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Introduction
Illumination of greenhouses in Northern latitudes requires Greenhouses at higher latitudes typically require large amounts of electricity for artificial illumination to ensure high and continuous production. This electricity is sourced from the net or produced locally, using local renewable energy production or combined heat and power (CHP) engines.
In the Netherlands, the standard in the early twenties 2020s is that a large fraction of the electricity consumption of the illumination is produced by combined heat and power enginesCHP. During summer, the illumination is not used . However, as the greenhouse still needs at least some heating but heating may still be necessary on many nights, the . The average grower with a will therefore choose to run the CHP-engine will run this engine in order to produce electricity for the public grid. Looking on a year round baseOn an annual basis, many greenhouses with illumination and CHP are more or less neutral in terms of electricity consumption. During : in winter there is net buying of electricity and in summer there is net selling of electricity.As a CHP-engine is typically running on natural gas, the CO2-emission associated with the electricity household of an illuminated greenhouse with CHP is substantially higher compared to a non-illuminated greenhouse
Impact of illumination and CHP on the greenhouse energy footprint:
Energy use: Artificial illumination typically increases energy demand, but reduces heating demand.
Without CHP, the greenhouse
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has to import a substantial amount of electricity. This electricity is then converted to light, but also to heat. Therefore the heating demand of an illuminated greenhouse is in general lower than that of a non-illuminated greenhouse. The decrement is however less than one might expect, which is caused by the fact that a crop requires a higher average greenhouse temperature when there is more light available for growth.
CO2 footprint: The use of CHP-engines typically increase the CO2 footprint of the greenhouse.
CHP-engines typically uses natural gas, the CO2-emission associated with the electricity use of an illuminated greenhouse with CHP will be substantially higher compared to a non-illuminated greenhouse.
Scenarios
In this case different combinations of illumination and sources of electricity are reviewed. The following scenarios are compared:
Not illuminatedNo artificial illumination
HPS lamps illumination. This is the ‘old fashioned‘traditional' type of greenhouse lighting
LED illumination. As LED lighting becomes affordable, many growers switch to this efficient system.
HPS lamps + CHP (combined heat and power). The CHP produces most of the electricity demand.
LED lamps + CHP. As LEDs use less electricity than HPS, the greenhouse becomes a net electricity producer.
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Tomato cultivation in a modern Venlo greenhouse in The Netherlands
RTR-based temperature control, aiming to a fixed ratio between temperature and radiation
Two energy screens
A heat buffer that allows for running the boiler or CHP during day time daytime for CO2 and electricity production
The CHP system has an electrical output of 50 W/m2, complemented by a standard boiler
The CHP system is running in heat demand modus
Illumination with 180 micromol/m2/s intensity
CO2 dosing from CHP (if available) and boiler flue gases
Electricity purchased from the public grid is regarded as emission free.
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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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Because of the higher efficiency, LED Illumination featuring LEDs consumes significant less electricity than HPS at the same illumination intensity. LED produces less heat which means that the CHP runs for more hours compared to HPS, due to their higher efficiency. This also means that less heat is produced by the lighting system. As a result, the CHP is used more frequently for heating and produces more electricity. The minus-sign in the table below means that the CHP produces electricity instead of consumes electricity.     |
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Illumination featuring LEDs produces less heat than HPS at the same illumination intensity, due to their higher efficiency. This results in a higher heat demand and therefore a higher boiler operation and CHP operation. In scenarios 1.4 and 1.5 the CHP produces a substantial amount of the heat demand, with two valuable byproducts: electricity and CO2.     |
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In the table below the consumption of different sources is shown. Because of the higher greenhouse temperature in combination with a higher PAR sum, the water use is higher for the illuminated scenarios, but when using LED compared.
The scenario with LED + CHP has a net negative electricity use, which means that on a yearly base the greenhouse sells more electricity to the grid than in buys. This is partly because LED’s require less electricity and partly because the greenhouse needs somewhat more heating compared to an illumnated greenhouse with HPS-lamps
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Performance
The overall performance is expressed in terms of economical feasibility and sustainability and compared in the Table below.
Illumination increases energy demand
The greenhouse without illumination 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.LED illumination has lower energy demand than HPS
When comparing
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the illuminated greenhouses, the cases with LED lighting clearly have lower costs than the greenhouses with HPS lighting. This benefit is
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to be expected
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, but has to be weighed against the higher initial investments for LEDs compared
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with HPS-lamps to determine the financial feasibility.
CHP decreases costs but increases CO2 emissions
Under the assumed economical conditions, options with CHP lead to lower costs, but higher CO2 emissions.
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Conclusions
Scenario with lowest energy use
Scenario 1.1 with no illumination requires the lowest energy use. Using illumination increases both energy use, variable costs and crop production significantly.The scenario with Scenario with lowest CO2 emissions in future energy net
Scenario 1.2 (HPS without CHP) has the highest electricity consumption (HPS and no co-generation) has , but the lowest CO2 emission. This is because it is emissions. In these scenarios it was assumed that the electricity from the grid is all coming stems from sustainable renewable sources. In case If the average CO2 emission of electricity in the public grid of the Netherlands (350 gram per kWh in the early twenties2020s) is assumed to be associated to the bought electricity, the HPS greenhouse without CHP Scenario 1.2 becomes the greenhouse with the highest CO2-emission, although the largest emission is then produced elsewhere. The CO2 emission of option 2 would then be : 51.5+234.6 x 0.350 = 134 kg/(m² yr). The dominant share of emissions is then produced elsewhere.Scenarios with lowest CO2 emissions in current energy net
Assigning this 350 gram CO2-emission to public grid electricity would mean that the CO2 - emission associated with option Scenario 1.4 would increase with 87.1 x 0.35 = 20 kg/(m² yr), whereas the CO2 -emission emissions associated with option Scenario 1.5 would decrease by 32.7 x 0.35=11.4 kg/(m² yr).
This shows that LED-lighting is always to be preferred compared to HPS-LEDs always outperform HPS lighting.
