
~n
684
RIAHI: EFFICIENCY OF HEAT STORAGE IN SOLAR ENERGY SYSTEMS
350°F. The need for heat storage came from the fact that a solar oven receives its heat from the radiation of the sun. Since there is no radiation between sunset and sunrise, the oven would have to keep its temperature hot throughout the night or at least a good portion of it. Reconsidering the minimum temperature for cooking and realizing that 350°F would be the moderate temperature for effective cooking, the results would change as well. Another important factor is that the material used for heat storage (pentaerythritol) has a phase-change temperature of 363°F (Table 2), which means temperatures lower than 363°F will bring the pentaerythritol to its original phase and, consequently, will not keep the heat for much longer. For the latter and previous reasons, the minimum effective temperature will have to be set at 360°F. By analyzing the data once more, it became evident that, for 14 h of receiving heat in the first run, the heat generator was able to keep the desired heat of 360°F up to 1 ~ h. As for the second run, it is obvious that, for only 8 h of heating the pentaerythritol, it stayed above 360°F for 16 h. The difference in behavior of the heat generator can be attributed to the following: change of insulation, change of the heater, and change in the speed of the pump. There are a number of methods that can be applied to improving the efficiency of the heat exchanger. As was observed by the change in the second run, perhaps the most important factor is the proper insulation of the box. Almost doubling the layers of fiberglass and mineral fibers in the second run, plus the fact that the layers were put together more tightly than before, contributed enormously to the improvement of the heat exchanger. The overall improvement from the first run to the second was approx. 90%. The second element involved was the heater. By using a submerging heat in the oil well, the heat loss between the heater and the circulating oil was eliminated. This also contributed to the length of time required for the circuit temperature to reach 420°F. It is observed from the figures that the required time was reduced from 14 to 8 h. The speed of the pump was set at 4.5 for the first run. Since the numbers on the pump represented no volumetric value, the pump's output had to be measured (Table 1). After the measurement, it was noticed that the volume of the oil circulated was 120 ml/s. For the second run, the speed was set at 3.2. The new number represented a circulation of 62 ml/s of oil. There were some other factors involved in the performance of this system that can be improved to obtain a more desirable result. First is the component sizes. By designing larger-surfaced fins and also increasing their numbers, the system can be more efficient. Second is the mass of the pentaerythritol in the heat generator. By reducing the mass of pentaerythritol to the exact amount that is capable of going through phase-change, the system becomes more time-efficient. The third factor involved in making improvements is the length of time needed by the system to reach the phase-change condition. Based on the available data, especially from the second run, the length of time that the heat generator kept the absorbed heat was approx. 16 h. Since the time required for the box to reach 420°F was 8 h, improvements need to be made to reduce that time to approx. 5 h instead; or even shorter. This is due to the fact that an average bright day of the year has approx. 4-6 h of effective radiation that can be collected for the purpose of this research. Improvements can take place by employing one or more of the methods mentioned above. In conclusion, this investigation proved that the idea of cooking food at night through using solar energy is feasible and can be done practically, as described. Acknowledgement--The author is grateful to the Departments of Physics and Technology at Northern Illinois University
for providing the necessaryfacilities for the present study. REFERENCES
1. National Iranian Oil Company, Annual Publication, 1979, pp. 110-114. Persian Translation, Tehran, Iran (1979). 2. R. M. Green and D. K. Otteson, Solar Energy in the Seventies, p. 34. OSHA and Ronald M. Green, New York (1976). 3. O. J. Kleppa, "Heat of Fusion of the Monovalent Nitrates by High Temperature Reaction Calorimetry." J. Chem. Engng Data 8, 331 (July 1963). 4. F. Baylin and M. Merino, A survey of sensible and latent heat thermal energy storage projects. SERI/RR-355-465, p. 386. Solar Energy Research Institute, Golden, Colo. (May 1981). 5. T. T. Bramlette et al., Survey of high temperature thermal energy storage (March 1976). 6. D. K. Benson et al., Materials research for passive solar systems: solid state phase change materials. SERI/TR-25511828, p. 124. Solar Energy Research Institute, Golden, Colo. (March 1985).
