1 Introduction
Ireland even though is not a tropical country gets enough sunlight to generate electrical energy from it. PV panels are becoming econom- ical which is a boost to this sector. South east Ireland comprises of counties such as Waterford, Wexford, Carlow, Kilkenny and South Tipperary.
Figure 1: Map of Ireland, south east region is highlighted
Average Irradiation in the south east of Ireland is around 995kWh/m2 per year. This figure shows that the south east of Ireland has enough potential to generate electricity from solar pv. Along with power generation comes the need of storing it. Many energy storage meth- ods are in use now. Choosing an appropriate one depends on many factors such as space, nearness to source and grid, location, safety, legislation etc. Following are some of the existing energy storage methods. From those, one of the technique will be short listed after checking its feasibility with the 40MW solar plant.
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2 Energy storage
2.1 Compressed air energy storage
Compressed air energy storage is a technique of storing energy as the potential energy of compressed gas. Normally, air is pumped into large storage tanks or naturally formed underground cavities. Whenever there is excess energy available, that energy is used to run an air compressor which pumps air into the storage tanks/caverns. This compressed air is then expanded through gas turbine expanders which are coupled with generators to produce electricity when short- age arises in the grid(Gardner and Haynes, 2007).This technique is suitable for solar pv as the excess energy produced during daytime can be stored and can be used during later peak hours. The follow- ing figure represents compressed air energy storage.
Figure 2: Conceptual representation of compressed air energy storage
One of the major advantage of this technique is that, excess energy from the grid can also be stored during off-peak hours. Gardner and Haynes (2007) states that this method becomes further advan- tageous when coupled with another intermittent source such an wind energy. One prime dis-advantage is the losses occurring during en- ergy conversion. Only lesser energy will make it eventually to the grid when passed through this system than transferring it directly to the grid. Another significant dis-advantage is the requirement of
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additional heating medium (natural gas in most cases) for the ex- pansion process. If the price of natural gas goes up, that will affect the economic viability of this system.
2.2 Flow batteries
Flow batteries are electrochemical storage systems which use elec- trolyte that is stored in a tank separated from the battery cell. Functioning of flow battery is based on oxidation and reduction re- actions in the cell. Flow battery consists of two parallel electrodes separated by an ion exchange membrane, forming two half cells. The electro active materials are stored externally in an electrolyte and is introduced into the device only during operation. The chemi- cal energy in the electro-active materials are converted directly into electrical energy, similar to a conventional battery and fuel cell.
Figure 3: Schematic of flow battery for renewable energy storage
In the energy conversion process, the electrodes do not undergo physical and chemical changes, leading to more stable and durable performance. In a flow battery there is inherent safety of stor- ing the active materials separately from the reactive point source. Other advantages are quick response times (common to all battery systems), high electricity-to-electricity conversion efficiency, no cell- to-cell equalization requirement, simple state-of-charge indication
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(based on electro-active concentrations), low maintenance, tolerance to overcharge and over- discharge, and perhaps most importantly, the ability for deep discharges without affecting cycle life(Nguyen and Savinell, 2010). Even though this technology has many advan- tages as stated above, it is fairly new and has just started gaining popularity. As per Nguyen and Savinell (2010) researches are still being carried out in areas such as :
• Low-cost, efficient, and durable electrodes.
• Chemically stable redox couples, having large potential dif- ferences, with high solubilities of both oxidized and reduced species, and fast redox kinetics.
• Highly permselective and durable membranes.
• Electrode structure and cell design that minimize transport
losses.
• Designs with minimal pumping and shunt current losses.
• Large scale power and system management and grid integra- tion.
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2.3 Pumped hydro energy storage
Pumped hydro energy storage (PHES) is an overwhelmingly estab- lished bulk energy storage technology with a global installed capac- ity of over 130GW. The fundamental principal is to store electrical energy in the form of hydraulic potential energy. Pumping of water to a height takes place during off peak hours. During the discharging process, gravitational potential energy is converted into mechanical energy and then into electrical energy by allowing water to flow down driving a turbine that in turn drives an electrical generator.
Figure 4: PHES plant layout
The amount of energy stored depends on the height difference be- tween the two reservoirs and the total volume of water stored. The rated power of PHS plants depends on the water pressure and flow rate through the turbines and rated power of the pump/turbine and generator/motor units(Figueiredo and Flynn, 2006). The storage duration for this technique is very high which makes it one of the most preferred energy storage method. The power capacity and life- time are higher than most of the other energy storage methods. One of the drawbacks is the space required to set up a dam as well as the closeness to electrical source(solar pv plant in this case) and grid. Losses may occur during transmission if the dam and water source is far away from grid and electrical source.
