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Energy Storage and The Integration of Renewable Energy Into The Grid. University of Colorado at Boulder Department of Electrical, Computer, and Energy Engineering Energy Storage Research Group. Frank S Barnes 303.492.8225.

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Energy Storage and The Integration of Renewable Energy Into The Grid

University of Colorado at BoulderDepartment of Electrical, Computer, and Energy EngineeringEnergy Storage Research Group

Frank S Barnes




The work leading to this talk was conducted by

  • Jonah Levine
  • Michelle Lim
  • MohitChhabra
  • Brad Lutz
  • Greg Martin
  • Muhammad Awan
  • TahaHarnesswala
  • Richard Moutoux
  • CameliaBouf
  • Kimberly Newman

Outline of Our Work

1. Potential Location of Pumped Hydroelectric Storage in Colorado

2. Issues in Compressed Air Storage at 1500m in Eastern Colorado

3. The use of battery storage for frequency control and voltage regulation

4. Feed In Angle for Solar Power

5. The Optimization of Energy Use in Water Systems.

6. Detection of Power Theft

7. Optimization of Energy Use in Water Systems


Obstacles to Integration of

Wind and Solar Energy

The Variability of Wind, Solar and Hydroelectric Power and Mismatch to the Loads

The Integration and Control of a Large Number of Distributed Sources in to the Grid

Lack of low cost convenient energy storage systems

simplified system model
Simplified System Model


Steam Generator


Gas Generator

Network Electric System



Wind Generators

Sbase= 600MVA


Energy Storage System (ESS)



Reference: [2]-[4]

input data
Input Data

Sbase = 600 MVA

11th Jan 2011: 1 pm

9th June 2011: 1 pm

frequency winter
Frequency - Winter

~32% wind penetration

frequency summer
Frequency - Summer

~29% wind penetration

power spectrum 1
Power Spectrum [1]

Turbine upper limit

small magnitude:turbine acts aslow-pass filter


Short-term Storage Time Scale :

≈ 10 sec – 3 hrs

Energy Storage

278 hours

27.8 hours

2.78 hours

16.7 min

100 sec

10 sec

2 sec


[1] J. Apt, “The spectrum of power from wind turbines”, Journal of Power Sources, v.169, March 2007

[2] G. Lalor, A. Mullane, M. O’Malley, "Frequency Control and Wind Turbine Technologies“, IEEE Transactions on Power Systems, v. 20, no.4, November 2005

[3] R. Doherty et al, “An Assessment of the Impact of Wind Generation on System Frequency Control", IEEE Transactions on Power Systems, v.25, no.11, February 2010

[4] P. Kundur, Power System Stability & Control, McGraw-Hill, 1994

case when wind energy exceeds capacity
Case When Wind Energy Exceeds Capacity.
  • Current Law Requires use of Wind Energy
  • The wind energy may exceed the amount of gas fired energy that can be shut off and require the reduction of heat rate to coal fired plants
  • This reduces electric power generation efficiency and increase emissions of SO2, NOx and CO2 for old plants
  • It is expected to up to double the costs of maintenance.
emissions for start up ramping and partial loads
Emissions for Start Up, Ramping and Partial Loads
  • IEEE Power systems Nov-Dec. 2013
cost of increasing wind energy penetration
Cost of Increasing Wind Energy Penetration

Gas Cost Impact of wind penetration with and without storage on Xcel’s electric grid

Cost Impact of increasing wind penetration on Xcel’s electric grid

lower bound on cycling costs
Lower Bound on Cycling Costs
  • IEEE Power Systems Nov-DEC 2013
approaches to solving the variability issues
Approaches to Solving the Variability Issues.
  • At low penetration grid spinning reserves.
  • Gas fired generators
  • Storage
    • Batteries, super capacitors, fly wheels
    • Pumped Hydroelectric systems, CAES
  • Demand Response
  • Biomass, geothermal
pumped hydro storage in colorado
Pumped Hydro Storage in Colorado

Wind Integration Study for Public Service of Colorado Addendum Detailed Analysis of 20% Wind Penetration

snapshot of pumped storage globally rick miller hdr dta
Snapshot of Pumped Storage Globally Rick Miller HDR/DTA

Pump Storage Units in Operation (MW) by Country/Continent

Pumped Storage Projects Under Construction (MW)

compressed air energy storage caes
Compressed Air Energy Storage CAES

Questions of Interest

  • Where can we locate CAES.?
  • Some Design Considerations
  • Value of Storage
  • When is it Cost Effective?
current and planned caes systems
Current and Planned CAES Systems

