Patent Application
20170010052
The invention relates to a method and a device for charging a stratified thermal energy store.
Stratified thermal energy stores make it possible to uncouple the time at which energy is generated from that at which it is used. In particular with fluctuating energy sources such as regenerative energy types, an uncoupling of the times of this kind provides security of the supply of energy, in particular of electrical energy. Stratified thermal energy stores can be coupled to heat pumps that pump thermal energy (heat) from a cold to a hot reservoir, the stratified thermal energy store, taking up electrical energy. By using a stratified thermal energy store that is coupled to a heat pump, it is thus possible to uncouple the time of generating thermal energy from the time of discharging it to a heat consumer, as a result of which for example peak loads in energy demand can be compensated, with the result that the security of supply improves overall.
Typically, a stratified thermal energy store is charged with heat by means of a heat pump. Here, the heat is transferred to the stratified thermal energy store through walls of a heat exchanger. To secure the transport of heat from the heat pump to the stratified thermal energy store, certain temperature differences are required as the driving force for the transport of heat. At the same time, said temperature differences limit the temperature of the heat that can be taken from the stratified store, that is to say its utilizable value. Furthermore, a constructional space that is not utilizable for storing thermal energy must be provided for the heat transfer surfaces of a heat exchanger.
A stratified thermal energy store having a heat exchanger which has heat transfer surfaces is charged by means of the heat pump in that a working fluid of the heat pump takes up heat at a low temperature on the primary side and, within the heat exchanger, on the secondary side transfers the heat of the working fluid at a relatively high temperature to a heat carrier of the stratified thermal energy store (secondary side).
It is known from the prior art, for taking up heat, to conduct the heat carrier of the stratified thermal energy store through a condenser on the secondary side, wherein this condenser is thermally coupled to the heat pump. It is further known from the prior art to guide the working fluid of the heat pump through a condenser on the secondary side, wherein this condenser is located within the stratified thermal energy store and is in thermal contact with the heat carrier of the stratified thermal energy store. In other words, the heat from the heat pump is always transferred to the stratified thermal energy store through a condenser in which condensation of the working fluid of the heat pump takes place, wherein the condenser is located outside the stratified thermal energy store in the first-mentioned case and within the stratified thermal energy store in the second-mentioned case and is always in thermal contact with the heat carrier of the stratified thermal energy store.
For efficient transfer of the heat from the working fluid to the heat carrier, the condensers of the prior art have heat transfer surfaces occupying a large space, which on the one hand require a large constructional space and on the other hand reduce the economic benefit of the stratified thermal energy store as a result of high investment costs.
One embodiment provides a method for charging a stratified thermal energy store, in which a working fluid of a heat pump is introduced in the gaseous state into a liquid heat carrier of the stratified thermal energy store at at least one point of introduction and is brought into direct material contact with the heat carrier, wherein the pressure in the stratified thermal energy store at the point of introduction is greater than or equal to the condensation pressure of the working fluid.
In one embodiment, working fluid that is condensed in the stratified thermal energy store is returned to the heat pump.
In one embodiment, a working fluid is used for which the density downstream of condensation in the stratified thermal energy store is greater than or equal to the density of the heat carrier.
In one embodiment, the working fluid in the liquid state and the heat carrier are the same fluid.
In one embodiment, the condensation pressure of the working fluid at a temperature of 100° C. is lower than 1 MPa.
In one embodiment, the working fluid includes at least one of the substances 1,1,1,2,2,4,5,5,5-nonafluoro-4-(trifluoromethyl)-3-pentanone, perfluoromethyl butanone, 1-chloro-3,3,3-trifluoro-1-propene, cis-1,1,1,4,4,4-hexafluoro-2-butene and/or cyclopentane.
In one embodiment, water is used as the working fluid.
In one embodiment, the working fluid, in the liquid state, is not miscible with the heat carrier.
In one embodiment, the gaseous working fluid is introduced into the heat carrier by means of a distribution device, wherein the distribution device distributes the working fluid homogeneously in a layer of the heat carrier that is at a constant temperature.
