Transcript Document

Passive Technologies and Other
Demand-Side Measures
Overview
• energy consumption in buildings
• passive demand reduction examples
–
–
–
–
insulation
thermal mass
natural ventilation
nat. vent alternatives
• demand management and “demand-shifting”
Energy Required (Revisited)
• space heating
• hot water
• electricity
– lighting
– appliances
– cooling
– … also for space heating and hot
water
demand in a typical commercial building
Energy Required (Revisited)
• “Typical” average energy consumptions for
dwellings:
• Source: Domestic Energy Fact File
Energy Required (Revisited)
• “Typical” average energy consumptions for offices:
• Source: ECGO 19
Illustration: Domestic Sector
• Using a simple housing stock model the C emissions for the
domestic sector are calculated for the current electricity
supply mix and post 2020 mix (0% nuclear, 40% RE, 60%
fossil fuel) for the following scenarios:
– continuing current trends (increasing heat and electricity demand)
– 30% reduction in heat demand
– 30% reduction in heat and electricity demand
• The desired reduction for carbon from the domestic sector is
also shown
Illustration: Domestic Sector
Carbon Emissions MtC
4.5
domestic emissions
only
4
Million Tonnes Carbon
3.5
emissions including
electrical related
emissions
3
2.5
supply:
0% nuclear
40% RE
60% fossil
2
1.5
1
target
demand:
static
0.5
0
current
2020
Illustration: Domestic Sector
Carbon Emissions MtC
4.5
domestic emissions
only
4
Million Tonnes Carbon
3.5
emissions including
electrical related
emissions
3
2.5
supply:
0% nuclear
40% RE
60% fossil
2
1.5
1
target
demand:
heat demand
reduced by 30%
0.5
0
current
2020
Illustration: Domestic Sector
Carbon Emissions MtC
4.5
domestic emissions
only
4
emissions including
electrical related
emissions
Million Tonnes Carbon
3.5
3
2.5
supply:
0% nuclear
40% RE
60% fossil
2
1.5
1
target
0.5
0
current
2020
demand:
heat and electrical
demand reduced by
30%
Example: Domestic Sector
• Only through reducing domestic heat and power demand do
we achieve any carbon savings
• Even with 40% renewables but with increasing demand
carbon emissions are still greater in 2020!
Energy Required Revisited
• fortunately given the poor energy
performance of most buildings in the
UK the scope for energy savings is
huge
• in this lecture we will cover passive
(design-driven) energy saving
measures
• … and aspects of load management
Passive Measures
Fabric Improvements
• improving the building fabric reduces the
thermal exchanges to/from the environment
e.g.:
– heat loss from inside to outside
– heat gain from outside to inside
• this can be achieved in a number of ways
– adding/improving wall insulation
– replacing old glazing systems (also
reduced unwanted infiltration)
•
improving air tightness (+ MV with heat
exchange)
• potential for 80%* reductions in heating-related
energy loads
* Olivier D, 2001, Building in Ignorance: Demolishing Complacency - Improving the Energy Performance of 21st
Century Homes, report published by the Association for the Conservation of Energy.
Fabric Improvements
• Source: EC
Fabric Improvements
• however there are potential pitfalls:
– increased risk of overheating (high internal loads)
– reduced air quality (reduced infiltration)
• overall fabric improvements are one of the most cost-effective
ways to reduce energy consumption and carbon emissions –
particularly in older buildings/retrofit projects
• Source: EST
Fabric Improvements
Savings ratio £/tonne (over 30-year life)
1000
100
10
1
Insulation
PV
Thermal Mass
• the use of exposed thermal mass is typically employed in
buildings (or spaces) likely to experience overheating:
– sunspaces
– areas of high occupancy
– areas with high equipment loads
• thermal mass acts like a sponge – absorbing surplus heat
during the day and releasing the heat during the evening
• however to work effectively the release of heat in the
evenings needs to be encouraged through flushing of the
air inside the building
Thermal Mass
daytime: Te > Tm
evening: Te < Tm
ventilation air
exposed mass
insulation
exposed mass
insulation
Thermal Mass
heat release from mass
end of night flush
start of night flush
Thermal Mass Temps.
