Chemical Storage of Solar Energy Using an Old Color Change Demonstration L. Gene Spears, Jr. Rice University, Houston, TX 77005 Larry G. Spears University of Houston Downtown, Houston, TX 77002 One of the major problems associated with the utilization of solar energy as a primary energy source for heating and cooling is the lack of a practical, efficient means of storing large amounts of energy after it has been collected ( I ) . Below we describe the results of a student research project that could he used as a class or erouo experiment a t the hiah school or first-year college leva to flluskate the potentialof hydrated salts for solar e n e m storsae. I t involves a demonstration often used t o illustrate ihe easewith which some transition metal ions can change their coordination numbers (2). The demonstration requires a solution, containing approximately 10 g CoC126Hz0per 100 mL of isopropyl alcohol, that has been made distinctly pink by the dropwise addition of water. At room temperature, the pink colorof the solution is due to the octahedral complex, C O ( H ~ O ) ~On ~ +heating . in a hot water bath, the solution changes to a deep hlue color due to the formation of the tetrahedral complex, C O C ~ ~This ~ - . system can be described by the equation CO(H~O+ ) ~4C1~+
+ heat
+
CoCL2- + 6H20
Since this reaction is endothermic, it provides a potential means of storing solar energy via the unhydrated Cock2- ion. On cooling (the reverse reaction), energy would he released based on the heat of hydration and the specific heat of solution. To determine the potential capacity of this system for solar energy storage it is necessary to know the following: (1)the solubility of CoCIz in isopropyl alcohol at different temperatures, (2) the heat of hydration for CoC142-, (3) the specific heat of the solutions, and (4) the effects of Hz0 and C1- on the equilibrium system. Based on these data and published data on solar energy systems and energy use, calculations can he made to determine the amount of CoC1~6H~O/isopropyl alcohol solution that would he needed to store the necessary energy required for a average residence in the United States. to other hvdrated The same ~roceduresmav also he applied .. salt systems in anhydrous solvents. The different sets of data needed to complete this proiect may be assigned to different students or groups. he only items of equipment needed to obtain the data are a balance, constant temperature bath, drying oven, and a visible-range spectrophotometer. Expanded polystyrene cup calorimeters can he used for determining the heat of hydration and the specific heat. The needed solar energy and energy use data can he obtained from a number of publications (3). Experlmental Procedures Determination of Calibration Plots In order to measure the amounts of Co(H20)fi2+ and CoC142- present in the test solutions, Beer's Law plots must be determined since it is necessary to use the Beer's Law exPan of Ihis paper was presentedat the Sinh International Conference on Chemical Education. University of Maryland at Co.lege Park, August, 1981.
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Journal of Chemical Education
Figure 1. The visible spectra of Co(HPb2+and COCI,~-,
pressions for a system with two ahsorbing species (4). A s~ectro~hotometric scan of both the pink and blue solutions indicated a maximum absorbance of the pink complex at 513 nm and a maximum absorbance of the blue complex at 655 nm (see Fig. 1). Calibration plots of absorbance versus molarity were made for CoCla2- at 513 and 655 nm and for Co(H~0)6~+ a t 513 and 655 nm, using a cell pathlength of 1.0 cm, and the following molar absorptivity values determined (P = pink complex and B = hlue complex) 513 nm
655 nm
a(L1mole-cm) a ~=' 18.0 ap' = 5.46 ap" = 0.60 a ~ =" 491
Since the molar absorptivity constant ap" is very small compared to ae", it can he approximated that a t 655 nm
However a t 513 nm the relatively large value for ae' requires that cp =
A513
- (IB'~B'CB
ap'bp' Thus, it is necessary t o determine c e before calculating the value for cp. Solubility Determinations The amount of cobalt chloride which can he dissolved in isopropyl alcohol solutions would he important t o the efficiency of a solar energy storage system; the more that can he dissolved, the greater the energy storage capacity. For these determinations twelve solutions were prepared using three different concentrations of water and heated at four different temperatures. Large amounts of CoC1~6Hz0were placed in each solution to insure saturation. After one hour, the values for ce and cp were determined from ahsorhance measure-
0
16
24
32
40 Tempntm, .C
64
56
48
Figure 3. ~ f f e cof t temperature and ti.0 on the COCI~~-ICO(H.O~~+ ratio
Figure 2. Lagarithmic plots ot the solubility of CoCi24H20)~ in isopropyl alcchal/H20 ~ o l ~ t i o atndifferent ~ temperatures.
