An easy way to increase thermal mass is to add barrels of water, painted flat black, along a wall where it will receive sunlight. Besides adding more “batteries” to the equation, these water barrels can also be used for plant irrigation, eliminating plant shock. The more dense the mass, the more solar energy it absorbs, ergo the more thermal energy that can be stored. The color of the mass affects the ability of the mass to absorb the energy, i.e., the speed at which it “charges.” In a sunken greenhouse, then, dark earthen walls absorb heat during the daytime, charging them with thermal energy.
A heat sink can also serve as storage for thermal mass, through the addition of a sunken pool of gravel/woodchips/etc. This medium heats in the warm day, and then equalizes as the ambient temperature cools, releasing heat.
