CO2 heat pump in the high temperature range

Heat pumps are used wherever a lot of hot or less hot water is needed. At best, a ground probe or a collector field are available. This is increasingly the case in the public sector, but also in commerce and industry. Typical areas of application are hotels, old people's homes, baths, schools, old building refurbishments, but also the meat processing industry and manufacturing companies with large heating support.

The F-Gas Regulation and its associated limitations on the refrigerant sector severely limit the choice of refrigerants available. If a natural and environmentally friendly refrigerant is to be used, which is also eligible, only propane (GWP 3) and carbon dioxide (CO 2 , GWP 1) are available. Propane has first-class properties as a refrigerant, but is not always a possible scenario because of its flammability and the associated safety concept, especially for indoor installation. CO 2as a high-pressure refrigerant with very good heat transfer properties, however, can be used by its classification as A1 refrigerant without major restrictions. Triple point and critical point can be achieved in the normal working range of a chiller. If one had to attach great importance to these features about 10 years ago, it has become the standard refrigerant due to the frequently used booster systems in commercial refrigeration. In the heat pump sector, CO 2 has so far rarely been used in Germany. On the one hand, this is due to the fact that the water inlet temperature on the warm side is extremely important for the efficiency of CO 2Cycle process (the lower the better) and, second, that the design and control of these machines are not yet familiar.

CO 2 heat pumps are generally operated transcritically throughout the year. This process above the critical point allows for sliding temperature delivery to a secondary medium (eg, water) in the gas cooler, and at significantly lower exergy losses compared to a subcritical process with constant condensing temperature in the condenser.

Figure 1 and Figure 2 show a transcritical heat pump process in the temperature-enthalpy diagram. The temperatures of the respective circuit can be read directly here. The length of the arrows also corresponds to the corresponding heat output. Figure 1 shows the heat pump cycle at an optimally low water inlet temperature and a CO 2 typical very high temperature. The "pinch point" (minimal temperature difference between refrigerant and secondary medium), which is very important for supercritical CO 2 applications for the design of the heat recovery or the gas cooler, must not be undercut (recommendation> 4 K). Figure 2 shows the heat pump process at CO 2Heat pumps quite atypically high return temperature. The cycle is just as reproducible, but at the expense of efficiency. If one were to use a larger 4-cylinder reciprocating compressor for both variants under the conditions shown and t 0 = 0 ° C, one would achieve a usable heat output of, for example, 140 kW at a COP of 4.7 for image 1. In Figure 2, the same compressor would only achieve a heat output of 60 kW with a COP of 2.1. The reason for this is primarily the positioning of the refrigerant-dependent two-phase region. If the vapor content after relaxation in image 1 was about 5%, it would be over 70% in image 2!

Image 1 

picture 2 

CO 2 heat pump cycles in the t, h diagram to illustrate the difference between low and high water return temperature in the gas cooler.

Typically, CO 2 heat pumps are designed in one stage without flash gas bypass (see Figure 3). Liquid separators and Sauggasüberhitzer are absolutely necessary, since the high-pressure control valve is the injection valve at the same time on the evaporator and this is operated quasi-flooded.

RI diagram of a typical CO 2 heat pump. 

picture 3

Compact Kältetechnik GmbH has developed the * carboHeat series for this system configuration . Typical heat output of * carboHeat at t g = 30 ° C, t 0 = -5 ° C, t water = 20/70 ° C, ph = 92bar (a):

The * carboHeat is equipped as a complete refrigeration circuit, each with a transcritical semi-hermetic reciprocating compressor (optionally with FU), one to three gas coolers, high pressure valve, evaporator, accumulator and an internal heat exchanger in intrinsically safe design with 120 bar on the high pressure side and 80 bar on the suction pressure side. In this case, no emergency cooling is required in case of standstill. With the control cabinet completely wired (control technology optionally with Siemens S7, Danfoss or Wurm), the customer receives a "Plug and Play" device. For support during commissioning or service and maintenance compact Kältetechnik is always available as a contact person.

CO 2 heat pumps are eligible under the BMU Refrigeration and Air Conditioning Directive. The range of compact refrigeration technology is listed at the BAFA. The calculation of the total working year is based on the VDI 4650 and achieves a fictitious total annual work figure of 4.3, for example for the * carboHeat18, which is also shown in Figure 4. Currently this CO 2Heat pump to a well-known mechanical engineering company in the Stuttgart area for heating support of the manufacturing plant. Designed as a monovalent water / brine heat pump, with design conditions of water 25/50 ° C on the warm side and ethylene glycol 30% + 12 / + 8 ° C as heat source this * carboHeat18 achieves a heating capacity of approx. 100 kW, with FU operation at 53 Hz. At the operating point, this heat pump achieves a COP of 4.54. In accordance with the BAFA funding described above, a subsidy of approximately € 8,500 was calculated for the operator. Measured by the slightly higher investment costs due to the high-pressure refrigerant CO 2 , the promotion results in noticeable cost advantages for the customer.

CO 2 heat pump of the type * carboHeat18 from compact Kältetechnik 

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Cooling machines from compact Kältetechnik GmbH cool, air-condition and freeze in various market segments across the globe. Since its founding in 1992, the company has established itself as one of the leading manufacturers of complex refrigeration machines and systems.

The experts from compact Kältetechnik are your competent partner from consulting, planning, conception and project planning to the delivery and commissioning of refrigeration plants and systems. Both tried-and-tested series products for a wide range of applications as well as customized solutions are available - using natural refrigerants as well as all common and F-gases compliant synthetic refrigerants. This results in specially designed systems with high energy efficiency that are tailored to the specific application. Whether composite systems, cascade systems, transcritical booster systems, condensing units, chillers or heat pumps - all compact products ensure top quality with high-quality components, well thought-out design and meticulous workmanship.

The complete planning and production of the systems according to ISO 9001 and machine and pressure equipment directive up to Cat. IV (H1) "Made in Germany" comes from Dresden and Scharfenstein in Saxony.

Summary:

1. CO 2 is predestined as a refrigerant for high-temperature heat pumps.
2. Transcritical process energetically beneficial for heat recovery and as a heat pump application (very small media to medium with less exergy losses than any other refrigerant)
3. There is another "optimal high pressure" for CO 2 heat pump processes than for chillers. This is directly dependent on the secondary temperatures and the resulting "pinch point" in the gas cooler.
4. CO 2 heat pumps are typically built fundamentally different than z. B. CO 2 -Kaltwassersätze (simple structure without Flashgasbypass with quasi-flooded evaporation).
5. Optimal conditions for a CO2 heat pump:

• steady water supply at a constant low temperature on the gas cooler
• high demand for warm water (eg at least 50 ° C) or low requirement for very hot water (up to 85 ... 90 ° C possible)
• Heat source should ideally be a geothermal probe / collector with year-round constant conditions.

1. Depending on the operating conditions, heating COP values ​​of 3 to 6 can be achieved, which is significantly higher than with conventional heat pumps.
2. With the use of the refrigerant CO 2 and its enormously high volumetric cooling capacity, it is possible to build space-saving and comparatively small machines for high performance requirements.
3. CO 2 heat pumps require less frequent service intervals and will not be restricted by the F-Gas VO in the future.

Written by: Dipl.-Ing. Stephan Leideck, Project Planning | Research & Development at compact Kältetechnik GmbH