A Development of a PV-Powered Aquaponics System

A Development of a PV-Powered Aquaponics System


Gunnar Shaffer, Cindy Achieng, Francisco Quezada, Chris Soenksen, Tim Gilmour, and Junseok Song
Division of Engineering
John Brown University
Siloam Springs, AR, USA
David Ready
Hidden Creek Opportunity Center
Little Rock, AR, USA




Abstract—Aquaponics systems use both aquaculture (raising aquatic animals such as fish) and hydroponics (cultivating plants in water) in a symbiotic arrangement to grow food. The techniques produce high food yield, but are nontrivial to implement, needing careful monitoring of pH, temperature, and other parameters. This paper presents the design and initial prototype testing of a complete photovoltaic-powered aquaponics system developed for the Hidden Creek Opportunity Center (HCOC) in Little Rock, AR. The goal of this system was twofold: to provide a sustainable business and educational opportunity for the clients of the HCOC, and to prototype a customizable unit which could be deployed worldwide to provide food and business opportunities in developing nations. The system was designed to meet multiple criteria: off-grid photovoltaic power (for worldwide use), full greenhouse enclosure (for longer growing season), modular components (for easy repair or customization), automated controller (with data collection and internet connectivity to aid remote troubleshooting), vertical column growing tubes (to minimize environmental footprint), and total system cost of less than $3,000. The design and data reported here will be beneficial for the next generation of inexpensive, robust, sustainable, and easy-to-use aquaponics systems.
Keywords—Aquaponics, hydroponics, photovoltaic, energy storage.
Introduction
An aquaponics system is a system that uses both aquaculture (raising aquatic animals such as fish) and hydroponics (cultivating plants in water) in a symbiotic environment to create food. The plants in the hydroponics filter the waste from the aquaculture to cultivate a variety of plants [1], using bacteria that colonize the plant roots. An aquaponics system may be beneficial to communities where there are limited spaces to grow plants. As many of rural areas in developing countries have limited access to electricity, this paper explores using photovoltaic (PV) modules to power necessary components to operate an aquaponics system.
System Design
The key parameters that must be regulated for the design of successful aquaponics operation are the water temperature, the pH, and the levels of ammonia/ammonium, nitrite and nitrate. The fish are much more sensitive to changes in temperature and chemical levels in the water than are the plants.
Chemical and thermal constraints
As ammonium (NH4) is toxic to fish, the levels of NH4 need to be kept under control for the fish to live. Fish have an internal pH of 7.4 [1]. Generally, as a rule of thumb, the whole aquaponics system needs to have a pH level between 6.8 and 7.0 [1]. The presence of useful bacteria in the water helps to maintain the pH at this optimum level.
Bacteria within the system use elements like carbon, oxygen, nitrogen, phosphorous, potassium, and calcium in order to catalyze chemical reactions. In turn, they get the energy they need to grow and thrive. The bacteria are vital to the system because they enable the nitrogen cycle to work which turns the ammonia produced from the fish gills into nutrients for the plants. When ammonia (NH3) is dissolved and absorbed in the water, it is consequently converted to nitrates through a process called nitrification. In this process, nitrosomanas bacteria convert ammonia to nitrites (NO2), and nitrospira bacteria oxidize nitrites to nitrates (NO3) as shown in Fig. 1. Nitrates enable healthy plant growth.
When ammonia is dissolved in water, the following chemical reaction occurs.
H3O+ + NH3 H2O + NH4+ (1)
Ammonia (NH3) is converted to ammonium (NH4+), and the degree to which NH3 is converted to NH4+ is determined by water temperature. If the pH is low, NH4+ is formed (which is harmless to the fish); but if the pH is high, NH3 is formed (which is toxic to the fish), demonstrating the reason the pH needs to stay within the 6.8 to 7.0 range [1] (see TABLE I).



