Absorption solar cooling systems using optimal driving temperatures

The optimum instantaneous driving temperature of a solar cooling facility is determined along a day. The chillers compared use single effect cycles working with NH₃/LiNO₃, either conventional or hybridised by incorporating a low pressure booster compressor. Their performances are compared with a H₂O/LiBr single effect absorption chiller as part of the same solar system. The results of a detailed thermodynamic cycle for the absorption chillers allow synthesizing them in a modified characteristic temperature difference model. The day accumulated solar cold production is determined using this optimum temperature during two sunny days in mid-July and mid-September, located in Madrid, Spain. The work shows the influences of operational variables and a striking result: selection of a time-constant temperature during all the day does not necessarily imply a substantial loss, being the temperature chosen a key parameter. The results indicate that the NH₃/LiNO₃ option with no boosting offers a smaller production above-zero Celsius degrees temperatures, but does not require higher hot water driving temperatures than H₂O/LiBr. The boosted cycle offers superior performance. Some operational details are discussed. - Highlights: • Instantaneous optimum driving temperature t_g,op for solar cooling in Madrid. • 3 absorption cycles tested: H₂O/LiBr and NH₃/LiNO₃ single effect and hybrid. • The t_g,op of the hybrid cycle is 16 °C lower than both single effect cycles. • The best fixed driving temperature can reach almost the same behaviour than t_g,op