Innovation Drives Sustainability Success at DENSO Manufacturing Tennessee Plant


Innovation is at the core of virtually every initiative at DENSO Manufacturing Tennessee, Inc. This includes a host of activities designed to allow the leading supplier of advanced automotive technology, systems and components, to realize its corporate vision of creating a sustainable automotive society.

Among key initiatives at DENSO’s Maryville, Tennessee, facility is the use of an innovative ice-storage system engineered to provide environmentally friendly comfort cooling to employees at the company’s main production facility. The system also allows Plant 101 to reduce cooling costs per ton by 44%, while providing a payback of less than four years. It also resulted in an annual carbon dioxide (CO2) reduction of 18,000 tons.


Taking a Lead in Environmental Sustainability with Eco Vision 2025

The Maryville facility began operation in 1988. It encompasses 2.6 million-square-feet under roof for the entire campus and its 230-acre campus is comprised of four major facilities and 13 buildings. Plant 101 – the main production facility where the ice-storage system is in operation – spans 708,575 square-feet. It is the largest production plant on the Maryville, Tennessee, campus. In addition to Maryville, DENSO operates a production facility in nearby Athens, Tennessee.

Denso Aerial

DMTN’s 230-acre campus in Maryville, Tennessee.

The DENSO Maryville operation produces starters, alternators, and instrument clusters, as well as various automotive electronic products and inverters for hybrid vehicles. The plant operates three shifts, seven days per week. In all, the plant employs more than 4,300 individuals and is ISO 14001, ISO/TS 16949, and QS 9000 certified.

Mike Wingo, a Section Leader who manages the Environmental Engineering Department at DENSO Maryville, said DENSO’s corporate program, Eco Vision 2025, drives DENSO’s commitment to sustainability. The impetus for this program was to decrease energy usage and CO2 generation. Wingo said the sustainability goals, as well as a desire to decrease energy costs and ongoing improvements in operational costs, led to the decision to upgrade Plant 101’s comfort cooling system, which was originally comprised of 21 direct expansion (DX) air-handling units.

“A number of factors went into the decision to improve comfort cooling at the plant as the operation and the cooling loads continued to grow, not the least of which is the continued opportunity to reduce our carbon footprint and energy consumption,” Wingo said.  

DENSO created Eco Vision in 1997, which is an action plan to help spark and invigorate a renewed awareness for environmental responsibility. The initiative also provides DENSO facilities with a blueprint to achieve a wide range of sustainability goals through various means, such as non-traditional methods and technologies designed to improve energy efficiency and waste reduction efforts.


Multiple Issues with Direct Expansion Air-Handling Units

As with any manufacturing plant, comfort cooling is crucial for optimal working conditions and employee morale, which also contributes to overall productivity. This is especially true in the depths of hot Tennessee summers.

Before the ice-storage system project, Plant 101’s HVAC system included 21, stand-alone and self-contained DX units manufactured by Webco to provide comfort cooling. The air-handling units are located throughout the plant rooftop. A DX unit consists of a refrigeration system, compressor and condenser, with air handling working through the evaporator or the cooling coil, air filter and blower. Air is directly chilled by the refrigerant (in this case R-22 or HCFC 22-Freon) via the unit’s cooling coil.

The DX units at Plant 101 – each 20-years or older – lagged compared to today’s more energy-efficient alternatives. Additionally, the plant’s heat load requirements steadily grew. From a cooling standpoint the units were struggling to keep up, yielding a limited cooling range of 85-95 °F. They also became costly to operate. The units’ refrigeration compressors required frequent replacements or rebuilds.

The air handling units also experienced frequent refrigerant leaks, driving the need to replace 2,600 pounds of R-22. From a sustainability standpoint, DENSO recognized an opportunity to eliminate R-22 and reduce CO2 emissions in keeping with Eco Vision 2025, which calls for a commitment to reducing CO2 emissions by half.  


