Alternative energy sources (part 2)

Question 1

Give 6 advantages of supercapacitors over batteries

Answer 1

+ High reliability and efficiency
+ No maintenance
+ Durable
+ Lasts for millions of charge cycles
+ Work in wide range of environments (temp, humidity)
+ Environmentally friendly, easily recyclable

Question 2

Give 2 disadvantages of supercapacitors

Answer 2

• Solvent in electrolyte degrades in 5-6 years (regardless of no. of cycles)
• High self-discharge rate

Question 3

What is SMES

Answer 3

Superconducting Magnetic Energy Storage. Use a superconducting magnet with a coil around it in a supercooled container. When the voltage is removed current will continue to flow since it is a superconducting material.
Energy is stored in the mag field.

Question 4

What are 6 advantages of SMES

Answer 4

\+ High efficiency (>90%)
\+ High capacity (20 MWh)
\+ High cycle rate
\+ Fast response time
\+ Long lifetime (~20 years)
\+ No moving parts

Question 5

Give 3 disadvantages of SMES

Answer 5

• High cost (main one)
• Low storage time (milliseconds to minutes)
• High maintenance

Question 6

Give 4 main advantages of PHS

Answer 6

+ Huge capacity (Largest are 1.5 GWh)
+ Long lifetime
+ Unlimited cycle stability
+ Fairly fast response time

Question 7

Give 3 disadvantages of PHS

Answer 7

• Cost (large initial cost)
• Needs the right topographical conditions, large land use
• Takes years to build

Question 8

How does the general operation of CAES work

Answer 8

Electricity is used to compress air into a chamber. When required, the air is mixed with natural gas and burnt and expanded in a modified gas turbine

Question 9

What are the 4 advantages of CAES

Answer 9

+ Very high capacity (largest are ~290MW)
+ Long lifetime
+ Unlimited cycle stability of the installation
+ Fairly fast response time

Question 10

Give 4 disadvantages of CAES

Answer 10

• Cost (very high initially)
• Low round trip efficiency (until Adiabatic process can be done)
• Depends on topography, and large land use
• Construction takes years

Question 11

Compare diabatic and adiabatic

Answer 11

Diabatic has low efficiency (<50%) but needs no extraneous power at startup.
Adiabatic has high efficiency (theoretical 100%, 70% in practice) but has not been properly tried and tested yet

Question 12

How does an adiabatic CAES system work

Answer 12

The thermal energy is captured and stored in some material (concrete, stone, hot oil, molten salt solution are all possible). Adiabatic is new and not really used properly yet

Question 13

What conditions give you the maximum energy storage capacity for a flywheel?

Answer 13

• Use a material with low density
• High tensile strength
• EITHER volume energy density OR mass energy density should be prioritised

Question 14

Compare high speed and low speed flywheels

Answer 14

Low speed flywheels:

• Well established tech
• Cheaper to build

High speed:

• Needed for most modern applications
• Maximise energy density using a small disc with a constant stress profile, built with low density and high strength materials