Interview with Robin McCarty

Featured Engineer

Robin McCarty

Robin McCarty - Member of Technical Staff, Product and Process R&D at Marlow Industries, Inc

How did you get into electronics/ engineering and when did you start?

Looking back on my childhood, I was very mechanically inclined. I put together model cars, planes and ships. I also took apart my sewing machine and put it back together. I always enjoyed math and science and went straight into mechanical engineering as an undergraduate. Once I took thermodynamics, I was hooked on heat and energy. I went on to further develop these skills in my master’s and Ph.D. programs. I worked on the master’s as I worked fulltime for a laser level company. After 5 years, I went back to school full-time for the Ph.D.t

Do you have any tricks up your sleeve?

I love to model complex thermal and energy systems using a combination of analytical simplifications and numerical methods—I usually write my own programs to predict the performance of these systems. I also love developing new test methods to acquire experimental data to further understand these systems. There is nothing better than dveloping a brand new model for a system, along with a new test fixture and method, and seeing the two results match.

What has been your favorite project?

During the Ph.D. program, I designed a test stand to measure the efficiency of thermoelectric generators (TEGs). It took a year to design and improve, but then I used it for years to take data. When Marlow hired me, they asked me to design a similar but enhanced fixture. It was a great opportunity to start fresh and incorporate all of the new ideas I had.

What are you currently working on?

We are targeting higher temperature energy harvesting applications. I think this is a niche for thermoelectric generators (TEG) where other energy harvesting technology can’t compete. In order to move our technology to higher temperatures, we have to target higher temperature materials, new device structures and fabrication methods, new test fixtures and new models. My focus is on the next generation TEG models and test fixtures.

Can you tell us more about Marlow Industries?

Marlow was founded in 1973. The core business has been thermoelectric coolers and systems but in recent years, thermoelectric power generation has emerged. A thermoelectric cooler (TEC) uses current input and produces heat pumping while a thermoelectric generator (TEG) takes a temperature difference and produces current. Marlow is the premier supplier of TECs, TEGs and systems. We are also among the world leaders in developing TE materials, technologies and new products.

How did the idea of EverGen come about?

Until a year ago, the existing thermoelectric technology that was used in energy harvesting applications for low power wireless sensors used thin-film thermoelectric materials. Because most energy harvesting applications will use a passive natural convection heat sink, a bulk thermoelectric material and device is a better thermal and mechanical match for these bulky, high thermal resistance heat sinks. Bottom line, I knew our devices would outperform the competition.

With the introduction of the low voltage DC-DC converters, our low electrical resistance, low cost, and high volume thermoelectric generators could be used for energy harvesting.

What are the goals of this project?

Produce more power and start working at lower temperature differences than the competition. Throughout the project, we investigated what makes an optimal thermoelectric energy harvesting system. Depending on the DC-DC converter technology used, the optimal TEG design (based on pure math and science) can be very similar to Marlow’s high volume, low cost thermoelectric coolers. It was a natural extension to use these types of TE devices for low cost, optimal thermal energy harvesting applications enabling Marlow to provide a cost effective thermal power source to replace batteries for wireless sensors.

How is this energy source able to power wireless sensors for the lifetime of the harvester?

The EverGen series takes advantage of small naturally occurring temperature differences that our customers encounter every day, and converts it into small, sustaining power sources. The unique quality of our products is that it starts to produce power with as little as 2°C temperature difference, much less than the current competition. This opens up a wide variety of applications such as providing power for sensors and actuators incorporated into smart buildings. With a perpetual power source, our customers do not need to worry about replacing batteries or other maintenance. The TEGs operating in their typical temperature ranges will be very reliable and virtually maintenance-free. Marlow leverages its long history producing extremely high reliability products for defense, space, telecommunications, medical and automotive applications.

What does the EverGen series kit offer?

Each system integrates a Marlow electrically-matched, thermally-optimized custom thermoelectric generator and heat sink with an ultra-low voltage DC-DC step-up converter to provide the customer with the tools and flexibility necessary to evaluate a wide range of test conditions. The customer can choose one of four regulated voltages depending on their application needs. It also has a 2.2V, 3mA LDO to power the external microprocessor. The kits have provisions for external energy storage. It also has a PGOOD logic out indicating output voltage is within regulation. And finally, it has built-in power and charge management prioritizing power to VOUT. When there is excess, it directs it to VSTORE.

Can you tell us about the different types of kit?

Each kit was designed to interface with common heat sources and heat sinks. One of the kits, EHA-PA1AN1-R02-L1, is designed to harvest heat from a hot solid surface such as a compressor to monitor if it is overheating. It dumps this heat to cooler ambient air through a natural convection heat sink. Two of our kits, the EHA-L50AN1-R02-L1 and EHA-L37AN1-R02-L1, harvest heat from hot pipes to cooler ambient air. An example of where these could be used is in our factory where we have warm waste fluid leaving our dicing-saw during machining. One could envision monitoring the temperature or pressure of this fluid if it is critical and wirelessly send this data. Our final two kits, EHA-L50L50-R01-L1 and EHA-L37L37-R01-L1, harvest heat between warm and cool pipes. Because this is in essence a water-to-water heat exchanger, the thermal resistance is quite low and we are able to pull much more heat through the TEG, therefore producing more power at lower temperature differences. These devices could be used wherever the customer has a warm and cold water pipe in near proximity to each other, such as the water pipes going to any commercial sink.

What are some benefits that the customers will see using this technology?

Customers will be able to produce small amounts of maintenance-free power wherever they have small temperature differences. By using EverGen products, our customers will see more power at smaller temperature differences, opening up a variety of applications previously not possible.

Where do you hope to see this technology implemented?

Because our bulk thermoelectric technology is so scalable, we can scale up our liquid-to-air harvester to address remote sensing applications for applications such as oil and gas monitoring that require Watts of power generation, multiple TEGs and heat sinks. This technology can be used as a stand-alone power source or work with other energy harvesting technologies.

What direction do you see your business heading in the next few years?

Marlow will continue to expand our thermoelectric technology into new and emerging markets for both cooling and power generation. For energy harvesting, we will be targeting higher power systems ranging from 100’s of micro Watts to multiple Watts and pushing our technology to higher operating temperatures.

What can we expect to see from Marlow Industries in the near future?

EverGen (liquid-to-air systems) will be expanding to higher power output (multiple Watts) and higher temperature harvesting (up to 500°C) for process plants and oil and gas sensing. These assemblies will utilize very similar TEG and converter technology but will be “super-sized” to include multiple TEGs and larger natural convection heat sink areas to accommodate pipes up to 12” in diameter.

What challenges do you foresee in our industry?

Most customers will need a custom thermoelectric solution to maximize power or efficiency of their systems. Thermoelectrics require specialized expertise to successfully incorporate them into systems. Educating our customers that these TEGs need to be electrically and thermally optimized for their systems is difficult. Most EE’s understand electrical matching but don’t appreciate thermal design; and most ME’s understand thermal design but might not understand electrical matching. How do we create standardized solutions that engineers from diverse backgrounds can easily implement?

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