John Donohoe
Renewable Energy Strategy for Ireland: Energy Storage Systems in Ireland
Abstract
This document aims to highlight the benefits of storage systems for Ireland's energy generation sector. Specifically it will look at the benefit that storage can bring to the renewable energy generation in Ireland and how it can convert unreliable sources of energy such as wind and ocean energy into more usable forms. First a quick overview of conventional generation techniques is given. The document will then detail the operation of various renewable generation technologies highlighting their main advantages and disadvantages. The document will then discuss the operation of various storage technologies and describe which renewable generation sources they are most suited to. Finally, both the renewable generation technologies and storage facilities will be considered in the case of Ireland. It will look at the proposed road map of renewable generation in Ireland and describe how storage solutions can add value to this market and help overcome some of the difficulties seen with some renewable generation technologies.
Overview
Storage is a key element in the efficient generation and usage of energy in the modern world. Storage is used in almost every electrical appliance in some form, be it batteries or capacitors. In transport, again batteries and fuels such as petrol and diesel are used, and in large scale electricity production such as water reservoirs for Hydro-electric plants and fossil fuels for power plants. The primary function of a storage system is to allow us to keep energy in a dormant state until we are ready to use it for some purpose. This allows us to manipulate energy flows to suit our purposes in day to day life.
The goal of this document is to show how we can benefit from the use storage systems in electricity generation. It will show how storage can be used to generate electricity more efficiently to meet demands and also how it can be used to improve the reliability of renewable energy sources. As the overall theme of this document is "Renewable Energy Strategy for Ireland", this document will focus more on how storage can be used with renewable energy sources to maximize the production of useable energy from these resources. The document will mainly focus on Ireland's energy needs and generation capabilities and how storage fits into these areas. It will take into account the renewable energy road map set out in the 2009 Renewable Energy Directive and try to show where storage can be used to help achieve and exceed the targets set out is this directive.
Justification for Energy Storage
Conventional Generation
Energy storage can be used to improve the overall efficiency of conventional electricity generation. It can be used in conjunction with typical coal, oil and gas power generators to help produce energy to meet the consumption demands. An example of how this is achieved is the pumped hydro-electric storage system (PHS) installed in Turlough Hill Co. Wicklow in Ireland. Electricity is generated in power stations and used to pump water from the lower reservoir to the upper reservoir at night time when electricity consumption is low. During the day, when the demand is higher, the water is allowed to flow back from the upper reservoir to the lower reservoir through a hydro-electric plant to generate electricity. This electricity is then supplied to the grid to meet the demand.
This increases the performance of the conventional generation in two ways. During the night, electricity demand is lower which means that the cost of electricity is lower when compared to the daytime rates. When the electricity is converted back during the peak electricity usage, there is a net profit on the generated electricity as it can be sold at a higher rate than when it was stored. The second advantage comes in the form of maintenance of conventional fossil fuel power generators. Before a conventional power station can start generating electricity, it must first ramp up its power production. The time this takes depends on the type of power station but can be anywhere from a few minutes to hours. It should be obvious that power stations do not need to be generating electricity when there is no demand for it so when this occurs the power generator is ramped down again. This is done to match the load on the system and cannot be entirely avoided. The process of ramping up and down is known as cycling. This cycling process takes its toll on the power stations in the form of required maintenance. Generally speaking, the more often a power station is put through this process the shorter the life expectancy of the plant.
The storage of electricity helps to smooth this process in two ways. When the station is ramping up, it can assist for a period of time allowing the power station to ramp up its output at the most cost effective and least damaging rate. The inverse occurs when the power station needs to ramp down its power output as the storage facility can be used to absorb excess generated electricity when ramp down rate is slower than the drop in demand. This helps to prolong the life of the power plant and improve the overall efficiency by not allowing as much electricity to go to waste.
Renewable Generation
Renewable energy generators can also benefit greatly from the use of storage systems. Unpredictable sources of electricity such as wind turbines and solar panels tend to have variable outputs over time. A Wind turbines output can vary from second to second due to turbulence, from minute to minute due to gusting conditions, hour to hour due to changes in air pressure systems, day to day and week to week due to changes in weather. Seasonal changes are also apparent due differences in weather during the winter and summer weather. All this variation makes it unsuitable for stable electricity supply. On top of all this variance it also suffers from a problem of mismatch between supply and demand. There is no matching between when electricity is in demand and when electricity is being generated by wind turbines which makes them even less attractive in terms of electricity supply. Solar suffers from similar variability as passing clouds can cause a drop in the electricity output from the cells. They also have seasonal variability like with wind generation.
Energy storage here again plays a part in increase the usability of the electricity generated from these resources as it can smooth out the variations in supply from these sources. Small scale storage like batteries can help smooth out short term variations over the course of seconds, minutes, and even days to allow the wind turbines and solar cells to produce a more stable source of electricity. These renewable energy sources can also be used with large scale storage systems such as PHS to allow the energy to be accumulated from a number of turbines / solar panels into one container which helps to smooth variations over a longer period of time. This provides benefits in two ways for solar and wind, the first being that they can be used as a stable electricity supply and the second means that the become a more valuable resource in the long term.
Capacity Factor and Availability Factor
When talking about energy generation and especially renewable generation, there are a number of technical terms referred to but not always explained. Two of these terms are the capacity factor and availability factor.
The capacity factor for an energy generating facility is ratio of the actual output of a generation facility compared to its potential output. This figures for a capacity factor can be calculated over various time lengths however it is usually taken over a period of a year in most cases. For example, a conventional plant may be rated at 5MW which means it is capable of producing 5MWh of energy in one hour. If the plant were to operate at a power of 5MW for an entire year, it would generate 43.8GWh of energy. However, there are a variety of reasons why the power plant may not be generating energy during the course of a year. Power plants need maintenance, suffer outages, and sometimes is it not economical for them to generate electricity constantly (in the case of peaking plants). This reduces the overall output of the power plant for a given year. Say in this example, the plant generated 20GWh of energy, that would mean the capacity factor for the power plant would be 20 / 43.8 = 0.45 or 45%. On average over the course of the year, the plant has generated 45% of the total possible for the entire year.
