IECs (Indirect Evaporative Coolers) have emerged as a promising cooling technology due to their lower energy consumption and carbon footprint, improved efficiency, and a zero GWP alternative by avoiding the usage of refrigerants. The purpose of this research is to assess the thermal performance of an HMX (Heat and Mass Exchanger) in the form of an IEC developed by Seeley International . Indirect evaporative coolers (IEC) cool the primary air without adding humidity to the indoor environment by using the latent heat of vaporization of water in the secondary air stream. By pre-cooling the secondary air stream before extracting heat in cross-flow with the primary air, the Maisotsenko Cycle (M-Cycle) design of an IEC has the ability to approach dew-point temperatures. A sectional mathematical model was developed by linking the heat and mass transfer equations in discretized control volumes and solved in Engineering Equation Solver (F-Chart Inc.). Results from the model were validated using experimental data from lab experiments for conditions with no condensation on the primary side. The simulation findings were found to be in good agreement with the experimental data, with an error of less than 10% in the primary exit temperatures and secondary outlet humidity ratios; and less than 8% in the secondary exit temperatures. The mean average errors (MAE) reported for the primary outlet temperatures, secondary outlet temperatures and humidity ratios were 0.78 C, 1.22 C and 0.0007 respectively. A parametric study was also carried out to evaluate the effects of some factors on the performance of the IEC, namely air flow rates, primary and secondary side channel lengths and heights, separation plate thickness and plate conductivity. Results concluded that the maximum impact on the performance parameters (dew-point and wet-bulb effectiveness and heat transferred) was exhibited by primary side air flow rate, channel lengths and heights of both primary and secondary side. The study corroborates the potential of IECs as a feasible cooling device besides suggesting the optimum design parameters.

With a goal to increase the percentage of grid energy produced by renewable sources, research and development efforts in solar, wind, and geothermal resources are progressing rapidly. In this work, the use of concentrated solar thermal power for energy production and thermal energy storage is presented for the supercritical carbon dioxide (sCO2) Brayton cycle. The use of sCO2 as the working fluid for concentrated solar thermal power generation allows for greater power output potential at a smaller footprint compared to steam power cycles. The present work is targeted at the design of next generation solar thermal receivers for such power cycles. The target design conditions for an sCO2 receiver for next-generation utility scale power generation systems include a design pressure of 200 bar, a fluid outlet temperature of 720C, and an incident concentrated flux surface of 100 W/cm2. The receiver must operate at these conditions for a 30 year lifetime. To operate at the needed high temperature and pressure conditions for next-generation sCO2 power cycles, the central receiver of the solar power plant needs to be made from a high temperature and high pressure materials such as nickel superalloys.

The manufacturing method of the solar thermal receiver becomes important as both pressure stresses and thermal stresses act on the internal features of the solar receiver. In prior work by the group, a microscale pin array receiver was developed and fabricated using microlamination methods. This fabrication method restricted the size of the unit cell of the pin array to 2.5 cm, requiring an elaborate and heavy header structure to route fluid in and out of multiple parallel unit cell pin array panels that comprise the central receiver. Headers were brazed to the diffusion-bonded core to bring fluid into and out of the unit cells. Several fabrication challenges were faced in the development of this design, including leaks in the brazed joints. Odele [30] showed, by numerical modeling, that by using metal Additive Manufacturing (AM), one could design longer unit cell pin arrays, that operate at similar efficiencies as the microlaminated design, resulting in a lighter header structure. Furthermore, the pin array core and headers could be built as a single panel monolithically, resulting in the elimination of braze joints.

In this thesis, detailed design of the header and its interface with the pin array core is developed. The design goals are to reduce weight of the microlamination-based headers, reduce stresses due to the high pressure and temperature, and design for maximal receiver life to 100,000 hours. The pin array core and the headers would constitute a unit cell panel. The panel would need to be designed to allow for placement of several panels in parallel without uncooled regions being directly exposed to the intense concentrated solar radiation in a power tower configuration.

The receiver pin array core makes use of correlations from previously validated experimental data. An iterative design campaign is performed using simulations in Ansys mechanical to meet the pressure stress design requirement of 150 MPa. After eliminating stress regions exceeding the design parameters, simulations were performed in Ansys Fluent to ensure that the flow from the headers to the pin array core was uniformly distributed and to confirm that the pressure drop was below 2% through the panel. The design was subsequently printed by the Rollett group at the NEXT Manufacturing Center at Carnegie Mellon University. The prototype was heat treated and pressure tested at operating temperatures at the UC Davis STEEL facility. Results of static and limited cyclic pressure testing at temperature indicate that a leak-free panel of the design can be fabricated using metal AM. Thermal stress simulations were performed on the receiver showing a few locations with high points. To mitigate these stresses, design changes were considered to insulate the outer edges and corners of the receiver from direct insolation.

