Master Thesis
HALMSTAD
UNIVERSITY
Master's Programme in Renewable Energy Systems
Using CHP plant to regulate wind power
Dissertation in engineering energy, 15 credits
Halmstad 2019-06-06
Arwa Elzubair
I
Abstract
Sweden is working on increasing the share of wind energy, but it comes along with many challenges, one of those challenges is the uncertainty of the wind power; CHP could be an option for better utilizing of wind power by adapting the power to heat ratio according to wind energy fluctuation.
The potential for utilizing installed wind energy in Sweden using CHP plant has been studied. A CHP plant installed in the South of Sweden was considered as studied case. To match the heat and electricity demand requested by the region with the output from the CHP plant two scenarios were simulated. Results showed that 5.3 MW of installed wind energy in Sweden could be adjusted and set to a level of 73.6 MW if the CHP plant alone were to cover the heat demand, and 25.4 MW of installed wind power in Sweden could be adjusted and set to a level of 54.2 MW with an additional heat supply of 8 MW in the studied case.
Keywords:
Integration of Fluctuating Sources, Power to Heat Ratio, Cogeneration Heat and Power, Wind Energy, Thermal Storage
II
Sammanfattning
I Sverige arbetar vi för att öka andelen vindkraft men det innebär många utmaningar. En av dessa utmaningar är vindkraftens väderberoende. En kraftvärmeanläggning kan vara ett alternativ för ett bättre utnyttjande av vindkraften genom att anpassa effektförhållandet till vindkraftens fluktuation.
Potentialen för att bättre nyttja installerad vindenergi i Sverige med hjälp av kraftvärme har studerats. En kraftvärmeanläggning installerad i södra Sverige valdes och prövades. För att matcha regionens el och värmebehov med effekten från kraftvärmeanläggningen simulerades två olika scenarier. Resultaten visade att 5,3 MW installerad vindkraft i Sverige kan anpassas till nivån 73,6 MW om man låter värmebehovet täckas av endast kraftvärmeanläggningen och 25,4 MW installerad vindkraft i Sverige kan anpassas upp till nivån 54,2 MW vid ytterligare värmeförsörjning på 8 MW effekt på den anläggningen.
III
Acknowledgments
It is a great pleasure to acknowledge my thanks and gratitude to prof. Mei Gong Ph.D., Docent, Associate Professor, School of Business, Engineering and Science Halmstad University for her supervision and comprehensive advice help.
I would like to express my sincere gratitude to Dr. Fredric Ottermo for his kind endless help Special thanks to the Swedish Institute which gave me the opportunity to be in Sweden and to study in Halmstad University.
IV
Dedication
Every challenging work needs support from people who believe in you My humble efforts are dedicated to
my loving Mother and Father My sister and my brothers
My aunt, my cousins My sweet Yasmin All my family and friends
Whose love, encouragement and prayers always make me success in what I am doing My life partner who has always being there for me, supporting me and helping me to get through
difficulties
V
Table of Contents
1 Introduction ... 1
1.1 background ... 1
1.2 Energy in Sweden ... 2
1.3 Wind power... 4
1.4 CHP plants ... 5
1.5 Thermal energy storage systems ... 5
1.6 Objectives... 6
2 Literature review ... 6
3 Methodology ... 8
3.1 Parameters ... 8
3.1.1 Variables constrained by the available data ... 8
3.1.2 Assumed variables ... 8
3.1.3 Calculated variables ... 8
3.2 Modelling ... 9
4 Case study ... 10
4.1 Input Data of a typical CHP plant ... 10
4.1.1 Heat demand ... 12
4.1.2 Thermal storage ... 12
4.1.3 Power to heat ratio ... 13
4.2 Data of wind power ... 13
5 Results from simulation ... 14
5.1 Scenario 1 ... 14
5.2 Scenario 2 ... 16
6 Discussion ... 18
7 Conclusion ... 20
8 References: ... 21
VI
List of Figures
Figure 1 Fuel shares in power generation [3] ... 1
Figure 2 Installed electricity generation units’ capacity in MW by the type of power source in Sweden [5] ... 2
Figure 3 The annual development of installed wind capacity in Sweden versus the world [5] ... 3
Figure 4 Heat supplied to district heating system in Sweden by type of method 1969-2015 [13] ... 3
Figure 5 Wind power production and electricity consumption for one week in 2015 in area DK1 in Denmark [14] ... 4
Figure 6 Schematic diagram of a CHP plant connected to district heating and district cooling system [19] ... 5
Figure 7 MATLAB code flow chart ... 10
Figure 8 Heat generation from CHP plant during January, February and March ... 11
Figure 9 Electricity generation from CHP plant during January, February and March ... 11
Figure 10 Hourly heat demand in January, February and March 2017 ... 12
Figure 11 Storage fill in January, February and March 2017 ... 12
Figure 12 Power to heat ratio during January, February and March 2017 ... 13
Figure 13 Wind power production in Sweden during January, February and March 2017 ... 13
Figure 14 Thermal storage fill simulation ... 14
Figure 15 Power to heat ratio simulation ... 15
Figure 16 Electricity contribution of wind energy and CHP plant ... 15
Figure 17 Thermal storage fill simulation ... 16
Figure 18 Power to heat ratio simulation ... 17
Figure 19 Electricity contribution of wind energy and CHP plant ... 17
Figure 20 Comparison between scenario 1 and 2 ... 18
Figure 21 Maximum regulated wind power contribution ... 18
Figure 22 Minimum regulated wind power contribution ... 19
VII
List of Tables
Table 1 Results for scenario 1 ... 14 Table 2 Results for scenario 2 ... 16 Table 3 Power to heat ratio and thermal storage difference when the amount of regulated power and set level are varied in scenario 2... 19
1
1 Introduction
1.1 background
Global warming is one the world’s biggest problems nowadays and a lot of work is being done to limit it, using more renewable energy to reduce the greenhouse gas emissions is an important agenda that the European union committed to work on it, as they set a goal to increase the share of renewable energies to 20% by 2020 [1]. Renewable energy resources can provide energy without climate changes and in a sustainable way, but beside their advantages they have some drawbacks.
