Absorption cooling – An analysis of the competition
between industrial excess heat, waste incineration, biofuelled CHP and NGCC.
Inger-Lise Svensson1, Bahram Moshfegh2
1,2
Energy Systems, Department of Management and Engineering, Linköping University,
S-581 83 Linköping, Sweden
1
PhD student, [email protected], +46(0)13-282767 (corresponding author)
2
Professor, [email protected], +46(0)13-281158
ABSTRACT: Utilization of excess heat from kraft pulp mills can improve the
energy efficiency of a system, but the excess heat may be used either for internal
purposes in the mill or externally, as district heating. Previous studies have shown
that the trade-off between internal and external use of excess heat from kraft pulp
mills depends mainly on energy prices and the demand for heat. The aim of this
study is to investigate how the trade-off between the investment options can be
altered when the option to produce district cooling through absorption cooling is
introduced.
The influence of absorption cooling on the trade-off between internal or external
use of excess heat is investigated through modeling the pulp mill and the Energy
Company (ECO) using the energy systems modeling tool MIND. To obtain a
broader system perspective, the kraft pulp mill and ECO are modeled within the
same system boundaries.
The results of the optimizations show that absorption cooling mainly benefits
investments in bio-fuelled CHP, but only influence the trade-off between internal
or external use of excess heat when the conditions for CHP are unfavorable, for
instance when the price of bio-fuels is high and the price of electricity certificates
is low. Compared to the baseline scenarios the same investments were profitable,
but the increased heat demand created a possibility for more electricity production
which not only increases the system revenue but also decreases the CO2 emissions
of the system.
Keywords: Absorption cooling, Industrial excess heat, CHP, Energy system
optimization
1.
INTRODUCTION
With growing concern for global
warming, energy efficiency has become
increasingly important. The Swedish pulp
and paper industry is a major Swedish
energy user and has potential for investments in new, efficient technology that can
make excess heat available [1-3]. Industrial
excess heat from kraft pulp mills can be
utilized both internally in e.g. processintegrated evaporation and drying of pulp
[2], and externally, to produce district
heating or cooling through an absorption
cooling process.
In an Energy Company (ECO) it is
possible to make several new investments,
including utilisation of industrial excess
heat. Examples of other possible investments are bio-fuelled combined heat and
power plants (CHP), natural gas combined
cycle plants (NGCC) or waste-fuelled CHP.
Gebremedhin [4] has shown that by
expanding the system boundaries to include
not only the ECO’s own utilities but also
the industry, existing plants and new
investments can be utilised more cost
effectively. This approach has been adopted
in a previous study by the authors [5, 6]
where it was concluded that the trade-off
between using excess heat from a kraft pulp
mill either externally, as district heating, or
for internal measures depends on multiple
factors. Among the most important factors
are: the composition of the already-existing
capacity in the district heating system, the
size of the district heating system, prices of
fuels and electricity and a number of policy
instruments, including the electricity
certificate system 1 . In the study
optimizations using future energy market
scenarios indicated that using excess heat
externally was more profitable in smaller
district heating systems. This is most likely
due to the fact that in larger district heating
1
The electricity certificate system is a policy
instrument to increase the electricity production
from renewable resources for example solar, wind,
water and bio-fuel, and was introduced in 2003.
systems, utilisation of excess heat competes
with large bio-fuelled CHP plants with high
electric efficiency.
When introducing absorption cooling in
a system, the trade-off between internal or
external use of excess heat may be altered
due to increased heat demand, especially in
the summer. Trygg et al. [7] argues that, in
spite of the low coefficient of performance
(COP) of absorption chillers, absorption
technology is a competitive alternative to
compression cooling when heat with low
production costs is available. Shadow
prices for heat are at the lowest level in the
summer when the need for cooling is as
highest, which favors the absorption
cooling process.
The objective of this study is to investigate how the introduction of absorption
cooling affects the trade-off between
utilizing industrial excess for internal
measures in a kraft pulp mill and/or to use
the excess heat to produce district heating
and cooling. The research questions
addressed in this study are:
• How does the introduction of
absorption chillers affect the
economic potential for external use
of industrial excess heat?
•
How does the choice of use for the
excess heat influence the system’s
global CO2-emissions?
2. METHODOLOGY
The study has been conducted through
optimizations using the energy systems
modelling tool MIND (Method for analysis
of INDustrial energy systems). The MIND
method uses mixed integer linear programming to minimise the system cost [8].
