Browsing by Author "Laukkanen, Timo"

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Advanced design and operation of Energy Hub for forest industry using reliability assessment
(2023-07-25) Talebjedi, Behnam; Laukkanen, Timo; Holmberg, Henrik; Syri, Sanna
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A large part of the refining heat production in the thermomechanical pulp mill can be recovered to supply the paper machine heat demand. This study introduces a novel approach for the heat integration of a thermomechanical pulp mill and paper machine using Energy Hub. An Energy Hub consisting of a steam generator heat pump and the electric boiler is integrated with the thermomechanical pulp mill to provide the heating demand of the paper machine. The advanced cost-efficient design and operation of the Energy Hub are investigated in this research by integrating thermo-economic analysis, reliability & availability assessment, and load profile prediction. The thermo-economic analysis combines economics and thermodynamics, which is necessary for energy system unit commitments. Reliability assessment will lead to more accurate modeling of real-life system operating conditions since system components' availability is considered in the design process. Load profile prediction estimates the Energy Hub load for the next hour, which helps with the optimal operation of the Energy Hub. Different state-of-the-art long-short-term memory (LSTM) neural network models have been developed to achieve the best time series model for refining heat prediction in the thermomechanical pulp mill. Results show that all the time series models are effective for refining heat prediction, while Bidirectional LSTM appears to perform better than others with the correlation coefficient and root mean square error of 0.9 and 0.15, respectively. In addition, the proposed Energy Hub design approach is compared with the conventional design method. The proposed design method offers a robust design that isn't impacted by unsupplied demand penalty rates. Depending on the penalty rates, the total system cost could decrease by 14%-28% utilizing the proposed design method.
Aktiivilieteprosessin häiriintyminen ja toipuminen lämpötila- ja pH-vaihtelusta
(2014-12-20) Rantanen, Noora
Kemiantekniikan korkeakoulu | Bachelor's thesis
Anaerobic digestion and further processing of pulp and paper mill biosludge
(2012) Hannukainen, Heikki
School of Chemical Engineering | Master's thesis
Anaerobic digestion is able to convert organic material into biogas, which can be utilized in energy production. Even though anaerobic digestion is a widely used sludge treatment method in municipal sector, it has not gained popularity in pulp and paper industry. Due to stringent environmental regulations regarding waste new means to utilize waste streams are being developed and assessed. In this thesis, the feasibility of anaerobic digestion of pulp and paper mill bio sludge (PPBS) was evaluated. The thesis consisted of literature review and experimental part, where the mesophilic anaerobic digestion was integrated in sludge treatment process in laboratory-scale. The research objectives in this thesis were to determine the effects of anaerobic digestion on PPBS dewater ability and to optimize the biogas production potential in anaerobic digestion process. The effects on PPBS dewater ability were evaluated in filtration tests. The anaerobic digestion of PPBS was evaluated in batch digestion and in continuously fed digestion. In batch digestion, the biogas productions of mechanically disintegrated and untreated PPBS were evaluated with manometer, which registered the pressure change in closed system. In continuously fed digestion, the formed biogas was collected in gas bags, and the amount of formed biogas was compared to the dry solids reduction. Based on the results, the feasibility of full-scale anaerobic digestion of PPBS was evaluated. In continuously fed anaerobic digestion experiments the organic fraction in PPBS from UPM Kaukas mill was reduced 9·18 % depending on the solids retention time. Based on the results, the optimal solids retention time for disintegrated PPBS seemed to be close to 15 days, where the daily biogas production per sludge feed was approximately 32 m 3/ton OS. The sludge disintegration was found to improve the biogas production by 30 % in manometric experiments. In addition, the sludge disintegration increased the methane content in biogas by 13 %. Even though anaerobic digestion is able to reduce the dry solids fraction In PPBS, it does not solve the problem related with the high water content. Both anaerobic digestion and sludge disintegration were found to decrease the PPBS dewater ability. The nutrient and heavy metal analysis indicated that the digested PPBS could be utilized into some extent for example in forest fertilizing.
