Refrigerated Warehouse

The following case study illustrates some of the savings that can be made within refrigerated warehouses. Some of the measures were implemented after (and during) an extended period of monitoring to ensure that the end requirement was still achieved i.e the temperature within each of the chill chambers was maintained at the required level.

Warehouse Layout

The warehouse being used in this case study has 6 chill chambers, 2 Azane chill chambers, 2 FRV Chambers and 1 ambient chamber. The chill chambers are to be kept at 3.5 deg C and FRV at 11degC. The chill & FRV chambers are cooled by the main plant, azane chill chambers cooled by 2 azane chillers.

The main plant has 6 compressors which are configured such that 2 are used for FRV and 4 are used for chill.

Energy Saving Measures Introduced

1. Control of Chamber Temperatures

Initially the control logic for the temperature control within the chambers was defined to work off an average of the temperature sensors within the chambers and a hard setpoint and diff were defined. It was found that chambers would constantly sit at 0.1 or 0.2 deg below the higher diff for a long period of time before any cooling was started and converse when the chamber had cooled.

LCS replaced the hard setpoint with a PID loop for the setpoint and diff which resulted in cooling being started earlier and not needing to have been running as long to cool the chamber. It has also lead to a smoother graph of the temperatures within the chambers with temperatures for all sensors not drifting as much from the required setpont.

2. Fans & Glycol Pumps Only

The initial control logic was configured to run a compressor as soon as any cooling within any of the chambers was required. 

Through some calculation, monitoring and testing LCS implemented logic that delayed the compressor start and used the residual temperature of the glycol (-3degC) and fans to provide some initial cooling and air movement before a compressor was required to run for the cooling of the Glycol. This resulted in energy savings from the reduced compressor run times.

3. Defrost based on Probability

The initial control logic was configured to defrost each of the coolers at a specific time. Combined with 1 and 2 above it was found that the coolers weren't running as often and not getting iced up as much. LCS implemented the defrost on a function of the cooler runtime which reduced the defrost cycles for each cooler leading to a reduction of heat being introduced into the chamber and the requirement of a compressor to be run to heat the hot glycol for defrost.

4. Compressor Start based on Cooler Demand

As described in 2 above the intial logic was to start a compressor when a chamber needed cooled. The logic in 2 above reduced this requirement and additional logic was introduced to determine when a compressor was started (based on number of coolers). In additon new logic was introduced to control when other compressors were required to start/stop due to additional demand including PID loops to control the loading and unloadng of compressors.

5. Comprosser Control based on Glycol Temperatures

Initial design and configuration was for the compressor control to be based on the return temperature of the glycol from the warehouse. This had the potential for the compressors overrunning and freezing the heat exchanger plates. LCS implemented additional glycol temperature probes on each of the main plant compressor and changed the control ogic for each compressor so that their control was based on the differential between the glycol temperatures at their own plates reducing the possibility of freezing the plates and redcing the need for that compressor to run.

6. Replacement of fixed speed drives with Variable Speed Drives

It was found that the coolers fans for the 2 Azane Chill Chambers were running approximately 100% of the time during summer months. LCS performed an analyses of the the effective air throughput within the azane chill chamber vs the reduction of fan speed vs electrical power usage and provided the refrigeration contractor and end customer with approximate cost savings and ROI of implementing variable speed drive controls for these fans.

The intial replacement resulted in an immediate 10% saving in electricity consumed and testing/monitoring is still being performed to determine if further savings could be made (expected up to 30% in general use with the potential for 50% dependant on crcumstances).

Summary

Energy savings in refrigeration systems are possible with some being easier/quicker to implement than others. To get the best savings the system needs to be considered as a whole and a longer term plan maybe required to implement all of the above.

LCS do not have direct access to the electrical usage for the plant above but through the steps above we have reduced the run hours of the FRV compressors from 1 per day during summer months to 1 occassionally and chill compressors from 2-3 per day during summer months to 1-2 per day during summer months.