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

Vapor-selective active membrane energy exchanger for high efficiency outdoor air treatment

As much as 40% of the total load on air conditioning systems can be attributed to condensation dehumidification. However, new water vapor-selective membranes present a unique opportunity to greatly reduce the power requirements for moisture removal by avoiding phase change and have thus been ranked as a top alternative to traditional HVAC systems. To date, however, all such systems have relied on the assumption of constant temperature, even terming the technology “isothermal dehumidification.” This work proposes a membrane-based air cooling and dehumidification approach, referred to as the Active Membrane Energy Exchanger (AMX), which is the first to provide simultaneous, yet decoupled, air cooling and dehumidification. The suggested AMX configuration uses two vapor-selective membrane modules with a water vapor compressor in between them, using the second membrane module to reject vapor into the exhaust stream. Cooling and heating coils in each membrane module channel move heat between the air streams using a vapor compression cycle. A detailed steady-state, thermodynamic model is presented for the AMX integrated within a 100% outdoor air conditioning system. The AMX’s limiting parameters and design considerations like compressor efficiency are systematically analyzed for a broad range of outdoor air conditions and compared against standard and state-of-the-art dedicated outdoor air systems. This new high efficiency approach is found to outperform all other standard and state-of-the-art systems, achieving 1.2–4.7 times the COP over conventional dedicated outdoor air treatment. Lastly, a building simulation case study predicted cooling energy savings as high as 66% in hospital buildings with 100% outdoor systems in hot, humid climates

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