Anaerobisen jätevedenkäsittelyprosessin kestävyys ja toipuminen pH- ja lämpötilashokin suhteen
(2014-11-30) Peltonen, Sofia
Kemiantekniikan korkeakoulu | Bachelor's thesis
Anammox-prosessin ominaisuudet ja käyttömahdollisuudet teollisuusjätevesien käsittelyssä
(2014-04-29) Vilpanen, Maija
Kemiantekniikan korkeakoulu | Bachelor's thesis
The Anoxic selector and dechloration in pulp and paper industry
(2011) Pellikainen, Paula
Kemian tekniikan korkeakoulu | Bachelor's thesis
AspenPlus-based techno-economic analysis of solar-assisted sorption-enhanced gasification for hydrogen and chemicals recovery from polyethylene terephthalate waste
(2024-04-15) Li, Shouzhuang; Laukkanen, Timo; Jiang, Dingyi; Vuorinen, Ville; Järvinen, Mika
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Although the recycling of polyethylene terephthalate (PET) bottles is well-operating in several countries, less than 10% of PET plastic waste was recycled in a closed loop globally, and the rest was discarded as leakage. A process to recover hydrogen and valuable chemicals through solar-assisted PET sorption-enhanced gasification was analyzed. PET steam gasification with CaO and solar-assisted calcium looping processes were already experimentally studied in our previous work to obtain optimized processing conditions. In this paper, the integrated process with product purification, steam power plant, and LiBr-H2O absorption chiller was simulated with Aspen Plus based on previous experimental results to investigate the techno-economic performance. Material flows, energy balance, exergy destruction, and economics were analyzed. Day and night mode operations at different seasons were evaluated based on energy balance and the weather conditions of Naples, Italy. The energy and exergy efficiencies of day and night modes varied between 62%–72%. The annual production yields of hydrogen and benzene were 684 t and 6286 t, respectively. Due to the high production of benzene, its higher price (>1092 €/t) would make the project feasible with a larger than 12% internal rate of return value and competitive break-even hydrogen price (less than 4 €/kg). The results show that this integrated process could be technically and economically feasible. It has the potential to be implemented for hydrogen and benzene recovery from PET plastic waste with sustainable solar heat source and zero fossil CO2 emissions.
Biomass based oxyfuel combustion in CHP power plant with opportunity of oxygen storage system for carbon capture and storage
(2012) Hasan, Mohammad Mahmodul
Insinööritieteiden korkeakoulu | Master's thesis
Carbon dioxide (CO2) emission from coal-based power plants is one of the major environmental concerns since coal will remain as a dominant source of energy for the next few decades. Therefore, the CO2 emission requires to be decreased and move towards renewable energy sources to meet the environmental and sustainability targets. However, it will not be able to meet the worldwide energy demand because of the limited commercialization of renewable energy sources. As coal is the most leading energy source, it is necessary to divert a considerable phase of research work in CO2 capture and utilization for coal-based power plants to achieve the global environmental targets. In this thesis, overall features for power plant process modeling and optimization with the provision of carbon capture, oxyfuel combustion and district heating have been analyzed. The process model was designed and simulated by Prosim software changing the key parameters of coal and biomass blending and various district heating loads. The simulation results for the proposed power plant explain the effect of biomass co-firing on net efficiency (heat and power) and power consumption for Air Separation Unit (ASU), oxygen storage linked with grid electricity price and analysis of power to heat ratio. From the analysis, increased net efficiency was originated with adding more biomass with coal. This research work also focuses the general economic evaluation for oxygen storage in relation to electricity and district heating price with optimization software GAMS. Most of the research works in this field are concentrating towards carbon capture and storage only. The approach behind the thesis focuses to incorporate different variables for a proposed system along with effect of selected key variables for the process optimization.
Biomembraaniprosessi puunjalostusteollisuuden aerobisessa jäteveden käsittelyssä
(2012-11-30) Loukola, Juuso
Kemiantekniikan korkeakoulu | Bachelor's thesis
Comparison of Indirect CO2-emissions of Different Renewable Transport Fuels
(2015) Holmberg, Henrik; Siitonen, Sari; Laukkanen, Timo; Tuomaala, Mari; Niskanen, Tuomas
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The European Union's goal is to increase the share of renewable energy sources to 20 per cent and that of liquid biofuels for transport to at least 10 per cent by 2020. Liquid biofuels for transport are, for example, biodiesel and bioethanol. Their use is not assumed to increase CO2-emissions in the atmosphere. However, production processes of transport fuels need energy causing indirect CO2-emissions. To evaluate the environmental burden of these biofuels it is important to consider indirect CO2-emissions in analyses, too. This study defines indirect CO2-emissions for Digestion process, Bioethanol process, FT-process (Fischer-Tropsch-process) and Bio-SNG-process and compares their environmental burden.
