With the imminent introduction of minimum energy efficiency requirements for household tumble dryers in the EU expected to promote the use of heat pump technology for drying applications, the use of hydrocarbons in heat pump dryers could be a means to achieve high product energy efficiency while minimising direct emissions at the same time.
The European Commission estimates that annual electricity consumption of household tumble driers amounted to 21 TWh in the European Union (EU) in 2005. Under business as usual scenarios this is expected to rise to 31 TWh in 2020.
Against this backdrop, a possible upcoming EU regulation will be setting ecodesign and therefore minimum energy efficiency standards for household tumble dryers, rendering the use of heat pump dryers more attractive. The scope of this regulation covers electric mains-operated and gas-fired household tumble dryers and built-in household tumble dryers, including those sold for non-household use, however it will not apply to household combined washer-dryers or household spin-extractors.
The latest publicly available working document proposes the following minimum energy efficiency requirements:
- One year after the entry into force of the regulation: the Energy Efficiency Index (EEI) of household tumble dryers shall be less than 85, while for household condenser tumble dryers the weighted condensation efficiency shall be not lower than 60%
- Five years after the entry into force of the regulation: the Energy Efficiency Index (EEI) of household tumble dryers shall be less than 76, while for household condenser tumble dryers the weighted condensation efficiency shall be not lower than 70%
Hydrocarbon heat pump dryer: good performance, lower TEWI
Heat pumps dryers available today on the market typically use R134a refrigerant. However the potential of using natural refrigerants hydrocarbons and CO2 is receiving interest, as indicated in a paper titled “Refrigerant selection for a heat pump tumble dryer” co-authored by Marek Zgliczynski and L. Novak of Embraco Slovakia s.r.o and J. Schnotale and A. Flga-Maryanczyk of Cracow University of Technology that was presented in August 2011 at the 23rd IIR International Congress of Refrigeration (ICR 2011), in Prague, Czech Republic.
The authors discuss theoretical simulations of a heat pump dryer with different refrigerants, namely R134a, R290 and R744, as well as actual performance measurements at 20°C ambient temperature of real heat pump laundry dryers employing piston compressors. Measured values of Specific Energy Consumption (SEC) are in agreement with those obtained from cycle simulations, which indicate almost identical SEC values for R134a and R290 and higher values for the R744 transcritical cycle.
The results from measurements of the real model were also used for carrying out a Total Equivalent Warming Impact (TEWI) analysis. Assuming among other things a leakage rate at the level of 0,003kg per year, an equipment lifetime of 7 years, a 90% recycling factor and considering the energy mix situation in Slovakia (0,171kgCO2/kWh), the authors found the R290 heat pump dryer to encompass the lowest TEWI factor:
- R134a heat pump dryer TEWI: 403,6
- R290 heat pump dryer TEWI: 332,5
- R744 heat pump dryer TEWI: 377,3
- Electric spiral dryer TEWI: 836,6
About heat pump dryers
A heat pump dryer uses a refrigerating system that heats up the process air for drying the laundry and then cools it down to extract the moisture and to recover energy. Hermetic reciprocating compressors used in heat pump dryers have similarities to those used in light commercial refrigeration applications. However, robust mechanical design and special features such as a strong motor are required for the compressor to be able to withstand more severe working conditions.