3 July 2010, De Ruijter Energy Consult
Energy and CO2 emission performance of various energy supply concepts for Organic
Grower BioVerbeek in Velden
BioVerbeek are considering a new energy supply for their organic greenhouse vegetable farm to
be expanded to about 12 ha. For an optimal growth of the crops (tomatoes, sweet peppers and
cucumbers), the greenhouses need to be kept at a specific optimal temperature (that may vary
over 24 hours) as well as possible. To obtain and/or maintain this temperature, heat needs to be
supplied to the greenhouse in the cold part of the year (or of the day). Another important
production factor is the CO2 concentration of the greenhouse air. The photosynthesis in the crop
will be better as the CO2 concentration increases (to a certain threshold concentration). For this
reason, CO2 is dosed among the crop in the daytime, when sufficient light enters the greenhouse.
The energy system of the farm preferably needs to meet both the heating need and the CO2 need
of the greenhouses. Such a combined heat and CO2 supply can be realised when a carbonaceous
fuel is used. This may, for instance, be a fossil fuel such as natural gas, but also a sustainable fuel
(biomass-based, such as for instance prunings). The organic grower would like an energy concept
that has a minimum environmental impact and uses primary resources (particularly primary energy
carriers such as natural gas) as little as possible (subject to certain economic preconditions). For
an informed choice, three energy concepts were compared with respect to CO2 emission to the
environment and primary energy consumption. The three concepts are:
1. biomass boiler (using wood as a sustainable fuel); capacity 6.8 MW th)
2. natural gas-powered combined heat and power plant (CHP, in this case 3 gas engines of 2 MW e
each)
3. gas boiler (natural gas-powered; capacity 13.2 MWth)
Because the thermal capacity of the first 2 plants is lower than the peak demand, these installations
also have a gas boiler for the peak supply of heat.
buffer
fuel
Natural gas /
wood
E = electricity
combined heat
and power
plant, gas
boiler or wood
boiler
boiler
house
heat
CO2
E
T, pCO2
greenhouse
Electricity feed into mains
For primary energy consumption, the annual (net) quantity of fossil fuel consumed (measured by
energy volume and expressed in GJ) is relevant. Not only the fuel consumption of one’s own
energy plant is important, but also the primary energy consumption related to electricity bought or
fed in. If electricity is bought, the primary energy needed to generate the electricity at the power
station should be added to one's own primary energy consumption. This quantity is calculated
using the average, specific primary energy consumption of the power stations of the Dutch
electricity production system, taking into account transport and distribution losses to supply the
electricity to the user. (For this specific consumption, 9 MJ per kWh of electricity is used for these
calculations1). If electricity is generated at the grower's farm and partly fed into the mains, this
means that less electricity needs to be produced in the Dutch power stations. Analogous to
electricity purchase, this means that, per kWh of electricity fed in, 9 MJ can be deducted from one's
own consumption. In this manner, a net primary energy consumption of the farm is calculated, in
which both one's own (fossil) fuel consumption and electricity purchase and (any) feed-in are
incorporated2. Beside the heat and CO2 demand in the greenhouses, the net consumption will
depend on the type of energy supply (and specifically on whether electricity is fed in or not). The
primary energy consumptions of the 3 energy supply concepts (for the farm expanded to 12 ha,
provided with a double screen combined with active dehumidification using (drier) outside air
suctioned in from outside3), are shown in the table on the next page4.
A similar reasoning as for primary energy consumption also applies to the net CO2 emission of the
farm. This consists of the emission related to:
- own (fossil) fuel consumption (in this case only natural gas, for boiler and/or combined heat
and power plant)
CO2 emission related to the purchased quantity of electricity produced in power stations5
Avoided CO2 emission: quantity of CO2 produced less in Dutch power stations because the
grower feeds electricity into the mains.
-
The net CO2 of the 3 energy concepts is also shown in the table on the next page.
1 This
value is also used in the Greenhouse Horticulture Decree (2002) for the calculation of the energy consumption for the energy
standard therein. The primary energy consumption is expressed in GJ and calculated based on the so-called upper value of the
specific calorific value of the fuel.
2A
sustainable fuel such as for instance wood or a different biomass is not included in the calculation of the primary energy
consumption because it is a renewable energy carrier, which is not part of the limited supply of fossil energy carriers.
3 for
instance based on the "ClimecoVent system".
4 From
the report "Globale haalbaarheidsstudie houtgestookte ketelinstallatie met CO2-afvangst op biologisch tuinbouwbedrijf
BioVerbeek", July 2010.
A specific emission of 0.566 kgCO2/kWh is used for this (from "Cijfers en Tabellen" by SenterNovem, 2007). This applies to a
representative mix of gas and coal-fired power plants.
5
Table 1
Overview of (year-round) fuel consumption of wood boiler, combined heat and power
plant, and gas boiler, respectively. Furthermore, the electricity consumption/production,
the (net) primary energy consumption and the (net) CO2 emission per energy concept are
shown.
