Part 2: Electricity and Cost
In part 1, we investigated the growth response of oak leaf lettuce in two methods of nutrient delivery: aquaponics and the Kratky method of passive hydroponics. The differences in weight between plants from one system to the other showed that the Kratky method led to a 71% increase in growth. There was somewhat less difference when comparing the dried weights of the plants, but since consumers typically purchase their lettuce ‘wet’- (ie not dried), we will consider this 71% increase the important number. Next we’ll look at how much energy was used by each of these systems, and more importantly, how much they cost.
To obtain the energy usage, we looked at two places. The first place we looked is the label on any device that is plugged in, which tells you the power consumption in watts (W) while it is running. The second place is to actually measure what the device is using. Surprisingly, these numbers can be quite different.
The labels for the aquaponics system water pump and air blower said they should be using 888 W and 672 W respectively. However when actually measured, these devices combined use only 540 W. How is this possible?
Both of these devices are powered by what are known as induction motors (meaning that they rely on a permanent magnet spinning within a larger electromagnet). The amount of electricity these motors use varies depending on the amount of work they are doing, so with more resistance (and therefore less water and air flow), less energy is consumed. For the water pump, we have the flow rate reduced to roughly half of its maximum capacity; and for the air blower, restriction from air lines, ceramic diffusers, and water depth restrict its flow considerably. This causes both devices to use less electricity than if they were to be run ‘free flow,’ ie with no restriction.
The other devices on both systems – the Happy Leaf Inc. LED lights – use exactly as much power as was stated on their labels (42 W each x 6 lights = 252 W for each grow bed).
The Kratky method system, being that it doesn’t use anything other than the lights, allows us to stop at 252 W. For the aquaponic bed, we divide the 540 W used for the pump and blower by 4 (540 W/4= 135 W), and add that to the 252 W for the lights.
Why divide by 4?
It is necessary to divide by 4 here because we want to approximate the amount of electricity used to operate just the one aquaponic bed, since we are comparing it to only one Kratky method bed. We are also dividing by 4 instead of 3 (one of the grow beds has been removed from the system in order to utilize the Kratky method) since we wanted to compare electricity usage in the system’s typical scenario.
What are watts anyway?
Watts of electricity is just an instantaneous measurement – the energy a device is using at that moment in time. What we really want to know is the energy used for the duration of the experiment, we have to convert that to kilowatt hours (kWh). In order to do this, we need to know how many hours were in the test period, and how many of those hours were devices operational. The pump and blower, for instance, were on 24 hrs/day for the entire experiment, whereas the lights (all connected to the same timer) were on for 16 hrs/day.
To start, we multiply by the number of hours in a day by the number of days in the test period (24 hours/day x 28 days = 672 total hours). We then multiply the number of hours the lights were on each day by the number of days in the test period (16 hours/day x 28 days = 448 total hours). This gives us the number of hours during the test period each set of devices was using electricity. Multiplying each device’s wattage by the number of hours in use and dividing by 1000 gives us kilowatt-hours (kWh) of electricity – a standard unit for defining electricity usage over time, and the number that appears on a utility bill for determining how much you owe the electric company.
At the time of this post (May, 2018), the average cost of electricity in Chicago is $0.075/kWh. Multiplying that out means that the aquaponic system cost $19.50 for the duration of the experiment, and the Kratky system cost $12.70.
Overall, it cost us $6.80, or 53% more to grow the plants aquaponically vs the Kratky method, but this only tells us part of the story. Back in part one, we estimated that the aquaponic system produced 454.4 oz, and the Kratky system 779.2 oz. If we now compare the cost to run each system with how much each system was able to produce, we get the following:
Now the difference is much more stark, as it cost us over 2.5X as much per ounce to grow plants in the aquaponic system vs growing them in the Kratky method system!
In part 3 we will estimate what the cost difference would be if we were to switch the entire farm to the Kratky method, this time not just from an electricity usage point of view, but also materials, supplies and labor as well.