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Posts from the ‘Thermal’ Category

Scientists Measure Thermal Properties in Famous Japanese Tomb

Named for the tall pine tree that sits at the top of the tumulus earth mound, Takamatsuzuka Tomb is located in the Asuka village, just south of Nara, Japan. Located within the tomb are some of the most beautiful and famous Japanese wall paintings. Discovered in 1972, the paintings are believed to have been made at the end of the seventh and beginning of the eighth centuries.

Thermal properties

Mural in the inner tomb.

Though it is unknown who is actually buried in the tomb, the murals are worthy of a nobleman. They depict a small-scale universe, including star constellations, the sun, the moon, and guardian gods, for the deceased.

In 2001 this national treasure became threatened by mold growing on the interior lime plaster walls. High humidity and high water content of the lime plaster walls are believed to be the main contributor to mold growth. As a short-term solution, a cooling system was put in the structure to prevent further growth. To optimize efficiency, scientists used the transient line heat source method to determine the thermal properties of the tomb and surrounding soil.

Thermal properties

Cooling system installed at Takamatsuzuka Tomb
to prevent fungal growth.

As a long term solution, the Agency of Cultural Affairs has decided to move the stone interior of the tomb to another location where the environment can be more easily controlled.

What Are Thermal Properties?

Thermal properties tell scientists important things about soil or other porous materials.  Thermal conductivity is the ability of a material to transfer heat. Thermal resistivity, the inverse of conductivity, illustrates how a well a material will resist the transfer of heat. Volumetric heat capacity is the heat required to raise the temperature of unit volume by 1℃, and thermal diffusivity is a measure of how quickly heat will move through a substance.

thermal properties

Thermal property measurements help scientists understand the effects of lasers, cauterization, or radiation on surrounding tissue.

Who Should Measure Thermal Properties, and Why?

Thermal property measurements are needed in varying industries and research fields. One example is underground power cable design. Electricity flowing in a conductor generates heat. Any resistance to heat flow between the cable and the ambient environment causes the cable temperature to rise. This can harm the cable and may even cause power outages in large sections of major cities. When cables are buried, soil forms part of the thermal resistance, and thus soil thermal properties become an important part of cable design.

Other popular applications for thermal property measurements include thermal conductivity of concrete, thermal conductivity of nanofluids, thermal resistivity of insulating material, and thermal properties of food. Unique applications range from measuring human tissue to adobe houses. 

The Transient Method is the Only Way to Measure Moist, Porous Materials

The standard technique for measuring thermal properties is called the steady state technique (guarded hot plate method). The steady state technique requires a needle to be heated until it comes to a steady state, at which time it measures the temperature gradient and determines the thermal properties of the measured material.

The transient line heat source method differs in that heat is only applied to the needle for a short amount of time, and temperature is measured as the material heats and cools.  The steady state technique is a good fundamental method because it uses the simplest equation.  However, it takes a full day to make a measurement because of the wait for steady state.  In addition, when measuring a porous material that contains moisture, heat flow will make moisture move away from the heated area and condense on the cold area.  Thus, the thermal properties of the material will change.  

This means there’s no way to measure the properties of moist, porous materials with the steady state method. The transient line heat source method, however, is able to measure the thermal properties of moist, porous materials, and it can even measure thermal conductivity and thermal resistivity in fluids.

Learn more about measuring the thermal properties of soils or other materials.

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Improved Methods Save Money in Future Borehole Thermal Energy Storage Design

Globally, the gap between the energy production and consumption is growing wider. To promote sustainability, University of California San Diego PhD candidate and ASCE GI Sustainability in Geotechnical Engineering committee member, Tugce Baser, Dr. John McCartney, Associate Professor, and their research team, Dr. Ning Lu, Professor at Colorado School of Mines and Dr. Yi Dong, Postdoctoral Researcher at Colorado School of Mines, are working on improving methods for borehole thermal energy storage (BTES), a system which stores solar heat in the soil during the summer months for reuse in homes during the winter. Baser says, “We are running out of finite energy resources. We need to come up with new strategies to use free and renewable energy resources such as solar energy for a sustainable future.”

borehole thermal energy storage

Baser’s BTES design.

