Question: What do I need to take into consideration when trying to create an energy-efficient refrigeration system?
Answer: Refrigeration systems are huge electricity consumers and contribute greatly to the costs of running a business with large cooling requirements. In fact, refrigeration alone can be responsible for up to 86% of total energy consumption, depending on the sector. Thankfully, technical improvements have the potential to reduce energy consumption by 15-40% saving you both energy, and costs.
Sustainability Victoria suggests that companies looking to create an energy-efficient refrigeration system should follow these 6 steps:
- Review refrigeration demand
- Review insulation
- Review coolant distribution system
- Review plant, controls, set points and heat rejection
- Optimize maintenance
- Measure the improvement
Using the 6 steps outlined above to guide you on your way is a great place to start when looking to create and maintain an energy-efficient industrial refrigeration system. The next step is understanding the technical changes that have the potential to save you a lot of money in the short, and long term. According to Energy Design Resources refrigeration systems usually comprise four major components: compressors, condensers, expansion devices, and evaporators. Here are some tips for optimizing these parts of your new (and soon to be improved) refrigeration system:
Compressors
When choosing a compressor, make sure you ask about efficiency over the range of expected loads. Consider high-speed, magnetic beating, variable-speed centrifugal compressors, external oil coolers and heat recovery.
Use multi-stage compressors with intercooling where high temperature lifts are required. Consider the efficiency profiles and capacities of all compressors in the system when determining optimal sequencing and loading strategies; screw compressors are more efficient when operating at full-load, however reciprocating compressors have higher part-load efficiency and a linear loading profile.
Condensers
An energy-efficient condenser option is a moderately oversized unit which lowers the saturated condensing temperature and reduces the work demand. While these units require larger fan motors and pumps (which consume slightly more energy), the amount saved is normally more significant than the increase in cost and energy consumption. Keep in mind, when sizing a compressor for your refrigeration system, it is important to consider the required head pressure (you can save even more if you operate a smaller compressor at near full load as opposed to a large compressor at part load.) Finally, to lower energy consumption, reduce condensing temperature by using a condenser with a high surface area.
Contemplating whether or not you should invest in a condenser or a cooling tower? Condensers will typically have a lower capital cost than cooling towers but are less efficient with heat rejection; cooling towers are typically the opposite. Cooling towers may be pricey but will pay themselves off with energy savings in the long-run.
Expansion devices
Valves: Use electronic expansion valves where possible on small systems. Link expansion valve control with head pressure controls (read more about head pressure control here).
Controls: Use a control system that is responsive to compressor head pressure. Aim to achieve the highest possible suction pressure and lowest possible head pressure at all times.
Floating Head Pressure Control: Use floating head pressure control to allow the condenser head pressure to float down to lower levels, taking advantage of an increase in the condenser’s cooling capacity during cooler conditions. Lowering the head pressure reduces the required pressure lift by the compressors, improving their operating efficiency.
Variable Frequency Drives: VFDs can be used in many ways to support an energy-efficient refrigeration system. VFDs or two speed motor controls can be used effectively on evaporator fans and are a very efficient control technology for compressor motors, as they control the motor speed based on required load by maintaining the suction pressure set point of a system.
Premium Efficiency Motors: Choose premium efficiency motors for the compressor motor, condenser fans, evaporator fans, and pumps. This is extremely important for systems that use oversized condensers due to large fan and pump sizes. Efficient motors are typically a bit more expensive, but the energy savings potential will outweigh the initial cost, especially given the high number of operating hours of industrial refrigeration systems.
Purgers: Non-condensable gases in your system can reduce both capacity and efficiency, and lead to wear and tear on your compressor. Purgers can minimize the effects of non-condensables by expelling them at points of accumulation throughout the system.
High Efficiency Lighting Fixtures and Controls: No surprise here: high efficiency lighting, and lighting controls, will reduce energy consumption and minimize heat gain from lighting (which can add to the system load). Take a look at using energy-saving fixtures like occupancy sensors, bi-level lighting, and timers to complement your energy-efficient lights.
Rapid-closing Doors: Consider installing automatic door closers, high-speed storage doors, and air curtains or plastic strip curtains to minimize energy losses.
Evaporators
Liquid overfeed evaporators are typically more efficient than direction expansion evaporators, and are common in many industrial refrigeration applications. Don’t be surprised however, to learn that liquid overfeed evaporators can have a higher installed cost because they generally require larger diameter refrigerant lines and piping insulation, and require more maintenance than the alternative.
One more tip? If you aren’t using a demand-based defrosting systems to maintain your evaporator coils, do so now. These systems determine when defrosting is required, based on temperature or pressure drop, frost accumulation, and humidity levels.
Take a look at your secondary distribution system
When it comes to considering the efficiency of your system, Save Power and EECA Business, suggest you pay extra attention to the secondary distribution system. This includes elements like pipework, valves and cooling liquid. Choose a cooling liquid with high heat capacity and good flow properties, and use the right size pipes to minimise pressure and prevent heat gain. Take a look at your system design, and keep an eye out for bends that can be shortened, throttle valves, T-intersections and other things that might restrict fluid and gas flow. Don’t forget to double-check the state of your insulation. Creating a building envelope will prevent heat transmitted through walls, ceilings and floors from adding external loads onto your system.
Needless to say, maintaining an energy-efficient system is the key to keeping costs low and seeing a quick payback after making the initial investment. Develop an ongoing maintenance plan and inform staff members about what to look for when it comes to inefficiency in the system (ice-build up on pipes for example) and encourage energy-saving behaviour like closing doors and making sure that seals are closed after loading.
Case study: Dreyer’s Grand Ice Cream
Making these types of changes can help you save thousands of dollars in energy costs. Take a lesson from companies like Dreyer’s Grand Ice Cream in Bakersfield, CA, a company that underwent large amounts of construction in order to boost ice cream production, tackling storage, distribution and R&D facilities.
During construction, the company installed a number of energy-saving measures like evaporative condensers; floating head pressure using variable set-point control strategy; premium efficiency compressor motors and air unit fan motors; oversize regeneration heat exchanger; lighting controls (occupancy sensors). They also reduced lighting power density in offices as well as lighting power density in dry warehouse. The result was a total energy savings of over 3.26 million kWh per year which translated into cost saving of $266,000 per year and a payback of 1.3 years.
Questions about creating an energy-efficient industrial refrigeration system? Leave them below!
image credit: rodney_619
