Simulation of Seasonal Performance of a Membrane Heat Pump System in Different Climate Regions

Membrane based heat pumps systems have attracted the attention of many research groups as a potentially more environmentally friendly alternative to conventional vapor compression systems that are being used for space cooling in 90% of the buildings in the United States. A membrane heat pump essentially combines an indirect evaporative cooler with a vacuum dehumidification process to provide sensible and latent cooling to a conditioned space. Membrane heat pumps potentially consume significantly less energy during the dehumidification process and do not use any refrigerant other than water, thereby eliminating the need of refrigerant compressor and the need for high global warming potential working fluids. Furthermore, membrane based systems effectively decouple latent and sensible cooling functions; allowing a possible novel control strategy to maintain thermal comfort as a function of dry bulb and relative humidity versus controlling to a fixed dry bulb temperature only. These features can lead to significant energy savings and increase in the system energy performance. Several prototypes systems have been developed, with reported claims of EER of 26. However, no detailed analysis is publically available which demonstrates the capability of these systems in different climate zones. Thus, the objective of this paper is to simulate seasonal performance in different climate regions within the United States. This is accomplished by developing a full thermodynamic cycle model of a representative membrane heat pump system, and then sizing the heat and mass transfer components to provide 5 tons of cooling at nominal rating conditions. Then, using the designed system, the seasonal performance of the system in different climate zones in the United States will be investigated. Energy performance ratings such as COP and EER as a function of time of year and location, as well as other utility parameters such as energy cost savings and annual water consumption will be evaluated. Finally, performance will be compared to conventional vapor compression systems

Parametric Evaluation of Governing Heat and Mass Transfer Resistances in Membrane Based Heat and Moisture Exchangers

To provide a healthy environment inside buildings, there must be some exchange of indoor conditioned air with fresh outdoor air. The outdoor air is then mechanically conditioned to a comfortable temperature and humidity. Research suggests human health and productivity increase with the amount of fresh air available in buildings. This conflicts with the desire to reduce building energy use since the conditioning process is energy intensive, especially in warm and humid climates. While sensible heat recovery using a standard heat exchanger is of some value, much of the energy consumption is due to dehumidification of air. More substantial reductions in energy consumption can be obtained by preconditioning the supply air with the previously conditioned exhaust air using a porous polymer membrane heat and moisture exchanger. As membrane technology improves, the convective heat and mass transfer resistances in the airstream can become the dominant resistance in these devices. This requires potentially new flow passage architecture to optimize heat and mass transfer, while maintaining acceptable pressure loss. Thus, the objective of this study is to develop an analytical model of a counterflow membrane based heat and mass exchanger with different internal flow geometries using the Engineering Equations Solver (EES) platform. The exchanger consists of multiple supply and exhaust air streams flowing in counterflow, separated by a thin membrane layer. The air flow passages are either a bare high aspect rectangular channel, or a high aspect channel with a pin-fin spacer inserted. The model is discretized along the length of the exchanger to more accurately calculate the air and vapor properties along the exchanger. In each segment the model considers the coupled convective heat and mass transfer resistances in each air stream. Heat and mass transfer coefficients are related through the Chilton and Colburn analogy, while representative values for membrane thermal conductivity and effective diffusivity are obtained from the literature. Conservation of energy and mass in each segment provides closure to the model. The model is then used to parametrically evaluate the effect of various exchanger dimensions and operating conditions on the dominant heat and mass transfer resistances, sensible and latent effectiveness, and pressure loss of the exchanger. The primary dimensions considered are hydraulic diameter and geometry (i.e., high aspect versus pin filled channels) of the channels, number of channels, membrane thickness and overall length, width and depth of the exchanger. The effect of operating parameters including air flow rate, water diffusivity of the membrane and allowable pressure drop on system performance and dominant transport mechanisms are also explored.

Measurement of Condensation Heat Transfer and Pressure Drop for Zeotropic Mixture R-454C and its Components R-32 and R-1234yf in a Horizontal Microfin Tubes

This paper compares the condensation heat transfer and pressure drop for zeotropic refrigerant R454C and its individual components, R32 and R1234yf, in a horizontal microfin tube. The microfin tube has a 4 mm outer diameter, 0.18 mm wall thickness, and a surface area ratio of 1.56. HFOs and HFC/HFO blends like R454C have low global warming potential and can be alternatives to HFC refrigerants when retrofitting a system or producing new equipment. However, there is an additional mass transfer resistance present during phase change for a zeotropic mixture, which results in reduced heat transfer performance. Microfin tubes enhance heat transfer through multiple mechanisms: they increase the internal surface area of the tube, the fins drain condensate from the fin tip to the trough region, and they produce secondary flow structures. Presently, there is limited data of HFO/HFC mixtures in microfin tubes. Thus, experiments are conducted for complete condensation of R454C, R1234yf and R32 for saturation temperatures of 40, 50 and 60 °C and mass fluxes from 100 to 600kgm−2s−1. Experimental heat transfer and pressure drop measurements are compared to wellestablished correlations from the literature. Heat transfer enhancement factors and pressure drop penalty factors are calculated for each refrigerant

