INTRODUCTION
 
 

Agricultural greenhouses have had very extensive application in past few decades. The main purpose of their use is to produce some agricultural products outside their usual cultivation season. In other words, application of agricultural greenhouses increase cultivation time, quality and yield of most of agricultural products. In addition, growing of some delicate plants can be achieved in the aeries with climate conditions which are not suitable for growing the particular plants.
 
 

In order to fulfill demands for satisfactory products quality and yield it is necessary to achieve suitable indoor conditions, especially in the periods and areas with extreme climatic fluctuations. Thus, the following parameters must be controlled:
 
 

• temperature
• humidity
• light intensity
• indoor air quality

The following benefits may be achieved using solar energy to satisfy energy demands in an agricultural greenhouse:
 
 

• Energy savings
• Clean environment
• Improved yield and period of cultivation
• " Collateral" benefits

ENERGY DEMANDS
 
 

Active solar heating systems for greenhouse heating, though energy effective, demand high maintenance costs and large land surfaces for solar collectors. On the other hand, passive solar systems, with suitable designed an selected storage, can offer a very significant reduction of energy costs.
 
 

Storage of solar thermal energy in phase change materials enable solar heat to be stored isothermally, at the temperature of reversible phase change (usually melting / solidification process). Solar heat is stored as latent heat of phase change when outside temperature is higher than the temperature of phase change. When outside temperature is lower the stored heat is released. In that way temperature is controlled automatically by the phase change process, provided that the materials and designs are appropriate and heat losses minimized.
 
 

Fig. 1. schematically describes the process of charge and discharge of a phase change storage unit.

The main difference between sensible and latent heat storage (in phase change materials) is that the latter unit absorbs heat isothermally until the total amount of material in the unit is transferred into the other phase. If outside temperature is increased further than the phase change material acts as sensible storage unit.

Needs for additional heating (i.e. fossil fuel consumption) are minimized and limited only for periods with extremely low ambient temperature.
 
 

THE IMPACT ON THE ENVIRONMENT
 
 

The substitution of conventional fuels for heating purposes with solar energy is in accordance with global trend to reduce concentration of CO₂ and other gases produced from burning fossil fuels. The photo synthesis process itself regulates an equilibrium concentration of gases in an open atmosphere. If the process occurs inside a greenhouse the equilibrium can be shifted towards temporarily higher concentrations of CO₂ and ventilation should be able to adjust the appropriate indoor air conditions.
 
 

Humidity and indoor air quality can be controlled by on-line control of relevant parameters and switching fens on/off when required. In some model units attempts were made to involve microcomputer control of these parameters.

In addition to the control of indoor air quality it is necessary to control the light intensity, particularly in the periods and attitudes with high intensity of solar radiation.

The control of the light intensity is usually attained using some sort of removable shades or paints.
 
 

In this presentation a new approach to the simultaneous control of temperature and light intensity will be shown.
 
 
 

ECONOMICAL EVALUATION
 
 

In order to obtain a fair economic balance it is necessary to obtain high quality and quantities of the agricultural products in the greenhouses at low investment and low energy costs. These two demands are usually met when passive solar heating is applied. However, for growing some delicate and expensive plants the pay off rate is higher even at higher investment costs.
 
 

There is now doubt that simplest plastic covered greenhouse proved to have very good output. With new and advanced technologies applied for relevant parameter control much better results are obtained at demonstration levels and, therefore are expected to be proved in practice.
 
 

In this presentation, a new procedure for simultaneous control of temperature and light intensity is described.
 
 

ADVANCED TECHNOLOGIES
 
 

The procedure described below has been developed in the Institute of Nuclear Sciences "Vinca", Belgrade [1,2].
 
 

The aim of this work was to consider a combined isothermal latent heat storage with thermochromic behavior of cobalt (II) chloride complex compounds dissolved in selected low melting mixtures for simultaneous control of temperature and light intensity in an agricultural greenhouse.
 
 

A large number of passive solar greenhouses uses latent heat storage materials undergoing reversible phase change solid-liquid [3]. The most frequently used phase change material for these purposes is CaCl2.6H2O with added thickening and seeding agents. The heat storage unit in such systems is effective only during periods when inside temperature is above melting point of the phase change material. In areas with fluctuating climate, even during sunny winter or early spring periods, the temperature inside the greenhouse can reach very high values, because the phase change material continues to act as sensible heat storage unit after the melting is completed. In such cases it is necessary to prevent overheating and damaging of some delicate plants. The general idea of this work was to attempt to apply the thermochromic behavior of selected metal complexes as an autoregulated shading in the south wall of a greenhouse, becoming effective above a the melting point of the phase change material used as a solvent for the thermochormic compounds.
 
 
 

The materials applied for the above procedure are required to satisfy the following criteria:
 
 

a)Thermophysical properties :
 
 

-The melting point close to ambient temperature

-High value of the enthalpy of fusion and
 
 

b) Spectroscopic properties
 
 

-The change of molar absorption coefficient of cobalt (II) complexes in visible spectral range from low values at room temperature to high values at temperature elevated up to 60oC).
 
 

Two phase change materials, satisfying both criteria were selected. Their compositions are the following:
 
 

PCM(1): Ca(NO3)2 . 4.06 H2O + 0.075 moles of CaCl2 . 6.11 H2O

PCM(2): CH3CONH2 - 0.1 Ca(NO3)2. 4 H2O

Most important thermophysical properties of PCM(1) and PCM(2) are presented in Table 1.
 
