Case 1: Illumination

Introduction

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 2020s is that a large fraction of the electricity consumption of the illumination is produced by CHP. During summer, the illumination is not used but heating may still be necessary on many nights. The average grower will therefore choose to run the CHP-engine in order to produce electricity for the public grid. On an annual basis, many greenhouses with illumination and CHP are more or less neutral in terms of electricity consumption: in winter there is net buying of electricity and in summer there is net selling of electricity.

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 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 increases the CO2 footprint of the greenhouse.
CHP-engines typically use 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:

No artificial illumination
HPS illumination. This is the ‘traditional' type of greenhouse lighting
LED illumination. As LED lighting becomes affordable, many growers switch to this efficient system.
HPS + CHP (combined heat and power). The CHP produces most of the electricity demand.
LED + CHP. As LEDs use less electricity than HPS, the greenhouse becomes a net electricity producer.

With the following assumptions:

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 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.

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

Detailed results

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

Greenhouse climate

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

Lighting with 180 µmol/(m² s) allows for a Daily Light Integral between 15 and 20 mol/m², which is roughly 60% of the light availability in summer in the Netherlands. This naturally increases production and allows for a more consistent production.
The average air temperature is higher for the illuminated scenarios, due to the higher PAR sum in combination with the RTR temperature control.
The PAR sum of the illuminated scenarios is naturally higher.
The average RH is higher for illuminated scenarios, due to increased crop transpiration when the lamps are on.
The use of LED naturally results in a lower electricity consumption than HPS, but also in a lower radiation load on the crop, leading to a reduced transpiration.

Electricity

Illumination featuring LEDs consumes significant less electricity than HPS at the same illumination intensity, 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.

Heat

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 both CHP Scenarios (1.4-1.5) the CHP produces a substantial amount of the heat demand, with two valuable byproducts: electricity and CO2.

Resource use

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

Greenhouses can achieve net negative electricity use.
Scenario 1.5 (LED + CHP) has a net negative electricity use. This mean that on an annual basis the greenhouse sells more electricity to the grid than in buys. This is the result of a higher efficiency of LEDs and increased CHP operation for heating, compared with HPS-lamps.
Illumination results in a higher water use.
The higher greenhouse temperatures in combination with higher PAR sums result in a higher water use for illuminated scenarios.
HPS results in a higher water use than LEDs.
When using LEDs instead of HPS, the water use is reduced. LEDs does not emit Near-InfraRed radiation to the crop, resulting in lower crop temperatures and consequently less transpiration.

Performance

The overall performance, expressed in some key numbers and sustainability, is 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 the illuminated greenhouses, the cases with LED lighting clearly have lower costs than the greenhouses with HPS lighting. This benefit is to be expected, but has to be weighed against the higher initial investments for LEDs compared 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.

Conclusions

Scenario with lowest energy use:
No artificial illumination logically requires the lowest energy (1.1). Using illumination increases energy use, variable costs and crop production significantly. When artificial illumination is applied LEDs result in the lowest energy use (1.3).
Scenario with lowest CO2 emissions in future energy net:
Scenario 1.2 (HPS without CHP) has the highest electricity consumption, but the lowest CO2 emissions. In these scenarios it was assumed that the electricity from the grid stems from renewable sources. 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 1.2 becomes the greenhouse with the highest CO2-emission: 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 Scenario 1.4 would increase with 87.1 x 0.35 = 20 kg/(m² yr), whereas the CO2 emissions associated with Scenario 1.5 would decrease by 32.7 x 0.35=11.4 kg/(m² yr).
This shows that LEDs always outperform HPS lighting.

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