Laurence Fyfe

Necessity for Storage Systems in Renewable Energy

Abstract

This report intends to outline the need for storage systems in renewable energy systems. The impact fossil fuels are having on the environment and different storage technologies will also be discussed.

Introduction

As we come to rely more and more on renewable sources of energy due to the diminish of fossil fuels and also due to the effect the burning of fossil fuels is having on out atmosphere we have a realization that a suitable energy storage system will be essential to integrate renewable sources of energy into the national grid. In particular here in Ireland where where the sun isn't always shining and the wind isn't always blowing we need to come with a solution to store energy that is produced and not needed immediately.

In this report different storage technologies will be compared and contrasted. There is a lot of important factors which must be taken into account when deciding on which storage technology to implement such as cost to implement a solution and the efficiency of the chosen solution.

Impact of Fossil Fuels on the Environment

The impact that the generation of fossil fuels is having on the environment is becoming more and more evident each year as our weather systems become increasingly erratic and unpredictable. Currently approximately 90% of the worlds electricity is produced by fossil fuels and this is a statistic that will rapidly change over the coming years. To sustain the earth we habitat for future generations suitable alternative "green" methods of producing energy must be incorporated into our electricity supply. There is continual debates and meetings of heads of state on the importance to reduce our reliance on fossil fuels. In 2009 Ireland, using wind turbines, produced 6.5 times the electricity needed to power the country for a year. This proves that we can implement renewable energy systems which can meet our energy requirements.

When producing electricity using fossil fuels it is the combustion of these fossil fuels that is the largest contributor to the release of greenhouse gases into the atmosphere. With the release of these greenhouse gases the effects include air and water pollution and general degradation of our planet which leads to erratic weather conditions and increased human illness. The effect of global warming due to the amount of carbon dioxide being released into the atmosphere is evident with ice caps melting in Antarctica. This is resulting in increases in the sea level and could possibly lead to the flooding of coastal areas, this could particularly be dangerous for an island nation such as Ireland. Smog is another side effect from the use of fossil fuels and this can cause sickness and can effect the production of crops on farms.

When coal is burned to produce electricity or burned for the purpose of heating a harmful gas called nitrous oxide is released. About 50% of the nitrous oxide in our atmosphere is naturally produced but the other 50% is a result of the amount of coal we are burning and this has to be reduce if we want to reduce the impact we are having on our atmosphere.

Our current reliance on fossil fuels is too high and we are having an extremely harmful effect on our atmosphere. Our demand for electricity is extremely high and this demand is expected to double in the next 20 years. As such we must find alternative methods of producing electricity which are not harmful to our atmosphere but at the same time these methods of energy production must be able to meet the demand.

Greenhouse Effect

Intermittent Renewable Energy Production

Renewable energy production can be intermittent due to various factors which can be outside of our control. A reliable storage system is one of the important issues in producing quality electricity networks which are reliable on renewable energy. At times of excess energy production or when demand is low the excess energy we generate needs to be stored for when demand is high or for emergency situations. For example here in Ireland we currently have approximately 150 wind farms and if these wind farms are producing more electricity than is needed for the grid it can be stored for future use. It can be seen in the graph below the faster the wind speed the higher the amount of watts which are output to the grid. If demand was on average 30,000 Watts anything above this production level could be stored and used at times when production output falls below 30,000 Watts.

Wind Energy Production

Comparing Storage Systems

When comparing and contrasting different storage solutions there are various factors which must be taken into consideration such as the cost of implementation, the efficiency of the system and also the amount of space and resources required if the system will occupy a large area.

Cost

The chart below compares the cost of implementing various storage technologies. It should noted that it is not always the best option to choose the cheapest technology. Although the initial cost may be low other factors such as maintenance and the life cycle of the technology should be taken into account.

