Renewable Energy

A short (and cynical) approach

Stephen Beck

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

Energy exchange processes

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).

Some pictures

Drilling holes for

vertical piles

Trenching for horizontal coils

Some pictures

Coils in trench

Spiral collectors

Some pictures

Domestic pump

unitUnder floor heating

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.

Outside heat exchanger

http://www.flickr.com/photos/goins/2484039387/

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).

Power station condensers.

http://www.flickr.com/photos/freefoto/3844250043/sizes/l/in/photostream/

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

Insulation Values

Insulation Values

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

The end

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