experimental and theoretical investigation of latent heat storage for water based solar heating...

~ Pergamon Energy Convers. Mgmt Vol. 36, No. 5, pp. 315-323, 1995

Copyright © 1995 Elsevier Science Ltd 0196-8904(94)00059-X Printed in Great Britain. All rights reserved

0196-8904/95 $9.50 + 0.00

EXPERIMENTAL AND THEORETICAL INVESTIGATION OF LATENT HEAT STORAGE FOR WATER BASED SOLAR

HEATING SYSTEMS

KAMIL KAYGUSUZ Department of Chemistry, Karadeniz Technical University, 61080 Trabzon, Turkey

(Received 29 September 1993; received for publication 23 November 1994)

Abstract--An experimental and a theoretical study has been conducted to determine the performance of phase-change energy storage materials for solar water-heating systems. Simulation techniques are used to determine the system performance over the entire heating season. Variations of tbe solar-supplied fraction of the load with storage mass and collector area for water-based systems with sensible and latent heat storage have been examined. The variation of the outlet fluid temperature with different values of NTU and the variation of the stored energy with time for a phase change material (CaC12 • 6H20) are also investigated.

Latent heat Energy storage Solar heating Phase-change material

N O M E N C L A T U R E u = Specific internal energy of PCM T = Temperature of PCM

T* = Melting temperature of PCM p = Density of PCM k = Thermal conductivity of PCM

Tr = Temperature of circulating fluid (water) Pr = Density of circulating fluid (water) Cr = Specific heat of circulating fluid (water) Ar = Flow area of circulating fluid (water) kf = Thermal conductivity of circulating fluid (water) m = Flow rate of circulating fluid Z = Position along storage unit in flow direction U = Overall heat transfer coefficient between PCM and water when m > 0

U" = Overall heat transfer coefficient between PCM and water when m = 0 x = Liquid fraction of PCM 2 = Latent heat of fusion of PCM C = Average specific heat of PCM

T~f = Reference temperature when PCM internal energy is zero NTU = Number of transfer units (NTU = UPL/mCf)

L = Storage unit length (m) P = Perimeter of storage material (P = 2nrcNc)

Vst = Inside volume of energy storage tank (m 3) Ast = Inside surface area of energy storage tank (m 2) N, = Number of cylindrical PVC containers rc = radius of cylindrical PVC containers

PVC = Polyvinyl chloride plastics Tn, = Collector fluid (water) inlet temperature (K)

Trout = Collector fluid (water) outlet temperature (K) Tow, = Outlet water temperature of store (K) Tind = Indoor air temperature (K)

T a = Ambient air temperature (K)

I N T R O D U C T I O N

I t is e s t i m a t e d t h a t , a t t h e p r e s e n t c o n s u m p t i o n r a t e o f fo s s i l fue l s , t h e w o r l d ' s foss i l fue l r e s e r v e s m a y b e d e p l e t e d c o m p l e t e l y in t h e n e x t 100 y r o r so [ i ] . T h e r e f o r e , e n g i n e e r s a n d r e s e a r c h e r s all o v e r t h e w o r l d a r e in s e a r c h o f n e w a n d r e n e w a b l e e n e r g y s o u r c e s . D e v e l o p i n g e f f i c i en t a n d i n e x p e n s i v e e n e r g y s t o r a g e s y s t e m s a n d d e v i c e s is, h o w e v e r , a s i m p o r t a n t a s d e v e l o p i n g n e w e n e r g y s o u r c e s .

315

316 KAYGUSUZ: WATER BASED SOLAR HEATING SYSTEMS

o

220

200

180

160

140

120

100

80

60

OOO Actual PCM _ energy storage

40 --

0 energy[s toragej

20 22 24 26 28 30 32 34 36

Temperature (°C)

Fig. 1. Performance comparison of PCM, water and rock storage system [2].

Thermal energy storage has always been one of the most critical components in residential solar space heating/cooling applications. Solar radiation is a time-dependent energy source with an intermittent character. The heating demands of a residential house are also time dependent. However, the energy source and the demands of a house (or building), in general, do not match each other, especially in solar heating applications. The peak solar radiation occurs near noon, but the peak heating demand is in the late evening when solar radiation is not available. Thermal energy storage provides a reservoir of energy to adjust this mismatch and to meet the energy needs at all times. It is used as a bridge to cross the gap between the energy source, the sun, the application and the building. So, thermal energy storage is essential in the solar heating system [2]. Thermal energy storage units may be classified into two groups: the specific-heat type and the latent-heat type. In the specific-heat type, heat is stored as a sensible heat. Water and rocks have been considered as the most practical heat storage materials, since they have higher heat capacity and negligible cost. Heat can be transferred from the working fluid to rocks by direct contact. In the case of water, the heat exchange is through a separating solid wall. The latent heat type heat storage

I -7)-J__ I

Fig. 2. Schematic diagram of the base solar energy system.

