Design for the Environment/Energy Storage for Wind Power Generation

A turbine generating power in a field of wheat.

Electricity generated through wind turbines is a highly intermittent power source and as a results its relative market penetration is quite low in order to avoid the risk of power shortages. As well, wind power is non dispatchable, meaning that it must be consumed as soon as it is produced and its production cannot be reserved for peak time either nor produced all day such as it can in base load plants. One way that both of these problems can be resolved is by storing the power generated as some other form of energy. Some of the ways this is accomplished are Pumped Storage, Flow Batteries and Compressed Air Energy Storage (CAES). Due to wind power's easy set up and relative availability, increasing its use would be a crucial step towards lowering CO2 emissions and bringing an end to global warming.

Currently, the most commonly used way of storing wind power currently is Pumped Storage. Pumped Storage is a method where excess electrical energy in the electrical grid is used to drive the turbines of a hydroelectric dam backwards so that water is pumped up into the reservoir thus adding hydraulic head and storing the power for future use. When it is needed (i.e. peak hours) the stored water will be brought back down the penstocks and through the turbines this time driving the generator and putting back into the grid. Flow batteries are a type of battery that converts electrical energy into chemical energy that is stored in an electrolyte. The charged electrolyte is stored in tanks and is passed through a reactor to convert the chemical potential into electricity. The most common type of flow battery is the redox flow battery where the electroactive compounds are dissolved in the electrolyte. The Vanadium Redox Battery (VRB) uses vanadium, in four different oxidation states, dissolved in the electrolyte to store the electrical energy and as a result does not require any other species in the electrolyte. CAES uses large, near air-tight geological formations such as salt caverns and aquifers to store compressed air, as is done for natural gas storage. Wind power is either used directly to drive a compressor and pump air into the storage facility or generated electricity is used to drive the compressors to the same effect.

Project Information

Section: 2 Group: B6

• Christopher Frankowski (C.frankowski)
• Dennis Ching-Liang Lin (Lindenni)
• Phillip Shin (YongKyun)
• Milos Uskokovic (M_uskokovic)

Highlights & Recommendations

Highlights

Pumped Storage, VRB and CAES all operate by transforming the electrical energy of wind into another form of potential energy; gravitational, chemical or pressure. All of these storage methods can start producing electricity within minutes of demand occurring. The main differences occur in size and location, Pumped Storage and CAES require a suitable natural location but are large enough that only a few are needed. On the other hand the VRB solution does not have any geographical restrictions but is considered a form of distributed power storage where one or two facilities would be present at each wind farm. The main advantages of pumped storage are the large storage capacity and no environmentally hazardous outputs during the use phase. The main disadvantage of pumped storage is that large areas of land need to be flooded. The VRB's main advantages include instantaneous electrical production, small environmental footprint, low maintenance and rapid implementation. However due to the fact that this is new technology, it is still rather expensive. The CAES system uses naturally occurring geological formations that are easy to convert into large compressed air storage tanks, since all of the caverns already exist and are underground there is no destruction of the land. The downside is that the system has to burn natural gas to heat the compressed air, this results in CO2 output. Increasing global warming potential.

Combined SLCA values

Facility Activity	Ecological Impacts			Energy Use			Solid Residues			Liquid Residues			Gaseous Residues			Sum
	(1)	(2)	(3)	(1)	(2)	(3)	(1)	(2)	(3)	(1)	(2)	(3)	(1)	(2)	(3)	(1)	(2)	(3)
Site Development	0	2	3	0	2	2	3	3	2	3	3	2	1	3	2	7	13	11
Materials and Product Delivery	2	0	2	0	3	2	3	4	2	3	0	3	2	3	2	10	10	11
Infrastructure Manufacture	3	4	2	0	3	2	3	3	3	3	3	2	2	2	1	11	15	10
Infrastructure Use	4	3	2	4	3	2	4	3	3	4	2	3	3	4	1	19	15	11
Refurbishment, Recycling, Disposal	0	3	0	0	4	3	0	3	3	4	3	3	4	4	3	8	17	15
Total	9	12	12	4	15	11	13	16	13	17	11	13	12	16	9	55	70	58

(1) Pumped Hydro (2) Vanadium Flow Battery (3) Compressed Air Energy Storage

The ideas presented here are three very different methods of accomplishing the same goal. All three processes are between 70% and 80% efficient and all three are improvements over the current method of fixing the intermittency of wind power which is by adding more natural gas turbines to provide peaking power if the wind is not blowing. All three methods also have quick start up so that short falls or peaks in the grid can be immediately supplemented.

