Thermal Efficiency Model Research Articles

Experimental Study to Characterize the Performance of Combined Photovoltaic/Thermal Air Collectors

Combined photovoltaic/thermal (PV/T) collectors show great potential for reaching the objective of net-zero energy consumption in buildings, but the number of products on the market is still very limited. One of the reasons for the slow market uptake of PV/T collectors is the absence of standardized methods to characterize their performance. Performance characterization is a challenge for PV/T collectors because of the interaction between the thermal and electrical yield. This study addresses this particular issue for PV/T air collectors used in either closed-loop or open-loop configurations. In particular, it presents the potential of the equivalent cell temperature method to determine the temperature of the PV cells in a PV/T air collector and validates models to predict the thermal performance and cell temperature for this particular type of solar collector. Indoor and outdoor experimental tests were performed on two c-Si unglazed PV/T modules. The indoor part of this procedure provided the thermal diode voltage factor and the open-circuit voltage temperature coefficient, two parameters that are essential in the calculation of the equivalent cell temperature. The outdoor procedure consisted of acquiring simultaneous electrical and thermal measurements at various inlet temperatures and flowrates. For the collector used in a closed-loop configuration, thermal efficiency models using the fluid inlet, outlet, or average temperature in the calculation of the reduced temperature provided similar results. For an open-loop configuration, a thermal efficiency model as a function of the fluid outlet flowrate was found to be more appropriate. Using selection of variable methods, it was found that a multiple linear regression model using the fluid inlet temperature, the irradiance, and the fluid outlet temperature as predictive variables could be used to estimate both the PV module back surface average temperature and the equivalent cell temperature. When using the PV temperature predicted by these models in the electrical efficiency model, both PV temperatures showed similar performance. In collectors where the PV back surface temperature is not accessible for temperature sensors mounting, the equivalent cell temperature provides a valuable alternative to be used as the PV temperature. The PV/T collector thermal and electrical performance in either closed-loop or open-loop configurations was found to be encapsulated with a series of five-plots.

99/01617 Health effects of indoor fluoride pollution from coal burning in China : Ando, M. et al. Environ. Health Perspect., 1998, 106, (5), 239–244

Even though ratio of air to fuel is the major factor in determining the thermal efficiency of a boiler, how to optimize the ratio in the real operation is still more like art than science, especially for a gas/solid multi-fuel combustion system. This study, taking operation data from a real gas/solid fuels co-firing boiler, is aimed to develop the thermal efficiency model. To cope with the complexity of combustion mechanism in a co-firing boiler, this study proposed a two-tier approach to modeling thermal efficiency. The first tier is to transform the plant input variables into the derived variables, which are physically and statistically meaningful to the model-building. In the second tier, this study proposed an adaptive modeling approach by employing nonnegative garrote variable selection and auto-regression integrated moving average correction. Both approaches are first time proposed in building the thermal efficiency model for boilers. The prediction error of boiler thermal efficiency made by the acquired model reaches less than 0.03%. The high accuracy of the proposed modeling approaches makes the implementation of the model-based control of ratio of air to fuel for improving boiler’s efficiency readily practicable.

Steamflooding Cold Lake Oil Reservoirs Through a Bottomwater Zone: A Scaled Physical Model Study

Summary A series of experiments was conducted in one-eighth of a five-spot-pattern, high-pressure, scaled physical model to evaluate the potential of steamflooding oil-sand reservoirs through a bottomwater zone in Cold Lake, Alta. During the experiments, steam was injected into the bottomwater zone at a constant rate until steam breakthrough occurred at the production well. The steam injection rate then was reduced to limit steam production. Results demonstrate that the process is influenced by the steam injection flow rate because of the important role played by gravity override. Increasing the steam injection rate beyond an optimum value results in decreased oil/steam ratios (OSR's) and reduced final oil recovery because steam channels to the production well. A delay in oil production was noticed in all experiments. A moving-heat-source, gravity-override, analytical model was used to investigate the mechanisms of reservoir heating in the presence of steam-gravity override. In addition, the thermal efficiency of the process, determined from the experiments and extrapolated to field conditions, was compared with predictions from Prats' thermal efficiency model. Prats' model predicted the measured thermal efficiency reasonably well at lower injection rates. As the injection rate increased, however, larger differences between Prats' model and the experiments were noticed.