Brian M. Fronk

Evolution of the Transition to a World Driven by Renewable Energy

The worlds energy supplies will continue to be pressured as the population grows and the standard of living rises in the developing world. A move by the rest of the world toward energy consumption rates on par with the United States is most probably unsustainable. An examination of population trends, current energy utilization rates, and estimated reserves shows that a major worldwide transition to renewable resources is necessary in the next 100 years. This paper examines one possible scenario of how energy usage and renewable power generation must evolve during this time period. As the global standard of living increases, energy consumption in developing nations will begin to approach that of the developed world. A combination of energy conservation and efficiency improvements in developed nations will be needed to push the worldwide energy consumption to approximately 200 million Btu per person per year. Fossil fuel resources will be exhausted or become prohibitively expensive, necessitating the development of renewable energy resources. At this projected steady state population and energy consumption, the required contribution of each type of renewable resource can be calculated. Comparing these numbers to the current renewable capacities illustrates the enormous effort that must be made in the next century.

Transcritical carbon dioxide microchannel heat pump water heaters: Part I – validated component simulation modules

Abstract An experimental and analytical study on the performance of carbon dioxide heat pumps for water heating was conducted. The performance of compact, microchannel, water-coupled gas coolers, evaporator, and suction line heat exchanger (SLHX) were evaluated in an experimental facility. Analytical heat exchanger models accounting for the flow orientation and changing CO 2 thermophysical properties were developed and validated with data. Heat transfer coefficients were predicted with correlations available in the literature and local heat duty calculated using the effectiveness-NTU approach. The gas cooler, evaporator, and SLHX models predicted measured heat duties with an absolute average error of 5.5%, 1.3%, and 3.9%, respectively. Compressor isentropic and volumetric efficiency values were found to range from 56% to 67% and 62%–82%, respectively. Empirical models for compressor efficiency and power were developed from the data. The resulting component models are implemented in a system model in a companion paper (Part II).

Single- and Multi-Constituent Condensation of Fluids and Mixtures with Varying Properties in Micro-Channels

The relative magnitudes of the forces governing the transfer of heat, mass, and momentum during microscale condensation are fundamentally different from those in macroscale geometries, primarily due to the increasing importance of surface tension. A systematic series of experiments on condensation flow regimes, pressure drop, and heat transfer was conducted using innovative visualization and measurement techniques for condensation of synthetic and natural refrigerants and their azeotropic and zeotropic mixtures through micro-channels with a wide range of diameters (0.1 < DH < 5 mm), shapes, and operating conditions. These experiments resulted in flow-regime-based heat transfer and pressure drop models with very good predictive capabilities for such micro-channel geometries.

Heat Transfer and Pressure Drop During Condensation of Ammonia in Microchannels

An experimental investigation of condensation heat transfer and pressure drop of ammonia flowing through a single, circular, microchannel (D = 1.435 mm) was conducted. The use of ammonia in thermal systems is attractive due to its high latent heat, favorable transport properties, zero ozone depletion (ODP), and zero global warming potential (GWP). At the same time, microchannel condensers are also being adopted to increase heat transfer performance to reduce component size and improve energy efficiency. While there is a growing body of research on condensation of conventional refrigerants (i.e., R134a, R404A, etc.) in microchannels, there are few data on condensation of ammonia at the microscale. Ammonia has significantly different fluid properties than synthetic HFC and HCFC refrigerants. For example, at Tsat = 60°C, ammonia has a surface tension 3.2 times and an enthalpy of vaporization 7.2 times greater than those of R134a. Thus, models validated with data for synthetic refrigerants may not predict condensation of ammonia with sufficient accuracy.The test section consisted of a stainless steel tube-in-tube heat exchanger with ammonia flowing through a microchannel inner tube and cooling water flowing through the annulus in counterflow. A high flow rate of water was maintained to provide an approximately isothermal heat sink and to ensure the condensation thermal resistance dominated the heat transfer process. Data were obtained at mass fluxes of 75 and 150 kg m−2 s−1, multiple saturation temperatures, and in small quality increments (Δx∼15–25%) from 0 to 1. Trends in heat transfer coefficients and pressure drops are discussed and the results are used to assess the applicability of models developed for both macro and microscale geometries for predicting the condensation of ammonia.Copyright

Application of Microchannel Condensers for Small Scale Kalina Waste Heat Recovery Systems

Thermally driven ammonia/water Kalina cycles have shown some promise for improving the efficiency of electricity production from low temperature reservoirs (T < 200°C). However, there has been limited application of these systems to exploiting widely available, disperse, waste heat streams for smaller scale power production (∼ 1 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 cost, while also reducing the working fluid inventory, enabling penetration of Kalina cycles into these new markets.To demonstrate this potential, a detailed heat exchanger model for a liquid-coupled microchannel ammonia/water condenser is developed. The heat exchanger is sized to provide the required heat transfer area for a 1 kWe Kalina system with a source and sink temperature of 150° and 20°C, respectively. An additional constraint on heat exchanger size is that the fluid pressure loss is maintained below some threshold value. A parametric analysis is conducted to assess the effect of different correlations/models for predicting the underlying heat and mass transfer and pressure drop of the ammonia/water mixture on the calculated heat exchanger area. The results show that accurately minimizing the size of the overall system is dependent upon validated zeotropic heat and mass transfer models at low mass fluxes and in small channels.Copyright

Evaluation of governing heat and mass transfer resistance in membrane-based energy recovery ventilators with internal support structures

Energy recovery ventilators are an effective way to reduce energy consumption in buildings while satisfying ventilation standards. As membrane technology and manufacturing methods improve, membrane-based energy recovery ventilators are seeing increased market penetration. In the present study, the feasibility of membrane energy recovery ventilators designed to fit in a commercial building wall cavity is investigated. A heat and mass transfer resistance and pressure drop model is developed for a low aspect ratio (width/height) exchanger and is used to evaluate the sensible and latent effectiveness of a counter-flow energy recovery ventilator with internal support structures. The performance of strip-fin and pin-fin structures are compared and dominant heat and mass transfer resistances are investigated. It is shown that for all cases the sensible heat transfer is dominated by the convective resistance while the dominant mass transfer resistance shifted to the membrane at smaller hydraulic diameters. The results suggest that as membrane technology improves, enhancements to the airside heat and mass transfer coefficients will be required to continue to realize performance gains.

