## 3 State of Technology
The \(\Delta\)T\({}_{\text{m,lift}}\) was determined as the difference of the logarithmic mean temperatures of the secondary fluids for heat sink and source using Equations (1) and (2):  
\[\Delta\text{T}_{\text{m,lift}}=\Delta\text{T}_{\text{m,log,Sink}}-\Delta\text {T}_{\text{m,log,Source}} \tag{1}\]  
\[\Delta\text{T}_{\text{m,log}}=\frac{\text{T}_{\text{secondary fluid,in}}-\text {T}_{\text{secondary fluid,out}}}{\text{In }\frac{\text{T}_{\text{secondary fluid,in}}}{\text{T}_{\text{secondary fluid,out}}}} \tag{2}\]  
For the total temperature lift of the ACHP cycle with single-stage solution circuit, values ranging from \(-5.6\) K to 38.7 K were achieved. As mentioned earlier, through the temperature glides occurring in the heat source and sink in combination with a large total temperature lift, a simultaneous use for cooling and heating demands is possible, as shown by [PERSON] (2005) [44]. COPs of 2.1 to 11.3 were achieved, whereby a direct comparison is difficult due to the different temperature levels and stages of optimization (insulation, compressor size, pinch in heat exchanger). Regarding the compressor design, oil-lubricated reciprocating compressors with single or two-stage compression were used in most investigations. Two times twin screw compressors with oil lubrication and oil cooling were used. Only four cases of oil-free operation involving three reciprocating compressors (two single-stage and one two-stage compression) and one screw compressor were tested. The oil-lubricated two-stage reciprocating compressors often achieved the highest temperature lifts, proportional to the required pressure ratio. Here, intercooling with an additional heat exchanger between the compression stages was frequently used to reduce the occurring discharge temperature. In the absorber and desorber design, an evolution from shell-and-tube to corrugated plate heat exchangers (PHE) can be seen, and the alignment changed from horizontal to vertical. In this context, several innovative approaches have been employed. [PERSON] and [PERSON] (1984) [48] investigated a subdivision of the heat exchanger surface with linking solution pumps. The connection of several PHEs to extend the effective heat transfer length in combination with different operation modes, described as falling film and bubble mode, as well as mixing techniques were evaluated during the investigations by several authors [20; 21; 41; 50]. The wet compression cycle has been investigated in five experimental studies. The first of these are industrial pilot plants with capacities of 500 kW and 1000 kW, which follow in time with the first installation of the single-stage ACHP pilot plants. Then a wet compression laboratory facility was investigated with one of the smallest capacities of all ACHP cycles at 1.4 kW. The tested temperature levels vary in all investigations with the shared feature of a larger temperature gradient over the heat sink. Single-stage compressors lubricated with the solution were used for all investigations, although a wide variety of different compressor types was tested. Except for the liquid ring compressor employed by [PERSON] (1998) [59], all types were considered suitable for use in wet compression cycles. However, for efficient and reliable use, the authors highlight the importance of modifications, such as selecting suitable bearings and the design of the liquid injection system [35; 57; 58; 60]. For the design of absorber and desorber, an evolution from horizontal to vertical arrangement was again observed. Additionally, [PERSON] and [PERSON] (1991) [58] tested a coaxial arrangement of a tube-to-tube heat exchanger. [PERSON] (1998) [59] employed plate-fin heat exchangers with the aim to increase the heat transfer area as a precursor to the commonly used corrugated PHE. The ACHP with a two-stage solution circuit and the ACHP with a single-stage circuit with DAHX have been studied only once by a group of researchers led by [PERSON] [39; 42]. The researchers modified an existing single-stage laboratory facility designed by [PERSON] et al.