Photovoltaics for evaporative cooling greenhouses

A group of researchers from Abdou Moumouni University in Niger has proposed to use photovoltaic energy for powering evaporative cooling greenhouses (ECGs) in Africa's Sahel region. “The technology is under review for a patent,” said corresponding author Dr. Alio Sanda M. Djibrilla to pv magazine.

ECGs are a type of greenhouses that cool the plants using the natural process of evaporation. The evaporation process is performed by pumping water onto special wet pads while fans push air through them.

“Under Sahelian dry and hot weather conditions, cooling is necessary in order to keep conducive microclimate,” explained the research group. “To avoid conventional costly and unsustainable cooling system in the Sahel with air conditioner connected to the grid or through generators fueled by carbonated fossil fuel, a standalone photovoltaic system was used to power the evaporative cooling system, that is the water pumping and distribution and ventilators.”

The experimental set-up was located in southwest Niger and covered an area of 50 m2, with a height of 3.66 m and a roof slope of 15 degrees. The water pumping and ventilators were powered by six PV modules with an output of 260 W each, using an inverter of 5 kVA. Four batteries of 150 Ah each were also used to enable smooth operation in case of a lack of irradiation.

As for the wet pads, the academics compared two different types in the same system. One was the commercially available Celdek pads (C-pads), while the other was the locally-made Hyphaene thebaica fiber pads (HF-pads). In both cases, pads were inserted along the greenhouse sides, with one ventilator per each pad.

“Field climatic parameters recorded in situ and thermophysical data obtained at the laboratory prototype level and from the installed PV-ECG were used to assess the alternative HF-pad cooling potential against conventional C-pad,” explained the academics. “The greenhouse key cooling performances  were derived from established energy and mass balance thermodynamic equations.”

In addition, after collecting the data from the real greenhouse and conducting a lab investigation of the pads, a computational fluid dynamic (CFD) analysis was conducted to gain a general understanding of the refrigerant fluid distribution in the greenhouse. Both the numerical model and the experimental data collection were conducted in the hot and dry seasons, with input temperatures of 35 C to 45 C and relative humidity of 10% to 15%.

“Cooler using HF-pad allows to keep the microclimate below 25 C, with maximum moisture rate up to 80%, under harsh ambient conditions,” the team stressed. “HF-pad had the highest cooling coefficient of performance (COP = 9 against 6 for C-pad), the best cost-to-efficiency ratio (CER = 5; 4 times less than C-pad), and the lowest outlet temperature (20.0 C).”

The scientists also highlighted that due to higher outlet air velocity, the C-pad cooler spread cool air up to 1.25 m farther than its counterpart, creating higher pressure in the atmosphere, with two times turbulent kinetic energy.

The group concluded that the HF-pad achieved cooling performances that compete with conventional pads. “Optimization of HF-pad frame engineering and the technology scaling up to industrial level can allow better thermal and economic performances,” they added.

Their findings were presented in “Improving the sustainability and effectiveness of photovoltaic evaporative cooling greenhouse in the Sahel,” published in Scientific Reports.