The availability factor of a power plant is the term used to describe the average availability of a power generator to generate energy over a period of time. Similar to capacity factor it is taken over the course of a year in most cases. If a power plant is offline for 2 months of the year due to maintenance then the availability factor is 10/12 = 0.83 or the plant was available to generate 83% of the year. Availability factor is an important contributor to the capacity factor as the time a plant is available to generate is directly related to how much it generates overall. The availability factor is important in the discussion of renewable energy sources as it indicates how much energy can be generated from a renewable generation technology such as solar. As the sun is only available for approximately half a day on average per year, the availability of the solar generator can only be 50% at best.
Conventional Electricity Generation.
As it stands, electricity is mostly generated as it is needed in power stations. This is mainly due to the nature of electricity itself, as it is a difficult form of energy to store on a large scale so the tendency is to store alternate forms of energy that can be easily converted to electricity when it is needed. This has led to the extensive use of fossil fuels such as coal, oil and natural gas as the mediums by which most electricity is generated. The energy density of fossil fuels allows quite a large amount of energy to be stored in a small area and they can be burned to quickly release heat energy needed in the production of electricity in power plants. This is useful when trying to meeting consumer demands for electricity which can vary throughout the course of the day.
Modern power grids cater for the variation in demand by quickly generating electricity when it is needed. Generally this is achieved by using three types of electricity generation plants. The first is a baseload plant which usually generates electricity at a constant output but does so efficiently. They are usually used to generate electricity and output at the minimum required electricity per day. The second type is a load-following plant which ramps up and down its electricity generation over the course of a day based on expected trends in electricity demand. The third type is a peaking plant which is used to offset the loading following generation of electricity to match demand. These types of plants tend to have a capability of ramping up and down their output very quickly but are not designed to generate electricity as efficiently as a baseload or load following plant.
Electricity Demand 12/12/2011
Data gather from Eirgrid website[5]
The image above shows the electricity demand for Ireland on 12/12/2011. This represents a typical daily demand profile where electricity is in least demand during the hours of 3am - 6am and rises throughout the day hitting a high point 6pm. In this scenario, baseload power plants would operate and output at the lowest demand which is approximately 2200MW indicated by the black line. A load following plant would begin ramping up at approximately 6am and follow the upward trend until 6pm indicated by the red line. The peaking plant would generate electricity to fill in the gaps between the load following plant and demand. This is only to give a rough idea of how electricity is generated to meet demand. It does not represent the exact operation of power plants as each plants can be operated at different levels to achieve higher efficiencies overall.
This situation can benefit from energy storage by generating more energy using the baseload power plants. If the baseload plants are allowed to operate at higher output to produce say 3000MW, there is excess electricity being generated between 12:00am and 7am. This excess can be diverted to storage unit until the demand rises above 3000MW. The stored energy can then be released gradually to follow demand. This means that the dependance on the load following and peaking generators is reduced while the storage system is releasing its energy. This process is used at Turlough Hill hydro-electric power station in Co. Wicklow Ireland to reduce the cost of producing electricity to meet demand.
Renewable Energy Technologies
Producing energy from renewable resources is a fast growing industry. Since the Kyoto Protocol was published, and the continuous rise in fossil fuel prices, many countries have started investing more money into alternative sources of energy. In fact the 2009 Renewable Energy Directive has set a target for Europe to generate 20% of all consumed energy from renewable sources by 2020. This is achieved through government policies encouraging the development of renewable energy resources within each country. This section will deal with introducing some of these renewable energy technologies. Their requirements, operation, advantages and disadvantages will be given.
Wind Generation
The idea of harnessing wind energy is not a new one. It has been around something that has been around for some time. In fact, some estimate that is has been around for over a thousand years. In the years 500-900 A.D, Persia saw the first vertical axis turbine used to grind grain and pump water[15]. It is only in the last century that we have looked to wind energy to generate electricity as our dependance on it has grown and only in recent years have we looked to generate electricity on a large scale from wind energy.
Wind Turbines[10]
The image above shows a standard horizontal axis wind turbine. The types of wind turbine can vary from this with turbines developed to have two blades or only one or even much more blades. Turbines also exist where the axis of rotation has been changed from horizontal to vertical. These types of turbines are known as vertical axis turbines. Regardless of the number of blades or axis of rotation, all these turbines share similar operation in that they use the air to to create a rotating motion. For the above wind turbines, this is achieved by angling the blades of the turbine slightly away from the direction the wind is blowing. This creates a higher air pressure on one side on the blade compared to the opposite side. This pressure differential creates a force on the blade trying to push it towards the lower pressure. The attachment of the blade to a central point causes the rotating motion.
Operating requirements
Wind turbines require large open areas to operate. Mountains, buildings, trees and other tall objects disrupt the flow of air and cause turbulence. This turbulence severely effects the performance of wind turbines as they need a constant flow of air to generate efficiently. This is partly the reason why turbines are installed in remote locations, far enough away from towns and cities that there disturbance to the air flow is minimal. Another reason is that more suitable areas for generating electricity tend to be in remote areas such as, on mountain and hill sides and in coastal regions where the wind can flow over the relatively flat sea water for miles before it reaches shore.
Wind speed has been found to increase with height from the ground. Generally, this means the taller the wind turbine, the more electricity it will produce. However, there has been some objection to wind turbines from local communities due to their size and noise pollution. This is another reason why wind turbines are preferably located in remote locations as it can be much more difficult to get planning permission for site development closer to villages, towns and cities.
The last requirement of wind turbine operation is spacing. When the air passes through a wind turbine, the turbine causes large amounts of turbulence. This turbulence can affect the operating efficiency of other wind turbines operating in close proximity to it. As a result, the turbines must be spaced quite far apart to avoid this problem. The recommendation is to separate the wind turbines by five times their rotating diameter[16](See chapter Wind II). This invariably leads to building in remote areas once again where land is cheaper than land closer to a town or city where there is more of a demand for the electricity generated.
Wind Energy Advantages.
Wind is a completely free and renewable resource, it exists everywhere on the surface of the planet meaning that any country can avail of this resource. Countries with large coastal regions are most likely to benefit greatly from use of this resource as off shore installations can provide larger operating efficiencies that those on land and do not consume large areas of land for the purpose of generating electricity. Generating electricity from wind has very small foot print in terms of carbon dioxide and other greenhouse gases. Also the unused land between wind turbines on a wind farm can be used for farming and agricultural purposes. Wind turbines can be used to generate electricity in remote locations such as areas of mountains that are inaccessible or un-economical for standard grid connection. This is useful for the likes of transmission masts which are generally located high up mountains overlooking large population centers.
