یک تحقیق تجربی از بهره برداری از یک مبدل حرارتی تماس مستقیم برای بهره برداری خورشیدی

The use of a direct contact heat exchanger (DCHX), if properly applied, will allow several benefits. Primarily these include the elimination of the cost of a closed heat exchanger and the ability to operate with much lower temperature differences. This paper examines the operation of a liquid–liquid type DCHX in harnessing the solar energy. Heat is delivered to the working fluid (heat transfer fluid) in the collection loop composed of solar collectors and a circulation pump. Two different kinds of working fluid were tested for their thermal characteristics that are immiscible with water. Texatherm 46 and diethyl phthalate (C6H4(CO2C2H5)2) and those that are experimented in the present analysis. Different schemes were used to introduce these fluids into the DCHX as they are either lighter (Texatherm 46) or heavier (diethyl phthalate) than water. A series of outdoor tests were conducted to determine the overall performance of DCHX as well as transient behaviors as the sun's energy is exploited. It is worthwhile to note that no thermal stratification was observed throughout the DCHX when in operation regardless of the working fluid. Stability and thermal performance, however, appear to improve when the working fluid is dispersed from the top of a DCHX. A difference of 8% is measured in the heat exchanger effectiveness, which gives a measure of the heat exchanger's overall ability in heat transfer.

The use of a direct contact heat exchanger (DCHX), if properly applied, could give a good solution to the freezing problem in wintry seasons, which has long been a major hurdle in harnessing the solar energy. It will not merely allow a technical solution to the problem but also several benefits. Primarily these include the elimination of the cost of a closed heat exchanger and the ability to operate with much lower temperature differences [1]. The temperature driving force required for the conventional heat exchanger is greatly reduced because of the direct contact between the two fluids without any intervening solid surfaces. Systems using DCHXs, however, are quite similar to systems using indirect heat exchangers in many respects. In these systems, the DCHX unit can be combined with the thermal storage unit, or alternatively, it can be used separately from the storage unit, much like an external (to storage) closed heat exchanger system. Hence either the DCHX system or the closed heat exchanger system can operate with the same number of pumps/loops. A DCHX generally relies upon the gravitational force to accomplish the fluid flow within the device. In most direct contact liquid–liquid heat exchangers, oil or hydrocarbon with a density less than water is normally used as the dispersed working fluid—the working fluid that is dispersed in a DCHX. In this case, the lighter fluid is injected into the DCHX (so-called, “spray column”) through a perforated device (or a sparger) at the bottom of the DCHX [2] and [3]. This arrangement sometimes requires the control of the interface at the top of the DCHX which is formed by the coalescence of the dispersed working fluid. The interface must remain fixed as water is introduced into the DCHX immediately below the interface. In addition, the rate of coalescence of the dispersed working fluid (arising as small droplets) has to be controlled appropriately as it influences the location of the interface. The rate of coalescence can be catalyzed by introducing a honeycomb structure (easily become wet by the dispersed phase) at the desired interface location. Apart from the arrangement above, a consideration could be made to use a liquid that is heavier and immiscible with water. Of course, the liquid should have higher boiling and lower freezing temperatures [4]. In such arrangement, it is possible to eliminate the internal structures to enhance the coalescence of the dispersed working fluid mentioned above. The perforated plate at the top of the DCHX evenly breaks the working fluid into small particles and uniformly distributes them before they start their journey through the water body contained in the DCHX. There is no need for a device to catalyze the coalescence of droplets or to adjust the interface with the water as in the case of the light working fluids. However, this does not mean that heavier fluids perform better than lighter fluids as there are many factors influencing the thermal behavior of a working fluid associated with the operation of a DCHX (Fig. 1). Full-size image (20 K) Fig. 1. Schematic diagram of the experimental setup. (a) A working fluid injected into a lighter fluid. (b) A working fluid dispersed in a heavier fluid. Figure options In the present investigation, two working fluids, Texatherm 46 (density: 0.872 at 15 °C) and diethyl phthalate (density: 1.117 at 20 °C), were examined for the operation of a liquid–liquid type DCHX in exploiting the solar energy. A series of outdoor tests were conducted measuring their performance during the hours of bright sunshine.

An experimental study was carried out to study the thermal performance of a liquid–liquid type direct contact heat exchanger (DCHX) in utilizing solar energy. Two sets of the experimental system were made that were composed of a solar collector, a pump and a DCHX. The system operates in such a way that a working fluid circulates between the solar collector and DCHX repetitiously. Two different kinds of working fluid were tested that are immiscible with water: Texatherm 46 and diethyl phthalate (C6H4(COOCH3)2). Texatherm 46 is lighter than water whereas diethyl phthalate is heavier. Observations were made regarding the hydrodynamic and thermal characteristics of these fluids as they travel through the water column of the DCHX in small drops. The mechanism of drop formation appears to be very important as it dictates the drop size and consequently affects the overall thermal performance of the DCHX. Sparging the working fluid from top with an air space was more effective in this regard as in the case of diethyl phthalate. The air space allows the working fluid to free fall and to collide with the water surface breaking up into small drops of 1 mm–2 mm in diameter appropriate for utmost heat transfer. No thermal stratification was observed in the water column regardless of the flow direction of the dispersed working fluid. When measured in terms of the heat exchanger effectiveness, there was a discrepancy of 8% where diethyl phthalate outperformed Texatherm 46.