Controlled Environment Agriculture Technologies

Controlled environment agriculture (CEA) is technology-based food production system where growers are able to manipulate the inside environment according to the growing requirements of the specific plants. A greenhouse is a closed space where fruits, vegetables, herbs and other plants can be grown. Growing seasons can be expanded to the whole year if the inside conditions are controlled properly according to the requirements of the plants. However, often the land and building space available is small, and currently there are no low-cost solutions available to grow food efficiently in small urban spaces. CEA technologies are used for the purpose of controlling the temperature, humidity, airflow and light in the building. The best possible orientation and structures of a greenhouse, heating, cooling, ventilation, lighting and glazing as well as insulation materials are discussed in this report. Technologies and methods are compared as needed. Results show that the most efficient and sustainable technologies are currently more expensive initially than others. Due to this fact, most of the time small urban farmers are not able to afford sustainable and energy-efficient technologies. Further development, testing, and analysis need to be done in order to make the CEA technologies affordable in low-income communities.

Controlled environment agriculture (CEA) is technology-based food production system where growers are able to manipulate the inside environment according to the growing requirements of the specific plants. A greenhouse is a closed space where fruits, vegetables, herbs and other plants can be grown. Growing seasons can be expanded to the whole year if the inside conditions are controlled properly according to the requirements of the plants. However, often the land and building space available is small, and currently there are no low-cost solutions available to grow food efficiently in small urban spaces. CEA technologies are used for the purpose of controlling the temperature, humidity, airflow and light in the building. The best possible orientation and structures of a greenhouse, heating, cooling, ventilation, lighting and glazing as well as insulation materials are discussed in this report. Technologies and methods are compared as needed. Results show that the most efficient and sustainable technologies are currently more expensive initially than others. Due to this fact, most of the time small urban farmers are not able to afford sustainable and energy-efficient technologies. Further development, testing, and analysis need to be done in order to make the CEA technologies affordable in low-income communities.


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Adrienn Santa<br />

Engaged Learning Fellowship<br />

Mentors: Dr. Ali Beskok, Dr. Eva Csaky<br />

April 1, 2018<br />

I would like to thank SMU, Engaged Learning and the Hunt Institute for supporting this<br />

research. I would also like to show my gratitude to Dr. Ali Beskok, Dr. Eva Csaky, and<br />

Corrie Harris for sharing their thoughts and wisdom to help improve my findings.

Table of Contents<br />

Abstract ............................................................................................................................................................. 4<br />

1. Introduction ............................................................................................................................................... 1<br />

2. <strong>Technologies</strong> used in controlled environment agriculture ........................................................................ 3<br />

a. Orientation and structure ....................................................................................................................... 4<br />

b. Heating .................................................................................................................................................. 5<br />

c. Cooling ................................................................................................................................................ 15<br />

d. Ventilation ........................................................................................................................................... 18<br />

e. Glazing and insulation ........................................................................................................................ 21<br />

f. Lighting ............................................................................................................................................... 24<br />

3. Evie, design of low-cost cooling system ................................................................................................. 26<br />

a. Importance of Absorption Refrigeration Cycle ....................................................................................... 27<br />

b. Experiment .............................................................................................................................................. 29<br />

c. Results ..................................................................................................................................................... 33<br />

d. Implications of this project ..................................................................................................................... 35<br />

4. Conclusion .............................................................................................................................................. 37<br />

5. References ............................................................................................................................................... 38<br />

Signature and Approval of Mentor: .................................................................................................................... 43

Abstract<br />

<strong>Controlled</strong> environment agriculture (CEA) is technology-based food production system where<br />

growers are able to manipulate the inside environment according to the growing requirements of<br />

the specific plants. A greenhouse is a closed space where fruits, vegetables, herbs and other plants<br />

can be grown. Growing seasons can be expanded to the whole year if the inside conditions are<br />

controlled properly according to the requirements of the plants. However, often the land and<br />

building space available is small, and currently there are no low-cost solutions available to grow<br />

food efficiently in small urban spaces. CEA technologies are used for the purpose of controlling<br />

the temperature, humidity, airflow and light in the building. The best possible orientation and<br />

structures of a greenhouse, heating, cooling, ventilation, lighting and glazing as well as insulation<br />

materials are discussed in this report. <strong>Technologies</strong> and methods are compared as needed. Results<br />

show that the most efficient and sustainable technologies are currently more expensive initially<br />

than others. Due to this fact, most of the time small urban farmers are not able to afford sustainable<br />

and energy-efficient technologies. Further development, testing, and analysis needs to be done in<br />

order to make the CEA technologies affordable in low-income communities.

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1. Introduction<br />

Food deserts are one of the biggest issues in low-income communities, not only in the Dallas<br />

area, but also in other parts of the world. According to the US Department of <strong>Agriculture</strong>, the term<br />

‘food deserts’ is defined as “parts of the country vapid of fresh fruit, vegetables, and other healthful<br />

whole foods, usually found in impoverished areas” [1]. In most cases there are no available quality<br />

grocery stores, farmers’ markets, or affordable fresh food nearby the affected area, which could<br />

result in serious health issues and other social consequences. According to the Economic Research<br />

Service of the USDA, an “estimated 2.1 million households, or 1.8 percent of all households (in<br />

the US) are in low-income and low access census tracts and are far from a supermarket and do not<br />

have a vehicle” [2]. Economic conditions often force grocery stores to relocate, which can result in<br />

having very few local stores. Some urban communities have access to public transportation, which<br />

can help residents to overcome the difficulties of accessibility of the nearest grocery store; however<br />

suburban and rural areas usually have very limited or no access to buses or trains. Climate change<br />

strongly affects the agriculture sector, as well. Farmers are unable to maximize their crop<br />

production with the unpredictable weather conditions. Thus, climate change forces farmers to<br />

switch to CEA in order to keep their crops healthy throughout the growing season.<br />

Figure 1: Store near San Antonio, Texas [3]<br />

The areas most affected by food deserts in the United States are around New Orleans (LA),<br />

Chicago (IL), Atlanta (GA), Memphis (TN) and Camden (NJ) [4]. Figure 2 shows the areas in the<br />

United States where a grocery store or a supermarket is further than a mile. It is evident that the<br />


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areas around New Orleans and Atlanta are critically affected by the issue of food deserts.<br />

Figure 2: Areas in the United States where the there are no available stores in a mile [1]<br />

Several movements and initiatives exist to reduce the number of food deserts and increase the<br />

availability of fresh food in impoverished communities. For example, with Amazon’s acquisition<br />

of Whole Foods, a major online grocery outlet was created, capable of delivering to all<br />

communities. However, this form of grocery shopping is currently very expensive, and in many<br />

cases, low-income populations cannot afford this type of service [5]. Digital distribution is another<br />

problem; many people affected by food deserts lack access to the Internet to take advantage of the<br />

service.<br />

Another potential solution to combat the issue of food deserts is to bring food production closer<br />

to the communities in need. This solution would create new jobs and opportunities for communities<br />

as well as make use of vacant lands and buildings. Often, the land and building space available is<br />

small, and currently there are no low-cost solutions available to grow food efficiently in small urban<br />

spaces. Existing solutions are not economically viable when used in small spaces because of their<br />

high costs of capital and operating, especially relating to energy. This report is a summary of the<br />

existing technologies such as heating cooling, ventilation, lighting systems, insulation, and<br />

materials used in controlled environment agriculture. I will first provide an overview of the most<br />

commonly used CEA technologies, then I will discuss a detailed design of a promising model for<br />


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low-cost cooling solution.<br />

2. <strong>Technologies</strong> used in controlled environment agriculture<br />

Greenhouses are promising solutions to provide better quality food for urban communities and<br />

protect plants against natural environmental effects on a year-round basis. The technologies used<br />

in greenhouses determine the installation and maintenance costs of the system. Various plants and<br />

seedlings have different optimal growing conditions (see Figure 3). They require different amounts<br />

of direct sunlight, temperature, humidity, and water.<br />

Figure 3: Germination and planting temperatures for some vegetables and fruits [6]<br />

The location of the greenhouse can make it difficult and costly to provide the required<br />

temperature for the plant. In Texas, greenhouses need to be cooled in the summer, while in very<br />

cold areas, heating has to be provided in order to keep the plants growing. This research is focusing<br />

on identifying and discussing the available climate control technologies used in controlled<br />

environment agriculture. First, the orientation and different structures are discussed followed by<br />

the heating technologies. Also, the usage of the various cooling, ventilation systems are analyzed<br />

as well as the most efficient ways to provide lighting for the plants, and the type of insulations<br />

used in controlled environment agriculture. The various aspects and technologies interact with<br />

each other, and the CEA system to create the optimal climate conditions.<br />


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a. Orientation and structure<br />