How it works

BTES systems are an approach to provide efficient renewable resource-based thermal energy to heat buildings. They are configured to store thermal energy collected from solar thermal panels during the summer and discharge the heat to buildings during the winter. They function by circulating a fluid within a closed-loop pipe network installed in vertical boreholes to inject heat collected from solar thermal panels. During winter, cold fluid is circulated through the heat exchangers to recover the heat from the subsurface and distribute it to the buildings. Baser explains, “The subsurface provides an excellent medium to store this heat due to the relatively lower thermal conductivity and lower specific heat capacity especially when the soil layer is in the vadose zone. Lower thermal properties allow us to concentrate the heat in a specific array and the heat losses to the environment are potentially low. These systems typically include an insulation layer and a hydraulic barrier near the ground surface to reduce heat and vapor losses to the atmosphere.”

borehole thermal energy storage

BTES construction.

Why do we need improved methods?

Baser and her team are trying to improve the understanding of heat storage mechanisms and evaluate changes in the rate of heat transfer and heat storage in the vadose zone where the soil is unsaturated. The goal of the project is improve conventional methods by generating models to fit different soil types and situations.  She says, “The European community introduced us to the borehole thermal energy storage systems to provide heat specifically for domestic use, but there is still a chance for us to design them more efficiently by having a full understanding of the thermal response of these systems that is specific to the ground material and subsurface conditions. The primary objective of this research is to understand the mechanisms of coupled heat transfer and water flow in unsaturated soil profiles during the heat injection and subsequent heat extraction into these different arrays and different dimensions of borehole heat exchangers.”

borehole thermal energy storage

Solar panels.

Baser and her team working on designing numerical models based on finite element method which improve some of the numerical models in the literature used to characterize the thermal response of the systems. The new models add new considerations, such as the heat pipe effect in different soil types. Baser explains, “Because thermal and hydraulic properties of soils are highly coupled and are specific to soils, the thermal response of a BTES system will be different when it is installed in different types of soils. For example, you see the heat pipe effect where there is evaporation and subsequent condensation in fine grained soils rather than coarse soils because in coarse grain soils the pore characteristics are different. The duration of the heat pipe effect (or convective cycle) is longer in fine grain soils. We conclude that considering coupled heat transfer and water flow in the thermal response of Borehole Thermal Energy Storage system is important.”

borehole thermal energy storage

In-ground heat exchanger

Experiments in the field and in the lab help verify the new models

To fully understand heat transfer mechanisms and water flow in unsaturated soils, the research team installed two different SBTS systems at different scales, one in Golden, Colorado School of Mines campus, and the other at the UC San Diego research campus.  Baser says, “The subsurface characteristics of both sites are different, and this gives us the opportunity to investigate the impact of the different soil layers on the thermal response experimentally in a full scale. In addition, the scales of each Borehole Thermal Energy Storage system are different, and we also apply different heat injection rates. We have used these data to further validate our coupled heat transfer and water flow model so that we can use it for design purposes.”

borehole thermal energy storage

Soil moisture sensor locations.

Baser started with laboratory heating experiments, in which soil in a large tank is heated by heat exchangers. She installed soil moisture sensors to measure volumetric water content and the temperature and then used the KD2 pro thermal property analyzer to monitor thermal properties during heating experiments to characterize the coupled thermo-hydraulic relationships. For the field experiments the team uses soil moisture sensors equipped with temperature sensors and the KD2 pro to monitor subsurface temperature fluctuation because during the summertime the air temperature is higher, thus ambient air temperature fluctuation and penetration may become significant.

Baser also uses thermistor strings that include six thermistors at different depths and thermistor pipe plugs, voltage input modules, and flow meters.  She says, “Thermistor pipe plugs and flow meters are used in the manifold to monitor the inlet and outlet fluid temperatures and flow rates in each loop to calculate heat transfer rate into the ground. Flow meters were installed to control flow in each loop because you don’t want to over or underload the borehole loops. The amount of energy that you collect from the solar loop and the amount of energy that you inject into the ground can be used to define the efficiency of the system.” Baser says thermistor strings help monitor the ground temperature during the summer heat loading at different depths. They’re also used to monitor borehole wall temperature over time. The team installed one thermistor string 9 meters away from the heat storage array to see if far field is affected by the heat transfer within the array.

borehole thermal energy storage

Insulation prevents heat loss to the environment.

The new models will save money in future Borehole Thermal Energy Storage design

Baser says building numerical models and solving them was very complicated and time consuming, but they’ve had good results. She explains, “We’ve recently proved, both experimentally and numerically, that considering coupled thermal and hydraulic relationships are very important for thermal response analysis. Thus, our recommendation is that it’s fine to use the analytical models and user-friendly numerical models that consider constant thermal properties in the design analyses for saturated soils. However, in unsaturated soils, there is a very high possibility that the contribution of heat transfer evaporation and condensation would be missing and the Borehole Thermal Energy Storage system would be oversized, costing a significant amount of money. When dealing with soils in the vadose zone, coupled thermo-hydraulic constitutive relationships in the modeling efforts need to be considered.”

