The electrical demand alternates during the whole day. For example, when a popular football match is being televised at half time, a vast number of people go to the kitchen to put the kettle on, causing a sudden peak in demand. This is called a "TV pickup". Therefore, if power stations don't generate more power immediately, there'll be power cuts all over the country, traffic lights will go out, causing accidents, and all sorts of other dangerous accident could occur [1].
The problem with most power generation systems is the amount of time required to power them up, such as a fossil fuel station takes approximately half an hour to power up and Nuclear power station would take even longer than that. Therefore, the requirement is of a system that could supply power immediately even from a power down stage and keep supplying power until required or until the power station if fully online. There are a few answers for this problem, such as a storage system.
Energy storage systems consist of substances that can store some form of energy that can be drawn upon at a later time to perform some useful operation. All types of energy storage systems can store either potential energy or kinetic energy. Energy storage systems make it easier to balance the supply and demand of energy. A few types of energy storage systems that are currently in use today are mechanical, electrical, chemical, biological, thermal and nuclear [2].
In recent times electricity has been used as it has been generated. It has not been stored on a major scale because it was not possible to store electricity on such major scale and did not have a requirement to store electricity but that may change as the weight of technology increases on our planet the usage of electricity will increase and it will be required to store it in abundance to meet consumer demands.
A few storage systems have been mentioned above but that's just an abbreviation to the actual amount of storage systems. As there are many storage systems in each of the energy sector mentioned above but only few that concern electricity storage in large scales will be explained in detail to inform of their characteristics.
2.1 Pumped Hydro Storage
Pumped hydro storage stores energy in the form of water and is a form of hydroelectric power [3]. The method stores energy in the form of water, pumped from a lower level reservoir to a higher level. The off-peak electric power is used to run the pumps, due the cost being less. Then in peak periods of high electrical demand, the stored water is released using the turbines. Although the costs of the pumping process makes the plant a net consumer of energy overall, the system increases revenue from selling more electricity during peak periods of electrical demand, when electricity prices are highest [4]. Pumped Hydro storage contributes over 90 GW worldwide which is approximately three per cent of global generation capacity [5]. Pumped storage is the largest-capacity form of grid energy storage now available [4].
Figure - 2.1.1Pumped Hydro Storage Plant [http://upload.wikimedia.org/wikipedia/commons/9/9a/Pumpstor_racoon_mtn.jpg]
Hydro pumped storage is proving increasingly beneficial to organisations for many reasons, such as the huge amount of energy that can be stored and a consequential power output which can be produced until the demand has been met. This form of energy storage has an advantage over previous storage methods because the fuel-based plants do not cope with immediate need of the energy, therefore would be wasting energy on keeping a regular production levels [3].
If the amount of evaporation losses from the water surface and also the conversion losses are taken into consideration approximately 70% - 85% of the electrical energy is used to pump water onto the higher level of reservoir could be recovered [5].
There are some concerns with the hydro pumped storage systems. Such as the initial cost to build a hydro pumped storage system is very high and the location of the system is very critical to future profit of the storage system, as the location must have a water capacity to insure a profitable system [3].
2.2 Flow Batteries
A flow battery is a chargeable, dischargeable and rechargeable battery. Most flow batteries consist of two electrolyte systems that act as the liquid energy carriers and are pumped simultaneously, the reaction cell separated by a membrane. The thin membrane between the half-cells restricts the electrolytes from mixing but allows ions to pass through to complete the process, redox reaction. Upon charging, the energy causes a chemical reduction reaction in one electrolyte and an oxidation reaction in the other electrolyte [6].
Figure - 2.2.1 Flow Batteries
[http://www.metaefficient.com/images/flow_battery.jpg]
A flow battery has electrolyte containing one or more dissolved electro-active flows through an electro-chemical. Therefore, the chemical energy is converted into electricity through by this process [7]. A flow battery can be recharged instantly by replacing the electrolyte liquid.
There are different classes of flow batteries such as reduction-oxidation flow battery and hybrid flow battery. In the reduction-oxidation flow battery all the electro-active components are dissolved, and if one or more electro-active component changes into a solid layer its known as the hybrid flow battery [8]. Another type is redox fuel cell battery, which is a normal flow battery that only operates to produce electricity, therefore is not electrically recharged. This kind of battery recharges by reducing the negative electrolyte and oxidising the positive electrolyte [9].
Capacity of a flow battery can be measured by the amount of liquid (litres) in the battery, also the recharging can easily be done by replacing the liquid and the response time to an immediate electrical demand would be rapid as it would just discharge the battery. Spare charged liquid can help deal with a load at peak hours.
