ADVANTAGES OF HIGH PRESSURE
- Climate-neutral operation, as opposed to heat pumps charged with special airconditioning gases to replace Freon, since the CO
2 charge gas extracted from the air is the heat transfer medium. Under the EU’s planned directive, all polluting „climate gases” will be phased out by 2030.
- Thanks to the high-pressure CO
2 technology developed during 6 years of research and development, the COP value (air-to-water mode, 3.8 on average) is extremely high, resulting in energy-efficient operation..
- It is unique in its ability to produce continuous hot water up to 90 °C in ambient temperatures between -25 °C and 43 °C 24 hours a day. This means that heating circuits disconnected from district heating can be connected
without any special modifications.
- 0 m
3 GAS consumption! The heating and hot water system can be independent from the gas supply.
- The high quality of the raw materials used as a requirement for the high pressure (120 bar) system ensures high reliability and long term trouble-free operation.
- Heat pumps can be in series, so that power requirements higher than the output of individual units can be easily met. The individual units can be disconnected during maintenance without shutting down the system (thus ensuring
a continuous supply of hot water).
- The control system ensures that only the required number of heat pumps are running, thus ensuring optimal electricity consumption.
In the context of current concerns about climate change and new European regulations on greenhouse refrigerants, it is also necessary to reconsider natural refrigerants such as NH₃ (R717) and CO₂ (R744), agents which compared to the freons currently used in the refrigeration industry, have virtually no impact on the environment. The Global Warming Potential (GWP) of NH₃ is 0 and CO₂ is 1, while the Ozone Depletion Potential (ODP) is 0 for both agents. The GWP of CO₂ can be neglected when used in technical applications as it is a by-product of many industrial processes.
The table below shows the ODP and GWP values for some refrigerants.
Both NH₃ and CO₂ were among the first substances to be used as refrigerants, as early as the 1850s. In 2008 a classification of refrigerants into four generations was proposed:
- First generation (whatever worked) (1830-1930): includes both NH₃ and CO₂;
- Second generation (safety and durability) (1931-1990): characterised by the switch to CFC refrigerants, but NH₃ remains representative of this period;
- Third generation (ozone protection) (1990 -2010): proposes HCFC-type agents in a transitional period and HFC-type agents for long-term use, in the context of the ﬁrst regulations on ozone layer protection. Natural agents, including NH₃ and CO₂ began to be reconsidered during this period;
- Fourth generation (global warming) (after 2010): removal of environmentally harmful synthetic agents. In this current context, both NH₃ and CO₂ are considered among the most viable alternatives.
Refrigeration cycles that work with both agents are well known and new improvements are continuously implemented, especially for CO₂.
Features of CO₂
CO₂ is a well-known and long-established refrigerant, it is non-toxic, non-ﬂammable, abundant (including in ambient air) and has a very low environmental impact compared to other refrigerants.
CO₂ is considered an excellent alternative to NH₃, especially in situations where toxicity and ﬂammability are problems to be avoided. These reasons may explain the success of CO₂ in areas such as the automotive industry or household and commercial applications. Recently, CO₂ has become a competitive agent also in air conditioning.
The main disadvantage of CO₂ is the low critical temperature value (tcr = 31.06 ºC), which causes transcritical, or supercritical, operation in many applications where condensation becomes impossible due to climatic conditions. Compared to NH₃, the energy efﬁciency of CO₂ cycles is lower, especially in the supercritical mode.
Although CO₂ was almost forgotten during the freon boom, it is being rediscovered and reconsidered in recent times because of its characteristics.
The CO₂ installation
In all cases where due to the too high temperature of the refrigerant (the hot source of the refrigeration cycle), it is not possible to condense CO₂, the operating cycles of installations with this refrigerant become supercritical, i.e. they operate at temperatures and pressures higher than those of the critical point
(tcr = 31.06 ºC; pcr = 73.834 bar). The maximum condensing temperature of CO₂ is the critical temperature (≈ 31 °C). In the case of supercritical operation of refrigeration plants and heat pumps with CO₂, since condensation no longer occurs, the heat exchanger that transfers heat energy to the hot source is no longer called a vapour cooler condenser.
In the ﬁgures below there are shown the schematics of the CO₂ plants with supercritical operation and the working cycle of these plants in the pressure-enthalpy diagram.
Schematic diagram of the classical CO₂ installation
Representation of the conventional (classical) supercritical cycle with CO₂