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In page Solar water heating:

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A bubble pump (also known as geyser pump) is suitable for flat panel as well as vacuum tube systems. In a bubble pump system, the closed HTF circuit is under reduced pressure, which causes the liquid to boil at low temperature as the sun heats it. The steam bubbles form a geyser, causing an upward flow. The bubbles are separated from the hot fluid and condensed at the highest point in the circuit, after which the fluid flows downward toward the heat exchanger caused by the difference in fluid levels.[1][2][3] The HTF typically arrives at the heat exchanger at 70 °C and returns to the circulating pump at 50 °C. Pumping typically starts at about 50 °C and increases as the sun rises until equilibrium is reached.[citation needed]