"A flow battery system costs $500 to $600 to store a KW/hr, the amount of electricity that sells for an average of 10.5cents in the USA. The system has a round trip efficiency of 65% - 75%. Therefore, it loses 25% - 35% of electricity put into the system [10]." One of the most recently developed and highly talked types of flow batteries is Vanadium Redox flow cell which is also abbreviated as VRB. They are rising candidate as a potential storage device due to high round trip efficiency and their broad spectrum energy capturing capability. This project discussed the benefit of Vanadium redox battery storage systems. A thorough investigation into theory, development and deployment of VRB had been carried during Chapter 3 of this dissertation.
2.3 Sodium-Sulfur (NaS) Batteries
Sodium-Sulfur (NaS) batteries are high capacity batteries which are developed for electrical power applications. A Sodium-Sulfur (NaS) battery has a high energy density of 367 Wh/l, high efficiency of charge or discharge 89% and a long life-cycle of approximately 15years [11][12].
"A NaS battery consists of liquid (molten) sulfur at the positive electrode and liquid (molten) sodium at the negative electrode as active materials separated by a solid beta alumina ceramic electrolyte. The electrolyte allows only the positive sodium ions to go through it and combine with the sulfur to form sodium polysulfides [13]."
Figure - 2.3.1Sodium Sulphur Battries [http://thefraserdomain.typepad.com/photos/uncategorized/single_nas_battery.gif]
The battery discharges as the positive Na+ ions flows through the electrolyte and electrons in the external circuitry of the battery which produces approximately 2volts. The charging and discharging process is fully reversible as charging causes the sodium polysulfides to release the positive sodium ions through the electrolyte to recombine as elemental sodium. This kind of a battery operates in conditions like when the both electrolyte active materials are liquid and also when the electrolyte is in solid state. At 300°C both active material react quickly and due to the internal resistance being low the NaS battery performs at a satisfactory level [13].
Table - 1 [Specification of NaS battery module] [11]
NaS battery is generally used in a structure of a battery module. A battery module has an electrical heater which can raise or maintain cell temperature as NaS battery operates at its best when the temperature is approximately 300° C.
Figure - 2.4.2Structure of NaS batteries [http://thefraserdomain.typepad.com/photos/uncategorized/nas_battery_module.gif]
"Cells are anchored into the module and are place by filling and solidifying them with sand to prevent fires [11]."
2.4 Lithium ion Batteries
Lithium-ion battery also known as Li-ion battery is a rechargeable battery in which the cathode is lithiated metal oxide and the anode is made of a graphite carbon with layer structure. Basic process of discharging and recharging is when the lithium-ions move from negative to positive electrode when discharging and from positive to negative electrode when recharging [14][15].
Fig - 2.4.1Lithium-ion Battries [http://electricity.ehclients.com/images/uploads/photo_liion_1large.jpg]
Lithium-ion battery is hugely used for small portable market but there are some concerns for manufacturing these at a large-scale batteries, main problem is the expensive costs of the batteries which is above $600/kWh [14].
Specification
Value
Specific energy
100-250 Wh/kg
Specific power
~250-~340 W/kg
Energy density
250-360 Wh/L
Cycle durability
400-1200 cycles
Cost
1.5 Wh/US$
Self-discharge rate
8% at 21 °C, 15% at 40 °C, 31% at 60 °C p/month
Nominal cell voltage
3.6 / 3.7 V
Table - 2 [Lithium-ion battery specification] [16][17][18][19]
2.5 Compressed Air Energy Storage (CAES)
Compress air energy storage uses the off-peak power from the grid and pumps air into a sealed underground reservoir. Then the pressured air generated is reserved in the underground reservoir for the peak-demand. When required the air kept can drive turbines as the air in the reservoir is slowly heated and released and generally the power produced is used for peak-demand. Just like a normal turbine plant the compressed air is merged with natural gas and they are burnt together, this process is more efficient [20][21].
Fig - 2.5.1Compressed Air Energy Storage
[http://www.pangeaexploration.com/Power Storage.gif]
There are several types of geologic structures that could be used to produce a compress air storage system. Geologic formations include, depleted gas reservoirs which are the most economical, naturally occurring aquifers, solution-mined salt caverns and constructed rock caverns. Salt caverns are formed by solution mining of salt formation and rock formation is formed by excavating solid rock, therefore salt formations are much cheaper then rock formations [20].
Compress air storage system generally costs approximately $60/kWh for large systems, which is less compared to other energy storage technologies. This is also a system which can provide regulated service for a small period of time [22].
2.6 Flywheel energy storage (FES)
A flywheel stores energy by rotating a mass and FES system works by accelerating a rotor to a very high speed and then maintaining the energy in the system as rotational energy. The flywheel has a vacuum system within itself to eliminate the friction from the air and suspended bearings to make the system stable. As the flywheel depends on the speed of the rotor, when any energy is taken from the system the rotor speed is decrease and if any energy is put in to the system the rotor speed is increased [23][24].
Fig - 2.6.1Flywheel energy storage [http://www.inference.phy.cam.ac.uk/sustainable/refs/storage/Flywheel.pdf]
The size and weight of a flywheel system largely depends on the output required. Generally in current products the importance is on high power and low energy designs capable of only short term supply [25].