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2.4 Flywheel energy storage system
Flywheel energy storage systems use electric energy input which is stored in the form of kinetic energy. A modern flywheel energy stor- age system is composed of five primary components: a flywheel, a group of bearings, a reversible electrical motor/generator, a power electronic unit and a vacuum chamber. The flywheel energy stor- age system is placed in the vacuum chamber to reduce wind shear and energy loss from air resistance. The amount of energy stored is dependent on the rotating speed of flywheel and its inertia(Luo et al., 2015). When short-term backup power is required when util- ity power fluctuates or is lost, the inertia allows the flywheel to continue spinning and the resulting kinetic energy is converted to electricity.
Figure 5: Flywheel energy storage facility
Electric energy input accelerates the mass to speed via an inte- grated motor-generator. The energy is discharged by drawing down the kinetic energy using the same motor-generator. Flywheel energy storage system have high energy density and substantial durability which allows them to be cycled frequently with no impact on perfor- mance. Response rate is very high for this technology(Association).
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2.5 Lead-acid battery
Lead-acid batteries are the most widely used rechargeable batteries. Cathode of this battery is made of lead oxide and anode is made of lead. Sulphuric acid is used as the electrolyte. When electrical energy is introduced into the battery, by chemical reactions, lead oxide and sulphuric acid is replenished and gets ready for discharge. At this stage they are said to be storing energy. The inequality of electrons between anode and cathode gives rise to the flow of current through external load for balancing this inequality. This process is called discharging of lead acid batteries. Lead-acid batteries have fast response time, small self discharge rates, relatively higher effi- ciencies and lower capital cost which makes them a favourable choice when it comes to energy storage(Luo et al., 2015).
Figure 6: Lead acid battery
However, there are only limited number of installations around the world as utility scale energy storage system mainly due to their rel- atively low cycling times, energy density and specific energy. More- over, they may perform poorly at low temperatures and hence ther- mal management system might be required which increases overall system cost(Paul, 2009).
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2.6 Lithium-ion battery
Lithium-ion batteries are extremely popular and versatile and can be found in many electronic devices such as laptop, cell phones etc. The anode of lithium-ion batteries are made of carbon and the cath- ode is made of lithium metal oxide. The electrolyte is normally a non-aqueous organic liquid containing dissolved lithium salts. When the battery is charging, the positively charged lithium ions are at- tracted to and move towards the cathode. Once it is bombarded with these ions, the cathode becomes more positively charged than the anode, and this attracts negatively charged electrons. The en- ergy of the negatively charged electrons flowing towards the cathode is used to generate power. When the battery is in charging mode, the lithium ions move in the opposite direction as before. As they move from the cathode to the anode, the battery is recharged for another use(Troiano).
Figure 7: Chemistry and principal components of Li-ion battery
Li-ion batteries are considered for applications where response time, small dimension and weight of equipment are important. Li-ion bat- teries also have high cycle efficiency. Another added advantage is that Li-ion batteries can produce a lot more electrical power per unit of weight than other batteries. This means that lithium-ion batteries can store the same amount of power as other batteries, but can accomplish this in a lighter and smaller package. However, Li-ion batteries take much longer to charge when compared to other batteries.
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3 Grid operation, transmission and distribution
The electricity grid is a complex and supremely significant system. It transmits power generated at various locations to end users over long distances. Non renewable and renewable technologies are used to generate electricity, and grid takes input from all of these. Cer- tain power plants such as coal and nuclear power plants have little short term flexibility in adjusting their electricity output; it takes a long time to ramp up or down their electricity output. Other plants, such as natural-gas fired plants, can be ramped up very quickly, and are often used to meet peaks in demand. More variable technologies, such as wind and solar photovoltaics, are generally used whenever they are available(of concerned scientists). They are also stored dur- ing off-peak time for later use when demand rises.
Transmission lines are used to carry high-voltage electricity over long distances and connect electricity generators with end users. Trans- mission lines are either overhead power lines or underground power cables. Overhead cables are not insulated and hence they are vul- nerable to the weather and climatic changes, but are less expensive to install than underground power cables. Underground lines are typically insulated. Overhead and underground transmission lines are made of aluminium alloy and reinforced with steel(of concerned scientists). Transmission lines carry electricity at high voltages as it reduces the amount of electricity lost in transit. Due to resistance in transmission lines, certain amount of electricity is dissipated as heat. Hence higher the voltage on transmission lines, lower the electricity losses. The typical voltage of transmission is around 110kV. Power generators can only produce electricity at lower voltages. A step up transformer is used to convert the generated electricity to higher voltage in order to make high voltage transmission possible. Once this electricity reaches close to customers, a step down transformer is used to convert it back to lower voltage(of concerned scientists).
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Figure 8: Basic structure of elctricity grid
The distribution network is the system of wires that picks up where the transmission lines leave off. These networks start at the transformers and end with homes, schools, and businesses. Distri- bution is regulated on the national level by a government authorised group who set the retail rates for electricity in each country(of con- cerned scientists).
