Performance testing of a novel porous concrete energy exchanger for agricultural buildings

Energy recovery in ventilation is one of the most challenging tasks in indoor farming and greenhouse operations. Traditional technologies can recover most sensible heat, but significant amounts of latent heat from outgoing air are lost. The energy wheel or other direct energy recovering technologies are not recommended to use in greenhouses as they transfer moisture directly between airstreams, whereas greenhouse supply air is normally ensured as dry (<50% of relative humidity) for any outdoor weather condition. In this study, a novel porous concrete heat exchanger was designed using three porous concrete wheels to recover both sensible and latent heat from the outgoing airstream, while continuously transferring it to the supply airstream. The three porous concrete rotary wheels (30 cm diameter and 2.5 cm thick; 30% porosity) were placed 30 cm apart from each other in a single shaft. The body frame was made of plywood and sheet metal connectors, to flow air using four air blowers. Heat transfer performance was investigated with inlet air temperature differences ranging from 10 to 60°C, with a wheel speed of 0-10 rpm. Data indicate that the porous concrete heat exchanger can provide continuous air supply at a set point temperature during northern hemisphere winter and spring seasons (Montreal, Canada) by recovering heat from the outgoing air with a maximum heat transfer rate of 4.4 kW. Maximum effectiveness is 0.99 at a 10°C temperature difference between hot and cold inlets. This eco-friendly energy exchanger could start new discussions around energy exchanger technologies that reduce the carbon footprint of traditional energy exchanger materials by providing high heat recovering capacity.