Application of Microscale Devices for Megawatt Scale Concentrating Solar Power Plants

Concentrated solar power (CSP) plants have the potential to reduce the consumption of non-renewable resources and greenhouse gas emissions in electricity production. In CSP systems, a field of heliostats focuses solar radiation on a central receiver, which is ultimately transferred to thermal electrical power plant at high temperature. However, the maximum receiver surface fluxes are low (30–100 W cm−2) with high thermal losses, which has limited the market penetration of CSP systems.Recently, small (∼ 4 cm2), laminated micro-channel devices have shown potential to achieve concentrated surface fluxes over 100 W cm−2 using supercritical CO2 as the working fluid. The present study explores the feasibility of using these microscale devices as building blocks for a megawatt scale (250 MW thermal) open solar receiver. This allows for a modular design of the central receiver with non-standard shapes customized to the heliostat field. The results show that the microscale unit-cells have the potential to be scaled to megawatt applications while providing high heat flux and thermal efficiency. At the design incident flux and surface emissivity, a global receiver thermal efficiency of > 90% can be achieved.Copyright

Improved Non-Equilibrium Film Method for the Design of High-Temperature-Glide, Mini- and Microchannel Condensers

High-temperature-glide (i.e., large difference in dew and bubble point temperature) zeotropic mixtures such as ammonia/water have the potential to improve efficiency as new working fluids in advanced energy cycles for heating, cooling and power. Furthermore, the high heat capacity of ammonia/water mixtures makes them particularly attractive for use in compact mini- and microchannel devices. The non-isothermal condensation process of zeotropic mixtures leads to coupled heat and mass transfer resistances in each phase, which are not accounted for by single-component in-tube condensation modeling and correlation techniques. Previous attempts to design zeotropic condensers have relied on use of non-equilibrium film theory or mixture resistance correction factors. The film theory models have been developed with many simplifying assumptions including annular flow, negligible condensate and vapor sensible heat loads, and/or laminar condensate film, while the correction factor approaches do not directly consider mass transfer resistances. In the present study of high-temperature-glide mixtures in small channels, these assumptions are relaxed, and a new design method for mini- and microchannel zeotropic condensers is introduced. The approach is validated with experiments conducted for a range of tube diameters (0.98 < D < 2.16 mm), mass fluxes (50 < G < 200 kg m−2 s−1) and mass fractions of ammonia (0.80 < xbulk < 0.96). The results can be used in the development of compact, highly efficient heat and mass transfer devices.© 2014 ASME

Analysis of Coupled Heat and Mass Transfer During Condensation of High Temperature Glide Zeotropic Mixtures in Small Channels

Condensing zeotropic mixtures have the potential to improve the thermodynamic performance of power generating, cooling, and heating systems. Further improvements can be realized by developing equipment using mini- and microchannels. However, it has been shown that the concentration gradients arising from the changing composition of the vapor and liquid phases during condensation introduce additional mass transfer resistances, degrading the overall heat transfer. Furthermore, the coupled heat and mass transfer in mixtures at this scale is not well understood. Results from experiments on condensation of ammonia/water mixtures at varying concentration (80–100% NH3), mass flux (G = 50 − 200 kg m−2 s−1), and tube diameter (D = 0.98–2.16 mm) are reported here. Other researchers have reported an apparent heat transfer coefficient for zeotropic mixtures, which fails to explicitly account for mass transfer. The present work quantifies the liquid film heat transfer coefficient by accounting for mass transfer through a film model approach, which allows the relative significance of the vapor and liquid heat and mass transfer resistances to be quantified.Copyright

Sustainability Impacts of a Program of Decommissioning Older Automobiles for Replacement With Newer, More Efficient Vehicles

This paper investigates a representative vehicle replacement scenario on a comprehensive basis, including energy for fabrication, operation and decommissioning, together with the associated environmental and economic impacts. It is postulated that to lessen the environmental impact of older, inefficient devices, legislation may be passed to require the decommissioning and purchase of newer equipment. However, the energy consumed, greenhouse gases emitted, and economic costs of building a new product and decommissioning the old product may outweigh the benefit of removing an inefficient device from service on a life-cycle basis. A representative case of retirement of automobiles from 1990 and earlier by comparable 2007 model internal combustion engine automobiles is considered. The energy use, greenhouse gas emissions, and economic cost to the consumer are compared for two cases, one that includes the production and operation of a new car and decommissioning of the older car, and a second case accounting for only the continued operation of the older car. It was found that from an energy and emissions standpoint, it is beneficial to require the decommissioning of older automobiles. However in the same time period, economic considerations from the customer’s perspective favor continued operation of the older automobile. Therefore, the study concludes that without such an economic basis, legislation and incentives would be required to enable the replacement of older automobiles with more efficient models.Copyright