A “once-through” solar water heating system

Solar Energy, Vol. 29, No. 6, pp. 541-547, 1982 0038-092X/821120541-07503.0010 Printed in Great Britain. Pergamon Press Ltd.

A "ONCE-THROUGH" SOLAR WATER HEATING SYSTEM

Y. F. WANG, Z. L. LI and X. L. SUN

Department of Thermophysics, China University of Science and Technology, Hefei, China

(Received 18 August 1981; accepted 20 February 1982)

Abstract--In China, solar water heater is being popularized and most existing solar water heaters are the natural circulation system. Due to some shortcomings of the natural circulation system, a "once-through" system is proposed. In a once-through system, the storage tank can be placed below the collector, therefore, the load on the roof will be cut down significantly. This system also has the advantages of no mixing of hot and cold water, no reverse flow and being able to provide hot water earlier, etc. Both theoretical and experimental investigations have been conducted to compare the collector efficiences between the once-through and natural circulation systems and they coincide with each other very well. The once-through system performs worse in the morning but better in the afternoon than the natural circulation system and the difference of daily collector etficiency between these two systems is negligible.

INTRODUCTION

Solar water heating is one of the most attractive applications of solar energy at present. Solar water heaters have become a standard means of providing domestic hot water in some parts of the world. In China, solar water heating is progressively spreading. Probably differing from many other countries, most Chinese solar water heaters are used to supply hot water to public bathrooms which usually belong to enterprises or est- ablishments and are open to their employees. Under this situation, the water heater system has a fairly concentrated load at late afternoon when people finish work and take a shower before going home. In some cases, solar hot water is also used in restaurants, barbershops, etc. self-evidently, the pattern of load for these is different from the former case.

A common solar water heater consists of a fiat-plate collector, storage tank, piping, valves and in some cases, controls and pumps. Basically, there are two kinds of solar water heater system, natural circulation (or thermosiphon) system and forced circulation system[l], although a number of versions have been proposed. Figures 1 and 2 are the schmatics of these two system types.

A natural circulation system needs no control, pump or check valve. It is reliable and less expensive. In China, most existing solar water heaters are natural circulation system and usually do not have an in-tank auxiliary heater. When further heating is needed the solar system acts as a pre-heater of a coal-fired boiler. In practical use, the natural circulation system does have some in- herent disadvantages.

1. The storage tank must be placed higher than the collector. In urban area of big crowded cities like Beijing (Peking) and Shanghai, the flat roof of a large building is the most likely place for locating the major components of a typical institutional solar water system. Therefore, the roof will bear all the loads of collector, piping, storage tank and support. The water tank causes the most serious problem due to its considerable weight and

the fact that it acts as a concentrated load. Considering that the capacity of the storage tank is up to 1001. for each square meter of collector area, the weight of a full-charged tank, for a 100 m 2 array, will exceed 10 tons. Roof structures are not always strong enough to support such a system. Sometime, this becomes a main obstacle to setting up a natural circulation system. Of course, this problem could be alleviated by dividing the whole water storage into several separated smaller tanks, but exces- sive dividing will cause extra complexity to the pipe fittings and consequently increase the total cost.

2. When the storage tank is put on the roof, thick insulation is required to limit heat loss.

3. Water circulates through the collector several times a day and the temperature rise at each time is small, therefore, this system often can not supply hot water at desired temperature in the morning. For some users of solar water heater like restaurants and babershops, this system cannot match the morning load unless an auxiliary heater is engaged. Continuous circulation also causes some mixing in the tank between upper region hot water and lower region cold water, despite the fact that efforts are made to maintain stratification in the tank.

The first two problems can be solved by using a forced circulation system, but such a system is more expensive because a pump, check valve and controls are necessary

6 HOT WATER

COLD WATER

Fig. 1. Schematic diagram of a natural circulation system.

SE Vol. 29, No. 6~3 541

542 Y.F. WANG et aL

. , • _ HOT WATER A K,L,AR - TO LOAD

Q N J i COLD WATER PUMP CHECK VALVE

Fig. 2. Schematic diagram of a forced circulation system.

and requires electricity supply to operate the pump. Based on the above facts, a once-through system is proposed.

