The Role Of Energy Storage

Published Date: 02 Nov 2017

INTRODUCTION

Climate change is a globally recognised problem with its root cause attributed to the presence of Greenhouse Gas (GHG) in the atmosphere, a result of increased anthropogenic activities (Grasso, 2010). In 2007, the intergovernmental panel on Climate Change (IPCC) released its fourth assessment report in which it established that based on the biological, chemical and physical data collated, the global warming of the earth was indisputable (IPCC, 2012). The IPCC, also made mention in its report that the most significant of all GHG which poses a threat to the environment is Carbon dioxide (CO2). This had increased from a pre-industrial level of 280ppm to over 380ppm during the last 650,000 years (IPCC, 2012).The concentration of CO2 in atmosphere is expected to increase as global economy continues to grow (ASME, 2009).

Typically, the associated human and environmental cost of addressing the impact of the increased concentration of CO2 on the environment is large (ASME, 2009). Also, the time scale for the intervention and resultant effect of climate change to be noticed is long (Edmunds et al., 2007). Hence, there is the urgent need for prudent and mitigating steps to be taken to tackle climate change and CO2 concentration increase. To address this, the United Kingdom (UK), in November 2008, passed the Climate Change Act, a legally binding commitment made by the country to reduce its carbon emission by at least 34% by 2020, and 80% by 2050 below its 1990 baseline (Climate Act, 2008). As a follow up to achieving this, the UK as part of a legally binding EU agreement committed itself to producing 15% of its energy from renewable sources by 2020 (Climate Act,2008). This commitment invariably means that 32-40% of the UK’s electricity will be generated from renewable sources by 2020 (reference).

The UK in following through with its 2007 commitment has made tremendous efforts in its quest to reduce CO2 emission as reflected in the UK Department of Energy and Climate Change (DECC) February 2013 statistical report on greenhouse gas emission. The key highlight of the report was a 7% decrease in GHG emission from the 594 metric tonnes carbon dioxide equivalent (MtCO2e) recorded in 2010 to 552.6 (MtCO2e) recorded in 2011 (DECC, 2013). This was followed by a 7.9% decrease in CO2 emission from 498 metric tonnes (mt) in 2010 to 459 mt in 2011 (DECC, 2013).

These figures show that while it is recognisable that the UK is making an effort to reduce emission much more will be required to achieve its 80% reduction target by 2050. This essay addresses the fundamental steps that the UK could take in achieving its target. Part one will discuss the various sources of atmospheric CO2 emission in the UK. It will pay particular attention to strategies that can be used to reduce CO2 emission from the transport and heating sector of the economy. Part two will discuss the variability of supply and demand for energy. It will pay special attention to the variability attributed to the use of renewable sources of energy such as wind and solar. Section three will analyse the extent to which energy storage can contribute to carbon reduction and the means by which this storage is provided. This will then be followed by a conclusion which will summarise all key facts and recommendations made in all the sections.

SOURCES OF CARBON DIOXIDE (CO2) IN THE UK

It is estimated that between now and 2050, the global economy will grow by a factor of four (OECD/IEA, 2008). It is also, projected that with this forecast in economic growth, there will be a 70% increase in the demand for oil invariably leading to a 130% increase in CO2 emissions by 2050 (OECD/IEA, 2008). Thus, the predominant source of CO2 emissions in the UK will be from the combustion of fossil based products used to produce energy, power vehicles and heat buildings and processes ( Hall and Bain, 2008)

1990

1995

2000

2005

2008

2009

2010

2011

2012 (p)

Energy Supply

241

210

203

216

213

190

195

182

192

Transportation

120

120

125

129

125

121

119

117

116

Business

113

107

107

97

90

79

76

76

79

Residential

79

81

87

84

80

75

87

66

74

Others

39

36

31

27

23

17

18

17

18

Total

592

554

553

534

531

481

498

459

479

Table 1: Sources of Carbon dioxide emissions, from 1990 to 2012 (Mt) (2012 figures are provisional estimates).

Source: Department of Energy and Climate Change (DECC) 2013.

Chart 1: Sources of CO2 emission in the UK

Source: Department of Energy and Climate Change (DECC) 2013.