ments, and then added to obtain the total concentration of dissolved cobalt chloride. The results of these measurements are shown in Figure 2. Determination of ProductlReactant Ratios
For the reaction in this study the equilibrium constant expression can be written as [COCI~~-][H~O]~ K - [CO(HZO)~~+][C~-]~ Due to the difficulty in measuring values for the free water concentration in this system, values for the ratio [COCI~~-]/ [Co(H20)62+]were used instead of K,,. Effects of HzO on the C O C I ~ ~ - I C ~ ( H ratio. ~ O ) ~Fifteen ~+ solutions were prepared with five different concentrations of added water and heated a t one of three different temperatures. After one hour of heating, the productlreactant ratios were determined and plotted as a function of temperature (Fig. 3). This data is important since it shows the sensitivity of the system to the water content of the solvent. Another set of measurements was made at room temperature to further illustrate the sensitivity of this equilibrium system to water. For this experiment seven solutions of varying water concentration and a constant total concentration of cobalt chloride were prepared and their CoCla2-/ C O ( H ~ O ) ratios ~ ~ + determined after one hour. The sample with the highest water concentration was selected as a reference and a plot of the per cent increase in the CoCb2-/ Co(H20)62+ ratio versus the per cent decrease in water concentration was made using the other six sets of data (see Fig.
)~~~ Figure 4. Effect of H P on the C O C I . ~ - / C O ( H ~ Oratio.
150r
4).
Effects of C1- on the C O C ~ ~ ~ - / C O ( Hratio. Z ~ )For ~ ~this + experiment five solutions of varying chloride concentration and a constant total concentration of cobalt chloride were prepared and their productlreactant ratios measured after one hour at room temperature. The sample with the lowest chloride concentration was selected as a reference and a plot of the per cent increase in the C O C I ~ ~ - / C O ( H ~ratio O ) ~ versus ~ + the per cent increase in Cl- concentration was made (see Fig. 5).
0
150
300
% rnerease in
450
600
a-cone.
Figure 5. Effect of Ci- on U?e CoC142-/Co(H20)s2+ ratio.
Volume 61
Number 3
March 1984
253
Determination of Specific Heat
Volume of E n e-. m Storage - Solution Needed
The specific heat describes the amount of heat a particular solution can absorb. A solution of 0.1 M cobalt chloride in isopropyl alcohol was selected as representative for this study, and a douhle-walled expanded polystyrene, student-type calorimeter used for the measurements (5).A value of 3.37 J I g P C was determined.
For these calculations it is assumed that t h r e n r r storage ~ solution is cirrulated onlvonre in a 24-h period and that the temperature of the cooled solution is 2 0 0 ~ and that of the heated solution is 60°C. The heat absorbed per liter of the solution is
Determination of the Heat of Hydration of C
a
The major reason for considering a hydrated salt system for energy storage is that the heat of hydration associated with the salt can store and release larger amounts of energy than just the specific heat associated with the heatingand cooling of a liquid such as water. Using a modified Nalgene" Dewar flask with cover for a calorimeter and a previously reported method for determining the heats of hydration (6), a value of 92.7 kJ1mole was obtained for the heat of hydration of CoC12. This value is higher than a reported literature value of 76.8 kJ1mole for the heat of solution of CoClo (7).and a value of 89.4 kJ/mole calculated based on the heats'of solution for CoC12 and CoC126H20 (8).
