WO2013088160A2

WO2013088160A2 - Heat absorption

Info

Publication number: WO2013088160A2
Application number: PCT/GB2012/053134
Authority: WO
Inventors: Saffa Riffat; Yuehong SU; Kevin Simpson
Original assignee: The University Of Nottingham; European Thermodynamics Limited
Priority date: 2011-12-14
Filing date: 2012-12-14
Publication date: 2013-06-20
Also published as: WO2013088160A3; GB201121520D0

Abstract

A method is disclosed of absorbing heat for subsequent re-use from a thermal source comprising: passing a high pressure stream of a working fluid through or close to the thermal source so as to absorb heat from the thermal source, thereby producing a higher temperature stream comprising a liquid component and a vapour component; and separating the vapour component from the liquid component; wherein the working fluid comprises a lubricant (for example EMKARATE RL 32H), in which a natural refrigerant (eg carbon dioxide) is dissolved. The thermal source may have a temperature of no more than 150°C, and optionally 40°C or more.

Description

HEAT ABSORPTION

The present invention relates to heat absorption. In particular, the invention relates to the absorption and/or recovery of heat from relatively low temperature thermal sources. The absorbed heat may be used in power generation and/or cooling applications.

Power generation from thermal sources has attracted great interests from both the academic and industrial communities in the past 30 years. This interest is due to the growth in the number of potential applications such as geothermal, solar thermal and waste heat power generation. Each of these applications is both commercially attractive and in-line with the EU legislative drivers to increase energy efficiency, reduce emissions and increase the use of renewable energy technologies. Geothermal energy has tremendous potential to allow the almost unlimited generation of heat and electrical power from a renewable source that could enable the EU to reduce C0₂ emissions and to increase energy security.

Traditionally, geothermal energy has been utilised in global regions where high grade (>200°C) thermal sources are available such as in the Pacific ring of fire . However, advances in technology which allow the production of electricity from lower temperature resources (thermal sources) using binary technology provide the opportunity to produce electricity in other geographic regions. However, current binary cycle power plants have relatively low thermal efficiencies of 10- 13 %, in comparison to up to 20% for high temperature fluids (> 180°C). For the production of electricity from low temperature geothermal resources (< 120°C) to become widespread, it must become cost-competitive with traditional forms of energy (typically, $0.03 to $0.05 per kilowatt-hour).

The majority of geothermal areas generate hot water at temperatures <200°C. Energy is typically extracted from these area using binary-cycle power plants where a hot geothermal fluid and a secondary "binary" fluid (with a much lower boiling point) pass through a heat exchanger. Heat from the geothermal fluid causes the secondary fluid to vapourise, driving a turbine. Because this is a closed-loop system, virtually nothing is emitted to the atmosphere . In low and medium temperature power generation applications, the Rankine cycle typically uses a low boiling point working fluid instead of water and is thus sometimes known as the organic Rankine cycle (ORC). ORCs are one of the most efficient technologies for producing electrical power from low temperature sources. ORCs have successfully been demonstrated for a number of practical applications. For example, Sawyer (Sawyer, S. L., Electricity generation from low temperature heat sources using organic Rankine cycle engine. Electric Energy Conf. Darwin, Australia, 10- 13 June, 1991) described the Enreco organic Rankine cycle heat engine which was developed as part of solar pond technology to generate electricity from brine at 80°C. However, at low temperature, <90°C organic Rankine cycles are inefficient. The overall power conversion efficiency of a solar-powered Rankine cycle may vary from less than 10% up to 20% depending on the operating temperature .