Wind Energy Dis-advantages
The output from wind turbine can be variable. The output can vary over seconds, minutes, hours, days, weeks, months and even years. These variations can be due to changes in weather, climate and local surroundings. This variability leads to much difficulty in providing electricity to consumers in a reliable way. Wind energy also suffers from a problem of mismatching supply and demand. The wind is not always blowing when someone wants to watch T.V. or listen to the radio so there may not be electricity available when it is needed. One way of reducing the variability of energy generated from a wind turbine is to build a wind farm consisting of a number of wind turbines. The short term variability over the course of seconds and minutes can be reduced as different wind turbines will experience different wind speeds at different times. Considering the overall output, the output of a single turbine becomes less significant giving a more consistent output. The variability over the course of hours and days and even weeks can be reduced by having multiple wind farms. While the wind may be low in one area, another area may have higher wind speeds. Again this reduces the average variability when viewed from an overall perspective.
Some people would argue that wind turbines are unsightly, this is of course personal preference but has caused previous objections to wind turbines being constructed. Another more evident problem with wind generation is noise pollution as the rotation of the large blades and operation of the internal mechanics such as gearboxes can generate substantial amounts of noise. The sound from the rotation on the turbine blades is generally only a problem at low wind speeds as at higher wind speeds, the wind drowns out the sound of the noise. For mechanical noise, special sound proofing materials can be used in the casing of the gear trains to reduce the noise output.
Suitability of location is another issue, wind can of course be harvested almost anywhere on the planet however the correct locations should be chosen to maximize the efficiency at which a wind turbine generates electricity. Optimum locations for generating electricity have found to be in large open areas, hills, mountains and coastal regions. The main difficulty with these regions is that they are usually away from population centers where electricity is required the most meaning that the electricity must be transmitted to the consumers incurring losses along the way.
Another issue that is related to the transmission of electricity generated from a wind turbine is the ability to dispatch the electricity. If the wind turbine is generating electricity at a time when there is high demand, there is a possibility that the electricity generated by the turbine cannot be transmitted within the electricity grid due to a lack of available transmission capacity. The transmission capacity can vary depending on the location and how much electricity is already being transmitted in the grid at any particular time. If the turbine is generating electricity when there is no capacity available to transmit it, then it is wasted.
Capacity factor and availability factor.
Location is an important variable in the value of capacity factor for a wind turbine or wind farm. The value obtained for capacity factor is generally given for a particular area rather than for a particular type of turbine. In the case of the west cost of Ireland, between 25% and 30% is generally the accepted capacity factor for wind generation. The availability factor for wind turbines comes down to the quality of the wind turbine itself. Most wind turbines are designed to run constantly and only very rarely taken offline for maintenance. While the wing is an unpredictable source of energy, wind turbines are generally designed to only begin generating electricity when a certain minimum wind speed is reached. As a result, maintenance can be performed on the turbines when wind speeds are below this minimum wind speed and thus not affecting the availability factor of the turbine itself. Generally speaking, availability factors for wind turbines tend to be quite high reaching 98%-100%.
Solar Generation
Harvesting the suns energy directly is the most tempting of all the renewable energy technologies as the sun supplies almost all the energy available on the earth. In fact, pretty much any type of renewable energy can be linked back to the sun as a source. For example, the wind and waves are the result of differential heating in the earths atmosphere on the earths surface leading to the flow of heated particles to cooler areas in the form of air and water currents. The creation of air currents leads to wind and wind blowing across the surface of the ocean mixed with gravitational energy from the sun and moon leads to waves and tides.
Solar energy arrives to the earth in the form of electromagnetic radiation. This can be visible light that our eyes use to see, infra-red radiation which we sense as heat or ultra-violet light which can cause sun-burn when we are exposed to it for too long. There are many forms of electromagnetic radiation but those most useful to us come in the form of visible light and heat from the sun.
Solar Thermal Generation
Solar Thermal Generation takes advantage of the heat radiation that arrives on the earth from the sun by using it to heat water. This is achieved by using highly polished concave mirrors to focus the radiation into a single point. Below is an image of a solar tower which is the focal point of all the surrounding reflecting mirrors. at the focal point is a container through which water is pumped and heated by the concentrated solar radiation. This water is usually heated and then pumped to a thermal storage facility or onto a steam turbine electricity generator where the heat energy in the water is converted to electricity and output to the grid. Solar thermal is also implemented on a smaller scale where the water can be heated and pumped to a housing estate or an individual house can supply its own hot water from the sun.
Solar Thermal Tower[11]
Solar Thermal Advantages
Solar thermal makes use of the suns energy which is freely available. Also the suns rays can be focused using relatively cheap equipment such as mirrors.
Solar Thermal Dis-Advantages.
Solar thermal relies heavily on direct sunlight which can be focused onto a single point using the reflective mirrors. The problem arises when there is low amounts of direct sunlight available, i.e. at night or when it is cloudy. This means the technology is only suited to areas where there are high amounts of direct sunlight and for long periods during the year. Also the technology can only be used during the daytime meaning that alternative sources of energy need to be made available during the night and during the winter season to provide electricity. Another problem is that a tracking system must be included to alter the angle of the mirrors to keep the light focused on a single point as the suns position in the sky changes over the course of a single day and over the course of the year.
Capacity factor and availability factor
The average capacity factor for solar thermal generation is usually around 20%. This seems quite low but can occur for a number of reasons. As mention, clouds can be problematic for solar thermal as diffuse light is not easily concentrated. Another issue with solar thermal is what the heat energy is used for. If the energy is used directly as a source of heat for homes, the capacity factor can be higher whereas using solar thermal power to generate electricity is a quite lossy process with conversion efficiencies of approximately 30%.
The availability factor for solar thermal is quite similar to wind in that it is quite high. During the nighttime only a storage and generation plant may be active meaning that collecting equipment can be maintained regularly without affecting the availability during the daytime. Multiple stores and turbine generators can be used to allow the storage and generation facilities to be maintained also without affecting the availability of the plant to produce power.