30
28
26
24
22
Temperature (C)
20
18
16
Ambient
Air Temp
Mass Temp
heat gain by mass
14
12
10
8
6
4
2
0
0
1
2
3
4
5
6
7
8
9
10
11
12
Hours
13
14
15
16
17
18
19
20
21
22
23
Thermal Mass
• useful in preventing
overheating however:
– slow response to plant
input
– more difficult to
accurately control internal
conditions (plant pre-heat
required)
– risk of under-heating on
colder mornings
– surface condensation risk
Thermal Mass
• thermally massive buildings
are highly dynamic thermal
systems
• typically rely on thermal
modelling to gauge the
effects on performance
• … particularly when also
dealing with night flush, etc.
Thermal Mass
• testing thermal mass +
night flush strategy
with ESP-r
Natural Ventilation
• ventilation type in most smaller UK buildings
• driven by wind pressure and density variations
– single sided ventilation (density driven)
– stack ventilation (density driven)
– cross flow ventilation (wind driven)
Natural Ventilation
• driving force will usually be a combination of wind
+ density (buoyancy) forces
• influenced by:
– wind direction
– wind speed
– ventilation opening location
– interior/exterior temp. difference
– internal gains
– building geometry
• results in highly variable flow (magnitude and
direction)
Natural Ventilation
the reality!
the drawing …
Natural Ventilation
• given the range of driving
forces and general
complexity of natural
ventilation (strongly
coupled with
temperatures) computer
modelling is often used to
assess natural ventilation
schemes
• gives an indication of the
variability of flow and the
influence on internal
temperatures, comfort and
air quality
Nat. Vent Alternatives
• if more control is required over the air flow
in a building an alternative is to employ
mechanical ventilation with heat recovery
(MVHR)
• the warm exhaust air is passed through a
heat exchanger to pre-heat incoming
ventilation air, reducing the overall building
heating load
• air flow rate is controlled by a fan – more
controllable than nat. ventilation but fan
consumes electricity
• In both nat. vent. and MVHR building must
be tightly sealed to minimise unwanted
infiltration
Nat. Vent Alternatives
• another alternative to natural ventilation is
so-called “dynamic insulation”
• ventilation is drawn through porous
insulation in the external wall cavity
• recovers heat that would otherwise be
conducted through the wall to the
environment
• interior of the building must be slightly depressurised in relation to the outside
• can significantly reduce the “U-value of the
wall”
de –pressurised
interior
Demand Shifting
• demand shifting is not the same as
demand reduction – bit both have a
role to play in the low-energy
buildings of the future
• both can be considered as elements of
“demand management”
load
(GW)
cost £
• with demand shifting we move
appropriate loads in time for an
environmental and/or an economic
benefit
• this is related to time-varying cost and
carbon content of electricity
• shifting can also be used to maximise
the benefit of local low carbon
technologies
CO2
g/kWh
Demand Shifting
Electricity Buy/Sell Price 10/09/08
120
100
£/MWh
80
System Sell Price
System Buy Price
60
40
20
0
0
12
24
1/2 hr period
36
48
Demand Shifting
•
different power
generation “mixes”
means different
electricity carbon
intensity during the
day
Demand Shifting
• with demand shifting we make use of “opportune” loads to move peak
demand out of peak cost or peak CO2 intensity periods
CO2 g or £/kWh
• note this does not reduce demand – only changes the demand profile
Demand Shifting
• opportune loads are loads that can be moved in time without
inconveniencing the user or causing adverse effects
Demand Shifting
• finally we can also use demand shifting to better match local loads to local
energy supplies
• e.g. with a PV system moving loads to the middle of the day when
generation is at a maximum
• this can also be done dynamically – with loads operating when power is
available - dynamic supply-demand matching
• this can also be done statically at the beginning of the design process,
reducing and levelling loads as far as possible and then selecting
appropriate renewable sources
• tools such as Merit (UK) and Homer (US) have emerged to assist in this
process