Sam V. Shelton

Design and testing of a solid-sorption heat-pump system

The solid-sorption refrigeration cycle was first commercialized for food preservation in the early part of this century. Renewed research interest in the 1980s focused on the development of the cycle for use in a natural-gas heat pump for residential space heating and cooling. Several prototype systems employing a high-efficiency regenerative cycle have been designed, built and tested. This paper presents recent experimental results from a two-bed system which uses the activated carbon/ammonia sorbent/sorbate pair. Seasonal performance based on this experimental data and computed using the ANSI Standard Z21.40 for performance rating of gas-fired heat pumps is presented. In addition, a design optimization methodology that has been developed is presented. New carbons being developed for methane storage offer advantages when applied to the solid-sorption refrigeration arena. The implications of using these new carbons and their effect on the design of the system are discussed.

Square wave analysis of the solid-vapor adsorption heat pump

Abstract A thermally driven heat pump using a solid/vapor adsorption/desorption compression process in a vapor compression cycle is thermodynamically analyzed. The cycle utilizes a simple heat transfer fluid circulating loop for heating and cooling two solid adsorbent beds. This heat transfer fluid loop also serves to transmit heat recovered from the adsorbing bed being cooled to the desorbing bed being heated. This heat recovery process greatly improves the efficiency of the single stage solid/vapor adsorption process without the complication of a two stage cycle. During the heating and cooling processes a thermal wave profile travels through the beds. This paper uses a square wave representation for the true shape of the thermal wave. However, this square wave is assumed to stop short of the bed ends to account for realistic finite waveforms. The square wave model is integrated into a thermodynamic cycle which provides detailed information on the performance of the beds as well as the COP and the heating and cooling outputs of the heat pump system. Significant cycle design and operating parameters are varied to determine their effect on cycle performance.

Gas fired sorption heat pump development

Abstract The heat-driven sorption heat pump cycle appears in U.S. patent literature as early as 1909, and refrigerators employing a solid sorbent were commercially available in the 1920s. This paper will describe a program to develop a gas-driven heat pump which demonstrates the simplicity and low cost potential of the original sorption cycle, while achieving high effieciency through regeneration. Regeneration is accomplished by circulating a heat transfer fluid through two solid sorbent beds being alternately heated and cooled. The current development program uses a proprietary “thermal wave” concept in which over 70% of the total bed input heat required per cycle is supplied by regeneration. The primary component requiring development is the sorbent bed containing the heat transfer fluid/sorbent heat exchanger. Considerable analysis has been carried out and several prototype beds have been built and tested. Analytical and experimental results for one bed design are presented and discussed in the paper.

Resedential space conditioning with solid sorption technology

Abstract Approximately 50% of residential energy use in the U.S. is for space heating and cooling. The most popular type of system installed in U.S. houses is a central air-circulating system combining a natural-gas-fired direct-air-heating furnace and an electric-driven vapor compression cooling system. Natural-gas-driven heat pump technology offers considerable benefits over these existing technologies including a reduction in energy use, improved environmental impact, and reduced investment in electric power plant construction. However, over the past 30 years, more than 100 major natural-gas-driven heat pump development projects have been undertaken without a currently successful product. These past projects have focused on engine-driven vapor compression refrigeration cycles and liquid sorption refrigeration cycles. New solid sorption technology offers significant advantages over these engine and liquid sorption technologies. The fundamental advantages of solid sorption refrigeration technology in natural gas residential heat pumps are presented and the societal benefits over existing residential systems discussed.

EXTERNAL FLUID HEATING OF A POROUS ED

This work deals with the thermal analysis of externally heated porous beds of finite length. A one dimensional model was developed that includes conduction and storage in both the fluid and bed, convective exchange between the fluid and bed, and the effect of adsorption/desorption in the bed. This model results in two coupled differential equations for the fluid and bed temperatures as functions of four independent dimensionless parameters. These equations were solved numerically using finite difference approximations. A truncation error analysis was carried out to maintain an accurate solution. The method of normalization is such that the results of this analysis are of use in bed design when the breakthrough characteristics in finite length beds are of interest. A method to measure bed thermal performance is defined and a means to optimize bed thermal performance is presented. An experiment was conducted to validate the numerically obtained results. A comparison of numerical to experimental results is p...

