Beat the Sun - Keep Your Equipment Cool
Solution: Solar Shade Wall/Roof Concept
Author:
Peter Brackett
Engineer, S&C Development and Quality Assurance
Chairman AREMA Committee 38
Phone: 403-319-7781
Email:
Canadian Pacific Railway
Operations, Engineering Services, Signals & Communications
5th Floor, Gulf Canada Square,
401, 9th Ave., S.W.
Calgary, Alberta
T2P 4Z4
Date: August 2003
Beat the Sun - Keep Your Equipment Cool
A “Solar Shade Wall/Roof” can reduce the temperature rise of buildings or equipment without using energy (fans or air-conditioning). Canadian Pacific Railway hopes, by sharing this information, that energy consumption can be reduced and the railways can get credit for improving the environment.
Most equipment is designed to operate within certain temperature ranges. When this same equipment must operate in conditions outside these temperature ranges the results are often unpredictable. For example: there is a considerable difference in the cost of designing equipment for a maximum ambient temperature of 40C versus designing for a maximum temperature of 70C caused by solar radiation. A small black metal housing can easily reach 70C in the summer sunshine of the southern latitudes.
Between 1999 and 2000, Canadian Pacific Railway constructed and deployed over a 100 new 8’by8’by16’ buildings to house trackside fiber and radio equipment. As per the railway norm, these buildings were made of welded aluminum and air- conditioning was installed to provide a controlled environment for this communication equipment. See Figure 1: Heat Transfer due to Ambient Temp. Differences.
Since these buildings (bungalows in Canadian railway terminology) house our mission critical fiber network (Alcatel SONET and Newbridge telephone/data multiplexing equipment), heating, ventilating and air conditioning (HVAC) units and temperature alarm monitoring were installed to provide the best environment for fibre functionality.
After initial installation, a series of high temperature alarms triggered an investigation into improved ways to maintain the temperature within the desired limits.
On a day that I will never forget, since it was the day after September 11, 2001, I visited Somers, Wisconsin. We recorded 125º F on the sun-exposed walls, versus 85º F on the non-sun exposed wall. Now, the expression “Hot Enough to Fry Eggs” took on new meaning with a measurement to back how hot “HOT” was. (Trivia note: Eggs fry at 158F). An extensive temperature modification and measurement program was initiated to determine the best way to keep the equipment within the desired temperature limits.
It was quickly determined that solar radiation heat gain was a key factor in causing the high temperature alarms. A measurement project was initiated to record multiple building temperatures and to determine the amount of solar radiation being absorbed by the buildings, and how this affected the inside temperature of the buildings.
One result of this investigation was to install a “Solar Shade Wall/Roof” on the roof and sun exposed sides of a test building in Calgary, Alberta, Canada. This “Solar Shade Wall/Roof” was able to prevent the solar radiation temperature rise and keep the sun exposed building surface at the same “ambient” temperature as the north- facing wall.
The Solar Shade Wall/Roof concept can reduce or eliminate the solar radiation heat gain of any building or enclosure. This reduces the energy consumption of air conditioning that is currently used to control the temperature rise caused by solar radiation.
Table of Content
Heat Transfer Principles:
Heat Transfer due to Ambient Temp. Differences
Heat Transfer Due to Solar Radiation
Solar Energy Daily Fluctuations
Requirement: Method to block solar energy heat transfer
Solution: Solar Shade Wall/Roof Concept
Shade Wall combined with natural Convection Thermal Chimney
Shade Wall/Roof Implementation at Canadian Pacific Railway’s Test Site
Thermal Chimney’s
Additional Solar Shade Wall Measurements
Building Data Comparing Temperatures without Solar Shade Wall/Roof to Temperatures with Solar Shade Wall/Roof
Plot of Data Comparing Building Temperatures
Conclusion:
Potential Railroad Applications:
Simple Implementations:
Further references:
The Field Guide for Energy Performance, Comfort, and Value in Hawaii Homes
Radiated Heat Transfer
Field Guide Chapter 7: Insulation and Radiant Barriers
Field Guide Chapter 8: Heat Mitigation in Roofs
Recommended Technique: Integrate roof strategies.
Radiant Barriers as referenced in the Field Guide.
