Scientists have developed a system that harvests rainwater running off PV panels for household use or hydrogen production. Their analysis showed that, in the southern Sahel, the system can meet both the energy and water requirements for electrolysis, with surplus rainwater covering up to 50% of a household’s daily water demand.

A European team of researchers has proposed a system that harvests rainwater running off PV panels for household use or hydrogen production.

“The combined water and energy harvesting by PV mini-grids may have the potential to increase water security by making more water available for domestic use or crop irrigation in the dry season,” said the researchers. “The potential of PV systems for simultaneous water and energy supply in the Sahel has not been studied yet.”

The water collection system the team had designed utilizes fixed-tilt PV arrays. Under each array, a primary gutter is placed, collecting the water that runs off the panel. Those are then directed to a secondary water conveyance system linking PV panel units with filtration and water storage facilities.

The team has assumed a 1 mm water storage loss on the panel, meaning rainwater that is trapped or evaporates from the panel before it can be collected. They also assumed a runoff coefficient of 0.9, meaning that 90% of the water that falls on the panel is being collected. They further assumed a first flush loss of 3 mm, meaning that the first few millimeters that fall on the panel after a dry spell are contaminated with soil and therefore could not be used. The analyzed area looks at two areas – the Sahel region and the Sahel proper.

After designing the system, the academics calculated its water harvesting and energy production potential in the Sahel region of Africa. Thoroughly supplying energy demand, such a system can contribute up to 7% of household water demand, they found.

The Sahel Region includes parts of Mauritania, Senegal, Gambia, Guinea-Bissau, Guinea, Sierra Leone, Mali, Algeria, Cote d’Ivoire, Burkina Faso, Ghana, Togo, Benin, Niger, Nigeria, Cameroon, Chad, Libya, Central African Republic, Sudan, Eritrea, Ethiopia, Djibouti and Somalia.

“The selected region also covers parts of Saudi Arabia, Yemen and Oman in the East, which are not in the Sahel. Furthermore, the northern part of the extent also includes arid parts of the Sahara desert,” the group explained. “The Sahel proper is defined here as the area within the 150 and 850 mm mean annual precipitation contours.”

Based on the data from the relevant countries, the researchers considered a 6.9-people household, which consumes 136 liters of water per day and 4.5 kWh of electricity. To meet the electricity demands of the Sahel Region, a PV surface of 4.1 m² is needed in the dry North, and close to 6.0 m² in the humid South. That surface area is then used as a factor in calculating how much water runs off the panel to the storage area. Precipitation and irradiance values were taken from weather databases.

“The net rainwater harvesting potential was negative in the Sahara Desert where interception losses and cleaning water demands exceed precipitation inputs,” the results showed. “The percentage of household water demand potentially provided by the rainwater ranged from about 1% along the northern boundary to about 7% along the southern boundary. South of the Sahel percentages exceed 20% due to higher precipitation inputs and the higher PV panel surface area.”

Furthermore, the group calculated the ability of such household PV systems to supply both electricity and rainwater to produce 1 kg of hydrogen per day. According to their analysis, in the southern Sahel, the system can meet both the energy and water requirements for electrolysis, with surplus rainwater covering up to 50% of a household’s daily water demand. Even higher surpluses — exceeding 100% of household water needs — were calculated for regions south of the Sahel, including Nigeria, Burkina Faso, Benin, and northwest Ethiopia.

Their findings were presented in “Rainwater harvesting potential from photovoltaic energy systems in the Sahel,” published in Water-Energy Nexus. Researchers from the Netherlands’ consultancy Acacia Water, the IHE Delft Institute for Water Education, and Spain’s water technology center Cetaqua contributed to the research.