Mechanical Engineering

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    Wave-Powered and Zero-Discharging Membrane-Distillation Desalination System: Conceptual Design and Analysis
    (MDPI, 2022-06-13) Kim, Gyeong Sung; Hwang, Yunho
    There are many islands without full access to electricity around the world. These energy-poor regions generally have drinking water supply issues too. Renewable energy-powered desalination units can convert seawater to freshwater by using such as oceanic wave energy to mitigate the water limitation in small islands. A novel wave-powered floating desalination system (WavoWater) was proposed for easy on-site deployment and minimal environmental impact. WavoWater can produce freshwater using a vacuum-applied air-gap membrane distillation (AGMD) system, and the heat needed for the AGMD is provided through a heat pump powered by wave energy. Small-scale experiments were conducted to estimate the water generation rate of the vacuum-applied AGMD, and the WavoWater system modeling was developed based on the experimental results and wave data observed near the City of Newport, OR, USA. Fast Fourier transform was applied to estimate the wave energy spectrum in a random sea wave state. It was evaluated that 1 m-diameter WavoWater can produce 12.6 kg of fresh water per day with about 3.1 kWh of wave energy. With the performance evaluation, the aspects of zero discharging and minimal environmental impact were also highlighted for the stand-alone wave-powered desalination system.
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    Review of Thermal Energy Storage Technologies and Experimental Investigation of Adsorption Thermal Energy Storage for Residential Application
    (2013) Li, Gang; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Thermal energy storage (TES) technologies can reduce or eliminate the peak electric power loads in buildings, and utilize benefits of waste heat recovery and renewable energy. This thesis work consists of TES literature review and experimental investigation of adsorption TES. Review work includes cold storage technologies for air conditioning and subzero applications, and heat storage technologies for residential application. Different technologies involving sensible, latent and sorption TES were compared and resolutions of their issues were summarized. In addition, adsorption TES was experimentally investigated and its energy and exergy flows were analyzed to evaluate the effects of different operating parameters, such as temperature and heat transfer fluid mass flow rate for different chambers on the system performance. Finally, a computer model was developed for the adsorption heat TES system integrated with a vapor compression heat pump to assess its performance. Simulation results showed that overall coefficient of performance (COP) and exergy-based COP are approximately 3.11 and 0.20, respectively.
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    Investigation of vapor injection heat pump system with a flash tank utilizing R410A and low-GWP refrigerant R32
    (2012) Xu, Xing; Radermacher, Reinhard; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Vapor injection technique has proven to be effective in improving heat pump system performance, especially for cooling application at high ambient and heating application at low ambient temperature conditions. Recent research on vapor injection technique has been mostly focused on the internal heat exchanger cycle and flash tank cycle. The flash tank cycle typically shows better performance than the internal heat exchanger cycle. However, the flash tank cycle control strategy is not yet clearly defined. Improper system control strategy would result in undesirable amount of liquid refrigerant injected to the compressor or poor system performance. In this research work, a novel cycle control strategy for a residential R-410A vapor injection flash tank heat pump system was developed and experimentally investigated. The proposed cycle control strategy utilizes an electronic expansion valve (EEV) coupled with a proportional-integral-derivative (PID) controller for the upper-stage expansion and a thermostatic expansion valve (TXV) for the lower-stage expansion, and applies a small electric heater in the vapor injection line to introduce superheat to the injected vapor thus providing a control signal to the upper-stage EEV. The proposed control strategy functions effectively for both transient and steady-state operating conditions. As global warming has raised more critical concerns in recent years, refrigerants with high global warming potentials (GWP) are facing the challenges of being phased out. R410A, with a GWP of 2,088, has been widely used in residential air-conditioners and heat pump systems. A potential substitute for R410A is R32, which has a GWP of 675. This research work also investigates the performance difference using R410A and R32 in a vapor-injected heat pump system. A drop-in test was performed using R32 in a heat pump system that is designed to utilize R410A, for both cooling and heating conditions. Through experimentation, it was found that there was improvement for capacity and coefficient of performance (COP) using R32, as compared to an identical cycle using R410A. The compressor, heat exchangers and two-stage vapor injection cycle have been modeled and validated against experimental data to facilitate an optimization study. Heat exchangers were optimized using 5 mm copper tubes and result in significant cost reduction while maintaining the same capacity. Compressor cooling was investigated to decrease the high compressor discharge temperature for R32.