The Kalina cycle has recently gained popularity for industrial waste heat recovery because the Kalina cycle uses the binary mixture NH₃/H₂0 as the working fluid and hence has a temperature glide in the boiling process to match the temperature change of the waste heat source to reduce thermodynamic irreversibility. The Kalina cycle is a so-called absorption power generation cycle. The performance of the Kalina cycle could be superior to the Rankine cycle for power generation from waste heat in cement plants, but involves a more complicated system. Direct use of a mixture as the working fluid in a Rankine cycle has been investigated as a simplified alternative approach to the Kalina cycle . However, the toxicity and flammability of NH₃ used in the Kalina cycle is a safety concern. In general, an absorption power generation cycle may use other working fluids such as LiBr/H₂0 and refrigerant (e.g., R134a and R123) based absorption working fluid. However, the turbine will be bulky for low pressure LiBr/H₂0 system and toxicity and environmental pollution is a concern for refrigerant-based absorption working fluids.

It has been shown that for low temperature heat sources (< 120°C), absorption power generation may be more advantageous than conventional (organic) Rankine Cycle power generation. In principle, absorption power generation may be able to achieve an efficiency of 20% in the low temperature range (< 120°C) when using working fluids such as LiBr/H₂0 and NH₃/H₂0. This efficiency is significantly higher than that which can be achieved by ORCs. This higher efficiency is due to the fact that the temperature glide in re-generation of an absorption solution matches with the temperature change of low temperature heat to reduce thermodynamic irreversibility in heat transfer. Absorption power generation technology therefore has massive potential for generation of electrical power from low temperature sources. However, there are still a number of significant limitations that have to be overcome before this potential can be realised. One significant barrier is the lack of a suitable working fluid. Current working fluids such as NH₃ have safety concerns due to toxicity and flammability issues and those based on fluorine containing gases such as R134a are to be phased out under the EU's F-gas regulations.

It is a non-exclusive object of the invention to provide an improved heat recovery technology, especially for low temperature applications, which overcomes or at least alleviates one or more of the problems associated with known technologies. It is another non-exclusive object of the invention to provide an improved absorption power generation technology, especially for low temperature applications, which overcomes or at least alleviates one or more of the problems associated with known technologies. A first aspect of the invention provides a method of absorbing heat for subsequent reuse from a thermal source comprising:

• passing a high pressure stream of a working fluid through or close to the thermal source so as to absorb heat from the thermal source, thereby producing a higher temperature stream comprising a liquid component and a vapour component; and

• separating the vapour component from the liquid component;

wherein the working fluid comprises a lubricant, in which a natural refrigerant is dissolved. The thermal source may have a temperature of no more than 150°C, preferably no more than 130°C or 120°C. The thermal source may have a temperature of 40°C or more, preferably 60°C or more .

In an embodiment, the thermal source may comprise one or more of a geothermal resource, a solar thermal source and/or a source of waste heat. In an embodiment, the high pressure stream may have a pressure of no more than 90 bar, preferably no more than 70 bar. The high pressure stream may have a pressure of at least 40 bar, preferably at least 50 bar.

The higher temperature stream may have a temperature of no more than 80°C, e.g. around 60°C. Preferably, the lubricant may be an oil. The oil may be an organic oil, e.g. vegetable or sunflower oil, or a mineral oil, e .g. a petrochemical oil. The oil may comprise a polyol ester.

The lubricant may comprise EMKARATE^® RL 32H.

The natural refrigerant may be C0₂. C0₂ may generally be preferred, but other natural refrigerants may be used, either as an alternative or in addition. Other suitable natural refrigerants may include hydrocarbons such as butane, propane, methanol and ethanol.

Preferably, the natural refrigerant may be present in the working fluid at the maximum possible concentration for a given pressure or at a concentration at least approaching the maximum possible for a given pressure. In general, as high a concentration as possible for a given pressure may be desirable .

Preferably, the vapour component comprises the natural refrigerant, e.g. gaseous C0₂.

The vapour component may subsequently be used to perform work in a power generation system, e.g. a turbine or an expander connected to an alternator, and/or in a cooling system, e.g. an ejector cooling system and/or an absorption cooling system. The method may further comprise re-combining the working fluid, and, preferably, pumping the re-combined working fluid to create a high pressure stream for heat recovery from the same thermal source or from another thermal source .

