New water use efficiency strategies to cope with climate change

Crop water requirements are important elements for food production, especially in arid and semiarid regions. These regions are experience increasing population growth and less water for agriculture, which amplifies the need for more efficient irrigation. Improved water use efficiency is needed to produce more food while conserving water as a limited natural resource. Evaporation (E) from bare soil and Transpiration (T) from plants is considered a critical part of the global water cycle and, in recent decades, climate change could lead to increased E and T. Because energy is required to break hydrogen bonds and vaporize water, water and energy balances are closely connected. The soil water balance is also linked with water vapour losses to evapotranspiration (ET) that are dependent mainly on energy balance at the Earth’s surface. This work addresses the role of evapotranspiration for water use efficiency by developing a mathematical model that improves the accuracy of crop evapotranspiration calculation; accounting for the effects of weather conditions, e.g., wind speed and humidity, on crop coefficients, which relates crop evapotranspiration to reference evapotranspiration. The ability to partition ET into Evaporation and Transpiration components will help irrigation managers to find ways to improve water use efficiency by decreasing the ratio of evaporation to transpiration. The developed crop coefficient model will improve both irrigation scheduling and water resources planning in response to future climate change, which can improve world food production and water use efficiency in agriculture.

Enhancing the efficiency of solar concentrators by controlled optical aberrations

The diameters of traditional dish concentrators can reach several tens of meters, the construction of monolithic mirrors being difficult at these scales: cheap flat reflecting facets mounted on a common frame generally reproduce a paraboloidal surface. When a standard imaging mirror is coupled with a PV dense array, problems arise since the solar image focused is intrinsically circular. Moreover, the corresponding irradiance distribution is bell-shaped in contrast with the requirement of having all the cells under the same illumination. Mismatch losses occur when interconnected cells experience different conditions, in particular in series connections. In this PhD Thesis, we aim at solving these issues by a multidisciplinary approach, exploiting optical concepts and applications developed specifically for astronomical use, where the improvement of the image quality is a very important issue. The strategy we propose is to boost the spot uniformity acting uniquely on the primary reflector and avoiding the big mirrors segmentation into numerous smaller elements that need to be accurately mounted and aligned. In the proposed method, the shape of the mirrors is analytically described by the Zernike polynomials and its optimization is numerically obtained to give a non-imaging optics able to produce a quasi-square spot, spatially uniform and with prescribed concentration level. The freeform primary optics leads to a substantial gain in efficiency without secondary optics. Simple electrical schemes for the receiver are also required. The concept has been investigated theoretically modeling an example of CPV dense array application, including the development of non-optical aspects as the design of the detector and of the supporting mechanics. For the method proposed and the specific CPV system described, a patent application has been filed in Italy with the number TO2014A000016. The patent has been developed thanks to the collaboration between the University of Bologna and INAF (National Institute for Astrophysics).