Power Generation from Renewable Energy Sources

Fall 2012Instructor: Xiaodong Chu

Email ： chuxd@sdu.edu.cnOffice Tel.: 81696127

Flashbacks of Last Lecture

• Systems that make use of solar energy by first concentrating solar radiation and then converting it to thermal energy

• Uses:– Electricity (solar thermal power)– Industrial process heat– Absorption cooling– Chemical processes

Why Energy Storage?

• Increase operational stability • Reduce intermittence• Increase plant utilization and capacity

factor• Provides “peak-shaving” ability (time-

shifted operation)• Reduce generation cost (as long as

storage is cheaper than increasing rated power)

Why Energy Storage

• Energy is stored to use it at a different time than when it was generated• The process of converting the energy to storable form means that some

energy is lost due to inefficiency and heat• Additional energy is lost when the energy is released or recovered due to

a second inefficiency• Ideally, storage is avoided to have a more efficient process• Time-of-day metering is likely in the future as metering becomes

electronic and inexpensive (like a thermostat)• Shifting the energy from usage peaks to low-use times helps the utility,

and customers would be rewarded by lower charges

• Renewable energy is often intermittent (like wind and sun), and storage allows use at a convenient time

• Compressed air, flywheels, weight-shifting (pumped water storage) are developing technologies

• Batteries are traditional for small systems and electric vehicles; grid storage is a financial alternative

• Energy may be stored financially as credits in the electrical grid

Why Energy Storage

Potential Values of Energy Storage

• “Capturable” in existing markets– Energy Arbitrage– Capacity– Frequency regulation– Spinning and non-spinning reserve

• Less-Capturable / Regulated Services – Black start– T&D deferral– All of the above in regulated service territories

Storage in Today’s Grid

• Can provide a variety of services• Proper valuation must consider multiple services

(energy + capacity)• Still economic, technical, regulatory & institutional

impediments to larger deployment• As with any potential solution it needs to meet

investment criteria and beat the competition• Storage is useful but not a required component of the

grid

Storage in the Near Future Grid

• Applications still dominated by whole grid applications• RE begins to have an impact

– Increase in A/S requirements– Increase in short-term price volatility– More hours of low-cost energy

• Storage increasingly attractive but not yet a required component of the grid

Supply Side Storage Technologies

?

CSP Thermal

Three Classes of Electricity Storage Devices

• Short-term storage for ancillary services• Distributed storage for local applications• Bulk energy storage

Short-term Devices (30 min or less)

• Devices that can provide frequency regulation and spinning reserve

• Flywheels, batteries & capacitors

Beacon Flywheel for Frequency Regulation

Distributed Storage (<10 MW or so)

• Provide both capacity and energy services• Local T&D appears to be a primary application• Primarily batteries

– Flow batteries– NaS– Other battery chemistries?

Bulk Energy Storage

(Courtesy of TVA)

Compressed Air Energy Storage (CAES)

Limited growth opportunities for pumped storage

End Use Storage

• Thermal Storage– Hot or Cold– Dispatchable– Low Cost

• Electric Transportation and PHEVs

Storage in the Future Grid • Inherent conflict between competing sources of low-carbon

energy– Wind and solar need flexible generation– IGCC and nuclear reduces system flexibility

• Flexibility will have to come from a variety of sources on both the supply and the demand side

• Storage is one of a large number of enabling technologies that can increase the utilization of RE generation

• Still unclear at what point storage becomes the best option, but its probably less than 100%!

Thermal energy storage

• A fraction of the thermal energy produced at the solar field is stored, increasing the internal energy of the storage medium– Sensible heat– Latent heat– Thermochemical

Types of thermal storage

• By utilization– Short term

• Provide operational stability

– Medium term• Increase capacity factor• Shift electrical generation hours

• By type– Direct (same substance as working fluid, does

not require heat exchange)– Indirect (different substance, requires heat

exchange)

Technical Requirements for thermal energy storage materials

• High energy density (per-unit mass or per-unit volume)

• Good heat conductivity• Good heat transfer between heat transfer fluid

(HTF) and the storage medium• Mechanical and chemical stability• Chemical compatibility between HTF, heat

exchanger and/or storage medium• Reversibility for a large number of

charging/discharging cycles• Low thermal losses• Easy to control

Thermal storage past experiences

thermal energy storage – STP commercial installations

• Short term: pressurized water– PS10 and PS20

• Mid term: Molten salt, 2 tank– Direct (CRS) – Gema Solar (Solar Tres)– Indirect (PT) – Andasol I

Short term thermal energy storage

• Pressurized water• Sliding pressure during

discharge• Pressure vessel

PS10 / PS20

PS10 thermal energy storage main

characteristics– Max. pressure: 40 bar– Thermal capacity: 20 MWh

(50 min at 50% load)– Total Volume: 600 m3– 4 tanks, sequentially

operated

Molten salt storage, 2 tank, direct

• Thermal capacity proportional to ΔT

• Hot – cold tank design• Commercial (salt widely

used in process industry)– High operation T

limited (salt decomposition)

– Need for heat – tracing (risk of freezing)

– Costly equipment (pumps, valves…)

Solar Two (Barstow, CA)

Molten salt storage, 2 tank, indirect

– Intermediate oil-to-salt heat exchange

– Freezing Temp = 220 oC

– High temp. limited by HTF

– Large volumes– Higher investment

costs

• Provides high storage capacity to PT power plants (Thermal oil)

