ENHANCED HEAT TRANSFER SURFACES FOR USE IN THE DEVELOPMENT OF HIGH PERFORMANCE SOLAR THERMAL SYSTEMS

Transcription

1 ENHANCED HEAT TRANSFER SURFACES FOR USE IN THE DEVELOPMENT OF HIGH PERFORMANCE SOLAR THERMAL SYSTEMS David J. Kukulka State University of New York College at Buffalo 1300 Elmwood Avenue Buffalo, New York USA Corresponding Author: Rick Smith Vipertex 658 Ohio Street Buffalo, New York USA ABSTRACT Many alternative forms of energy are being evaluated in the search for replacement energy sources and solar energy is proving to be one of the most promising forms of alternative energies. Solar thermal systems are a developed form of solar technology and provide an opportunity for design enhancement. Most of solar thermal designs involve the transfer of energy across a solar absorber surface; and most absorbers are flat, unenhanced absorbers. These surfaces utilize old unenhanced surface technology and that makes them prime candidates for redesign and improved process performance. Heat transfer enhancement has become popular recently in the development of a wide range of high performance thermal systems. New solar designs are desired that are lighter and more efficient; with a smaller footprint. Vipertex developed the EHT series of solar surfaces that provide: an enhanced energy exchange surface; is lighter; and provides the same or greater structural rigidity as classic absorbers. Enhanced heat transfer surfaces meeting those requirements are produced through material surface modifications that result in: additional heat transfer surface area, increased energy absorption, increased fluid turbulence, generation of secondary fluid flow patterns, and disruption of the thermal boundary layer. Vipertex EHT surface design criteria includes: maximization of the overall energy transfer; a minimization of any friction increases that might occur in the flowing fluid; minimization of material usage; reduction in absorber weight; and a structurally sound surface. Development of the enhanced surfaces included computational fluids dynamics (CFD) modeling and experimental evaluation of the developed solar surface. CFD modeling allowed a comparison of the flow patterns for several alternative surfaces. Experimental results of the enhanced solar surfaces show a 20% increase in energy transfer for a range of conditions. These enhanced surfaces recover more energy and provide an opportunity to advance the design of many solar thermal and PV/T products. 1. INTRODUCTION Solar energy production is an important source of green energy that utilizes various thermal designs. Most of these designs involve the transfer of energy across a solar absorber surface; and most absorbers are unenhanced and flat. These surfaces utilize old technology, making them prime candidates for redesign and improved process performance. Enhanced surfaces increase heat transfer through surface modifications that result in increased surface area and optimized fluid flow. Increased heat transfer is accomplished using the newly developed Vipertex three dimensional, enhanced heat transfer surfaces. Energy transfer for both smooth and enhanced solar absorbers has been evaluated over a wide range of solar intensities for a variety of fluid flow rates. Many alternative energy processes involve the transfer of heat energy. These processes provide a source for energy 1