A comparison of waste heat recovery from LuxTurrim 5G poles (in the Finnish energy system)
(2020-08-17) Perna Vila, Mireia
Insinööritieteiden korkeakoulu | Master's thesis
The aim of this project is to economically and energetically compare different power generation systems that use renewable sources as fuel to increase the temperature of a water line. This line is used to cool the LuxTurrim 5G Poles, which will be implanted in the Kera Area and in the Nokia Campus, in order to use the heat acquired by its electronic components, for the Low Temperature District Heating Network of this area. The extra heat provided to the water line after the poles, comes from a heatgeneration system thatuses renewable sources as fuel,therefore, somescenarios are proposed using different renewable sources and technologies to increase the temperature levels of the waterlineto thelevels of the network. Some of theproposedsystemsaremathematically modelledinMatlab and others taken from other studies, to determine the energy production per year and also, an economic analysis of each system is madeto determine the costs of production, maintenance and start-up. Finally,the Levelized Cost of Energy of each systemis calculated, in order to compare them both energetically and economically. The objective is to find out which system is the most suitable to implement in combination with the poles and to cover part of the heat demand of the district heating network, studying the energy and cost of production of eight scenarios using different combinations of technologies and fuels. It should be noted that the scope of this project does not cover the modelling of all systems, therefore some data has been taken from previous studies.
District heat network as a short-term energy storage
(2019-06-15) Kouhia, Mikko; Laukkanen, Timo; Holmberg, Henrik; Ahtila, Pekka
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
In this article, we show how a revised district heat network control strategy can be employed to utilize the storage capabilities of the network. An optimization problem is formulated, with minimum operating costs as the objective. Allowing the district heat supply temperature to vary freely within given boundaries results in approximately 2% reduction in annual heat provision costs, in comparison to a reference control scheme in a case study. The benefits of added heat storage functionality in the network are greatest when there is a large difference between district heat generation costs from the available heat sources. In general, supply temperature optimization results in increased operational hours of those plants, whose variable costs are the lowest. (C) 2019 Elsevier Ltd. All rights reserved.
Effect of biomass-based carbon capture on the sustainability and economics of pulp and paper production in the Nordic mills
(2023-01) Kuparinen, Katja; Lipiäinen, Satu; Vakkilainen, Esa; Laukkanen, Timo
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Bioenergy with carbon capture and storage (BECCS) is one of the key negative emission technologies (NETs). Large-scale implementation of BECCS has been criticized of the associated increase in land use. The existing large Nordic pulp and paper production units enable BECCS deployment without additional land use, as they currently release large amounts of bio-based carbon dioxide (CO2). The application of BECCS in pulp mills has been found technically feasible in earlier studies. This study explores key factors that affect the propensity to invest in BECCS in different types of existing European pulp and paper mills. The results give fresh understanding on the effects of BECCS on the market price of pulp and paper products and the required level of incentives. Based on statistical data, the marginal carbon dioxide credit (€ per ton CO2) to make BECCS profitable was derived. The results show that the required level of credit greatly depends on the mill type and details and that the feasibility of BECCS does not clearly correlate with the economic performance or the measured efficiency of the mill. The most promising mill type, a market kraft pulp mill, would find BECCS profitable with a credit in the range of 62–70 €/tCO2 and a credit of 80 €/tCO2 would decrease pulp production costs by 15 €/tproduct on average if 50% of CO2 emissions was captured. The EU Emission Trading System (ETS) is the main policy instrument to achieve the climate targets related to fossil energy use, but does not yet contemplate bio-based emissions.
