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Introduction
Greenhouse horticulture has expanded to hot and humid climate, heating is hardly an issue. Also water availability is mostly not an issue as humid tropical climates normally have periods of heavy rain fall. If managed well, rain climates, for instance South-East Asia. These locations feature periods of heavy rainfall and abundant solar radiation due to their location near the equator. Water availability is generally not an issue and rain water can be used to replenish water reserves, if managed well. Instead of artificial illumination or heating, cooling becomes the main challenge.
Cooling, however, is very difficult in such conditions. Taking the typical weather in Singapore as an example, the scenarios below show that greenhouses with natural ventilation, but also greenhouses with pad and fan cooling cannot be cooled sufficiently to grow Adiabatic cooling, pad and fan cooling, or mechanical cooling can normally be used to cool greenhouses.
Adiabatic cooling requires natural ventilation, which is frequently insufficient in dry and humid climates. The outside temperature and humidity are frequently higher than desired for the greenhouse. The air exchange is further impeded by the lack of buoyancy driven exchange.
Pad and fan cooling uses forced ventilation to pull air through evaporative cooling pads. High humidity levels negatively affect the system’s efficiency: The higher the humidity, the less effective evaporation will be. As a result, pad and fan systems typically cannot cool sufficiently to produce common greenhouse crops like tomato or lettuce. Only air conditioned greenhouses provide temperatures low enough to grow the typical greenhouse crops. And, just like in the dry hot climates, the performance of these air conditioned greenhouses improves a lot when CO2 dosing is applied. In this case heating scenarios are reviewed for a Dutch greenhouse using different heat sources. The .
Mechanical cooling uses AC-units provided with cold water from a chiller to accurately control temperature. Mechanically cooled greenhouses limit natural ventilation and thus can recuperate the water transpired by the crop, which leads to a drastic reduction in net water use. On the flipside, chillers with pumps and fans increase electricity use. Furthermore, CO2-supplementation is required in closed greenhouses to maintain CO2 levels and plant growth.
Impact of cooling systems on the greenhouse energy footprint:
Energy use: Additional cooling increases energy demand, but is essential for operation in hot and humid climates. The efficiency of each cooling technology will impact the total energy use and their capacity, design and control will determine their efficacy.
CO2 footprint: The use of electricity for cooling typically increases the CO2 footprint of the greenhouse, when it does not stem from renewable energy. Fossil-free fuel sources can substantially reduce CO2 footprint.
Water use: The design and control of the cooling and ventilation systems will directly impact net water use. In arid regions, this could provide a climatological advantage and reduce the associated CO2 footprint of water use.
Scenarios
In this case the following scenarios are compared:
Natural ventilation
Pad and fan
Mechanical cooling without CO2 fertilisation
Mechanical cooling with CO2 fertilisationsupplementation
With the following assumptions:
Non-illuminated tomato cultivation in a modern glass greenhouse inShanghai
As light-conditions are equal in all scenarios, the temperatures to be achieved in the greenhouse are equal in all scenarios , China
Scenarios 1 to 3 are without CO2 supplementation
The temperature setpoints are equal across scenarios (a consequence of RTR-based temperature control)Because of low temperatures in winter, a , as light conditions are equal across scenarios
A boiler on LPG in combination with a thermal screen is used for the low temperatures during winter
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The configuration differences between the scenarios are shown in the table below.
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The simulation results are grouped into realised realized climate, CO2, electricity and heat. Expand each topic for detailed results.
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First, let’s look at the realized greenhouse climate.
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Each cooling system differs significantly in their electricity use. Pad and fan systems (4.2) have a significantly lower electricity use than mechanical cooling (4.3-4.4). The chillers with pumps and fans used for mechanical cooling dramatically increase electricity use.   |
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The cold production is expressed below as net sensible cooling power provided to the greenhouse by each cooling system. The pad and fan system also produces latent cooling power via evaporative cooling. Evaporative cooling is hindered by the high external humidity.   |
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The water use of the natural ventilation Scenario (3.1) provides an indicator for comparison. The pad and fan system (3.2) requires only moderate amounts of water, as it is hindered by the high external humidity. The closed greenhouse allows the mechanical cooling system to regain a significant share of transpiration (3.3-3.4). As a result, the pad and fan system requires ~3.5 times more water than the mechanical cooling system.   |
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In the Table below the consumption of different resources is compared.
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Performance
The overall performance is , expressed in terms of economical feasibility some key numbers and sustainability, is compared in the table below.
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Conclusions
Because of the high outside humidity, pad and fan has unsufficient cooling capacity
Scenario with lowest energy use:
Natural ventilation (4.1) logically requires the lowest energy use. Using cooling can increase energy use, variable costs and crop production significantly. When cooling is applied, pad and fan systems (4.2) require the lowest energy use but suffer from insufficient cooling capacity due to the high ambient humidity.Scenario with lowest CO2 emissions in future energy net:
The CO2 footprint is the direct result of the CO2 footprint of the local energy mix, as no fossil fuels were used for the operation of these greenhouses. In a fully renewable energy network, the CO2 footprint would remain zero across Scenarios. Water use is at the moment not included in the CO2 footprint of the greenhouse.Scenario with lowest CO2 emissions in current energy net:
The CO2 footprint is the direct result of the CO2 footprint of the local energy mix, as no fossil fuels were used for the operation of these greenhouses. In a mixed energy network, the CO2 footprint is proportional to the energy use. Water use is at the moment not included in the CO2 footprint of the greenhouse.
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