Computer Aided Control for Solar Drying

Copyright © IFAC Identification and System Parameter Estimation . Budapest. Hunga ry 1991

COMPUTER AIDED CONTROL FOR SOLAR DRYING J. Miiller*, T. Conrad*, M. Martinov** and M. Tesic** *Institutefor Agricultural Engineering in the Tropics and Subtropics. Hohenheim University. Stullgart. Germany **Institutefor Mechanization. University of Novi Sad. Novi Sad. Yugoslavia

Abstract. An exclusively solar heated dryer in modular design for medicinal plants and spices has been developed at the Institute for Agricultural Engineering in the Tropics and Subtropics of Hohenheim University. Solar air heaters have been incorporated into the roof of a plastic film covered greenhouse in which a batch dryer is located. The dryer can be operated temporarily with recirculated air by closing the air inlet resulting in a rise of maximum temperature from 50°C to 70°C. A computer aided control system opens the air inlet as soon as the additional drying effect of the higher temperature is compensated by the reduction of drying rate caused by gradual increase of air humidity. Using the control system the capacity of the dryer is increased by 30%. Keywords. Solar energy; food-processing; drying; microcomputer-based control; computer simulation; humidity control; temperature control.

However, as a minimum mass flow passing the drying material is required, this option is strongly restricted in its application.

INTRODUCTION For several reasons solar energy is appropriate for drying of medicinal plants and spices.

In the presented work the problem is solved by partially re circulating exhaust air from the batch dryer leading to a rise of drying temperature, but also to higher humidity of the drying air. Thus, to obtain optimum drying conditions a system is needed which controls air recirculation.

So, the production of active substances is correlated to solar irradiation. Therefore, medicinal plants and spices are mainly cultivated in regions with high solar irradiation. As most of the active substances are sensitive to high temperature, medicinal plants and spices have to be dried at low temperature level which can be achieved by using simple solar air heaters. Conventional dryers, burning fossil fuels, are working far from their optimal efficiency at such temperatures.

OBJECTIVES The control system has to manage air re circulation in order to obtain the highest drying rate available at the existing conditions, such as irradiation, ambient temperature and humidity as well as exhaust temperature and humidity. To develop such a system the following objectives have been established:

Furthermore, as medicinal plants and spices are usually harvested at 80% and stored at 11 % moisture content w.b., the drying of this crop requires a high energy equivalent of 1 to 2 I fuel oil per kg of dried material. Thus, the substitution of fuels by solar energy promises to be economical, already at today's oil price.

- Investigation of the influence of drying temperature and air humidity on drying time and product quality in laboratory tests.

Since 1987, the prototype of a greenhouse-type solar dryer has been operated in Yugoslavia, showing that solar energy can successfully be utilized for drying of medicinal plants and spices (Muller. J and M. Tesic, 1988; Tesic et. aI., 1988; Muller et. aI., 1989 a.; Muller et. aI., 1989 b.).

- Development of a computer aided control system, based on data resulting from these investigations. Examination of system effectiveness in practical operation.

However, as the solar air heaters are dimensioned in a way, not allowing air temperature to exceed the specific maximum temperature of the drying material, the dryer is often working at temperatures below. Therefore drying rate of the solar dryer is smaller compared to conventional dryers.

DESIGN AND FUNCTION OF THE SOLAR DRYER To keep investment costs low the drying equipment has been incorporated into a standard plastic film greenhouse with the roof used as air heater area and a batch dryer being installed in the interior space. The greenhouse has been covered with UV-stabilized air-bubble foil.

One possibility to increase drying rate is to rise drying temperature in times of low irradiation by decreasing the air flow rate.

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Underneath this transparent cover an absorbing surface and a second air-bubble foil for insulation have been installed, thus converting the pane of the roof into a solar air heater. To facilitate maximum efficiency, the roofridge is oriented west-east and the south-bound air heater area is extended by enlarging it downwards to ground level (Fig. 1). Total width of the plant is 15 m and the pitch of the roof has 22° at a height of 4.4 m. Due to its modular design, the dryer's length can be varied in steps, corresponding to the spacing of the trusses. Each of these spacings constitutes an autonomous module with 32 m2 air heater area and a dryer surface of 10 m2 . For ventilation, a 450 W radial fan is operated, providing 3900 m3 /h air mass flow at a pressure of 40 Pa. The prototype plant, situated in Yugoslavia, consists of 6 modules and has a total load of 3 to 5 tons fresh mass per batch.

