radiant heated bridge and thermosyphon

7/22/2019 Radiant Heated Bridge and Thermosyphon

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Second Semester Project: Radiant Heated Bridge and Thermosyphon

Gavin AveryApplied Science Research

Dr. DannSpring 2013

Abstract:The purpose of this project was to construct a thermosyphon (solar water heater) and a

model bridge. These were then connected into a bridge heating system. The model bridge,named Realbridge, was built out of a concrete block with copper tubing curving through it,and the thermosyphon was constructed from a 2 by 4 box with an aluminum fin covered

grid of copper tubing, which was spray painted black, sealed by plexiglass, and secured at a45 angle.Water was frozen over Realbridge and tests were then run to determine first, ifthe system could successfully melt the ice on Realbridge and second, the effectiveness ofthe thermosyphon in heating water. Temperature and concrete (no pun intended) data

were recorded and analyzed in evaluating the success of the system. The Thermosyphonalso proved to heat water effectively to its boiling point. The primary result is that the

system succeeded in melting the ice atop Realbridge. The secondary result is that thethermosyphon successfully boiled water.


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History:

The concept of radiant heating can be traced back to the Romans, who heated pipesunder their baths to keep the baths warm around the year. But the Romans, as well as othercivilizations that adopted radiant heating, faced the problem of collecting natural

resources, such as wood, to use as fuel for their heating systems.It was not until 1767 that people began to recognize the potential heating power the

sun held. Swiss naturalist Horace de Saussure was the first to create and test a hot box(seen in Figure 1). He speculated, It is a known fact, and a fact that has probably beenknown for a long time, that a room, a carriage, or any other place is hotter when the rays ofthe sun pass through glass.1De Saussure found that the temperature inside the hot boxreached well above the boiling temperature of water.

Figure 1: De Saussures hot box design.2

De Saussures design was the prototype for solar water heaters to come. Over ahundred years went by before Clarence Kemp, an American plumbing and heatingmanufacturer, created the first solar water heater by building a hot box similar to DeSaussures and placing a black watertank inside of it (see Figure 2). He began tomanufacture his solar water heater, named The Climax, and attempted to sell them acrossthe East Coast.

1John Perlin, Solar Thermal History, California Solar Center,http://www.californiasolarcenter.org/history_solarthermal.html (accessed May 14, 2013).2The History of Solar Water Heating, sinoyin, http://www.sinoyin.com/Flat-Plate-Solar-Collector/art/2011/01/05/29/ (accessed May 13, 2013).


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Figure 2: an advertisement for Kemps Climax Solar Water Heater.3

Unfortunately, the East Coast experienced little sun and was rarely sunny during thewinter months. This made the solar water heater impractical for use on the East Coast.Luckily he began to market them on the West Coast, especially in places like California,where it is sunny year around. In following years, other inventers continued to improve onthe design so that the water could be stored inside the house, where it would stay warmover night. But in the 1920s, large amounts of natural gas and oil were discovered inCalifornia and heating water through combustion became less expensive. Although solar

water heaters are not as popular in America today, they have been adopted in placesaround the world where the sun shines bright. In Israel, households heat over 90% of theirwater with the sun.4

Introduction:

In recent years, radiant heating has been adapted for outdoor environments, such asdriveways, where snow and ice can compile.5The concept of a solar radiant heated bridgehas been implemented before. In 1994, the Swiss Government funded SERSO, a projectwith the goal of recovering heat from the surface of the asphalt bridge, storing that heat,

and utilizing the heat during the winter months to heat the bridge. As seen in figure 3, theSERSO system pumps cool water to the pavement in the summer months to be heated. Thiswater is then sent to be stored in the Rock Store. During the winter months, the system

3The History of Solar Water Heating, sinoyin, http://www.sinoyin.com/Flat-Plate-Solar-Collector/art/2011/01/05/29/ (accessed May 13, 2013).4ibid5ibid


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works in reverse, with hot water being pumped into the asphalt coils to heat up the roadand cool water returning to be stored in the Rock Store.6

Figure 3:SERSO heated bridge design during the winter months.7

The SERSO system was proven to be so effective that it was recommended for use inairports, parking lots, and other asphalt projects. As seen in figure 4, SERSO also discovereda surplus of energy in the Rock Store that could be used to power other utilities, such asstreetlights. During all four winters represented on the chart, the amount of energy

reserved plus the amount of energy used to prevent ice buildup is substantially more thanthe amount of energy lost during both the summer and the winter, proving that the systemis energy efficient.