To investigate the influence of absorption
chillers on the trade-off between internal or
external use of excess heat, the model is
optimized
for
different
boundary
conditions, in this study modelled as energy
market scenarios. Using future energy
market
scenarios,
different
energy
efficiency investments can be evaluated
from an economic as well as an environmental point of view.
In the text following, data concerning
the model is displayed. For a more detailed
description of the model see previous work
by the authors [6].
2.1.
Model
The studied energy system consists of
two parts; a model of an average
Scandinavian kraft pulp mill and a model
of an ECO. The general structure of the
model is displayed in Figure 1. Broken
lines indicate investment options and solid
lines already existing capacity in the
system.
Figure 1. The general structure of the
model.
The model of the kraft pulp mill
includes the present energy balance along
with technically possible investments in
new, efficient technology. The model of the
ECO consists of the existing heat, power
and cooling capacity in the system and
possibilities for investments in new
capacity, including the option to invest in
an absorption cooling plant and to buy
excess heat from the mill. The two parts are
integrated and optimized as one system.
Through optimizing the mill and the ECO
within the same system boundary, the
optimal solution from a common investment perspective is obtained.
In the optimization the system cost for
the total system consisting of both the kraft
pulp mill and the ECO is minimized. The
objective function includes the costs of
investments, fuel costs and income from
sales of electricity and received electricity
certificates.
2.2.
The mill
The kraft pulp mill in the model is based
on data from the national Swedish research
programme “Future Resource Adapted Pulp
Mill” (FRAM) and is a model of an average
Scandinavian kraft pulp mill [9]. Using
data from the FRAM mill, several possibilities for energy efficiency within the
modelled mill have been identified [2, 10].
The suggested new investments are, e.g.
process-integrated evaporation, a new conventional evaporation plant, an increase of
the dry solid content, a retrofit of the hot
and warm water system, steam savings at
the wood yard and a new three-stage flash.
Some of the new investments decrease the
steam demand and therefore make more
steam available. In this study the steam is
assumed to be used to increase electricity
production. To be able to increase electricity production, further investments in new
turbines are needed. Some of the new investments generate excess heat of temperatures that are suitable for district
heating and others, like process-integrated
evaporation, use excess heat [2, 9]. The
mill has the option of selling the excess
heat to the ECO, but can also use it for internal purposes; hence there is a trade-off
between using the excess heat internally or
externally. In addition to the excess heat
recovered through the previously mentioned measures it is possible to use heat
pumps to increase the temperature of
excess heat of lower temperatures and use
this heat as district heating. Another possibility is flue gas heat recovery, the heat
recovered from the flue gases can be used
either for internal processes or as district
heating.
Table 2. Investment options for new
district heating and cooling capacity
Heat production
*
2.3.
The ECO
The modelled ECO is based on data
from existing Swedish district heating
systems located near kraft pulp mills [6].
The total heat demand and the different
heat production plants for the studied
district heating system are displayed in
Table 1.
Table 1. Existing heat production in the
district heating system.
Heating
*
Top load (MW)
Heat demand (GWh) *
Waste CHP (GWhheat)
Bio-fuel CHP (GWhheat)
Coal boiler (GWh)
Top load, oil boiler (GWh)
*
483
1651*
616
591
115
329
Cooling
Cooling demand (GWh)
Compression cooling
34
34
*
Information about the heat load was received from the local
energy company Tekniska Verken in Linköping
District cooling is not as common as
district heating in Sweden, but there is a
rising demand for cooling in, e.g. office
buildings and industrial processes. The
cooling demand in the model is based on a
study made by Trygg et al. [7]. The cooling
demand is assumed to originally be covered
by compression chillers in the baseline
scenarios.
For the ECO there are several possible
investment
options,
including
new
combined heat and power plants, buying
excess heat from the kraft pulp mill and
absorption chillers. The investment options
are described in
Table 2. The capacities of the possible
investments are variable, so that an optimal
size of the investment may be chosen.
Excess heat 100 °C (MW)
*
Excess heat 40/60°C (MW)
*
FGHR (MW]
**
Bio-fuel CHP (MWheat)
**
NGCC (MWheat)
Cooling production
0-13
0-43
0-4
18.5-235
18.5-126
Absorption cooling (MWcooling)
0-5
*
In accordance with [2]. COP of the heat pumps is 6.05 for
60°C and 3.63 for 40°C.