~ ] Conditioned air

o" 0

Return air

KAYGUSUZ: WATER BASED SOLAR HEATING SYSTEMS 317

Table I. Water-based system parameters

Collector Number of glass covers 1 Thickness of glass cover 0.004 m Refractive index 1.45 Collector plate absorptance 0.90 Collector emittance 0.85 Collector efficiency factor 0.85 Back and side losses 1.20 kJ/h m 2 K Mass flow rate 40 kg/h m 2 Total collector area 30 m 2 Number of collectors 18

System circuit pipe Length 40 m Diameter 0.04 m Heat loss 20 kJ/h K Fluid density 1000 kg/m 3 Fluid specific heat 4.197 kJ/kg K Ambient temperature 18°C

Energy storage tank Volume 3.65 m 3 Thermal loss 0.210 W/(m 2. °C) Shape (L/D) 2.46 Ambient temperature 18°C

unit uses materials which melt at a moderate temperature level and store heat as their heat of fusion. Low-cost salt hydrates with high latent heats of fusion are employed for this purpose.

The use of phase change materials (PCMs) for thermal energy storage in solar heating and cooling systems has received considerable attention. The motivation for using phase change energy storage (PCES) materials is the reduction in storage volume which can be achieved compared to sensible heat storage systems (see Fig. 1). Abhat [3] reviewed low temperature PCMs in the temperature range 0-120°C and investigated their melting and freezing behaviour. Short term heat storage systems, utilizing salt hydrates which melt in their crystallization water, have been proposed by many researchers [4-7]. The most studied PCMs include Glauber's salt, which melts at 32.2°C, sodium thiosulfate pentahydrate, which melts at 56°C, sodium carbonate decahydrate, which melts at 33.5°C, disodium phosphate dodecahydrate, which melts at 36.2°C and calcium chloride hexahydrate, which melts at 28.4°C. Calcium chloride hexahydrate has especially received considerable attention because the commercial calcium chloride hexahydrate (produced by Dow Chemical Co.) is cheaper and has better thermal stability and resistance to corrosion than the other salt hydrates [8-12].

In the present study, an experimental set-up was constructed to determine the performance of the thermal energy storage materials, collectors and energy storage tank that is filled by the PCMs, as used for domestic heating. In addition, the original SOLSIM computer program[13] was modified to include some parameters in the experimental set-up, and it was used to calculate the performance of the collectors, the energy storage capacity of the phase change material (PCM), the heating load of the laboratory building and the solar-supplied fraction of the load for Trabzon

Table 2. Some features of the calcium chloride hexahydrate

Melting point 28'~C Density 1500 kg/m 3 Heat of fusion 45 kcal/kg Supercooled degree I'C (by nucleating agent) Heat storage capacity 7.6 x 104 kcal/m s Price 0.35 $/kg Number of heat cycles 850 (experimental) Poisonous non Specific heat

Solid 0.34 kcal/kg C Liquid 0.50 kcal/kg C

Conductivity Solid 1.085 W/(mK) Liquid at 50 C 0.550 W/(mK)


Table 3. Construction properties of the laboratory building

Window area (single glass, U = 4.8 W/m 2 °C-day) 75 m 2 Wall area (single brick, U = 1,6 W/m z °C-day) 60 m 2 Floor area (concrete, U = 2.5 W/m 2 °C-day) 75 m 2 Ceiling area (concrete + flat metal, U = 2.0 W/m2-day) 75 m 2 Effective UA [(kWh/(°C-day)] 0.800 Comfort temperature 22°C Average degree days for heating season 2500 Average total heating loading in heating season (kWh) 19,500

Table 4. Climatic conditions of Trabzon in heating season

Average outdoor temperature Minimum outdoor temperature (February) Average minimum outdoor temperature Maximum outdoor temperature (May) Average maximum outdoor temperature Average relative humidity Average wind velocity Average solar insolation

9.35°C - 3.40°C

2.30°C 29.10°C 22.91°C 74.85% 2.50 m/s 5.10 MJ/m 2 day

by using meteorological data and experimental results during the heating season in 1991 (from December to May).