1.Huntorf Germany 1978

290 MW for 2 to 3 hours per cycle

2. McIntosh, Alabama

110 MW ,19 million cubic feet and 26 hours per charge

3. Others that have been under discussion for a long time

A. Iowa Stored Energy Park

B. Norton Ohio (2700 MW)

4. Others?

caes characteristics
CAES Characteristics

1. It is a hybrid system with energy stored in compressed air and need heat from another source as well.

2. Require 0.7 to 0.8 kWh off peak electrical energy and 4100 to 4500 Btu (1.2 -1.3 kWh) of natural gas for 1 kWh of dispatchable electricity

3. This compares with ~ 11,000 Btu/kWh for conventional gas fired turbine generators.

4. Efficiency of electrical energy out to electrical plus natural gas energy in ~ 50%

caes characteristics1
CAES Characteristics
  • Another way to calculate efficiency is comparing to the normal low efficiency of natural gas turbines with heat rate of 11000 Btu/kWh yielding 0.39 kWh of electricity and adding 0.75 kWh off peak electricity to get 1.14 kWh’s to get 1 kWh of dispatchable electricity
  • This gives an efficiency of 88%
  • There are two types of CAES systems
    • Underground CAES
    • Above ground CAES
underground ceas
Underground CEAS
  • Potential for large scale energy storage –

100 to 300 MW for 10 – 20


  • Effective in performing load management, peak shaving, regulation and ramping duty.
  • Less capital cost compared to other large scale energy storage options.

Main components of underground CAES

challenges associated with underground caes
Challenges associated with Underground CAES
  • Identification of suitable site for setting up a underground facility.
  • Optimizing the compression process to reduce the compression work required.
  • Thermal management – efficiently extracting, storing and reusing the available heat of compression, thus improving the efficiency of the system.
  • Understanding the effect of cyclic loading and unloading on the structural integrity of the underground cavern.
deep caes
  • Deep compressed air energy storage is an underground CAES facility where the cavern is formed at depths of >4000 ft. as against 1000-2000 ft. in case of conventional facilities.
  • The main advantage of going deep are,
    • Maximum permissible operating pressure of a cavern increases

with depth.

- A good approximation will be 0. 75 to 1.13 psi/ft. based on the local geology.

    • Hence going deep helps store air at higher pressures in much

smaller cavern volume, hence higher energy density.

The possibility of setting up a deep compressed air energy storage facility in Eastern Colorado is being currently investigated by Energy Storage Research group at CU, Boulder.

challenges associated with deep caes
Challenges associated with Deep CAES

Deep CAES brings in additional challenges, which are

  • Utilizing the high pressure compressed air effectively.

Most of the off-the-self gas turbines operate in the range of 70-100 bars, hence it is necessary to design the system such that high pressures can be utilized.

    • Understanding the effect of high pressure & temperature on the cavern structure.
    • Identifying suitable equipment's / material to operate at high pressure .
    • Potential for leakage through faults.
criteria for site selection
Criteria for Site Selection

1. Tight Cavern

2. Adequate natural gas

3. Ability to withstand 600 to 1200psi for conventional &

2000 – 5000 psi for deep CAES.

4. Proximity to Wind or Load to minimize transmission

line losses.

5. Appropriate geology

6. A report by Cohn et al. 1991 “Applications of air saturation to integrated coal gasification/CAES power plants. ASME 91-JPGC-GT-2 says that this can be found in 85% of the US.

possible geologies
Possible Geologies

1. Abandoned Natural Gas fields.

2. Old Mines

3. Dome Aquifers

4. Porous Sandstone

4. Salt Domes

5. Bedded Salt

why salt beds domes
Why Salt Beds / Domes?

Salt beds are more desirable for setting up new Caverns because of the following reasons,

  • Easy to be solution mined
  • Salt has good Elasto-plastic properties resulting in minimal risk of air leakage
  • Salt deposits are widespread in many of the subsurface basins of the continental US, including western states (Colorado, West Texas, Utah, North Dakota, Kansas)
salt beds in eastern colorado
Salt Beds In Eastern Colorado
  • 1. Salt beds from 4100 ft to 6,800 ft.
  • 2.Thickness from 3 to 292 ft.
  • 3. Required Operating pressures in the range of 4000 to 7000 psi.
  • 6 At about 6,000 psi we need about 14,400 cubic meters per gigawatt hour energy storage or a cavern of about 30 x22 x 22 meters
need for thermal management
Need for thermal management
  • When air is compressed - up to 85% of the energy supplied is lost in the from of heat.
  • Even with isothermal compression 50% of the energy may be lost as heat

Polytrophic compression

Figure showing the fraction of work stored in compressed air Vs. the pressure. Rest dissipated as heat.

