Energy Efficiency Reference/Refrigeration/Recommendations/Minimze Excess Refrigeration Loads

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Recommendation: Minimize Excess Refrigeration Loads[edit]

Summary: Compressor power can be directly related to refrigeration load by the COP. Reduce excess refrigeration loads to reduce compressor power and save energy. This recommendation commonly applies to refrigeration loads in freezers and coolers.

When to Apply: When excess heat gain is a substantial part of the refrigeration load, from:

  • Lights
  • Open doors
  • Poor insulation
  • Door weatherstripping
  • Defrost
  • Floor heating

Key Engineering Concepts:

  • Refrigeration load in freezers and coolers consists of the desired refrigeration effect and heat
  • Gains from lights, and through doors, walls, floors and ceilings.
  • Efficient lights produce more lumens per watt and introduce less energy to the cold area
  • Fast acting, well-insulated weatherproofed doors reduced heat gain from outside air
  • Improved insulation reduces heat gain through doors, walls, floors and ceilings.
  • The COP relates refrigeration effect to compressor power

Preparation: Tools Required

  • Power Meter
  • DMM
  • Clamp-on Ammeter
  • Stopwatch
  • Therometer
  • Psychrometer
  • Refrigeration Data Sheets

Data Required

  • Room Dimensions (Measure, take from plans or step off approximate dimensions)
  • Insulation thickness (Use plans or approximate)
  • Wall, ceiling and door temperatures (Measure)
  • Outside temperature (Measure)
  • Doors dimensions (Approximate)
  • Number and type of doors (observe)
  • Time the doors spend open (Approximate)
  • Number, type, power, and operating hours of lights
  • Lighting level in room (Measure with light meter)

Analysis Process: 1a) Calculate Existing Lighting Power A simple approximation for the lighting load is to consider instantaneous power dissipated by the lamps in the room, multiplied by the use factor. This approximates the time-varying heat flux from the lamps as a constant flux to be absorbed by the refrigeration system.

Existing Lighting Power Count the light fixtures in the space and determine the input power rating for each fixture, including lamps and ballasts. Sum input power for all fixtures to calculate the average power dissipated into the room.

  • Existing lighting power = number of fixtures x input wattage x use factor

1b) Determine the Proposed Lighting Configuration Replace existing lamps with more efficient lamps that emit more lumens per watt. Lamps in order of increasing efficiency are:

  • Incandescent
  • Halogen
  • Compact Fluorescent
  • Fluorescent
  • Mercury Vapor
  • Metal Halide
  • High-pressure Sodium

Take care to use only fixtures that will work in low temperature areas.

Consider lighting system design to ensure that lighting level is adequate with a reduced number of fixtures or lamps.

1c) Calculate Proposed Lighting Power Once you select a new lighting type and the number of fixtures, calculate the proposed lighting power.

Proposed Lighting Power Use the proposed number of fixtures and input walls to calculate the proposed lighting power.

  • Proposed lighting power = number of fixtures x input watts x use factor

1d) Determine the Load Reduction due to Efficient Lighting

Lighting Power Savings Subtract the proposed from the existing lighting power to calculate the lighting power savings.

  • Lighting power savings = existing power - proposed power

2a) Calculate Heat Gain through Walls, Floor and Ceiling Heat flows through the walls, floor, and ceiling of a cold room driven by a temperature difference between the cold space and the outside air. You will need dimensions and extermal temperatures of each. Heat transfer is by convection on the outside and inside of the wall, and by conduction through the wall and insulation. Unless you can measure temperature throughout the year, you will have to use bin weather data to calculate the heat transfer through the year. In the calculations below, only average heat transfer rates are shown.

Calculate Existing Heat Gain through each Surface Consider radiative and convective coefficients on the outside of the surface, conduction through the surface components and insulation, and finally convection to the cold space. Refer to a heat transfer text for convective and radiative heat transfer rates.

Once all heat-transfer coefficients have been approximated, calculate the total heat transfer to the cold space. The total heat transfer into the room through the walls, ceiling, and floor is the sum of the individual heat transfer rates for each surface.

  • Heat Gain through Surfaces = sum (Heat transfer coefficient x area x temp difference)

2b) Determine Insulation Thickness Increasing insulation thickness or replacing saturated insulation will decrease the heat gain to the cold space. However, there are practical limits to the amount of insulation that should be applied. Reaching and maintaining desired temperature is generally a high priority if it affects product quality. Reasonable payback may also affect the amount of insulation to be installed.

2c) Calculate Proposed Heat Gain Recalculate heat-transfer rates with the proposed insulation installed, and recalculate total heat gain:

  • proposed surface gain = sum (new heat transfer coefficient x area x temp difference)

2d) Determine insulation savings Subtract proposed from existing surface heat gain to calculate the reduction.

  • insulation heat reduction = existing surface heat gain - proposed surface heat gain

3a) Calculate heat gain through doors When a cold space door is opened without air or strip curtains, buoyant forces draw warm air into the space. Heat gain through an open door will be mainly by convection, unless a curtain is used. Unless you can measure temperature throughout the year, use bin weather data to calculate the heat transfer through the year. In the calculations below, only the average heat transfer rates are shown.

Calculate Heat Gain through Doors Heat gain through doors varies with similar to lighting load. To approximate and average heat flux through the doors, estimate the percentage of time that the doors spend open then calculate the heat transfer die to convection into the cooler.

  • Door heat gain = percent of time open x calculated heat transfer

3b) Calculate proposed heat gain through doors To reduce the heat transfer through the doors of a cold space, reduce either the time the doors spend open or the heat transfer rates. Install air or strip curtains when doors do not have them. Install fast acting doors or others automatic closers to reduce open time. Calculate the proposed heat transfer through the doors as above, using the proposed time spent open or heat transfer rates. Calculate proposed heat gain through doors.

Using the new valve for percent of time spent open, calculate the proposed heat gain through the doors. The heat transfer rate will change if a new air door is installed.

  • proposed door gain = New percentage of time open x calculated heat transfer

3c)Calculate door savings Calculate door savings. Subtract the proposed from the existing heat gain to calculate door load reduction.

  • door load reduction = existing - proposed door load

3d)Calculate total heat reduction Calculate the total reduction from lighting, surfaces, and doors.

Total reduction in refrigeration load is:

  • total load reduction = lighting reduction + surface load reduction + door load reduction

4) Calculate COP Calculate COP

Coefficient of Performance is a measure of efficiency, calculated as the ratio of refrigeration effect to compressor power. COP is unit less, so convert power and refrigeration capacity to common units using the following conversion factors:

  • COP = Refrigeration Effect / Compressor Power
  • Where 1 ton of refrigeration = 12,000 BTU/hr
  • 1 hp = 2,546 BTU / hr

Calculate Compressor Power Savings Calculate the average reduction in compressor power based on the decrease in average refrigeration load and the coefficient of performance.

  • PS = TRR / COP

Calculate Auxiliary Power and Energy Increase Installation of an air curtain will increase the system power because the air requires a blower. The power and energy used by the blower depend on the size of the freezer door and the air curtain chosen for application.

Calculate Cost Savings Calculate cost savings by multiplying energy savings by energy cost from the utility bills.

Estimate implementation cost Implementation cost is the total cost for purchase and installation of the new equipment. The choice of new equipment will be plant specific.

Determine Payback Determine simple payback period by dividing the implementation cost by the annual savings.