The present application relates generally to air conditioning, capturing combustion contaminants, desalination, and other processes using liquid desiccants.
The term “air conditioning” generally refers to the treatment of air going into a building space, including to the heating, cooling, humidity adjustment, or purification of air that enters or leaves the space. It is well known that air conditioning is an enormous source of energy use and that summer cooling in particular can lead to electricity grid problems. Air conditioning is often the largest operating cost in a building.
Current air conditioning systems for cooling are generally based on the compression of a gas such as Freon and the expansion of the compressed gas through a valve assembly. However, in order to reach the required humidity levels of the entering air into a building, the air generally needs to be overcooled in order to condense water vapor into liquid water. This dehumidification (latent cooling) generally uses more energy in an air conditioning system than the physical lowering of the air temperature (sensible cooling). Oftentimes re-heaters are employed within an air conditioner requiring even larger amounts of energy.
Air conditioning for heating air is typically done by combustion of natural gas or some other fuel. The combustion often heats a heat transfer fluid that is then directed to a fan coil unit where the entering air is heated. In many buildings, such sensible only heating results in humidity levels that are too low for comfort. Oftentimes humidifiers are integrated with the heating system. Such humidification however results in the cooling of the air, which means that additional heating will have to be applied to counteract the cooling effect of the humidifier.
Solid desiccant systems have been used for many years primarily for summer cooling. However, the heating effect that occurs when air is dehumidified in an adiabatic fashion (no heat is added or removed) requires large amounts of sensible cooling post-dehumidification and as a result limits the energy savings that can be obtained.
Absorption chillers such as units manufactured by Yazaki Energy Systems typically utilize a low pressure vacuum vessel in which a desiccant material is contained (in Yazaki's case LiBr2 and water, but systems using Silica gel have also been developed). However, the use of low pressure vacuum systems significantly increases the cost and complexity of the equipment and increases the requirements for maintenance. Also, each transition (from air to a heat transfer fluid to the desiccant) utilizes heat exchangers, fans, and pumps and thus results in higher costs. And importantly, such transitions result in larger temperature requirements since each transition is not perfectly efficient. As a result, absorption chillers require higher temperatures to operate making them less suitable for integration with systems that employ waste heat or low grade heat.
More recently systems have been introduced that employ other methods for dehumidification of the air. Liquid desiccant systems such as the systems manufactured by DuCool and Agam use a strong desiccant material such as a CaCl2 and water or LiCl2 and water solution to absorb water vapor in the air. The liquid desiccant is directly exposed to the air unlike the previously discussed absorption chillers that do not have air to desiccant direct contact. After the desiccant absorbs moisture from the air stream, it is heated to release the excess water. In winter, such desiccants can be used to recover heat and moisture from the leaving air and transfer it to the incoming air.
Liquid desiccant systems however have traditionally suffered from the risk of desiccant carry-over into the air stream resulting in sometimes severe corrosion problems in the building since the desiccants that are used are typically strongly corrosive to metals.
Furthermore, liquid desiccant systems are typically spraying a liquid desiccant on a filter media to increase the surface area of desiccant exposed to the air. The spraying increases the risk of desiccant carryover into the air stream. Oftentimes additional mist eliminator filters are used to capture any airborne desiccant particles. However, these mist eliminators require frequent maintenance and replacement. Furthermore, the process of using a filter media is inherently energy inefficient. The filter media is an obstruction in the air flow and thus generally requires large fan power. Also, the filter media typically are thermally non-conductive which makes the dehumidification process adiabatic resulting in undesirable heating of the air. To counteract the heating effect, one can increase the flow rate of the desiccant and one can pre-cool the desiccant to achieve some level of sensible cooling at the dehumidification stage in the filter media. Increasing the flow rate of course increases the risk of desiccant carry over and requires more liquid pump power. The liquid desiccant typically “rains” down from the filter media into a liquid desiccant collection pan. This generally prevents the liquid desiccant system from using a vertical air flow and requires more costly duct work to be used during the installation of the system on a buildings roof, where air is typically handled vertically. Furthermore, the drain pans do not easily allow the system to be set up as a “split” system wherein the conditioner and regenerator are located in physically separate locations. In addition, the drain pans do not easily allow for the system to be expandable: one has to increase the size of the pan—which means a new design, rather than adding capacity through a scalable design.
AIL Research has developed a low flow desiccant system that overcomes some of the objections mentioned above. The use of a heat transfer fluid in-situ to where the desiccant is dehumidifying the air results in better thermal performance and lower fan and pump power. However this approach still utilizes a horizontal air flow—which makes it much harder to integrate to a rooftop installation—and a very complex conditioner design that has a desiccant drain pan at the bottom, but does not allow for a counter flow between the air and the liquids. This system also still has the risk of desiccant carryover since the desiccant is still directly exposed to the air flow.
The source of heat and the required temperature for regeneration of the desiccant is also an important consideration in the design of a solar air conditioning system. It should be clear that the lower the regeneration temperature of the desiccant is, the easier it should be to find a source of such (waste) heat. Higher regeneration temperatures necessitate higher quality (temperature) heat sources and thus are less easily available. At worst, the system has to be powered by a non-waste heat source such as a hot water furnace. Yazaki absorption units have been powered by evacuated tube solar thermal modules that are able to generate heat as high as 100° C. Concentrated solar thermal modules are able to achieve even higher temperatures, but oftentimes do so at higher costs. Glazed flat plate solar thermal collectors typically operate at somewhat lower temperatures of 70-80° C., but also lose a significant portion of their efficiency at higher temperature, which means that the array size needs to be increased to generate adequate power. Unglazed flat plate solar thermal collectors have higher efficiencies at lower temperatures, but generally lose a lot of their efficiency at high temperatures and are usually not able to achieve temperatures higher than 60° C., making them unsuitable for integration with absorption chillers.
None of the solar heat sources mentioned above (concentrated solar thermal, evacuated tube collectors and glazed and unglazed flat plate collectors) generates electricity at the same time as generating heat. However, all air conditioning systems still require electricity for fans and liquid pumps. Electricity is oftentimes much more expensive per unit of energy than fuels used for heat. It is therefore desirable to have an energy source that can provide heat as well as electricity.
It is known that solar Photo-Voltaic Modules (PV modules) heat up significantly in direct sun exposure with temperatures approaching 70-80° C. Such temperatures have a deteriorating effect on the performance of the module since module performance degrades with an increase in temperature. Applying a thermal transfer fluid to the back of the PV module (a module known as a PVT (PV-Thermal) module) effectively draws the heat from the module, lowering its temperature and increasing its efficiency. The thermal transfer fluid (typically water or water and propylene or ethylene glycol) can reach temperatures and thermal efficiencies typically between those of a glazed and an unglazed solar thermal module.
From a cost perspective, solar thermal systems augmented with conventional PV modules are less cost effective than PVT modules and take up more space than PVT modules. However, PVT modules generally supply lower temperatures and efficiencies than pure solar thermal systems. But beneficially they generate more electricity than conventional PV modules.