IN
DEGREE PROJECT ENERGY AND ENVIRONMENT,
SECOND CYCLE, 30 CREDITS
STOCKHOLM SWEDEN 2021,
An assessment of the solar and battery storage potential in
Hammarby Sjöstad and its grid impact
ERICA ERIKSSON
KTH ROYAL INSTITUTE OF TECHNOLOGY
SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT
Master of Science Thesis Department of Energy Technology
KTH 2020
An assessment of the solar and battery storage potential in Hammarby Sjöstad and its grid impact
TRITA: TRITA-ITM-EX 2020:537
Erica Eriksson
Approved
2021-01-19
Examiner
Björn Laumert
Supervisor
Monika Topel
Industrial Supervisor Contact person
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Abstract
Urbanization, technological development, increasing population and fast transition towards electricity in both industries and the car sector bring challenges to the Swedish power network. The increasing demand of both electricity and capacity within the cities create demand of the networks. The Swedish power system has a history to be secure, reliable and sustainable. That there was lack of power and grid was news for many people in 2019. It is not tomorrow’s problem that can be solved easily, but rather todays’ tough challenge that will affect the development of cities in Sweden.
The local network owner in Stockholm have already faced the problem, they can’t soak enough capacity from the transmission network in order to fulfill their customers’ demands. Reinvestments and developments of the transmission network are the main solutions, but it takes years before the power system have cached up with the fast growing demand. Local actions, within the city boundaries, that decrease the capacity demand from the transmission network are required for a sustainable development of the Swedish cities. At the same time is it a high interest from both cities and property owner to become self-sufficient by produce, utilize and store renewable energy locally.
This study aims to evaluate the potential of solar and battery storage in Hammarby Sjöstad, Stockholm, and its grid impact. Key areas within the city district that have highest potential are identified. It is also evaluated to what extent Hammarby Sjöstad can be self-sufficient and how much of the produced electricity that can be self-consumed. The two technologies impact on the power and energy demand as well as if it can help alleviate the capacity problem are analyzed. .
An estimation of the suitable roof area for solar were made for each building to evaluate the potential of PV and storage. Three scenarios were developed, Reference scenario, which only included loads, Sun in Sjöstan that had 5.57 MW of PV and PV-BESS in Sjöstan, which also included 3.72 MW of storage. A dispatch strategy was developed to decrease the peak power demands. From power flow analysis of Hammarby Sjöstads power network and real based electricity data was results of how the power grid was affected received. The potential to reduce power and energy demand in order to have a positive effect on the challenge of grid capacity were also evaluated for a week in May and October.
The potential of PV in Hammarby Sjöstad is large, where some areas are more beneficial than others.
During spring months when there are good weather conditions can both the power and energy demands be reduced significantly. With PV can the high morning peaks be decreased. Together with storage is it possible to decrease the demand of bought electricity during times when the PV doesn’t produce any electricity. Evening peaks, that occur when the sun has set, can only be reduced by storage. The reduction of power and energy are less in October since the weather conditions are worse for PV then. Less sunshine hours and lower irradiation lead to that less electricity are produced. With PV can only the smaller morning peaks be reduced, when it’s actually the evening peaks that are the main challenge. Due to very high power demands are almost all solar power utilized onsite and self- consumption of nearly 100% in both May and October. Because of this won’t the batteries increase the self-consumption or reduce the bough energy. Instead, together with PV have it a high potential to reduce the peaks, which are the challenge of today’s power system. Even if all suitable roofs are covered with solar and all produced electricity is utilized can only a fraction of the total electricity demand in Hammarby Sjöstad be covered. It takes more measures than PV and storage in order to become a more sustainable and self-sufficient city district.
The conclusion of this thesis are that there is high potential for PV and battery in Hammarby Sjöstad and together can it reduce the power peaks and help alleviate with capacity demand. For future recharge is it recommended to develop other discharge strategies of the batteries and size them for each network area. By scaling up the result for the entire Stockholm is it possible to evaluate the potential to help alleviate the capacity problem on a higher level.
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Sammanfattning
Urbaniseringen, den teknisk utveckling, populationsökningen och den snabba övergången till elektricitet inom både industrier och bil flottan medför stora utmaningar för Sveriges elsystem. Det ökade behovet av både elektricitet och kapacitet i städerna sätter stor press. Historiskt sett har det svenska elsystemet alltid varit säkert, tillförlitligt och hållbart. Bristen av effekt och nätkapacitet är inte morgondagens problem, som enkelt kan lösas, utan effekterna märks av i våra städer redan idag. För att fortsätta en hållbar utveckling och inte låta dess följder bli allt för stora krävs åtgärder redan nu.
Ellevio, det lokala nätbolaget i Stockholm, har märkt effekterna av ett förlegat elsystem. De kan inte ta ut tillräckligt stor kapacitet från det övre tranmissionsnätet för att möta deras kunders behov.
Investeringar och utbyggnad av tranmissionsnätet är lösningar på problemen, men dessa lösningar tar lång tid att genomföra. Det är många år till dess att elnäten har utvecklats tillräckligt för att kunna möta det allt ökade behovet. Därför krävs det lokala åtgärder inom städernas regionala och lokala elnät som minskar behovet av nät kapacitet från transmissionsnätet. Det finns för bli själv försörjande på energi genom att producera, använda och lagra förnybar energi lokalt.
Syftet med studien är att utvärdera potentialen för solceller och batterilager i Hammarby Sjöstad, Stockholm, samt analysera vilka effekter det har på elnätet. Vilka områden som är mest lämpade för systemen utvärderas. Egenanvändningen av solel samt stadsdelens potential till att bli självförsörjande utvärderas också. Till sist utvärderas ifall två teknologier kan minska nätkapacitetsbristen.
För att beräkna potentialen av solceller och batterilager i Hammarby Sjöstad gjordes en sammanställning över vilka tak som var lämpliga för solceller. Tre olika scenarier utformades;
Reference scenario som endast innefattade elkonsumtion, PV in Sjöstan som inkluderade 5.57 MW solceller samt PV-BESS in Sjöstan där 3.72 MW batterier också ingick. Strategi för urladdning av batterierna togs fram och modellerades. Genom simuleringar för elnätet i Hammarby Sjöstad med riktig data för elbehovet, kunde resultat för hur elsystemet påverkades i de olika scenarierna påvisas.
Potentialen till att minska effekt- och energibehovet i syfte att ha en positiv effekt på nätkapacitetsbristen utvärderades därefter för de olika scenarierna för en vecka i maj och oktober.
