Identification of waste heat sources in Uppsala - with potential use in Bergsbrunna as a case study

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Master of Science Thesis TRITA-ITM-EX 2018:623 KTH School of Industrial Engineering and Management

Energy Technology

Division of Heat and Power Technology SE-100 44 STOCKHOLM

Identification of waste heat sources in Uppsala - with

potential use in Bergsbrunna as a case study

Malin Frisk

Elise Ramqvist

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Master of Science Thesis TRITA-ITM-EX 2018:623

Identification of waste heat sources in Uppsala - with potential use in Bergsbrunna

as a case study

Malin Frisk Elise Ramqvist

Approved 2018-08-29

Examiner

Anders Malmquist

Supervisor

Anders Malmquist

Commissioner

Vattenfall R&D

Uppsala kommun

Contact person

Nader Padban Linda Nylén Kristina Starborg

Abstract

Reducing energy losses within the energy system is essential for a sustainable future. Waste heat usage could be a part of an increased energy efficiency and a sustainable use of resources. Uppsala Municipality aims to become a climate positive municipality in 2050, with negative net emissions of CO2. Increasing waste heat usage represent one possible measure in order to achieve this goal. Vattenfall AB is the local supplier of heat, cooling, steam and electricity in Uppsala and has a strong ambition for a sustainable future.

The main objective of this work is to identify, quantify and classify low and high temperature waste heat sources within Uppsala Municipality. Also, the objective is to assess the potential contribution from low temperature waste heat sources for a low temperature district heating network in Bergsbrunna, a planned urban area in Uppsala. The contribution was evaluated based the technical and economic feasibility.

To reach the objectives, a survey on the waste heat and waste heat generating processes within different businesses in Uppsala Municipality was created and sent to 374 businesses of different type within the Municipality. The selection of targeted businesses types was based on the findings of potential waste heat within these businesses in the literature and limited to available contact information.

This work contributes with profiles of the waste heat transfer rate from a number of businesses on an hourly basis, which can be applied to any area to estimate the waste heat potential. Waste heat profiles were developed for grocery stores of different sizes, a restaurant, a hotel, an ice rink, and an indoor swimming pool. In addition to this, a decision-making matrix was created to facilitate comparison of the waste heat sources. The considered waste heat parameters are quantity, temperature, daily and seasonal variations and distance to the present district heating network.

Calculations of the theoretical amount of low temperature waste heat sources in Uppsala Municipality have been made based on the developed waste heat profiles and the number of identified businesses. The results show that the quantified amount of low temperature waste heat within Uppsala Municipality amount to approximately 62 GWh annually, which is available at temperatures between 22°C to 55°C.

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From the developed waste heat profiles, it was found that a grocery store has a potential or delivering between 1,200 MWh and 3,500 MWh waste heat annually depending on its size. A restaurant could potentially deliver 90 MWh waste heat annually, whereas a hotel has the potential of 80 MWh. Additionally, an ice rink and an indoor swimming pool could potentially deliver 1,400 MWh and 600 MWh of waste heat, respectively.

By means of the decision-making matrix, grocery stores and ice rinks were presented as the most prominent low temperature waste heat sources in Uppsala Municipality. Mostly due to the continuity of waste heat delivery, but also thanks to favorable geographic positions.

When evaluating the contribution of waste heat sources to a low temperature district heating network in Bergsbrunna, it was seen that the waste heat contributed to almost 14% of the heat demand if the waste heat temperature was raised to 65°C with heat pumps. However, the economic assessment shows that the lowest cost is approximately 0.34 SEK/kWh for raising the temperature to 65°C. Additionally, it was seen that the temperature of the waste heat could be raised to 85°C to be utilized in the conventional district heating network. However, the associated production cost where higher in comparison with the cost of utilizing the waste heat in a network with a lower design temperature, where the lowest cost is approximately 0.39 SEK/kWh.

It should be mentioned that a number of assumptions have been made to calculate the waste heat potential.

The most important assumption is addressed to the fact that the potential is based on secondary data of an average energy use in different buildings on a national level, which was not intentionally collected for calculating waste heat potential. The urban planning used in the case-study of Bergsbrunna is based on several assumptions. Thereby, it is not certain that this represents Bergsbrunna in the future or another area of the same size. Also, the heat production cost only includes approximated investment and installation costs of the heat pump and the electricity costs, which are based on historical data.

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Sammanfattning

Att hushålla med jordens resurser är en av det viktigaste faktorerna för en hållbar framtid. Tillvaratagande av spillvärme kan vara ett sätt att öka energieffektiviteten och utnyttjandegraden av resurserna. Uppsala kommun har som mål att vara en klimatpositiv kommun år 2050, vilket innebär negativa utsläpp av koldioxid. Spillvärmetillvaratagande presenteras som en potentiell åtgärd för att uppnå visionen om en klimatpositiv kommun.

Vattenfall AB är värme-, kyl-, ång- och eldistributör i Uppsala och har ett starkt mål inom hållbarhet.

Vattenfall är en samarbetspartner till Uppsala och ser ett intresse i möjligheterna för spillvärme i ett framtida energisystem.

Det här examensarbetet undersöker vilka spillvärmekällor som finns i Uppsala kommun och approximerar den teoretiska mängden lågtempererad spillvärme från typiska verksamheter med spillvärmegenererande processer. Dessutom undersöks hur stor mängd av spillvärmen som kan nyttjas i den planerade stadsdelen Bergsbrunna och hur stor del av värmebehovet i stadsdelen som spillvärmen kan täcka genom ett lågtempererat fjärrvärmenät.

I syfte att undersöka hur mycket och i vilken form spillvärme samt vilka spillvärmealstrande processer som förekommer inom olika verksamheter i Uppsala kommun skapades en enkät, vilken skickades ut till totalt 374 olika verksamheter. Urvalet av de olika verksamhetstyperna baserades på den spillvärmepotential som tidigare studier visat samt begränsades av tillgänglig kontaktinformation till de identifierade verksamheterna.

Timbaserade spillvärmeprofiler togs fram för ett antal verksamheter, vilka är livsmedelsbutiker, hotell och restauranger samt ishallar och simhallar. Dessa profiler kan nyttjas som bas när spillvärmepotentialen ska approximeras i ett område där en eller flera av dessa verksamheter finns. Dessutom togs en bedömningsmatris fram som förslagsvis används då spillvärmekällans olika parametrar ska summeras och potentialen jämföras med andra spillvärmekällor.

Utifrån de framtagna spillvärmeprofilerna kunde en teoretisk potential av de låggradiga spillvärmekällorna i Uppsala kommun beräknas. Resultaten visar att det approximativt finns 62 GWh tillgänglig låggradig spillvärme årligen inom kommunen samt att dess temperatur varierar mellan 22°C och 55°C.

