The Einstein Refrigeration Cycle

Completed Work - A. Delano's M.S. Thesis
Completed Work - A. Delano's Ph.D. Dissertation [ HTML Version ][Zipped PDF Version (1.2 MB)]
Completed Work  - S. Stewart's M.S. Thesis  (Buble Pump Design & Performance)
Publications - Design Analysis , Second Law Analysis , Bubble Pump Design
Summary -

Currently, electric motor driven vapor compression refrigeration cycles dominate air conditioning, heat pump, and refrigeration applications. However, direct thermally driven refrigeration cycles are receiving increasing attention for several reasons: environmentally benign refrigerants, reducing electric utility summer peak demand, silent operation, and efficient utilization of waste heat.

The thermally activated refrigeration cycles receiving the most attention are all dual pressure cycles requiring an electric pump to move the liquid solution from low pressure to high pressure. A recently uncovered patent by Albert Einstein issued Nov. 11, 1930 discloses a single pressure thermally driven refrigeration cycle which does not require a pump. It accomplishes this by using a triple mixture of butane, ammonia, and water. Butane acts as the refrigerant, and ammonia acts as an inert gas creating low partial pressure for the butane in the evaporator to provide low temperature refrigeration. Water serves as the absorbent to separate the ammonia from the butane. A literature search has provided only one brief reference to this unique cycle. The analysis of the cycle was accomplished via a computer model which calculates operating characteristics such as efficiency, operating temperature ranges, and specific cooling output. Parametric studies with the model provide insight on the effect of various design parameters on cooling output and efficiency as well as the potential of this cycle and its optimum design.

The computer model of the cycle assumed ideal gas mixtures and ideal solutions. Vapor-liquid equilibrium was calculated using Raoult's law.

Analysis of this cycle shows the potential COP to be quite high relative to the ammonia-water-hydrogen cycles widely used today. Current ammonia-water-hydrogen cycles have COP's around 0.1, and the COP's calculated for the Einstein cycle were as high as 0.4.

Analysis of the cycle also shows an interesting limitation between the evaporator and condenser. The maximum lift ( the difference  between the condenser and evaporator temperatures) is restricted by the saturation temperatures of ammonia and butane at the system pressure. Fortunately, an evaporator temperature of below freezing is obtainable with the reasonably high condenser temperature of
107 F.

Zeotropic Refrigerant Mixtures

Completed Work - L. A. Schaefer's M.S. Thesis
ASME Publication - [ PDF Version ][ HTML Version ]
Summary -
Recent protocols have reduced, and will eventually eliminate, the production and use of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). While it has been possible to find single-component replacements for CFCs such as R-12, replacements for HCFCs are generally refrigerant mixtures. These mixtures may be azeotropic, in which case single-component behavior is exhibited, near-azeotropic or zeotropic. Azeotropic refrigerant mixtures are uncommon, so it is of interest to study the characteristics of zeotropes and near-azeotropes.

The temperature of a zeotropic mixtures does not remain constant throughout a heat exchanger. Furthermore, zeotropes often exhibit a nonlinear temperature/enthalpy relationship. These factors contradict some of the assumptions that are made in deriving the Log Mean Temperature Difference (LMTD), a calculation that is used to compute the size of a heat exchanger (UA). In order to develop a more precise mean temperature difference, the derivation of the LMTD must be reëvaluated. Since the temperature is not an easily-determined function of enthalpy, the heat transfer process must be discretized and the properties determined at each point.

An ammonia-water mixture is examined first, since it is known to display a highly nonlinear temperature glide. Synthetic mixtures that are being studied by the refrigeration industry as replacements for HCFC-22 are also analyzed. In each case, the actual UA is compared to the UA found using the LMTD, and representative error scales are developed. It is found that these errors can cause a heat exchanger to be undersized by as much as a factor of fifty.

Finally, the advantages of zeotropes are also discussed, as are cycles that can utilize those advantages. Recommendations are made that the effect of the modified UA calculations on these cycles should be studied, and that the benefits of natural over synthetic refrigerants should also be investigated.

