Experimental results of a latent-heat solar-roof, used for breeding chickens

Solar Energy Vot, 26, pp. 347-359, 1981 Printed in Great Britain. All rights reserved

0038-092X/811040347-135~.0010 Copyright © 1981 Pergamon Press Ltd

EXPERIMENTAL RESULTS OF A LATENT-HEAT SOLAR-ROOF, USED FOR BREEDING CHICKENS C. BI~NARD Laboratoire des Signaux et Systbmes, C.N.R.S./E.S.E., Plateau du Moulon, 91190--Gif-sur-Yvette, France D. GOBIN GR 14 (C.N.R.S.) Ecole Centrale 92290--Chatenay-Malabry, France and M. GUTIERREZ Hualalache, Talavera, Provincia de Andahuayias, Peru

(Recieved 18 January 1980; revision accepted 8 October 1980) Al~tract--The experimental results presented here have been obtained on a solar chicken brooder built in the Peruvian mountains (altitude ~ 3000 m, latitude ~ 13°S). This installation is made of adobe and part of its roof is a solar collection-storage system consisting of two tanks of paraffin-wax located below glass panes. We study the short-range and long-range fluctuations of the variables characterizing the behaviour of the system and its fitness to its purpose: breeding young chickens that must be kept in a given range of temperature, with the air regularly renewed. l. INTRODUCTION

1.1 Presentation o[ the experiment The solar installation studied here has been built in 1976 in the Peruvian Sierra (3000m high, - 1 3 o South lat.). It is designed to raise chickens from age 1 day to 3 or 4 weeks in environmental conditions suitable to their upbringing. In this region the heating problem for this type of chicken breeding is presently solved by using electricity or kerosene. To allow the large number of countrymen living in villages, where these facilities are

not available, to participate to this activity, an experimental chicken-brooder using only solar energy for heating purposes has been realized. In the present paper the final results and conclusions about this chicken brooder are given on the basis of all the measurements performed in 1977 and 1978. It is interesting to underline that, in spite of the rustic construction (Fig. l) and of the rather poor instrumentation, the analysis of the behavior of the system could be carried out very satisfactorily. In building the chicken-brooder, the traditional model

Fig. I. View of the installation; the patio is on the I.h.s. and the solar enclosure on the r.h.s. 347

348

BI~NARDet

C.

storing radiation, in order to regulate the enclosure temperature between - 2 2 and -30°C[2]. In equilibrium regimes, the stored energy is used to carry over through the night and, in non-equilibrium regimes, through sequences of several days of bad or very hot weather. Exchange between the tanks and the enclosure takes place through radiative transfer from the blackened bottom of the tanks to the enclosure walls and floor. A first version of this installation was experimented with from June to August 1977 with a rather had glass roof and inside mirrors, but good top insulation at night (from April 1977). Moreover, the patio was not roofed over yet but the door leading from the enclosure to the patio was very well insulated (from April 1977)[1]. Though the results were quite acceptable, the enclosure temperature was found to have an important daily mean oscillation from 16 to 33°C (in the coldest month, i.e. June) during equilibrium regimes, and the minimum enclosure temperature to fall sometimes below 16°C after 2 and 3 days of bad weather during the coldest season. The results analyzed in this paper have been obtained

of a Peruvian house was kept as much as possible[l]. The overall horizontal dimensions of the installation are 4.90 x 2.80 m. It is divided into two connecting parts, a patio and a heated enclosure (Figs. 1 and 2). The 2 x 1.80 me patio is identical to a usual room in a Peruvian house, with 0.5 m thick adobe walls and a corrugated asbestos roof. The 1.56 x 1.78 m 2 heated enclosure is lower in height (distance from the floor to the glass roof = 1.20m); two tanks containing 42 kg of paraffinwax each, with transparent glass upper face of 0.5 m2 each, are located below the glass roof, with surrounding mobile mirrors (Fig. 3). The intermediary wall between the heated enclosure and the patio is 0.3 m thick. An external mobile mirror looking Northward to the sun allows the impinging radiation on the paraffin to be increased if necessary. At night, inside polyurethane insulators, 0.1 m thick, are placed between the glass roof and the paraffin tanks, and the external mirror is lowered onto the glass roof. The paraffin-wax, colored red with a melting point at around 58--60°C, plays the two roles of collecting and

I ,l~! i

.......................

aL

4.90 m- -~ . . . . . . . . . . . . . . . . . . . .

i~ i

,V F / / I

k.~ _ ~_,~.___

I

_.

r/A ki ,o°,

/ \,

V X! I.,o.smq

,DI,, tank I/

I//,I

"~.A,.,\\\~<,:\,

i i i i

//

1

,/

Patio

.

I/

/ < V A/, ~~"," "//~,v I

Mobile mirrors (day position) Fig. 2. Horizontal cross-section of the installation.

Glossroof \\

Paraffin tanks /~

\

1

i,,

/

V/A-t'/E";'sure

I/lid ;

l I

V

Thermocouples

Mobile mirrors (doy position)

"3,-"/",' i ' \

;:l~eated

"'''""~/A

~---GIoss

r//d

i ;

'

Fig. 3. East-west vertical cross-sectionof the solar enclosure as it is from April 78.

