From the point of economic, environmental and social view our current trends in energy supply and use are unsustainable. Our current energy and climate direction must be changed. Renewable technologies will play crucial role in this change. In order to address these challenges International energy agency (lEA) is developing number of roadmaps for different technologies that can have effects in C02 reduction. There are also roadmaps for renewable technologies. One of them is: "Roadmap for Energy Efficient Heating and Cooling" according to which 20% of increasing Coefficient of Performance (COP) in renewable technologies is planned by the year 2020 and global target in reducing carbon dioxide (C02) for 50% from current levels till 2050. (lEA, 2010) Phase change material (PCM) storages can represent a real challenge to design. One of the challenges is choosing a PCM that is compatible with the system that we want to optimize, and the another is choosing the exact heat exchanger that will give optimal surface area during charging and discharging, due to the fact that PCMs have low thermal conductivity. Clearly, water can be used as a storage medium, but it does not give us the freedom to maintain the system at the desired temperature. In order to gain maximum thermal power from their potential, proper geometrical design needs to be used for each kind of PCM thermal energy storage (TES). The Nusselt equations for heat exchangers used for air and water cannot be applied to PCMs. Extensive research therefore needs to be undertaken in order to investigate the different geometrical configurations of heat exchangers and their interactions with different kinds of PCMs. This thesis summarizes the results from 3 different types of experiments. The first type was with a single tube in water and PCM storage. The second type of experiment was with a multi-tube heat exchanger in PCM-TES. The third type of experiments was with a heat pump system coupled to water/PCM TES. In total 66 tests were performed: 6 experiments with single tube, 42 experiments with multi-tube and 18 experiments with heat pump system. The main contribution presented in this thesis was novel Nusselt number correlations for multi-tube heat exchanger, as heat transfer fluid refrigerant was used and as a storage medium PCM was used.

Phase pure cuprous oxide (Cu2O) is desirable as an absorber material to realize efficient thin photovoltaic cells based entirely on 'cheap, environmentally benign and earth abundant metal oxides' for terawatt-level deployment. One of the major barriers to make efficient Cu2O based solar cells is the difficulty to grow stoichiometric as well as defect free interface with suitable photoanodes (for example, Zinc Oxide(ZnO)). This thesis mainly focuses on the growth of Cu(I) oxide thin films on variety of substrates produced by pulsed laser deposition (PLD) with varying growth temperature(Tsub: 25 °C - 400 °C), oxygen content(O2pp: 1-10 mTorr) and laser pulse energy(LP: 25-35 mJ), in order to control the structural, optical and electrical properties. The deposited films were characterized by variety of characterization techniques including, X-ray diffraction(XRD), Transmission electron microscopy(TEM), Raman spectroscopy and room temperature Photoluminescence (RT-PL), variable angle spectroscopic ellipsometry, UV-Vis-NIR spectroscopy, four point co-linear probe/Hall measurement and photoelectrochemical measurements. The structural analysis revealed that PLD films grown at 25 °C ≤ Tsub ≤ 200 °C with O2pp ≈ 3mTorr and LP ≈ 25mJ are single phase polycrystalline Cu2O while deposited on amorphous quartz substrate; and partially oriented pure and twinned Cu2O film with {001}Cu2O || {001}NaCl with < 110 > Cu2O || < 100 > NaCl epitaxy while grown on NaCl(100) substrate. All phase pure Cu2O films exhibited an optical bandgap in the range 2.15-2.40 eV with high absorption coefficient above 105 cm-1 near the band edge. The thin films grown at Tsub ≥ 300 °C with O2pp ≥ 3mTorr and LP ≈ 25mJ exhibited a predominant CuxOy species with the Cu2O structure, confirmed by XRD, TEM and Raman analysis, with a 2-3% lattice parameter expansion compared to the pure Cu2O. The RT-PL spectra exhibited an intense luminescence peak near 760 nm which has been attributed to a distinct Cu3O2 phase. The large bandgap observed in CuxOy species compared to pure Cu2O phase is attributed to the stoichiometric difference. It is concluded here that this is not a new phase, rather an oxygen-rich defect structure of Cu2O. Thin films grown at Tsub ≥ 200 °C and under more oxygen rich (≥5mTorr) conditions revealed mixtures of cupric (Cu (II)) and cuprous (Cu(I)) oxide phases. The electrical resistivity of Cu(I) oxide films grown at Tsub ≈ 25 °C(RT) and Tsub ≈ 200 °C(HT) on quartz substrate were found to vary in the ranges 3 mΩ.cm - 20 kΩ.cm and 80 mΩ.cm - 13 kΩ.cm respectively depending on the oxygen content. The Hall mobility (carrier concentration) of RT- and HT-grown films were estimated to be ~2.08 cm2 V-1s-1(~1019 cm-3) and ~22.21 cm2 V-1s-1 (~1013 cm-3) respectively where lower mobility is attributed to the scattering of holes in the grain boundaries of the nanocrystalline films. All Cu2O films demonstrated a p-type conductivity confirmed by positive Hall voltage as well as cathodic photocurrent nature. The external quantum efficiency of the best HT-grown found to be ~10 times higher compared to the best RT-grown film measured in a Photoelectrochemical cell. A preliminary study of a solid junction with a PLD-grown ZnO electrode demonstrated photoresponse but with very low efficiency, attributed to the presence of an amorphous layer at the Cu2O/ZnO interface.

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