SIMULATION ANALYSIS OF SOLAR STERILIZATION SYSTEMS
Tadashi Takakura1, Stephen Kania, and William J. Roberts
Dept. of Bioresource Eng., Rutgers, The State University of New Jersey
20 Ag. Eng. Way, New Brunswick, N. J. 08901-8500
Abstract: A two-dimensional simulation model of a greenhouse-type solar
sterilization system, which consists of a small tunnel with mulching, has been
developed. Several parameters in the model were adjusted through model
verification with experimental data. The model was then used to analyze effects
of thermal properties on temperature increases at various soil depths. Higher
emissivities or absorptivities of both tunnel and mulch films can result in higher
soil temperatures. Absorptivity of the soil has a positive effect on maximum and
a negative effect on minimum soil temperatures. Drier soil achieves higher
maximum temperatures and lower minimum temperatures. Degree-hours above
a particular temperature which kills soil-borne pathogens can easily be calculated
by the model.
Keywords: Solarization, Computer simulation, Sterilization, Mulching
Introduction
Soil sterilization is carried out in some areas by using agricultural chemicals or steam. It
is apparent that the usage of chemicals such as methyl bromide (the use of methyl bromide will be
prohibited after the year 2001) is not favorable because of their negative environmental impact, and
steam is not favorable because of the great energy input required for its use. Mulching has been
extensively studied for soil sterilization by solar energy (solarization) and has been thoroughly
reviewed, although mostly from the pathological standpoint (Katan, 1981; Stapleton, 1996). It has
also been reported that there is a large difference between promising results achieved in greenhouses
and failures seen under field conditions (Katan, 1981). Now a greenhouse-type soil sterilization
system has been built and investigated experimentally (Kania and Roberts, 1996). It is clear that the
term "greenhouse effect" originated from greenhouses, and in this respect, greenhouses are not only
places for plant production but also can be systems to collect solar energy.
It can be said that greenhouses have not been well-studied in this aspect, although a high
potential of the system to utilize solar energy is anticipated from an economic viewpoint. We have
only a limited ability to experimentally test various types of sterilization systems under the many
possible test site conditions because of the expense and time. In order to design the best system,
it is necessary to test many types of systems. Simulation is an advantageous solution because many
types of systems can be tested quickly on the computer. A simulation model for mulching was
developed and a spatial soil temperature regime under transparent polyethylene mulch was
analyzed (Mahere and Katan, 1981). The model was relatively similar to an earlier model, and did
not take into account condensation which often occurs on the film surface and is linked with a large
amount of energy transfer. The effectiveness of differing mulch widths was mainly discussed in the
associated work.
In the present research, a two-dimensional model of a greenhouse with mulching has
been developed by using the computer simulation language called CSMP (Continuous System
1
Present address: College of Environmental Studies, Nagasaki University, Japan
Modeling Program) which is available on PCs as well as mainframes and is a model-oriented
language which is easy to use. Water balance including condensation on the film surface is taken into
account in the present model. After the model was verified, a sensitivity analysis was conducted
for the purpose of system design.
Simulation Model Description
A two-dimensional model of a greenhouse-type solar sterilization system with mulching has
been developed (Fig. I)2. The soil surface is covered by plastic film ('mulch'), and then a tunnel is
constructed over it. Tunnel temperature is represented by the variable TT and mulch by TC in the
figure. Temperature and humidity ratio of air space in the tunnel are TI and WI, respectively.
Temperature and humidity ratio of air space in the mulch are TB and WB, respectively. Arbitrary
soil layers are divided in a two-dimensional way. There are five vertical layers (ZO through Z4),
and five horizontal layers (XO through X4), which assume the system is symmetrical.
Temperatures and the humidity ratio of the inside surface layer are TOI, T02, T03, and WF,
respectively. Those outside the tunnel are T04, T05 and WE, respectively. The temperature of each
soil block is defined as shown in the figure. Inputs to the system are outside hemispherical solar
radiation (RAD), air temperature (TO), humidity ratio (WO), and dew-point temperature (TD)
which is a variable for emissivity of the atmosphere. Wind speed is assumed as constant but can
be input as a variable if the data is available. Heat transfer coefficients outside (HO) and at the
tunnel surface as well as ventilation rate (QH) can be derived.
