WO2009021090A1

WO2009021090A1 - Carbon dioxide based heat pump for water purification

Info

Publication number: WO2009021090A1
Application number: PCT/US2008/072417
Authority: WO
Inventors: David J. Bayless; V. Gowreesan; Chaminda Perera
Original assignee: Ohio University
Priority date: 2007-08-07
Filing date: 2008-08-07
Publication date: 2009-02-12
Also published as: WO2009021090A8

Abstract

A method and an apparatus that purifies drinking water by boiling water and immediately cooling it while consuming relatively lower energy compared to other boiling methods. In this system, the heating side of a refrigeration system using CO2 refrigerant is first used to boil the water while the cooling side of the system is subsequently used to cool the previously boiled water. Specifically designed heat exchangers are used for the transfer of heat from CO2 to water and from water to CO2. This invention can be modified for commercial or domestic use. The temperature of the output water can be adjusted by simple modification of the system parameters.

Description

TITLE: CARBON DIOXIDE BASED HEAT PUMP FOR WATER PURIFICATION

BACKGROUND OF THE INVENTION

[0001] 1. Field Of The Invention

[0002] This invention relates generally to methods and apparatuses for boiling water, and more particularly relates to a method and apparatus that boils water and subsequently cools the water.

[0003] 2. Description Of The Related Art

[0004] It is estimated that one-third of the population of the world does not have access to safe drinking water, which leaves billions of people vulnerable to water-borne diseases.

Boiling water is a common method used to make water suitable for drinking, because boiling is an effective method to kill bacterial, parasitic, and viral causes of diarrhea in drinking water.

[0005] Traditionally, water is boiled using electrical resistance heating or the heat released by combusting a fuel. Conventional boiling methods have several disadvantages, including the fact that heat generated by combustion and electric resistant heating is not an energy-efficient method of boiling water. Furthermore, most households in geographic areas where safe water is unavailable do not have dedicated burners for boiling water. If the burner is required for other purposes, handling and removal of hot water may be difficult or even dangerous.

[0006] Still further, with conventional boiling methods, boiled water may require significant time to cool before use or consumption. Use of a separate refrigerator for cooling requires additional energy, which makes the process even less energy efficient.

[0007] The need exists for an apparatus and method in which water can be boiled and subsequently cooled in a continuous, energy-efficient manner.

BRIEF SUMMARY OF THE INVENTION

[0008] The invention includes, in one embodiment, a novel water purification system using CO₂ refrigerant. This embodiment includes an inline water supply in conjunction with a heat pump using CO₂ as a refrigerant for heating water. In the embodiment, the cold side of the CO₂ heat pump is part of a heat exchanger used for subsequent cooling of the just-boiled water. If a continuous water supply is available, this embodiment of the invention can produce purified, drinkable water continuously and very rapidly.

[0009] Thermodynamic calculations have shown that the method and apparatus of the invention are more energy efficient than conventional heating methods, because the invention has a very high coefficient of performance (COP). The temperature of the output water can be set to a safe range, thereby eliminating the need for a burner and the problems of handling and storage of hot water. The system can be modified to suit the application.

[0010] A feature of the invention is the use of a heat exchanger on the heating side of the

CO₂ refrigeration cycle to heat water to the boiling point where it destroys most of the water born bacteria. Another feature is the use of another heat exchanger on the cooling side of the same CO₂ refrigeration cycle to reduce the temperature of the boiled water immediately after boiling the water, thus making the output water fit for immediate consumption. This continuous process eliminates the need for a separate refrigerator or other cooling process.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0011] Fig. 1 is a schematic illustrating a preferred embodiment of the present invention.

[0012] Fig. 2 is a Pressure-Enthalpy (P-h) diagram for CO₂ including numerical points from Fig. 1 on the corresponding part of the diagram.

[0013] In describing the preferred embodiment of the invention which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific term so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word connected or term similar thereto are often used. They are not limited to direct connection, but include connection through other elements where such connection is recognized as being equivalent by those skilled in the art.

DETAILED DESCRIPTION OF THE INVENTION

[0014] For the purposes of illustration, a system 8 was designed for domestic usage and is illustrated schematically in Fig. 1. The values given in the following explanation are cited as examples for the particular system, and can readily be modified as will be understood by a person having ordinary skill in the technology in order to adapt the apparatus to different circumstances. Additionally, although CO₂ is described as the refrigerant, it is contemplated that other refrigerants, now known or hereafter discovered, that are equivalent to CO₂ can be used. [0015] The system 8 was designed according to the following criteria. The temperature of the water output by the apparatus is desirably approximately 17°C (63°F). The power input desired for the system is below 1500 Watts. The efficiency of the heat exchangers is about 0.85 (85%). The purified water flow from the system is determined based on typical usage of a household (consumption per adult of approximately 2.0 liters of water per day). The temperature of the water coming into the apparatus is assumed to be approximately 25°C (77 ⁰F). A CO₂ refrigeration cycle with a high pressure side of 12 MPa and a low pressure side of 4 MPa was selected to match the criteria.