In one embodiment, a regulated pressure accumulator is used as the stratified thermal energy store.
In one embodiment, heat from the stratified thermal energy store is supplied to the working fluid before it is introduced into a compressor of the heat pump.
In one embodiment, the heat carrier that has been separated off from an evaporator of the heat pump by means of a droplet separator is returned to the stratified thermal energy store.
In one embodiment, the heat carrier is conducted to a heat consumer for the purpose of utilizing its heat, wherein the heat carrier is conducted through a separator before it is utilized in the heat consumer.
In one embodiment, a phase change material is used in the stratified thermal energy store for storing thermal energy.
Another embodiment provides a device including a stratified thermal energy store with a liquid heat carrier and a heat pump with a working fluid, wherein the stratified thermal energy store and the heat pump are constructed and coupled such that the working fluid is introduced in the gaseous state into the heat carrier at a point of introduction and is brought into direct material contact with the heat carrier, wherein the pressure of the stratified thermal energy store at the point of introduction is greater than or equal to the condensation pressure of the working fluid.
Example aspects and embodiments of the invention are described below with reference to the drawings, in which:
Embodiments of the present invention may provide improved charging of a stratified thermal energy store with thermal energy.
Some embodiments provide a method for charging a stratified thermal energy store, wherein a working fluid of a heat pump is introduced in the gaseous state into a liquid heat carrier of the stratified thermal energy store at at least one point of introduction and is brought into direct material contact with the heat carrier, wherein the pressure in the stratified thermal energy store at the point of introduction is greater than or equal to the condensation pressure of the working fluid. , the working fluid of the heat pump is introduced in the gaseous state directly into the liquid heat carrier of the stratified thermal energy store, as a result of which there is a direct material contact between the heat carrier and the working fluid. The direct material contact results in condensation of the gaseous working fluid. This is the case because the pressure in the stratified thermal energy store at the point of introduction of the gaseous working fluid, or in a partial region of the stratified thermal energy store at which the gaseous working fluid is introduced, is greater than or equal to the condensation pressure of the working fluid. The condensation pressure of the working fluid here depends on the temperature at the point of introduction, and should be adjusted according to said temperature. The term “condensation pressure” means the pressure at which the gaseous working fluid of the heat pump changes from the gaseous to the liquid state, namely the temperature that is present at the point of introduction of the working fluid in the stratified store. In other words, the condensation point of the gaseous working fluid is reached at the point of introduction, or in a partial region of the stratified thermal energy store. As a result of the direct material contact between the gaseous working fluid and the liquid heat carrier of the stratified thermal energy store, and the consequent condensation of the working fluid, the condensation heat that is released in the process of condensation of the working fluid is transferred directly to the heat carrier of the stratified thermal energy store. Additional condensers, heat exchangers and/or heat transfer surfaces are thus dispensed with. As a result of the inventive dispensing with condensers, heat exchangers and/or heat transfer surfaces, additional losses of thermal energy in and/or on said components can be avoided, as a result of which the efficiency of the stratified thermal energy store is increased.
A further advantage of the direct material contact between the gaseous working fluid and the liquid heat carrier of the stratified thermal energy store is that there is no need for large temperature differences between the working fluid and the heat carrier for efficient transfer of heat. If the stratified thermal energy store is charged by means of a heat pump having a compressor, an initial pressure at the compressor may consequently be reduced, as a result of which the consumption of electrical energy by the heat pump is advantageously reduced.
The disclosed device for charging a stratified thermal energy store includes a stratified thermal energy store with a liquid heat carrier and a heat pump with a working fluid, wherein the stratified thermal energy store and the heat pump are constructed and coupled such that the working fluid is introduced in the gaseous state (as superheated vapor or as saturated vapor) into the heat carrier at a point of introduction and is brought into direct material contact with the heat carrier, wherein the pressure of the stratified thermal energy store at the point of introduction is greater than or equal to the condensation pressure of the working fluid.
