Patent Application
20090223236
The present invention pertains to the art of producing potable water. More particularly, the present invention pertains to the production of potable water by extraction of water vapor from air with subsequent condensation of the vapor to obtain liquid water.
Providing water in remote locations is often quite difficult. For example, soldiers in the field require between 1.5 and 7 gallons of water per day for drinking, washing, and food preparation. Supplying this water to widely distributed ground troops presents a significant logistical burden to the U.S. Military. In some instances, soldiers can obtain water from local water supplies (e.g., civilian supplies, rivers, and lakes), but in cases where no local water is available or where it is potentially contaminated, trucks, helicopters, and other vehicles deliver water to the forward-deployed soldiers. Similar logistical problems face non-governmental organizations performing relief work in remote areas.
The logistical burden of water delivery could be mitigated if soldiers could instead produce their own water directly from water vapor in ambient air. Atmospheric air contains water at concentrations typically between 0.003 and 0.03 kg of water vapor per kg of air. Extraction of logistically significant quantities of water from the air can require processing large air volumes. However, the potential reduction in the burden of delivering water to soldiers in the field makes extraction of water from air worthy of consideration.
Numerous techniques have been developed to obtain potable water in remote locations. One basic technique is the “solar still” in which a clear barrier is extended over source of moisture (a pit dug into the soil or a source of non-potable water), solar radiation is used to evaporate water from the source, and potable water is condensed and collected from the underside of the barrier. This technique has limited applications since it cannot produce large amounts of potable water and depends on both solar energy and a vaporizable source of moisture. The utility of solar stills has generally been limited to emergency or survival situations.
Another technique has been to condense moisture in the air by forcing moisture-laden air over a refrigerated coil with a fan and collecting the condensed water. This method has typically been used by dehumidifiers, but suffers from the relative inefficiency of the refrigeration cycle as well as the growth of contaminants on the exposed condensation surfaces. For further details, refer to the examples of U.S. Pat. No. 6,755,037 and U.S. Pat. No. 6,588,225.
A further method for extracting water from air is to compress the air to the point where water vapor condenses to form liquid water. This method typically requires large amounts of energy and equipment involving many moving parts including seals that must withstand high pressures. The cost and complexity of this method makes it unattractive. For further details, refer to the examples of U.S. Pat. No. 6,453,684, U.S. Pat. No. 6,360,549, and U.S. Pat. No. 6,230,503.
Yet another technique, likewise used for both dehumidification and water production, has been the extraction of water from air via adsorption with a desiccant. Some of these desiccant systems used to produce potable water use liquid desiccant, which require complicated controls. For example, refer to U.S. Pat. No. 6,156,102. Other such systems use a fixed desiccant, such as silica gel or zeolite, in a batch process. These systems are limited in that the batch process limits the time the systems are used. For example, refer to U.S. Pat. No. 4,344,778, U.S. Pat. No. 4,342,569, U.S. Pat. No. 4,219,341, and U.S. Pat. No. 4,146,372. To overcome this limitation, some water producing systems have used plural desiccant beds in an alternating batch process. For further details, refer to the example of U.S. Pat. No. 4,304,577.
While rotating desiccant wheels have been more commonly employed in dehumidifying air conditioning systems that require continuous operation, the desorbed water is left in the waste streams in these systems. For further details, refer to the examples of U.S. Pat. No. 6,099,623, U.S. Pat. No. 5,931,015, U.S. Pat. No. 5,709,736, U.S. Pat. No. 5,526,651, U.S. Pat. No. 5,242,473, U.S. Pat. No. 5,170,633 and U.S. Pat. No. 3,844,737.
It has been proposed to use a rotating desiccant wheel for the production of liquid water from moisture in the air. Such a system adsorbs water from an incoming air stream on a portion of an intermittently rotated desiccant wheel. The wheel rotates to align with a desorbing section in which a recirculating air supply is heated with an electric heater, passed through the wheel to desorb water and regenerate the desiccant, and then passed over a condenser to condense liquid water. The energy requirements of the condenser and the heater in this system limit its efficiency and utility. For further details, refer to the example of U.S. Pat. No. 4,365,979.
Water absorbed inside a desiccant bed is forced into vapor phase by using energy from an energy conversion device (ECD). An ECD is defined as any device that converts potential energy to electrical energy and/or heat energy. The potential energy can come from numerous sources including (but not limited to) chemical, mechanical, or solar sources. The energy (electrical and/or heat) produced by the ECD is useful for collecting water from humidity in the ambient air.