Thermal Performance Evaluation of a Residential Solar/Gas Hybrid Water Heating System

In climate regions with lower average daily solar radiation, such as the Pacific Northwest, a solar energy collector might not economically satisfy year-round domestic water heating demands, requiring an auxiliary unit, such as a natural gas water heater. Previous studies of such hybrid systems have shown that the efficiencies achieved while running in combined solar/gas mode was lower than expected. This inefficiency was attributed to a reduction in gas burner efficiency when the process fluid was partially pre-heated by the solar input. To predict the actual energy and cost savings under various design conditions, the performance of solar/gas hybrid systems must be better understood. In this work, the performance of a commercial hybrid solar/gas system is experimentally characterized to evaluate individual component and overall system efficiency. The hybrid water heating system consisted of three flat plate collectors arranged in series (total area = 6.44 m2), and a 22.3 kW natural gas burner. Under different temperature lifts and solar insolation values, the system was operated at three different modes of heating: solar, gas, and combined solar/gas mode. Efficiency value for each mode was calculated. Based on the experimental efficiency results, a configuration that would provide higher efficiency for combined solar/gas heating is suggested

Conceptual Design of a Manufacturing Process for an Automotive Microchannel Heat Exchanger

Calls for higher fuel efficiency in the United States and Europe are driving the need for waste heat recovery in automotive markets. While conventional heat exchangers can be designed to meet the heat duty requirement, the resulting volume, weight, and thermal mass are too large for rapid transient response and packaging of the device. The lightweight, compact form factor of microchannel heat exchangers with submillimeter flow passages is attractive for automotive applications. However, the industrial use of microchannel heat exchangers continues to be inhibited by high manufacturing costs. The objective of this paper is to develop a microchannel heat exchanger concept capable of meeting the cost and performance goals for an automotive application. So-called printed-circuit microchannel heat exchangers are produced using a stacked-lamina approach in which individual metal laminae are photochemically machined and diffusion bonded. Here, the conceptual design of a microchannel heat exchanger produced using more conventional stamping and joining technologies is discussed for an automotive application. The device is sized to provide waste heat recovery from an exhaust stream to engine coolant for a representative passenger vehicle with acceptable pressure loss. Using the specified design, a process-based cost model is presented showing cost modeling efforts to date including the capital investment and cost-of-goods-sold as a function of annual production volume. The initial results show a pathway for the cost effective integration of compact microchannel heat exchangers into advanced vehicle thermal management systems

Simulation of membrane heat pump system performance for space cooling

Membrane based heat pumps systems have attracted the attention of many research groups as a potentially more environmentally friendly alternative to vapor compression systems for space cooling. Several prototype systems have been developed, with reported claims of Energy Efficiency Ratios (EER) approaching 26. At present, no detailed analysis is publicly available which simulates the capability of these systems in different climate zones. In this work, a thermodynamic cycle model of a representative membrane heat pump system is developed and heat and mass transfer components are sized to provide 3 t of cooling at nominal rating conditions. The performance of the 3 t system is then simulated in various climate regions using representative cooling loads calculated from a building energy modeling software. The simulated membrane heat pump system had an EER of 16–20. Compared with the performance of a commercial vapor compression system, the membrane system performs better at higher dry-bulb temperature and shows energy saving potential in all climate regions examined. While the membrane system shows potential from a thermodynamic perspective, there are many practical challenges that must be addressed prior to commercialization including the design of an efficient vacuum pump, mitigating membrane fouling and reliability issues, and developing advanced controls to maintain desired sensible and latent heating capacities

Optimization of 3D printed liquid cooled heat sink designs using a micro-genetic algorithm with bit array representation