The disclosed device enables a direct material contact between the gaseous and consequently also the condensed (liquid) working fluid and the liquid heat carrier. This gives like and equivalent advantages to those of the method according to the invention that has already been described.
In a further embodiment of the method, the working fluid that is condensed in the stratified thermal energy store is returned to the heat pump.
By returning the condensed and thus liquid working fluid, a particularly advantageous circulation process for charging the stratified thermal energy store is made possible. It may be provided, before it is returned to the working cycle of the heat pump, for the condensed working fluid to be conducted through a separator, which separates residues of the heat carrier that are present in the condensed working fluid, with the result that no or almost no heat carrier is discharged into the working cycle of the heat pump. The material separation of working fluid and heat carrier that is to be performed downstream of condensation of the working fluid is not restricted to the use of a separator, and may be performed using devices that are known from the prior art and/or equivalents thereof.
According to an embodiment of the method, there is used a working fluid whereof the density downstream of condensation in the stratified thermal energy store is greater than or equal to the density of the heat carrier, wherein a density that is really greater at all times may be preferred.
The density of the condensed working fluid that is greater than that of the liquid heat carrier has the advantage that the working fluid can be introduced or put close to the upper end of the stratified thermal energy store. As a result of the action of gravity prevailing at the location of the stratified thermal energy store, the working fluid, which is denser than the heat carrier, will fall during and/or after its condensation, from the point of introduction to a lower end of the stratified thermal energy store. In this context, the relative terms “upper” and “lower”, as is known, relate to the prevailing direction of gravity. Typically, the heat carrier in the stratified thermal energy store will have the highest temperature at the upper end thereof.
The advantage of the greater density of the condensed working fluid and the resulting fall of the working fluid is that the working fluid is subcooled to the temperature of the stratified thermal energy store prevailing at the lower end, as a result of which the heat carrier and consequently the stratified thermal energy store are charged with additional heat.
It is a further advantage that the condensed working fluid is almost completely condensed by the fall and the associated constant material contact with the heat carrier. After the condensed working fluid has fallen and accumulated at the lower end of the stratified thermal energy store, for example at the base, it can be returned from there back to the heat pump.
In an embodiment, one and the same fluid is used for the working fluid in the liquid state and the liquid heat carrier.
Advantageously, as a result additional separators that separate the working fluid from the heat carrier, for example before it is returned to the heat pump or to a heat consumer, can be dispensed with.
In a further embodiment, there is used a working fluid that, at a temperature of 100° C. (373.15 K), has a condensation pressure lower than 1 MPa.
Working fluids that, at a temperature of 100° C., have a condensation pressure lower than 1 MPa are called low-pressure fluids here. An advantage of such low-pressure fluids is the fact that, in combination with known stratified thermal energy stores, they make it possible to use the disclosed method. This is the case because stratified thermal energy stores that are typical in the prior art, in particular stratified stores using water, are at a pressure lower than 1 MPa and in particular in the range from 0.3 MPa to 1 MPa. Typical working fluids that are used in heat pumps, such as the fluids R134a, R400c or R410a, have a condensation pressure in the range from 2 MPa to 4 MPa at 100° C. The condensation pressure of said working fluids is thus significantly greater than the pressure that typically prevails in stratified thermal energy stores, with the result that when the working fluid is introduced at a temperature of 100° C. no condensation of the working fluid occurs. Low-pressure fluids, by contrast, have a condensation pressure that is in the range of pressures that prevail in stratified stores, with the result that they condense when they are in contact with the liquid heat carrier of the stratified thermal energy store.
In some embodiments, the working fluid includes at least one of the substances 1,1,1,2,2,4,5,5,5-nonafluoro-4-(trifluoromethyl)-3-pentanone (trade name Novec™ 649), perfluoromethyl butanone, 1-chloro-3,3,3-trifluoro-1-propene, cis-1,1,1,4,4,4-hexafluoro-2-butene and/or cyclopentane.
