How much do you spend each month or year to keep your perishables fresh? From meats to dairy and frozen foods, to fresh produce and floral, it has been estimated to that between 30% and 50% of the electricity used in food retail is consumed by the refrigeration system. Ambient conditions that differ from the original design and equipment selection conditions adversely affect the energy used in refrigeration. Then, other inefficiencies creep in over time, such as dirty coils, burned up fan motors, etc., all the while driving up energy costs. Systems that were put in as the state of the art systems a few years ago must be refined, tweaked, managed, updated and replaced on a regular basis because of newer, more efficient control schemes that were not available at the time of installation.
What is the first step to improving your refrigeration system efficiency? Well, it is not upgrading or replacing the control system. The first step is system recommissioning. Over time, with service calls and owners keeping the system in operation, your systems drift away from the original specifications and set points. Recommissioning resets all of this and gives you some level ground to work from when looking at the controls system.
Establish Control Strategies
The next step is where monitor and control systems including energy management system come into play. There are many new technologies that help automate refrigeration control, monitoring and maintenance scheduling.
Floating Suction Pressure Control
Adjusting and maintaining suction pressures allows the system pressure to float above established set points and maintain case or walk-in temperatures. This is called Floating Suction Pressure Control (FSPC). For each 1 PSI increase in suction pressure, it is possible to save 2% in compressor power. The control systems read the systems temperatures and pressures then adjust the set points to ensure that the refrigerant leaving the coils reach the superheat design parameters.
Floating Head Pressure Control
An additional control strategy is reducing the compressor discharge pressure or head pressure. This strategy is known as Floating Head Pressure Control (FHPC). By allowing the control system to reduce the head pressure, the system could save as much as 0.5% per PSI reduction. At least one study found 14% savings of combined compressor and condenser energy consumption for floating head pressure controls with variable frequency drives.
Control systems do a great job of making the system operate more efficiently, but it can also perform other cost saving measures for your store. This is where load shedding comes into play. Suppose there are some functions that are not required to operate during a peak load time of day, such as anti-sweat door heaters, these loads can be turned off for that period of time. That will not only save some energy, but more importantly, it will reduce the electrical consumption at higher rate times. Control systems also can coordinate different aspects of the refrigeration system so some large loads do not operate at the same time, if possible, thus reducing the increased demand charges.
Selecting the right energy management and control system can also unify other systems besides refrigeration, such as lighting and HVAC. A unified energy management and control system allows the building HVAC management system and the refrigeration control system to work together to keep the systems from fighting over the temperature and humidity conditions in the space. It can control the lighting within the store to conserve energy when spaces are unoccupied and when the store is closed, or there is less traffic.
Approach to Control System Upgrades
The key to a successful upgrade of any refrigeration system comes from a thorough, front-end engineered approach. The methodology for the upgrade can be in the form of 1) a phased migration, 2) complete system replacement, or 3) a system upgrade. An important point to remember is that any new user interface must communicate with any existing controllers on a continuous basis.
A phased migration is often the best approach to large scale systems such as large grocery stores. This approach eliminates risk of system failures or inoperable systems for any period of time, while providing a fallback position with the existing control system, should a failure occur. While phased migration does have its drawbacks in terms of cost and time, the offsetting benefits include reduced risk and less downtime.
The approach of control system upgrades or complete replacement offers benefits which include: 1) increased asset protection, 2) increased reliability, 3) improved efficiency, 4) faster information access, 5) better interface functionality, 6) improved component communication, 7) reduced service and implementation costs, and 8) lower component costs over legacy systems.
Significant savings are possible
Some controls manufacturers have stated that controls systems can save at least 15% on the electric bill due to better capacity control, set point shifting, scheduling, load shedding and enterprise wide energy management. The payback for such systems can be 1½ years through energy savings alone, not to mention the reduction in unplanned maintenance calls.
Could your facility use an extra $35,000 added to the bottom line each year?
One study found in one of the largest supermarket chains, a survey of 50 stores showed the majority had control systems but that the control strategies were not implemented correctly. These misgivings provide an opportunity for savings of up to 335,000 kWh annually per site. This opportunity to save money was hidden because the building operators believed they already had the full benefits of controls.
Control systems have come a long way over the last few years. Is your system doing all that it can for you? Are you missing out on significant operational savings due to your current control system, or maybe just from the control scheme? A small investment could allow you to reap large paybacks to the bottom line.
The first step to improve your operational costs is to contract with a commissioning agent or firm to conduct a systems commissioning, then work with an experienced controls engineer to develop the controls system strategy that is configured to best serve your system needs.
Miguel Purdy, P.E. is a licensed mechanical Engineer who has over 25 years of refrigeration system engineering design expertise. Miguel serves as the Program Manager for Refrigeration at HFA can be reached at Miguel.Purdy@HFA-AE.com or 479-273-7780.
1 Based on 335,000 kWh x $0.1045/kWh = $35,007. National average electricity cost = $0.1045/kWh
 Supermarket Controls and Commissioning: Uncovering Hidden Opportunities, Diane Levin and Lawrence Paulsen, Portland Energy Conservation, Inc., 2006 ACEEE Summer Study on Energy Efficiency in Buildings.