Optimization of the orientation, structure, shape and location of a greenhouse have significant<br />

effect on energy consumption. The best possible orientation is where the greenhouse has the<br />

maximum interception to sunlight, which provide adequate nutrients for crops. To find the optimum<br />

orientation of a greenhouse, the climate conditions, latitude, hemisphere of the Earth, and time of<br />

the year for maximum production need to be considered. The most common type of orientations<br />

is east-west and south-north. Snezana Dragicevic from University of Kragujevac made a<br />

comparison about the two different types of orientation for each month of the year at different<br />

latitudes in the northern hemisphere and computed the total solar radiation input though each wall<br />

[7]. According to her findings, the uneven-span shape receives the most radiation of all the<br />

compared shapes. This type of greenhouse is suggested to have an east-west orientation greenhouse<br />

at latitudes of 44°N and 54°N “as it receives less solar radiation in summer with small differences<br />

in receive solar radiation in winter month” and “with increase of the latitude angle, the difference<br />

in the radiation received during winter month increase as compared to lower altitudes” [7]. Another<br />

researcher, Wael M. El-Maghlany recommends orienting the greenhouse with respect to south<br />

direction in latitudes between 24° and 31.2°; however, in latitudes below 24° it is better to align<br />

the greenhouse towards east and west. By choosing the most optimal orientation of the enclosed<br />

place, total heat gain can be maximized, and radiation heat can be used as supplementary heating<br />

which could reduce the running cost of the heating system [8].<br />

The structure of the greenhouse plays an important role in efficiency and productivity. The<br />

growing structure provides shelter for the crops from unforeseeable weather conditions. Selecting<br />

the type of the greenhouse depends on the crops produced inside, use of space, and extension plans<br />

[9]. There are three types of greenhouses: lean-to, detached, and gutter connected (ridge or furrow).<br />

The most favored type of structure is the lean to, but very few of them are used in commercial food<br />

production because of its limitations. The detached greenhouse stands by itself, and the most<br />

common type of detached greenhouse is the Quonset. Solid walls support the end of the structure<br />

while arched rafters are found throughout the body. This is not the most efficient type of<br />

greenhouse, as near the walls the growing area is restricted for smaller crops [10]. Ridge or furrow<br />

greenhouses are “two or more greenhouse structures joined together at a common intermediate<br />

gutter” which increases their efficiency [9]. There are several different shapes used as gutterconnected<br />

greenhouse, such as arch or gothic, A-frame or even-span, sawtooth, Venlo and open<br />


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roof.<br />

Figure 4: Greenhouse structures: A-Gutter connected, B-Quonset, C-Single gable [10]<br />

b. Heating<br />

According to Jim Rearden an estimated $250 million is spent annually on commercial<br />

greenhouse heating in the United States [9]. In most parts of the world, it is necessary to provide<br />

some form of heating to the plants in either winter or summer (depending on the location of the<br />

greenhouse on Earth) because of the heat loss through the walls and the ground. Taking Texas<br />

weather into consideration, the average low temperature in January 2018 was -1.3°C, and the<br />

lowest temperature reached was -15°C [11]. The light winter and a good greenhouse structure can<br />

help eliminate the usage of heating. However, this is not always the case. Before choosing the<br />

proper heating design, variables such as heat source, availability, and cost need to be considered [9].<br />

Heat sources can be: natural gas, liquified petroleum gas, fuel oil, wood and electricity. The<br />

dynamic environment of the greenhouse needs to be taken into consideration as well in order to<br />

choose the most appropriate form of heating. Greenhouses tend to lose heat in a very short time,<br />

but they also gain heat very quickly because of their solar collector properties [9]. Moreover, being<br />

able to control where the heat is going is very important, as not all the zones are necessary to be<br />

heated inside the greenhouse. For example, the temperature requirement can change according to<br />

the stage of growing. Consideration of all of these factors can create an energy efficient and cost-<br />


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effective way for raising the temperature. Furthermore, 30% less energy is required for heating if<br />

insulation and other coverage are utilized to protect the greenhouse from the cold days.<br />

There are many options to maintain the required temperature inside the building during the<br />

cold days, but the two major types are hot air or hot water. Many times, the combination of different<br />

heating systems result in the most efficient solution.<br />

Unit heaters are one way to create supplemental heating in a greenhouse. “In the United States,<br />

unit heaters enjoy a market dominance of approximately 60% of all the heating equipment dollars<br />

spent on greenhouses” [9]. On the other hand, in the other parts of the world, the water heating<br />

systems are more popular. There are several unit heater types available for use, but all of them<br />

have basically the same function. They generate hot air then distribute it in the available space with<br />

a help of an internal fan. Unit heaters mainly utilize propane, natural gas, or oil, but electric units,<br />

hot water, and steam styles are also available on the market [9]. There are two major types of unit<br />

heaters used for greenhouse heating: vented and unvented. “The traditional vented, gas-fired unit<br />

heater transfers heat from the combustion gases to the air through a heat exchanger and exhausts<br />

the combustion gases outside the greenhouse through a flue pipe” [12]. Yet, in an unvented unit<br />

heater, all combustion gases are exhausted straight in the greenhouse after burning the gas, which<br />

means that all of the heat is utilized for raising the temperature of the inside air. The overall<br />

efficiency of most unit heaters is rated between 50-90%.<br />


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Table 1: Advantages and Disadvantages of unit heaters [9] [12]<br />

Advantages<br />

Disadvantages<br />

Low initial/capital cost<br />

Lack of focused heating<br />

Low installation cost<br />

Longevity can be poor<br />

High reliability<br />

Undesired shadows by cast<br />

Low cost for lots of BTU<br />

Can be noisy<br />

Quick response<br />

Limit the ability to hang plants near heaters<br />

Ease of zoning houses<br />

Water condensation<br />

Ease of staging multiple heaters<br />

Potential fungal diseases<br />

Can be used for CO 2 enrichment Byproducts of combustion (ethylene, sulfur<br />

dioxide, nitrous oxide, carbon monoxide) can<br />

be harmful for plants and humans<br />

- Plants are unlikely to use all generated CO 2<br />

Figure 5: Unit heater in a greenhouse [13]<br />

Unit heaters are typically mounted near one of the end walls of the greenhouse. This position<br />

enables the system to deliver heat towards the middle section of the building. Most of them are<br />


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placed high, especially the gas-fired units, to avoiding damaging the crops. The cost of the unit<br />

heaters depends on the size and type that will be installed. The average cost of a design with<br />

minimal complexity in the northern region in the United States ranges from $0.40 to 0.80/ft 2 for a<br />

1-acre application [9]. Table 2 represents an example of the manufacturer suggested retail price<br />

for the specified vented unit heaters as well as their thermal and seasonal efficiencies.<br />

Table 2: Heater efficiencies and costs of vented unit heaters with propeller fans and<br />

stainless-steel heat exchangers and burners [12]<br />

Another type of heating system which is usually installed near the peak of the greenhouse is<br />

infrared heating systems. They are classified into two categories: high-intensity infrared heaters<br />

and low-intensity infrared heaters. The system runs near the roof through the length of the whole<br />

greenhouse and reflects down the heat energy to the plants below it, the infrared rays passing<br />

through the air. A metal heating tube that runs throughout the whole heater system is heated with<br />

combusted fuel (natural gas or propane). Infrared pipes can be used for the same purpose. The<br />

system efficiency can be increased by installing fins of the infrared pipes. “Low-intensity infrared<br />

heaters are safer to use in a greenhouse than high-intensity infrared heaters, because the pipes are<br />

heated up to 1100°F (594°C); the pipes of a high intensity infrared heater can be heated as high as<br />

1800°F (982°C)” [14].<br />


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The heat radiated from the pipes can be adjusted by changing the height of the heating system.<br />

The average cost of an infrared system with minimal complexity in the northern region in the<br />

United States ranges from $1.00 to 1.60/ft 2 for a 1-acre application [9]. This type of heating is<br />

cleaner, more environmentally friendly, and more effective at heating than the traditional heating<br />

systems.<br />

Figure 6: Infrared heating system [15]<br />

Table 3: Advantages and disadvantages of infrared heating [9]<br />

Advantages<br />

Energy efficiency<br />

Dry leaves<br />

Reduction in diseases such as botrytis<br />

Reduced heat loss<br />

Disadvantages<br />

Shadowing<br />

Proximity-dependent heat transfer<br />

Uneven heat transfer<br />

Limited lifespan<br />

Hot water systems are the most common type of heating systems used across the world in modern<br />

greenhouses because of their ability to be managed. The thermal properties of the water enable it to<br />

carry large amounts of energy over long distances with only a small loss to efficiency. “Water can<br />

carry 3,500 times the energy that air can carry,” which means that heating air is far less efficient<br />

than heating water [9]. The heat carried by hot water can be transferred in a multitude of ways. The<br />


