Enthalpy

Enthalpy in greenhouse practice is the energy content of moist air, the sum of sensible heat in dry air and latent heat in water vapor. It gauges ventilation effectiveness and evaporative cooling needs, typically expressed as kJ/kg or kJ/m3.

Enthalpy is most commonly determined from psychrometric data, combining the dry-bulb temperature (sensible heat) and humidity ratio (latent heat) to yield energy content. In the greenhouse, this metric clarifies how much energy can be removed by ventilating air or how much cooling is achievable through evaporative methods, depending on the indoor outdoor enthalpy difference. It integrates with dew point, relative humidity, and air density, since changes in humidity or density alter kJ/kg and kJ/m3 values. Practical use includes sizing fans, misting systems, and heat exchangers and guiding control algorithms that optimize energy balance while protecting transpiration and disease risk. Synonyms and abbreviations: specific enthalpy, enthalpy of moist air, energy content of air, total heat content, h (symbol), specific enthalpy (h).

Enthalpy integrates climate control decisions across ventilation, dehumidification, humidification, and evaporative cooling. In practice, greenhouse operators use indoor–outdoor enthalpy differences to size exhaust fans and determine when misting will meaningfully reduce air temperature without pushing humidity to levels that promote disease. Because enthalpy fuses temperature and moisture content, it helps predict energy removal by air exchange and water vapor addition by misting. Sensor networks measuring dry-bulb temperature, humidity ratio, and pressure feed into a psychrometric model, enabling control systems to target specific enthalpy ranges suited to crop stage and season. This has implications for disease risk management, as maintaining high RH with low vapor pressure deficit can raise foliar pathogen pressure; conversely, lowering indoor enthalpy can improve leaf transpiration and water-use efficiency. The relationship to related metrics, such as dew point and humidity ratio, means that small weather changes can shift the energy balance quickly; operators therefore favor automation that responds to real-time enthalpy rather than static temperature targets. The notes also emphasize practical limitations: enthalpy calculations rely on accurate inputs and corrections for pressure or altitude, and measurement errors can mislead control actions. Overall, enthalpy-centric climate strategies support crop quality, energy efficiency, and the feasibility of advanced automation in modern greenhouses.

1 kg dry air at 20 °C with 5 g water vapour has enthalpy ≈ 32.5 kJ/kg and illustrates the energy content of air used in a calculation during a greenhouse energy audit; a 2,000 m2 tomato greenhouse uses the indoor–outdoor enthalpy difference to size exhaust fans and adiabatic cooling strategy for hot afternoons; a humid coastal greenhouse maintains a limited net cooling effect from ventilation and relies on misting to reach target leaf temperature without excessive RH; a winter lettuce house employs a heat exchanger to recover energy when the outdoor enthalpy is favorable, reducing fuel use; an automated climate system tracks constant-enthalpy lines on the psychrometric chart to schedule humidification cycles and venting sequence.

Enthalpy sits at the center of psychrometric climate control in greenhouses, guiding how setpoints are chosen and how algorithms decide when to vent, humidify, or dehumidify. The indoor–outdoor enthalpy difference informs energy transfer and the effectiveness of adiabatic cooling, influencing both energy use and crop health. Operators rely on accurate dry-bulb and humidity readings to compute enthalpy and to avoid conditions that favor foliar diseases or uneven transpiration. In practice, enthalpy-aware control improves crop quality, reduces energy costs, and enhances automation performance by tying ventilation rates to measured energy content rather than temperature alone. These concepts underpin setpoint strategies, project ROI, and risk mitigation, with psychrometric charts providing a visual framework for planning humidification, dehumidification, and adiabatic cooling. The references provide practical guidance and case studies for applying this in real greenhouses. Sources — Books: Greenhouse Cultivation Manual — GreenhouseCultivationExpert. Sources — Online: https://extension.psu.edu/psychrometric-chart-use/; https://pdhonline.com/courses/m135/m135content.pdf; https://ceac.arizona.edu/sites/default/files/asae_-_heating_ventilating_and_cooling_greenhouses.pdf; https://msu-prod.dotcmscloud.com/floriculture/uploads/files/Section%20_3.pdf