In some embodiments, said substances may be used in combination with stratified thermal energy stores that are known from the prior art. For example, at a temperature of 100° C. Novec™ 649 has a condensation pressure of 0.45 MPa, perfluoromethyl butanone a condensation pressure of 0.89 MPa and cyclopentane a condensation pressure of 0.42 MPa. At 100° C., therefore, the condensation pressure of said fluids is significantly below the condensation pressure of, for example, R134a, which has a condensation pressure of around 3.97 MPa.
A further advantage of said substances is their ease of technical handling. They are characterized by good environmental compatibility and by their properties relating to safety, such as a lack of flammability and a very low greenhouse gas potential. In general, the substances Novec™ 649 and perfluoromethyl butanone are allocated to the substance class of fluoroketones, while cyclopentane is allocated to the substance class of cycloalkanes.
According to a further embodiment of the method, water is used as the working fluid.
As a result, advantageously additional separators that are able to separate the working fluid from the heat carrier can be dispensed with. In the case of hydrostatic stratified thermal energy stores, which build up the pressure in the stratified thermal energy store solely by way of the hydrostatic pressure of the water column, the height of the point of introduction of the working fluid into the heat carrier is therefore not significant. In particular, the stratified store using water may be charged at its upper end with the introduced gaseous and subsequently condensed working fluid, as a result of which advantageously there is only a small time lag between charging the stratified store that uses water and reaching the temperature that is desired at the upper end.
In a further embodiment of the method, there is used a working fluid that, in the liquid (condensed) state, is not miscible with the liquid heat carrier.
In other words, the condensed working fluid and the liquid heat carrier form a two-phase liquid, wherein the one phase is formed by the condensed working fluid and the other phase by the liquid heat carrier. It is also possible to provide a working fluid that has low miscibility with the heat carrier in the liquid state.
As a result of the mixture of working fluid and heat carrier being present in two phases, it is possible to perform a material separation of said fluids in a simple manner, in particular if the condensed working fluid and the liquid heat carrier have different densities. For example, the already mentioned low-pressure fluids Novec™ 649, perfluoromethyl butanone and cyclopentane are poorly soluble in water, which is particularly suitable as a heat carrier, and so are only miscible with water in small quantities. For example, only 20 ppm of water may be dissolved in Novec™ 649.
In a further embodiment, the gaseous working fluid is introduced into the heat carrier by means of a distribution device, wherein the distribution device distributes the working fluid homogeneously in a layer of the heat carrier that is at a constant temperature.
Stratified thermal energy stores, such as stratified stores using water, have a layered construction in respect of the temperature of their heat carrier, wherein each layer has a particular temperature and density. As regards the efficiency of heat transfer from the working fluid of the heat pump to the heat carrier, it is thus advantageous to distribute the gaseous working fluid uniformly or homogeneously in a layer of the heat carrier. The terms “uniform” and “homogeneous”, and the temperature or density of a layer, are in all cases to be understood as approximate.
Typical stratified stores are oriented vertically—relative to the gravity prevailing at the stratified store—such that the individual layers of the stratified store extend horizontally. As a result of the uniform distribution of the gaseous working fluid in a layer of the liquid heat carrier, the surface of material contact (contact surface) between the heat carrier and the working fluid is increased, as a result of which the efficiency of the heat transfer from the working fluid to the heat carrier is improved.
As a result of uniform distribution of the working fluid in a horizontal layer of the stratified thermal energy store, furthermore a distribution of the pulses of introduced working fluid is made possible, with the result that undesired mixing procedures that could result in the layers becoming completely mixed can be prevented.
Possible distribution devices are for example horizontal distributor pipe systems such as are used in stratified stores. In particular, the distribution devices that are known there result in a reduction in the input rate of the working fluid into the heat carrier (cf. Göppert et al., Chemie Ingenieur Technik, 2008, 80, No 3). Furthermore, the input rate of the gaseous working fluid may be regulated by altering the cross-sectional surface area of input holes in the distribution device. A further advantage of regulating the cross-sectional surface areas of the input holes is that a primary bubble size of the gaseous working fluid can be set.