Solar Photovoltaic
Solar Photovoltaic utilizes the suns energy in a different manner. It uses semi-conducting photo cells to generate electricity directly from the suns light rays. The process by which the solar panels create electricity is known as the photovoltaic effect. This is similar to the photo-electric effect in which electrons are emitted from the cells when they are exposed to radiation. A quick summary of what happens in the photo-electric effect is that the electrons in the cell material absorb the energy from the photons in the radiation. The energy they gain from these photons gives them enough energy to break free of the material and move about freely. In Photovoltaic, the semiconductor materials such as silicon undergo a process known a doping in their manufacture. This doping process creates two types of doped silicon. P-type and N-type. The N-type material has excess electrons and the P-type has a lack of electrons. When a P-type and N-type are combined, whats known as a PN junction is created. When the PN junction is created, some of the electrons from the N-type move into the P-type material in an effort to balance the distribution of charge. This results in the creation of a depletion region. This region is where there is a balance between the number of electrons and the number of holes (lack of elections).
This region results in the formation of what is known as the valence band and conduction band. The energy difference between these two bands is the energy gap or band gap of the semiconductor. When the PN junction is exposed to radiation, the electrons in the valence band absorb the energy from the photons in the radiation and move from the valence band to the conduction band. This results in a the creation of a electron - hole pair which are negatively and positively charged respectively. These charges then move to opposite ends of the cell. This results in the build up of charge creating a potential difference (Voltage) which can be used for electrical purposes.
Solar Photovoltaic[12]
Solar Photovoltaic Advantages.
Similar to solar thermal, photovoltaic makes use of the sun as its primary source of energy which is freely available. However, it is not as reliant on direct sunlight and can still generate electricity in cloudier conditions.
Solar Photovoltaic Dis-advantages.
Solar cell efficiency is not very high. The effective conversion rates for commercial solar panels reach up to approximately 17%. This can be viewed on a solar cell comparison site here: http://sroeco.com/solar/table/. This means that 83% of the light that shines on the panel is not converted into electricity. Some solar cells have been produced which exceed this conversion efficiency, however, they are still quite new and expensive. Solar cells also do not function at night time meaning that depending on location, solar cells may not be a viable source of electricity. Materials used in the production of some solar cells can be toxic as well as limited in supply. Cells made of silicon remain the most common due to the high availability of the material.
Capacity factor and availability factor
Ocean Generation
Generation of electricity from the seas and oceans of the world is a technologically challenging prospect however, there is much to be gained from harnessing the power of the waves and tides. Sea and ocean tides are created from the gravitational forces between the earth sun and moon. The vast amounts of water on the earths surface is attracted towards the moon as it spins around the earth and towards the sun as the earth rotates around the sun. These forces of attraction lead to the movements of water known as tides. Waves are produced as a result of wind blowing across the surface of the water. This pushes the surface water back causing it to build up and form a wave.
Tidal Energy
The energy of the tides can be harnessed in a few ways but probably the most common is shown in the image below which is know as a tidal dam or barrage. The dam holds back the incoming water of the tide to create a difference in water pressure between both sides of the dam. This pressure difference causes water to flow through the under water tunnel and through an electricity generating turbine. When the tide goes back out, the water flows the opposite way through the tunnel generating more electricity.
Tidal Generation[13]
Tidal energy advantages
The movement of the tides are predictable and reliable so assuming everything is functioning correctly, accurate predictions can be made on the energy generated from tidal generators. This has its advantages over other renewable technologies such as Solar and Wind which do not share this same reliability.
Another advantage of tidal energy is that most of the infrastructure is either in the water or on the coast. This means that less land which can be costly in certain locations can be used for other purposes.
Tidal energy disadvantages
Tide change only occurs every 6 hours approximately and time at which it occurs changes from one day to the next. This means there is not matching between supply of electricity and demand. Tidal generators are costly, large capital investment is required and ongoing maintenance costs can be high due to wear and tear on components from sea water. Tidal would be considered to be at higher risk than land based generators due to unforeseen incidents such as large storms creating larger than expected waves which could damage or destroy equipment.
Wave Energy
Wave energy is a form of ocean energy that has yet to be capitalized on but there are many technologies being researched currently to try and utilize the power of the waves. The below image is an example of such a technology which is intended to be deployed of the coast of Ireland. The technology uses under water wave energy capturing devices to pump seawater to the mainland at high pressure. This high pressure water can then be used within generator to generate electricity or be stored in an onshore water reservoir where it can be used with a seawater hydroelectric plant to generate electricity as needed.
Wave Generation[14]
Storage Technologies
Now that the renewable generation technologies have been introduced, we will look at some storage technologies and how they function. In particular, the requirements, benefits and scalability of the storage solution will be looked at and also a brief mention of which renewable generation technologies they are best suited to.
Pumped Hydro-Electro Storage (PHS)
PHS is a storage system which utilizes water as a store and source of energy. It operates in much the same was a hydro-electric dam. The main features include two reservoirs, upper and lower, and a generation / pumping station between the two. It has two basic modes of operation, pumping and storing. When the PHS is operating as a pump, it pumps the water from the lower reservoir to the upper reservoir. In generation mode it allows the water to flow from the upper reservoir to the lower reservoir through a power generation plant to create electricity. In electricity generation, it serves two general purposes, it is used as a store when more electricity than is needed is being produced and can be used as a source when demand exceeds supply.
Pumped Hydro Storage Facility[7]
Requirements
The image above shows the typical layout of a Pumped storage plant. Pumped hydro storage systems have large requirements in terms of land and water resources. They are generally used for large scale applications to supply large amounts of electricity over an extended period of time. The installations themselves require large amounts of capital investment to build with some estimates in the region of €1100 per kW to €1500 per kW for a typical installation[17] and require a number of years to construct. Another requirement is the grid infrastructure as they need the transmission capacity available to accept and store the energy by also to generate and transmit.
Benefits
PHS plants offer many benefits from their installation and operation. The can provide a fast reaction to increases and decreases in demand when operating as either a generator or a store. They also provide quite useful mechanisms in system stability and frequency regulation of the transmission grid. They can be used with almost any kind of electricity generator and is ideally suited to renewable generation such as wind, solar and ocean technology.