E X P E R I M E N T A L S E T - U P

The water-based system investigated in this experimental study is shown in Fig. 2, and the system parameters are listed in Table 1. As shown in Fig. 2, this system consists o f the solar collector, energy storage tank, water-to-air heat exchanger, auxiliary electrical heater, water circulating p u m p and other measuring and control equipment. Whenever solar energy is available, it is collected and transferred to the energy storage tank that is filled by 1500 kg encapsulated phase change material (PCM). The thermal properties o f the storage media used in the present study are given in Table 2. Whenever a space heating load is present, it is met by the energy storage tank and the auxiliary energy source.

From heat source * D To load

i- L -!

Fluid i n ~ PCM . _ X ~ r e Fluid out

Tfl m ~ - -- " - ~ - -~/t; ~ Tf°

Fig. 3. Schematic configuration of the energy storage tank.


In this study, for the heating purpose, we used a laboratory building with 75 m 2 floor area. The structural properties of the laboratory building are given in Table 3. Table 4 summarizes the climatic conditions of Trabzon, Turkey in 1991.

C O N F I G U R A T I O N O F T H E S T O R A G E T A N K

Figure 3 shows the configuration chosen for the storage tank. It consists of a vessel packed in the horizontal direction with cylindrical tubes. The energy storage material (CaCI2 • 6H2 O) is inside the tubes (the tubes or containers are made of PVC plastic) and the heat transfer fluid (water) flows parallel to them. The storage tank contains cylindrical PVC containers filled with PCM. The void fraction (the ratio between the fluid volume and the storage tank volume) is 0.3. The inside volume and inside surface area of the energy storage tank are, respectively, given by Vs~ and Ast. The number of cylindrical PVC containers inside the storage tank is N c. The radius of the cylinder containers is re, and the length of the cylindrical tube containers is given by L. Also, the radius and length of the energy storage tank are given by Rst and Lst , respectively. The r c and L are 0.03 and 3.0 m, respectively. Also, the Rst and L~ are 0.65 and 3.0 m, respectively. The rc/L is 0.01 and this ratio is small enough to minimize radial heat conduction in the storage material.

T H E O R E T I C A L ANALYSES

In this study, we used a theoretical model developed by Morrison and Abdel-khalik [4] because the supposed flow model of Morrison and Abdel-khalik and our flow model in the experimental study for PCM are approximately the same. So, the following assumptions are made:

(a) The PCM behaves ideally, i.e. such phenomena as property degradation, supercooling and crystallization are not taken into account, and the PCM is assumed to have a definite melting temperature.

(b) Heat losses from the storage unit to the surroundings are negligible. (c) Infinite thermal conductivity for the PCM in the direction normal to the flow direction, which

means that the Biot number is sufficiently low that the temperature gradients in the storage material normal to the flow direction can be ignored.

(d) During the flow mode, axial conduction in the circulating fluid is neglected. (e) The number of transfer units of the storage unit, NTU, is sufficiently large so that the local

film temperature difference between the fluid and the PCM can be ignored.

Based on these assumptions, energy balances for the PCM and the circulating fluid (water) during the flow mode yield the following equations:

60 F . ,o ~ _O ~ ~ o o~O

los G O,o t ,o

o / - . j /

-~ o / o/ / Trabzon = Water-based system

20 -- e / / CaCI2. 6H20 (Tri = 60°C) o/

l0 L I L I I I I 2 4 6 8 10 12 14

T i m e (h)

Fig. 4. Variation of outlet fluid temperature with different values of NTU for calcium chloride hexahydrate.

320 K A Y G U S U Z : W A T E R B A S E D S O L A R H E A T I N G S Y S T E M S

5 -

NTU = co

4 - - ° / o / ° ~ ° ' ~ ° ~ ° / NTU = 20

_ / _o~O --'---°~° 3 - / o ~ ° / "

o 2

0 I I 0 I 2 3 4 5 6 7

T i m e (h)

Fig. 5. Var ia t ion of s tored energy with t ime for CaC12 - 6H20 .

and

dU k d2T UP dt p dZ 2 + - ~ (Tr - T)

(l)

dTr m dTf UP

d T + - - pfAf d Z - prArCr ( T Tf).

The boundary conditions for the above equations are:

dT

{2)

(3)

and

Tr(0, t) =f i t ) . (4)

Equations (3) show that there are no end losses from the PCM. In equation (4), the inlet fluid temperature is a function of time, f ( t ) , to be determined by simultaneously solving the governing equations o f the different system components.