Potentialen för solceller i Hammarby Sjöstad är väldigt stor, där några områden lämpar sig mer än andra. Under vår och sommaren när vädret är gynnsamt för solceller minskas både effekt- och energibehovet avsevärt. Denna tid på året kan några av de problematiska effekttopparna reduceras med endast solceller. Under hösten är förutsättningarna för produktion av solel sämre vilket gör att minskningen av effekt och el är mindre. Färre soltimmar och lägre sol instrålning leder till lägre produktion av effekt och energi. Under denna tid på året kan endast mornarnas effekttoppar reduceras med bara solceller, när det stora problemet är de höga effekterna kvällstid. Effekttoppar som sker tider då solen inte är uppe kan endast reduceras med batterilager. På grund av högt effektbehov året runt så är egenanvändningen nästintill 100 procent i både maj och oktober. Därför så bidrar inte batterilagren till lägre energibehov, istället så reducerar de behovet av effekt vid hög last. Något som egentligen är ett av de stora problemen i dagens elsystem. Trots att solceller installeras på alla lämpliga tak kan endast en väldigt liten andel av det totala elbehovet täckas av solel. Detta påvisar att det krävs många fler åtgärder än endast solceller och batterilager för att bli en självförsörjande stadsdel.
Om solceller och batterilager installeras i stor skala i hela Stockholm kan de bidra till att dämpa följderna av följderna av nätkapacitetsbristen. Det rekommenderas att i framtida forskning ta fram och studera resultateten av andra strategier för urladdning av batterierna samt att välja storlek på batterierna utifrån varje nätverksområde. Genom att skala upp resultaten från denna studie till hela Stockholm är det möjligt att utvärdera potentialen att lokalt kunna bidra till att minska effekterna av nätkapacitetsbristen.
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Acknowledgements
This is my last word as a student in the Energy and Environment program at KTH Royal Institute of Technology. I would like to thank Monika Topel for her assistance, knowledge and patience with my work. Besides not only have the opportunity to do a thesis in a subject I’m really interest in, I have also learnt a lot in the process. To my supporting friends and family, thanks for always being by my side during these five years and especially now when I finalize with this thesis.
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Table of Contents
Abstract ... 2
Sammanfattning ... 3
Acknowledgements ... 4
Table of Contents ... 5
List of Figures ... 8
List of Tables ... 10
Abbreviations ... 11
1 Introduction ... 12
1.1 Background ... 12
1.2 Aim and objectives ... 13
1.3 Research questions ... 13
1.4 Scope and limitations ... 13
1.5 Thesis disposition ... 13
2 Methodology ... 15
2.1 Theory and previous research ... 15
2.2 Collection of data ... 15
2.2.1 Dividing of network areas ... 15
2.2.2 Electricity data ... 16
2.3 Formulating scenarios ... 16
2.4 Modeling of case study ... 16
2.4 Software ... 17
3 Theory ... 18
3.1 Swedish power grid ... 18
3.1.1 Swedish power system ... 18
3.1.2 Grid capacity challenges ... 19
3.2 Hammarby Sjöstad ... 20
3.3 Solar PV ... 20
3.3.1 Solar cell ... 20
3.3.2 Solar cell performance ... 21
3.3.3 PV module ... 23
3.3.4 Test conditions ... 23
3.3.5 Solar PV systems ... 23
3.3.6 Maximum power point (MPP) tracker ... 24
3.4 Solar fundamentals ... 24
3.5 PV in Sweden ... 26
3.6 Key performance indicators for solar systems ... 26
3.6.1 Self-consumption ... 26
3.6.2 Self-sufficiency ... 27
3.7 Energy storage ... 27
3.7.1 Electrochemical storage ... 28
3.7.2 Terminology of energy battery storage ... 29
3.7.3 Applications and benefits of battery energy storage ... 29
4 Data collection ... 31
4.1 Estimation of suitable roof area in Hammarby Sjöstad ... 31
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4.2 Directions of the systems ... 31
4.3 Hammarby Sjöstad meteorological data ... 31
4.4 PV technical input parameters ... 32
4.5 Battery technical input parameters ... 33
5 Model development ... 35
5.1 PV Modeling ... 35
5.1.1 Step 1. Potential DC capacity ... 35
5.1.2 Step 2. Sizing of PV system ... 36
5.1.3 Step 3. Solar time ... 38
5.1.4 Step 4. Solar position angles ... 38
5.1.5 Step 5. Solar radiation for fixed angles ... 39
5.1.6 Step 6. Electricity yield ... 40
5.2 Projection of electricity consumption ... 42
5.3 Scenarios ... 44
5.3.1 Reference scenario ... 45
5.3.2 Sun in Sjöstan ... 45
5.3.3 PV-BESS in Sjöstan ... 45
5.4 System integration ... 46
5.5 Battery modelling ... 46
5.5.1 Discharge and recharge strategy ... 46
5.5.2 Number of batteries ... 48
5.6 Indicators ... 49
5.6.1 Number of times the power limit is exceeded ... 49
5.6.2 Self-consumption ... 49
5.6.3 Self-sufficiency ... 50
6 Results ... 51
6.1 Potential for PV in Hammarby Sjöstad ... 51
6.2 Yearly electricity production ... 51
6.2.1 Hammarby Sjöstad ... 51
6.2.2 Network areas ... 52
6.3 Network areas with highest potential for solar ... 54
6.4 PV production for the weeks in 2025 ... 54
6.4.1 PV production in May ... 54
6.4.2 PV production in October ... 55
6.5 Power reduction ... 55
6.5.1 May ... 55
6.5.2 October ... 58
6.6 Energy reduction ... 60
6.6.1 May ... 60
6.6.2 October ... 60
6.7 Self-consumption & self-sufficiency ... 60
6.7.1 May ... 61
6.7.2 October ... 61
7 Discussion ... 62
7.1 Capacity ... 62
7.2 Energy and power production ... 62
7.3 Projections of electricity usage ... 63
7.4 Weather impact ... 64
7.5 Demand and reduction from external grid ... 64
7.6 Self-consumption and self-sufficiency ... 65
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8 Conclusion ... 66
9 Future work ... 67
10 Bibliography ... 68
11 Appendix ... 75
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List of Figures
Figure 1. Overview of the study approach ... 15
Figure 2. Network areas in Hammarby Sjöstad ... 16
Figure 3. Illustration of case study development ... 17
Figure 4. Nordic power system ... 18
Figure 5. Bidding areas of NordPool Market ... 19
Figure 6. Capacity balance in the Stockholm grid ... 20
Figure 7. C-Si solar cell structure and working principle ... 21
Figure 8. Single diode circuit ... 21
Figure 9. IV and Power curve for solar cell ... 22
Figure 10. IV-curve for different temperatures ... 22
Figure 11. IV-curve for different irradiances ... 22
Figure 12. Solar PV module parts ... 23
Figure 13. IV-curve for PV modules in single, series or parallel ... 24
Figure 14. PV plant with two strings in one array ... 24
Figure 15. Elevation angle, solar azimuth angle and solar zenith angle (left). Declination angle and hour angle (right) ... 25
Figure 16. Surface tilt angle, surface incident angle and surface azimuth angle ... 25
Figure 17. Load, PV production, self-consumption ... 27
Figure 18. Five main types of energy storage ... 28
Figure 19. Services of energy storage ... 30
Figure 20. Global irradiation in January (left) and June (right) in 2020 ... 32
Figure 21. Tilted panels and its angles and lengths ... 35
Figure 22. Future scenarios of electricity demand in Sweden ... 43
Figure 23. Historical correlation between yearly peak power and electricity consumption ... 43
Figure 24. Power demand a week in may in 2016 and 2025 for Hammarby Sjöstad ... 44
Figure 25. Power demand a week in October in 2016 and 2025 for Hammarby Sjöstad ... 44
Figure 26. Summary of the three scenarios ... 45