6SERSO System, Polydynamics Engineering Zuerich,http://www.polydynamics.ch/e/r_d/page_e_serso.htm(accessed February 11, 2013)7SERSO Pictures, Polydynamics Engineering Zuerich,http://www.polydynamics.ch/e/r_d/page_e_s_pic.htm (accessed February 11, 2013)


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Figure 4: Energy data from the SERSO system.8

Since the SERSO project, other countries, universities, and companies have becomeinterested in using solar energy to radiantly heat roads and bridges. Pave GuardTechnology, Inc., with the funding of the Missouri Department of Transportation, has builttwo bridges near Kansas City that are heated during the winter by solar energy collectedand stored during the summer.9 Due to the bridges short term of operation thus far, noconclusions have been made about the effectiveness of the radiant heated bridge program.

The technology used in radiantly heated bridges is the future of winter road safety.From 2008 to 2010 at least 1000 people died in traffic accidents due to icy roads and

bridges.10

Although icy roads do not immediately threaten the Bay Area, many residentsdrive to places such as Lake Tahoe, where icy roads can lead to a plunge off a cliff. Byinstalling new, radiant heated bridges that run on solar energy, states can both save moneyand protect their citizens from the dangers of icy roads. Along with roads and bridges, thistechnology can be implemented at airports. Every year, airports are shut down due towinter storms. After these storms, airports often stay closed due to the ice and snowbuildup on the runway, causing flights to be cancelled. If runways were radiantly heated,the snow and ice would not build up and airports reopen earlier. Radiant heating systemshave great potential when it comes to transportation safety during the winter.

8SERSO Pictures, Polydynamics Engineering Zuerich,http://www.polydynamics.ch/e/r_d/page_e_s_pic.htm (accessed February 11, 2013)9Pave Guard Solutions, Pave Guard Technologies, Inc.,http://www.paveguardtech.com/solutions/paveguardsolutions.html (accessed February11, 2013)10Icy Road Fatality Statistics, Icy Road Safety, http://icyroadsafety.com/fatalitystats.shtml(accessed February 11, 2013)


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Theory:

All three methods of heat transfer are present in the thermosyphon system. First,radiant heat transfer occurs inside the insulated thermosyphon black box. The box trapsand absorbs the thermal radiation, a type of electromagnetic wave, emitted by the sun. The

box must be black because black can absorb all wavelengths of radiation. Aluminum sheetsare also added to the box because the amount of heat absorbed is proportional to thesurface area of the conductor, as seen in Equation 1. The electromagnetic radiation willthen heat up the air and the aluminum inside of the black box. As heat is added to the airinside the box, the airs molecular kinetic energy is increased.11This creates a temperaturegradient inside the box going from the hot air and aluminum to the cold water through theconductive copper pipes.

Equation 1:q = eAT4; where q = the heat transfer rate, e = the emissivity of asubstance, = Stefan-Boltzmann constant, A = surface area, and T = absolute

temperature12

The water in the copper pipes is then heated through conduction, as seen in Figure5. The air then transfers its thermal energy to the water through microscopic diffusion andmolecular collisions with the copper pipes. The rate at which this occurs is known as theheat transfer rate, which can be found from Equation 1.