Fig. 1. Nitrogen cycle in fish tank [1]

System Sponsorship and Specifications
The Hidden Creek Opportunity Center (HCOC) in Little Rock, AR is an employment center interested in implementing a sustainable business and educational system through the use of aquaponics in developing nations in the future. The project specifications were as follows:
The aquaponics system needed to be powered by PV modules with battery backup to be independent from the local power grid.
The system needed to use a vertical design to minimize the environmental foot print.
The water temperature could not go above 90°F or below 55°F (optimal: 84°F). The system pH could not change more than 0.2 per day and had to be kept within the range of 6.5-7.5.
All materials were required to be food-safety grade.
TABLE I. Change in pH with a change in water temperature [1]
pH
20°C (68°F)
25°C (77°F)
6.5
15.4
11.1
7
5
3.6
7.5
1.6
1.2
8
0.5
0.4
8.5
0.2
0.1

The system needed to be modular so that it could be easily upgraded or minimized in order to meet the needs and specifications of users in the different parts of the world.
The system needed to include an automated controller which would be able to provide ability to collect data and monitor data remotely so that these could be used in system regulation and troubleshooting.
The system needed to use a greenhouse to aid climate control, thereby extending the growing season.
The system needed to cost less than $3,000.
Physical Design


Fig. 2. Physical design of the proposed aquaponics system (side view)
Fig. 2 shows the side view of the physical design (the top view is shown in Appendix). The selected design housed the pump in the bottom of the 100 gallon fish tank, with 0.5 in. diameter tubing to transport the water through the system. A dripper valve was secured into the tubing directly above each of the 9 plant towers, allowing 7 gal/hour of water to flow through each tower, thus adhering to the rule of thumb that the total amount of water in the fish tank should circulate through the system once every hour [1]. The water tubing and the plant towers were connected to the wood frame with hangers, allowing easy replacement or removal to bring a growing tower to market. Each tower had the capacity to hold 22 plants.
The fish tank was buried in the ground so the top of the tank was at ground level, allowing the ground to help regulate the temperature and using gravity to return the water back to the fish tank through gutters so that only one pump was needed in the system.
TABLE II. Power and energy demand
Device
Power (W)
Hours
Wh/day
%
Pump
25
6
150
64
Aerator
1.5
24
36
15
Electronics
2
24
48
21
Total Demand


234
100

The fish used were tilapia because they are hardy, edible, omnivorous, and have low oxygen requirements. The recommended water temperature for the fish was 60-95°F [1]. In addition, the medium inside the grow towers was chosen to be hydroton because it is lightweight, inexpensive, and neutral in pH. Moreover, hydroton is also more eco-friendly than synthetic materials because it is not made from petroleum.
PV-Energy Storage System Design
In order to provide energy even when there is no insolation, e.g., during nighttime, a PV-energy storage system was designed to operate components that use electricity. TABLE II shows the power and energy demand for the operation of the proposed aquaponics system. 12 V was chosen for the system voltage, which would give the maximum steady-state current of 4.58 A and the desired DC input value of 19.5 Ah/day as shown in (2). DC input value (Ah/day) for the load was used to suggest the size of energy storage needed for the aquaponics system's operation.

Desired DC input = 234 Whday12 V=19.5Ahday (2)

In order to design the system that would be able to operate in the worst month in terms of insolation, the peak sun hours in December (location: Siloam Springs, AR)—2.9 hours/day—was used. According to [2], the number of storage days that would provide 95% availability for 2.9 peak sun hours is approximately 5, which means the energy storage would need to store a 5 day amount of energy that can power the aquaponics system without any insolation. 95% was chosen by authors in order to simplify the calculation; however, this number could be arranged in a way that meets users' desired availability, e.g., 90% availability would significantly lower the required energy storage capacity.
As (3) suggests, the system needs to have a 97.5 Ah energy storage in order to meet 95% availability.

