A prototype of a self-sufficient greenhouse which uses photonics to increase its efficiency has been made for use in an European project called PhabLabs 4.0. PhabLabs 4.0 is a project which aims to achieve a larger and better skilled photonics workforce with great innovation capacity to support the revolution in digitization. In the future people will be able to participate in a competition for building small sized greenhouses with added photonic component to increase efficiency. The prototype will serve as an example design.
In this intructable you'll learn how the greenhouse was build and how to build it yourself.
Step 1: Step 1: Building the Greenhouse
The final design for the greenhouse prototype is based on the SOCKER greenhouse by IKEA. The area on which the plants are grown has been tilted to an angle of 30 degrees. This way the area receives more sunlight per unit of area compared to when the area is horizontal.
The tilted area consists of three ridges so the cups in which plants are grown can be distributed over three different height levels. The dimensions of the ridges have been made based on the small dishes used to grow the samples during the experiment.
The choice has been made to place the LEDs pretty close to the samples. The closer the LEDs are placed, the higher the lux received by the plants. Measurements concluded that a maximum distance to the samples of 6cm was enough to use 1 LED per circular area with a 3,2 cm diameter. Placing the LEDs closer to the sample means more LED bars are needed as the illuminated area is smaller. However, the advantage is this uses a lot less current when the LEDs are turned on.
The LED bars are made as thin as possible to avoid the blocking of incoming sunlight as much as possible. The little bit of shading which is produced by the LED bars outweighs the importance of energy conservation during the nighttime. The charging system could not yet be fully connected due to some components being delivered too late.
The angle of the solar panel array can be adjusted by placing it in a different indent on the support. These indents have been made so the solar array can be set perpendicular to the average inclination of the sun for each month of the year. This way the solar panels can be set in a position which maximises the energy output.
Step 2: Step 2: the Electrical Circuit
If the solar cells were able to run at peak performance throughout 12 hours of the day this would generate 4,56 Ah. To make sure the system will not be overcharged a battery bigger than 4,56 Ah must be used. The solar cells will never run for 12 hours at peak performance, but these numbers have been chosen to add some room for safety due to overcharging the battery.
For the current design a total of 84 red LEDS need to be powered. The battery has a voltage of 12V, used to power fourteen strings of six LEDs each. For each string of LEDs a current draw of 20 mA is required resulting in a total current draw of 280 mA. When the battery is charged to 4,56 Ah is able to power these LEDs for 16,3 hours. This number should be halved to get the expected number of hours the LEDs are able to light up after a clear summer day.
In less ideal light situations like during winter time, the solar panel will generate much less energy. In the Netherlands December is the worst month to generate solar energy. The total energy generated in December is only about 15 - 20% of the energy generated in June.. When taking 15% of 4,56 Ah this results in only 0,684 Ah. This is enough energy to run all LEDs for 2,85 hours. This is a very optimistic estimation, and in reality the actual hours of the lights can be lit up shall be closer to half this value.
Charging the 12V battery requires a constant voltage of roughly 14,4V. To avoid any current leakage from the batteries back to the solar cells a Schottky diode of is added. To make sure the solar panels deliver a constant output voltage an adjustable step-up converter is added into the circuit. This converter is able to change the variable input voltage delivered by the solar panels into a constant output equal to the sum of 14,4 V and the forward voltage of the Schottky diode.
To keep the diagram well-organized, only one string of three LEDs is depicted, while the actual number would be fourteen strings of six red LEDs each. The resistance value for each string of LEDs is 82 Ω. These strings have been successfully tested with a power supply in the voltage range of 11,5 V to 14,5 V. The LEDs did not burn out, but a slight reduction in intensity was observed on the lower end of the voltage range. The actual magnitudes of the resistance values for both voltage dividers and the LDR could not be tested due to the delivery of some components being delayed by four weeks, so these values are not displayed in the diagram.
A microprocessor has been added to control the LEDs and the charging of the battery. This microprocessor is only able to handle a voltage up to 5V. A second voltage converter is added to make sure the voltage on pin 8 of the microprocessor will never exceed 5V. Connected to this node is a voltage divider. This voltage divider is made by using a Light Dependent Resistor (LDR) and a normal resistor. The LDR is used to measure the intensity of the light of the surroundings. The resistance of the LDR will be higher in a dark environment, causing the voltage at the middle of the divider to change.
The other voltage divider connected to pin 5 is used to monitor the charge level of the 12V battery, which would be around 14,5 V when fully charged. When both the LDR is placed in a dark environment and the measured battery voltage is above 12 V, the microprocessor will give a high signal output on the base of a transistor. In this scenario the transistor is able to conduct a current. This means a current will be able to run through the LEDs towards ground, causing the LEDs to light up. The programmed microprocessor has been positively tested on a breadboard, so it should work correctly when connected into the circuit.