Optimization of a Novel Combined Power/Refrigeration Thermodynamic Cycle

A novel combined power/refrigeration thermodynamic cycle is optimized for thermal performance in this paper. The cycle uses ammonia-water binary mixture as a working fluid and can be driven by various heat sources, such as solar, geothermal and low temperature waste heat. It could produce power as well as refrigeration with power output as a primary goal. The optimization program, which is based on the Generalized Reduced Gradient (GRG) algorithm, can be used to optimize for different objective functions. Examples that maximize second law efficiency, work output and refrigeration output are presented, showing the cycle may be optimized for any desired performance parameter. In addition, cycle performance over a range of ambient temperatures was investigated. It was found that for a source temperature of 360K, which is in the range of flat plate solar collectors, both power and refrigeration outputs are achieved under optimum conditions. All performance parameters, including first and second law efficiencies, power and refrigeration output decrease as the ambient temperature goes up. On the other hand, for a source of 440K, optimum conditions do not provide any refrigeration. However, refrigeration can be obtained even for this temperature under non-optimum performance conditions.

Revising and Validating Spectral Irradiance Reference Standards for Photovoltaic Performance Evaluation

In 1982, the American Society for Testing and Materials (ASTM) adopted consensus standard direct-normal and global-tilted solar terrestrial spectra (ASTM E891/E892). These standard spectra were intended to evaluate photovoltaic (PV) device performance and other solar-related applications. The International Standards Organization (ISO) and International Electrotechnical Commission (IEC) adopted these spectra as spectral standards ISO 9845-1 and IEC 60904-3. Additional information and more accurately representative spectra are needed by today’s PV community. Modern terrestrial spectral radiation models, knowledge of atmospheric physics, and measured radiometric quantities are applied to develop new reference spectra for consideration by ASTM.

A Novel Combined Power and Cooling Thermodynamic Cycle for Low Temperature Heat Sources: Part II — Experimental Investigation

A combined thermal power and cooling cycle proposed by Goswami is under intensive investigation, both theoretically and experimentally. The proposed cycle combines the Rankine and absorption refrigeration cycles, producing refrigeration while power is the primary goal. A binary ammonia-water mixture is used as the working fluid. This cycle can be used as a bottoming cycle using waste heat from a conventional power cycle or an independent cycle using low temperature sources such as geothermal and solar energy. An experimental system was constructed to demonstrate the feasibility of the cycle and to compare the experimental results with the theoretical simulation. Results showed that the vapor generation and absorption condensation processes work experimentally, exhibiting expected trends, but with deviations from ideal and equilibrium modeling. The potential for combined turbine work and refrigeration output was evidenced in operating the system. Analysis of losses showed where improvements could be made, in preparation for further testing over a broader range of operating parameters.

Photocatalytic Oxidation of Toluene in Water From an Algae Pond With High Dissolved Oxygen Content

Water in well-mixed ponds containing photosynthetic algae has been observed to have an extremely high Dissolved Oxygen (DO) content. Up to four times saturation levels of DO have been recorded. Since DO is known to have an important role in the photocatalytic oxidation of organic contaminants in water, it was hypothesized that a faster rate of contaminant destruction would be observed in water drawn from algae ponds supersaturated with DO. In order to verify this hypothesis a bench scale, batch type photoreactor was constructed. Some baseline tests were performed to investigate the influence of UV intensity, water pH and DO content on the photocatalytic destruction of toluene in water. An array of ultraviolet “blacklight” lamps in a lamp box was used to simulate solar ultraviolet radiation. First-order reaction rate constants were calculated from the destruction data, using a kinetic model proposed earlier. The reaction was found to proceed forward equally fast at pH 4 and 10. A power law relation was derived for the reaction rate dependence on UV intensity. Presence of DO in the water was found to be required for the reaction to go forward. Water from an algae pond, supersaturated with dissolved oxygen was spiked with toluene and destruction tests were then conducted in the same reactor. These tests did not show the expected improvement in destruction rates.

Analyzing Two Federal Building Integrated Photovoltaics Projects Using ENERGY-10 Simulations

A new version of the ENERGY-10 computer program simulates the performance of photovoltaic systems, in addition to a wide range of opportunities to improve energy efficiency in buildings. This paper describes two test cases in which the beta release of ENERGY-10 version 1.4 was used to evaluate energy efficiency and building-integrated photovoltaics (BIPV) for two Federal building projects: a 16,000-ft2 (1,487 m2) office and laboratory building at the Smithsonian Astrophysical Laboratory in Hilo, Hawaii, and housing for visiting scientists [three 1400-ft2 (130 m2) and three 1564-ft2 (145 m2) houses] at the Smithsonian Environmental Research Center in Edgewater, Maryland. The paper describes the capabilities of the software, the method in which ENERGY-10 was used to assist in the design, and a synopsis of the results. The results indicate that ENERGY-10 is an effective tool for evaluating BIPV options very early in the building design process. By simulating both the building electrical load and simultaneous PV performance for each hour of the year, the ENERGY-10 program facilitates a highly accurate, integrated analysis.

Measurement of High Thermodynamic Temperatures in the DLR Solar Furnace by UV-B Detection

The correct selection of the operating wavelength is essential for a precise pyrometric temperature measurement on solar irradiated samples, as the measurement may be disturbed by reflected solar radiation. Atmospheric conditions and particularly the emissivity as basic material property determine the amount of this reflected and the emitted radiation from a sample under investigation. An approach to solve this problem by using a monochromator system for temperature measurement in the UV-B range was developed and experimentally tested. With this system, temperature measurements were possible beginning at 1320°C and ranging up to about 2400°C. Two calculation methods are described and compared. The influence of the calibration temperature on the quality of the temperature measurement is shown. Measurements on a blackbody up to 1500°C were performed for calibration purpose. Temperature measurements on a real solar heated magnesia sample up to 2400°C are presented and discussed. These spectral measurements on hot bodies irradiated in the DLR Solar Furnace led to the final specification of the measurement wavelengths to be in the range from 280 nm to 293 nm.

