Some Notes on the Introduction of Industrial Excess Heat and Absorption Cooling Process in a CHP System

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1 The 3 rd International Conference on Sustainable Energy and Environment (SEE 29) May 29, Bangkok, Thailand Some Notes on the Introduction of Industrial Excess Heat and Absorption Cooling Process in a CHP System Inger-Lise Svensson 1,*, Bahram Moshfegh 1 1 Division of Energy Systems, Department of Management and Engineering, Linköping University, Linköping, Sweden * Corresponding author: inger-lise.svensson@liu.se Abstract: Introduction of excess heat in a district heating system can provide an opportunity for reduction of carbon dioxide emissions. Combined heat and power plants produce both heat and electricity and introduction of heat-driven absorption cooling in an integrated district heating and cooling system can increase the potential for electricity production which in its turn may reduce carbon dioxide emissions when replacing other, more carbon intensive, electricity production. Depending on what assumptions are made for marginal electricity production the carbon dioxide emissions of the system will vary. In this paper the introduction of industrial excess heat and absorption cooling is analyzed for an integrated district heating and cooling system using three different types of fuel and two sources of electricity production. The results show that introduction of excess heat will reduce the use of resources but depending on what other electricity production will be replaced in the surrounding system, the carbon dioxide emissions are not necessarily reduced. Absorption cooling increases the heat demand and thus increases the use of resources but can still reduce the carbon dioxide emissions if the increased production of electricity is assumed to replace electricity from coal condensing power plants. If the increased electricity production replaces electricity from average Nordic electricity production, introduction of absorption cooling will increase the carbon dioxide emissions. Keywords: excess heat, resource effectiveness, carbon dioxide emissions, CHP, absorption cooling process 1. INTRODUCTION Industrial excess heat is still an unexploited resource in Swedish energy systems. District heating systems can provide an excellent opportunity to use the industrial excess heat and in many cases contribute to carbon dioxide reductions as well as a more efficient use of energy resources. In Sweden the amount of available excess heat that potentially could be used for district heating has been estimated at about 9.5 TWh [1]. Today about 4 TWh [2] of industrial excess heat is used in the Swedish district heating systems, which corresponds to about 7 % [2] of the total district heating demand. The industrial excess heat used comes from multiple types of industry such as the pulp and paper industry, the steel industry and the manufacturing industry. Excess heat is preferably used as base load in a district heating system which can cause a conflict with competing technologies such as combined heat and power (CHP). 1.1 Combined heat and power CHP plants produce both electricity and heat. To attain the highest possible economic efficiency in a CHP plant it should preferably be running at a high heating load for most of the year. Due to the difference in heat demand between summer and winter, this is usually not the case, but the CHP plants are used as base load in the district heating systems. Previous studies by the authors [3, 4] have shown that in a situation with a high price of electricity and a low price of fuel (e.g. biomass, natural gas, etc.), investing in a CHP plant is more economically profitable than investing in the use of excess heat as district heating. 1.2 Absorption cooling The demand for space cooling and cooling of industrial processes is a growing issue which is satisfied mainly by using compressor cooling machines. Compression chillers require electricity to produce cooling, which results in a significant environmental impact. Figure 1 An integrated district heating and cooling system. 1