In one embodiment of the method, a regulated pressure accumulator is used as the stratified thermal energy store.
Advantageously, with a regulated pressure accumulator the pressure inside the stratified thermal energy store can be regulated to a particular pressure range. By regulating the pressure in the pressure accumulator, the pressure inside the pressure accumulator can be adjusted to the condensation pressure of the working fluid, with the result that condensation of the working fluid occurs regardless of the temperature that prevails at the point of introduction. For example, as a result the gaseous working fluid may be introduced at a point of introduction in the stratified store that is as high up as possible. In this arrangement, the temperature of a layer of the stratified store or pressure accumulator is correlated to the height of the layer, with the result that a point of introduction that is at the greatest possible height corresponds to a greatest possible temperature.
According to a further embodiment of the method, heat from the stratified thermal energy store is supplied to the working fluid before it is introduced into a compressor of the heat pump.
This is particularly advantageous if working fluids whereof the condensation curve has an overhang are used. The heat that is required with such working fluids, which serves to superheat the working fluid before it enters the compressor, can thus be taken from the stratified thermal energy store.
In a further embodiment, heat carrier that has been separated off from an evaporator of the heat pump by means of a droplet separator is returned to the stratified thermal energy store.
As a result of the direct material contact of the working fluid with the heat carrier of the stratified thermal energy store, it is in principle not possible to prevent the heat carrier from being introduced into the working fluid and thus into a cycle of the working fluid within the heat pump. Thus, liquid heat carrier that has not (also) evaporated accumulates in particular in the evaporator of the heat pump. This heat carrier that accumulates in the evaporator is advantageously removed from the evaporator by means of a droplet separator and returned to the stratified thermal energy store.
According to an embodiment, the heat carrier is conducted to a heat consumer for the purpose of utilizing its heat, wherein the heat carrier is conducted through a separator before it is utilized in the heat consumer.
Conducting the heat carrier through a separator is provided in particular when the heat carrier is removed directly from the stratified thermal energy store. When the heat carrier is removed directly, as a result of the inventive material contact between the working fluid and the heat carrier, some of the working fluid is discharged with the heat carrier. In this arrangement, the working fluid may be discharged in droplet form (as an emulsion) or indeed as a constituent that is dissolved in the heat carrier (as a solution).
Advantageously, by means of the separator it is ensured that the discharged portions of the working fluid do not reach the heat consumer and where appropriate may be returned to the stratified thermal energy store and/or the heat pump. Suitable for the separation are for example active droplet separators and/or coalescing separators. A further possibility for preventing working fluid from being discharged is to reduce the solubility of the working fluid in the heat carrier as a result of the reduced temperature of the heat consumer. This is the case for substance mixtures that have a higher solubility at higher temperature. As a result of the reduced temperature of the heat consumer, the working fluid is precipitated and can thus be materially separated from the heat carrier.
In the case of indirect removal of the heat for a heat consumer, for example by way of a heat exchanger, a separator of this kind that is on the heat consumer side and separates the working fluid from the heat carrier may be dispensed with.
According to a further embodiment, a phase change material (PCM) is used in the stratified thermal energy store for storing thermal energy.
The stratified store thus includes two heat carriers, wherein the further heat carrier takes the form of a phase change material.
Phase change materials or phase change stores may be preferred, since they can store thermal energy with low losses and with numerous repeat cycles and over a long period of time. In particular, a phase change material whereof the melting point (phase change temperature) is lower than the condensation point of the working fluid (at condensation pressure) may be preferred. For example, the condensation point of the working fluid may be 130° C., so a melting point of 125° C. of the phase change material may be preferred. Thus, a melting point that is at most 5% lower than the condensation point may be preferred.
