Thermal Performance Evaluation of a Residential Solar/Gas Hybrid Water Heating System

In climate regions with lower average daily solar radiation, such as the Pacific Northwest, a solar energy collector might not economically satisfy year-round domestic water heating demands, requiring an auxiliary unit, such as a natural gas water heater. Previous studies of such hybrid systems have shown that the efficiencies achieved while running in combined solar/gas mode was lower than expected. This inefficiency was attributed to a reduction in gas burner efficiency when the process fluid was partially pre-heated by the solar input. To predict the actual energy and cost savings under various design conditions, the performance of solar/gas hybrid systems must be better understood. In this work, the performance of a commercial hybrid solar/gas system is experimentally characterized to evaluate individual component and overall system efficiency. The hybrid water heating system consisted of three flat plate collectors arranged in series (total area = 6.44 m2), and a 22.3 kW natural gas burner. Under different temperature lifts and solar insolation values, the system was operated at three different modes of heating: solar, gas, and combined solar/gas mode. Efficiency value for each mode was calculated. Based on the experimental efficiency results, a configuration that would provide higher efficiency for combined solar/gas heating is suggested

Evaluation of Heat and Mass Transfer Models for Sizing Low-Temperature Kalina Cycle Microchannel Condensers

Waste heat driven ammonia/water Kalina cycles have shown promise for improving the efficiency of electricity production from low-temperature reservoirs (T < 150 °C). However, there has been limited application of these systems to utilize widely available, disperse, waste heat streams for smaller scale power production (1–10 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 material cost, while also reducing the working fluid inventory, enabling penetration of Kalina cycles into these new markets. However, accurate methods of predicting the heat and mass transfer in microscale geometries must be available for designing and optimizing these compact heat exchangers. In the present study, the effect of different heat and mass transfer models on the calculated Kalina cycle condenser size is investigated at representative system conditions. A detailed heat exchanger model for a liquid-coupled microchannel ammonia/water condenser is developed. The heat exchanger is sized using different predictive methods to provide the required heat transfer area for a 1 kWe Kalina system with a source and sink temperature of 150 °C and 20 °C, respectively. The results show that for the models considered, predicted heat exchanger size can vary by up to 58%. Based on prior experimental results, a nonequilibrium approach is recommended to provide the most accurate, economically sized ammonia/water condenser

Experimental investigation of supercritical carbon dioxide in horizontal microchannels with non-uniform heat flux boundary conditions

Supercritical carbon dioxide (sCO2) experiences a drastic change in its thermophysical properties near the thermodynamic critical point. A non-linear thermophysical property variation can influence the heat transfer behavior of sCO2 which is not predicted well by conventional single phase heat transfer theory. This can become a major hindrance in the effective design of heat exchangers using sCO2 as a heat transfer fluid and operating in the vicinity of the critical point. Previous investigations of sCO2 heating have been primarily focused on macroscale, circular and uniformly heated channels at relatively low heat fluxes. It is unclear if models and correlations developed from large circular tube data can be scaled down to the microscale, non-circular channels subject to non-uniform heating. The present study experimentally investigates the turbulent heat transfer performance of sCO2 in a microchannel heat exchanger operating in a horizontal configuration with a single wall non-uniform heat flux boundary condition. The test section has five parallel channels with a 0.75 mm hydraulic diameter and an aspect ratio of 1. The channels are fabricated using computer numerical control machining and the test section sealed using a diffusion bonding approach. Data analysis techniques which employ 2-D and 3-D heat transfer models of the experimental test section are developed to calculate the average heat transfer coefficients for a given set of experimental conditions. Data are obtained over a wide range of experimental parameters including test section applied heat flux, mass flux , reduced pressure, and inlet temperatures. The heat transfer data were screened for the presence of buoyancy and flow acceleration effects and then compared against correlations developed for turbulent subcritical and supercritical fluid flows

Investigation of the combined efficiency of a solar/gas hybrid water heating system

