Can One Preserve Sunshine?
Who hasn’t recalled swimming in 35˚C-weather during the previous summer while freezing in the winter cold and wished to find at least a little summer heat in their backpack? Wouldn’t that be a great Idea?
But above all, a lot of oil, gas, and coal could be saved if summer heat were used as a heat source in the winter. 2 EJ (that is 2*1018 J!!!) of heat is needed each year to heat homes, schools, offices, and other buildings in Germany alone. This requires, for example, 68 million t of coal or 48 million t of oil. On some buildings today, you see not only photovoltaic systems, which turn sunlight into electricity, but also solar collectors, which use the rays of the sun to heat water. Water does indeed have the highest specific heat capacity, i.e. it can store large amounts of heat per kg, however in an isolated vessel (e.g. thermos) it gives off its heat to the colder surroundings within a couple of hours.
Heat may be stored in microporous materials (e.g., silica gel, zeolites, or activated carbon) without losses due to cooling. The pores of these materials readily take up water vapor from the air; they adsorb water. Since the water molecules in the pores can no longer move as they would in the air, they must give off some of their energy as heat. This can result in temperatures of up to 300°C. This describes the discharging process of such an adsorption storage unit (Fig. 1). In a closed system, water vapor is generated in an evacuated evaporator, thereby also providing a cooling effect (evaporative cooling).
To recharge the storage, the porous material needs to be heated, whereby, the water molecules require more space than the pores provide. They are desorbed (Fig. 2).
The evaporator now works as a condenser, turning the desorbed water vapor back into water. The valve is closed and can be opened again once heat is needed, allowing water vapor into the pores and releasing heat.