In conventional Venlo greenhouses, heating and cooling components are distributed haphazardly throughout the cultivation zone: lower heating rails (Schienenrohrheizung) under gutters, upper grow-tube heating loops, horizontal circulation fans (HAF) hung from roof purlins, and high-pressure fogging lines suspended over plant rows. While functional, this dispersed hardware obstructs PAR light transmission, creates non-uniform microclimate micro-zones, and exposes high-voltage equipment to corrosive foliar humidity.
The Semi-Closed Corridor ATU concentrates all climate conditioning inside an isolated, double-glazed or sandwich-panel service corridor located along the gable end or central drive aisle of the greenhouse. Each climate cell (typically spanning 4 to 8 Venlo bays, 16m to 38.4m wide) is serviced by an ATU module comprising:
In hot and temperate growing regions worldwide, insect-vectored diseases represent the single largest catastrophic financial risk to protected vegetable crops. The introduction of Tomato Brown Rugose Fruit Virus (ToBRFV) and Tomato Spotted Wilt Virus (TSWV)—transmitted mechanically by bumblebees, thrips (Frankliniella occidentalis), and whiteflies (Bemisia tabaci)—has wiped out entire multi-hectare operations.
In open Venlo greenhouses, attempting to exclude microscopic thrips requires installing ultrafine insect netting with a mesh opening of $\le 0.15\text{ mm}$ (approx. 50–70 mesh) across all continuous roof vent openings. However, fluid dynamic drag across such dense screens is catastrophic:
With porosity $\epsilon < 0.40$, net natural ventilation capacity drops by $45\%\text{ to }60\%$. Under summer solar irradiance ($>800\text{ W/m}^2$), greenhouse temperatures spike beyond $40^\circ\text{C}$, causing severe flower abortion, pollen sterility, and crop collapse.
Semi-closed greenhouses eliminate roof insect screens entirely. Instead, the ATU EC fans continually pump filtered air into the greenhouse envelope, establishing a controlled positive static pressure differential:
Whenever roof pressure-relief dampers or worker service doors are opened, the pressure differential forces air outward through the aperture at velocities exceeding $2.5\text{ m/s}$:
Because the maximum flight speed of western flower thrips is only $0.3\text{ to }0.5\text{ m/s}$, and whiteflies fly at $<0.2\text{ m/s}$, insects encountering the outward air plume are violently blown away from the opening. Chemical insecticide applications are reduced by up to $82\%$, while daylight $CO_2$ loss is reduced by $90\%$ compared to open ridge vents.
Air conditioning inside the ATU chamber is governed by the conservation of mass and thermal enthalpy on the Mollier h-x Diagram. When the recirculation air fraction $\alpha$ ($0.0 \le \alpha \le 1.0$) is blended with outdoor fresh air $(1 - \alpha)$, the mixture state point $(T_{mix}, x_{mix})$ lies precisely on the straight line connecting the two parent states:
When high-pressure fogging ($70\text{ bar}$) is activated inside the ATU mixing chamber, the evaporating micro-droplets absorb sensible heat from the air and convert it into latent heat. Because no heat is added or removed from the chamber boundary ($Q_{external} = 0$), the transformation follows a line of constant enthalpy ($h \approx \text{const}$) toward the saturation curve ($\phi = 100\%$):
Where $T_{wb,mix}$ is the wet-bulb temperature of the mixed air and $\eta_{evap}$ is the saturation efficiency ($\approx 0.88\text{--}0.92$ for an engineered ATU). On a hot summer afternoon ($T_{amb} = 34^\circ\text{C}, 35\%\text{ RH}, T_{wb} = 21.2^\circ\text{C}$), the ATU delivers supply air at $22.5^\circ\text{C}$ directly to the crop roots—achieving over $11.5^\circ\text{C}$ of sensible cooling purely through adiabatic evaporation, with zero mechanical compressor power.
Experience interactive 3D WebGL airflow simulation, Mollier h-x psychrometric balance, and 1-click VenloCAD parametric CAD generation:
Supplying conditioned air from the corridor ATU into a 100-meter-long cultivation row without microclimate stratification requires rigorous fluid dynamics. Semi-closed greenhouses utilize continuous extruded polyethylene (PE) air distribution tubes—known in German engineering practice as Folientransportschläuche—suspended directly beneath hanging crop gutters.
