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  • Writer's picture3R Technology

New generation of sorption refrigeration machines and heat pumps

Naum Mirmov, Dr. S., Chief of Science (Company "3R-Technology" – Israel)

David Glazer, M. E., CEO, (Company "3R-Technology" – Israel)

Sorption refrigeration machines and heat pumps are the most promising direction for using secondary energy resources, solar and geothermal thermal energy in absorption refrigeration machines, the sorption process is carried out in the entire volume of the absorbent (at the boundary of the liquid and vapor phases), and in adsorption refrigeration machines, on the surface of the adsorbent located in the solid phase. Sorption refrigeration machines can operate in cooling or heating mode (heat pumps). The refrigeration machine uses external energy in the form of heat to operate.


Sorption machines include: absorption (AbRMSm) and adsorption refrigeration machines (AdRMSm). In the technical literature, such machines are called sorption heat converters (SHC) [Sorption Heat Converter] and belong to the group of heat-using machines and installations [1, 2, 3].


The advantage of sorption machines is that they do not consume electricity to implement the refrigeration cycle. Electricity is consumed only to move cold or warm media, that is, to operate pumps and fans. Absorption refrigeration machines are widely used in industry to produce process cold and for air conditioning systems. In all cases, the production of artificial cold is carried out due to the removed secondary heat of technological processes, the heat of thermal power plants during the non-heating period, and solar energy. Often, it is economically feasible to use combustion-heated absorption machines. The presence on site of an autonomous boiler house (gas or running on any fuel) for refrigeration using absorption or adsorption refrigeration machines has no alternative.


Table 1 provides a comparative analysis of the energy consumption of compressor and sorption refrigeration machines.


Table 1:



As can be seen from Table 1, with all data on performance and temperature conditions being equal, the energy consumption of sorption refrigeration machines is less.


Currently, there are only water-ammonia low-temperature sorption (absorption) refrigerating machines on the market. They allow obtaining cooling temperatures from ‒10°C to ‒40°C. Lithium bromide absorption refrigeration machines and adsorption refrigeration machines, in which the water is a refrigerant, operate in the air conditioning temperature range. For the temperature range from ‒18ºС to ‒5°С there are no sorption refrigeration machines on the market. This circumstance is due to the fact that existing freon-based refrigerants can only work with organic absorbents (absorption refrigeration machines), for example, DME-TEG (tetra ethylene glycol dimethyl ether - C10H22O5). The possibility of using freon-based working solutions was tested at the AB-AD Cooling Systems company (Israel) by specialists from the Scientific-Engineering Center (City Ma'alot, Israel). Checking the performance of the R22 / DME-TEG binary system showed that the organic absorbent decomposes after periodic heating and cooling. Decomposition products contaminate the heat exchange surface of the evaporator and drastically impair its performance.


Our company has developed a new generation of absorption refrigeration machines (AbRMSm) - which operate on the basis of a solution of salt in alcohols. Using a working solution of lithium bromide + methyl alcohol (LiBr / CH3OH) allows you to obtain the boiling point of the refrigerant (methanol) in the evaporator from +10°C to −18ºC. Also in development is a family of adsorption refrigeration machines (AdRMSm) with the refrigerant CH3OH (methanol) with the same temperature ranges as absorption machines. For sorption machines with methyl alcohol refrigerant, a heating source with a temperature of about 68-75°C is required.


Taking into account the classification of refrigeration machines and heat pumps adopted in refrigeration engineering, it is planned to develop a model range of sorption machines:

Absorption refrigeration machines AbRMSm 10-350 (10, 18, 35, 70, 150, 200, 350 kW);

Adsorption refrigeration machines AdRMSm 8-75 (8, 15, 25, 40, 60, 75 kW).


