On the selection of fin profiles for OTEC plate-fin evaporators

This paper considers the plate-fin heat exchanger for an OTEC power plant on account of its compactness. Ammonia is selected as the working fluid for a plant of 1 MW gross output. Choice of seawater side velocity is made on the basis of pressure drop and biofouling. Fourteen different fin profiles are considered in order to choose a suitable configuration. The selection of the plate-fin heat exchanger depends on minimum exchanger volume, minimum pumping power requirement and low ammonia side pressure drop.     

Effects of working fluids on the performance of a bi-directional thermodiode for solar energy utilization in buildings

A series of experiments were conducted to investigate the effects of different working fluids on the behavior and performance of a bi-directional thermodiode. The thermodiode was made up of two rectangular loops mounted between a collector plate and a radiator plate. The loops were filled with a working fluid for effective heat transfer when the thermodiode was forward biased. Five different working fluids were tested with thermal conductivity values ranging from 0.1 to 0.607 W/m-K, thermal expansion coefficient values ranging from 2.54 × 10⁻⁴ to 1.43 × 10⁻³ 1/K, and kinematic viscosity values ranging from 6.5 × 10⁻⁷ to 1 × 10⁻⁴ m²/s. The thermodiode was heated by a radiant heater consisting of 88 halogen lamps that generated a heat flux of about 10³ W/m² on the collector surface. Experimental results indicated that the onset time for natural convection to be induced throughout the diode system did not differ considerably when different working fluids were used. On the other hand the required fluid temperature differences in the loops for the onset of throughflow were quite different and depended strongly upon the viscosity and other properties such as thermal expansion coefficient and specific heat of the working fluid. Of the five fluids tested, water and low-viscosity silicon oil had the highest heat transfer rate. An analytical model was developed to predict and analyze the steady operation of the diode system when different working fluids are used.     

Heat transfer design in adsorption refrigeration systems for efficient use of low-grade thermal energy

Adsorption refrigeration and heat pump systems have been considered as important means for the efficient use of low-grade thermal energy of 60–150 °C. Sorption systems are merely thermodynamic systems based on heat exchangers, and therefore a good design to optimize heat and mass transfer with reaction or sorption processes is very important, for which the notable technique is the use of expanded graphite to improve both heat and mass transfer in the chemisorption beds. Studies have also shown the need to enhance the heat transfer in adsorption bed by matching with the efficient heat transfer of thermal fluids. Heat pipes and good thermal loop design coupled with adsorption beds could yield higher thermal performance of a sorption system. A novel design with passive evaporation, known as rising film evaporation coupled with a gravity heat pipe was introduced for high cooling output. It has also been shown that the performance of traditional heat and mass recovery in the sorption systems is limited, and novel arrangement of thermal fluid and refrigerant may improve the performance of sorption systems. Based upon the above researches, various sorption systems have been developed, and high performances have been reached.     

Economic assessment of the engineering basis for wind power: Perspective of a vertically integrated utility

If wind park configurations are globally coordinated across the service area of a power utility, then electricity can be generated for the grid with substantial cost advantages. Based on this premise, the paper introduces a model by which large scale assessment of grid connected wind based power generation may be undertaken for a utility service area. The model can be useful to the policy maker for decisions regarding suitable wind portfolio standards (WPS) definition. The utility on the other hand, may use the model to study its service area for prospective wind based generation. Aspects of the problem modelled include cost-of-energy from individual generating units, daily load variations for the utility with emphasis on limited penetration, features of wind at prospective installation sites, makes of wind energy conversion systems (WECS) available, and recovery of expenditure through revenue. Application of the model to an assessment exercise for the state of Andhra Pradesh (India) is presented as an example.     

Economic analysis of power generation from parabolic trough solar thermal plants for the Mediterranean region—A case study for the island of Cyprus

In this work a feasibility study is carried out in order to investigate whether the installation of a parabolic trough solar thermal technology for power generation in the Mediterranean region is economically feasible. The case study takes into account the available solar potential for Cyprus, as well as all available data concerning current renewable energy sources policy of the Cyprus Government, including the relevant feed-in tariff. In order to identify the least cost feasible option for the installation of the parabolic trough solar thermal plant a parametric cost–benefit analysis is carried out by varying parameters, such as, parabolic trough solar thermal plant capacity, parabolic trough solar thermal capital investment, operating hours, carbon dioxide emission trading system price, etc. For all above cases the electricity unit cost or benefit before tax, as well as after tax cash flow, net present value, internal rate of return and payback period are calculated. The results indicate that under certain conditions such projects can be profitable.     

Modeling the coupling between free and forced convection in a vertical permeable slot: Implications for the heat production of an Enhanced Geothermal System

The aim of the hydraulic stimulations in the Soultz-sous-Forêts, France, Enhanced Geothermal System (EGS) project was to create, in crystalline rocks, a fractured reservoir 750 m high, 750 m long and 35 m thick interconnecting the injection and production wells. Increasing the permeability in a zone with a high geothermal gradient will trigger free convection, which will interact with the forced flow driven by pumping. A systematic numerical study of the coupling between forced and free convective flows has been performed by considering a large range of injection rates and Rayleigh numbers. The simulations showed that if there is weak or no free convection in an EGS reservoir, economic exploitation of the system will rapidly end because of a decrease in produced fluid temperature. The maximum injection rate preventing such a temperature drop increases with the Rayleigh number and the height of the stimulated domain. The model establishes constraints on the conditions for achieving optimal heat extraction at the Soultz-sous-Forêts EGS site. It was also shown that, although mineral precipitation may partially close or heal some open fissures, it does not lead to a major decrease of the hydraulic conductivity in the stimulated reservoir.