Equation 2:q = -kA(T/L); where q = the heat transfer rate, k = a materials thermalconductivity, A = the cross sectional surface area, T = the change in temperature over

the gradient, and L = the thickness (length) of the conductor13

Figure 5: Diagram of one dimensional heat flow through a conductor14

11Heat Transfer, Grade 9 to Engineering,http://www.g9toengineering.com/resources/heattransfer.htm (accessed March 22, 2013)12ibid13ibid14Heat Transfer, Grade 9 to Engineering,http://www.g9toengineering.com/resources/heattransfer.htm (accessed March 22, 2013)


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Copper makes for a good conductor because of its specific heat capacity of .385J/(g*K), which is low among metals. As shown in Equation 2, a lower heat capacity meansless heat is needed to change the temperature of a substance. The less heat needed tochange the temperature of a substance is more efficient because it requires less energy. The

copper pipes also act as a good conductor because the thickness of the tubings walls is only.035 inches. The less length of the conductor, the higher the heat transfer rate, as shown inEquation 2. Water is not a good conductor because it has a high specific heat capacity of4.186 J/(g*K).

Equation 2: C = Q / T; where C = heat capacity,Q = change in heat, andT = change in temperature15

When the heat is transferred to the water inside the copper pipes, the watermolecules begin to move faster due to the increase in kinetic energy. When the watermolecules begin to move at faster speeds, they bump into each other more frequently and

bounce farther away from each other. The increase in speed of the molecules increases thetemperature of the water, and the increase of space between the fast moving watermolecules results in a decrease in the density of the water.

Figure 6: Graph of water density vs. water temperature.16

15ibid16Density vs. Temperature, Department of Civil, Architectural and EnvironmentalEngineering at The University of Texas at Austin,http://www.ce.utexas.edu/prof/kinnas/319lab/Book/CH1/PROPS/densgif.html (accessedMarch 22, 2013)


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When the density of the water is decreased, it rises through the system, creatingconvection currents that bring hot water to the top of the system and return cool water tothe bottom. The convection currents transfer heat through diffusion and macroscopicmovement of fluid. The reason why less dense water floats to the surface is because it has

less weight (which means less mass because gravity is constant), as shown on Figure 7.According to Archimedes Principle, the buoyancy force on the hot water is equal to theweight of the regular water it displaced.17The resulting net force is slightly upward, whichwould push the hot water to the top of the system. At the top of the system, the pipes passthrough a concrete block at a quarter inch below the concretes surface. On the surface ofthe concrete is a sheet of ice. This is where conductive heat transfer reoccurs, but with thetemperature gradient switched. Now the temperature gradient goes from the hot water inthe pipes to the cool concrete, and the ice above the concrete. As the heat in the water istransferred, the temperature of the water will decrease. With the heat transferring out ofthe water, the kinetic energy of the water molecules decreases, which causes the density ofthe water to increase. This water will then sink through the system because it is denser.

The water at the bottom will be reheated by the thermosyphon and the process begins anew.

Figure 7: Chart of density and weight of water at different temperatures.18

17Why do wood, cork, and ice always float?, UCSB Science Line,http://scienceline.ucsb.edu/getkey.php?key=94 (accessed March 22, 2013)18Howard Periman, Water Density, U.S. Geological Survey Water Science School,ga.water.usgs.gov/edu/density.html (accessed March 22, 2013)


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The three methods of heat transfer that where used in the thermosyphon system areshown in Figure 8:

Figure 8: The three forms of heat transfer.19

Results:

Protobridge:

Protobridge was made in the form of a concrete block with copper pipes in it (seeAppendix E for CAD drawing). A thin layer of water was then frozen over the concrete blockand hot water was sent through pipes at different distances from the surface. There wereseveral setbacks during the testing of the prototype in the form of leaks in the outer woodstructure. Despite the setbacks, the prototype was successfully built and the tests haveproven the radiant heating system to work, as seen in Figure 9.

Figure 9:Prototype test results on pipes below (right) and below (left) surface ofconcrete.