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water can be utilizing for floor heating, bench heating, top heating, perimeter heating, snow and ice<br />

removal, or preheating irrigation water. Usually, the central boiler system is used to heat the water<br />

between 120°F and 180°F, and then it is distributed through a pipe system. However, solar<br />

collectors can be installed on the roof to use the energy from the sun for heating purposes. Pumps<br />

and valves help to provide even distribution in the system and can control the amount of heat added<br />

to the building [16]. The pipe system can be placed closed to the crops under the bench (Figure 8),<br />

in the soil, or the hot water can be circulated in a floor heating system (Figure 9).<br />

Figure 7: Piping of hot water heating system [16]<br />


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Figure 8: Under bench heating distribution [16]<br />

Figure 9: In-floor heating distribution [16]<br />


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Another way of utilizing hot water is heating a coil connected to a fan to blow hot air out into<br />

the greenhouse. Hot water systems are more energy efficient as the heat is radiated from below<br />

and it heats up only the close environment of the plants instead of heating up all or most of the air<br />

in the greenhouse. This is especially important in the different vegetation stages. This system is<br />

recommended for larger scale buildings due to the higher initial costs. The average cost of a hot<br />

water system with minimal complexity in the northern region in the United States ranges from<br />

$1.50 to $4.00/ft 2 for a 1-acre application [9].<br />

Table 4: Advantages and disadvantages of hot water systems [9] [14]<br />

Advantages<br />

Disadvantages<br />

Energy efficient<br />

Takes time to reheat the space<br />

Quiet<br />

Higher Initial cost<br />

Good manageability<br />

Complexity<br />

Carry great amount of energy in long Special skills needed for proper installation<br />

distances<br />

Can transfer the energy in different ways -<br />

<strong>Environment</strong>ally friendly -<br />

Good control of temperature -<br />

Flexibility -<br />

Other type of heating methods used in controlled environment agriculture include electric<br />

heaters and burning wood in a furnace or boiler. Electric heaters are not very popular, as it is one<br />

of the least efficient ways to heat a greenhouse due to their significant operating costs. Wood is a<br />

great source of fuel in areas that it is widely available. It can be used in a furnace to create hot air<br />

or in a boiler to heat water. In recent years, boilers have been more commonly used.<br />

Renewable energy sources have become more popular in the controlled environment<br />

agriculture sector in the past couple years. The effects of global warming increase the heating and<br />

cooling loads for buildings which result in greater energy requirements and the relatively high<br />

price of fossil fuel makes owners to move towards other alternative energy sources. Solar panels,<br />

solar collectors, heat pumps, biomass and cogeneration systems are the most utilized ones.<br />


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A solar greenhouse is a passive way to help maintain the required temperature inside the<br />

building by maximizing the southern glaze exposure to the sun by orienting the greenhouse in such<br />

a way to reduce the amount of fossil fuels used for heating. The solar greenhouse collects a mass<br />

of thermal heat from the sun during the day and the insulation prevents the loss of heat at night.<br />

The northern side of the building is usually well-insulated and has very minimal glazing. “The heat<br />

storage and thermal insulation of greenhouse is one of the key technologies for improvement of<br />

solar energy utilization and the reduction of manual concurrent heating power consumption” [17].<br />

This type of method can require supplemental heating such as usage of solar collectors or heat<br />

pumps.<br />

The easiest way to generate renewable energy is photovoltaic solar panels. The photovoltaic<br />

panels convert the sunlight into electricity using semiconducting materials that exhibit the<br />

photovoltaic effect. The panels can be mounted somewhere high or on the roof of the greenhouse<br />

for maximum exposure of the sunlight. The optical lenses can be mechanically operated to track<br />

the sunlight which could generate 15% more electricity over the standard photovoltaic system<br />

according to James Holloway’s testing [18]. The greenhouse’s equipment can be powered by the<br />

electricity generated by the solar panels. Off-grid photovoltaic systems need to be equipped with<br />

banks of batteries to be able to store the excess power and utilize it when there is very little or no<br />

sun. These systems are recommended for operations in which the nearest electrical grid is far.<br />

While solar panels do have high initial costs, the system does not require fuel, they have low<br />

carbon-dioxide emissions, and they require very little maintenance. The system payback period<br />

depends on the size and the amount of exposure to sunlight. The utilization of free energy from the<br />

sun is a sustainable way for heating and cooling as well.<br />


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Figure 10: Solar panel system on the roof of a greenhouse [19]<br />

Geothermal energy uses a thermal mass of soil underground to store (cooling) and dispense<br />

energy (heating). The tubing system is installed at least 4-6 feet underground so that air can be<br />

circulated back and forth with the building. Deeper, at around 10-12 feet, the soil has a more even<br />

temperature which can be useful if the plant has a small range of optimal temperature. In summer,<br />

humid hot air is circulated in the pipes where the cool temperature of the earth reduces the air<br />

temperature coming from the greenhouse and condenses the humidity on the wall, which is<br />

transferred to the soil around the tubes. The ground becomes a store of energy which can release<br />

the heat on cold days. In the winter, the higher soil temperature raises the temperature of the air<br />

coming from the greenhouse by drawing off the stored summer heat. Similarly, a liquid system can<br />

be installed in the soil. Through heat exchanges, the heat can be released or absorbed according to<br />

the season. This type of heating and cooling is very sustainable in long term. The initial cost of the<br />

system is very high, but the “free” heating and cooling eventually outweighs the initial costs [20].<br />

This is one of the most economical options that currently exists for temperature regulation of a<br />

greenhouse.<br />


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Figure 11: Geothermal energy used for heating and cooling [20]<br />

c. Cooling<br />

Due to high solar radiation, the average high temperature in Texas in August is 96 °F [11], and<br />

the temperature inside a greenhouse can be several degrees higher. In order to keep the crops<br />

growing and provide fresh vegetables and fruits year-round in the southern regions of the United<br />

States, the temperature must be reduced inside the greenhouse during the hot summer days. In<br />

northern areas, ventilation techniques provide enough air circulation so that the plants can<br />

evaporate enough (cool themselves), and additional cooling becomes optional. There are several<br />

active and passive ways to maintain the temperature cool inside.<br />

One of the passive ways to block some of the sunlight is using shading. This can be done by<br />

shade paints or shade cloth. Paints are a very quick and cost-effective way to protect the plants<br />

from the strong direct sunlight. The number of layers can be adjusted according to the temperature.<br />

This solution is not always applicable; therefore, it can be replaced with shade cloth like nets and<br />

blinds. Shade cloth can lower the temperature up to 10 degrees [21]. Blinds can be internal or<br />

external. With the external blinders the sunlight is filtered before it passes through the glass and<br />

traps inside which make this type of blinders more efficient. Internal blinders can be installed in<br />

front of the windows. Both types are removable when their usage is not necessary and can be<br />

automated if the budget allows [22]. The cost of these depend on their size and the type that is<br />

needed. It is also important to note that too much shading can result in reduction of plant height<br />

and number of trusses [23].<br />


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Figure 12: Shading on a greenhouse roof [22]<br />

Another passive method to reduce the average summer temperature inside the growing space<br />

is trenching the greenhouse. By trenching the greenhouse about 36 inches beneath the ground level,<br />

where the soil temperature is around 60-70 degrees, the energy requirements can be reduced. The<br />

inside ambient temperature will be lower than usual due to the utilization of lower soil temperature,<br />

and this will decrease the amount of heat that needs to be removed from inside [24].<br />

Evaporative cooling pads are one of the most common ways to provide cooling in a greenhouse.<br />

The system is based “on the conversion of sensible heat into latent heat through evaporation of<br />

water” [23]. The operation of evaporative cooling pads requires air flow through a pad medium.<br />

The pad can be placed on one side of the greenhouse and exhaust fans, which help pull the air<br />

through the pad, can be placed on the opposite wall. Temperature reduction can be achieved by<br />

releasing the energy from the air. “The air released to the outside has absorbed 8,100 BTU of heat<br />

energy for each gallon evaporated” [9]. Water distribution helps the liquid circulate through the<br />

material. The original evaporative cooling systems were about 2 inches thick, while the modern<br />

pads are 4-6 inches thick. The newer cooling pads are made of corrugated cellulose material which<br />

helps prevent rotting. The medium usually sits in an aluminum or plastic trough. Although it is<br />

simple to operate the system, it has high installation and operating costs as well as amplified<br />

possibility for fungal diseases [23]. The ability of a well-design system is capable to reduce the<br />

dry bulb temperature inside to around 85% of the difference between the outside dry bulb and wet<br />

bulb temperature. The evaporative cooling system needs to be oversized in areas where the<br />