2 recovery. Solar is considered to be the dominating renewable energy source since it has such a large potential; it is non-polluting and is readily available. The major applications of solar energy can be classified into two categories: solar thermal systems, which convert solar energy to thermal energy, and photovoltaic (PV) systems, which convert solar energy to electrical energy. Thermal energy conversion is most often used in solar hot water systems or solar air heaters. The simplest method to utilize solar thermal energy is for heating air in space heating or for use in drying applications. Heated water systems are a significant user of energy, making the use of solar energy in hot water solar systems an excellent alternative. Solar air heaters are less complicated than solar water heaters, with no need to worry about freezing or corrosion; making them a good choice for space heating. A PV-Thermal (PV/T) collector is a module in which the PV is not only producing electricity but also serves as a thermal absorber. Usually, these systems (PV and solar thermal) are used separately, but in some hybrid systems both energy forms are utilized. It is well known that photovoltaic (PV) cells suffer an efficiency drop as their operating temperature increases [Brinkworth and Sandberg (1)]. Cooling the PV panel is a simple method of increasing the efficiency of the PV system and the air / water utilized for the thermal cooling can recover energy that can be utilized for additional heating purposes. [Sarhaddi et al. (2), Tonui and Tripanagnostopoulos (3), Zondag (4)]. One of the first PV/T designs was the Solar One house, which was built in the 1970 s at the University of Delaware, and many others since then. Enhanced heat transfer surfaces use a modified surface to enhance the rate of heat transfer to the fluid flowing near the surface. This is accomplished through disturbing the laminar sub-layer, close to the surface. Enhanced heat transfer surfaces create a combination of: increased heat transfer surface area; increased turbulence; secondary flow generation; disruption of the boundary layer; all causing an earlier transition to turbulence. These factors lead to an increase in heat transfer and an enhanced/prolonged product life of the PV cells. A variety of enhanced surface studies have been previously performed including : Nikuradse (5); Nunner (6); Prasad and Saini (7); Webb and Eckert (8); Han (9); Hosni et al. (10); Gupta and Kaushik (11); Karmare and Tikekar (12); Kumar and Rosen (13). Economics is a major consideration in justifying the use of energy devices and for this application includes: product life, increased power production, initial development cost, capital cost, operating cost, and maintenance cost. New solar designs are desired that are lighter and more efficient with a smaller footprint. Vipertex developed the EHT (Enhanced Heat Transfer) series of solar surfaces that provide: an increased heat exchange surface area; an enhanced energy exchange surface (radiation and convection); and a surface which is lighter while providing the same structural rigidity as conventional absorbers. Enhanced heat transfer surfaces meeting those requirements were then produced by Vipertex through material surface modifications Several Vipertex enhanced solar surfaces were studied here, with transient observations and heat transfer measurements of the surfaces presented. The patented three dimensional Vipertex enhanced heat transfer surface produced by Rigidized Metals Corporation show heat transfer performance gains in excess of 22% for the EHT series of enhanced surfaces that were evaluated. The purpose of this study was to optimize the Vipertex 4EHT surface in order to obtain better performance for PV/T applications. As a result, the Vipertex surfaces enhance heat transfer, minimize cost and save energy; they can even be used to increase heat transfer at low flow rates, where previously it was difficult to achieve such gains. 2. EXPERIMENTAL DETAILS Vipertex EHT solar surfaces were tested under a solar test stand in order to evaluate the effect of enhanced surface design on thermal solar properties. Twenty Osram Ultra Vitalux lamps provided the solar source for the test stand. This setup is capable of varying solar intensity from 200 to 1000 W/m 2. A SolaData irradiance meter and readout device (calibrated for use with Ultra Vitalux lamps) was used to measure solar intensity. A data acquisition and temperature measurement system using Omega wireless transmitters and thermocouples were utilized in order to obtain the data, with tests running until the plates reached steady state conditions. Table 1 shows the Vipertex EHT solar surfaces evaluated in this study. The Vipertex 4EHT surface was then modelled and studied to modify its design in order to provide better heat transfer results. Enhanced surfaces were then evaluated in a PVT application for a variety of conditions. Measurements included cooling fluid temperatures and flowrates, ambient temperature and PV output voltage. 2

3 TABLE 1: VIPERTEX EHT SERIES OF SOLAR SURFACES EVALUATED IN THIS STUDY 3EHT 4EHT 5EHT Fig. 1: Comparison of the transient temperature response on the backside of various Vipertex EHT solar surfaces for a solar intensity of 300 W/m RESULTS Vipertex EHT solar surfaces are created to form a three dimensional textured alloy surface that is produced in a variety of configurations. Several EHT surfaces with various surface designs were evaluated for heat transfer with the results compared to a smooth, unenhanced surface. In Figure 1, it can be seen that a smooth material/surface would reach a lower steady state temperature than an enhanced surface. Figure 1 also shows that the Vipertex 4EHT had performance gains over smooth surfaces in excess of 20%. The objective of this study is to model and further improve the heat transfer characteristics of these surfaces. Surface characters that make up the Vipertex 4EHT surface (see Figure 2) were evaluated to examine the fluid interaction that takes place with the process surface. This allowed Vipertex to produce a flow optimized Vipertex 4EHT.2 enhanced solar surface that provides maximum energy transfer while minimizing both the increase of frictional losses and material usage. Fig. 2 (a): Top Surface View of the Vipertex 4EHT enhanced solar surface. 3