An apparatus embodying the present invention has access to an ECD, and it has a rotating desiccant rotor that is divided into adsorption and desorption portions. The apparatus also has an ambient air blower, a circulating desorption fan, a heat exchanger, a water condenser, and a pre-heating module. The desorption portion has two sections, a pre-heating section and main desorbing section. The adsorption portion also has two sections, a heat recovery section and main adsorbing section.
During operation ambient air is driven by the ambient air blower and forced through the main adsorbing section of the desiccant rotor, which absorbs water from the ambient air. Simultaneously, another fan circulates air through a plurality of desorbing channels. In the desorbing channels, the air temperature is elevated by energy from the ECD prior to entering the desorbing section of the desiccant rotor. This high temperature/low-humidity-air then extracts the water from the desiccant rotor and passes it through a heat exchanger and a condenser. The condenser may be cooled by ambient air or an active cooling system. Once the system reaches a steady-operating condition, the relative humidity (RH) of the desorbing air downstream of the condenser reaches 100% and liquid water is collected.
One aspect of the present invention is the use of a rotating desiccant rotor to adsorb water from air and to then desorb water vapor from the desiccant rotor using desorbing air that has been heated by energy from an energy conversion device.
Another aspect of the present invention is the use of heat that has been recovered from elsewhere in the system to heat the desorbing air.
Another aspect of the present invention is the use of a closed channel-loop to keep the relative humidity of air at about 100% downstream of the condenser.
Another aspect of the present invention is the pre-heating of a portion of the rotating desiccant rotor to increase the water production rate.
Another aspect of the present invention is the recovery of heat from a portion of the rotating desiccant rotor to increase the water production rate.
According to one embodiment of the present invention, an apparatus extracts water from ambient air. The apparatus has a desiccant rotor, an adsorption fan disposed to force ambient air to contact a section of the desiccant rotor that lies within a main adsorption flow path, and a desorption fan disposed to force a closed loop of desorping air to contact a section of the desiccant rotor that lies within a main desorption flow path. A condenser is disposed to contact the desorping air so that water vapor in the desorping air is condensed as liquid water. A main heat exchanger is disposed to contact the desorping air so that the temperature of the desorping air is raised by heat energy transferred via the main heat exchanger. The apparatus also has a second heat exchanger, which has a hot side and a cold side. A pre-heating circulation fan is disposed to force pre-heating air to contact a section of the desiccant rotor lying within a pre-heating flow path, and to contact the hot side of the second heat exchanger. A heat recovery circulation fan is disposed to force heat recovery air to contact a section of the desiccant rotor lying within a heat recovery flow path, and to contact the cold side of the second heat exchanger.
According to another embodiment of the present invention, a method is provided for extracting water from ambient air. The method has a step of forcing ambient air to contact a section of a desiccant rotor lying within a main adsorption flow path, and a step of forcing a closed loop of desorping air to contact another section of the desiccant rotor lying within a main desorption flow path. The desorping air is passed over a condenser so that water vapor in the desorping air is condensed as liquid water. The desorping air is caused to contact a main heat exchanger disposed to so that the temperature of the desorping air is raised by heat energy transferred via the main heat exchanger. Pre-heating air is forced to contact a section of the desiccant rotor lying within a pre-heating flow path, and to contact a hot side of a second heat exchanger. Heat recovery air is forced to contact a section of the desiccant rotor lying within a heat recovery flow path, and to contact a cold side of the second heat exchanger.
According to a further embodiment of the present invention, an apparatus is provided that extracts water from ambient air. The apparatus has a desiccant rotor and a means for forcing ambient air to contact a section of the desiccant rotor lying within a main adsorption flow path. The apparatus also has a means for forcing a closed loop of desorping air to contact another section of the desiccant rotor lying within a main desorption flow path. Additionally, the apparatus has a condenser and a means for passing the desorping air over the condenser so that water vapor in the desorping air is condensed as liquid water. The apparatus further has a main heat exchanger and a means for contacting the desorping air with the main heat exchanger disposed to so that the temperature of the desorping air is raised by heat energy transferred via the main heat exchanger. The apparatus also has a second heat exchanger, a means for forcing pre-heating air to contact a section of the desiccant rotor lying within a pre-heating flow path, and to contact a hot side of the second heat exchanger, and a means for forcing heat recovery air to contact a section of the desiccant rotor lying within a heat recovery flow path, and to contact a cold side of the second heat exchanger.