Accordingly, the method may be repeated, preferably continuously, in a cyclical manner. A second aspect of the invention provides a working fluid for use in heat recovery from a thermal source, the working fluid comprising a lubricant with a natural refrigerant dissolved therein.

A third aspect of the invention provides the use of a working fluid comprising a lubricant with a natural refrigerant dissolved therein to absorb heat from a thermal source. Preferably, the heat may be re-used in power-generation and/or cooling. A fourth aspect of the invention provides a method of generating electricity comprising use of a working fluid comprising a lubricant with a natural refrigerant dissolved therein to absorb heat from a thermal source and subsequently using a vapour stream separated from the working fluid to carry out work, e.g. in a turbine or expander.

The turbine or expander may be connected to an alternator. Electricity generated by the method may be transmitted to a location remote from where the work was carried out to generate the electricity, e.g. in the turbine or expander. The electricity may be transmitted by a conventional grid or a super grid.

The location may be a power point in a domestic property, a commercial property, an industrial establishment, a public amenity or a public space .

A fifth aspect of the invention provides a method of cooling comprising use of a working fluid comprising a lubricant with a natural refrigerant dissolved therein to absorb heat from a thermal source and subsequently using a vapour stream separated from the working fluid to carry out work, e.g. in an ejector cooling system and/or in an absorption cooling system. A sixth aspect of the invention provides a system for generating electricity and/or providing cooling, the system being adapted to carry out a method according to the invention. For instance, the system may comprise a pump operable to pressurize the working fluid to provide a high pressure stream, means to transport the high pressure stream through or close to the thermal source, a heat exchanger arranged to effect absorption of heat from the thermal source into the working fluid, thereby producing a higher temperature stream, and a separator for separating the high temperature stream into a vapour component from a liquid component.

The working fluid of the present invention has the following advantages:

1. In use, the pressure level with the or a turbine or expander may be such that a conventional turbine or expander may be used.

2. In use, there may be no need for a rectifier. A rectifier is usually necessary when NH₃/H₂0 is used as a working fluid.

3. Compared with refrigerant (e.g., R134a and R123) based absorption working fluid, the C0₂/lubricant does not cause environmental pollution.

The invention may be particularly applicable in the following areas:

1. Geothermal power: According to the KPMG World Geothermal Market and Outlook report (02/02/201 1) 24 countries are utilising geothermal energy for electricity production with the world's total installed capacity being was 10.715 MWe in 2010. Traditionally geothermal development has been limited to areas such as the Pacific ring of fire and the Atlantic ridge where natural hydrothermal systems that provide high quality heat are available for exploitation. However, recent advances in binary technology have enabled the generation of electricity from lower temperature resources which has increased the feasibility of geothermal electricity production from EU countries such as Germany and Spain. The application of the invention for low temperature (< 120°C) geothermal applications may be 20% more efficient than current binary technologies ( 10- 13% in the low temperature range).

2. Solar thermal: The EU solar thermal market has 2,586 MWh (3 694 940 m²) of newly installed capacity. Most solar thermal systems are used for water or space heating and use water as the working fluid and so temperatures are typically < 100°C.

3. Waste heat energy recovery: Industries such as the chemicals industry, steel, food, cement, ceramics and paper manufacturing along with power stations and incinerators produce huge amounts of industrial waste heat is discharged into the environment, most of this is low quality heat at a temperature of < 100°C. The invention may be used to recover waste heat, e.g. in exhaust emissions, generated by vehicles driven by internal combustion engines. The invention may have utility in commercial scale power generation.

Advantageously, the invention may open up new possibilities for commercial geothermal electricity production, because the invention may enable more efficient exploitation of low enthalpy, e .g . low temperature, resources that it may not otherwise be economically viable to exploit.