1. The main differences between the proposed sorption machines and heat pumps

1.1. Absorption refrigeration machines and heat pumps

During the development of a new generation of sorption refrigeration machines, several designs of absorption machines were produced. Several samples used a recognized layout, which were tested for performance with working solutions of R22 / DME-TEG and LiBr / CH3OH. The test results are presented in detail in [5]. Based on the analysis of the research results, we settled on the prospect of using a LiBr / CH3OH working solution in absorption refrigeration machines. Taking into account the thermophysical and thermodynamic properties of methanol as a refrigerant and the properties of the working solution, basic layout diagrams and some designs of the main elements of absorption refrigeration machines were developed [7, 8, 9].

Figure 1 shows a diagram of an absorption refrigeration machine with a capacity of 175–350 kW, which operates on a LiBr / CH3OH solution and is designed for a boiling temperature in the evaporator up to –12°C.




Fig.1.

Block (A): 1 – Generator; 2 – Pipe-bend; 3 – Condenser; 4 – Automatic valve; 5 – Refrigerant receiver;

6 – Regenerative solution heat exchanger; 7 – Generator tube bundle; 8 – Condenser tube bundle;

Block (B): 9 – Absorber; 10 – Evaporator;11 – Hydrodynamic blower; 12 – Storage receiver; 13 – Pump for strong solution; 14 – Absorber tube bundle; 15 – Evaporator tube bundle; 16 – Circulation pump; 17 – Pump for refrigerant; EV – Expansion valve; SCV – Automatic valve; C1-C3 – Refrigerant lines; C4-C7 – Working solutions pipelines; C8-C10, C13-C16 – Water pipelines; C11-C12 – coolant pipelines


The AbRMSm design has a number of distinctive features:

1.1.1. The refrigeration machine is equipped with a hydrodynamic jet-type supercharger, which supplies a strong solution from the generator to the absorber. In the hydrodynamic blower, an intensive process of absorption of refrigerant vapors by the working solution LiBr / CH3OH takes place, which makes it possible to reduce the dimensions of the absorber by 25-30%. The hydrodynamic blower provides the specified boiling point of the refrigerant in the evaporator by sucking steam from the evaporator.

1.1.2. The refrigeration machine is additionally equipped with a solution subcooler, with a combined receiver, to which a pump for supplying a strong solution from the generator to the absorber is connected.

1.1.3. The evaporator is connected to the absorber by the blower in such a way that the suction chamber of the blower is located on the steam pipe of the evaporator, and the diffuser of the blower is fixed to the inlet pipe of the absorber.

1.1.4. The body of the absorber, generator, evaporator and condenser is made of square or rectangular shape, which allows to reduce the weight of the refrigeration machine by 30-35% and reduce its overall dimensions compared to the traditional double-casing machine.

Figure 2 shows a diagram of an absorption refrigeration machine, which served as the upper stage of a two-stage refrigeration machine.


Fig.2.

This is a compact refrigeration machine with a generator and a vertical condenser. A standard type evaporator with the dimensions of the steam pipe, which takes into account the thermophysical properties of methanol vapor. A compact absorber, which is connected to the evaporator by a hydrodynamic blower. Compact absorber, which is coupled to the evaporator with a hydrodynamic supercharger. This refrigeration machine has been tested as a high temperature stage for a two-stage combination type refrigeration machine. This machine can be used as an upper stage for a cascade refrigeration machine A two-stage refrigeration machine is presented in more detail in [6]. The main advantage of combined type refrigeration machines (CRMS) is their higher efficiency compared to traditional counterparts. CRM consumes 40-42% less electricity, and their coefficient of performance is 2.0 times higher.

Comparative data of a standard refrigeration machine with a combined type machine are presented in table 2.

Table 2:



Symbols given in the table: Two-stage RM – two-stage refrigeration machine; U-B – upper branch of the cascade; L-B lower branch of the cascade; Hp-S – upper compression stage; Lp-S – lower compression stage; Q0 – cooling capacity; t0 – boiling temperature in the evaporator; Nec – effective power; Nic – installed power; Ntp – total power; ε – cooling coefficient;


These refrigerating machines can be manufactured in a wide range of capacities and temperature ranges in accordance with any requirements of various users. The operating temperature range of the CRM is from -45°C to -80°C. An experimental test showed the high efficiency of CRM for blast freezing of various products, for example, dumplings, ice cream, etc. In mass production, combined type machines have the same price as two-stage refrigeration machines that are on the market.