RIAHI:
EFFICIENCY OF HEAT STORAGE IN SOLAR ENERGY SYSTEMS
685
7. C. T. Lynch (Ed.) CRC Handbook of Material Science, Vol. II, p. c-451. CRC Press, Cleveland, Ohio (1983). 8. CRC Handbook of Chemistry and Physics, 50th edn, p. c4/52. CRC Press, Cleveland, Ohio (1976/1977). 9. B. M. Cohen, Comstoek and Wescott Inc., 765 Concord Avenue, Cambridge, MA 02138, Private communication. 10. W. A. Beckman and J. A. Duffle, Solar Engineering of Thermal Processes, p. 231. Univ. of Wisconsin, Madison, Wis. (1984). 11. SERI/TR-631-647, p. 212. Solar Energy Research Institute, Golden, Colo. (March 1983). 12. J. W. Mullin, Crystallization, 2nd edn. CRC Press, New York (1979). 13. J. O. M. Bockris, Energy Options, p. 234. Wiley, New York (1980). 14. S.S.E.A., Performance Table of Materials, The Society of Solar Engineers of America, Golden, Colo. (1979).
0196-8904/93 $6.00 + 0.00 Copyright © 1993 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
EFFICIENCY
OF
HEAT
STORAGE
IN SOLAR
ENERGY
SYSTEMS
MOHAMMAD RIAHIt Department of Technology, Northern Illinois University, Dekalb, IL 60115, U.S.A.
(Received 6 April 1992; received for publication 18 December 1992)
Abstract~The purpose of this research was to develop an original method of measurement of the efficiency of heat storage in solar energy systems. The method was tested experimentally by using an electric heater instead of the heat from solar collectors. A rectangular-shaped box was made to hold the material that is capable of holding heat. In this research, pentaerythritol was chosen as being potentially the most effective material for this experiment from all aspects. This report presents a theoretical justification of the methods and the results of the error analysis, Different means and ways of improving the efficiency of a heat storage box, as well as the whole system, are suggested. The report concludes by analyzing the data that the material used is capable of storing heat at 360°F for up to 16 h. Heat storage
Solar
Energy
Efficiency
NOMENCLATURE T* ~ Temperature of heat transition Cs -~ Heat capacity of solid phase CL = Heat capacity of liquid phase 2 = Latent heat of phase transition T, = Temperature of oil entering oil well To = Temperature of oil leaving oil well 0i -- Temperature of oil entering heat generator 0o = Temperature of oil leaving heat generator m = Mass of pentaerythritol in box v -- Volume of hot oil in circuit SAE60 = A heavy duty lubricant oil that is capable of receiving heat up to 570°F without burning [6]
INTRODUCTION
Energy materials such as oil, natural gas and coal are being consumed at an ever increasing rate. By the most generous estimate, the known quantity of oil and natural gas resources will run out only two generations from now [1]. The everlasting source of energy is solar. Employing the radiation of the sun and successfully converting it into some other form of energy will be the answer to the shortage of energy in the future. For centuries, the sun has been viewed only as a light source. It has been only in recent decades that analytical studies of sunlight have come to exist for scientific purposes. One such purpose is to collect the radiated energy and then use it for heating a place. As soon as this hypothesis proved practical, different applications of converted radiation from the sun have emerged. Heating houses, offices, and greenhouses, as well as boiling water for different purposes, are but a few examples of applying the radiated energy from the sun into regular day-to-day usage. As useful and practical as it may seem, solar energy cannot be relied upon as the mere source of energy and possesses its deficiencies. One such deficiency is the night time where there are no rays of radiation, and because of that, all equipment working with the radiated beams from the sun become useless at night or severely diminished on cloudy days. Studies have been conducted on finding materials capable of storing heat for short periods of time [2, 3]. There are a number of materials known to scientists that can hold heat for times long tPresent address: Department of Technology, School of Applied Science, University of Southern Maine, 37 College Avenue, Gotham, ME 04038, U.S.A. 677
678
RIAHI: EFFICIENCYOF HEAT STORAGEIN SOLAR ENERGY SYSTEMS