Spillvärmeprofilerna visar dessutom att en livsmedelsbutik har en årlig spillvärmepotential mellan 1 200 och 3 500 MWh, beroende på butikens storlek. En restaurang och ett hotell skulle potentiellt kunna leverera 90 MWh respektive 80 MWh spillvärme årligen. En ishall har en potential att leverera 1 400 MWh spillvärme medan en simhall har en årlig spillvärmepotential på 600 MWh.

I den planerade stadsdelen Bergsbrunna kan spillvärmemängden från 14 spillvärmekällor i form av livsmedelsbutiker, en ishall, en simhall, ett hotell och restauranger täcka nästan 14% av det årliga värmebehovet om spillvärmetemperaturen höjs till 65°C med värmepumpar. Dessutom höjdes temperaturen av spillvärmen till 85°C för att kunna användas i dagens fjärrvärmenät. Den ekonomiska analysen visar att den lägsta produktionskostnaden uppgår till ungefär 0.34 SEK/kWh för temperaturhöjning till 65°C, jämfört med den lägsta produktionskostnaden som uppgår till ungefär 0.39 SEK/kWh för temperaturhöjning till 85°C.

Flera antaganden har gjorts för att beräkna spillvärmepotentialen i Uppsala. Det viktigaste antagandet är att andrahandsdata av medelenergiförbrukning i olika typer av verksamheter kan användas för att beräkna spillvärmepotentialen. Dessa mätvärden var inte initialt uppmätta för att beräkna spillvärmepotentialen.

Stadsplaneringen av Bergsbrunna är också baserat på flera antaganden och därav är det inte säkert att den representerar den framtida stadsdelen eller ett annat område av samma storlek. Slutligen, de ekonomiska beräkningarna inkluderar endast investerings- och installationskostnaderna av värmepumpen och elkostnaden för att höja spillvärmetemperaturen, som är baserad på historisk data.

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Acknowledgement and foreword

This master thesis is a part of the Sustainable Energy Engineering Master’s Program at the KTH School of Industrial Engineering and Management within the division of Heat and Power Technology. The thesis has been carried out at the Vattenfall Research and Development department in the Process and Chemistry team in collaboration with Uppsala Municipality.

Firstly, special thanks are addressed to our supervisors at Vattenfall R&D, Nader Padban and Linda Nylén, your expertise and guidance has been invaluable throughout this work. We would also like to thank all employees at the Process and Chemistry team for the inspiring work environment and support throughout our time at Vattenfall.

Moreover, we want to express our appreciation to Kristina Starborg at Uppsala Municipality, who has provided us with internal knowledge about the Municipality and contact information to businesses of importance to this work. We would also like to thank Magnus Åberg at Uppsala University, your knowledge and time has been beyond expectations. Also, thank you Ove Borg and Sofia Petersson at ÅF for your support regarding investments and estimations of costs.

To our supervisor at KTH, Anders Malmquist, thank you for your time and supervision for this project. We appreciate all the effort and patience you put up for us and making this master thesis possible.

Furthermore, we would like to show our gratitude to all the participants in the survey and all the people we have been in contact with throughout this work. Thank you for taking the time to answer the survey and share knowledge and information about your businesses.

Lastly, we would like to thank our family and friends for all the support throughout these intense and inspiring five years at KTH.

Malin Frisk and Elise Ramqvist Stockholm, August 2018

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Nomenclature

In this section definitions, abbreviation and indices used in this work will be presented.

Definitions

Waste heat, also defined as excess heat or surplus heat, is unexploited heat in gaseous or liquid form, rejected from a process that requires cooling or is released to the ambient.

High temperature waste heat, is generally released from industrial and manufacturing processes. The temperature level is above the supply temperature of the district heating network. High temperature steam and flue gases are examples of high temperature industrial waste heat.

Low temperature waste heat, or low-grade waste heat, is waste heat at temperatures below the supply temperature of the district heating network. Sources of low temperature waste heat are refrigeration processes and hot ventilation air for example.

Abbreviations

1GDH First generation district heating

2GDH Second generation district heating

3GDH Third generation district heating

4GDH Fourth generation district heating

CHP Combined Heat and Power

CO2 Carbon dioxide

COP Coefficient of performance

DDM Decision-making matrix

DH District heating

EAC Equivalent Annual Cost

IPCC Intergovernmental Panel on Climate Change

LCOH Levelized Cost of Heat

LTDH Low temperature district heating

MDB Multi-dwelling buildings

NZEB Near zero energy building

OTDB One- and two dwelling buildings

SCB Statistics Sweden

SEK Swedish krona

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List of figures

Figure 1: Structure of the research design used in the master thesis. ... 2

Figure 2: Seasonal differences in heat generation at Vattenfall in Uppsala. Taken from [11]. ... 5

Figure 3: Overview of the Uppsala Municipality in 2050, where Börejtull, Gränby, Gottsunda-Ultuna, Bergsbrunna and Innerstaden constitute the new city nodes. Taken from [13]. ... 7

Figure 4: Planned tramway in Uppsala, printed as the red line on the map. Taken from [15] ... 8

Figure 5: A graphic figure of waste heat recovered from a grocery store. 1 is the cooling system in the store, 2 is the district heating network and 3 is the district cooling network. Taken from [26]. ...10

Figure 6: Schematic set up for waste heat recovery in the data center Bahnhof Pionen. ...11

Figure 7: Input of energy in district heating 2016. Taken from [33]. ...13

Figure 8. Direct (left) and indirect (right) refrigeration systems used in ice rinks [38]. ...14

Figure 9. Solutions of heat recovery from different refrigeration system configurations [35]. ...15

Figure 10. Pressure-enthalpy diagram of a vapor compression cycle with indicated condensation and desuperheating. Taken from [37], modified by the authors. ...15

Figure 11. Mass and energy balance for a swimming pool [41]. ...16

Figure 12. Ventilation system with heat exchanger [45]. ...17

Figure 13. Indirect system layout in supermarket refrigeration [48]. ...18

Figure 14. Suggestion of heat recovery solution [48]. ...18

Figure 15: Comparison of the different district heating concepts. Taken from [67]. ...21

Figure 16: Schematic figure of a CHP plant. Taken from [68], modified by authors. ...21

Figure 17: Temperatures in the DH network of Vattenfall. Taken from [60], modified by authors. ...22

Figure 18: District heating network coverage in Uppsala, supplied by Vattenfall. Taken from [70]. ...22

Figure 19: An overview of the district heating network in Uppsala, where the red dots illustrates the centralized heat production units. Provided by personnel at Vattenfall [71]. ...23