Energy Calculations Some basic properties of a solar heating system using cobalt chloride as a heat storage medium were calculated based upon data collected in this study and from various published documents. Daily Heat Energy Requirement for the Average U.S. Residence
Assuming that the average U.S. residence of 1500 ft2 requires 1000 therms of heat per year (31, the daily heat requirement is (1000thermslyr) X (1.055 X 108Jltherm) X (1yrI365 days) = 2.89 X 108Jlday Concentration of Cobalt Chloride
The solar collector was assumed to concentrate incoming heat a t a 4:l ratio, and to operate a t a mean temperature of 60°C. Using Figure 2, the concentration of cobalt chloride in a saturated solution of 100% alcohol was determined to be 10.47 M a t 60% Heat Content of Solution
The heat content of the solution is due to both the heat of hydration and the specific heat. In Figure 3, it is seen that for 100% alcohol a t 60°C, the CoC142-/Co(H20)62+ ratio is 0.64 and a t 20°C it decreases to 0.44. From these values it can be determined that a t 60°C, 39% is unhydrated, and a t 20°C, 30.6% is unhydrated. Thus on cooling from 60°C to 20°C, 8.4% of the C o C l P undergoes hydration. (92.7 kJ/mole) X (10.47moles/l) X (0.084) = 81.5 k J L T h e specific heat of 1.0 L of a 10.47 M solution is approximately (3.37 JIgPC) X (0.793 g1mL) X (1000 mL/L) = 2.67 k J P C L
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Journal of Chemical Education
81.5 kJ/L + 2.67 kJIoC/L X 40'C
= 188 kJ/L
Since the amount of heat needed per day is 2.89 X los Jlday, the required volume of solution needed per day is (2.89 X 105 kJ1day) X (1Ll188 kJ) = 1537 Lldsy = 1.067 Llmin. Volume of Storage Tank Required
T h e volume of the heat storage tank required to store a one-day supply of heat using the above conditions is thus 1537 L o r 1.537 m? If a 50% efficiency factor for heat storage is assumed, a 7.9-ms tank could store a three-day supply of enerpy. 11only watrr wrre t o be usrd for energy storage under these conditions. n volume of 10.4 m.'would be required. Obviously. iithe solution were circulated faster than 1.067 Llmin. more energy would be available and thus a smaller storage volume would be required. Circulation rates of 10-40 Llmin are commonly used for aqueous systems. ~
~~
Solar Heat Collector Surface Area Required
Assuming that the solar collector is exposed to 2.05 X 104 k J of heat per square meter of surface area per day, an accepted figure for the southern United States (3), the available solar energy per square meter per minute would be 14.2 kd. Earlier we found that 2.89 X lo5kJIday, or 200 kJ/min, of heat are needed for the average residence. Assuming a 100% collector efficiency, the area of collector surface needed is Assuming a more realistic figure of 50% efficiency for the collector, 28.2 m2 of collector surface area would be required. Using a 4:l heat concentration ratio and a 1%collector tubing area to collector surface area ratio, 42.3 m2 of collector area would be needed for the solar heat collector. Possible Oil Dollars Saved by Using a Solar Heating System
Assuming that the residential heating requirements for the above residence are obtained only from imported oil, the oil dollars saved by using only solar energy would be (1.066 X 10" Jlyr) X (1barre113.06 X 109 J ) X
($34lbarrel)= $1,184
Literature Cited i l l Wentworth. W. E.,etal..SoiorEnorgy,26,141 (1981). (21 Chon, Philip S.,"Entertainincand Fducationsl Chemical Dcmonlrstions."Stockton santa ~ eCA , i974,p. 36. ~ r s d prim. e (81 Lnftness, Rokn L., ''Energy Handbook:' Van Norstrand Reinhdd Co., N w York.
