heat and power sources for buildings. overview energy requirements of buildings traditional energy...
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Heat and Power Sources for BuildingsHeat and Power Sources for Buildings
Overview
• energy requirements of buildings• traditional energy sources• carbon emissions calcs• LZC energy sources
– low-carbon energy sources– renewable (zero-carbon) energy sources
•space heating•hot water•electricity
– lighting– appliances– cooling–… also for space heating
and hot water
Energy Required
distribution: cables, ducts, fans, pumps, piping, etc.
delivery: radiators, underfloor heating, lights, diffusers, etc.
environmental system
control: thermostats, dampers, valves, timers, PID controllers, etc.
sources: boilers, chillers, electricity supply
Traditional Energy SourcesTraditional Energy Sources
• space heating – gas, oil or solid fuel boilers, direct electric, electric storage heating
• hot water - gas, oil or solid fuel boilers, direct electric heating
• electrical equipment and appliances – power from the grid
• … ultimate energy source typically fossil fuels
BoilersBoilers
• the main function of the boiler is to convert the potential energy of a fuel to heat
• In the UK this is typically in the form of hot water or steam (larger systems)
• boilers can be:
– condensing (recover latent heat from flue gases)
– combination (instant hot water)
• typical device efficiencies range from 70-90% depending upon age, features and fuel type
• fuels: natural gas, oil, solid fuel
GridGrid
• grid electricity ultimately comes from large central power stations:
– combined cycle gas turbine (η=50+%)
– coal/oil power station (η=35%)
– nuclear power station (η=35%)
• grid electricity carbon intensity: 0.53 kgCO2/kWh (DEFRA)
EmissionsEmissions
• how do we calculate emissions?
• example – natural gas:
• CH4 + 2O2 → CO2 + 2H2O
• (16) → (44) or 1 kg → 2.75 kgCO2/kgCH4 or x (12/44) = 0.75 kgC/kgCH4 (CO2 and Carbon coefficients resp.)
• energy content of nat. gas 93MJ/m3 or 51.12 MJ/kg or 14.2 kWh/kg
• so for an 80% efficient boiler, C emission for 1kWh of heat
• C = (energy/(efficiency x energy content)) x carbon coefficient
• C = (1/(0.8 x 14.2)) x 0.75 = 0.07 kg C/kWh = 0.24 kg CO2/kWh
EmissionsEmissions
• Similarly ….
• so for an 35% efficient coal power station C emission for 1kWh of electricity
• C = (energy/(efficiency x energy content)) x carbon coefficient
• C = (1/(0.35 x 10)) x 0.9 = 0.26 kg C/kWh = 0.94 kg CO2/kWh
distribution: cables, ducts, fans, pumps, piping, etc.
delivery: radiators, underfloor heating, lights, diffusers, etc.
environmental system
control: thermostats, dampers, valves, timers, PID controllers, etc.
LZC sources: CHP, PV, solar thermal, etc.
sources: boilers, chillers, electricity supply
Low Carbon Energy Systems
Combined Heat and Power (CHP)
• CHP (combined heat and power) is the simultaneous generation of heat and power from a single conversion device
• CHP technologies:
– ICE – internal combustion engine
– SE – stirling engine
– gas turbine
– fuel cell (SOFC)
CHPCHP
• CHP is classed as low carbon as it makes use of the waste heat produced by a thermodynamic cycle
• this is not done in conventional power generation – the heat is typically rejected to atmosphere
CHPCHP
25electricity
65heat
100 fuel
7 waste
72 fuel
83 waste
108 fuel
180 fuel total
90% eff. boiler
30% eff. power station
90% eff. CHP
10 waste
CHP
– the CHP prime mover depends upon the application
1kWe
>1MWe
Stirling
ICE (gas)
ICE (diesel)
Gas turbine
CHP
• typical device efficiencies : 85-95%
• heat/power ratios:
– 8:1 stirling engine;
– 2:1 ICE;
– 1:1 gas turbine
• fuel cell CHP is still a research area with lots of work to be done before these devices appear on the market
CHP
CHPBUFFER
TANK
DHW TANK
RAD NRAD A
T
T
T
T
CHPBUFFER
TANK
DHW TANK
RAD NRAD A
T
T
T
T
• CHP device coupled into heating system
Heat Pump
• heat pumps move heat energy from a low temperature heat reservoir to a high temperature reservoir (e.g. the building) using a refrigerant cycle
• heat pumps can use the ground, water or even the air as the low temperature reservoir
• the cycle is driven by a compressor, which consumes electricity
Heat Pump
• heat pump performance is measured using a quantity known as the coefficient of performance (COP)
• COP = useful heat output ÷ energy consumed by compressor
• so for a COP of 4 (typical) 1kWh of heat will require 0.25 kWh of electricity
• the cycle can also be reversed to surplus heat from the house can be returned to the ground (e.g. summer cooling)
• heat pumps (arguably) have the greatest carbon saving potential of any low carbon technology
• if powered using renewable electricity heat pumps become zero carbon devices
Heat Pump
Zero Carbon Sources
Photovoltaics• photovoltaic devices (PV) convert sunlight
directly to electricity
• PV is based on semiconductor technology
• the most common material used is silicon
• the basic unit of a PV system is the cell:
Photovoltaics• individual cells are wired together and
encapsulated in a panel
• groups of PV panels installed on a building are called an “array”
• silicon PV is typically 12% efficient
• so an incident solar intensity of 600W/m2 falling on a 1m2 panel will generate 72W
• typical energy yields are ~100kWh/m2/yr
• conversion efficiency is dependent upon:
– the PV material used
– temperature
– solar intensity
– the load
Photovoltaics• PV power is intermittent – the
amount being produced being determined by the solar intensity
• PV produced DC electricity – which can be used directly for battery charging
• connecting to AC loads requires the power from the panel is inverted
• PV is usually connected to the building’s electrical system via a power electronic interface
• this maximises the PV efficiency and converts ac → dc
Photovoltaics
Micro Wind• micro wind power devices generate
electricity from air flow around a building
• typical devices are horizontal axis machines – smaller versions of large scale machines
• typical device ratings are 1-5kW (@5-6 m/s)
• however the rated wind speed is rarely achieved in urban areas in practice (2-3 m/s)
• better suited to more isolated buildings or unobstructed air flow
Micro Wind• flow in urban areas is highly turbulent
and not ideal conditions for turbines
• wind speed and direction can vary wildly in short distances
• proper siting is critical to achieve the best yield
Micro Wind• the best site for a turbine can be
predicted …
Other Zero Carbon
• solar thermal
– flat plate
– evacuated tube
• biomass/biogas boilers
• hydrogen fuel cell