An experimental test of absorption refrigeration machines with a working solution of LiBr / CH3OH (methanol refrigerant) showed the effectiveness of its use. In such machines, the thermal energy conversion coefficient - ηht is significantly higher than lithium bromide and water-ammonia machines and reaches 0.81-0.84. The use of the above working solution makes it possible to obtain negative temperatures at low vacuum (not lower than 0.04 bar at t0 = −10°C) and to abandon heavy freons.


In addition, AbRMSm, which runs on LiBr/CH3OH solution, is 18-20% lighter than existing lithium bromide refrigeration machines.


1.2. Adsorption refrigeration machines and heat pumps

1.2.1. In the proposed refrigeration machines, methyl alcohol (methanol) is used as a refrigerant. Using methyl alcohol as a refrigerant instead of water allows you to increase the productivity of the refrigeration machine (heat pump) by 2 times (with the same overall dimensions of the adsorbers).

1.2.2. The adsorbent is sintered silica gel or a modified analogue of activated carbon, which is applied to the heat transfer surface of the adsorber tubes. The adsorbent applied to the heat exchange surface of the adsorber tubes makes it possible to reduce the overall dimensions of the adsorber by 20-25%. A modified analogue of activated carbon, which has high adsorbing properties, makes it possible to use adsorption refrigeration machines in transport technique.

1.2.3. To eliminate the periodicity of operation of the adsorption refrigeration machine, it uses two adsorbers with a vertical arrangement of heat exchange tubes.

1.2.4. The basic design of an adsorption refrigeration machine is a unified module with a capacity of 15-20 kW.

1.2.5. The design of refrigeration machines of the high-capacity model range involves an arrangement of standardized modules.

1.2.6. Adsorption refrigeration machines operate with a shallow vacuum, which, depending on the set boiling point of the refrigerant (methanol) in the evaporator, is 35-45 mm Hg. This makes it possible not to increase the thickness of the walls of the devices.

Figure 3 shows a schematic diagram of an adsorption air conditioner with a heat accumulator and a solar panel on heat pipes.



Fig.3. Adsorption air conditioner with heat storage and solar panel on heat pipes


1 - A solar panel; 2 – Container for water; 3 – Water pump; 4 – Heat accumulator; 5 – Adsorber; 6, 7, 8 – Fans; 9 – Condenser; 10 – Receiver; 11 – Evaporator; 12 – Solar panel heat pipes; 13 – Heat accumulator heat pipes; 14 – Electric contact valves; 15 – Cool of the adsorber; 16 – Photovoltaic panel; BPV - Check Valve;

EV – Expansion Valve; PR - Pressure Regulator


A distinctive feature of the air conditioner is that all air conditioning units and adsorbers are air-cooled. The air conditioning circuit uses a combined type solar panel. The panel contains a heating part and a photovoltaic part to generate electricity. Electricity is used to operate fans, pumps and electric contact valves. Round-the-clock operation of the air conditioner is ensured by a heat accumulator. The structural layout of the air conditioner allows it to be well adapted when used in individual buildings and small industries where air conditioning temperatures are required.



2. Areas of use:

Potential users of the proposed sorption (absorption and adsorption) refrigeration machines are:

2.1. Enterprises of the food and meat industry, chemical and petrochemical industries;

2.2. Greenhouses for growing vegetables and fruits;

2.3. Warehouses for storing food, vegetables and fruits;

2.4. The proposed sorption refrigeration machines, with methanol refrigerant, can be used in installations for low-temperature air drying and technological low-temperature air conditioning.

2.5. Sorption refrigeration machines can be used to cool the body of an automobile refrigerator and for air conditioning in the cabin of cars and buses.