Website URL additional References:
Permissions for quoted material:
List of Figures
Figure 1: Heat Transfer due to Ambient Temp. Differences
Figure 2: Heat Transfer Due to Solar Radiation
Figure 3: Solar Energy Daily Fluctuations
Figure 4: Solar Radiant Energy Blocking Solution
Figure 5: Shade Wall = Solar Radiant Energy blocking
Figure 6: Shade Wall/Roof combined with Natural Convection Thermal
Figure 7: Shade Wall Framework being added to Test Site
Figure 8: Bungalow view from East with Wall and Roof Solar Shade Wall Visible
Figure 9: Thermal Chimneys - Ducts made by Outer Skin
Figure 10: Dramatic Proof of Temperature Differences
Figure 11: Aug 29th Clad vs July 16 Non-Clad Temp. Comparisons
Figure 12: Temperature Comparison Table
Figure 13: Bare Roof Temperatures and Reduced Temperature under Shade Roof
Figure 14: Ambient North Wall Temperatures comparing July16th to Aug29th
Figure 15: Chart of Temperature Comparison Table
Figure 16: Wall/Roof Temperature measurements July 2003
Figure 17: Simple Solar Shade for Equipment Enclosure
Figure 18: Modes of Heat Transfer
Figure 19: Radiated Heat Transfer
Figure 20: Insulation: Barrier to Heat Transfer
Figure 21: Hawaii Field Guide's "Solar Shade Wall"
Heat Transfer Principles:
Heat Transfer is directly proportional to Temperature Difference (for Steady Heat Conduction). It is easy to understand if you use the Thermal Resistance Concept. The Equation for heat conduction through a plane wall is:
(W)
Where
Rwallis the thermal resistance of the wall against heat conduction or simply the conduction resistance of the wall.
Note that the thermal resistance depends on the geometry and the thermal properties of the wall.
The equation above for heat flow is analogous to the relation for electric current flow I, expressed as
Where
R is a function of the per unit electrical resistance times the length of wire
V1 _ V2 is the voltage difference across the resistance
Analogy between thermal and electrical resistance concepts.
- the rate of heat transfer through a wall corresponds to the electric current
- the thermal resistance corresponds to electrical resistance
- the temperature difference across the wall corresponds to voltage difference across the resistor
For more details on the Thermal Resistance Concept, see Heat Transfer: A Practical Approach, 2/e, Yunus A. Çengel, University of Nevada-Reno
To understand the Solar Shade Wall/Roof concept, heat transfer should be separated into two separate transfers to best understand the Solar Shade Wall/Roof Effect.
a) Heat Transfer due to Ambient Temp. Differences
b) Heat Transfer Due to Solar Radiation
Heat Transfer due to Ambient Temp. Differences
Figure 1: Heat Transfer due to Ambient Temp. Differences
The temperature difference between the outside wall surface and inside wall surface drives heat energy through the walls. Heat energy transfer is directly proportional to this temperature difference.
Heat Transfer due to Ambient Temperature Differences
We can identify one component of this heat transfer as energy driven through the wall by the difference between the outside wall’s ambient temperature (temperature directly affected by outside air touching the outside walls) and the inside wall of the bungalow. This is recorded as the temperature difference between the outside wall’s temperature (that is not exposed to the sun) and the inside wall’s temp.
This heat transfer rate is a function of the surface area of all the walls of the building.
Heat Transfer Due to Solar Radiation
Figure 2: Heat Transfer Due to Solar Radiation
The second component of heat transfer is driven by the outside wall’s temperature rise above the outside ambient temperature. This temperature rise is directly related to the absorption of solar radiation energy on the sun exposed wall and roof surface area. On sunny days, the solar radiation temperature rise over ambient is very significant but the difference in surface area affects the balance between the two components of energy transfer.
The strength of this solar radiation heat transfer varies constantly as a result of:
a)Atmospheric - cloud cover
b)Sun angle versus time of day and angle above horizon versus time of year
c)Air movement over the building’s outside surface (affects heat loss)
To understand how variable the solar radiation heating is see Solar Energy Daily Fluctuations below.
Solar Energy Daily Fluctuations
Figure 3: Solar Energy Daily Fluctuations
Requirement: Method to block solar energy heat transfer
From practical experience, it is evident that shaded surfaces are cooler and sun exposed surfaces
Figure 4: Solar Radiant Energy Blocking Solution
Planting trees is not feasible for buildings along the railway for many reasons.
