Impact of Mechanical Ventilation and Indoor Air Recirculation Rates on the Performance of an Active Membrane Energy Exchanger System

As concern for indoor air quality grows, many buildings will likely opt to provide higher rates of outdoor air than would traditionally be specified. This imposes a challenge on air conditioning systems since the latent loads associated with ventilation air are much higher than those associated with recirculated air. Membrane-based technologies, which enable mechanical separation of water vapor from air, have recently emerged as promising candidates for efficiently providing dehumidification, however, limitations remain. To date, most modeling work on these types of systems has focused on 100% outdoor air configurations that employ isothermal dehumidification designs. However, we have proposed a design referred to as the Active Membrane Energy Exchanger (AMX) that integrates cooling and membrane dehumidification into one device (thus non-isothermal) for a range of benefits. This work presents a specific application of the AMX in a system configuration that includes the treatment of both outdoor ventilation air and recirculated air. The system’s performance is analyzed over a broad range of ambient conditions and the effect of ventilation rates on the system performance is studied in detail. This configuration is found to be capable of providing three times the ventilation air of conventional systems with comparable or less energy consumption for the given conditions. Additionally, the optimal membrane module-outlet air temperature is found to be 18-20 ℃. Lastly, a case study using EnergyPlus building simulations shows that this configuration can reduce annual cooling energy requirements by as much as 34% in hot and humid cities for buildings with high latent loads and high ventilation rates

Impact of Spray Coating on the Performance of Hydrophobic Membranes

Membrane distillation (MD) is a rapidly emerging water treatment technology used to combat the global water crisis. Membrane pore wetting is a primary barrier to widespread industrial use of MD. The primary causes of membrane wetting are membrane fouling and an exceedance of liquid entry pressure. The development of different types of polymer membranes and the use of pretreatment have led to significant movement towards the prevention of wetting in MD. We sought to take a new approach to combat membrane wetting that involves coating these membranes with hydrophilic chemical compounds, which consequently would decrease their air permeability. Pulling data from our HVAC group’s latest papers, we used two different compounds for our coats: Graphene Oxide (GO) and Pebax 1657. After heating and mixing, these compounds were spray coated onto polypropylene membranes at 10 mL, 20mL, and 30 mL worth of solution. 7 membranes with area 38mm x 44 mm were created, including one uncoated for control, and placed into a porometer to measure the gas permeability. We discovered that 30 mL of Pebax and GO made an equal, strong difference in combating wettability. In the future, these membranes can be used in membrane distillation to measure the performance of their coats’ ability to combat wetting. These experiments serve as a big step in the move toward the industrialization of membrane distillation with the goal of overcoming the freshwater shortages of the world

Improved Batch Reverse Osmosis Configuration for Better Energy Effiency

Recent progress in batch and semi-batch reverse osmosis processes such as CCRO have shown the promise to be the most efficient desalination systems. Despite their progress, it is critical to further increase their efficiencies, and reduce the downtime between cycles that worsens their cost performance. In this study, we model in new detail a further improved batch desalination system that uses a high pressure feed tank with a reciprocating piston. A high-pressure pump fills the inactive side with the following cycle’s feedwater, providing two main benefits. First, no tank emptying step is needed because feed is already present, thus reducing downtime. Second, the tank fully empties each cycle, thus avoiding the small energy losses from brine mixing with the new feed that past best designs had. The modeling methodology is the most thorough yet for batch processes, as it uses a discretized module that includes transient mass transport equations for salt boundary layers, membrane permeability effects, and minute salt permeation through the membrane. Comparing the new configuration to standard reverse osmosis with and without energy recovery, the new process vastly outperforms, with the potential to be below 2 kWh/m3 for seawater. The new process has less downtime too, around 2% of cycle time, compared with 10% for CCRO or 16% from past batch studies

ULTRAPERMEABLE MEMBRANES FOR BATCH DESALINATION: MAXIMUM DESALINATION ENERGY EFFICIENCY, AND COST ANALYSIS

Reducing the energy consumption of membrane desalination is critical to reducing its cost of water and minimizing desalination’s CO₂ emissions. Hybrids of reverse osmosis (RO) with ul trapermeable membranes promise to address the efficiency, rejection, and fouling issues. In a batch reverse osmosis (BRO) process, salinity is varied over time so that the applied pressure better matches osmotic pressure, increasing efficiency. In this paper, the impact of ultrapermeable membranes in BRO are modelled, and a cost analysis is performed. The results show energetic advantages for the BRO over the best continuous RO configurations. Batch RO systems offer significant cost savings, and saves more energy than the use of ultrapermeable membranes in continuous RO systems. The two combined, BRO and ultrapermeable membranes, has the potential for the most efficient desalination systems yet proposed. However, low membrane cost is needed for ultrapermeable membranes to be viable

