Thermal Energy Storage Reaps Financial Benefits
Viking Cold Solutions, Inc. conducted a Measurement and Verification (M&V) study of its thermal energy storage (TES) technology installed in an industrial low-temperature cold storage warehouse. The objectives of the M&V study were to determine the effectiveness of TES on energy efficiency and temperature stability with an ammonia-based refrigeration system.
Facility and Electricity Rate Plan
The host site for the M&V study was a frozen food distribution center owned and operated by Dreisbach Enterprises in Richmond, California. Viking Cold’s patented TES system was installed in a 93,000-square-foot low-temperature freezer, part of a facility also comprised of mediumtemperature refrigerated storage, dry storage, and office space. Refrigeration for the low- and medium-temperature cold storage rooms was provided by a central ammonia refrigeration plant composed of multiple staged screw compressors and water cooled condensers. Peak period consumption charges and peak demand charges account for nearly 50 percent of annual energy costs at the Dreisbach facility.
Phase Change Material
Phase Change Material (PCM) is a substance with a high latent heat of fusion that remains near a constant temperature while storing and releasing large amounts of energy. During
its transition between solid and liquid states, Viking Cold‘s PCM absorbs up to 85 percent of heat infiltration and maintains more stable temperatures to better protect food product.
This environmentally friendly, food safe PCM is engineered by Viking Cold to freeze and thaw at the desired temperature setpoint of the customer’s freezer.
M&V Study Design
The M&V study was designed to isolate the temperature and energy benefits of TES in low-temperature cold storage facilities. Three operational variables were measured: temperature as a function of time, power consumption (kWh), and power load (kW). Since Viking Cold has implemented TES systems in many other facilities with significant positive results, this study was designed to understand the efficacy of TES under the conditions and challenges of large ammonia refrigerated freezers. To establish the baseline performance of the freezer, all refrigeration equipment ran under normal operating conditions without Viking Cold intervention for three weeks. During this period, the Viking Cold controls equipment measured and captured temperatures, energy consumption, peak demand, and refrigeration equipment status.
Thermal Energy Storage Installation and Configuration
After the gathering of baseline data was completed, the TES modules containing phase change material were installed in the freezer. The TES modules were installed on top of the
highest cross members of the product storage racking in the direct airflow path of the refrigeration evaporator fans. The integrated Viking Cold control system was configured to optimize the operating sequence of refrigeration. Multiple temperature sensors within the freezer provide new inputs to the existing refrigeration control sequence to effectively leverage the thermal energy stored in the TES modules. Viking Cold’s proprietary algorithms calculate how to best manage the refrigeration system to minimize energy consumption while maintaining stable desired temperatures inside the freezer. The key strategy to reducing energy consumption and peak demand is to run the refrigeration equipment fully-loaded during the lower cost off-peak hours when ambient temperatures are lower, which yields maximum mechanical and condensing efficiency as well as lower energy costs. As ambient temperatures increase during peak hours of the day and the heat rejection of the condenser is less efficient, the control algorithms reduce or stage the run time of the refrigeration system and allow the fully charged TES modules (frozen PCM) to absorb the heat infiltration and maintain temperature stability inside the freezer. Temperature and energy data are collected at five-minute intervals and securely stored in a cloud-hosted database. Live data is accessible by the operator to monitor conditions
inside the freezer with automated notifications and alarms triggered by events set to customer-defined parameters. The Viking Cold algorithms are programmed by default to prioritize temperature compliance above energy savings and will automatically restore mechanical refrigeration if necessary to maintain temperatures within acceptable ranges.
One year later, the same data set that was recorded during the baseline was captured again and analyzed to conduct the M&V study. In doing so, seasonal weather and business
cycle variables were determined not to be a significant factor in the results.
Energy Consumption Reduction
Looking at total energy consumption across the entire facility, the entire facility experienced a weekday energy consumption reduction of 20 percent after implementation of the TES system. This was slightly offset by a 5 percent increase in off-peak weekend consumption by the freezer in order to fully recharge the TES modules in preparation for the following week. The net result was a 13 percent reduction of energy consumption for the entire facility each week. When measuring the decrease in energy usage of just the freezers where the TES was installed and in operation, refrigeration energy consumption was reduced by 35 percent. Peak Period billing occurs Monday through Friday, 8:30 am through 9:30 pm, and accounts for nearly 50 percent of annual energy costs for the facility. These expensive peak period hours were targeted for reduction to maximize dollar savings. Peak period kWh consumption across the entire study period was reduced by 43 percent.
Peak Demand Reduction
Peak demand charges are calculated by determining the peak load (kW) of the facility at any point during the peak period. By using the stored thermal energy of the TES system, freezer refrigeration load could be reduced for long periods of time. The TES system lowered peak refrigeration load (low and medium temperature) by 251 kW, a 29 percent reduction for 13 hours each day.
Greater Temperature Stability
The energy savings and cost reductions detailed above were accomplished simultaneously with better maintenance of operating temperatures within the customer’s specified
range. The data showed that the temperature stability inside the freezer improved with the addition of the TES system. The temperatures measured during the M&V study showed significant improvements in two statistics – hourly standard deviation and rate of rise. Even during periods when the refrigeration equipment is not operating, the release of stored thermal energy can better control temperatures in the freezer.
Standard Deviation The hourly standard deviation during pre-TES baseline measurement duration is consistently about 4° F throughout the day. The hourly standard deviation after
TES installation, where the system prioritized performance during the peak period is approximately 2° F, a 50 percent improvement in temperature stability. This demonstrates that even during periods when the refrigeration equipment is not operating, the release of stored thermal energy can better control temperatures in the freezer.
Rate of Rise
This M&V study also conducted an experiment to quantify the rate of temperature rise with and without TES inside the freezer. Temperatures were measured in multiple locations within the freezer. One set of temperature monitors was positioned vertically at the middle pallet position, and the second set was positioned vertically at the top pallet position to identify and quantify temperature stratification. The same experiment was performed during the baseline period before TES was installed and again after the TES system was installed. For each experiment, refrigeration was completely shut off and temperature data was recorded for 15 hours. During the baseline rate of rise test without TES, the average temperature in the room exceeded the upper customer specified limit after six hours. Note that the upper pallet positions recorded temperatures above the limit within minutes after refrigeration was shut off. After TES was installed, the average temperature in the room remained within the limit for the entire 15-hour duration of the test, and the upper pallet positions remained within the upper limit even with refrigeration shut off. These results illustrate how the TES system is able to maintain the temperature integrity of the products despite temperature stratification inside the freezer while reducing energy usage of the facility for over 15 hours in this case.
M&V Study Conclusions
The results of the M&V study demonstrate the ability of TES to reduce energy consumption by 43 percent and reduce peak demand by 29 percent while maintaining 50 percent more stable temperatures in a warehouse. TES technology has significant operational and financial benefits to cold storage opera-tors, to the cold storage industry, and to the utilities that are challenged to manage the peak loads on the electrical grid.