Experiments results of a latent-heat solar-roof, used for breeding chickens with an improved installation which was modified so as to get: • A smaller daily oscillation of the enclosure temperature • A higher minimum temperature, around 22°C • A lower maximum temperature, around 30°C • Greater stability during bad weather sequences • Better ventilation. To achieve these purposes, the improvements were as follows • For better overall efficiency, the initial glass roof was changed to a new one which is more solidly attached so that no hot air escapes • In order to lower the maximum enclosure temperature, the inside mobile mirrors have been totally remade so that no direct insolation is possible at all in the enclosure during the day • The patio was covered over • Four ventilators were made to help not only to bring fresh air into the enclosure but also to push the hot air from above the tanks down into the lower part of the enclosure, where the chickens should be. They are located symmetrically in the East and West walls of the enclosure. Two of these are wind ventilators (built in Sept. 1977), that is to say, air ducts into which natural wind is allowed to enter and then is channeled slowly upwards until it escapes between the paraffin tanks and the glass roof. The other two are Trombe-walls (built in April 1978) working alternatively in the morning and in the afternoon with the hot air also exiting above the paraffin tanks (Fig. 3).

349

• Ibidem at the level of the storage upper surface, T4,. and T4~ • Ibidem at 7 cm from both the upper surface and the side walls of the storage (Fig. 3). In the first part of this paper (Sections 2-4) the behavior of the inputs and outputs only are studied. In the second part, such a behavior is correlated with the state-variable behavior (Sections 5-7). 2. SHORT-RANGE BEHAVIOR OF THE INPUT AND OUTPUT OF THE SYSTEM

The daily fluctuations and the correlation of the inputs and outputs of the system are analyzed for different periods of the year, running from April to November, when the new version of the installation was used in the absence of chickens. During three of these periods, Period I (27 Sept.~ 13 Oct. 1977), II (18-30 Nov. 1977), III (1 ~ 15 May 1978), the night insulation was correctly used (see Table 1). During Period IV (27 April~l May 1978), no night insulation was used. The main features of the local climate, which have already been studied previously[3,4], are summed up in Table 1 for the periods of interest. It should be emphasized that the solar daily total irradiation meanvalue varies little during the year (the minimum in June is around 2.1 × lip kJ/m2/day, the maximum in November and December is around 2.7x lip kJ/m2/day). But its S.D. reaches 0.54 x liP kJ/m2/day during the highest intensity season, which is also the rainy season, and drops down to 0.28 x liP kJ during the lowest intensity season, which is the dry season (Fig. 4).

2.1 Short-range correlation between input and output 2.1.1 Correlation between the daily solar irradiation and the enclosure minimum temperature. The curves (Fig. 5) allow only to note immediately that the daily variations of Tm are not correlated to the daily variations of the outside temperature but only to the daily variations of I. The daily variations of Tm follow the daily variations of I with a l-day day delay, smoothing them out considerably. The importance of this smoothing effect, on the average, can be analyzed with the help of the two ratios ar~/(Tm) and o'i//I), which are calculated for each of the three periods of interest, tr is the period's empirical S.D., and (...) indicates the period's empirical mean-value. The first ratio, which characterizes Tin, oscillates be--Inputs: • Outside minimum and maximum temperatures Tern tween 0.05 and 0.06 according to the period considered, while the second oscillates between 0.14 and 0.19. and T,M 2.1.2 Behavior of the enclosure maximum • Daily solar total irradiation I. temperature. The estimation from the data of the intercorrelation function between Tm and TM, the enclosure --Outputs: • Enclosure minimum and maximum temperatures minimum and maximum temperatures, is not very precise and no correlation effect can be seen. This result, T,, and T~. indeed, was expected since T~ depends mainly on the solar irradiation during the few hours around noon, on --State-variables: • Morning and evening paraffin-wax temperatures at the very same day, and not on the solar irradiation on the previous day. The maximum solar irradiation may oscilthe bottom of the storage, T~,, and T~M • Ibidem at 14 cm under the upper surface of the late strongly and for this reason the standard deviation of TM is much more important than that of T,, and even storage, TErn and T2~ T,M, the outside maximum temperature. • Ibidem at 7 cm, T3,,, and T3~

1.2 Analysis of the experiment The variables that are used to analyse the collectionstorage system behavior are the inputs and outputs of this system and its state-variables. The inputs are the outside temperature, the solar intensity, the wind velocity. The output is the enclosure temperature. The statevariables are the temperatures in different parts of the storage material. For all these variables, the time-scale of the measurement sampling ranges, according to the variable, from a few hours (temperatures) to 1 day (daily integrated solar intensity). Thus, in fact, the system behavior will be described with the help of the daily sampled variables that follow:

14

June I I-.9,June 30

(1977)

(1978)

-..~May

Apr. 21

Nov. 18 --~Nov. 30 (1977)

(1977)

S e p t . 28 - - ~ O c t . 13

PERIOD

5.7

10.47

13.

9.75

Mean

Min~

1.2

(~.3S)

.55

1.3

2.3 (0.58)

St -dev.