Fig. 1. Two-dimensional model representation of a greenhouse-type solar
sterilization system with mulching.
Since the air gap between the mulch film and soil surface is small, it is assumed for
simplicity that the temperature TB is the arithmetic average of TC and TF, and the humidity ratio
WB is equal to WF. The inside soil surface layer was symmetrically divided into five blocks and the
outside into two blocks as shown in Fig. 1. Then, the temperature TF was introduced as an average
of T01, T02, and T03 for radiation exchange and for the calculation of WF. TE is an average of
2
A full model on a floppy disk is available from the first author on request.
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T04 and T05 for the outside conditions of radiation and WE (humidity ratio of the outside). Then,
29 differential equations are formed for the 31 unknown variables, including TT, TI, WI, TC, WF,
WE, T01, T02, ... ,T11, ... , T15, ... , T41, ... , T45 (see Fig. 1). Two additional equations to
define WF and WE are derived from the relationship between saturated humidity ratio and
temperature. Wetness factors to describe the degree of unsaturation of the soil surface are
introduced. Numerical integration techniques are available in CSMP, hence the differential equation
is converted into integration and can be written symbolically in the program code.
Inputs such as solar radiation and outside temperature are described as functions which are
based on actual measured values. Some thermal properties of the system, such as transmissivity of
the film, have been experimentally determined and are introduced as functions by the use of function
generators (AFGEN). Function generators in CSMP operate on pairs of values which constitute xy
tables.
Model Verification
Model verification has been conducted using experimental data obtained from a solarization
greenhouse built and instrumented for this purpose. The site and the greenhouses are described in
the paper by Kania and Roberts (1996). All input data which was available experimentally were
used, and some parameters which were not available experimentally were adjusted to match
temperature patterns of inside air and soil layers. Simulation duration was 48 hours. It is apparent
that outside conditions can be more simply determined if fewer parameters are used. An outside
output from the system was soil temperature at 3 cm depth (T15). This temperature was measured
a distance from the greenhouse, since surface temperature is a most difficult factor to measure
experimentally. Furthermore, this temperature was used as an indicator to evaluate the agreement
between the measured values and the simulated ones. In the present model, the thickness of the soil
surface layer is assumed to be 1 cm, and the temperature of this layer is assumed to represent the
surface temperature of the soil. Model variables representing important physical properties of the
experimental solarization site include the following: the atmospheric emissivity (based on dew-point
temperature); the absorptivity of the soil surface for solar radiation, and the heat transfer coefficient
due to convection at the soil surface. These parameters were adjusted to obtain good agreement
between the measured and simulated data.
Timc(hr)
Fig. 2. Measured and simulated temperatures of outside soil and inside air.
2
A full model on a floppy disk is available from the senior author on request.
A typical simulated result showing outside soil temperature at the 3 cm depth is shown as the
lower curve (measured values in open triangle symbols) in Fig. 2. It is clear that the simulated curve
is higher than the measured values at all times for the outside soil temperature. There are several
ways to decrease the simulated values in order to attain better agreement. These include using a
lower atmospheric emissivity, a greater heat transfer coefficient or a lower absorptivity value at the
soil surface. However, the former two affect inside conditions of the greenhouse and decrease
inside temperatures as well. For the inside, it is clear that the simulated air temperature is already
lower than measured values in the daytime, and so it is not practical to lower this simulated
temperature further. A very low absorptivity for the outside soil surface (which assumes a very
reflective soil surface) was already used in the existing simulation since experimental data was not
available. Therefore, the simulated data shown in Fig. 2 was considered to be 'best- fitted' with the
measured data.
Some parameters of the inside conditions were then adjusted in order to match the simulated
inside air temperature with the measured values, which are shown by the upper curve and dots,
respectively, in Fig. 2. Inside air temperature was chosen as an indicator of the agreement between
the simulated and measured values because the measured values of inside air temperature were a
spatial average and had little distribution variation. In the nighttime, measured and simulated
results agreed relatively well. In the daytime, however, the measured values were several degrees
centigrade higher than the simulated results.