[0016] The flow of CO₂ refrigerant through the conventional tubing of the refrigerant circuit in the system 8 of Fig. 1 is indicated by the Arabic numerals 1-6, while the flow of water through the conventional tubing of the water path is indicated by the Roman numerals i-iv. Compression of CO₂ takes place at the compressor 12 between points 1 and 2. The compressor 12, which is a conventional compressor, raises the pressure of the CO₂ refrigerant from about 4 MPa to about 12 MPa, which in turn raises the temperature of the CO₂ from about 25⁰C to about 116⁰C. The heat of the compressed CO₂ is used to boil incoming water in the heat exchanger 20, which is between points i and ii of Fig. 1. The heat exchanger 20 is a conventional heat exchanger designed to withstand the pressure of compressed CO₂ and boiling water while transferring much of the thermal energy of the CO₂ to the water to be boiled. The heat exchanger 20 separates the flow path of the water from the flow path of the CO₂ by a solid, highly thermally conductive wall that prevents the fluids from mixing, but permits the conduction of thermal energy through the wall. The temperature of the CO₂ coming out of the heat exchanger 20 at point 3 is calculated to be about 40⁰C, and the temperature of the water coming out of the heat exchanger 20 at point ii is about 100 ⁰C.

[0017] A pump (not illustrated) impels the water through the water path of the system 8 if the pressure of the incoming water of the source is not sufficient to do so. The boiled water from the heat exchanger 20 flows to a conventional convective radiator 22 to remove excess heat from the water by passing ambient air through the radiator 22, such as by an electric fan. In the radiator 22, the water circulates through exposed pipes with fins or other means of increasing surface area so that as air passes over the pipes, fins and other means, the air removes thermal energy by convection in the manner of an automobile radiator.

[0018] The hot CO₂ flows from the heat exchanger 20 through the heat exchanger 30 where heat is transferred to the incoming CO₂ (CO₂ that has just absorbed heat from the water as described below) from the CO₂ evaporator 50. This process is calculated to reduce the temperature of the already-compressed CO₂ gas to about 25⁰C while increasing to about 25⁰C (from 5.3⁰C) the temperature of the CO₂ that is to be compressed in the compressor 12. [0019] The compressed CO₂ gas coming out of the heat exchanger 30 is then throttled between points 4 and 5 by the expansion valve 40. This process decreases the pressure of the CO₂ to about 4MPa, and reduces the temperature to about 5.3⁰C, thereby liquefying the CO₂ refrigerant. Liquid CO₂ is conveyed through the evaporator 50, which is a conventional heat exchanger in which thermal energy is transferred from the water (coming from the radiator 22 at about 75⁰C) to the CO₂. The liquid CO₂ absorbs thermal energy in the evaporator to become gaseous CO₂ with a temperature of about 5.3⁰C. The phase change of the refrigerant causes the refrigerant temperature to remain substantially the same despite absorbing heat from the water. The CO₂ that flows from the evaporator 50 at about 4MPa and 5.3⁰C then pre-cools the higher pressure (12MPa) CO₂ from about 4O⁰C to about 25⁰C in the heat exchanger 30 before the higher pressure CO₂ is throttled at the expansion valve 40 as described above. [0020] In summary, the CO₂ refrigerant is compressed to about 12MPa and about 116⁰C just before entering the heat exchanger 20 where it boils the non-potable water and is thereby cooled. The refrigerant is then further cooled in the heat exchanger 30 by the lower pressure and lower temperature refrigerant that flows from absorbing heat from the potable water in the evaporator 50. The further cooled refrigerant is then expanded, which dramatically cools it and permits it to absorb heat from the potable water in the evaporator. This refrigerant flows to the heat exchanger 30 and then to the compressor 12 where the cycle starts again. [0021] The water flows from the heat exchanger 20, where it is boiled, to the radiator in which thermal energy is removed to the ambient air through convection. This water then passes through the evaporator 50, which functions much like the heat exchanger 20 inasmuch as thermal energy is transferred between the water and the refrigerant without the fluids mixing. However, in the evaporator 50, the water gives up heat to the refrigerant, and exits the evaporator 50 at about 17⁰C. This water is chemically safe to drink, because it has been boiled, and it is cool enough that it can be consumed immediately.

