A heat pipe includes an elongated, heat conductive shell containing heat transfer fluid which is vaporizable and condensable, and cooling means within the shell to condense the heat transfer fluid from the vapor form to the liquid form and thereby to cool the shell to approximately the same temperature throughout its length. The cooling means can include a conduit or conduits running longitudinally inside the shell of the heat pipe and carrying a circulating coolant which can be a liquid or a gas. The substantially uniform cool surface temperature of the heat pipe adapts it for use as an efficient solar energy collector with the circulating coolant serving as a heat extracting medium. Alternatively, a row of photovoltaic cells may be mounted on the uniformly cool heat pipe shell and an elongated lens used to refract solar rays to impinge concentrated or high intensity solar energy onto the targets of such cells.