Photon Use Efficiency (PUE) & Electric Power to Fresh Biomass Kinetics in Vertical Farming

1. Energy Economics of Artificial Lighting

In vertical plant factories (PFAL), electrical lighting accounts for 60% to 75% of total variable operating costs. Maximizing economic viability requires engineering the conversion cascade from grid electricity (\(kWh\)) to emitted photons (\(\mu mol\)), photon interception by canopy (\(mol/m^2\)), and ultimately harvestable fresh biomass (\(g\)).

2. Mathematical Formulations of PUE and Biomass Yield

Benchmark performance metrics across commercial vertical farm crops:

• Butterhead / Romaine Lettuce (Lactuca sativa): Target PUE = 6.5 – 8.5 g fresh weight / mol PAR. With modern 3.0 \(\mu mol/J\) LED fixtures, energy yield reaches 70 – 90 g/kWh.
• Sweet Basil (Ocimum basilicum): Target PUE = 4.0 – 5.5 g fresh weight / mol PAR, heavily dependent on secondary metabolite accumulation and essential oil synthesis.
• Microgreens (Brassica spp.): High packing density allows PUE up to 9.0 – 12.0 g/mol due to short 10-day crop cycle and high cotyledon leaf area index (LAI).

3. Canopy Light Interception and Dynamic Dimming

During the early propagation and seedling stage, canopy coverage is under 20%, resulting in 80% of emitted photons wasting energy on empty floor space. Implementing dynamic LED dimming or zoned photoperiod scheduling scales PPFD linearly with canopy expansion, reducing total electrical expenditure by up to 28% per growth cycle.