During drying, this temperature cannot be reached, because part of the sensible heat is transformed into latent heat, due to evaporation. Furthermore, using the recirculation mode, drying rate only increases as long as the accelerative effect of temperature rising is not compensated by the derating effect of rising humidity. To decide if this balance is reached and therefore the vent has to be opened, a special control system is needed. PSYCHROMETRIC ACTION AT RECIRCULATION MODE Figure 3 shows the psychrometric action of air passing air heater and bulk.

oS1.n

'1'=100%

.c.

>a.

o

:S c

UJ s ol or Olr hea te r

Fig. 1:

batch dry er

pas sage

Design of the solar greenhouse dryer Absolute humidity, x

At the northern eaves air is sucked by fans and forced between transparent cover and absorber. At the southern eaves it is turned round and conducted to the batch dryer via an air duct. The humidified air ascends in the inside space of the greenhouse and is exhausted through a side wall vent. To open this vent the plastic cover of the northern side wall is lowered along its whole length by an electric winch.

Fig. 3:

If the vent is opened, ambient air at a temperature of &0 is sucked into the air heater where temperature is raised to &l' temperature difference a & being proportional to global irradiation. As absolute humidity of the air does not change on its way through the air heater, humidity decreases, increasing saturation deficit of the air. Passing the bulk, the air takes up water vapour, thus being cooled adiabatically to &2'

80 ,-------,-------,-------,-------, ~

60~------~------~~----~_+------~

At recirculation mode exhaust air from the batch dryer, instead of ambient air, is heated by the air heater, which leads to a higher mean air heater temperature. Therefore, heat losses to the ambience do increase. Hence, temperature rise a & is decreasing with every heating cycle until a maximum temperature is reached asymptotically. At this point, incoming global irradiation is compensated by the heat losses

oi

" ~

Psychrometric action during the drying process.

40~------~--~~~~----~~~~--~

Q)

a.

E Q)

~ 20~==~~~~--~-=~~~~==~ OL-------~------~-------L------~

0.00

6.00

12 . 00

18.00

24.00

The rise of temperature and humidity are mainly influenced by water vapour absorption of the air. To elucidate this fact, a model for steady-state meteorological parameters is used to describe the psychrometric action. For simplification of the model, the fact that water absorption changes with changing air condition is neglected.

Time of Day

Fig. 2:

March of temperature at recirculation and ambient air mode.

At ambient air mode, for the thermal output and the thermal losses can be stated:

To operate the dryer at recirculation mode the side wall vent is closed. In this way, the hot, humidified air is recirculated causing higher drying temperature. As shown in Fig. 2, at ambient air mode, temperatures of up to 50°C have been measured inside the empty dryer, whereas at recirculation mode, the temperature was rising up to 70°C.

The product of absorption coefficient a and transmission

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Vapour absorption of 5 g/kg even leads to lower temperature at recirculation mode, in comparison to ambient air mode, because evaporative cooling exceeds the heating effect of the air heater.

coefficient f of the transparent cover yields the optical efficiency of the air heater. Heating of the air mass m.ir represents the thermal output. Thermal losses are determined by the heat transition coefficient le, which takes into account convective losses as well as radiation losses. At re circulation mode, air heater inlet temperature corresponds to batch dryer exhaust temperature. Eq. (1) then changes to:

100 80

(2)

en GA

= rh., Cp.., (OI .n-

02.n_l) + kA (OI.n + °2.n-l -

2

001

x ~ 109 / kg

;f. >- 60

U

E

Derived from Eq. (2), the temperature at the air heater outlet 0 I' which is at the same time the batch dryer inlet temperature, can be stated for the nth cycle as:

:J

I

20 0 20

(3)

The batch dryer outlet temperature depends on adiabatic cooling of the air, which is dependent on water absorption II x : fI

2".

h n - r (x + llx) = ---"------C . + c (x + llx) P.""

40

Fig. 4:

px (0.622 + x) , Ps(fI)

70

Psychrometric action at vapour absorption rates of 1 to 5 g/kg; G = 600 W /m2, flo = 30°C, x = 10 g/kg.

FUNCTION OF THE COMPUTER AIDED CONTROL SYSTEM Recirculation can only be useful, if the batch dryer outlet temperature fl2n is higher than ambient temperature fl o. If conditions meet this requirement, the vent is closed and a control system has to find the point where the effect of increasing drying rate by rising the temperature is exceeded by the decreasing effect due to raising humidity. For the design of such a control system it is mandatory to know the influence of temperature and humidity on the drying process of the concerned drying material. Hence, these parameters have been systematically varied in laboratory drying tests with different varieties of medicinal plants. Exemplary, Fig. 5 shows the dependency of drying time on humidity at different temperatures for peppermint.

