WO2010035051A1

WO2010035051A1 - Ammonia centrifugal heat pump

Info

Publication number: WO2010035051A1
Application number: PCT/GB2009/051274
Authority: WO
Inventors: Abdulsalam Al-Mayahi; Mohammed Aljohani
Original assignee: Abdulsalam Al-Mayahi; Mohammed Aljohani
Priority date: 2008-09-29
Filing date: 2009-09-29
Publication date: 2010-04-01
Also published as: GB0820893D0; GB0817672D0; GB0820892D0

Abstract

A heat pump, suitable for use with ammonia as a working fluid and water as an absorbent fluid, comprises a centrifuge arranged to dissolve the working fluid in the absorbent fluid, and to pressurise the resultant solution; a heat recovery unit adapted to recover heat from the solution; means for evaporating the working fluid from the solution; and an energy recovery unit adapted to recover energy such as hydraulic energy from the solution.

Description

AMMONIA CENTRIFUGAL HEAT PUMP

Field of the Invention

The invention relates to heat pump systems, and in particular to heat pump systems for use in relatively extreme temperature environments.

Background to the Invention

While there are many prior proposed heat pump systems, a recurring problem is that the optimum temperature at which they operate is often not well matched to the prevailing conditions. This is particularly the case in cold climates, where heating has generally been achieved by the burning of copious quantities of fossil fuels. Available low grade heat sources, such as rivers, seas, the sub-soil and the atmosphere may be only a few degrees above the freezing point of water. Unless there is an available source of geothermal heat or solar heat at a higher temperature, existing heat pump systems will often not work well. As will be explained below with reference to a preferred embodiment, a heat pump system capable of operating with a heat source that may be but a few degrees above freezing is provided. In hot climates, this invention could be used for space conditioning achieving low-temperature cooling level. This invention could be used for refrigeration applications with a wide range of cooling temperatures.

Heat pumps are reverse heat engines (see Fig. 1 of the accompanying drawings) in which Work W_1n is applied to take in Heat Q_1n from a source at temperature T_C0|_d to transfer Heat Q_0Ut to a heat sink at a higher temperature T_hot- The reverse heat engine of Fig. 1 may be used to heat the sink in which case its Co-efficient of Performance (COP) as a heat pump will be given by the formula COP_hp= CW W_1n. Alternatively, the reverse heat engine of Fig. 1 may be used as a refrigerator to abstract heat from the refrigerator source and transfer it, for example to atmosphere, at the sink. Its Co-efficient of Performance as a refrigerator will be given by the formula COP_ref = Q_1n / W_1n. Moreover, in this system Q_out - Q_1n = W_1n. Apart from the relevant Co-efficient of Performance, heat pumps used for heating and refrigerators used for cooling are essentially the same apparatus, and the term "heat pump" is used herein below as the generic term for such apparatus. Although heat pumps are based on a variety of heat engine cycles, the most common such cycle is the vapour compression cycle, as employed in refrigerators. A simplified refrigeration heat pump cycle is illustrated in Fig. 2 of the accompanying drawings.

A working fluid circulates around a closed cycle. The working fluid as gas is compressed in the compressor where Work W_1n is applied. The compressed gas at a higher pressure and hence higher temperature passes to a condenser, where the compressed vapour is condensed into a saturated or sub-cooled liquid, releasing Heat Q_out. The working fluid then passes through a throttling or expansion valve which expands the fluid, so reducing its pressure and its temperature. As a result, the working fluid may absorb Heat Q_1n at an evaporator where the fluid returns to its vapour state before passing again to the compressor. The effect of this cycle is to continuously absorb Heat at the evaporator, to continuously give off Heat at the condenser and to apply Work to the cycle at the compressor.

Absorption heat pumps work on different principles, being thermally rather than mechanically driven. Absorption heat pumps used for space conditioning are often gas-fired, while high-pressure steam or waste water has been used in industry to drive such heat pump installations. Absorption systems utilize the ability of liquids or salts to absorb the vapour of the working fluid. The most common working pairs are: water (working fluid) and lithium bromide (absorbent); and ammonia (working fluid) and water (absorbent).

Traditionally, the most common working fluids for closed cycle compression heat pumps have been CFCs (chlorofluorocarbons) and related chemicals, usually identified in the refrigeration industry as CFC-12, CFC-114, R500, R502 and HCFC-22.