Figure 27. System integration ... 46
Figure 28. Flowchart of discharge/recharge strategy for battery ... 47
Figure 29. Plot boundary map of a part of Hammarby Sjöstad ... 48
Figure 30. Hourly power production from PV in Hammarby Sjöstad ... 52
Figure 31. Yearly electricity production per network area ... 52
Figure 32. Yearly electricity production and demand per network area ... 53
Figure 33. Yearly electricity production as percentage of total load demand per network area ... 53
Figure 34. Generated PV power and power demand a week in May ... 54
Figure 35. Generated PV power and power demand a week in October ... 55
Figure 36. Power demand from external grid a week in May in 2025 for Reference scenario ... 56
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Figure 37. Power demand from external grid a week in May for Reference and Sun in Sjöstan scenario ... 56 Figure 38. Power demand from external grid a week in May for Reference, Sun in Sjöstan and PV- BESS scenario ... 57 Figure 39. Power demand from external grid a week in May for Sun in Sjöstan and PV-BESS as a
percentage of the maximum limit of 8.5 MW ... 57 Figure 40. Power demand from external grid a week in October for Reference scenario ... 58 Figure 41. Power demand from external grid a week in October for Reference and Sun in Sjöstan
scenario ... 58 Figure 42. Power demand from external grid a week in October for Reference, Sun in Sjöstan and PV- BESS scenario ... 59 Figure 43. Power demand from external grid a week in October for Sun in Sjöstan and PV-BESS as
percentage of the maximum output limit of 10.5 MW ... 59
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List of Tables
Table 1. Characteristics of different secondary batteries ... 28
Table 2. Technical information of PV module Sunpower SPR-X21-350-BLK ... 33
Table 3. Technical information of inverter SG30CX ... 33
Table 4. Summary of used technical parameters of battery energy storage ... 34
Table 5. Coefficients of the Equation of time ... 38
Table 6. Used parameters in the discharge/recharge strategy of battery ... 48
Table 7. Potential of installed DC capacity per network area ... 51
Table 8. Electricity production from PV in Hammarby Sjöstad the week in May ... 54
Table 9. Electricity production from PV a week in October ... 55
Table 10. Number of times the power from external grid exceeds the limit for the week in May ... 57
Table 11. Number of times the power from external grid exceed the limit for the week in October ... 59
Table 12. Energy demand and reduction for the three scenarios a week in May ... 60
Table 13. Electricity demand and reduction for all three scenarios a week in October ... 60
Table 14. Self-consumption and self-sufficiency a week in May ... 61
Table 15. Self-consumption and self-sufficiency a week in October ... 61
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Abbreviations
A – Ampere
AC – Alternating current
BESS- Battery energy storage system DC – Direct current
EV – Electric Vehicles GWh – Gigawatt hours kW – Kilowatt
Li-ion – Lithium Ion MW – Megawatt
NOCT – Nominal operating cell temperature conditions STC – Standard test conditions
PV – Photovoltaic
PV-BESS – Photovoltaic battery energy storage system V – Volt
VTB – Vehicle to building VTG – Vehicle to grid
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1 Introduction
The introduction is conducted in order to give the reader the background of the study and its importance. It also establishes the objective, research questions and limitations of this study as well as the structure of the report.
1.1 Background
In 2018, Sweden took the ambitious environmental goal of zero net emissions of greenhouse gases by 2045 (Swedish Environmental Protection Agency, 2020). The goal after that is to achieve negative emission by collect and store them. In addition to Sweden’s climate goals the City of Stockholm have decided to take it one step further, both for the organization and the city. One is that Stockholm is a fossil-fuel free and climate positive city by 2040 (Stockholm’s Stad, 2020). Ambitious targets require smart solutions and measures that do not have a negative impact on the climate, where electrification and digitalization are believed to be two of the main solutions (Stockholm Stad, 2020a).
Because of the urbanization, population growth, large infrastructure projects and the electrification in both industries and the car sector, the demand of power and electricity has been increasing in Sweden (Sweco, 2019). Projections indicate that even with energy efficiency the demand will continue to increase in the future. The electricity in Sweden is mainly produced by hydro and nuclear, the two combined represents up to 80% (SCB, 2020a). 38% of the electricity production is placed in the north but approximately 85% of the Swedish residents live in the third southern part of Sweden (SCB, 2016) (SCB, 2020b). As a consequence, a lot of power and electricity is transmitted long distances, which leads to energy losses in the transmission lines. The cables can also only transmit up to a certain amount of capacity. This puts challenge on the transmission network to keep up with the quickly increasing demand. As a result, congestion within the network occurs which prevents electricity to be delivered to the customers. Some of the largest cities in Sweden, such as Västerås, Stockholm, Uppsala and Malmö are already facing a lack of capacity. These problems can prevent the cities from developing as planned (Swedish Energy Agency, 2020a).
Due to lack of grid capacity, the Swedish transmission system operator (TSO), Svenska Kraftnät, has decided to implement capacity limitations for different regions in Sweden. Ellevio, the local system operator in Stockholm, has not experienced issues until the year 2020 regarding increasing the capacity consumption from the transmission network. But the limit has been reached, the transmission network can only deliver 1525 MW to Stockholm, which is not enough to meet the demand (Ellevio, n.d).
Solutions to this problem are investments and development of the transmission network. Due to the fact that these processes take very long time, the TSO envision the demand will be fulfilled by 2028, at the earliest. Local actions are required in order to still ensure a sustainable development of the Swedish cities. Some solutions that can be believed to reduce the problem are local power generation and storage within the city boundaries (Ellevio, n.d). Since it could be placed within the city, it would not contribute to the issue of congestion in the transmission network. The result would be a decreased demand on Svenska Kraftnät. Installations of solar photovoltaic (PV) in Sweden have increased five times from 2016 to 2019 (Swedish Energy Agency, 2020b). Real estate owners, house associations and private house owners, they all produce their own electricity. The self-consumption, which is how much of the produced solar power that is utilized onsite is a highly important factor when the size of the solar system is decided (Swedish Energy Agency, 2020c). An overproduction of electricity and selling it to the grid is not as beneficial in Sweden as using it oneself. Therefore, there are many PV systems in Sweden sized by the self-consumption rather of how much area that is available on the roofs.
