# Category Archives: heat pipe

## Webinar on Heat Pipes and Vapor Chambers

Advanced Thermal Solutions, Inc. (ATS) is hosting a series of monthly, online webinars covering different aspects of the thermal management of electronics. This month’s webinar will be held on Thursday, Nov. 29 from 2-3 p.m. ET and will cover the role of heat pipes and vapor chambers in heat transfer. Learn more and register at https://qats.com/Training/Webinars.

## Technical Note: Effect of Vacuum and Fill Ratio on Performance of Heat Pipes

(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)

The discussion generally arises as to what would be the heat pipe performance as a function of liquid fill ratio and vacuum. Before we show some results, let’s see how a heat pipe works. The thermodynamic cycle of a heat pipe is shown in figure 1 in a T-S diagram. [1]

Fig. 1 – Thermodynamic cycle of a heat pipe. [1]

Liquid at state 1 enters the evaporator and after absorbing the heat vaporizes to a mixture at 2 or to a saturated vapor at 2’. This vapor travels through the length of the heat pipe and enters the condenser at state 3. This vapor after losing its heat in the condenser exits at state 4 which is saturated liquid and upon travelling through the wick loses its temperature until it reaches point 1, which the cycle begins. If one looks at the phase diagram for a liquid, for example water in figure 2, it is apparent that the state of the liquid should be very close to the liquid vapor line in order for the liquid to promptly changes phase from liquid to gas upon heating. The triple point of water is at 4.58 Torr at temperature of 0.0075℃.

Fig. 2 – Phase diagram of water.

The state of liquid pressure should be in the region below atmosphere (vacuum) and above the complete vacuum. To obtain lower operating temperature the heat pipe pressure should be decrease and vice versa. The state of liquid after the condenser has to be saturated liquid, so the wick can create the liquid motion.

Lin, et al. [2] have shown that maximum heat transfer in a heat pipe is an exponential function of vacuum pressure according to the following formula:

Where,
Qmax,0 = maximum heat transfer at 0 pressure (no condensable gas)
PNCG = pressure of the non-condensable gas
∆Pcg = pressure drop difference between capillary and gravity

This equation clearly shows that by decreasing vacuum pressure, Qmax increases.

Another important factor is the amount of liquid in the heat pipe which is commonly called the fill ratio or inventory. If there is too much liquid, evaporation will not happen or delayed, and if there is not enough liquid, the dry out condition will happen. The rule of thumb is the volume of the liquid should be higher than the volume of the pore volume of the wick.

Figure 3 shows that as the pressure decreases from 10 Torr to 1 Torr the Qmax increases. The graph also shows that as the inventory (fill ratio) increases from 0.7 ml to 1.1 ml, Qmax peaks at 0.8 ml. This corresponds to a fill ratio of 26.4%, which is the ratio of the liquid volume to total volume of the heat pipe when it is empty. This graph shows the importance of fill ratio. If the fill ratio is not optimized as is shown for example for 1 Torr, Qmax drops from 8 W to 4 W, a 50% drop that can be catastrophic for the application. Mozumder et al. [3], in their experiment, measured the thermal resistance of a heat pipe for different fill ratios and power.

Fig. 3 – Qmax as a function of vacuum pressure and fill ratio for a heat pipe. [2]

Figure 4 shows that as the fill ratio increases from dry run to 85% (in their experiment fill ratio is defined as the volume of liquid to the volume of the evaporator section), thermal resistance decreases, but then increase with further increase of fill ratio.

Fig. 4 – Heat pipe thermal resistance as a function of fill ratio. [3]

The aforementioned arguments demonstrate that the fill ratio and vacuum pressure are very important in the proper design of a heat pipe. There is not much data in the literature about the effect of these two factors on the performance of the heat pipe. And it appears that most heat pipe manufacturers either resort to a try and error procedure or use the information from past experience. This topic needs further research.

Advanced Thermal Solutions, Inc. (ATS) is an industry-leader in heat pipe technology and has recently expanded its offering of high-performance, off-the-shelf heat pipes to provide the broadest selection on the market. Use the heat pipe selection tool to find the right fit for your project and avoid the cost and time for custom solutions. Learn more at https://qats.com/Products/Heat-Pipes.

