How utility demand response works
In recent years, many utilities have started offering demand response programs designed to cut electric consumption during peak times of the day when electricity is in high demand. Each program is influenced by many factors, including the transmission system, the individual utilities involved, and the technology used to trigger a response. When electricity demand is at its peak, utilities either have to buy power from other utilities or employ their least efficient generating equipment. By avoiding these costly alternatives, both the utility and the customer save money.
Despite being a proven solution, demand response programs are not widely used by supermarkets in the US. According to the US Department of Energy’s ENERGY STAR® program, the average supermarket uses 50 kilowatt-hours (kWh) of electricity and 50 cubic feet of natural gas per square foot per year. That translates to an average energy cost of $4 per square foot, with up to 60% of that cost spent on refrigeration. A carefully considered demand response program with the appropriate control systems can achieve substantial energy savings while still protecting food integrity by optimizing refrigeration system management.
There are several options available for use in a demand response system, which vary according to regionality, the utility offering the program, and customer preferences.
Thermal storage is a way to optimize thermal potential by shifting electricity use from expensive peak rates during the day to lower rate periods during the night. With a typical thermal storage system, a liquid medium is chilled at night when electricity prices are low. The chilled medium—usually glycol or ice—is stored in tanks. During the day, the medium absorbs heat from the display cases, reducing the need to run mechanical equipment and improving its efficiency.
Thermal storage capacity can also be used to produce a revenue stream when it’s connected to other stores through a district heating and cooling (DHC) system. DHC systems use a central plant to produce steam, hot water, and/or chilled water that is piped underground to specific areas within a district (the district can be as large as several buildings or as small as part of a mall or shopping center). As a result, there is no need for each area in the district to have its own source of thermal energy.
Heat recovery can also be utilized by a DHC system. With heat recovery technology, the heat that would otherwise be rejected by the refrigeration system’s condensers is used by the facility. However, this type of DHC system is only viable with refrigeration systems that use refrigerants under high pressure, such as CO2, which can have an exit temperature as high as 175 °F, hot enough to replace an oil or gas boiler. While CO2 as a refrigerant is prevalent in Europe, it is only slowly being adopted by the rest of the world as a “green” alternative to HFC refrigerants. For stores using CO2 with a heat recovery system, the size of the separate heat source can be greatly reduced or even eliminated.
Smart electric grid
A smart electric grid incorporates high-tech digital devices in transmission, distribution, and customer equipment to optimize electric usage by metering electricity consumption so customers can make informed energy decisions.
Smart thermal grid
A smart thermal grid is a broader concept that integrates all sources of heating and cooling coming from thermal storage, heat reclaim, and unused compressor capacity. Used in smart city concepts designed by European urban planners, a smart thermal grid can accommodate any changes in the supply and demand of thermal energy and facilitates the highest system efficiencies.
When viewed all together, demand response, DHC and smart grid concepts give supermarkets fresh alternatives on how their thermal potential can be used profitably in the not-so-distant future.
The good news is that to implement these tactics, the technology exists now in the form of intelligent central refrigeration control systems. While many supermarket owners use a central management controller to connect multiple cooling cases, compressors, lighting, etc., they may not be aware that some system controllers can use demand response, DHC, and smart grid opportunities. For example, the Danfoss ADAP-KOOL® System Manager AK-SM 800 Series not only features a full web interface for remote monitoring and data management, it also incorporates built-in demand response capabilities that can take advantage of utility incentive programs. It also supports heat reclaim technology and a variety of refrigerants, including CO2.
In one European case study, a supermarket using an intelligent system controller for a CO2 system was able to reclaim 40% of its heat loss and utilize it within a DHC network. It produced a revenue steam of up to $9,000 per year based on a heat value in that location of $28 per mWh above 150 °F, resulting in a payback of just a year and a half.
By using demand response and DHC systems to unlock the thermal potential inside their stores, supermarkets can not only reduce their energy costs, but can even generate new revenue. Regulators can encourage the move towards these systems by allowing utilities to offer incentives that reward supermarkets for using energy-conserving technologies that benefit themselves, the public, and the planet.