You can learn more about Tugce Baser’s research here.

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Understanding Avalanches: Thermal Conductivity of Snow (Part 2)

In a continuation of last week’s article “Understanding Avalanches,” we find out what conclusions Dr. Ed Adams and his colleagues in Montana State University’s avalanche studies program were able to make about measuring the thermal conductivity of snow.

In order to study the thermal properties of snow samples, the research team wanted a way to measure thermal conductivity in three directions.

In order to study the thermal properties of snow samples, the research team wanted a way to measure thermal conductivity in three directions.

In order to study the thermal properties of snow samples, the research team wanted a way to measure thermal conductivity in three directions. That ruled out flux plates. Thermal probes were an obvious alternative, but they brought a different set of challenges. Snow has a very low thermal conductivity, and as Shertzer explains, “if you add a lot of thermal energy to snow, since it’s very insulative, you’ll tend to raise the temperature. Not only do we want to avoid melting the snow in the neighborhood of the probe, but we want to prevent the probe from artificially inducing the same thermal processes we’re measuring—the ones that cause the crystals to change size, and shape, and orientation.”

Shertzer read an article about measuring thermal conductivity in liquids, where if you add too much heat, you induce convection. “Our situation is similar to that,” he explains. “Heating the needle induces local phase change.” The article gave him some ideas about delivering low levels of heat for a relatively long period of time, and he contacted Decagon to see if that option was a possibility.

thermal properties

Snow barriers in the Alps

Unbeknownst to him, Decagon’s research scientists had just completed a year-long project focused on reducing the contact resistance errors that occur when using the large TR1 needle to measure thermal conductivity in large-grained samples.  This made the TR1 needle a good candidate for measuring thermal conductivity in snow.  The scientists were excited about modifying KD2 Pro firmware to produce a low power version that would work in snow. The resulting modification has given Shertzer some good data.

“I can definitely say that the anisotropy is there [in the snow samples]. It’s measureable and it’s significant. As the crystals reorient in these depth hoar like chains, the ice network is more conductive than the air in between. The orientation of the chains follows a direction of increased conductivity, and the directions that are perpendicular to the chains tend to decrease in conductivity. Qualitatively, it’s always made sense, and we were just looking for a way to actually relate it to properties like conductivity. Using needles to measure in three different directions simultaneously has given us the ability to measure those properties like conductivity. We expect that this orientation also affects other properties like strength and stiffness.”

thermal properties

Skiers look at signs of an avalanche

Thermal conductivity studies may ultimately lead to a better understanding of the conditions that cause the snowpack to fracture and trigger an avalanche–and information that may help save lives among the growing number of people who ski and snowboard the backcountry.

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Understanding Avalanches: Thermal Conductivity of Snow (Part 1)

Reading through our archives the other day, I came across this article about thermal conductivity and snow. It’s a unique application for this instrument, an interesting story, and something that may ultimately even save the lives of backcountry skiers and snowboarders.

thermal conductivity

Rich Shertzer, who finished a PhD in the program at Montana State, thinks snow may be unique among natural materials because “the thermal environment it’s exposed to every day can cause pretty remarkable changes in its microstructure.”

When Wired Magazine wrote up Dr. Ed Adams and his colleagues in February 2011, they didn’t refer to them as a team of civil engineers studying granular mechanics. Instead, they named them one of seven teams of “Mad Scientists” and called them “Snow Bombers.”

It’s not hard to find articles about Montana State University’s avalanche studies program. Just describing a typical field study makes for a good story: to investigate real-world avalanche conditions, MSU researchers sit in an outhouse-sized shack bolted to the side of a mountain while colleagues trigger an avalanche up-slope.

But this isn’t just a story about explosions and extreme sports. At its heart, it’s a story about the microstructure of a very fascinating and difficult material. Rich Shertzer, who finished a PhD in the program at Montana State, thinks snow may be unique among natural materials because “the thermal environment it’s exposed to every day can cause pretty remarkable changes in its microstructure.”  A cold, sunny day in the mountains can cause significant changes in snow crystals. It can change their size and shape, but more significantly it can cause a directional orientation in snow layers.

thermal conductivity

Signs of a recent avalanche.