So we had to develop another alternative and hence Canadian Pacific Railway designed the following.
Solution: Solar Shade Wall/Roof Concept
Figure 5: Shade Wall = Solar Radiant Energy blocking
Shade Wall combined with natural Convection Thermal Chimney
The outer skin blocks the solar radiation and heats up. If the air filled cavity between the new outer skin and the original inner skin was sealed at the top and bottom, the closed wall cavity would just act as another insulating layer. Heat can still be transferred across the cavity by conduction (contact) transfer of heat from the inside of the outer skin to the trapped air, which creates a convection (air movement) transfer to the inner surface; this, in turn conducts the heat energy to the original building skin.
To minimize this heat transfer, the natural convection or “Thermal Chimney” concept was used.
Figure 6: Shade Wall/Roof combined with Natural Convection Thermal
Shade Wall/Roof Implementation at Canadian Pacific Railway’s Test Site
Figure 7: Shade Wall Framework being added to Test Site
This building is aligned east-west. This view is from the northeast looking southwest.
The shade wall’s support framework utilized 2’by4’s and had an open air space at the top and bottom.
Since this was an experimental test site, the framework was designed to be removable without leaving any holes in the existing welded aluminum building skin.
The roof is a hip roof design, ie: 4 slopes. For ease of installation, only the north-south centre roof sections where covered with the shade Wall/Roof.
Figure 8: Bungalow view from East with Wall and Roof Solar Shade Wall Visible
Natural convection draws air up the “Thermal Chimney” created by the vertical studs on walls and roof and the extra new aluminium outer skin (Solar Shade Wall/Roof).
Thermal Chimney’s
Roof Thermal Chimney /
Wall Vertical Thermal Chimney
Figure 9: Thermal Chimneys - Ducts made by Outer Skin
Solar Shade Wall/Roof Temperature Measurements Changes Prove It Works
Figure 10: Dramatic Proof of Temperature Differences[CPR1]
Temperature of Outside Solar Shade Wall/Roof
versus
Temperature of Original Roof under Solar Shade Wall/Roof
Measurement Results: Solar Shade Wall Reduces Original Roof Temp. from 136F/58C to 93F/34C
Additional Solar Shade Wall Measurements
Since the exterior building surface is kept at the outside ambient temperature (versus outside ambient plus solar radiation temperature rise), much less cooling energy is required.
Amount of energy reduction is dependent the amount of solar surface temperature rise on the sun exposed building surfaces less the ambient temperature of the non-sun exposed reference wall.
Figure 11: Aug 29th Clad vs July 16 Non-Clad Temp. Comparisons[CPR2]
Building Data Comparing Temperatures without Solar Shade Wall/Roof to Temperatures with Solar Shade Wall/Roof
Without outer skin-Solar Shade Wall/RoofTemperature Measurement Locations / Temp. / Degree-Hours
Temperatures as recorded 1 PM / C / F / 14 hr Integration Window
Outside Bare Roof Temp / 54 / 129 / 567
Outside Roof Temp without Solar Shade (bare roof) / 56 / 133 / 579
North Wall’s Ambient “Air” Temp (includes sun in PM) / 28 / 83 / 423[CPR3]
Inside Ceiling Temp Between Ribs / 28 / 83 / 388
Inside Ambient Air Temp / 27 / 80 / 349
HVAC’s daily running hours / 12.5 hrs
(above temperature readings on from July 16/02 08:39 AM to 10:35 PM)
With outer skin-Solar Shade Wall & Thermal Chimney
Temperature Measurement Locations / C / F / Degree-Hours
Outside Bare Roof Temp / 55 / 131 / 530
Outside Roof Temp Under Solar Shade / 31 / 88 / 383
North Wall’s Ambient “Air” Temp / 32 / 90 / 373
Inside Ceiling Temp Between Ribs / 27 / 80 / 374
Inside Ambient Air Temp / 27 / 81 / 366
HVAC’s daily running hours[CPR4] / 5.8 hrs
(above temperature readings on Aug 29/02 from 08:39 AM to 10:35 PM)
Key Differences / % Differences
HVAC running time reduction / 6.7 hrs / 54%
Solar Shade Wall Outer Skin to Roof Temp. Reduction / 147hrs / 138%
Comparison Adjustment
Outside Ambient Degree-Hour difference between days
(non-shade wall day appears hotter-but includes sun heating in PM) / 50 hrs / 12%
See: Figure 11: Aug 29th Clad vs July 16 Non-Clad Temp. Comparisons.