During 1970’s fossil fuels were supplying approximately 86% of the world’s energy, in this period the fossil fuel prices have dramatically rose, seeking for new energy sources began and this refreshed the sector of renewable energy [2]. Only biomass and hydropower were competitive to the fossil fuels during the beginning of 20th century, nowadays many renewable energy technologies are economically competing with fossil fuels and their role in energy production is increasing.
In 2017 the share of renewable energy in total global power production increased to be 8.4%, strong growth for solar, wind and natural gas contributed to a 17% increasing in renewable energy [3]. The growth in renewables over the time can be seen in Figure 1, it shows the trends of fuels share in the world.
Figure 1 Fuel shares in power generation [3]
Electricity generation using fossil fuels is expected to rise while electricity generation using renewable resources is expected to fall, in a cost wise renewable energy resources is a genuine alternative for energy production, but the issue they are uncontrollable [4]. Those resources are dependent on natural resources, which do not suit the required demand.
As renewable energy becomes more outstanding, future energy system need to be more flexible to mitigate problems due to uncertainty of renewable resources; these problems include grid stability problems and harmonic creation.
2
1.2 Energy in Sweden
Since 1980s Sweden basically was depending on nuclear power and hydropower, but in the last 10 years the wind power share has increased significantly as shown in Figure 2 [5].
Figure 2 Installed electricity generation units’ capacity in MW by the type of power source in Sweden [5]
Sweden climate and energy policy aims to increase the share of renewable energy, also there are some political objectives aiming to reduce the nuclear power production share and to settle down the new hydropower production and to develop more cogeneration heat and power production. [6]
Sweden along with Finland, Denmark and Norway are sharing same Nordic electricity market. In Sweden it is the supplier responsibility to supply as much electricity as costumers need, the balance should be made by planning the production based on forecast of consumption and it is controlled by the control room, the power production also should not be too much or too low and it is
economically regulated [7].
Installed wind energy capacity is increasing every year in Sweden and the number of employees involved in wind energy industry is also increasing, more energy polices have been reviled and more countries are interacting in this field; as shown in Figure 3. There is great development in installed wind energy between 2006 and 2017 in Sweden as well as in the world [8]. The total installed wind power in Sweden in 2017 is 6615 MW [7].
The first Combined heat and power (CHP) plant in Sweden was introduced in Karlstad in 1984 by converting thermal power plant to CHP plant, it was providing heat to industrial facilities; during 1950 many CHP plants were introduced to provide heat to residential sector, as can be seen in Figure 4 CHP plants growing along with other district heating methods [9].
3
Figure 3 The annual development of installed wind capacity in Sweden versus the world [5]
Figure 4 Heat supplied to district heating system in Sweden by type of method 1969-2015 [13]
It is important to have secure and sufficient energy system while reducing the greenhouse gases.
The wind energy generation is not predictable and always change over time, the determination of the contribution of CHP plant to regulate certain amount of wind energy is a matter of importance.
CHP plants together with renewable energy are basis to implement the objectives of responding to the climate change Europe [10].
4
1.3 Wind power
Using wind to produce power firstly started in the beginning of the 20th century, and it became more popular after the oil crisis in 1970. Today the main motivation of wind power development is the global warming issue [11].