References
1. Ong, S., “Heat Pipes”, Jurutera, April 2008.
2. Lin, W., Chao, C., Calvin, Y., Hsu, G., Chou, S., “Effect of vacuum pressure and the Working Fluid Inventory to the Maximum Heat Loading (Qmax) of the Heat Pipe”, 10th IHPS, Taipei, Taiwan, Nov. 6-9, 2011.
3. Mozumder, A., Akon, A., Chowdhury, M. Banik, S., “Performance of Heat Pipe with Different Working Fluids and Fill Ratios”, Journal of Mechanical Engineering, Vol. ME 41, No. 2 December 2010.

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

## ATS Power Solutions on Display at APEC 2018

Advanced Thermal Solutions, Inc. (ATS) recently attended APEC 2018 in San Antonio to showcase the company’s thermal solutions for power electronics, meet with industry representatives, and learn the latest trends in the power industry.

ATS has been a member of the PSMA (Power Sources Manufacturers Association), which is the sponsoring organization for APEC, for the past three years because ATS has a strong connection to the power industry and extensive expertise in keeping the industry cool.

Product Engineering Manager is interviewed about ATS cold plates by Alix Paultre, the Power Editor at Aspencore, in the ATS booth at APEC 2018 in San Antonio. (Advanced Thermal Solutions, Inc.)

Vice-President of Sales and Business Development Steve Nolan and Product Engineering Manager Greg Wong manned the ATS booth during the show and took visits from sales representatives, distributors, and engineers from across the industry, some were familiar faces and some were learning about ATS expertise in thermal management of power electronics for the first time.

The highlighted products were ATS liquid cold plates, which boast 30% better thermal performance than comparable products on the market and are easily customizable to meet a variety of applications, and the line of ATS high-performance round and flat heat pipes, which is expanding by the end of 2018 to give ATS the broadest offering of off-the-shelf heat pipes on the market.

“When people start having thermal issues, it’s because they’re dissipating a lot of power and then you start to need things like heat pipes and liquid cold plates,” said Wong. “In most of these applications, people are talking about custom designs, which is where we have a lot of strength working with the customer and designing these custom applications.”

ATS cold plates were also featured in the Texas Instruments (TI) booth as part of a “98.5% efficiency, 6.6-kW Totem-Pole PFC Reference Design for HEV/EV Onboard Charger.” The base of the design was silicon carbide (SiC) MOSFETs with a C2000 microcontroller with SiC-isolated gate drivers, according to information presented by TI.

Underneath the prototype that was on display at the TI booth was a custom ATS cold plate to meet the charger’s thermal requirements.

ATS cold plates were on display at the TI booth, as a thermal solution for a new TI design. (Advanced Thermal Solutions, Inc.)

“It’s a great example of how we can customize our cold plates to meet a particular application,” Wong added. “A lot of people were taking pictures of that piece at the TI booth and a lot of people were talking with TI about it. The booth was mobbed every time I went over there.”

ATS participation in the TI booth at APEC 2018 is a continuation of the strong working relationship between the two companies. ATS has also been a key reference design supplier of heat sink solutions for TI’s audio evaluation module.

Wong and Nolan also learned a lot about the future of power electronics, including the prevalence of SiC and gallium nitride (GaN) components in the industry and the increasing popularity of liquid cooling, to keep ATS current on industry trends and ensure that the innovative thermal solutions that ATS provides can meet ever-rising power demands.

While there is a lot that is new in the industry, IGBT designs continue to be popular and the standard IGBT footprint matches perfectly with ATS off-the-shelf cold plates to make an easy fit for engineers designing liquid cooling solutions.

“If people are still making devices in that IGBT footprint then it will bolt directly to the cold plate, which is good news because that package is very popular, so it’s good to have those standard products,” Wong explained.

Watch the video below as Greg Wong of ATS was interviewed by Alix Paultre of Power Electronics News at APEC 2018 about ATS heat pipes and cold plates.

ATS has the expertise, products, and resources to provide off-the-shelf and customized thermal solutions for the power electronics industry. Learn more about the full line of products at https://www.qats.com or contact ATS at ats-hq@qats.com.

