CIVE40010 Energy & Environmental Engineering Flashcards
which countries are currently produce the most amount of nuclear energy ?
- USA
- France
- China
- Russia
- South Korea
what percentage of UK energy need is currently supplied by nuclear ?
17 %
what is a PWR ?
nuclear
pressurised-water reactor
- pure water heated to very high temp. through fission
- kept under v. high pressure
- converted to steam by a steam generator
- turbines -> generators -> electrical power
describe nuclear fission
- neutron induced process
- Nucleus fragments into two or more smaller
elements (fission products), generating heat (KE -> heat through collisions with fuel lattice) and other neutrons, - Only happens for large nuclei (eg Uranium, Plutonium, etc) known as fissile (particular isotopes only)
- chain reaction (fission event produces servral neutrons), however likelihood of chain reaction depends on neutron velocity and the fuel content (fission cross-section).
what is the fuel for nuclear fission reactors ?
usually, uranium (either U235 or U238 isotope)
how do you increase likelihood of nuclear fission chain reactions ?
- increase amount of fuel, enrichment (~4%)
- slow down neutrons, moderation
two types of nuclear fission reactors ?
Thermal Reactors (most used)
- high velocity neutrons slowed into thermal region using a moderator material (eg, graphite, light water H20, heavy water D20)
Fast Reactors
- no moderation required, use of enrichment
How is nuclear fission rate described ?
Effective neutron multiplication factor
(keff): how many neutrons from one
fission, cause another?
* keff > 1 fission rate increases exponentially
* keff < 1 fission rate decays
* keff = 1 criticality – the operating state of a
power reactor.
Describe the structure of a fission reactor?
- uranium contained as uranium dioxide (UO2) stacked in fuel rods and installed in fuel assemblies which form the reactor core
- control rods (material that absorbs nuetrons so keff<1) to stop reaction when required)
- enclosed in reactor pressure vessel
- pump coolant in (cold) and comes out hotter (in PWR, water)
what are the three sources of nuclear waste ?
mining & fuel fabrication
radioactive waste
storage, GDF
what are the five main reasons for waste arising ?
nuclear
- mining and milling of uranium ores
- spent fuel discharged from reactors
- reprocessing of fuel
- Decommissioning
- Military
- Medical and engineering applications
what are the three categories of waste ?
- LLW (low level waste) 91% radioactive waste - discarded equipment, tools, protective clothing
- ILW (intermediate level waste) 9% radioactive waste- stripped/leached remains of cladding or PCM
- HLW (high level waste) 0.1% radioactive waste - fission products
depending on radiation levels
(note : uk classification has vllw (very low level waste))
describe radioactive waste management
HLW requires immobilisation followed by** storage underground**, methods include :
Vitrification in glass
- Commonly borosilicate glass
- Some components are immiscible in glass
Encapsulation in ceramics
- SYNROC is the most common (composite)
- Fluorite, Pyrochlore , Phosphate
Mixtures of glass and ceramic - composite
what are key functions of deep geological nuclear disposal ?
1) Isolate waste from near-surface
processes and human activities,
2) Protect the biosphere,
3) Limit release from progressively
degrading waste package,
4) Disperse and dilute flux of longlived radionuclides.
what are different fuel types ?
nuclear
- Natural uranium metal
- Uranium dioxide
- Thorium, other
note : PWR uses uranium oxide, thermal neutron cycle and light water as coolant
what types of reactors are there ?
nuclear
- PWR pressurised light water reactor (high safety, loss of coolant = loss of moderator = reaction shut down)
- AGR advanced gas cooled reactor ( uranium dioxide fuel with graphite core as moderator, using gas as coolant )
- BWR boiling water reactor ( lower pressures, cheaper, no steam generator)
- PHWR pressurised heavy water reactor
currently all use thermal neutron cycle with moderator
future tech :
- advanced reator (Gen IV), using closed fuel cycles (‘fast reactors’)
- small modular reactors
describe difference between closed & open fuel cycles
nuclear
open : fuel used once & discarded
modified open : part separation of actinides and fuel reprocessing
closed (full recycle) : full separation of actinides from fuel, therefore waste only contains fission products
half-life significantly reduced for actinide separation
what is ccs ?
CO2 can be captured
from a process or the
atmosphere and
permanently stored
geologically
how much ccs do we ideally need ?
To meet COP21 targets, we
need to capture a total of ~10 Gt
CO2 per year.
~2000x 5 MMtpa projects?
(currently at about 20 plants)
what are the ccs categories
Post-combustion capture
– Separate CO2 from gas mixture after combustion
Pre-combustion capture
– Separate CO2 from the production of H2, combust the H2
Oxy-fuel combustion
– Combust fuel in O2, no further separation needed
what is a TRL
technology readiness levels
- must advance through series of scale-up steps
- TRL 9 is ‘ready to go’
- from initial research to commercial readiness
Describe post-combustion capture methodology ? (inc. advantages & disadvantages)
amine-based chemical absorption
- uses 2x columns solvent regeneration & co2 scrubbing columns
- outputs compressed CO2 for storage
advantages :
* Retrofittable
* Mature technology
* Experience with ‘large-scale’ in the O&G industry
* Flexibility – range of operating conditions
disadvantages :
* High CapEx – large gas volumes → large equipment
* Parasitic energy (uses ~ 20% energy output)
* Solvent disposal
describe amine degradation ?
