METHOD OF THERMAL ENERGYTRANSFER

Technical Field

The invention relates to heat pumps and methods of thermal energy transfer for moving thermal energy from a lower temperature heat source to a higher temperature heat sink and can be used in the heat supply, air conditioning units, cooling systems and other fields.

Background Art

There are known absorption heat pumps and methods of moving thermal energy from a lower temperature heat source to a higher temperature heat sink, which comprise vaporization of the working substance by means of evaporator, absorption of created vapour in the absorbent, pressure increase in the working substance and absorbent solution, separation of the working substance from the absorbent by thermal energy supply to the working substance and absorbent solution, condensation of the working substance by thermal energy transfer, pressure decrease in the working substance and absorbent. There are widely used absorption heat pumps with pairs of substances: H ₂ O - working substance;

LiBr water solution - absorbent; NH ₃ - working substance; H ₂ O - absorbent.

Disadvantage of the known engineering solutions is the necessity for high potential thermal energy for the separation of the working substance and absorbent solution as well as the necessity for thermal energy transfer, condensing the working substance, what limits the achievable coefficient of performance of absorption heat pumps and methods of thermal energy transfer.

The closest known engineering solution, is the liquid phase separation method in the absorption refrigeration units, (US Patent 4283918, 18.08.1981, "Liquid phase separation in absorption refrigeration", SKI ³ F25B 15/00) according to which the chosen working substance of absorption heat pump is the substance, which is fully miscible in the

absorbent at relatively low absorption temperature and partially miscible in the absorbent at elevated temperatures, allowing to separate the working substance from absorbent solution in the liquid state separating two solutions from each other. This method of thermal energy movement comprises the following consecutive steps: increasing the pressure of the working substance and absorbent solution to prevent vaporization of the working substance during separation; heating the solution of working substance and absorbent to a temperature above that of complete miscibility; separation of the working- substance-rich solution from the absorbent-rich solution; separately cooling the working substance solution and the absorbent solution; reducing the pressure of the working- substance-rich solution to the pressure at which vaporization of the working substance and absorption of thermal energy to produce cooling effect is possible; reducing the pressure of the absorbent-rich solution to the pressure of the evaporator; absorption of vapour of the working substance in the absorbent-rich solution. The method set forth in the US Patent 4283918 allows to decrease the amount of thermal energy required to perform the working cycle of absorption heat pump since it, unlike the widely used methods, can be performed by merely liquid heating rather than vaporization. Liquid heating requires lower amount of energy than vaporization. However, the disadvantage of the known method is the necessity of additional thermal energy supply in order to achieve the state of partial miscibility for solution separation that limits the achievable coefficient of performance of the absorption heat pump and the thermal energy transfer method. Besides that, the known method performance is hindered by the choice of such pair of substances, which would correspond to specified criteria.

Disclosure of Invention

The aim of the invention is to increase the coefficient of performance of the absorption heat pump and the method for thermal energy transfer.

The proposed aim is achieved by the use of such working substance and absorbent in the heat pump, which partially dissolve in each other at all stages of thermal energy transfer process and allow to separate the liquid working substance from the absorbent, letting settle or separating solutions without additional thermal energy supply.

In accordance with the proposed method, the absorbent, in comparison with the working substance, has a higher boiling temperature at the working pressure of the unit, lower vapour pressure in the range of working temperatures of the absorber and different density. The vapour pressure of the working substance and absorbent affects the specific power and the possible difference of working temperatures of the unit - the bigger the difference between the vapour pressures of both substances, the more working substance is dissolved in the absorbent and the less circulation of solutions is necessary. Between pairs of substances that can be used for the proposed method realization, the following pairs can be mentioned: water - transformer oil; Freon 134a - water; Freon 134a - transformer oil; water - hexadecane; water - octadecane; water - oleic acid.

The method of moving thermal energy from a lower temperature heat source to a higher temperature heat sink is explained with attached figures: Fig. 1 and Fig. 2 show thermal energy transfer flow diagrams (variants).

