internal heat gains and small dwellings
TRANSCRIPT
Internal gain assump/ons and building size !
Nick Grant, E-‐Mail [email protected] @ecominimalnick
Alan Clarke, E-‐Mail [email protected] @AR_Clarke!
Interna/onal Passivhaus conference Aachen 2014
Thanks to members of the Passivhaus Trust for technical review.
Small is beau6ful but tricky in PHPP
Our model building: 2 floors Square in plan 6m fixed height 0.5m wall, roof and floor thickness L= 3.6m to 13.3m
PloWng form factor v Floor area
Mean detached German Passivhaus
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0! 50! 100! 150! 200! 250! 300!
Form
fact
or!
Total floor area (m2)!
Form factor = heat loss area / floor area!
Prop
osed
dwelling
PloWng Annual heat v floor area smaller dwellings appear less efficient 0.1 U values, PH vent and glazing, simplest form etc, same for all sizes
Even
a cub
e won
’t work
Almost any shape and orienta6on works in PHPP
But small area so should s6ll use less?
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20! 70! 120! 170!
kWh/
(a.p
)!
Total floor area!
Calculated annual heating demand per person by PHPP (35m2/p) !
Does it really cost more to heat a small home of the same specification?!
This doesn’t fit my anecdotal experience: 15m2 living off grid for 7 years, 100mm insula6on & double glazed but cosy warm with twigs and body heat. 1/3 not 3x the fuel to heat our larger super-‐insulated house
What about a 6ny envelope, in an extreme climate?
Na6onal Geographic
Is high form factor compensated by higher specific gains?
About a third ± PH hea6ng is from IHGs
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16.1
11.5
14.9
5.8
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0.0 0.0 0.0 4.3
5.2 0.0
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Losses Gains
Hea
t flo
ws
[kW
h/(m²a
)]
.
Energy balance heating (annual method)
Non useful heat gains
Exterior wall - Ambient
Roof/Ceiling - Ambient
Floor slab / Basement ceiling
Windows
Ventilation
Solar gains
Internal heat gains
Heating demand
2.1 W/m2 IHG breakdown:
Per person: Metabolic, ligh6ng people, drying towels, some appliances ≅ 54W/p
Per m2: Aux electric, ligh6ng space, DHW distribu6on, pot plant evapora6on? ≅ 0.1W/m2
Per dwelling: Fridge, freezer, appliances, boiler DHW base storage ≅105W/dwelling
Calculated in PHPP IHG sheet (DHW not included):
But how many people? Occupancy v TFA
SAP 2005
SAP 2012 (based on 30,000 homes)
Is UK occupancy data relevant?
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0.5!
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3.5!
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50!
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Peop
le/h
ome!
m2!
Average m2/p!
Average m2/dwelling!
PHPP35m2/p!
P/dwelling!
Linear (P/dwelling)!
IHG calculated using PHPP assump6ons with BRE occupancy rate.
y = 71.32x-0.73!R² = 0.99!
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20! 40! 60! 80! 100! 120! 140! 160! 180! 200!
Inte
rnal
hea
t gai
n (W
/m2 )!
Total floor area (m2)!
calculated W/m²!
W/m2 PHPP occupancy for comparison!curve fit W/m²!
2.1 W/m2 is about right for typical detached PH in Germany!
PHPP 2.1 W/m2
IHG with BRE occupancy to obtain curve fit
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5.0!
10.0!
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20.0!
25.0!
30.0!
35.0!
40.0!
20! 40! 60! 80! 100! 120! 140! 160! 180! 200!
kWh/
(m2 .a
)!
Total floor area!
Heating demand for simplified dwelling!
variable W/m² internal gain!fixed 2.1 W/m² internal gain!
Annual Heat demand Using IHG = 71TFA-‐0.73
IHG assump6ons will never be correct over the life of building but an improved heuris6c* model will beker reflect reality for smaller and larger buildings. The downsides of overes6ma6ng IHGs for small buildings are less than for underes6ma6ng.
“All models are wrong, some are useful” George Box
*Rule of thumb/trial and error
‘The importance of hot water system design in the passivhaus’ Clarke & Grant Dresden 2010
Losses contribu6ng to hea6ng
Wasted losses
Assume state of the art DHW Dwelling will probably have either: 1. Hot water distribu6on from central system: Say 15m x 15mm pipe with 25mm insula6on = 45W ignoring conduc6on to outlets etc.
Or: 2. Storage: 3W/K PHPP default = 120W 2W/K minimum in PHPP = 80W
Plus distribu6on losses and actual DHW use hea6ng space Can we agree 40W/dwelling is a low es6mate?
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1.0!
2.0!
3.0!
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7.0!
8.0!
20! 40! 60! 80! 100! 120! 140! 160! 180! 200!
Inte
rnal
hea
t gai
n (W
/m2 )!
Total floor area (m2)!
calculated W/m²!
W/m2 PHPP occupancy for comparison!curve fit W/m²!
IHG including 40W/dwelling DHW losses
PHPP 2.1 W/m2
Curve fit: IHG = 100TFA-‐0.77
Adding 40W/dwelling DHW gains IHG = 100TFA-‐0.77
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25.0
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20 40 60 80 100 120 140 160 180 200
kWh/(m
2 .a)
Total floor area
variable W/m² internal gain fixed 2.1 W/m² internal gain
Curve should pass through origin 0-‐0!
Small buildings do need less heat/p
0!
200!
400!
600!
800!
1000!
1200!
20! 70! 120! 170!
kWh/
(a.p
)!
Total floor area!
Overall annual heating demand per person BRE occupancy, ignoring DHW gains!
Curve should pass through origin 0-‐0!
Conclusions • kWh/(m2.a) metric validated, (usually cri6cised as
favouring larger dwellings). • Small homes work beker than large ones. • No relaxa6on is needed for small dwellings. • Implica6ons for summer overhea6ng calcula6ons. • More realis6c occupancy assump6ons have
implica6ons for primary energy calcula6ons.
Schools (in brief)
Δ Metabolic Heat Gains Only School Children TFA m2 m2/child Bushbury Hill (UK) 240 1707 7.1 Oakmeadow (UK) 450 2205 4.9 Montgomery (UK) 446 2367 5.3 Swillington (UK) 240 1344 5.6 Wilkinson (UK) 459 2500 5.4 LH Hannover (D) 300 3507 11.7 Gronau (D) 336 2953 8.8 Reidberg (D) 500 5540 11.1 Average for UK examples 5.7 m2/child Average for German examples 10.5 m2/child
Difference +1.32W/m2 + 5-‐6 kWh/(m2.a) of useful hea/ng Against 15kWh/(m2.a) target
Less reliance on solar gain
4.2
6.3
3.9
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0.2
12.9
17.7
8.2
14.7
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Losses Gains
kWh/
(m2 .
a)
Annual Heat Balance
Heat Solar IHG Vent Doors Opaque Window Ground Roof Walls
3.8W/m2 v 2.8W/m2 IHG
Means we designed a different building 50% v 60% g glass No additional south glazing just to meet 15kWh/(m2.a)
Schools Conclusions • Design is very sensi6ve to IHG assump6ons. • Too low an IHG assump6on favours passive
solar design with associated high cost and increased overhea6ng risk.
• Custom IHG calcula6ons could lead to game playing but occupancy density and 6me should be factored in.
• More detailed analysis could not be fiked into this paper.