The treatment of the energy within the greenhouse will depend on the time of the year and the crop produced. In many parts of the world, in summer, growers will be venting to remove excess heat from the greenhouse while in winter steps are taken to conserve the energy and to prevent unnecessary heat loss.
Solar energy is the most common source of energy in the greenhouse with over 70% of the solar energy hitting the greenhouse transmitted into the greenhouse through the cover. Seventy percent of this heat energy is used to evaporate water from the leaves by the process of transpiration. Heat is also lost through the floor of the greenhouse by conduction. The greenhouse heats up due to the heat released by objects in form of long wave radiation which glass is opaque to. Long wave radiation will pass through most polyethylene used in greenhouse skins hence plastic greenhouses are usually cooler than a glasshouse of a similar size in the same location. Twin-skin greenhouses are more efficient in retaining heat than glasshouses and modern greenhouse films have improved in their ability to retain heat.
A greenhouse (glass and plastic) will lose heat by convection to the night air, heat is also lost by conduction directly through the skin and single skin covers are very efficient conductors at night. This is why twin skin greenhouses are better heat traps at night than single skin houses as heat moves across the two skins more slowly.
There are various other forms of energy transfer within the greenhouse including the evaporation of water from the leaves which removes heat from the leaves and gives it to the air and then the condensation of the moisture on the greenhouse roof releases this heat to the cover.
Two percent of energy available for crop production is typically used in photosynthesis by tomato plants. The rest needs to be vented in summer or maintained in winter. Ventilation also has to be adequate all year round to allow for adequate CO2 to enter for photosynthesis to occur.
Some principles of energy conservation within the greenhouse
Summer removal of heat:
- Fan ventilation us used by some greenhouse growers to move the excess heat from the greenhouse but energy is consumed in rotating the fans so instead use the natural free energy of hot air, which rises to the roof of the greenhouse, can be mixed with (free) wind. This of course assumes their are roof vents.
- Misting systems are also used in some areas of New Zealand such as the East Coast which can be dry (low humidity) in summer with high winds at times. The misting is needed to replace moisture, which is lost from the greenhouse due to excess ventilation. The ventilation is required to remove excess heat increasing in the greenhouse due to the sunny weather. The misting adds moisture to the air and the addition of this moisture will cool the air as it evaporates as well as raising the greenhouse humidity level. The rise in humidity and cooling of the air will reduce moisture stress on the plants. Thus allowing them to photosynthesize and so grow rather than closing stomata due to the moisture stress thus reducing the rate of photosynthesis. An alternative option is the use of water sprinklers on glasshouses roofs which keeps the glass cool thus cooling the warm greenhouse air as it rises and comes in contact with the glasshouse roof. This is an indirect form of greenhouse cooling and is probably not as
efficient as an internal greenhouse misting system.
Winter heating principles.
Greenhouses are heated in winter for the obvious reason to prevent frost damage in areas where this is a risk. But many crops will produce inefficiently at such low temperatures and traditionally this is when the market will provide high prices for many crops. Also high humidity at this time will increase the risk of winter fungal diseases and plant quality disorders, so the heating is required to boost production and to keep the greenhouse environmental parameters within the required crop production limits.
There is however a law of diminishing returns with heating and greenhouse crop production so conservation of the heat in winter, so reducing heating costs, has some obvious advantages.
1. Reduce air leakage, a typical old wooden glasshouse in New Zealand will have about four air changes per hour, which can easily increase up to ten with wind. Lining the inside walls with clear polythene will help to reduce this leakage and so add a couple of degrees celcius to the average air temperature.
2. Sealing the laps between glass panes in old glasshouses with small glass panes will help to reduce leakage.
3. Twin skinning old glasshouses with a quality greenhouse plastic film will provide you with an efficient greenhouse and should be considered if the glasshouse roof has structurally sound woodwork. Twin skin roofs are thermally more efficient than old glasshouses. Some of the modern Venlo style glasshouses with 4mm thick glass, which are wide and long are more thermally efficient than the old design style of glasshouse
4. Use a well designed heating system, which delivers heat evenly across the greenhouse. Place the warm air of hot water pipe systems at the base of the crop (some flower crops use aerial heating systems) and not above. Placement of heating pipes above the crop will result in warmer air temperatures above the crop than below with increased heat loss from the greenhouse, which is obviously expensive. There is an economic penalty (poor and uneven production) if the heat is not delivered evenly across the greenhouse.
5. Accurate control systems with even a one-degree celcius error in the control temperatures can be expensive with excessive heating fuel used to maintain the set points. Thermostats must be reliable and placed in an aspirated screen where outside influences (sun) will not affect the reading. If thermostats are left in the open then the thermostat can be chilled by the night time air due to long wave radiation lost to the night time air. Alternatively they can be heated by the sun resulting in the vents opening while the greenhouse is still below the greenhouse air temperature. The airflow through the aspirated screen due to the small internal fan will ensure that the thermostat changes to changes in the greenhouse air.
6. Thermal screens can be pulled across at night to reduce heat loss. These come in various grades and are technically advanced. They are usually quite expensive to install so careful economic analysis is required to ensure that the investment is worthwhile.
7. Computer control of the greenhouse will provide for efficient use of the energy resources with accurate and intelligent maintenance of the set points. Overseas research shows that some crops respond to mean set points rather than accurate mean temperatures so computer control can be used so as to maintain mean night temperatures while achieving maximum energy savings.
Setting a winter heating budget can help to give some control over the annual heating budget and so possibly prevent excessive energy use. This is based around the concept of having a monthly budgeted quantity of energy that can be used to heat the greenhouse and to power any pumps, fans and vent motors. If the budget is reasonable and based on local weather records it can provide a starting point in reducing energy use and heat loss.
Summer control
Shading with screens or with whitewash to reduce the greenhouse temperature by reducing the solar radiation entering the greenhouse. This is useful in cooling the greenhouse air temperature but has a disadvantage in that the rate of photosynthesis will be reduced due to the loss in solar radiation. Misting has the advantage that it will lower the temperature, increase the relative humidity of the greenhouse without impacting on the solar radiation level thus maintaining the rate of photosynthesis.
