Session: 11-42-01: Heat and Mass Transfer in Heating, Cooling, and Power Systems

Paper Number: 119316

119316 - Static Conversion of a Salinity Difference Into a Temperature Difference: A Heat and Mass Transfer Investigation 

Space cooling in buildings is expected to increase due to the growing demand for thermal comfort worldwide, requiring increasingly more sustainable and cost-effective cooling technologies. Nowadays vapor compression technologies are used for fulfilling most of the cooling demand despite their significant electricity consumption and their possibility of causing seasonal peaks in the energy consumption. As an example, air conditioning is responsible for almost 16% of the total primary energy needed in buildings in the United States. In addition, vapor compression technologies typically involve refrigerants with high global warming potential. In recent years, several evaporative cooling technologies have been developed, representing a viable alternative to traditional air conditioning systems. However, the cooling potential of evaporative solutions strongly depends on the inlet conditions of ambient air, thus presenting substantial limitations when used in humid climates. Environmentally friendly and possibly scalable approaches that provide a net cooling capacity without resorting to electricity may therefore be highly beneficial to reduce the energy consumption and alleviate the environmental impact of current cooling technologies.

In this work, we model and design, realize, and experimentally test a passive multi-stage proof-of-concept cooling device, which operates without moving mechanical parts or auxiliary equipment. The net cooling load is generated, at a nearly ambient pressure, by a salinity disparity between two aqueous solutions (here represented by distilled water and brine). Those solutions, kept separated by a hydrophobic microporous membrane, can only exchange water vapor. Under proper conditions, the difference in water activity generates a net non-isothermal vapor flux from the distilled water (evaporator) toward the brine (condenser). In particular, the evaporator is made of a hydrophilic porous material connected to a distilled water reservoir, thus allowing a constant supply of distilled water by capillary action without the need of an active pump. The brine, however, is held in a cavity connected to a high-salinity reservoir. The above describes only one core unit of the tested device. Several units may be stacked so that the heat flux received by the lowest condenser is transferred to the second lowest evaporator, thus recovering the enthalpy of condensation. This multistage configuration allows an increase in the maximum temperature difference and cooling capacity achievable by a single-stage device. 

The developed laboratory-scale prototype is able to provide a maximum cooling capacity of nearly 100 W m⁻² when it operates with a sodium chloride solution at 3.1 mol/kg and a maximum cooling capacity of approximately 170 W m⁻² with a calcium chloride solution at the same concentration, with zero temperature difference. A lumped parameter model is used to interpret the experimental results and to estimate the performance of the device under different operating conditions (e.g., different ambient temperatures, a different number of stages, or the presence of an air gap above the membrane), thus allowing the optimization of the device design to match specific working conditions. To provide perspective, if the system is successfully scaled up, it can help meet at least part of the cooling needs in hot and humid regions with naturally available salinity gradients.

Presenting Author: Matteo Morciano Politecnico di Torino

Presenting Author Biography: I received B.S. and M.S. in Mechanical Engineering in 2015 at Polytechnic of Turin. I carried out my master thesis in the Complex Multiscale Systems group in the Department of Chemical Engineering at Imperial College London. Then, I received a PhD in Energetics in 2019 at Polytechnic of Turin. In this context, I worked in the multi-Scale ModeLing Laboratory group. During this period, I visited the research Nanoengineering group at the Massachusetts Institute of Technology (MIT). I am currently working as researcher (tenure-track) at the Department of Energy (Polytechnic of Turin) in collaboration with the Clean Water Center. My experience includes both modelling and experimental techniques as well as analysis and practical implementation of energy conversion systems.

Authors:

Matteo Morciano Politecnico di Torino
Matteo Fasano Politecnico di Torino
Pietro Asinari Politecnico di Torino
Eliodoro Chiavazzo Politecnico di Torino