The following matrix shows the results of the different analyses between all of the alternatives.

Comparison of all alternatives, using different analyses

System	Total SLCA score	EIO-LCA (GWP MTCO2E)	EIO-LCA Total Releases/Trans kg	EIOLCA Total Energy (TJ)	Cost Analysis ($/kWh)
Pumped Hydro	55	1420000	837600	1005.3	148.95
VRB	80	62200	206000	448	466.67
CAES	58	5070	3160	43.3	281.33

Obviously each method has its advantages. Pumped Hydro is cheapest to implement for the especially in the long run but at great cost to the environment. Pumped hydro also requires a large capital cost. VRB is very environmentally friendly according to the SLCA but quite expensive. However, the EIOLCA results for VRB are significantly better than those for Pumped Hydro for the same energy output, up to four times better in fact. The CAES system scored better than pumped hydro in the SLCA and was between Pumped Hydro and VRB in cost. However, the EIOLCA results appear to be too low for a system that uses natural gas especially for a product with a long life time such as this and especially when compared to a system like the VRB which uses no hydrocarbons for fuel and has no air or toxic emissions.

Recommendation

Therefore the best design is the VRB. A system that can be implemented easily on the wind farm site, is safe to operate and does not displace large groups of people or emit CO2 is not only a logical choice but an economically and commercially viable. As well, the use of any technology will only further its development and thus make large improvements in VRB energy capacity as well as lowering system cost.

Functional Analysis

Pumped Storage

A cross section of a hydroelectric power generating station.

The storage of energy through pumped hydro is a simple solution that relies on the potential energy that can be gained and lost by moving a mass through a gravitational field. Energy is used to pump water to an upper reservoir, increasing its potential energy. When the energy is needed, it can be dropped to its original position to regain the potential energy. The facilities that perform this function are largely similar to those used in hydroelectric electric power generation. Energy storage is performed when water is pumped to an upper reservoir that could be contained behind a dam, or a large, natural impermeable basin. When the energy is needed, the water is released back to the lower reservoir after passing through a turbine connected to a generator that can turn the kinetic energy of the falling water into an electric current. The advantages of this sort of solution is that it does not have any major impacts on the environment during the use phase, requiring almost no inputs and producing virtually zero outputs. The disadvantages are the massive land requirements and the resulting destruction of wildlife habitat, along with the fact that pumped storage plants are characterized by long construction times and high capital expenditure .

Compressed Air Energy Storage (CAES)

Compressed air energy storage (CAES) technology uses the electricity generated by wind turbines available overnight to run the compressors (electric motors) during off-peak times. The compressors operate by driving air down into a natural aquifer or a cavern, then extracted from the aquifer/cavern when power is needed. The air flowing back up through the pipes must be pre-heated and burnt in the combustion chamber using natural gas in order to turn the expansion turbine.

One of the significant benefits of a CAES plant is its separation of the compression cycle from the combustion cycle, which presents a number of advantages. First of all, it provides high thermal efficiency in the production mode, which is less than 4,500 Btu/kWh for a typical heat rate. This makes CAES one of the most efficient power plants for producing electricity. Another advantage is its high ramp rates(Explain), as it is not restrained by compression requirements. Also, CAES has a stable heat rate at low capacity. In general, it requires the generators to operate lower than 100% output in order to maintain responsive capacity. The heat rate degradation will only occur when CAES plants operates below 50% of the generating capacity. Moreover, high output in high ambient temperatures is another advantage of CAES plants. Since the temperature and pressure of the air flowing into the combustion turbine is constant, there will be no change in the mass flow. Thus, CAES plants can produce as much power as possible regardless of what the temperature is outside, which makes them very versatile. On the other hand, there are a few disadvantages of CAES such as geological structure reliance, loss of the input energy is lost each time the air is pumped into and extracted out of the cavern, and it requires additional heating in the expansion process. Although the consumption of fuel is minimal, its influences in the future cost is still a major concern due to increasing prices of natural gas.