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4 Feasibility Analysis
Among the various energy storage methods discussed in section 2, one of them will be chosen for the 40MW distributed solar pv plant in the south east of Ireland, considering various parameters.
Location
With an estimated total population of over 2,10,000 [as per 2015 census], cities in south east of Ireland are considered as urban and is technically advancing day by day. Since these areas get sufficient solar irradiation, harnessing electricity from solar pv is a viable op- tion. Storing the excess power generated during off-peak by use of pumped hydro energy storage is a feasible option as these regions have rivers flowing through them. The proximity to water bodies is advantageous and can be made use of if properly planned. Even though this advantage is in place, space for building a dam can put this technology behind other energy storage methods. Storing using compressed air energy method will involve piling during construc- tion, which cannot be done in populated urban areas. Since, these areas are near to the sea, they are relatively cooler than most of the other places in Ireland. Lead-acid battery does not perform well un- der cool conditions and hence using them will is not technologically viable.
Proximity to network
All the major counties in south east Ireland have a 110kV substation and hence transmission losses will be minimised. Hence the major losses to be considered is the loss incurring during energy conversion.
Safety
Even though batteries are most widely used for storing solar pv output, they are not the most safest technology in existence. The disposal or recycling of dumped batteries must be considered if toxic chemical materials are used(Author, 2004). In this case, pumped hydro, compressed gas and flywheel energy storage technologies are considered safe.
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Environment
Since the technology used to generate electricity is 100% renewable and does not have a negative impact on environment, the technique used to store that electricity must also not have any negative im- plication on environment. The chemicals used in batteries are con- sidered toxic and hence they are not environment friendly. Among the discussed energy storage methods, pumped hydro and flywheel energy storage system are the ones which does not have a huge en- vironmental impact.
Economics
The capital cost for pumped hydro energy system is higher than the rest of the technologies used. But when considering the long run, pumped hydro have low maintenance cost whereas batteries and compressed air storage method have high maintenance and over- hauling cost.
By analysing the various energy storage systems, it is clear that pumped hydro energy storage technology is most suitable for this scenario. Compressed air energy storage requires underground cav- erns which cannot be built in urban and populated areas. The requirement of natural gas for energy storage process is also a draw- back for this system. Flow battery technology is still in its initial developmental stages and has not yet gained wide popularity. Re- searches are still being carried out in this area and it can only be deployed once it is technologically advanced like other energy storing techniques. Flywheel system can be integrated with batteries to im- prove the system output and elongate the batterys operational life- time(Bolund et al., 2007). But this means, cost will increase while we integrate both technologies together. Flywheel energy storage system, even though has fast response characteristics which make them suitable in applications involving renewable energy sources for grid frequency balancing, using them as a stand alone energy stor- age system is not fully effective. Both Li-ion and lead acid batteries are used mostly for storing energy from intermittent energy source like solar pv. Li-ion batteries have higher depth of discharge, higher power and energy density, longer lifespan and higher efficiency when compared to lead acid batteries. They are lighter and is more com- pact than lead acid batteries. But using batteries for storing such
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high amount of energy is not viable as numerous number of batter- ies will be required for storage. The price of Li-ion battery [efficient among other batteries] is high and hence it is not economically fea- sible to use battery for this specific pv plant. Pumped hydro energy system, even thought it has relatively low power/energy densities, for a 40MW solar pv plant, using this technology will be econom- ically viable. Even though the initial cost is high, due to high cy- cle efficiency [70%-80%], a lifetime of more than 40 years and less maintenance cost, pumped hydro energy storage is found out to be the best technology for this particular project. The suitable energy storage duration rangers from few hours to days or months when compared to batteries which can store only for a maximum of few hours to days. Nearness to water bodies in the project location is an added advantage to use this technology.
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5 Advantages of using pumped hydro technol- ogy
Few advantages of PHES is listed below:
• Pumped hydro energy storage is suitable for high capacities with longer life span.
• This technology has more maturity in terms of technological development and knowledge of application when compared to batteries.
• Initial cost is high when compared to all other technologies, but lesser maintenance cost and longer lifespan than all other available energy storage technologies.
• Pumped hydro energy storage plant does not require flowing water supply like conventional hydro electric power plants.
• These storage plants can be operated all over the year in all seasons.
• Off-peak pumping of water to reservoir reduces cost implica- tions.
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6 Conclusion
From the overview, it is clear that there will be different decision- making factors involved while choosing suitable energy storage op- tions for deployment. For the national grid, the level of technologi- cal maturity, reliability and potential environmental impacts such as the toxic chemical materials used in batteries may be considered as the main parameters and cost-effectiveness may not be particularly important. For individual or private networks, in addition to the above stated factors, the capital cost and the payback period will also be the governing factors.
For this specific project, among the many energy storage technolo- gies mentioned here, through detailed analysis and comparison, pumped hydro storage system was found to have a leading edge than other technologies.