Winter climate and heat demand

In Western Europe, winter is characterised by wet, dark and relatively mild days, occasionally interrupted by drier and colder periods. Unlike summer, daily outdoor weather variations have fairly limited impact on day-to-day greenhouse management due to reduced solar radiation. Even on very sunny days in winter, lighting is required almost 100% of the time. And with temperatures remaining below 10°C for most of the winter, there is consistently substantial heat demand, resulting in little variation in daily heat requirements.

The energy dynamics of a greenhouse in winter are entirely different from those in summer. This is caused by the energy source, the sun. In midsummer, the daily radiation sum ranges from at least 500 J/cm² on very cloudy days to 3000 J/cm² per day on sunny days. However, in December, this drops to only 50–300 J/cm² per day, a mere 10% of summer levels. Furthermore, winter months experience more frequent cloud cover compared to summer.

The two most prominent factors affecting greenhouse climate in winter are wind and outgoing radiation. When there is no cloud cover, significant cooling occurs due to outgoing radiation. The impact of wind is far greater, as strong winds effectively blow out all heat from the greenhouse. For comparison, when the outdoor temperature is 10°C and the weather is stormy, the heat demand is higher than when it is clear and calm at -10°C. These two cooling factors also contribute to temperature and humidity differences in the greenhouse. In this article, we delve into the effect of wind and outgoing radiation on our greenhouse climate and how we can best manage these conditions.

Outgoing radiation explained

Outside the greenhouse:
When significant outgoing radiation occurs, it is often under a clear sky. The Earth’s surface radiates substantial heat outwards, causing surfaces to cool more than the air. For instance, greenhouse roofs, cars or grass can experience this effect. These surfaces may become wet from condensation, and frost can even form on them during periods of intense outgoing radiation.

In the greenhouse:
Outgoing radiation in a greenhouse depends on the temperature of the greenhouse roof, which is influenced by cloud cover, outside temperature, wind speed and precipitation. The colder the roof, the greater the heat emission from the plant to the roof. A rule of thumb is that for every degree of temperature difference between the plant and the screen (or greenhouse roof if the screen is open), there is approximately 5 W/m² of outgoing radiation. This outgoing radiation can be mitigated through the use of energy-saving screens. A single screen has a significant effect, while a second or even third screen can prevent the majority of outgoing radiation. In cases of high outgoing radiation, it can be beneficial to increase heating through the overhead heating system, which heats the lower screen and prevents excessive temperature differences between the plant and the screen.

Energy balance of the plant:
All these factors are compounded in the energy balance of the plant. A plant receives energy from the sun, lighting and heating while losing energy through evaporation, radiation, reflection, convection and photosynthesis (see the graphic below).

Measurements
Instruments are available to measure outgoing radiation. Many growers already use radiometers, specifically pyrgeometers, which measure longwave radiation outdoors. Another useful instrument is a net radiometer at plant level, which measures both incoming and outgoing radiation on the plant, indicating whether the net radiation is positive (net incoming radiation) or if there is net outgoing radiation. It is also useful to measure the temperature above the screen. These types of instruments can provide a comprehensive overview of the situation in the greenhouse and simplify the selection of appropriate actions. Finally, infrared plant temperature measurements can provide a good indication of plant temperature.

The key is maintaining energy balance — energy input must equal energy output (see the graphic below). When this is achieved, the temperature of the plant remains constant. The balance is the sum of convection (C), evaporation (E) and the sum of incoming and outgoing radiation (∑R). If this sum equals zero, there is a balance, and the temperature of the plant will remain stable.

Actions

• Before net incoming radiation falls below zero, close the screen and turn up the overhead heating system. This reduces the temperature difference between the plant and the screen, thereby reducing outgoing radiation.
• When net incoming radiation rises above zero or when the temperature above the screen is less than 5°C different from the temperature below the screen, open the screen. It is no longer necessary to keep it closed.
• Without measurements, open the screen at >100 W/m² (assuming no light needs to be screened out). This obviously depends on climate factors such as wind, outside temperature, precipitation and radiative heat loss.
• When lighting is switched off at the end of the day, a significant source of radiation and heat is removed. This is also the case when LED lighting is switched off. To minimise cooling, it is advisable to close the screens before turning off the lamps.

The ‘Greenhouse as an Energy Source’ website features a radiation monitor that provides further insight into this topic (in Dutch): https://www.kasalsenergiebron....

Plant temperature

Plant temperature can drop significantly as a result of outgoing radiation. This has several disadvantages:
- Reduced evaporation leads to a decrease in the transport of water and nutrients within the plant

- Plants may develop weaker cell walls

- Increased susceptibility to diseases

- Reduced stem elongation

- Potential condensation, which results in faster mould development

Plant temperature is influenced by the ambient temperature and the sum of incoming and outgoing radiation. For the sake of simplicity, we have omitted the evaporation factor here. When there is NET outgoing radiation, plant temperature is usually below ambient temperature and vice versa for NET incoming radiation.

Climate uniformity in the greenhouse

When there are significant temperature differences between the greenhouse and the outside environment, substantial temperature variations can occur within the greenhouse. This is exacerbated by cooling factors like wind and outgoing radiation. Furthermore, the cooling effect is unevenly distributed throughout the greenhouse. Cold façades can lead to air currents, and wind can worsen this effect. Various measures can be taken to ensure optimal climate uniformity.

1. The most obvious measures: Seal openings in the structure (broken windows, draughts near doors, poor sealing of façade screens etc.) and ensure adequate façade insulation and heating.
   
2. To measure is to know: Use wireless sensors throughout the greenhouse. These provide insights into temperature and humidity differences. A rule of thumb for measurements is to have at least one measuring point per 1,000 m².
   
3. Screens: When the temperature above the screen is very low and there is a lot of wind, temperature differences can occur if there are gaps between the screens. Fully closing the screens can largely prevent this. If humidity or temperature in the greenhouse is too high, ventilation should occur above the closed screen.
   
4. In sloped greenhouses (often for proper gutter drainage): Air above the screen may move from high to low points. When the air reaches the façade, it goes down into the greenhouse through gaps, causing significant temperature differences in the greenhouse. Installing baffle screens can greatly reduce the cold air flows that cause these differences.
   
5. In cold weather: Growers are less inclined to use ventilation on the side exposed to the wind. However, using ventilation on both sides can reduce temperature and humidity differences in the greenhouse, creating a highly stable airflow, especially when combined with a properly closed and insulated screen.
   
6. Temperature differences due to cooling: This creates air currents that further increase these differences. A common phenomenon in windy conditions is heat accumulation on the façade exposed to the wind. The façade experiences the greatest cooling due to wind, but the resulting air current causes heat to accumulate there. This is not easy to counteract. Fully closing the lower screen (without any gaps) can help mitigate this by limiting cold airflow from the unheated ridge downwards. Installing baffle screens can counteract airflow above the screen.