"One such example is an industrial 1100kW / 4.6kWh unit named the POWERBRIDGE and manufactured by Piller to serve as a bridge unit when moving high power systems from mains supply to diesel generators during a power cut. This unit is an 1800-3600rpm, 6000kg design [25]."
Most FES systems use electricity to accelerate and decelerate the flywheel, but devices that directly use mechanical energy are being developed. [26] A flywheel system has a long life-cycle, low maintenance and has a fast response [25].
2.7 Super Capacitor Energy Storage
Super capacitor is also known as a double layer capacitor. Super capacitors consist of two electrodes, a separator and an electrolyte. The two electrodes, made from carbon provide a high surface area. The energy is stored by a transferable charge at the edge of the electrode and electrolyte. The amount of energy stored is critical to the amount of surface available to electrode and electrolyte. The two electrodes are separated by a thin membrane, which allows ions to be charged ions and restricts electronic contact [26][27][28].
Figure - 2.8Super Capacitor Energy Storage [http://upload.wikimedia.org/wikipedia/commons/8/83/Supercapacitor_diagram.svg]
Generally super capacitors are divided into two separate types. One is double layer capacitor and the other being electrochemical capacitor. The specifications of Super Capacitors defined by leading manufacturers will shown in Table-3 below.
Sl.No.
Manufacturer
Specifications of Super capacitors
1
Power Star China Make
(single Unit)
50 F/2.7V,
300F/2.7V,
600F/2.7 V, ESR less than 1mΩ.
2
Power Star China Make
(single Unit)
0.022-70F, 2.1-5.5V,
ESR 200 mΩ-350 Ω
3
Maxwell Make
(Module)
63F/125V, 150A ESR 18 mΩ
94F/75 V, 50 A, ESR 15 mΩ
4
Vinatech Make
10-600F/2.3V, ESR 400 -20 mΩ,
3-350F/2.7, ESR 90-8 mΩ
5
Nesscap Make
(module)
15V/33F, ESR 27 mΩ
340V/ 51F, ESR 19 mΩ
Table - 3 [Super Capacitor specification] [27][28]
The price table for super capacitors had been given below Table-4.
Sl.No.
Year
Cost / farad ($)
Cost/kJ($)
1
1996
0.75
281.55
2
1998
0.40
151.23
3
2000
0.01
32
4
2002
0.023
7.51
5
2006
0.010
2.85
6
2010
0.005
1.28
Table - 4 [Super Capacitor Cost Development] [27] [28]
Super capacitor costs have fallen rapidly as table-4 shows with cost per kilo joule [Cost/kJ ($)] dropping quicker than cost per farad. As of 2006 the cost of super capacitors was 1 cent per farad and $2.85 per kilo joule, and was expected to drop further. [27][28]
2.8 Hydrogen Storage
Hydrogen storage is a process for storing H2 for later use, when the demand is high. There are three different types of processes included in hydrogen storage; Compressed hydrogen, Liquid hydrogen, and Slush hydrogen storage system. The three processes may use high pressures, cryogenics, and chemical compounds to store the hydrogen for later use and that reversibly release H2 upon heating. Hydrogen storage is a topical goal in the development of a hydrogen economy [29][30].
Figure - 2.9Hydrogen Storage
[http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/46719.pdf]
Compressed hydrogen is gas state of the element hydrogen, which is kept under pressure. It's used for mobile hydrogen storage [29][31][32][33][34].
Liquid hydrogen is the liquid state of the element hydrogen. To be as liquid hydrogen must be pressurised above and cooled below hydrogen's critical point. But for the hydrogen to be in fully liquid state without boiling of it needs to cool to 20.28 K (−423.17 °F/−252.87°C) still pressured [29][34][32].
Slush hydrogen has similar characteristics to liquid and solid hydrogen, with lower temperature and higher density than liquid hydrogen. It's created by bring the liquid hydrogen down to approximately the melting point (14.01 K or -259.14 °C) that increases density [29][32].
2.9 Plug-in Hybrid Vehicles
Plug-in hybrid is a hybrid vehicle which has rechargeable batteries that can recharge to fully by to an external power plug or source. A plug-in hybrid shares the attributes of both a normal hybrid electrical vehicle having an electrical motor and an internal combustion engine. A plug-in hybrid also has a plug to connect to the electrical grid. The cost for electricity to plug-in hybrids for an all-electric process has been estimated at less than one quarter of the cost of gasoline / fuel [35].
2.10 Energy storage characteristics
The following table shows attributes of the afore-mentioned storage options available in the market at present. But more emphasis will be based on a type of a flow battery which arguably is the best option. As technologies get more sophisticated and the planet is in need of more technology based applications, the requirement of immediate power becomes increasingly demanding. A power storage option has to be looked into and also developed to meet the future requirements.