Of these, all but HCFC-22 are CFCs. These chemicals are harmful to the global environment having both a high Ozone Depletion Potential and a high Global Warming Potential. Since January 1996, the use of CFCs has been phased out, and these chemicals are now prohibited for use as refrigerants. HCFCs (hydrochlorofluorocarbons), of which the most commonly used has been HCFC-22, which has been used in virtually all refrigerators since 1996, may be less harmful than CFCs, but they still have significant Ozone Depletion and Global Warming Potentials. Since January 2004, the use of HCFCs in the European Union is required to be reduced, and they are required to be phased out by January 2015. As a consequence, the industry is turning to HFCs (hydrofluorocarbons) such as R-134a, R-152a, R- 32, R-125 and R-507, which may not contribute to ozone depletion, as they have no chlorine content, but still possess significant Global Warming Potential. However, special attention must be given to the use of lubricants, as mineral oils are non-miscible with these refrigerants, so that any mineral oil residues must be completely removed, which is a serious problem in retrofitting, only special expensive ester-based lubricant oils recommended by the manufacturers being useable.

These problems have held back the development and application of heat pumps on a wider scale, notwithstanding the apparent economic attractiveness of heat pumps, whether used for refrigeration or for heating, that they commonly have a Co-efficient of Performance that are in theory substantially greater than unity (in other words: when used for heating the systems produce more Heat then the energy put in as applied Work, and, when used for cooling/refrigeration, the Heat abstracted by cooling is again greater than the energy supplied as Work put into the system).

A possible alternative to the otherwise almost ubiquitous use of fossil fuels or electricity derived from fossil fuels or nuclear energy for space heating and for cooling might be heat pumps based upon alternative "natural" working fluids but, as explained below, all such fluids used to date suffer from significant disadvantages.

So-called "natural" working fluids are substances that naturally exist in the biosphere and so do not represent a significant Global hazard. They will generally have essentially negligible Global environmental drawbacks (zero or near zero Ozone Depletion or Global Warming Potentials). Examples of natural working fluids that have been employed in heat pump cycles include ammonia (once widely used as a working fluid for refrigeration), hydrocarbons, carbon dioxide and water.

Water appears at first sight to be an excellent choice of working fluid for high- temperature industrial heat pumps due to its generally favourable thermodynamic properties, particularly its high latent heat of evaporation, and the fact that it is neither flammable nor toxic.

Water has mainly been employed as a working fluid in open and semi-open mechanical vapour recompression systems. The major disadvantage of water, however, is its low volumetric heat capacity. This requires large (and hence expensive) compressors, especially when it is operating at low temperatures. A further disadvantage is that water freezes which means that the evaporator in a closed cycle cannot be exposed to temperature below O⁰C, which rules water out for use in refrigerators.

The problem with hydrocarbons is their high flammability, requiring precautions to be taken in enclosure of the heat pump, the adoption of fail-safe ventilation systems, and the addition of tracer gas to the working fluid and/or use of gas detectors.

Carbon dioxide is a potentially attractive working fluid being non-toxic, non-flammable and compatible with most normal lubricants and construction materials. Its volumetric refrigeration capacity is high. However, the theoretical Co-efficient of Performance of a conventional heat pump cycle employing carbon dioxide as the working fluid is rather poor. As a result, the use of carbon dioxide has largely been restricted to use as a secondary refrigerant in cascade systems.

The problem with ammonia is its relative toxicity and the fact that many construction materials are incompatible with its use. In high-temperature industrial heat pumps ammonia is not suitable because efficient high-pressure compressors able to operate at the pressure required when using ammonia as the working fluid are simply not available. Ammonia has a high latent heat of vaporization value 1369 KJ/Kg in comparison with other working fluids, e.g. 2260 KJ/Kg for water at 100⁰C, and has a lower density than air.

As will be explained below with reference to a preferred embodiment, a heat pump system may be provided that is capable of operating with a heat source that may be but a few degrees above freezing and could be used for cooling and refrigeration applications.

According to a first aspect of the present invention, there is provided a heat pump, suitable for use with ammonia as a working fluid and water as an absorbent fluid, comprising: a centrifuge arranged to dissolve the working fluid in the absorbent fluid, and to pressurise the resultant solution; a heat recovery unit adapted to recover heat from the solution; means for evaporating the working fluid from the solution; and an energy recovery unit adapted to recover hydraulic energy from the solution.

According to a further aspect of the present invention there is provided an apparatus for Ammonia Centrifugal Heat Pump, said apparatus comprising:-

(i) a centrifuge; (ii) a heat recovery unit means to take the heat out from said centrifuge;

(iii) a flash evaporator or boiler; and

(iv) an energy recovery device in fluid connection with high pressure stream coming out from said centrifuge or said heat recovery unit.

Preferably a centrifuge means is provided to pressurise a liquid in by a rotation mechanism.

Preferably a centrifuge is configured to provide pressure distribution alongside its radius.

Preferably a centrifuge is configured to rotate by an electrical motor.

Preferably a centrifuge is configured to rotate by means of a rotating compartment as a part of energy recovery device.