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humidity is constantly high in the summer, such as in Dallas [9].<br />

Figure 13: Evaporative cooling pad [21]<br />

Fog (mist) cooling and humidifying systems tend to cost less than the evaporative cooling pads.<br />

The system is getting more popular in the agricultural sector due to its ability to provide even<br />

temperature reduction and levels of humidity. These advantages persist in both humid and dry areas<br />

as well. The principles of the fogging system are very similar to the evaporative cooling system,<br />

with the only difference being how efficiently the water is utilized. In the fog cooling system, water<br />

is fogged and sprinkled over the crops from the sprinkler system installed close to the roof. The<br />

fog is evenly distributed throughout the greenhouse and “because of the flash evaporation<br />

characteristics of fog, 100 % efficiency is achieved” [9]. The minimum lowering temperature can<br />

be as low as 1°F with the fogging system, while the minimum temperature differential in the<br />

evaporative cooling system is 7-10 °F. The mist cooling system consists of a high-pressure pump<br />

and fine fog nozzle which requires proper maintenance [9]. High-pressure fog systems can<br />


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change the environmental conditions quickly. Fog systems are designed for all types of greenhouse<br />

configurations and all types of plants [25].<br />

Cooling with water through pipe system has the same principles as heating with water. The<br />

water must be cooled and pushed through a pipe system which is installed close to the crops. This<br />

is one of the less common types of cooling systems used in controlled environment agriculture.<br />

Geothermal heating and cooling are paired together in a single package; if you install the<br />

system you will get both. Low grade geothermal cooling has a high initial cost but is very<br />

sustainable and economical in a longer term. On hot days, the air from the greenhouse is circulated<br />

through underground pipes, and when the water vapor condenses from the hot, humid air on to the<br />

outside of the pipe, energy gets released into the soil.<br />

As discussed previously in the heating section, the solar-powered air conditioning system is a<br />

sustainable choice to achieve heating or cooling. Solar energy can be attained by solar collectors<br />

or solar panels. Solar collectors use water to store the energy, while solar panels generate electricity<br />

which can be used for heating, cooling, lighting, or powering other equipment around the building.<br />

Further explanation about this system can be found in the heating section.<br />

An absorption refrigeration cycle is another sustainable way to provide cooling in controlled<br />

environment agriculture. The advantages and disadvantages of the system is discussed in detail in<br />

section 3, but in summary, this system is a promising low-cost cooling solution and is similar to<br />

the regular vapor compression air-conditioning system.<br />

d. Ventilation<br />

Ventilation is necessary year-round in order to maintain the required humidity and CO2 levels for<br />

the crops. Before discussing the various ventilation methods, it is important to highlight the<br />

difference between open and closed greenhouses. An open greenhouse has some form of air flow<br />

from outside of the structure and has more interaction with outside air. In contrast, a closed<br />

greenhouse is similar to indoor growing, and there can be more control over the inside conditions<br />

as there is no direct contact with the outside air. The open greenhouse mostly relies on natural<br />

convection, but it can be equipped with supplemental ventilation system. In the case of natural<br />

ventilation, the differences in air pressure move the air through the space. On the other hand, the<br />

forced ventilation method is primarily used in closed greenhouses and utilizes fans to move to air.<br />


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On an average summer day in the United States, the noontime solar radiation input can be around<br />

300-350 BTUs per hour/ft 2 of floor area. This heat contributes to the temperature and humidity rise<br />

inside, resulting in a higher evaporation rate from the plants [9]. Too significant a humidity level<br />

can result in moldy surfaces, pests, and diseases on the plants.<br />

Fan ventilation systems can be used in all weather conditions. The most common recirculation<br />

fan used in controlled environment agriculture is the overhead horizontal airflow fan [9]. Fans<br />

create a positive air flow through the greenhouse. Bottom air flow systems and vertical airflow<br />

fans are commonly used in dense plant canopies. Exhaust fans help release the heated air from the<br />

greenhouse, and by creating a vacuum, they bring cool, fresh outside air inside the building through<br />

inlets like open doors or cracks. “Cooling effectiveness of the greenhouse reduces as it travels<br />

across a greenhouse, resulting in a temperature gradient between the air inlet and exhaust” [9].<br />

This problem can be solved by designing a shorter greenhouse and increasing the airflow velocity<br />

of the fans. The most effective distance between the inlet and the fan outlet is 150 feet or less. It is<br />

more efficient to have a few larger fans than several smaller fans. The maximum distance between<br />

the installed fans can be 25 feet in order to provide uniform airflow in the greenhouse. Fans are<br />

usually selected based on static air pressures; the average value used is 0.125 inches of water<br />

column. “The volume of air to be moved (or exchanged) is calculated as the number of volumetric<br />

air changes per minute (usually 1.0-1.5/min)” [9]. The most energy-efficient fans have properly<br />

sized motors and are maintained regularly. Fans are fairly cheap to install and operate, and they<br />

can be used as supplemental equipment with evaporative cooling pads, infrared heating, or other<br />

heating and cooling methods as well. The power for the fan operation can be generated by<br />

photovoltaic panels.<br />


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Figure 14: Exhaust fan on the side of the greenhouse [21]<br />

Desiccant-based ventilation systems are very useful in reducing humidity of the inside air in<br />

colder weather. Desiccant is a medium which absorbs moisture from the air and can be recycled in<br />

the desiccant wheel. The collected moisture evaporates with heat as it travels through the wheel<br />

[24]. “Rotary desiccant wheels can be used in HVAC systems to reduce the relative humidity in an<br />

incoming airstream through the adsorption of water molecules to the pore surface of a desiccant<br />

material, lowering the relative humidity” [26]. The regeneration temperature of the system depends<br />

on the desiccant material used, and while it could work in up to 120 °C, the development of new<br />

material can potentially reduce this temperature to 80 °C. Due to the high temperature of the<br />

regeneration process, additional heat energy needs to be added to the system. For airstreams, the<br />

pressure drop can be as high as 150 Pa with the typical rotary desiccant wheel structure [26].<br />

Further reduction in regeneration temperatures and pressure is essential for the more efficient<br />

operation. This system is used for cooling purposes as well, and it is more environmentally-friendly<br />

than conventional cooling if the system is powered by free, or solar, energy. According to Amel<br />

Rjibi, the system powered with solar energy can save up to 24% of the electric energy and reduce<br />

CO2 emissions by 16% [27].<br />


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A refrigerant-based dehumidification system is built similarly to the regular air-conditioning<br />

system, but it is designed for removal of water vapor from the air instead of cooling. It is equipped<br />

with an evaporator, condenser, expansion valve, water receptacle and motor-compressor unit (see<br />

Figure 15). Humid air enters on the evaporator side, where the cold coils of the element cool the<br />

air below its dew point. Subsequently, the removed moisture can be released to the water<br />

receptacle. The cold air is pushed through the condenser to increase its temperature, then released<br />

back to the room with higher temperature but less humidity [28]. This system uses about one<br />

quarter of the energy per pint when compared to the desiccant-based dehumidification [24].<br />

Figure 15: Elements of the refrigerant-based dehumidification system [38]<br />

e. Glazing and insulation<br />

The glazing is the outside cover material or the “skin” of the greenhouse. The type of glazing<br />

used directly influences the amount of solar energy that enters the building. The solar energy both<br />

increases the temperature in the closed place and provides essential light for the plants [9]. Before<br />

choosing the right cover material, there are several factors that need to be considered: the type of<br />

plant being grown, the cost of the material and its maintenance, the durability, transparency and<br />

lifespan of the material, the location of the building, and the weather conditions of the area. The<br />