In climate regions with large seasonal variations in solar radiation, such as the Pacific Northwest of the United States, a solar thermal energy collector might not economically satisfy year-round domestic water heating demands, requiring an auxiliary unit, such as a natural gas-fired water heater. Previous studies have shown that the burner efficiency of a gas-fired water heater varies depending on the log-mean temperature difference between the cold fluid (water) and the hot fluid (combustion gases). In a solar/gas hybrid water heating system, where solar collectors are used in conjunction with a gas-fired heater, the partial heating of water provided by solar input reduces the log-mean temperature difference value for the gas heater, reducing the efficiency of the gas burner. Since this efficiency reduction varies depending on the amount of pre-heating provided by solar energy input, it is difficult to accurately predict the actual cost and energy savings offered by solar/gas hybrid water heaters in different climates and operation scenarios. Hence, to predict the actual energy and cost savings under various design conditions, the performance of solar/gas hybrid systems must be better understood. The objective of this work is to experimentally determine the thermal performance of a solar/gas water hybrid water heating system with a 6.44 m2 flat plate solar collector array and a 22.3 kW natural gas burner in Corvallis, Oregon, USA. Under different temperature lifts and solar insolation values, the system was operated at three different modes of heating: solar, gas, and combined solar/gas mode. The overall system thermal efficiency value for each mode is calculated. The efficiency of the solar collector heating system was found to be 41.97%, 39.82%, and 35.05% at initial water temperature of 20, 30, and 51.5 °C, respectively. For initial water temperatures of 20, 30, and 51.5 °C, the efficiency of the gas burner was found to be 69.2%, 66.4%, and 65.5% at the HHV, and 76.7%, 73.6%, and 72.6% at the LHV of natural gas, respectively. In the combined solar/gas heating mode, the efficiency of the gas burner decreased with increasing solar fraction. For solar fractions of 4.93%, 9.40%, 11.39%, and 14.27%, the efficiency of the gas burner in terms of the HHV of natural gas was found to be of 69.08%, 66.80%, 66.17%, and 65.18%, respectively. Based on the experimental results, a configuration that would provide higher overall system efficiency for combined solar/gas heating is suggested

Numbering-Up of Microscale Devices for Megawatt-Scale Supercritical Carbon Dioxide Concentrating Solar Power Receivers

Concentrated solar power (CSP) plants have the potential to reduce the consumption of nonrenewable resources and greenhouse gas emissions in electricity production. In CSP systems, a field of heliostats focuses solar radiation on a central receiver, and energy is then transferred to a thermal power plant at high temperature. However, maximum receiver surface fluxes are low (30–100 W cm−2) with high thermal losses, which has contributed to the limited market penetration of CSP systems. Recently, small (∼4 cm2), laminated micro pin-fin 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 unit cells as building blocks for a megawatt-scale (250 MW thermal) open solar receiver through a numbering-up approach, where multiple microscale unit cell devices are connected in parallel. A multiscale model of the full-scale central receiver is developed. The model consists of interconnected unit cell and module level (i.e., multiple unit cells in parallel) submodels which predict local performance of the central receiver. Each full-scale receiver consists of 3000 micro pin-fin unit cells divided into 250 modules. The performance of three different full-scale receivers is simulated under representative operating conditions. The results show that the microscale unit cells have the potential to be numbered up to megawatt applications while providing high heat flux and thermal efficiency. At the design incident flux and surface emissivity, a global receiver efficiency of approximately 90% when heating sCO2 from 550 °C to 650 °C at an average incident flux of 110 W cm−2 can be achieved

The Example of State Forestry Information System (SFIS) Data Application to Prognosis of Changes in Pine Forests

W pracy pokazano zastosowanie danych z podsystemu «Las» Systemu Informatycznego Lasów Państwowych (SILP) w Polsce do prognozowania zmian lasów sosnowych. Zaprezentowano specjalnie opracowany blok «GAP TAKSACYJNY» modelu komputerowego pozwalający na automatyczne wczytywanie uśrednionych danych taksacyjnych dla wydzieleń leśnych i przeprowadzenie prognoz na przykładzie nadleśnictwa Tuczno z Wielkopolski.The paper shows the application of data from the subsystem «Forest» State Forestry Information System (SILP) in Poland for forecasting changes in pine forests. There was presented a specially designed block «GAP TAKSACYJNY» of computer model that allows the automatic loading of sub-compartment forests data and carrying out prognosis on the example of forest district Tuczno in Wielkopolska

Micro Structured Sensors for Neutron Detection

The shortage of 3He gas, identified as a problem several years ago, initiated research into alternative neutron detectors for various applications. One such technology is the microstructured semiconductor neutron detector (MSND). These compact detectors have microstructures etched deeply into the substrates that are subsequently backfilled with neutron reactive material. Single sided devices typically have thermal neutron detection efficiencies exceeding 30%, while double sided microstructured semiconductor neutron detectors (DS-MSND) have yielded \u3e69% thermal neutron detection efficiency. Both MSNDs and DS-MSNDs have been integrated into compact low-noise and low-power electronics modules. Dosimetry calculations indicate that these detectors can be used as active wearable neutron dosimeters. A discussion on the physics, performance and instrumentation of these MSNDs will be presented. The radiation environment in a nuclear reactor precludes the use of semiconductor detectors for in-core sensors, leading to the invention of another miniaturized neutron detector, the micro- pocket fission detector (MPFD). The detectors were developed for real time reactor power monitoring and also for pulse tracking for power excursion experiments. These miniaturized fission chambers have gas pockets on the order of 1 mm3 with a small concentration of uranium electrodeposited inside the gas chamber. The detectors are composed of radiation hard materials and assembled without adhesives. The small geometries can be assembled in arrays to transmit reactor power at various locations. Stable device operation was confirmed by testing under steady-state reactor conditions. Reactor power transients were observed in real-time. Design details and performance of MPFDs will be presented