16.8

17.97

18.15

19.25

0.S

(0.41)

1.84

1.1

1.3 (0.32)

St-dev.

Maximum Mean

Outside temperature

16.1

22.3

23.6

23.45

Mean

0.9

(0.26)

1.18

1.48

1.47 (0.37)

st-dev.

Minin~am

33

29.2

29.3

30.35

Mean

Maximum

Enclosure t e ~ r a t u r e

1.9

(0.61)

2.75

1.97

2.31

St-dev.

11 .I

7.5

5.15

9.S

Out

16.9

6.9

5.7

6.9

In

variation

Temperature

2.1 x104

x104

2.2

2.7 x104

2.6 x104

Mean

0.28 x104

x104

0.31

0.52 x104

0.47 x104

St-dev.

irradiation

Daily total

Table 1. For Periods I-III of experimentation of the new version of the installation and for Period V when the former version was working, the outside minimum and maximum temperature empirical mean-values and S.Ds are given (columns 1-4) as well as for the enclosure temperatures (columes 5--8) and the daily total irradiation (columns 11, 12). The temperature variation of columns 9 and 10 is the mean daily difference between the enclosure maximum and minimum temperatures on the same day. The units used are respectively °C and Id/m2/day

.......

¢~

z

Experimental results of a latent-heat solar-roof,used for breeding chickens

kd/mZ/doy

Daily

total

radiation

..~¢~**+~'%

293x104 2 50 2 09

~"~ ~'~*~'*~"~¢

167

Peru Lat. 12"S Air. 3 3 0 0 m

0

2

3

4

5

6

7

8

9

IO II

12

Fig. 4. Meandaily total solar irradiation (lm(d)) in kJ/m2/dayas a function of month m (continuous line), aim is the S.D. of Ira(d) considered as a function of day d for month m; crosses indicate respectively (Is)+ o'lm and (Im)-al,,, (total fluctuation). The dash give (I,.)+EI,. and (Im)--~lm where ~ I m is the S.D. of I m considered as a function of the year (slow fluctuation).

15%

I0

L

5

r~

25"C

22 19

. . . .

T

. . . .

i

. . . .

'

2 93

XlO

209

-'-

.~ ..,f--r- ~.

X104 kJ/m2/doy

125

~

~

,4~_ o ~'~-.~

33°C

r~ 3O 27 24

22°C

r,. 16

SeIDt 27

0¢1 2

7

12

Doys

Fig. 5. Period I, 27 Sept.-13 Oct. 1977. (a) Outside minimum

temperature T,m as a function of the day. (b) Enclosure minimum temperature Tin,as a function of the day. (c) Estimation of the daily total irradiation I, as a function of the day. (d) Enclosure maximum temperature TM, in terms of the day. (e) Outside maximum temperature T,M,in terms of the day.

The dependence of TM upon the daily maximum solar irradiation produces a loose correlation between TM and I, the solar irradiation integrated over the entire day for the same day. A correlation is also observed between the daily fluctuations of TM and T,M, caused by the common dependence of these variables upon fluctuations of the solar irradiation at its maximum, on the same day. On the

351

contrary, no short-range correlation can be detected between T,, and T,m, the outside minimum temperature. 2.1.3 Response o[ the system to a l-day oscillation o[ the solar irradiation. A l-day oscillation of the daily solar irradiation I, far from a stable level that had been maintained for three or more days and that resumes after the oscillation for a few more days does not basically affect the behavior of Tin. Depending upon the stable level that had been reached, T,, reacts more or less strongly but the reaction lasts only one day, i.e. the day following the solar irradiation oscillation. If the storage is well loaded, after a good sunny sequence, the Tm response is considerably smoothed out. As an example, the large oscillation of I on the 4th and 5th of October (curve C of Fig. 5) gives rise, on the 5th and 6th, only to the small oscillation of Tin, of 0,5°C (curve B of Fig. 5). On the contrary, if the storage has been progressively unloaded by a streak of bad weather, Tm reacts much more strongly to a l-day oscillation of I, but this does not affect its subsequent behavior much. For instance, the 12 Oct. oscillation of I (curve C of Fig. 5) results in the 13 Oct. oscillation of Tm of 4.5°C. 2.1.4 Response o[ the system to a several-day oscillation of the solar irradiation. Progressive load (or unload) of the paraffin storage can be observed after 3 or 4 days of good (or bad) weather. For instance, the long good weather sequence from 27 Sept. to 4 Oct. makes T,, rise slowly from 21.3 to 25°C in 7 days (curve B of Fig. 5). The bad weather sequence from 8 to 11 Oct. makes T,, drop from 9 to 12 Oct. (curve B of Fig. 5). The mean daily temperature drop over such bad weather sequences is around I°C except when extremely bad weather, as on 11 Oct., makes it necessary to close the collector, producing a 3°C/day drop. 2.1.5 Influence of working conditions. The short-range correlations between variables during Period IV when the system works without night insulation, or during Period V when the system was operating in its initial configuration, are very similar to those observed during Periods I-III. Only the mean level of T,, and T• is changed. For instance, the only effect of not insulating at night is to make T,, drop by about 2°C (indeed (Tin)= 20.5°C and (TM) = 28.6°C, during Period IV). By comparing Periods I-III to Period V (Table I) we immediately notice that the mean daily temperature difference (TM)- (Tin) drops from 16.9°C for Period V to 6 or 7°C for the others. This effect comes at the same time from the drop of (TM), which was achieved b37 having better mirrors that prevent the sun from getting into the enclosure under the paraffin tanks, and by better ventilation during the maximum insolation hours, due to the Trombe-walls and the wind ventilators, and from the rise of (T,,) coming from a better collection (new glass roof and new mirrors) and a better night insulation (roof over patio). In the new version of the installation, the average difference between the outside and enclosure minimum temperatures is also improved, that is to say. increased, as one can see from Table 2.