In order that the invention may be well understood, it will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 shows a schematic view of a power generation system according to the invention;

Figure 2 shows the pressure-temperature curves of C0₂/RL32H, and an illustration of an absorption power generation cycle;

Figure 3 is a graph showing temperature glide during re-generation and cooling processes; Figure 4 shows a 4-vane expander that may be employed in a system according to the invention;

Figures 5a, 5b and 5c show the working process of a single screw expander that may be employed in a system according to the invention;

Figure 6 shows a schematic view of an embodiment of a combined power generation and cooling system according to the invention; and

Figure 7 shows a schematic view of another embodiment of a combined power generation and cooling system according to the invention.

A two-phase working fluid comprising C0₂ and a commercially available refrigeration compressor lubricant, EMKARATE^® RL 32H, may be a particularly suitable absorption working fluid for power generation from low temperature heat. EMKARATE^® RL 32H is a polyol ester. It is available from Lubrizol and is primarily used as a lubricant. This absorption working fluid has a large temperature glide in the re-generation process to give a good matching with the temperature change of low temperature heat. Advantageously, the absorption power generation cycle using C0₂/lubricant as the working fluid does not need a rectifier, which is common in an NH₃/H₂0 Kalina cycle . Thus, the cycle may be simple . In addition, the level of C0₂ pressure may be suitable for the use of some commercially available turbines or expanders.

Figure 1 shows a schematic view of a power generation system 10 according to the invention. The above-described working-fluid may be used in this power generation system.

The system 10 comprises a generator 1 1 , a vapour/liquid separator 12, an absorber 13, a heat recovery unit 14, a turbine (or expander) 15, an alternator unit 16 and a circulation pump 17. The C0₂/lubricant working fluid is re-generated in the generator 1 1 to release C0₂. After passing through the vapour/liquid separator 12, gaseous C0₂ enters the turbine or expander 15 to rotate the alternator 16 to produce electricity. The re-generated working fluid from the separator 12 enters the absorber 13 to absorb the gaseous C0₂ from the turbine or expander 15. After absorption, the working fluid is pumped by pump 17 back to the generator 1 1 to complete the cycle. Heat H may originate from flue gas or solar hot water at a temperature of up to ~ 120°C. The fluid may have a temperature of around 60°C on leaving the generator 1 1 en route to the separator 12. The generated electricity may be transmitted to a location remote from the alternator 16.

As noted above, a commercial refrigeration compressor lubricant, EMKARATE^® RL 32H, is a promising solvent for C0₂. Figure 2 shows the pressure-temperature curves of C0₂/RL32H, and an illustration of an absorption power generation cycle . The cycle may operate between a re-generation pressure of -65 bar and an absorption pressure of -45 bar. The cycle has a large temperature glide from 120°C to 60°C in the regeneration process ' 1 -2' (points in cycle also marked in Figure 1 , Figure 6 and Figure 7) while the temperature glide in the absorption process '3-4' (points in cycle also marked in Figure 1 , Figure 6 and Figure 7) is from about 60°C to about 40°C. The temperature glide in the re-generation process ' 1 -2' has a good matching with the temperature change of low temperature heat to reduce the thermodynamic irreversibility in heat transfer. This is illustrated in Figure 3. The heat recovery is between the cooling process '2-3 ' and pre-heating process '4- 1 ' .

The actual operating pressures for the system may be optimised for different working fluids.