  • Storing and re-using the heat of compression would result in increasing the overall efficiency of the CAES system and result in reduced or no fuel consumption.
isothermal caes
Isothermal CAES
  • Another approach to keep the temperature constant during compression is to slow down the pumping process (As it results in efficient heat dissipation thus constant temperature).
  • Such a system can be used for small scale applications.

Isothermal CAES developed by SustainX

The SustainX system operates at 0 to 3000 Psi and provides 1 MW for 4 hours at an expected efficiency of 70%.

recent developments in aa caes
Recent developments in AA - CAES

RWE group, Germany in collaboration with GE are developing an AA-CAES project. (Started 2010)

They propose no fuel operation with a target efficiency of 70%


Feasibility study has shown that such high efficiencies can be achieved by system optimization and suitable equipment development.


R&D is being carried out to develop Turbomachinary & Thermal energy storage to achieve the above goals.

storing re use of compression heat
Storing & Re-use of compression heat
  • Heat of compression can be stored in two ways
    • With the help of thermal energy storage facility
    • By storing the heat in compressed air itself
    • There are two options for utilizing the stored energy
    • Using the stored heat + Fuel for preheating the air
    • Only utilizing the stored heat (No Fuel) also called as Advanced Adiabatic CAES (AA-CAES).
thermal time constants
Thermal Time Constants
  • Thermal time constants vary with the surface to volume ratio.
  • For a Sphere
  • For a Cylinder
  • For a cube
  • For a rectangle
major cavern design parameters
Major Cavern Design parameters
  • Cavern geometry & volume
  • Depth of the cavern – as the overburden pressure increases with depth
  • Cavern Minimum operating pressure – as inside pressure of the cavern acts as a static lining to the cavern contour
  • Cavern maximum operating pressure – must be fixed to avoid gas infiltration and cracking of the surrounding rock mass
  • Cavern operation pattern
  • Distance between adjoining caverns
cavern operating pressures
Cavern operating pressures

Operating pressure of the cavern depends on the,

  • Depth of the cavern
  • The in-situ stresses in the surrounding rock formation.
  • The maximum operating pressure of the above ground equipment.
effect of cyclic loading on cavern
Effect of cyclic loading on cavern
  • Increase in cavern inside pressure causes increase in deviatoric stresses, this in turn results in increase in creep rate.
  • Its has been found by laboratory experiments that the overall creep rate decreases in case of cyclic loading, thus resulting in reduced convergence – good for CAES. But the stresses in the rock mass increases.
  • Charging & discharging of cavern is associated with rise and fall in temperature inside the cavern as well as the rock surrounding it. Heating of rock salt creates thermal induced compressive stresses, cooling of rock salt creates thermal induced tensile stresses.

*Results of experiments conducted by University of Technology, Clausthal-Zellerfeld, Germany

effect of cyclic loading on cavern1
Effect of cyclic loading on cavern

Transient effect of cyclic stresses on the salt cavern (cycle period – 5 days)

The graph shows the reducing creep rate & increase in stress with time.

*Results of experiments conducted by University of Technology, Clausthal-Zellerfeld, Germany

effect of cyclic loading on cavern2
Effect of cyclic loading on Cavern

Survey of the Huntorf cavern contour conducted in 1984 & 2001 show negligible convergence of the cavern in spite of continuous cyclic operation.

Change in contours of the Huntorf Caverns between 1984 & 2001

Visualization of thermally induced cracks in salt rock


Effect of cyclic loading on Cavern

Further work needed:

  • Understanding the thermo-mechanical effects (convergence & creep) on surround rock at high pressures & temperature.
  • Effect of different charging and discharging periods / operation patterns
  • Effect of having a deep cavern at atmospheric pressure for maintenance work.
system integration
System Integration

One of the suitable configurations to utilize the maximum available pressure


1. Costs

A. A little more than conventional gas fired generators at $4oo to $500/kW

B. CAES estimates at $600 to $700/kW (Note these numbers could be low depending on the site etc)

C. Low Operating Costs

2. Value

A. Smooth out wind fluctuations

B. Match to transmission line limits.

C. Match to loads increasing capacity factor.


2. Value

D. Absorb Energy when the wind power exceeds transmission or load. This is in contrast to gas fired generators

E. Arbitrage , buy wind or other energy low and sell high.

F. Ancillary services , frequency control, black start etc.

G. Reduced natural gas consumption by approximately two thirds.

  • Factors effecting CAES capital cost
    • CAES site selection

Depth of Cavern

Local geology

Proximity to transmission network

Availability of Natural gas

    • Presence of Thermal energy storage.
    • Factors effecting CAES Operating cost
    • Cost of off peak energy and/or Wind energy generation cost.
    • Natural gas requirement based on TES availability