Case 4: Cooling in a warm and humid climate

[ 1 Introduction ] [ 2 Scenarios ] [ 3 Performance ] [ 4 Performance ] [ 5 Conclusions ] [ 6 Simulate ]

Introduction

Greenhouse horticulture has expanded to hot and humid 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. 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.
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
Mechanical cooling with CO₂supplementation

With the following assumptions:

Non-illuminated tomato cultivation in a modern glass greenhouse in Shanghai, China
Scenarios 1 to 3 are without CO₂supplementation
The temperature setpoints are equal across scenarios (a consequence of RTR-based temperature control), 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

Average climate in Shanghai, China

The configuration differences between the scenarios are shown in the table below.

Performance

The simulation results are grouped into realized climate, CO2, electricity and heat. Expand each topic for detailed results.

First, let’s look at the realized greenhouse climate.

The realized greenhouse climate differs significantly for all Scenarios.
Natural ventilation and pad and fan systems (4.1-4.2) result in higher average, maximum and exceedances of temperatures. Natural ventilation is not considered an option due to the high exterior temperatures. Evaporative cooling with the pad and fan system is not an option because of the high external humidity.
Mechanical cooling controls the greenhouse temperature more accurately, resulting in fewer transgressions of RTR temperature. Humidity setpoints are realized less frequently.
CO2 concentration drops marginally when mechanical cooling is used in a largely closed greenhouse (4.3). CO2 supplementation is still considered beneficial for this system (4.4).

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.

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.

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.

In the Table below the consumption of different resources is compared.

Pad and fan cooling results in lower electricity use.
Natural ventilation (4.1) and pad and fan (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.
Mechanical cooling results in a lower water use.
The closed greenhouse allows the mechanical cooling system to regain a significant share of transpiration, reducing water use (4.3-4.4).
Mechanical cooling necessitates CO2 supplementation.
Plant production is optimized by supplementing CO2 in the closed system, featuring mechanical cooling (4.4).

Performance

The overall performance, expressed in some key numbers and sustainability, is compared in the table below.

Conclusions

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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