The present invention relies upon thermal-swing adsorption, a process in which air is passed over the surface of an adsorbent, and water is collected from the air, typically in an adsorbed state inside small-diameter pores. Recovery of the water from the adsorbent requires addition of heat typically in excess of water's latent heat of vaporization, which is roughly 2400 J/g at room temperature. With sufficient heat addition, though, the water will desorb and form a high-humidity, high-temperature mixture of water vapor and air. Passing this mixture through a suitable condenser (e.g., air-cooled) allows the water to be collected as liquid.
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A pre-heating circulation fan 3 moves air in a closed loop through the pre-heating section of the desorption portion of the rotating desiccant rotor 10 and the pre-heat/heat recovery heat exchanger 7. A heat recovery fan 4 moves air through the heat recovery section of the adsorption portion of the rotating desiccant rotor 10 and the pre-heat/heat recovery heat exchanger 7. This arrangement causes transfer of heat energy between the pre-heating section and the heat recovery section via the pre-heat/heat recovery heat exchanger 7.
The auxiliary heat exchanger 6 in the closed loop of desorbing air is optional. Although not strictly required to practice the invention, including the auxiliary heat exchanger 6 enhances overall system efficiency.
For ease of explanation and illustration, the side of the desiccant rotor 10 through which ambient air enters for adsorption is defined as being the upstream side of the rotor, and the opposite side is defined as the downstream side.
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One particularly efficient way to operate an apparatus according to the present invention is to utilize waste heat (such as that produced by a combustion engine, an air conditioner, or a solar energy collector) to heat the desorbing air.
One optional feature that adds to the usefulness of a system embodied according to the present invention is a filtration device that filters the water produced. The filtration device is advantageously disposed between the condenser structure (where the water is condensed into liquid form) and a reservoir for holding the water for later use. Another optional feature that adds to the usefulness of a system embodied according to the present invention is a taste improvement module. The taste improvement module is advantageously disposed at or inside the water reservoir.
The present invention has been practiced using a 440 mm diameter and 200 mm depth silica-gel desiccant rotor. An electric heater was used to produce heat for the desorping air path. Cold-water was used for the cold side of the condenser. This implementation produced 1.8 liter/hr of water.
The present invention has been practiced using a 100 mm deep and 270 mm diameter silica-gel rotor. An electric heater was used to produce heat for the desorping air path. The air flow rate through the main adsorption section was about 60-70 cfm and through the main desorption section was 20-25 cfm. This implementation produced 6 L/day with 500 W of energy being input to the desorption air path, and with the ambient air conditions of 40% relative humidity and T=23° C. Water production rose to 8-9 L/day with 500 W being input, and with the ambient air conditions of 70% relative humidity and T=30 C. Maintaining the ambient air conditions of 70% relative humidity and T=30 C, water production rose to 10 L/day with energy input of 600 W, and rose further to 12 L/day with energy input of 800 W.
The present invention has been practiced using a diesel generator as the ECD. This system produces 1.5 to 2.0 L/hour of water using waste heat from a small diesel engine to desorb the collected water from a continuously rotating desiccant wheel. For every gallon of fuel burned, this system produces roughly 2 gallons of water.
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Three additional design options may be implemented to substantially improve the water-generation efficiency of the present invention. These options are not implemented in Working Example #3. These design options are summarized as:
The electrical generator on the Kubota engine used for the system of Working Example #3 is not intended to take full advantage of the all the mechanical power produced by the engine. Instead, it provides only about 1 kW of continuous electrical power. An electrical generator designed to fully use the Kubota engine power will produce in excess of 4 kW. It is preferable for the water-from-air system of the present invention to use all the available mechanical engine power to produce electricity.
Additional efficiency may be gained if an external burner is used to generate additional heat because an engine-driven generator will typically produce more electrical energy than embodiments of the present invention can utilize based solely on the excess heat from the engine. It is preferable to capture waste heat from the engine exhaust and coolant system, but this heat will not generally be commensurate with the water-production potential of the available electrical power. Adding an external fuel burner to provide additional heat allows one to match the total heat generation to the engine's electrical-generation capacity.
As illustrated according to Working Example #3, the approach of the present invention uses a continuously rotating desiccant wheel to collect water from ambient air. In a preferred embodiment, the ECD is an internal combustion engine or an internal combustion engine with an auxiliary fuel burner such that heat from the burning of a hydrocarbon fuel (whether generated directly by combustion or as waste heat from an engine) is used to desorb water from the desiccant. The air movement required to bring about water-vapor condensation is provided by a series of electrically driven fans, which are powered by an engine-driven generator. With careful attention to heat recovery and the kinetics of water adsorption/desorption, this method can yield up to about 7 gallons of water per gallon of fuel burned when the system is operated under baseline ambient conditions of 25° C. and 30% relative humidity.