Table - 5 [Energy Storage Characteristics] [36]
The table below provides high level overview of the main characteristics of nine different technologies.
Table- 6 High Level Comparison of Energy Storage Technologies[37]
CHAPTER 3
Vanadium Redox
CHAPTER 4
CASE STUDIES
Case Study 1: Off Grid House
This model analyzes the options for providing power to an off-grid house in Montana, USA. Case study Modelled by Tom Lambert of HOMER Energy had been modified by installation of VRB-ESS in order to analyse the potential benefit of the storage technology and cost benefit analysis had been carried out.
4.1 Introduction of system under study:
The Two power sources of supply had been considered: a 1 kW wind turbine (WT), and a 2.6 kW gasoline generator (GG), as well as battery storage. When the load is connected to a gasoline generator and a wind turbine without consideration of any storage technology than, the schematic of the considered equipment shown on HOMER was as follows;
Figure 4.1.1 Load connected to Gasoline generator and Wind-turbine
The figure below shows the sensitivity results the black region shows that at low wind speeds the load can only be supplied with GG. On the other hand blue region which covers the vast area of graph tells that at higher wind speeds and or when it is possible to supply renewable generation than WT/GG is of course most economical and most suited mean to supply the load.
Figure 4.1.2 Sensitivity Results
At low gasoline prices as $0.2/L and low wind speed as 3m/s HOMER suggests running the GG (Gasoline Generator) due to high initial capital cost of GG $900 as compared to $5550 of GG/WT combination.
Figure 4.1.3
4.2 Installation of storage technology into system:
Now, a system is installed with VRB-ESS. The schematic is shown in figure below.
Figure 4.2.1 System is installed with VRB-ESS
It increases the Net present Cost of the system alongside the initial capital. But the VRB-ESS stores excess energy at the times of high wind speed i.e. excess wind generation and utilizes it when the wind speed is low and or oil prices are high.
Figure 4.2.2 Sensitivity test based on wind speed and prices of gasoline
A sensitivity test had been carried on the wind speed and the price of gasoline. The optimal system type graph shown in figure 6.2.2 tells that HOMER recommends the generator and the batteries combination for supply throughout the sensitivity space, and some renewable power for almost all the sensitivity space. Only for very cheap gasoline and very light winds does HOMER recommend the generator/battery system without renewable power. That is because the generator power is quite expensive due to short lifetime and fairly low efficiency.
4.3 Detailed description of case under consideration and data entry:
The energy output from a wind turbine depends on the time of the day, month of the year; height above the ground, site and it varies with all of these factors. In order to match the electrical energy supply to the energy demand and have a reliable power supply, a hybrid system with storage needs to be in place. Village hybrid energy systems employing renewable energy offer an attractive and practical approach to meet the electrical power needs in rural communities around the globe.
[V. C. Nelson, R. E. Foster, R. N. Clark and D. Raubenheimer, Wind hybrid systems technology characterization, NREL, 2002.]
HOMER software had been employed to find the optimum combination of wind-diesel hybrid system with vanadium redox flow cells as a mean to storage. The HOMER software works considering the resource availability constraints and simulates the multiple combinations of technologies to determine the lowest cost system to meet the expected electricity demand.
The three main parameters in the model included; Resource availability, Electrical Demand (Load) and Input costs.
4.4 Resource Availability:
The average wind-speed data that had been fed to HOMER in wind resource input window can be seen in figure 4.4.1 below.
Figure 4.4.1
4.5 Electrical Demand (Load):
The hourly electrical demand data which is actually KWh usage by load can be fed to HOMER figure 6.2.4 by entering 24 hourly values in the load table, each of the 24 hourly values in the table define the demand for the single hour of the day. The scaled annual average had also been entered. HOMER replicates the profile throughout the year unless different load profile had been defined.
Figure 4.5.1
4.6 Input Costs:
4.6.1 Wind Turbine:
The cost of wind turbine for the project had been taken as $3900. The operation and management (O&M) cost every year had been taken as $100/yr which is 2.6% of the capital cost. The figure 6.2.5 below shows the power curve and cost curve for the wind turbine.
Figure 4.6.1a
Power curve shows that maximum rated output of 1.24 kW from wind turbine can be obtained at high wind speeds between 11-12 m/s. The wind turbine had been chosen according to the frequency distribution graph of wind speeds at the location shown in Figure 4.6.1a
Figure 4.6.1b
The wind turbine life-time was taken as 20 years.
4.6.2 Gasoline Generator:
A 2.6 kW gasoline generator had been chosen for the project with capital cost of $900 and O&M of 0.040 $/hr. Figure 6.2.7 For the sensitivity analysis the fuel costs had been taken as $0.2, $0.4, $0.6, $0.8 per litter.
Figure 4.6.2
4.6.3 Storage:
For the storage purposes VRB-ESS had been chosen and data entries had been detailed in figure 6.2.8. The battery cost data had been obtained from prudent energy website. What is unique about VRB-ESS is that energy and power capacity can be sized separately. HOMER has got an option to customise this.