Hammarby Sjöstad is famous for their Hammarby Model, which is a model for the usage of energy, waste and water (Svane, 2007). Every year, many people from all over the world visit the city district to observe the implemented sustainable technologies (Hammarby Sjöstad 2.0, n.d.a). The citizen initiative ElectriCITY is active in Hammarby Sjöstad with the goal to make the city district climate neutral by 2030. By working with the local house associations, companies that deliver sustainable solutions, the municipality as well as academia, it is possible to have an influence on the citizens to make sustainable choices and measures (Hammarby Sjöstad 2.0, 2018). The actions made by the citizens reduce the
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climate impact of the city district. ElectriCITY has several initiatives and projects in Hammarby Sjöstad with regards to energy, transport, digitalization and sharing economy. A project called Energy at Home teaches the house associations how they can reduce their energy consumption in the buildings and make more sustainable choices (Hammarby Sjöstad 2.0, n.d.b). Many of the house associations in Hammarby Sjöstad have changed their heating system from district heating to ground source heat pumps based on both economic and environmental reasons. This adjustment reduces the bought energy but at the same time increases the demand of power, especially during the winter season when Sweden already facing lack of power. Clean and locally produced electricity is required in order for the measures to not have a negative impact on the power system. Also, to reach the local environmental target, the emissions of greenhouse gases by the citizens have to be reduced. With this background, Hammarby Sjöstad is a suitable area for the case study.
Local actions are vital in order to reduce the effects from the lack of grid capacity. This is a societal issue that prevents development of Swedish cities. Never before have the potential for rooftop PV or energy storage on a larger scale been evaluated for a part of Stockholm. When the installations of both solar and energy storage are increasing, it is also important to know what impact these systems have on the network. Therefore, this study will evaluate the potential of solar and battery storage in Hammarby Sjöstad. The contribution of technologies for the reduction of capacity and energy demand, and to what extent the district can be self-sufficient is researched.
1.2 Aim and objectives
The aim of the study is to estimate and evaluate the potential of PV and battery storage in Hammarby Sjöstad and which areas that is most promising for it. Furthermore, this study develops and model PV and battery storage system for each area and evaluates the impact of the technologies on the power network by power flow calculations. The aim is also to evaluate to what extent the city district can be self-sufficient of electricity.
1.3 Research questions
● What is the potential of solar PV in Hammarby Sjöstad in total and per network area?
● Which areas are most promising for PV?
● How much energy can the solar systems generate during a year and how much of the load demand can be covered?
● What effects will the PV and PV-BESS has on the electrical grid regarding power and energy demand a week in May and October in 2025?
● What are the self-consumption and self-sufficiency in Hammarby Sjöstad a week in May and October in 2025?
● Can solar PV and battery storage help alleviate the lack of capacity in Stockholm?
1.4 Scope and limitations
● Assumptions and results are based on available data.
● Modeling of case study was made for each network area and not for each building.
● The assumption above leads to the electricity consumption is modeled as one large load for each network area connected to a substation. The power production was also modeled for each network area.
● Projection of power demand in 2025 is based on other projections as well as assumptions, therefore can the real consumption in 2025 be different.
1.5 Thesis disposition
The study is structured into 8 chapters. The disposition is as follows:
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Chapter 1 Introduction. This chapter gives the reader the context to the thesis as well as the aim, objectives and limitations of the study.
Chapter 2 Methodology. This chapter describes the methodological steps that were made in order to do the study.
Chapter 3 Theory. This chapter gives background about the subject of the thesis and theoretical knowledge that is necessary for development of case study and to understand and analyze the result.
Chapter 4 Data collection. This chapter presents the collection of data that was required in the case study.
Chapter 4 Model development. In this chapter is each step of the case study described.
Chapter 5 Results. In this chapter is each research question answered as well as the result of the power flow calculations in Pandapower presented.
Chapter 6 Discussion. The result is discussed and analyzed. Assumption that could affect the result is also investigated as well as how the result would look like for other scenarios. Improvements of the method and study are also brought up.
Chapter 7 Conclusion. This chapter concludes the research.
Chapter 8 Future work. This chapter presents what could be done in order to complement this study with future work to expand the field.
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2 Methodology
The main part of the research was the case study development. To make it theoretical knowledge was required based on literature review. Figure 1 shows the main approach of the study.
Figure 1. Overview of the study approach
2.1 Theory and previous research
In order to know the necessary steps that had to be done in the case study a review of the fundamental theory for solar and energy storage as well as previous research in the field was made. It was collected and summarized to the theory chapter of this study, which is mainly based on articles, journals and reports from Science Direct and Google Scholar. Reports from both international and national agencies were used, such as The Swedish Energy Agency, The Swedish Energy Market Inspectorate and Power Circle. Some previous written thesis from the database Diva in similar subjects as this study was also used for the fundamental equations of produced solar energy. First when the necessary background, theory and research were known could the case study be conducted.
2.2 Collection of data
Different data was required to do the case study, such as electricity and metrological data as well as information about the power network in Hammarby Sjöstad.
2.2.1 Dividing of network areas
In Hammarby Sjöstad there are twenty substations, each of these substations supplies several buildings within one area with electricity. These areas are called network areas in this study. Location and size of each network area were given be the supervisor, Monica Topel (Topel, 2020). In this study, the loads of electrical data were received for each substation and not for each building. Therefore were estimations of suitable roof top area, electricity production and electricity demand modeled for each network area and not for each building in Hammarby Sjöstad. The network areas of Hammarby Sjöstad are shown in Figure 2.
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Figure 2. Network areas in Hammarby Sjöstad (Topel, 2020)
2.2.2 Electricity data
Electricity data for each hour and each network area were received by the supervisor and used as input in this study. It was used to calculate the projected power and electricity demand in 2025.
2.3 Formulating scenarios
Three scenarios were done in order to evaluate the impact the two systems, PV and PV-BESS had on the electricity network and compare what impacts different scenarios has on the results. The three scenarios were based on projections and assumptions and are more detailed explained in 6.4 Scenarios.
2.4 Modeling of case study
The case study was done in order to receive results and answer the research questions. Hammarby Sjöstad was chosen as the area for the case study because both the authors knowledge about the area and the possibilities to receive real electricity data. The main steps that were done for the case study are illustrated in Figure 3. Real local conditions for solar as available rooftop area and number of buildings within one network area was mapped. It was used as input for the calculations of PV system sizing to evaluate the possible installed capacity within each network area. Models were developed in Matlab and Python for the produced solar electricity, battery energy storage discharge and recharge strategy, electricity network in Hammarby Sjöstad in order to receive results of the different scenarios with the combined systems. More detailed explanations of each step are described in chapter 4 Model Development.
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Figure 3. Illustration of case study development
2.4 Software
Several software was used in order to develop the different models that were described above. In this section are the used software briefly described.
Matlab
Matlab is a programming platform specially designed for engineers and scientists developed by MathWorks (The Mathworks Inc, n.d). In this software is it possible to analyse data, develop algorithms, create models and applications and plot functions. The software was chosen since it is an easy and fast tool to work with. Matlab was used for the modelling of the PV systems, solar time, solar radiation on fixed surface and inputs of parameters that was later used in Python.
Python
Python is an open-source, interpreted, high level programming language, which is easy to learn and use (Python, n.d). Python include many function and model packages in the extensive standard library as well as from third parties. Python was selected since it access Pandapower to be used, which was required to built up the electricity network.