80 -- A = 100m 2(theoretical) c Q_

/ e ~ e , , - - - - ' e ~ e ~

~ o A, = 50 m 2 (theoretical) " 60 O / c , . . . . . O . . . , ~ o _

o ~ o ~ O ' ' ' ~O

• 2 -- J A c = 30 m (experimental) '3 40

..,..~ / o e / e ~ e . . . . ~ o e • •

20 / o J Water-based system CaCI 2 . 6H20 Trabzon

0 I I I I I 10 20 30 40 50

S t o r a g e m a s s pe r un i t c o l l e c t o r a rea ( k g / m 2)

Fig. 6. Var ia t ion of solar-suppl ied fract ion of the load with s torage mass and col lector area for water-based systems with PCES in Trabzon.


80

| . / " ~ ~ ~ ~ (theoretical) ~, 6o I-- / / ; " . ~ - . . . . . . . . : .

1 . / f~ /" . . ~ : : ,0 m= I / / , " - . eoret,cal,

'~ 40 / ~ / " f " ~ £ = 30 m 2

~" / / / / ~ ' ~ Trabzon (ttleoretlca,) o / J / " / / " Water-based system

r~ 20 / / / ~ . ~ . CaCI2.6H20 / . . . . Na2SO 4 • 10H20

. . . . Water I I I I I

0 l 0 20 30 40 50

Storage mass per unit co l lector area (kg/m 2)

Fig. 7. Comparison between the variation of solar-supplied fraction of the load with storage mass and collector area for water-based systems with sensible and latent heat storage in Trabzon.

The specific internal energy U is related to the temperature T and liquid fraction X by the following equations as

so that

U = c ( T - T,~r) + X " 2

X = 0 for T < T*

0~<X~<I for T = T *

X = 1 for T > T*. (5)

In equations (5), supercooling effects are neglected, i.e. the PCM is assumed to behave ideally with a constant melting temperature and latent heat.

During the conduction mode, the working fluid (water) ceases to flow through the storage unit. The governing equation for the circulating fluid becomes as follows

dTr kf da2Tr U ' P T dt =Cfpf dZ 2 q-Cr---~fhf ( - T r ) . (6)

The governing equations and boundary conditions for the PCM during the conduction mode are similar to those for the flow model [equations (1) and (3)] with U replaced by U'. The boundary conditions for equation (6) are

1000 --

800

600

"" 400

200

,g

, o / - : g ' < - , , / 4 0 ~ O O'-- /Lo _ "~

/l•" \ ~o.

~ / / \ ' ,

December 1990 - - - January 1991 - - . - - February 1991

o I I 10 12

Time of day

Fig. 8. Radiation distribution throughout the day.

o ~ \

I • i l 14 16


dZ .

d T f -'=1 dZ

(7)

RESULTS AND D I S C U S S I O N

For water-based solar heating systems, an experimental and a parametric study for the energy stored was conducted because significant differences between the finite and infinite NTU models were expected due to the relatively large heat capacity of the water. Figure 4 shows the variation of the fluid outlet temperature with time for CaCI2 • 6H20 at different values of NTU. These figures have been generated for a constant inlet fluid temperature (T~ = 60°C) during the heating mode only. As the NTU increases at a given time, the outlet fluid temperature becomes smaller, so more heat is removed from the fluid, and then more heat will be stored in the material. This figure shows that there is a significant difference between the finite and infinite NTU models (this result is approximately the same as the previous studies [4, 6]). In the theoretical analysis, two models were considered; the finite NTU model and the infinite NTU model. However, in the finite N TU model, two conduction heat transfers in the PCES are considered; the axial conduction which is called the one-dimensional (l-D) and both axial and radial conduction which is called the two-dimensional (2-D) finite NTU models as described by Ghoneim [6].

Figure 5 shows the variation of the stored energy with time for calcium chloride hexahydrate. There is a little difference between the finite NTU model l-D, and the finite N TU model 2-D, but there is a significant difference between the values of the stored energy obtained by using the infinite NTU model and the finite NTU model of about 25%.

The effects of the storage capacity on the long term performance of water-based systems with PCES are shown in Fig. 6 for Trabzon. These experimental (for Ac = 30 m 2) and theoretical (for Ac = 50 and 100 m 2) results are based on simulations and experiments for a 6-month heating season from December to May in 1991. As seen from Fig. 6, the solar-supplied fraction of the load is increased with the increase of the storage mass per collector area (kg/m 2) and collector area (At). Therefore, the optimum storage mass per collector area should be determined by a detailed economical analysis.

Figure 7 shows the comparison between the variation of the solar-supplied fraction of the load with the storage mass and collector area for water-based systems with sensible and latent heat storage in Trabzon. This theoretical result shows that the solar-supplied fraction of the load is increased with the increase of the storage mass and collector area for water-based systems with sensible and latent heat storage.

