Methodology for assessing carbon footprints of horticultural products

5.2 Combined heat and power (CHP) in greenhouse horticulture
5.2.1 Problem description
A significant proportion of Dutch greenhouse horticulture businesses produce energy as well as crops. A
CHP plant produces heat, carbon dioxide and electricity from natural gas. The heat and carbon dioxide and
part of the electricity are used in the greenhouse, the remaining energy (often in the form of electricity) is
sold. If another product besides the crop is produced, the question that arises is how the greenhouse gas
emissions from the CHP unit should be allocated between the co-products.
5.2.2 Review of solutions
Dividing the process up into smaller processes in which no co-production occurs is not possible when a
horticultural business operates a CHP unit because the production of heat and electricity by a CHP unit are
inextricably linked. There are therefore two options for allocating the greenhouse gas emissions of a
horticultural business between the crop and other products:
1. System expansion
2. Physical or economic allocation
For the system expansion option we describe the PAS 2050 method and a variant of this method that is more
consistent with the allocation principles we have formulated for horticultural products. For the allocation
option we describe, compare and assess three methods. In the subsequent sections we make
recommendations for the protocol, the calculation tool and further research. We illustrate the various options
using two example businesses: a tomato and a rose producer, both of which have a CHP unit and supply
electricity to the national grid (Table 5.2). In the example calculations we include only the greenhouse gas
emissions arising from the consumption and supply of electricity. The greenhouse gas emissions that are
associated with materials use, fertilisation and transport make a relatively limited contribution and so we
ignore these here.
Table 5.2 Production and energy inputs of the example rose producer and tomato producer
Parameter Unit Rose Vine tomato
Luminance lux 8000 0
Income from horticultural product €/kg 7.4 0.8
Income from electricity production €/kWh 0.08 0.08
Combined heat & power MWe/ha 0.55 0.5
Yield of horticultural product kg/m 2 12.5 ≤ 250 stems/m 2 56.5
Supply of electricity (in peak/off-peak hours) kWh/m 2 66 (47/19) 178 (127/51)
Gas consumption CHP m 3 /m 2 83.9 49.7
Gas consumption boiler m 3 /m 2 0 15
Electricity purchases kWh/m 2 92 10
Allocation option 1: System expansion
The principle of system expansion is that the supply of the co-product avoids the production of a comparable
product elsewhere. This method is inherent to consequential life cycle analysis. With regard to CHP, PAS
2050 expressly prescribes the method of system expansion to offset the co-production of electricity. PAS
2050 is not clear about which electricity production is avoided by applying a CHP. For the Dutch situation in
2007, we interpreted the protocol as such that the emissions avoided by the supply of electricity by growers
with a CHP unit are 463 g CO2 equivalent per supplied kWh, assuming that the average electricity production
in the Netherlands is avoided, rather than electricity supply (which includes import).
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