Heat rejection and primary energy efficiency of solar driven absorption cooling systems

Efficient heat rejection is crucial for the overall primary energy balance of sorption systems, as it dominates the auxiliary energy consumption. Low ratios of cooling power to auxiliary electricity of 3.0 or less are still common in sorption system, so that the primary energy efficiency is not always higher than for conventional compression chillers.Whereas dry heat rejection systems require electricity for fan operation, hybrid or wet cooling systems in addition need pumping energy for the cooling water and the water itself. The energy efficiency can be improved for heat rejection to the ground, where only pumping energy is needed for the geothermal heat exchange.Dynamic simulation models were used for a single effect absorption chiller powered by solar thermal collectors via a hot storage tank. The chiller models were coupled to a three dimensional numerical ground heat exchanger model or to cooling tower models. The models were validated with operating data of a 15 kW solar cooling system installed in an office building.Primary energy efficiency ratios were determined for different heat rejection systems and improved control strategies were developed. The installed system primary energy ratios varied between 1.1 and 2.2 for auxiliary heating and between 1.2 and 2.5 for auxiliary cooling depending on the heat rejection and control strategy chosen. The low electrical energy consumption of the geothermal heat rejection saves 30% of auxiliary electricity and results in an electrical coefficient of performance of 13. The maximum primary energy ratios for solar fractions up to 88% are 2.7 for auxiliary heating and 3.2 for auxiliary cooling, i.e. nearly three times higher than for the reference electrical compression system of 1.2.

► Cooling tower control improves the primary energy efficiency ratio of an absorption chiller system from 1.1 to 2.2.
► Solar fractions increase from 70 to 88% using optimised control strategies.
► An electrical coefficient of 13 can be reached with geothermal heat rejection.
► A primary energy efficiency of 3.2 can be obtained with heat rejection to the ground.