Figure 4.6.3
The table below gives the information about the cost of battery.
Cost
Capital ($)
Replacement
O&M ($/yr)
Cell Stack
Size 1 kW
150
0
70
Electrolyte
Size 1 kWh
0
150
0
Table 4.6.3
If, the comparison between the present cost of the system and the system with included VRB-ESS is carried out, the results obtained are discussed below.
4.7 Cost benefits analysis of supplying the load with gasoline and VRB-ESS:
At the low wind speed as 3 m/s and low gasoline prices as $0.2 if the system is supplied with GG only than the net present cost of the project to supply the load over the year was $31,381/yr. while the NPC can be reduced to as low as $23,421/yr with an inclusion of VRB-ESS. On the other hand the cost of energy supply per kWh can be reduced from $0.805/kWh to $0.601kWh. This was because of savings on fuel cost and also the saving on maintenance cost of generator. Even though the capital cost of supplying with generator and battery is high i.e. $4650 as compared to generator alone of $900 but because of low NPC the calculations made by HOMER the pay-back for the investment will be obtained in 3 years. On the other hand if the fuel costs are high as $0.8 than the payback time reduces to 1 and three quarters of a year. Which means the maximum load will be supplied by battery and hence it will cover the deficit.
Figure 4.7.1
4.8 Cost benefits analysis of supplying the load with gasoline, wind generator and inclusion of VRB-ESS:
It had been seen that when VRB-ESS was installed into system supplied by wind turbine and gasoline generator the return on the investment will be obtained in a year's time by considering that the wind speed and gasoline price was fixed at 3m/s and $0.8 for the whole year. For high wind speed and high gasoline prices the pay back will be received in less than a year's time. While the system without VRB-ESS at high wind speed and high gasoline price pays back in nearly 3 years.
Gasoline running hours were reduced by 94% from 8760hrs to 511 hrs when the payback on an investment is obtained in nearly two years when 70 kWh VRB-ESS couples with 1.24 KW wind turbine and gasoline generator based on gasoline price of $0.8/L.
Figure 4.8.1 Monthly Average Electric Production
The benefit of connecting VRB-ESS with a gasoline generator and renewable source was that it removed high maintenance cost of replacing the traditional battery systems and by reducing the generator operation hours, huge saving can be made on generator replacement. It cuts the use of Gasoline generator saving costs associated with maintenance, fuel consumption and fuel transportation to remote areas.
4.9 Frequency Histogram:
Frequency histogram of battery SOC (state of charge), for four different scenarios are discussed below.
4.9.1 High Wind Speed and High Fuel Cost:
It can be seen in the figure below that at high wind speeds and high fuel costs the battery maintains its state of charge.
Figure 4.9.1 Frequency Histogram High Wind Speed and High Fuel Cost
The discharge frequency is very low because the maximum load is supplied by wind generation with minuet contribution from battery.
4.9.2 Low Wind Speed and Low Fuel Cost:
It can be seen from the figure below that at low wind speed the contribution to supply the load through battery was high.
Figure 4.9.2 Frequency Histogram Low Wind Speed and Low Fuel Cost
The battery discharges at higher frequency to fulfil the demand. The monthly statistics graph shows that at average the battery maintains 40% state of charge.
4.9.3 High Fuel Cost and Low Wind Speed:
It can be seen from the figure below that when the wind speed is low and fuel cost are high at that time the battery discharges at higher frequency i.e. supplies the load coupling with gasoline generator.
Figure 4.9.3
4.9.4 Low Fuel Cost and High Wind Speed:
It can be seen from figure below that at higher wind speeds the battery maintains its state of charge and attains the maximum value i.e. 100% to utilize it at periods of low wind speed.
Figure 4.9.4
4.9.5 Installation of PV-Array into the system:
Now PV-Array had been installed into the system. The cost benefit analysis had been carried out and the results obtained are discussed below.
4.9.5a PV-Inputs:
The figure 4.9.51 below shows the PV-input window. The capital cost for had been chosen as $4000/kW. The replacement cost was $3500/kW. To get an optimized solution the maximum size of 4.000 kW had been considered. The figure 6.1.17 below shows the PV-input window.
Figure 4.9.5a
4.5.9b Solar Resource:
The solar resource data can be fed to HOMER under the solar resource window. As shown in figure 6.1.15. HOMER used the solar resource inputs to calculate the PV array power for each hour of the year. The latitude, longitude and the average daily radiation can be entered. The solar resource file for this case study had been provided by TOM Lambert of HOMER energy.
Figure 4.9.5b Solar Resource input Window
The sensitivity analysis carried by HOMER is shown in figure 4.9.51 below.
Figure 4.9.51
It can be seen from above sensitivity areas that HOMER recommends battery combination with all generating options. Wind generation is only recommended during the high winds or at high fuel costs as seen in the above plot in blue and green areas.