CARBON EMISSION REDUCTION STRATEGIES

Fossil fuel generates two–thirds of the world’s electricity and in doing so, produces one–third of the world’s global CO2 emission (AME, 2009). This same scenario can be applied to the demand of heating and transport energy in the UK being met by the combustion of fossil fuel (RAEng, 2010). A look at the country’s 2008 energy flow chat shows that a large chunk of the energy supplied was from fossil fuel (198GW) with the transport and heating sector consuming 78GW and 81GW respectively.

Diagram 1:UK energy flow (2008)

Source: The Royal Academy of Engineering (RAEng) 2010

Bearing this in mind, it becomes exceeding difficult for the UK to ignore the fact that if it hopes to achieve its 2050 carbon emission reduction target, it will have to incorporate a combination of strategies that will shift its transport and heat dependent demand on fossil fuel to low carbon sources of energy.

To achieve a reduction in carbon emission from the transport and heat sectors, the following strategies are suggested;

TRANSPORT

As stated earlier, the UK’s 2008 energy flow diagram, shows that of the 198GW of energy generated by fossil fuel, 78GW was consumed by the transport sector (RAEng, 2010). This conclusively shows that the transport sector is greatly dependent on fossil fuel combustion. The reduction of CO2 emission from this sector is reliant on three key strategic objectives namely: displacing the consumption of petroleum based products such as fossil fuel by increasing the energy efficiency of vehicles and use the of low carbon alternatives; reducing the probable life cycle emissions that will be associated with the electrification of the sector, and finally, increasing the variety of energy resources that will power the sector (USDT, 2010).

Achieving the aforementioned objectives will require the UK to address a few technological challenges such as developing and deploying advance battery and storage technologies for the adoption of plug-in and electric vehicle technologies ( REFERENCE). secondly, strategies that will enable end users of energy use present and future biofuels technology in an environmentally sustainable and cost effect manner need to be developed (REFENCE). Finally, the country will have to develop technological systems that will allow the easy incorporation of low carbon fuels into multiple modes of transport (REFRENCE)

Meeting this technological challenges and at the same time reducing carbon emission through other means will involve.

Improving fuel efficiency with advancement in engine technologies: The development of alternative propulsion technologies (such as improved engine design and efficiency are important for reducing carbon emission. The essence of this development will be to reduce the amount of fuel consumed per unit thus resulting in a reduction in carbon emission (reference). This will involve the deployment of hybrid and electric vehicles .

Alternative energy sources: This technique of making use of alternative energy sources in place of fossil fuel is called fuel switching’ (refrence). It involves the use of fuels with low carbon content such as hydrogen, biofuels, electricity that can be gotten from renewable sources such as solar, wind, hydro and tidal waves (reference). This fuels will generate little or no carbon emissions compared to what would have been obtained from fossil combustion.

Avoiding Vehicle trips and shortening trip distance: with the advent and rapid development of low energy information and communication technology such as the internet and mobile phones, the physical and intensive movement of people can be avoided (Woodcock et al, 2007). In addition to avoiding trips, trip chaining (use of a single trip to replace multiple trips) and vehicle occupancy increase can be used as strategies to reduce the emission that would otherwise have been produced if these technological alternatives to transportation were not in place (reference). There is also the alternative of active transport which offers the greatest potential for reducing energy transport use which invariably results in carbon emission reduction.

Conclusion

From the forgoing, it is evident that the most viable option for the transport sector to reduce its emission will involve its electrification. However, this will only be made possible by behavioural changes in the way producers produce vehicles and in the way consumers use them before technological solutions will play a role in carbon emission reduction.

