Prof. beck 'renewable energy- a short (and cynical) approach
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Transcript of Prof. beck 'renewable energy- a short (and cynical) approach
The total energy in a system may comprise all or a combination of the following:
potential energy - mgh
kinetic energy - ½ mv2
static pressure - pressure x volume (pV)internal energy - uheat - energy transfer due to temp differencework - integral force with distance
They all have units of Joules (J)
Types of Energy
HEAT
Heat is the form of energy that is transferredbetween two systems (or a system and itssurroundings) by virtue of a temperaturedifference.
The amount of heat transferred into a givensystem during the process between two statesis denoted by the symbol Q ( kJ).
WORK
Work is the energy transfer associated with a forceacting through a distance
The amount of work done by a system is known asW (kJ)
The base unit of work in the SI system is theNewton metre (Nm)
1 Nm is also known as 1 joule (J)(Due to the scale involved in the real world, the kJ (1000 J)is usually used)
As these are all the energies that we mustconsider, we can change between them.
The energy between the beginning and end, andthe heat and work can be added up:
Energy = Heat in - Work out= change in Potential Energy +
change in Kinetic Energy + change in Internal Energy +
change in (Pressure Volume)OR
11221212
2
1
2
2 )()(2
1 VpVpuumzzmgCCmWQ
The first Law of Thermodynamic
The second law of
thermodynamics.Imagine a flywheel spinning in an insulated
box that is filled with gas.
Flywheel
State A: Flywheel
spinning, gas cool
State B: Flywheel
Stationary, gas
warm
Flywheel
Work into heat, or heat into
work?Is it better to heat something up with a spinning
flywheel or with a gas heater? Producing the
work to get the flywheel going is difficult.
Flywheel
Gas
Heater
Some Equations for Entropy (i)There is entropy increase associated with
heat transfer, and friction, but not with work.
Entropy decreases when a system is cooled
Entropy increases when heat is added to a system.
Thus, entropy increase implies either Heat Input or an
Irreversible Process (due to friction).
The total entropy change is thus made out of the
entropy change due to the irreversibility and that due
to the heat transfer:
S SQ
Tirrev
Some Equations for Entropy (ii)There is also entropy increase associated
Disorder
Boltzmann
where k = 1.38×10−23 J K−1 and is Boltzmann's
constant and W is the frequency of occurrence of a
macrostate, the number of (unobservable) ways the
(observable) thermodynamic state of a system can be
realized by assigning different positions and momenta
to the various molecules. In other works, the
complexity of the system.
WkS log
Let’s see what it means
There is a lot of information going around
about renewable energy, putting in a few
numbers helps us to understand what it really
means.
Here are a few examples.
Wind•Basically:Expected energy output per year can
be reliably calculated when the wind
turbine's capacity factor at a given
average annual wind speed is known.
The capacity factor is simply the wind
turbine's actual energy output for the
year divided by the energy output if the
machine operated at its rated power
output for the entire year. A reasonable
capacity factor would be 0.25 to 0.30. A
very good capacity factor would be
0.40.
3
21 vAW
http://www.awea.org/faq/basicen.html
Do the Cameron
• B and Q Windsave Wind
Turbine System -
WS1000PS T2 £1498 1KW
at 12.5 m/s (30MPH)
• Expected Safe Life:10 years
(depended upon actual
conditions the system has
been subjected to)
http://www.diy.com/diy/jsp/bq/nav/nav.jsp?action=detail&fh_secondid=9330400&fh_
search=wind&fh_eds=%c3%9f&fh_refview=search&ts=1174393542806
Dear editor
• Mine was installed on 27th November, and after
10 weeks it has produced only 48 kW despite
being mounted on the gable end apex of my
house with uninterrupted winds from the SW.