Scalability
PHS systems are generally quite large requiring large amounts of land to store the water in both the upper and lower reservoir. The storage capacity of the facility in terms of kWh is directly related to the amount of water that can be stored in the upper reservoir and the hydraulic head which is the difference in height between the upper and lower reservoir. Larger storage capacity means that either the height of hydraulic head or volume of water storage must be increased. There is somewhat a trade-off between these two values however as a smaller reservoir can be used if the corresponding height adjustments are made between the upper and lower reservoirs. The output power of the facility is determined from the hydraulic head and the flow rate of water through the system. Larger flow-rates typically mean a larger generating facility and larger water pipes through which the water can flow. The output power determines how quickly the storage facility can generate electricity and deliver it to the grid. This means that the major factor affecting both the deliverable power and over storage capacity is the hydraulic head of the PHS system.
Suitable Generation Technologies.
PHS is quite suitable for most renewable technologies which produce electricity. Their primary function is to utilize electricity to pump the water from the lower store to the upper store. The requirements for water however make them more suitable for Wind Generation and Ocean Generation rather than solar as solar requires large amounts of sunlight which tends to occur in places where there is less water such. Wind generation is ideally suited as PHS systems can be built into mountains where a natural hydraulic head already exists between the upper and lower reservoirs. Mountains tend to provide suitable locations for wind energy generation as greater wind speeds exist at higher locations. Ocean generation is suited due to PHS due to the availability of water in the ocean. It does however involve the use of salt water within the hydro facility increasing the cost of the components to make them resistant to salt water corrosion.
Compressed Air Energy Storage (CAES)
The CAES system uses air as a storage medium and as the name implies, compresses the air into a storage container using electricity when storing energy. When electricity is needed, the compressed air is released through turbines to generate electricity. Typically the air is compressed into an underground cavern. CAES systems tend to be used in conjunction with conventional natural gas powered turbine generators to create electricity. In a natural gas power plant, the gas is combined with heated compressed air before it is burned to generate electricity. This requires the air to be pre-compressed before it is combined with the gas which is typically done using electricity generated from the plant. This process can be quite taxing on electricity generated from the plant and reduces the overall efficiency. The use of the CAES facility allows the air to be compressed and stored using electricity at off-peak times when it is much cheaper. The compressed air from the store can then be used instead of using a pre-compressor when the station is required to generate electricity thus allowing a electricity to be generated more efficiently from the plant.
Compressed Air Energy Storage[8]
Requirements
The compressed air storage system has little requirements in comparison to PHS. The solution is highly scalable allowing for large and small installations to suit requirements and can be paired with most types of electricity generation. The land requirements are not as large as those of PHS.
Benefits
The storage system uses air as a storage medium which freely available and has no direct environmental impact on the operation of the storage system or the surrounding environment. Storage facilities for the air can include salt domes as shown in the image above or man-made structures however there is an increase capital cost associated with the latter. Installations are typically cheaper than PHS solutions with installations ranging around the €350 per kW mark[17]. CAES is the only technology with a facility to provide energy storage on the same scale as PHS however CAES has a distinct advantage over PHS in that it uses air and not water. This means that while PHS is restricted to areas where there is an abundant supply of water, the CAES system can be installed in almost any location.
Scalability
As previously mentioned, the CAES system is scalable to suit requirements. Larger installations can be developed for purposes of electricity generation in gas turbine generators where baseload power of renewable generators can be used to pre-compress the air for storage. The system can also be scaled down for use as an uninterruptable power supply (UPS) in the likes of a building. This system is known as a thermal and compressed air storage system (TACAS). The air store is a number of compressed air cylinders connected to a thermal storage and turbine generator capable of providing a supply of back up power in case of an outage. This could be used in conjunction with a small scale wind turbine or solar generator to charge the storage system when not in use.
Suitable Generation Technologies.
CAES is suited to almost any type of renewable generation. As CAES is more flexible in terms of location, it can be placed almost anywhere meaning that it can take advantage of solar, wind, and even ocean power however, it would probably be more practical to use hydro in the case of ocean technology.
Batteries
Batteries are the most well known and popular form of energy storage. Almost any device that uses electricity uses a battery or could be configured to use a battery. Computers, televisions, radios, mobile devices and vehicles are to name but a few of the devices batteries are used in. There are two main reasons for the popularity of batteries. These are energy density and the ability to instantaneously deliver power. Energy is stored in a battery in the form of chemical energy, this chemical energy is the result of the materials used in the production of batteries. Batteries are composed of two chemical substances called electrolytes. There is both a negatively and positively charged electrolyte which cause the accumulation of positive and negative charges at the positive and negative ends of a battery cell which are known as the anode and cathode. Different numbers of stacks of these cells are used to create batteries with different voltages. The image below shows the basic structure of a battery.
Basic Battery Structure[18]
Types
There are two general types of batteries available in the market, primary batteries and secondary batteries. Primary batteries are disposable batteries from which users get a single use before the battery is disposed of. The chemical reaction that occurs in the production of electricity from the battery is usually not a reversible reaction meaning that once all the chemicals within the battery have been used up, the battery is not capable of producing any more electricity.
Secondary batteries are re-chargeable batteries which can be used over and over again many times once they are recharged after the become depleted. The chemicals used in the batteries react in such a way as to allow chemical process to be reversed by applying electric current to the battery. These types of batteries are generally more costly than primary batteries but offer more capabilities in terms of their uses.
Redox batteries (flow batteries) are rechargeable batteries which generate electricity by passing and energized electroactive liquid through the electrolytes to produce positive and negative charge. The batteries feature a storage tank where the electroactive substance is stored and a cell stack where the electricity is produced. The energy in a flow battery is contained within the electroactive liquid which is passed through the cell stack at a constant rate to produce a constant power. Once substance has been passed through the cell stack it is returned to a storage container where it can be re-energized. This allows the energy storage capacity of the battery and generating capacity (capacity factor) of the battery to separated from each other. This is advantageous because it allows a constant power output from the battery in comparison to standard batteries where the power output can fall as the energy contained in the battery is used up. It also means that the battery can be recharged as it is discharging as once the electroactive liquid passes through the cell stack it can be instantly sent to be re-energized. This may seem counter intuitive as the battery is both charging and discharging at the same time but operating in this way allows for frequency control of the generated and absorbed electricity.