Electricity generation from wind power has been increasing. Globally, electricity generated by wind power has increased by 25% every year between 1994 and 2015. In 2014 the total share of wind power electricity production reached more than 700 TWh, which was roughly 3% of the total global electricity production. In 2017, wind provided more than 50% of the renewable energy growth [3].
International Energy agency is expecting the wind power production to reach 18% of the total electricity production globally in 2050.
Wind energy is considered as the most economical energy among all the renewable energies that’s why it has a greater importance and attention than the other renewable energy resources [12]. It is a free cost energy and has a lower environmental impact comparing to the other energy resources.
It has also many other advantages, wind energy is inexhaustible, wind provides power with stable prices, wind creates energy security [13].
Furthermore, wind energy speed has a random probability pattern and it can change at any time.
Accordingly, it produces uncertain power output [1], it does not follow the consumption pattern as can be seen in Figure 5. Therefore, this generation uncertainty should be treated by several methods for example: using energy storages and regulating wind power.
Figure 5 Wind power production and electricity consumption for one week in 2015 in area DK1 in Denmark [14]
5
1.4 CHP plants
CHP plants produce electricity and heat simultaneously using the same thermodynamic process at the same location and usually have better thermal efficiency than the traditional thermal power plant [15]. CHP are used to convert the fuels chemical bounding energy in to heat and electrical energy with about 90% overall energy efficiency [16]. Renewable fuels can be used as well as fossil fuel in the CHP process, and when green fuels like biomass are used this can help in reducing carbon dioxide emissions [17]. Fossil fuels are not preferred because of the effect of global warming and climate change, organic waste and biofuel are highly preferred [18].
Figure 6 shows a schematic diagram of a CHP plant that provides electricity to the grid, heat to district heating system and cooling to district cooling system.
Figure 6 Schematic diagram of a CHP plant connected to district heating and district cooling system [19]
The heat produced from CHP plant is used to fulfill the district heating demand and can be
considered as the primary product, and the electricity produced may be considered a byproduct, but it has a higher exergy and economic value than the heat [19].
CHP plants are very important generation source in recent years and have experienced rapid
developments recently, when comparing the CHP plant and convenient plants that produce heat and electricity separately, for the same output the CHP plant has the potential to consume less fuel by 20-30% [20]. Many studies [20,21,22] have investigated how to optimize CHP technologies and the relation with the changing demand of district heating, such studies also worked with optimization of heat storage systems.
Thermal storages have been suggested as a good solution to convenient operation of CHP plants.
Some studies stated that the plants’ output could be economically improved by using the thermal storages. [23,24]
1.5 Thermal energy storage systems
Thermal energy storages are used to match between energy demand and energy generation by storing heat or cold to be used when it is needed through a three basic steps process: charge,
6
storage and discharge. There are two main concepts of thermal storage systems, active where the convection heat transfer is forced into the storage material and passive where the heat transfer fluid is only passing through the storage to charge and discharge a solid material [25].
There are many benefits of using energy storage systems:
• Economic benefit: less capital and operational costs
• Efficiency improvement: the energy used more efficiently
• System performance improvement and more reliability [25]
The investment cost of thermal storage is 1-5 euro/kWh while the cost for batteries starts from 200 euro/kWh and for pumped hydro storages starts from 50 euro/kWh, thus thermal storages may be a good option to match the consumption and production [26].
Installing thermal storages make the CHP plant more flexible, thermal storages can separate the power production from the heat production [23]. Many studies [23,24, 27] focused on optimization of CHP plants operation with thermal storages, and state that thermal can make CHP plants more profitable.
1.6 Objectives
Objectives in this thesis are:
• to study the thermal energy balance between the demand, the production and the storage to find the maximum amount of electricity-regulating contribution from the CHP to wind energy with different power to heat ratio.
2 Literature review
Regulating the fluctuation of renewable energy sources specially wind power has been area of interest of many recent researches. The study in [14] focused on developing the flexibility of energy system in Denmark through using model to analyze the ability of utilization and integration more wind energy, they found out that the share of wind energy can be increased from 20% to 40% by adding heat pumps to CHP plants.
Similarly, in [28] Levihnpresented the potential of CHP plant combined with large heat pumps district heating system to balance renewable energy sources, he presented Stockholm district heating system as an empirical example.
Thermal storages represent a good solution to make CHP plants more flexible, it has a lower cost and more environmentally friendly than the electricity storage systems. Chen et al. [18] studied the impact of introducing heat boilers and thermal storages on the flexibility of the CHP plant and wind curtailment through proposing a linear dispatch model to integrate CHP and wind energy, the results showed that using electrical boilers combined with thermal storages can improve the wind
curtailment.
Using the existing hourly data to check the possibility of balancing the wind energy using CHP plants in Finland was done in [26], Rinne and Syri checked this possibility by using larger thermal storages than the current storages; they used energyPLAN method to optimize the usage of the available electricity and heat production and they found that using the optimal thermal storage can increase the CHP generation by 15% in case of 24% of the electricity production is provided by wind energy.