The effect of hydrodynamics on the bed-to-wall heat transfer in a circulating fluidized bed
(1999) Laukkanen, Timo
Helsinki University of Technology | Master's thesis
Tässä diplomityössä tutkitaan hydrodynamiikan vaikutusta pedin ja seinämän väliseen lämmönsiirtoon kiertoleijukerroksessa. Tämä tapahtuu mittaamalla seinämän läheisyydessä alaspäin liikkuvien partikkelien nopeus, halkaisija sekä vertikaalitason ja partikkelin kulkeman suunnan välinen kulma. Lisäksi seinämän lähellä oleva aukko-osuus mitataan. Näiden mitattujen tietojen avulla voidaan laskea partikkelin terminaalinopeus, kontaktiaika, siis aika jonka partikkeli kulkee seinämän läheisyydessä, kontaktimatka ja poikkipinnan keskimääräinen aukko-osuus. Kaikkia näitä tietoja käytetään lämmönsiirtomallissa, josta kaasun ja partikkelien konvektiiviset lämmönsiirtokertoimet voidaan määrittää. Mittaukset suoritettiin Foster Wheeler Energia OY:n Karhulan tehtailla kahdessa eri koereaktorissa, joista toinen oli kylmä malli, joka toimi noin 25 °C:n lämpötilassa ja jonka pohjan pinta-ala on 250*1000 mm ja korkeus 7000 mm. Toinen oli kuuma malli, joka toimi noin 850 °C:n lämpötilassa. Sen pohjan pinta-ala on 650*650 mm ja korkeus 13400 mm. Mittaukset suoritettiin kuvaamalla tapahtumia videonauhalle seinämän välittömässä läheisyydessä. Nauhoituksissa partikkelit valaistiin joko yksivalaistustekniikalla, jolloin aukko-osuudet saatiin laskettua tai kolmoisvalaistustekniikalla, jolloin kukin yksittäinen partikkeli näkyi samassa kuvassa kolmena eri pisteenä. Täten saatiin partikkelin liike kuvattua. Kylmässä mallissa mittauskorkeuksia oli kaksi, 2.25 m ja 2.75 m ja leijutuskaasun nopeuksina kummallakin korkeudella käytettiin neljää eri nopeutta, 1.63 m/s, 2.12 m/s, 2.61 m/s ja 3.10 m/s. Kuumassa mallissa oli mittauskorkeuksia myös kaksi, 3.95 m ja 6.55 m, mutta vain yhtä leijutuskaasun nopeutta, 4.49 m/s, oli käytettävissä. Kaikki mitatut ja lasketut parametrit, aukko-osuuksia ja lämmönsiirtokertoimia lukuun ottamatta, esitetään jakaumina, joita verrataan normaalijakaumiin. Kaikki mitatut ja lasketut parametrit vastaavat hyvin kirjallisuudessa esitettyjä vastaavia arvoja. Lämmönsiirron osalta voidaan todeta, että partikkelin halkaisija sekä suspensiotiheys ovat keskeisimmät parametrit, jotka vaikuttavat konvektiiviseen lämmönsiirtoon. Kylmässä mallissa kaasun konvektiivinen lämmönsiirtokerroin oli selvästi suurempi kuin partikkelien konvektiivinen lämmönsiirtokerroin. Kuumassa mallissa tilanne oli sama, tosin suhde-ero ei ollut aivan yhtä suuri. Kuuman mallin niin kaasun, partikkelien kuin näistä saatavan kokonaiskonvektion arvot olivat selvästi suurempia kuin vastaavat arvot kylmässä mallissa. Tähän vaikuttivat se, että niin partikkelinopeudet kuin -halkaisijat, sekä näistä johtuvat kontaktiaika ja -matka, olivat pienempiä kuin vastaavat kylmässä mallissa. Koska käytetty kuvaus- ja analysointitekniikka sekä lämmönsiirtomalli antaa vastaavanlaisia tuloksia kuin kirjallisuudessa esitetyt, antaa se uusia näkökulmia hydrodynamiikan vaikutukselle pedin ja seinämän väliseen lämmönsiirtoon kiertoleijukattilassa.