Ice-Storage System Examined

The DENSO team analyzed common cooling system options, which included the replacement of the DX units with new units. Another option was to invest in traditional chillers. Yet the planning process in partnership with Trane® and CALMAC led to the discovery of another option in the form of Trane’s IceBank® Thermal Energy Storage system.

DENSO also regularly explores innovative ways to achieve sustainability goals and save costs, pointing to its ongoing participation in the Tennessee Department of Environment and Conservation’s Tennessee Green Star Program. The voluntary program recognizes companies in Tennessee committed to sustainable best practices. (Read more about the program at

“We are actively in involved in TGSP,” Wingo said. “The program and its members are committed to going above and beyond state and federal requirements geared toward sustainability – and it provides a great platform for sharing sustainability ideas.”

A key idea of the plant was thermal energy storage, which is known commonly as ice-storage. A typical ice-storage system includes a chiller(s), ice-storage tank(s), pumps, cooling coils, and heat-transfer fluid. The heat-transfer fluid, commonly a mixture of glycol and water, is circulated inside the heat exchanger within the storage tank in the process of creating ice. The purpose of the system is to save on energy costs for cooling by using the stored thermal energy to supplement traditional forms of process cooling.

An ice-storage system produces ice during off-peak hours and normally at night when utility rates are typically lowest. The ice is stored in ice-storage tanks. When the chilled water (heat-transfer fluid) is needed to supplement cooling during on-peak hours, the heat-transfer fluid is routed throughout the storage tanks at a temperature above freezing point, causing the ice to melt – and creating the chilled water at the desired temperature.


Shown are Trane’s IceBank® ice-storage tanks as part of the ice-storage system at DMTN Plant 101.


Unique System Includes Ice-Storage and Retrofitted DX Units

At the DENSO Maryville facility, the team opted to use the ice-storage system to provide chilled water to the DX units. Installed in 2018, the system includes five air-cooled chillers from Trane, each with a 500-ton capacity. All chillers have the ability to perform traditional cooling. Three of the five chillers serve as primary chillers used for comfort cooling in the plant. The remaining two chillers are able to make ice while also providing comfort cooling.


One of five Trane air-cooled chillers as part of the ice-storage system at Plant 101.

Also key to the system are the ice-storage tanks. There are four CALMAC Ice Bank® Model-1320CSF ice tanks with each unit rated at 324 ton-hours (1140 kWh); and three CALMAC Ice Bank® Model-1500CSF, each of which is rated at 486 ton-hours (1,710 kWh) for a total of 2,754 tons of cooling capacity. The system also includes piping, pumps and a glycol-based heat transfer fluid that contains 25% glycol in addition to water.

Mark Johnson with CALMAC Portfolio said the air-cooled chillers are an excellent choice for the ice-storage system at Plant 101.

“While traditionally water-cooled chillers are more efficient than air-cooled chillers, the latter with the use of ice storage can deliver similar energy costs to water-cooled chillers because they do most of the work at night when ambient air temperatures outside are cool and the chillers are running during off-peak hours when energy rates are low. Air-cooled chillers also eliminate the need for cooling towers, water make-up due to drift and blowdown, and water treatment.”

The system of five chillers is designed as an N+1 system for redundancy to allow for system availability during routine maintenance, or the unlikely event of component failure.

The use of an ice-storage system drove the need to retrofit the DX units. To do so, the team switched out the refrigerant coils of the units with chilled-water coils, which essentially allows them to do what they’ve always done: deliver cool air where needed – but without the use of a refrigerant. The retrofit also included piping to connect the ice-storage system to the DX units. While typically steel piping is used, the team opted to use polyethylene piping given its comparatively lower weight and price and proven performance in cooling applications.


Flexible Comfort Cooling Strategy

Most ice-storage systems are a supplemental method of cooling used in combination with traditional chillers. This is the case with Plant 101 where two chillers are dedicated icemakers, and three chillers – referred to as primary chillers – function as any traditional chillers to provide cooling water. While all five chillers can function in this capacity, the two ice-making chillers also have the ability to run at lower set points to produce ice.