Figure 20: A graph of the legionella bacteria growth depending on the temperature. Taken from [72]. ...24

Figure 21: Schematic overview of the feed-in technique between a waste heat source and the district heating network. Taken from [74], modified by authors. ...25

Figure 22: Schematic overview a heat pump cycle. Taken from [76] ...25

Figure 23: Heat exchanger between waste heat source and district heating. Made by authors. ...26

Figure 24. Electrical power supply to the compressors of the refrigeration system in a Swedish ice rink during approximately one day [34]. ...29

Figure 25. Measurements of the tap water flow during one day in Husbybadet in Stockholm [43]. ...31

Figure 26. Electrical supply in kW to the compressors in the refrigeration system of a grocery store [46]. 32 Figure 27. Electrical power (kW) supply to the compressor(s) in the refrigeration system in a commercial kitchen [49]. ...33

Figure 28. Block diagram of the inputs, constraints and outputs of the calculation model. ...35

Figure 29: Schematic overview of the heat pump used to raise the waste heat temperature in order to be fed into the DH network. Taken from [76], modified by authors. ...38

Figure 30: Historical electricity price at Vattenfall for companies in price region 3, south of middle Sweden. Taken from [84]. ...40

Figure 31: Shares of interested in selling waste heat within each type of category. ...44

Figure 32: Geographic location of the manufacturing industries. ...46

Figure 33: Geographic position of Thermofisher, AMO, Uppsala Business Park and Fresenius Kabi. ...47

Figure 34: Waste heat output from the condenser at an ice rink per year. ...48

Figure 35: Waste heat output from the superheater in an ice rink. ...48

Figure 36: Waste heat output from the pool water in an indoor swimming pool over a year. ...49

Figure 37: Waste heat output from the shower water in an indoor swimming pool over a year. ...49

Figure 38: Geographic location of the sport facilities. The red dots are indoor swimming pools and the blue dots are ice rinks. ...50

Figure 39: Annual waste heat output in a large-sized grocery store. ...51

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Figure 40: Annual waste heat output in a medium-sized grocery store. ...51

Figure 41: Annual waste heat output from a small-sized grocery store. ...52

Figure 42: Identified grocery stores in Uppsala. The green dots are large stores, the red dots are medium stores and the yellow are small stores. ...52

Figure 43: Waste heat output per year in a hotel. ...53

Figure 44: Identified hotels in Uppsala. The green dots are hotels with restaurants and the red dots are hotels without restaurant. ...53

Figure 45: Annual waste heat output in restaurants. ...54

Figure 46: The red dots illustrates the geographic position of the identified restaurants. ...54

Figure 47: A graphic overview of the annual waste heat delivery and heat demand in Bergsbrunna...56

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List of tables

Table 1: Estimated number of students and the need of schools in a newly built area with 12 000 households

in Uppsala. Based on [17]; [18]. ... 8

Table 2: Restrictions regarding specific energy use per m² in newly built buildings, where heating includes space heating and heating of water. Taken from [28] and [29], edited by authors. ...11

Table 3: Waste heat sources and potential processes within the business. ...12

Table 4: Number of businesses included in the survey, divided by the SNI 2007 code. ...28

Table 5: Limitations for identifying the number of businesses with waste heat potential in Uppsala. ...29

Table 6: Design of the DDM. ...34

Table 7: Score system for the DDM. ...34

Table 8: Assumptions regarding input values for households in Bergsbrunna, based on information stated in section 2.2.3. ...36

Table 9: Energy use in premises during 2017. Taken from [29]; [82]. ...36

Table 10: Assumed amount of different services in Bergsbrunna. Based on information stated in section 2.2.3, 2.4, 2.5.2, 2.5.3 and 2.5.4. ...37

Table 11: Input data to the calculation model of waste heat potential in Bergsbrunna. ...37

Table 12. Network tariffs N4 for electricity cost for a company in south of Sweden. Taken from [79]. ...40

Table 13: Monthly average of electricity market price, calculated based on historical data. ...40

Table 14: Respondents in each industry type of the two most interesting questions in the survey...43

Table 15: Amount of quantified waste heat available in different businesses. Based on answers in survey.45 Table 16: Compilation of identified manufacturing industries in Uppsala and the waste heat generating processes, the medium of the waste heat and the potential temperature of the waste heat. ...46

Table 17: Average temperature of waste heat per month in grocery stores ...50

Table 18: Quantified amount of waste heat and average temperature from small scale waste heat sources in Uppsala Municipality...55

Table 19: Amount of delivered heat from different waste heat sources in Bergsbrunna. ...56

Table 20: Results linked to the design of the rated power of the heat pumps. ...57

Table 21: Costs related to raising the waste heat temperature to 65 ºC in grocery stores. ...57

Table 22: Costs related to raising the waste heat temperature to 65 ºC in hotels and restaurants. ...58

Table 23: Costs in ice rinks and swimming pools related to raising the waste heat temperature to 65 ºC. ..58

Table 24: Change of electricity market price with +10% and -10%. ...59

Table 25: Results of a variating COP between 3 to 5, with 0.5 as a step. ...60

Table 26. The heat demand and production for an internal rate of 20%, 40% and 60%...60

Table 27: Compiled results of increasing heat pump rated power in the businesses. ...61

Table 28: Cost related to raising the waste heat temperature to 85°C. ...62

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1 Introduction

In this chapter, the objective of this master thesis will be presented. The research methodology and the limitations of this work as well as background to the research topic will be presented and discussed.

1.1 Background

This master thesis is carried out for Vattenfall Research and Development in collaboration Uppsala Municipality.

Vattenfall AB is a local supplier in Uppsala of heat, electricity, steam and cooling. Almost 95% of all properties in Uppsala are supplied with heat from the district heating network. Vattenfall’s combined heat and power (CHP) plant in Uppsala generates electricity, produces district heating, steam and cooling. The facility has an installed heat capacity of 828 MW and an electricity capacity of 146 MW. The main fuel consists of domestic and industrial waste Additionally, the bio fueled CHP plant in Boländerna in Uppsala has soon reached its lifetime and will, by 2021 be replaced by a new facility, Carpe Futurum. The implementation of Carpe Futurum is an essential step for Vattenfall to reach net zero emissions of CO2 by 2030 [1]. Vattenfall is participating in Uppsala Municipality’s Climate Protocol, which is a collaborative project for reduced climate impact.

The vision of Uppsala Municipality is to create a future climate positive energy system, i.e. an environmentally sustainable, climate resilient and economically viable energy system where the net emissions of CO2 is negative. In order to reach this target, Uppsala Municipality requires integrated and dynamic future urban supply chains with optimized flows of energy and materials. To optimize the energy utilization, no energy can be wasted. Thus, quantifying the amount of currently wasted heat within Uppsala and estimating its potential to add value to the future energy system are essential [2].