Vanadium Redox Battery (VRB)

A schematic of VRB.

A flow battery is a form of rechargeable battery that stores electrical energy by converting it into chemical potential. Flow batteries can increase the usefulness of the wind power plant in that the stored energy can be used to supply consistent power output and to increase the maximum power rating of the wind power plant during peak electricity demand hours.

Vanadium Redox Battery(VRB), a widely used flow battery for wind power application, stores and generates electricity through oxidation and reduction of a Vanadium electrolyte. The system consists of two electrolyte tanks, pumps to circulate the electrolyte and a chemical reactor that converts chemical energy into electricity.

In VRB chemical energy is stored in different ionic forms of vanadium in a dilute sulphuric acid electrolyte. The difference in electric potential creates current flow through the electrode, which is available to an external circuit. The system is designed for unattended operation with minimal maintenance cost. The system has round-trip efficiencies of 65% - 75%.

The advantages that VRB offers over other conventional storage systems are its flexible system layout and the modular design between the maximum power rating and total storage capacity. Increasing the volume of the electrolyte tanks increases the total storage capacity, whereas increasing the number of reactors increases the maximum power rating. Several disadvantages of the VRB are its high cost and the use of large volume of sulphuric acid during its use phase.

SLCA

Pumped Storage

The information used to score pumped storage for this SLCA is gathered from several implementations of pumped storage around the world. . The data is a compilation of 4 pumped storage solutions: the Seneca plant near Warren, Pennsylvania, the Ludington plant in Ludington, Michigan, the Kruonis plant in Kruonis, Lithuania, and the world’s most powerful pumped storage plant in Bath County, Virginia.

SLCA for Pumped Storage

Environmental Stressor
Facility Activity	Ecological Impacts	Energy Use	Solid Residues	Liquid Residues	Gaseous Residues	Sum
Site Development	0	0	3	3	1	7
Materials and Product Delivery	2	0	3	3	2	10
Infrastructure Manufacture	3	0	3	3	2	11
Infrastructure Use	4	4	4	4	3	19
Refurbishment, Recycling, Disposal	0	0	0	4	4	8
Total	9	4	13	17	12	55

Site Development

The key factor in determining the environmental responsibility of a societal infrastructure is the choice of site, and minimization of ecosystem disturbance . Alarge amount of land is lost and due to the destruction of the natural habitat of the wildlife in the area of the creation of the upper storage reservoir. The areas that are suitable for use in a pumped storage solution are usually located away from areas of dense human populations, and thus long spanning connections are needed to join the power grid that would not necessarily be needed for the other options. Based on the construction process at Bath County, any earth and rock fill that was removed to facilitate construction was used within the dam structure. Solid, liquid and gaseous residues associated with site development would be minimal.

Materials and Product Delivery

Due to the remoteness of most pumped storage solutions, the choice of materials and delivery methods is of high importance. The majority of the material needed is the concrete needed to produce the reservoir, either dam or pool bed, and all of the associated piping. The rock fill used during construction can be locally excavated, but the concrete and steel need to be delivered. The steel that is used to reinforce the concrete can come from a recycled source, as 75% of steel is recycled. Because of the speciality required to produce the massive turbines and electric generators, these components are manufactured off site, and sometimes internationally. These massive steel components, along with the massive quantity of building materials needed for such large products, require large amounts of energy to deliver to the construction site. The solid shipping materials that package the components are all common and widely reusable and recyclable. The liquid and gaseous emissions related to transport come largely from the emissions of transport vehicles, instead of packing material. The gaseous emissions from all of these vehicles have a large GWP. This option has a much higher amount of transportation and uses a much larger volume of material compared to the other solutions.

Infrastructure Manufacture

The production of the dam and reservoir relies largely on reusable material. The components for the large amount of cement needed for construction must excavated, but are all commonly occurring. The remainder of materials needed for concrete can be excavated locally, and the reinforcing steel can come from a recycled source. The manufacturing of the reservoir is a large energy burden, and the production of the turbine, housing, and connectors are also highly energy intensive.

Infrastructure Use

This category is the highlight of the pumped storage option. The process of hydroelectric generation requires minimal inputs, and produces virtually no outputs. There is a small energy input involved with operating the facility associated with the 70-85 % efficiency of the system. Loses occur due to evaporation of water from the upper reservoir, head loses involved with the penstocks, as well as loses related to the turbine. These efficiencies are comparable or better than the other storage options discussed. There are no solid or liquid wastes associated with the operation of a pumped storage solution.