(5)

(~

• 10

t

.237.3

. 0.66(9)

(6)

The time interval of a cycle follows from the volume of the inside spacing divided by the volume flow rate: llt

60

(4)

The saturation vapour pressure Ps(fI) can be calculated with an approximate equation derived by Tetens (1930):

= 1.3329

40 50 Temperature . ·C

P.""P

The humidity according to these air conditions is given by Eq. (5):

Ps (fI)

30

=

~

(7)

VeW

36

Figure 4 shows the calculated changes of air conditions exemplary for a summer morning at the experimental site in Yugoslavia. Ambient air temperature was 30°C, global irradiation 600 W/m2 and absolute humidity 10 g/kg. Water vapour adsorption has been varied between 1 and 5 g/kg.

L

L

aj

24

~

•

Ol

c

':;" L

0

~ l' 4 0 ' C

45

12 50

•

As plotted in Fig. 4, ambient air is heated in the air heater

60

up to 40°C at constant absolute humidity. At recirculation mode, humidity raises, depending on water vapour absorption from the drying material. When water vapour absorption is low, e.g. at the end of the drying process, temperature is rising fast, due to the low heat of evaporation required by the drying process. After operating the installation at recirculation mode for 10 minutes, the temperature reaches 51°C and after another 10 minutes saturation point temperature of 53°C is almost reached. At higher vapour absorption rates the temperature rise is lower.

0 10

30

50

70

Humidit y. %

Fig. 5:

165

Dependency of drying time on humidity of the drying air at different temperatures for peppermint.

Drying time ends as soon as the desired storage moisture content of 11 % is reached. With raising humidity, drying time first increases at linear slope until the slope becomes exponentially by approaching equilibrium humidity. At 40°C the linear section only keeps up to 50% RH., whereas at 60°C it ends at 65% RH. Due to the exponential increase of drying time, the control system has to prevent humidity exceeding this limit:

Whereas 0 1 and
(10)

with

From Fig. 5, couples of humidity and drying temperature leading to the same drying rate can be selected. In the area of linear slope the influence of drying temperature is significantly higher than the one of humidity. To keep drying time constant, humidity is allowed to raise 7% RH. per 1 K temperature rise, starting at 50°C. At 40°C the relationship equals 10% RH. per 1 K. For a temperature range from 25 to 55°C this substitution ratio s can be approximated as:

s

=

62 - 20 1

+

0.02 O~

40 =f(G)

(11)

Humidity
(8)

Xl = 0.622 Now, in a

(12)

The simulated humidity for the ambient air mode
(9)

Now, with the simulated values for the air condition at ambient air mode, the variable limit of humidity
Operating the dryer at conditions, represented by couples on this line, drying time will be equal to conditions at ambient air mode. As long as the actual air condition P(OI,
Furthermore, to save essential oils of the medicinal plants and spices, which are sensitive to temperature, a plant specific maximum temperature has to be guaranteed by the control system. If this temperature is exceeded, the side wall vent will open also, to take up colder ambient air:

100.-----,,----- .------.------.------.

The marked region in Fig.6 indicates the range of control for drying peppermint, where the drying rate is increased by recirculation mode without reducing product quality. Temperature is limited to 50°C, as higher temperature leads to blackening of the material.

BO ~----_+------~~~-~~----~----~

;f.

x-lO g I kg

~ 50 ~~--_4------_+-3 ---~~----_4------~ is

E

£

40

For the weather conditions described above and at vapour absorption rates below 2 g/kg, re circulation mode is only limited by the temperature of the drying air. At higher absorption rates, primary the invariable limit of humidity
~----~._----_+__h7.~fffl (1--"'"'-- "-...+--------1

20 1-----+--

-

1'

OL-----~-----L----~~----~----~

20

30

40

50

50

70

Te'T1perature . ·C

Fig. 6:

Figure 7 shows a flow chart of the control system. As the temperature rise in the air heater is, apart from global irradiation, dependent on many other factors being difficult or expensive to measure, e.g. dirt accumulation on the transparent cover, a self-learning component is added to the program. This component updates the regression every time, the side wal\ vent is opened.

Range of control for drying peppermint.

166

Using the control system, drying could be finished after 76 hours at 11 % moisture content. Without re circulation, at the same time the moisture content of the drying material still was 30% and drying had to be continued for two more days. Hence, drying time was shortened by 30% due to the control system, i.e. drying rate could be increased by the same percentage. Thus, using the control system, the same throughput can be managed by a smaller dryer with less modules, saving investment costs.