## What is Geothermal Cooling and Heating Technology and How Does it Work

By Norman Quesnel
Senior Member of Marketing Staff

If you cross-sectioned the Earth, it would show a multi-layered structure with a solid iron core spinning in a sea of liquid iron and sulfur. You would also find great quantities of flowing heat starting at the very center and moving outward.

The flow of heat from Earth’s interior to the surface is estimated at 47 terawatts, i.e. 47 trillion watts, and comes from two main sources in roughly equal amounts: the radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust and the primordial heat left over from the formation of the Earth 4-1/2 billion years ago.

In a recent study, scientists estimated the temperature of the center of the Earth at 6,000°C (10,800°F) – about as hot as the surface of the sun. [2]

Fig. 1. Earth’s Outermost Layer, the Crust, and Comprises Just 1% of Our Planet’s Mass. [2]

In fact, more than 99% of the inner Earth is hotter than 1,000°C (1,800°F). Unlike the sun, the Earth is much cooler at its outer crust and on its outside surface. But even at cooler temperatures, the shallow depths of the Earth’s crust provide a geothermal resource for both heating and cooling structures built above the ground.

By drilling deeper down, but still within the crust, much higher temperatures can be harnessed to help generate power that can in turn provide industrial and even community-wide levels of heating and cooling. High-temperature geothermal heat has tremendous potential because it represents an inexhaustible, and virtually emissions-free, energy source. [3]

Fig. 2. Simple Diagram of Near Surface Heating and Cooling Geothermal System. [4]

Near Surface Heating and Cooling Systems

The ground absorbs nearly half of the solar energy the planet receives. As a result, the Earth remains at a constant, moderate temperature just below its surface year-round. However, air temperature varies greatly from summer to winter, making air source (traditional) heating and cooling least efficient when you need it the most. [5]

Geothermal heat pumps take advantage of the moderate temperatures typically found at shallow depths to boost efficiency and reduce the operational costs of architectural heating and cooling systems. Unlike conventional heating and air conditioning systems, which use the outside air to absorb and release heat, geothermal systems use heat pumps to transfer heat from below the surface.

Fig. 3. Four Basic Types of Geothermal Heat Pump System Shown in a US Department of Energy Illustration. Open Loop Systems Can Use Either a Man-made or Natural Water Reservoir. [6]

The pumps connect to closed loops of plastic pipes buried either horizontally or vertically in the ground below the frost line (about 100-200 meters), where the temperature is consistently between 40-80°F depending on location. Called ground loops, the underground pipes are filled with water and sealed tight except where they are connected to the geothermal heating and cooling system inside the building.

In winter, water running through the loops will absorb heat from the ground and pipe it into the home, while the system will run in the opposite direction to keep things cool during the scorching summer months. The pipes are connected to a heat pump and water heater inside the house and users can control the indoor climate through a smart thermostat.

There are four basic configurations for geothermal heat pump ground loops. Three of them are closed-loop systems that use a self-contained water and an anti-freeze solution. The open-loop system uses ground water or water from a well. [6]

Fig 4. Geothermal Pumps Can Efficiently Heat and Cool Homes and Commercial Buildings. [7]

Newer geothermal systems can be installed at shallower depths, less than 50 feet. Even at these levels, the ground can provide a heat source in colder weather and serve as a cooling heat sink when surface temperatures are hot. [8]

There are several providers of geothermal heating and cooling systems. Google’s parent company, Alphabet, is among them with its newly created startup, Dandelion. Originally conceived at X, Alphabet’s innovation lab, Dandelion is now an independent company offering geothermal heating and cooling systems to homeowners, starting in the northeastern U.S. [9]

To put in the ground loops, Dandelion uses its “clean drilling technology” to dig a few small holes in the yard, each only a few inches wide. Then a technician will install the other components inside the house, and the system should be up and running in two or three days. After that, the only regular maintenance is an air filter change every six to 12 months.