Some types of flow batteries are: Zinc Bromine, Cerium Zinc, Vanadium redox
Scalability
Battery storage is quite scalable in small to medium sized applications. Battery banks can be built to suit almost any size application except those targeted by large scale storage such as PHS and CAES where the cost of the storage medium is of much higher importance in the overall cost of the system. PHS uses water and CAES uses air which are both freely available where as batteries use specific chemical substances which must be produced and refined at a cost. There is also the matter of safety. Battery banks on a scale to compete with PHS and CAES would involve putting large amounts of highly toxic chemicals into one location and would be subject to far higher standards of safety and security. Outside of the large scale storage market, batteries feature quite heavily in storage solutions. Most notably on medium scale applications where they are used as backup supplies for data centers and buildings when power outages occur.
Suitable Generation Technologies.
Battery storage is similar to CAES as it is not location dependent. It's scalability however makes it less suited to large scale applications such which can be targeted using PHS or CAES. It is most suited for solar and wind technologies where it can be used to smooth out the variability in these types of generation.
Thermal Storage
Thermal storage as the name implies is the storage of heat energy. This is generally achieved using water contained in tanks of varying sizes. Generally the water is heated by some process be it by an immersion heater in a house or by a solar tower in a solar thermal generating plant and moved to an insulated storage tank. The water can then be pumped from the insulated storage tank to the desired location for the desired purpose.
Solar Array combined with thermal storage[9]
Requirements
There are few requirements for a Thermal storage solution. Some kind of water pumping infrastructure is required to move the water to and from the charging area where the water is heated and then to and from the discharging area where the heat is used. The size of the storage container dictates the size requirements of the pumping infrastructure required and how much energy can be stored within the container.
Benefits
A major benefit of thermal storage is the the avoidance of a lossy energy conversion if heating is the desired end product. For example, in a house, a small scale solar thermal collector can be used with a thermal storage system to create hot water or heating for the house without the need to burn fuels or use electricity to heat the home. This can also be scaled up to provide hot water for a number of homes if a large scale solar collector is used with a larger thermal store.
Scalability
Thermal storage is quite scalable in its applications however, its generating capability is not on par with large scale PHS as heat energy is very prone to losses. On a small to medium scale however it is quite useful an a practical form of storage where heat is needed as the end product. It can also be combined with a turbine based generator to generate electricity on a larger scale.
Suitable Generation Technologies.
Thermal storage is ideally suited to heat based energy generation such as solar thermal. As the output of solar thermal plant is heat it can be stored directly without the need for inefficient conversions. It can be applied to wind, solar photovoltaic and ocean generation however, these technologies are better suited other forms of storage as they are used to generate electricity which would have to be converted to heat. Energy conversion to heat is typically more efficient than the conversion of heat to another form of energy, thermal storage is better suited to heat based applications.
Combining Renewable Generation and Storage
In the previous sections, a number of forms of renewable generation and storage technologies have been introduced. Each renewable technology has been examined in terms of its requirements, advantages and disadvantages and has been looked at in terms of its compatibility with various storage solutions. In this section, we will examine how these technologies can be combined to benefit Ireland's energy generation capabilities.
Ireland's Renewable Generation
Renewable generation capacity 2011
As of the 1st of November 2011, fully connected and operational energy generation from renewable resources totaled approximately 1830 MW of electricity generating capacity. This is predominantly comprised of wind generation which totaled 1592.8 MW and Hydro electric power which totaled 237.4 MW. Small contributions from solar and other sources comprised the rest[20].
Storage capacity
Total storage capacity for the purposes of electricity generation totaled 292 MW which is contained within the Turlough Hill pumped hydro storage facility located in Co Wicklow Ireland. This facility still remains for the moment the only large scale energy storage facility in use in Ireland currently[20].
Road map of Renewable Generation
The 2009 Renewable Energy Directive has given Ireland a target of 16% of all energy consumed within Ireland to come from renewable resources by 2020[1]. This has been divided into three areas, Electricity, Transport and Heating / Cooling. The target for electricity consumption has been set at 40% while transport and heating are 10% and 12% respectively[1]. As of 2009, Ireland had reached 14.4% of overall energy consumption from renewable energy and 10% of overall energy consumption was generated from Wind Energy[2]. It is intended that the Ireland will continue to increase its dependance on wind energy and proposed the inclusion of ocean energy as another source of energy generation in the future. The Irish government has set a target of 33% of energy consumption being generated from renewable sources by 2020[3].
Ireland is uniquely positioned within Europe to take advantage of both wind and ocean energy due to the Geographical location. The image below shows the average winds speeds across Europe[4]. Ireland has average wind speeds of 4.5 - 6 m/s with the west cost of the country experiencing winds upwards of 6m/s on average. As can be seen from the graph, the west coast of Ireland receives the the highest average wind speeds in Europe making it an ideal location for wind generation technology.
Wind speeds across Europe[4]
Ocean Energy in Ireland is also very abundant in Ireland with the country being situated on the edge of the Atlantic ocean. Ocean technology is still under development but could possibly supply a large percentage of Ireland's energy needs if enough of it could be harnessed. In a publication released by the Sustainable Energy Authority of Ireland, projections of between 20TWh per year and 120TWh per year by 2050 were given taking into account both baseline and ambitious growth in the sector[19]. Wave technology is currently being tested in Galway Bay in Ireland.
Additional storage solutions for Ireland
Wind and Ocean energy generation seem to be the planned way forward for Ireland's renewable energy generation. Here the benefits of adding storage to these solutions will be estimated.
Benefits of adding storage.
The fundamental problem with both wind generation and ocean generation is a mismatch in supply and demand. While tidal energy is predictable to a certain degree, the tides do not match up to the demand profiles for electricity over the course of a day and so storage will be key for the future of this technology. Wind energy which currently supplies almost 15% of the energy consumed in Ireland is currently absorbed by the grid as it is generated. Very few if any wind farms in Ireland use storage solutions to store the energy generated by wind farms. The electricity for the most part is conditioned for the grid and delivered as it is generated. While this is fine at reasonable low levels of energy generation, if Ireland is to increase its dependence on renewable energy sources to between 30% - 40%, then the variability of wind will play a larger factor it the stability of energy supply. There are two minds of thought on wind generation, one is that large scale storage should be used and the other is that backup power units such as gas power plants should be put in place to generate when the wind is not available.
