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Uzakov Gulom Norboyevich, professor, doctor of technical sciences Karshi engineering-economics institute, Head of "Heat energy" department E-mail: [email protected] Davlonov Khayrulla Allamurotovich, post-doctorate researcher of "Heat energy" department Karshi engineering-economics institute E-mail: [email protected]
MODELING OF THE HEAT AND MOISTURE BALANCE OF THE HELIO-GREENHOUSE WITH THE PYROLYSIS HEATING INSTALLATION AND THE HEAT ENERGY UTILIZATION
Abstract: The physico-mathematical model of the heat and moisture balance of a solar heater with a pyrolysis heating installation under steady-state conditions is developed in the article. The developed mathematical model of thermal and material balances of the greenhouse allows to determine the heat load for heating and the required water flow for irrigation taking into account specific conditions.
Keywords: heat balance, helio-greenhouse, solar energy, pyrolysis installation, heat flow, heat, heating system.
Currently, much attention is being paid to the issues of energy saving and increasing the energy efficiency of thermal power systems for agricultural purposes. One of the ways to save traditional fuel and energy resources in agricultural facilities and facilities is the use of alternative and renewable energy sources (RES).
The production of fruit and vegetable products in the structures of protected soil requires high energy costs. In many countries of the world, for example in the Netherlands, Germany, energy consumption in greenhouses is 1 ^ 1.5% of the national energy consumption and reaches 20 ^ 35% of the total energy consumption in agriculture [1]. Energy costs in greenhouses are the determining factor for reducing the cost of greenhouse products. Now even in the developed countries of the world, the share of energy costs in the structure of the cost of vegetable products of protected soil is up to 70% [2]. In Central Asia, the energy intensity of vegetables grown in greenhouses is also very high. The production of 1 kg of vegetables in greenhouses consumed 10-13 kg.u.t. [3; 4].
Greenhouses are biological and heat engineering devices and they can be greatly improved if they are turned into solar greenhouses. Solar energy in an ordinary greenhouse is used mainly for the process of photosynthesis, in which plants absorb and accumulate up to 10% of the energy of the incident solar radiation [4; 5; 6]. The solar greenhouse itself is a passive solar heating system. The conducted studies show that the passive solar heating system of greenhouses in winter mode provides only 30% of the heating load [7; 8]. Simulation and study of the thermal balance of a solar greenhouse with a passive heat accumulator was considered in [9]. The authors in [10] presented the results of modeling the unsteady
temperature regime of helio-heaters with short-term heat accumulators.
Thus, in the solar greenhouses, especially in the night mode, in winter an additional alternative energy source is required. World experience in the development of greenhouse production points to the need to introduce energy-saving technologies and heating methods on the basis of non-traditional renewable energy sources (NER).
With the purpose of solving the above-mentioned problems, we have developed a combined heating system for solar greenhouses with a pyrolysis heating plant and a heat recovery device (condensing heat of the condenser of the pyrolysis unit cooler).
To assess the effectiveness of the proposed system, let us consider the heat balance of a solar greenhouse with a pyrolysis heating installation (POU) and thermal energy utilization (UTE). In this case, the structure is considered as a single energy system, which includes heating, ventilation, humidification, utilization and accumulation of thermal energy and heat engineering of enclosing structures.
The computational scheme of the energy balance of the greenhouse with the UU and UTE is presented in (Fig 1).
Based on the developed calculation scheme, we compile a system of equations for the energy balance of the structure, which is a physico-mathematical model for the formation of the energy regime in the greenhouse.
The equation of the heat balance of the greenhouse as a whole has the following form:
Qot + Qp + Qmy = Qogr + Qvent + Q,nf +
p y g n , Vt (1)
+ Qpm + Qgr + Qrast + Qconi + Quvl + Qdr + Qeks
Figure 1. The calculation scheme of the energy balance of the greenhouse with pyrolysis heating installation and heat energy utilization; 1 - soil of the greenhouse; 2 - fencing; 3 - heating battery; 4 - subsoil ground; 5 - utilizer of thermal energy - heat accumulator; 6 - ventilation opening; 7 - plants
where, Qot - heat transfer (thermal power) of the heating system, W; Qp - penetrating solar radiation, Vt; Qmy - heat flow from the heat energy recovery system, Vt; Qogr - loss of heat through the fence, Vt; Qvmt - loss of heat with ventilation air leaving the greenhouse through the exhaust ventilation opening, Vt; Qinf - heat loss through leakage in the enclosure (infiltration), W; Qpoc - heat exchange with soil, W; Qgr - heat loss through the ground, W; Qat - heat exchange with plants, W; QKond - is the heat released during the condensation of water vapor and the inner surfaces of the transparent coatings of the greenhouse, W; Quvl - heat, introduced by moisturizing water from the air humidification system, W; Qdr - heat flux, released by breathing plants, W; Qeks - operational heat inflows, Wt.