Electric Water Heating

GHGT Publication
Slide Presentation
Summary -
Currently, approximately 38% of U.S. residential water heating is provided by electric resistance water heaters (Federal Technology Alert , 1996). These units exhibit low energy factors (EFs). An EF is defined as the energy content of the hot water delivered divided by the total energy used to heat the water (Koomey et al., 1995). Electric heat pump water heaters (HPWHs) transfer heat rather than create it, and so use fifty to seventy percent less electricity than resistance heaters, returning EFs greater than 100%. Unfortunately, new water heating technology must not only be efficient and environmentally-friendly, it must also be cost-effective if it is to interest the consumer. HPWHs have high initial costs, and their average payback period is six years. However, recent innovations by the Electric Power Research Institute and the Crispaire Corporation may make it possible to cut that time in half (Levine et al., 1995).

Improvements to electric HPWHs will also be studied. Even greater national impact is expected with improved technology for the larger gas water heating market. Existing technology can be utilized to develop cycles such as a gas HPWH, which should yield EFs greater than 100%. Single-pressure absorption cycles with no electrical requirements are presently used in a variety of refrigeration applications, so adapting those configurations to heating should not prove difficult.

In addition to system efficiency, the nature of the power supply and its indirect TEWI contribution must also be considered. The type of fuel used to generate electricity varies throughout the U.S., but the majority of electric power still comes from coal. It should be evident that the TEWI of a HPWH in a single-family dwelling in the Pacific Northwest (where hydropower is widely used) will differ from the TEWI of a multi-family dwelling in east Texas (where oil and coal are used in power production) for a variety of reasons. The effects of climate, power source, hot water usage, and system efficiency must each be analyzed. These aspects can then be given the appropriate weights in optimizing overall improvement. The merits of each system will also be evaluated on the basis of cost per unit of TEWI reduction.

In the past twenty years, more than fifty percent of all increases in global carbon dioxide emissions due to increased energy use have been from electricity generation (Levine et al., 1995). Water heating accounts for approximately 16% of total primary residential sector energy consumption, so improving heater efficiency is obviously important both from an environmental and an economic viewpoint. Improved water heating technology can also have a significant impact on larger industrial applications, so those systems will also be evaluated.

Federal Register (1994). "Proposed Rules," Department of Energy59(43): 10464-10507.

Federal Technology Alert (1996). "Residential Heat Pump Water Heaters," URL:http:// www.pnl.gov/fta/3_res.htm

Koomey, J. G., Dunham, C., and Lutz, J. D. (1995). "The Effect of Efficiency Standards on Water Use and Water-Heating Energy Use in the U.S.: A Detailed End-Use Treatment," Energy20 (7): 627-635.

Levine, M. D., Koomey, J. G., Price, L., Geller, H., and Nadel, S. (1995). "Electricity End-Use Efficiency: Experience with Technologies, Markets, and Policies Throughout the World," Energy 20(1): 37-61.

Hybrid Chiller Systems

Completed Work - D.E. Lilly's M.S. Thesis
Summary -
Deregulation of the natural gas and electric utility industry along with the advancement of gas cooling equipment has prompted HVAC designers and building operators to consider alternatives to the typical electric chiller system. Price fluctuations between peak demand periods and off peak periods for both gas and electricity are expected to increase. These price fluctuations for gas and electricity are out of phase. This suggests studying combinations of different types of chillers.

When considering commercial building chiller systems comprised of two equal size chillers, the base loaded chiller provides 90 percent of the buildingThe trend in deregulated energy rates combined with the different load characteristics for base and peaking chillers suggests looking at hybrid systems, which use a combination of electric and gas chillers. This is the primary focus of this study.

Using a building cooling load profile for a 400,000 square foot hotel or other 24 hour/day facility, actual weather data, and electric and gas rates for Atlanta, Georgia, a computer simulation model for a 1000 ton chiller/cooling tower system is developed. A 1000 ton electric chiller, a 1000 ton single stage absorption chiller, a 1000 ton double stage absorption chiller, and a 1000 ton gas engine driven chiller are first modeled. For hybrid systems, 500 ton chillers of each of these chiller types are also modeled and used in several combinations with each other. Different operating strategies are also considered.