4.10 Cost Benefit Analysis:
It had been seen that when the system had been installed with PV array the return on an investment can be obtained in 4 and half year's time by considering that the wind speed and gasoline price was fixed at 3m/s and $0.8 for the whole year. For high wind speed and high gasoline prices the pay back will be received in 9 year's time if the comparison between supplying combination of PV/WT/GG/VRB-ESS to WT/GG/VRB is carried.
It was notable that HOMER recommended 50kWh of storage for combination of WT/PV/GG/VRB-ESS as compared to 70kWh for WT/GG/VRB-ESS combination to achieve optimization.
Figure 4.10.1
Because of this increased storage requirement the NPC for WT/GG/VRB-ESS increased.
Figure 4.10.1a Monthly Average Electric production by GG+ Wind+ PV
4.10.1b Monthly Average Electric production by GG +Wind +PV
Gasoline running hours were reduced by 5% from 511hrs to 22 hrs while the payback on an investment is obtained in nearly 6 and half years when 50 kWh VRB-ESS couples with 1.24 KW wind turbine, 2kW solar cells and gasoline generator based on gasoline price of $0.8/L.
It means if the solar panels are installed alongside wind generator and VRB-ESS than operational cost will be $969 per years as compared to operational costs of $1476 per year without PV cells.
The following table gives the specified case summary for all the three solutions;
Supplying the load with gasoline generator and wind turbine
Supplying the load with gasoline and wind turbine + VRB-ESS
Supplying the load with wind turbine + VRB-ESS and integration of PV-Array
Cases
Without Battery
With Battery
With Battery
Wind turbine
1.24 kW
1.24 kW
Mean output (kW)
0.43
0.43
0.43
Capacity factor (%)
34.5
34.5
34.5
Hours of operation per year
7459
7459
7459
Batteries
-
10 kW- 50kWh VRB-ESS
10 kW- 50kWh VRB-ESS
Unit
-
1
1
Expected life (yrs)
-
20
20
Gasoline generator
Size (kW)
2.6
2.6
2.6
Hours of operation per year
5889
560
314
Number of starts/year
579
20
238
Capacity factor (%)
14.2
5.18
0.718
Operational life (yrs)
0.84
8.93
15.9
Fuel consumption (L/yr)
3026
567
158
Specific fuel consumption(L/kWh)
0.935
0.481
0.969
PV Array
---
---
Mean output kW
---
---
0.18
Capacity Factor
---
---
18.2
Hours of operation per year
---
4371
Electrical
Wind (%)
54
76
68
Generator (%)
46
24
3
PV (%)
---
---
29
Economics
Total net present cost ($)
27453
20196
21776
Levelised cost of energy ($/kWh)
0.705
0.518
0.559Table 4.10.1
It had been seen during the whole of the case study that maximum benefit from integration of wind turbine and PV- Array can be obtained with inclusion of some storage medium like flow cells such as vanadium redox are the most compact solution and thus so far the best to buffer the output from wind-turbine and or PV cells.
In appendix D the graphs for monthly data i.e. daily average wind speed, output from wind, daily battery storage had been given.
It can be seen and clearly observed from those graphs that battery maintains its high state of charge during the periods of high wind-speed and batteries percentage state of charge reduces during the periods of low wind speed thus the energy will be utilized during those low wind generation periods.
4.11 Frequency Histogram:
Frequency histogram of battery SOC (state of charge), for four different scenarios after including PV-array into the system are discussed below.
4.11.1 High Wind Speed and High Fuel Cost:
It can be seen in the figure below that at high wind speeds and high fuel costs the battery maintains its state of charge.
Figure 4.11.1 Frequency Histogram High Wind Speed and High Fuel Cost
The discharge frequency (frequency of supply from battery) is very low because the maximum load is supplied by wind generation as well as the solar panels with minuet contribution from battery. It can also be assed that % SOC for battery had also improved after inclusion of PV array.
4.11.2 Low Wind Speed and Low Fuel Cost:
It can be seen from the figure below that at low wind speed the contribution to supply the load through battery was high.
Figure 4.11.2 Frequency Histogram Low Wind Speed and Low Fuel Cost
The battery discharges (supplies) from right to left i.e. at higher frequency to fulfil the demand. The monthly statistics graph shows that at average the battery maintains 40% state of charge. There was not much of improvement on the battery state of charge by inclusion of PV-Array if the comparison is carried between the figure 4.11.2 and
4.11.3 High Fuel Cost and Low Wind Speed:
It can be seen from the figure below that when the wind speed is low and fuel cost are high at that time the battery discharges at higher frequency i.e. supplies the load coupling with gasoline generator.
Figure 4.11.3
4.11.4 Low Fuel Cost and High Wind Speed:
It can be seen from figure below that at higher wind speeds the battery maintains its state of charge and attains the maximum value i.e. 100% to utilize it at periods of low wind speed.
