Agriculture & Future Food Systems · Article

Lighting, ventilation and heat transfer

Connect the light supplied to a crop with air exchange, heat removal and water movement in its growing environment.

Preparation and related reading

Shared Foundations

This article helps you

Separate electrical energy, crop light and thermal conditions.
Distinguish circulating air from exchanging it with outside air.
Read a simple sensible-heat calculation within its explicit limits.

Lighting changes more than brightness

Lights turn on above a greenhouse crop, and the air warms. Opening a vent may lower the temperature, but it also changes the exchange of moisture and gases. One equipment action therefore affects several growing conditions.

Begin with the crop's needs and the actual environment. This article follows equipment and transfer pathways; the earlier light and growth articles explain plant responses. A thermostat reading alone does not describe the light reaching leaves or the temperature everywhere in the structure.

Watts are not a plant-light measurement

Electrical power describes how quickly a fixture takes energy from its supply. The light delivered to the canopy also depends on its spectrum, optical distribution, position and losses. Equal electrical power does not establish equal plant light.

A fixture also releases heat through its components and surroundings. LEDs require heat management; they are not heat-free because their light beam differs from another lamp's radiation. Follow the energy within a stated boundary rather than converting all incoming electricity into an assumed immediate leaf-temperature rise.

Air movement can perform different jobs

A circulation fan moves air within the structure and can reduce local differences. Ventilation replaces some indoor air with outside air. Natural exchange depends on wind and buoyancy; fans can drive exchange through planned openings.

The entering air's condition matters. In the worked case, incoming air warms as it crosses the greenhouse, carrying sensible heat out. If incoming air is warmer than indoor air, that same simple pathway cannot supply sensible cooling. Ventilation alone does not set a temperature independently of the outside conditions.

Moisture transfer adds another balance

Evaporation takes energy and moves water into the air. Plant transpiration, wetted cooling equipment and condensation consequently connect thermal and moisture conditions. Ventilation may remove moisture or introduce it, depending on the incoming air.

Evaporative cooling relies on the air's ability to take up more water vapour. Its effectiveness changes with humidity; it is not a fixed temperature reduction available in every climate. The sensible-heat example deliberately omits these effects, so it cannot be treated as a complete cooling-system design.

Check the crop environment

Place temperature measurements where they represent the conditions being controlled, with appropriate protection from direct radiation. Check readings against suitable reference instruments. A sensor near a heater or an unrepresentative opening can steer the wrong response.

Light distribution likewise needs observations across the growing area. Maintenance of fans, openings and lighting affects performance. Coordinated controls should account for the way ventilation, shading, heating and lighting interact, rather than assuming each setting produces its advertised result independently.

Calculate one contribution honestly

The shared energy and heat-transfer principles organise the equipment balance. Distinguish energy entering or leaving from energy stored, and distinguish rates from accumulated quantities. The diagram shows only the stipulated sensible heat carried by a moving air stream.

For a complete assessment include the other relevant paths, operating hours, fan and pump demand, weather and crop outcome. A smaller electricity bill can be useful evidence, but compare the same delivered growing service before concluding that the production system improved.

Follow the stipulated air stream

Original hypothetical contribution, with specified density and heat capacity. Other transfers and crop-temperature response are outside this example.

Read the diagram as text

The idealised steady air stream warms by 10 K while crossing the greenhouse, carrying 1.2 kW of sensible heat out.

Parts

Incoming air: 20 °C: 0.10 m³/s; density 1.2 kg/m³
Air warms by 10 K: 0.12 kg/s × 1.0 kJ/(kg·K) × 10 K = 1.2 kW
Exiting air: 30 °C: Sensible heat only; no humidity change stipulated

Connections

Incoming air: 20 °C → Air warms by 10 K: enters
Air warms by 10 K → Exiting air: 30 °C: leaves

Worked examples

An air stream carries sensible heat

In an idealised steady case, 0.10 m³/s of air enters at 20 °C and exits at 30 °C. Assume density 1.2 kg/m³, specific heat 1.0 kJ/(kg·K), and no humidity change. What sensible-heat rate leaves through warming that air?

Mass flow is 0.10 m³/s × 1.2 kg/m³ = 0.12 kg/s.
The temperature increase is 10 K. Heat carried is 0.12 kg/s × 1.0 kJ/(kg·K) × 10 K = 1.2 kJ/s.
1.2 kJ/s = 1.2 kW. This omits latent heat, fan heating, other transfers and storage.

The stipulated air stream carries 1.2 kW of sensible heat out. This is one contribution, not a crop-temperature prediction or fan-sizing recommendation.

Keep power and energy distinct

An invented lighting installation draws a constant 2.0 kW for 8 hours. What electricity does it use, and what does that result leave unknown?

Electrical energy is 2.0 kW × 8 h = 16 kWh.
This accounts for fixture electricity over the stated period.
Canopy light distribution, crop response and heating/cooling demand remain separate measurements.

16 kWh of fixture electricity; no yield, photon delivery or total climate-system demand is established.

Check your understanding

Does air circulating inside automatically mean fresh outside air is exchanged?
Show answer and reasoning
No.
Internal circulation and ventilation describe different flows.
Does using LEDs eliminate the need to account for equipment heat?
Show answer and reasoning
No.
Electrical energy still has thermal pathways; fixture and room boundaries matter.
Does the 1.2 kW example include energy associated with evaporating water?
Show answer and reasoning
No.
No humidity change is stipulated; it is explicitly a sensible-heat calculation.

Key ideas

Measure crop light and thermal conditions separately from electrical power.
Distinguish air circulation, air exchange and evaporation.
Build the complete balance from defined contributions and measured operating conditions.

References

Sources supporting the explanations and examples in this article.