Why Energy Balance Beats VPD for Greenhouse Control

Most greenhouse control systems use Vapour Pressure Deficit (VPD) as the primary driver for climate decisions. It's intuitive — VPD tells you how much the air 'wants' to pull moisture from the plant. But as a control input, VPD has a fundamental limitation: it's a symptom, not a cause.

Energy drives evaporation

The Plant Empowerment framework establishes a different hierarchy: radiation provides the energy that drives transpiration. VPD is a confirmation signal — useful for monitoring, but not for leading control decisions.

In practice, this means Ghyll's physics engine looks at the energy balance in your greenhouse first. How much radiation is entering? How much is being absorbed by the crop? What's the partition between sensible and latent heat? These questions determine when to ventilate, shade, or fog — not VPD alone.

When VPD misleads

VPD can mislead in several common scenarios. On a cloudy monsoon day with low radiation but moderate VPD, a VPD-first system might still trigger fogging — even though there's no energy driving transpiration. The fog just adds humidity without purpose, increasing condensation risk.

Conversely, on a high-radiation morning with temporarily low VPD (after a cool night), a VPD-first system might delay ventilation — even though the energy load is building rapidly and the crop needs relief.

How Ghyll handles this

Ghyll uses energy balance as the primary input and VPD as a secondary confirmation signal. The agent continuously monitors both, but defers to radiation data for timing decisions. This approach produces more stable crop environments, fewer unnecessary actuations, and better resource efficiency.