352

C. BI~NARDet aL

Table 2. Outside and enclosure minimum temperature average difference for each period Period

25'4

" (~Tem>

I

13.7 °C

II III

10.6 °C

V

10.4 °C

12. ! °C

2(

This indicates a better overall efficiency for the system in its new version. Finally a drastic change in the operating conditions may also be considered in which the system is kept closed. Its behavior, under such circumstances, is summed up on Fig. 6. T,, is shown to drop rather regularly and from operating level of 23.7°C to 16°C, with a daily temperature drop ranging from ~ 14°C at the beginning to I°C at the end for an outside minimum temperature of 13°C. 3. LONG-RANGE COVARIATION OF THE INPUT AND OUTPUT

The climatic conditions, which are different during Periods I-III[3, 4], may be characterized in the present paper by using only the empirical mean-value and S.D. for the period involved, (T.,,), (T,u), (I), (rr.,., (rr, u, o"i (see Table 1). The average behavior of the system during each of the three periods will be defined by the meanvalues and standard deviations of T,. and Tu, (T,.), (Tu), ~rr.,, Vru (see Table 1). The purpose here is to follow the covariation of the mean-values and S.Ds of the variables from 1 period to the next. A first important result is that (T,,) varies much less than (I) from 1 period to the next. Indeed, if Ax is the maximum difference of x among the 3 periods,

15

t

t

i

i

t

2

3

4

5

6

i

Days

Fig. 6. Enclosure minimumtemperature as a function of the day, when the system is closed (Nov.).

4. CHICKEN INFLUENCE AND BEHAVIOR

In the presence of chickens (100) the behavior of T= and TM is slightly modified. Indeed the mean values of T= and Ten, on Periods I and II are (T=)ui=23.5°C

and

(T~=)i.n=ll.4°C

whereas, during Period VI (15 Oct-8 Nov 1977), between Periods I and II, when chickens are present in the brooder (T,.)v=25.7°C

and (T,,,)v=il.8°C.

Thus, without chicken

{Tm)I,I1 - ( Tern)1,11 ---- 12.1 oc and, with chickens

A(T.)I(T=)=O.05

(1) {T,, )w-(T,,~ )v, = 13.9°C.

and A(I)/(I) = 0.20.

(2)

It should be stressed that this stability of (Tin) is achieved by the system itself and does not come from any other influence compensating for the variation of the solar irradiation. On the contrary, (Ten,) follows (I) and also contributes to the variation of iT,,). ~rrm follows the variation of a~ from one period to the next but its fluctuation is less important (Table 1). This result once more underscores the main correlation effect which occurs between T,. and I as well as the smoothing effect of the paraffin storage. As could already have been deduced from the shortrange results, the long-range variations of Tm are not correlated with those of Tu or T,,. (Table 1). As for the behavior of (Tu), it seems to result from the complex, mixed influence of (T,u) and {I). Finally ~rrM does not covary with ,r~.

The chickens make the outside-inside temperature difference rises of about 1.8°C, during the high intensity periods. During the lowest intensity periods (Period III without chicken and Period VII (30 May-25 June 1978) with chickens) the outside-inside temperature differences are respectively

(Tm )II1 - ( Tern)ili = 11.8°C and ( T. )v. - ( T.. )v. = 15.2°C. Thus a 3.4°C difference between the two periods III and VII is attained. The relatively lower gain, due to the chickens, during period VI (1.8°C against 3.4°C) can be mainly explained by less care in keeping doors closed. Indeed the S.Ds of

Oct. 30 P-~Nov. 19 (1978)

May 30 ---)~June 25 (1978)

Oct. 1 5 Nov. 8 (1977)

PERIOD

11.83

~an

1.41

1.67

St-dev.

Minin~n

15.64

18.65

Mean

1.48

I .51

St-dev.

Maxim~n

Outside temperature

21.30

22.20

25.73

Mean

1.69

I .07

2.46

St-dev.

Minimt~

30.35

29.76

33.20

Mean

2.39

2.20

3.60

St-dev.