**
In accordance with [11]
Absorption chillers use excess heat to
produce cooling. The COP of absorption
chillers is low (about 0.7) compared to
compression chillers (1-3) but the fact that
they utilize heat instead of electricity makes
them competitive with compression chillers
when heat with low production cost is
available. In this study the absorption
chillers are assumed to have a COP of 0.7
and the compression chillers a COP of 3.
Absorption chillers use a small amount of
electricity for pumping (in the present study
it is assumed to be negligible) and
compared to compressions chillers the
amount of electricity used can be reduced
substantially [7] . A schematic overview of
an absorption chiller’s role in a district
heating and cooling system is shown in
Figure 2.
Figure 2. An absorption chiller in an
integrated district heating and cooling
system.
2.4.
Economic conditions
The investment costs of the possible
measures in the model are described in
more detail in a previous article by the
author [6].
In order to simulate and evaluate
decisions for future investments, future
energy market prices have been estimated
using energy market scenarios. The energy
market scenarios reflect four different
combinations of level of fuel prices and
CO2 charge as can be seen in Table 3.
Table 3. Input data for the energy market
scenarios used, based on Axelsson et al.
[12].
2006
Future energy market scenario:
1
2
3
4
Policy
instruments
CO2 charge
(€/tonne)
27
27
44
27
44
Electricity cert.
(€/MWhel)
22
16
5
16
5
Transport cert.
(€/MWhfuel)
0
0
0
0
39
Electricity price
(€/MWhel)
40
55
60
59
63
Fuel prices
including CO2
charge
(€/MWhfuel)
Oil, EO5
40
29
34
41
46
Natural gas
36
26
30
34
38
Coal
15
15
20
16
21
Bio-fuel, chips
15
17
24
18
34
-21.8
-21.8
-21.8 -21.8 -21.8
Wastea
CO2emissionsb
Bio-fuel
(tonne/GWhfuel)
329
329
329
329
159
Electricity
(tonne/GWhel)
779
374
136
723
136
Marginal
Coal NGCC Coal
Coal
Coal
technology for
power
with power with
power production
CCS
CCS
Marginal
technology for
Coal
Coal
Coal
Coal DME
power power power power prod.
bio-fuel use
a
The price of waste is negative due to the fact that waste
incineration plants are often paid to take care of waste from
several other municipalities [11]
b
Well-to-gate from marginal use
The future energy market scenarios and
prices from 2006 are applied to the model
of the kraft pulp mill and the ECO. Each
scenario has a corresponding baseline
scenario where the cooling demand is
covered by compression chillers.
3. RESULTS
In all of the studied scenarios, all
possible investments in energy efficiency in
the kraft pulp mill that have no need for
excess heat are economically profitable.
These measures do not compete directly
with the option of using the excess heat
externally, but an external use of excess
heat will lead to a decreased electricity
production within the mill. The measures
that use excess heat of the same quality as
district heating and absorption chillers are
influenced by the increased heat demand in
the ECO, but since the heating demand for
the absorption chillers is relatively small
compared to the total heating demand, the
trade-off between using excess heat internally or externally is not altered compared
to the baseline scenarios except when the
price of electricity and electricity certificates is too low to make increased CHP
production profitable.
3.1.
Use of excess heat
The trade-off between using excess heat
either internally or externally is in all cases
but one not altered when introducing
absorption cooling, compared to the
baseline scenarios where only compression
cooling was available. The only scenario
where the increased heat demand is covered
by excess heat is scenario 2, where the
price of bio-fuel is high combined with a
moderate electricity price and a low electricity certificate price, which makes biofuelled CHP less profitable than in the
other scenarios. The fact that excess heat is
used externally as district heating influences the investments in the mill that are
dependent on access to excess heat, and
less steam will be made available for
electricity production. In scenarios 2006, 1,
3 and 4 the increased heat demand is
mainly covered by a small increase of heat
production in the waste CHP and the biofuelled CHP which does not affect the
possible investments in new technology
within the mill. The distribution of the
utilities used to cover the heat demand in
each scenario and the corresponding
baseline scenarios are displayed in Figure
3.