Pandapower
Pandapower is a power flow system analysis tool that is open source and based upon Python (Cornell University, 2018). It is mainly developed for calculations, modeling, optimization and analysis of transmission, sub transmission and distribution systems. The problem solver that aims to statistical analysis balanced power systems is based on the Newton-Raphson method, formerly based on the power system analysis toolbox PYPOWER. The software includes a large standard type library for creations of lines, transformers, buses and much more which makes it easy to model networks. Former work has been done on the electrical grid in Hammarby Sjöstad and the network of Hammarby Sjöstad has been modeled in Pandapower, which is the reason why this program was chosen to work in for this study.
PVlib Python
PVlib Python is an open source-modeling tool for PV system models (pvlib-python, n.d). It includes a big library of functions, data and other parameters for simulating the performance of PV system. In this software it is possible to calculate the current-voltage couple for maximum power point based on the Newtons method by given the single diode equation coefficients, which was the main reason to use this software.
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3 Theory
This chapter aims to provide the fundamental knowledge and theory about the subjects that the report further explore and evaluates, so that it is possible to follow the case study and understand and analyze the results. Initially is the Swedish power grid down to the network in Hammarby Sjöstad described.
Information and summary about the discussion of transmission congestion in Stockholm is thereafter presented. The two main types of technologies that are used in this study, PV and energy battery storage are described with their fundamental technology, usage in Sweden today and their potential application and services.
3.1 Swedish power grid
In this section the overview of the Swedish grid, the network in Hammarby Sjöstad and challenges the Swedish power grid is facing will be presented.
3.1.1 Swedish power system
The Swedish power system is a part of the Nord Pool market and integrated in the Nordic power system that consists of Denmark, Estonia, Finland, Latvia, Lithuania, Norway and Sweden (NordPool, n.d). The market make it possible for electricity to transport to and from the countries, so that it is ensured that the consumption is meet even thought the production is less within the own country. The Swedish power system is a network with cables in three different levels. The transmission network has the highest voltages and transports electricity for very long distances from the power plants (Svk, 2020). On the next level are the regional networks that have medium voltages where some large electricity consumers as industries are directly connected. But mostly is the regional network used to transport electricity from the transmission network to the local network. The local network has the lowest voltage level where most of the consumers are connected. The Swedish power system is shown in Figure 4. Since 2011 is the power system in Sweden divided into four geographical bidding areas, SE1 in north to SE4 in south (Svk, n.d). That leads to that it can be different electricity prices within the country. The bidding areas of the Nord Pool market countries are shown in Figure 5.
Figure 4. Nordic power system (Svk, 2020)
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Figure 5. Bidding areas of NordPool Market (NordPool, n.d)
Hammarby Sjöstad is located in bidding area SE3, network areas STH and the local distribution network owner is Ellevio AB (Svk, 2010). The network to Hammarby Sjöstad is the regional network with a distribution electricity of voltage between 33-130 kV. The local network with a voltage level of 12 kV is distributed to different substations with transformers where the voltage decrease to 400 V, which makes it possible to the consumer to use the electricity.
3.1.2 Grid capacity challenges
In Sweden is a big part of the electricity generation placed in the northern part of the country but most of the consumption occurs in south, that leads to electricity are transmitted long distances (Ingeberg, n.d). Yearly there is an export of 10-20 TWh of electricity, but during a few times per year Sweden import electricity due to that the power produced is less than the demand (Svk, n.d). The population growth, urbanisation and fast transition to electricity in industries lead to a growing demand of capacity and electricity in especially the larger cities (Ingeberg, n.d). Grid capacity and lack of power limits this development. When the capacity demand at a lower network level are higher than the grid capacity of the higher network level congestions within the network occurs. Mostly is it the grid capacity of the transmission networks that are lower than the demand of the regional network. It can be seen as the cables of the transmission grid is not thick enough do deliver the wished capacity. Investments and developments of the regional and local networks are done to meet the higher demand but
reinvestments and building out the transmission network is a very large and expensive task, which takes long time. The regional network owned by Ellevio in Stockholm is allowed to extract 1525 MW from the transmission grid own by Svenska Kraftnät. In addition to that there is possible for Ellevio to use 320 MW of electricity from Stockholm Exergi, which has several combined heat and power plants in Stockholm. Other than that is the local power generation in Stockholm very low (Ellevio, n.d.b).
Due to the limit and low local power generation could new large consumers be rejected to connect to the grid and it can be difficult for cities to develop as planned. According to Svenska Kraftnät there will most probably be a lack of capacity in Stockholm up until 2028. By then, they believe that the
transmission network is developed enough to meet the demand. This is shown in Figure 6. The main solution to lack of grid capacity is reinvestments and developments of the transmission network. One factor that leads to that this solution takes so long time is the long permit process of the power lines.
Until the time when Svenska Kraftnät can meet the demand is it up to Ellevio and the other network owners in Stockholm take actions in order to decrease the negative effect by the lack of grid capacity.
Some examples of these actions are demand side management, user flexibility and changes in the cost tariffs, that make it profitable for the customers to use electricity during times of the day when the capacity demand is low (Ellevio, n.d). Local power generation as well as local energy storage solution could also have a positive effect.
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Figure 6. Capacity balance in the Stockholm grid (NEPP, 2019a)
3.2 Hammarby Sjöstad
Hammarby Sjöstad is a city district in the south of Stockholm, Sweden. The area became a residential area around 2000 and even today is it not completed (Stockholm Stad, n.d.a). Hammarby Sjöstad was meant to be the Olympic village in Stockholm if Sweden received The Olympic Games in 2004, but Sweden was not the organizer and instead a residential district was built. An environmental program was taken for Hammarby Sjöstad, one of the goals was regarding energy and be twice as good than what was already built. That meant that the energy consumption should be half of that it was for a typical Swedish building. Even though the result did meet the goal and the energy consumption is much higher than expected, Hammarby Sjöstad it still world known as a sustainable district. Hammarby Sjöstad was the reason that Stockholm became the first European Green Capital in 2010 (European Commission, n.d). The area is 200 ha with 21.000 residents today and with more being built. It is estimated that there will be 27.500 inhabitants, 12.700 apartments, 400.000 square meter business spaces and 150.000 square meter working spaces when it completely built (Stockholm Stad, n.d.a) Since it was meant to be a sustainable district a few buildings had solar energy systems installed already from the start, some on the roof and one building even on the facades. Many of the housing
associations care a lot both about their economy but also about the environment. Therefore make many of them measures that reduce their energy consumption and increase the economy of the association (Hammarby Sjöstad 2.0, n.d.b).
3.3 Solar PV
In this chapter the fundamental concepts and theories for PV such as the technologies, types of cells, configuration and system, operation as well as orientation is presented.
3.3.1 Solar cell
The solar cell is the element of PV system that converts the radiation from solar energy into electrical energy (Khartchenko, 2014). The cell consists of semiconductors of type n-junction and p-junction.
When light enters the cell the atoms in the semiconductor absorbs the energy, hv, from the light. This process leads to electrons from the cells negative layer releases, flow through external circuit to the positive charged layer, which then create a electric current of the type direct current, DC.
