WO2016076779A1

WO2016076779A1 - Vapour-compression heat pump using a working fluid and co2

Info

Publication number: WO2016076779A1
Application number: PCT/SE2015/051181
Authority: WO
Inventors: Joachim KARTHÄUSER
Original assignee: Climeon Ab
Priority date: 2014-11-13
Filing date: 2015-11-09
Publication date: 2016-05-19
Also published as: EP3338036A1; EP3338036A4

Abstract

A method for conversion of a low temperature heat (5) into a high temperature heat (6) employing the use of a working fluid comprising an absorbent for a working gas comprising CO2 (carbon dioxide) and at least one of an alkali metal hydroxide, ammonia (NH3) or at least one amine, characterized by the steps of: - a) providing said low temperature heat (5) having a temperature in the range of 20-80 °C, - b) converting said low temperature heat (5) to said high temperature heat (6) having a temperature in the range higher than 80 °C by liberating said working gas by contact said working fluid with said low temperature heat (5), - c) compressing said working gas by a compression device (2), and - d) reacting said working gas with said absorbent immediately after compression, wherein a maximum pressure is below 40 bar, preferably below 20 bar and most preferably below 16 bar.

Description

Vapour-compression heat pump using a working fluid and CO₂ Field of the invention This invention relates to compression heat pumps and the generation of useful energy (heat, cold, electricity, stored energy) .

Background of the Invention and Prior Art

Heat pumps are generally known (see various types as described in Wikipedia) and are used among others for domestic heating. Electrically powered compression heat pumps use refrigerants (butane, CFC etc.), whereas fuel-powered absorption heat pumps use two chemicals (such as lithium bromide LiBr and water H₂0) which generate heat/cold upon combination/separation.

The chemistry used in the invention is described in various disclosures, including WO 2012/128715, PCT/ SE2013 / 051059 , and SE 1400027-7 and SE 1400160-6 (not published), SE 1400514-4 (not published), and SE 1300576-4 (not published), all hereby incorporated by reference. The latter document discloses a heat pump involving this chemistry and an extra turbine for energy recovery. Further, the Japanese patent application S55-149 641 or

JPS55149641 (Masuda and Tsurumi, 1980, assignee Toray

Industries) discloses a heat energy recovery method whereby a gas such as CO₂, H₂S or SO₂ is absorbed in an absorbing medium such as amines, however, no details are given regarding specific and preferred working media, advantages versus prior art, pressures, useful temperature ranges or applications. Two aspects triggered further work: a heat pump should be simple, robust and low in capital expenditure, further current heat pumps are not ideal for converting waste heat to

industrially useful temperatures such as above 110 or above 120°C, mainly because the operating pressure gets too high using conventional working gases or refrigerants.

Surprisingly, it was found that the properties of amine/CC>₂ mixtures, such as high absorption enthalpies and pressure vs. temperature profiles as function of CO₂ loading, which make the use of these chemicals challenging for a Rankine type process are almost ideal for use in a compression heat pump. This will be illustrated in the following section.

Summary of the Invention

By the invention is achieved a method for conversion of a low temperature heat into a high temperature heat employing the use of a working fluid comprising an absorbent for a working gas comprising C0₂ (carbon dioxide) and at least one of an alkali metal hydroxide, ammonia (NH₃) or at least one amine, characterized by the steps of:

- a) providing said low temperature heat having a temperature in the range of 20-80 °C,

- b) converting said low temperature heat to said high

temperature heat having a temperature in the range higher than 80 °C by liberating said working gas by contact said working fluid with said low temperature heat,

- c) compressing said working gas by a compression device, and

- d) reacting said working gas with said absorbent immediately after compression, wherein a maximum pressure is below 40 bar, preferably below 20 bar and most preferably below 16 bar.

Brief Description of the Drawings The invention is described in more detail below in the form of a non-limiting example, reference being made to the

accompanying drawing, in which

- Figure 1 is a schematic drawing of a compression heat pump according to the invention.