Energy storage = 19.5Ahday ×5 days=97.5 Ah (3)

In addition, as the maximum depth of discharge (MDOD) is approximately 80% for lead-acid batteries [2] and assuming that the capacity rating at 25°C is 97% at C/20 rate, the following equation can be used to find the nominal battery capacity.

Nominal capacity = 97.5 Ah0.80 × 0.97=125.6 Ah (at 12 V) (4)

As the capacity of energy storage was determined using the December peak sun hours and desired availability, the desired capacity of PV power was evaluated so that more than the required amount of energy is supplied from PV modules. An ALT 120-12P [3] 120 W PV module was used for the system design. This PV module has a maximum power point (MPP) voltage of 18 V and MPP current of 6.67 A [3]. Assuming the de-rating factor (DF) and coulomb efficiency (η) are both 90% and PV module is operating at MPP, the required Ah per string can be calculated with the value of MPP current (IMPP) and peak sun hours using (5).

Ah per string = IMPP×peak sun hours × η×DF (5)

With 15.67 Ah per string from (5) and the 19.5 Ah/day desired DC input from (2), the number of the required PV module parallel strings (N) can be identified using (6).

Number of parallel strings (N) = Desired DC inputAh per string (6)

Equation (6) results 1.24 parallel strings, which means that the system needed at least 2 parallel strings of PV module(s). Moreover, as the MPP voltage (18 V) is greater than the system voltage (12 V), there was a need for only one PV module per string. Thus, two modules were needed for the whole aquaponics system to supply energy for the system operation.
The above calculations were confirmed with the calculation of the energy storage output. In order to determine the energy storage output, (7) can be used.

Energy storage output = PV output×η (7)

where PV output can be found as follows.

PV Output = N×IMPP×peak sun hours×DF (8)

Using the aforementioned values, (7) and (8) indicate that the energy storage output would be 31.3 Ah/day (=2×6.67×2.9×0.9×0.9). As the result shows, two PV modules provide a greater amount of energy (31.3 Ah/day) than the required energy (19.5 Ah/day) shown in (2).
In order to meet the required capacity for energy storage (125.6 Ah) mentioned in (4), two 12 V 100 Ah Trojan deep cycle AGM batteries [4] were chosen for the system. Deep cycle batteries were selected since they can be charged and discharged with relatively small loss of efficiency and long lifespan, e.g., 80% depth of discharge (DOD) is expected after 2,800 cycles of charge and discharge.
Controller
As Fig. 3 shows, the control system used a Beaglebone Black (BBB) [5] running custom Python code as the master controller, with an Arduino Mega 2560 [6] also used to assist with interfacing with the sensors. The sensor data was read by the Arduino and transmitted by a serial connection to the BBB. The BBB then stored all data into a CSV file. If any readings were outside their proper ranges, the BBB sent an "alert" email to the user, using an external email server. The BBB can also be used in the future to run a web server and provide real-time monitoring of the data, but this feature has not yet been implemented.
The Arduino module also showed the current status and readings of the system on a 20 character 4 line LCD module. The sensors connected to the Arduino were an Atlas Scientific pH sensor [7], a TSL2561 luminosity sensor [8], a DHT22 air temperature and humidity sensor [9], and a waterproof water temperature sensor as shown in Fig. 4.


Fig. 4. Assembled control system unit with sensors
Experimental Results

Fig. 3. Control system unit for the proposed aquaponics system
A theoretical analysis and design were verified through the constructed aquaponics system prototype. The prototype was built in Siloam Springs, AR with less than $3,000 material cost and was tested from February to April. This time of the year was chosen for the relatively warm weather; however, there were a few times when the air temperature was below freezing during which some fish were not able to survive.
Figures 5 through 8 show the collected data sets of pH, ammonia, nitrite (NO2), and nitrate (NO3), respectively as these were separately monitored to ensure a livable environment for fish. In addition, Fig. 9 shows the energy storage capacity throughout the operation of the aquaponics system. As the prototype was tested during the spring season of the year, the