It may be preferred to employ a multi-vane expander as the power generator in the system shown in Figure 1. Compared with the turbines, piston-type expanders and scroll expanders, the vane-type expanders are often comparatively simple in design. Figure 4 shows a commercial four-vane expander produced by GAST UK Ltd. The left-hand drawing shows the expander in cross-section, while the right-hand drawing shows the expander with its outer casing in place. The expander comprises an inlet 41 , a rotor 42 comprising four vanes and an outlet 43. In use, C0₂ flows into the expander through inlet 41 , causes the rotor 42 to rotate and then exits the expander through the outlet 43. The rotor 42 drives a shaft 44. The rotor 42 of the expander is mounted eccentrically in a cylinder bore, and the vanes slide radially in the rotor slots as it turns. The vanes are thrown outwards by centrifugal force and pressed inwards by the cylinder. Expansion of high pressure vapour entering at the intake port pushes the vanes to sweep the cylinder and a torque is produced on the shaft 44. The exhaust vapour is discharged at the other side. The shaft work could be converted to electricity using an alternator. A small scale test rig comprising a combination of a 4-vane expander and an automotive alternator has produced an electricity output of up to 200W. Alternatively, a single screw expander may be employed as the power generator in the system shown in Figure 1. In comparison, the screw expanders may be slightly more complicated, but may be more reliable and may be suitable for both small-scale and large-scale applications. Single screw expanders are different from the conventional double screw compressors which have been widely used in refrigeration, air conditioning and gas compression. A single screw expander is a rotary positive displacement expansion device including a main helical screw 5 1 and two supporting rotors 52, 53, as shown in Figures 5a, 5b and 5c. The process in a single screw expander includes three stages, namely filling, expansion and discharge, which are illustrated in Figures 5a, 5b and 5c respectively. Compared with the common two screw expanders, use of two supporting rotors make the single screw expander better balanced in mechanical load, so it runs more quietly and reliably. The single screw expander may also have a relatively small size and be relatively light in weight. Therefore, it may be especially suitable for small scale applications. Figure 6 shows a system 10', which provides cooling as well as power generation. Like features from the system of Figure 1 are indicated with like reference numerals, but with a prime, in Figure 6. Such features function essentially as described in relation to Figure 1 above . As shown in Figure 6, a portion of the high pressure C0₂ vapour stream from the generator 12' is directed to an ejector 18, while the remainder of the stream is directed to the turbine or expander 15 '. The gaseous C0₂ expands through the nozzle of the ejector, which entrains C0₂ from an evaporator 19 to provide cooling. The mixed C0₂ is then pressurized by the ejector 18 and flows into the absorber 13 '. A valve 20 is provided in a line going from the absorber 13 ' to the evaporator 19.

It will be appreciated that ejector cooling may be provided by the invention in addition to power generation, e.g. as shown in Figure 6, or instead of power generation.

Figure 7 shows another system 10", which provides cooling as well as power generation. Like features from the system of Figure 1 (and Figure 6) are indicated with like reference numerals, but with a double prime, in Figure 7. Such features function essentially as described in relation to Figure 1 above .

As shown in Figure 7, a portion of the high pressure C0₂ vapour stream from the generator 12' is directed to a second absorber 21 , while the remainder of the stream is directed to the turbine or expander 15 ". The gaseous C0₂ is dissolved in the second absorber 21 to form a diluted solution, which is then supplied to an evaporator 23. Evaporation of C0₂ from the diluted solution in the evaporator 23 provides cooling . A valve 22 is provided in a line going from the second absorber 21 to the evaporator 23. A pump 24 is provided in a line going from the evaporator 23 to the second absorber 21. It will be appreciated that absorption cooling may be provided by the invention in addition to power generation, e.g. as shown in Figure 7, or instead of power generation. It will be appreciated that absorption cooling may be provided by the invention in addition to ejector cooling, i.e. a system may provide both types of cooling.

The invention provides several advantages, some of which are summarised in Table 1 below.

Feature Advantage

/. Large Increases the cycle efficiency and allow for the use of natural

Temperature refrigerant/lubricants, e.g. C0₂/lubricant, as the working glide. fluids.

2. Working The application of C0₂/lubricant as the working fluid in an Fluid absorption power generation system for use in geothermal, solar thermal and waste heat recovery has advantages over the current state of the art. For instance, compared with NH₃/H₂0 absorption power generation systems (the Kalina cycle) the C0₂/lubricant does not have issues of flammability and toxicity. Power generation systems based on fluorine containing gases such as R134a are harmful to the environment and will to be phased out under the EU's F-gas regulations.