~ximum

Enclosure temperature

8.6

6.8

Out

7.5

7.5

In

variation

Ter~perature

2.72 x]04

2.12 x104

2.67 x104

Mean

0.48 x104

0.30 x}04

0.48 x104

St-dev.

irradiation

Daily t o t a l

Table 3. The same temperatures as in Table I are given for Periods VI (15 Oct.-8 Nov.) and VII (30 May-25 June) when breeding chickens. The last line characterizes a traditional kerosene brooder

E

C. BI~NARD et aL

354

both T,, and TM are much higher during Period VI than during other periods (Table 3). Another interesting feature of the solar brooder is that the first results obtained are at least as good as the ones of a traditional kerosene brooder. Indeed, by comparing them for the solar brooder (Periods VI and VII) and for a kerosene brooder,we notice that the critical temperature (T,,) for the solar-brooder is higher than the corresponding temperature for the traditional brooder. As a consequence the daily temperature oscillation ((TM)-(T,,)) of the solar brooder is smaller than the corresponding temperature oscillation for the traditional brooder. Another salient feature lies in the different configuration of the solar brooder and the traditional brooder. Indeed, in a traditional brooder, the chickens generally stand at the periphery of the heated brooder, which is located in a large room. The temperature given on the last line of Table 3 are these maximum and minimum periphery temperatures. During daytime, the chickens tend to get away from this peripheric region to scatter in the room which is at a lower temperature TR. On the contrary, at night, they gather around or under the brooder. In the solar brooder, there is a whole uniformally heated area where the young chicks can wander without having to suffer important temperature drops, that would make them eat more. And whenever they need more room, during daytime, when they are older, they may go to the patio, whose temperature is rather high because of the ventilation from the solar enclosure to the patio. Indeed the maximum patio temperature Teu is higher than the maximum room temperature TRM of the traditional brooder (see Table 4). The benefit of this cont~guration can be checked on the chickens growth related to their food consumption. These quantities are given in Table 5, from which it can be deduced that the conversion rate 0, for a whole breeding period of 24 days, is 0 = chicken weight increase (kg) = 0.35. food consumption (kg)

5. MEAN ENERGY BALANCE OF THE SYSTEM--F~T1MATION OF THE CHARACTERISTIC PARAMETERS

5.1 Loading period The daily loading of the paraffin storage is characterized by the heat AQ stored daily, between 8 a.m. and 4.30 p.m. (insolation period) per kilogram of paraffin-wax, on one hand, and by the daily storage yield RA, on the other hand:

I× S

'

5.2 Night behavior 5.2.1 Heat used during the night. The heat used during the night by kilogram of paraffin-wax from day i - 1 to day i is qNi = qM(i- I) -- qral

(3)

M is the total mass of paraffin (84 kg) and S the total collection surface (2.76 m2), including mirrors. The values taken by AQ can be seen for Periods I and II on curves 7 and 8. These values have been estimated

(4)

that is to say the difference between qu,-~), the heat stored in 1 kg of paraffin-wax at 4.30p.m. on day ( i - 1), and q,,i the heat left in 1 kg of paraffin wax at 8 a.m. on day i. Obviously the mean value of qN on a given period is equal to the one of AQ. For the period running from the 28 Sept. to 9 Oct., (qN)= 173kJ/kg (S.D. cr~N----" 30 kJ/kg). Thus, on the average, the whole storage gives back (0N) = 84(qN ) -- 14,500 kJ,

that is a good conversion rate [2].

RA = A Q × M

from the temperatures measured by 6 thermocouples located in the storage as indicated on Fig 3, on one hand, and from the specific heat C,(T) of the paraffin-wax (Fig. 9) on the other hand. The results for Period I show that when the solar intensity I varies between 2 and 3 x 104kJ/M 2 ("Fair weather"), as on 27-30 Sept. and 3, 4 and 7 Oct. (Fig. 7), the storage yield RA fluctuates between 0.21 and 0.24. Let us stress that the fluctuations of AQ are relatively weaker than the ones of I. Indeed, for the considered set of days, ~ao/(AQ)~-O.07 and ~i/(I)=0.10 (see [1] and [2]). A threshold effect can be observed whenever I climbs higher than 3 × 104 up to 3.3 or 3.5 x 104 kJ/M 2. Such events lower RA down to -0.18. The low intensity behavior depends strongly on the recent past of the storage (compare 5, 6 and 9, 10 Oct. on Fig. 7). But, anyway, RA tends to fall down quickly. For instance, the average storage efficiency from 27 Sept. to 7 Oct., a period that includes bad days (Fig. 7), drops down to 0.20, and the relative fluctuations of AQ are larger than the ones of I. Indeed, on the considered set of days ~'ao/(AQ) ~-0.22 whereas ~d(I) "=-0.16.

(5)

on every night of this period. ON is used either for heating purposes or through losses. 5.2.2 Night losses through the upper surface. A first estimation of the night losses through the upper surface can be performed which is valid if the night radiation can be considered as neglegible. The upper glass is then assumed to be at the outside air temperature. In such conditions, the 10 cm-thick polyurethane insulation, whose conductivity is - 0.025 W/m/°C and surface 2.76 m2, loses around 28 W. Indeed its bottom temperature ranges around 55°C and its top temperature around 8-9°C, maintaining a temperature difference AT close to 50°C. Thus, during 15 hr (5 p.m.8 a.m. on the next day) the loss QP is - 1500 ld. Nevertheless, during the dry period, the night radiation is quite important because of the high altitude. If it lowers the top temperature of the insulator down to - 10°C, AT reaches -65°C and QP ~ 2000kJ. 5.2.3 Useful heat. Let us call QU the useful heat given back to the enclosure between 4.30 p.m. and 8 a.m.

r

z P

M

o<

i Total water consumption during the 24 days : 13.5 liters.

x 9 died from pullorosis, which was brought from outside, with the chickens.