Figure 4.11.4a
Chapter 5
UK Electricity Market
5.1 Brief Introduction:
The main principles of BETTA (British Electricity Trading and Transmission Arrangements) are based on trading the electricity between the suppliers and the consumers at the prices agreed upon by both the parties.
5.1.1 Imbalance settlement
They define and provide the arrangements and mechanism needed for clearing and settling the imbalances that may occur between the prediction of supply or demand and the actual market position ensuring that the overall system stays in balance in real time.
The following diagram shows all then arrangements made by the BETTA:
Figure 5.1 Brief review of BETTA
[2010 NETS Seven Year Statement: Chapter 10 - Market Overview PAGE 2]
http://www.nationalgrid.com/uk/sys_06/default.asp?action=mnch10_2.htm&sNode=1&Exp=N
5.1.2. The gate closure:
It is the moment at which the system operator will be notified by the market participants (generators, suppliers, traders and consumers) of their final position, and is set at one hour ahead of delivery no further contract notification can be made afterwards. The generators take the imbalancing risk by contracting on ETSA (Electricity Trading Service Arrangements). They pay penalties if they do not supply the amount of energy predicted and also, if the production is higher than the prediction, the excess will only be bought at SSP (System Sell Price) which is determined by the grid every thirty minutes. The main advantage of contracting on ETSA for the generators is that the amount of energy will be sold more expensively, in order to assure the correct supply of electricity as the supplier takes the risk of imbalance. The same process occurs for customers with large electricity requirements who contract on ETSA. Electricity consumed without any contract will be bought at SBP (System Buy Price) which also changes every thirty minutes.
5.2 The Balancing Mechanism:
The balancing mechanism involves submitting offers (proposed trades to increase the generation or decrease the demand) and/or bids (proposed trades to decrease the generation or increase the demand). The price of electricity is mentioned within the bid and is immediately paid the process operates on a 'pay as bid' basis. National Grid balances the system by resolving transmission constraints and purchasing offers, bids and other balancing services to match supply and demand. The security of supply is ensured by National Grid by assessing the physical position of all the market participants as, the market moves closer to the delivery time. A final physical notification has been supplied to National Grid by the participants before the gate closure declaring their current position and status. [2010 NETS Seven Year Statement: Chapter 10 - Market Overview PAGE4- 5 url: http://www.nationalgrid.com/uk/sys_06/default.asp?action=mnch10_2.htm&sNode=1&Exp=N
5.3 Settlement and Imbalance:
The energy imbalance is settled at the cash out price after determining the magnitude of any imbalance between participants' contractual positions notified at gate closure time including accepted offers and bids, physical flow between the suppliers and consumers calculated by the settlement administrator which can be a company such as Elexon in UK. The system sell price (SSP) is paid to parties with a surplus and without a contract and the deficit will be charged at the system buy price (SBP). These prices are designed to reflect the cost of operating the balancing mechanism or purchasing short term energy ahead of gate closure in the forward and spot markets. The electricity is bought in this way in the case of an unpredicted output power, such as the one from renewables. These incremental costs are derived by taking the average cost of the marginal 100MWh of priced action actions that the National Grid has taken to resolve the energy imbalance. If there are less than 100 MWh of priced actions, all the priced volume is used to calculate the replacement price. Imbalance settlement arrangement had been changed on 5th November 2009. The National Grid as transmission operator NETSO 'flags' when the bid-offer acceptance might resolve the transmission constraint along with forward trade actions and certain System Operator to System Operator actions over interconnectors which resolve a transmission constraint, or which are used to avoid other adverse effects on the systems joined by the interconnection. [2010 NETS Seven Year Statement: Chapter 10 - Market Overview PAGE4- 5]
Figure 5.3 Imbalance Price Calculations
[KEMA (2007) Outline description of studied international markets.
[SSP & SBP prices every 30 minutes, data provided by Smartest Energy, electricity trading company and Elexon, a settlement administrator.]
5.4 System Sell Price:
The System Sell Price indicates the evolving electricity prices. Indeed, as supply contracted on ETSA is bought more expensively than SSP, it has to follow its evolution.
The following graphs were obtained from Balancing Mechanism Reporting Service (BMRS) website:
Figure 5.4.1a
http://www.bmreports.com/bsp/bsp_home.htm [22:05 Date: 20/09/10]
Average SSP variation along the day is given in the graph below:
Figure 5.4.1b
These prices (SSP) fluctuate every thirty minutes. Each value represents the average price per year at the considered time. These fluctuations explain why traders divide days in trading blocks. Indeed, each block represents 4 hours. These blocks represent the main variations of the SSP. The following table illustrates it:
Trading Block
Average SSP (£/MWh)
block 1
18.03
block 2
18.54
block 3
28.07
block 4
27.68
block 5
32.89
block 6
22.97
Table 5.4.1 Variation of SSP per block
The table above shows that the best period to sell electricity is during the block 5, called the "peak time" period. The electricity is sold 30.3% more expensive than the daily average SSP during this block 5. For this reason, it can be really interesting for a storing technology to save energy during the cheap blocks as it can be seen from table 4.1 that block 1 and block 2, the electricity is 24% less expensive than average SSP and deliver it during peak hours.