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following table summarizes the type of materials used in controlled environment agriculture and<br />

discusses their advantages and disadvantages:<br />

Table 5: Comparison of the different cover materials [29]<br />

- Advantages Disadvantages<br />

Glass<br />

Well-known, many different types to<br />

choose from, aesthetic, retain its<br />

transparency for long time, great<br />

thermal insulator,<br />

Durable<br />

Fragile, can be costly, the<br />

infrastructure needs to be<br />

able to support it<br />

Polystyrene (Rigid<br />

Plastic)<br />

Polycarbonate (Rigid<br />

Plastic)<br />

Glass-like panels, cheaper than glass,<br />

requires less structural support,<br />

strength and translucency are like<br />

glass, not easily damageable, fair<br />

insulator<br />

Strength and durability are second<br />

only to glass, leading option among<br />

plastics, double-wall configuration,<br />

admits up to 90% of sunlight, long<br />

lifespan (up to 20 years), cheaper than<br />

glass<br />

Lifespan is shorter than glass,<br />

not the strongest plastic, not<br />

the least expensive, clouding<br />

over a longer period<br />

More expensive than other<br />

plastics, may need additional<br />

insulation, can be an<br />

incubator for moisture,<br />

residue, dirt, or bacteria<br />

Polyethylene (Film) Cheapest option Easily damaged, short<br />

lifespan (usually less than 18<br />

month), requires frequent<br />

replacement, limited<br />

insulation possibilities<br />

Copolymer (Film) Cheaper than rigid plastic and glass Easily damaged, short<br />

lifespan (3 years), requires<br />

frequent replacement, limited<br />

insulation possibilities, can<br />

become brittle and crack in<br />

very cold weather<br />

Polyvinyl (Film)<br />

Most durable plastic film, cheaper<br />

than rigid plastic and glass<br />

Short lifespan (5 years),<br />

limited insulation possibilities<br />


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Chris Beytes made a general comparison of the different covering materials. More detailed<br />

specifications need to be requested from the manufacturers. This comparison can be found in Figure<br />

16.<br />

Figure 16: Greenhouse Coverage Comparison [9]<br />

The proper coverage material can help reduce heating and cooling costs during the winter and<br />

summer, respectively. Some techniques, such as installation of a double layer of film plastic while<br />

inflating it with a blower, can be both cheap and efficient insulation methods to apply. Usage of<br />

insulation material is required to increase the efficiency of a greenhouse system. Before choosing<br />

the right option, it is important to research the available types and understand their differences.<br />

Some insulation types can be left up all year around, while some require installation before the<br />

winter or summer arrives and removal at the end of the season. Double glazing is the most efficient<br />


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way to insulate the entire greenhouse. This type of insulation necessitates the installation with the<br />

greenhouse itself. It is not removable and is expensive, but this method provides the most benefits<br />

of any insulation method [30].<br />

A clear bubble plastic cover can reduce heating costs by 35%. It is made of a double or triple<br />

layer of transparent plastic and is covered with air cells. This high UV- translucent insulator works<br />

just as effectively as double glazing and helps to prevent the greenhouse from overheating during<br />

hot summer days. The insulation works well with all types of glazing and can be left on yearround,<br />

but it is recommended to replace the bubble plastic cover every 3-4 years [31].<br />

Figure 17: Clear bubble insulation [32]<br />

The third type of insulation is thermal screens. This insulation method consists of translucent<br />

material or clear plastic like cross laminated fabric. The material is attached to the eaves with wires<br />

and restricts heat from rising above the eaves by trapping the warm air inside the building. These<br />

screens are typically used overnight to conserve energy [30]. It is also possible to cover the<br />

greenhouse only partially and leave the remainder without insulation, which could be necessary if<br />

different plants are grown in each end of the structure. Finally, insulation can be installed on the<br />

floor as well. Base cladding blocks push out the incoming air at the intersection of the walls and<br />

floor. Polystyrene panels are often utilized for this form of seasonal insulation.<br />

f. Lighting<br />

Light energy is one of the major factors necessary for plant growth. In areas where natural<br />

sunlight is insufficient, some form of supplemental lighting is required to sustain the plants.<br />

Supplemental lighting enables the farmers to increase the growing season, plant productivity, and<br />

plant quality. There are several types of possible light sources, including high-pressure sodium<br />


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lamps and other metal halide lamps, fluorescent lighting, incandescent plant-grow lights, and LED<br />

lights [33].<br />

Figure 18: LED supplemental lighting [34]<br />

High-pressure sodium (HPS) lamps and other metal halide lamps are not very efficient, as<br />

they generate significant amounts of heat through their operation. The HPS lamps have an orangered<br />

glow which can help trigger hormones in the plants, but it cannot be installed in close proximity<br />

to the plants because of its high heat generation. The average lifespan of the HPS lamps is 18,000<br />

hours, while the metal halide lamps produce light in the blue spectrum and have an average lifespan<br />

of 10,000 hours [33] [35]. The supplement of the metal halide lights can produce green leafy<br />

growth. HPS lamps produce up to 140 lumens per watt, while the metal halide lights only produce<br />

up to 125 lumens per watt. These values are significantly higher than those for incandescent or<br />

fluorescent light bulbs. A significant disadvantage of HPS lamps is that they are deficient in the<br />

blue spectrum, which is essential to the even growth of the plants. If using this type of lighting, the<br />

plants will require an additional source of blue light [35].<br />

Fluorescent lights contain filaments that require replacement from time to time. This type of<br />


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lamp consumes lots of electrical power and generate a great amount of heat, though less than the<br />

incandescent, metal halide, or HPS lights. The average lifespan of fluorescent light bulbs is<br />

estimated to be around 20,000 hours [33]. Because of their size limitations, it is only recommended<br />

to use for seedlings. Fluorescent bulbs provide a wide spectrum of light and have a better colorrendering<br />

property. To achieve the best results, it is recommended to place them no closer than a<br />

couple feet from the plants. The standard fluorescent grow light produces about 39 lumens per<br />

watt, but the newer versions are more energy-efficient and can produce up to 90 lumens per watt<br />

[35].<br />

Incandescent bulbs are both the least expensive and the least efficient solution to supply<br />

artificial lighting in a greenhouse. The standard incandescent lightbulb can produce only 18 lumens<br />

per watt, much less than the other lighting options discussed [35].<br />

LED is a type of semiconductor diode for which the spectral composition and light intensity<br />

can be adjusted according to the time of the day and the type of plants. Each type of plant has<br />

different requirements of light intensity, light quality (spectral distribution) of the radiation, and<br />

light duration (photoperiod) to perform photosynthesis. Spectral distribution affects the shape,<br />

development, and flowering of the plant. Plants are very selective about the wavelength of light that<br />

they will absorb, and they show the most reaction to red and blue lights [33]. One of the numerous<br />

advantages of LED light is the possibility to control its spectral composition and intensity. These<br />

characteristics can be adjusted according to the plant being grown and its developmental stage.<br />

Their lifespan is also significantly higher than the other light bulbs, reaching 30,000 to 50,000<br />

hours or beyond. Due to low-radiant heat production, they can be placed close to the crops without<br />

damaging them. Though LED lights have high capital costs, they are the most energy-efficient and<br />

sustainable method of supplemental lighting.<br />

3. Evie, design of low-cost cooling system<br />

A solar-powered absorption refrigeration cycle is a sustainable way of cooling commercial<br />

buildings. According to the size constraints of Evie (a mobile greenhouse built by the Hunt Institute<br />

for Engineering & Humanity at Southern Methodist University), a design model was fully<br />

completed by the ARC design team and compared with the regular vapor compression refrigeration<br />

cycle.<br />


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a. Importance of Absorption Refrigeration Cycle<br />

Figure 19: Evie at the State Fair (Photo Credit: Laura Graham)<br />

An absorption system has five main components: the generator, condenser, evaporator,<br />

absorber, and heat exchanger. Instead of using electricity as in vapor-compression systems,<br />

absorption refrigeration systems use a heat source to provide the energy needed for cooling. The<br />

absorption refrigeration system uses two different fluids and their mixtures in order to achieve<br />

cooling in a specified space. One fluid is used as a refrigerant, while the other is used as an<br />

absorbent. The two most common types of fluid pairs used for the cycle are ammonia-water and<br />

water-lithium bromide. In this design, ammonia was chosen as the refrigerant and water as the<br />

absorbent.<br />

The absorption refrigeration cycle has multiple advantages compared to the conventional vapor<br />

compression refrigeration cycle. One of its most important advantages is the method of<br />

compression of the refrigerant. In the regular vapor compression system, a compressor is used for<br />

the purpose of circulating the refrigerant in the cycle. This could be done by a centrifugal,<br />

reciprocating, or rotating type of compressor [36]. In the vapor absorption refrigeration cycle the<br />


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compression of the refrigerant is done by the absorber (water in this case), which absorbs the vapor<br />

refrigerant coming from the evaporator. Phase change can be observed in the absorber.<br />