Moreover, Uppsala Municipality is expanding, and the number of inhabitants is forecasted to reach 300,000 in 2045 [3]. To meet this increase of inhabitants, several new urban areas are planned to be constructed in Uppsala. These new urban areas are supposed to be in line with the vision of a climate positive energy systems, in which waste heat could have a favorable impact [2].

Waste heat utilization allows for an essential opportunity for improving the efficiency of global energy systems according to the International Energy Agency (IEA). The potential is yet difficult to quantify as its magnitude is a function of the amount of heat available and its quality, i.e. temperature and pressure. In addition, potential field of applications for waste heat resources are dependent on cost and energy-efficient solutions. The European Union Energy Efficiency Directive calls for member countries to accomplish comprehensive assessment of national heating potentials from resources including waste heat [4].

1.2 Objectives

The scope of this work consists of two main research objectives. The first objective is to identify the waste heat sources in Uppsala Municipality and to quantify these regarding their physical properties and characteristics. The second objective is to evaluate the potential of low temperature waste heat sources in a low temperature district heating (LTDH) network in an urban development area. In order to reach the two research objectives, the following intermediate targets have been defined.

- Identify sources of high and low temperature waste heat within Uppsala Municipality.

- Quantify the waste heat of the identified sources

- Categorize the waste heat sources based on temperature level, medium and its state of matter, distance to distribution network and level of continuity.

- Examine the potential value of upgrading low temperature waste heat in a 60/30°C LTDH network.

- Size the main system components of the LTDH distribution.

- Evaluate the system performance in regard to economic parameters with the waste heat usage rate.

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1.3 Methodology

The objectives of this work, stated in the section above, are approached from an objective perspective with an exploratory type of research in order to investigate the potential of waste heat usage in Uppsala Municipality. In [5], it is explained that a quantitative research strives to be specific and well-structured, while a qualitative research focuses on understanding and explaining situations. The wide scope of this work was tackled by using a combination of these two methodologies, where the qualitative part results in a profound knowledge and understanding of the waste heat awareness and attitude in Uppsala Municipality.

The quantitative part is used to quantify the amount of waste heat structurally. Performing physical measurements at site is one approach to quantify the waste heat potential. This approach was rejected due to the time limit and the wide geographical area of investigation. The quantitative approach is mainly based on secondary data from previous studies and empirical tests regarding the topic of this work. This method was chosen as no measurements at site was included in the scope of this work. Moreover, the qualitative part is mainly generated from first hand contact with business included in this study and personnel with expertise in the area of waste heat recovery using unstructured interviewing. As secondary sources are used, and the problem formulation was reformulated based on the qualitative part, an inductive reasoning is applied in this work. The chosen research strategy, architecture and structure can be seen in Figure 1.

Figure 1: Structure of the research design used in the master thesis.

Firstly, an exploratory and unstructured approach was applied to identify the waste heat potential and knowledge about waste heat within different businesses in Uppsala Municipality. To accomplish the research goal and to collect the necessary data, a survey with both closed and open-ended questions was used as a research tool to collect information on current sources of waste heat. This method can be motivated by the large target group of the survey, both by the number of businesses and the variety of business types. As an identification of waste heat sources in Uppsala Municipality has not been carried out before this work, a survey was chosen as a productive data collecting method. In accordance with theories regarding questionnaire as a data collecting technique presented in [5], a survey is a time effective method to gather data and a conventional research tool. Also, previous studies in the field of investigating waste heat potential in different businesses, represented in [6] and [7], use surveys as a data gathering tool.

In order to achieve the objective of quantifying the amount of waste heat in Uppsala Municipality, the aim of the survey was to collect the amount of waste heat within different businesses and to generalize the waste heat generating processes. By assessing the survey, it was discovered that interest and knowledge regarding waste heat in Uppsala were not satisfactory, thus the survey did not meet the purpose. This lead to an attempt to identify and contact businesses by telephone and e-mail, without great success. Subsequently, a structured approach was used to further investigate unidentified sources of waste heat and was assessed by means of secondary data in the literature and in primary data provided by personnel at Vattenfall. This method is motivated by the hypothesis that waste heat sources with similar waste heat generating processes can be generalized and does not vary in a large extent from one business to another. Also, this method was used in [8], where the waste heat potential from both the industrial sector and public premises in Sweden was quantified by the Swedish Energy Agency.

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By quantifying data of different low temperature waste heat sources and generalizing patterns of waste heat generating processes, a vast work regarding data transformation has been carried out. The data deconstructing analysis technique aimed to transform different quantitative data from secondary sources into rejected waste heat per hour for some selected types of businesses, represented on average. Focus was concentrated to businesses with low temperature waste heat, commonly present in a municipality. By studying low temperature waste heat generating processes within the typical businesses and by assuming average sizes and characteristics of the businesses, profiles of the released waste heat on an hourly basis for a normal year were estimated. The different profiles were then used in the quantification of waste heat in Uppsala Municipality and applied in the heat demand and supply calculations of the newly development area Bergsbrunna.

Secondly, calculations were carried out to estimate the potential quantity of waste heat in Uppsala Municipality. The input values of the calculation model included the compiled waste heat profiles and the number of identified business with waste heat generating processes. This method was chosen with the purpose of quantify the amount of waste heat potential in a standardized way, where the calculation model could be applicable in other regions with similar constraints. In order to systematically analyze the potential of different waste heat sources, a weighted decision-making matrix (DDM) has been created. The purpose of the matrix was to classify the feasibility of waste heat usage in the current energy system of Uppsala.

Thirdly, the waste heat profiles together with an approximated heat demand profile of the future urban area Bergsbrunna in Uppsala Municipality were evaluated from a system perspective, by comparing potential heat supply from waste heat sources with the demand profile. This method is used in [9], where the waste heat potential from low temperature waste heat sources in a newly developed area in southern Sweden is investigated. The urban planning of Bergsbrunna and the setup of the new city node was compiled by a qualitative research of other newly developed areas in Sweden and the guidelines for newly developments in Uppsala. A synthesis was carried out by combining multiple assumptions regarding urban planning and data gained from the first phase, which was used to quantify the features of the calculation model. The calculations made in this part of the work, aimed to demonstrate a possible application of low temperature waste heat sources of various size in a LTDH network. Also, this method can illustrate the waste heat potential and impact on the centralized heat production. The feasibility of the application was measured with the levelized cost of heat (LCOH), to compare the results of this work with other application areas.

This method was chosen since LCOH is a conventional approach and the outcome is comparable to other heat production techniques and investments coupled to the technique.

