In heat storage systems it is a requirement to take heat from one source, transfer it to a storage medium and then derive the heat when required from storage. Poor efficiency in energy storage and transfer has caused many systems to be marginal in performance or inoperative. Some of the most pronounced deficiencies are in the nature of the storage medium and the transfer mechanism for getting heat in or out of the storage medium. Wherever a media interface occurs or a system interface there is a boundary matching problem to overcome. Thus, for example, transfer of heat from a liquid such as hot water to a gas such as air produces an interface where the amount of heat transfer is critical to the heating efficiency of the two systems or in the ultimate media.
Similarly efficiency and transfer problems exist in the storage of heat. Hot water may be used as storage medium, but it is temperature limited since it becomes steam at boiling temperature. Solids with good heat retention characteristics can be used as storage media. Salts such as common sodium chloride table salt thus might be used.
One serious problem that is often met in handling hydrated salt products is the tendency of the salt particles to cake or bind together. This is often troublesome in bulk storage or in barrelled products but is most serious in those cases where salt crystals are disposed in packages or in systems where a humid gaseous stream flows through the particles. The difficulty is particularly serious in changing both the physical surface area afforded by granules and the physical relationship of the salt with water.
Remedies to prevent caking have met with little success in the prior art, perhaps because of restrictions of the nature of impurities that may be involved. For example, table salt has been dusted with magnesia or tricalcium phosphate to prevent caking. Also, flake grade calcium chloride has been dusted with anahydrous calcium chloride in an attempt to prevent caking, all with limited success.
Also, caking has been partly prevented by dusting the crystals with powdery material by prior art methods, again with limited results.
Another serious problem with such salt products is their corrosiveness. They will tend to rust or pit metals and cannot be stored in or used around steel or iron in particular.
Whenever salts take on moisture and go into solution the corrosiveness spreads, creeps and contaminates surrounding areas.
These properties have limited use and storage of the salt products in favor of substitute materials when available.
Other solid heat storage materials such as stones or sand present interface problems and tend to settle or pack. The ability to effectively use available heat by appropriate transfer into the heat media is limited not only by the material heat storage properties but the nature of the heat transfer interface. Thus with stones or other granular particles packed together in a storage container the amount of surface presented to the transfer medium is critical as well as the nature of the particle-to-particle contact if that be the transfer medium.
By very nature the retention of heat in a storage medium detracts from its physical properties in efficiently receiving and giving up the stored heat. Thus, in heat system construction it has been difficult to provide both high capacity heat storage capabilities and high efficiency operation in transfer of heat into and out of the storage medium. This problem is particularly accentuated when low cost heat storage materials are dictated in a system, such as sand or salt.
It has now been found that by practice of the present invention, many difficulties and disadvantages of prior art attempts to produce efficient heat storage and transfer systems have been overcome in a simple highly efficient manner.