[0022] The benefits of the system 8 described herein are evident. First, much of the thermal energy used to boil the water is removed from the water, thereby cooling the water to make it usable for drinking, and "recycling" the heat to be used to boil the untreated water. This makes the system energy-efficient compared to resistance heating systems and fuel-combusting systems, because not all of the thermal energy used to boil water is generated, but instead is reused. Furthermore, little energy is used to cool the water, because the normally cooler portion of the heat pump cools the water, which not only recycles the thermal energy, as described immediately above, but renders the water suitable for immediate drinking. Still further, the fact that a heat pump is used to heat the water makes the system highly energy-efficient due to the nature of heat pump devices.

[0023] Figure 2 illustrates the heat pump cycle of the system 8 of Fig. 1 on the p-h diagram. The cycle was plotted on the p-h diagram for Carbon Dioxide Refrigerants created by Urieli (http://www.ent.ohiou.edu/~thermo/me328/Potter/CO2.html), as will be understood by the person having ordinary skill. The calculated COP of the system is approximately 3.4. [0024] The system can be modified for several applications. A water purification system can be developed for domestic use. This system consumes power at a level comparable to that of other household electric appliances, while producing a sufficient amount of purified water more efficiently and feasibly than any known conventional method.

[0025] Water purification can also be performed at a commercial scale provided compressors with suitable capacity are used. At the same time, some boiled water can be extracted directly after boiling for other applications, such as cooking, bathing or industrial processes while the rest of the treated water is cooled. The heat lost due to the removal of hot water can be compensated by reducing the heat extraction from water at the radiator 22. [0026] In an alternative embodiment, instead of using a radiator to transfer heat to the environment, another heat exchanger is used to transfer the heat from the boiled water (to be cooled) to the incoming water to be boiled. Thus, the thermal energy can be removed from the water that is about to be cooled by transferring the thermal energy to the water that is about to be boiled, thereby pre-heating the water that is about to be boiled.

[0027] If the incoming water temperature is much less than 25°C, one of the above methods can be used to compensate for the additional heat required to boil the water. Furthermore, other refrigerants having a different boiling temperature or pressure can be substituted for CO₂

[0028] If the system is to be used where there is not access to electric power, compressors directly connected to an externally heated free piston Stirling cycle engine can be used. Such a system has the flexibility of using different fuels such as natural gas, biomass, etc. [0029] The system 8 can be mounted on a vehicle, trailer or other transportable structure and used for water purification for emergency relief operations, such as floods. In such circumstances, the system 8 can be operated by coupling the compressor with the vehicle engine. Contaminated water can also be filtered prior to purification with the system 8 by commercially available particle removal filters.

[0030] Because refrigeration systems with CO₂ refrigerant are not yet commercially popular, other than the "Eco-Cute" (Mitsubishi Electric Corporation - Melco), the components are relatively expensive. However, the long-term savings should justify the relatively higher initial cost, and new developments in CO₂ refrigeration and heat pump systems are expected to reduce the price of the components as a result of competition.

[0031] It is understood by the person of ordinary skill that under normal conditions, CO₂ functions well as a refrigerant for the embodiments of the invention described above for raising the temperature of water above the boiling point, and then substantially lowering the temperature, under normal conditions. It will be understood that the invention can alternatively be used with different temperature and pressure parameters when, for example, attempting to raise and then lower the temperature of fluids other than water. For example, in an elevated region, a system using a different refrigerant may be necessary to account for the difference in atmospheric pressure. Likewise, when water at a different ambient temperature than that described above is being input to the system, a different refrigerant can be used. Still further, under extremely different conditions, such as aboard spacecraft or underwater where ambient pressure is significantly different, still different refrigerant may be used.

[0032] This detailed description in connection with the drawings is intended principally as a description of the presently preferred embodiments of the invention, and is not intended to represent the only form in which the present invention may be constructed or utilized. The description sets forth the designs, functions, means, and methods of implementing the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and features may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention and that various modifications may be adopted without departing from the invention or scope of the following claims.

Claims

1. An improved apparatus for heating water to at least a predetermined temperature including a compressor, an expansion valve, a condenser and an evaporator disposed along a refrigerant circuit through which a refrigerant flows, the improvement comprising:

(a) a water path adjacent at least a portion of the refrigerant circuit, in which water that is unsafe to drink flows from a source into the water path;

(b) a first heat exchanger adjacent the condenser through which compressed refrigerant flows separated from a portion of the water path by a thermally conductive wall that permits thermal energy to flow from the refrigerant to the water, thereby raising the water temperature to at least the predetermined temperature, but prevents mixing of the refrigerant and the water;

(b) a second heat exchanger adjacent the evaporator through which expanded refrigerant flows separated from a portion of the water path by a thermally conductive wall that permits thermal energy to flow from the water to the refrigerant, thereby lowering the water temperature substantially, but prevents mixing of the refrigerant and the water.