All the above studies considered using thermal storages to handle the heating needs, by making the power and heat production separated from the heat need, the CHP plant should run to meet the
7
power need to regulate the fluctuating energy while the thermal storage is helping to cover the heat demand, this principle is also used in this study.
Some studies discussed the economical aspects. Shao et al. [23] developed an optimization model of CHP plants that operate in a flexible way to satisfy and integrate with the wind energy while
satisfying the end users with lower overall cost. The idea in this study is taking advantage of
substituting between electricity and heat for space heating. Simulation results showed that up to 5%
of the operational costs can be saved due to this integrated optimized the heat from a necessary thermal power production could theoretically handle a large share of the European space heating needs also in energy system. On the same context Lund et al. [29] discussed the possibility of using flexible CHP plants to balance renewable energy sources could be more profitable. In [20] Andersen and Lund focused on software and computer tools and organizational setup that allow the
partnerships between small CHP plants to replace the large-scale power plants and integrate the fluctuating energy supply from renewable energy resources to the electricity systems, they presented tools and method to calculate those partnerships bidding prices for the power market.
Similarly, Hong et al. [30] discussed utilizing flexible power plants instead of nuclear and coal to balance the wind power integration into Jiangsu’s electrical system.
In [1] Papaefthymiou analyzed the impact of integration the stochastic generation in to the power system using modelling methodologies and they applied the stochastic bounds methodology.
Similarly, Spiecker and Weber in [31] analyzed the European electricity market development in the presence of the stochastic power sources, they investigated the investments within 5 scenarios up to the year 2025
In [32] Connolly et al. investigated how the pumped hydroelectric energy storage can help the integration of fluctuating wind energy on the Irish energy system, in terms of cost, size and
operation. The result was the share of the wind energy in the Irish energy system can be increased up to additional 20% without additional operational cost.
8
3 Methodology
In this chapter, the modelling of a MATLAB flow chart will be presented. To determine the amount of CHP contribution, certain percentage of wind energy is to be assumed and checked
3.1 Parameters
3.1.1 Variables constrained by the available data 3.1.1.1 CHP production
The energy production from CHP plant could be assumed to be constant, by assuming using the same fuel. Electricity and heat product vary according to the power to heat ratio but the summation of them is constant.
CHP energy output = electricity product from CHP + heat product from CHP
=constant
(1)
CHP production remains the same when regulating wind power.
3.1.1.2 Heat demand
Heat demand is derived by a change of weather conditions, such as outdoor temperature and magnitude of solar radiation. It is also influenced by hot tap water consumption. Heat demand, which is fulfilled by the CHP during normal operation, should also be fulfilled while regulating wind power.
According to the available data, heat demand from CHP plant can be calculated using:
Heat demand from CHP= heat product from condensation in CHP - thermal storage level
(2)
3.1.2 Assumed variables 3.1.2.1 Setting level
Setting level is the level that assumed to be the limit of the regulated wind power, through guessing different values and finding the possible highest level.
set level = regulated wind power + electricity product from CHP (3) 3.1.2.2 Regulated wind power
Regulated wind energy is taken as a percentage of the total installed wind power in Sweden, the method used is to assume specific amount of wind power to be regulated and checking the
constrains that presented later in Chapter 6.2, the target is to find the maximum possible amount of wind power that can be regulated.
3.1.3 Calculated variables 3.1.3.1 Electricity production
Electricity produced from CHP is constrained by the amount of the regulated wind power and the setting level as shown in equation below, it varies according to wind fluctuating and assumed setting level.
Electricity production from CHP = setting level- regulated wind energy (4) 3.1.3.2 Heat production
Heat production from CHP is calculated according to the electricity production, as the total output should be the same the heat production is defined in equation 5.
9
Heat production = CHP output - electricity production from CHP (5) 3.1.3.3 Power to heat ratio
Power to heat ratio is the parameter which indicates the amount of electricity production versus the amount of heat production in a CHP plant, it is defined by equation 6 where 𝑃𝐻 is power to heat ratio.
𝑃
𝐻 = electricity production 𝐻𝑒𝑎𝑡 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛
(6)
For each power plant there is an applicable range for this value, the factors defining this range are the temperature and the pressure of the superheated steam[22].
Minimum value ≤ 𝑃𝐻 ≤maximum value 3.1.3.4 Thermal storage
Thermal storage is designed to trap and store the access heat that produced by the CHP plant, the thermal storage filling is changing according to the heat production and heat demand, the storage discharge when the heat demand is more than the heat production, and charge when the produced heat is more than the needed heat.
