Energy efficient heat pump dryer

This article explains how replacing a conventional electric clothes dryer with a heat pump dryer will considerably reduce energy consumption as well as energy bills. 

Conventional electric dryers are wasteful:

Conventional electric dryers use a large electric heating element to warm the air used to dry laundry. Conditioned air from the building enters the dryer and passes through the large electric heating element. The warm air, then enters the clothing drum and removes moisture from the laundry. The warm, damp air, then exits the clothing drum and immediately exhausts to the outdoors. This process continues until the moisture level within the clothing drum is low, at which point the laundry is dry.

This conventional process of drying laundry has three major energy wastes.

  1. The electric dryer is continuously taking condition air from the building throughout the entire drying process and exhausting it outside. It does not matter where you live or what session it is, you have already used energy and money conditioning this building air.  You may have used air conditioning to cool down the temperature or a furnace to warm up the temperature within the building. This results in unconditioned outdoor air replacing the conditioned air exiting the building through the dryer exhaust vent.
  2. The dryer is continuously consuming large amounts of power, heating the electric element to warm the air. It is typical for conventional dryers to consume 5000 watts of power continuously throughout the drying process.  The exception to this is when the dryer switches off the electricity to the heating element to monitor the moisture levels of the laundry.
  3. Air exiting the heating element only passes through the clothing drum once to remove moisture from the laundry before exiting the building. The air that exhausts from the dryer to the outdoors is still warm, however, the air is also damp.  Conventional dryers are not able to separate moisture from the air and therefore wastefully exhaust this air.

Heat pump dryers are efficient

A heat pump dryer typically uses one-third to one-half the energy per load compared to conventional dryers. Achieving these energy savings derive by replacing the large power consuming heating element with a closed loop refrigerant system. The air within the heat pump, laundry dryer is continuously re-circulating throughout the entire drying cycle. The evaporator coil within the closed loop refrigerant system removes the moisture from the air. Since the air is re-circulating within the heat pump dryer, there is no longer a need to exhaust warm damp air outdoors. Additionally, there is no longer a need to use conditioned air from within the building. This results in the buildings air conditioning system or furnace no longer needing to recondition new air entering the building for the purpose of laundry.


How heat pump dryers work

How heat pump dryers work
How Heat Pump Dryers Work

Heat pump dryers operate with the use of two closed loop cycles. The first is a closed loop air system, where the same air continuously cycles from warm to cool.  The second is a refrigerant closed loop system, where the refrigerant continuously cycles from warm to cool.  The closed loop refrigerant circuit functions similarly to that of an air conditioner.  However, it removes moisture as well as warms the air to dry laundry.

Heat pump dryer air closed loop cycle

Hot, dry air enters the drum and absorbs moisture from the laundry.  The warm, damp air that exits the drum passes through a main filter and a fine filter, to remove lint, before it enters the evaporator coil.  The evaporator coil, which is part of the closed loop refrigerant circuit, cools and removes moisture from the air as the air passes through it.   Moisture condenses on the evaporator coil because the dew point temperature of the air is higher than the evaporator coil temperature. 

A condensate pump, then removes this collected moisture from the dryer.  This is why heat pump dryers have drain lines that connect to the same drain plumbing as the washing machine.  The cool, dry air exiting the evaporator coil then passes through the condenser coil.  The condenser coil, which is also part of the closed loop refrigerant circuit, warms the dry air as the air passes through it.  The warm dry air then enters the drum again, repeating the cycle until your laundry is dry.

Note

The same air is used throughout this cycle, eliminating the need for an exhaust vent required by conventional dryers.

Heat pump dryer refrigerant closed loop cycle

The refrigerant temperature that enters the dryer’s evaporator coil is cool.  The pressure is low and the state of the refrigerant is a mixture of both liquid and gas.  When warm/damp air from the dryer comes in contact with the evaporator coil, the remaining liquid of the refrigerant evaporates. The refrigerant then changes from a mixed gas / liquid state to a pure gas state.  The temperature of the refrigerant is now slightly warmer because heat from the dryer air has been transferred to it.  The refrigerant in its gas state, then enters the compressor.  This is where the refrigerant compresses from a low pressure to a high pressure gas.  This compression generates heat and the refrigerant is now at its highest temperature within the system.

The refrigerant then enters the condenser, which works on the principle that heat transfers from warmer to cooler objects.  The current refrigerant temperature is warmer than the dryer air exiting the evaporator coil.  The cool/dry air exiting the evaporator coil cools the refrigerant enough for the refrigerant to condense and change from a gas to a liquid state.  The liquid refrigerant then passes through a capillary tube.  The capillary tube causes an immediate reduction in refrigerant pressure.  This instant pressure drop changes both the refrigerants state and temperature.  The refrigerants state changes from a liquid to a mixture of liquid and gas.  The refrigerant temperature drops, restarting the cycle within the closed loop system.

Conventional electric dryer measurements proving energy waste

A laundry load consisting of the same eight towels was placed in both a heat pump dryer and a conventional heating element dryer.  A power quality analyzer was connected to monitor the energy consumed by each dryer type.  A trend was also captured to show the kilowatt consumption throughout the entire drying cycle.

Power & energy conventional dryer
Power & Energy Conventional Dryer
Power consumption trend conventional dryer
Power Consumption Trend Conventional Dryer

Red: Line 1 – kW trend
Green: Line 2 – kW trend
Black: Line 1 & Line 2 Combined kW trend

The conventional heating element dryer’s cycle ran for 47 minutes and 45 seconds and consumed:
3.185 kWh of real power
3.274 kVAh of apparent power
0.451 kVARh of reactive power

Note

Both Line1 (red) and Line 2 (green) supply the resistive heating element.  Line 2 has the additional load of the motor required to rotate the dryers drum.  The resistive heating element consumes a large amount of power throughout the majority of the drying cycle. The dryer switches off the heating element three times to monitor the moisture levels of the laundry.

Heat pump dryer measurements proving energy savings

Power & energy heat pump dryer
Power & Energy Heat Pump Dryer
Power consumption trend heat pump dryer
Power Consumption Trend Heat Pump Dryer

Red: Line 1 – kW trend
Green: Line 2 – kW trend
Black: Line 1 & Line 2 Combined kW trend

The heat pump dryer’s cycle ran for 1 hour 36 minutes and 13 seconds and consumed:
1.229 kWh of real power
1.440 kVAh of apparent power
0.583 kVARh of reactive power

Note

Both Line1 (red) and Line 2 (green) supply the heat pump’s compressor motor.  Line 1 has the additional load of the motor required to rotate the dryers drum.  The heat pump dryer consumes a low constant amount of power throughout the majority of the drying cycle. The dryer continuously monitors moisture levels of the laundry.

Conclusion

Replacing a conventional electric dryer with a heat pump dryer will considerably reduce energy consumption as well as energy bills.  There are additional benefits, two of which are that a dryer exhaust vent and the buildings conditioned air are no longer required.   No longer requiring a dryer vent will allow for the building to be more air tight.  No longer requiring conditioned air will decrease run time of either the air conditioner or furnace.  Eliminating the need for these wastes, results in the increased total overall efficiency of the building.

The only dryer more efficient than a heat pump dryer is a warm summer day and a clothes line.

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