Desalination is used in over 120 countries worldwide and is a viable option for providing fresh water for many regions worldwide. The chief approaches to desalination are Thermal Desalination (TD) and Reverse Osmosis (RO). While there have been significant improvements in membrane technology to enhance the efficiency of the RO process, TD research has been relatively less developed. In this dissertation, the prospect of improving TD is discussed with a specific focus on Multistage Flash (MSF) desalination. In MSF, the brine flows through a bank of tubes in multiple stages, in which steam flashes from the preheated brine. The steam condenses on the outer surface of the tubes and is collected as fresh distillate water. Condensers and the stages are made of exotic alloys to withstand harsh operating conditions and are capital intensive. MSF desalination plants have heat transfer bottlenecks related to phase change heat transfer external to the tubes and internal flow within the tubes. In this work, the potential to reduce the footprint of the tube condenser, and hence the size of the stages, by use of multi-port minichannel condensers is explored. The condenser footprint reduction is achieved by two methods: by tightly packing condenser plates and enhancing the overall heat transfer coefficient. While the former reduces the condenser footprint, the latter directly reduces the condenser heat exchange area and hence the condenser material cost. An analytical model is developed to predict the performance of a baseline tube bank condenser and proposed multi-port minichannel condensers. A parametric study using the model is undertaken to assess the impact of variations in geometrical parameters on the multi-port condenser performance, results of which are used to optimize the condenser architecture. The results of the parametric study point to the possibility of a 90% size reduction corresponding to a 17% material cost reduction. A numerical model is developed to test the effect of plate spacing on condensation. From the results, it is concluded that for the geometry of the proposed condenser plate spacing of 6 mm does not adversely affect condensation. An experimental study on condensation using multi-port mini-channels is performed, and the results validate the model within a mean average error of 7 percent. Improvements in the performance of the condenser are observed using hydrophobic coatings. Preliminary long-term salt deposition experiments are performed for CaCO_3 to quantify the potential adverse effects of fouling on condenser performance. These initial results do not show any change in pressure drop over 48 hours of testing at 60 °C. Overall, a potential path to a reduction in the size of MSF condensers and stages is demonstrated in this work.

Liquid-to-air heat exchangers are key components in residential cooling applications. They are typically made from metal (e.g., finned tube heat exchangers, such as water coils), but there has been a drive to use polymers due to the potential to make less costly products that are not as prone to fouling and corrosion as their traditional metal counterparts. The lower thermal conductivity of polymers, however, can compromise performance. Hence, design innovations must be considered to develop heat exchangers with comparable or improved performance since extended surfaces such as fins cannot be relied upon to enhance heat transfer. This research describes the design, fabrication, and experimental characterization of a few iterations of a novel, high-effectiveness Microchannel Polymer Heat eXchanger (MPHX). This heat exchanger implements a counterflow configuration that, while not prevalent in the literature due to the challenges associated with preventing water headers from obstructing airflow, has the potential to enhance the heat transfer over traditional water coils (which are arranged in crossflow). The microchannels for fluid flow increase the heat transfer surface area and thin walls address the issue of the lower thermal conductivity of polymers. Another advantage of the final iteration of the MPHX design presented is that it is easy to scale up or down to suit various applications. A sectional model accounting for potential condensation in the air stream is developed and validated with experimental data. The model predicts performance well for the first-generation MPHX, although it shows greater divergence from the second-generation MPHX, possibly due to limitations of the model and the experimental measurements. Overall, heat transfers of up to 433 W and 886 W, respectively, were achieved for the first- and second-generation MPHX units. A parametric study is performed to examine the impact of varying different parameters which could help inform future design iterations. The sectional model is implemented in a case study in conjunction with a Sub-Wet bulb Evaporative Chiller (SWEC) to assess the system performance of an energy-efficient cooling scheme for dry climate conditions. Additionally, a process-based cost model is developed for the MPHX to evaluate the viability and cost-competitiveness of the design, as well as identify opportunities to reduce the cost.

Phase Change Materials (PCMs) have the potential to reduce the cooling and heating energy consumption of buildings. The high latent heat of PCMs helps them to absorb and release large amounts of energy when the PCM changes phase from solid to liquid and from liquid to solid respectively. The climatic conditions and the thermophysical properties of the PCMs play a crucial role and thus, understanding the impact using a particular PCM in a particular climatic condition needs to be understood thoroughly before adding PCM to the building envelope. Previous research has shown that addition of PCM to building envelopes does not always reduce energy consumption.

