4.1. Assessment of the water demand
The monthly average water demand for the growing of Alfalfa on a sandy soil and the trend of the monthly average precipitation are presented in Fig. 3. It is clear that the trend of the daily water demand for irrigation is affected mainly by the evaporation related to the growing phase and rainfall. The evapotranspiration registered a peak during the sunniest months of the year whereas the precipitation registered the highest values in the period May–September. The irrigation season for the crop chosen is five months, this study interested then only the months from May to September. In this work it was assumed that in May takes place the development phase, in June and July the intermediate phase and in August and September the final phase. The Alfalfa water demand trend shows then a peak during the month of June of 47 m3/ha and it decreases during the remaining months. The minimum daily water demand estimated for this period corresponded to the water requirements in May which is equal to 10.4m3/ha. The irrigation turn estimated by the model was 10 days. In this work an irrigated area of 1 ha was considered. The validation of the results obtained from the water demand model was carried out through personal communication with field expert and with the results obtained in field studies con- ducted in the same region [24]. The former proved that the daily maximum water requirement for irrigation is 50m3/ha. Whereas the results obtained in previous field studies showed an irrigation duty of 600 m3/ha for an irrigation turn of 14 days corresponding to 40m3/ha day.
P.E. Campana et al./Applied Energy 112 (2013) 635–645 639
4.2. Solar energy assessment
The available solar radiation and its variation with the tilt angle and system technology are shown in Fig. 4. In this study it was as- sumed to usea fixedsystemwith an azimuthangle equal to 0 that corresponds to solar array oriented towards south. The results of the simulations show that for the fixed system the best tilt angle on annual basis was 30 with a corresponding collectable solar radiation of 1870 kW h/m2 year. For the simulations carried out only during the irrigation season, from May to September, the best tilt angle resulted in 10 collecting 854kW h/m2 season. The 10 tilted solar array was then used in our study. As regards the fully tracking system, the annual collected solar radiation on the plane surface was 2490kW h/m2 year whereas 1120 kW h/m2 during the irrigation season. This corresponds to a collected solar energy 30% higher compared to the optimal fixed system. The power output from fixed and fully tracking PV system with a capacity of 1 kWp during a sunny day in June is shown in Fig. 5. The energy collected by the 10 tilted system was 7.0 kWh/m2 whereas the solar energy collected by the fully tracking array it was equal to 10 kWh/m2 corresponding to 40% more energy than the fixed system. The better performances of the sun fully tracking system are mainly due to the system, varying continuously its tilt and azimuth angle in order to follow the sun, optimizes the har- nessing of available solar radiation guaranteeing a wider range of working hours at higher power output compared to the fixed system. It is clear that the solar generator power output depends on the variation of the available solar power and is mainly sensitive to the variation of ambient temperatures. The typical effect of the hourly variation of ambient temperature on the power output of solar ar- ray is presented in Fig. 6 for 1 kWp PV array. The power output from the solar generator without considering the temperature effect, is the power at the reference temperature of 25 C and depends only on the available solar radiation, PV modules optical efficiency and incidence angle modifiers. As it is shown, the temperature affects the power generation of the solar
array during the sunniest and warmest hours of the day due to the difference between cell temperature and reference tempera- ture. The maximum drop of the efficiency and the subsequent drop of power generation were registered at 1 pm and it was equal to 198W representing a loss of 17%. The high value of power waste was due to the theoretical approach used in this study to perform the effect of temperature on the PV modules efficiency. The previous approach tends to overestimate the power losses due to temperature, usually in the range of 10%, on behalf of guaranteeing more accurate water supply forecasts.