J.M. Gordon’s research while affiliated with Ben-Gurion University of the Negev and other places

Although most commercial chillers are designed and installed to operate at constant coolant flow rates, one can alter the power consumption of their compressors by modifying the design to incorporate variable coolant flow rates. The issue addressed here is how to model that explicit flow-rate dependence within an analytic chiller model. We expand our earlier thermodynamic model to explicitly account for the influence of condenser coolant flow rate, and validate model predictions against an extensive set of experimental measurements from a large commercial centrifugal chiller.

The thermodynamic performance of real irreversible cooling and refrigeration systems (chillers) can be summarized in simple rectangular temperature-entropy diagrams, in analogy to classic pedagogical examples for idealized reversible devices. The key to translating complex dissipative losses into this graphical framework is the process average temperature—a factor that can be calculated from nonintrusive experimental measurements, for converting entropy production into lost work. An uncomplicated thermodynamic model is used to transform the governing chiller performance equations into an easily-interpreted graph. Examples based upon actual data from commercial work-driven (reciprocating) and heat-driven (absorption) chillers are presented, and are used to highlight the predominance of internal dissipation in determining chiller efficiency. With the thermodynamic diagram representation, the relative roles of each irreversibility source, as well as the reversible and endoreversible limits, become transparent. © 1999 American Institute of Physics.

Most large solar concentrators designed for high flux concentration at high collection efficiency are based on imaging primary mirrors and nonimaging secondary concentrators. In this paper, we offer an alternative purely imaging two-stage solar concentrator that can attain high flux concentration at high collection efficiency. Possible practical virtues include: (1) an inherent large gap between absorber and secondary mirror; (2) a restricted angular range on the absorber; and (3) an upward-facing receiver where collected energy can be extracted via the (shaded) apex of the parabola. We use efficiency–concentration plots to characterize the solar concentrators considered, and to evaluate the potential improvements with secondary concentrators.

Absorption chillers operate well below their reversible or endoreversible limits because their thermodynamic behavior is dominated by internal dissipation, a significant part of which occurs in the chiller’s heat exchangers. This fact has summarily been omitted from earlier analyses. It translates into incorrect values for the refrigerant process-average temperature (PAT), and leads to noticeable errors in chiller diagnostics and optimization. Using experimental measurements from an absorption chiller, in concert with a computer simulation code and an analytic thermodynamic model, we fortify these claims with quantitative examples. The correct PAT is derived and its significance in chiller analysis is highlighted. Aspects of chiller optimization that are unique to absorption technology, as opposed to conventional vapor-cycle reciprocating chillers, are also illustrated. We also substantiate that commercial absorption chiller technology has empirically evolved to close to optimal operating conditions.

The thermodynamic behavior of conventional chillers (generalized air conditioning and refrigeration systems) is acutely sensitive to internal dissipation. Previous chiller analyses have excluded entropy production in the evaporator and condenser heat exchangers, on the assumption of its being negligible. With experimental measurements from a commercial chiller, we demonstrate that heat exchanger internal dissipation is not inconsequential, and that ignoring its contribution can lead to substantial errors in chiller diagnostics and in the prediction of chiller performance curves. To evaluate the impact of this dissipation on chiller efficiency, one needs to define a proper process average temperature (PAT). In addition to discussing the fundamental significance of the correct PAT, we will show that earlier conventional definitions of PAT, where internal irreversibilities in chiller heat exchangers have been overlooked, result in inaccurate and sometimes unphysical predictions. © 1998 American Institute of Physics.

For the conversion of absorbed sunlight into useful thermal power, we demonstrate that the profiles of absorber temperature, fluid temperature and thermal power delivery along linear solar collectors can be solved in closed form even when the collector heat-loss coefficient is far from constant over the collector operating range. This analytic solution eliminates the errors inherent in earlier approximate solutions, and makes the dependence of collector performance on component properties transparent. An example for a realistic solar concentrator illustrates the improvement in prediction accuracy.

The efficiency of chillers (refrigeration and heat pump devices) is limited by the dissipation from their principal components: compressor, throttler, and heat exchangers at the condenser and evaporator. Developing a generalized finite-time thermodynamics model for reciprocating chillers, we derive analytic formulae for how the fixed finite resources of cycle time and heat exchanger inventory should be allocated so as to optimize chiller performance. Our predictions for optimal operating schemes are compared with detailed experimental data from two different commercial chillers. The agreement between theory and actual performance data attests to the empirical wisdom that has evolved in chiller manufacture. Besides quantitatively documenting the individual sources of irreversibility, we show how the limitations of currently-available chiller components affect optimal chiller design, as well as how potential steps to improve chiller efficiency can be evaluated within a universal thermodynamic framework.

Absorption chillers, heat pumps and heat transformers exploit thermal power input to satisfy a variety of refrigeration and heating applications, as well as to boost the temperature of low-grade heat. Can one arrive at accurate predictions of system performance with a simple analytic irreversible thermodynamic model? In capturing the essential physics of the problem, that model would be required to provide a predictive and diagnostic tool and to permit determination of optimal absorption system operating conditions. We derive such a model and check its validity against experimental data and computer simulation results from a variety of commercial absorption units. We reinforce the observation that commercial units appear to have evolved empirically such that maximum efficiency is realized under design conditions. The failure of a host of previously-published endo-reversible thermodynamic models to account for fundamental qualitative features and accurate predictions of system behavior is documented with computer simulation results and experimental performance data.

We develop a simple analytic diagnostic model for reciprocating chillers. With only a handful of non-intrusive, in situ measurements, one can then ascertain quantitatively how chiller performance changes with time or after a prescribed modification. We derive how reciprocating chillers can be characterized by just three parameters with clear physical significance. We then verify the correspondence between theory and reality with detailed experimental measurements. It is also demonstrated how this model can be used to establish optimal operating conditions for reciprocating chillers, and to evaluate potential improvements that stem from changes in operating conditions or the distribution of heat exchanger inventory. Again, comparisons with actual performance data from commercial chillers are provided. We give quantitative expression to the contribution to chiller efficiency of internal dissipation from compression and throttling, and consequently reinforce the fact that endoreversible chiller models are far off the mark.

We propose, analyze and offer sample designs and results for a high-flux photovoltaic concentrator comprised of a large-aperture paraboloidal-dish primary concentrator, and a second-stage kaleidoscope flux homogenizer. The following key design aims are all satisfied: (1) highly uniform irradiance on the solar cell absorber; (2) maximum collection efficiency; and (3) not exceeding the prescribed target flux level (for illustrative purposes here taken to be 500 suns), despite the dish being capable of much higher concentration. As a result of recent advances in the low cost and ease of production of large dish concentrators, the kaleidoscope-based design offers an intriguing alternative to other high-concentration optical designs developed to date. Admissible kaleidoscope geometries are identified. We generate quantitative results for a compact practical design that incurs low optical losses, and produces a highly homogeneous flux map.