The major power-consuming device in the regular vapor compression cycle is the compressor,<br />

while a pump is needed for the absorption refrigeration cycle. The compressor requires a large<br />

amount of power in order to run it, and the consumption increases as the size of the refrigeration<br />

system itself increases. The compressor is considered a major moving part in the vapor<br />

compression cycle but gets eliminated in the absorption refrigeration cycle, meaning that the only<br />

moving part is a pump, which requires significantly less power [36].<br />

The type of energy used for the two aforementioned refrigeration cycles also differs.<br />

Mechanical energy is required to run the compressor in the regular vapor compression refrigeration<br />

cycle, while in the absorption refrigeration cycle, heat is used as an energy input. This heat could be<br />

residual heat coming from an engine, such as from a car, generated by solar collectors, etc. This<br />

project utilizes solar panels to generate electricity, which is used to provide power to the heating<br />

element. The heating element in the generator provides the required heat to separate the refrigerant<br />

from the mixture solution.<br />

The vapor compression cycle requires a lot of power, as it runs solely on electricity. Thus, the<br />

running cost of the system is expensive. The absorption refrigeration cycle depends on a natural<br />

energy source in this project, as solar panels provide the required power. This method of heat input<br />

minimizes the running cost of the refrigeration system.<br />

The compressor in the regular vapor compression refrigeration cycle usually operates at very<br />

high speeds, meaning significant amounts of vibration and noise. As the compression process is<br />

done by a generator and absorber in the absorption refrigeration cycle, the noise and vibration of<br />

the compressor is eliminated. Compressors also need strong foundations to resist the generated<br />

vibration and remain intact under the high pressure of the refrigerant. Thus, there are multiple<br />

advantages to replacing the compressor in our cycle [36].<br />

The maintenance of the regular vapor compression cycle is more complicated and more costly<br />

than the maintenance of the absorption refrigeration cycle. Because the compressor is considered<br />

the main moving part in the vapor compression cycle, the regular lubrication and potential defects<br />

have to be checked regularly. Over time, it also requires replacing the piston, cylinder liner, piston<br />

rings, etc. Potential failure, and subsequent replacement, of the compressor is very expensive as<br />


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well. This type of maintenance also requires special care. In the absorption refrigeration cycle, on<br />

the other hand, the only moving part is the pump, which rarely fails and therefore requires<br />

significantly less maintenance.<br />

The absorption refrigeration cycle requires the usage of a mixture fluid. In this project,<br />

ammonia is used as the refrigerant and water as the absorbent fluid. Ammonia is an accessible and<br />

cheap refrigerant, while the halocarbons used in the regular vapor compression cycle are very<br />

expensive. The halocarbons are also dangerous to the environment, as they contribute to the<br />

greenhouse effect. Additionally, there could be multiple leaks in a vapor compression cycle because<br />

of the use of the compressor and its moving parts. In the case of leakage, the refrigerant has to be<br />

recharged regularly, which could be very costly. The absorption refrigeration cycle has very little<br />

or no potential for leakage, as there are no moving parts in the system other than a pump. This<br />

eliminates the potential cost of refrigerant recharge [36].<br />

Some disadvantages of the absorption refrigeration cycle are its higher initial cost, the low<br />

coefficient of performance of the system, and the higher heat rejection rate.<br />

b. Experiment<br />

A thermodynamic analysis was performed on the cycle to achieve the working conditions of<br />

the absorption refrigeration cycle for Evie, the mobile greenhouse. The outdoor and indoor<br />

temperatures were assumed (36°C and 20°C, respectively) according to Dallas weather data and<br />

the required growing temperature for most plants [37]. Heat transfer calculations were done in<br />

order to obtain the heat load calculations on Evie. Some assumptions were made for the heat gain<br />

calculations, such as that Evie is made out of only glass like a regular greenhouse.<br />

Circuit analysis was utilized to study the required electricity for the system. As a result, the<br />

appropriate heat source could be picked to run the cycle.<br />

The theoretical cycle was modified according to the size of Evie. Since the mobile greenhouse<br />

has a limited space which requires cooling in the summer, the absorption refrigeration cycle had<br />

to be sized in accordance with its requirements.<br />


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The engineering specifications can be seen in Table 6, Table 7 and Table 8.<br />

Table 6: Objectives<br />

Objectives<br />

Cost-efficient<br />

Cool the given space<br />

Operate off-grid<br />

Details<br />

In about 5 years of operation, the<br />

overall cost should be less than the<br />

cost of operating a regular AC<br />

system<br />

Maintain a temperature of 20 °C inside<br />

of Evie<br />

Utilize solar-panels<br />

Have low capital cost Initial cost around $5000<br />

Table 7: Constraints<br />

Constraints<br />

Low maintenance cost<br />

Be able to operate within<br />

the given cooling load<br />

System size<br />

Details<br />

Must be less than regular AC system<br />

($300)<br />

The given cooling load for our system is<br />

1.68kW<br />

The size of the system including the solar<br />

panels should be less than 6.8 m 2<br />

Table 8: Functions<br />

Operate safely<br />

Functions<br />

Details<br />

Pressure should not exceed 2.85 MPa<br />

Operate efficiently The coefficient of performance is 0.2<br />

The cycle can be broken into different flows based on the presence of the ammonia-water<br />

mixture, ammonia gas, and ammonia vapor. States 1 to 6 are the cycle of the ammonium hydroxide<br />

solution, ammonia in liquid phase constitutes states 7 and 10, and states 8 and 9 contain ammonia<br />

vapor. The basic operation of an ammonia-water absorption refrigeration cycle is as follows.<br />


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The solution at state 1, rich in refrigerant, is pumped by higher pressure through the heat<br />

exchanger at state 2 into the generator at state 3, which contains the solution of ammonia-water,<br />

rich in ammonia. Heat is then added to the generator, causing high-pressure ammonia vapor to<br />

desorb the solution. The solution containing less refrigerant at state 4 is sent back through the heat<br />

exchanger to the absorber. The high-pressure ammonia vapor flows to the condenser, where it then<br />

loses heat to the surrounding environment by convection as it leaves the condenser. Then, the highpressure<br />

ammonia liquid (8) goes through a valve restriction to the low-pressure side (9) of the<br />

cycle. This liquid, at low pressure, boils and evaporates in the evaporator where heat from the given<br />

space is absorbed. The ammonia liquid (10) then returns to the absorber, where it is absorbed into<br />

the weak solution coming from the generator. The solution in the absorber, now once again rich in<br />

ammonia, is pumped to the generator (1), where the process starts again. The schematics of the<br />

absorption refrigeration cycle with the flow of solution mixture and ammonia vapor can be seen<br />

in Figure 20.<br />

Figure 20: Schematics of the Absorption Refrigeration Cycle<br />

The conditions at different states in Figure 20 is stated below, and the calculation can be<br />

found in Table 9 and Table 10. The cost of each element can be found in Table 11.<br />

1. Strong solution leaving the absorber (X1 =55.3% water, T1 = 30 °C)<br />

2. Strong solution entering the heat exchanger (X2 = 55.3% water, T2 = 30 °C)<br />


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3. Strong solution entering the generator (X3 = 55.3% water)<br />

4. Weak solution leaving the generator (X4 = 56% water, T4 = 60°C)<br />

5. Weak solution leaving the heat exchanger (X5 =56% water)<br />

6. Weak solution entering the absorber (X6=56% water)<br />

7. Refrigerant vapor leaving the generator which is heated at T7= 60°C, P7 = 2.61 MPa)<br />

8. Refrigerant liquid leaving the condenser (Saturated at 60°C, T8 = 60°C, P8 =2.61 MPa)<br />

9. Refrigerant liquid entering the evaporator (Saturated at 16°C, T9 = 16°C, P9 = 0.75 MPa)<br />

10. Refrigerant vapor entering the absorber (Saturated at 16°C, T10 = 16°C, P10 = 0.75 MPa)<br />


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c. Results<br />

Temperatures(T), pressures(P), enthalpies(h), mass flow rate of the fluids (m) and the quantity<br />

of water in the mixture fluid (X) were calculated at each state identified by a number in Figure 20,<br />

and the results are summarized in Table 9 and Table 10.<br />

Table 9: Thermodynamic Cycle Calculations 1<br />

Table 10: Thermodynamic Cycle Calculations 2<br />


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Table 11: Cost of the Elements<br />