Refurbishment, Recycling, Disposal

Hydroelectric facilities are rarely designed with disposal in mind, but are usually designed to allow for easy upgrading or refurbishment. With the possibility of upgrading turbines when developments lead to more efficient designs, the high capital cost of construction and the importance of its function to society, the need for destruction of this infrastructure is minimal, as would be true for all options. The removal of the structure would result in millions of cubic meters of concrete to dispose of requiring huge amounts of energy, and if a dam reservoir is used, once again great damage to the surrounding ecology. For these reasons pumped storage had generally low scores but due to the simple and clean operation of the plant, there will be little to no liquid or gaseous residues, so these elements scored highly.

Compressed Air Energy Storage (CAES)

SLCA for Compressed Air Energy Storage Storage

Environmental Stressor
Facility Activity	Ecological Impacts	Energy Use	Solid Residues	Liquid Residues	Gaseous Residues	Sum
Site Development	3	2	2	2	2	11
Materials and Product Delivery	2	2	2	3	2	11
Infrastructure Manufacture	2	2	3	2	1	10
Infrastructure Use	2	2	3	3	1	11
Refurbishment, Recycling, Disposal	3	3	3	3	3	15
Total	12	11	13	13	9	58

Site Development

Material Choice: If a natural geological formation is used, the infrastructure will be developed with minimal destruction of natural areas.

Energy Use: The infrastructure design requires the use of natural gas. It requires a certain amount of energy to construct the site.

Solid Residue: Since a natural aquifer is used, there will not be much solid residue created by site development.

Liquid Residue: The infrastructure site can be made operational with only minimal production of liquid resides.

Gas Residue: The possible release some left over gasses trapped in the aquifer results in a score of 2.

Materials and Product Delivery:

Material Choice: No scarce materials are acquired from the suppliers, thus there will be minimal ecological impacts. But since not all the materials are recyclable, a score of 2 is given.

Energy Use: The infrastructure design requires the use of natural gas. It requires a certain amount of energy to ship and extract the natural gas.

Solid Residue: Extraction of natural gas creates a certain amount of solid wastes such as mud.

Liquid Residue: Liquid shipping and installation residues are minimized.

Gas Residue: A small amount of greenhouse gases are generated in shipping.

Infrastructure Manufacture:

Material Choice: The main construction requires common materials such as metal and concrete.

Energy Use: Building the CAES plants do not requires lots of energy; therefore a score of 2 is given. Solid Residue: The infrastructure does not require the use of materials whose extraction involves the production of large amounts of solid residues or toxic/radioactive materials. In fact, some of the materials used can be recycled, thus a score of 3 is given.

Liquid Residue: Water is required for the construction of plant. Used water will form liquid residue from combining with dusts.

Gas Residue: A major gas residue created during the construction process is dust. A score of 1 is given.

Infrastructure Use:

Infrastructure use requires natural gas to reheat the cool air before it enters the expansion turbine, but it has been minimized for the CAES system. Also, Natural gas is not renewable; therefore, a score of 2 is given.

Energy Use: Combustion chamber requires some energy to heat up the cool air and to operate the turbines.

Solid & Liquid Residue: The periodic disposal of solid and liquid materials associated with the use and maintenance of the CAES plants has been minimized. Also, the operational impacts on the environment regarding water and soil wastes are not significant, thus given a score of 3.

Gas Residue: The greenhouse gas emission from the combustion turbine leads to global warming which has a significant environmental impact but the amount of emission is minimized.

Refurbishment, Recycling, Disposal:

All the categories are given a score of 3 due to its relatively low disposal impacts on the environment and the following reasons.

Material Choice: The Site and structures can be reused with negligible impact on natural areas or existing ecosystems.

Energy Use: It does not require much energy for the infrastructure closure.

Solid, Liquid, & Gas Residue: A small amount of solid, liquid and gas waste will be produced by infrastructure closure.

