Case 1: Illumination

Introduction

Illumination of greenhouses in Northern latitudes requires large amounts of electricity. In the Netherlands, the standard in the early twenties is that a large fraction of the electricity consumption of the illumination is produced by combined heat and power engines. During summer, the illumination is not used. However, as the greenhouse still needs at least some heating on many nights, the average grower with a CHP-engine will run this engine in order to produce electricity for the public grid. Looking on a year round base, many greenhouses with illumination and CHP are more or less neutral in terms of electricity consumption. During 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.
Without CHP, the greenhouse imports a substantial amount of electricity. This electricity is 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.

Scenarios

In this case different combinations of illumination and sources of electricity are reviewed. The following scenarios are compared:

1. Not illuminated

2. HPS lamps. This is the ‘old fashioned' type of greenhouse lighting

3. LED illumination. As LED lighting becomes affordable, many growers switch to this efficient system.

4. HPS lamps + CHP (combined heat and power). The CHP produces most of the electricity demand.

5. LED lamps + CHP. As LEDs use less electricity, 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 day time 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 realised 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 gives of course a higher production, but also a much more constant production level.

• Because of the RTR temperature control the average air temperature is higher for the illuminated scenarios because of the higher PAR sum.

• Of course the PAR sum of the illuminated scenarios is higher

• Because of the increased crop transpiration when the lamps are on, the average RH is higher for illuminated scenarios.

• The difference between HPS and LED is of course the lower electricity consumption of the latter, but LED also leads to a lower radiation load on the crop, leading to a reduced transpiration.

 Electricity

Because of the higher efficiency, LED 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. The minus-sign in the table means that the CHP produces electricity instead of consumes electricity.

 Heat

 Sources

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 instead of HPS, the water use is a little lower. This is because the LED does not emit Near Infra Red radiation to the crop, leading to crop temperatures a little lower and hence less transpiration.
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.

Performance

The overall performance is expressed in terms of economical feasibility and sustainability. The greenhouse without illumination has the lowest energy costs, but has also a substantial lower production.
When comparing the the illuminated greenhouses, the cases with LED lighting clearly have lower costs than the greenhouses with HPS lighting. This is of course to be expected and has to be weighed against the higher initial investments for LEDs compared to HPS-lamps.
Under the assumed economical conditions, options with CHP lead to lower costs, but higher CO2 emissions.