its all about conserving energy - nvidia · 2015-08-12 · solar irradiance and atmospheric...

Ian Williams, Director of Applied Engineering, NVIDIA

Its all about Conserving Energy How predictive rendering can avoid creating an accidental Death Ray

2

Agenda

The Problem

A Quick Physics Recap

Simulating the Death Ray in Iray

Deriving Temperature from Irradiance

Comparing Predictions with Measurement

Conclusions

3

The Problem August 29th 2013

Martin Lindsay parked his Jaguar car on Eastcheap, London around noon

2 hours later he returned to find parts of the car had melted

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The Scale of the Problem

“On Tuesday afternoon, I was sent out to see if I could fry an egg in the heat, a task that I presumed was impossible on an overcast September day. But, not only was it possible, I had to run out of the death ray that was slowly cooking my egg, because the thinning hairs on my head started to catch fire. ”

“On Monday, the air temperature in the concentrated beam, reached 69.8C, which in old money is 158F. To put that in context, the world’s hottest temperature was recorded in Death Valley at 56.7C (134F) over a century ago.”

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Death Ray’s Apparently are Fairly Common

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Agenda

A Quick Physics Recap

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Electromagnetic Radiation

Electromagnetic Radiation consists of waves that are synchronized oscillations of electric and magnetic fields that propagate at the speed of light

Produced whenever charged particles are accelerated, and these waves can subsequently interact with any charged particles

EM waves carry energy, momentum and angular momentum away from their source particle and can impart those quantities to matter with which they interact

Electromagnetic spectrum: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays

A Quick Recap

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How Does Sunlight Heat Things Up?

Radiation is both absorbed and emitted

Convection

Conduction

Three Main Modes of Energy Transfer

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Governing Laws

The law of conservation of energy states:

That the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed

The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic systems, and states:

That the change in the internal energy of a closed system is equal to the amount of heat supplied to the system, minus the amount of work done by the system on its surroundings

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Quantifying Energy Transfer in the Electro Magnetic Spectrum

Radiant Flux per unit area received by a Surface

Units = watts per square metre W/m2

Hemispherical quantity

Irradiance

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Solar Irradiance and Atmospheric Absorbtion

The electromagnetic radiation emitted by the sun incident on the top of the atmosphere

50% in region beyond visible light

40% in visible light region

10% in ultraviolet region

On a clear day approximately 50% of solar energy absorbed by atmosphere

Irradiance

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Agenda Simulating the Death Ray in Iray

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Simulating the Physics of the Death Ray

Walkie Talkie Building and Environment were extensively modelled in detail using AutoDesk 3D Studio Max

Used Iray Renderer to:

Simulate time of day and year using calibrated Sunlight model

Calculate Irradiance incident on specific surface

Iray

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The Model

Movie

20 Fenchurch Street and Surroundings, London

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Time of Day Analysis

Movie

29 August 2013

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Predicted Irradiance from Iray

Movie

29 August 2013

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The Energy Contained in the Death Ray

Direct Sunlight on a clear day has an Irradiance of ~110K Lux or ~1.1 KW/m2

Predicted Irradiance at street level outside the Death Ray was ~98K Lux or ~1.1 KW/m2

Predicted Irradiance at street level inside the Death Ray was ~700K Lux or ~7.5 KW/m2

=6x more powerful than the Sun!

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It Could Have Been Worse...

Slight modifications to the building geometry, in the form of between 5 to 10 degrees more curvature, increased Irradiance from:

~700K Lux or ~7.5 KW/m2

To

~3M Lux or ~32.3 KW/m2

=30x more powerful than the Sun!!!

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Agenda Deriving Temperature from Irradiance

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Calculating Temperature from Irradiance Solar Heat Transfer to a Surface

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Temperature of the Surface

Assuming system is steady state with no phase changes, then from 1st law of thermodynamics the energy balance equation is:

Energy Balance

Ee . α = ε . σ [ Tsurf-air4 - Tair-amb

4 ] + hsurf-air [Tsurf-air – Tair-amb] + k . [Tsurf-air – Tsurf-other] / s

Absorbed Energy Re-Radiated

Energy

Energy transfer through Convection

Energy transfer Through Conduction

Surf -> Atmosphere

Atmosphere -> Surf

Ee = Irradiance (Wm-2) α = Absorptivity ε = Emissivity σ = Stefan-Boltzman constant = 5.670373x10-8 (Wm-2K-4) Tsurf-air = Temperature at air surface boundary (K) Tair-amb = Ambient air temperature (K) Tsurf-other = Temperature at other side of surface (K)

hsurf-air = Heat transfer coefficient between surface and air (Wm-2K-1) k = Thermal Conductivity of surface material (Wm-1K-1) s = Surface material thickness