HEATING

Currently, 85% of UK homes are heated by natural gas, with the remaining 15 % being heated by electricity and the combustion of oils (RAE, 2012). From the forgoing, it can be seen that the combustion of natural gas used for water and space heating dominates the residential sector and as such, this is what needs to be addressed if the UK hopes to meet its 2050 emission reduction target. (Palmer et al, 2006). This recommendation is evidenced by the heating sectors consumption of 81GW of energy out of the 198GW of fossil fuel combusted in 2008 ( RAEng, 2010). To achieve emission reduction in this sector, the under listed suggestions are put forward;

Energy Efficiency Improvement: Incorporating zero carbon standards in the construction of new building will go a long way in reducing carbon emission from total housing stock by 10% in 2030 (Lillicrap and Ashford, 200). This will leave 90% carbon emission which can be taken care of by reducing the heat demand in buildings through a process of widespread insulation of solid walls (which will reduce demand for heat by 3% by 2030), energy efficient glazing and solid floor insulation(will reduce emission by 11 MtCO2e)( Joosen and Blok, 2001). In addition to this, retrofitting of old houses is also recommended as a strategy to reduce heat demand which ultimately results in the reduction of energy demand (reference)

Low and zero carbon emission heat technologies deployment: Space and water heating from gas accounts for 80% of energy consumed in the residential sector and 56% of energy consumed in the public and commercial sector (reference). The deployment of heat decarbonisation technologies such as bioenergy heat pumps, combined heat and power (CHP) will play an important role in helping the country achieve its 2050 carbon reduction target (reference). Electricity energy from heat pumps can generate 2.5-4.5 units of heat as against the conventional electric heating system such as immersion heating that produces less than one unit of heat (reference). Examples of heat pumps include; ground source heat pump, air source heat pump and heat pump with storage. The use of bioenergy such as biomass and biogas can also serve as alternative sources for low carbon heat generation for residential, industrial or non-residential purposes (references). Also, CHP as a low carbon heat producing technology works to increase the energy production efficiency of a system by making use of wasted heat energy from thermal combustion either directly from industrial process or through district heating system

Note: Natural gas CHP thermal efficiency is usually in excess of 80% while combined cycle gas turbine (CCGT) is 50% efficient; biomass CHP is 40-60% efficient (reference)

Conclusion

From the forgoing, it can be concluded that the way forward in reducing emission from the heating sector will require the electrification of the sector.

VARIABILITY OF ENERGY: Supply and Demand

The variability of energy can be defined as the degree to which an energy source can exhibit uncontrollable or undesirable changes in output (Graham, 2005). Also, in relation to energy demand, this represents the changing energy requirements by users depending on the seasonal weather conditions. The unpredictability of these two factors represents a major challenge to the low carbon future especially in the presence of a deficiency in energy storage systems.

Supply Side Variability

Solar energy: The amount of solar energy produced is largely dependent on day/night cycles, geographic locations, weather conditions and seasonal cycles (reference). While weather conditions and seasonal cycles are dependent on average solar content and the variation between minimum and maximum content after wards, geographic location and day/night cycles affect energy output on a short time scale (reference). In the UK, peak energy is usually produced at summer time where the sun irradiance lasts for between five to twenty one hours a day (reference). However, the level of energy production efficiency of a solar panel cell starts to drop from a level of 100% at 12 hours to 70% at 16.5 hours and finally to 20% at 18 hours (reference). The extent to which this variability becomes an issue is dependent on the extent to which the solar energy generated is unable to meet demand.

Wind Energy: The production of electrical energy from a wind turbine is dependent on the turbine characteristics, wind speed, and air density (reference). A wind turbine operates maximally at a wind speed of between 2.5 to 25 metre per second (m/s); at values below this range, the wind will not drive the turbine to generate electricity, and at above this range, the turbine automatically shuts down to prevent damage (reference). The production of electrical energy from wind turbines displays a seasonal, diurnally and hourly variability with more electrical energy being produced in the winter when the air is cooler and denser, than in the summer time when the air is hot and light (reference). Diurnal and hourly variation in electricity production is highly dependent on the daily variations in temperature (reference). The output from a single turbine varies greatly and rapidly as wind, speed varies from location to location, but as more turbines are created and connected over large areas, the power output becomes less variable.

Tidal Energy: Tidal energy is captured from either tidal streams of water or by the storage and release of water into barrages (reference). As the most predictable of all renewable energy sources, tidal waves lasting a little bit less than 12 hours 30 minutes a day is used to generate electricity on both ebb and flood tides or just on ebb tides alone(reference). The block of energy produced by tidal waves is intermittent, and the size and time is dependent on the lunar cycle (reference).

