A Comparative Analysis of Pulsated Heat Pipe Solar Collectors: Development of Solar Water Heater utilizing Super-critical Carbon Dioxide as a working fluid. As compared to pulsated heat pipe with water as the working fluid that generates heavily pulsated discharge driven by natural convection, Super-critical CO2 with its volatile natural convection at a significantly lower temperature provides a more consistent solar thermal energy conversion and a greater hot water discharge even in regions with low sun-light and a colder climate.