DESCRIPTION OF "ONCE-THROUGH" SYSTEM

Two once-through systems are shown schematically in Figs. 3 and 4. The name of "once-through" follows the name given to a once-through boiler. It indicates that cold water enters the system from one side and hot water flows out the other side without any circulation in the system.

If a temperature controlled valve (for instance, a temperature sensor controlling an adjustable valve) is installed (Fig. 3), a constant temperature at the upper header can be maintained by adjusting the flow rate. Obviously, in this system, the driving force for the flow is the head of the cold water supply, usually the tap water.

Hot water at servicing temperature is collected in an insulated tank, which can be placed anywhere below the collector array. It is often possible to put the storage tank inside the building and still have enough head for showering when the showerbath is on the ground floor.

A "once-through" system without the temperature controlled valve could be employed (Fig. 4), when a variable outlet temperature of the collector is acceptable. Such application include the solar system being used as a pre-heater or the temperature of collected hot water being higher than service temperature as when the solar hot water is expected to be mixed with cold water.

, COOTROL ] L-- T- ----~

- - COLD WATER

FVENT

1 I HOT WATER

Fig. 3. Schematic diagram of a once-through system with temperature controlled valve.

f VENT mALL

J R I 'c°'°'''"

c AD,OSTA,LE I rVEOT

FLO, UDT E WATa

TO LOAD OIL BOOSTER

Fig. 4. Schematic diagram of a self-adjusting once-through system.

In this case, the density difference between riser tubes (leg CF) and the pipe connecting head tank and lower header (leg BC) forms the driving force of the flow. Pipe BC operates at approximately the temperature of cold water, and the mean temperature of riser leg CF is higher due to absorbed heat from solar radiation. If the heights of water columns at both sides are the same, i.e. A and F at the same level, then density difference will cause the water to flow through the collector and to enter the tank whenever there is sufficient solar radiation available. Attention must be paid so that the head tank is not placed higher than the highest point of the hot water pipe and that a vent is located at the highest point in the hot water pipe. The flow occurs if and only if there is solar radiation available. A vent at the storage tank is also needed in a "once-through" system to assure that the tank is open to atmosphere, otherwise water can not flow during a non-loaded period.

The density difference between the two legs only depends on temperature difference so the water flow rate is a functin of collector heat gain. It means the stronger the insolation, the greater the flow rate. Therefore, this system is inherently self-adjusting to the outlet temperature to some extent. Lof and Close [2] have reported that in a natural circulation system, the temperature increase of the water flowing through the collector tends to remain constant under a wide range of conditions. It is expected that a well designed and installed "once- through" solar water heating system can supply hot water at reasonably constant temperature without an expensive controlling device. However, an adjustable flow-resistance, say, a globe valve, can be used to match different demands of servicing temperature and weather conditions. In practice, only a very few times of manual operation are needed each day, and this is quite acceptable at present in China. The once-through system has the following advantages:

1. The storage tank can he set below collector, in many

A "once-through" solar water heating system

cases, it leads to a significant reduction of the load on the F =

roof. 2. The mixing of service hot water and collector inlet

cold water is completely avoided. 3. During a sunny day, this system supplies hot water

much earlier than the natural circulation system. 4. Reverse flow can not occur in this system. 5. Compared with natural circulation system, the size

of piping work can be cut down considerably. 6. No insulation is required on the cold water pipe as it

always operates at the temperature of tap water. 7. On a cloudy day, this system produces some hot

water at determined temperature during sunny periods while the natural circulation system supplies a greater quantity of luke-warm water.

8. When a "drain back" method is used for freeze protection, less water has to be drained because the storage tank is placed indoors.

COMPARISON OF COLLECTOR EFFICIENCIES BETWEEN NATURAL CIRCULATION AND ONCE-THROUGH SYSTEMS

A relevant evaluation must include efficiency com- parisons, since the once-through system operates at constant low inlet but relatively high outlet temperature (service temperature), and its flow rate is considerably smaller than that in a common natural circulation system.