325 --

I ~osO~O-O~O~.o o , .° ".o x

315 / ° / - - ' o ' ° - ° ' 8 ~ e . ~ . ./e ..o...o - e , o o , o " , * 2 o - ° . o - o - o - o . o x o o.,

/ ..- % . - . . 305 e . ~ • ' ' e ~ir..8.~8

285 9.00 1100 13.00 15.00 17.00 19.00

Time of day

Fig. 9. Variation of temperature with time of day: (I) Tt~,u~; (2) To~; (3) T~i,; (4) Ti,~; (5) T~,


Figure 8 shows the variation of the total solar radiation on the tilted surface with time of day for December, January and February. As is shown in Fig. 8, the total solar radiation is a maximum in February around solar noon, and its value is approx. 900 W/m 2.

Figure 9 shows the variation of T~n, Trout, Tows, T~nd and Ta with time of day for the solar water heating system with energy storage at a typical day over the heating season in 1991.

C O N C L U S I O N S

An experimental solar heating system with solar energy storage in encapsulated phase-change material packings was presented and investigated during the heating season (from December to May in 1991) for domestic heating in Turkey. Also, a theoretical investigation was made by using the model that was presented by Morrison and Abdel-khalik [4]. In the theoretical study, a simulation program was used to determine the effects of the phase-change energy storage on the solar heating system.

During the heating season, the measured values of the mean collector and storage efficiencies are 0.60 and 0.70, respectively, with 30 m 2 water-cooled solar collectors. The solar-supplied fraction of the load was not as good as the collector and storage efficiencies for the same collector areas with PCES, and its maximum value is around 0.30-0.35 (see Figs 6 and 7) because our region has more cloudy days.

On the other hand, the solar water heating system simulation with SOLSIM for the Trabzon climate shows that the solar-supplied fraction of the load is increased with an increase of the collector area and storage mass for water-based systems with sensible and latent heat storage. Therefore, the opt imum collector area and storage mass should be determined by a detailed technical and economical analysis.

Based on these simulations, we find that the solar heating system with Na2SO 4- 10H20 has slightly higher F values than those with the same mass of CaCI2.6H2 O, provided that the thermal properties of the PCM do not degrade from the cyclic operation of the system. Little gain in F is realized for increases in storage capacities per unit collector area above 20 kg/m 2, while the F values drop below 5 kg/m 2. These results are valid for the water-based system with PCES in Trabzon.

From the experimental and theoretical investigations, we concluded that heat storage is an important component in moderate climatic conditions such as those encountered in Trabzon, and for this purpose, a PCES using calcium chloride hexahydrate or sodium sulfate decahydrate can be used as a thermal energy storage unit and would provide a desirable alternative to rock and water storage systems. Especially in a solar-assisted heat pump system for domestic heating, the PCM stores energy from the solar collectors as a latent heat at a nearly constant transition temperature (during melting and solidification) so it can be used more preferable as a heat source than the water and rock storage for a heat pump because the energy storage temperature of the PCM is around 25-35°C for both calcium chloride and sodium sulfate. These temperature intervals are suitable for solar-assisted heat pump applications in our region.

R E F E R E N C E S

1. S, Kakaq, E. Paykoq and Y. Yener, NATO ASI Series, pp. 129-161 (1989). 2. B. K. Huang, M. Toksoy and Y. A. Cengel, Sol. Energy 37, 279 (1986). 3. A. Abhat, Sol. Energy 311, 313 (1983). 4. D. J. Morrison and S. I. Abdel-khalik, Sol. Energy 20, 57 (1978). 5. J. J. Jurinak and S. I. Abdel-khalik, Sol. Energy 22, 355 (1979). 6. A. A. Ghoneim, Sol. Energy 42, 209 (1989). 7. A. A. Ghoneim and S. A. Klein, Sol. Energy 42, 441 (1989). 8. M. Telkes, ASHRAE J. 16, 38 (1974). 9. B. Carlsson, H. Stymne and G. Wettermark, Sol. Energy' 23, 343 (1979).

10. F. C. Parisini, Sol. Energy 41, 193 (1988). 11. N. Giiltekin, T. Ayhan and K. Kaygusuz, Energy Convers. Mgmt 32, 311 (1991). 12. A. Brandstetter, Sol. Energy 41, 183 (1988). 13. J. R. Howell, R. B. Bannerot and G. C. Vliet, Solar-Thermal Energy Systems, pp. 330-350. McGraw-Hill, New York

(1982).

experimental and theoretical investigation of latent heat storage for water based solar heating...

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