Sorption-Based Systems for Sustainable Carbon Dioxide Capture and Humidity Pumping from Indoor Spaces

Abstract

With the majority of human activities conducted indoors, the need for better air quality in occupied enclosed spaces has become more stringent. Achieving such acceptable indoor air quality (IAQ) levels is very challenging since it involves the control of a wide range of gaseous species such as water vapor, CO2 and VOCs to within healthy ranges. Among these species, special care should be given to water vapor and CO2. On one hand, elevated CO2 concentrations deteriorate the cognitive well-being of occupants. On the other hand, inadequate indoor air humidity levels damage the building structure and hinder the occupants perceived thermal comfort. Conventionally adopted techniques resorted to the use of large amounts of cooled and dehumidified outdoor air to dilute indoor generated H2O and CO2. These techniques are known to be energy intensive, especially in regions with hot and humid climates. Therefore, this thesis work aims towards designing different energy-efficient alternatives to maintain the needed IAQ in occupied spaces and is divided into two main parts.

The first part tackles different techniques to maintain indoor CO2 levels within healthy ranges. Air revitalization system (ARS) is proposed as an alternative solution, where excess H2O and CO2 are sequentially removed from the recirculated air using total volume ventilation (TVV) that integrates solid adsorbents such as silica gel for H2O and metal organic frameworks-based (MOFs) for CO2. This reduces the amount of outdoor air intake to ultra-low minimum levels needed to maintain indoor O2 and VOCs levels. This has been especially appealing since the heat of adsorption of CO2 is lower than that of H2O, which results in lower regeneration energy. Another system is proposed to affect the indoor air treatment (carbon capture + dehumidification) in a localized manner using a multi-functional personalized ventilator (MFPV). A smaller amount of indoor air is thus treated and supplied to the occupant breathing zone (microclimate) while its surrounding air (macroclimate) has relaxed conditions of species concentration. This reduces the needed mass of adsorbent and consequently the system size and energy consumption. The system is integrated within thermoelectric cooling modules that provide simultaneously the cooling needs for the supply air and the regeneration heat of the adsorbent.

The second part tackles different techniques that are adopted to regulate the indoor humidity levels without supplying dried outdoor air. Instead, the water vapor is directly pumped from space using hygroscopic materials that act as active moisture buffering materials. In the first proposed system (SHP), direct removal of the indoor generated moisture is conducted via moisture migration through an actively ventilated building façade. The system integrates a MOFs desiccant dehumidifier to supply dry air to the ventilated building façade provided with water permeable insulation. This creates a water vapor pressure gradient between the indoor and dry air to actively drive the indoor moisture to the outdoor environment, irrespective of its humidity conditions. In the second system (LHP), the water moisture is pumped outdoors using a thermosyphon-driven membrane-based liquid desiccant system without active cooling. In this system, indoor water vapor is directly absorbed by the liquid desiccant which is circulated using thermosyphon mechanism. The latter makes use of the density gradient induced by the temperature difference of the desiccant between the dehumidifier and regenerator sides of the system. Furthermore, new liquid absorbents, such as weak acids, have been developed to absorb water vapor at elevated temperatures compared to conventional halide salts. As a result, the desiccant solution does not require active cooling prior to entering the indoor space, reducing the system complexity.

For the different proposed systems, mathematical models are developed for the heat and mass transfers in the different components, which are then validated with published data in the literature as well as in-house experiments conducted at the American University of Beirut. The models are used to properly size and operate the systems to meet the occupied space IAQ constraints at minimal initial and running costs. The performance of each system is then compared to the conventional systems that use vapor compression cooling systems either as standalone systems or integrated with desiccant dehumidifiers to determine their impact on the building energy consumption.

On one hand, the ARS-TVV was evaluated for high (classroom), and low (residential) buildings located in the predominantly hot and humid climate of Qatar. A comparative economic analysis showed that, over the entire hot season, the ARS-TVV decreased the outdoor air intake by 91 % and 71 %, for the classroom and house, respectively, leading to savings of 30 % and 24 % in the operating costs and CO2 emissions as well as payback periods of 5 and 2 years compared to the conventional system. The decentralized MFPV with a co-flow air terminal was tested for office spaces application. Different numerical simulations were conducted to determine the optimal system operation. The latter was achieved with a purge-to-supply flowrate of 40 %, adsorbent mass of 125.3 g of Lewatit® VP OC 1065, along with two TEC modules with a power of 7.2.8 Wh and a cycle time of 7.5 min. The MFPV resulted in 22 % and 58 % energy savings and CO2 emission reduction compared to conventional co-flow and single flow PV system using treated outdoor air, respectively.

On the other hand, the SHP system performance was simulated for a case study of a space with high latent load located in the hot and humid climate of Beirut. A comparative analysis with the conventionally used hybrid desiccant – vapor compression cooling system showed that the proposed system resulted in 66 % reduction in the investment cost and 86 % in the operating cost. The performance of the LHP was evaluated through a parametric analysis under humid and dry climate conditions to determine the effect of the channel height and air conditions surrounding the system. It was found that the system was able to pump the air moisture from high to low and low to high humidity areas. Four cases were considered with outdoor conditions raging from moderate and hot and humid to semi-arid climates. The latent load removal density per unit area of the exposed membrane varied between 4 W/m2 to 35 W/m2 over the entire range of considered cases at required heat inputs varying between 150 W to 570 W respectively. The heat input increased with the channel height and the air humidity surrounding both system sides. The added sensible load depended on the heat sink temperature and was more than 65 % lower than removed latent load in most considered cases.