4 new surface characters that produce the enhanced solar heat transfer surface. Table 2 compares images of the new 4EHT.2 Vipertex EHT surface to the original surface, enhancements can easily be seen. Additional designs were also considered that provide higher heat transfer values at the expense of increased frictional losses and costs. Those results are the subject of a future study. Fig. 3 (a): Top Surface View of the Vipertex 4EHT.2 enhanced solar surface. TABLE 2: IMAGES OF THE VIPERTEX 4EHT ENHANCED SOLAR SURFACE AND THE OPTIMIZED VIPERTEX 4EHT.2 SOLAR SURFACE 4EHT 4EHT.2 Fig. 4: Transient response of the power produced by a PV/T panel with a Vipertex 4EHT.2 enhanced solar absorber after a change in the cooling water temperature to a temperature less than the ambient temperature; for a cooling water flow of 0.8 GPM; with a solar intensity of 300 W/m 2. For the conditions considered, the optimized surface can then be compared in design to an unenhanced surface. Various models of surfaces, that are capable of being produced, were identified and surfaces analyzed for specific conditions using CFD analysis. Pattern results were visualized and characteristics combined to achieve the desired results. Several patterns were evaluated for the desired objectives (total heat transfer, radiation and convection, frictional loss, low flow, turbulent flow, etc) and these patterns combined to produce the combined results desired. Figure 3 shows the Vipertex 4EHT.2 surface that was produced in order to maximize convective heat transfer, minimize frictional increases and minimize the use of material. Figure 3 details the Figure 4 details the effect of cooling the backside of the PV panel that is part of the PV/T module tested. An increase in output of approximately 10% is shown when the panel is cooled with a fluid whose temperature is less than the ambient temperature. Figure 4 shows the rapid increase in the power output when there is a sudden change in the cooling of the PV module. Figure 5 and 6 detail PV output voltages on the right axes and the difference in temperature (between the inlet temperature of the cooling liquid and the ambient temperature) on the left axes. In comparing the two figures, a 10% increase in the PV output is observed when cooling the back of a cell. In addition there is also thermal energy in the cooling fluid that can be utilized as 4

5 discussed previously. Table 3 presents a summary of the tests performed as part of the enhanced absorber plate evaluation. An increase in output voltage is even observed at very low flows. It is this ability to operate at low flows that is an important feature of the Vipertex surfaces. TABLE 3: POWER OUTPUT SUMMARY FROM A PV/T SYSTEM USING A VIPERTEX 4EHT.2 ABSORBER SURFACE Flow Rate (kg/min) Cooling Water In ( F) Ambient ( F) Power Output (volts) Fig. 5: Transient response of the power produced by a PV/T panel using a Vipertex 4EHT.2 solar absorber with the cooling water temperature greater than the ambient temperature; for a cooling water flow of 0.8 GPM; with a solar intensity of 300 W/m CONCLUSIONS Through the use of computational fluid dynamic methods, optimized three dimensional, enhanced Vipertex heat transfer surfaces were developed. Enhanced Vipertex EHT solar surfaces are able to increase energy transfer for some conditions by more than 22%. Additionally, PV/T solar surfaces have been designed to enhance convective heat transfer coefficients of enhanced solar surfaces. Vipertex EHT surfaces can be used for a variety of thermal solar applications (solar thermal, PV and PV/T). This study optimized a previously used Vipertex EHT solar surface for use in PV/T applications. The surface enhancement pattern and geometry was modified through CFD modelling to increase the overall heat transfer and minimize frictional increases. Future plans include a more extensive experimental evaluation of the optimized 4EHT.2 enhanced solar surface to further validate its design. Additionally, other designs that incorporate Vipertex EHT enhanced surfaces in other energy applications are being evaluated in order to increase the total energy exchange, minimize total costs and conserve energy. That information will be the subject of another study. 5. REFERENCES Fig. 6: Transient response of the power produced by a PV/T panel using a Vipertex 4EHT.2 solar absorber with the cooling water temperature less than the ambient temperature; for a cooling water flow of 0.8 GPM; with a solar intensity of 300 W/m 2. (1) Brinkworth, B.J., Sandberg,M., Design procedure for cooling ducts to minimise efficiency loss due to temperature rise in PV arrays, Solar Energy, 80:89 103, 2006 (2) Sarhaddi, F., Farahat, S., Ajam, H., Behzadmehr, A., Mahdavi Adeli, M., An improved thermal and 5

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