Introduction
Greenhouses located in colder climates typically require a high amount of additional heating to maintain the optimal temperature for plant growth. Heating demand typically decreases during summer, but may still be present on many nights. Greenhouse heating is generally produced locally using boilers, CHP, heat pumps, heat exchangers or any combination of the four. Each technology operates on various fuel sources, which contributes significantly to their CO2 footprint.
In the Netherlands, boilers are traditionally used for generating heat by burning different types of fossil fuels such as natural gas or LPG. Alternative heating technologies may provide the key to realize fossil-free greenhouses, significantly decreasing the CO2 footprint along the way. The key technologies that are discussed in this Case use (latent) heat recovery, waste heat or geothermal heat and biomass. For low-grade energy a heat pump will be necessary.
Impact of heating systems on the greenhouse energy footprint:
Energy use: Additional heating increases energy demand, but is essential for most modern greenhouses.
The efficiency of each heating technology will impact the total energy use and their operational temperature will determine their efficacy.CO2 footprint: The use of fossil fuels for heating typically increases the CO2 footprint of the greenhouse.
Boilers and CHP-engines typically use natural gas or LPG, with a high associated CO2 footprint. Alternative, fossil-free fuel sources can substantially reduce CO2 footprint.
Scenarios
In this case the following heating scenarios are reviewed.
Heating with a boiler as reference case
Heat pump with heat recovery and seasonal heat storage
Geothermal heat
Additional biomass boiler
With the following assumptions:
Tomato cultivation without artificial illumination in a modern glass greenhouse in The Netherlands
As light-conditions are equal in all scenarios, the temperatures to be achieved in the greenhouse are equal in all scenarios (a consequence of RTR-based temperature control)
Two energy screens to limit the heat demand
Dehumidification for Scenarios 2.1, 2.3 and 2.4 is performed by forced ventilation of outside air
CO2-enrichment from flue gases, supplemented with pure CO2
No variable costs for biomass
The sensible and latent heat recovery system is outlined below. The greenhouse air is dehumidified while the recovered latent heat is stored in a daily or seasonal heat storage. The cooled air is re-heated with the condenser side of the heat pump.
The configuration differences between the scenarios are shown in the table below.
Detailed results
The simulation results are grouped into realized climate, electricity and heat. Expand each topic for detailed results.
The realized greenhouse climate is nearly the same for all scenarios.
The only significant difference in the greenhouse climate control is the CO2 dose and realized concentration. The decrease in boiler exhaust gasses in Scenarios 2.2-2.4 results in an increase in the amount of pure CO2 supplemented in Scenario 2.3 and 2.4. This increase is not seen in Scenario 2.2, because water is regained and less CO2 is lost through ventilation.
Artificial illumination is the main electricity consumer in all Scenarios. The heat pump and geothermal Scenarios (2.2-2.3) require electricity for heating. Geothermal heat requires significantly less electricity than heat pumps do.
In the base situation all heat is produced by the boiler. In the other Scenarios it is solely used as a backup system, in case the main system cannot meet the heat demand. The biomass boiler (2.4) is capable of covering almost all of the greenhouse heat demand. The heat pump and geothermal heat are capable of covering most of the heat demand (2.2-2.3).
In the table below the consumption of different resources is compared.
Fossil fuel use can be reduced by switching heating technology.
The alternative heating technologies (2.2-2.4) have significantly reduced the use of fossil fuels by switching to different methods of heat production. Maintaining a boiler as a backup system allowed for greenhouse with an accurate and consistent climate control, as well as realistic capacities for the heat pump, geothermal heat and biomass boiler.Electricity use can be reduced by utilizing renewable sources.
The use of available geothermal energy naturally reduces the electricity used for heating via a heat pump. Alternatively, the energy requirement can be covered using biomass.
Performance
The overall performance, expressed in some key numbers and sustainability, is compared in the table below.
Consistent production can be achieved using various heating technologies.
Crop production can be maintained at a consistent level using different heating systems, if the necessary capacities are installed.Geothermal heat has lowest energy demand.
When comparing the heating systems, the geothermal system clearly has a lower energy use than the greenhouses with heat pumps or boilers. This benefit is to be expected, but has to be weighed against the higher initial investments for geothermal systems to determine the financial feasibility.Fossil fuel use and CO2 emissions can be reduced by switching heating technology.
All alternative scenarios result in a significant reduction of the fossil fuel use and CO2 emissions. As no CO2 emission was attributed to biomass, Scenario 2.4 results in the lowest emission.Heat pumps decrease CO2 emissions but increase costs.
The heat pump with latent heat recovery has the highest variable costs and electricity use.
Conclusions
Scenario with lowest energy use:
Scenario 2.3 using geothermal heat requires the lowest energy use. Using available natural sources of energy reduces energy use and variable costs.Scenario with lowest CO2 emissions in future energy net:
All alternative scenarios result in a significant reduction of the CO2 emission. As no CO2 emission was attributed to biomass, Scenario 2.4 results in the lowest emission. As it was assumed that the electricity from the grid stems from renewable sources, electricity use has no effect on CO2 emissions.Scenario with lowest CO2 emissions in current energy net:
All alternative scenarios result in a significant reduction of the CO2 emission. As no CO2 emission was attributed to biomass, Scenario 2.4 results in the lowest emission. In the current energy net, electricity use has a greater effect on total CO2 emissions. If the average CO2 emission of electricity in the public grid of the Netherlands (350 gram per kWh in the early 2020s) is assumed, Scenario 2.2 has the second-highest CO2 emission (10.0+77.8 x 0.350 = 37.2 kg/(m² yr)), followed by Scenario 2.3 with 16.1 kg/(m² yr) and 2.4 with 3.9 kg/(m² yr).
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