The variable low frequency drone can be heard
throughout the house. In gusty conditions the
stair rods rattle. I have estimated my pay-back
time at around 50 years. Needless to say, I will
be contacting Windsave for an explanation.
http://www.bettergeneration.co.uk/wind-turbine-models/the-windsave-ws1000.html
Financial benefits?
http://www.masterresource.org/2012/08/microwind-consumer-reports/
At the rate the is
delivering power at our
test site, it would take
several millennia for the
product to pay for itself in
savings—not the 56
years it would take even
with the 1,155 kWh
quote we received.
http://savonius-balaton.hupont.hu/128/wind-tronics-inc-canada
Cost and payback
• 6KW Proven about £30K
Site Average Wind Speed m/s
4.0 5.0 6.0 7.0 8.0 9.0
mph 9.0 11.2 13.4 15.7 17.9 20.2
Av. Yearly Energy Output (kWh)
6,765 11,622 16,900 21,944 26,216 29,467
Saving (12p/kWh) £812 £1,395 £2,028 £2,633 £3,146 £3,536
http://www.solarwindworks.com/Products/Wind_Turbines/Proven/Proven_Output/proven_output.htm
http://www.esru.strath.ac.uk/EandE/Web_sites/01-02/RE_info/photovoltaics.htm
Site Average Wind Speed m/s
4.0 5.0 6.0 7.0 8.0 9.0
mph 9.0 11.2 13.4 15.7 17.9 20.2
Av. Yearly Energy Output (kWh)
6,765 11,622 16,900 21,944 26,216 29,467
Saving (12p/kWh) £812 £1,395 £2,028 £2,633 £3,146 £3,536
current costs for onshore wind in good sites
are in the region of from 2.5–3.0 p/kWh
Solar panels
• Get hot water
Clear Cover plate
Air space
Black Absorber
Plate
Circulating Fluid
Insulation
Cost of Solar Panels• The cost of a commercial flat plate
system, including installation, for an 'average'
house ranges from about £2,000 to £4,000.
• Paul Jones of the EST says the average
saving on electricity bills would be roughly £60
to £92 a year, though he stressed that this is
"dependent on property and usage". This does
not sound a great deal of money, so it seems
the only real reason to install the system is for
environmental purposes.
http://www.cat.org.uk/information/catinfo.tmpl?command=search&db=catinfo.db&eqSKUdatarq=20020210164613
http://news.bbc.co.uk/2/hi/programmes/moneybox/2002080.stm
Photovoltaics
• Convert photons directly
into electricity
• Efficiency 4-20%
www2.northumberland.gov.uk/reports/nreg/about.asp
Photovoltaics - cost
• BRITISH PV ASSOCIATION say that PV
technology has a long way to go before
establishing itself competitively with
conventional electricity and other Renewables.
Photovoltaic technology costs typically range
from 60-70p/kWh and is viewed by the
government as a long term project with
anticipated price by 2020 of 10–16 p/kWh, with
the possibility of becoming cost competitive with
retail electricity in the UK around 2025.http://www.esru.strath.ac.uk/EandE/Web_sites/01-02/RE_info/photovoltaics.htm
Cost of Solar PV
http://blogs.scientificamerican.com/guest-blog/2011/03/16/smaller-cheaper-faster-
does-moores-law-apply-to-solar-cells/
Feed in Tariff
• Don’t even get me started on this…
• Too late!
• These were too high (46p per kWh) and
turned into a scam. The Fit should reflect
the price.
• Dropping 23p per kWh April 2012 (now
15.44p per kWh).
As can be seen from measurements dating as
far back as to the 17th century, the
temperature below a certain depth (neutral
zone, at about. 15-20 m depth) remains
constant over the year. Without extraction!
Underground temperatures
at the Royal Edinburgh
Observatory, average
1838-1854 (after data from
Everett, 1860)
Ground
Source
Heat pumps
Horizontal heat recovery
For the ground heat
collectors with dense
pipe pattern, usually
the top earth layer is
removed
completely, the pipes
are laid, and the soil
is distributed back
over the pipes.
Vertical heat recovery
The need to install
sufficient heat
exchange capacity
under a confined
surface
area, favours
vertical ground heat
exchangers
(borehole heat
exchangers).
Measurements and simulations can
visualise the temperature development
during operation as well as the
Long term problems
thermal
recovery
after
operation:
COPHP
Useful heat
Work input
As well as moving
COP times the amount
of heat that was put
into the compressor as
work, the amount of
heat that will end up in
the house is one more
than this: COP COPHP R 1
The
Performance of a
Heat Pump
Coefficient of Performance (COP)
COPHP
Useful heat
Work input
For refrigeration systems we define how good
they are by how much heat (Watts) can be
moved from a cold place to a warm one for each
Watt (electricity generally) work input.