Thermal storage filling when regulating wind power should follow the same pattern of the thermal storage during normal operation. At the end of the period it is necessary to have the same amount of stored heat.
storage fill(k) = storage fill(k−1) + heat production(k−1) – heat demand(k−1) +extra heat from external source
(7)
3.2 Modelling
The proposed modelling is written in MATLAB code and allows the power to heat ratio to be changed according to the amount of electricity needed to regulate the wind energy while suppling the city with the enough needed heat.
The MATLAB code work to regulate specific percentage of the wind power to a set level and then checking the constrains.
The constrains in modelling are listed below:
• The storage at the end of the period should be enough.
• The minimum and the maximum power to heat ratio should be within the applicable range for the CHP plant.
• Only considering the normal operation and skipping the unrepresentative values, such as values during maintain periods.
The input data to the modelling is hourly based and are summarized below:
• Wind energy production (MW)
• Heat power generation from condenser in the CHP plant (MW)
• Electricity power generation from the CHP plant (MW)
• Thermal storage filling (MW)
As described in Figure 7 after entering the data, all the unrepresentative values are skipped;
reasonable guess were made for wind energy to be regulated as a percentage of the total wind
10
energy generation and set value. The way of calculation is to keep the summation of the generated electricity from the CHP plant and the regulated wind energy is constant; the model task is to calculate the new values of power to heat ratio and the thermal storage level. Then the power to heat ratio values are controlled within the allowed range; also, the thermal storage at the end of the period is to be checked to keep it at the minimum difference from the original value.
Figure 7 MATLAB code flow chart
4 Case study
4.1 Input Data of a typical CHP plant
The studied CHP plant is located in southern Sweden, the biofuel used consists of different types of waste to produce electricity and heat, the fuel can be assumed to have the same characteristics.
Thermal storage with total capacity of 12 000 MW is attached to this plant.
input data
•heat production
•electricity production
•thermal storage level
exclude unrepresentative
data
•(due to CHP shutdowns)
•(due to sudden failures or problems)
calculate constrained variables
•CHP production (electricity and heat)
•heat demand
assume variables
•setting level
•regulated wind power
calculate variables
•new electricity production
•new heat production
•new power to heat ratio
11
The CHP plant provides heat to the district heating grid and electricity to the electricity grid, amount of produced energy has a slight variation over time; during the normal operation, the heat
production is larger than the electricity production and it is controlled according to heat demand.
Unrepresentative values were skipped in this study, only data during normal operation hours are included.
In order to study the potential of this CHP plant to regulate wind energy, two scenarios were developed:
Scenario 1 is all the required heat demand from the CHP plant is covered by CHP heat production and thermal storage without using external source.
Scenario 2 assumed that there is a heat source contributes with the CHP plant to supply the heat demand. The heat source was chosen to be 8.5 MW to help covering the hourly heat demand from CHP (6% to 29% of the hourly heat demand).
Figure 8 and Figure 99 present the heat and electricity generation variation over time respectively.
Figure 8 Heat generation from CHP plant during January, February and March
Figure 9 Electricity generation from CHP plant during January, February and March
12 4.1.1 Heat demand
The heat demand data was taken from a Swedish CHP plant actual data on an hourly basis during January, February and March 2017. Heat demand is fulfilled by CHP heat production and thermal storage discharging.
The data is summarized in Figure 10, heat demand rapidly change from 139.7 MW and to 28.7 MW.
These are used as basic values in this study
Figure 10 Hourly heat demand in January, February and March 2017
4.1.2 Thermal storage
The thermal storage charging and discharging data was also taken from the Swedish CHP plant actual data on an hourly basis during January, February and March 2017.
The thermal storage fill is shown in Figure 11 it varies with the heat demand and heat production;
and when comparing it with the heat demand in Figure 10, they have inverse relationship when the heat demand increases the thermal storage level decreases because the access heat which used to charge the thermal storage become less.
The storage level at the end of the period is 3950MW.
Figure 11 Storage fill in January, February and March 2017
13 4.1.3 Power to heat ratio
For this CHP plant the power to heat ratio is almost constant (approximately 0.47) during the normal operation as shown in Figure 12. Power to heat ratio change slightly during this period to reach maximum of 0.62.
Figure 12 Power to heat ratio during January, February and March 2017
For this CHP plant and after analyzing the data and with using the same biofuel, it showed that it has the potential to work with a power to heat ratio between 0.4 and 1.0.
0.4 ≤ 𝑃𝐻 ≤ 1.0
4.2 Data of wind power
The data of wind energy production in 2017 was from Swedish power net (Svenska kraftnät), which is the system operator of electricity in Sweden, the data used in this study is for January, February and March 2017.
