Ground source heat pump (GSHP) is an innovative and perspective technology able to use the ground as a thermal sink or heat source. If combined with system operating at relatively low temperature, it represents a high efficiency solution for the heating of buildings. Complementarily, during cooling operation it has a good advantage with respect to air-cooled systems, because the ground temperature is stably lower than the outdoor air one. Geothermal heat pump systems are able to reduce the environmental impact of buildings for space heating and cooling by using the ground as an energy renewable source. This paper presents a review on the GSHP systems presenting both a summary of different ground-source typologies of heat pumps and a thermodynamic approach for their modeling. The irreversible thermodynamic approach is here summarized and exposed for a complete GSHPs system. This analytical approach is particularly useful for implementing an optimization design tool for GSHP systems. Recently many works have been published about exergy analysis of these systems. Those works suggest that future lines of development may be considered: a) the optimization based on the transient performance of GSHP systems and not on the sole design condition; b) the integration of irreversible thermodynamic optimization approach into the algorithms of control systems. The diffusion of optimized GSHP systems is essential in order to reduce fossil fuel consumption and CO2 emissions, complying with the EU's directive

Direct solar absorption has been often considered in the past as a possible solution for solar thermal collectors for residential and small commercial applications. A direct absorption could indeed improve the performance of solar collectors by skipping one step of the heat transfer mechanism in standard devices and having a more convenient temperature distribution inside the collector. Classical solar thermal collectors have a metal sheet as absorber, designed such that water has the minimum temperature in each transversal section, in order to collect as much solar thermal energy as possible. On the other hand, in a direct configuration, the hottest part of the system is the operating fluid and this allows to have a more efficient conversion. Nanofluids, i.e., fluids with a suspension of nanoparticles, such as carbon nanohorns, could be a good and innovative family of absorbing fluids owing to their higher absorption coefficient compared to the base fluid and stability under moderate temperature gradients. Moreover, carbon nanohorns offer the remarkable advantage of a reduced toxicity over other carbon nanoparticles. In this work, a three-dimensional model of the absorption phenomena in nanofluids within a cylindrical tube is coupled with a computational fluid dynamics (CFD) analysis of the flow and temperature field. Measured optical properties of nanofluids at different concentrations have been implemented in the model. Heat losses due to conduction, convection and radiation at the boundaries are considered as well.