Lastly, as assumptions are made, and secondary data is used in this work, a sensitivity analysis was essential.

The sensitivity analysis was carried out to identify sensitive parameters in the application of using waste heat sources in a heat system of a newly developed area. Both technical and economical parameters were tested, and the results were analyzed and discussed by comparing these to the reference case.

1.4 Limitations

The main scope is to identify the potential sources of waste heat and quantify the amount of waste heat in Uppsala Municipality. The considered sources of waste heat in this work consist of heat bound in gaseous or liquid form. The assessment of the potential sources has been limited to the theoretical investigations only and no measurements have been carried out. Thereby, the calculation of the waste heat potential within different business is limited by the availability of appropriate data in the literature or by data provided by the businesses included in the survey.

Waste heat profiles were developed for a limited number of business types, typically found in a municipality.

Thus, not all different business types typically found in an urban area were evaluated. The number of investigated businesses were limited by time constraints and the business types were selected based on their potential of waste heat described in previous studies. Thus, the business types that were described as most prominent in the literature, were selected and their waste heat profiles were developed.

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The businesses included in the survey sample group with waste heat sources are based on previous studies and waste heat audits in other geographical areas. Since the geographical area of interest in this work is large, a large sample group was created. Nevertheless, some sources of waste heat within the Municipality might have been unintentionally neglected.

The amount of waste heat available is calculated for the existing technical systems and their limitations, no future progresses of the systems have been considered. Also, heat demand profiles and energy use are based on historical data and is assumed to follow the same pattern in the near future.

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2 Literature study

A literature study has been carried out to study different research areas needed for further investigations within this work.

2.1 Current energy demand in Uppsala

In [10], a vast compilation of the energy situation in Uppsala has been carried out. The data is mainly based on Statistics Sweden (SCB), both for Uppsala and Sweden in general, and data from the distributors of electricity and heat in Uppsala. The district heating is by far the most common heating solution in multi dwelling buildings (MDB), where this accounts for nearly 96% in 2011. In contrast to this, about 35% of one- and two dwelling buildings (OTDB) uses district heating, 28% electrical driven heat solution, 17%

wood as energy source and 10% geothermal energy. Since Vattenfall AB is the owner of the district heating network in Uppsala, the heat production can illustrate the seasonal variation of heat demand of households in Uppsala. As seen in Figure 2, the heating demand varies between seasons and by Swedish summer the heating demand in Uppsala can be covered by the waste incineration plant but during winter the CHP plant is needed [11].

Figure 2: Seasonal differences in heat generation at Vattenfall in Uppsala. Taken from [11].

The total demand of district heating in Uppsala amount to 1.3 TWh in 2016 [12].

2.2 Vision of Uppsala 2050

Uppsala is a fast-growing region and presents a vision of a climate positive municipality in 2050, with negative net emissions of CO2. In this section, the visions and predictions of the municipality is described.

This chapter includes both targets, visions and urban planning in Uppsala Municipality until 2050. Also, some of the strategies to reach their ambitious climate targets are mentioned.

2.2.1 Energy and climate targets

The climate and energy targets in Uppsala Municipality are presented in the part below.

Climate targets

Uppsala Municipality has formulated climate targets in line with the Fifth Assessment Report of Intergovernmental Panel on Climate Change (IPCC) from 2013 and the Paris Agreement. Compared to the global timeline, Uppsala has set a shorter timeline to reach the climate targets. The climate targets for Uppsala are:

• The local energy use in Uppsala is fossil free and renewable in 2030

• Uppsala is climate positive for the overall greenhouse gas emissions in 2050

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The local emissions considered in the first target include energy generation and distribution, energy use and transportation. The second target considers the non-energy related emissions allocated the agricultural and industrial sector within the municipality and the emissions from the citizens’ long distance travelling [2].

Energy targets

Uppsala Municipality has formed a regulatory document, the “Energy program 2050”, that provides guidelines and strategies for reaching the targets of the future energy sector. In 2050, Uppsala is climate positive with an energy system facilitating sustainable environmental, social and economic functions and development within the society.

Three main targets for the energy sector are formulated to develop and reach a climate positive Uppsala in 2050, which described in the following section.

Energy target 1: Integrated social system resulting in synergy profits

The objective of integrating the energy system with other systems within the municipality is to increase the resource effectiveness and to create synergies between several sectors in the society. In particular, the energy and the waste management systems, as well as the water systems, will be integrated to a great extent. The future energy sector will take advantage of waste flows, such as waste heat and electricity within the energy sector and from other sectors to distribute and supply the waste flows to other technical systems in the municipality. Energy storage is also suggested as a part of the future local energy system.

According to Uppsala Municipality, the greatest potential for enhancing synergies in today’s systems can be found in unnecessary high-grade energy carriers, for example where electricity usage can be replaced by use of local waste heat. To enable these synergy profits, Uppsala Municipality has developed strategies of the future system with regards to the advantage of integrated and connected systems so that synergies are possible. Lowering the temperature levels in heating systems in order to enable usage of waste heat is one example of development which Uppsala Municipality acknowledges as desirable in the future.

Energy target 2: Resource effective energy supply with high use of local resources and closed cycles

Uppsala Municipality proposes that resource effectiveness will be reached by means of efficient use of energy and by high performance buildings with integrated solar collectors, for example. Moreover, the future energy resources are renewable, where the major part of the resources are locally or regionally available.

Uppsala Municipality aspires to a future society of closed cycles where biogas is produced from sewage sludge and municipality waste and waste energy is used. By recycling unutilized waste flows of energy, Uppsala Municipality intends to reach a system of circular economy with an efficient use of resources. In agreement with these visions, the aim is to secure supply without compromising any climate targets.

Therefore, the Municipality emphasizes that both distribution and storage must be possible in future energy networks [2].

Energy target 3: Available, secure, equal and integrated energy system

Historically, the energy system in Uppsala Municipality has been characterized by large-scale centralized energy generation with a linear and one-way distribution network between the production unit and the consumer. The development towards an integrated and non-linear energy system in Uppsala have been facilitated through Vattenfall’s combined heat and power plant (CHP) and the utilization of waste heat from Uppsala Vatten och Avfall. The vision of Uppsala Municipality is that the future energy system will incorporate several energy production units of different size and for different kind of energy carriers to secure the supply and availability. Moreover, additional small-scale and local producers and users will share the flows and storage capacity of local systems to decrease the load of the centralized network and to increase resilience. However, the local energy systems will operate in relation to the main systems [2].

The Municipality desires an increased flexibility within the energy system, which will contribute to equal availability of affordable energy for all consumers, whom can also act as producers in the local system.