14 c~

-->

11 30.100

14.990

82

24

13 cm

~>

10.100

82

17

10

5.970

82

9

16.600

4.540

83 x

5

Average size of the animals ("castellana Negra" race).

10 cm 5.100

4

92

1

day

2.46

Stand-Dev.

°C

0.99

Stand-Dev.

°C

7.5 ~

2.550

of the chickens

food, from the I

Total weight of

chickens

Total weight of st

24.66

Mean-value

Maximl~n patio termperature TpM

23.45

Mean-value

Number of

Table 5. Solar brooder balance

1.01

Stand-Dev.

°C

1.50

Stand-Dev.

~laxim~n room te~Derature TRM

Days of

12.10

~an-value

Minimum patlo temperature Tpm

14.45

Mean-value

oC

breeding

Solar Brooder

Traditionnal kerozene brooder

Minim~an room temperature TRm

Table 4. Comparison of the room temperatures for a traditional kerosene brooder and the solar brooder

g

o

m"

C. BI~NARD et al.

356 I

2°9

terized by the heat exchange coefficient h

L x104 kd/m2/d°Y r" . . . . . I-

~ ~-

.~. ,,... I

AOlkj/k0 200

I "N ,

/"---

h=

i

-

I

'

'

0¢,.2

'

'

'

I

. . . .

12

'

Fig. 7. Daily stored heat AQ as a function of the day (Period I).

[

,o/..%oy

[

4 2 93 xlO ~-%.

!

I

i

i

I

p

I

-

~ -,/,, i

/

./' ,,~

i

i

i

i

~

L

i

I

i

i

i

kd/kq 200-

I

IO0

i

i

I

lov 18

,

25

(QU)

= 11 W/mZ/°C. (9)

(T,.))S'x 15 x 3600

6. TIME ANALYSISOF THE DAILY FLUCTUATIONS OF THE STORAGEVARIABLES

I

I000

SIpl '27 '

((T,,.)-

28

Fig. 8. Daily stored heat AQ as a function of the day (Period ll).

In the present chapter, our purpose is to estimate the time-constants of the storage system that range from 1 day up to several days, on one hand. On the other hand, we will analyze the smoothing effect of the storage system by comparing the slow (period ~>half a day) fluctuations of the input and output. These estimations and analyses are performed on the basis of the experimental measurements of the input, state-variables and output of the system realized with a time scale of the order of half-a-day or 1 day (see Section 1.2). 6.1 Time-constants o/the collection-storage system 6.1.1 Working conditions without melting. From the values of the specific heat of the paraffin wax, Cp(T) (Fig. 9) and of the thermal conductivity (k = 0.2 W/m/*C), we can estimate the order of magnitude of characteristic time-constants, according to the range of temperatures where the storage system works. Such time-constants of thermal elements are defined by electrical analogy as RC-constants where R is the thermal resistance and C the heat capacity of the considered elements. When no melting occurs (storage temperature < 58°C), C,(T) = 2 kJ/kg/°C and the time-constant r of 1-cm-thick horizontal slab of the storage material (10 I.) is ¢--~ 103 S.

on the next day

(10)

In the same conditions, the time-constant ~', of the whole storage material (84 kg) is

QN = QP + QU

(6)

QU --- 12,500 kJ. In order to estimate the relative importance of the convective and radiative transfers that take place between the storage and the enclosure, let us calculate the order of magnitude of the radiative night losses QR, during 15hr, that would take place between the enclosure and the storage if they were black bodies:

es = 105 s (= 28 hr).

(11)

These estimations are confirmed by the fact that, when AQ is close to zero (bad weather), the stored heat is sensible heat only and that it drops down to nearly zero within 2 days (e.g. on 11 and 12 Oct., as can be seen in Fig. 7).

QR = x [ ( T f , . ) - ( T,.4)] x S ' x 15 x 3600

= 7300 kJ.

Cp(T)

(7)

kd/kg

836

where X is the Stefan constant and S' the interface between the storage and the enclosure. We may estimate a mean linear exchange coefficient ha hR -_

4

4

,~[(T tin) -- (Tin)]

(T,.,)-(T,,,)

416

_ ~fi ........... ,4 W/m2/Of ",

-

(8)

From the fact that (QU) is much higher than OR, we may conclude that the draught effectcreated between the patio and the enclosure gives rise to a non-neglegible convective transfer. The total transfer may be charac-

209 . . . .

i

.

38i T

,

,

.61 50

r

~6 60

i

I

I 70

*C

Fig. 9. Specific heat Cp(T)of the parat~n-wax,as a function of temperature T.