The stratified store may include further heat carriers that are in the solid state. In this arrangement, the porosity of the solid heat carriers may be adapted to their purpose. For example, the porosity may be selected such that it becomes possible to lower the condensed working fluid, which has a greater density than the liquid heat carrier.
The heat pump 6 includes a compressor 14, an evaporator 16, an expansion valve 20, a separator 18, a droplet separator 15 and a nonreturn valve 22. The working fluid 4 circulates counterclockwise 36 in the heat pump 6.
Further visible in
The gaseous working fluid 4 is introduced into the heat carrier 10 downstream of the compressor 14 at the height 8 on the pressure accumulator 2, by way of the distribution device 12, and is thus brought into direct material contact with the heat carrier 10. Here, the temperature of the pressure accumulator 2 at the introduction height 8 is for example 130° C. If for example Novec™ 649 is used as the working fluid, the pressure in the pressure accumulator 2 must be at least 0.9 MPa so that immediate condensation of the gaseous working fluid 4 takes place.
One advantage of the regulated pressure accumulator 2 is that the working fluid 4 can be introduced at a point on the pressure accumulator 2 that is as hot as possible. This is the case because the condensation pressure of the working fluid 4 at the introduction height 8 can always be exceeded by adjusting the pressure in the pressure accumulator 2. In general, the heat from the pressure accumulator 2 is removed for a heat consumer (which is not shown) at the point of highest possible temperature. By introducing the working fluid 4 at said point, the pressure accumulator 2 can reach the temperatures required by the heat consumer efficiently and in little time with a low thermal load.
In the exemplary embodiment that is shown in
If a working fluid 4 that has a lower density than water 10 is used, for example cyclopentane (C5H10), which has a density of 650 kg/m3, the working fluid 4 rises after condensation and must thus be removed at an upper end of the pressure accumulator 2.
The nonreturn valve 22 prevents heat carrier 10 from being discharged into the compressor 14 and thus into the working cycle 36 of the heat pump 6.
Unlike a pressure accumulator 2, in a hydrostatic pressure accumulator 3 the pressure inside the store 3 is generated solely by the hydrostatic pressure of the heat carrier 10, in this case water 10. In other words, the pressure in the pressure accumulator 3 is generated solely by way of the liquid column of water 10. If once again Novec™ 649 is introduced as the working fluid 4 into the hydrostatic pressure accumulator at a temperature of 110° C., a pressure of at least 0.6 MPa is required for the working fluid 4 to condense. The result of this is that the point of introduction or height 8 at which the working fluid 4 is introduced into the pressure accumulator 3 must be selected such that at least 50 m of water 10 lies above the introduction height 8 of the working fluid 4. In general, the pressure can be selected in accordance with the introduction height 8 of the working fluid 4.
In order to increase the pressure in the hydrostatic pressure accumulator 3 further without making the liquid column of the heat carrier 10 taller or the introduction height 8 lower, a cold water layer 32 is placed at the upper end of the pressure accumulator 3. The cold water layer 32 is separated from the water 10 of the hydrostatic pressure accumulator 3 by a separation device 34. Placing the cold water layer 32 at the upper end of the hydrostatic pressure accumulator 3 ensures that the pressure at the introduction height 8 exceeds the condensation pressure of the working fluid 4 as it is introduced, and condensation of the working fluid 4 occurs. In this way, the introduction height 8 of the working fluid 4 may be made higher, as a result of which the temperature at the introduction height 8 may be increased.
As was already the case in
Although the invention has been closely illustrated and described in detail by way of the preferred exemplary embodiments, the invention is not restricted by the disclosed examples, and alternatively other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2014 202 849.3 | Feb 2014 | DE | national |
This application is a U.S. National Stage Application of International Application No. PCT/EP2015/051130 filed Jan. 21, 2015, which designates the United States of America, and claims priority to DE Application No. 10 2014 202 849.3 filed Feb. 17, 2014, the contents of which are hereby incorporated by reference in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/051130 | 1/21/2015 | WO | 00 |