Let us now have a look at the variation of the SSP over several years. This is plotted on the following graph:
Figure 5.4.1c Variation of SSP along the previous year
It can be seen on the graph that the SSP slightly increased since 2000. However, the most important thing to notice is that the SSP always goes up during winter. It is even easier to notice this by plotting the price over one year:
Figure 5.4.1d Variation of SSP along last year
These prices are the average peak time system sell prices for every day. It can be seen that they only reach and go over £50/MWh between October and March. For this reason, storage can be considered in summer in order to deliver energy in the winter. However, this would imply enormous reserves that cannot be reached with current storage technologies.
Chapter 6
Future Scenarios
The previous literature review described the main objective of the project, the purpose was to assess if flow batteries can be used to store energy produced by wind farm this energy will be utilised afterwards for several periods of time. This storage technology seems to be the most suitable one for storage of electricity produced by renewables. The Flow batteries can store huge amount of energy at GW levels as it had already been reviewed that storage size depends on the size of the cell stacks, and the storage capacity can be increased (expressed in kWh) by the volume of electrolyte. The flow batteries appear worthwhile looking into when storage has to be considered as long term applications and without any special requirement.
According to the structure of UK electricity market, there is a real need for sound and long term predictions to sell electricity at higher price than SSP by making valuable bids.
Therefore, the different possible ways of using a flow battery as a storage device for a wind farms will be accessed which, will provide knowledge and foundation for future work. It would be required to define control system and strategy for these scenarios to come true. These can be simulated and then a cost analysis can be carried out for each one of them. The simulations would considering an initial size of the battery than, SOC (state of charge) and the output of the battery can be calculated.
This chapter explains the simulation processes. The behaviour of a considered battery can be simulated over a period of ten months. The objective was to calculate the SOC and the output power of the flow battery after each time step. The benefit of using the battery can be estimated and, therefore the net present cost can be calculated after the lifetime of the project. The optimum size of battery for each configuration can be defined optimisation process should be used afterwards aiming to minimise the net present cost by changing the size of the battery.
The configurations would match the requirements of the British electricity market and in order to work on realistic scenarios, from the literature review, three main cases appeared:
Delivering energy during peak-demand periods
Delivering a constant power for each trading block of 4 hours.
Delivering a constant power for time periods of half an hour.
The algorithm representation either linear programming approach or non-linear programming approach should be designed, the SOC (state of charge) of the battery and the energy stored and discharged out of the battery has to be determined.
6.1 Choice of the scenarios:
6.1.1 Delivering energy mainly during peak-time periods:
The variation of the demand of electricity shows that the biggest consumption of electricity takes place during the "peak-time hours". During this period, electricity is at its most expensive, and can represent a good opportunity to pay back the flow batteries involved. In this case, the income should be much higher than in the following scenarios, since most of the electricity produced will be sold at the highest SSP. However, storing energy twenty hours a day requires a battery with a very big capacity. For this reason, another scenario will be set up: delivering a high constant power during peak-time hours and a low constant power the rest of the time. This should reduce the size of the battery and therefore its costs. With this second scenario, contracts to sell the electricity can be made 24 hours ahead in order to take advantage of the power prediction and then get higher bids when selling the electricity.
6.1.2. Delivering a constant power in blocks of 4 hours:
The electricity market is basically organised into trading blocks of 4 hours, as explained previously, and the prices of electricity mainly vary according to these blocks. This configuration of the market inspired this scenario, where the battery delivers a constant output power, matching that of the grid, for a period of four hours. The value of this power may be estimated from the prediction of the wind farm power output, from the changing rate of the wind farm power output, or from the energy stored inside the battery. This scenario will consequently reduce the size of the battery as it will store energy for a shorter period of time. However, this period is suitable for short-term contracts. Indeed, one hour after the closing gate time, the output power of the battery can be predicted and therefore electricity is still going to be sold more expensively because of the prediction and the risk of imbalancing taken on.
6.1.3. Delivering a constant power for a period of half an hour:
SSP and SBP prices change every 30 minutes. Therefore, it would be very interesting to consider a scenario delivering a constant power during this period of time. This will smooth the output of the wind farm and increase the quality of the power delivered. Currently, the period considered is too short to make a short-term contract. However, this situation may change in the future; therefore, additional revenue for the quality of the electricity was still considered. For each of the above scenarios, simulations will be run in order to optimise the size of the battery to maximise its benefits. The details of this optimisation process and the feasibility of the scenarios will be given in the financial analysis of the project.
Chapter 7
CONCLUSION