Uppsala Municipality visions end users, such as the industry or residential sector, contributing to the flexibility by admitting control of their energy use to some extent. For example, the heating system of several

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buildings can be controlled centrally through an aggregator to shave the peak hours to maintain the balance of the energy system [2].

2.2.2 Demography

The population growth trend of the municipally of Uppsala is positive, with an increasing number of newborns and people moving to Uppsala. The number of inhabitants is forecasted to reach 300,000 around 2045, which is based on previous trends [3]. Until 2050 the population is forecasted to increase around 50%

from year 2016. Thereby, the demand of job opportunities and residential buildings also increases. To manage the population growth successfully, Uppsala plans to build 3,000 new households and create 2,000 new job opportunities each year the upcoming decades [13].

The municipally will be divided into five city nodes in 2050, where the city center will account for the main node. The other city nodes included in the strategy are Gränby, Gottsunda-Ultuna, Bergsbrunna and Börjetull. An overlook of the position of the city nodes in 2050 can be seen in Figure 3. Each city node should include every day services, such as residential buildings, schools, parks and job opportunities. The public transport and extended bike lanes connects the different city nodes, where the connection to the main node will be prioritized [13].

Figure 3: Overview of the Uppsala Municipality in 2050, where Börejtull, Gränby, Gottsunda-Ultuna, Bergsbrunna and Innerstaden constitute the new city nodes. Taken from [13].

2.2.3 Urban planning - Bergsbrunna

As mentioned above, Uppsala Municipality is one of the fastest growing municipalities in Sweden. Thereby, new urban areas in Uppsala are being planned to be constructed. Bergsbrunna is one example of a new urban area in Uppsala, placed in the south part of the city, and will be further described in this section.

Transportation

The new station for commuter trains and inter-city trains in Bergsbrunna will be an important connecting point between the city centre of Uppsala and Stockholm, which requires an expansion of both railways and public transportation. At latest in year 2029 the railway between Uppsala and Bergsbrunna will consist of four railroad tracks to handle the increased number of commuters.

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Also, a new tramway, that is illustrated as a red dotted line in Figure 4, will be developed between Bergsbrunna, Ultuna, Gottsunda and Uppsala C. In 2033 the part between Bergsbrunna and the border of Stockholm County will be extended to a four railroad tracks [14].

Figure 4: Planned tramway in Uppsala, printed as the red line on the map. Taken from [15]

Urban environment

About 10,500 new households are included in the urban planning of the surrounding area, about 1 km from the new train station in Bergsbrunna. Approximately 5,000 to 10,000 new workplaces could be established in Bergsbrunna. The urban planning of Bergsbrunna is at early stage with few guidelines. However, it is mentioned that the area reserved for different services and businesses amount to 650,000 m² [15].

In [16], a study was carried out to identify possible outcomes, which needs to be considered in the urban planning of Uppsala. Two different future scenarios, “base” and “high”, are examined. The “base” scenario present Uppsala as dependent on the job possibilities in Stockholm, whereas the “high” scenario present Uppsala as an independent city. The key figures presented in this study indicate that the share of MBD and OTDB is 75% and 25%, respectively, for both the “base” and “high” cases in 2050. The inhabitants per household are 2 for MDB and 2.3 for OTDB, these key figures also vary very little or nothing between the cases. In MDB the floor area per inhabitant is 37 m² and 60 m², for both cases in 2050.

These key figures and other outcomes of the study are used in the extended overview plan for another area under development in Uppsala, called “Södra staden” [17]. This part of Uppsala has come further in its development and progress of the urban planning and could thereby be useful for the approximation of the structure in Bergsbrunna. For instance, the number of schools needed per 12,000 households is proposed in the overview plan and by a compilation with the program for school facilities in Uppsala, an approximation of the needed area reserved for schools is stated in Table 1.

Table 1: Estimated number of students and the need of schools in a newly built area with 12,000 households in Uppsala. Based on [17]; [18].

Type of school Schools Students Area per student [m²] Total area [m²]

Pre school 13-20 1,900-2,100 8.3-8.7 15,770 – 18,270

Lower school 4-5 1,200-1,400 9.0 10,800 – 12,600

Grammar school 3 950-1,100 7.5-8.0 7,125 - 8 800

Junior high school 3 950-1,100 8.0 7,600 – 8,800

Senior high school 1 950-1,100 8.0 7,600 – 8,800

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According to [19], Uppsala had 0.36 grocery stores per 1,000 inhabitants in 2011. However, the amount of grocery stores in Bergsbrunna also depend on the average number of potential customers. In other newly developed areas, such as the Royal sea port in Stockholm with a plan of 12,000 new household until year 2030, only two grocery stores have been established upon now [20]. Other services, such as shopping centers, hotels and restaurants, could also depend on the number of potential customers in the area. As Bergsbrunna is a commuter node, these services could be favorable in the area.

Energy system

In the overview plan of Uppsala Municipality, Bergsbrunna is described as an area with an innovative energy plan and that new energy technologies should be encouraged [13]. A strategy of energy innovation could be a LTDH network, with small scale producers of heat included. Since this area is not yet built, the buildings can be designed with a lower temperature use, than conventional, from the district heating network. As mentioned above, the urban planning of Bergsbrunna is not yet planned in detail and this includes the energy system as well, which is not planned at all.

2.3 Previous projects

In this section, different examples of waste heat utilization in previous and ongoing sustainable urban development projects will be presented. Also, some examples of low temperature waste heat utilization in district heating network are highlighted.

2.3.1 Stockholm Royal Seaport

Stockholm Royal Seaport is a new city district that will be fully developed in 2030. In 2010, Stockholm City Council decided that the area was going to be a sustainable urban district and an international model for sustainable urban projects [21].

In Stockholm Royal Seaport, source-separating wastewater systems is an ongoing initiative, which aims to optimize the utilization of resources and heat within the wastewater in order to create synergies over system boundaries. The wastewater flow is separated into food waste, toilet water and other wastewater, which simplifies the utilization of resources and heat in wastewater. By separating the high temperature waste water from the remainder waste water, the possibility to utilize waste heat from the waste water is increased [21].

Installations of heat exchangers within the system boundaries of the building are promoted to recover heat from the building’s wastewater. FTX systems, i.e. exhaust and supply air ventilation with heat recovery, have been installed in the low-energy buildings [20].

Stockholm City Development Administration manages the development of Stockholm Royal Seaport and the cooperation with all stakeholders. However, the different property developers are responsible of reaching the required maximum level of energy use per square meter (kWh/m²) in the residential buildings by means of taking measures such as using heat recovery technologies for example [22].