Cost

Efficiency

The efficiency of a storage technology is an important factor to take into account when deciding on which storage technology is to be utilized. The chart below compares various storage technologies. The amount of cycles with an 80% depth of discharge is compared with the efficiency of the technology. If a low efficiency storage technology with a short life time is chosen the overall cost of the project will be increased due to the low utilization of the stored energy and the increase in operation and maintenance costs.

Efficiency

Size and Weight

The size and weight of a storage technology is anther important parameter which must be taken into consideration when choosing which technology to implement. The chart below compares the weight and size of various storage technologies

Size and Weight

Energy Storage Systems

There are generally 2 different applications for the use of energy storage in renewable systems. Energy Management requires technologies with a high level of storage and are used to balance the difference between the current production of energy and the current demand of energy. Power Management is used to ensure the continued supply of power if generation of the power has stopped.

The table below compares various storage technologies and outlines which applications they are best suited for.

Application of Storage Technologies

Mechanical Storage

Mechanical Energy Storage is mainly based on kinetic and potential energy. Kinetic energy is an energy of motion. The equation that defines Kinetic energy is KE = 1/2*m*v² where m stands for mass and v stands for velocity. As can be seen from this formula kinetic energy is increased with the velocity squared.

Potential Energy is energy which is stored, the energy is stored within a gravitational field. The formula for potential energy is PE = mgh where m stands for mass in kilograms, g stands for the gravitational acceleration of the earth, and h stands for the height above the earths surface.

Pumped Storage Hydroelectricity

The pumped hydro storage plant is the oldest method of electricity storage. The logic is to have two separate water sources, one at a greater height than the other. A vertical tunnel connects the two water sources together. During off-peak hours when the demand for electricity is normally low, water from the lower source is pumped up to the higher source. During peak hours or whenever electricity is needed the pump is switched off and water flows from the higher source to the lower source to generate the required electricity.

The electricity is produced by generators which are activated when the water is flowing from the upper reservoir to the lower one. The principal operation of such a generator is the rotation of magnets inside coils of wire which moves electrons and produces electricity. There is normally a number of generators like these installed in such a system.

There is only one Pumped Storage Hydroelectricity plant currently in Ireland and this is located on Turlough Hill in County Wicklow. This power station is owned and operated by the Electricity Supply Board and can generate up to 292 megawatts of electricity. An artificial reservoir was created near the peak of Turlough Hill which is 681 metres high. This is connected to Lough Nahangan at the base of the mountan via a pressure tunnel which has 4 turbines installed in it for the generation of electricity when water is flowing from the upper reservoir to the lake below. During times of peak electricity generation water is pumped up the mountain to the upper reservoir where it is stored and during times of peak demand this plant can be activated to supply electricity to the grid.

Pumped Storage Hydroelectricity

Flywheel

A flywheel is an electromechanical device which is used to store energy for short duration's. A flywheel is charged and discharge via a motor, this motor is powered by the renewable energy source to spin the rotor of the flywheel. When electricity is required from the flywheel, the kinetic energy stored in the rotor is transformed to DC electric energy by the generator.

Flywheels can be used to provide continuity in the generation of electricity from a renewable source.

Flywheel

Compressed Air Energy Storage (CAES)

In a Compressed Air Energy Storage System air is stored during periods of low energy consumption. The CAES can store energy generated from renewable sources which can be used in times of high demand. In periods when power is needed from this system the pressurized air is mixed with natural gas which is burned in a turbine and this generates electricity which can be supplied to the grid. is released through a turbine to produce electricity. 1m³ of space in the cavern can store up to 5 kWh of energy and the minimum pressure is approximately 1200 psi. There is currently research being undertaken to eliminate the need for the use of natural gas in this sort of system. This involves the storing of the heat of compression and reusing this to heat the compressed air before expansion.

Compressed Air Energy Storage

Thermal Storage

Solar Thermal Power Plant

Solar thermal electric power plants are power plants which generate heat by using lenses and relectors to concentrate the suns energy. The suns heat energy is then focused on a pipe which usually contains a liquid such as oil. This fluid is then circulated through the pipe and is used to drive a conventional generator to produce electricity.