With this system cooling can be delivered in a number of ways depending upon system demand. During hot summer days or during on-peak high-heat demand, chillers and ice may be needed. However, during spring and fall when temperatures are milder, and heat loads are lower, the ice can exclusively cool the facility. In all, the system at DENSO allows for the following combinations of operating strategies:

1. Cooling with the chillers only.
2. Cooling with the supply from the ice-storage system only.
3. Cooling with a combination of chiller and a supply from the ice-storage system.

Ryan Miles, Facilities Mechanical Engineer at DENSO Maryville explained, “Typically, when the cooling water from the ice-storage system alone can’t fulfill the cooling need at Plant 101, like during peak times, the three primary chillers work on a lead/lag scenario to provide cooling by chilled-water to the DX units.”

The ice-making chillers have the capability to run for comfort cooling just as the three primary units; however, they can also run at lower set points to produce ice.

“These two chillers also rotate on a weekly lead/lag basis,” said Miles. “During off-peak utility hours and when the plant heat load is lower, the ice-making chillers are isolated from the main process-cooling loop to the building so the system can cycle chilled water through the ice tanks to form ice. In this way, the N+1 system can operate up to four chillers at once.”


Continued Sustainability Progress

Following the completion of the ice-storage system project in the summer of 2018, the DENSO team saw immediate results, including a more comfortable production environment since the system delivers cooling temperature at 78 °F, which is considerably cooler than the capabilities of the original DX units.  

In keeping with Eco Vision 2025, the installation has also allowed Plant 101 to reduce the power consumption per ton of cooling by 44%. What’s more, the cutback in power consumption creates an annual CO2 reduction of 18,000 tons. The high costs of maintenance with the DX units have also been eliminated.

While the ice-storage system continues to deliver results, DENSO’s Maryville plant continues to make progress on multiple Eco Vision initiatives.

Just one example is landfill reduction. Since 2000, the plant has reduced 99.7 percent of all waste. In 2017, it diverted nearly 30 million pounds to landfill and replicated this by reducing 27.5 million pounds in 2018. On average, annual waste headed to local landfills is now nine tons versus 2,928 tons per year before the plant adopted best practices in waste reduction.

“We’re proud of progress we’ve made with all of our initiatives in support of achieving our Eco Vision,” said Wingo. “The new cooling system allows us to achieve virtually every goal we set out to achieve. Looking ahead we’re excited to see how the ice-storage system data shapes out in coming years, and we hope to see it lead to other ice-storage system initiatives. In the meantime, we remain committed to continuing to keep pace with our landfill diversion program.”

All photos courtesy of DENSO.

To read similar Chiller and Cooling System Assessments articles, visit

Thermal Energy Storage System: A Closer Look

The following describes how Trane’s IceBank® Thermal Energy Storage system works.

Step 1

During nighttime, off-peak hours, water that contains 25% ethylene or propylene glycol is cooled by a chiller. That solution circulates inside the heat exchanger within the IceBank tank, freezing 95% of the water that surrounds the heat exchanger inside the tank. The water surrounding the heat exchanger never leaves the tank.

Step 2

Ice is created uniformly inside the IceBank tank via CALMAC’s, counter-flow-heat exchanger tubes. As ice forms, water still moves freely, which prevents damage to the tank. To fully charge an IceBank tank takes from six to 12 hours.

Step 3

During daytime on peak hours, the glycol solution circulates through the ice storage tanks to deliver the stored energy to the building to augment or offset electric chiller cooling. The cold glycol is delivered at the proper temperature to the cooling coil in an air handler.

Step 4

A fan blows air over the coils to deliver cooling to the occupant spaces. People feel cool and comfortable and never know ice storage is being used to save money on cooling costs.