2 The 3 rd International Conference on Sustainable Energy and Environment (SEE 29) May 29, Bangkok, Thailand Production of cooling using a heat-driven absorption machine integrated with a CHP plant can generate new possibilities. An absorption machine, which uses heat to produce cooling, could increase the heat demand in the CHP system, thus enabling increased electricity production in a CHP plant (see Figure 1). Thus the replacement of compressor cooling machines with absorption machines will decrease the carbon dioxide emissions. Previous studies [5, 6] have shown that introduction of absorption cooling is the most efficient way to supply industries and district cooling systems with cooling when CHP is available in the district heating system. 2. OBJECTIVE The objective of this paper is to explore the potential for carbon dioxide reductions and energy resource savings as well as the increase in electricity production which can be obtained through introducing excess heat and heat-driven absorption cooling in a district heating system supplied by a CHP plant. The research questions addressed are: How will introduction of industrial excess heat influence resource use and carbon dioxide emissions? How will introduction of absorption cooling influence resource use and carbon dioxide emissions? 3. METHODOLOGY The carbon dioxide emissions, resource usage and electricity production will be analyzed using a systems approach. The carbon dioxide emissions for electricity will be calculated assuming electricity used in the system originates from one of two different sources: 1) marginal electricity from coal condensing power plants or 2) average Nordic power plants Electricity produced in the system will be assumed to replace electricity produced from the same source as the used electricity. The use of resources and electricity will be analyzed through comparing the use of fuel and electricity before and after introducing industrial excess and/or absorption cooling in the system. The carbon emissions from fuels will be calculated from the use of fuel resources in each system. The carbon dioxide emissions emitted from each type of fuel is presented in Table 1. Biomass is often considered to have no carbon dioxide emission, being a renewable resource, but it can also be considered a limited resource which, used elsewhere, could replace coal in coal condensing power plants. To consider this aspect, biomass has been assigned carbon dioxide emissions corresponding to the reduction of carbon dioxide that would occur if it was used in a coal condensing plant. Table 1 Carbon dioxide emissions used in calculations Marginal electricity producer CO 2 emissions [kg/mwh el ] Coal condensing power plant 933 Average Nordic power plant 11 Marginal biomass use CO 2 emissions [kg/mwh fuel ] Coal condensing plant Fuel CO 2 emissions [kg/mwh fuel ] Natural gas 23 Waste 9 1 [7] 3.1 Systems analyzed Three types of fuel for CHP production have been considered in the calculations: biomass, natural gas and waste. Each fuel scenario is analyzed by comparing the effects of introducing excess heat and/or absorption cooling in the existing system. Absorption cooling is assumed to replace compression chillers with a COP of 3. The absorption chillers are assumed to have a COP of.7. Biomass CHP plants are considered to have a electricity to heat ratio, or so called α- value, of.43 [8], natural gas is assumed to be used in a combined cycle with an α-value of 1.1 [8] and waste CHP plants are assumed to have an α-value of.3 [8]. The analyzed scenarios are displayed in Table 2. 2

3 The 3 rd International Conference on Sustainable Energy and Environment (SEE 29) May 29, Bangkok, Thailand Table 2 Analyzed systems Scenario 1a 1b 1c 1d Fuel in CHP Biomass Biomass Biomass Biomass Excess heat No Yes No Yes Compression cooling Yes Yes No No Absorption cooling No No Yes Yes Scenario 2a 2b 2c 2d Fuel in CHP Natural gas Natural gas Natural gas Natural gas Excess heat No Yes No Yes Compression cooling Yes Yes No No Absorption cooling No No Yes Yes Scenario 3a 3b 3c 3d Fuel in CHP Waste Waste Waste Waste Excess heat No Yes No Yes Compression cooling Yes Yes No No Absorption cooling No No Yes Yes 4. RESULTS Introducing excess heat in a district heating system could replace the use of other resources such as waste, natural gas or biomass. The excess heat is available throughout the whole year, and if introduced in the district heating system it would cause a conflict with the production of CHP, which is preferably used as base load in a district heating system. The impact on the heat demand available for CHP production is presented in Figure 2. The decreased heat demand for CHP reduces the possibility for electricity production in the system. Heat production in a districy heating system (4 % excess heat) MW CHP Excess heat hours Figure 2 Distribution of heat production in a district heating system Absorption cooling increases the heat demand in the system, as is displayed in Figure 3. The increased heat demand can be used to increase the electricity production in the CHP plants as well as increase the opportunity for introduction of excess heat. M W District heating demand hours M W Heat demand including absorption cooling hours Figure 3 District heating demand with and without absorption cooling 3