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Simplifying Things

Assume worst case scenario where energy transfer/cooling through conduction is negligible

Assume radiation from atmosphere back to surface can be considered negligible

Energy Transfer Equation be re-written:

Which equals:

Which is a quartic of the form:

Ee . α - ε . σ Tsurf-air4 - hsurf-air [Tsurf-air – Tair-amb] = 0

ε . σ . Tsurf-air4 + hsurf-air .Tsurf-air – [ Ee . α + hsurf-air .Tair-amb] = 0

a . x4 + b . x3 + c . X2 + d .x – e = 0

a = ε . σ

d = hsurf-air

e = Ee . α + hsurf-air .Tair-amb

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General Solution To A Quartic Equation

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In Our Case…

b = c = 0

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So The Formula Becomes…

Δ0 = 12ae

Δ1 = 27ad2

Q = [ (Δ1 + (Δ1

2 - 4 Δ03)1/2 )/2]1/3

S = ½ [ (1/3a) * (Q + Δ0/Q) ]1/2

q = d/a

a = ε . σ

d = hsurf-air

e = Ee . α + hsurf-air .Tair-amb + ε . σ .Tair-amb4

Tsurf-air = X1 or X2 or X3 or X4

Depending on which root is a +ve real number

Where:

x1,2 = -S ± ½( -4S2 + q/S )1/2 x3,4 = S ± ½( -4S2 – q/S )1/2

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Assumed Values…

Ee = Irradiance (Wm-2) P(W) = Ev(lx) × A(m2) / η(lm/W) , where η = Lumic Efficacy = 0.93 for sunlight α = Absorptivity ε = Emissivity σ = Stefan-Boltzman constant = 5.670373x10-8 Wm-2K-4

Tsurf-air = Temperature at air surface boundary (K) Tair-amb = Ambient air temperature (K) = 300K hsurf-air = Heat transfer coefficient between surface and air (Wm-2K-1) = 10.45 –v + 10v1/2 , where v is velocity of object in air (source engineering toolbox)

= 10.45 Wm-2K-1 for static object

Absorptivity Emissivity

Asphalt 0.93 0.93

Polished Aluminum 0.09 0.09

Magnesium Oxide Paint 0.09 0.09

Chromium 0.2 0.2

Sherwin Williams White Paint 0.89 0.87

Concrete 0.6 0.88

Red Brick 0.94 0.93

Polythene Black Plastic 0.94 0.92

Black Paint 0.94 0.94

Human Skin 0.97 0.97

(source engineering toolbox)

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Calculated Temperatures Ambient Temperature = 27C, 80F

NORMAL SUNLIGHT

INSIDE DEATH RAY

Asphalt 82C, 179F 276C, 530F

Asphalt Light Breeze 57C, 135F 208C, 407F

Black Plastic 82C, 179F 277C, 531F

Black Plastic Light Breeze 58C, 136F 209C, 408F

Chrome 44C, 113F 146C, 295F

Chrome Light Breeze 34C, 94F 82C, 180F

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Melting Point of Plastics

Material Degrees(F) Material Degrees(F)

Acetal (CoPo) 400 PBT 500

Acetal (HoPo) 425 PCT 580

Acrylic 425 Peek 720

Acrylic (Mod) 500 PET 540

ABS (MedImp) 400 Polycarbonate 550

ABS (HiImpFR) 420 Polyetherimide 700

CelAcetate 385 Polyethylene (LD) 325

CelButyrate 350 Polyethylene (HD) 400

CelPropionate 350 Polypropylene 350

EVA 350 Polystyrene (GP) 350

LCP 500 Polystyrene (MI) 380

Nylon (6) 500 Polystyrene (HI) 390

Nylon (6/6) 525 Polysulfone 700

Polyamide-imide 650 PPO 575

Polyarylate 700 PVC (Rig/Flex) 350/325

TFE 600

Source: http://plastictroubleshooter.com/ThePlasticTroubleshooter/melt_temps.htm

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Observations on Calculated Temperatures

Some temperatures seem believable, eg:

