Applications of Energy Transfer Energy Efficiency and heat exchanges

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Applications ofEnergy Transfer (Energy
Efficiency and heat exchanges)

• Dr Neil J Hewitt
• School of the Built Environment
• University of Ulster

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Rationale
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Course Content

• Introduction
• Thermal Equilibrium
• Conduction, Convection, Radiation
• Heat Transfer
• U-Values
• Temperature Control
• Energy efficiency Renewable Energy

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The Environmental Issues
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The Environmental Issues
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The Environmental Issues
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The Environmental Issues
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The Environmental Issues
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World-wide Energy Use

• http//www.bpamoco.com

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The energy users
1 exaJ 1018J
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Energy Use in the UK
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Energy Use in the UK
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Energy Temperature

• Hot and cold?
• Subjective terms
• One system is hot/cold relative to another
• TEMPERATURE
• Study of temperature
• THERMODYNAMICS

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What is Thermodynamics

• Study of work, heat and energy on a system
• Heat
• Energy that flows due to a difference in
  temperature
• Three Laws of Thermodynamics
• Zeroth Law
• First Law
• Second Law

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Zeroth Law
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Zeroth Law
Thermal Equilibrium is used to define temperature
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Temperature

• A method of assigningarbitrary numbers
  todescribe state of thermalequilibrium
• If the two objects are inthermal equilibrium
• Assigned the same numbers
• If the objects are not in thermal equilibrium
• Heat flows from hotter to colder

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Heat Transfer

• Heat transfer
• Rate of Energy Transfer
• Promotion of heat transfer
• Boilers, heaters, heat exchangers
• Prevention of heat transfer
• Insulation etc
• Three types of Heat Transfer
• Conduction, Convection Radiation

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Conduction

• Why do substancesconduct?
• Solids
• Heat gives molecules energy
• Increased vibration about theirmean positions
• Insulators
• Molecules tightly held -little movement
• Conductors
• Electrons free to move - heat, electricity etc.

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Conduction

• Why do substances conduct?
• Liquids
• Single molecules are able to move long distances
• Many collisions transferring energy
• Gases
• Single molecules are free to move long distances
• Conductivity is lower than liquids or solids
• Very long MEAN FREE PATH

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Conduction

• Fouriers Law of Heat Conduction
• Assume a steady flow of heat
• Independent of wall thickness in one dimension
• Heat flow is proportional to
• Area of the flow
• Temperature difference
• Heat is inversely proportional to
• Wall thickness

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Conduction

• Fouriers Law of Heat Conductionwhere Q
  heat (J), t time (s), A area (m2), T
  temperature (K) and k thermal conductivity (W/m
  K), x thickness (m)

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Conduction

• ExampleA steel bar is heatedat one endk 14
  W/mKHow much energyis conducted in40
  seconds?Q/t 14 x 2 x (500-25)/10Q/t 1330
  watts (J/s)Q 1330 x 40 53200 J

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Conduction

• Typical values of Thermal Conductivity
• Foam 0.010 W/mK
• Air 0.026 W/mK
• Wood 0.150 W/mK
• Water 0.600 W/mK
• Glass 0.800 W/mK
• Concrete 1.100 W/mK
• Aluminium 240.000 W/mK

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Convection

• Bulk movement of thermal energy in fluids

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Convection

• Forced convection
• Caused by pump or fan etc
• Natural convection
• Thermally induced temperature gradient
• Fluid is in motion and affected by
• Type of flow
• Turbulent or laminar ie velocity
• Viscosity and density

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Convection

• Fluid is in motion
• Velocity distribution
• Velocity increases withdistance from the wall
• This leads to a temperature distribution
• Thermal boundary layer

Wall
Tf
Tw
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Radiation

• Conduction and convection require a medium to
  transfer heat
• Radiation passes throughvacuum of space
• Electromagnetic radiation
• X-rays
• light
• radiowaves etc

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Radiation

• Stefan-Boltzmann Law of Radiationwhere
  emissivity E lt 1A area (m2)T temperature
  (K)and o 5.67 x 10-8 W/m2K4

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Radiation
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Heat Transfer

• If energy (Q) is transferred to a mass (m) the
  temperature change is dt such that Q m c
  dtwhere c is the specific heat capacity of a
  substance (provided there is no phase change)

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Heat Transfer

• Specific heat capacity
• Heat required to raise 1 kg of a substance by 1C
• e.g. specific heat capacity water 4.18 kJ/kg
  Kspecific heat capacity of lead 0.13 kJ/kg K
• Units
• Heat (Q) Joules, mass (m) kg, Temperature
  difference (dt) Kelvin