4 The 3 rd International Conference on Sustainable Energy and Environment (SEE 29) May 29, Bangkok, Thailand The amount of resources used in the system is reduced by introducing excess heat, which ultimately means a reduction in carbon dioxide emissions, see Figure 4 and Figure 6. When introducing absorption cooling, the use of resources is increased due to the increased heat demand, but depending on which fuel is used and what level of carbon dioxide emissions are assigned to the electricity used and produced in the system, the impact on carbon dioxide emissions will vary. Resource use 2 15 GWh 1 5 1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d Scenario Figure 4 Resource use, scenarios 1a-1d are fuelled by biomass, 2a-2d by natural gas and 3a-3d by waste. The amount of electricity produced in the system depends on the α-value of the CHP plant used and the heat demand available for CHP production. The scenarios where excess heat is present (1b, 1d, 2b, 2d, 3b, 3d) has a lower heat demand available for CHP which results in lower electricity production. Absorption cooling will increase the heat demand (1c, 1d, 2c, 2d, 3c, 3d), thus enabling a higher production of electricity. The natural gas combined cycle in scenarios 2a-2d has a very high α-value which results in a high electricity production in comparison to the biomass CHP (scenarios 1a-1d) and waste CHP (scenarios 3a-3d). The electricity production for each scenario is displayed in Figure 5. Electricity production 5 4 GWh a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d Scenario Figure 5 Electricity production from CHP in a district heating system The carbon dioxide emissions in the system depend on three factors: the use of electricity to produce cooling, the use of fuel to produce heat and the production of electricity that may replace other electricity production in the electricity grid. Electricity is mainly used for compression cooling (1a, 1b, 2a, 2b, 3a, and 3b); even though some electricity is needed for absorption cooling it is negligible in comparison to compression cooling. The fuels used for CHP are biomass, natural gas and waste, each of which will give different results in carbon dioxide emissions, but the emissions also depend on the heat efficiency of each technology, which impacts the levels of resource use (see Figure 4). Electricity is produced in all scenarios, but depending on the electricity efficiency of the technology and the heat demand available for CHP production, the level of production varies. Scenarios 1c, 1d, 2c, 2d, 3c, and 3d include absorption cooling, which provides a larger heat demand, making increased electricity production possible. Thus, when assuming that produced electricity replaces marginal electricity, the carbon dioxide emissions are decreased. 4

5 The 3 rd International Conference on Sustainable Energy and Environment (SEE 29) May 29, Bangkok, Thailand ktonnes CO Total carbon dioxide emissions 1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d Scenario Figure 6 Carbon dioxide emissions Marginal electricity Average Nordic As a comparison of the effects of a lower excess heat potential in scenarios 1b and 1d as well as a lower COP of the cooling machines in scenarios 1a and 1b have been analyzed. A lower excess heat potential (see scenarios 1b 2 % excess heat and scenario 1d 2% excess heat) will result in a higher use of resources, e.g. biomass, compared to the scenarios where more excess heat is available (see scenario 1b and 1d). However, the reduced potential for use of excess heat allows an increased production of electricity in the CHP plant. If a lower COP is assumed for the compression cooling machines in scenario 1a and 1b the CO 2 benefits of conversion of absorption cooling will be further emphasized. The lower COP results in higher electricity use in the compression cooling machines which causes higher emissions of CO 2. The effect of the lower excess heat potential and COP is displayed in Table 3. Table 3 Effect of different excess heat potential and COP value for the compression cooling process on resource use, electricity production, CO 2 emissions. 1a 1a (COP=2) 1b 1b (COP=2) 1b 2% excess heat 1c 1d 1d 2% excess heat Resource use (GWh) Electricity production (GWh) CO 2 emissions (ktonnes) CO 2 emissions (ktonnes) Marginal electricity from coal condensing power plant 2 Marginal electricity from average Nordic electricity production 5. CONCLUDING DISCUSSION Although biomass (scenarios 1a-1d) is a renewable resource, the access to biomass is limited and the biomass is therefore assigned carbon dioxide emissions that correspond to the reduction of carbon dioxide that would have occurred if the biomass had been used in a coal condensing plant. Assigning biomass with carbon dioxide emissions reduces the climate benefit of using biomass CHP considerably in comparison to