Operating systems with natural refrigerants for energy efficiency
Whether energy transition in Germany or the Energy Efficiency Act in Austria, from an environmental and economic point of view, it is becoming ever more important for companies in Europe and worldwide, to use energy efficiently. An increasing number of planners, builders and operators of refrigeration systems and air-conditioning systems are recognising the cost reduction potential that energy efficient refrigeration technology has to offer. Efficient system performance is evident in the operating costs and is also dependent on the choice of refrigerant. Natural refrigerants can score highly here: they are available in unlimited quantities, and are sustainable. Moreover, they are very inexpensive and can be used in virtually any refrigeration application. With an ODP of 0 and a global warming potential between 0 and 3 they are also environmentally friendly. In the following interview Michael Elsen, sales manager of eurammon member Kreutzträger Kältetechnik GmbH & Co KG (pictured), explains why the use of natural refrigerants has a positive effect on the energy efficiency of refrigeration systems.
1. The refrigerant is the central work and operating material of each refrigeration system and thus plays a significant part in their performance. How large an influence does the refrigerant have on the energy efficiency of the system?
The refrigerant has influence on the energy efficiency of the refrigeration system. The thermodynamic properties determine the compressor size and the associated loss ratios, which also have a share of the total power requirement of the refrigeration system. The power requirement of the auxiliary drives such as pumps and the fans of the heat exchanger is by no means to be ignored.
Direct cooling systems in which the refrigerant evaporates directly in the evaporator by heat absorption have a clear advantage over indirect systems that use a cold transfer medium, as these require an additional temperature difference for the heat transport, and pumping capacity for transporting the cold transfer medium.
Modern plant designs allow the use of direct cooling systems with natural refrigerants in minimal quantities. Even with large industrial refrigeration systems in the megawatt range, costly approvals under the Federal Pollution Control Act (BImSchG) can be avoided.
2. What are the physical properties of natural refrigerants which ensure that systems run energy efficiently?
Natural refrigerants are characterised by a number of physical properties, especially with regard to energy efficiency. For example, an efficient refrigerant has a preferably high specific enthalpy of vaporisation value. That is the amount of heat required to produce 1kg of refrigerant to evaporate at a constant temperature.
Furthermore, considerable variations in the cooling capacity can be deduced from the volumetric cooling capacity which depends on the material properties. The volumetric capacity illustrates how much cooling capacity can theoretically be realised with 1 m³ refrigerant. The higher the value, the less refrigerant has to be circulated for a certain cooling capacity. This in turn means that the energy losses in system components are also reduced.
The isentropic exponent is the quotient of the specific heat at constant pressure divided by the specific heat at constant volume. With refrigerants that have a high isentropic exponent, high discharge gas temperatures occur, which is an advantage especially for the heat recovery.
A low pressure ratio between the suction and condensing pressure has a positive effect on the volumetric efficiency and the energy demand.
3. Where in refrigeration and air conditioning applications is the use of natural refrigerants particularly energy efficient and why?
Using natural refrigerants is economically feasible in particular in industrial refrigeration systems. In systems with a cooling capacity over 100kW, the efficiency advantages justify the necessary increased investment needs. These costs are usually redeemed within two to five years compared to systems with synthetic refrigerants.
The range of applications for refrigeration systems with natural refrigerants is constantly expanding. Even in applications where one would have resorted to synthetic refrigerants in prior years, natural refrigerants such as ammonia, carbon dioxide and propane are the first choice today. At the moment, systems with natural refrigerants are being used primarily in industrial cooling applications. For example: in logistics, freezing tunnels, blast freezers, breweries, dairies and in food processing establishments (slaughterhouses, ice cream factories, industrial bakeries etc). There is also growing use in applications including heat pumps and supermarkets.
In ammonia-operated systems for water cooling that are equipped with evaporative condensers or with air-cooled condensers, the extremely high vaporisation enthalpy of ammonia can be used for free cooling, meaning for heat transmission to the environment without compressor operation, as soon as the outside temperature allows this.
4. The natural refrigerant ammonia is, in many areas of application, the most economical refrigerant. Why is this, what makes ammonia particularly energy-efficient?
The high specific vaporisation enthalpy of ammonia, low operating pressures in refrigeration systems, relatively low density and excellent heat transfer properties have a positive effect on pipe cross-sections, heat exchangers and refrigerant compressors. This also has a positive impact on the total power requirement of the system compared to synthetic refrigerants.
Due to the high isentropic exponent of ammonia, the compressor discharge temperature can be up to 150°C, depending on the design of the system components. This temperature level is ideal for hot water production. Thus, hot water temperatures of about 60°C at a heating capacity of about 15 per cent of the total cooling demand can be achieved without additional energy requirement.
5. In many industries, refrigeration system operators face the challenge of reducing greenhouse gas emissions of their commercial refrigeration systems or consider it necessary within the framework of energy policy to increase the energy efficiency of their systems. For which industry and business is the change to a natural refrigerant a particularly effective option?
Ultimately, the use of natural refrigerants is not limited to particular industries and refrigeration systems. On the contrary the thermodynamic properties of the refrigerant, whose application limits must be considered with regard to the application, additionally provide numerous advantages in terms of environmental performance and energy efficiency. When choosing the refrigerant, the efficiency of the overall system concept should not be forgotten. A restriction to certain applications would be fundamentally wrong.
That is why we find systems with natural refrigerants in almost every capacity and application area these days. In order to develop the optimal solution for the operator, professionally qualified and experienced companies are essential, to objectively develop the tailored solutions to meet the needs of the operator, while taking environmental and economic factors into account.
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