Based on a theoretical calculation, a comparison of collector efficiency between natural circulation and once- through systems has been done by Wang and Li[3]. In their work, the following conditions have been set to achieve a fair comparison.

1. Both systems comprise identical fiat-plate collectors. 2. No hot water is extracted during the day. 3. In the early morinig, the natural circulation system

starts at a temperature equal to the constant inlet temperature of the once-through system and achieves a final average system temperature which is just the same as the consant outlet temperature of once-through system. Therefore, the two systems operate at the same temperature level.

Both instantaneous and daily collector efficiencies have been calculated for a clear day for which sine functions can be employed to simulate insolation and ambient temperature variations.

For the once-through system, as inlet and outlet temperatures are fixed and the variations of solar radiation and ambient temperature are known, the collector efficiency only depends on its structural characteristics. The following equations are used[l],

Q. = A , F r [ S - UL(tr . , - t.)] (1)

: Q./(A~H) (2)

"7, = f G dO/ (A~lH dO) (3)

GCv e (tJLF'/~C,,] Fr = ~ [1 - (4)

F' I/UL (5) W{l/[ UL(D + ( W - D)F)] + l/Ch + l/(rrO, hr~)}

543

tanh [ m ( W - D)/2] m( W - D)12

(6)

In order to solve the above system equations, the method of trial and error is employed: assuming a mass flow rate G, using eqns (1) and (4)-(6), useful energy gain Q, can be determined. The calculated value should be equal to the value obtained from the equation of heat balance,

Q, = AcGCp(t:.o - b , ) (7)

If not, the assumed value for G is changed and the calculation is repeated, until the difference of Q, values from eqns (1) and (7) is less than a given error. There- fore, the appropriate Q, and G are obtained.

In the natural circulation system, the situation is more complex, because the inlet temperature of the collector depends on the tank temperature distribution, which varies constantly. Therefore, it is impossible to calculate instantaneous or daily collector efficiency without putting the whole system under consideration.

Close, Gupta and Ong et a1.[4-7] have investigated the thermal performance models for a fiat-plate solar water heater with natural circulation. Because of its simplicity and also its good agreement with experiment, Close's model is adopted. In order to consider the fin efficiency for the absorber of the collector, a minor modification has been introduced to this model. The following assumptions are made:

1. Temperature distributions of water inside the collector tubes and in the storage tank are linear.

2. The water from the collector rises to the top of the tank.

3. There are no heat losses from connecting tubes. 4. As an inference from the above assumptions, the

mean temperatures of collector and storage tank are equal.

If the collector thermal capacity is neglected and a clear day is assumed, this simplification will not induce significant error. Based on the above assumptions, the following instantaneous equations are obtained,

rhCp(h.o - ~.,) = A c F ' [ S - UL(tm - 6,)] (8)

mcp(ti.,, - trl) : QL, + C dt" dO (9)

From the above equations one gets,

A,F ' [S - UL(t.. - t.)] = QL, + C dt----v-~ dO

(10)

where,

S = H,.(ra) sin w, 0 (11)

t,, = t,, + tL sin o~0 (12)

and F' can be determined from eqn (5). o~, oJ2, to and tL are constants used to approximate

the solar radiation and ambient temperature.

544 Y.F. WANG et al.

Heat loss from storage tank can be written as, Therefore

QL, = GA,( tm - t,)

Substituting (11)-(13)into (9) and rearranging,

(13) 128 v l t h , 8Kth 2 hi = ~ - + ~ .

Rearranging eqn (8), one gets,

(22)

dt___~_~ + [AcF'UL + GAd tm d0 C

_ AcF ' ( za )H , , sin o~104 [AcF'UL + u,a,]f i , sin to20 C C

(14)

[A ,F 'UL + GA,]to + C

The first-order linear differential eqn (14) can be solved for t,. as a function of time 0.

After the mean system temperature t,~ is obtained, the following procedure is employed to determine the mass flow rate in the system.

