I. Kubo

Review of Solar Cooling Technologies

Increasing awareness of global warming forces policy makers and industries to face two challenges: reducing greenhouse gas emissions and securing stable energy supply against ever-increasing world energy consumption, which is projected to increase by 71% from 2003 to 2030. In addressing these two issues simultaneously, renewable energies prove themselves attractive, as they are independent from the fossil fuel supply and do not contribute to greenhouse gas emissions. Along with the global warming impacts and climate changes, the demands for air conditioning and refrigeration have increased. Therefore, providing cooling by utilizing renewable energy such as solar energy is a key solution to the energy and environmental issues. This paper provides a review of the available cooling technologies assisted by solar energy and their recent advances. Thermally powered cooling technologies were classified into three categories: closed cycles, open cycles, and thermo-mechanical cycles. Adsorption and absorption cycles represent the closed cycle. The adsorption cycle typically needs lower heat source temperatures than the absorption cycle. Solid and liquid desiccant cycles represent the open cycle. The liquid desiccant cycle has a higher thermal COP than the solid desiccant cycle. The ejector cycle represents the thermo-mechanical cycle, and has a higher COP but requires a higher heat source temperature than other cycles. Based on the thermal COP of each cycle, the absorption cycle is preferred to the adsorption cycle, and the liquid desiccant system is preferred to the solid desiccant system. Although these cycles have similar thermal COP values, advanced solar collector technologies such as the mini dish or trough collectors open a new opportunity of achieving higher overall system efficiency with these thermally powered cooling technologies and enable the solar thermal system to outperform the solar electric system for the time being. Therefore, next to improving efficiency and compactness of solar-driven cooling technologies, research on advanced solar collectors is the most important research topic.