Vanadium Redox Battery (VRB)

Streamlined LCA method is used to assess the environmental impacts of VRB during different lifestages according to a generallized scoring guideline. The following result was obtained:

SLCA for VRB Storage

Environmental Stressor
Facility Activity	Ecological Impacts	Energy Use	Solid Residues	Liquid Residues	Gaseous Residues	Sum
Site Development	2	2	3	3	3	13
Materials and Product Delivery	0	3	4	0	3	10
Infrastructure Manufacture	4	3	3	3	2	15
Infrastructure Use	3	3	3	2	4	15
Refurbishment, Recycling, Disposal	3	4	3	3	4	17
Total	12	15	16	11	16	70

Vanadium is a non-toxic element with availability of greater than 98%. Large construction and site development are not necessary in order to install VRB because the system does not occupy significant amount of land. Most materials used in VRB such as PVC pipes or polypropylene electrolyte tanks can now be made with much less energy than the metal parts. The large volume of sulphuric acid required in the system is one of its disadvantages but the concentration of the sulphuric acid is about 30% lower than the lead acid battery.

During the use phase, operation of VRB is fully automated. There are virtually no residues created during the long use period. Most components of VRB are recyclable or reuseable at the end of its life such as electrolyte tanks.

EIO-LCA

Pumped Storage

The data that was used to perform the EIO-LCA of pumped storage was based on the Dinorwig pumped storage solution in North Wales. The focus of this assessment is on the impact of initial construction since effects are minimal in the use stage and would be comparable to those of the other options.

Environmental Impact of Pumped Storage

Impact	Total	Highest Contributor
Air Pollutants(mt)
SO2	3,600	Power Generation and Supply (70.4%)
CO	9,670	Truck Transportation -> Cement Manufacturing (81.3%)
NOx	4,069	Power Generation and Supply (63.5%)
VOC	605	Iron and Steel Manufacturing / Cement Manufacturing (88.1%)
GWP(MTCO2E)	1,420,000	Power Generation / Cement Manufacturing (86.7%)
Toxic Release(kg)
Air	176,000	Power Generation / Cement Manufacturing (76.3%)
Water	37,600	Iron and Steel Mills -> Steel Manufacturing (69.8%)
Land	624,000	Iron and Steel Mills -> Steel Manufacturing (34.4%)

The Results of the EIO-LCA point out areas that are most harmful to the environment, but also show us where the largest improvements can be made. The use of recycled steel as reinforcement in the concrete of the dam, as well as in the piping and turbines would greatly reduce the amount of energy required as well as in the production of solid, liquid, and gaseous releases in the production of steel.

Another point that can be made from the EIO-LCA is that a large amount of pollution stems from the transportation of materials to the work site. Using more locally sourced materials as well as using more efficient modes of transportation for moving raw materials would greatly increase the scores of pumped storage.

Lastly, the production of concrete from cement is a highly developed process, and so developments that would drastically reduce the emissions are unlikely. Pumped storage will always have relatively high gaseous emissions because of the cement manufacturing process and the fact that enormous amounts of concrete are needed to create a pumped storage solution.

Compressed Air Energy Storage (CAES)

Conventional Air Pollutants

Conventional Air Pollutants

Conventional Air Pollutants

The facility is chosen to have the capacities of 7MW for the amount of power that can be released when in operation and 70MWh for the total storage capacity. The above table and figure illustrate the amount of conventional air pollutants released by different sectors used to manufacture the CAES facility . Based on the results, substantial amounts of NOx are also released in cement manufacturing. This is due to the burning of coal which results in the production and release of NOx. Moreover, the highest level of emission of power generation is SO2, which is produced from fuel combustion. It also shows a moderate level for CO, NOx, and VOC as power generation is closely related to natural gas distribution. Truck transportation has the highest total amount of releasing air pollutants, which is mostly due to the amount of CO emission. However, mining is concluded to be the main factor of the emission of air pollutants, as most activities in the table above are closely related to it.

Greenhouse Gases

Greenhouse Gases

Greenhouse Gases

The above table and figure show the greenhouse gas emission for different sectors. First of all, GWP and CO2 emissions are very high for power generation, cement manufacturing, this is due to the fact that natural gases are burned during combustion in power generation and the process of calcinations and chemical reactions from heating limestone creates about 60% of the CO2 emission in cement production. Furthermore, gas extraction contributes to a moderate amount of CO2 due to the possible leakage during natural gas extraction process. Pipeline and truck transportations are also responsible for total CO2 emission since leakage during transportation is possible plus the emission of greenhouse gas of the truck. Based on the results, it is concluded that natural gas combustion and leakage of gas are the most significant factors to greenhouse gas emission.