The basic equation is

h, = h I. (15)

1 tr,, - ti., = ~ A , F ' [ S - UL(t,, - t,,)] (23)

Substituting eqns (16), (22) and (23) into eqn (15), we get

16 y~ 1_._.__.._~ rh z _~ Y g 7"i'2 d4 th 3 + K 16 K Cp

A , . F ' [ S - UL(t,. - G)]" (2C, t,. + Cz)f(h) = 0. (24)

Since the mean temperature tm as a function of 0 has been already determined from eqn (14), the flow rate rh at any time during the day can be obtained from the above equation and then the inlet and outlet temperature of collector can be easily found. Consequently, by using eqns (1)-(3), both instantaneous and daily efficiencies can be determined.

The thermosiphon buoyancy head h, is a function of system temperature distribution and the system layout [4],

h, = (rt-~ - fro) l (h) 2

= (t[, - tt.o) (2C, tm + C2)f(h). 2

C, and C2 are the constants in the parabolic relationship between density of fluid y and temperature as follows,

and also

y = CltZ+ C2I+ C3

](h) = 2 ( h 3 - h , ) - ( h 2 - h , ) - ( h 3 - h , )

- (h3- h~)2/(h~- he) ( 1 8 )

where h~, h2, h3, h5 and h~ are the heights of corresponding points of the system above a arbitarily selec- ted datum (see Fig. 1).

Friction head can be calculated as,

where

1 U 2 /t 2

hi = ~I3~g + ~K~g

64 64u f = R e = u--d-

U = rh

"/'r 2 Y~- d

RESULTS AND ANALYSES OF THE CALCULATIONS

Calculations have been carried out for both once- through and natural circulation systems, each comprising a 2 m 2 tube-and-sheet collector. The storage tank in the natural circulation system has a capacity of 2001. In- solation and ambient temperature corresponding to the

(16) winter conditions in Hefei are chosen. Initial temperature of the natural circulation system and the inlet temperature of the once-through system is considered to have a value of 20°C, and the final temperature in both systems to be 40°C. The detailed structural data of collector, tank and piping are omitted here.

(17) Figure 5 shows the assumed insolation and ambient temperature profiles, the temperature variations in both systems, and also the obtained collector efficiency variations with time.

Daily collector efficiencies for both systems can be determined from eqn (3). For the natural ciurculation system, the daily collector efficiency was found to be 46.5 per cent, and for the once-through system the daily efficiency was 46.7 per cent. As can be seen, the difference between the daily efficiencies of the two systems is quite negligible.

For the once-through system, as compared with (19) natural circulation system, the greater temperature rise

and small flow rate are unfavourable to its collector performance, but the constant low inlet temperature provides compensation to improve the efficiency to an equivalent value,

(20) Figure 5 shows that in the morning, the natural cir- culatin system has a higher efficiency. Due to the increase of ambient temperature and solar radiation, its instantaneous collector efficiency increases in the first

(21) two hours despite the gradual climbing of the inlet temperature. Efficiency reaches to its highest value at some-

A "once-through" solar water heating system 545

• 7o0

=:" 501

3og

I( .,...,

0

40

- 30

.5,

.4

.3

J

't., -OT|

t f . i -OTS

9:00 II:OO 13:00 15:00

TIME

Fig. 5. Assumed insolation and ambient temperature profiles and obtained temperature, collector efficiency variations with time in both systems: NCS--natural circulation system. OTS--once

through system.

time around 10.30 a.m. and then declines rapidly in the afternoon.

The circumstances are different for the once-through system. Because of the constant inlet and outlet temperature, its instantaneous collector performance largely depends on the intensity of solar radiation and ambient temperature. The pattern of its efficiency curve is very similar to that of the insolation curve. Since the afternoon ambient temperature is higher than that in the morning, the summit of the efficiency curve is achieved sometime after midday.