In this research project, the performance and heat transfer characteristics of two PCMs integrated in a typical residential wall in California was characterized using simulations. PCM 1 is a spackle type product that can be applied to the inner wall and consists of 53% PCM. PCM 1 is a blend of different PCMs and has a melting range from 280K (6.85˚C) to 300K (26.85˚C). PCM 2 is a salt based PCM and is encapsulated inside panels of plastic material. PCM 2 has melting 294.15K (21˚C) to 294.95K (21.8˚C). The typical residential wall with no PCM integrated was used as a baseline and a total of 8 PCM-integrated wall designs were studied using simulations, and their impact on reducing cooling and heating energy consumption in comparison to the baseline case was analyzed. The Sacramento weather was used in these studies and representative days of Winter, Summer, and Shoulder were tested. Addition of PCM 1 to the inner wall surface did not produce any significant energy savings for the Winter day and Shoulder day. But adding PCM 1 to the inner wall surface produced 6.7% drop in the cumulative heat leaving the inner wall surface and entering the house for the Summer day and thus, has the potential to reduce cooling energy consumption. Addition of PCM 2 to the inner wall surface does not reduce the energy consumption of the Winter day and the Summer day to have any significant impact. PCM Next to Studs was a design tested where the PCM was placed between the stud and the insulation. PCM inside Insulation+Stud Layer was another design tested where cubes of PCM were uniformly distributed inside the Insulated+Stud blended layer. For the PCM Next to Studs and PCM inside Insulation+Stud layer the behaviors of PCM 1 and PCM 2 are significantly different but results in reduced heating energy consumption for the two Winter representative days simulated. The results indicated that the for all the different wall designs tested in the Sacramento weather no significant energy savings are generated by use of the two specific PCMs characterized in this study.

Design, fabrication and experimental characterization of the performance of a molten salt (MS)-to-supercritical carbon dioxide (sCO2) heat exchanger (HE) for concentrating solar power (CSP) applications using laser powder bed fusion (L-PBF) additive manufacturing (AM) of nickel-based superalloys is presented in this work. The hypothesis is that such additively manufactured HEs will result in a monolithic, compact, low-pressure drop HE that is durable under cyclic operation at high temperature and high pressure in a corrosive salt environment, making them suitable to withstand the stringent operating conditions of the US Department of Energy’s Generation 3 technologies roadmap for concentrating solar power systems.The primary heat exchanger (PHE) is designed to handle temperatures up to 720 oC on the MS side and an internal pressure of 200 bar on the sCO2 side. Two designs (generation 1 and 2) of the heat exchanger were developed by a co-design effort considering mechanical stresses, thermal and fluidic performance, and AM fabrication constraints such as overhangs, powder removal, and build dimensions. Additional design considerations included creep, fatigue, and thermal stresses. Two Nickel-based superalloys were originally considered to manufacture the PHE- Haynes 230 (H230: 57% Nickel, 22% Chromium, 14% Tungsten, 2% Molybdenum) and Haynes 282 (H282: 57% Nickel, 20% Chromium, 10% Cobalt, 8.5% Molybdenum, 2% Titanium). Ultimately, Haynes 282 was down selected for the HE fabrication based on its better dimensional tolerance, better corrosion resistance to chloride salt, and higher creep strength. An experimentally-validated, correlation-based sectional PHE core thermofluidic model is developed to study the impact of flow and geometrical parameters on the PHE performance, with varied parameters including the mass flow rate, surface roughness, and PHE dimensions. The model results show that a heat exchanger with a power density of 16 MW/m3 (including sCO2 header volume) and effectiveness of 0.93 can be achieved at a heat capacity rate ratio of 0.83. To experimentally demonstrate the performance of the HE, a 20 pair unit was built and characterized using heated air as a surrogate to the chloride salt on the hot side and sCO2 in the 100-200 bar range was used on the cold side. The results of exit temperatures and heat transfer rate were compared against the model. The model was able to predict experimental results within 1.00% percent for heat transfer rate, 1.26% percent for exit sCO2 temperature, and 1.01% percent for exit air temperature. The validated HE model was used to determine the performance of a scaled-up design. With a 20 cm tall build that covers 24 cm x 24 cm of the build plate, a 167.5 kW HE can be built in a single laser machine with a bed size of 25 cm x 25 cm. The impact of system-level variations in inlet temperature on the performance of this HE was established.

