Pedro J. Mago

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.

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

Heat and mass transfer on a cylinder surface in cross flow under supersaturated frosting conditions

In this paper a semi-empirical model describing heat and mass transfer on a cylinder surface in humid air cross flow under supersaturated frosting conditions is presented. The lack of psychrometric data in the supersaturated zone of the psychrometric chart has historically impeded the ability of researchers to accurately predict heat and mass transfer in supersaturated air. The work described in this paper has been partially made possible by developing a systematic procedure to compute the properties of supersaturated moist air, especially in the low temperature zone of the psychrometric chart. Development of such a capability will allow us to predict the amount of frost collected on a coil, the frost deposition and coil heat transfer rates, frost thickness and frost surface temperature, and other important coil frost parameters under supersaturated conditions.

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 study of the performance of a hybrid liquid desiccant cooling system using lithium chloride

This paper presents field test of a hybrid solar liquid desiccant cooling system conducted at a test house at the University of Floridas Energy Research and Education Park. These tests consisted of operating the air conditioning system at the test house in two configurations: the conventional vapor compression system and the hybrid desiccant system. Experiments were conducted to study the influence of the air mass flow rate, temperature of the inlet air, temperature of the desiccant, and desiccant mass flow rate on the performance of both system configurations. Based on the field test results, it was found that the hybrid desiccant system improves the air conditioning performance in the field test house by decreasing the outlet humidity and temperature of the air.

Modeling the Cooling Process Path of a Dehumidifying Coil Under Frosting Conditions

Whenever humid air comes in contact with a cooling coil whose temperature is below both the dew-point of water vapor in air and the freezing point, frost will form. The nature of the frost forming on the coil will depend to a large measure on the psychrometric conditions prevailing inside the freezer and whether the air around the coil is subsaturated or supersaturated. Psychrometric theory and the apparatus-dew-point calculating procedure assume that the cooling process path as the air passes through the coil is a straight-line on the psychrometric chart. The actual path is however a result of a much more complex series of processes and is therefore a curve. While researchers have calculated the actual process path on a dehumidifying coil, none has attempted to do the same for a frosted, multi-row coil. It is believed that determining the actual conditions leaving a given row in a multi-row freezer coil is a crucial step in identifying the coil location in the vicinity of which the transformation from the subsaturated zone to the supersaturated zone occurs. This will prove a key step in identifying a demarcation line between the unfavorable snow-like frost and the more traditional (and more favorable) frost formation patterns. Thus, the objective of this paper is to calculate the air path on an actual industrial-size finned-tube, multi-row coil utilizing experimentally derived data and correlate the shape of the path with the prevailing psychrometric conditions in the freezer in the hope of identifying the demarcation line in question.

Analysis of combined cooling, heating, and power systems operating following the electric load and following the thermal load strategies with no electricity export

Technologies such as cogeneration and trigeneration have great potential for energy and emission reduction because these technologies make better use of fuels by recovering waste heat to satisfy thermal loads. Operation of a system that involves several types of equipment operating as one unit, that at the same time interact with the building to meet its energy demand, requires an operational strategy that makes the system operate properly. This means the system must be able to respond to the building energy demand while having the best performance within the constraints imposed by the operational strategy. When a cogeneration system (combined heating and power) or trigeneration system (combined cooling, heating, and power) operates at partial load, the operational strategy has particular effect on the performance of the system. Two common operational strategies to operate these systems are following the electric load and following the thermal load. This article presents a methodology that allows selecting the right operational strategy based on the ratio between the building electric and thermal loads, and the ratio between electricity demand and size of the power generation unit when exporting electricity is not an option. Results show that the following the thermal load strategy seems to be better than the following the electric load strategy for most cases. Therefore, the methodology presented in this article is a decision-making tool for the selection of the right operational strategy.

Analytic Solutions for Optimal Power Generation Unit Operation in Combined Heating and Power Systems

This paper presents an analytic approach for defining optimal operation decisions for a power generation unit (PGU) in combined heating and power (CHP) systems. The system is optimized with respect to cost, and the independent variables are the thermal load and the electric load. Linear programming is a common tool used to find the optimal PGU operation for a given combination of thermal and electric loads, but these methods are more computationally intensive than the analytical approach proposed in this paper. The analytic process introduced in this paper shows that the optimal PGU operation for all possible thermal and electric loads can be decided by simple and explicit equations even when the efficiency of the PGU is allowed to vary with PGU loading. Moreover, the analysis reveals that for all possible load conditions, the optimal CHP system operation is based on either following the electric load (FEL) or following the thermal load (FTL) strategies. The cost ratio, i.e., the ratio of the electricity price to the fuel price, is introduced as the key parameter used for making optimal decisions. Cost ratios in Chicago, IL and Philadelphia, PA are used as case studies to show the effect that different cost ratios have on the optimal operation decisions for each possible input load.