Greenhouse Ventilation and Cooling
Greenhouse Shading
Ventilation alone is frequently insufficient to control the greenhouse temperature, especially during warm months. It should be recognized that many of our greenhouse crops perform exceptionally well in full outdoor sunlight; therefore, shade is usually provided for temperature control rather than for reducing the light delivered to the plants. There are two primary options for providing shade: greenhouse shading compounds (i.e., whitewash) or shade cloth either used externally or as movable refractive screens or curtains inside the greenhouse. Each of these options has distinct advantages and disadvantages, which warrant consideration when deciding how to reduce light and temperature in the greenhouse.
Interior versus Exterior Curtains
Several factors must be considered when designing a shade curtain system for greenhouses, including shade placement, shade curtain materials, and system configuration. The first decision to be made is whether to place shade curtains on the interior or exterior of the greenhouse. Shade curtains in temperate climates are generally installed inside the greenhouse (Figure 6.11). Although shade curtains are relatively expensive, they allow light to be more precisely controlled, which can improve crop quality. The ability to retract screens during periods of low light is an important attribute of this technology. When installed inside greenhouses, curtains are suspended from the overhead structure and installed in sections.
Operation of Greenhouse Shade Curtains
Greenhouse curtains can be controlled manually or automatically by a time clock or smart controller that can detect solar levels. In this way, screening is applied only during the hours when it is needed. Manual operation of screens would require constant vigilance 7 days a week, which is out of the question for most operations. The advantage of a time clock or a smart controller is that no one must be present to have the curtain open or close. The obvious downside in using a time clock is that shade is provided regardless of the solar radiation. A time clock makes the most sense for situations where shade is likely to be needed daily regardless of the outdoor environment. The smart controller has an advantage over a time clock because it detects solar radiation level and can control the curtain to optimize greenhouse cooling.
Shading Compounds (White Wash)
Greenhouse temperatures can be reduced by applying semi-permanent shading compounds, or white wash to the outer glazing of the greenhouse. Application of whitewash is the traditional and low-cost method of shading greenhouses. The shading factor (or percentage reduction in light transmission) is determined by the concentration of compound mixed with water, as well as the volume applied. Although shading compounds have some limitations, they are useful in situations when other shading alternatives cannot be incorporated into the existing greenhouse infrastructure, do not already exist, or are not economical.
Click on the following topics for more information on greenhouse ventilation and cooling.
