Heat pumping for cloudy weather and night solar water purification

Abstract

As a result of rapid population growth, and the expansion of urbanization, industrial and agricultural activities, the stress on fresh water resources has drastically increased. The limitation of natural drinkable water resources has a negative impact on safety, security and socioeconomic conditions of human societies. The traditional water treatment processes are associated with controversial matters such as environmental issues and economic decline; however, the advanced processes required need a developed infrastructure facility and high initial investment cost. This research work presents a method that could help individuals in rural areas and small communities to produce their daily needs of potable water. Two types of solar powered heat pump systems were designed and developed to be combined into conventional solar still models, and therefore to overcome the main drawback of conventional solar stills, which is low efficiency. The advanced solar stills make use of the available solar energy to generate electrical and thermal energies, so that heating up water more quickly and recovering the energy losses is possible. The first proposed system was solar still integrated with a DC vapour compression heat pump system. Outdoor experimental results showed that in summer season of Cape Town, the comparison between the conventional solar still and the proposed one indicated that on a typical day of November (summer season in the southern hemisphere), the water temperature in advanced solar still reached 73 ⁰C while in conventional one was 57.5 ⁰C. The daily distillate yield was 6.21 and 2.67 L/m2 for the advanced and conventional solar still respectively. In the winter season, the conventional solar still performed poorly on a partly cloudy cold day, the maximum water temperature was about 38.5 ⁰C and the daily distillate yield was 0.215 L/m2, while the performance of the advanced still was satisfactory, the daily distillate yield was 4.2 L/m2 at a maximum water temperature of 58.9 ⁰C. The average of the coefficient of performance (COP) of the vapour compression heat pump system was 3.5. The second proposed system was a solar still integrated with thermoelectric heat pump system. Outdoor experimental tests were performed to investigate the performance of the system. The results obtained show that the daily distillate yield of the second proposed system was about 4.4 L/m2, while the conventional one produced 2.1 L/m2. The maximum value (COP) of the thermoelectric heat pump according to energy balance equations and the principle of thermodynamics for a single-stage heat pump was about 1.9.