EXPERIMENTAL RESULTS

Experiments have been carried out to compare the performances of these two systems [8]. In order to make a fair comparison, identical fiat-plate collectors were employed for each system and the two collectors were placed side by side (Fig. 6), to insure the same insolation

Fig. 6. Arrangement of test apparatus: Left side--natural circulation system. Right side----once-through system.

and ambient conditions. Efforts were also made to make the two systems operate at the same temperature level. The flow rate in the once-through system was manually controlled to maintain a reasonable constant outlet temperature.

Tube-and-fin collectors were used, each having an area of 0.73 × 0.54 m z, mounted south facing, at an inclined angle of 320 . Storage tanks and connecting pipes were all insulated. Collector inlet and outlet temperatures and temperature distributions in both storage tanks were monitored by calibrated thermocouples. Water flow rate in the once-through system was determined by weight measuring, and in the circulation system a dye-injection method was used. Solar radiation was measured by a pyranometer installed in the collector plane.

Repeated tests show good agreement with previous conclusions from theoretical analyses that the daily collector efficiencies of these two systems are about the same (see Table l). On three days, the inlet and outlet temperature differences between these two systems were less than l°C. The maximum deviation of the collector efficiencies between these two systems was 1.8 per cent. The collector efficiences of the natural circulation system were slightly higher than that of the once-through system

Table 1. Part oftestresults

Platural circulation s y s t e m Once-through system

Date In i t i a l t m Final t m n d Average t f . i Average t f . o n d

o C o C % °C oc %

1981.5.17 21.2 a4.8 70.6 23.2 47.5 69.0

1981.5.21 21.0 45.4 73.0 22.0 47.1 72.2

1981.5.31 23.6 38.2 69.0 23.1 38.8 68.2

1981.6.3 22.2 38.6 71.0 22.5 39.4 72.0

1981.6.5 23.8 49.2 73.4 24.0 48.8 71.6

546 Y. F. WANG et aL

on two days, but on another day, the contrary results were obtained. On two days, although natural circulation system performed a little better, it was attributed to the lower inlet and outlet temperature. A rough estimation shows the influence of temperature on collector efficiency. Assuming that the single glazed black paint collector has a efficiency formula of ?7 = 0.80 - 8 AT/H, if H=700W/m 2, then a I°C drop of AT increases the collector efficiency by 1.1 per cent. With this consideration, the once-through system has a better performance.

From Figs. 7 and 8, one can see that before midday, the instantaneous collector efficiency of the once-through system is lower than that of natural circulation system. In the afternoon the result turns out contrary to that in the morning. It agrees with the results from calculations.

Tabor[9] has stated that for natural circulation systems, one-pass and multi-pass systems have roughly the same efficiency, assuming no mixng in the tank. Although Tabor's approach differs from the present work, his statement could be ragarded as indirect evidence of our conclusions. Because in the once-through system the storage tank is separated from the cold water supply, the operation of the collector is exactly the same as that in a one-pass natural circulation system with ideal stratification in the storage tank.

There are at least two operating once-through solar water heaters is Belling and a couple of these systems are under installation. The users of the first once-through systems are satisfied by its performances. Unfortunately, due to the lack of instrumentation, no quantiative data are available.

9O0

700

500

3O

- - 25

2O

~2 I

0

,-, 5~

3O

H

.I

£

.5 9:00 11:00 ~3:00 15:~

TIME

Fig. 7. Experimental results, 1981.5.17.

•fBB• 901

701 .

m 5Ol

. 25

2O

1

51 h -0TS

so tf.i -0IS 20 -

.?

. i ~ OTS

B:N II:H 13:0Q lS:OO

TIME

Fig. 8, Experimental results, 1981.5.21.

C O N C L U S I O N S

The once-through solar water heating system has many advantages over the conventional natural circulation system. The storage tank can be placed below the collector and the load on the roof will be greatly cut down which has practical significance in the installation of the solar heater system. The size of the piping can be considerably smaller than that of the natural circulation system. There is no mixing of hot and cold water and no reverse flow in this sytem. These factors make the once- through system an attractive alternative to the conventional circulation system.

Theoretical analyses show that the natural circulation system performs better in the morning but the once- through system has a higher efficiency in the afternoon and that the difference of daily efficiency between these two systems is negligible. Repeated experiments agree with above conclusion satisfactorily.