Component Details Quantity Unit Price Total price<br />

Generator We need 36 " × 48 "<br />

&<br />

Aluminum sheets with<br />

1 $182 $182<br />

Absorber [38] thickness of 0.2”<br />

Condenser [39] Already exist in market.<br />

The cooling capacity is<br />

1 $718 $718<br />

1.5 tons<br />

Evaporator [39] Already exist in market.<br />

The cooling capacity is<br />

1 $ 254 $254<br />

1.5 tons<br />

Heat exchanger Already exist in market. 1 $150 $150<br />

Solar Panels [40]<br />

250-watt solar micro<br />

inverter kit 2 $579 $1158<br />

Battery 12 V/200 Amp-hr 3 $269 $807<br />

Steel pipe [41] 1/2" SCH 40.<br />

6 ft. long 1 $12.46 $12.46<br />

Pump [42] Red Dragon 5 ECO 25<br />

Watt 4.0m³ DC Pump 1 $458.3 $458.3<br />

Expansion Valve<br />

[43]<br />

90˚ Flanged<br />

Elbow [44]<br />

Heating element<br />

[45]<br />

TEA thermostatic<br />

expansion valve<br />

Smith Cooper 304<br />

Stainless Steel 1/2 in.<br />

90° Long Radius Elbow<br />

Weld Fittings - Sch 40<br />

1 $130 $130<br />

4 $3.70 $14.8<br />

material: stainless<br />

power:100 W 1 $10.44 $10.44<br />

Total $3895<br />

After preliminary design analysis and cost analysis, the ARC team has determined that this system<br />

will not meet the requirements set forward by our customer. The system with the solar panels will<br />

be too heavy to be placed on top of Evie, and the system components will be bigger than the given<br />

size requirement. Given Evie’s small size (about 7 m 2 ) and the requirement for a solar-powered<br />

system, we have concluded that our design will not be more efficient than the compression AC<br />

system that is already in place. However, from the design research, it is evident that this system is<br />


<strong>Controlled</strong> <strong>Environment</strong> <strong>Agriculture</strong> <strong>Technologies</strong><br />

Adrienn Santa<br />

feasible and could be efficient for a larger-scale cooling system where the cost of compression AC<br />

system is too high and sources like steam, geothermal energy, and natural gas could be used as heat<br />

input to the system. When used at such a large scale, this system will surpass the overall cost of<br />

regular AC system in about three to five years.<br />

d. Implications of this project<br />

Global Impact<br />

Upon success, this absorption cooling system could provide a significant advantage for<br />

farmers worldwide. This system provides an efficient, cost-effective method of cooling a<br />

greenhouse to an optimal temperature. Millions of people in the United States alone live in areas<br />

that face a lack of fresh, healthy food options and could greatly benefit if this system is used to<br />

produce food in a greenhouse setting. Moreover, there are many remote areas and households that<br />

do not have proper access to electricity, and this cycle could be used to provide AC in their homes<br />

or even cool their greenhouse.<br />

Since this system does not rely on electricity to operate and could be run on energy sources<br />

like solar, gas, geothermal water, woods, kerosene etc., a broad set of applications for this system<br />

are possible. One option may be to use this system to make portable solar freezers which could be<br />

used to store blood and vaccines. This will be useful in the remote parts of the world where, due to<br />

limited access to electricity and refrigeration systems, establishing a blood bank has not yet been<br />

possible. It also has promising possible applications for fishermen and small dairy producers in the<br />

developing world who often lack access to electricity and require cooling for their highly<br />

perishable products. Without such technologies, they are under pressure to sell their product<br />

quickly, no matter the price.<br />

Economic Impact<br />

With this system running in their greenhouse, farmers will face a lower variable cost of<br />

cooling their greenhouse after the cost of initial setup. Since this system has only a few moving<br />

parts, there will be less maintenance required, also contributing to its cost-effective nature. With<br />

these greenhouses running efficiently, farmers can sell their food locally and bolster their local<br />

economy.<br />


<strong>Controlled</strong> <strong>Environment</strong> <strong>Agriculture</strong> <strong>Technologies</strong><br />

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If the efficiency of this system could be increased to a level where it matches the efficiency<br />

of the AC systems currently used in households, this system could completely replace the<br />

compressor AC system. It can have a significant economic impact in the worldwide HVAC market<br />

since there would be great demand for a low-cost, low-maintenance system.<br />

If this system could be successfully implemented in cars to replace the compression AC<br />

system with a solar powered absorption cooling system, the car could maintain a constant<br />

temperature even while not running. Not only would car users save money on gas, they would also<br />

not have to waste time and energy waiting for their car to heat or cool to their desired temperature.<br />

<strong>Environment</strong>al impact<br />

By using the absorption refrigeration system, one could use renewable, clean energy<br />

instead of using electricity, one of the leading sources for greenhouse gas emissions. In this system<br />

we can use sunlight, wind, or water to generate power to run the system with minimal to no<br />

negative impact on the environment.<br />


<strong>Controlled</strong> <strong>Environment</strong> <strong>Agriculture</strong> <strong>Technologies</strong><br />

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4. Conclusion<br />

This research focused on assessing the available technologies used in controlled environment<br />

agriculture and identifying a promising model for a low-cost cooling method. Based on the available<br />

literature, a detailed discussion is included on heating, cooling, and ventilation technologies as<br />

well as the best possible orientation, structure, insulation, glazing materials, and supplemental<br />

lighting methods for the greenhouse. A detailed design of a solar powered absorption refrigeration<br />

cycle model was designed for Evie, taking into consideration its size and limitations. The results<br />

show that the proposed design would be efficient in a larger scale, with costs lower than a regular<br />

air-conditioning system over a 5-year period. According to these findings, solar and geothermal<br />

energy are the most sustainable ways to heat and cool a greenhouse, yet currently their initial and<br />

installation costs are very high. Small-scale urban farmers and low-income communities are unlikely<br />

to be able to afford these technologies. Passive solar heating can be a great solution to reduce the<br />

heating costs during cold days, but this method requires some form of insulation. Clear bubble<br />

insulation has a relatively lower price when compared to the other insulation methods. The least<br />

expensive way to achieve ventilation is by natural convection, but natural convection is not always<br />

sufficient depending on the weather conditions of the area, the size of the greenhouse, and the type<br />

of plants grown. Light is one of the most important conditions for the adequate growth of the<br />

plants. While LED lighting has higher capital costs than other lighting methods, its numerous other<br />

advantages make it the most economical, sustainable, and energy-efficient solution to provide<br />

supplemental lighting to the plants. Communities interested in producing their own produce need<br />

to invest in these technologies to enjoy their long-term benefits. Further development, testing, and<br />

analysis are needed to improve the efficiency of the current technologies used in controlled<br />

environment agriculture and make them accessible to low-income communities.<br />


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5. References<br />

[1] “USDA Defines Food Deserts.” American Nutrition Association,<br />

americannutritionassociation.org/newsletter/usda-definesfood-deserts.<br />

[2] “Documentation.” USDA ERS - Documentation, www.ers.usda.gov/data-products/foodaccess-research-atlas/documentation/.<br />

[3] Bornstein, David. “Time to Revisit Food Deserts.” The New York Times, The New York<br />

Times, 25 Apr. 2012, opinionator.blogs.nytimes.com/2012/04/25/time-to-revisitfood-deserts/.<br />

[4] Goldschein, Eric. “10 American Food Deserts Where It Is Impossible To Eat Healthily.”<br />

Business Insider, Business Insider, 12 Oct. 2011, www.businessinsider.com/fooddeserts-urban-<br />

2011-10#the-duwamish-river-in-seattle-needs-more-stockbox-grocers-<br />

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[5] Karsten, Jack, and Darrell West. “How the Amazon-Whole Foods Merger Shrinks Food<br />

Deserts.” Brookings, Brookings, 29 Aug. 2017,<br />

www.brookings.edu/blog/techtank/2017/08/29/how-the-amazon-whole-foodsmerger-shrinks-<br />

food-deserts/.<br />

[6] LaLiberte, Kathy. “When Is It Warm Enough to Plant?” Gardeners,<br />

www.gardeners.com/how-to/when-is-it-warm-enough-toplant/9029.htm<br />

[7] Dragicevic, Snezana. “Determining the Optimum Orientation of a Greenhouse on the Basis<br />

of the Total Solar Radiation Availability.” Thermal Science, vol. 15, no. 1, 2011, pp.<br />

215–221., doi:10.2298/tsci100220057d.<br />

[8] El-Maghlany, Wael M., et al. “Optimum Design and Orientation of the Greenhouses for<br />

Maximum Capture of Solar Energy in North Tropical Region.” Energy<br />

Conversion and Management, vol. 105, 2015, pp. 1096–1104.,<br />

doi:10.1016/j.enconman.2015.08.066.<br />

[9] Beytes, Chris, and Jim Nau. Ball Redbook. 18th ed., vol. 1, Ball Pub., 2011.<br />

[10] “Greenhouse Structures | Ornamental Production.” Aggie Horticulture®, aggiehorticulture.tamu.edu/ornamental/greenhouse-management/greenhousestructures/.<br />