Brief Description of the Invention The invention is described by using a system comprising

dibutylamine (DBA)+ CO₂, as example, and referring to figure 1. Briefly, heat, e.g. using 60-70 °C water, is supplied to a heat exchanger HX 1. A liquid comprising dibutylamine, CO₂ and optionally a solvent such as Carbitol ™ is heated in the HX 1, thereby generating gaseous CO₂ by decomposition of the

carbamate formed by the DBA-CO₂ reaction. For simplicity, a heat pump employing 1000 kg of DBA (molecular weight 129 g/mol) is considered. These 1000 kg DBA could absorb 171 kg CO₂ given that 2 moles DBA are required to bind 1 mole C0₂ (note that this description is an example, other amines may be employed including NH₃, and amines may well react to inorganic carbonate in the presence of water, or they may react to organic carbonates, both with 1:1 stoichiometry, in the

presence of organic alcohols. The text provides some non- limiting examples.

DBA-CO₂ carbamate is in reversible equilibrium with DBA and CO₂. At 60°C, 70% loaded carbamate generates about 1 bar CO₂ pressure, at 120°C, about 16 bar CO₂ pressure is generated.

These pressures are dependent on the CO₂ loading degree, however, for constant loading, the slope of the pressure- temperature profile, more exactly of the In (pressure) versus 1/T graph (van't Hoff plot) is an expression of the reaction or absorption enthalpy. In the case of amines and CO₂, these enthalpies are often in the order of 1000-3000 kJ/kg CO₂. High values are preferred, because the release of this enthalpy is the driving force of the heat pump according to the invention.

Assume the 1000 kg DBA are pre-reacted with 50% of the maximum load of CO₂, i.e. with about 85 kg, and that 6 kg CC^/min are generated in the heat exchanger HX 1 at about 1 bar. The generation of these 6 kg CC^/min extracts 8500 kJ (140 kW) from waste heat 5 supplied to HX 1. CO₂ is compressed from 1 to about 16 bar using a compressor 2 which consumes 1200 kJ (20 kW) for isothermal compression according to W^A^^B =

nRT*ln (V_B/V_A) . Compressed CO₂ reacts in a heat exchanger HX 3 with amine (about 50% or higher loaded) releasing roughly 8500 kJ (140 kW) . Via a pump 9 amine is re-circulated within HX 3 to maximise loading of amine. The heat generated in HX 3, calculated as delta T= energy/ (mass*average heat capacity) and being in the range +50°C compared to the prevailing temperature in HX 1, is extracted by e.g. 90°C water 6 which is converted to low pressure steam, e.g. 110°C at 1-2 bar.

Loaded or rich amine is fed through a pipe 8 to HX 1. Lean amine, after CO₂ liberation, is collected in a vessel 4 from which it is pumped by a pump 7 via a pipe 10 to HX 3.

For pumping, it is very advantageous to use displacement pumps and to use the pressure energy in the liquid in HX 3 to partly power the pump 7 whose main task it is to pump liquid from vessel 4 at 1 bar to HX 3 at 16 bar. Saving energy for liquid pumping is important because some 100 liter of amine or amine solution has to be pumped or exchanged between HX 1 and HX 3 for every 6 kg of C0₂. Given 140 kW thermal input and output and 20-25 kW electricity consumption (compressor and balance for liquid pumping), a COP (coefficient of performance) of 6-7 is calculated. Heat losses, such as caused by transfer of hot, rich amine from HX 3 against colder lean amine from HX 1 are not

considered because they essentially cancel out, not

considering radiation and similar losses. However, it is an option in some embodiments to heat-exchange these flows, albeit at the cost of an extra heat exchanger.

Various solutions or designs of non-standard equipment, especially HX 3 can be useful, such as spraying amines into HX 3, or injecting amines according to SE 1400514-4 and related PCT/SE2015/051 121 (not published) . Removal of non-condensable gases, using standard deaerators or preferably using SE

1400182-0 (not published), is typically needed (not shown in the figures ) . The functionality of HX 3 may be realized technically by employing a separate amine absorption vessel and a separate heat exchanger without deviating from the spirit of the invention. In the following, various embodiments are

disclosed .