Hydroelectricity Power Generation: The capacity for a hydropower plant to generate electrical energy from water is dependent on the water cycle providing seasonal rain and runoff from snow packs (reference). Seasonal variation in rainfall determines the water content of the river, its flow and ultimately its ability to produce energy (reference). The variability of electricity supply using hydro-plants becomes an issue in periods of drought, especially when such periods correspond with periods of high demand for energy.

Demand Side Variability

The variability of demand for energy

Solar and wind energy are referred to as variable and sometimes intermittent energy sources because their electricity production varies base on the availability or non-availability of sunlight and wind respectively(IEA,....). However, in the energy grid system, they are not the only cause for concern when it comes to variability. The demand for electricity by end users also varies and the grid is designed to manage such situations when they occur (reference). It is easy to predict long-term changes in electricity demand over a duration of several hours. Example: There is a widely known daily schedule of early morning electricity demand pick-up and an evening electricity demand drop-off. However, this is not the case in short-term changes in demand which span from seconds to minutes. The system is however designed to manage such changes when they occur because they are usually small and attributed to unsystematic events that changes the demand for electricity to a different direct (reference)

The major difference between supply and demand variability is that it is usually easy to understand the reason for a change demand than it is to understand a change in the supply of energy. In some cases however, certain aspects of the variability of an energy supply might be understood and predicted such as; the electricity generation capacity of an individual wind turbine is highly variable while the collection of a group of turbines at a single site is pointedly less variable (reference). Also the collection of a large number of turbines over a large geographical site becomes less variable and this progresses as the number of turbines increases within a specific location (reference). However, while the variability of a large-scale turbine over a period of seconds to a minute is small and posses no great threat to energy generation this is not the case when the variability of the turbine extends to hours. (Reference)

http://www.nrel.gov/electricity/transmission/images/graph_output_56520.jpg

A 9 hours comparison of second-to-second variability of wind production between a wind plant with 15 turbines and a wind plant with 200 turbines.

To buttress the above point on variability, the same scenario can be applied to solar variability. The variability in the ability of a solar panel to generate electricity is dependent on sunrise and sunset. However, it becomes exceedingly difficult to predict when cloud covers occur (refernce

http://www.nrel.gov/electricity/transmission/images/graph_variability_socal.jpg Comparison of solar power production variability in southern California

Addressing variability of energy supply and demand

In a bid to deal with the challenges posed by the variability of renewable energy and demand-supply balance on a large scale it bores on the UK government to consider several strategies which includes: having a backup supply capacity that is based on storable fossil fuels or biomass, developing technologies that will produce more controllable and reliable electricity across a wider range of weather conditions and also, ensuring the availability of a wider regional dispersion of variable renewable plants to cancel the variability among sites (reference). In furtherance to these suggested strategies, the UK can also create interconnections between the grid and energy storage resources, which will increase the grids ability to stabilise the problems (IEA, 2006)

ROLES OF ENERGY STORAGE

The principal options for low carbon electricity generation needed by the UK to meet its emission reduction targets comes 2050 are; nuclear power, renewable energies, coal fired power plants with carbon capture and sequestration (CCS). These considered options have limitations In their application within the grid. Electricity demand within the Grid system is constantly changing and it is required that supply technologies meet that demand at every point in time (reference). However, with the application of low carbon energy sources as those listed above that will be a tall order. This is because the CCS and nuclear power plant technologies are designed to run continuously and thus cannot adapt to changing electricity demand (reference). The renewables such as wind and solar because of their intermittent and in-dispatchability nature can only generate electricity when the weather is favourable (when there is wind and sunlight)(references). From the forgoing, it can inevitably be said that a collection of all the best low carbon electricity generating technologies cannot produce a flexible electricity grid system without the integration of an energy system that will help store energy when it is produced In excess and release it when demand is at its peak (Yang and Williams, 2009).

Energy Storage: an enabling technology for future low-carbon electricity

The inability of solar and wind electricity generators to control their electricity generating timing, in addition to their intermittent and in-dispatchable framework, pose a disadvantage for their competitiveness within the electricity market (reference). The effect of which might be a limitation to their expansion and use in the National Grid System.