| Duct Diameter ($\Phi$) | Typical Row Length | Recommended Airflow | Inlet Velocity ($v_{in}$) | Target Canopy Air Velocity |
|---|---|---|---|---|
| $\Phi 600\text{ mm}$ | $40\text{--}60\text{ m}$ | $4,500\text{--}6,000\text{ m}^3/\text{h}$ | $4.4\text{--}5.9\text{ m/s}$ | $0.8\text{--}1.2\text{ m/s}$ |
| $\Phi 750\text{ mm}$ (Standard) | $60\text{--}85\text{ m}$ | $7,500\text{--}10,500\text{ m}^3/\text{h}$ | $4.7\text{--}6.6\text{ m/s}$ | $1.2\text{--}1.6\text{ m/s}$ |
| $\Phi 800\text{ mm}$ (High-Flow) | $80\text{--}110\text{ m}$ | $10,000\text{--}14,000\text{ m}^3/\text{h}$ | $5.5\text{--}7.7\text{ m/s}$ | $1.4\text{--}1.8\text{ m/s}$ |
As air travels down the closed poly-duct, friction along the plastic walls causes pressure loss. Simultaneously, as air discharges through laser-perforated side holes, the longitudinal mass flow decreases, causing duct air velocity to decelerate ($v_2 < v_1$). According to Bernoulli's principle, this deceleration converts dynamic velocity pressure into static pressure—a phenomenon termed Static Pressure Regain:
If holes were drilled with uniform spacing and diameter, the static pressure would rise toward the end of the row, discharging up to 40% more air at the far end than at the corridor inlet! To guarantee an identical air discharge velocity ($1.2\text{ to }1.8\text{ m/s}$) along all 100 meters, hole perforation density (hole area per linear meter) is mathematically tapered using Darcy-Weisbach flow modeling:
Traditionally, designing a semi-closed greenhouse required disjointed engineering steps: structural steelwork drafting in generic BIM tools (AutoCAD, Revit), HVAC duct calculations in independent spreadsheet models, and manual quotation compilation. Inwoovation Lab has unified this workflow through VenloCAD 3D—a browser-native WebGL parametric design platform.
The aerodynamic and psychrometric equations powering VenloCAD 3D are fully accessible via the OpenCEA Python micro-framework. Developers and climate computer engineers can query the /v1/climate/atu-mollier REST API endpoint to programmatically retrieve enthalpy mixing points, wet-bulb cooling limits, and duct static pressure regain profiles directly into automated climate computers.
The table below compares commercial high-wire greenhouse topologies across energy, climate precision, pest control, and economic yield metrics:
| Metric | Standard Open Venlo | Insect-Screened Venlo | Semi-Closed ATU Venlo (Vol. 74 SOTA) |
|---|---|---|---|
| Insect Ingress Protection | None (Pests enter freely) | Moderate (0.15mm mesh) | Absolute (+20~25 Pa Outward Barrier) |
| Summer Natural Ventilation Loss | $0\%$ (Unrestricted) | $-45\%\text{ to }-60\%$ (Severe drag) | $0\%$ (Mechanically Driven ATU) |
| Daylight $CO_2$ Retention (PAR > 800) | $420\text{ ppm}$ (Vents open) | $450\text{ ppm}$ (Vents open) | $900\text{--}1,200\text{ ppm}$ (Vents 100% Sealed) |
| Summer Canopy Temperature Rise | $+4\text{ to }+8^\circ\text{C}$ above ambient | $+7\text{ to }+12^\circ\text{C}$ (Heat trap) | $-2\text{ to }-6^\circ\text{C}$ (Adiabatic Flash Cooling) |
| Crop Yield (Tomatoes, kg/m²·yr) | $65\text{--}75\text{ kg/m}^2$ | $55\text{--}65\text{ kg/m}^2$ (Heat stress) | $88\text{--}105\text{ kg/m}^2$ ($+25\text{ to }+35\%$) |
| Annual Pesticide Spray Applications | $18\text{--}24\text{ cycles/yr}$ | $12\text{--}16\text{ cycles/yr}$ | $2\text{--}4\text{ cycles/yr}$ ($-82\%$) |
| Electrical Fan OPEX | $1.5\text{ kWh/m}^2\cdot\text{yr}$ | $2.0\text{ kWh/m}^2\cdot\text{yr}$ | $18\text{--}25\text{ kWh/m}^2\cdot\text{yr}$ |
| CAPEX Premium over Standard | Baseline (€0) | $+€12\text{--}€18/\text{m}^2$ | $+€48\text{--}€68/\text{m}^2$ |
| Payback on Additional CAPEX | N/A | Negative (Yield dropped) | 3.1 to 4.2 Years |
Air treated inside the ATU chamber must never enter the poly-ducts in a supersaturated state. The climate control algorithm modulates high-pressure fogging to maintain supply air relative humidity at $\le 92\%\text{ RH}$. Furthermore, high-pressure 70-bar nozzles produce micro-droplets ($10\text{--}15\text{ }\mu\text{m}$) that flash-evaporate within $1.2\text{ seconds}$ inside the turbulent fan mixing plenum, ensuring completely dry air delivery into the perforated tubes.
Yes. Retrofitting involves constructing an external modular ATU corridor along the end gable, sealing ridge vent drive shafts with brush seals, installing under-gutter poly-ducts, and updating the climate computer software. As long as post height is $\ge 5.5\text{ m}$ to accommodate duct geometry beneath crop gutters, existing Venlo glasshouses can achieve 90% of the performance of a newly constructed semi-closed facility.
In winter, the ATU operates in 100% recirculation mode ($\alpha = 1.0$) with outdoor dampers tightly sealed. The EC fans continuously draw warm air that naturally rises to the ridge and push it back down to the crop roots through the floor ducts. This completely eliminates vertical thermal inversion, reduces attic thermal loss through the roof glass, and decreases winter heating fuel consumption by $18\%\text{ to }28\%$.