[11] “Climate Winters - Texas.” U.S. Climate Data,<br />

www.usclimatedata.com/climate/winters/texas/unitedstates/ustx1482/2018/1.<br />

[12] Sanford, Scott. “Greenhouse Unit Heaters: Types, Placement, and Efficiency.” Urban<br />

<strong>Agriculture</strong> Manual, University of Wisconsin Extension, 15 Mar. 2016,<br />


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Adrienn Santa<br />

urbanagriculture.horticulture.wisc.edu/2016/01/21/greenhouse-unit-heaters-typesplacement-<br />

and-efficiency/.<br />

[13] King, Kerry. “Greenhouse Heating Buyer's Survival Guide.” Growers Supply, 4 Dec. 2013,<br />

growerssupply.wordpress.com/2012/12/03/greenhouse-heating-buyers-survival-guide/.<br />

[14] Lopez, Jose Chen. “Pointers on Greenhouse Heating and Energy Conservation.” PRO-MIX,<br />

12 Sept. 2017, www.pthorticulture.com/en/training-center/pointers-on-greenhouseheating-and-<br />

energy-conservation-part-2-heating-systems/.<br />

[15] “Greenhouse Infrared Tube Heaters.” Roberts Gordon,<br />

www.robertsgordon.com/greenhouse-infraredheating.<br />

[16] Berkum, Erik van. “Greenhouse Heating Systems.” Geek in Asia, 31 Oct. 2014,<br />

blog.maripositas.org/horticulture/greenhouse-heating-systems.<br />

[17] Esen, Mehmet, and Tahsin Yuksel. “Experimental Evaluation of Using Various Renewable<br />

Energy Sources for Heating a Greenhouse.” Energy and Buildings, Elsevier, 19 June<br />

2013, www.sciencedirect.com/science/article/pii/S0378778813003563.<br />

[18] Holloway, James. “Solar Panels Keep Greenhouses Cooler in Summer, Let the Sun through<br />

in Winter.” New Atlas, New Atlas, 13 Jan. 2012, newatlas.com/umla-solargreenhouse/21090/.<br />

[19] Greenhouse Solar Photovoltaic Remodeling,<br />

www.solarbuildingtech.com/Greenhouse_Sunroof_Remodeling/greenhouse_solar_pv_s<br />

unroof_r emodeling.htm.<br />

[20] Storey, Nate. “Low Grade Geothermal Heating and Cooling for Greenhouses.” Zipgrow, 24<br />

Sept. 2013, blog.zipgrow.com/low-grade-geothermal-heating-cooling-greenhouses/.<br />

[21] Storey, Amy. “The 4 Best Ways to Cool a Greenhouse.” Powered by Plenty, 31 May 2016,<br />

university.upstartfarmers.com/blog/the-4-best-ways-to-cool-a-greenhouse.<br />

[22] Vanheems, Benedict. “How to Keep Your Greenhouse Cool in Summer.” GrowVeg, 23 July<br />

2015, www.growveg.com/guides/how-to-keep-your-greenhouse-cool-in-summer/.<br />

[23] Mutwiwa, Urbanus Ndungwa. Cooling Greenhouses in the Tropics. Lap Lambert Academic<br />

Publ, 2011.<br />

[24] Brown, Kyle. “'Digging Deep' Into Greenhouse HVAC.” Cannabis Business Times,<br />

magazine.cannabisbusinesstimes.com/article/july-2016/digging-deep-into-greenhousehvac.aspx.<br />

[25] Parsons, Gene. “Grower 101: Greenhouse Cooling Options By Gene Parsons.” Greenhouse<br />

Product News, Oct. 2007, gpnmag.com/article/grower-101-greenhouse-coolingoptions/.<br />


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Adrienn Santa<br />

[26] OâConnor, Dominic, et al. “A Novel Design of a Rotary Desiccant System for Reduced<br />

Dehumidification Regeneration Air Temperature.” Energy Procedia, vol. 142, 2017,<br />

pp. 253– 258., doi:10.1016/j.egypro.2017.12.040.<br />

[27] Rjibi, Amel, et al. “A Solid-Desiccant Cooling System for Greenhouses.” Proceedings of<br />

2016/53.pdf.<br />

[28] Jones, Tiffany, et al. The Dehumidifier System, 19 Nov. 1996,<br />

435-1.html.<br />

[29] Inhofe, Maggie. Site and Structure: An Exploration of Greenhouse Viablity at SMU. 2016<br />

[30] “4 Greenhouse Insulation Options.” DoItYourself.com, 17 Oct. 2009,<br />

www.doityourself.com/stry/4-greenhouse-insulation-options<br />

[31] “Greenhouse Insulation.” Growers Supply,<br />

www.growerssupply.com/farm/supplies/cat1%3Bgs_greenhouse_building_materials%3<br />

Bgs_insu lation.html.<br />

[32] “Energy Conservation | Solar Greenhouses.” Greenhouse Garden,<br />

Engineering and Technology, pp. 320–325., ipco-co.com/PET_Journal/Acecs-<br />

chem.engr.utc.edu/Webres/435F/Dehumidifier/Dehumid/R5-<br />

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[33] Singh, Devesh, et al. “LEDs for Energy Efficient Greenhouse Lighting.” Renewable and<br />

Sustainable Energy Reviews, vol. 49, 2015, pp. 139–147.,<br />

doi:10.1016/j.rser.2015.04.117.<br />

[34] “Spectrum Graph Mode.” UPRTEK ®, www.uprtek-europe.com/project/uprtek-mk350-<br />

spectrum-graph/.<br />

[35] “Indoor Plant Grow Light Guide.” ACF Greenhouses,<br />

www.littlegreenhouse.com/guide3.shtml.<br />

[36] “Vapor Compression Refrigeration Cycle versus Absorption Refrigeration.” Brighthub<br />

Engineering, 17 Mar. 2010, .<br />

[37] Albert, Steve, et al. “Tomato Growing: Six Requirements for Success.” Harvest to<br />


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Table, 10 Mar. 1970,<br />

www.harvesttotable.com/tomato_growing_six_requirement/.<br />

[38] Metals, Online. “0.25’ Aluminum Plate 6061-T651.” OnlineMetals.com,<br />

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p_cat=60.<br />

[39] “Goodman GSX140181 GSX14 - 1.5 Ton - Air Conditioner - 14 Nominal SEER - Single-<br />

Stage - R-410A Refrigerant.” EComfort, www.ecomfort.com/Goodman-<br />

GSX140181/p26109.html.<br />

[40] “1500 WATT SOLAR MICRO INVERTER KIT.” The Inverter Store,<br />

theinverterstore.com/product/1500-watt-solar-micro-inverter-kit/.<br />

[41] “Steel Pipe.” MetalsDepot® - Buy Steel Pipe Online!, www.metalsdepot.com/steel-<br />

products/steel-pipe.<br />

[42] Specialty, Aquarium. “Royal Exclusiv Red Dragon 5 ECO 25 Watt 4.0m³ DC<br />

Pump.” Aquarium Specialty, www.aquariumspecialty.com/royal-exclusiv-red-dragon-5-eco-25-<br />

watt-4-0m-dc-pump.html.<br />

[43] “TEA Thermostatic Expansion Valve, Complete Valves - Code Number Selector.” Danfoss<br />

Products, products.danfoss.com/productrange/list/refrigeration/thermostaticexpansion-<br />

valves/thermostatic-expansion-valves-exchangeable-orifice/tea/teacomplete-valves/?Conn--<br />

size=1-2-in#/.<br />

[44] “Smith Cooper 304 Stainless Steel 1/2 in. 90° Long Radius Elbow Weld Fittings - Sch 40.”<br />

40-90- long-radius-elbow-weld-<br />

fittings/?gclid=Cj0KCQiA6enQBRDUARIsAGs1YQgYXiM87jjaNVehOP-<br />

Lca_AioLs1t7qSCteZBRSQ_7LlglFliY3H3AaAjtTEALw_wcB.<br />

[45] “2017 Four U Type Electric Heating Tube,U Shape Heating Element, 4u Cartridge Heater<br />

Tube,Liquid Heating Pipe, Tubular Heater Elements From Gdlees, $3.75.” DHgate,<br />

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www.dhgate.com/product/four-u-type-electric-heating-tube-ushape/153428861.html.<br />



Signature and Approval of Mentor:<br />

I have reviewed and approve of this final report.<br />

Ali<br />

Digitally signed<br />

by Ali Beskok<br />

12:53:50 -05'00'<br />

Dr. Ali Beskok, Mechanical Engineering Department Chair<br />


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