enough to be useful in this work. However, in this particular case, two factors are involved. One is the high temperature that the material must hold [3]. Second is the length of time involved for storing the heat, which should be no less than 6-10 h for the obvious reason that there is, on average, a 12 h pause in the shining of the sun in every 24 h. It is clear that, for a normal daily use in a household, a temperature up to 500°F is needed for cooking and up to 150°F is needed for hot water and heating of the house [2]. The concept of sensible heat storage represents the current state of thermal energy storage technology. This type of storage system can be designed and engineered, using available technology and materials, with a relatively low cost of time and money. Systems using water as the storage medium are limited in maximum temperature by the high vapor pressure, with costs of these systems being inflated by the need for high pressure containment. A significant decrease in storage cost is projected through the use of underground tanks or aquifers; although the latter is restricted to a relatively low temperature due to the limited artesian pressure head [4]. The vapor pressure problem can be alleviated by the use of heat transfer oils or molten salts [5]. Although reduction in containment cost is offset by higher material costs, these materials offer the possibility of an alternative for higher temperature storage operation [5]. If significant reduction in material costs can be realized, the oil or salt systems will become much more attractive [6]. The use of a solid storage material has the advantages of high temperature and low pressure operation, direct contact heat exchanger, and very low material costs [6]. Hybrid systems consisting of a packed bed of solid particles through which a heat transfer oil or molten salt is flowing have been shown to reduce significantly the cost of an all-liquid system [6]. The high temperature solid storage concept appears quite attractive, however reliable cost estimates are needed [5]. DESIGN
There were three main parts in the test system circuit that needed to be designed: (a) the storing heat exchanger, (b) the electric pump, and (c) the connecting pipes of the apparatus and the insulation. (a) The design of the heat exchanger was the main task of this experiment. At the beginning of the research, attempts were made in regard to making the shape of the box in a rectangle or square or possibly even a sphere [4]. Since there was no available information pertaining to the practicality of those shapes suggested from previous experiments, rectangular was chosen for its simplicity of making. Also, for fins, aluminum plates of a square shape were drilled at the center so that the l~in. dia copper pipe containing hot fluid would pass through them. Four 18 in. fin tubes were made, and they were connected to the copper pipeline. The box had a size of 18 x 18 x 5 in. The fins had plates of nearly square shape, with dimensions of 5~ x 4¼in. After the copper pipes were connected in the way shown in Fig. 1, the box was filled with pentaerythritol. The direction of the flow of the hot oil was designed to keep a balance for the distribution of heat. Q~ = m[(T* - Ti)Cs + 2 + (To - T*)CL]. In order to create as uniform a temperature as possible inside the heat exchanger, the hot oil was first sent to the first quarter and then to the third quarter. From the third quarter, the hot oil went to the second and then to the fourth quarter so that the decline in the temperature of the oil would be gradual and even. The box was then filled with 21 kg of pentaerythritol, and then the cap of the box was fitted. (b) The electric pump and heater were both purchased according to the needed specifications. The pump used had a continuously variable speed control which required calibration in order to do quantitative work. The pump was tested, and its output was measured at different speeds. Table 1 indicates the output of the pump per unit of time. A submerged electric heater (listed output of 750 W at 120 V) was employed in direct contact with the oil so that the heat generated would only go to the oil and would, therefore, provide a known input power to the oil stream. An aluminum pot in the form of a cylinder was designed to contain the heater. The radius of this pot was 6 in., and the length was 18 in.
