Abstract: A highly effective modular passive solar energy heating unit for heating an enclosed space employs phase change heat storage material, which is optically transparent to visible light when in high-stored-energy liquid state enabling a viewer to see through clearly and which is translucent milky white when in low-stored-energy solid state for providing pleasant illumination to the enclosed space when first illuminated by sunlight in the morning. This phase change material, in a sealed container having two opposite surface areas transparent to sunlight, is polyethylene glycol having a heat of fusion of about 33 to 37 cal./gm. (4,170 to 4,670 Btu's per cubic ft.), a melting point of about 13.degree. to 25.degree. C. (55.degree. F. to 77.degree. F.), a specific gravity of about 1,125 at 20.degree. C., and desirable isotropic freezing and melting characteristics with only very modest volume change during phase change.

Abstract: A modular solar greenhouse incorporates rigid curving frames overhead with screw and nut tracks for ease of assembly and east side and west side end modules and advantageously employs a thermally broken glazing system; that is, the exterior and interior portions of the aluminum frame extrusions are bonded together by strong plastic material and then the metal bridge is removed for thermally isolating the interior frame portion from the exterior portion. At least one overhead heat storage unit is carried by the rigid frames, with insulated roofing being located above and also being supported by the frames, so that low-angle solar radiation in the wintertime can travel directly into the upper region of the greenhouse below the roof for directly impinging upon and supplying heat energy to the thermal storage units.