CONCLUSIONS Solar energy has proved to be appropriate for drying of medicinal plants and spices. A solar dryer, developed at the Institute for Agricultural Engineering in the Tropics and Subtropics of Hohenheim University (F.R.G.) and operated since 1987 at an experimental site in Novi Sad (Yugoslavia) showed good results, however having maximum efficiency only at high irradiation during few hours of the day. To increase drying rate, the development of a control system has been targeted, which allows to raise drying temperature by recirculation of drying air.

Radiat ion Temperature auts,de Humidity outSide Temperaturt inside Humidity inside

Calculation Temperature inside Hum,dlt y Inside Set va lue

"': = U"'o . ~1-' I

"i =11"'; . xl "var = m

.

I"' ,·"','· '"

", '" "v or

•

Therefore, two opposing effects have to be considered. Apart from increasing drying rate by rising temperature, recirculation also leads to a rise of absolute humidity, leading to a decrease of drying rate, if equilibrium humidity is reached.

no

", '" "mo x "', "''''max

yes

f-------.,

Fig. 7:

Hence, a control system has to determine which effect is predominant at actual conditions i.e. whether drying with recirculated air or drying with ambient air increases the perfornance of the solar dryer.

Calculation". ~ fiG)

To investigate the influence of these parameters on the drying rate of different products, laboratory tests have been conducted. Based on results from these tests a control system has been developed, performing the following features:

Control algorithm. EFFEcrs OF TIlE CONTROL SYSTEM ON TIlE DRYING PROCESS

Primary, a product specific maximum temperature limit is kept to protect the drying material from damages due to high temperature.

To examine the effects of the control system in practical operation, tests have been carried out at the experimental site in Yugoslavia. Therefore, one dryer module has been operated at ambient air mode and another one at recirculation mode using the control system. Figure 8 shows the results exemplary for peppermint.

Also an invariable limit of humidity is kept to avoid equilibrium conditions. To increase the drying rate, a variable limit of humidity and temperature is kept, being dependent on actual weather conditions and actual conditions inside the batch dryer. To perform this topic, conditions which would occur at ambient air mode are simulated .

100 .ri ~

;l-

75

e

'-

-;::,...

/~

without recirculahon

ID

C 50 0

~

;A::.:

U ID

with recirculotion

~

:::J III

0

Tests at the experimental site in Yugoslavia showed, that using the control system, performance of the solar dryer could be improved by 30%.

25

~"-

\

~

o o

2

3

4

NOMENCLATURE

Time , h

Fig. 8:

A G V

March of drying for peppermint with and without control system.

V 167

area, m2 global irradiation, W' m"2 volume, m3 volume flow, m3 . h"\

cp h k m

m s p r x

a 1

,H -& IP

heat capacity, kJ · kg· 1 enthalpy, kJ · kf k-value, W ' m- . KI mass, kg mass flow, kg' h- I substitution ratio pressure, mbar heat of evaporation, kJ · kg-I absolute humidity, kg · kg-I absorption coefficient transmission coefficient temperature difference, K temperature, °C humidity

Subscripts air abs max n vap var

air absolute maximum number of cycle vapour variable

S

saturation point

o

ambient dryer inlet dryer outlet

1 2

REFERENCES Miiller, J. and M. Tesic (1988). Drying of medicinal plants in a greenhouse-type solar dryer. Proceedings of CNRE Technical Meeting_ CNRE Bulletin No. 19, Food and Agriculture Organization of the United Nations, Rom (Italy), pp. 130-143. Tesic, M., M. Martinov, W. Miihlbauer, J. Miiller, J. Kisgeci and E. Kota (1988). Greenhouse-type solar dryer for drying medicinal plants. Proceedings of the International Conference on "Alternative Energy Sources Today and for 21st. Centmy", Brioni (Yugoslavia), pp. 379-386. Miiller, J., G. Reisinger, J. Kisgeci, E. Kota, M. Tesic and W. Miihlbauer (1989). Development of a greenhouse-type solar dryer for medicinal plants and herbs. Solar and Wind Technology. 6, No. 5, pp. 523-530. Miiller, J., G. Reisinger, W. Miihlbauer, M. Martinov, M. Tesic, J. Kisgeci (1982). Trocknung von Heil- und Gewiirzpflanzen mit Solarenergie in einem Foliengewachshaus. Landtechnik. 44, No.2, pp. 58-65. Tetens, O. iller einige meteorologische Begriffe. Geophysik. 6, No. 6, pp. 297-309.

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