Deeper Down Geothermal Systems

The Kola borehole, on Russia’s Kola Peninsula is the deepest mankind has ever drilled into the Earth’s crust. After nearly 15 years of boring, the hole was 12,262 meters (40,230 feet) – over 12 kilometers or 7.6 miles deep. At that depth, the temperature was 185°C (356°F). [10]

Fig. 5. Very Hot, Deep Underground Thermal Energy Can Convert Water to Steam to Power Turbine Generators in Power Plants. [11]

Even at half that depth, there can be much more heat than a near-surface geothermal system can access. The Norwegian company Rock Energy wants to be an international leader in high power geothermal heat and energy. A pilot plant has been planned for Oslo that will collect heat from 5,500 meters deep. The high temperatures from this depth can heat water to 90-95°C (194-203°F) and can be used in district heating plants. [12]

Rock Energy is planning to drill two wells, an injection well where cold water is pumped down, and a production well where hot water flows back up. Between these will be so-called radiator leads that connect the wells. The water is then exchanged with water in a district heating plant managed by the Norwegian power company Hafslund. [13]

Figure 6. Water is Superheated by Deeply Located Hot Rocks and Pumped to the Surface Where It Converts a Separate Liquid to Turbine-Driving Steam. [14]

A hot dry rock system potentially allows geothermal energy to be captured from hot rocks, 3-5 km (1.8-3.1 miles) underground. In operation, cold water is pumped at high pressure down into the very high-temperature, fractured hot rock. The water becomes superheated as it passes through the rock on its way to the extraction boreholes.

In Figure 6, the diagram of a hot dry rock system shows hot water emerging from the borehole and directed through a heat exchanger. After giving up its heat, the cooled water is recycled back down the injection borehole in the hot rock bed. The working fluid, a low boiling point liquid, circulating through the secondary circuit of the heat exchanger is vaporized by the heat extracted from the well water and used to drive the power plant’s turbines. [14]

At both shallow depths and miles down, the Earth offers thermal energy that can harnessed for heating, cooling and power generation. Compared to most other processes, geothermal energy is cleaner, continuous and, as technology advances, a low cost alternative to fossil fuels or to solar and wind-powered systems.

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

## Case Study: Designing Air-to-Air Heat Exchanger With Heat Pipes

Advanced Thermal Solutions, Inc. (ATS) engineers were tasked by a client to design an air-to-air, aluminum heat exchanger with multiple copper heat pipes that could meet high power demands (more than 400W) with a thermal resistance requirement of 0.046°C/W and could withstand a wide range of ambient temperatures from -40°C to 60°C. Also, the separation between the heat pipe’s evaporator and condenser sections needed to be air tight.

ATS engineers were tasked with designing an air-to-air heat exchanger with heat pipes that would fit inside an enclosure. (Advanced Thermal Solutions, Inc.)

Using analytical modeling, ATS engineers calculated the system pressure drop from the heat pipe to the fin block to the flow turn and also the thermal performance of the fins in ducted flow to determine the proper amount of fins to avoid over pressurizing the fans, while at the same time meeting the thermal resistance demands of the system. It was calculated that a maximum of 14 fins per inch could be used, while the overall size was well within the client’s requirements.

Challenge: To design an air-to-air heat exchanger that could handle high power demands of more than 400W and specific requirements on thermal resistance (0.046°C/W).

Chips/Components: Electronics junction box that requires internal air cooling.

Analysis: ATS engineers conducted analysis of the pressure drop across the system from the heat pipe to the fin block to the flow turn section, as well as analyzing the thermal performance of the entire heat exchanger. This analysis included calculating the ducted flow, heat transfer coefficient, and the fin and heat pipe resistance of the exchanger. The analysis also explored the difference between designs with copper and with aluminum fins.

Design Data: The data showed that thermal resistance and pressure drop of the CFD model were within 16% of the analytical model. The thermal performance of the heat exchanger with heat pipes was 0.044°C/W, meeting the client’s requirements.

Solution: The ATS design was optimized for four heat pipes and a suggestion was made to enhance the heat exchanger by using copper fins, rather than aluminum, because of a higher fin efficiency and lower thermal resistance.

Net Result: The customer was supplied with a production design of a heat exchanger block with heat pipes that could fit into the enclosure and provide the necessary forced convection cooling to maintain the proper temperature for the system. ATS also supplied the heat exchangers from the prototype stage to production.

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.