19Heat Transfer, Grade 9 to Engineering,http://www.g9toengineering.com/resources/heattransfer.htm (accessed March 22, 2013)


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Along with the concrete (no pun intended) results in Figure 9, the data recordedduring a test on the tube proved that the system worked well. As seen in Figure 10,when hot water was run through the pipe, the surface temperature of the concrete began toincrease. As the water continues to run, the surface temperature continued to rise until it

was high enough to melt the ice. Unfortunately, the data was not very accurate due tohuman error and the complexity of the infrared temperature sensor. This test will beredone on the model bridge once it is completed. A regular different type of temperaturesensor will also be used to record data. One problem with having the tubing so close to thesurface is it would not be as applicable to real bridges because the weight of cars drivingover the bridge would compress the concrete near the surface, thus crushing the pipes.

Figure 10: Graph of surface temperature of concrete vs. time for piping of an inch belowconcrete.

Thermosyphon:

A thermosyphon was built in the form of a 2 by 4 by 8 box. Inside of the box is a

grid of copper tubing connected to a sheet of wood and topped with aluminum fins.Beneath this was placed a 1 layer of insulation. All of these pieces were spray paintedblack in order to increase the amount of heat absorbed by the box. These were bothsecured into the box with glue and a 1/8 thick piece of plexiglass was screwed on as the

top of the box. The box was then tilted to a 45 angle and supports were added to the box (See CAD diagram in Appendix E).


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Figure 11: Picture of thermosyphon.

Once the thermosyphon was completed, testing began to determine its effectivenessat heating the water. The thermosyphon was placed in the sun in order to allow UVradiation from the sun to enter the box and be absorbed. It is imperative that thethermosyphon be completely facing the sun in order for the maximum UV exposure. After

20 minutes in the sun, the air in the thermosyphon reached a temperature of 70C. Thetemperature rose at an average rate of .024676C/S. At 36 minutes the temperature of the

air inside the thermosyphon reached 96.1C. Unfortunately, there was no way to measurethe temperature of the water inside the tubing of the thermosyphon because the tubingsystem was closed to ensure no water could leak out. Despite this, when the bridge wasdetached from the thermosyphon, boiling water and steam spurted out, which suggeststhat the water temperature inside the thermosyphon exceeded the boiling point of water,which is 100C. One possible explanation for the discrepancy between the temperature of

the water and the temperature of the air in the thermosyphon is the water was in acompletely closed system while the air in the box could leak out because the plexiglassbegan to bend and bow under the extreme temperature.

Realbridge:

The purpose of Realbridge is to mimic a section of a real bridge (if the name didntgive it away). Realbridge was made in the form of a cement block with copper tubing in it.The copper tubing was laid is a winding formation below the surface of the cement(seeFigure 12 for pictures, see Appendix E for CAD diagram). Thermistors were placed at eachcurve and in a corner to measure the change in resistance, which is used to calculate the


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change in temperature (see Equation 4). The sides of the block and any holes in the blockwere sealed to ensure water does not leak from the box while freezing. Once Realbridgewas completed, water was poured on the surface of the cement and Realbridge was put inthe freezer in create the desired frozen conditions.

Figure 12: Images of Realbridge with and without cement.

Equation 4:T = T0 / (+ T0 Ln[R/R0]); where T = temperature,= thermistormaterial constant; T0= starting temperature, R = resistance, R0= starting resistance20

Sample Calculation:T = T0 / (+ T0 Ln[R/R0]); resistance is 33630at 273K,= 4038K

T = 4038K 273K / (4038K + 273K Ln[119000 / 33630])

T = 251.51K = -21.51C

20Negative Temperature Coefficient Thermistors for Temperature Measurement, PortlandState Aerospace Society, psas.pdx.edu/RocketScience/Thermistors.pdf (accessed May 13,2013).


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System:

The thermosyphon was connected to Realbridge by copper tubing covered withtubing insulation wrap in order to prevent heat lose between the two components. Thesystem is almost completely filled with water and closed to ensure no water leaks out. A

box was also placed over Realbridge to prevent the sun from directly melting the ice. SeeFigure 13 for a complete diagram of the system.