natural gas and waste. If on the other hand a local Nordic perspective is adopted, using average Nordic electricity production and considering biomass as an unlimited resource, biomass CHP causes lower carbon dioxide emissions than the other two analyzed fuels (see Figure 6). Introduction of excess heat in the district heating system will increase the carbon dioxide emissions in a marginal electricity perspective due to the fact that less electricity is produced in the CHP plant. Introduction of absorption cooling will reduce the carbon dioxide emissions, although increasing the resource use in both a Nordic and a marginal electricity perspective. The reason for this is the increased electricity production in the system. In the Nordic view biomass is considered an unlimited renewable resource, thereby the produced electricity has no carbon dioxide emissions. The natural gas combined cycle (scenarios 2a-2d) has a high α-value and thus benefits from a view where the electricity produced from natural gas is assumed to replace electricity produced from coal. However, a view using average Nordic electricity production will not benefit natural gas combined cycles, since the direct emissions from natural gas will be high and since the technology requires a higher resource use to cover the heat demand. Introducing excess heat in a NGCC system will cause a reduced possibility for electricity production, causing higher carbon dioxide emissions in a marginal electricity perspective. However, the reduced use of fuels will compensate for most of the increase in carbon dioxide emissions. For a Nordic perspective the reduced use of natural gas will reduce the carbon dioxide emissions to a 5

6 The 3 rd International Conference on Sustainable Energy and Environment (SEE 29) May 29, Bangkok, Thailand lower level than before introducing excess heat. This is due to the fact that the electricity replaced by electricity from the NGCC has lower emissions than a NGCC. Introducing absorption cooling increases the electricity production in the system, this results in higher emissions using a Nordic perspective contrary to the marginal electricity perspective which would result in lower emissions. Waste CHP (scenarios 3a-3d) has a lower α-value than both biomass CHP and NGCC due to limitations applied on the boiler using waste as fuel. This means that the benefits of reduced carbon dioxide emissions from electricity production are less obvious than for a technology with a higher α-value. Waste is to a large extent considered to be renewable, which results in rather low carbon dioxide emissions associated with the use of resources. Introduction of excess heat in the district heating system will have the same result as in the NGCC system, while the carbon dioxide emissions will increase in a marginal electricity perspective but decrease in a Nordic perspective. The introduction of absorption cooling will increase the carbon dioxide emissions and resource use in a Nordic perspective, but from a marginal electricity perspective it provides an opportunity for increased electricity production and thereby reduced carbon dioxide emissions. 7. ACKNOWLEDGMENTS The work has been carried out under the auspices of The Energy Systems Programme, which is primarily financed by the Swedish Energy Agency. 8. REFERENCES [1] SDHA (22) Industriell spillvärme-processer och potentialer, Swedish District Heating Association Report No. FVF 21149, In Swedish. [2] SDHA (29) Statistik 27-excelfil, Swedish District Heating Association. [3] Jönsson, J., I.-L. Svensson, T. Berntsson, and B. Moshfegh (28) Excess heat from kraft pulp mills: Tradeoffs between internal and external use in the case of Sweden--Part 2: Results for future energy market scenarios, Energy Policy: 36(11): [4] Svensson, I.-L., J. Jönsson, T. Berntsson, and B. Moshfegh (28) Excess heat from kraft pulp mills: Tradeoffs between internal and external use in the case of Sweden--Part 1: Methodology, Energy Policy: 36(11): [5] Trygg, L., Amiri, S. (27) European perspective on absorption cooling in a combined heat and power system A case study of energy utility and industries in Sweden, Applied Energy: 84(12): [6] Ericsson, M., Lindmark S., Martin, V., Vamling, L. (23) Systems studies of absorption and compression chillers in a combined district cooling and district heating system, International congress of refrigeration 23, Washington DC, USA. [7] Axelsson, E., S. Harvey, and T. Berntsson. (27) A tool for creating energy market scenarios for evaluation of investments in energy intensive industry. In Proceedings of ECOS 27, Padova, Italy, June, 27, p [8] Hansson, H., Larsson, S.E., Nyström, O., Olsson, F., Ridell, B. (27) El från nya anläggningar-27, Report No. Elforsk rapport nr 7:5, In Swedish. 6