According to the implementation of Working Example #3, heat is not used as efficiently as it might be. Before it is reheated, the scavenger air exiting the condenser can be preheated by the hot, humid scavenger air exiting the desiccant. This is not done in the system of Working Example #3. The scavenger air can be further preheated by the engine exhaust gas. Better heat recovery within the system reduces the need to burn additional fuel for heat production and, thereby, increases system efficiency. This improvement can be accomplished by adding several additional compact heat exchangers to the system.
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To raise the temperature of the scavenger air stream and keep the energy requirements low, a heat exchanger (located in the extraction loop) uses waste heat from the engine cooling system as well as the engine exhaust. To further reduce power requirements and limit system size, the condenser is placed in-line with the primary system fan and up stream of the adsorption section of the desiccant wheel. In this configuration, the condenser and the adsorption section of the desiccant wheel utilize a single air mover that may be driven either by an electric motor or by the shaft power of the engine.
A diesel-powered burner is incorporated in the scavenger-air extraction loop. The burner elevates the temperature of the desorption stream above the upper limit of the engine heat. This approach makes it possible to increase the fuel efficiency and process airflow by maximizing the engine's electrical energy output and using the energy to power the system fans.
Like Working Example #3, this alternate embodiment can be powered by a Kubota diesel engine. However, this alternate embodiment uses a commercially available 3.5 kW electric generator supplied by Phasor Marine. For further details, refer to the Internet web page at URL http://www.phasormarine.com/lp1-2-2 kw.htm.
When operating in ambient conditions of 25° C. and 30% relative humidity, it is estimated that certain embodiments of the present invention are capable of generating up to 11 L/hr of potable drinking water. Operating and physical parameters of the system can be modified to maximize water production and minimize fuel use during operation in these conditions. Table 1 below lists expected performance metrics for a typical system embodied according to the present invention.
Table 2 below lists nominal physical parameters for a typical system embodied according to the present invention.
The performance metrics and physical parameters shown above are based on experimental data collected during operation of the system of Working Example #3 and on feasibility studies of the primary system components. In the system of Working Example #3, approximately 3.2 kW of heat from the engine cooling system was used to extract water from the desiccant. An improved embodiment can recover 20% of the energy from the desorption process and utilize approximately 85% (or 3.6 kilowatts) of the available waste energy generated by the improved system engine. The combined available energy will be 7.5 kilowatts. Based on the performance data collected during operation of the system of Working Example #3 it has been determined that 1.6 kW of heat is needed to generate 1.0 liter per hour of water. Therefore, using only the waste engine heat to drive the desorption process, approximately 4.7 liters/hour of water can be generated.
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To assure production of safe drinking water, the present invention will typically incorporate a filtration component in the effluent water stream. The preferred filter component can combine hollow-fiber membranes, granular activated carbon, and magnesium oxide to improve the taste of the product water and prevent contamination with biological pathogens. Such an effluent filter is envisioned as a low-cost, long-life consumable and can designed to meet EPA efficacy standards for bacteria, protozoa, and viruses. Due to the relatively slow water-production rate, the effluent filter will typically operate via gravity drip and therefore require no power for operation.
To increase the production rate, a Hydronic Coolant Heater distributed by ESPAR will be incorporated into the design (for additional details, please refer to the specifications found at http://www.espar.com/htm/Specs/water/D9Wspec.htm). The Hydronic Coolant Heater can provide an additional 11 kilowatts of heat to use for desorption, resulting in a combined total of 18.3 kilowatts. The maximum fuel consumption for the EPSPAR burner is 1.2 L/h. Fuel consumption for the generator is approximately 0.95 L/h.
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A system and method for extracting water from ambient air has been described in terms of various examples and embodiments. It will be understood by those skilled in the art that the present invention may be embodied in other specific forms without departing from the scope of the invention disclosed and that the examples and embodiments described herein are in all respects illustrative and not restrictive. Any reference to claim elements in the singular, for example, using the articles “a,” “an,” or “the” is not to be construed as limiting the element to the singular. Those skilled in the art of the present invention will recognize that other embodiments using the concepts described herein are also possible.
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided by the terms of TACOM contract DAAE07-02-C-L054.