It should be pointed out that for a once-through system with an outdoor storage tank and other components the same size as that of the natural circulation system, the whole once-through system might be less efficient because of more heat losses from the storage tank which results from higher temperature in the tank over most time in the day (see Figs. 7 and 8).

Acknowledgements--The authors greatefuly acknowledge the help from Mr. W. W. S. Charters and Dr. A. Akbarzadeh at University of Melbourne who reviewed the manuscript. Special thanks are due to Prof. Ge, X.S. at China University of Sci. & Tech. for his relevant advice.

Ac At

Ct, etc. C

N O M E N C L A T U R E

collector area, m 2 tank surface area, m 2 constants in eqn (17) total thermal capacity of the storage tank and water,

J / C °

Cb bond conductance, W/m°C Cp specific heat, J/kg°C d inner diameter of connecting piping, m D outside tube diameter, m /9,. inside tube diameter, m

e base of natural logarithm f friction factor F standard fin efficiency for straight fins with rectangular

profile

A "once-through" solar water heating system 547

F' collector efficiency factor FR collector heat removal factor

g gravitational constant, g = 9.81 m/s 2 G mass flow rate per unit area, kg/m 2 s

Hm maximum solar radiation on the collector plane, W/m 2 H total solar radiation on collector plane, W/m 2

hli heat transfer rate inside the tube, W/m2°C h I friction head opposing flow, kg/m 2 ht thermosiphon head causing flow, kg/m 2 k thermal conductivity of the fin, W/m°C

K equivalent number of velocity heads lost by the flow in passing through bends, tees and restrictions

/ equivalent length of piping through which fluid passes, m

rh thermosiphon mass flow rate, kg/s m defined as rn 2= Udk8

Qu heat loss from tank, W Q~ total useful energy gain of the collector, W Re Reynolds number

S absorbed solar energy per unit area collector, W/m 2 to ambient temperature,°C

tl.i inlet water temperature, °C tt. o outlet water temperature, °C tm mean system temperature, °C

to, h constants,°C UL overall heat loss coefficient from collector, Wlm2°C Ut overall heat loss coefficient from tank, W/m2°C u thermosiphon velocity, m/s v wind velocity, mls

W distance between tubes, m y density of the fluid, kg/m 3

thickness of the fin, m "0 instantaneous efficiency of the collector

r~a daily efficiency of the collector 0 time, s v kinematic viscosity, m2/s

ra transmittance-absorptance product of the cover ~o~,~o2 constants, 1/s

REFERENCES

1. J. A Duffle and W. A. Beckman, Solar Energy Thermal Pro- cesses. Wiley, New York (1974).

2. G. O. G. Lof and D. J. Close, Low temperature engineering applications of solar energy. Solar Water Heater, ASHRAE, New York (1967).

3. Y. F. Wang and Z. L. Li, A Comparison between the "Once- through" and the "Thermosiphon" solar water heaters and the calculation of their thermal efficiences (in Chinese). Acta Energiae Solaris Sinica 2, 174 (1981).

4. D. J. Close, The performance of solar water heaters with natural circulation. Solar Energy 6, 33 (1962).

5. C. L. Gupta and H. P. Garg, System design in solar water heaters with natural circulation. Solar Energy 12, 163 (1968).

6. K. S. Ong, A finite-difference method to evaluate the thermal performance of a solar water heater. Solar Energy 16, 137 (1974).

7. K. S. Ong, An improved computer program for the thermal performance of a solar water heater. Solar Energy 18, 183 (1976).

8. Z. L. Li and X. L. Sun, An experimental investigation of collector performance comparison between once-through and thermosiphon systems. (in Chinese). Paper presented to the Syrup. China Solar Energy Soc. (1981).

9. H. Tabor, A Note on the Thermosiphon Solar Hot Water Heater. Bulletin, Cooperation Mediterraneene pour L'Energie Solaire (COMPLES), No. 17, 33 (1969).

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A “once-through” solar water heating system