Energy consumption is still growing worldwide and due to the problems of global warming, sustainable developments become of more importance. Heat pumps are one of the technologies that can contribute to energy savings of industrial processes, for example by heat recovery from waste water streams. The focus of this research is to improve the performance of these heat pump systems.
The compression-resorption heat pump (CRHP) can be distinguished from a traditional vapor-compression heat pump (VCHP) by operating with non-azeotropic mixtures and an incomplete evaporation in the desorber. The working fluid enters the compressor as a two-phase mixture. This type of compression is known as wet compression. The efficiency of the wet compression process has significant influence on the performance of the complete heat pump system. Ammonia water mixtures are often used in this type of heat pump systems. A new mixture, NH3-H2O-CO2, has been proposed in order to increase the performance of CRHPs. However this improvement can only be reached if the compressor efficiency is not negatively influenced by the addition of CO2.
A wet compressor model operating with NH3-H2O-CO2 is developed in order to predict the efficiency of the compressor. The model indicates improvements on the isentropic efficiency of the compressor with NH3-H2O-CO2 compared to ammonia water up to 3.5 %. With the compressor model heat pump applications can be investigated by taking into account the efficiency of the compressor, predicted with the model. Especially for heating applications, improvements on the heat pump system operating with NH3-H2O-CO2 are indicated. Experimental validation of the compressor model is the next important step at this point.

Excessive consumption of non - renewable resources in the recent times has negative effects on the environment, one of the important of which include global warming. This suggests an urgent need for adaption to the sustainable use of the resources, for which the thermal technologies show a lot of scope. Absorption heat pump is a type of heat pump which uses a thermal compressor instead of a mechanical compressor as in the case for the conventional heat pumps. Thus, such heat pumps are indeed an alternative energy conversion technology. This thesis focuses on the use of an inventive fluid as an absorbent in the absorption thermodynamic cycles: ionic liquids. In this thesis, the process of absorption of NH₃ into an NH₃ + ionic liquid solution is studied. [emim][SCN] was selected as the ionic liquid to be used as an absorbent based on its efficiency, availability and the beneficial economic factors. In the first part of this thesis, an existing numerical vapor - liquid equilibrium model for the working pair of NH₃ - [emim][SCN] developed by Wang (2019) was validated. This is done by comparing the results of the model with the experimental data obtained by Yokozeki and Shiflett (2007) for the same working pair. The second part of this thesis includes experimental and numerical studies of the absorption process in question. For the experimental study, the absorption setup located in Process and Energy laboratory at TU Delft was used. This setup does not accommodate the complete absorption heat pump thermodynamic cycle, but just the absorption process. Firstly, H₂O - H₂O convective heat transfer experiments were performed inside the setup, for both the lower range Reynolds numbers (40-140), and for the higher range Reynolds numbers (150-360). It was ascertained that the derived correlations governing the heat transfer for both of these ranges, along with the average correlation covering all the ranges of the Reynolds number, agree with the previous literature, to ascertain that the setup worked correctly. Consecutively, the absorption experiments were performed with NH₃ - [emim][SCN] working pair. The amount of vapor flow recorded during these experiments was quite low (maximum upto 0.1\%), suggesting that most of the heat was transferred to the cooling water by means of the sensible heat transfer. A numerical model for the absorption + sensible heat transfer process developed by Wang (2019) was used as a reference to design a similar numerical model in the present work. The data obtained from the experiments was used as an input to the numerical model. It was observed that the overall heat transfer coefficient for the absorption process is indeed more than that for the convective heat transfer process. The numerical results agree with the experimental results including a moderate error margin. Therefore, the numerical model was considered validated. The conceptual reasoning behind the error margin between the numerical and the experimental results are also presented. Little can be said about the validity of the empirical correlations governing the absorption process estimated by Wang (2019), since absorption only took place during a small part of the length of the absorber (maximum 10.5\%). However, it is expected that these correlations work satisfactorily for those cases where the solution is subcooled before entering the absorber. It is concluded based on the numerical results that the correlation governing the convective heat transfer derived from the water - water experiments is considered acceptable for the present case. Finally in the last part of this thesis, an attempt was made to predict the accurate value for the viscosity of NH₃ + [emim][SCN] solution by an experimental study. A separate set of convective heat transfer experiments was performed with NH₃ + [emim][SCN] solution and the cooling water. The experimental data obtained was analyzed using the correlations obtained from the H₂O - H₂O convective heat transfer experiments. The results of this analysis were compared with the solution viscosity values predicted by Wang (2019) with a logarithmic correlation, wherein the solution viscosity is a logarithmic function of the viscosities of the individual fluids present in the solution. The reasonable agreement (with on an average 32.05 \% accuracy, as was estimated by Wang (2019)) between the viscosity values obtained by the analysis and the values predicted by Wang (2019) confirmed the validity of this logarithmic function. Recommendations are presented in the last part of this thesis for future studies.

The main objective of this study was to develop a quick model for an oil free twin screw compressor. The computational time of the model was to be kept as low as possible so that the model could be used for quick estimations and optimizations of the process. This was done by using a simple geometry focusing on the volume curve and port sizes, that can easily be changed by scaling the process as needed. The whole model was done in Matlab, with thermodynamic properties of the ammonia water mixture implemented by Rattner and Garimella. The model was validated by comparing general trends seen in oil free twin screw compressors using different working fluids.
The quick computational time allows for many different features to be examined in a short time to get a general idea of how the compressor would react to them. While using models with more detailed geometries one test can take hours compared to minutes with the model of this study. From the model search it was concluded that for this level of model a finite volume method would be the best option for this study. Various methods were explored to solve the governing equations, counting a few integrated solvers in Matlab and some numerical procedures for ODEs. The method that was chosen as the best for this system was the Euler method. All methods had in common was that the step size needed to be very small, for smaller geometries they needed to be even smaller in some sections of the compressor. To account for that the model was adjusted to accept variable steps. The variable steps allowed for smaller steps around the beginning and end of compression and thus lowering computational time. The main output of the model is the efficiency of the compressor and the Coefficient Of Performance (COP) of the cycle.
The model was tested for various operating conditions to determine how it, and the compressor, would respond. The main result is that the vapor quality should be kept rather low, around 40%, to achieve the highest efficiencies. The performance is also better with higher rotational speeds, however higher speeds also mean more mas flow through the whole system and increased mechanical losses. This means that up to a certain point the rotational speed can be increased until it starts decreasing the efficiency. The ideal ammonia concentration is around 33-35wt%, at higher or lower values the coefficient of performance decreases.
As some preliminary work for future experiments the experimental cycle was set up using pumps and heat exchangers to simulate the compressor. This included verifying the calibrations of sensors, confirming that correct data was read from sensor to LabView and getting the system leakproof. This cycle was also implemented in Matlab to be tested. From those tests the conclusion is that after going through the separator and the vapor and liquid streams being mixed back together the temperature decreases around 60 degrees usually, with a small increase in vapor quality around 2-3%.