2.3.2 Albano

In the northern part of Stockholm city, Albano is a campus area under development with the objective to reach net zero emissions of CO2 and net zero purchases of energy. Albano is a cooperation project in which Akademiska Hus (property developer), Svenska Bostäder (property developer), Stockholm University, KTH Royal Institute of Technology in Stockholm and Stockholm City.

After evaluation of possible energy system solutions, the most promising system for Albano includes a centrally located borehole thermal energy storage (BTES) and a local low temperature distribution network in combination with local heat pumps.

The feasibility study of the project evaluated the potential of integrating solar thermal collectors in the area.

It was concluded that almost half of the produced heat during summer would be wasted due to unbalance in production and load. Thus, efficient seasonal thermal energy storage is required in order to decrease the thermal losses from the solar collectors [23]. An IT/data center with installed air-to-water heat pump system

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to recover waste heat from the data servers will be located on the campus area. The waste heat is recovered from the cooling need of the sever units and the waste heat is used to heat the circulating water to approximately 35°C to 40°C. The output is connected to the central heat distribution substation on the campus area. In this way, more than 23,000 MWh of energy per year is saved and the purchased district heating is reduced by 25% [24]. Additionally, the Research Institute of Sweden (RISE) have evaluated heat exchangers for wastewater recovery and have concluded that they can reach an efficiency of 25%. In the project of Albano, an efficiency of 23% was assumed and it was concluded that the annual heat savings can reach 380 MWh [23].

2.3.3 Hyllie

In Malmö a new city district, called Hyllie, is under construction with the aim of being the climate smartest area in the region of Öresund. The district heating network allows a supply temperature down to 65°C and the return temperature around 30°C. In [9], a study has been made to investigate the contribution of small scale waste heat sources to the district heating network. The waste heat sources are called prosumers since they both consume heat and produce heat. The amount of buildings included as district heating customers in the system was 43 buildings, where the investigated prosumers was a shopping center, a grocery store and an ice rink. The study shows that the small-scale waste heat sources has a potential of covering 50% of the annual heating demand. It is also concluded that prosumers with year around cooling demand, such as ice rinks and grocery store have the best potential to deliver waste heat that can cover the heat demand.

However, it is also concluded that if the ratio of residential buildings increases the waste heat potential from small scale waste heat sources will decrease [9].

2.3.4 Stockholm Exergi – Open District Heating

The heating company Stockholm Exergi, former Fortum, has developed a business model, which allows small scale waste heat sources to deliver and sell waste heat to the district heating network. In general, a process that can deliver 1 MW of continuous waste heat annually at the required district heating temperature could get 1.5 million SEK in revenue per year. This results in an approximative income of 0.17 SEK/kWh [25].

Grocery stores is an example where the cooling demand is high, and the cooling device runs with high load, a constant demand and are often located near the city. Thereby, the potential to recover waste heat can be successful. One example is the grocery store Coop Rådhuset in Stockholm, that has 50 kW fridges and 20 kW freezers installed. These supplies both the district heating and the district cooling network with waste heat, which can be seen in Figure 5. In this case, the district cooling and the district heating network is connected to the cooling system in the grocery store. Thereby, the waste heat can be recovered in the network and the grocery store can sell the waste heat to the district heating company [26].

Figure 5: A graphic figure of waste heat recovered from a grocery store. 1 is the cooling system in the store, 2 is the district heating network and 3 is the district cooling network. Taken from [26].

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Another waste heat source available in the Open District Heating concept is data centers, which have a potential to deliver waste heat due to the cooling demand all year round. The data center Bahnhof Pionen is connected to the Open District Heating, where the waste heat from the cooling machine is delivered to the district heating network. In Figure 6, the waste heat recovery and cooling system can be seen. The number 1 is the heat pumps connected in series, number 2 is the cooling coil batteries, number 3 is the existing cooling machine used for back up and number 4 is the district heating network connection. The heat pumps have a total cooling output of 694 kW and a total heating output of 975 kW. The waste heat delivered to the district heating network has a temperature of 68°C [27].

Figure 6: Schematic set up for waste heat recovery in the data center Bahnhof Pionen.

2.4 Heat demand in new development of building

In 2021 all new development of buildings should follow the commission targets of the European Union and be labelled as “nearly zero-energy buildings” (NZEB) [28]. The requirements regarding the specific energy use is compiled by the Swedish building regulations unit and can be seen in Table 2, where the domestic hot water is based on the normal use according to the Swedish building regulations [29].

Table 2: Restrictions regarding specific energy use per m² in newly built buildings, where heating includes space heating and heating of water. Taken from [28] and [29], edited by authors.

Type of building NZEB

Heating Domestic hot water

[kWh/m2Atemp] [kWh/m2Atemp]

OTDB 55 20

MDB 15 25

Non-residential premises 18 2

In the regulations of the specific energy use the amount of energy needed for heating is declared separated and include both space heating and domestic hot water. The energy agency of Sweden has evaluated the methods to standardize the amount of energy used for water heating in residential buildings by measuring the total water use in 44 households and the amount of hot and cold water use in 10 households. Thereafter, the measured values are compared to the suggested methods to calculate the hot water use in various reports.

According to this comparison, the amount of hot water use should not be calculated by assuming a percentage of the total energy use. Hence, the building insulation and energy efficiency actions concern the space heating foremost. The recommended method to calculate the hot water use is by calculating the energy needed per square meter heated floor area. Even though, data from individual hot and cold-water meters is addressed as soon available data in the report, written six years ago, no new data is available regarding this

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topic [30]. However, the Swedish building regulation uses 20 kWh/m2Atemp as energy needed for domestic hot water in OTDBs 25 kWh/m2Atemp for MDBs and 2 kWh/m2Atem in non-residential premises. Schools usually use around 4 kWh/m2Atemp, this could change depending on whether the schools have a kitchen or not. These values are used as guidelines for calculations of the energy performance in new development of buildings, but variations between households could occur [29].

2.5 Sources of waste heat

Waste heat can be generated in many different processes, which results in both useful and non-useful heat.

This section describes the different areas where waste heat is most common and describes the waste heat generating processes of typical low temperature waste heat sources, such as sport facilities, grocery stores, hotels and restaurants. By compiling information from both [8] and [31], Table 3 was created. The table illustrates the processes which generates waste heat and the potential within the different businesses. Waste heat is conventionally generated within the industrial sector and different industries have different waste heat sources. The waste heat sources can be divided into cooling or heating processes. Examples of cooling systems include indoor cooling, cooling in data centers and manufacturing processes, food refrigerators and freezers among others. Examples of heating processes consist of generating steam and processes with hot flue gases, processes including drying and evaporation et cetera [8].

Table 3: Waste heat sources and potential processes within the business.