Figure 13 Wind power production in Sweden during January, February and March 2017
As can be seen in Figure 13 wind power varies widely over time on hourly scale with a minimum production of 217 MW and maximum production of 5524 MW during January to March.
14
5 Results from simulation
In order to find out the amount of wind energy can be regulated, the simulation of two scenarios were performed for 3 months in 2017. The aim was that the heat demand is fulfilled in each hour while balancing the wind energy.
MATLAB simulation conditions and constrains were set according to Chapter 3.2. All the data is on hourly basis and the simulation was done according to it. MATLAB code calculates the following parameters:
• Power to heat ratio
• Thermal storage difference
The simulation model was run using different values for the regulated wind energy and set level to find the best values for each scenario.
5.1 Scenario 1
All the heat demand is assumed to be supplied only from the CHP plant without additional heat from external source. The results show in Table 1.
Table 1 Results for scenario 1
Electricity set level (MW)
Regulated wind power (MW)
𝑷
𝑯 range Thermal Storage difference (MW)
37.60 5.30 0.40 - 0.54 69.96
In this simulation the storage level at the end of the period was set to be as closest as possible to the storage level during normal operation.
As can be shown in Figure 14 the thermal storage fill during the normal operation and in the case of regulating the wind energy follow the same pattern.
Figure 14 Thermal storage fill simulation
15
Figure 15 Power to heat ratio simulation
By using the MATLAB code and the available data, an analysis of the amount of wind energy that can be regulated is done; different values of the wind energy and set level were performed.
In Figure 16 simulation results of regulating 5.30 MW of installed wind energy to 37.60 MW set level is presented.
Figure 16 Electricity contribution of wind energy and CHP plant
As the heat demand should be fulfilled the thermal storage difference was calculated to be 69.96 MW. Power to heat ratio range are from 0.40 to 0.54
16
5.2 Scenario 2
Simulation for this scenario was done by adding 8.5 MW heat source as described in Chapter 3. The simulation result shows that 25 MW of wind power can be regulated to 54.7 MW set level as shown in table 2.
Table 2 Results for scenario 2
Electricity set level (MW)
Regulated wind power (MW)
𝑷
𝑯 range Thermal Storage difference (MW)
54.7 25 0.4019- 0.9977 53.07
Figure 17 shows the simulation of the thermal storage filling, as can be shown the thermal storage fill considering regulating wind energy is higher than the storage fill during normal operation. At the end of the period the thermal storage level considering regulating wind energy is lower than the thermal storage during normal operation.
Figure 17 Thermal storage fill simulation
As shown in Figure 18 the power to heat ratio is rapidly changing while regulating wind energy in a wider range than the normal operation, it has higher values than power to heat ratio during normal operation because more electricity is produced to balance the wind energy.
17
Figure 18 Power to heat ratio simulation
Figure 19 presents the electricity contribution from wind power and CHP to maintain the set level, at some point the share of wind power is larger than the share of CHP and at another point the share of CHP is larger.
Figure 19 Electricity contribution of wind energy and CHP plant
18
6 Discussion
The increasing amount of installed wind power require considerable amount of balancing and regulating or using electricity storages, since the electricity storage prices are high, CHP plants are good option to handle it.
CHP plants with thermal storage have the possibility to be flexible and change the production of electricity and heat depending on the highest need
The simulation was done for only 3 months in 2017. This may have some effect on the result, since the wind energy production and the heat demand profiles may change from month to month and from year to year.
As shown in Figure 20 the regulated wind power in scenario 2 is much larger than regulated wind power in scenario 1, and the thermal storage at the end of the period is less in scenario 2. That is due to the additional heat source in scenario 2, the CHP plant in scenario 2 is responsible for handling the heat demand along with the heat source which give the chance for more electricity production and therefore more wind power regulation.
Figure 20 Comparison between scenario 1 and 2
As can be seen in Figure 21 maximum share of regulated wind power in scenario 1 is 12% of the set level which is 6.56 MW while the maximum share of regulated wind power in scenario 2 can reach 39% of the regulated wind power which is 21.33 MW.
Figure 21 Maximum regulated wind power contribution
0 5 10 15 20 25 30
Regulated wind power (MW) scenario 1 scenario 2
0 10 20 30 40 50 60 70 80
thermal storage shortage scenario 1 scenario 2
88%
12%
scenario 1
electricity from CHP regulated wind power
61%
39%
scenario 2
electricity from CHP regulated wind power
19
Figure 22 Minimum regulated wind power contribution
Figure 22 shows the minimum amount of wind power regulated in this period, most of the production to maintain the set level is from the CHP plant in both cases.
Part of simulation result for scenario 2 are presented in Table 3, some of the values are not
applicable because they are out of the required range as described before in section 3.2; the table 3 can show the principle used to choose the final result, by refining the values to have high set level as possible while storage difference is minimum.