Concentrated solar power is being investigated by the U.S. Department of Energy (DOE) as a renewable energy source for meeting baseload and peak load electric power. The gas receiver pathway, utilizing supercritical CO2 as the working fluid, has been identified as a potentially viable source of implementing CSP technologies because of recent interest in supercritical carbon dioxide Brayton cycles [1]. In past studies, lab-scale experiments and on-sun test done on a micro-pin-array solar receiver utilizing sCO2 as the heat transfer fluid have shown the ability to absorb heat flux up to 100W/cm2 at thermal efficiency above 90 percent[2], [3]. The feasibility of using microscale unit cells, as building blocks for a megawatt-scale (250MW thermal) open solar receiver through a numbering-up approach, where multiple microscale unit cells are connected in parallel, has been explored by Zada et al.[4]. In the previous studies, a microlamination approach (in which the pin array is chemically etched in a sheet and diffusion bonded to a flux plate) is used in the manufacture of these receivers. The limitation of this manufacturing method has a consequence in limiting the pin array length of the receiver. These short pin array lengths would require more unit cells in the receiver, and thus a more complicated header system. Also, a notable assumption from previous studies is that a uniform heat flux is imposed on the receiver module(s) over a period, whereas in a heliostat field, the flux distribution is highly non-uniform and varies temporally and spatially. In this study, additive manufacturing of the receiver is presented as an alternative means of manufacturing the receivers. Additive manufacturing enables longer pin array lengths, thus reducing the complexity and mass of the header network. In order to study the performance of the receivers, and account for the spatial variation of properties throughout the receiver, a numerical code with a 2-dimensional discretization is developed and the performance of the additively manufactured pin array receiver (AM2PAR) and the microlaminated pin array receiver (µLPAR) are compared at their respective design lengths. Furthermore, this work explores the impact of non-uniform heat flux on a AM2PAR central tower receiver. The heat flux data for a select geographical location has been modeled with NREL's SolarPILOT to model hourly heat flux distribution over a central receiver on a typical hot summer day. The flux data from SolarPILOT is used in numerically modeling the thermal and hydraulic parameters in the modules of a central receiver with an area of 250m2 (7.5m radius and 10m height). The non-uniformity effects are compared with the results of a uniform-flux model. The study results present the estimated mass flow distribution for the novel receiver design needed to heat supercritical CO2 from 550℃ to 720℃ with a maximum permissible pressure drop of 4 bar. The central receiver's surface temperature distribution is assessed to highlight non-uniformity in surface and fluid temperature distribution due to the flux non-uniformity. Thereafter, a novel receiver concept of using variable height pin arrays: VPH-AM2PAR is explored. In such a receiver, the modules will have varied pin heights in accordance with the respective flux on the module. It is shown that the VPH- AM2PAR is effective in reducing the peak surface temperature on the receiver. The study addresses the thermofluidic operating adjustments required and the total thermal power generated to ensure the creep life of the receiver meets a 30 year lifetime requirement.

Passive two-phase fluid cooling systems are of major interest for cooling electronics in terrestrial and micro gravity environments due to their compactness and minimization of moving parts. In this thesis, the use of asymmetry for bubble ebullition and growth on a heated surface to passively generate lateral motion of fluid within an open-ended channel is discussed. The asymmetry is achieved by locating reentrant slot cavities on one face of a mm-scale 30/60-degree ratchet. Two such ratcheted walls with cavities in every third ratchet form the vertical walls of an open-ended channel. The ratcheted walls are heated using a serpentine thick-film metallic heater. The open-ended channel is located within a quiescent pool of a dielectric fluid. Visualization studies show that bubbles and slugs tend to move in a preferential direction within the open channel. This direction corresponds to the 30-degree slope face of the ratchet, in which the reentrant cavity is located. Prior studies have alluded to two potential mechanisms for this preferential lateral motion- (1) bubble pump model developed by Kapsenberg et al. [14], which attributes the lateral motion to the momentum imparted by the growing bubble to the surrounding liquid, and (2) asymmetry in the curvature of the slug that spans several ratchet lengths, resulting in a net surface tension force along the 30-degree slope of the ratchet. In order to assess the importance of the bubble pump model, bubble ebullition and growth from the nucleation sites for different heat fluxes and subcooling temperatures are captured using high speed videos and are analyzed using custom image processing of high-speed videos. The major purpose of the image processing is to detect the bubbles in the frames and obtain height and diameter of the bubbles attached to the ratchet in each frame. Active contouring and segmentation techniques are used to detect bubbles in the frames. The velocity imparted by the growing bubble on the ratchet to the surrounding liquid is calculated using this data with the semi empirical model of Kapsenberg et al. This predicted velocity is compared against velocities of detached small (Stokes) bubbles in the field of view obtained using particle (bubble) tracking velocimetry. This comparative analysis has revealed that the semi empirical formula predicts the horizontal velocity imparted by the growing bubble on the ratchet with a deviation of ±15% of the horizontal velocity obtained from the particle tracking velocimetry except for high subcooling mid heat flux condition. Overall, it was observed that around 20-30 mm/s of lateral velocity is imparted in the surrounding fluid by the bubble growing on ratchet. These results validate the bubble pumping model and the semi empirical formula developed by Kapsenberg in the nucleate boiling regime.