Fig. 5. Collected pH data


Fig. 6. Collected ammonia data


Fig. 7. Collected nitrite (NO2) data


Fig. 8. Collected nitrate (NO3) data


Fig. 9. Energy storage capacity during operation


Day 1 Day 11 Day 18 Day 25

Fig. 10. Growth of a lettuce (Day 1 shows the lettuce which
was transplanted after spending 3 weeks in the growing bed)



Fig. 11. Aquaponics system prototype built in Siloam Springs, AR


energy storage capacity stayed relatively high during the operation. These levels may go down when the prototype is used during the winter season; however, the energy storage capacity would be greater than enough to operate the system, as the design was performed for the worst case scenario.
Figure 10 shows the lettuce grown with the proposed aquaponics system. Various vegetables, such as lettuce, were tested with vertical plant growth towers. Plants were originally germinated and grown in a growing bed for a few weeks before being transplanted to the growth towers. As shown in Fig. 11, a greenhouse was utilized to protect the fish tank from various weather conditions. In particular, avoiding rain is critical, since rain can greatly impact pH, which can cause problems in controlling the water parameters. While the greenhouse somewhat reduced the interior insolation level, there was no issue with the plants' growth during the operation.
Conclusion
This paper presented the technical design and experimental results of a developed PV-powered aquaponics system that cost less than $3,000. The focus of the design was to make a system requiring minimal labor and maintenance since this system may be used in the future for implementing a sustainable business and educational system in developing nations. The PV aspect of the design was chosen in order to make the system self-sufficient in terms of a power source by not relying on the power grid, as well as to provide a renewable energy component for sustainability and limit the long-term system operating cost. The addition of an energy storage system provided back-up for the entire system in time of minimal or no insolation.
A working prototype of the system was built, and showed successful reduction of the nitrite levels during the initial startup period. Healthy vegetables were successfully grown in the grow-towers, and after an initial adaption period where several fish died, the fish were able to successfully survive in the tank.
One limitation of our current work is that due to time constraints we were not yet able to take long-term recordings using the Beaglebone controller, measuring system parameters such as temperature, pH, insolation, and battery voltage and make them available over the network. This network presentation feature should be readily feasible, and is the logical next step for this research project. Another useful future development would be to commercialize the system as a set of separate interconnected modules, so that people could buy and operate an arbitrary number of fish tanks, grow towers, pumps, and other modules as part of a single connected system.
In summary, a complete PV-powered aquaponics system has been developed and begun initial testing. The system holds promise in making aquaponic farming more widely available in areas without a reliable power grid and without access to chemical measurement labs, such as developing countries. This may provide better local food access to rural communities, and at the same time increase employment opportunities for those communities, as a dual humanitarian application of engineering.
Acknowledgment
The authors would like to thank the HCOC for their financial support to conduct this project.
References
S. Bernstein, Aquaponic Gardening: A Step-by-step Guide to Raising Vegetables and Fish Together. Gabriola Island, BC, Canada: New Society Publishers, 2011.
G. M. Masters, Renewable and Efficient Electric Power Systems, 1st ed. Hoboken, NJ: John Wiley & Sons, 2004.
http://www.altestore.com/mmsolar/Others/ALT120-12P_alte-solar-modules-spec-sheet.pdf [Accessed: Nov. 14, 2014]
http://www.trojanbatteryre.com/pdf/agm_trojan_productlinesheet.pdf [Accessed: Nov. 14, 2014]
http://beagleboard.org/Products/BeagleBone+Black [Accessed: Nov. 14, 2014]
http://arduino.cc/en/Main/arduinoBoardMega2560 [Accessed: Nov. 14, 2014]
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https://www.sparkfun.com/datasheets/Sensors/Temperature/DHT22.pdf [Accessed: Nov. 14, 2014]



















































VII. Appendix




































Fig. A-1. Physical design of the proposed aquaponics system (top view)