 
 
 

Table 1. Thermophysical properties of PCM(1) and PCM(2)
 
 
 

Mixture	Tm (oC)	Tc (oC)	D Hm (kJ/kg)	D Hc (kJ/kg)	cp (JK-1g-1) (m.p.)	Storage  density (MJ/m3)
PCM(1)	35.6	32.4	155	130	2.96	265
PCM(2)	27.7(22.3)	25.1	141	127	2.51	168

 

* Value in brackets correspond to the metastable crystal form.
 
 

In order to satisfy the required spectroscopic properties and to obtain the desired thermochromic behavior in the desired temperature range (relevant for passive solar applications), it is necessary to add appropriate amount of a transition metal salt together with appropriate amount of ligand. Thus, chemical equilibria between various complex species is established in the phase change material. The change of color is a result of the shift of chemical equilibria with change of temperature.
 
 

In this work cobalt chloride salt was used with added chloride ligands to obtain the desired spectroscopic properties. The color changes from light pink with low molar absorbance through purple to dark blue.
 
 

The most indicative thermochromic behavior in PCM(1) was obtained with cobalt (II) chloride solution containing 0.648 mol/kg excess chloride added as CaCl2.
 

In PCM(2) the adequate chloride ligand concentration was 0.294 mol/kg.
 

In both thermochromic mixtures dissolved in PCM(1) and PCM(2) the molar absorption coefficient in the visible wavelength range 380 - 740 nm increased about 4 times when temperature was raised from room temperature to about 60⁰C.

The thermochromic behavior of importance for passive solar applications can be presented in more realistic way. The overall solar energy transmitted through the wall made of a transparent material with a gap of 1 cm filled with phase change and thermochromic material can be calculated as follows. The spectral transmittances of the wall filled with absorbing medium are multiplied by solar spectral irradiances in steps of 5 nm for the spectral range studied. The area borded by these values is compared with corresponding area without absorbing media. The results expressed as integrated opacity/shading (O = 1-T, where T is transmittance) in the wavelength range 380-740 nm), are presented in Table 2.
 
 

Material	[Co2+] mol/kg	[Cl-] mol/kg	T/ 0C	O(%)
PCM(1)	0.0107 "	0.648 "	23 58	22 38.8
PCM(2)	0.0047 "	0.294 "	23 61	5.7 13.8

 

From Table 2 it is evident that a remarkable shading effect is attained in all three materials. Measurements of relevant thermotechnical parameters on a model of a greenhouse with an appropriate design and orientation are necessary in order to make conclusions on the energy efficiency in such a device.
 
 

In Figs 2-4 schematic presentations of an inteligent green house are given, with thermochromic light intensity regulations and latent heat temperature regulations. Thermochromic complex mixture in the phase change material (1) is  placed  in the south wall (2). Heat exchanger tubes (3) are placed through the soil deep in the ground. Another phase change material without thermochromic additives (4) is placed in the north wall (5). In order to obtained stabilized temperature range inside the object it is proposed to place some  capsulated  phase change material (6) in the tubes for heat exchange. It is preferable that the phase change material placed in the ground has lower transition temperature than those used in the walls.

Fig.2. Schematic presentation of the greenhouse with temperature and light intensity regulation
 
 

"COLLATERAL" BENEFITS

During the spring 1999. a new vocabulary has become popular. Two most extraordinary expressions to be remembered were the following:

· Legitimate targets 
· Collateral damage

A reader  might ask himself what is the connection between the two "popular" expressions and the passive solar greenhouse. The explanation is very simple:

It has not been reported that any greenhouse was on the list of "legitimate" targets, which means that growing vegetables in a greenhouse is safe from air bombing, at least from the hight of 5 000 feet.  Therefore a conclusion might be drown that unlike bridges, roads, schools, hospitals, embassy buildings etc, it is considered that greenhouses are to be used strictly for civil purposes. Hence, the use of greenhouses is highly recommended not only for the areas with fluctuating climate but also for the areas with fluctuating number of independent sovereign states, fluctuating number of mutual peace keepers and other factors relevant to the subject.

On the other hand, it has always been popular here to consider that vegetables is much more tasty when it is grown in an open field. However, after the mission of "Merciful Angel" which took a large number of chemical plants as legitimate targets, placed in the most fertile land fields, it has become a hit of the season to put a strange advert on an open market counter in Belgrade:" The vegetable sold here is grown in my greenhouse" .  That can obviously be considered as nothing but "Collateral" benefit. 
 
 
 
 
 

REFERENCES
 

[1]   R. Nikolic, Yu Patent Appl., No P-734 (1995), FRY Bull. Intellectual Propeties, No.4. 
       p.574, 1998.  

[2]   M. Marinkovic, R. Nikolic, J. Savovic, S. Gadzuric, I. Zsigrai,  Solar Energy Materials 
        &  Solar Cells, 51 (1998) 401- 411

[3]   M. Santamouris, C.A.Balaras, E. Dascalaki and M. Vallindras, Solar Energy, 53        
 (1994) 411 - 426.
 
 
 

Ruzica Nikolic

bruza@rt270.vin.bg.ac.yu
rufizhem@EUnet.yu