Solar Air Conditioning Systems With PCM Solar Collectors

This paper investigates the technical feasibility of using a compact, air-cooled, solar absorption air conditioning system when coupled to an innovative array of solar collectors. The particular absorption system of study is a single effect that uses lithium bromide and water as the absorbent and refrigerant fluid pair. The geographical location of interest is Puerto Rico and similar subtropical regions. The heat input to the absorption system generator is provided by an array of novels flat plate solar collectors that integrate the thermal storage component into them. The proposed collectors have a phase change material (PCM) integrated into them as a storage mechanism. The PCM-integrated solar collector eliminates the need of conventional storage tanks reducing cost and space. The present work uses a paraffin-graphite composite as the PCM to increase the conductivity of the PC matrix. The paraffin’s melting point is around 89°C that is appropriate for use in absorption systems. The mathematical model that describes the thermal process in the PCM is presented and differs from the analysis of conventional flat plate solar collectors. The proposed model for the PCM considers the temporal changes but not the spatial variations. The resulting set of equations for the fluid flow, the PCM, and the collector’s surface are solved simultaneously. Results for the collectors’ thermal performance are presented along with the effects of the composition of the PCM material. The thermal performance of an absorption machine coupled to an array of the proposed PCM’s solar collectors was investigated for nominal cooling capacities of 10.5, 14, and 17.5 kW. These cooling loads are suitable for a typical house or a small business building in Caribbean Islands. Computer simulations were conducted to evaluate the overall system’s performance when subjected to dynamic cooling loads. Within the computer model, heat and mass balances are conducted on each component of the system, including the solar collectors, the air-cooled condenser, and the air-cooled absorber. Comparisons are made with an absorption air conditioning system that uses a cooling tower with conventional flat plate collectors instead of air-cooled and PCM components. Useful information about physical dimensions of collectors, number of collectors needed, and efficiency of the overall system is presented.

Short-Term Measurement of a Photovoltaic/Fuel Cell Remote Hybrid Power System at Golden Gate National Recreation Area

This paper reports short-term performance measurement of a hybrid photovoltaic/fuel cell power supply system at Kirby Cove Campground in Golden Gate National Recreation Area, California. The system operated reliably for two years from June 1999 to July 2001. During this period, the campground host load was met with a combination of solar power and power from the fuel cell. In August of 2001, reports of power outages justified an in-depth investigation. Data is reported over 13.5 days from September 2 to September 15, 2001. Over this period, energy delivered by the photovoltaic array totaled 42.82 kWh. Energy delivered by the fuel cell totaled 1.34 kWh, and net (out-in) energy from the battery totaled 6.82 kWh. After losses in the battery and inverter, energy delivered to the campground host totaled 34.94 kWh, an average of 2.6 kWh/day. Photovoltaic efficiency was measured at 8.9%. Fuel cell efficiency was measured at 42%, which is a typical value, but fuel cell power output was only 35 W instead of the 250 W rated power. Replacing a burnt fuse restored fuel cell power to 125 W, but several cells measured low voltage, and the fuel cell was removed for repair or replacement. Ordinarily, load in excess of the PV capability would be met by the fuel cell, and 22 cylinders of H2 (261 scf, 7,386 sl each) were consumed from April to August 2001. After failure of the fuel cell, load in excess of the solar capability resulted in discharged batteries and eight power outages totaling 48 hours in duration. Thus, overall system availability was 85% when relying only on solar power. This paper describes daily system operation in detail, presents component performance indicators, identifies causes of performance degradation, and provides recommendations for improvement.

Measured Versus Predicted Performance of Building Integrated Photovoltaics

The lack of predictive performance tools creates a barrier to the widespread use of building integrated photovoltaic panels. The National Institute of Standards and Technology (NIST) has created a building integrated photovoltaic (BIPV) “test bed” to capture experimental data that can be used to improve and validate previously developed computer simulation tools. Twelve months of performance data have been collected for building integrated photovoltaic panels using four different cell technologies – crystalline, polycrystalline, silicon film, and triple-junction amorphous. Two panels using each cell technology were present, one without any insulation attached to its rear surface and one with insulation having a nominal thermal resistance value of 3.5 m2·K/W attached to its rear surface. The performance data associated with these eight panels, along with meteorological data, were compared to the predictions of a photovoltaic model developed jointly by Maui Solar Software and Sandia National Laboratories (SNL), which is implemented in their IV Curve Tracer software [1]. The evaluation of the predictive performance tools was done in the interest of refining the tools to provide BIPV system designers with a reliable source for economic evaluation and system sizing.

A Novel Combined Power and Cooling Thermodynamic Cycle for Low Temperature Heat Sources: Part I — Theoretical Investigation

A combined thermal power and cooling cycle proposed by Goswami is under intensive investigation, both theoretically and experimentally. The proposed cycle combines the Rankine and absorption refrigeration cycles, producing refrigeration while power is the primary goal. A binary ammonia-water mixture is used as the working fluid. This cycle can be used as a bottoming cycle using waste heat from a conventional power cycle or an independent cycle using low temperature sources such as geothermal and solar energy. Initial parametric studies of the cycle showed the potential for the cycle to be optimized for first or second law efficiency, as well as work or cooling output. For a solar heat source, optimization of the second law efficiency is most appropriate, since the spent heat source fluid is recycled through the solar collectors. The optimization results verified that the cycle could be optimized using the Generalized Reduced Gradient method. Theoretical results were extended to include realistic irreversibilities in the cycle, in preparation for the experimental study.