The heat balance equation for the soil surface in the greenhouse:
Qpoc = QLnv + Ql + Qlp + Qn ,Vt (2)
where, Q1onv - convective heat heat flow from soil to air in the working area of the greenhouse, Vt; Ql - the radiation is the radiant heat flux from the soil surface in the greenhouse, Vt; Qlp - is the heat flux characterizing the heat expended on evaporation of moisture from the soil, Vt; Qn - is the heat absorbed by the soil of the greenhouse, Vt.
The equation of heat balance on the surface of the fence of the greenhouse:
Qk + Ql + QHd = Q0n + Ql, vt (3)
where, Q°v - is the heat flux as a result of convective heat exchange ofthe inner surface ofthe enclosure with air in the green-
house, Vt; Ql - radiant heat flux from the inner surface of the fence,; Ql" - he is the heat flux as a result of convective heat exchange of the outer surface ofthe enclosure with the surrounding air, Vt; Ql - the radiation is the radiant heat flux from the outer surface of the enclosure, Vt; Qlond - heat flow, which characterizes the evolution of heat during the condensation of water vapor on the inner surface of the enclosure, Vt.
Heat losses through the greenhouse fence can be determined from the heat transfer equation;
Qogr = KFg (tnv - tbb )Vt (4)
or
Qogr = Fg (1 + Pmf ),Vt (5)
where, K - is the heat transfer coefficient of the greenhouse Vt
fence, 2 ; Fogr - total greenhouse fence area, m2; tnb -temperature of outside air, °C; tbb - air temperature in the greenhouse, °C; togr - temperature of the outer surface of the
enclosure of the greenhouse, °C; Rt = Rop +— - thermal
_ ,eheettta„8ferofthee„Rosur: ^ -
Vt
heat transfer coefficient of the outer surface of the enclosure,
—2-; Pinf - coefficient of infiltration of outside air, for
m ■ K
greenhouses can be taken equal to 0.2.
The radiant heat flux Qllc I from the heat exchange between the soil surface and the inner surface of the greenhouse enclosure, provided that > tgr and ty 12 = 1 (all thermal ra-
diation from the surface of the soil 1 falls on the inner surface of the enclosure 2 of the greenhouse) is determined by the formula.
( T„
Qluc Co^np12Fpc
( Т Л
pov
v ÎÔÔ ,
v100 У
Vt
(6)
where, co - is the emissivity of the absolute black body, 5.67 Vt
7—2—tz > enpn; - reduced relative coefficient of thermal radia-
(m ■ K )
tion of the soil surface 1 and the inner surface of the enclosure 2 of the greenhouse; Fpov - the surface area of the soil 1, m2. Absolute surface temperatures and fences:
T = t + 273.15 K (7)
pov pov v '
T = t + 273.15 K (8)
ogr ogr v '
The convective component of the heat exchange between the internal air and the inner surface of the fence of the greenhouse 2 can be determined according to the Newton-Rich-mann law:
QO =ahn (tbb - togr )• Fogr, Vt, (9)
where, ahn - coefficient of heat transfer from the inner surface
Vt
of the enclosure of the greenhouse, —^^.
The heat flow from the outer surface of the enclosure to the ambient air is also calculated according to the law of con-vective heat transfer
Q0n =anap ( - tnh ) Fogr , Vt , (10)
Loss of heat with outgoing ventilation air Qvent is numerically equal to the heat flow that goes to heating the supply air entering the greenhouse through its ventilation opening, i.e.
Qvent = Gb ((bb - lnb )'Vt
(11)
or
Qvmt = Gb ■ Cp (( - tnb ),V (12)
where, Gb - mass flow of supply air, participating in the air exchange of the greenhouse, kg/s; ibb - inb - respectively, the specific enthalpy of indoor and outdoor air, DJ/kg; cp - is the specific isobaric heat capacity of air, DJ/kg • °C.
