With great power comes great… cost. In the case of vertical farming, it’s the cost of electricity - one of the largest expenses for operating an indoor or vertical farm. A method that we’ve used to mitigate this for the Glens Falls project is a time of use meter. Below, I will break down the cost savings for a single light. For context, National Grid is the power supplier for our area. We are also using lighting supplied by Fluence.
Before we begin, here are a list of definitions that will help as we go through this article:
Daily Light Integral (DLI) – Amount of light a plant receives in one day Photosynthetic Active Radiation (PAR) – Amount of light available for photosynthesis, which is light in the 400 to 700 nanometer wavelength range Photosynthetic Photon Flux (PPF) – Amount of PAR light that is emitted from the light Photosynthetic Photon Flux Density (PPFD) – Amount of photosynthetic photons that are actually impacting the grow area |
A single Fluence by Osram Razr-x LED takes 90W to operate. To determine how long a light needs to be on we need a DLI formula and some conversions. Young lettuce requires a DLI of ~10 mol/m-2/day while adult heads can receive upwards of ~17 mol/m-2/day. Each light will have a PPF listed. For the Razr-x light the PPF number is 210 μmol/s.
Now we need to go from PPF to PPFD. PPF is the total output of the lamp, but not all of this light reaches the canopy. The dimensions of the canopy need to be taken into account for the density portion of PPFD as well. With an output of 210 PPF, length of 4’, width of 2’ and a 90% efficiency of light reaching the target the PPFD is 254.30 μmol/m2/s). The formula for DLI is (PPFD x light hours per day x 3600) / (1,000,000). To achieve a DLI of 10 mol/m-2/day with a Razr-x the light would need to be on for 10.92 hours. To achieve a DLI of 17 mol/m-2/day the light would need to be on for 18.57 hours. While this may seem complicated, it is rather simple for an indoor farm. For a greenhouse one would need to average the light level received throughout a day and then adjust seasonally to further calculate necessary levels of supplemental light.
Now that we know how long the light needs to be on (18.57 hours) let us calculate the potential savings of a time of use meter. National grid incentivizes off-peak energy use to help them balance the grid load. Typically, a flat rate for our area of 6.440 cents per kWh is charged. So, if we had our single light on for 18.57 hours a day, consuming 90 W per hour, it comes to $3.28 a month. For a time of use meter, National Grid charges slightly more for peak power (8.007 cents), but significantly less for non-peak power (1.088 cents). Peak power is defined as 7am to 11pm, while non-peak is 11pm to 7am. For us needing light 18.57 hours we are able to have the light off for 5.43 hours of peak time. The 8 hours of non-peak would cost $0.24 a month, while the remaining 10.57 would cost $2.32 a month, together this amount is $2.56 to run our single light for a month. This is a $0.72 reduction per month or 24.7% reduction. While this may seem small it is only a single light. We currently have 48 lights with room to expand and add an additional 64 lights making it 112 total. This would equate to a monthly savings of $80.64. A breakdown of the cost can be found below
Hours of Light Use @ Peak Time/month |
10.57 |
Cost of Light Use @ Peak Time/month |
$2.32 |
Hours of Light Use @ Non-Peak Time/month |
8.00 |
Cost of Light Use @ Non-Peak Time/month |
$0.24 |
Total Cost of Light Use (for one light)/month |
$2.32 |
Total Cost of Light Use Without Time of Use Meter (for one light)/month |
$3.28 |
Max Number of Fluence Lights Used/month |
112 |
Total Cost of Light With Time of Use Meter (for max num of lights)/month |
$259.84 |
Total Cost of Light Without Time of Use Meter (for max num of lights)/month |
$367.36 |
For young seedlings that only require ~10 DLI, the savings is even greater. For a normal rate being on 10.92 hours a day, it would be $1.93 per month. If we use a time of use meter, the 8 hours of non-peak would be $0.24 a month, combined with 2.92 hours of peak which is $0.64 a month for a total of $0.88 or a 74.73% difference. For our planned 112 lights that would mean a monthly savings of $117.60.
While this only looks at lighting, it can also be applied to the other energy consuming entities. For example, turning off the lights during peak times when the sun is out and the room will be the hottest is a great practice. Not only will this reduce the amount of time the HVAC is running on peak, but it will instead have it run more frequently during off peak. We also can stage equipment that can come on during non-peak. An example of this is our RO system. It can use a timer to make a stock of RO water from 11pm-7am and store this water for use during peak. All of these techniques collectively can achieve significant cost savings and increase profitability for the project.
You can follow more insights from our partnership with Glens Falls and others committed to the Glens Falls Vertical Farm Public Pilot here.
By Josh Fabian, Glens Falls Farm Manager