The three most commercial used types of solar cells are crystalline silicon (C-Si) solar cells, thin-film solar cells and multi-junction solar cells (Khartchenko, 2014). This study has chosen to use data for crystalline silicon solar cells since it is the most used one of these three. There are two main types of C- Si solar cells called mono-crystalline and poly-crystalline. Mono-crystalline solar cells are made from one whole crystal of silicon and poly-crystalline solar cells are instead made of several fragments with
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different sources of silicon that are blended together (EnergySage, 2020). It leads to the slightly higher efficiency of mono-crystalline solar cells. Figure 7 shows the structure and working principle of a crystalline silicon solar cell.
Figure 7. C-Si solar cell structure and working principle (Circuit Globe, n.d)
3.3.2 Solar cell performance
Figure 8. Single diode circuit (PV Performance, n.d)
Figure 8 shows the single diode circuit for a solar cell. By the use of the single diode circuit is it possible to measure the current and voltage at maximum power point. Which is when the solar cell delivers the highest power output, also known as the rated power of the solar cell.
The current-voltage curve, IV-curve, represents the correlation between current I delivered by the solar cell and the voltage V that is generated by that current. In Figure 9 below the current and voltage that results to the highest power, the rated power, are stated as the current and voltage at maximum power point. Power is the product of the current and voltage, which makes it possible to plot the power curve as a function of current and voltage. Normally voltage and current do not occur at maximum power point and therefore is the produced power less than the rated power. The short circuit current is the highest possible current that can occur in the cell, it happen during short circuit. The open circuit voltage is in the other hand the highest voltage between the solar cells terminals and it occurs when there is no flow of current in the external circuit.
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Figure 9. IV and Power curve for solar cell (Seaward, n.d)
The current and voltage are strongly dependent on the cell temperature as well as the irradiance from the sun. The temperature of the cell is highly determined on the ambient temperature of the air. The effect changes in irradiance and cell temperature is normally visualized in the IV-curve.
Figure 10. IV-curve for different temperatures (McFadden, 2015)
Changes in cell temperatures affects the current and voltage strongly at fixed irradiance as Figure 10 shows. Higher temperatures lead to a slightly higher current and a linear decrease of the open circuit voltage.
Figure 11. IV-curve for different irradiances (McFadyen, 2015)
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In Figure 11 the IV-curve for different irradiance with the fixed cell temperature are presented. Lower irradiance leads to a linear decrease of the short circuit current. This curve shows why location with higher irradiance can produce more power as long as the cell temperature does not increase too much.
3.3.3 PV module
Figure 12. Solar PV module parts (Clean Energy Reviews, 2019)
A solar PV module consists of several solar cells connected in series and parallel (Khartchenko, 2014).
Normally there are 36, 72 or 96 cells connected in series so that the desired voltage is achieved. A panel includes several layers of different components to both protect the solar cells as well as increase the efficiency of the system (Clean Energy Reviews. 2019). The aluminum frame is used to make the module less fragile and keep everything in place but also to protect the sides of the laminated layer where the solar cells are placed. The tempered glass is placed there to protect the solar cells from different wind conditions. Solar modules are placed outside all year around for many years and therefore are it very important that the modules can hold against all type of conditions. The
encapsulation EVA, ethylene vinyl acetate, film layer is placed on both the front and backside of the cells to protect them from dirt and water ingress. The junction box on the back is where the
interconnection with the cables between the panels attaches to each other.
3.3.4 Test conditions
The test conditions are the conditions where the solar panels are tested to verify the characteristics during performance. Standard test conditions (STC) are at a cell temperature of 25 degrees, a solar irradiation of 1000 W/m2 and air mass AM1.5 that is at sea level (Khartchenko, 2014). Nominal operating cell temperature conditions (NOCT) are at air temperature of 20 degrees, an irradiance of 800 W/m2 and a wind velocity of 1 m/s (PV Education, n.d). The results from these test made at exact same conditions makes it possible to compare different solar cells and solar modules against each other.
3.3.5 Solar PV systems
A solar PV system is several PV modules interconnected together in series and parallel to make one or several arrays. One mono-crystalline cell with an area of 100 cm2 can produce voltage of 0.6 V and power between 1.8-2.4 W (Khartchenko, 2014). So little power is not any use for practical applications.
With a PV system of several PV modules is it possible to achieve high enough current and voltage to use if for practical application. When designing a system is it important to remember that the voltage increases in series and the current increases in parallel, which is shown in Figure 13.
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Figure 13. IV-curve for PV modules in single, series or parallel (Alternative Energy Tutorials, n.d) Modules are connected in series in what’s called strings to increase the voltage level to high enough.
Two or more strings connected in parallel makes a PV array with the preferable current. An array with two strings in parallel with four modules in series is shown in Figure 14.
Figure 14. PV plant with two strings in one array (Damiando et al, 2014)
3.3.6 Maximum power point (MPP) tracker
As presented in 3.3.4 Solar cell performance, the solar panel will have a IV-curve correlating for each irradiance and cell temperature. There is only one value of current and voltage that deliver the maximum power. By only using solar panels the voltage will not be regulated and just be a constant voltage level, that leads to that the power output often can be less than the rated power. A maximum power point tracker is an electrical device that let the maximum available energy be transmitted from the PV modules by regulating the voltage (Xhafa, Leu, Hung, 2017). In this study is the energy production modeled including maximum power point trackers to achieve the best current and voltage couple for different conditions.
3.4 Solar fundamentals
The amount of irradiation that reached a specific place on the earth depends highly on the sun’s position in the sky relative to the surface and the time and day. It is important to determine the geometry of the solar movements across the sky for sizing of PV systems (McEvoy, Markvart, Castaner, 2012).
The solar position angles are angles for how the sun is positioned to earth for specific latitude, longitude, day and time. For this calculation the solar time !! , which depends on the local longitude, the time zone meridian, the Georgian calendar day, the local clock time and the equation of time have to be calculated. The equation of time (∆!"#) is a correction for the eccicentry of the Earth’s orbit
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and the Earth’s axial tilt (Marco & Guiseppe, 2018). The solar time is calculation of movement of time based on the sun’s position in the sky.
The solar position angles are defined by the solar azimuth angle !! , the solar zenith angle !! and the solar elevation angle !! (Marco & Guiseppe, 2018). Solar azimuth angle is the angle between the projection of the sun on the horizontal plane and due south direction in clockwise. !! is the angle of the sun between the normal vector of the earth surface and the position of the sun and !! is the angle between the horizontal plane and the position of the sun. These angles are defined by the hour angle
! and the declination angle ! . The hour angle is defined as the number of degrees that the sun has moved across the sky for the solar time. Solar noon is at zero degrees and for each hour the earth rotates 15 degrees. The declination angle is the angle of tilt the earth has with its vertical axis during the year. All these five angles can be seen in Figure 15.