Energy storage as one of the most favourable options for addressing electricity supply gaps, can be used to provide a solution for both the intermittency and indispatchability of solar and wind power. As a complete solution to the variability in demand for these renewables, energy storage once coupled with a wind farm will provide a stable output of electricity to meet demand (reference). It will also, address the issue of dispatchability of output by dispatching the stored electricity generated in times of peak demand (lee and Gushee,2008)

Energy storage within an electric grid system can serve other purposes apart from deploying renewable electricity. It can also be integrated into the electricity generating system of a nuclear power plant and CCS plant. Seeing as both plants produce large scale-stable base output of electrical energy , but are incapable of adapting to changing demand, the building of these stable-base output all over the country to meet peak demands will be an expensive venture (reference). Presently variable demands for electricity are being met by inefficient carbon intensive fossil fired peaking plants that can generate electricity without delay but with carbon emissions occurring (reference). Instead of this occurring, energy storage can be used to slightly increase the CCS and nuclear base-load capacity by storing the excess electricity generated by both plants at night when demand is low and then releasing them during peak periods of the day (reference).

Energy storage can also be used to provide low carbon dispatchable peak-load electricity in a carbon constrained economy irrespective of the blend of CCS, nuclear and renewables. Stored renewable energy, can be used to meet both peak-load electricity demand and base-load electricity demand (reference). This use of energy storage will be of great benefit to the renewable-plus-storage operators (RPSO). Seeing as peak-load electricity cost more than base-load electricity, it is expected that RPSO’s will want to sell their output at peak-load rate first (reference). However, if in the future, renewables such as wind extend beyond just meeting peak-electricity load demand, then storage can also be used to serve as base-load source of electricity (reference)

Energy storage Technologies.

It is expected that with the planned increase in the use of low carbon electricity generating sources such as renewables to meet future electricity demand come 2050, current energy storage technologies for renewables will evolve from the existing grid-tied storages. The current application of grid-tied energy storages is divided into "peak shaving"(PS) and "power quality"(PQ) (reference).

The PS energy storage stores excess off-peak, low-value power for hours and releases into the grid as high-value-peak-load power (reference). The PQ on the other hand stores energy for seconds and minutes and releases when momentary outages within the grid is about to occur (reference). Such momentary outages within the grid will only give cause for concern when large scaled grids systems are being powered by renewable energy resources such as wind and solar which are dependent on the weather. (reference). The PS and PQ are used to enhance the grids capacity to manage variable demands (smith et al.,2007)

In conclusion, it also ………. For me to say that in addition to the afore mentioned roles of energy storage in the grid system, energy storage also serves to improve the reliability and flexibility of the grid system and also acts as an effective substitute for transmission upgrades (reference)

Technological options for future energy storage.

At present there are only three commercially available energy storage technologies for sale in the world (reference). They are listed as ; Sodium sulphur (NaS) battery technology, Pumped hydroelectric storage (PHS) and Compressed-air energy storage (CAES). However, there is on-going research taking place in the development of new storage technologies. The propose energy storage technologies for a low carbon economy are listed below;

Pumped hydroelectric Storage (PHS).

The PHS is the only large scaled storage technology that is in wide use (Ibrahim et al., 2008). The PHS uses off peak power to pump water uphill to a reservoir. At periods of peak demand for electricity, it releases the water to flow downward into a reservoir stationed at the bottom of the hill. It is this flow of water downhill that drives the turbines to generate electricity ((Denholm and Kukcinski, 2004)). The benefits of this storage system for future low carbon economies like the UK is that it is made available at any scale and it has a discharge time ranging from several hours to days. It has a road trip efficiency of between 60% and 80%. The PHS has a potential of around 1675GW in a type-one application ((Ibrahim et al., 2008)

http://www.fhc.co.uk/images/pics/pumped_storage.jpg

Diagram 2:Pumped Hydroelectric Storage (PHP)

Source: First Hydro Company ()

Compressed Air Energy Storage (CAES)

At present the CAES is not as widely spread as the PHS. There are only two operating CAES systems in the world. One is located in Germany and the other in the United States. CAES, uses off peak electricity to compress air into an underground storage air-tight cavern. When energy is required, the pressurised air is then released and fed into the gas-driven turbine to generate electricity. The case has an energy efficiency of 71% and a life time of 40years (Denholm and Kukcinski, 2004)

http://news.cnet.com/i/bto/20090826/compresssed-air-energy-storage_att.jpg

Diagram 3:System description of Compressed Air Energy Storage

Source: Renewable energy website (REUK, 2009)

Batteries Energy Storage.