The losses of heat through the leakage of barriers (infiltration) on the basis of experimental studies are calculated depending on the heat loss through the fence and constitute an average of 20% of heat loss through the fence:
Q _ a (tbb - tn, " Q,nf _ -
100
■Qo:
(13)
or
Q- f = 0,2 ■ Q , Vt
^inf ' ogr '
(14)
Convective heat exchange between the soil surface 1 and the internal air of the greenhouse obeys the Newton-Rich-mann law:
Qkonv =apoc (tpov — thh ) ' Fpov , Vt , ( 15)
where, a - soil is the heat transfer coefficient of the soil sur-
w Vt
face, —2-•
m2 • K
The heat flux consumed by the evaporation of moisture from the soil surface is determined by the formula
QSp = G"sp • r, Vt, (16)
Heat losses through the soil are calculated by the formula [4; 5]:
t -1,
Q _ p0V ПЬ _ F
^gr RCp П
(17)
where, Rf is the average resistance of heat transfer through the soil in the greenhouse.
The temperature regime of a helio-heater depends not only on the effects of solar radiation and heating and ventilation systems, but also on the interaction of the air environment with soil and plants.
Convective heat exchange, due to the temperature difference between plants and air, can be determined by the formula:
Qrast = aratt * Frast (trast - thh ) Vt , (18)
where, amst - rest is the heat transfer coefficient from the plant
rasvt
surface, —2—> Fmtt - surface of plants, m2; trast is the average m • K
temperature of plants, °C.
Heat transfer by transpiration (evaporation of water by plant leaves)
Qr, = mrFrast ,Vt where, m - speed of transpiration, 0,03 ^0,3-
(19)
(m2-C )
r - is the heat of vaporization, r = 2257 kDJ/kg.
The work also takes into account the influence of the heat of breathing of plants on the heat balance of the greenhouse. The heat flux released during the respiration of plants is calculated by the formula [11].
Qd r = 0,2s = 0,2 Vt / m2
(20)
Heat generated by condensation of water vapor on the inner surface of a film coating
Qkond = Gkond ■ r, Vt , (21)
where, Gkond - is the condensate flow rate, kg/s; r - latent heat of condensation, DJ/kg.
Heat that penetrates the humidifying air through the humidification system:
Quvl = Guvl.v ' Cp (tuvl .v — hb) (22)
Operational heat fluxes are determined by the formula:
Qeks = Qejv + Qov + Qfyi + Qpr , Vt (23)
where, Qedv - heat inflow from the operation of electric motors of fans and pumps in the greenhouse, Vt; Qov - heat input from the lighting system, Vt; Qyd - heat input from working personnel, Vt; Qpr - other operational heat fluxes, Vt.
Heat losses from electric motors of fans, pumps and other electrical equipment are determined by the formula [11].
Qedv = Ne ■ n, Vt, (24)
where, Ne - electric motor power, Vt; n - the number of electrical equipment, pcs. Heat input from lighting [11]:
Q0Sv = A • F, Vt, (25)
where, A - the amount of heat emitted by the lighting devices per 1 m2 of the greenhouse area, A = 4 Vt/m2; F - is the area of the greenhouse, m2.
Heat supply from working people in the greenhouse [11]: Qfyd = 350 m, Vt, (26)
where, m - number of employees in the greenhouse; 350 Vt -heat dissipation from one person at an average intensity ofwork.
Heat flow from the surface of the heating batteries:
Qt =a ■ Fot (t„.ot - thh) Vt, (27)
where, aot - coefficient of heat transfer from the surface of Vt
G.p = Gsp = Kp pp (Ppov - Pbb ) • Fpov, kg / s (32)
the heating batteries,
m2 • K
; F0t ; - the surface of the heating
batteries, m2; tn.ot - average temperature of the surface of the heating batteries, °C.
Inflow of solar radiation inside greenhouses according to the formula [4]:
Qp = qp ■ Kprop ■an ■ F, Vt, (28)
where, qp - the power of the total incident solar radiation, Vt/ m2; Kpmp - the transmittance of solar radiation of the light-impregnated greenhouse fence; an - coefficient of absorption of solar radiation of air; F - is the area of the greenhouse, m2.