Useful chemical couples comprise CO₂ and alkaline materials such as an alkali metal hydroxide in water (NaOH, KOH) , ammonia NH₃, and at least one amine. A range of useful amines are disclosed in WO 2012/128 715, PCT/SE2013/051059, and SE 1400027-7. For the compression heat pump according to the invention, non-volatile amines are preferred as only gaseous C02 needs to be compressed, but volatile amines and NH3/water may be used alternatively, see below for specific advantages. Solvents including water, at least one alcohol, at least one ketone, at least one paraffin etc may be employed.

An advantage of the technology disclosed here is that the total system pressure can be kept below 40 bar, preferably below 20 bar and specifically below 16 bar. Many heat

exchangers are designed for operation below 16 bar. The CO₂ loading degree in amine/CC>₂ systems can be adjusted such that 16 bar are not exceeded at any stage in the system, such that equipment costs can be held at reasonable levels.

The technology can be used to "lift" the temperature of a medium, e.g. water, by 10, 20, 30, 40, 50°C or more. Typically, the COP will decrease slightly with increasing lift. The starting or heat source temperature can be 40-50°C or higher, as it is desired that HX 1 generates a pressure of close to 1 bar. Waste heat is often available in the form of water having a temperature of 20-80°C. Here, it is highly useful to lift this temperature by 50-60°C in order to generate low pressure steam, e.g. 2 bar at 110 or 120°C. However, even generating almost boiling water, i.e. water having temperatures just below 100°C, may be useful, too. Even generating water having a temperature of above 80°C is also useful. The heat pump according to the invention may be used in connection with district heating systems, e.g. to increase the temperature locally or to assist at peak demand. The heat pump may specifically be used in order to generate hot water for energy storage purposes: excess electricity may thus be converted to hot water, and hot water may be used later to generate electricity, e.g. using ORC (Organic Rankine Cycle) or similar Rankine cycles. Hot water is easy to store, and has a high energy density. For lower heat source temperatures, it is preferred that volatile amines or ammonia are employed in order to generate pressure. Further, such systems are "auto-extractive" with the further benefit that the volume of liquid streams is reduced. However, crystallization risk of amine/CC>₂ in cold sections needs to be controlled.

In one embodiment, the technology is used for generation of cold. The removal of CO₂ from rich amine/CC>₂ requires heat which may be taken from the medium passing through HX 1.

Preferably, rather cold amine/CC>₂ is supplied to HX 1 from HX 3. Ammonia is preferred for cold generation as the vapour pressure of amine/CC>₂ systems is in the millibar range even for highly loaded carbamate systems.

In one embodiment, the working fluid may comprise a low boiling solvent comprising at least one alcohol such as methanol, ethanol, acetone, isopropanol or butanol on the one hand at a concentration of at least 5% by weight, and at least one amine such as disclosed in above mentioned documents and C0₂, and the at least one amine of the working fluid may be selected from at least one dialkylamine, where alkyl is methyl, ethyl, propyl, and butyl, preferably diethylamine or dibutylamine , or the at least one amine is chemically bonded to solid substrates such as zeolites, or where a mixture of ammonia NH₃ and water is used as working fluid or absorbent of C0₂. In some embodiment, the absorption vessel may consist of various stages, especially if slowly reacting amines are employed. Further, the amine may be recycled by a pump in order to increase the loading thereof. Also, instead of a standard compressor which compresses the gas phase, a pump which is stable against gas cavitations may be employed for compression. Such pumps are usually cheaper than compressors. In a further embodiment the inventive method may comprise a . step of transferring hot CC>2-loaded "rich" absorbent from step of reaction to step of converting said low temperature heat 5 to said high temperature heat 6, and a step of replacing cold, less CC>2-loaded "lean" absorbent from the step of reacting said working gas with said absorbent immediately after compression. Said step of transferring is made optionally via a separate heat exchanging step, whereby it is preferably being carried out such that the pressure of the "rich" absorber is used to save energy for transferring "lean" absorbent, e.g. by using mechanically or otherwise coupled displacement pumps.

