Simulation of a standalone photovoltaic system powering an atmospheric water generator

Nowadays, water scarcity and lack of infrastructures such as the electric grid are two of the main issues that different countries must face. Both issues are treated between the seventeen Sustainable Development Goals (SDGs). In relation to lack of water supply, several technologies to produce water have been developed. Among them, there are technologies to produce water from atmospheric air. These devices are usually called atmospheric water generators. Aquaseek, spin-off of Polytechnic of Turin and start-up of Princeton University, develops an atmospheric water generator whose operation is based on adsorption-regeneration cycles. Heat must be supplied during the regeneration phase to extrapolate water from adsorbent material, with maximum temperature around 60-70°C. Given the required low thermal grade, low temperature heat sources can be used such as solar energy through thermal collectors. Another way, analyzed in the following work, is based on an electrical heater and consequently an electric source could be implemented. Moreover, other auxiliary components require electricity. To guarantee operations of the atmospheric water generator independent from electric infrastructures which are missing in several locations, a stand-alone photovoltaic system is considered. Adsorption-regeneration cycles repeat several times during the entire day and electricity must be supplied also during night hours in which photovoltaic production is absent. The use of accumulators is necessary and photovoltaic production should be sufficient to satisfy the load during the day and charge the battery to supply demand during the night. The project has the aim to evaluate the evolution of principal parameters of the system (PV production, state of charge of the battery, stored and released battery energy) during an entire year knowing the load profile and location. A model based on energy considerations is implemented using the MATLAB environment and used to simulate different configurations of the system. Meteorological data such as irradiance and temperature of the location are downloaded using the National Solar Radiation Database of NREL. The model is based on power balance equations involving photovoltaic production, battery charge and discharge power and load request. Constraints on battery operation are applied to prevent battery aging, battery degradation and to guarantee safety operations. Over-charging and over-discharging of batteries are limited by the charge controller. When the battery reaches the maximum state of charge, the PV system is disconnected from the battery to avoid over-charging. The amount of waste energy is evaluated. In case of minimum state of charge, the inverter used to convert DC current into AC current is off and the load couldn’t be supplied. The impossibility to satisfy the load is evaluated. Loss of load hours (LOLH) is the number of hours per year in which the inverter is switched off by the charge regulator. LOLH is a reliability parameter that must be minimized to guarantee autonomous operation. Loss of load of probability (LOLP) and energy missing are related to LOLH. Different locations are considered and for each of them evaluation of different parameters, such as number of panels and nominal capacity of batteries, is performed. Further technical considerations are evaluated for the other components. Finally, multiplying factors are used to scale the load request or to adapt it to the power source characteristics. Changes in the size of the system are evaluated.