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MIDDLE EAST TECHNICAL UNIVERSITY DEPARTMENT OF PHYSICS PHYS 471 SOLAR ENERGY -1

MIDDLE EAST TECHNICAL UNIVERSITY DEPARTMENT OF PHYSICS PHYS 471 SOLAR ENERGY -1. INDUSTRIAL PROCESS HEAT by Savaş GÜMÜŞTOP Instructor: Prof. Dr. Ahmet ECEVIT 2004-1. 1. Introduction.............................................................................................. 1

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MIDDLE EAST TECHNICAL UNIVERSITY DEPARTMENT OF PHYSICS PHYS 471 SOLAR ENERGY -1

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  1. MIDDLE EAST TECHNICAL UNIVERSITYDEPARTMENT OF PHYSICSPHYS 471SOLAR ENERGY -1 INDUSTRIAL PROCESS HEAT by Savaş GÜMÜŞTOP Instructor: Prof. Dr. Ahmet ECEVIT 2004-1

  2. 1. Introduction.............................................................................................. 1 2. Components of Industrial Process Heat System.................................... 5 3. Solar Collector Technology.......................................................................7 3.1 Flat Plate Collectors.........................................................................9 3.2 Compound Parabolic .Concentrators.............................................10 3.3 Evacuated Tubular Collectors.......................................................12 3.4 Parabolic Through Collectors........................................................14 3.5 Solar Ponds.......................................................................................16 4. Industrial Process Heat Systems..............................................................19 4.1 Industrial Solar System Without Heat Storage............................22 4.2 Industrial Solar System With Heat Storage..................................23 5. Industrial Process Heat System Design...................................................25 5.1 Hot Water Industrial Process Heat System..................................26 5.2 Hot Air Industrial Process Heat System.......................................29 5.3 Steam Industrial Process Heat System ........................................30 TABLE OF CONTENT

  3. 6. Guidelines for Evaluation and System Design.........................32 6.1 Feasibility Analysis...........................................................33 6.1.1 Sellection of Appropriate Interfaces for the Coupling of a Solar System......................................................34 6.1.2 Influence of the Working Temperature................35 6.1.3 Continuity of the Load and Storage......................36 6.2 Guidelines for System Design...........................................37 6.2.1 Solar Collector Field...............................................38 6.2.2 Storage......................................................................39 7. Conclusion....................................................................................40 References....................................................................................41 PAGE

  4. 1. Introduction The industrial sector is a major energy-consuming sector in our country, using about 50% of the total commercial energy. A major portion of industrial energy consumption is in the form of thermal energy. And primary sources of this thermal energy are fossil fuels like coal, lignite, oil and gas. But upon combustion, these fuels release large quantities of pollutants [1].

  5. Solar technology may replace fossil fuels. It offers various cost-effective enduses without endangering the environment. Commercial and industrial buildings may use the same solar technologies - photovoltaics, passive heating, daylighting, and water heating - that are used for residential buildings. These nonresidential buildings can also use solar energy technologies that would be impractical for a home. These technologies include ventilation air preheating, solar process heating, and cooling [2].

  6. Industrial process heat is the thermal energy used directly in the preparation or treatment of materials and items manufactured by an industry. Large portion of industrial process heat is at sufficiently low temperatures which can easily be supplied by solar energy [1].

  7. Beyond the low temperature applications there are several potential fields of application for solar thermal energy at a medium and medium-high temperature level (80ºC -250ºC). The most important of them are: heat production for industrial processes, solar cooling and air conditioning, solar drying, distillation and desalination [3].

  8. 2.Components of Solar Industrial Process Heat System Solar process heating systems are designed to provide large quantities of hot water or space heating for nonresidential buildings. A typical system includes solar collectors that work along with a pump, a heat exchanger, and/or one or more large storage tanks [4].

  9. Components • Collectors • Pump • Heat exchanger • Storage tanks

  10. 3.Solar CollectorTechnology Solar energy collector is the most important component of any solar energy utilization device. Different types of collectors and systems are used in process heat industries. Due to the needs and opportunities several types can be use. Here are some of them.

  11. *Flat-plate *Compound Parabolic Concentrator (CPC)*Evacuated Tubular Collectors *Parabolic Through Collectors*Solar Ponds

  12. 3.1 Flat Plate Collectors • Flat-plate collectors are characterized by durability, dependability, simplicity, and high solar collector efficiency. At low temperatures, the flat-plate collectors operate at high optical and thermal efficiency compared to concentrators. However, as the collection temperature goes on increasing, the efficiency of a concentrator decreases very slowly while the flat plate collector efficiency decreases very fast. Therefore, the most obvious choice is flat plate collectors for applications below 80 ºC [5].