RIAHI:
Heater control
EFFICIENCY OF HEAT STORAGE IN SOLAR E N E R G Y SYSTEMS I
Heat e x c h a n g e r with p h a s e c h a n g e material × 0o
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(c) Connection and insulation of parts. After the box was ready with an inlet and an outlet, a copper pipe was connected to the outlet of the box and then connected to the pump. This pipe was continued to the pot where it poured from the outlet of the box into the pot. From the inlet of the box, a copper wire was directly connected to the pot or "oil well". Therefore, circulation of the hot oil took place by the suction produced by the pump (Fig. 2). To provide for the insulation of the circuit, four layers of l in. fiberglass were used [7]. However, after running the experiment for the first time, it was noticed that the heat loss was higher than expected [8]. Consequently, for the second run, eight layers of insulating materials (three mineral fiber and five fiberglass [7]), each with a thickness of 1 in. were used. By using wood and long screws, the whole insulation was tied together [9]. This time, the heat loss was minimal, and the result was more satisfactory. Also, up to 8 in. of fiberglass was used around the oil well, and the pipes were tied together with fence wire around them [5]. To place the thermocouple wires, before closing the cap of the heat generator, a 14 in. drill bit was used to make 10 different holes at different locations [4]. There were two wires placed at 6 in. depth inside the box, two at 9 in., two at 12 in. depth and the remaining four at Ti, To, 0i, 0o. One single unit analog thermometer was located in the oil inside the containment vessel [10].
Table 1. Speed of the pump Speed of pump 2.0 2.5 3.0 3.2 3.5 3.8 4.0 4.5 5.0 5.5 6.0 6.5 7.0
Time (s)
Volume (ml)
30 15 10 10 I0 10 5 5 5 5 5 3 3
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680
RIAHI:
E F F I C I E N C Y OF HEAT STORAGE IN SOLAR E N E R G Y SYSTEMS Solar p o w e r on c o l l e c t o r a p e r t u r e area = m
TO
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EXPERIMENTAL APPARATUS
The equipment used in this research was as follows: (1) Electric variable-speed pump. An electric pump designed to perform at high temperature (up to 500°F) for the circulation of the liquid. (2) Electric multireader thermocouple. A 10 channel thermocouple that could detect temperatures at 10 different locations at all times and record them with selected repetition of time interval. Also, two single thermocouples for separate readings. (3) Copper pipes. One-half inch copper pipes that were used to connect the whole circuit. (4) Insulating materials. Fiberglass and some asbestos that were used for insulating the heat generator and the pipes. (5) Electric floating heater. A 1500-W floating heater that was place inside the boiling pot to heat the fluid. (6) Boiling pot. A cylinder used for boiling the fluid to be circulated in the circuit. EXPERIMENT
The main principle of this research was to build an oven for cooking purposes that would be capable of holding the heat it received for at least 6-10 h. There were three elements involved in this design. First, the heat storage (box), which was made of galvanized steel. The shape of the heat storage could be in any geometrical form. However, since there was no information available in regard to the performance of one form compared to the other, a rectangular shape was selected for the purpose of practicality. The second element in the design was the material used for filling the heat generator, known as the box. This material had to have the capability of storing heat in it for the length of time desired and up to 500°F. From presently available information on materials, the most practical one for this research appeared to be pentaerythritol [11]. Pentaerythritol has physical properties, such as solid to solid phase change and large latent heat, that are very suitable for this experiment [12]. In addition to that, pentaerythritol with 98% purity would be economically attractive [6, 11]. The third element involved was the insulation of the box. Since no heat loss of any amount should take place, the insulation had to be as effective as possible. For this reason, different layers of fiberglass were used. Also, for the purpose of heat exchanger performance measurement, instead of using heat collected from the sun, an electric heater was used in the pumping circuit. This was done to minimize the control problems involved. An electric heater with specific output was used so that the heat generated by it would be equivalent to that being received through the solar collectors [13]. There were two different runs involved in this research. From the 10 thermocouples involved, two of them were malfunctioning; and after calibration, they were found to be out of order. The