Experimental results of a latent-heat solar-roof, used for breeding chickens

6.1.2 Working conditions with melting. When melting occurs, the heat capacities that have to be used either when a "small" melting slab (thickness of 1 or 2 cm) is considered or when the whole storage is considered, are totally different: in the first case, the whole slab is very close to the melting temperature and the specific heat can be taken to be the fusion latent heat divided by the temperature spread of the slab, that is ~ 30 kJ/kg/°C. In the second case, the storage always exhibits a higher temperature difference between top and bottom of about 30°C. Thus, during the daily fusion-solidification cycle, an important part of the storage does not change phase and only exchange sensible heat. On a fair weather sequence, Q~, the heat stored at night, ranges around 300kJ/kg and the average night temperature (T3~) is about 60°C higher than the reference temperature of 13°C. Thus the mean specific heat can be estimated to be of the order of 5 kJ/kg/°C. As a consequnce, the time-constant ~"of a I cm melting slab (101.) ranges around 1.6 × 104 s (= 4.5 hr). As for the time-constant ~'~of the whole storage, it is of about 50 hr. Such time-constants explain very well the response of the storage-system to the slow time fluctuations of the input as we shall see in the next subsection.

2 9 3 r"-xl04 --e-- --,.- --e" / , a ~

~

209 -xiO 4

I 25

1

~ ~,'¢'''%

kJ/~/~oy

:x,o"

357

90~ -

P'"~

V

%--,, */I

[ .,L

. .......... . .....

...."T ......

'..

'w

70 ~

'..

r~.

...4, ,.= ,.

6 0 ~ &...')~ '..¢,....X_. el. • .X"" ' ^ " . . x . . . . . .1 x.. "4~..-)~"" 40

-

J

%.

.. ".

-..

l 23"c

_

_

I~ , , Slpf 27

,

,

i i 0¢L2

i

i

i

i 7

T.,, -L

i

,

i

i

I 12

Fig. 10. Daily fluctuations of I, T3m, T~., T3w, T~M and T. (Period I).

6.2 Daily covariations of input, output and state vari-

ables The daily fluctuations of the variables may be observed in Figs. 10 and 11. For instance, during Period I (Fig. 10), from 27 Sept. to 5 Oct., the heat load is very high, and the response of T3,, to the fluctuations of the solar intensity I are considerably smoothed out, with a time delay of about 1 day. The response of TI,, is still weaker, with a timedelay of 2 days. Let us emphasize that the system can endure 2 days without melting (5, 6 Oct.) with a trifling temperature drop (Fig. 10). Let us consider another case, when the heat load is a little bit smaller. The fluctuations of the solar intensity I from 21 to 25 Nov. are closely followed by T3m with a very small smoothing effect and a I-day delay (Fig. 11). TI,, follows with a much more important smoothing effect and a 2-day delay (Fig. 11). Finally, if the storage-system is nearly totally unloaded, the response amplitude of the state-variables are very large and much quicker. Indeed, the solar intensity oscillation of 27 Nov. is followed, on the very next day, by 7"3,,, T~,, and Tm simultaneously (the time-constant is much weaker than in the previous cases) (Fig. 11). The same behavior of T3m, T~m and T,, may be observed during the bad weaker sequence of 9-11 Oct. (Fig. 10). 7, COMPARISON BETWEEN DWFERENT STABLE OPERATION LEVELS

During the two Periods I and II, the storage system behaves in quite different ways. Indeed, during Period I, the storage is considerably melted, sometimes totally melted, playing its regulation role by limiting the temperature rise of the whole installation. During Period II generally its heat capacity is not totally used. A smaller part of the storage-system is melted and a weaker tem-

~ kJ/m2/day 2"3~ 4--

"~,

'K

, i '

A

2.09~ Xl04~

/i A

/

I ~ ' ~ .~,~ . . . .

I

. . . .

!

L: ..........

I~

/

xr.-." "' -x....~..x---.~.. -

2~

I

,

:

......

~

...*

~

r~ z3

20[ ~ ~ Nov. IO

Fig.

11. Daily

fluctuations

,

~

[

,

,

25

,

L

i

~

L

28

of I, T3,,, T~,,, T3M, TIw and T= (Period

II).

perature difference between T,, and T,,. is maintained. Indeed Period I iT,. - T , , ) = 13.75 (t7 =2.42).

(12)

iT,. - L,.) = 10.60 (o- = 1.59).

(13)

Period II

358

C. BENARDet al.

This 3°C difference between the two periods means a larger heat exchange between the storage and the enclosure during Period I. Indeed Period I

us stress that it is a little bit smaller than the value of 5 kJ/kg established for sunny sequences. Indeed the meanvalues presented here include bad days. 7.3 Overnightuseful heat capacity understable operation

conditions ( T , m - Tin) = 11.53 (o-=4.15).

(14)

(T,,.-Tm)=9 (~ = 3.29).

(15)

Period II

Now, it is also interesting to define the average night useful specific heat that is available for the very same night. Indeed, under stable operating conditions, the storage system oscillates only between T3., and T3M without dropping down to Te,., so that it is interesting to define

Thus, QN, the heat used during the night, is clearly more important during Period I: Period I

QN = 137 kJ/kg.

c " - (T3M)(q") - (T3,.)'