Chrome in Normal Sunlight – 44C, 134F

Ashpalt in Light Breeze – 57C, 135F

Some temperatures too high, eg:

Asphalt in the Death Ray – 276C, 530F

Asphalt in a Light Breeze - 208C, 407F

Conclusion energy transfer model is too simplistic

As energy transfer from Irradiance increases

Reality will be greater dissipation through Convection and Conduction

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Agenda

Comparing Predictions with Measurement

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Calibration Scene

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Calibration Scene in Iray

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Physical Measurements

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Comparing Measured and Calculated Temperature Ambient Temperature = 71F, Windspeed ~0 Knots

CALCULATED FROM

IRRADIANCE

MEASURED 3-11-2015 1:05 PM

MEASURED 3-11-2015 1:51 PM*

1 Asphalt (Horizontal) 122F 106F 87F

2 Concrete (Horizontal) 108F 98F 87.5F

3 Concrete (Vertical, ~south) 116F 104F 89F

4 White Painted Steel (Vertical, ~south) 130F 86.5F 76F

5 White Painted Steel (Vertical, ~east) 66F 78.5F 74.5F

6 Black Painted Steel (Vertical, ~south) 131F 99F 80F

7 Black Painted Steel (Vertical, ~east) 114F 92.5F 80F

* Full Cloud Cover for > 30 mins

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Observations Between Measured and Calculated Temperature

CALCULATED FROM

IRRADIANCE

MEASURED 3-11-2015 1:05 PM

MEASURED 3-11-2015 1:51 PM*

Asphalt (Horizontal) 122F 106F 87F

Concrete (Horizontal) 108F 98F 87.5F

Concrete (Vertical, ~south) 116F 104F 89F

White Painted Steel (Vertical, ~south) 130F 86.5F 76F

White Painted Steel (Vertical, ~east) 66F 78.5F 74.5F

Black Painted Steel (Vertical, ~south) 131F 99F 80F

Black Painted Steel (Vertical, ~east) 114F 92.5F 80F


Significant heat loss after full sunlight is obscured by

cloud

- Up to 20% temperature drop

Clearly significant heat transfer occurring through:

- Radiation

- Convection

- Conduction

36

Observations Between Measured and Calculated Temperature (cont.)

CALCULATED FROM

IRRADIANCE

MEASURED 3-11-2015 1:05 PM

MEASURED 3-11-2015 1:51 PM*









Results are expected to be higher since

conduction of heat away from the surface

is being ignored by the model

These are large uniform surfaces so local

heat transfer effects will not occur

Over prediction is approximately ~9-13%

37


CALCULATED FROM

IRRADIANCE

MEASURED 3-11-2015 1:05 PM

MEASURED 3-11-2015 1:51 PM*









These surfaces are in very close proximity so

local heat transfer effects will occur and the

model assumes steady-state energy transfer

at a single surface. It doesn’t, for example,

model heat flow from one surface to another

when both are subject to different Irradiance

levels.

The materials for surfaces 4 thru 6 are also

highly conductive (steel) and very thin (surface

4 & 5 < ⅛”, surface 5 & 6 < ¼”) so the impact

of conduction will be increased.

38


CALCULATED FROM

IRRADIANCE

MEASURED 3-11-2015 1:05 PM

MEASURED 3-11-2015 1:51 PM*









The above inaccuracies with the

model are almost certainly causing

this under predicition

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Summarizing Temperature Derived from Irradiance

Temperature could be calculated from Irradiance in certain limited situations

Large uniform surfaces with consistent, steady state exposure to sunlight

Most real-world situations extremely complex

Assumptions in energy transfer model easily break down

For more accuracy heat transfer needs to be modelled with significant detail

Heat transfer through/between solids

Multiple surfaces exposed to different Irradiance levels

Etc etc

40

Agenda

Conclusions

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Overall Conclusions

Miscalculating design impact at either design or engineering stage can lead to significant and expensive consequences

Knowledge is Power - ability to understand full implications of design choices critical

- Issues may not be observable in required validation points

Interactive rendering combined with calibrated sun model within in Iray facilitated identifying the Death Ray problem literally in minutes

Irradiance prediction within Iray is an extremely good indicator of potential heat problems

Deriving temperature from predicted Irradiance extremely complex challenge

- requires significant research, investigation and calibration

Thank You!

its all about conserving energy - nvidia · 2015-08-12 · solar irradiance and atmospheric...

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