Preferably a heat recovery unit means comprises a heat exchanger to remove the generated heat from the centrifuge.

Preferably a heat recovery unit means provides heating product when a said heat pump is used for heating applications.

Preferably a centrifuge is configured to be connected with a heat recovery unit.

Preferably a centrifuge is configured to be connected with a built-in heat exchanger to recover any generated heat.

Preferably a centrifuge is configured to be in contact with a suitable liquid bath working as a heat exchanger to recover any generated heat inside the centrifuge.

Preferably a heat recovery unit and a centrifuge unit are connected by a recycle line.

Preferably an energy recovery device unit means transfers the hydraulic energy from the outlet stream of the heat recovery unit to a rotating compartment which could be used to rotate the centrifuge.

Preferably the outlet stream from a heat recovery unit and after transferring its hydraulic energy enters a flash evaporator. Preferably a flash evaporator means comprises a heat exchanger.

Preferably a flash evaporator unit means comprises a heat exchanger to provide the heat to the heat pump.

Preferably a flash evaporator unit means provides cooling product when a said heat pump is used for cooling applications.

Preferably a flash evaporator unit means comprises a vapour outlet stream connected to the middle of the centrifuge.

Preferably a flash evaporator unit means comprises a liquid outlet stream connected to the middle of the centrifuge.

According to a yet further aspect of the present invention, there is provided a heat pump system comprising: a centrifugal ammonia absorber unit (water contactor) where ammonia vapour can be absorbed in water, a heat removal unit (heat exchanger) which is a part of the centrifuge or is connected to the outlet stream from the wall of the centrifugal unit and delivers the high concentrated ammonia solution at a higher temperature, optional energy recovery device that converts the hydraulic energy from the cooled outlet stream from the centrifuge or from the heat removal unit to mechanical energy to enhance the rotation of the centrifuge and a flash evaporator with suitable heat exchanger to provide a heating source.

Further features of the invention are characterised by the dependent claims.

The invention extends to methods and/or apparatus substantially as herein described with reference to the accompanying drawings.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa.

The invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 (PRIOR ART) shows the general thermodynamic concept behind a heat pump;

Figure 2 (PRIOR ART) shows a conventional vapour compression heat pump; Figure 3 shows a schematic diagram of an ammonia centrifugal heat pump system according to the present invention; and

Figure 4 shows a schematic diagram of an ammonia centrifugal heat pump system according to the present invention.

As shown in Figure 4 there is a heat pump system with a centrifugal ammonia absorber unit (water contactor) 10 where ammonia vapour can be absorbed in water, a heat removal unit (heat exchanger) 20 which is connected to the outlet stream 1 1 from the outside wall (i.e. the periphery) of the centrifugal unit and delivers the high concentrated ammonia solution at a higher temperature. Alternatively, the heat removal unit can be integral to the centrifuge, i.e. a centrifuge with a built-in heat exchanger. Or else, it can be a liquid bath that the centrifuge is mounted in. The energy recovery device 25 converts the hydraulic energy, from the cooled outlet stream 23 from the heat removal unit, to mechanical energy. This mechanical energy is used to rotate the centrifuge, or to assist the motor or the like in rotating the centrifuge. The flash evaporator 30 is provided with a suitable heat exchanger to provide a heating source.

Fig.3 describes the main units of the heat pump whereas Fig 4 describes more details.

Below is the key to figure 4.

KEY TO FIGURE 4

Item Description

10 Centrifugal unit: Centrifugal vapour liquid contactor - ammonia water absorber (condenser) (the motor or other moving parts are not shown in this drawing)

20 Heat recovery unit: Heat exchanger, to take out the generated heat from the centrifuge. Heating product can be collected from here.

30 Flash evaporator (boiler) with heating source. Cooling product can be collected from here. 25 Energy Recovery Device (turbine) that transfers the hydraulic energy from the high-pressure stream to rotate the centrifuge. 1 1 High pressure liquid stream: High pressure line (pipe) between the wall of centrifugal and the heat recovery unit. 21 Heating product: Output line from which heating product can be collected in case of heating applications. 22 Input cooling line for the heat recovery unit.

23 Cooled, high pressure and high - ammonia concentration outlet line.

24 Recycle: Recycle high pressure line between the centrifugal and the heat recovery unit

25 Energy recovery device: Energy recovery turbine (device) that converts the hydraulic energy from line 23 to a mechanical rotating device joined with the centrifuge.

31 Vapour stream: Ammonia vapour line transfers the separated vapours from the flash evaporator to the centre of centrifuge which is at a lower pressure. 32 Liquid stream: Ammonia liquid solution line which transfers the low- concentration ammonia solution to the centre of centrifuge which is at a lower pressure.