  13. 3.2 Compound Parabolic Concentrator (CPC) To reduce the heat losses of a solar collector consists in reducing the area ofabsorber with respect to the collecting area, since the heat losses are proportional to theabsorber area and not to the collecting area. This concentration can beobtained using reflectors that force the radiation incident within a certain angle into thecollector aperture in direction to the absorber after one or more reflections.Compound parabolic concentrator is shown in figure 1.

  14. Fig. 1 : Compound Parabolic Concentrator [6].

  15. 3.3 Evacuated Tubular Collectors • An evacuated-tube collector is a shallow box full of many glass, double-walled tubes and reflectors to heat the fluid inside the tubes. A vacuum between the two walls insulates the inner tube, holding in the heat. Evacuated tubular collectors are shown in figure 2.

  16. Fig. 2 : Evacuated Tubular Collectors [6].

  17. 3.4 Parabolic Troughs Collectors • Parabolic troughs are long, rectangular, curved (U-shaped) mirrors tilted to focus sunlight on a tube, which runs down the center of the trough. This heats the fluid within the tube. Some parabolic trough collectors are shown in figure 3.

  18. Fig. 3: Parabolic Trough Collectors [6].

  19. 3.5 Solar Ponds • A solar pond is a body of water that collects and stores solar energy. Solar energy will warm a body of water (that is exposed to the sun), but the water loses its heat unless some method is used to trap it. Water warmed by the sun expands and rises as it becomes less dense. Once it reaches the surface, the water loses its heat to the air through convection, or evaporates, taking heat with it.

  20. The colder water, which is heavier, moves down to replace the warm water, creating a natural convective circulation that mixes the water and dissipates the heat. The design of solar ponds reduces either convection or evaporation in order to store the heat collected by the pond. They can operate in almost any climate[5].

  21. Types of Solar ponds * Nonconvecting ponds, which reduce heat loss by preventing convection from occurring within the pond. * Convecting ponds, which reduce heat loss by hindering evaporation with a cover over the surface of the pond [5].

  22. 4. Industrial Process Heat Systems The economic and technical feasibility of any solarindustrial process heat (SIPH) system depends on four factors [1]. • Heat must be supplied in sufficient quantity, • Heat must be of adequate quality, i.e. at an appropriate temperature, • Heat must be transferred directly from the solar collector to the process where it is to be used, and • Solar energy must be used profitable.

  23. Each industrial plant has unique requirement and hence the SIPH system is to be carefully designed. Because of the specific intermittent nature of solar radiation, SIPH must be backed up with alternate fossil-fuel system so that the industry gets uninterrupted supply of process heat. Generally SIPH has one of the following two possible modes : • Solar Augmentation without energy storage, and • Solar Augmentation with energy storage [1].

  24. 4.1 Industrial Solar System Without Heat StorageIn most of the industries heat demand is so high that there is no need to store heat. Eliminating storage cost it is possible to build a low cost solar system. The simplest case is an industrial solar system supplying heat for a process with a continuous operation and a load alwayshigher than the solar gains (process operating at least 12 hours per day during daytime). In these cases, the solar system can be conceived without storage. The solar heat produced will be fed directly to the process or to the heat supply system[7]. Figure 4showssolar system without storage.

  25. Fig. 4: Solar System Without Storage [4].

  26. 4.2 Industrial Solar System With Heat Storage If, as it is mostly common, the industrial process operates only 6 or 5 days a week and it isidle during the weekend, the system can be designed considering storage of the energycollected during these weekend-breaks. The collected energy will be used during the restof the days of the week. Storage may also be necessary if there are strong fluctuations of the process heatdemand during the operational periods (demand peaks, short breaks of operation) [7]. Figure 5showssolar system with heat storage.

  27. Fig.5 : Solar System With Heat Storage [7].

  28. 5. Industrial Process Heat System Design The process heat in various industries is supplied generally in the following three modes [1]. • Process hot water, • Hot air,and • Process steam.

  29. 5.1 Hot Water Industrial Process Heat System • In hot water process systems both the direct solar water system where the heated water from the solar collector is directly supplied as process heat and indirect solar hot water system where a heat exchanger is used between the collector loop and delivery loop are used. In cold climates, an indirect water system is used with some antifreeze mixtures in the collector and storage loop. Direct systems although work at higher efficiency are preferred only in hot climates or during the day time or in special process industries or with some precautionary measures for protecting it against damage due to freezing.

  30. In industries large amounts of hot water in the temperature range of 50-100°C is required for applications like cooking, washing, bleaching etc. The solar pre-heated water can also be used as feed water to boilers [1]. Schematicdiagram of the solar energy system is shown in figure 6.

  31. Fig. 6 :Schematic Diagram of The Solar Energy System[4].

  32. 5.2 Hot Air Industrial Process Heat System • Hot air systems are employed for drying or dehydration processes in industries and such systems are safe from damage due to freezing. The hot air if sufficiently heated by Solar Energy can be directly supplied for drying/dehydration or can be further heated by an auxiliary heater before it goes to process load. An alternative to direct hot air system is the use of liquid collectors (since they are better than air collectors) and a liquid-to-air heat exchanger (which reduce the efficiency) and finally heated air can be supplied to the process load [4]. Heated air can be directly used for ventilation and heating such application in Fed-ex Denver can be seen in figure 7.