The advancement of additive manufacturing enables significantly greater freedom of design compared to conventional techniques. Of significant interest is the potential improvements in the design of cold plates and heat sinks for electronics cooling. Greater design freedom could enable new designs that reduce thermal resistance and hydraulic resistance, enabling the usage of higher power systems while maintaining an equivalent heat sink volume. Parts made by using additive manufacturing are built layer by layer, and can be conceptualized as a bit array in a 2D plane. Therefore, this work explores the feasibility of combining computational fluid dynamics and micro-genetic algorithms to produce optimum shapes for liquid cooled heat sinks represented as bit arrays. A 50 mm × 10 mm area is discretized into a grid space. An initial bit array geometry is developed using a heat flux dependent probability function based on two representative chip power distribution profiles. One distribution is a symmetric flux map, and the other uses a non-symmetric map, more representative of a real processor. The performance of generated designs is determined using a commercial computational fluid dynamics code, after which an optimization algorithm is applied. Solution ranking in each generation is based on the system entropy generation rate, which is dependent on the fluid pressure drop and overall heat sink thermal resistance. Crossover, mutation and elitism operations are used to create new generations of designs. Results after one hundred generations are compared with a baseline straight finned cold plate to determine the advantages of designing for an unrestricted manufacturing process. Results indicate that the optimization methods reduced entropy generation rate by 26.4% and 21.7% for the symmetric and non-symmetric power maps, respectively

Numerical Analysis of Time Required for De-stratification in Warehouses

A significant source of energy consumption comes from maintaining desired indoor environment conditions in warehouses and other industrial facilities. To combat the raising energy costs, studies into more efficient heating and cooling strategies has been a topic of consideration for a number of years. One of the areas of investigation is the implications of a thermally stratified environment. In heating, removing the stratification phenomena has been linked to savings in the cost of fuel to heat an environment. Whereas in cooling a highly stratified environment is desired. The primary method of de-stratification is the utilization of ceiling fans. The use of fans reduces the overall savings of de-stratification for heating purposes. A solution to offset the reliance on grid power is the use of solar powered ceiling fans. The challenge with utilizing solar power during heating seasons is a reduction in the time the sun is available to charge and store energy to run the fans. While there are studies on the impact of thermal stratification, with air as a medium in an indoor environment, there is a lack of information on the frequency at which the ceiling fans need to operate to maintain a de-stratified environment. The determination of a fan operation frequency, to maintain a de-stratified environment, informs potential designers on the viability of installing solar powered fans as an alternative to grid powered fans. In the event that solar powered fans were not a viable option, it also provides information on the frequency that a grid powered fan would need to run to maintain de-stratification. To determine a fan operating frequency, a numerical analysis will be performed. This numerical analysis will assess the time required to de-stratify an environment based on inputs such as flow rate and spatial considerations. In order to establish the quality of the numerical analysis, two experiments have been conducted to observe the impact of de-stratification. One experiment is located at a large retail warehouse distribution center, the other is a small classroom. The data collected from these experiments will be compared to the models developed to predict the change in stratification with time

High-Flux Thermal Management With Supercritical Fluids

A novel thermal management approach is explored, which uses supercritical carbon dioxide (sCO2) as a working fluid to manage extreme heat fluxes in electronics cooling applications. In the pseudocritical region, sCO2 has extremely high volumetric thermal capacity, which can enable operation with low pumping requirements, and without the potential for two-phase critical heat flux (CHF) and flow instabilities. A model of a representative microchannel heat sink is evaluated with single-phase liquid water and FC-72, two-phase boiling R-134a, and sCO2. For a fixed pumping power, sCO2 is found to yield lower heat-sink wall temperatures than liquid coolants. Practical engineering challenges for supercritical thermal management systems are discussed, including the limits of predictive heat transfer models, narrow operating temperature ranges, high working pressures, and pump design criteria. Based on these findings, sCO2 is a promising candidate working fluid for cooling high heat flux electronics, but additional thermal transport research and engineering are needed before practical systems can be realized

Evaluation of Heat and Mass Transfer Models for Sizing Low-Temperature Kalina Cycle Microchannel Condensers

Waste heat driven ammonia/water Kalina cycles have shown promise for improving the efficiency of electricity production from low-temperature reservoirs (T < 150 °C). However, there has been limited application of these systems to utilize widely available, disperse, waste heat streams for smaller scale power production (1–10 kWe). Factors limiting increased deployment of these systems include large, costly heat exchangers, and concerns over safety of the working fluid. The use of mini- and microchannel (D < 1 mm) heat exchangers has the potential to decrease system size and material cost, while also reducing the working fluid inventory, enabling penetration of Kalina cycles into these new markets. However, accurate methods of predicting the heat and mass transfer in microscale geometries must be available for designing and optimizing these compact heat exchangers. In the present study, the effect of different heat and mass transfer models on the calculated Kalina cycle condenser size is investigated at representative system conditions. A detailed heat exchanger model for a liquid-coupled microchannel ammonia/water condenser is developed. The heat exchanger is sized using different predictive methods to provide the required heat transfer area for a 1 kWe Kalina system with a source and sink temperature of 150 °C and 20 °C, respectively. The results show that for the models considered, predicted heat exchanger size can vary by up to 58%. Based on prior experimental results, a nonequilibrium approach is recommended to provide the most accurate, economically sized ammonia/water condenser