Figure 15. Elevation angle, solar azimuth angle and solar zenith angle (left). Declination angle and hour angle (right) (Guédez, 2019a)
When the sun position relative to the earth for the specific location is known, is it possible to decide what the irradiation would be on that location for a tilted surface. The surface tilt angle !! is the angle between the zenith of earth and the normal vector of the surface. The surface azimuth angle !! is the angle between the projection of the normal vector of the surface on the horizontal plane due south. When these two angles are known is it possible to decide the surface incident angle ! , the angle between the beam radiation, the sun, and the normal vector of the surface. That angle corrects the beam irradiance for a tilted surface. Figure 16 illustrates these three angles.
Figure 16. Surface tilt angle, surface incident angle and surface azimuth angle (Guédez, 2019a) As a rule of thumb is it best to choose a tilt angle as the latitude of the location and let the panels face due south. Then is the surface azimuth angle zero degrees, which maximize the received irradiation
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during a year. Depending on the characteristics of the load curve, if the panels are placed on flat roof, tilted roof or ground and other aspects could the angles be chosen differently.
3.5 PV in Sweden
It could be believed that PV systems are not so efficient in Sweden since it is high up in the north and do not have so many hours of sun during the year, but that is not true. PV are efficient in Sweden and south of Sweden has 1100 hours with sunlight each year, as much as Germany which has most solar systems in Europe (Fortnum, n.d.a). The development of solar in Sweden has increased and the installed capacity of PV connected to the external grid was almost five times more in 2019 compared to three years before in 2016 (Swedish Energy Agency, 2020b). In 2019 over 19.000 new systems with a capacity of 287 MW became connected to the grid (Svensk Solenergi, 2020). The big mayor of these installations was smaller than 20 kW and would most likely be rooftop solar systems (Swedish Energy Agency, 2020c). By the end on 2019 it were totally 44.000 grid connected PV systems with a total installed capacity of 698 MW in Sweden. Even though the installed capacity has increased a lot the last years electricity from solar stand for less than 1% of the total produced electricity in Sweden (Swedish Energy Agency, 2020d).
There are different projections how the Swedish energy production will look like in the future.
According to The Royal Swedish Academy of Engineering Sciences the installed capacity of solar could be as much as 50 TWh in 2040, from today’s 0.1 TWh. That would be the result if all for this purpose suitable roofs where installed with solar panels. The capacity could be even higher if land based solar energy was included (IVA, 2016a). The Swedish Energy Agency believes in their short-time forecast that the installed capacity of PV will increase from 0.3 TWh to 1.3 TWh by 2022 (Swedish Energy Agency, 2019).
3.6 Key performance indicators for solar systems
When sizing a solar system not only the size and direction of the roofs is taking into consideration, but also how much of the generated electricity that is used onsite is an important factor for the profitability of the system.
3.6.1 Self-consumption
It is often preferable to use the own produced PV electricity since it costs less per kWh than electricity bought from the external grid. This is often the main reason that property owners, house associations, companies or private people invest in solar systems. Self-consumption is defined as the onsite power production utilized directly onsite over the total onsite power production during a specific time period (Luthander et al, 2015). It is a number in percentage how much of the generated power that consumed onsite. A higher self-consumption is often preferable in Sweden, since the sold power to the grid has a low feed-in rate. So it is often more economic to have a smaller system and not sell to much to the grid than to have an oversized system and feed more to the grid. Depending of the objectives of a PV system can the self-consumption either have a big or small impact on the size of the system. Another way to utilize more of the produced power and increase the self-consumption can be done by shifting the loads and use electricity during when the modules produces power rather then during morning or evenings (Luthander et al, 2015). For some applications as dishwasher, washing machine and charging of electric vehicles this can quite easily be done. But for many other applications is it not possible to change time when it is used and therefore is this way to increase the self-consumption limited. Another way to increase the self-consumption is to integrate energy storage to the system (Nyholm et al, 2016).
It can both lead to a higher self-consumption but also decrease the bought electricity during the times when the battery is discharged. A self-consumption can by obvious reason not become greater than 100%. According to the research is it otherwise the energy capacity and power capacity of the storage as well as the capacity of the PV system that limits the increase.
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Figure 17. Load, PV production, self-consumption (Luthander et al, 2015)
In Figure 17 is area A the load demand, area B the onsite power production and area C the directly utilized power production from PV.
3.6.2 Self-sufficiency
The self-sufficiency is in many ways similar as the self-consumption but the big difference is that it states how much of the load that the produced and utilized onsite power can cover (Luthander et al, 2015). Private people do not use this factor so often but for companies it can be a goal to produce 10%
of their total electricity demand. The City of Stockholm have the goal that 10% of the electricity comes from solar by 2040 (Stockholm’s Stad, 2020).
3.7 Energy storage
By store and later dispatch energy in energy storage is it possible to use energy at other times than it is produced. The interest for energy storage has increased the last decades due to the implementation of intermittent energy production from renewable energy sources for example solar and wind. There is different types of energy storage that stores the energy in different ways, have different characteristics and can be used for different applications. In Figure 18 are the five main types of energy storage and their different types presented. For this study is only the electrochemical battery storage investigated for the potential of integration in buildings energy systems.
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Figure 18. Five main types of energy storage (EASE & EERA, 2017)
3.7.1 Electrochemical storage
Electrochemical storage or more often called battery, is the type of storage where electricity is stored as chemical energy (Johnsson & Wingren, 2018). A battery can consist of one or several electrochemical cells where each is made of two electrodes that are divided by an electrolyte (Bhatt et al, n.d). The electricity that can be discharged from the battery is made from the flow of electrons from the negative charged anode to the positive charged cathode, also called a redox reaction or reduction-oxidation reaction. At the anode the electrons are produced and accumulate by a reaction between the anode and the electrolyte. This reaction is a loss of electrons and called oxidation. At the cathode a reaction occurs that lead to that the electrons is accepted by the electrode, gains of electrons is called reduction. The electrodes can be different metals and the electrolyte different substances. Batteries are divided into two different groups, primary- and secondary batteries. The primary batteries are non-rechargeable and secondary batteries are rechargeable (Nilsson, 2017). This study evaluates PV-BESS systems where the batteries are recharged therefore are only secondary batteries considered. There are several types of secondary batteries where the metals of the electrodes are different. Some examples are lead-acid battery, lithium-ion battery, nickel metal hydride battery and flow batteries. Lithium-ion battery has been most developed for the last years and the installed capacity has also increased mostly due to the development of electric vehicles where lithium-ion batteries are on of the most used types of batteries (IVA, 2015). In Table 1 a comparison between some batteries and their characteristics are shown.
Table 1. Characteristics of different secondary batteries Gravimetric
energy density
Cycle life Discharge time
Efficiency [%]
References
Lithium Ion Li-ion 80-200 200-25.000 Hours 80-90 (IVA, 2015)
Lead-acid Lead-acid 50 500-2.8000 Hours 75-90 (Büngeler et
al, 2018).