Process Medium Temperature

Industry

Drying Refrigeration Process cooling Indoor cooling Water heating Evaporation Condensation Process air Ventilation air Water Steam Flue gases 20-50ºC 50-70ºC Over 70ºC

Manufacturing x x x x x x x x x x x x x x x

Grocery stores x x x x x x x

Hotel and restaurants x x x x x x x x

Sport facilities x x x x x x x x x

Other premises x x x x x x x

Waste heat in smaller scale is historically not economically beneficial and thereby not as common within waste heat recovery. However, with a new law conformed to the district heating network in Sweden, the opportunities for smaller producers of waste heat have increased. The new law encourages the district heating companies to investigate the potential of waste heat delivery from external actors on the market and increase the economic incentives to recover waste heat [32]. Two of the largest heating distribution companies in Sweden, Vattenfall AB and Stockholm Exergi, have started business opportunities to recover waste heat from different industries. Examples of collaborations with the business model Open District Heating at Stockholm Exergi was mentioned in section 2.3.4.

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13 2.5.1 Industrial waste heat

Waste heat recovery used in the energy systems of today primarily consists of industrial waste heat. During 2016 about 4.8 TWh waste heat was supplied to the district heating network in Sweden, which mainly originated from manufacturing industries. This amount of waste heat composed 7.9 % of the total energy input for district heating and the share of different energy inputs can be seen in Figure 7 [33].

Figure 7: Input of energy in district heating 2016. Taken from [33].

In Swedish industries a major fraction of waste heat is below 70°C or in the range of 70°C to 90°C, which are considered as low temperatures [7]. Waste heat from energy intensive industries, which usually delivers waste heat at higher temperatures, has traditionally been more interesting for the district heating. In general, industries with high energy demand, usually has a greater potential of delivering waste heat to an external party. In contrast to this, the system can be more resilient if there are several smaller suppliers than few large suppliers, which could increase the interest for all types of suppliers to contribute. In Sweden, the pulp and paper industry, iron and steel industry and the chemical industry was in 2007, the most prominent suppliers of waste heat to the district heating network [8]. In Uppsala Municipality, the largest manufacturing industries consist of pharmaceutical industries and a saw mill. These industries and the waste heat generating processes will be further described in this section below

Pharmaceutical industry

The pharmaceutical industry is generally not as energy intensive as the overall chemical industry and usually has a batch production instead of continuous production. At site, steam production is usually necessary in order to sterilize products. Evaporation, process cooling and reactor heating could also be included in the process. In pharmaceutical industries, high regulations concerning quality and hygiene usually makes the companies prioritize improvements within those areas rather than questions regarding energy and waste heat recovery. Thus, this does not decrease the quantity of the waste heat within the industry. However, the potential of waste heat within pharmaceutical industries might decrease due to a varying production over the year [8].

Wood processing – saw mill

Within the forest industry, wood processing is included. Drying is an essential process within all wood processing, around 70% of the energy use stands for drying in a saw mill and is usually a continuous production. The heat from the drying process has a potential for waste heat recovery. The heat used for drying is usually produced at site by a biofuel fired hot water boiler, where the biofuel is wood waste from the production. Thereby, waste heat by flue gas condensation could be another process with waste heat potential within the wood processing industry [8].

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In 2009, the Swedish Energy Agency publicized a report on national energy statistics for sport facilities [34], in which 44 indoor ice rinks and 17 indoor swimming pools in different areas in Sweden were examined. It was concluded that on average, the refrigeration systems in ice rinks have an electrical energy use of 84 kWh/m² [34]. The results of energy use presented in [34] are weighted at the national level and corrected to a normal year with the degree day method.

Ice rink

In order to estimate a potential amount of available waste heat in ice rinks, the ice rink refrigeration system need to be studied. The cooling capacity of a typical ice rink should be around 300 to 350 kW and there is a permanent need for cooling during the opening season [35].

Electricity powered vapor compression refrigeration cycle is the most conventional refrigeration system used in ice rinks. The refrigeration unit can be directly or indirectly connected to the ice sheet to maintain the ice at approximately -5C. In Swedish ice rink arenas, indirect vapor compression refrigeration cycles with ammonia (R717) as refrigerant is the most common refrigeration method [36]. A schematic view of a direct and an indirect refrigeration system is presented in Figure 8. The main system components are the condenser, evaporator, expansion valve and the compressor. Several compressors can be coupled in parallel to improve the refrigeration system performance [37].

Figure 8. Direct (left) and indirect (right) refrigeration systems used in ice rinks [38].

The refrigeration capacity, Q^C, in a vapor compression cycle is the useful cooling at the evaporator, which is dependent on the system cooling efficiency, the coefficient of performance (COPC) and the compressor electrical power and can be expressed as

𝑄_𝐶 = 𝐸_{𝑐𝑜𝑚𝑝}∙ 𝐶𝑂𝑃_𝑐, Eq. 2.1

where Ecomp is the compressor power.

The refrigerant is compressed at high pressure and temperature in the compressor, after which the refrigerant is condensed at constant pressure and temperature releasing its latent heat to a cooling medium in the condenser. Before the refrigerant enters the condenser, it can be desuperheated and reject high temperature heat. The heat transfer rate, 𝑄_𝐻, at the condenser can be expressed in terms of the cooling capacity and the compressor power as

𝑄_𝐻= 𝐸_{𝑐𝑜𝑚𝑝}+ 𝑄_𝐶, Eq. 2.2

according to the second law of thermodynamics.

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There are different heat recovery solutions for ice rink refrigeration systems and the waste heat recovery rate varies for different ice rinks in Sweden. Some examples of waste heat recovery strategies in different system designs can be seen in Figure 9.

Figure 9. Solutions of heat recovery from different refrigeration system configurations [35].

In the system configuration displayed in the middle of Figure 9, heat is released to the ambient environment.

The top-left system includes heat recovery by means of a desuperheater, which enables a high discharge temperature. The top-right layout is a heat pump cascade solution for subcooling, in which the heat is recovered in a subcooler after the condenser to increase the efficiency of the refrigeration system. The bottom-left system is also a heat pump cascade solution where low grade heat is recovered from the condenser and delivered to a heat pump, which upgrades the temperature. The bottom-right configuration includes a fixed-head pressure heat recovery system, where a coolant transfers heat released from the condenser to the HVAC system [35].

In Figure 10, the condensation and desuperheating processes are emphasized in the pressure-enthalpy diagram of the vapor refrigeration cycle. The exit temperature of the compressor is normally at 80C for R717 at the thermodynamic state point indicated with a in Figure 10.

Figure 10. Pressure-enthalpy diagram of a vapor compression cycle with indicated condensation and desuperheating. Taken from [37], modified by the authors.