The annual energy consumption and the annual energy costs for ten chiller systems are calculated using deregulated gas rates and hourly real-time-pricing electric rates representative of expected deregulated electric rates. Equipment capital costs and annual maintenance costs are also considered.

The results indicate that most of the multiple chiller systems have lower energy costs than the single chiller systems, primarily due to reduced cooling tower pump and fan energy use. With the real time electric rates, the total gas fired absorption systems offer little or no operating costs advantage over total electric chiller systems. The systems that use a gas engine driven chiller alone, or in combination with an electric chiller, show dramatically reduced energy costs. This is particularly true when their engine heat recovery option is utilized. These chillers, however, have much higher capital costs than the electric chillers.

Even though the systems using engine chillers show much lower energy costs, they have long capital cost payback periods. These engine systems also have high maintenance costs due to the high maintenance aspect of the engine.

The energy purchasing flexibility of a hybrid gas/electric system, however, may be advantageous in the future deregulated environment. Energy prices will be volatile, and predicting future gas/electric prices is problematic. Even today in Atlanta, the hourly real time pricing electric rates which will be in effect tomorrow are not known until after 4:00 PM today. This makes chiller system design optimization problematic.

Single Pressure Absorption Heat Pump Analysis

Completed Work - L. A. Schaefer's Ph.D. Dissertation [ PDF Version ][HTML Version (Coming Soon)]
Summary -
Thermally-driven absorption heat pump cycles have several advantages over conventional vapor compression cycles. They do not require a costly electric power plant and they use environmentally-benign natural fluids. Current absorption systems are dominated by dual-pressure cycles using a solution pump (which still requires a small electrical power source). Single-pressure cycles remove the need for a pump and any electrical power. This makes them portable, inexpensive, reliable, and silent. An entirely different approach to a single-pressure absorption cycle was taken by Einstein and Szilard in the 1930s. Their cycle decouples the temperature lift from the generator temperature, which allows the selection of a refrigerant that matches the application temperature requirements.

The configuration of the Einstein cycle was examined, and changes were made to increase the coefficient of performance. These changes were primarily implemented on the generator side. The bubble pump performance was increased through selection of optimum operating parameters. An external heat exchanger was added between the generator and the condenser/absorber to improve heat recovery, and the partial internal heat exchanger in the generator was expanded to a full internal heat exchanger in order to minimize entropy generation.

The Einstein Cycle has been modeled using two separate property models: 1) a corresponding states/ideal solution property model, and 2) a Patel-Teja/Panagiotopoulos and Reid property model. The first model was used to predict which parameters would increase the COP and the second model was used to more accurately predict the behavior the cycle

For this study, three temperature conditions were evaluated: 1) T _{condens-absorb} = 325 K and T_evaporator = 295 K, 2) T _ca = 325 K and T_evap = 306 K, and 3) T _ca = 316 K and T_evap = 295 K. As outlined in the introduction, each of these temperature levels are suitable for using the Einstein cycle for gas heat pump water heating. The highest COP for the first set of temperatures, which can produce 125 F water from ambient conditions (flue gases could be used to help to maintain the evaporator at 72 F), is 1.51. The best COP for the second set of temperatures, which require a hotter evaporator, is 1.88. Finally, the COP for the third set of temperatures, where the Einstein cycle would be used as a preheater, is 1.76.

With a COP of 1.5, an Einstein cycle gas heat pump water heater would cut the operating costs of a conventional gas water heater by 33%. This could result in large economic and environmental savings.
 

Micro-Cogeneration Optimal Design For Service Hot Water Thermal Loads

Completed Work - S. Jones' M.S. Thesis [ HTML Version ][PDF Version (390 KB) ]
Summary -
Deregulation of the gas and electric industries are rearranging electric versus gas energy pricing and changing optimal energy systems design. In addition, small gas turbine technology is becoming available which is appropriate for micro co-generation; i.e., less than 50 kW electrical power output. Economic evaluation of micro co-generation of electricity and thermal energy in small commercial applications is thereby necessary.