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Heat Transfer

• Watts J/s
• Therefore Energy M c dt where M mass flow
  rate (m/s)
• Calculate heat transferred by moving fluids
• e.g. water to be heated from 10C to 40C at 1
  litre/s (density 1 kg/litre)Energy 1 x 4.18
  x (40-10) 125.4 kW

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Heat Transfer

• Layered systems e.g. buildings
• Need to calculate
• THERMALTRANSMITTANCE (U)
• U VALUES
• Units W/m2K
• Overall coefficient ofheat transfer

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Heat Transfer

• Heat transfer calculated by Resistance Method
• Sum of thermal resistances of wall components
• In series
• RTR1R2R3RN

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Heat Transfer

• For a wall of a single material of thermal
  conductivity k and thickness Landwhere h
  is the film/surface conductance

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Heat Transfer

• Film/surface conductance
• Why does glass feel cold?
• For a wall with an air space made of two
  materials (k1 and k2) and thicknesses (L1 and L2)
  separated by an air space of conductance C

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Heat Transfer

• Calculation of temperature at each interface
• Temperature drop proportional to
  resistanceand thus

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Energy and Temperature

• Energy efficiency depends on accurate temperature
  control
• Temperature control
• Temperature measurement linked to heating system

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Control of Temperature
Toast
Press the lever down
FOOD!

Preset the temperature/ scale
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Control of Temperature
Space Heating
Boiler
Fuel
Heat Losses
Room Thermostat
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Control of Temperature
On-Off Control versus PID Control
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The Energy Problem

• Solutions
• Energy Efficiency
• Both in generation and in end use
• Cleaner Combustion of of fossil fuels
• Electricity generation, transport
• Renewable Energy
• Displacing fossil fuels

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Domestic Energy Efficiency

• Main energy uses
• Space Water heating
• Cooking Lighting
• Solutions
• Insulation draft proofing
• Passive solar gain
• Choice of lights, appliances and heating

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Domestic Energy Efficiency
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Domestic Energy Efficiency
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Industrial Energy Efficiency

• High usage of heat and electricity
• Combined Heat and Power (Cogeneration)
• Heat recovery
• Heat exchangers, heat pumps etc
• Energy efficiency in buildings
• Air-conditioning versus solar gain versus heating

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Transport

• Energy Reduction Scenarios
• New Technologies
• Electric/Hybrid vehicles
• Higher efficiency turbo diesel engines
• Engine management Tyres
• Use management
• Public transport
• Bicycles
• Energy taxes

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Sustainability

• Maintaining current/better situation for future
  generations
• Need energy need environment
• CONFLICT
• Solutions
• Energy Efficiency
• Renewable Energy
• Cleaner use of fossil fuels

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Social Problems

• Political and economic tensions
• Wars fought over energy
• Centralisation
• Vulnerable to attack
• Nuclear Proliferation
• Spent Uranium leads to plutonium
• Very unstable
• Easily exploded BY ANYONE!!

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Energy Efficiency

• Possible to make up to 50 savings in energy
• Energy is inexpensive!!
• However
• Energy/environmental laws are tightening
• Emissions
• Green taxes are a possibility

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Renewable Energy

• Energy that flows naturally and repeatedly in
  the environment
• Direct from the sun
• Solar power, solar panels, PV cells, passive
  solar design
• Indirectly from the sun
• Wind, waves, running water, biomass
• Indirectly from the Moon
• Tidal

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Renewable Energy

• Energy from within the Earth
• Geothermal
• Energy from our lifestyle
• Energy from waste
• MSW sewage sludge
• Renewable Energy
• maximum 50
• Current 5
• 2010 12 in the EU

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Integration
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Renewable Energy

• Most promising technologies
• Wind
• Biomass
• Solar
• Hydro
• Energy from waste
• However renewable energy cannot do it all!!

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Clean Combustion of Fossil Fuels

• Coal reserves in UK for next 200 years
• Conventional power station efficiency 35
• Fluidised bed combustion
• 38-39 cleaner combustion
• Coal gasification
• Efficiency 45 CLEANER COMBUSTION

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The Future?

• Are environmental issues a problem?
• What if?
• Do we have enough energy to meet future demands?
• No
• HOW?
• Energy Efficiency, Cleaner Use of Fossil Fuels,
  Renewable Energy...