(20)

(16)

For Period I, C"= 8.85 kJ/kg/°C. Period II QN = 82 kJ/kg. (17) For Period II, C" = 6.63 kJ/kg/°C. Obviously, during stable sequences, a better use of the latent heat is made, especially for Period I, than when the The corresponding heat exchange coefficients are h = 9.7 W/m2/°C (Period I) and h = 7 W/m2/°C (Period II). storage temperature drops down because of bad weather. Indeed, during Period II the heat exchange is limited by Nevertheless, an optimal utilization of the heat capacity of 30 kJ/kg (Section 5.12) is not attained. Indeed, for the the low conductivity solid paraffin layer. application considered here, we can't think of having this 7.1 Morning useful heat capacity /or bad weather rather high temperature-melting storage used in the melting-zone only. sequences For a given time interval, the average useful specific heat may be defined as the ratio between the heat quantity really exchanged by the whole storage system on one hand and the product of the weight of the storage material with the temperature drop on the other hand. For instance, during bad weather sequences, (q,,), the heat that is available in the morning for the following days shifts from 101.SkJ/kg for Period 1 to 79kJ/kg for Period II. But for both bad weather sequences, C, the average useful heat capacity is the same: C=

(qm)

(T3m)-ITm)

---2.40 kJ/°C/kg.

(18)

Indeed, in both cases, the storage works only on sensible heat. Let us emphasize that result (18), which has been obtained by thermal balance on our installation, corroborates the Cp(T) measurements made (Section 5.1.1).

8. CONCLUSION

In spite of a rough installation and a rough instrumentation, very encouraging results have been obtained and could be analyzed. This positive outcome has already led to the construction of a new brooder, a little bit larger, with ventilation and storage optimized on the basis of this experiment.

Acknowledgements--The authors are extremely happy to take the opportunity of the present publication to thank the staff of

the Instituto T6cnico Superior de Andahuaylas (Peru) namely Don. A. Lerzundi and H. Valer for their participation and support, all along these experiments, in spite of all the difficulties that were met.

NOMENCLATURE

7.2 Night useful heat capacityfor bad weathersequences At night, the mean temperature of the storage system, that is the mean-value of T3m, rises up to 67.56°C (Fig. 10) during Period I and up to 57.8&C (Fig. 11) during Period II). The average useful specific heat available at night for the following days is

C ' = (T3,,)-(Tem)" (qM)

(19)

For Period I, q~ = 238.62kJ/kg, C'=4.12kJ/kg/°C. For Period II, qM = 169.47 kJ/kg, C' = 3.78 kJ/kg/°C. C' is slightly weaker during Period II, because, as previously noticed, during this period the storage material is not totally used. As for the order of magnitude of C', let

Period I Period II Period III Period IV Period V Period VI Period VII

27 Sept.-->13 Oct. 1977 (insulated) 18 Nov. ~ 30 Nov. 1977 (insulated) l May~ 15 May 1978 (insulated) 27 April~ 1 May 1978 (no night insulation) June 1977 (lst version) 15 Oct.-,8 Nov. 1977 (chickens) 30 May ~ 25 June 1978 (chickens) ITX empirical S.D. of the daily varying quantity X on a given set of days (x) empirical mean-value of X, on a given set of days AY maximum difference between different values taken by a fluctuating quantity Y time-constant of l-era-thick horizontal slab of the storage material ¢s time-constant of the whole storage material

aQ daily stored heat (from 8 a.m. to 4.30p.m.) 4.30 p.m.) R A daily storage yield cat) specific heat of the paraffin-wax z melting level of the paraffin-wax

Experimental results of a latent-heat solar-roof, used for breeding chickens q~ qN; ON qu Qu q,~ Q,,,

QP QU OR C C'

heat given back by I kg of paraffin-wax during the night (4.30 p.m. ~ 8 a.m.) value taken by qN on the night from day i - 1 to day i 84qN heat present in I kg of paraffin at 4.30p.m. 84qM heat present on I kg of paraffin at 8 a.m. 84q,,, night losses of the whole storage through the upper surface useful heat give back by the storage to the enclosure between 4.30p.m. and 8a.m. on the next day radiative night losses of the storage if it were a black body morning useful specific heat of the storage material, for bad weather sequences night average useful specific heat of the storage material, for bad weather sequences

359

C" overnight useful specific heat under stable operation conditions

REFERENCES

1. C. B~nard, D. Gobin and A. Wirgin, Low-cost solar energy storage regulation systems for raising young chicken in a subsistence level economy. ISES Cong., New Delhi, India (Jan. 1978). 2. J. A. Torrijos G6mez, La cria del polio de carne. Editorial Aedos, Barcelona (in Spanish) (1976). 3. C. B~nard, Y. Body, A. Wirgin and D. Gobin, Caract6risation de la stabilit6 de I'intensit6 solaire par I'analyse temporelle de s6ries at6atoires, en France et au P~rou. La M~t(orologie VI 13, 53 (1978). 4. E. Boileau, Discussion d'un module statistique en m~t6orologie solaire. La Revue de PhysiqueAppliquie 34 (Jan. 79).