The amount of heat from the heat recovery system is calculated from the heat transfer equation:
Qut = K ■ Fut (tuLv - tbb ), Vt, (29)
or
Qut = an.ut ■ Fut (t*.ut - tbb ), Vt, (30)
where, K - is the heat transfer coefficient,
Vt
-Fut - area of
heat transfer of heat energy utilization, m2; mut - temperature of hot water in the utilizer, °C ;anMt - coefficient of heat trans-
Vt
fer from the heat recovery surface, —2— tcmym temperature
m • K
of the heat exchanger wall, °C.
The equation of the material balance of the helio-green-house can be represented by the following equation:
G,sp + Guvl + Gkond = Grmt + G.nf , kg I s (31) where Gkp - evaporation of moisture from the soil surface, kg/s; Gvent - loss of moisture with exhaust air leaving the greenhouse through the ventilation line, kg/s; Ginf - loss of moisture from the greenhouse through air infiltration, kg/s; Guvl - is the moisture inflow, i.e. the amount of moisture entering the greenhouse through the humidifying air, kg/s; Gkond -condition the amount of moisture released during the condensation of water vapor on the inner surfaces of the greenhouse fences, kg/s.
Evaporation of moisture from the soil surface can be determined by the mass transfer equation:
For
pov
where Kor =--soil irrigation coefficient.
pov
If Kor = 0, then the irrigation of the soil is completely absent. If Kor = 1 - the entire soil surface is irrigated in the greenhouse; F'pov - is the area of the irrigated part of the soil surface, m2; Fpov - the surface area of the soil in the greenhouse, m2. Pp - is the average mass-transfer coefficient of the soil surface, kg }
—2-I; PPov and Phh - partial pressure ofwater vapor, re-
i^Q 'C' Pa J
spectively, at the surface of the soil and far from it, Pa.
The amount of moisture coming in with humidifying air (through the supply air duct)
Guv, = Wuv, (dn -dd), kg I s (33)
where Wym - moisture content of air after humidification, kg/s; dn - isture content of air before humidification, kg/s; dd - moisture content of air before humidification, kg/s.
The loss of moisture with the air leaving the greenhouse is determined by the following formula
Gvent =Wb (dbb - dnh), kg I s (34)
where Wh - is the exhaust air flow in the ventilation system, dhh - and dnh - is the moisture content of the indoor and outdoor air, g/kg.
The equation of material balance on the soil surface has the following form:
Gpol =Gpogl.r + Gsp , kg 1 s (35)
where Gpol - is the flow of water going to watering the soil, kg/s; Gpogl.r - the expense of water absorbed by plants, kg/s.
Equation of material balance (31) and (35) is also an important condition for determining the temperature tbb and relative humidity fhh in the internal air.
In (Fig. 2) shows the possible heat balance of the helio -greenhouse in the natural and climatic conditions ofKarshi. The analysis of the obtained results shows that in the radiation - climatic conditions of Karshi, the energy supply of film solar heaters due to solar energy is 25-30% (in winter). Based on the calculations and experimental studies carried out, it is established that due to combined use of solar radiation and biomass energy, the heat demand of the greenhouse for heating is fully provided. At the same time, the heat recovered from the condenser of the pyrolysis plant covers 29.5 ^ 44.7% of the heat load.
Figure 2. Dynamics of changes in heat fluxes in a helio-heater: 1 - total solar radiation (Qpr) is the simplest in a helio-heater; 2 - total heat losses (Qtp) in the greenhouse; 3 - solar energy absorbed by soil (Qpoc); 4 - heat supplied to the greenhouse from the heat energy reclaimer
The energy flows in the helio-heater under consideration are shown in (Fig. 3).
Figure 3. Scheme of energy flows in a helio-heater with a pyrolysis unit: PR - pyrolysis reactor-boiler; K - capacitor-cooler; UT heat recovery; EG power generator
Thus, the developed mathematical model of heat and moisture balance makes it possible to determine for the he-
lio-greenhouse such important parameters as the required heat capacity of the heating system with a pyrolysis plant,
energy savings in the utilization of the VED pyrolysis plant, and the necessary water flow for irrigating the soil in a helio-greenhouse under specified specific conditions. The results obtained serve to establish and regulate the required thermal
conditions of helio-heaters. It is established that the utilization of the thermal energy of the condenser of the pyrolysis plant for heating the greenhouse significantly increases the efficiency of the energy use of biomass.
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