  33. Fig. 7 Solar system used for ventilation and heating [4].

  34. 5.3 Steam Industrial Process Heat System In industries the largest share of process heat (two thirds of all industrial process heat) is met by steam. Significantly different approaches is used for producing steam using solar energy then that for air or water process heating. Following three possible ways to supply steam with solar collectors are tried : • Circulation of pressurized water in the collectors with subsequent flashing to steam in a flash tank, • Use of high temperature fluid in the collectors with heat transferred to an unfired boiler, and • Boiling of water in collectors[1]. Figure 8 shows schematic diagram of the solar process steam system using a flash tank.

  35. Fig. 8. Schematic Diagram of The Solar Process Steam System Using AFlash Tank [1].

  36. 6. Guidelines for Evaluation and System Design 6.1 Feasibility Analysis 6.1.1 Selection of Appropriate Interfaces for the Coupling of a Solar System First of all most appropriate interfaces (processes) of coupling a solar system to the existing heat supply have to be selected. The selection criteria are the following [7].

  37. Low temperature level: Solar heat at temperatures above 150  C is technically feasible but not economically reasonable at present system costs. Applications at low temperature (<60  C) are best, • Continuous demand (otherwise storage is needed), and • Technical possibility of introducing a heat exchanger for the solar system in the existing equipment or heat supply circuit [7].

  38. 6.1.2 Influence of the Working Temperature The upper limit for the working temperature depends on the climate. As a rule thumb, it can be stated that solar systems for temperatures above 100 °C are only recommendable in high radiation regions (southern regions). In the northern regions only low temperature systems should be considered. It has to be taken into account that working temperature in the solar system is always somewhat higher than the required process temperatures , due to losses in the piping and the temperature drop in heat exchangers.

  39. 6.1.3 Continuity of the Load and Storage In order to obtain a reasonable economic performance, solar systems should be designed close the ideal of 100% utilization. This means that the heat demand should always be higher than the maximum possible output of the solar system. Otherwise, and if no storage is used, the useful heat drawn from the solar system is reduced [3].

  40. 6.2 Guidelines for System Design • 6.2.1 Solar Collector Field While selecting collector type, operating temperature is most imported aspect. Other aspects such as the possibility of roof integration or system size have to be considered as well . By an adequate design of flow rates, pipe diameters and pipe insulation, the electricity consumption for fluid circulation can be below 1% of the overall heat gains. Thermal losses in the piping and storage should not be above 5% of the overall heat gains for medium and large size systems [3]. Table 1 shows the selection criteria of collector type for different applications.

  41. Table 1. The selection criteria of collector type for different applications [6].

  42. 6.2.2 Storage • Short-term heat storage is recommended whenever a mismatch between available solar radiation and heat demand occurs. For short-term storage (several hours) storage volumes about 25 liter /m2 are recommended. Short-term storage may even be recommended for continuously operating process, in order to lower the mean working temperature of the solar system and thereby improving its efficiency, especially if low cost solar collectors with high thermal loss coefficients are used. The larger the system’s size the more effective the heat storage over longer periods (e.g. weekends) [3].

  43. 7.Conclusion • The industrial sector is a major energy-consuming sector in every country, using about 50% of the total commercial energy. In general, industry is highly energy-intensive and its energy efficiency is well below that of othersectors. Moreover, on account of high specific fuel consumption, it becomes difficult for the developing countries products to be competitive globally. A major portion of industrial energy consumption is in the form of thermal energy.

  44. And primary sources of this thermal energy are fossil fuels like coal, lignite, oil and gas. But upon combustion, these fuels release large quantities of pollutants such as suspended particulate matter, SO2, NOx, CO2 and CO. Thus, there is an urgent need to find alternative technologies that not only address ever-worsening energy situation but also are enviromentally benign. Solar technology is one of such options. It offers various cost-effective enduses without endangering the environment. By virtue of having built-in energy storage, it can be used irrespective of time and season. In industry, where there is a demand of thermal energy in a number of energy intensive processes, SIPH can offer cost-effective solutions.

  45. References • [1] Advances in Solar Energy Technology, Garg H.P, Volume 2 (Industrial Application of Solar Energy), D.Reidel Publishing Company, 1987 • [2] http://www.teriin.org/division/eetdiv/reta/docs/abs02.htm • [3] Poship Final Report :http://www.aiguasol.com/poship.htm • [4] http://www.nrel.gov/clean_energy/solarprocessheat.html • [5] http://www.eere.energy.gov/consumerinfo/factsheets /aa8.html • [6] http://www.solarnetix.com/vacuumtubesolar.htm • [7] http://www.eere.energy.gov/consumerinfo/factsheets /aa8.html

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