Performance analysis of ultralow grade waste heat upgrade using absorption heat transformer

The present paper aimed at exploring absorption heat transformer (AHT) to upgrade ultralow grade waste heat in the temperature range of 40-60 °C. The performance of AHTs with different configurations, including single stage, double stage and double effect, were numerically analysed and compared in terms of temperature lift, coefficient of performance (COP) and exergy coefficient of performance (COPe). The most influential and crucial factor for the studied AHTs is the recirculation flow ratio (FR), the increase of which results in an increasing temperature lift but gradually declining COP. The COPe can achieve its maximum value with a certain FR, and such a state can be considered as the optimal working condition. Within the studied waste heat temperature range, the optimal FR in single stage AHT is in the range of 10-12, at which the system can deliver 17.1~34.7 °C temperature lift with COP at 0.471~0.475. The best configuration amid the studied four different double stage AHTs has a temperature lifting capacity of 31.8~68.6 °C with a COP around 0.30. The double effect AHT compromises its temperature lifting capacity for the highest COP among all the AHTs studied, which can reach about 0.65 though necessitates relatively higher waste heat temperature and higher strong solution concentration to drive the cycle; the double effect AHT is not recommended for the upgrading of ultralow grade waste heat.