The discrepancy between measured values of outside soil temperature and simulated ones
is considered to be due to the lack of measured data of soil thermal properties such as thermal
conductivity, volumetric heat capacity, absorptivity for solar radiation, and the initial spatial
distribution of soil temperatures. Inside conditions in the daytime can be adjusted by parameters such
as heat transfer coefficients inside and outside, and absorptivity of solar radiation. If better
agreement is needed, modifications to these parameters based on experimental data should be made.
There were some discrepancies between measured and simulated values in the present model, but
the model itself can be used for sensitivity analysis for design purposes since its behavior described
the system well. The discrepancies do not change the results of the sensitivity analysis when used
for design purposes.
Sensitivity Analysis and Discussion
To design the system it was necessary to evaluate the effects of thermal characteristics of the
films and soil properties on the system performance. If soil structures are homogeneous, the
temperature regime in the soil layer has a typical shape, i.e., amplitude decay and phase delay vary
according to the depth. In the present study, since physical characteristics of the system are
paramount, comparison of maximum and minimum temperatures of the surface soil layer between
inside and outside is a good indicator of system performance. The ratio of soil temperature of the
inside surface layer to soil temperature of the outside surface layer has been used to evaluate the
system in the present study.
Choice of films for tunnel glazing and inside soil mulching
Since lower emissivity of the plastic house glazing results in lower inside temperature, it is
very important to first examine the effect of film emissivity on soil temperature. The effect of film
emissivity on the ratio of maximum inside soil surface temperature to maximum outside soil surface
temperature is summarized in Fig. 3-1. Fig. 3-2 shows the ratio of minimum temperatures. It is
clear from Fig. 3-1 that the emissivity of mulch film has less effect than that for tunnel film, but both
emissivities have positive relations with the ratio of maximum temperatures. Suppose the outside
soil surface temperature is 40°C. Then the inside soil surface can reach 40°C x 1.8 = 72°C in this
case when emissivities are 0.9 for tunnel and 0.8 for mulch. Films with high emissivities are
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recommended for tunnel glazing, but this is not necessary for mulching film.
The trend is very similar to the ratio of minimum temperatures in Fig. 3-2, although in this
case, emissivity has less impact. Since emissivity is ckisekt related to long-wave radiation
exchange, it is surprising that the emissivity has little effect on the ratio of minimum temperatures.
This is especially remarkable at night, because emissivity is one of the dominating factors in the
nighttime temperature regime.
Fig. 3-1. Ratio of maximum temperatures of inside and outside soil surfaces.
Fig. 3-2. Ratio of minimum temperatures of inside and outside soil surfaces.
Table 1. Effect of absorptivity on temperature ratios(min. temp./max. temp, ratios).
Mulch
0.01
0.08
0.01
1.12/1.67
1.14/1.72
0.08
1.13/1.70
1.15/1.76
Absorptivity
Tunnel
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Table 1 shows the effect of the absorptivities of tunnel and mulch films. These absorptivity
coefficients are based on the amount of solar energy absorbed by the films. The values used in the
present study are the correct order of magnitude, namely between 1 and 8 %. Higher film
absorptivity values reduces the energy impacting the soil surface and is not effective in achieving
higher soil temperatures. For example, when absorptivity is increased from 1 % to 8%, the changes
in the ratio of temperatures are very small: the ratio of maximum temperatures rises slightly from
1.67 to 1.76 and the ratio of minimum temperatures increases only from 1.12 to 1.15. Therefore,
although higher absorptivity increases mulch and tunnel film temperatures for both day and night,
the overall effect is negligible.
Infiltration effects
When properly installed, soil mulching with plastic film can be very airtight. Row tunnels
placed above mulched soil are generally much less airtight. For the purposes of solarization, tunnels
should be as airtight as possible because infiltrating air is at much lower temperatures and this,
combined with the air movement from infiltration will have a negative effect on soil temperature.
