Passive Technologies and Other Demand-Side Measures.

Passive Technologies and Other Passive Technologies and Other Demand-Side MeasuresDemand-Side Measures

Overview

• energy consumption in buildings• passive demand reduction examples

– insulation– thermal mass – natural ventilation– nat. vent alternatives

• demand management and “demand-shifting”

• space heating• hot water• electricity

– lighting

– appliances

– cooling

– … also for space heating and hot water

Energy Required (Revisited)

demand in a typical commercial building

• “Typical” average energy consumptions for dwellings:

Energy Required (Revisited)

• Source: Domestic Energy Fact File

• “Typical” average energy consumptions for offices:

Energy Required (Revisited)

• 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 SectorCarbon Emissions MtC

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Mil

lio

n T

on

nes

Car

bo

n

domestic emissionsonly

emissions includingelectrical relatedemissions

current 2020

supply: 0% nuclear40% RE60% fossil

demand:static

target

Carbon Emissions MtC

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Mil

lio

n T

on

nes

Car

bo

n

domestic emissionsonly

emissions includingelectrical relatedemissions

current 2020

supply: 0% nuclear40% RE60% fossil

demand:heat demand reduced by 30%

Illustration: Domestic Sector

target

Carbon Emissions MtC

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Mil

lio

n T

on

nes

Car

bo

n

domestic emissionsonly

emissions includingelectrical relatedemissions

current 2020

supply: 0% nuclear40% RE60% fossil

demand:heat and electrical demand reduced by 30%

Illustration: Domestic Sector

target

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 ImprovementsFabric 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 ImprovementsFabric 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 ImprovementsFabric Improvements

1

10

100

1000

Insulation PV

Savings ratio £/tonne (over 30-year life)

Thermal MassThermal 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 MassThermal Mass

insulation

exposed mass

daytime: Te > Tm

insulation

exposed mass

evening: Te < Tm

ventilation air

Thermal MassThermal Mass

Thermal Mass Temps.

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Hours

Tem

per

atu

re (

C)

Ambient

Air Temp

Mass Temp

start of night flushend of night flushheat release from mass

heat gain by mass

Thermal MassThermal 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 MassThermal 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 MassThermal Mass

• testing thermal mass + night flush strategy with ESP-r

Natural VentilationNatural 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 VentilationNatural 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 VentilationNatural Ventilation

the drawing …

the reality!

Natural VentilationNatural 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 AlternativesNat. 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 AlternativesNat. 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 de-pressurised 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”

• 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

load (GW)

cost £

CO2

g/kWh

Demand Shifting

Electricity Buy/Sell Price 10/09/08

0

20

40

60

80

100

120

0 12 24 36 48

1/2 hr period

£/M

Wh

System Sell Price

System Buy Price

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

• note this does not reduce demand – only changes the demand profile

CO

2 g

or £

/kW

h

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