Industrial Solar Thermal Energy 1
Industrial Heating And primary sources of 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, SO 2, NO x, CO 2 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, solar thermal can offer cost effective solutions. 2
1. SolarThermal inindustries Theindustrialsector sector isa majorenergy 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 ilfuels like coal, lignite, it oil and gas. But upon combustion, these fuels release large quantities of pollutants t [1]. 3
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]. 4
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) ºC). The most important of them are: heat production for industrial processes, solar cooling and air conditioning, solar drying, distillation and desalination [3]. 5
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]. 6
Components Collectors Pump Heat exchanger Storage tanks 7
3SolarCollector 3.Solar Technology 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. 8
Flat plate Compound Parabolic Concentrator (CPC) Evacuated Tubular Collectors Parabolic Through Collectors Solar Ponds 9
31Flat 3.1 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]. 10
3.2 32Compound Parabolic Concentrator (CPC) To reduce the heat losses of a solar collector consists in reducing the area of absorber with respect to the collecting area, since the heat losses are proportional to the absorber area and not to the collecting area. This concentration can be obtained using reflectors that force the radiation incident id within a certain ti angle into the collector aperture in direction to the absorber after oneor or more reflections. Compound parabolic concentrator is shown in figure 1. 11
Fig. 1 : Compound Parabolic Concentrator [6]. 12
33 3.33 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. 13
Fig. 2 : Evacuated Tubular Collectors [6]. 14
3.4 34Parabolic Troughs Collectors Parabolictroughsare are long, rectangular, curved (U shaped) mirrors tilted to focus sunlight on a tube, which h runs down the center of the trough. This heats the fluid within the tube. Some parabolic trough collectors are shown in figure 3. 15
Fig. 3: Parabolic Trough Collectors [6]. 16
3.5 35Solar 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. 17
The colder water, which is heavier, moves down to replace the warm water, creating a natural convective circulation that mixes the water and dissipates i 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]. 18
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]. 19
4. IndustrialProcess Heat Systems Theeconomic and technical feasibility of any solar industrial 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 Solarl energy must beused profitable. 20
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 fuelfuel system so that the industry gets uninterrupted supply of process heat. Generally SIPH has one of the following two possible modes : Solarl Augmentation without ih energy storage, and Solar Augmentation with energy storage [1]. 21
4.1 Industrial Solar System Without Heat Storage In 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 alow cost solar system. The simplest case is an industrial solar system supplying heat for a process with acontinuous operation and aload always higher 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]. 22
4.2 IndustrialSolarSystem System WithHeatStorage If, asit is mostly common,the industrial i process operates only 6 or 5 days a week and it is idle during the weekend, the system can be designed considering storage of the energy collected during these weekend breaks. The collected energy will be used during the rest of the days of the week. Storage may also be necessary if there are strong fluctuations of the process heat demand during the operational periods (demand peaks, short breaks of operation) [7]. Figure 5 shows solar system with heat storage. 23
Solar System With Heat Storage Fig. 5 : Solar System With Heat Storage [7]. 24
5. IndustrialProcess Heat System Design The process heat in various industries is supplied generally in the following three modes [1]. Process hotwater, Hot air, and Process steam. 25
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. 26
In industries large amountsof 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]. Schematic diagram of the solar energy system is shown infigure 6. 27
Fig. 6 : Schematic Diagram of The Solar Energy System [4]. 28
5.2 HotAir IndustrialProcess Heat System Hot air systems are employed dfor drying or dehydration d 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 ex Denver can be seen in figure 7. 29
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 ihsolar 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. 30
Fig. 8. Schematic Diagram of The Solar Process Steam System Using A Flash Tank [1]. 31
6. Guidelines for Evaluation and System Design 6.1 Feasibility Analysis 6.1.1 Selection of Appropriate Interfaces for the Coupling of asolarsystem 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]. 32
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 supplycircuit it[7] [7]. 33
6.1.2 Influenceof the WorkingTemperature 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. 34
6.1.3 13 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 shouldalways 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]. 35
6.2 Guidelines for System Design 6.2.1 Solar Collector Field While selecting collector type, operating temperature t is most important 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 1shows the selection criteria of collector type for different applications. 36
Table 1. The selection criteria of collector type for different applications [6] (CPC Compound Parabolic Concentrating). 37
622Storage 6.2.2 Short term heat storage is recommended whenever a mismatch between available solar radiation and heat demand occurs. For short term term storage (several hours) storage volumes about 25 liter /m 2 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 s sizethe more effective the heat storage over longer periods (e.g. weekends) [3]. 38
7Conclusion 7.Conclusion The industrial i 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. 39
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 / hi [4] http://www.nrel.gov/clean_energy/solarprocessheat.html [5]h http://www.eere.energy.gov/consumerinfo/factsheets / /f h /aa8.html [6] http://www.solarnetix.com/vacuumtubesolar.htm htm [7] http://www.eere.energy.gov/consumerinfo/factsheets /aa8.html 40