Figure 13:diagram of completed system

The system successfully melted the ice on the bridge (see Appendix C for pictures ofice melting on bridge). As the temperature in the thermosyphon rose, the temperature ofthe water in the thermosyphon increased and the density of the water decreased. Thisallowed for the hot water to rise through the system into Realbridge, where it released itsheat and heated up the concrete, which in turn heated up and began to melt the ice on thesurface. For a more detailed explanation of this process, please refer to the theory section.Some of the water in the system boiled and converted to steam. Steam is more desirable forthe system because it rises more quickly and stores more energy than simply hot water. As

seen in Figure 14, the temperature of the cement increased over time. Another importantthing to notice is the thermistors near the input of Realbridge heated up more quickly thanthe thermistors near the output of Realbridge. This is proven by the discrepancy betweenthe slopes of the fitted lines (in C/S) for thermistor 1 and thermistor 7.For the data chartof all thermistors, please refer to Appendix D.


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Figure 14: Graph of temperatures of thermistors at set time intervals. Temperaturesderived from Equation 4).

There were several problems with the system; the most significant is that withoutflow of the water, the water in Realbridge began to freeze. Freezing the entire bridge atvery low temperatures for an extended period created this problem. A possible fix to thisproblem is installing a water pump into the system. Although it would require electricity, itwould prevent the water from freezing by keeping it moving. Another possible source oferror was the air temperature. In a situation where this device would be used, the airtemperature would be below freezing level. The system was tested on a warm day, whichmeans the warm air temperature may have contributed to the melting of the ice. Thecement surface of Realbridge is not flat, which means the ice sheet was not a consistentdepth. The ice over the higher parts of the bridge is thinner and melts more easily. Despitethese sources of error, the system proved successful in melting the ice on Realbridge.

Acknowledgements:There are many people who helped me on this project in one-way or another. Firstis Dr. Dann, who encouraged me during the project, and in return we found out what finswere. Mr. Del Carlo was a huge help with all the building that went into my project. Hehelped me with designing the support for the thermosyphon. I would like to thank Kien forfinding the connecters I needed for the thermosyphon. And everyone else who helps meknows who they are and I thank them.Appendices:


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Appendix A: Parts list. Check in back of binder for receipts.

Part Description Needed For Cost Where Ill Buy It3/8 copper tubing X 50 ft Radiant heating system $50 newegg.com

Quickrete Model bridge $10 Ace HardwareAluminum Sheets thermosyphon $20 Ace Hardware3/8 copper piping thermosyphon provided Workshop

Flat black paint thermosyphon $10 ACA HardwarePlastic couplings X 16 thermosyphon $40 Plumbing store

Thermistors X 8 bridge provided ASR labplexiglass thermosyphon $28 Home Depot

Spray paint thermosyphon $6 Ace HardwareHeat Sink adhesive X 4 thermosyphon $20 AmazonTubing insulation wrap thermosyphon $5 Ace Hardware

Appendix B: Chart of Thermistor TemperaturesSensor # Frozen 15 min 20 min 40 min 55 min

1 -18.3C -2.89C -1.85C .575C 2.44C

2 -20.9C -5.11C -3.87C -1.25C -.256C

3 -23.5C -6.88C -6.22C -3.78C -2.85C

4 -23.3C -6.37C -5.66C -3.48C -2.47C

5 -22.7C -4.95C -4.50C -2.75C -1.80C6 -21.5C -3.91C -3.11C -1.45C -.840C

7 -21.9C -4.63C -4.17C -2.85C -2.19C

8 -19.9C -3.87C -3.21C -1.50C -.943C

Notes Watercompletelyfrozen, right outof freezer

No ice melting Ice beginningto melt onedges

Ice melting onedges

Ice meltingeverywhere,cementemerging


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Appendix C: Image of ice melting on the bridge.


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Appendix D: Thermistor arrangement and Spec Sheet


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Appendix E: CAD drawings


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Appendix F: Febuary & January papers

radiant heated bridge and thermosyphon

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