A facility category with a relatively uniform domestic hot water thermal energy load is large athletic/health clubs. It also has a co-incident large and uniform electrical lighting load. These loads appear appropriate for the newly emerging micro co-generation technology.

This study will collect energy and water use data on a large athletic club. The data will be compiled and statistically analyzed. A co-generation system will be parametrically optimized using statistical analysis of the thermal and electrical load profiles. Thermal and electric load following operating strategies will be investigated. Design optimization methodologies using statistical analysis of co-generation system loads is not currently available.

The environmental global warming impact of the resulting micro co-generation design will be determined, and the economics of using this technology to reduce global warming evaluated. Its potential national impact will be analyzed with respect to the Kyoto International Global Warming Treaty.

The results will be generalized to determine the economics of optimum micro co-generation designs and operating strategies using new micro gas turbine technologies in the emerging energy environment.
 

Flat Plate Heat Exchanger Design Optimization

Completed Work - C. Seeton's M.S. Thesis
Summary -
Current refrigeration evaporator heat exchangers are designed to maintain the heat transfer fluid in the turbulent flow regime. This study will focus on studying the performance and design of laminar flow flat plate heat exchangers, which is rarely considered in the literature. In the laminar regime, the Nusselt number is relatively constant; meaning that the product of the heat transfer coefficient and the hydraulic diameter is constant. This study will investigate designing flat plate evaporator heat exchangers for refrigeration systems with very small hydraulic diameters to obtain large heat transfer coefficients. The trade-off of pressure drop and heat transfer coefficient will be optimized using the Second Law of Thermodynamics.

Single phase laminar flow heat exchangers will first be analyzed. Two-phase evaporator heat exchangers operating at small hydraulic diameters will also be investigated. Experimental testing of a flat plate heat exchanger operating with liquid flow, vapor flow, and two-phase vapor and liquid will be conducted to validate the theoretical study. Water, air, and an air-water mixture will be used in the tests.
 

R-410A Plate-Fin-And-Tube Condenser Performance and Design

Completed Work - M. Wright's M.S. Thesis (950 KB)
Completed Work - S. Stewart's Proposal
Summary -
Current residential air-conditioners and heat pumps use the hydrochlorofluorocarbon refrigerant, R-22, as the working fluid. In accordance with the Montreal Protocol, a production ban of all equipment utilizing R-22 will begin in 2005, and a total ban on the production of R-22 is also impending. A binary zeotropic mixture, R-410a, is a strong candidate for R-22 replacement due to its many favorable performance characteristics; e.g., non-flammability, high working pressures, and good cycle efficiency.

Since R-410a has significantly higher working pressure and vapor densities than R-22, current air cooled finned tube condenser designs are not appropriate. The optimum condenser and other high-pressure-side components are expected to employ smaller diameter tubes, which will affect other design parameters. At this time, there is limited information about condenser coil design and optimization using R-410a as the working fluid. Furthermore, the heat transfer and friction data are also limited.

This work includes an examination of the available refrigerant-side two-phase flow heat transfer and pressure drop models for refrigerants. A model based on first principles is used to predict the performance of a unitary air-conditioning system with refrigerant R-410a as the working fluid. The seasonal coefficient of performance of the air-conditioning system is used as the figure of merit. The primary objective of this research was to provide guidelines for the design and optimization of the condenser coil for two distinct criteria: (1) fixed condenser frontal area (size constraint), and (2) fixed condenser material cost (capital cost constraint).

This study concludes that for both design criteria, the velocity of air flow over the condenser ranges between 7.5 ft/s and 8.5 ft/s while the optimum sub-cooling of the refrigerant exiting the condenser is approximately 15°F. It is also concluded that condensers employing tubes of smaller diameters yield the best system performance. Recommendations for further research into the modeling of the in-tube condensation of refrigerant R-410a are outlined. An exhaustive search optimization study could not be performed due to computational speed limitations, therefore more advanced optimization search techniques are also recommended for further study.