Table 3 Power to heat ratio and thermal storage difference when the amount of regulated power and set level are varied in scenario 2
Electricity set level MW
Regulated wind power (% of the installed
capacity) MW
𝑃
𝐻 range Thermal Storage difference
MW
54.6 0.0039 0.4002-0.9940 000.00
54.7 0.0037 0.4000-0.9900 053.07
54.7 0.0040 0.3926 - 0.9965 000.00
54.7 0.0038 0.4114 - 0.9989 458.95
54.8 0.0039 0.4036- 1.0100 233.87
99.997%
0.003%
scenario 1
electricity fom CHP regulated wind power
99.97%
0.013%
scenario 2
electricity fom CHP regulated wind power
20
7 Conclusion
In this study the potential of CHP plant to balance installed wind power in Sweden has been assessed. By changing the operation of the CHP plant according to the wind fluctuation, the wind energy reliability can be improved.
A typical CHP plant in the South of Sweden with a thermal storage was investigated in 3 months running pattern to check its capacity to handle the wind power variation. The results show that using additional heat source with the CHP can increase the amount of regulated power.
Simulation results in scenario 1 of total heat demand is fulfilled by the CHP plant show that 5.3MW of the installed wind energy can be regulated to the level of 37.6 MW, in this case heat storage of 69.96 MW will be needed to cover all the heat demand.
In scenario 2, how well CHP plant work together with additional heat source to balance wind power fluctuation, simulation results show that 25.8 MW of the installed wind power in Sweden can be regulated to 54.7 MW set level.
Further studies should include technical considerations like ramping rate and how readiness for ramp up can affect the CHP plant; also, economical considerations should be included, and profitability should be assessed.
CHP plants needed to be running all the time to regulate wind power and that is not the real case, annual maintenance and check for CHP plants are usually done in the summer time when heat demand is low, this should be further studied.
21
8 References:
[1] Papaefthymiou, G. et al. (2006) ‘Integration of stochastic generation in power systems’, International Journal of Electrical Power and Energy Systems, 28(9), pp. 655–667. doi:
10.1016/j.ijepes.2006.03.004.
[2] International Energy Agency. (2009). ‘IEA Scoreboard 2009: 35 Key Energy Trends over 35 Years. Available at:
https://webstore.iea.org/search?q=iea+scoreboard+2009
[3] BP. (2018). ‘Statistical review of world energy 2018’. Accessed February 10, 2019, available at:
https://www.bp.com/en/global/corporate/energy-economics/statistical-review-ofworld- energy.html?
[4] Connolly, D., Lund, H., Mathiesen, B., Pican, E. and Leahy, M. (2012). ‘The technical and economic implications of integrating fluctuating renewable energy using energy storage’. Renewable Energy, 43, pp.47-60. Available at:
ttp://www.sciencedirect.com/science/article/pii/S0960148111006057
[5] Swedish Energy Agency, ET 2018:17, December 2018, Energy in Sweden 2018.
Available at:
https://energimyndigheten.a-w2m.se/Home.mvc?ResourceId=5774
[6] Salomón, M. et al. (2011) ‘Small-scale biomass CHP plants in Sweden and Finland’, Renewable and Sustainable Energy Reviews, 15(9), pp. 4451–4465. doi:
10.1016/j.rser.2011.07.106.
[7] Svenska Kraftnat. (2019). ‘operation and market’. Accessed 6 April. 2019, Available at:
https://www.svk.se/en/nationalgrid/operations-and-market
[8] Peter Enevoldsen and Finn-Hendrik Permien (2018) ‘Mapping the Wind Energy Potential of Sweden: A Sociotechnical Wind Atlas’, Journal of Renewable Energy, Vol 2018 (2018).
22 doi: 10.1155/2018/1650794.
[9] Werner, S. (2017) ‘District heating and cooling in Sweden’, Energy, 126, pp. 419–429.
doi: 10.1016/j.energy.2017.03.052.
[10] Lund, H. and Münster, E. (2006) ‘Integrated energy systems and local energy markets’, Energy Policy, 34(10), pp. 1152–1160. doi: 10.1016/j.enpol.2004.10.004.
[11] Davis, C.J., 2012. ‘Computational modeling of wind turbine wake interactions (Doctoral dissertation’, Colorado State University).
[12] Maynard, J.E., Lovecraft, A., Rose, C. and Chapin III, T., 2010 ‘Factors influencing the development of wind power in rural Alaska communities’ (Doctoral dissertation, University of Alaska Fairbanks).
[13] Nyamdash, B. and Denny, E. (2013) ‘The impact of electricity storage on wholesale electricity prices’, Energy Policy, 58, pp. 6–16. doi: 10.1016/j.enpol.2012.11.033.