Consumptions and productions
Plant-->
Nominal thermal capacity
Thermal capacity when using CO2 dosing
Electrical capacity
Wood consumption per day (at max. production)
Number of freight movements (at max. production)
6.77
6.23
30 tons each
Number of operating hours per year
of which regular hours (for electricity)
of which off-peak hours (for electricity)
Total wood consumption moisture content
Total gas consumption boiler
Total gas consumption CHP
Total gas consumption
Wood boiler
62.2
2.1
6721
40%
11880
405316
Total greenhouse heat demand
Total heat produced by wood boiler and/or CHP
Total heat produced by gas boiler
Total heat produced
Surplus heat production
Surplus heat production as percentage of greenhouse demand
14,92 kWh/m 2
Electricity consumption (when using ClimecoVent)
Electricity production
Total primary energy consumption (fossil fuel consumption)
GJ.bw/yr Primary energy related to E-consumption
9 GJbw/MWhe
GJ.bw/yr Primary energy related to E-feed-in
9 GJbw/MWhe
Net primary energy consumption incl. electricity purchase and feed-in
in accordance with Greenhouse Horticulture Decree
Total local CO2 production
Total fossil CO2 production
4146 hours
hours
hours
31669
25764
6780
32544
875
2.8%
31669 MWh,th/yr
MWh,th/yr
31663 MWh,th/yr
31663 MWh,th/yr
-6 MWh,th/yr
0.0% MWh,th/yr
1910
1790
22157
1790 MWhe/yr
MWhe/yr
14255
17191
31446
232900
16111
-199414
49596
13227
11721
6320 tonCO 2/yr
717
11721
6320 tonCO 2/yr
125581
16111
0
141692
GJ.bw/yr
GJ.bw/yr
GJ.bw/yr
GJ.bw/yr
tonCO 2/yr
0,5660 kgCO 2/kWh
1081
Avoided CO2 emission in central electricity plants (by feed-in)
Source
3774
3171
603
31669
31044
3594
34639
2970
9.4%
93.9%
*)
t/d
a day
405316
Fossil CO2 emission reduction percentage (compared to reference)
by feeding electricity into the mains
Net emission fossil CO2 at national level
MWth
MWth
MWe
tons/yr
3570696 Nm 3/h/yr
Nm 3/h/yr
3570696 Nm 3/h/yr
11004
CO 2 related to electricity consumption (purchase)
Unit
764593
5857519
6622112
Fossil CO2 emission reduction (compared to reference)
CO 2 emission factor electricity*
Reference Reference2
CHP
Gas boiler
6.83
8.533
6.83
13.23
5.87
0
1799
1013
1013 tonCO 2/yr
-12541
0 tonCO 2/yr
193
7333 tonCO 2/yr
Cijfers en tabellen 2007
The table shows that the wood boiler plants scores best with regard to primary energy
consumption. It consumes about 31,446 GJ for 12 ha. This is only about 22% of the primary energy
consumption of the (single) gas boiler plant (141,692 GJ). The combined heat and power plant is
also considerably better than the gas boiler plant: 49,596 GJ (that is about 35% of the use of the
single gas boiler). With respect to the aspect of net CO2 emission (which contributes to global
warming, according to climate experts), the combined heat and power plant scores even better
than the wood boiler plant (which largely uses sustainable fuel without fossil CO2 emission). The
net CO2 emission of the CHP is only 193 tons of CO2 per year compared to 1,799 for the wood
boiler plant6. In this case, of the three options, the gas boiler again causes the highest
environmental load: the net emission is 7,333 tons of CO2 per year. For the CHP, this is only 2.6%
of that emission. The differences between the net primary energy consumptions and between the
net CO2 emission (and the components contributing thereto) are shown in diagrams in Figures 2 and
3, respectively.
Comparison of primary energy use of energy supply options (for 12 ha)
Own use, effect E-purchase and E-feed-in and net primary energy consumption
300000
250000
net
200000
GJ per year
150000
100000
Wood boiler
50000
Reference CHP
0
Reference2 Gas
-50000
boiler
-100000
-150000
-200000
-250000
Total primary energy
consumption (fossil
fuel consumption)
Primary energy related
to E-consumption
Primary energy
related to Efeed-in
Net primary energy
consumption incl. electr.
purchase and feed-in
Wood boiler
14255
17191
Reference CHP
232900
16111
-199414
49596
Reference2 Gas boiler
125581
16111
0
141692
31446
elements in net primary energy consumption
Figure 2 Comparison of primary energy consumption of energy supply concepts
The table and the figures show that the wood boiler scores best with regard to net primary energy
consumption (slightly better than the CHP), but that the CHP (gas engines) is the best with regard
to net CO2 emission. The gas engine combined heat and power plant (with electricity feed-in) is an
attractive option with regard to environmental load and use of natural resources, which is an overall
match for the energy supply using a wood boiler. Taking into account this good environmental
performance, the CHP option is the best choice given the difference in capital cost and operating
cost.
6 The
CHP plant achieves a better score here because the national electricity production system has a relatively high specific CO2
emission, resulting in a relatively high deduction for electricity feed-in. The specific emission is so high because in the Netherlands a
considerable part of the electricity production is generated using coals, producing more CO2 per unit of energy than generation
using natural gas.
Comparison of CO2 emission of energy supply options (for 12 ha)
Production at local level, effect of E-purchase and E-feed-in and net CO2 emission
15000
net
10000
Wood boiler
Reference CHP
tons of CO2 per year
5000
Reference2 Gas
boiler
0
-5000
-10000
-15000
production at farm
CO2 production
in power plants
due to electricity
consumption
Total fossil CO2
Wood boiler
Avoided Co2 emission in Net emission of fossil CO2
central electricity system
due to feed-in
at national level
717
1081
0
Reference CHP
11721
1013
-12541
193
Reference2 Gas boiler
6320
1013
0
7333
elements in net emission
Figure 2 Comparison of CO2 emission of energy supply concepts
1799