Both are reliable technologies as Pumped-hydro storage has been in use in Ireland since 1974 and conventional power stations have been around much longer. However, to comply with Europe's requirements in the 2009 directive, Ireland should focus on large scale storage as the solution rather than backup conventional generators as we are required to reduce our CO2 and other greenhouse gas emissions also. On top of reducing greenhouse gas emissions and improving the dependence of wind energy. Storage can also add value to the wind market. Currently, any energy generated by a wind turbine outside of demand requirements is curtailed or wasted. Any energy that is generated inside demand is used as it is generated. If the wind turbine operator is lucky, the wind generated will fall within the hours of peak demand where they can get the highest price for their electricity. The window of opportunity to sell electricity at the price high is low, maybe 1-2 hours before the price drops back to its original price.
Data taken from the All Island Market website on the 13th of December 2011 showed a daily low price of approximately €27 and a daily high price of €262 per MWh of electricity. The daily high price was only available for a period of approximately two hours around 6pm in the evening.
Below is a simple calculation of how energy storage can add value to the wind generation market. Using approximated values for the prices above, say €25 and €250 and time periods of 22 and 2 for the availability of these prices. So the price per MWh is €25 for 22 hours and €250 for 2 hours of the day. The example uses a 1MW wind farm.
Initial calculations.
1MW wind farm will generate 1 * 24 * 365 = 8760MWh of energy in one year. At the low price this is €219000 worth of electricity. At the high price: €2.19 million. Factoring in both prices: [(22*25) + (2*250)] * 365 = €383250
Factoring in a capacity factor of 30% which is typical of wind generation in Ireland.
1MW wind farm will generate 1 * 24 * 365 * 0.3 = 2628MWh of energy in one year. Low price only : €65700, High price only: €657000. Combination of prices: €115000.
A wind farm operating without storage must either export the generated electricity to the transmission grid or curtail the generated electricity. In fact, in reality, if there is no transmission capacity available, the wind turbine may be shut off as it is of no use to generate electricity that cannot be used. Assuming that there is more than enough transmission capacity, these figures are reasonably accurate however, a major problem in Ireland is that the transmission grid does not always have the available capacity to transmit the generated electricity. In the west of Ireland, the transmission capacity is lower as the grid was developed to transmit electricity to population centers rather than from isolated wind farms.
As a result, a curtailment factor should be considered in the above calculations. For example, at peak hours, the transmission capacity will be much lower than at hours of lowest demand meaning the hours when the highest price for electricity is available, there is restricted amounts of transmission capacity available. Below is the typical demand profile for electricity in Ireland. For the sake of example, we will assume that below 3000MW there is enough capacity available to transmit all the electricity as it is generated from the wind farm. Between 3000MW and 4000MW there is only 50% transmission capability I.E. 50% of the time it is possible to transmit the electricity generated and 50% of the time it is not. Above 4000MW we will assume there is only 20% transmission capability. These numbers are completely fictional and do not represent actual data.
If we assume that this demand profile is the same for the entire year then the money generated from the wind farm can be calculated as follows:
Time below 3000MW = 00:00 - 07:00 = 7 hours
Time above 4000MW ~ 12:00 - 20:00 = 8 hours
Time between 3000MW and 4000MW = 9 hours
Daily high price occurs between 17:00 - 19:00 = 2 hours
Daily low price occurs at all other times
Below 3000MW
1 * 7 * 25 * 365 * 0.3 = €19100
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Between 3000MW and 4000MW
1 * 9 * 25 * 365 * 0.3 * 0.5 = €12300
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Above 4000MW
Daily high rate: (1 * 2 * 250 * 365 * 0.3 * 0.2) = €11000
Daily low rate: (1 * 6 * 25 * 365 * 0.3 * 0.2) = €3300
Total Above 4000MW = €14300
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Overall Total = €45700
Now consider adding storage to the wind farm. Assuming we are using a PHS facility that is capable of storing all the energy generated by the wind farm this allows electricity to be transmitted when it is most economically viable to do so. The electricity stored in the PHS facility does not come freely however, the efficiency of a typical PHS plant is generally in the region on 75% - 85%. 80% is a generally accepted value for operating efficiency.
On daily operation, we will assume that the PHS is capable of storing all the energy generated from the wind farm and delivering it back to the grid during the peak hours. We will also assume that the energy generated after peak hours will be stored until the next day's peak prices occur.
Assuming all this energy can be and is delivered to the grid at peak electricity price:
24 * 250 * 365 * 0.3 * 0.2 = €131400
This is unfeasible of course as this would constitute transmitting 2600MWh of energy to the transmission grid in a 2 hour period. A more realistic approach could be to transmit electricity off peak when guaranteed that there is transmission capacity available or when peak prices are available. This would mean that the mid section (between 3000MW and 4000MW) of the calculation can be removed.
Off peak: 22 * 25 * 365 * 0.3 = €60200
Peak: 2 * 250 * 365 * 0.3 * 0.2 = €11000
Total €71200
While the number is significantly smaller, almost 50%, this return would be far more stable as transmission is guaranteed except for the peak hour transmission. There is almost a €25000 increase on the amount of return when compared to a turbine operating without storage. Disregarding pricing, this would be a far more optimal mode of operation as it would mean less energy is wasted.
Cost of Storage
Storage facilities do not come cheaply though with typical PHS facilities running into the hundreds of millions or even billions. PHS is however ideally suited to Ireland as was highlighted by the "Spirit of Ireland" group. They estimated that around 50 glaciated valleys in west of Ireland could conceivably be converted into PHS facilities at a reduced cost due to the utilization of natural formations and the ocean as the lower reservoir. They approximated that these facilities could be produced at a cost of €800 million per facility and 2-3 installations could provide enough storage capacity for enough wind turbines to provide 80% of Ireland electricity generation needs.
800 million is quite a large sum of money considering that the above calculations will give between €72000 and €130000 per year. Factoring in operational costs and maintenance, these numbers will quickly diminish. The idea proposed by the Spirit of Ireland group was to have more than one wind farm connected to each storage installation however. They proposed approximately 50. This reduces the cost of the installation to approximately 16 million per wind farm.