[14] Nordpoolgroup.com. (2019). ‘Historical Market data’. Accessed May 18, 2019, Available at:
http://www.nordpoolspot.com/historical-market-data/
[15] Fragaki, A. and Andersen, A. N. (2011) ‘Conditions for aggregation of CHP plants in the UK electricity market and exploration of plant size’, Applied Energy, 88(11), pp. 3930–
3940. doi: 10.1016/j.apenergy.2011.04.004.
[16] Vögelin, P. et al. (2017) ‘Heuristic approach for the economic optimisation of combined heat and power (CHP) plants: Operating strategy, heat storage and power’, Energy, 121, pp.
66–77. doi: 10.1016/j.energy.2016.12.133.
[17] Mattei, G. (2018) ‘Cogeneration and electricity generation’, Salem Press Encyclopedia.
Accessed May 6, 2019. Available at:
http://search.ebscohost.com.ezproxy.bib.hh.se/login.aspx?direct=true&AuthType=ip,uid&db
=ers&AN=89475043&site=eds-live&scope=site
[18] Karlsson, J., Brunzell, L. and Venkatesh, G. (2018) ‘Material-flow analysis, energy
23
analysis, and partial environmental-LCA of a district-heating combined heat and power plant in Sweden’, Energy, 144, pp. 31–40. doi: 10.1016/j.energy.2017.11.159.
[19] Amiri, S. (2013) ‘Economic and Environmental Benefits of CHP-based District Heating Systems in Sweden’. Accessed May 18, 2019. Available at:
http://search.ebscohost.com.ezproxy.bib.hh.se/login.aspx?direct=true&AuthType=ip,uid&db
=edsndl&AN=edsndl.oai.union.ndltd.org.UPSALLA1.oai.DiVA.org.liu-92807&site=eds- live&scope=site.
[20] Andersen, A. N. and Lund, H. (2007) ‘New CHP partnerships offering balancing of fluctuating renewable electricity productions’, Journal of Cleaner Production, 15, pp. 288–
293. doi: 10.1016/j.jclepro.2005.08.017.
[21] Bhatt, M.S., (2001). ‘Mapping of general combined heat and power systems’. Energy Conversion and Management, 42(1), pp.115-124.
[22] Blok, K. and Turkenburg, W.C., )1994(. ‘CO2 emission reduction by means of industrial CHP in the Netherlands’. Energy Conversion and Management, 35(4), pp.317-340.
[23] Shao, C. et al. (2016) ‘Optimal Coordination of CHP Plants with Renewable Energy Generation Considering Substitutability between Electricity and Heat’, Energy Procedia, 103, pp. 100–105. doi: 10.1016/j.egypro.2016.11.256.
[24 Bogdan, Ž. and Kopjar, D., )2006(. ‘Improvement of the cogeneration plant economy by using heat accumulator’. Energy, 31(13), pp.2285-2292.
[25] Cabeza L. F.(2015). ‘Advances in thermal energy storage systems’, Elsevier, UK.
[26] Rinne, S. and Syri, S. (2015) ‘The possibilities of combined heat and power production balancing large amounts of wind power in Finland’, Energy, 82, pp. 1034–1046. doi:
10.1016/j.energy.2015.02.002.
[27] Rolfsman, B. (2004) ‘Combined heat-and-power plants and district heating in a deregulated electricity market’. Applied energy, 78(1), pp. 37-52
24
[28] Levihn, F. (2017) ‘CHP and heat pumps to balance renewable power production:
Lessons from the district heating network in Stockholm’, Energy, 137, pp. 670–678. doi:
10.1016/j.energy.2017.01.118.
[29] Lund, H. et al. (2014) ‘4th Generation District Heating (4GDH): Integrating smart thermal grids into future sustainable energy systems’, Energy, 68, pp. 1–11. doi:
10.1016/j.energy.2014.02.089. [30] Hong, L., Lund, H. and Möller, B. (2012) ‘The
importance of flexible power plant operation for Jiangsu’s wind integration’, Energy, 41(1), pp. 499–507. doi: 10.1016/j.energy.2012.02.038.
[31]Spiecker, S. and Weber, C. (2014) ‘The future of the European electricity system and the impact of fluctuating renewable energy – A scenario analysis’, Energy Policy, 65, pp. 185–
197. doi: 10.1016/j.enpol.2013.10.032.
[32] Connolly, D. et al. (2012) ‘The technical and economic implications of integrating fluctuating renewable energy using energy storage’, Renewable Energy, 43, pp. 47–60. doi:
10.1016/j.renene.2011.11.003.
PO Box 823, SE-301 18 Halmstad Phone: +35 46 16 71 00
E-mail: registrator@hh.se www.hh.se
Mechanical engineer from Sudan, 4 years working experience in power generation and interested in clean and renewable energy