In this model, air infiltration through the tunnel is taken into account, although the soil mulch is
assumed to be airtight. The air infiltration rate was first set at QH = 0.03 m3m"2min~' which is
equivalent to one volume air change per hour. During the sensitivity analysis QH was increased to
0.1, 1.0, and 3.0 (100 times higher than the first estimate). The temperature ratios do not change
at all for QH = 0.1, and increase only slightly at QH = 1.0. The maximum and minimum
temperature ratios are 1.83 and 1.17, respectively, foraQH of 0.1. These become 1.82 and 1.16
at a QH of 3.0. At present, the heat transfer coefficient at the inside surface is assumed to change
with QH because high infiltration rates increase the coefficient. The heat transfer coefficient at
the inside surface, HI, was set to 10 kJm~2hr"l0C"' for two early cases and was set to 15 in two later
cases. HI is the sole variable affecting convective heat transfer between the soil mulch and tunnel
films and the tunnel air. Internal circulating fans can be used to increase HI. Therefore, an airtight
tunnel is desirable but not mandatory. As the sensitivity analysis results show, infiltration can be
100 air changes per hour without significantly affecting the mulched soil temperature.
Soil properties
Soil water content was thought to have a significant effect on the soil temperature regime,
because of the large specific heat and high thermal conductivity of water. The affected soil
properties are its volumetric heat capacity and its thermal conductivity. Fig. 4 shows the change
of temperature ratios for maximum and minimum soil surface temperatures according to the change
of volumetric heat capacity of soil. As the soil becomes drier, its volumetric heat capacity decreases,
the ratio of maximum soil surface temperatures increases and the minimum temperature ratio
decreases slightly.
The effect of water content on the thermal conductivity of the soil (KS) is very similar.
Increasing soil water content increases its thermal conductivity. The thermal conductivity in the
model is called 'apparent thermal conductivity' because it is really a heat transfer coefficient which
takes into account heat flow due to water movement in the soil layers. When KS is doubled from
2.5 to 5.0 kJm"'hr"loC"1, the ratio of maximum soil surface temperatures decreases about 10%, and
the minimum temperature ratio shows a negligible 1% decrease.
The effect of soil absorptivity, which affects the amount of energy received directly from
solar radiation, is shown in Fig. 5. The ratio of maximum soil temperature is over 2.0 when its
absorptivity is 0.8, which means the inside maximum soil surface temperature is more that twice
that of the outside. The effect on the ratio of minimum soil surface temperatures is similar in
direction but is of greatly reduced magnitude. The change in soil water content affects not only heat
flow but also the gaseous composition of the soil. Dry soil inhibits the growth of nematodes and
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other soilborne pathogens (e.g., Katan, 1981). Further investigation into the merits of solarizing and
relatively dry soil should be made from a plant pathology viewpoint.
2.5
2.5
Max.
?
o
aj
1.5
i-i
O,
I
e
a;
H
0.5
2
Kj
l-l 1.5
ci
E
Min.
D
Min.
H 0.5
0
0
2000
4000
5000
Volumetric heat capacity(kJ/m 3 /C)
Fig. 4. Effect of soil water content on
temperature ratios.
0.8
0.6
0.4
Soil a b s o r p t i v i t y
Fig. 5. Effect of soil absorptivity on
temperature ratios.
Horizontal heat flow in soil layer
All thermal effects in the soil need to be considered including horizontal heat flow. For long
tunnels, horizontal heat flow along the long dimension can be neglected. Depending upon tunnel
width, horizontal heat flow along the short dimension may become as important as vertical heat
flow. Since soil temperatures in the solarized tunnel are much higher than those outside, horizontal
heat flow exists. The impact of this heat flow can be reduced in several ways. One way is to make
a wider structure (either a wider single-span type or multispan). For a solarized greenhouse, the
insulation board can be buried along its perimeter.
Depending upon the individual situation, these two methods can be sufficient to reduce the
heat flow problem to one dimension. The model predicted a smaller temperature gradient than was
observed experimentally. The reason for this is not yet clear. Possibly, it was due to some unknown
experimental error, but more probably, the differences arose because most of the soil thermal
properties were not precisely known, as discussed previously.
Degree-hour concept for thermal mortality
The effect of thermal treatment on soil-bome plant pathogens has been studied and the
degree of thermal mortality depends on both the temperatures achieved as well as exposure duration
(e.g., Katan, 1981; Pullman et al., 1981). A linear relationship was found when the logarithm of
exposure time required to kill 90% of soil-bome plant pathogens was plotted against temperature
(Pullman et al., 1981), but the dynamic temperature change in the soil layer was not discussed.
Soil temperature change is dynamic, that is, dependent on time. Tolerance of soil- borne
pathogens can be expressed by the time duration of exposure above the minimum temperature
required. This concept can be summarized with a degree-hour expression, where is temperatures
above a particular threshold are integrated over time, hence 'degree-hours', which is easily calculated
by the model for a particular condition.
Greenhouse thermal efficiency
A greenhouse itself is a vessel for plant production, and by its thermal merits (even when
unheated) it can extend the season of plant production. In addition, its thermal efficiency is further
enhanced when the soil within is mulched with plastic (nonpermeable) film. A modified computer
model which does not have a greenhouse structure over the soil mulching, has been tested in order
to help analyze the thermal efficiency of greenhouses. In the present model, it was difficult to find
125
a significant difference between the overall thermal efficiency of the greenhouse system and just
a soil mulch. Maximum soil temperature is lower in the greenhouse system because of the reduced
energy input of solar radiation transmitted through the greenhouse glazing. However, the
greenhouse becomes a warm air reservoir which prevents lower soil temperatures at night. The
model was configured for a 48-hour simulation to analyze the effect of the thermal properties of
the materials used. The thermal advantage of the greenhouse over the mulched soil is that it is a slow
but effective means of long-term energy accumulation in the soil solarization system.
Since the thermal efficiency of the greenhouse system depends on the properties of the
tunnel covering materials, it is clear that the thermal properties of the tunnel cover should be
optimized. Theoretically the best system is a 'greenhouse' which is a movable screen system that
remains open in the daytime (to accumulate energy efficiently) and closes at night to attenuate
energy loss. Further investigation is needed to confirm this hypothesis.
Conclusion
A two-dimensional simulation model of a greenhouse-type soil sterilization system, which
consists of a small tunnel with mulching has been developed. The model has been verified by
experimental data, although some discrepancies exist in soil temperatures because not all soil
parameters were measured. The model was then used to analyze the effects of thermal material
properties on temperature at various soil depths.
It is recommended that tunnel and mulch films with higher emissivities be used, which result
in higher soil temperatures. P VC or IR-resistant PE film is better than ordinary PE film but glazings
with much higher emissivities need to be developed. Condensation droplets on the inside film
surface increases emissivity. Increases in the absorptivity coefficient of both tunnel and mulch films
up to 8% result in rises in soil temperatures on the order of 1%. Absorptivity higher than 8% does
not give better results. The properties of the tunnel cover have a greater impact on soil temperature
than those of the soil mulch. Therefore, more attention should be paid to the design of tunnel films.
Absorptivity of the soil has a positive effect on maximum and a negative effect on minimum soil
temperatures. Drier soil gives higher maximum temperatures and lower minimum temperatures.
Airtight coverings are recommended, even though infiltration does not have a significant
effect on soil temperature (within certain limits). In general, the still air provided by airtight
coverings acts as good insulation for the mulching. Degree-hours above a certain temperature at a
certain soil depth, which kills soil-bome pathogens, can be easily calculated by the model.
1.
Literature Cited
Kania, S. and W. J. Roberts. 1996. Solarization study of soil in plastic greenhouses.
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Katan, J. 1981. Solar heating (solarization) of soil for control of soilborne pests. Ann. Rev.
Phytopathoi, 19: 211-236.
Mahrer, Y. and J. Katan. 1981. Spatial soil temperature regime under transparent
polyethylene mulch: Numerical and experimental studies. Soil Sci., 131: 82-87.
Pullman, G.S., J. E. DeVay, and R. H. Garber. 1981. Soil solarization and thermal death:
A logarithmic relationship between time and temperature for four soilborne plant pathogens.
Phytopathology, 71: 959-964.
Stapleton, J.J. 1996. Fumigation and solarization practice in plasticulture systems. Hort
Technology, 6: 189-193.
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Engineering. Kluwer Academic Publishers, 155 pp.
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Dynamic Simulation of Plant Bio-