Solvent susceptible to chemical degradation in O2, SO2, CO2, high temperatures
Solvent losses to environment – vapourisation, entrainment
Degradation products could present health risks
– Amides, aldehydes, nitrosamines, nitroamines
Release mitigation measures
* Reclaimers to reverse thermal degradation (more energy required)
* Water wash and demisters to minimise entrainment
* Waste management practices – treatment/disposal
what is adsorption ?
another method of post-combustion capture
* Adhesion of species/molecules to a solid surface
* Governed by electrostatic interactions
* Porous solids are used in a cyclic process to separate
gas mixtures
– Pores give more surface area per unit volume
– Enables re-use of the adsorbent
Adsorption is proportional to pressure/concentration
– Reducing the pressure reverses the process
– Pressure/vacuum-swing adsorption (PSA, VSA, PVSA)
* Adsorption is exothermic
– Heating the adsorbent reverses the process
– Temperature-swing adsorption (TSA) (much faster, less co2 recovery)
* Solids (beads) are traditionally packed in a vessel to contact with gas
– Pressure drop limits gas throughput per vessel
explain pre combustion co2 capture
- React fuel with steam & small amount of O2
– Produces ‘syngas’ – CO2, CO, CH4, H2
– Syngas further reacted to produce CO2 and H2 - CO2 is separated from H2
- H2 is used as a fuel
- First part of process is identical to conventional H2 production process
‘hydrogen production’ , complex
options:
– Pressure swing adsorption
– Cryogenic separation
– Physical absorption
– Chemical absorption
advantages/disadvantages of pre-combustion carbon capture
Advantages
* Uses processes that are already commercially used
* Lower energy penalty than post-combustion capture
* Overall process can be very efficient (60 – 65%)
Disadvantages
* High CapEx
* Complex process – low flexibility
* No commercial scale demonstration of pre-combustion capture for power generation
describe oxy-fuel combustion
- O2 separated from air
- Mixture of O2 and CO2 used to combust fuel
- No further separation of CO2
required
what factors affect the cost of CCS
Location in the world
* Available resources (e.g. land, water)
Technology:
* Brownfield vs greenfield
* Technology maturity
Labour:
* Rates
* Unionised
Commercial:
* Risks
* Contingencies
* Warranties and insurances
* Price of CO2
Capture process:
* Technology choices
* Chemicals and fuel cost
Greatest impacts :
Transport:
* Mode of transport
* Route distance
* Flow rate through pipeline
* Pressures
Storage:
* “Finding costs” / exploration
* Capacity
* Injectivity
* Containment
however, costs will reduce over time as more plants are buit
describe different greenhouse gas removal technologies (GGR)
Bioenergy with carbon capture & storage (BECCS)
Direct air capture (DACCS)]
lots of options but limited readily available options
explain Bioenergy with CCS (BECCS)
generating energy from biomass (by combustion) and capturing and storing the carbon dioxide (CO2) emissions
Feasible technology
* Social aspects
* Life cycle analysis
* Plenty of BE examples, no BECCS examples
explain Direct air capture (DAC) with CCS (DACCS)
- Adsorption based (Climeworks, Global Thermostat)
- Absorption based (Carbon Engineering)
- Technically feasible
- Economically questionable
- Socially acceptable
- No large scale demonstrations
what barriers are there to deployment of ccs
Policy and economics
* System integration and operation
- public funding required
what is a band gap
solar energy
energy to excite an electron from a
valence to a conduction band (electrical current!)
how does a solar photovoltaic work
Incident sunlight – provides energy (photons) for
electrons to cross band gap
what is the limitation of solar energy
semiconductors
only about ~35% can be harvested,
~47% excess & ~ 18% not of the right wavelength
loss causes : not in correct bandgap, thermal losses, relaxation to band edges
Shockley & Queisser Limit
what is the most common type of solar cell
crystalline silicon
- moderate efficiency
- high embodied energy
- relatively high production cost
explain recent developments in solar cells
thin-film technology – “2nd generation” based on silicon wafer
- lower efficiency
- low cost
- low material usage
- low embeded energy
multijunction cells – utilize different parts of the solar
spectrum – “3rd generation”
- higher efficiency
- aim of being more cost effective
- low material usage
- low embodied energy
-
what is concentrated solar power ? (csp)
- use mirrors to reflect and concentrate sunlight onto a receiver
- generate electricity by converting energy from sunlight to power a turbine
- limited by rate of heat collection, absorbity & emissivity & geometry, convection
explain thermal energy storage
- relatively easy and cheap, at range of scales
- Concentrating solar power with storage complementary to other
technologies - dispatchable (on demand)
- storage capacities can provide continuous, “baseload” supply
- coupling solar power with storage led to reduction in LCOE (cost advantage)
different types of marine renewables ?
