Development of a tracking compound parabolic collector for a solar-driven diffusion absorption cycle

Abstract

In an aqua ammonia absorption refrigeration system, the heat required by the generator can be delivered by a renewable source, such as solar energy. A compound parabolic collector (CPC) can deliver medium to high temperatures with relatively low heat loss, making it a good candidate for use in such a system. A CPC has been designed that can provide a temperature of 90°C to the bubble-pump generator of an in-development aqua ammonia absorption refrigeration system. Previous studies in this field focused mainly on stand-alone CPCs, as well as stationary CPCs specifically for use in solar-driven absorption systems. From the literature it was gathered that an East-West orientation of a collector results in more absorbed energy, and that sun-tracking also increases the amount of energy absorbed. No literature was found regarding the use of a sun-tracking CPC for use in a solar-driven absorption system. A mathematical model was developed to predict the amount of useful energy gained with a particular design. The model included determining the path of the sun in the sky throughout the year, determining a CPC profile, gathering solar radiation data, and determining the thermal performance of the CPC. The modelled sun-tracking CPC with a concentration ratio of 2.09, receiver radius of 16 mm, and total collector area of 9.84 m2, was then constructed and tested. Testing took place from 29 March 2017 to 30 March 2017 and showed that, on a cloudy day, a maximum temperature of 109°C was achieved. The results translate to an actual useful energy gain of up to 4462.33 W, varying with weather conditions. Compared to the theoretical useful energy gain of 3681.45 W for a cloudy day, and 4806.83 W for a clear sky day, the actual results correlate well with these figures. As the collector produced temperatures above 90°C, it would be able to provide the bubble-pump generator with the heat it needs to drive the aqua ammonia absorption refrigeration system. Recommendations include converting the tracking mechanism to an electronic system, as well as using an electronic system to control the flow rate through the collector. It is also recommended that any future measurements on the collector be taken only when the entire refrigeration system is fully assembled.