Case 3: Cooling in a warm and dry climate

Introduction

Greenhouse horticulture has expanded to hot and arid climates, for instance the North American deserts and the Arabian peninsula. These locations feature abundant solar radiation due their clear skies and their location near the equator. Instead of artificial illumination or heating, cooling becomes the main challenge. Adiabatic cooling, pad and fan cooling, or mechanical cooling can be used to cool greenhouses.

Adiabatic cooling may be adequate if the crops perform at temperatures around 23 - 25 °C and natural ventilation is sufficient. However, natural ventilation is frequently insufficient in dry arid climates, as the outside temperature is higher than the desired greenhouse air temperature. The air exchange is further impeded by the lack of buoyancy driven exchange.
Pad and fan cooling uses forced ventilation to overcome these issues. These systems need to be carefully designed and controlled. Their performance does not necessarily increase proportionally with system capacity, but water and electricity consumption does.
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 arid 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 CO₂supplementation
Mechanical cooling with CO₂supplementation

With the following assumptions:

Tomato cultivation without artificial illumination in a modern glass greenhouse in The United Arab Emirates
No shading screens are used
The temperature setpoints are equal across scenarios (a consequence of RTR-based temperature control), as light conditions are equal across scenarios

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

Detailed results

The simulation results are grouped into realized greenhouse climate, cold production, water use and resource use. Expand each topic for detailed results.

Greenhouse climate

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

The realized greenhouse climate differs significantly for all Scenarios.
Natural ventilation results in the highest average, maximum and exceedances of temperature. The high outside temperatures reduce its efficacy, resulting in extremely high greenhouse temperatures. Natural ventilation is therefore not considered an option.
Pad and fan cooling achieves a lower maximum temperature than mechanical cooling, but exceeds RTR temperature far more frequently. Pad and fan also causes the greenhouse temperature to fall below RTR temperature more frequently.
Mechanical cooling controls the greenhouse temperature more accurately, resulting in fewer transgressions of RTR temperature. Humidity setpoints are realized less frequently.
CO2 concentration drops significantly when mechanical cooling is used in a largely closed greenhouse (3.3). CO2 supplementation is therefore considered a requirement for this system (3.4).

Cold

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. The combined sensible and latent cooling power of pad and fan (3.2) was comparable with the sensible cooling power of mechanical cooling (3.3-3.4).

Water

The water use of the natural ventilation Scenario (3.1) provides an indicator for comparison. The pad and fan system (3.2) requires tremendous amounts of water, which are lost to the exterior air. 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 ~5 times more water than the mechanical cooling system.

Resource use

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

Pad and fan cooling results in lower electricity use.
Pad and fan (3.2) has a significantly lower electricity use than mechanical cooling (3.3-3.4). 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 (3.3-3.4).
Mechanical cooling necessitates CO2 supplementation.
Crops can make use of surrounding air’s CO2 in a ventilated system. Additional CO2 supplementation is required in a closed system, featuring mechanical cooling (3.4).

Performance

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

Crop production is greatly influenced by cooling systems.
Crop production varies greatly between the cooling systems. The temperature transgressions and extremes resulted in a compromised production for natural ventilation (3.1) and to a lesser extent for pad and fan (3.2). When adequately supplemented with CO2, mechanical cooling realizes the best climate for crop production (3.4)
Cooling increases energy and electricity demand.
The greenhouse without active cooling (3.1) naturally has the lowest energy costs, but has also a substantial lower production. The lower production will have a detrimental impact on the financial feasibility and was therefore excluded from further consideration.
Pad and fan systems have the lowest energy and electricity demand.
Pad and fan (3.2) has a significantly lower electricity use than mechanical cooling (3.3-3.4). Electricity is only required to establish air flow for evaporative cooling in pad and fan systems. The chillers with pumps and fans used for mechanical cooling dramatically increase electricity use.
CO2 emissions will be directly related to the local energy mix.
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.

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 (3.2) require the lowest energy use.
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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