Transcript Document

Heat and Power Sources for Buildings
Overview
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energy requirements of buildings
traditional energy sources
carbon emissions calcs
LZC energy sources
– low-carbon energy sources
– renewable (zero-carbon) energy sources
Energy Required
•space heating
•hot water
•electricity
– lighting
– appliances
– cooling
–… also for space heating
and hot water
sources: boilers, chillers,
electricity supply
distribution: cables, ducts,
fans, pumps, piping, etc.
delivery: radiators,
underfloor heating, lights,
diffusers, etc.
control: thermostats,
dampers, valves, timers,
PID controllers, etc.
environmental system
Traditional 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
Boilers
• 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 7090% depending upon age, features and fuel
type
• fuels: natural gas, oil, solid fuel
Grid
• 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)
Emissions
• 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
Emissions
• 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
sources: boilers, chillers,
electricity supply
LZC sources: CHP, PV,
solar thermal, etc.
distribution: cables, ducts,
fans, pumps, piping, etc.
delivery: radiators,
underfloor heating, lights,
diffusers, etc.
control: thermostats,
dampers, valves, timers,
PID controllers, etc.
environmental system
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)
CHP
• 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
CHP
83 waste
100 fuel
108 fuel
180
fuel total
30% eff. power
station
25
electricity
90% eff. boiler
65
heat
72 fuel
7 waste
10 waste
90% eff. CHP
CHP
– the CHP prime mover depends upon
the application
1kWe
Stirling
ICE (gas)
>1MWe
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
T
T
RAD A
T
CHP
BUFFER
TANK
T
DHW
TANK
• CHP device coupled into heating system
RAD N
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
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so for a COP of 4 (typical) 1kWh of heat will
require 0.25 kWh of electricity
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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