Investigating Membrane Material Alternatives for Air Revitalization in Space

Recently, NASA’s ultimate goal has been to launch a crewed Mars mission. However, the current system used for carbon dioxide (CO2) removal in air revitalization in the International Space Station (ISS) is not equipped to handle beyond low-earth-orbit missions. The Carbon Dioxide Removal Assembly (CDRA) is a complex system that relies heavily on sorbent materials and faces challenges in reliability, energy efficiency, and material degradation. Although the CDRA has operated well in the ISS for the past two decades, health effects from high CO2 levels are amongst the most common complaints from and challenges for astronauts. Recent developments in membrane technology prove to be a promising alternative to sorbent-based systems for CO2 removal. Maintaining high selectivity for CO2 with a reasonable permeability, at such low partial pressures and in the presence of water, is among the main challenges of using membranes in this application. In this work, we have created a membrane-based model with appropriate conditions to identify the membrane technology for this application. We expect to determine a working range of critical parameters such as permeability, selectivity, and membrane area for successful CO2 separation. We will also be comparing the thermodynamic efficiency of a membrane-based process to that of the CDRA to pin-point areas of improvement

Wetting prevention in membrane distillation through superhydrophobicity and recharging an air layer on the membrane surface

Although membrane distillation offers distinctive benefits in some certain areas, i.e., RO concentrate treatment, concentrating solutions in the food industry and solar heat utilization, the occurrence of wetting of the hydrophobic membrane hinders its potential industrial applications. Therefore, wetting prevention is a vital criterion particularly for the treatment of solutions with lower surface tension than water. The present work examines the effect of recharging air bubbles on the membrane surface for the wetting incidence when a surfactant (sodium dodecyl sulfate, SDS) exists in a highly concentrated NaCl aqueous solution. This study shows that the presence of the air bubbles on the surface of the superhydrophobic membrane in a direct contact membrane distillation setup inhibited the occurrence of wetting (similar to 100% salt rejection) even for high concentrations of the surface-active species (up to 0.8 mM SDS) in the feed solution while no undesirable influence on the permeate flux was observed. Introducing air into the feed side of the membrane displaces the liquid which partly tends to penetrate the macro porous structure with air bubbles and therefore increases the liquid entry pressure, and in addition, the simultaneous use of a superhydrophobic membrane enhances the solution contact angle

Temporally Multi-staged Batch Counterflow Reverse Osmosis for High Recovery Desalination

Osmotically assisted reverse osmosis (OARO) or counterflow reverse osmosis (CFRO) are recent RO configurations that uses saline streams on both sides of the membrane in counterflow. This reduces the osmotic pressure difference that needs to be overcome for permeation and allows water recovery from high salinity feeds at regular RO pressure. Batch RO is a new, transient RO configuration that closely follows the osmotic pressure profile of the feed and is marked by high energy efficiency. In this work we extend a transient version of CFRO, Batch CFRO for high recovery (~74%) desalination of seawater using a temporally multi-staged version of the process for the first time. In doing so, we introduce the first configuration to achieve Batch CFRO using entirely available components, including a pressure exchanger rather than high pressure tanks. Using a reduced order model, the terminal salinity of the brine leaving the system is calculated to be 183 g/kg. The key feature of this new configuration is that it is multi-staged in time rather than space. As such it can use the same hollow fiber membrane module for the different stages and hence reduce the component (pumps and pressure exchangers) count of the process. The brine produced in each stage is stored in inexpensive atmospheric pressure tanks. This is in contrast with other multi-stage processes where the number of flow devices usually scale with the number of stages needed for higher recovery and usually leads to high cost. Notably, the choice of membrane type can make a significant difference, as common narrow hollow fibers can experience large pressure drops that become significant. This leads to the conclusion that module design must be key to achieve the top-energy numbers of other batch CFRO configurations by the team, such as spiral wound membranes, turbulence-inducing spacers, or using feed on the shell side of the fibers