RIAHI: 500
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E F F I C I E N C Y OF H E A T S T O R A G E IN S O L A R E N E R G Y S Y S T E M S Power off Cooling starts • •*•
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remaining eight were recorded on the same print the thermocouple printed. Also, the other two singie-wire thermocouples had functioned properly. Since the reading of those two wires was not crucial to the outcome of this research, the main focus became concentrated on the main thermocouples with eight wires. When the run started, all the probes involved were reading the room temperature which was at about 75°F. Then the heater and the pump were turned on. The thermocouples recorded the temperatures every 5 min as programmed and showed its reading every 5 s digitally. This procedure went on for a period of nearly 12 h. At that time (after 12 h), the pump and the heater were both turned off. The thermocouples then continued printing at every 10 min. The graphical data shown in Fig. 3 was collected during a 6-h heating and 24-h cooling process. The second run took place about 2 months later. The same procedure took place. This time, 3 thermocouples out of 10 were not functioning correctly. Therefore, the data collected were based on the readings from 7 thermocouples. This time, the pump and the heater were turned off after 8 h. The cooling position of the data shown on Figs 4 and 5 were collected for a period of 39 h after the pump was turned off. ANALYSIS
Results of the first experiment, shown in Fig. 3, indicated that there were two rather undesirable curves in both achieving the desired temperature of over 360°F and maintaining it at that level for a considerably long period of time, e.g. 20 h. Items to be considered for analyses of the curves in Fig. 3 were the following: lack of proper insulation, too much resistance between the heater and the fluid [5], the use of a small powered heater [13], and the mass of pentaerythritol that filled the heat exchanger box [14]. However, despite the deficiencies mentioned above, the overall result was satisfactory. One result that could be drawn from the graph is that, after turning the heater off, the temperature of the heat exchanger remained above 350°F for a period of 12 h and, thus, achieved the original purpose of this research. Therefore, although in later runs, better results were 500
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RIAHI:
E F F I C I E N C Y O F H E A T S T O R A G E IN S O L A R E N E R G Y SYSTEMS
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achieved. Nevertheless, considering the overall circumstances, the first run did accomplish precisely what the whole experiment was designed to accomplish. In the second run, some changes took place that could be looked upon as the main reasons for the better results. New insulation was installed that, in addition to having two more layers of fiberglass, covered the whole circuit, including the pipes. Also, the amount of pentaerythritol was increased [14]. It could be deduced from Figs 4 and 5 that the latent cooling has increased from 12 to nearly 19 h, which is a 53% improvement over the previous run. Also, the sensible cooling, extends to approx. 