Efficient energy solution for buildings: Leveraging long-term heat storage for optimal operation and cost savings
(2023-12-11) Tanny, Nusrat
Sähkötekniikan korkeakoulu | Master's thesis
As of 2020, Residential heating accounted for a substantial 39 TWh of energy consumption in Finland, primarily sourced from district heat, wood, and electricity, constituting 82% of heating sources. Despite the vital role of indoor heating in addressing weather challenges and high heating demands, its reliance on fossil fuels poses significant obstacles. Amid the transition towards sustainable energy, Thermal Energy Storage (TES) emerges as pivotal for improving energy efficiency. Recently phase change materials (PCM), particularly cold-crystallizing materials (CCM), have gained attention as a potential long-term heat storage solution. CCM, composed of erythritol within a polymer matrix, exhibits the capability to store heat for the long term without efficiency loss. This material achieves heat storage by cooling to a deeply supercooled state and releases stored energy through cold crystallization, making it a promising candidate for a scalable and efficient energy storage medium. This thesis aims to optimize electricity consumption costs for a year by strategically charging and discharging during low-price periods and discharging during power price variations. This study utilizes a Mixed-Integer Linear Programming (MILP) model considering three storage units based on CCM. The findings showcase potential cost reductions of 2.58%, 4.74%, and 5.89% with one, two, and three storage units, respectively, compared to a non-storage system. Current research highlights the potential possibility of CCM-based storage as a means of reducing power consumption costs and promoting energy efficiency. However, additional research and experimentation are necessary to validate these findings and address the limitations of this approach.
Electricity demand time series forecasting based on empirical mode decomposition and long short-term memory
(2021) Taheri, Saman; Talebjedi, Behnam; Laukkanen, Timo
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Load forecasting is critical for a variety of applications in modern energy systems. Nonetheless, forecasting is a difficult task because electricity load profiles are tied with uncertain, non-linear, and non-stationary signals. To address these issues, long short-term memory (LSTM), a machine learning algorithm capable of learning temporal dependencies, has been extensively integrated into load forecasting in recent years. To further increase the effectiveness of using LSTM for demand forecasting, this paper proposes a hybrid prediction model that incorporates LSTM with empirical mode decomposition (EMD). EMD algorithm breaks down a load time-series data into several sub-series called intrinsic mode functions (IMFs). For each of the derived IMFs, a different LSTM model is trained. Finally, the outputs of all the individual LSTM learners are fed to a meta-learner to provide an aggregated output for the energy demand prediction. The suggested methodology is applied to the California ISO dataset to demonstrate its applicability. Additionally, we compare the output of the proposed algorithm to a single LSTM and two state-of-the-art data-driven models, specifically XGBoost, and logistic regression (LR). The proposed hybrid model outperforms single LSTM, LR, and XGBoost by, 35.19%, 54%, and 49.25% for short-term, and 36.3%, 34.04%, 32% for long-term prediction in mean absolute percentage error, respectively.
Energy Efficiency Analysis of the Refining Unit in Thermo-Mechanical Pulp Mill
(2021-03-17) Talebjedi, Behnam; Laukkanen, Timo; Holmberg, Henrik; Vakkilainen, Esa; Syri, Sanna
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A refining model is developed to analyses the refining process's energy efficiency based on the refining variables. A simulation model is obtained for longer-term refining energy analysis by further developing the MATLAB Thermo-Mechanical Pulping Simulink toolbox. This model is utilized to predict two essential variables for refining energy efficiency calculation: refining motor-load and generated steam. The conventional variable for presenting refining energy efficiency is refining specific energy consumption (RSEC), which is the ratio of the refining motor load to throughput and does not consider the share of recovered energy from the refining produced steam. In this study, a new variable, corrected refining specific energy consumption (CRSEC), is introduced and practiced for better representation of the refining energy efficiency. In the calculation process of the CRSEC, recovered energy from the refining generated steam is considered useful energy. The developed model results in 160% and 78.75% improvement in simulation model determination coefficient and error, respectively. Utilizing the developed model and hourly district heating demand for CRSEC calculation, results prove a 22% annual average difference between CRSEC and RSEC. Findings confirm that the wintertime refining energy efficiency is 27% higher due to higher recovered energy in the heat recovery unit compared to summertime.
Energy modeling of a refiner in thermo-mechanical pulping process using ANFIS method
(2020-10-01) Talebjedi, Behnam; Khosravi, Ali; Laukkanen, Timo; Holmberg, Henrik; Vakkilainen, Esa; Syri, Sanna
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
In the pulping industry, thermo-mechanical pulping (TMP) as a subdivision of the refiner-based mechanical pulping is one of the most energy-intensive processes where the core of the process is attributed to the refining process. In this study, to simulate the refining unit of the TMP process under different operational states, the idea of machine learning algorithms is employed. Complicated processes and prediction problems could be simulated and solved by utilizing artificial intelligence methods inspired by the pattern of brain learning. In this research, six evolutionary optimization algorithms are employed to be joined with the adaptive neuro-fuzzy inference system (ANFIS) to increase the refining simulation accuracy. The applied optimization algorithms are particle swarm optimization algorithm (PSO), differential evolution (DE), biogeography-based optimization algorithm (BBO), genetic algorithm (GA), ant colony (ACO), and teaching learning-based optimization algorithm (TLBO). The simulation predictor variables are site ambient temperature, refining dilution water, refining plate gap, and chip transfer screw speed, while the model outputs are refining motor load and generated steam. Findings confirm the superiority of the PSO algorithm concerning model performance comparing to the other evolutionary algorithms for optimizing ANFIS method parameters, which are utilized for simulating a refiner unit in the TMP process.