Wave Energy
* Tidal Energy (Tidal Range vs Tidal Stream)
* Offshore Wind Energy
* Ocean Thermal Energy
What is LCOE
Levelized Cost of Energy (LCoE)
- standard way in which industries
and governments compare the cost of different energy systems
- the lowest price that consumers need to pay for the electricity generated to break even at the end of the project lifetime
( operational - capital ) expenditure / energy produced in design lifetime
define ocean waves
main characteristics of a liner ocean wave ?
a wave theory (simplest)
1. Direction of propagation
2. Crest
3. Trough
4. Wavelength
5. Particle motion
6. Negligible motion (water depth larger than the wavelength)
the vertical distance from the crest to the trough is the wave height.
The wave amplitude is defined as half of the wave height
how does wave power vary with distance from shore ?
Near shore: lower values (depending on bottom slope, etc.)
- Deep water: 6 – 70kW/m
- (It is considered that a minimum wave energy flux of 20-25 kW/m is
required for wave energy extraction to be economically viable at a site)
how do seasonal variations affect wave power
has important consequences for WEC design, since we need to
ensure that the WEC will survive the most energetic sea states while keeping
the cost of energy as low as possible
- much larger in the Northern Hemisphere than in the
Southern Hemisphere
how is wave energy absorbed ?
A Wave Energy Converter (WEC) is a device that absorbs part of the
ambient incident wave energy
- incorporates a mechanical system commonly called Power Take-Off
(PTO) that transforms mechanical energy into electricity
- Wave energy absorption can be regarded as a wave interference process, whereby the waves generated by the motion of the WEC cancel incident waves
what is the relationship in wave energy conversion between natural & applied frequency ?
In wave energy conversion the objective is exactly the opposite, i.e. wave
energy absorption is maximized when the WEC operates at resonance
what is the principal issue with wave energy converter technologies
no design convergance
(hence why LCOE is so relatively high)
what are the 3 categories of wave energy converters
- Oscillating water columns (fixed, floating)
- Oscillating bodies (heaving, pitching-floating or submerged)
- Overtopping devices (fixed, floating)
define tide
periodic vertical rise and fall of water in the oceans
define tidal range
The difference in sea level between high and low tide
(Electricity generation from tidal range capitalizes on the artificial height
differential of two bodies of water created by a dam or barrier)
higher tidal range = more energy generation
describe terminology for tidal flow
tidal current (or tidal stream)
incoming (flood) or outgoing (ebb) horizontal
flow of water
how are tides generated
Tides are generated by the action of the Moon and the Sun, in combination
with the rotation of the Earth on its axis.
The oceans are acted upon by two forces of opposite signs:
* The gravitational force (variable)
* The centrifugal force (constant)
what is the semi-diurnal cycle
existence of two low-tides and two high-tides per day
- caused by the rotation of the Earth along its own
rotation axis (not the rotation of the moon around the Earth!)
- in reality, the two
daily high-tides are not identical.
what’s the difference between the spring & neap tide ?
Spring tide: the Moon and the Sun are aligned
Neap tide: the Moon and the Sun are perpendicular
- cycle of neap/spring tides has a period of 14 days
define a tidal stream
an incoming (flood) or outgoing (ebb) horizontal
flow of water
- In nearshore locations tidal currents are accelerated around headlands or
through constrictions such as channels between islands
what is the difference between tidal range and tidal stream/current energy generation
tidal range : gravitational potential energy difference between the two bodies of
water is used to drive a hydroelectric turbine
tidal stream : Electricity generation from tidal currents capitalizes on the kinetic energy of the free flowing water, using sub-surface turbines
describe the different types of tidal range technologies
power plants normally take two forms
- tidal barrage (spans entire river width)
- tidal lagoon (encloses an area of coastline with a high
tidal range behind a breakwater)
their main components include :
- sluice gates
- bulb turbines
- impoundment
what are environmental effects of tidal energy schemes
- disturbance to marine ecosystems
- co-existance with other industries, e.g. fishing, boating
- sediment/sediment transport
- changing existing tidal currants
- water quality
what are first/second generation tidal generation devices
Based on the location on the water column, tidal stream devices are
classified as first or second generation
explain first gen tidal generation devices
First generation support structures for horizontal-axis tidal turbines
* Resist sliding due to turbine drag force
* Resist bending moment due to vertical lever arm
In order to resist these forces, conservative design approaches have been taken :
- expensive due to being huge structures
- limited to shallow sites
explain second generation tidal devices
Second generation support structures
* Use buoyancy modules to support the turbines
* Use mooring systems that operate in tension rather than compression
* They can be installed at an optimal position in the water column
what are the main advantages of tidal energy ?
- predicatable (tidal patterns)
- out of sight
- spatially effitient (not displacing other land uses)
- can combine infrastructure