GWh
GWh
1800
1200
1600
Oil
boiler
1400
1000
Baseline
800
Coal
boiler
1200
Excess
heat
1000
600
400
With
absorption
cooling
200
800
Bio
CHP
n
0
2006
600
400
New
Bio
CHP
200
Waste
CHP
4
4(b)
3
3 (b)
2
2 (b)
1
1 (b)
2006
2006 (b)
0
Scenario
Figure 3. Heat supply in the district heating
system. Scenarios marked with (b)
distinguish the baseline scenarios.
3.2.
Amount of electricity produced and
used
The amount of electricity produced in
the system is increased in scenarios 2006,
1, 3 and 4 compared to the baseline
scenarios. The increase is due to the larger
heat demand which is covered by CHP
plants in the ECO. In scenario 2 the
increased heat demand is covered by excess
heat which also replaces some of the heat
produced in CHP plants in the baseline
scenario, leading to an overall reduction of
electricity production. This is due to the
high price of bio-fuel and a high CO2
charge which makes an increased use of
excess heat as district heating more profitable. Some of the reduction in electricity
production in scenario 2 is also related to
the reduced electricity production in the
mill. The production of electricity is
displayed in Figure 4.
1
2
3
4
Scenario
Figure 4. Electricity production in the
system.
The amount of electricity used in heat
pumps and chillers is reduced in all cases
except scenario 2 (see Figure 5), where the
increased heat demand for absorption
chillers is covered by excess heat. The
excess heat is to some extent heat pumped
to increase the temperature to a suitable
level for district heating which increases
the total use of electricity.
GWh
70
60
Baseline
50
40
30
With
absorption
cooling
20
10
0
2006
1
2
Scenario
3
4
Figure 5. Use of electricity in heat pumps
and chillers.
3.3.
System revenue and CO2 emissions
The total system revenue increases or
remains unchanged in all scenarios. The
increased revenue is due to increased
electricity production in the ECO, see Table
4. The magnitude of the system revenue
increase depends on the given energy
market scenario. For almost all of the
studied scenarios the global CO2-emissions
will
decrease
when
implementing
absorption cooling due to the fact that
absorption cooling creates a larger heat
demand. The increased heat demand makes
it possible to produce more electricity with
CHP that can replace marginal electricity
production, as is shown in Table 4. The
exception in scenario 1 is due to the fact
that bio-fuel is used to cover the increased
heat demand. In scenario 1 bio-fuel is
strained with rather large CO2 emissions
since the marginal users of bio-fuel are coal
condensing plants [12].
Table 4.
emissions.
2006
1
2
3
4
L:2006
L:1
L:2
L:3
L:4
System
revenue
and
CO2
System revenue (M€)
Baseline
EE
24.6
25.0
46.8
47.7
33.8
34.2
48.0
49.1
25.6
26.8
CO2 emissions (ktonnes)
Baseline
EE
-296.6
-304.3
128.7
133.3
408.1
355.9
-305.1
-772.5
77.4
55.4
4. CONCLUDING DISCUSSION
The results of the optimizations shows
that implementation of absorption cooling
is a profitable investment independent of
which scenario is used to analyze the
system. The increase in system revenue is
related to the increase in heat demand
caused by the absorption chillers. The
larger heat demand makes it possible to
produce more electricity with CHP, but the
increase in electricity production is more
favourable when the price of electricity is
high and thus the greatest rise in electricity
production is seen in scenarios 1, 3 and 4.
The increased production benefits not only
the system revenue but also the CO2 emissions of the system. The CO2 emissions are
reduced compared to the baseline scenarios
due to the reduced use of electricity in the
compression chillers, but also as a result of
the increased electricity production in the
ECO which can replace marginal electricity
production.
The use of excess heat is influenced only
to a smaller extent by the introduction of
absorption cooling. The cooling demand is
relatively small compared to the heating
demand and in only one of the studied
scenarios (scenario 2) is the use of excess
heat altered compared to the baseline
scenario. As long as the prices of electricity
and electricity certificates are rather high,
an increase of the CHP production is more
beneficial than to cover the increased heat
demand caused by the absorption chillers
with excess heat.
Introduction of absorption cooling in a
district heating and cooling system benefits
the CHP production in the system.
Presently the cooling demand in Sweden is
relatively small compared to the heating
demand but with a rising demand for
cooling in industries and public buildings,
absorption cooling can help to reduce the
CO2 emissions.
Acknowledgements
The work has been carried out under the
auspices of The Energy Systems
Programme, which is primarily financed by
the Swedish Energy Agency.
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