The maximum COP is limited by the second
law of thermodynamics such that
lh
hHP
TT
TCOP (max)
Where Th and Tl are the top and bottom
temperatures of the cycle in Kelvin
Typical COP values for a GSHP
system
• Under floor and hot air heating
(35 C), COPHP = 4
• Low temperature radiators (45 C) COPHP = 3
• Hot water (65 C) COPHP = 2.5
Air Source Heat pumps
• The idea is the same, but instead of the
heat coming from the ground, it is
extracted from the air.
• This is similar to an air conditioner run in
reverse.
ASHP Typical COP values
Due to engineering issues and heat transfer,
real COP’s are much lower.
• Under floor and hot air heating (35 C), from
air at -10 C COPHP = 3 (3.5)
• Hot water (65 C) from air at 0 C COPHP = 2.0
• Conventional Radiators(75 C), from air at -
10 C COPHP = 2 (1.5)
There are also issues about condensation etc.
Getting the electricity (I)If the electricity is produced burning fuel, there is
an inherent inefficiency in conversion, limited by
the second law of thermodynamics.
Where Th and Tl are the top and bottom
temperatures of the cycle in Kelvin
Typical conversion rates ( ) vary from 30% for
open cycle gas turbine systems through 42% for
coal fired plant to 53% for the latest gas fires
combined cycle stations.
h
lh
T
TT
Getting the electricity (II)
As the GSHS system needs electricity for
run, this needs to be taken into account.
It will be seen that savings are possible, but
this needs to be taken into account.
If however, the electricity is produced using
renewable resources (hydro, wind) then
will be unity and the energy savings far
greater. We will be getting far more heat
than we are putting in as primary energy.
Getting the electricity (III)
1
11
RCOP
COPHP of GSHS
2 3 4 5
0.3 -0.67 -0.11 0.17 0.33
0.4 -0.25 0.17 0.38 0.50
0.5 0.00 0.33 0.50 0.60
1 0.50 0.67 0.75 0.80
Proportional saving
So
• If you need to heat by electricity, then this
is better that resistance heaters.
• However in cold weather, you’re better off
with a gas system,
• Or why not…
Biofuels or Biomass
• Burning fossil fuel releases locked in CO2..
• If you grow something, burn it and replant
it then this is a renewable.
Typical Fuels
• Straw
• Willow
• Pellets
• Offcuts.
Typical
biomass
boiler
Moving
grate
De-asher
Primary
air valve
Boiler
Heat
exchanger
Rotation
combustio
n chamber
Moving
grate
De-asher
Feed screw
Primary
air valve
Boiler
Heat
exchanger
Rotation
combustio
n chamber
Moving
grate
De-asher
Primary
air valve
Boiler
Heat
exchanger
Rotation
combustio
n chamber
Biofuel Boiler
Costs
Capital costs depend on the type and size of
system you choose. Stand alone room heaters
generally cost £1500 - £3000 installed. A typical
20kW (average size required for a three-
bedroom semi-detached house) pellet boiler
would cost around £5000 installed, including
the cost of the flue and commissioning.
Running costs: Unlike other forms of renewable
energy, biomass systems require you to pay for
the fuel. This is a bit cheaper than oil, and gas.
http://www.est.org.uk/myhome/generating/types/biomass/
But―Environmentalists are also concerned at new
subsidies for burning wood pellets in power
stations. They say the huge scale of imported
wood is unsustainable.
Oxfam's policy adviser Tracy Carty said the MPs'
decision made no sense because it would only
increase the burning of harmful biofuels in UK
power plants.
"Biofuels, like palm oil, produce more carbon
emissions than they save, fuel land grabs and
increase global food prices," she said.‖http://www.bbc.co.uk/news/science-environment-21692673
The Rankine Cycle.
Invented in the late 19th century, this is a cycle
that uses steam to run.
