CO2 Scrubbers for Controlled Atmosphere Refrigerated Warehouses
Refrigerated Warehouse: CO2 Scrubbers vs. Molecular Sieve Absorbers (MSAs)
Devices using activated carbon to remove carbon dioxide which results from the respiration of fruit stored in controlled atmosphere refrigerated warehouses.
Item ID: 455
Process Loads & Appliances--Industrial Processes
Controlled Atmosphere (CA) refrigerated warehouses are used to store tree fruit after harvest. The fruit is then withdrawn throughout the year to meet product demands. A refrigeration system is used to remove field heat and then tightly control temperature over the course of the storage interval. Control of temperature, oxygen and carbon dioxide yields high edibility and visible fruit quality---for instance, for Golden Delicious apples, the optimum oxygen concentration is about 2% while the optimum CO2 concentration is about 1.5%. Staying at optimum concentrations provides for a 6 to 8 month storage life without storage scald or internal browning. Even under controlled conditions, CO2 is released due to fruit respiration. This "excess" CO2 must be removed to protect fruit quality. With a "feed and bleed" approach, nitrogen gas is admitted to a warehouse storage room with a corresponding volume of CO2-enriched gas removed.
Activated carbon CO2 scrubbers are a more energy-efficient method for CO2 control than molecular sieve absorber (MSA) units. Energy savings from the use of CO2 scrubbers is dependent upon the baseline method used for gas composition control. If nitrogen gas is fed into the storage rooms, savings are dependent upon the technology used to produce the pure nitrogen gas. Often, delivered gas is available or warehouse operators produce the gas on-site using air compressors and molecular sieves or bundles of hollow membranes. CO2 scrubbers are available and work well. Typically, MSA units require an average input power of 21 kW. One CO2 scrubber may replace two MSA units while requiring only 5 kW. Advanced scrubber on-demand controls provide additional savings as they cycle scrubbers off when CO2 concentrations are within acceptable levels.
Baseline Description: Controlled Atmosphere Warehouse with Molecular Sieve Absorber Units
Baseline Energy Use: 529200 kWh per year per Facility
The atmosphere from rooms in a controlled atmosphere fruit storage warehouse is held under controlled conditions. Carbon dioxide produced from fruit respiration can be circulated through a molecular sieve absorber (MSA) to maintain CO2 within allowable limits. The MSA unit contains a material that absorbs CO2. Carbon dioxide captured by the absorber material is periodically removed by heating the absorber material and then releasing the gas that evolves into the atmosphere. In the manner, the absorber material is regenerated. A typical MSA unit would have a 42 kW electrical heating element that cycles on and off every 15 minutes (leading to an average power requirement of 21 kW). A large fruit warehouse may have multiple MSA units (three or four). With an annual operation of 6,300 hours/year, the baseline energy use for a single MSA unit is about 132,300 kWh/year. Assuming four MSA units per warehouse gives a total MSA energy use of 529,200 kWh annually.
Manufacturer's Energy Savings Claims:
Currently no data available.
Best Estimate of Energy Savings:
"Typical" Savings: 87%
Energy Savings Reliability: 5 - Comprehensive Analysis
CO2 scrubbers can be installed or retrofitted into a fruit storage warehouse to eliminate the need to operate MSA units or greatly reduce the horsepower and operating hour requirements of nitrogen generators (the nitrogen generators are still required as large volumes of nitrogen gas are used to purge oxygen from a room when it is initially filled with fruit and sealed). In one MSA replacement project, four MSA units were replaced by two CO2 scrubbers that had a power requirement of only 10.6 kW. Energy savings are (1 - 10.6/(4 x 21)) x 100% = 87.4%
A single scrubber provides CO2 removal for multiple controlled atmosphere rooms. CO2 sensors monitor concentrations in individual rooms with gases circulated through the scrubber by fans as needed to maintain optimum CO2 concentrations based upon the fruit present. The scrubber fan can be turned off when no CO2 removal is required. Some scrubber models are equipped with variable frequency drives to reduce cycling and maintain setpoints.
Energy savings are much greater for warehouse facilities that "feed and bleed" Nitrogen for atmosphere control. Energy savings for this scenario are determined by examining the baseline input power requirements of existing compressors and MSA units (if present) versus the input compressor power requirements and runtimes and total scrubber energy use after CO2 scrubber installation. One case study shows that the annual energy savings with CO2 scrubbers placed into operation at a controlled atmosphere warehouse facility is about 1,800,000 kWh. The bulk of the reduction in annual energy use is from a decrease in compressor energy consumption from about 4,000,000 to 2,200,000 kWh/year. The initial energy use of the standard air compressor/molecular sieve nitrogen production system of 4,000,000 kWh/year is reduced by 57%. Since we don't know the number of warehouses using each type of atmosphere concentration control approach, only savings from displacing the operation of existing MSA units are taken into account in this evaluation.
Energy Use of Emerging Technology:
68,796 kWh per Facility per year
Energy Use of an Emerging Technology is based upon the following algorithm.
Baseline Energy Use - (Baseline Energy Use * Best Estimate of Energy Savings (either Typical savings OR the high range of savings.))
Potential number of units replaced by this technology:
The number of controlled atmosphere refrigerated warehouses in the Northwest Region was determined for the NEEA Evaporator Fan VFD Initiative (which was conducted in the 1999--2002 time frame). Market assessments conducted for the initiative indicate a total of 236 controlled atmosphere refrigerated warehouses in the Northwest. (From: Andy Eckman, NEEA, "Focus on Cold Storage Evaporator Fan VFDs Is a Market Transformation Success", and Macro International "Evaporator Fan VFD Initiative: Market Baseline Evaluation Report", prepared for NEEA, April, 1999. See: http://www.eceee.org/library/conference_proceedings/ACEEE_industry/2003/Panel_4/p4_9/paper
Regional Technical Potential:
0.11 TWh per year
Regional Technical Potential of an Emerging Technology is calculated as follows:
Baseline Energy Use * Estimate of Energy Savings (either Typical savings OR the high range of savings) * Technical Potential (potential number of units replaced by the Emerging Technology)
Currently no data available.
Simple payback, new construction (years): N/A
Simple payback, retrofit (years): N/A
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.