33 Cooling product: Output line from which cooling product can be collected in case of cooling applications. 34 Heating line for the flash evaporator (boiler).

This arrangement allows ammonia vapour to be dissolved (absorbed) in water at the centre of the centrifuge where the pressure is at a lowest value. Accordingly, both vapour and liquid streams leaving the flash evaporator can be introduced to the centrifuge where the pressure is low. The pressure at the centre of the centrifuge is at the lowest value whereas it is at the highest at the outside wall of centrifuge based upon the centrifugal mechanism. In this example, the centrifuge is approximately 500mm in diameter and rotates at approximately 5000rpm. The pressure distribution along the radius of centrifuge allows ammonia concentration to be varied correspondingly so that the concentration near the wall will be at the highest value. The generated heat due to ammonia absorption (condensation) inside the centrifuge allows a temperature distribution respectively, which means that the temperature at the wall of centrifuge is higher. Generally, pressure, concentration and temperature gradually increase alongside the distance from the centre of the centrifuge to the wall at the periphery of centrifuge. Therefore, at the centre there is low temperature, concentration and pressure and higher values at the wall.

The centrifugal separation process is a well known technology and used in different industrial fields to separate fluid constituents based on their density difference such as solid- liquid separators and liquid-liquid separators. Generally, in case of liquid phase centrifuges, the pressure varies from the centre of the centrifugal unit to the wall of the centrifuge. Centrifugal effect may create a vacuum pressure at the centre of centrifuge whereas it can create a very high pressure on the wall of the centrifuge depending on many factors including; the rotating speed, the radius of centrifuge, fluid density and the centrifuge shape design. The centrifugal part of the heat pump could be similar to those liquid phase centrifuges that can create a pressure gradient alongside the radius when it rotates.

Heat pumps are reverse heat engines, and most of the power consumption is in the compressor. Therefore, by reducing the power required in the compression stage by utilising a centrifuge the Coefficient of Performance is increased.

It will be understood that the invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Claims

1. A heat pump, suitable for use with ammonia as a working fluid and water as an absorbent fluid, comprising:

a centrifuge arranged to dissolve the working fluid in the absorbent fluid, and to pressurise the resultant solution;

a heat recovery unit adapted to recover heat from the solution;

means for evaporating the working fluid from the solution; and

an energy recovery unit adapted to recover energy such as hydraulic energy from the solution.

2. A heat pump according to Claim 1 , wherein said energy recovery unit is further adapted to utilise said recovered hydraulic energy to power said centrifuge.

3. A heat pump according to Claim 2, wherein said energy recovery unit is further adapted to transfer said hydraulic energy to a rotating compartment utilised to rotate said centrifuge.

4. A heat pump according to any of Claims 1 , 2 or 3 wherein an outlet from said centrifuge is located at its periphery.

5. A heat pump according to any of Claims 1 to 4, wherein said energy recovery unit recovers energy from the solution exiting said heat recovery unit.

6. A heat pump according to any of the preceding claims, wherein said heat recovery unit comprises a heat exchanger.

7. A heat pump according to any of the preceding claims, further comprising a recycle line from an outlet of said heat recovery unit to said centrifuge.

8. A heat pump according to any of the preceding claims wherein said heat recovery unit is adapted to provide heating product when said heat pump is used for heating applications.

9. A heat pump according to any of the preceding claims, wherein said means for evaporating the working fluid comprises a heat exchanger.

10. A heat pump according to any of the preceding claims, wherein said means for evaporating the working fluid comprises a vapour outlet line in fluid communication with the centre of said centrifuge.

1 1. A heat pump according to any of the preceding claims, wherein said means for evaporating the working fluid comprises a liquid outlet line in fluid communication with said centrifuge.

12. A heat pump according to any of the preceding claims wherein said means for evaporating the working fluid is a flash evaporator.

13. A heat pump according to any of the claims 1 to 11 , wherein said means for evaporating the working fluid is a boiler.

14. A heat pump according to any of the preceding claims, wherein said means for evaporating the working fluid is adapted to provide cooling product when said heat pump is used for cooling applications.

15. A heat pump according to any of the preceding claims wherein an outlet from said energy recovery unit is in fluid communication with said means for evaporating the working fluid.

16. A heat pump according to any of the preceding claims wherein said heat recovery unit is integral to said centrifuge.

17. A heat pump according to Claim 16, said heat recovery unit being a heat exchanger built-in to said centrifuge.

18. A heat pump according to Claim 16, said heat recovery unit being a liquid bath suitable for use as a heat exchanger to recover heat from said centrifuge.

19. A heat pump according to any of the preceding claims, further comprising an electrical motor arranged to power said centrifuge.

20. A heat pump substantially as herein described with reference to Figure 4.