Andasol (Granada, Spain)

Andasol storage -Technical characteristics

• Type: 2-Tank Molten Salt Storage• Fluid: Nitrate salt mixture

(60% NaNO3 and 40% KNO3)• Melting Point: 223°C • Storage Capacity: 1,010 MWh

(~7.5 hrs full load operation)• Storage Tank Size: 14 m height 37 m

diameter • Salt Mass: 27,500 tons • Flow Rate: 953 kg/s• Cold Tank Temperature: 292° C• Hot Tank Temperature: 386°C

ANDASOL, Molten salt 2-tank thermal energy storage

Thermal energy storage – advanced experiences

Thermal energy storage – advanced experiences

Thermal energy storage costs and benefits

• Improves plant controlability and operability, expanding the range of possible operating strategies

• Facilitate thermal energy storage dispatchability• If adequately designed, can improve

– The efficiency of the plant– The profitability of the project

• Extends lifetime of equipment (reduces the number of strat-stop cycles)

• Increases investment– Oversized solar field– Tanks, heat exchange, molten salt management equipment,

heat tracing, safety

• Increases O&M costs

Electrochemical Batteries

• Batteries (groups; from artillery guns) of cells are used separately or in a case containing several cells; a 12V car battery has six 2V cells inside the case

• Large batteries are often use separate 2V cells placed next to each other in a rectangle

• Various cell chemistries are used– Lead-acid; Nickel-cadmium; Lithium– Nickel-metal hydride– Zinc-air

• Best suited to storage periods of 1 second to 60 days • Self-discharge and sulphation occur with time• Desulphator circuits can reduce sulfates for longer life

Flow Batteries

• Flow batteries use pumped electrolytes that move outside of the battery case– Polysulfide Bromide (PSB), Vanadium Redox (VRB), Zinc

Bromine (ZnBr), and Hydrogen Bromine (H-Br) batteries are examples

• A “filling station” could exchange spent electrolyte for new “charged” electrolyte

• The power and energy ratings are thus independent since the power is from the battery electrodes while the electrolyte may be replaced periodically

Flywheels

• Flywheels store energy as angular momentum• Best suited to storage periods of 1 second to 10 minutes • High temperature superconducting bearings reduce

bearing friction to 2% of speed drop per day• Ball bearings are so inexpensive that the performance

gains of magnetic bearings are irrelevant• The flywheel case is designed with a shield to contain a

failed rotor and its pieces if it shatters and blows up• Batteries are much cheaper than flywheel systems• Test buses used flywheels that were spun up by electricity

at bus stops; no wires along streets

Flywheels & Trains

• This trackside flywheel system provides stabilization of voltages on the track system by being both motor and generator

• Similar types are used to stabilize renewable energy outputs

• Buses have been operated that use flywheels charged by electricity at the bus stops, thus avoiding the cost of overhead trolley wires

Ultracapacitors

• Ultracapacitors are very high capacitance units • Best suited to storage periods of 0.1 second to 10

seconds • Stored energy is 0.5 C V2

• Capacitances now reach 2.7 kF (kilofarad)• Carbon electrode surface areas 1000m2 to 2000m2 per

gram provide high capacitance• Electrolytes are sulfuric acid or potassium hydroxide

Hydro Pumped Storage

• Special turbines can run either to spin an alternator or to act as a pump

• This reversibility allows excess electrical energy to be used to pump water to a higher storage reservoir to be used as an energy source later

• Since 2.31 ft of elevation has a bottom pressure of one pound per square inch (psi), a head height of 200 ft is equivalent to 86 psi

• Worldwide, pumped hydro is about 90GW, ~3% of total storage, the most widespread high-energy storage technique

Compressed Air Pumped Storage

• "The world's first compressed air energy storage plant was in Germany," Lee Davis (plant manager for the Compressed Air Energy Storage (CAES) Power Plant in McIntosh, Alabama). "The Alabama CAES plant was the first in the United States when it opened in 1991.“

• Electrical motors compress air to 1078 psi within underground salt caverns (100 MW); heat is lost in the cavern

• On release, natural gas is burned to heat the air again, which then passes through a turbine, spinning an alternator (326 MWe)

• The Norton Energy Company plans a similar site using an abandoned limestone mine 35 miles south of Cleveland, Ohio

Compressed Air Energy Storage

                           

          

Compressed H2 and NG Storage

• Hydrogen is normally stored in 8-inch tubes and tanks– H2 pressures range from 2000 to

10,000 psi– Nickel-metal hydride is a solid

pellet or powder storage

• CNG or compressed natural gas is stored at 3000 psi

Liquid Air Energy Storage

• Mitsubishi Heavy Industries is developing LASE (Liquid Air Storage Energy)

• The system makes liquid air at nights and weekends for vaporization and electricity generation

• The turbine is based upon a rocket motor pump• This load-shifting provides the economic incentive to use

the system• Could also be done with liquid nitrogen storage

Superconductors

• Since a superconductor has essentially zero resistance, a current once started will flow “forever”

• At a later time, energy could be extracted from the superconductor

• Since the superconductors must be kept far below usual air temperature (~20K to 80K), energy must be used to compress the gas and make it liquefy

• Newer superconductors are being investigated to find ones with a higher critical temperature near room temperature

Superconductor Example

• A current is induced in the superconductor toroid by inserting a magnet briefly

• Once replaced in the liquid nitrogen, the current circulation can be detected by a compass

• Current decay is on the order of 50% in 1020 years