Etikkahappopitoisen lauhteen anaerobinen käsittely
(2013-10-15) Lotti, Outi
Kemian tekniikan korkeakoulu | Master's thesis
In this Master’s Thesis the anaerobic treatment of Stora Enso Heinola Fluting mill condensates was examined and its benefits for the mill assessed. The anaerobic treatment of the condensate could reduce the load towards waste water treatment plant and produce biogas. The secondary condensate from semi-chemical pulp and board mill carries high load of fast degrading organic matter especially acetic acid, thus it could be suitable for biogas production. In the 10-day laboratory scale batch tests the biogas production rate of only 0.19 m3/kg COD, instead of expected rate of 0.38 m3/kg COD, was measured with condensate. The methane-carbon dioxide ratio at the time was 1.7. It was observed that the pH remained optimal without any neutralizing, as the organic acids were decomposed during the process. In the following phase the condensate, and the mixture of the bio sludge, condensate and the bio sludge filtrate was treated anaerobically for one and a half month in a completely mixed, semi-continuous batch reactor. In the arrangement feeding of the waste water was set continuous with a timer while the removal was performed batch wise by first separating the waste water from the biomass with a centrifuge. All samples presented a methane-carbon dioxide ratio greater than three while the proportion of methane from the composed biogas was ca. 75%. Despite of the acidity of the feed the pH remained between 6.8 and 7.5 without neutralizing. With the condensate, the COD reduction measured 75 % and somewhat lower with other fractions, except for the sample containing pre-treated bio sludge for which the reduction was ca. 85 %. According to the literature study, for the detailed dimensioning and cost assessment, IC, EGSB and AnMBR technique were selected as anaerobic process solutions. The selection between five different anaerobic condensate treatment processes was made by comparing different variables describing efficiency and cost. With an IC or EGSB reactor the production of biogas was estimated to be between 5160 and 6240 m3/d. Information about anaerobic treatment of condensate with different process solutions were received from various equipment suppliers and these proposals were used in order to assess possible laboratory measurements as well as when evaluating the benefits for the mill. Impact assessment took into account the anaerobic treatment’s energy and chemical consumption impacts to existing waste water treatment plant at Heinola Fluting Mill. All this was done parallel taking into account the potential biogas applications either for energy production at the mill site, as a vehicle fuel or selling the gas as unpurified. According to the results, the condensate is feasible for anaerobic treatment bringing savings for the mill as the organic load at the waste water treatment plant is reduced by 40%. According to the Thesis, either IC or EGSB technique would be feasible for Heinola Fluting Mill as an anaerobic process. The best option at the moment for the use of biogas is the energy production for internal consumption which would reduce the use of peat.
Evaluation of design objectives in district heating system design
(2019-01-15) Kouhia, Mikko; Laukkanen, Timo; Holmberg, Henrik; Ahtila, Pekka
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Energy system design is complex and the long utilization time of plants and the variance in economic parameters induce uncertainty into the outcome. In this article, different design objectives in a mid-sized district heating system design are evaluated. A mixed integer linear multi-objective optimization model is formulated and solved for maximum profit, minimum exergy losses, minimum CO2 emissions, minimum district heat primary energy factor and minimum district heat primary exergy (PeXa) factor. Energy system design should include metrics that take externalities into account. A combination of profit, CO2 and primary energy factor is recognized as a feasible set of design objectives. Exergy losses do not represent sustainability viewpoint well in energy system design - PeXa method expands it to fit system evaluation better. Optimization models such as this may provide relevant information for system operators, and for setting policy actions. (C) 2018 Elsevier Ltd. All rights reserved.