Nickel- metal hybrid
NiMh 70 500-800 Hours 70-80 (H.Zhu et
al, 2013) Sodium
Sulfur
NaS 60 4.500 Hours 80 (IVA, 2015)
Nickel-
cadium Ni-Cd 45-80 500-2000 Hours 70-75 (N.Omar et
al, 2014)
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3.7.2 Terminology of energy battery storage
Depth of discharge (DoD)
The depth of discharge stands for how much of the battery’s total capacity in percent should be discharged during cyclic operation before it recharged again (IVA, 2015). If the depth of discharge is 20 percent it means that 20 percent of the total capacity of the battery can be discharged. A battery cycle is when a battery has been totally, 100 percent discharged. With a DoD of 20% it requires 5 discharges and recharges before it has completed a whole cycle (Jibril, 2018). Batteries have shorter lifetime with a frequent discharge and recharge. A higher DoD often means fewer discharging cycles.
State of Charge (SoC)
The State of Charge, SoC, is the opposite of DoD. With a DoD of 80% the SoC would be 20%. The State of Charge is in percentage how much capacity of the total capacity the battery has left to discharge for a specific time (Gustafsson, 2017).
Discharge rate/C-rate
The discharge rate, or C-rate is how long time it takes to discharge the battery by a current with a specific ampere. If a battery has a C2 capacity for 300Ah, it has after two hours a capacity of 300A and with a discharge current of 150A it could take two hours to discharge it (Spiritenergy, n.d).
Cycles
One cycle is a complete charge and a complete discharge of the battery (Battery University, n.d). In the datasheet for a battery the numbers of possible cycles is stated as a measure for how many cycles it can last.
Specific energy
The specific energy of the battery is defined as the battery’s energy density in weight or volume with a unit of Wh/kg or Wh/l (Battery University, n.d). If the weight is used it’s called gravimetric energy density and when volume is used it’s called volumetric energy density
Capacity
The capacity of a battery represent the amount of specific energy the battery can hold (Berg, 2015). It is often measured in Ah and is by that the product of the discharge current, discharge rate and number of hours the battery can hold that discharge rate.
Cycling life
The cycling life of a battery is a measure of the ability to withstand numbers of recharges and discharges and still provide the minimum capacity (Berg, 2015). It is often stated as number of cycles the battery can do before it’s capacity degrades.
Self-discharge
Self-discharge is the losses within a battery due to internal chemical reactions that occur even though the battery are not connected to an external circuit (Berg, 2015).
Self-discharge rate
The self-discharge rate measures the time that the battery still can provide the minimum capacity by just being stored.
3.7.3 Applications and benefits of battery energy storage
Rocky Mountain Institute investigated in 2015 the potential for energy storage and the services it can provide to the electricity grid. According to the report battery energy storage could provide thirteen fundamental electricity services for three actor groups, customers, utilities, or independent system operators/regional transmission organizations (Fitzgerald et al, 2015). The battery storage can be
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placed on three different levels of the system, behind the meter, at the distribution level or at the transmission level. The battery provides some or all thirteen services on each level. Figure 19 below shows the thirteen services that could be serve to the electricity grid by battery energy storage.
Figure 19. Services of energy storage (Ftizgerald et al, 2015)
As further down in the electricity system or in other words as close to the customer as possible, the more services the battery can provide to the system. By placing the battery behind the meter, which is at the customer’s side of the energy metering inside the buildings, the battery can provide all of the thirteen services and be a benefit both for the customers as well as the power system. As seen in Figure 18 the service that can be provided to customers are demand charge reduction, time-of-use bill management, backup power and increased PV self consumption. In Sweden there has been a subsidy of 60% on batteries that is installed in homes or buildings up until 2020. Between 2016 and 2018 only 360 persons were granted and by those only 60 actually installed batteries and received money (Wallnér, 2018).
In Power Circles report Local energy storage or traditional network reinforcement they list fourteen benefits and applications of battery storage in the network system (Power Circle, 2020). As well as RMI, Power Circle states that the benefits of the battery highly depend on where the battery is placed in the network system. According to both studies it is also difficult to calculate the profitability of the battery since it can serve so many different purposes to different stakeholders at the same time. Some of the benefits and applications that Power Circle present that’s for the user is increased self-consumption decrease in peak demand, emergency power and better quality of the power. The Swedish Energy Market Inspectorate published in 2016 the report The market conditions for electrical battery storage in Sweden which states benefits as lowered peak demand, increased self consumption and emergency power but also support to local micro grid and cost optimization based on tariffs (Swedish Energy Market Inspectorate, 2016). Some of the benefits on the distribution level that The Swedish Energy Market Inspectorate presents are increased power quality, reactive power compensation and manage the stability and congestion problems in the grid.
Before installing a battery is it very important to decide the objectives of the battery, since the characteristics highly depends of the application of the battery.
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4 Data collection
This part of the study presents the collection of data that was required to have in order to do the case study and later answer the research questions. Available rooftop areas, directions of the PV systems, technical parameters of solar panels and battery storage, as well as the meteorological data for Hammarby Sjöstad were collected.
4.1 Estimation of suitable roof area in Hammarby Sjöstad
In order to estimate the suitable roof area some assumptions had to me made. In Sweden there is a regulation to not install solar modules closer than one meter to end of the roof
(Taksäkerhetskommittén, 2019). Normally the panels are placed on tilted or flat roofs facing east, south or west and roofs facing north are discarded (Lopez et al, 2015). Therefore were tilted roofs facing north neglected in this study. The number of suitable roofs and their sizes were found by using online maps as Google Earth, Eniro and Google. The online maps were some years old and because of that some buildings that was constructed the last years were neglected in this study.
A suitable area was decided to:
• Facing east, south or west.
• Was more than 1 m from the edge of the roof
• Was more than 1 m or more from any object on the roof that could lead to shading.
• Could either be flat or tilted. For the flat roofs were the modules tilted.
In Appendix 1 are estimations of suitable roof area for each network and group presented.
4.2 Directions of the systems
For the calculations in 6.1.5 Step 5 Solar Radiation on Fixed Surfaces the surface azimuth angle was required to be known. As Figure 2 in 3.4 Dividing of Geographical areas shows are most of the streets in Hammarby Sjöstad quite straight. Because of that are almost all buildings within one network area facing north south and east west almost the same. It was therefor assumed that all buildings within one network area were facing north south and east west the same. Thereby instead of calculating the power generation for each building the power generation was calculated for network area that includes four groups of PV system with different directions. Each group had the same surface azimuth angle and the groups were south, east, west and south with tilted panels. To know the direction of each area influenced by a substation a visit to Hammarby Sjöstad was done and with the use of compass, directions at several locations were decided. In Appendix 2 are the directions for each group and network area presented.
4.3 Hammarby Sjöstad meteorological data
Hammarby Sjöstad has the latitude and longitude of 59.3050 and 18.1008 (Google Earth, n.d). The number of sunshine hours varies a lot in Stockholm, in average there are 1821 hours per year but in 2018, which is known as the summer with really good weather, Stockholm had over 2254 sunshine hours (Stockholm Stad, n.d.b). The sunshine hours is defined as the times when the direct irradiance is higher than 120 W/m2. For calculations of the electricity production it is required to know the radiation at the location of the PV system. In Sweden the irradiation varies a lot during the year, which is seen in Figure 20 where the global irradiation in Sweden in January 2020 and June 2020 are shown.