Energy

Energy

The energy consumed in each sector is shown in the above table. It is clear that although most of the sectors do not require high energy, power generation is still more energy intensive compared to other activities due to combustion of coal. Therefore, this result indicates that the process during CAES installation is not energy intensive.

Toxic Releases

Toxic Releases

As the results from above table, cement manufacturing and steel mills contribute to the major amount of toxic releases during the manufacturing stage. Also, it is discovered that the top 10 sectors are mostly related to metal production and mining, as possible use or production of toxic chemicals may occur during the process.

.

Vanadium Redox Battery (VRB)

In order to assess the environmental impact of VRB systems, EIO-LCA tool provided by the Green Design Institute has been used. The analysis is based on a 7MW VRB system with 70MWh capacity.

Environmental Impact of VRB

Impact	Total	Highest Contributor
Air Pollutants(mt)
SO2	122	Power Generation and Supply(89.3%)
CO	265	Ferroalloy and related product manufacturing(29.2%)
NOx	69.9	Power Generation and Supply(70.2%)
VOC	11.3	Truck Transportation(42.6%)
GWP(MTCO2E)	62200	Ferroalloy and related product manufacturing(46.0%)
Toxic Release(kg)
Air	29200	Ferroalloy and related product manufacturing(64.0%)
Water	4780	Ferroalloy and related product manufacturing(76.6%)
Land	156000	Copper, nickel, lead, and zinc mining(47.7%)

From the above chart, carbon monoxide output is identified as the greatest among all conventional air pollutant in the VRB manufacturing sector, most of which is from the ferroalloy and related product manufacturing sector. Ferrous alloy manufacturing sectors and power generation sectors are related by energy intensive manufacturing processes. In order to reduce the amount of conventional air pollutants, replacing metal parts with plastic or other materials is necessary. The highest contributor to the green house gas emissions is also ferroalloy and related product manufacturing sector.

The total toxic release caused from manufacturing of 7MW VRB is 189980kg, most of which is solid waste. The mining of copper and other metals is the highest contributor to toxic releases. Since the eio-LCA is heavily based on assumptions of many conditions, the result should not be entirely trusted, but rather be used as a guide for qualitative analysis.

Cost Analysis

Pumped Storage

The information on the four pumped energy examples used in the SLCA was also used to analyse the initial cost of construction a pumped energy storage solution. To begin with, the initial costs were adjusted based on the US GDP inflation from the years of construction to a present date cost using figures provided by [16]. This information is summarized in the table below.

Capital Cost Estimates

Site	Year of Construction	Initial Cost (Mil. USD)	Adjusted to Present Cost (Mil. USD)	Storage Capacity (MWh)	Output Capacity (MW)	Cost for Storage Capacity (Mil. USD / GWh)	Cost of Output Capacity (Mil. USD / MW)
Seneca	1965	108	560	NA	400	-	1.4
Ludington	1973	327	1,227	14,400	1,800	85.2	0.682
Dinorwig	1984	1080	1,865	8500	1,700	219.4	1.10
Bathcounty	1985	1700	2,845	20,000	2,100	142.25	1.35

This is a much larger initial cost for construction, but the operational and maintenance costs are much smaller compared to a flow battery solution, and comparable to that of CAES.

Compressed Air Energy Storage (CAES)

Capital Cost Estimates (Using various types of Expansion and compression equipment)

CAES	Expansion/ Compression Equipment ($/kW)	Storage Cavern ($/kW)	Surface Reservoir ($/kW)	Balance of Plant ($/kW)	Indirect Costs ($kW)	Total ($/kW)
1	35.5	65.8	5.5	80.0	116.8	303.6
2	43.4	58.4	4.7	80.0	116.5	303.0
3	49.0	41.6	3.4	70.8	103.3	267.8
4	38.6	41.6	3.4	70.8	96.5	250.9

The capital costs of a CAES system construction consist of the expansion and compression equipment, the storage cavern, the surface reservoir, the balance of plant, and the indirect costs. The estimations using different types of expansion and compression equipments are shown in the table above. The total cost of the storage cavern is calculated based on the costs of the cavity, the air and water shafts, development and mobilization, and completion. Also, the motors, recuperator, fuel storage, intercoolers, and the combustors are counted in the total cost of the balance of plant. Based on the results, it is indicated that the total capital cost of CAES is relatively around 300$/kW, which is close to the results from Table 3.

