KASPRO horticulture simulator
Finding the ideal subtropical greenhouse system: using the KASPRO simulator
What is the optimum greenhouse design for vegetable growing in Taiwan? To answer this question, we have used a systematic design procedure to study the differences between possible solutions, using our greenhouse simulation model KASPRO.
Authors: Bas Speetjens, Wageningen UR; Silke Hemming, Wageningen UR; Dennis Wang, Tainan DARES; Jyh-Rong Tsay, Taiwan Agricultural Research Institute. Source: Wageningen UR
In Taiwan open field vegetable production is threatened by subtropical climatic disasters, such as high wind speeds and heavy rainfall, which can cause the destruction of whole crops. Next to that, vegetable production is threatened by pests and diseases resulting a high need for pesticides.
Greenhouse production systems are able to provide protection for the crop. However, in current protected tomato production in Taiwan yields are extremely low (max. 4 kg cherry tomato m-2 per season, though with high Brix). Next to a deficiency in modern crop management, there is a lack of knowledge of what is the optimum greenhouse design for vegetable growing in Taiwan.
Our goal is to realise an environmentally friendly production system with low energy input and a high water use efficiency. The possibility of solar energy is examined. The ideal greenhouse should have a high crop yield and at the same time provide a high product quality and food safety. Moreover, the proposed greenhouse should be economically viable.
Greenhouse design has been shown to be a multifactorial problem, which can be approached with a systematic design method (Van Henten et al., 2012). The method is based on the use of different models, which predict a suitable greenhouse design. The local climatic conditions (outside radiation, temperature, humidity, wind speed) are used as input for the greenhouse climate model, which is used to evaluate different greenhouse designs and technologies. The climate and crop growth inside the green¬house can be simulated.
These data serve as input to an economic model, which can be used to come to the economic suitable greenhouse design. This greenhouse design approach was applied for Taiwanese climate conditions.
In this approach, all sub-functions within the greenhouse system are mapped and possible solutions are identified. To study the effect of these different solutions for the sub-functions of the greenhouse, we have employed our greenhouse simulation model KASPRO. This is a dynamic model that accurately mimics the behaviour of a greenhouse, resulting in hourly data on –amongst others temperature, relative humidity, crop growth and CO2 concentration. The model is based on the computation of relevant heat and mass balances.
An economic model is used to study the economic performance of the proposed solutions. Benefits and cost are calculated on a yearly base. Crop yield data is taken from the KASPRO simulations. The calculation of the investment cost is based on both data from literature as well as data supplied by the horticultural industry. To be able to quickly compare different greenhouse types, the simple payback time is calculated.
Weather data for three locations in Taiwan were supplied by Tainan Dares. The yearly solar radiation sum in Tainan is around 6 GJ (compared to 3.9 GJ in The Netherlands). As plant growth is largely determined by solar radiation, the potential crop yield for vegetable production in Taiwan is higher than in the Netherlands. However, due to the higher radiation intensities in Taiwan, the temperature in the greenhouse will be higher, which will reduce crop growth. The relative humidity of the air is very high and often exceeds 95%. Another important consideration in the design of the greenhouse is the existence of typhoons in Taiwan. The greenhouse structure should not be permanently damaged by these typhoons.
As said before, a dynamic greenhouse model was used to simulate the effects of different types of equipment in the greenhouse. In an iterative process with the economic mode, the following technologies were examined:
- Ventilation and insect nets A greenhouse for sub-tropical climate in Taiwan should be equipped with a well-designed ventilation system to avoid excessive temperatures inside the greenhouse. The power consumption of mechanical ventilation is high, so we advise to use natural ventilation. The vents should be equipped with insect net to keep whitefly out. To provide sufficient ventilation capacity, even with insect nets, the surface of the vents should be at least 0.5 m2 per m2 greenhouse ground surface. In that case, the temperature inside the greenhouse is close the outside temperature. Natural ventilation is not able to decrease the temperature below the outside temperature level.
- Greenhouse cover material and shading screens We advise to use a plastic film that is diffuse and has a high transmission of light (>75%). It should also have a high transmission of infrared radiation, which helps to reduce high greenhouse temperatures. Shading screens are useful to decrease crop temperature during periods with high irradiation. Due to the lower light transmission, the potential crop production is lower than without the application of a screen. As Taiwan has many cloudy days, the effect of shading is limited. However, a screen does provide an additional way of reducing crop stress, so we advise to install an external screen with a shading percentage of 30%. This type of screen does not limit the ventilation too much and reduces the risk of crop damage by too much light.
- Adiabatic cooling We advise installing a fogging system with a net capacity of at least 300 g/m2/h. This system will decrease the temperature inside the greenhouse during the hottest hours of the day and will contribute to a less stressed crop. We do not advise a pad and fan system, because of the higher energy cost and inhomogeneous temperature distribution inside the greenhouse.
- Heating system and CO2 dosing A heating system is recommended to avoid cold hours that have negative impact on the crop growth. Without heating system in the greenhouse, plants will survive. However, crop production is higher with a heating system. The greenhouse may be heated either with fossil fuels or by solar energy (or a combination of both). Heating the greenhouse by means of solar energy is more sustainable and has low running cost. However, it is not the most economical option (which may change in future, as energy prices keep raising). If the greenhouse is only heated with solar energy, we advise to install a solar collector of 0.7 m2 collector/m2 greenhouse ground surface. In this case, the buffer size should be in the order of 200 m3/ ha greenhouse ground surface. The capacity of a boiler (if it is the only heat source) should be around 100W/m2 to keep the greenhouse warmer than 12o With a buffer, the boiler capacity may be substantially reduced. Unless CO2 is free/cheaply available, it is not economically viable to supply CO2 to a ventilated greenhouse in Taiwanese weather conditions. If, in future projects, a connection can be made to industrial (waste) CO2, it is worth investigating the possibilities again.
- Closed greenhouse A closed greenhouse provides optimal growing conditions for the crop, resulting in very high crop yields. Unfortunately, investment costs of these greenhouses are high (expensive greenhouse and cooling equipment is needed). Also the skills of the grower to fully exploit the benefits of the technology must be very well developed. For an average Taiwanese vegetable grower, the transition from the currently used greenhouses to a closed greenhouse is (probably too) large. The concept of closed greenhouses is more suitable as a demonstration and research project in the near future than to be used for commercial vegetable production.
More information: See http://edepot.wur.nl/238571 for the full paper