Louay M. Chamra

Performance analysis of different working fluids for use in organic Rankine cycles

Abstract This article presents a second-law analysis for the use of organic Rankine cycle (ORC) to convert waste energy to power from low-grade heat sources. The organic working fluids were selected to investigate the effect of the fluid boiling point temperature on the performance of ORCs. The working fluids under investigation are R134a, R113, R245ca, R245fa, R123, isobutane, and propane, with boiling points between 243 and 48 °C. The results are compared with those of water under similar conditions. A combined first- and second-law analysis is performed by varying some system operating parameters at various reference temperatures. Some of the results demonstrate that ORC using R113 shows the maximum efficiency among the evaluated organic fluids for temperatures >430 K; R123, R245ca, and R245fa show the best efficiencies for temperatures between 380 and 430 K; and for temperatures <380 K, isobutane shows the best efficiency. Also, it is shown that the organic-fluid boiling point has a strong influence on the system thermal efficiency.

Pressure drop and heat transfer comparison for both microfin tube and twisted-tape inserts in laminar flow

Abstract Experiments were carried out to compare pressure drop and heat transfer coefficients for a plain, microfin, and twisted-tape insert-tubes. The twisted-tape experiments include three different twist ratios each with two different widths. The data were taken at Reynolds numbers well in the laminar region. The heat transfer data were obtained in a single shell-and-tube heat exchanger where steam is used as a heat source to obtain a uniform wall temperature and the working fluid in the tube is oil. The twist ratio and the width of the tape seem to have a large effect on the performance of the twisted-tape insert. The results demonstrate that as the twist ratio decreases, the twisted-tape will give better heat transfer enhancement. The loose-fit (W=10.8 mm) is recommended to be used in the design of heat exchanger where low twist ratios (Y=5.4, and Y=3.6) and high pressure drop situations are expected since it is easier to install and remove for cleaning purposes. Other than these situations, the tight-fit tape gives a better performance over the loose-fit tape. For the microfin tube tested in this paper, the data shows a small increase in both heat transfer and pressure drop. This type of microfin tube is not recommended to be used in laminar flow conditions.

Cooling, heating, and power energy performance for system feasibility:

Abstract Buildings with heating and cooling energy requirements are usually supplied by separated systems such as furnaces or boilers for heating, and vapour compression systems for cooling. For these types of buildings, the use of cooling, heating, and power (CHP) systems is an alternative for energy savings. Different investigations have claimed that the use of CHP systems reduces the energy consumption related to transmission and distribution of energy. However, most of these analyses are based on the reduction of operating cost without measuring the actual energy use reduction. In this study, the definition of building primary energy ratio (BPER) is introduced as a new parameter to evaluate the CHP energy performance. BPER measures the variation of the building primary energy (BPE) when the building is operated without a CHP system versus the BPE when a CHP system is used. Results show that using the thermal efficiency alone is not the best approach to describe the CHP system energy performance and that using the BPER provides a more comprehensive CHP evaluation. For this investigation, values of BPER greater than 1 indicate that primary energy is being saved for that specific time, which makes this concept a reliable tool for the CHP design and operational control.

Second Law Analysis and Optimization of Organic Rankine Cycle

This paper presents a second law analysis and optimization for the use of Organic Rankine Cycle “ORC” to convert waste energy to power from low grade heat sources. The working fluids used in this study are organic substances which have a low boiling point and a low latent heat for using low grade waste heat sources. The organic working fluids under investigation are R134a and R113 and their results are compared with those of ammonia and water under similar operating conditions. A combined first and second law analysis is performed by varying some system operating parameters at various reference temperatures. Some of the results show that the efficiency of ORC is typically below 20% depending on the temperatures and matched working fluid. In addition, it has been found that organic working fluids are more suited for heat recovery than water for low temperature applications, which justifies the use of organic working fluids at the lower waste source temperatures.Copyright

Supervisory Feed-Forward Control for Real-Time Topping Cycle CHP Operation

This paper presents an energy dispatch algorithm for real-time topping cycle cooling, heating, and power (CHP) operation for buildings with the objective of minimizing the operational cost, primary energy consumption (PEC), or carbon dioxide emission (CDE). The algorithm features a supervisory feed-forward control for real-time CHP operation using short-term weather forecasting. The advantages of the proposed control scheme for CHP operation are (a) relatively simple and efficient implementation allowing realistic real-time operation, (b) optimized CHP operation with respect to operational cost, PEC, or CDE, and (c) increased site-energy consumption resulting in less dependence on the electric grid. In the feed-forward portion of the control scheme, short-term electric, cooling, and heating loads are predicted using the U.S. Department of Energy benchmark small office building model. The results are encouraging regarding the potential saving of operational cost, PEC, and CDE from using the control system for a CHP system with electric and thermal energy storages.