It’s long been empirically understood that avalanches tend to form above “weak layers” of snow. Shertzer and his colleagues are studying how the orientation of snow crystals correlates with weak layers. Most models of granular mechanics assume that the material’s microstructure is randomly arranged. However, snow layers seem to show a regular arrangement.

As Shertzer explains, “Qualitatively, people have known for a while that when you look at certain snow layers, chains of these ice grains seem to be forming. What I was trying to mathematically model is how that might affect the material properties [of snow], including thermal properties.”

thermal conductivity

Avalanche on Mt. Everest.

In order to study the thermal properties of snow samples, the research team wanted a way to measure thermal conductivity in three directions. That ruled out flux plates. Thermal probes were an obvious alternative, but they brought a different set of challenges. Snow has a very low thermal conductivity, and as Shertzer explains, “if you add a lot of thermal energy to snow, since it’s very insulative, you’ll tend to raise the temperature. Not only do we want to avoid melting the snow in the neighborhood of the probe, but we want to prevent the probe from artificially inducing the same thermal processes we’re measuring—the ones that cause the crystals to change size, and shape, and orientation.”

Read about how the team addressed these problems next week in part 2 of “Understanding Avalanches.”

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Small Company, Big Mission: The Phoenix Mars Lander & TECP Sensor

On May 25, 2008 NASA’s Phoenix Lander successfully landed on the surface of Mars and used a robotic scoop arm to deliver regolith samples to the suite of instruments on the deck of the Lander–with one exception. The Thermal and Electrical Conductivity Probe (TECP), designed by a team of Decagon research scientists, was mounted on the knuckle of the robotic arm and made direct contact with the regolith. It measured thermal conductivity, thermal diffusivity, electrical conductivity, and dielectric permittivity of the regolith, as well as vapor pressure of the air.

But, that’s starting at the end of the story.  The fact is that TECP almost didn’t get started.  After seeing a thermal properties needle at the American Geophysical Union meeting in San Francisco, Mike Hecht (project leader on the Mars Environmental Compatibility Assessment (MECA) instrument suite) encouraged his colleague Martin Buehler to call Decagon to see if we’d be willing to participate in the Phoenix Lander project. When Martin called one Friday afternoon, announcing that he was from JPL and wondering if we would be willing to fly our sensor on the Phoenix Lander, I was instantly intimidated. I knew JPL was associated with NASA, and I couldn’t imagine why they would be calling Decagon.  I always thought there was a fundamental relationship between NASA and Lockheed Martin, Northrop Grumman, and other major companies that did NASA work.  I told him that Decagon (much smaller in those days) didn’t have the capacity to develop instrumentation for space flight. He suggested they come up for a visit and at least consult with us on what they would need to do to obtain this measurement.  The following Monday, we were talking Martian science and inexorably hooked on the idea of joining the team.

Phoenix Lander

I knew JPL was associated with NASA, and I couldn’t imagine why they would be calling Decagon.

Deciding to put one of our sensors on Mars did nothing to lessen the intimidation factor. But, working with Mike and his team at JPL/NASA taught us that doing amazing science can be an inspiring and collaborative effort. I’d always imagined NASA as a group of uber-scientists and engineers sitting in glass offices dreaming up and executing great projects that would be impossible for mere mortals.  The reality is that sending something to Mars and having it do real science requires the combined effort of thousands of smart, dedicated people who are not that much different from the rest of us.

This idea was really brought home when we finally visited JPL. Although the things they were doing were amazing and on a much grander scale, they weren’t that much different from the things we do at Decagon.  They had testing facilities, development facilities, production facilities, and support personnel all working together on projects, just like us.  However, the projects were pretty amazing. We watched the robot arm being tested in a lab for the ability to dig martian soil analogs. We observed an ice probe working in a 55 gallon drum trying to prove it could melt its way down through the thick martian polar ice caps. We were mesmerized by prototypes of Mars rovers being programmed and executing maneuvers on Martian surface analogs.

It was fun to discover who the Jet Propulsion Lab is and how enjoyable it is to collaborate with people that are thinking about new applications of technology.  This collaboration also benefitted Decagon’s thermal properties instrument because the mathematical models we developed for Mars made this sensor much more accurate and effective. The Mars project expanded both the depth of our understanding and the breadth of our perspective.  Even so, it was fun to find out that scientists who work at JPL have to put their pants on one leg at a time, just like all of us.

See a virtual seminar where Dr. Mike Hecht talks Mars, poetry, and Decagon’s involvement in the Mars Phoenix Lander Mission.

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