2. The apparatus in accordance with claim 1, wherein the predetermined temperature is the boiling point of water.

3. The apparatus in accordance with claim 1, further comprising a third heat exchanger disposed in the water path between the first heat exchanger and the second heat exchanger.

4. The apparatus in accordance with claim 3, wherein the third heat exchanger comprises a radiator in which water flows along the water path through thin-walled tubes across which ambient air can flow to remove thermal energy from the water.

5. The apparatus in accordance with claim 3, further comprising a fourth heat exchanger interposed along the refrigerant circuit between the first and second heat exchangers in which refrigerant flows from the first heat exchanger to the expansion valve separated from refrigerant that flows from the second heat exchanger to the compressor by a thermally conductive wall that permits thermal energy to flow from warmer refrigerant to cooler refrigerant, but prevents mixing of the refrigerant.

6. The apparatus in accordance with claim 5, wherein the refrigerant is carbon dioxide.

7. An apparatus for heating water to at least a water boiling point, the apparatus comprising:

(a) a compressor disposed along a refrigerant circuit through which a carbon dioxide refrigerant flows, the compressor configured to compress the refrigerant in the circuit;

(b) a first heat exchanger disposed in the refrigerant circuit through which compressed refrigerant flows from the compressor, the first heat exchanger having a thermally conductive wall disposed against the refrigerant circuit and against the opposite side of which is disposed a water path, the thermally conductive wall configured to permit thermal energy to flow from the refrigerant to the water while preventing mixing of the refrigerant and the water, thereby raising the water temperature to at least the water boiling point;

(d) a second heat exchanger disposed in the refrigerant circuit through which refrigerant flows from an expansion valve, the second heat exchanger having a thermally conductive wall disposed against the refrigerant circuit and against the opposite side of which is disposed the water path downstream from the first heat exchanger, the thermally conductive wall configured to permit thermal energy to flow to the refrigerant from the water while preventing mixing of the refrigerant and the water, thereby lowering the water temperature substantially.

8. The apparatus in accordance with claim 7, further comprising a radiator disposed in the water path between the first heat exchanger and the second heat exchanger, wherein thin-walled tubes form a radiator portion of the water path that is configured to permit ambient air to flow over the tubes for removing thermal energy from water in the thin- walled tubes.

9. The apparatus in accordance with claim 8, further comprising a third heat exchanger disposed in the refrigerant circuit between the first and second heat exchangers, the third heat exchanger having a thermally conductive wall disposed against a portion of the refrigerant circuit in which refrigerant flows from the first heat exchanger to the expansion valve and against the opposite side of the thermally conductive wall is disposed a portion of the refrigerant circuit in which refrigerant flows from the second heat exchanger to the compressor, the thermally conductive wall configured to permit thermal energy to flow from warmer refrigerant to cooler refrigerant.

10. A method of heating water to at least a predetermined temperature, and subsequently cooling the water, using a heat pump including at least a compressor, an expansion valve, a condenser and an evaporator disposed along a refrigerant circuit through which a refrigerant flows, the method comprising:

(a) compressing the refrigerant sufficiently to raise its temperature to at least the predetermined temperature of the water and conveying the compressed refrigerant through a first heat exchanger in the condenser;

(b) conveying water that is unsafe to drink through the first heat exchanger separated from the refrigerant by a thermally conductive wall that permits thermal energy to flow from the refrigerant to the water, but prevents mixing of the refrigerant and the water, thereby raising the water temperature to at least the predetermined temperature;

(c) expanding the refrigerant sufficiently to lower its temperature and conveying the expanded refrigerant through a second heat exchanger in the evaporator;

(d) conveying water through the second heat exchanger separated from the refrigerant by a thermally conductive wall that permits thermal energy to flow from the water to the refrigerant, but prevents mixing of the refrigerant and the water, thereby lowering the water temperature substantially.

11. The method in accordance with claim 10, further comprising removing thermal energy from the water after it has been conveyed through the first heat exchanger, and before it is conveyed through the second heat exchanger.

12. The method in accordance with claim 11, wherein the step of removing thermal energy comprises conveying water through a radiator.

13. The method in accordance with claim 12, wherein the step of conveying water through a radiator further comprises directing ambient air across a plurality of tubes on the radiator through which the water flows.

14. The method in accordance with claim 11, further comprising conducting thermal energy from the refrigerant that exits the first heat exchanger to the refrigerant that exits the second heat exchanger.

15. The method in accordance with claim 14, wherein the step of conducting thermal energy comprises conveying refrigerant exiting the first heat exchanger through a fourth heat exchanger separated from refrigerant exiting the second heat exchanger by a thermally conductive wall that permits thermal energy to flow from the warmer refrigerant to the cooler refrigerant, but prevents mixing of the refrigerant.