Methodology to perform a non-conventional evaluation of cooling, heating, and power systems

Abstract Cooling, heating, and power (CHP) is a distributed generation technology that can provide electricity and heat while improving the overall thermal energy efficiency of a building. The evaluation and comparison of this technology versus conventional technologies cannot be limited to economical considerations only. Therefore, a non-conventional evaluation, based on non-economical aspects, is necessary to show the additional benefits that can be obtained from the CHP technology. A non-conventional evaluation includes aspects such as: environmental quality, energy-efficient buildings, power reliability, power quality, fuel source flexibility, etc. Some benefits of these non-conventional evaluations can be factored into an economic evaluation but others give intangible potential to the technology. The current paper presents a methodology to evaluate CHP systems based on two non-conventional aspects: energy-efficient buildings and emission of pollutants. Using the methodology described in the current paper it can be demonstrated that the use of CHP systems could improve the Energy Star rating of a building in more than 50 points. The improvement on the Energy Star rating is significant on the Leadership in Energy and Environmental Design rating as a building can score up to ten points of the 23 available in the energy and atmosphere category on energy efficiency alone. As much as eight points can be obtained in this category due to the Energy Star rating increment from the use of CHP systems. Also, using the proposed methodology it can be demonstrated that CHP systems have the ability to significantly reduce emission of pollutants. For carbon dioxide a reduction around 50 per cent can be reached, for nitrogen oxides the reduction can be in the order of 75 per cent, while for sulphur dioxide the reduction is higher than 90 per cent.

Exergy analysis of a combined engine-organic Rankine cycle configuration

Abstract This article presents an exergy analysis of a combined engine-organic Rankine cycle configuration (E-ORC) using the exergy topological method. A detailed roadmap of exergy flow is presented using an exergy destruction chart to clearly depict the exergy accounting associated with each thermodynamic process. The analysis indicates that an ORC combined with an engine not only improves the engine thermal efficiency but also increases the exergy efficiency. Different organic fluids are evaluated in this article. Depending on the organic fluid employed, the thermal and exergy efficiencies could be increased by approximately 10 per cent. Parameters such as the thermodynamic influence coefficient and degree of thermodynamic perfection are identified as useful design metrics to assist exergy-based design of devices. This article also examines the effect of the pinch-point temperature difference (PPTD) on the E-ORC performance. Results show that the lower the PPTD the higher the thermal and exergy efficiencies.

A Simplified Model of Heat and Mass Transfer Between Air and Falling-Film Desiccant in a Parallel-Plate Dehumidifier

This paper presents a simplified model to predict the heat and mass transfer between air and falling-film liquid desiccant during dehumidification in a parallel-plate absorber. First-order, ordinary differential equations are used to estimate the heat and mass transferred, and explicit equations are derived from conservation principles to yield the exiting absorber conditions for different flow arrangements. The developed model uses a control volume approach that accounts for the change in film thickness and property values. The model results were within 5% of a more complicated parallel-flow model currently available in literature. The model was also in good agreement with existing experimental data for a counterflow absorber.

First and second law analysis of a Stirling engine with imperfect regeneration and dead volume

Abstract This article discusses the thermodynamic performance of an ideal Stirling cycle engine. This investigation uses the first law of thermodynamics to obtain trends of total heat addition, net work output, and thermal efficiency with varying dead volume percentage and regenerator effectiveness. Second law analysis is used to obtain trends for the total entropy generation of the cycle. In addition, the entropy generation of each component contributing to the Stirling cycle processes is considered. In particular, parametric studies of dead volume effects and regenerator effectiveness on Stirling engine performance are investigated. Finally, the thermodynamic availability of the system is assessed to determine theoretical second law efficiencies based on the useful exergy output of the cycle. Results indicate that a Stirling engine has high net work output and thermal efficiency for low dead volume percentages and high regenerator effectiveness. For example, compared to an engine with zero dead volume and perfect regeneration, an engine with 40 per cent dead volume and a regenerator effectiveness of 0.8 is shown to have ∼60 per cent less net work output and a 70 per cent smaller thermal efficiency. Additionally, this engine results in approximately nine times greater overall entropy generation and 55 per cent smaller second law efficiency.

Hybrid-cooling, combined cooling, heating, and power systems

Abstract Combined cooling, heating, and power (CCHP) systems have the ability to optimize fuel consumption by recovering thermal energy from the prime mover of the power generation unit (PGU). Design of a CCHP system requires consideration, among other variables, of CCHP system components size and type. This study focuses on the analysis of hybrid-cooling, heating, and power (hybrid-cooling CCHP) systems that have an absorption chiller (CH) and a vapour compression system to handle the cooling load. The effect of the size of both cooling mechanisms is analysed in conjunction with the PGU size and efficiency. For better energy performance analysis simulations, results are presented based on the building-CCHP system primary energy consumption (PEC). Hybrid-cooling CCHP systems yield higher primary energy reduction than CCHP systems with an absorption CH alone. To account for the effect of climate conditions, hot and cold climates were considered by performing simulations for Tampa and Chicago weather conditions. The results are presented in tabular form to show the value of the PEC reduction as a function of the PGU size and efficiency, and the size of the absorption CH.