In yet a further embodiment the inventive method may comprise removing of non-condensable gases such as air. According to the inventive method it is also possible to generate cold, especially comfort cooling, in the range -20 to +15 °C.

It should be understood that above embodiments are merely examples of useful sequences and constructions to achieve the objective of the invention, namely to convert a heat stream to a higher temperature. In particular, the process, here

described as continuous process, may very well be constructed as batch process, e.g. if amines coupled to zeolites are employed ("swing bed process") . Most often, a continuous process is more economic, but a batch process may have

advantages in certain applications, especially for generation of cold. Similar arrangements and modifications which are obvious to the expert should be seen as falling under the spirit of this invention.

Claims

1. A method for conversion of a low temperature heat (5) into a high temperature heat (6) employing the use of a working fluid comprising an absorbent for a working gas comprising CO₂ (carbon dioxide) and at least one of an alkali metal

hydroxide, ammonia (NH₃) or at least one amine, characterized by the steps of :

- a) providing said low temperature heat (5) having a

temperature in the range of 20-80 °C,

- b) converting said low temperature heat (5) to said high temperature heat (6) having a temperature in the range higher than 80 °C by liberating said working gas by contact said working fluid with said low temperature heat (5),

- c) compressing said working gas by a compression device (2), and

2. The method according to claim 1, wherein the alkali metal hydroxide of the working fluid is selected from NaOH and KOH in water, and wherein the at least one amine of the working fluid is selected from at least one dialkylamine, where alkyl is methyl, ethyl, propyl, and butyl, preferably diethylamine or dibutylamine , or the at least one amine is chemically bonded to solid substrates such as zeolites, or where a mixture of ammonia NH₃ and water is used as working fluid or absorbent of CO2.

3. The method according to claim 1 or 2, wherein the working fluid further comprises a solvent selected from the group consisting of water, at least one alcohol such as methanol, ethanol, isopropanol and butanol, at least one ketone such as acetone, and at least one paraffin, and wherein the

concentration of the solvent is at least 5% by weight.

4. The method according to anyone of the preceding claims, wherein the low temperature heat (5) has a temperature of 20- 80 °C and where the high temperature heat (6) produced is in the range 80-150 °C, more preferably in the range 100-120 °C .

5. The method according to anyone of the preceding claims, wherein the working gas pressure in step b) is in the range 0.1-2 bar, and the pressure in step d) is in the range 1-40 bar, more preferably 2-20 bar, most preferably under 16 bar.

6. The method according to anyone of the preceding claims, comprising the further steps of:

- e) transferring hot CC>2-loaded "rich" absorbent from step d) to step b) , and

- f) transferring cold, less C0₂-loaded "lean" absorbent from step b) to step d) such that an essentially closed loop process results.

7. The method according to claim 6, wherein said step e) and f) is made optionally via a separate heat exchanging step, whereby said step e) and f) is preferably being carried out such that the pressure of the "rich" absorber is used to save energy for transferring "lean" absorbent, e.g. by using mechanically or otherwise coupled displacement pumps.

8. The method according to anyone of the preceding claims, further comprising a step of g) removing non-condensable gases such as air.

9. The method according to anyone of the preceding claims, wherein said at least one amine is a non-volatile amine.

10. The use of the method according to anyone of the preceding claims for the purpose of generating low pressure steam of above 100 °C and above 1 bar, or for storing energy in the form of hot liquid such as hot water, e.g. for later electricity production using ORC or similar Rankine cycles, or in

combination with district heating schemes, e.g. for lifting heating water temperatures.

11. The use of the method according to any one of claims 1-9 for generation of cold, especially comfort cooling in the range -20 to +15 °C .