3. Economic feasibility of using and manufacturing sorption refrigeration machines

3.1. For the developer and manufacturer, economic efficiency is determined as follows:

3.1.1. Serial production of sorption refrigeration machines (heat pumps) with a capacity of 10-20 kW with a wide operating temperature range (from −12°C to +12°C) on a modular basis reduces manufacturing costs by 15-18%.

3.1.2. The company, a developer and manufacturer of sorption machines, provides supervision of installation and commissioning of the cooling (heating) system at the site of their operation, which provides the company with additional profit.

3.1.3. The company, a developer and manufacturer of sorption machines, organizes professional training for maintenance and repair personnel.

3.1.4. To increase the profitability of the production of sorption machines, an enterprise can organize the production of efficient dry-type cooling towers and coolers for various purposes based on heat pipes.

3.1.5. The production of sorption refrigeration machines (heat pumps) can be established in any country and in any region if there are standard machine-building enterprises with a wide range of profiles.

3.1.6. The payback period for an enterprise for the development and manufacture of sorption refrigeration machines, depending on the quantity and refrigeration performance, is 4-5 years.


3.2. For a company or enterprise that purchases one of the modifications of sorption refrigeration machines, economic efficiency consists of the following indicators:

3.2.1. Sorption refrigeration machines operate on waste (secondary) sources of thermal energy, so the main economic efficiency is large savings in electrical energy.

3.2.2. In AbRMSm, electricity is consumed only to drive the circulation pump and the solution supply pump from the generator to the absorber through a jet-type supercharger.

3.2.3. For AbRMSm with a capacity of 120 kW with specified temperature conditions (boiling point −5°C, condensing temperature +30°C), the electricity consumption is 6.5 kW. For the same compression refrigeration machine, a 56-kW electric motor is required. Thus, the energy consumption of AbRMSm is 8.6 times less than that of a compression refrigeration machine. Annual savings are about 360,000 kW h.

3.2.4. Adsorption refrigeration machines in a cooling system with a capacity of 40 kW (for the same temperature conditions as for an absorption machine) have only three additional 0.8 kW fans for pre-cooling the water removed from the adsorber during the desorption process. A compression refrigeration machine of this capacity has an electric motor of 16-18 kW. Annual savings are about 106,000 kW*h.

3.2.5. The simplicity of design and reliability of adsorption machines reduces operating costs significantly compared to compression machines. The service life without repairs is 7-10 years.

3.2.6. The payback period for using sorption refrigeration machines, depending on the installed refrigeration capacity, is 3-4 years.


List of used literature:

1. Badylkes I. S., Danilov R. L. Absorption refrigeration machines. M., Food Industry, 1966, 178 pp.;

2. Orekhov I. I., Timofeevsky L.S., Karavan S.V., Absorption heat converters, - L.: Chemistry, 1989, 208 pp.;

3. Galimova L. V. Absorption refrigeration machines and heat pumps. Tutorial. Astrakhan; Ed. ASTU, 1997, 226 pp.;

4. Mirmov I. N., Mirmov N. I. Use of solar energy and secondary heat sources for obtaining cold. // Kholodilnay technika, 2011, №. 9, pp. 42-48;

; 5. Mirmov N. I., Mirmov I. N. Absorption refrigeration machines for obtaining negative temperatures. News of BSTU, Scientific journal. Issue 1. № 2 (198) 2017, pp. 328-339;

6. Mirmov I. N., Mirmov. N. I., Shiptsov S. A. Two-stage refrigerating machines of combined type. // Kholodilnay technika, №. 9, 2018, pp. 42 – 46;

7. Patent of Israel № 248315 dated 02/05/2018 “Absorption refrigeration machine with methanol/lithium bromide working solution”

8. RF Patent № 2690896 dated 06/06/2019 “Low-temperature absorption refrigeration machine based on a solution of salt in alcohols”.

9. RF Patent №. 2784763 dated November 29, 2022 “Compact low-temperature absorption refrigeration machine”.




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