Steam engines, coal and early nuclear power
stations used this
Boiler (A)
Turbine
(B) Pump
(D)
Condenser
(C)
W out
W in
Q in
Q out
4
1
3
2
T
s
TH
TL
1a
3 4
2
1
A
D C
B
The four
processes
are:
A) Heat addition at
constant pressure in a
boiler from the
compressed water
region to the saturated
vapour point
(W = 0, P = 0), (1-2).
T
s
TH
TL
1a
3 4
2
1
A
D C
B
The four
processes
are:
B) An isentropic
expansion in a turbine
from the saturated
vapour line to the wet
region (Q = 0, S =
0), (2-3).
T
s
TH
TL
1a
3 4
2
1
A
D C
B
The four
processes
are:
C) Heat removal in a
condenser from the
wet region to the
saturated liquid point
(W = 0, P = 0), (3-4).
T
s
TH
TL
1a
3 4
2
1
A
D C
B
The four
processes
are:
D) An isentropic
compression using feed
pumps from saturated
liquid point to the
compressed water region,
(Q = 0, S = 0), (4-1)
Getting the electricity (I)Once again the efficiency is limited by the second
law of thermodynamics
Where Th and Tl are the top and bottom
temperatures of the cycle in Kelvin
Typical conversion rates ( ) vary from 30% for
open cycle gas turbine systems through 42% for
coal fired plant to 53% for the latest gas fires
combined cycle stations.
h
lh
T
TT
Combined heat and power
• Basically an electricity generator that uses
its hot exhaust gases to heat buildings and
processes.
• Total energy use varies from 70-90%
http://www.bartonwillmore.co.uk/townplanning/project_sheet.asp?id=61
Power and fuel
Cycle Engine Fuel Exhaust Temp. ( C) Power
Diesel Internal
combustion
Oil/biodiesel 400 50-200kW
Brayton Gas turbine Oil/gas 600 5-100MW
Rankine Steam turbine Anything 100 50-2000MW
http://www.greenpeace.org.uk/contentlookup.cfm?
CFID=1044260&CFTOKEN=&ucidparam=20020225085523&MenuPoint=G-A
25-02-2002
A team of Greenpeace volunteers
today shut-down and occupied
Britain’s ―flagship‖ waste incinerator
in south London to protect the
health of Britain’s children.
Building Insulation
As well as looking at ways of
acquiring energy from renewable
resources, let’s look at where the heat
goes
Conduction
heat transferConduction is heat transfer
through a material due to a
heat difference between its
two surfaces it is:
(W), where
T1 - T2 (or T) is the
temperature difference
between the two surfaces,
A, is the surface area
UA is the conductance
(W m-2 C-1).
)( 12 TTAUQ
Conduction through multiple
materials.
T1
T2
With two or more
materials, the temperature
drops linearly through
each material from the hot
side to the cool side.
The gradient of the
temperature drop is
greater through a poor
conductor.
Conduction through multiple
materials.
The in the conduction heat transfer
equation can be equated to a thermal
resistance for the wall.
where
U = Heat transfer coefficient (W m-2 C-1)
d = Thickness of each of the materials (m)
k = Thermal conductivity of material (W m-1 C-1)
U
1
....1
c
c
b
b
a
a
k
d
k
d
k
d
U
Thermal conductivity at 25 C
Material Thermal conductivity
(W m-1
C-1
)
Density (kg m-3
)
Air (stationary) 0.0263 1.2
Aluminium Alloy 170 2780
Brick, common 0.72 1920
Concrete 1.4 2300
Concrete block 0.67 -
Glass 1.4 2500
Glass fibre 0.036 105
Plaster 0.22 1680
Plywood 0.12 545
Polystyrene, expanded 0.027 55
Steel, mild 61 7854
Vermiculite flakes 0.063 80
Payback periods (very approx)
Effect Cost Payback
Filling cavity walls £400 3 Years
Double glazing £4000 50 years
Roof insulation £400 2 Years
Low energy lightbulb
99p 4 months
Payback periods (very approx)
Effect Cost Payback
Filling cavity walls £400 3 Years
Double glazing £4000 50 years
Roof insulation £400 2 Years
Low energy light bulb
99p 4 months
Wearing jumper £20 (lower thermostat) 4 months