With thermal storage the power plant will still be able to produce electricity even if there is no solar energy available. The main method used in this technique is to use two tanks and a storage medium such as molten salt. The extra energy which is collected from the sun during a time of peak solar production is used to heat up the molten salt. The molten salt is then pumped from one tank which is cold to the other tank which is hot. If there is not enough solar energy to drive the generator the molten salt can be pumped back from the hot tank to the cold tank and this will heat up the oil and electricity will continue to be produced.

Solar Thermal Power Plant with Thermal Storage

Electrical Storage

Batteries

Batteries are the original and still the most popular method of storing energy. All batteries contain cells and a cell is made up of three main parts, a positive electrode, a negative electrode and an electrolyte between these two electrodes. An electrolyte can be in liquid or solid form. When a battery is placed in an electric circuit, a chemical reaction takes place in the electrolyte which causes ions to flow through the battery one way. Electrons flow through the outer circuit of the battery and this flow of ions and electrons generates an electric current and provides us with power.

The Amp-hour (Ah) is the term which quantifies the amount of energy that is stored in a battery.

Battery

Lead Acid

Lead Acid Batteries can be used to store energy which is produced from renewable sources. This energy can then be used at time of demand. The deep-cycle lead acid battery is the main battery used for renewable energy applications but there is also another type of lead-acid battery called a starter battery. Multiple deep-cycle batteries can be connected together to form a battery bank for efficient storage of energy generated from renewable sources.

A lead-acid battery uses a reversible chemical reaction to store energy. Lead plates or grids and an electrolyte which can consist of sulfuric acid is used to create electrical energy from the stored energy.

Lead Acid Battery

Lithium Ion

A lithium ion battery is a rechargeable battery. To produce electricity from the stored energy lithium ions move from the negative electrode to the positive electrode, to charge a lithium ion battery back up the reverse takes place and lithium ions move from the positive electrode back to the negative one. Lithium Ion batteries are currently very popular in consumer electronics such as mobile phones and mp3 players.

Lithium Ion Battery

Supercapacitors

A capacitor is made up of two metal plates which are seperated by a distance, in the distance between the plates lies a dielectric material and this can be used for storing electrical charge. Capacitance is the amount of charge which is stored between the two metal plates with a potential difference existing across the plates. The capacitance C is defined as: C = Q (Amount of Charge) / V (Potential Difference) and is measured in farads.

Supercapacitor

Conclusion

It is clear that the need for renewable sources of energy is of extreme importance and one of the main things which needs to be considered when implementing a renewable system is the necessity for an efficient storage technology. There was various storage techniques outlined in this report and which solution is implemented will depend on the scale of the renewable project which is being undertaken. In particular here in Ireland we have the capability to provide for electricity needs using wind power but this can only be implemented properly if a storage technology is implemented alongside the construction of wind farms. This would be beneficial not just to the environment but also to the economy as our reliance on importing fossil fuels would be greatly reduced.

A solution such as the pumped storage hydroelectricity could work well due to our mountainous terrain and utilizing existing lakes such as the case with the existing system in County Wicklow would bring down the overall cost of such a solution. The location of the majority if our wind farms are located in mountainous areas so it is logical to implement this type of storage solution close by to existing wind farms.

Bibliography

"Ultracapacitors for Renewable Energy Storage" - Mark Daugherty

"ENERGY STORAGE TECHNOLOGIES" - Alfred J. Cavallo

"Sustainable Energy Ireland" - http://www.seai.ie/

"Wikipedia" - http://www.wikipedia.org/

"Environment 360" - http://e360.yale.edu

"Green Tech Media" - http://www.greentechmedia.com

"David Darling" - http://www.daviddarling.info

"Electricity Storage Association" - http://www.electricitystorage.org