Environmental costs
The environmental costs of the storage systems must be considered when choosing renewable technology and storage systems. Large scale pumped hydro systems need vast amounts of water to function. The proposed sea-water based systems eliminates effects on fresh water supplies in Ireland however, uses of seawater increase equipment costs compared to those for fresh water as sea-water is more corrosive that freshwater. The impact of pumping large amounts of seawater into the proposed valleys on the surrounding wildlife is not known either as only one such system exists in the world in Japan. Wildlife inhabitants in Ireland are different to those in Japan so only general comparisons can be drawn without a full investigation.
The proposed building of these storage systems would however providing many employment opportunities with Electronic, Mechanical and Civil engineers required to build the devices and numbers of engineers required to run the facility once constructed. This would also create more jobs by creating a knock on effect. Construction of local housing would be required and also local shops and other business would be created as a direct result of the construction of the storage facilities.
Upgrades to the Irish Grid
Building these storage systems however will require substantial upgrades to the Irish electricity grid if they are to go ahead. The storage facilities will require large 400kV transmission lines to transmit the stored energy to the grid as it is required. Currently the distribution of 400kV lines in Ireland are shown in red in the below image. The black lines represent 110kV lines and 220kV lines are represented in green.
Irish transmission grid[21]
400kV lines: Red
220kV lines: Green
110kV lines: Black
As can be seen, there are two main 400kV lines connecting Dublin to the power station in Money-point and else where 220kV lines and 110kV lines have been used to connect the rest of the country. This has functioned for Ireland up until now as transmission has been based around where the energy has been generated and where it is in demand. The proposed upgrade to the grid by the Spirit of Ireland is to provide a 400kV backbone network along the west cost of Ireland into which the wind and ocean based technologies can connect. Incorporating storage into the mix would almost certainly require 400kV lines to be provided to the facilities with lesser grid connections supplying individual wind farms with a connection to the storage facilities.
Conclusions
Large scale storage is required for Ireland if it is to increase its dependence on renewable resources such as wind and ocean energy technology. These storage facilities will most likely be in the form of large scale PHS systems which are most suited to Ireland's topography especially along the west coast. The availability of these storage facilities will increase the dependency of Ireland's wind and Ocean energy thus increasing the value to the Irish economy of these generators. It will also remove the necessity to provide backup generation in the form of fossil fuel burning generators and reduce Ireland's overall reliance on Imported energy. This reduction will help secure Ireland's supply of energy and make the countries economy less susceptible to variation in fuel prices on the global market.
A number of challenges still need to be addressed however, the Irish transmission grid needs to be re-wired to provide the transmission capacity for these renewable resources to become economically viable and the locations, size and cost of the storage facilities needs to be agreed upon.
Appendix
References
[1] http://www.seai.ie/Publications/Statistics_Publications/Statistics_FAQ/Energy_Targets_FAQ/#What_are_Irelands
[2] http://www.seai.ie/Publications/Statistics_Publications/SEI_Renewable_Energy_2010_Update/RE_in_Ire_2010update.pdf
[3] http://www.dcenr.gov.ie/NR/rdonlyres/54C78A1E-4E96-4E28-A77A-3226220DF2FC/30374/EnergyWhitePaper12March2007.pdf (Section 3.10.10)
[4] http://www.windatlas.dk/Europe/landmap.html
[5] http://www.eirgrid.com/operations/systemperformancedata/systemdemand/
[6] http://www.seai.ie/Publications/Statistics_Publications/EPSSU_Publications/Energy_in_Ireland_Key_Statistics.pdf
[7] http://bravenewclimate.com/2010/04/05/pumped-hydro-system-cost/
[8] http://www.scotland.gov.uk/Publications/2010/10/28091356/4
[9] http://www.volker-quaschning.de/articles/fundamentals2/index.php
[10] http://www.tcd.ie/Communications/news/news.php?headerID=1617&vs_date=2010-11-1
[11] http://www.solarthermalmagazine.com/2010/07/04/washington-listens-backs-largest-concentrated-solar-thermal-project-in-the-world/
[12] http://solarphotovoltaicsite.com/
[13] http://www.bigelow.org/virtual/handson/water_level.html
[14] http://www.oceanpowermagazine.net/2009/04/30/commercial-demonstration-wave-energy-project-announcedwashington-state/
[15] http://telosnet.com/wind/early.html
[16] David J.C. MacKay. Sustainable Energy – without the hot air. UIT Cambridge, 2008. ISBN 978-0-9544529-3-3. Available free online from www.withouthotair.com.
[17] Richard Baxter (2006). Energy Storage, A Nontechnical Guide. United States of America: PennWell Corporation. Page: 60-124.
[18] http://www.pctechguide.com/mobile-computing-components/battery-technology-for-mobile-computers-laptops-notebooks-and-webbooks
[19] http://www.seai.ie/Renewables/Ocean_Energy/Ocean_Energy_Information_Research/Ocean_Energy_Publications/Ocean_Energy_Roadmap_to_2050.pdf
[20] http://www.eirgrid.com/media/Summary%20%28All%20Generators%29%20-%2030%20Sep%202011.pdf
Bibliography
Websites:
Irish Policies on renewable energy: Sustainable Energy Ireland: http://www.seai.ie/
Irish transmission grid & renewable generation data: Eirgrid: http://www.eirgrid.com/
Irish electricity pricing information: Single Electricity Market Operator: http://www.sem-o.com/Pages/default.aspx
General Information: http://www.wikipedia.org/
Books:
Godfrey Boyle (2004). Renewable Energy. Open University: Oxford University Press.
Freris, L. L; Infield, D. G (2008). Renewable Energy in power systems. Chichester U.K.: John Wiley and Sons.
Richard Baxter (2006). Energy Storage, A Nontechnical Guide. United States of America: PennWell Corporation.
David J.C. MacKay. Sustainable Energy – without the hot air. UIT Cambridge, 2008. ISBN 978-0-9544529-3-3. Available free online from www.withouthotair.com.
Douthwaite, R. J (2003). Before the wells run dry: Ireland's transition to renewable energy. Dublin: FEASTA in association with Green Books [and] the Lilliput Press.