Batteries are one of the most widely used energy technologies in the world. They store electricity in the form of electrochemical energy. Among the most prominent batteries in the world are the flow batteries and the Sodium-sulphur(NaS) battery. The NaS has a shelf life of between 10-15 years, a high operating temperature of 350oC , energy efficiency of 85% and energy density of 151KWh/m3 (Kazempour et al; 2009). NaS batteries can be used to meet momentary demand in energy or in most cases meet longer demand for energy.

http://www.aheadenergy.org/uploads/4/6/2/3/4623812/9195693_orig.jpg

Diagram 4:Walking principle of a battery storage system.

Source: Sandia Nation Laboratories

Flywheel Energy Storage System (FESS)

Flywheels are electrochemical systems that store energy in the form of kinetic energy (Diaz-Gonzalez et al., 2012). A flywheel has an electric motor that accelerates the movement of a rotating disc to charge the storage device. The motor acts as a generator in the reverse, slowing down the disc and producing electricity in the process (Diaz-Gonzalez et al., 2012). Flywheels have the advantage of requiring low maintenance and are suitable for frequent discharge of electricity (Gotchall and Kamath, 2003). A flywheel has an energy capacity of about 19MWs and can deliver 10s of ride-through at 1.65MW load(Diaz-Gonzalez et al., 2012).

http://www.dg.history.vt.edu/images/image2_41.gif

Diagram 5:Flywheels Energy Storage Systems (FESS).

Source:

Hydrogen-based Energy Storage System (HESS)

Hydrogen can be obtained from numerous sources among which is the electrolysis of water, gasification of glass, from renewables like wind or solar form coal and fuel among others (Conte et al., 2001). Hydrogen produced from wind power, it can be transported to end users through pipes to produce electricity or used directly in fuel cells (Sherif et al., 2010).The storage of hydrogen is through a technology called Regenerative Fuel Cell (RFC)(113). The technology used in producing hydrogen is clean one because neither the production nor the use of hydrogen in producing electricity result in the production of pollutants. Hydrogen cells have a useful life of 15 years, 20,000 charge and discharge cycles. It has a low energy efficiency of 42% (Diaz-Gonzalez et al., 2012).

Image of Linde liquefied hydrogen storage tank. Parts of the tank are identifies as follows: filling port, liquid extraction tube, gas extraction tube, filling line, level probe, super insulation, inner vessel, outer vessel, suspension, safety valve, shut-off valve, cooling water heater exchanger, reversing valve (gaseous/liquid), electrical heater.

Diagram 6: Hydrogen-based Energy Storage System (HESS)

Source:

Ultracapacitor energy storage systems

The ultra-capacitors have a lower energy when compared with batteries but are able to store more energies than batteries (to the tune of tens of thousands). They store energy as electrostatic charges on opposite surfaces of an electric layer (Baxer 2006). They have a round trip efficiency of between 80% to 95%. They are however, only suitable for Their round trip efficiency is about 80-95%. Are suitable for power quality application because of their short discharge duration ( Mufti et al., 2009).

http://ars.els-cdn.com/content/image/1-s2.0-S1364032112000305-gr6.jpg

Diagram 7: Ultracapacitor energy storage systems

Source:

Superconducting magnetic energy storage (SMES)

The SMES is a relatively new technology. It’s functionality is based on storing energy in a magnetic field, which is created by a DC current through a large superconducting coil at a cryogenic temperature. (reference). The maximum amount that can flow through the coil is dependent on temperature (reference). They however, have the ability to absorb or inject large amount of current within a short space of time. The energy capacity of an SMES ranges from 100KW to 10MW. It is only commercially applied for They are commercially used for power quality purposes (reference).

http://www.bnl.gov/cmpmsd/AdvancedEnergyMaterials/images/ARPA-E%20SMES.jpg

Diagram 8: Superconducting magnetic energy storage (SMES)

Source: