The SOLGEM model is a numerical engineering model which solves the flow and energy balance equations for the air flowing through a growing environment, assuming quasi-steady state conditions within the system. SOLGEM provides a dynamic simulation of the controlled environment system in that the temperature and flow conditions of the growing environment are estimated on an hourly basis in response to the weather data and the plant growth parameters. The flow energy balance considers the incident solar flux; incoming air temperature, humidity, and flow rate; heat exchange with the roof and floor; and heat and moisture exchange with the plants. A plant transpiration subroutine was developed based plant growth research facility, intended for the study of bioregenerative life support theories. The results of a performance analysis of the plant growth module are given. The estimated energy requirements of the module components and the total energy are given

Jakob, F. E.

Landstrom, D. K.

Stickford, G. H., Jr.

English

NASA Technical Reports Server

AN ENGINEERING ANALYSIS OF A CLOSED CYCLE 
PLANT GROWTH MODULE 
George H. Stickford, Jr. 
Frank E. Jakob 
D. Karl Landstrom 
BATTELLE 
Columbus Laboratories 
505 King Avenue 
Columbus, Ohio 43201 
ABSTRACT 
The SOLGEM model has been applied to calculate the per- 
formance of a plant growth module similar to the module under 
study at NASA Ames. The plant growth module is a ground- 
159 
An engineering computer model has been developed at 
Battelle to simulate the performance of a controlled environ- 
ment agriculture system. This computer model, called SOLGEM, 
was developed to provide a dynamic performance simulation 
of an enclosed growing system in which the ventilating and 
air conditioning power required to maintain ideal growing 
conditions was provided entirely by solar energy. The model 
is made up of 1 1  primary subroutines which simulate the per- 
formance of the major components of the system, including 
a plant growth and evapotranspiration model, ventilation 
and flow model, evaporative cooling and solar-driven absorp- 
tion cooling, and photovoltaic power generation. 
https://ntrs.nasa.gov/search.jsp?R=19860010448 2020-03-20T15:11:48+00:00Z
based plant growth research facility, intended for the study 
of bioregenerative life support theories. 
This report presents the results of a performance analysis 
o'f the plant growth module. The estimated energy requirements 
of the module components and the total energy are given. 
The water- balance and estimated plant evaporation are presented. 
An analysis of the effect of design alternatives of component 
sizing and energy use is discussed. 
INTRODUCTION 
.- The SOLGEM model is a numerical engineering model which 
solves the flow and energy balance equations for the air 
flowing through a growing environment, assuming quasi-steady 
state conditions within the system. SOLGEM provides a dynamic 
simulation of the controlled environment system in that the 
temperature and flow conditions of the growing environment 
are estimated on an hourly basis in response to the weather 
data and the plant growth parameters. The flow energy balance 
Considers the incident solar flux; incoming air temperature, 
humidity, and flow rate; heat exchange with the roof and 
floor; and heat and moisture-exchange with the plants. 
A plant transpiration subroutine has been developed 
to simulate the heat and moisture transfer between the plants 
and the air. This routine provides a realistic model of 
transpiration and leaf temperature in terms of incident 
solar flux, air temperature, air-flow velocity,.roof and 
floor temperature, andl humidity. Important plant character- 
istics such as stomata1 opening, convective and radiative 
heat exchange, and canopy development are all included in , 
the model. . . 
The purpose of the SOLGEM dynamic simulation model 
is to provide an engineering estimate of the performance 
of a controlled-environment growing system. The required 
input to the model is a complete physical description of 
the system, i.e., size of all components, performance character- 
istics of the energy subsystems, and the weather data. 
For a given system design and a specific location, the hourly 
air temperature, humidity, and solar intensity experienced 
by the plants are determined.. Thep results indicate whether 
or not the growing enviromnent maintained by the system 
was satisfactory for optimum plant growth. 
The simulation model also determines the quantities of mass 
and energy flowing between components. The sizing of compo,nents 
can be accomplished by monitoring the component output and . 
noting excesses or deficiencies. In most cases, the perfor- 
mance of the overall system will be affected by varying 
the size until the desired system performance is achieved. 
The model can also be used to conduct parameter sensi- 
tivity studies. By varying the value of a specific input 
and repeating the simulation run with all other parameters 
fixed, it is possible to determine the sensitivity of the 
system performance to changes in this parameter. Sensitivity 
studies can be used to determine the most important design 
variables, and to indicate the benefits possible by making 
changes in system design. 
162 
APPROACH TO ANALYSIS OF PLANT GROWTH MODULE 
Plant Growth Module 
The SOLGEM model has been used to conduct an engineering 
analysis of a simplified version of a plant growth module. 
A sketch of the module is presented in Figure 1. The module 
simulated consisted of a growing space 1m wide, 1m tall, 
and 5m long. A bank of grow lights provided radiation. 
The air was circulated by a single fan which passed air 
over a cooling coil before cycling it back to the growing 
space. The cooling coil removed the thermal energy added 
by the grow lights, and condensed and removed the moisture 
transpired by the plants. 
Modeling Plant Heat Transfer 
In an enclosed growing environment the plants are a 
major factor in determining the energy balance of the system. 
The plants absorb the incident radiant energy and convert 
it to thermal energy, and to latent heat in the form of 
water vapor. ( A  small percentage of the energy is converted 
to chemical energy by the plants. This energy is negligibly 
small and consequently has not been included in the analysis.) 
FIGURE 1. SKETCH OF PLANT GROWTH MODULE 
The transpiration and leaf temperature model of Gates 1,2 
was used to calculate the energy balance of the plants. 
The Gates model considers three energy transfer mechanisms: 
o Convection 
o Radiation 
o Transpiration 
The convection of energy between the leaf and the air 
stream is governed by the temperature difference between 
the leaf and the air, and by the air flow velocity. The 
convection coefficients depend on the leaf geometry and 
are given by Gates. 
The radiative exchange between the leaf and its surround- 
ing is determined by considering five radiation sources as 
shown in Figure 2. The primary radiant energy input is 
QSOL, which in this case is the radiation supplied by the 
grow lights. Radiative exchange with the roof and the floor 
is included, as is the reflected radiation from the floor. 
Radiation from surrounding leaves is also considered in 
the energy balance. 
Transpiration by the leaves is actually a mass transfer 
mechanism and is a significant term in the plant energy 
balance. The amount of energy that is absorbed by trans- 
piration can range from 1/3  to 1/2 of the incoming radiation. 
The Gates transpiration model depends on leaf temperature, 
sun w nding 
Ler- \ (E LIISIGHAI(TL)4/ Volume 
(EF) (SIGMA) ( T F ) ~  Reflected Solar Energy 
FIGURE 2. R A D I A T I V E  HEAT TRANSFER TERMS 
INCLUDED I N  THE LEAF ENERGY BALANCE 
166 
air temperature and humidity, air flow velocity, and a leaf 
stomatal resistance term. 
The stomatal resistance is an attempt to simulate the 
opening and closing of the plant stomate, shown in Figure 3. 
The stornatal resistance was selected so the predicted water 
transpiration rate of a mature plant canopy matched the 
known performance of a greenhouse. The nighttime stornatal 
resistance of 500 sec/cm was recommended which virtually 
eliminated transpiration at night. The daytime value of 
5 sec/cm was found to result in a transpiration rate sufficient 
to absorb 1/2 of the incoming solar radiation by a mature 
plant canopy during full sun conditions. 
Description of Component Models 
Cooling Coil. The cooling coil consists of a liquid- 
to-air heat exchanger using chilled water as the liquid 
cooling medium. The amount of air passing through the heat 
exchanger was varied by bypassing a portion of the recircula- 
tion air around the heat exchanger. The two air streams 
were then mixed together before reentering the growing zone. 
This simple system is capable of providing dehumidifica- 
tion and removal of thermal energy in one process. By vary- 
ing the chilled water temperature and the fraction of the 
recirculation air that is bypassed, it is possible to achieve 
MESOBHYLL 
TISSUE 
ATMOSPHERIC- 
BOUNDARY v 
LAVER 
/ H 2 0  
FIGURE 3. LEAF CROSS SECTION 
168 
desirable levels of temperature and humidity in the growing 
zone for all realistic conditions of radiation input and 
plant transpiration. A detailed discussion of this process 
follows in a later section. 
Grow Lights. The grow lights provide radiant energy 
input to the plant canopy. In addition to triggering the 
growth process, which is not included in the SOLGEM model, 
the radiant energy affects the leaf temperature and the 
amount of water vapor transpired by the plants. 
For the purposes of this simulation, th,e 'gcow lights 
8 i l t l ,  
were assumed to be 80% efficient, meaning that 80% of the 
energy input to the grow lights was emitted as radiation, 
". . > 
and 20% was given off as thermal energy. Th6 radiant energy 
was transmitted to the- plants, then to the air b; convection 
and transpiration.   he thermal energy was transmitted directly 
to the air by convection from the lights. 
Circulation Fan. The electric energy supplied to a 
fan motor is transfered to the air stream in three distinct 
forms: kinetic, potential, and thermal. Kinetic energy 
is imparted to the air by the fan blades creating a moving 
air stream. Potential energy is imparted to the air by 
the fan blades in the form of the pressure rise across the 
fan. Thermal energy is imparted to the air stream in several 
ways. Viscous frictional heating of the air stream occurs 
because of the interactions between the air and the fan 
surfaces. Conduction to the air stream of the thermal energy 
generated by mechanical friction and electrical resistance 
also occurs. 
For a particular air flow rate, the kinetic energy 
is proportional to the velocity squared. The air velocity 
through the fan can be reduced significantly by using larger 
diameter fans. The required flow rate and pressure rise 
can be achieved by selecting the appropriate fan speed. 
. , 
Thus by using large-diameter, variable-speed fans the kinetic 
energy can be minimized. 
Potential energy considerations lead to the need to 
minimize the product of flow rate and pressure rise. The 
flow rate requirements are fixed by the thermal energy input 
. 
of the grow lights. However, the pressure rise is a function 
of the system design. Thus a low pressure rise system is 
desirable. 
The temperature rise of the air through the fan system 
I 
results from frictional effects and inefficient hardware. 
Thus fans and motors having the highest efficiency should 
be selected for the plant growth module. 
Given a particular plant growth module design(i.e., 
the pressure drop is a function of the air flow rate), the 
fan energy must be minimized by minimizing the air flow 
requirements, and by choosing the appropriate number and 
size of fans. 
Temperature and Humidity Control Strategy 
The air passing through the growing zone absorbs the 
heat generated by the grow lights and increases in tempera- 
ture. The air also absorbs the moisture transpired by the 
plants. All the thermal energy and moisture absorbed by 
the air must be removed as the air passes through the recir- 
culation ducts. 
Air Flow Velocity. The air flow velocity can be increased 
by increasing the recirculation fan speed. As the air flow 
velocity increases the temperature rise across the growing 
zone decreases. Thus the air flow velocity can be used 
to control the allowable temperature variation across the 
growing zone. 
Chiller Control. Two chiller control variables are 
required in order to condition the recirculation air. By 
varying the chilled water temperature and the fraction of 
air bypassing the chiller, the temperature and humidity 
171 
o f  t h e  r e c i r c u l a t i o n  a i r  c a n  be  c o n ' t r o l l e d .  L o w e r i n g  t h e  
c h i l l e d  water t e m p e r a t u r e  a n d  i n c r e a s i n g  t h e  b y p a s s  i n c r e a s e s  - 
t h e  amount  o f  water removed r e l a t i v e  t o  t h e  amount  o f  s e n s i b l e  
h e a t .  R a i s i n g  't'he c h i l l e d  water t e i n p e r a t u r e  w h i l e  r e d u c i n g  
t h e  b y p a s s  w i l l  r e s ' u l t  i n  more  s e n s X b l e  h e a t  r e d u c t i o n  a n d  
less water removed .  ' 
." 
RESULTS OF ANALYSIS 
An analysis of the plant growth module was conducted 
2 for an illumination level of 1000pmol/sec/m . The tempera- 
ture of the air exiting the growth zone was fixed between 
25 C and 26 C. The relative humidity was held between 60 
and 70 percent. 
The air temperature rise through the 5 m plant growth module 
is shown in Figure 4. For the mature plants, the temperature 
3 rise is less than 1 C at an air flow rate of 60 m /min. 
This is equivalent to 1 m/sec flow velocity. 
For seedlings the amount of water transpired is signif- 
icantly less. Thus the temperature rise at a given air 
flow rate is greater than for mature plants. 
Figure 5 presents the average leaf temperature as a function 
of air flow rate. The average leaf temperature appears 
to be slightly less sensitive to air flow rate than the 
air temperature. Thus increasing the air flow velocity 
is not as beneficial to reducing plant temperatures as would 
be indicated by calculating air temperature only. 
The effect of cooling water temperature on the humidity 
5 
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MATURE 
PLANTS 
I I I I I I J 
0 10 20 30 40 50 60 70 
A I R  FLOW RATE, M ~ / M I N  
F I G U R E  4 .  A I R  TEMPERATURE R I S E  THROUGH P L A N T  GROWTH MODULE . 
F I G U R E  5. AVERAGE L E A F  TEMPERATURE 
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A I R  FLOW RATE, M ~ / M I N  
I I I 1 I I 
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I I I I 1 I I b 
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u 
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COOLING WATER TEMPERATURE, C 
FIGURE 6. HUMID ITY  LEVEL VERSUS COOLING WATER TEMPERATURE AND LATENT 
HEAT R A T I O  
level in the growing zone is presented in Figures 6 and 7. 
For practical levels of the latent heat ratio(the ratio 
of energy absorbed by transpiration to the total energy 
absorbed by the air stream), it is possible to achieve humidities 
down to 65 percent with a cooling coil. At lower transpiration 
rates characteristic of young plants, the humidity could 
be controlled with water1 near room temperature. 
Figure 7 shows the effect of air temperature on cooling 
water temperature. For higher air~temperatures in the grow- 
ing zone, the allowable cooling water temperature is higher. 
Higher cooling water temperatures result in lower energy 
requirements for humidity control. 
Energy Requirements. The energy requirements for the 
plant growth module are given in Table 1. Shown are the 
> 
estimates for a 5 m and a 30 m module. Lighting is the 
major energy user, although the energy required for cooling 
is significant. The fan energy is relatively small for 
the 5 m module, but becomes sizable when scaled up to 30 m. 
The 30 m module is assumed to have the same cross section 
area in the air return ducts as the 5 m module and the same 
air flow velocity. The fan energy could be reduced by 2 or 3 
by enlarging the air return ducts and cooling coil. 
Water Usage. The total water transpired by the plant 
I I I I 
- LATENT - 1 - 
- - 
TOTAL 3 
L - 
- - 
- 
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- - 
* I I I I 
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M 80 
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5 
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CL 50 
40 
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0 10 20 I 3 0  
COOL1 NG WATER TEMPERATURE, C 
F I G U R E  7. H U M I D I T Y  L E V E L  VERSUS COOLING WATER TEMPERATURE AND A I R  TEMPERATURE 
~ 
-- 
I 
Table 1. Plant Growth Module Energy Requirements. 
Total Lighting Cooling Fan 
Power, kw % % % 
5 meter 2.5 69 27 4 
30 meter 16.0 65 25  10 
growth module filled with mature plants is estimated to 
be approximately 850 gm/hr. This level of transpiration, 
coupled with the light energy input, results in a latent 
heat ratio of 1 / 3 .  
C O N C L U S I O N S  
The temperature and humidity of a closed-cycle plant 
growth module can be controlled with a circulating fan and 
a chilled water cooling coil. The fan speed controls the 
temperature rise of the air passing through the growing 
zone. The amount of thermal energy removed from the recir- 
culating air can be controlled by the fraction of air passing 
through the cooling coil, the remainder being bypassed. 
The temperature of the circulating water controls the amount 
of moisture removed, thus controlling the humidity level 
in the growth zone. 
Minimizing the energy requirements of the plant growth 
module will require minimizing the air flow rate, and reducing 
the pressure drop through the recirculation ducts. It will 
also require operating the cooling coil in the most energy 
efficient mode. If the chilled water is generated by a 
heat pump, the higher the chilled water temperature, the 
more efficiently the heat pump will operate. Thus it will 
be most efficient to operate the plant growth module at 
the highest acceptable temperature and humidity. 
REFERENCES 
1. Gates, D. M., "Transpiration and Leaf Temperaturev, 
Annual Review of Plant Physiology, 1968 
2. Gates, D. M., and Dapian, L. E., Atlas of Energy Budgets 
of Plant Leaves, Academic Press, London-New York, 1971 


An engineering analysis of a closed cycle plant growth module

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