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Energy Comparison

Energy use comparison of liquid desiccant air conditioning versus conventional approaches.

Example 1 - Conventional Dedicated Outdoor Air System

Outside Air (OA) of 3000 cfm at Boston summer cooling conditions of 90°F dry bulb and 73°F wet bulb is to be conditioned to room neutral 73°F and 45% relative humidity. Using the psychrometric chart, estimate the total energy required for a traditional cooling coil and re-heat coil.




Estimated Total Energy Qtotal = Qpre-c + Qreheat

Qtotal = 204,718 Btu/hr + 69,935 Btu/hr

Qtotal = 274,653 Btu/hr

Calculation Method

Locate the Outside Air and Leaving Air points and record the OA specific volume and LA humidity ratio and enthalpy for both points.
      OA Specific Volume = 14.156 cu.ft./lba
      OA Enthalpy = 36.642 Btu/lb
      LA Enthalpy = 26.034 Btu/lb
      LA Humidity Ratio = 54.5 gr/lb

Locate the point (CC) where the LA humidity ratio line intersects the saturation curve and record the Enthalpy.
      CC Enthalpy = 20.548 Btu/lb

The cooling coil energy can be estimated by the following equation:
      Qpre-c = (hOA - hCC) x (cfm x 60) / spec.vol.OA
      Qpre-c = (36.6 - 20.5) x (3000 x 60) / 14.156
      Qpre-c = 204,718 Btu/hr

The heating coil energy for air re-heating to the desired dry bulb temperature can be estimated by the following equation:
      Q = (hLA - hCC) x (cfm x 60) / spec.vol.OA
      Q = (26.0 - 20.5) x (3000 x 60) / 14.156
      Q = 69,935 Btu/hr


Example 2 - Solid Desiccant Wheel System

Outside Air (OA) of 3000 cfm at Boston summer cooling conditions of 90°F dry bulb and 73°F wet bulb is to be conditioned to room neutral 73°F and 45% relative humidity. Using the psychrometric chart, estimate the energy required for a solid desiccant, assuming a typical 33% regeneration cycle efficiency and a heat carryover ratio of 20% (0.2).



Estimated Total Energy Qtotal = Qpost-c + Qregen

Qtotal = 192,370 Btu/hr + 252,218 Btu/hr

Qtotal = 444,588 Btu/hr


Calculation Method

Locate the Outside Air and Leaving Air points and record the OA and LA humidity ratio and enthalpy for both points To find the moisture load on the desiccant.
      Qlatent = CFM x (Gri - Grf) x .68 Btu/hr
      Qlatent = 3000 x (95.2-54.4) x .68 Btu/hr
      Qlatent = 83,232 Btu/hr

To find the regeneration specific heat input (RSHI)
      Qregen = RSHI = Qlatent/Efficiency
      Qregen = 83,232 Btu/hr / 0.33 = 252,218 Btu/hr

Find point DH, first approximate the solid desiccant process as a constant enthalpy process and locate the intersection of the OA enthalpy and the LA humidity ratio.
      Enthalpy = 36.642 Btu/lb
      Humidity Ratio = 54.5 gr/lb
      Intersection Dry Bulb Temperature = 116.6 °F

Then add the heat carried over from regeneration Qcarryover = RSHI x heat carryover ratio
      Temp rise from heat carryover = Btuh / CFM / 1.085
      Temp rise from heat carryover = 50,443 / 3000 / 1.085
      Temp rise from heat carryover = 15.5 °F
      DH Dry Bulb temp = 116.6 °F + 15.5 °F = 132.1 °F

The solid desiccant post cooling energy can be estimated by the following equation:
      Qpost-c = CFM x (ti-tf) x 1.085 Btu/hr
      Qpost-c = 3000 x (132.1 °F - 73 °F) x 1.085 Btu/hr
      Qpost-c = 192,370 Btu/hr


Example 3 - Liquid Desiccant System

Outside Air (OA) of 3000 cfm at Boston summer cooling conditions of 90°F dry bulb and 73°F wet bulb is to be conditioned to room neutral 73°F and 45% relative humidity. Using the psychrometric chart, estimate the energy required for a liquid desiccant process.



Estimated Total Energy

Qtotal = 134,783 Btu/hr


Calculation Method

Locate the Outside Air and Leaving Air points and record the OA specific volume and the enthalpy for both points.
      OA Specific Volume = 14.156 cu.ft./lba
      OA Enthalpy = 36.642 Btu/lb LA
      Enthalpy = 26.034 Btu/lb

The liquid desiccant energy for dehumidification and cooling can be estimated by the following equation:
      Q = (hOA - hLA) x (cfm x 60) / spec.vol.OA
      Q = (36.6 - 26.0) x (3000 x 60) / 14.156
      Q = 134,783 Btu/hr

No Additional Regeneration Specific Heat Input (RSHI) required: Due to the lower RSHI of liquid desiccants, these systems have the ability to be completely regenerated by the condenser heat coming from the cooling process. In this example, after exerting 134,783 Btu/hr of cooling, the system would be rejecting about 170,000 Btu/hr of heat from the condenser, which is more than sufficient to regenerate the desiccant. This eliminates the need for any external energy input beyond that needed for the cooling process alone.



How to use the Psychrometric Chart:

LINES AND SCALES

Dry Bulb Temperature Scale
Humidity Ratio Scale
DewPoint Scale
Enthalpy Scale
Sensible Heat Ratio Index
Sensible Heat Ratio Index Origin
Vapor Pressure Scale
Dry Bulb Temperature Line
Wet Bulb Temperature Line
Humidity Ratio Line
Relative Humidity Line
Specific Volume Line
A - Humidification
B - Heat and Humidity
C - Heating
D - Solid Desiccant
E - Liquid Desiccant
F - Cooling Coil
G - Evaporative Cooling


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