Site Specific Optimization of Rotor/Generator Sizing of Wind Turbines

Economics, including all incentives, is the primary factor that drives the development of wind farms. Optimizing the wind turbine generator size-to-rotor size design based on an economic figure of merit shows that maximum wind turbine capacity factor does not yield the best economics for a given wind resource. A large rotor on a small generator will have a high capacity factor but a low annual output of electrical energy. For the same capital investment a different configuration would produce more electricity making the project more economically sound. This study varied rotor-to-generator size at a fixed capital cost and used a modified blade element momentum model to predict annual electrical energy production for each design at a given wind resource. Optimal design was the design that resulted in the highest annual electrical energy production. This was done at a series of fixed costs and a series of wind resources defined by the Weibull distribution parameters. The results indicated the following: At larger turbine sizes, (higher capital cost per turbine), the economics shifted toward a larger generator and smaller rotor (relatively). This exact relationship is dependent on the wind resource. At large turbine sizes, greater flexibility is shown in optimum generator sizing vs. rotor sizing. Having multiple generator size options for the same rotor size allows developers to more closely match and capitalize on the characteristics of their wind resource. The end result of the research is a set of diagrams developers can use to select the best turbine based on economics for their wind resource. This provides an additional tool they can use to make their projects more cost effective.Copyright

Solid-Vapor Heat Pump Technology

ABSTRACT A reliable, low cost, high efficiency thermally-driven heat pump for residential and commercial heating and cooling has been the focus of considerable international research for many years. This research has generally been in (1) engine-driven vapor compression systems and (2) liquid/vapor absorption systems. Both concepts have been limited in their ability to successfully compete with the electric heat pump. While the engine driven systems have demonstrated high efficiency, capital and maintenance costs have limited their use to applications having high thermal loads. On the other hand, the liquid/vapor absorption systems have suffered from low efficiency. Efforts to improve the efficiency have increased costs beyond economically allowable limits. The complexity and size of liquid absorption systems also tends to limit their use in residential applications. Solid/vapor adsorption systems were first commercialized in the 1920s and hold great promise for overcoming the limitations of the engine-driven vapor compression and the liquid/vapor absorption systems. The basic solid/vapor adsorption cycle utilized in the period from 1920–40 was simple but inefficient. Recent research indicates a potential for high efficiency, simplicity, and low cost which would make the solid/vapor heat pump an innovative and competitive technology in the residential and commercial sectors. This article describes a system with performance and simplicity advantages, not evident in previous studies, which may lead to the development of a fossil fuel driven heat pump to compete with the electric heat pump.

The Usefulness of Entropic Average Temperatures

The second law Carnot efficiency, entropy balances, and many other principles of the second law are stated with assumed constant temperature heat sinks and reservoirs; i.e., assuming heat transfer across a boundary at a constant temperature. However, real world heat exchangers reject and receive heat transfer to a flowing fluid with a varying temperature making the application of many aspects of the second law inappropriate or complex. For such varying temperature cases, an entropic average temperature can be defined and easily calculated that can be substituted for the varying temperature heat sink or source temperature. The constant temperature restricted second law statements can then be used with this entropic average temperature. This entropic average temperature concept is simple to understand and is very useful in the presentation of thermodynamic concepts to new students, making it seem less abstract and more applicable to real world processes with which they are familiar. This paper serves to develop the concept and details of the entropic average temperature and show its usefulness while emphasizing its benefit for inclusion in engineering thermodynamics syllabi.Copyright

Analysis of a nozzle condensation drying cycle

Of the total energy required for U.S. manufacturing processes, a significant portion is used for industrial drying processes, estimated at 12%. Often these industrial dryers operate without any provisions for heat recovery. This means that almost all the energy consumed by the dryer is essentially thrown away. Without the ability to recover the latent heat of the water vapor in the air, significant reductions in dryer energy usage cannot be made. In an attempt to recover the latent heat of the water vapor, a closed drying cycle is proposed, which uses a nozzle as the means to recover this energy from moist-saturated air. This nozzle condensation drying cycle (NCDC) is composed of the following components: dryer, nozzle, diffuser, air-water separator, heat exchanger and compressor. Analysis of this drying cycle shows that ideally this cycle requires a third of the energy compared with an ideal open cycle heated dryer or a heat pump dehumidification cycle. However, when equipment isentropic efficiencies are included, the energy costs are of the same order as that for the other two non-ideal cycles, but with as much as six times less heat exchange requirements. The cycle can be operated at varying conditions, allowing for a single design to be utilized for various applications. Other than the advantage of lower heat exchanger equipment costs, other advantages are in applications that require drying of hazardous liquids from a product.

Design Study Comparison of Plain Finned Versus Louvered Finned-Tube Condenser Heat Exchangers

Enhanced fins are widely used in residential air conditioning system finned-tube condenser designs. While this heat transfer augmentation technique increases the heat transfer coefficient in the heat exchanger, it also increases the air side frictional pressure drop. These two effects compete with each other, making it difficult to determine the relative goodness between plain fin versus enhanced fin designs with realistic constraints. In the past, this design tradeoff has been largely determined by experimental trial and error or heuristic figures of merit. No studies are available showing the effect of fin augmentation on overall system performance under consistent cost and frontal area constraints. The residential air conditioning system model calculates all component and system performance parameters. The condenser design requires the specification of approximately ten design parameters. A search method is used to vary these ten parameters and reach an optimum design based on a COP (efficiency) figure-of-merit with condenser cost and other appropriate constraints. It was found that when optimized, louvered fin designs always show better system performance than the optimum plain fin design for the cases studied. However a decrease in system efficiency can result if louvers are merely added to a plain fin optimum design.© 2003 ASME