Arizona summers are brutal and Salt River Project (SRP), one of the oldest and largest public power utilities in the United States, understands this better than anyone. Serving more than 1 million customers across three Arizona counties, including the city of Phoenix, SRP is accustomed to dealing with high energy loads during summer months. As SRP customers come home at the end of a summer day, they turn on their air conditioning to cope with the heat, leading to peaks in the electric grid system in the late afternoon.
Like the increase in traffic on freeways during rush hour, utilities experience their own rush during peak hours. During this time, historically from 3 p.m. to 7 p.m., residential customers are cranking up their air conditioners to cool their homes. SRP has addressed peak energy usage through energy-efficiency programs and time-of-use price plans, which encourage customers to use less energy during on-peak hours. SRP is also piloting demand response and home energy management programs to help households manage their demand, but some commercial customers are more restricted in their approach to demand management, as they must continue to use energy during peak times, as well.
In October 2017, SRP announced its 2035 sustainability goals, one of which was to reduce CO2 emissions by 33% (per megawatt-hour) from 2015 levels. SRP intends to achieve that goal by reducing fossil-fuel generation, shifting from coal to natural gas and increasing the use of zero-emission energy technologies. Currently, SRP’s diverse mix of sustainable resources accounts for 17% of retail energy needs, with the goal of achieving 20% by 2020.
Another goal is to modernize the energy grid by helping residential and commercial customers incorporate private energy resources, such as rooftop solar, demand response and energy-efficiency options. Considering the spikes in energy demand during the summer months, advanced metering and enhanced communications infrastructure is another important component of the strategy for reaching the 2035 goals.
Large Commercial Loads
An effective approach to managing peak energy demand is in helping large commercial customers lower their energy usage during peak hours. This will help the utility avoid purchasing expensive energy off the market at higher rates. It also helps avoid reliance on natural-gas-powered peaker plants, aligning with the 2035 goals of reducing CO2 emissions. Large industrial freezer warehouses are unable to take advantage of time-of-use price plans or demand response. This is because they must keep their products at a constant temperature 24 hours a day, 7 days a week to ensure food safety and quality.
SRP recognized that thermal energy storage (TES) technologies might make the most sense for these types of customers by providing a peak-load-reduction approach with lower-cost and longer-lasting discharge times than other storage technologies. SRP wants to help its customers control their energy usage and costs more effectively during on-peak hours, which is why it partnered with Viking Cold Solutions on a TES project. When the TES provider for the frozen (low-temperature) cold storage industry was first introduced to SRP, the utility found its abilities to reduce energy consumption and peak loads aligned well with the 2035 goals.
TES leverages the heat transference properties of phase-change materials (PCMs) to absorb heat in large cold storage facilities. At the heart of the system are modules containing its proprietary PCM. During off-peak hours, a facility’s existing refrigeration equipment freezes the non-toxic, environmentally safe PCM. During peak hours, the facility can reduce its use of high-energy-consuming, grid-dependent refrigeration systems and rely on the PCM to maintain temperature stability. During this time, the PCM absorbs 300 times more thermal energy than the food and 85% of all heat infiltration into the freezer.
SRP decided to implement a test pilot project — the first of its kind in the state — in partnership with Viking Cold and the Bashas’ Family of Stores, an Arizona-based chain of grocery stores. SRP has previously partnered with Bashas’ on other energy-efficiency pilot projects. Customers like these, who are willing to evaluate new energy-efficiency technologies, are important for utilities in helping to develop new policies and programs.
The TES system was installed in the low-temperature refrigerated warehouse of a Bashas’ distribution center. The system contains a specialized formulation of PCMs engineered to freeze at -18ºF (-27°C), temperatures necessary for safely storing ice cream prior to shipment to stores, and one of Viking Cold’s coldest installations (its PCM formulas range from -18ºF to 32ºF [-27°C to 0°C]). The energy and peak-load-reduction performance of the PCM modules is maintained by an intelligent control system with proprietary algorithms that determine when and how to run the refrigeration system for maximum efficiency and temperature stability. The controls incorporate intelligent defrost and fan cycling into the refrigeration unit run cycles, which result in lower energy consumption. The controls also run the refrigeration equipment fully loaded during periods of lower ambient temperatures, also known as off-peak hours, which allows the compressors to achieve their optimal refrigeration efficiencies and remove the same amount of heat with lower total energy consumption.
The proprietary monitoring and predictive software provides an estimate of the yearly energy use and the effect on electrical demand. The heat load estimate considers the physical characteristics such as the size of the freezer, the freezer temperature and the average weather conditions expected for the location. It also estimates the heat load contribution of air infiltration through door openings, incoming product, lighting, equipment, employees and material handling equipment. Evaporator fan motor and defrost direct energy usage are added to the yearly kilowatt-hour usage, as is the corresponding increased refrigeration heat load. Lastly, the model’s evaporator, compressor and defrost assumptions are adjusted to match projected operation after installation of the TES system, and a comparison to the original model predicts the corresponding reduction in projected electrical demand profile and yearly kilowatt-hour usage.
The purpose of this pilot project is to find methods for SRP customers to shift the energy demand outside of the 2 p.m. to 7 p.m. peak period, which can cost as much as 40% more than off-peak hours for large general service customers (those consuming more than 300,000 kWh for three consecutive months). This will help keep down costs for customers and defer infrastructure investments for SRP.
The Bashas’ TES pilot project was launched in January, and data collection will continue for at least one year. This type of a system also presents an opportunity to evaluate alternatives to battery technologies, which are still uneconomical for the heavy load and long duration of demand in cold storage applications. With TES technology, cold storage facilities will still be able to maintain stable temperatures while reducing mechanical run time and overall energy consumption. Additionally, the TES system is easy to install and operate because it is a non-mechanical system that integrates with existing refrigeration, controls and racking. It is low maintenance, it does not interfere in day-to-day operations, and its design and installation do not reduce product storage space.
Bashas’ is one of the largest frozen food distribution warehouses in SRP’s service territory, but SRP has many cold storage customers for whom Viking Cold’s technology might be applicable. Measuring the demand-shift effectiveness of TES will be critical to the success of this pilot.
To that end, SRP is collaborating with Arizona State University’s School of Sustainable Engineering and the Built Environment, along with Nexant’s Utility Services Group, to measure the results of the project and extrapolate the potential impact of this technology on additional low-temperature refrigeration loads in the Phoenix area. It is especially beneficial to have the local university’s faculty and graduate students be aware of the challenges the utility industry faces and how it is addressing them. These future engineers will be able to see first-hand the effectiveness of the technology and how it can apply to other industrial customers in the area. In addition to reducing peak demand, this study will help train SRP’s future workforce about sustainable energy management.
For SRP’s cold storage customers, TES is one of very few realistic peak load-reduction approaches using an energy storage technology. If this project goes well, SRP will evaluate how this technology compares to other peak-load-reduction customer-incentive programs and consider adding TES to shift even more load to off-peak hours. ♦
Alejandra Mendez is a senior research engineer at Salt River Project in Phoenix, Arizona. She specializes in research focused on energy management and power generation in partnership with EPRI and state universities. Mendez has 17 years of electric utility experience specialized in power generation. She holds a BSCE degree from New Mexico State University and is a registered professional engineer in the state of Arizona.
Check out the May 2018 issue for more articles, news and commentary.