For the realization of the thermal energy transfer method, low-grade thermal energy is delivered to the working substance evaporator thus evaporating the working substance, the vapour of which is delivered to the absorber, in which absorbent is placed (Fig. 1). Absorption of vapour of the working substance in the absorbent takes place according to the Henry's law - at a constant temperature, the amount of vapours dissolved in a given volume of liquid is directly proportional to the partial pressure of the vapour. If the vapour pressure of the working substance exceeds the equilibrium vapour pressure of the working substance and absorbent above the absorbent, what is determined by the low vapour pressure of the absorbent and low miscibility of the working substance in the absorbent at the absorption temperature, then the amount of the working substance in the absorbent at the absorber outlet exceeds the equilibrium miscibility (that does not depend on a pressure in the absorber, but only on solution temperature), therefore the working substance ooze out as liquid drops in the absorbent layer. Depending on substance properties, absorption (condensation) heat evolves when the working substance vapour dissolves in the absorbent solution or working substance oozes out from the absorbent solution. In both cases the temperature of the absorbent solution at the absorber outlet increases. Increase of temperature can cause vaporization of the working substance, if it exceeds the temperature of the working substance evaporator. Until the working substance in the absorbent solution

remains in a form of small drops, the working substance vaporization does not happen due to influence of interfacial surface energy of the liquids (until a certain increase of temperature). Since the absorbent and the working substance have different densities, the drops of the working substance consolidate and try to create a separate layer of the working substance having small surface tension force, which does not limit the trend of the working substance to vaporize (secondary boiling). The trend of the working substance to vaporize at an elevated temperature can be limited by forces of interfacial surface energy, which increase the boiling temperature of working substance, if the working substance is below the absorbent layer. The trend of the working substance to vaporize at an elevated temperature can also be limited by increasing the pressure of the working substance and absorbent emulsion by means of a liquid pump to the pressure that corresponds to the pressure of the working substance vaporization at the temperature of the absorber. The trend of the working substance to vaporize can be limited by reducing the liquid temperature to the boiling temperature of the working substance, if the liquid from the absorber is separated as emulsion (for example, by means of centrifuge), and the boiling takes place in the already separated layer of the working substance.

Therefore the working substance separation from the absorbent (desorption) takes place spontaneously - the working substance is separated as a layer in the settler - separator to the concentration, which corresponds to the miscibility of the working substance at the temperature the absorbent solution is cooled to, removing condensation/absorption heat. In accordance with proposed method the working substance separation from the absorbent can be also performed in the active type separator such as a centrifuge.

The absorbent saturated with the working substance is moved from the settler to the absorber inlet. The working substance from the settler is again moved to the working substance evaporator and the cycle is repeated. If the pressure increase is used for the limitation of the working substance vaporization then each solution line shall be equipped with a pressure-reducing valve. Having chosen the pair of substances: water - transformer oil, the thermal energy transfer can be performed by using the flow diagram shown on Fig.l, whereas with the pair of substances: Freon 134a - water, the thermal energy transfer can be performed by using the flow diagram shown on Fig. 2.

Therefore, unlike the prior art, the proposed method does not require additional heat generator and heat transfer for the working substance and absorbent separation, which considerably increases the coefficient of performance of the heat pump and the thermal energy transfer method.

Example of Realization of the Invention

For the realization of the thermal energy transfer method the chosen pair of substances was: working substance - water, absorbent - transformer oil. The unit working by the proposed method consumes heat from the heat source at temperature of 70°C for the vaporization of the working substance - water - in the vapour generator; pressure 31 kPa. Water vapour from the vapour generator is entered into the absorber, where the vapour is absorbed in the absorbent - the transformer oil, because the vapour pressure in the vapour generator (31 kPa) is several times bigger than the water vapour / oil vapour pressure - 5 kPa, which is in equilibrium with oil at the absorber temperature - 100 ⁰ C (data of the experimental unit; lower pressure is possible when the vacuum is deeper). Water miscibility at 100°C in the transformer oil - 772 ppm in weight units; at 70 ⁰ C - 331 ppm; increase of miscibility, observing the Henry's law: [331 ppm * (31 kPa / 5 kPa)] - 772 ppm = 1280 ppm. By means of the liquid pump the absorbent is moved back to the absorber through the water separator and the heat sink with temperature 100 ⁰ C and water content of 772 ppm. Water is separated from oil in the settler and is removed to the vapour generator. Therefore, taking into account that the working substance separation from the absorbent takes places spontaneously, the realization of this method does not require additional thermal energy to separate substances from each other. Consequently the coefficient of performance of the proposed method of thermal energy transfer increases 2.5 to 3 times.