July 16th’s non-shade ambient degree-hours were 50 hrs (423non-shade-373shade) greater that Aug 29th’s shade.
Since July 16th was warmer, the actual HVAC run time should be reduced by 5-20% to~ 50%.
But 50% reduction in HVAC energy usage is still significant.
Figure 12: Temperature Comparison Table
Plot of Data Comparing Building Temperatures
Figure 13: Bare Roof Temperatures and Reduced Temperature under Shade Roof
The total daily solar radiation amounts are similar for both days, with Aug 29th peaking higher but rising slower in the morning (AM) and falling faster and afternoon (PM). It is very difficult to get highly comparable data when dealing with the high fluctuations of temperature data.
Figure 14: Ambient North Wall Temperatures comparing July16th to Aug29th
This shows the daily north wall temperature profile versus the shaded Aug 29th roof temperature.
It also shows the effect of the sun heating the north wall on July 16th. For the Calgary latitude, the summer sun sets to the north of an east-west line, so it shines on the North wall in the early AM and late PM. It does not affect the results in a significant manner when the east wall temperatures were substituted for the 3 pm to 10:35 pm readings. The reference wall degrees hours only dropped from 423 to 390.
Figure 15: Chart of Temperature Comparison Table
This chart [CPR5] shows the difference in air conditioning demand between the day the building had the Solar Shade Wall/Roof installed and the day without the Solar Shade Wall/Roof.
During the day without the solar shade protection the air conditioner had to run continuously. This is determined by monitoring the HVAC’s output air temperature. When the unit is working the air temperature drops approximately 16 C from the interior temperature. Per above, on the non-protected day the HVAC started running almost continuously from 11:31 AM until after 9 pm. This indicates the heat rise in the building is exceeding the HVAC’s cooling capacity.
For the solar shade protected building on Aug29th, the HVAC oscillated on and off indicating it was able to control the internal building temperature and that it only had to run approximately 50% of the time.
The conclusion is the solar shade can reduce air-conditioning by 50% or better on the hot days.
Effects will be less dramatic on days with less solar radiation.
In summary, the Solar Shade Wall works
Temperature Measurement Locations / C / F / /Bare Roof (No Shade wall on South East corner) / 52 / 125
South Solar Shade Wall (The Outer skin) / 34 / 94
Top of Airspace between Walls / 28 / 83
South Wall Original Bungalow Skin / 26 / 78
South Roof Original Bungalow Skin / 26 / 78
North Wall Ambient Reference (in Shade) / 26 / 78
Interior Wall / 22 / 72
Recorded on July 25, 2003 at 2:47 PM
Figure 16: Wall/Roof Temperature measurements July 2003
Because
a)The outer solar shade Wall/Roof absorbs the Solar Radiation (short wave Infrared) and heats up ( to 34C /94F in this example)
Note: Unprotected roof heated to 52C/125F.
From previous tests, the roof’s Solar Shade skin would be approximately the same temperature as the bare roof.
The roof’s shade skin was not measured on for this date but from previous recordings the temperatures do track.
The sidewall absorbs less energy than the roof because of the angle of the sun relative to the roof and wall respectively.
b)The heated outer solar shade wall transfers energy to the air in the cavity (Thermal Chimney) by a solid to gas conduction. The hot outer surface heats air touching it.
c)The heated outer aluminum solar shade wall also re-transmits the energy from both its surfaces to the air and from the inner surface to the original bungalow skin (to 26C/78F) by re-radiating the energy at long wave Infrared wavelengths.
Aluminum does not absorb a lot of short wave radiation compared to other materials and hence does not have as much energy to re-radiated as long wave infrared radiation (near infrared).
Possible explanation of why the original bungalow’s wall temperature remains at the ambient temperature.
d)The heated air in the between wall cavity rises up the thermal chimney (28C/83F at top of chimney) but this natural convection air movement flows out the top of the chimney faster than the time needed for this warmed air to transfer its energy to the inner wall (by gas to solid conduction).
e)The long wave radiation emitted by the outer wall into the cavity is partially reflected by the polished aluminum wall surface of the original bungalow wall which reduces the long wave radiation heating effect. See Radiant Barrier technology described below.