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Abstract
Absorption chillers are a potential technology for using solar energy or waste heat for cooling and air-conditioning purposes. State-of-the-art working pairs require elaborate system designs and an appropriate material selection to deal with such problems as formation of non-condensable gases, corrosion, and crystallization. The investigation of alternative working pairs is one important possibility to reduce or avoid such problems. A new approach is to use linear- or hyper-branched polymers in combination with water as a working pair. A selected polymer used in this work has the capability to absorb water at suitable vapor pressure levels. Furthermore, the new working pair is expected to potentially diminish the problems of corrosion, formation of non-condensable gases, and damage of the machine due to crystallization and to relax at least the pricing situation. On the other hand, the high viscosity and the low heat conductivity compared to conventional working pairs are crucial for its application. To prove the feasibility of the polymeric-based working pair, experiments have been carried out in a lab-scale single-stage absorption chiller. The lab-scale chiller used falling-film heat exchangers and had a cooling capacity between 60 and 100 W. It is shown that under equal operating conditions, the new working pair is able to reach about one-third of the cooling power compared to lithium bromide/water at vaporizer temperatures suitable for cooling ceilings applications. If used as a drop-in replacement, the cooling power of the new working pair does not reach the performance of lithium bromide/water. Nevertheless, the usage in adapted machines for specific applications should be discussed in the future.
Acknowledgment
The authors want to thank Ernst Hofmann for critical discussions.
Nomenclature
| cp | = | specific heat (J kg−1 K−1) |
| COP | = | coefficient of performance (—) |
| d | = | hydraulic diameter (m) |
| D | = | diffusion coefficient (m2 s−1) |
| h | = | specific enthalpy (J kg−1) |
| L | = | length (m) |
| m | = | mass (kg) |
| p | = | pressure (Pa) |
| = | heat transfer rate (W) | |
| T | = | temperature (°C) |
| V | = | volume (m3) |
| η | = | dynamic viscosity (Pas) |
| λ | = | thermal conductivity (W m−1 K−1) |
| ϱ | = | density (m3kg−1) |
| χ | = | mass fraction (—) |
Subscripts
| abs | = | absorber |
| AHP | = | absorption heat pump modus |
| amb | = | ambient |
| des | = | desorber |
| E-A | = | evaporator–absorber-modus |
| eva | = | evaporator |
| rt | = | room temperature |
| sol | = | solution |