3. Simplified In addition, the lubricant may not evaporate along with C0₂ absorption during re-generation, so there is no need for rectification, system which, however, is common in NH₃/H₂0 systems. This may without make the system according to the invention simpler.

rectification

Increased The invention may provide a large temperature glide in the overall re-generation (boiling) process to match the temperature efficiency of change of the heat source to reduce thermodynamic heat recovery irreversibility. This may increase the overall efficiency of at low heat recovery. The invention may provide for the generation temperatures of power from 120°C to 60°C in comparison to Rankine (60-120°C) cycles that typically can only do this from 120°C to 90°C.

Even if the same efficiency of cycle (and the present invention should in practice be more efficient) is assumed, for the same volume of waste heat source, then the present invention could produce twice as much power as a Rankine cycle system (( 120-60)/( 120-90) = 2) . It is estimated that the proposed system may be able to achieve 20% efficiency in comparison to an ORC efficiency of - 10% in the low temperature range.

Table 1

Claims

1. A method of absorbing heat for subsequent re-use from a thermal source comprising:

passing a high pressure stream of a working fluid through or close to the thermal source so as to absorb heat from the thermal source, thereby producing a higher temperature stream comprising a liquid component and a vapour component; and

separating the vapour component from the liquid component;

wherein the working fluid comprises a lubricant, in which a natural refrigerant is dissolved.

2. A method according to claim 1 , wherein the thermal source has a temperature of no more than 150°C.

3. A method according to claim 1 or claim 2, wherein the thermal source has a temperature of 40°C or more.

4. A method according to claim 1 , claim 2 or claim 3, wherein the thermal source comprises one or more of a geothermal resource, a solar thermal source and/or a source of waste heat.

5. A method according to any one of the preceding claims, wherein the high pressure stream has a pressure of up to 90 bar.

6. A method according to any one of the preceding claims, wherein the lubricant is an oil.

7. A method according to any one of the preceding claims, wherein the natural refrigerant is C0₂.

8. A method according to any one of the preceding claims, wherein the vapour component comprises the natural refrigerant.

9. A method according to any one of the preceding claims, wherein the vapour component is used to perform work in a power generation system, e.g a turbine or an expander connected to an alternator, and/or in a cooling system, e.g. an ejector cooling system.

10. A method according to any one of the preceding claims, further comprising re- combining the working fluid.

1 1. A method according to claim 10, further comprising pumping the re-combined working fluid to create a high pressure stream for heat recovery from the same thermal source or from another thermal source.

12. A method according to any claim 1 1 , wherein the method is repeated, preferably continuously, in a cyclical manner.

13. A working fluid for use in heat recovery from a thermal source, the working fluid comprising a lubricant with a natural refrigerant dissolved therein.

14. A working fluid according to claim 13, wherein the lubricant is an oil and the natural refrigerant is C0₂.

15. Use of a working fluid comprising a lubricant with a natural refrigerant dissolved therein to absorb heat from a thermal source .

16. The use of claim 15, wherein the absorbed heat is re-used in power-generation and/or cooling.

17. A method of generating electricity comprising use of a working fluid comprising a lubricant with a natural refrigerant dissolved therein to absorb heat from a thermal source and subsequently using a vapour stream separated from the working fluid to carry out work, e.g. in a turbine or expander.

18. A method according to claim 17, wherein the turbine or expander is connected to an alternator.

19. A method according to claim 17 or claim 18, further comprising transmitting electricity to a location remote from where the work was carried out to generate the electricity.

20. A method of cooling comprising use of a working fluid comprising a lubricant with a natural refrigerant dissolved therein to absorb heat from a thermal source and subsequently using a vapour stream separated from the working fluid to carry out work, e.g. in an ejector cooling system and/or in an absorption cooling system.

21. A system for absorbing and re-using heat from a thermal source substantially as described herein with reference to the accompanying drawings.

22. A working fluid for absorbing heat from a thermal source substantially as described herein.

23. A method of absorbing and/or re-using heat from a thermal source substantially as described herein with reference to the accompanying drawings.