Operating Cost Estimates (Using various types of Expansion and compression equipment)

CAES	Capital charge (mills/kWh)	Fuel Cost (mills/kWh)	Electricity off-peak (mills/kWh)	Operating & Maintenance (mills/kWh)	Total (mills/kWh)
1	25	9.5	10.9	2.0	47.4
2	24.9	10.0	8.9	2.0	45.8
3	22.0	11.8	6.3	2.0	42.1
4	20.6	11.8	6.3	2.0	40.7

The operating costs of a CAES plant include the capital charge, fuel costs for the combustors, the off-peak electricity costs required to run the air compressors, and also the operating and maintenance costs as shown in the table above. The total estimations indicate that the operating cost is about 45mills/kWh (4.5 cents/kWh). The capital charge is estimated based on the capital charge rate of 18% a year, assuming the operating time is 2190 hours a year. Also, the cost rate used to calculate the fuel cost $8.53/103kWh, and 1.5cents/kWh for the charge rate of the cost of electricity.

Replacement Assumptions

Storage System	Replacement period (Years)	Replacement Costs ($/kWh)
CAES	30	0
Pumped hydro	30	0
Lead-acid battery	6	150
Hydrogen fuel cell	6	100

The CAES system does not require significant replacement costs during a 20-year lifetime based on the table above. The assumption cost is $0/kWh. This indicates that the replacements cost is relatively small due to some salvage and minimal needed cleanup.

Vanadium Redox Battery (VRB)

The direct cost of VRB ranges between $300-650, with approximately $0.008/kWh maintenance cost. The disposal cost of VRB is considered not significant because most parts can be reused or recycled.

In order to be able to compare the cost of the three storage methods, a 7MW VRB storage model with 70MW capacity is analyzed. The direct cost of the VRB infrastructure is evaluated by considering the capital, operating and disposal costs of the infrastructure.

Cost of 7MW VRB with 70MWh storage capacity

Capital Cost	Operating Cost	Disposal Cost	Direct Cost(total)	Cost per kWh
$29,400,000	$958,306.91	$5,276.10	$30,363,583.01	$466.67/kWh

Societal Impacts

Pumped Storage

Because of the similarities between pumped storage and hydroelectric power generation, and the use of both processes in parallel when possible, society’s view of pumped storage mirrors that of its perception of hydroelectric power generation. The ability to create power from minimal inputs and producing minimal outputs is seen as a huge benefit to society, and the structures are seen as marvels of modern engineering where the benefits during the long use period far outweigh the initial environmental damages cause during construction. Because of the settings or pumped storage solutions, the surrounding areas and water bodies are used for general public recreation . The Ludington plant even goes as far as placing a 4km long barrier net surrounding the intakes to ensure that fish species are preserved in the surrounding area. The downside is that large projects have become controversial due to the number of people who are displaced by the rising water that is needed for hydroelectric power.

Compressed Air Energy Storage (CAES)

CAES plants are geographically limited to locations with salt caverns and aquifers. However, even if a population centre were to be found near a CAES site the population would probably be unaware of much going on as the infrastructure is similar to that of gas turbine peaking plants found all over the country. The only social stigma that arises is due to the use of natural gas to heat and expand the air. With more and more emphasis being placed on renewable energy, using any sort of hydrocarbon-combustion based process may be frowned upon by tax payers or investors.

Vanadium Redox Battery (VRB)

Vanadium is a non-toxic element that is highly unlikely to cause a significant damage to human body in case of exposure . The system is relatively easily implemented with little disturbance to the natural habitat. During the use phase, the operation of the storage system is automated and the noise created by the system is expected to be minimal. For the most part VRB is considered an environmentally friendly solution to replace conventional stationary battery application. However, for large VRB applications the volume of the sulphuric acid electrolyte may raise safety concerns of the inhabitants near the area in case of an accident. For this reason VRB storage system may need to undergo public review or government assessment prior to installation in some countries .