38 h after the heater was turned off, which is considerably longer than the sensible cooling of the first run. Figure 3 represents the data collected by a thermocouple that was placed 4 in. deep in the middle of the heat exchanger box in the first run. Figure 4 shows the temperatures of a thermocouple placed 8 in. deep inside the box and next to the front cover. Figure 5 is the temperature of the outlet of the fluid in the second run. CONCLUSION The main goal in performing this research was to conclude from the collected data whether the idea of storing heat was feasible or not. Therefore, the outcome was dependent upon the numbers collected from the thermocouples placed in the circuit. To make assurances that the readings of the thermocouples were sufficiently accurate, two calibrations were conducted. In the first run, by observing the digital numbers that appeared on the thermocouple, it was noticed that some of the reported temperatures did not agree with the theoretical physics laws. As a result, all the wires were calibrated. In order to calibrate, all 10 wires were wrapped together with a band. All of their thermojunctions were separated from each other by bending them from one end, and the wires were placed in the oil inside the oil containment vessel. The wires were connected to the thermocouple on the other end. Also, another single-wired thermocouple was connected to the inside of the oil well, where the other wires were placed. The heater was turned on, and after it reached 300°F, the attached thermocouple was stopped. It was noticed that the temperature read from the smaller thermocouple, and seven of the other thermocouples agreed at any given time. Therefore, the three defective ones were discarded. Also, in the second run, the same procedure with the calibration of the thermocouples took place. This time as well, the inaccurate thermocouples were removed. As mentioned earlier and the results are indicating, the second run was much more efficient than the first, in that, for the first run, an effective storage time of 14 h was recorded for the temperature over 360°F, whereas in the second run, the reported temperature stayed above 369°F for 23 h constantly. Another important point needs to be mentioned, which is to notice that, in the first run, the circuit was operating for nearly 14 h before it was turned off. However, in the second run, the time that the circuit was in operation and the heater and the pump were running did not exceed 8h. Since the ultimate goal was to build an oven for household or commercial use that would stay hot long enough to cook or bake for at least 6-10 h; the required temperature had to be at least
Reference
[10, 1I] [13, 14] [14] [4] [131 [13, 8, 9] [7] [7] [6] [12] [13, 5] [4] [4] [5, 10]
PCM for thermal storage
NaCLO 3 *LiNO3 MgCI2 • 6H20 • Pentaerythritoi {2-2-Bis hydroxymethyl 1,3-propanediol} CsHt204 2,2-Bis(hydroxymethyl) (proprionic acid) 2-Amino-2-(hydroxymethyl) 1,3-propanediol Adipic acid Thermkeep {91.8"/, NaOH, 8°/0 NaNOj, 0.2% MnO2} • Draw salt {50°/. NaNO3, 50% KNO3} • K-salt (50% NaNOs, 35% KNO3, 15% NaNO2) NaOH-KOH (50%, 50*/,) Solder (Pb-Sn, 40%, 60%) Anthracene Urea (NH2CONH2) Phthalimide (C6I-I4 (CO)2NH) Hydroquinone ~453
363 306-311 268-273
Tss (°F) 491 482 242 498 381-387 381-387 306 559 440 390 338 361 424 271 453 342
TSL (°F)
1.328
0.9
1.87 1.89
2.38
Specific gravity
Table 2. Phase-change storage materials
68.946
Molecular wt (g)
64 ~ 1! 38.5 57.6 47.9 58.8
49.7 87.8 41.2 8.8 6.4 6.0 59.6
Lsg (cal/g)
98
72.5 68.6 67.6
Lss (eal/g)
78.1
209
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684
RIAHI: EFFICIENCY OF HEAT STORAGE IN SOLAR ENERGY SYSTEMS
350°F. The need for heat storage came from the fact that a solar oven receives its heat from the radiation of the sun. Since there is no radiation between sunset and sunrise, the oven would have to keep its temperature hot throughout the night or at least a good portion of it. Reconsidering the minimum temperature for cooking and realizing that 350°F would be the moderate temperature for effective cooking, the results would change as well. Another important factor is that the material used for heat storage (pentaerythritol) has a phase-change temperature of 363°F (Table 2), which means temperatures lower than 363°F will bring the pentaerythritol to its original phase and, consequently, will not keep the heat for much longer. For the latter and previous reasons, the minimum effective temperature will have to be set at 360°F. By analyzing the data once more, it became evident that, for 14 h of receiving heat in the first run, the heat generator was able to keep the desired heat of 360°F up to 1 ~ h. As for the second run, it is obvious that, for only 8 h of heating the pentaerythritol, it stayed above 360°F for 16 h. The difference in behavior of the heat generator can be attributed to the following: change of insulation, change of the heater, and change in the speed of the pump. There are a number of methods that can be applied to improving the efficiency of the heat exchanger. As was observed by the change in the second run, perhaps the most important factor is the proper insulation of the box. Almost doubling the layers of fiberglass and mineral fibers in the second run, plus the fact that the layers were put together more tightly than before, contributed enormously to the improvement of the heat exchanger. The overall improvement from the first run to the second was approx. 90%. The second element involved was the heater. By using a submerging heat in the oil well, the heat loss between the heater and the circulating oil was eliminated. This also contributed to the length of time required for the circuit temperature to reach 420°F. It is observed from the figures that the required time was reduced from 14 to 8 h. The speed of the pump was set at 4.5 for the first run. Since the numbers on the pump represented no volumetric value, the pump's output had to be measured (Table 1). After the measurement, it was noticed that the volume of the oil circulated was 120 ml/s. For the second run, the speed was set at 3.2. The new number represented a circulation of 62 ml/s of oil. There were some other factors involved in the performance of this system that can be improved to obtain a more desirable result. First is the component sizes. By designing larger-surfaced fins and also increasing their numbers, the system can be more efficient. Second is the mass of the pentaerythritol in the heat generator. By reducing the mass of pentaerythritol to the exact amount that is capable of going through phase-change, the system becomes more time-efficient. The third factor involved in making improvements is the length of time needed by the system to reach the phase-change condition. Based on the available data, especially from the second run, the length of time that the heat generator kept the absorbed heat was approx. 16 h. Since the time required for the box to reach 420°F was 8 h, improvements need to be made to reduce that time to approx. 5 h instead; or even shorter. This is due to the fact that an average bright day of the year has approx. 4-6 h of effective radiation that can be collected for the purpose of this research. Improvements can take place by employing one or more of the methods mentioned above. In conclusion, this investigation proved that the idea of cooking food at night through using solar energy is feasible and can be done practically, as described. Acknowledgement--The author is grateful to the Departments of Physics and Technology at Northern Illinois University
for providing the necessaryfacilities for the present study. REFERENCES
1. National Iranian Oil Company, Annual Publication, 1979, pp. 110-114. Persian Translation, Tehran, Iran (1979). 2. R. M. Green and D. K. Otteson, Solar Energy in the Seventies, p. 34. OSHA and Ronald M. Green, New York (1976). 3. O. J. Kleppa, "Heat of Fusion of the Monovalent Nitrates by High Temperature Reaction Calorimetry." J. Chem. Engng Data 8, 331 (July 1963). 4. F. Baylin and M. Merino, A survey of sensible and latent heat thermal energy storage projects. SERI/RR-355-465, p. 386. Solar Energy Research Institute, Golden, Colo. (May 1981). 5. T. T. Bramlette et al., Survey of high temperature thermal energy storage (March 1976). 6. D. K. Benson et al., Materials research for passive solar systems: solid state phase change materials. SERI/TR-25511828, p. 124. Solar Energy Research Institute, Golden, Colo. (March 1985).
RIAHI:
EFFICIENCY OF HEAT STORAGE IN SOLAR ENERGY SYSTEMS
685
7. C. T. Lynch (Ed.) CRC Handbook of Material Science, Vol. II, p. c-451. CRC Press, Cleveland, Ohio (1983). 8. CRC Handbook of Chemistry and Physics, 50th edn, p. c4/52. CRC Press, Cleveland, Ohio (1976/1977). 9. B. M. Cohen, Comstoek and Wescott Inc., 765 Concord Avenue, Cambridge, MA 02138, Private communication. 10. W. A. Beckman and J. A. Duffle, Solar Engineering of Thermal Processes, p. 231. Univ. of Wisconsin, Madison, Wis. (1984). 11. SERI/TR-631-647, p. 212. Solar Energy Research Institute, Golden, Colo. (March 1983). 12. J. W. Mullin, Crystallization, 2nd edn. CRC Press, New York (1979). 13. J. O. M. Bockris, Energy Options, p. 234. Wiley, New York (1980). 14. S.S.E.A., Performance Table of Materials, The Society of Solar Engineers of America, Golden, Colo. (1979).