MIDDLE EAST TECHNICAL UNIVERSITYMIDDLE EAST TECHNICAL UNIVERSITYDEPARTMENT OF PHYSICSDEPARTMENT OF PHYSICS

PHYS 471PHYS 471SOLAR ENERGY -1SOLAR ENERGY -1

INDUSTRIAL INDUSTRIAL

PROCESS HEATPROCESS HEATby by

Savaş GÜMÜŞTOPSavaş GÜMÜŞTOP

Instructor: Prof. Dr. Ahmet ECEVITInstructor: Prof. Dr. Ahmet ECEVIT

2004-12004-1

TABLE OF CONTENTTABLE OF CONTENT1. Introduction.............................................................................................. 11. Introduction.............................................................................................. 12. Components of Industrial Process Heat System.................................... 52. Components of Industrial Process Heat System.................................... 53. Solar Collector Technology.......................................................................73. Solar Collector Technology.......................................................................7 3.1 Flat Plate Collectors.........................................................................9 3.1 Flat Plate Collectors.........................................................................9 3.2 Compound Parabolic .Concentrators.............................................103.2 Compound Parabolic .Concentrators.............................................10 3.3 3.3 Evacuated TubularEvacuated Tubular Collectors.......................................................12 Collectors.......................................................12 3.4 3.4 Parabolic Through CollectorsParabolic Through Collectors........................................................14........................................................14 3.5 Solar Ponds.......................................................................................163.5 Solar Ponds.......................................................................................164. Industrial Process Heat Systems..............................................................194. Industrial Process Heat Systems..............................................................19 4.1 Industrial Solar System Without Heat Storage............................22 4.1 Industrial Solar System Without Heat Storage............................22 4.2 Industrial Solar System With Heat Storage..................................23 4.2 Industrial Solar System With Heat Storage..................................23 5. Industrial Process Heat System Design...................................................255. Industrial Process Heat System Design...................................................25 5.1 Hot Water Industrial Process Heat System..................................265.1 Hot Water Industrial Process Heat System..................................26 5.2 Hot Air Industrial Process Heat System.......................................295.2 Hot Air Industrial Process Heat System.......................................29 5.3 Steam Industrial Process Heat System ........................................305.3 Steam Industrial Process Heat System ........................................30

PAGEPAGE 6. Guidelines for Evaluation and System Design.........................326. Guidelines for Evaluation and System Design.........................32 6.1 Feasibility Analysis...........................................................336.1 Feasibility Analysis...........................................................33 6.1.1 Sellection of Appropriate Interfaces for the 6.1.1 Sellection of Appropriate Interfaces for the

Coupling of a Solar System......................................................34Coupling of a Solar System......................................................34 6.1.2 Influence of the Working Temperature................356.1.2 Influence of the Working Temperature................35 6.1.3 Continuity of the Load and Storage......................366.1.3 Continuity of the Load and Storage......................36 6.2 Guidelines for System Design...........................................376.2 Guidelines for System Design...........................................37 6.2.1 Solar Collector Field...............................................386.2.1 Solar Collector Field...............................................38 6.2.2 Storage......................................................................396.2.2 Storage......................................................................397. Conclusion....................................................................................407. Conclusion....................................................................................40 References....................................................................................41References....................................................................................41

1. Introduction1. Introduction

The industrial sector is a major energy-The industrial sector is a major energy-consuming sector in our country, using about consuming sector in our country, using about 50% of the total commercial energy.50% of the total commercial energy.

A major portion of industrial energy A major portion of industrial energy consumption is in the form of thermal consumption is in the form of thermal energy. And primary sources of this thermal energy. And primary sources of this thermal energy are fossil fuels like coal, lignite, oil energy are fossil fuels like coal, lignite, oil and gas. But upon combustion, these fuels and gas. But upon combustion, these fuels release large quantities of pollutants [1].release large quantities of pollutants [1].

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

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

Beyond the low temperature applications there are Beyond the low temperature applications there are several potential fields of application for solar several potential fields of application for solar thermal energy at a medium and medium-high thermal energy at a medium and medium-high temperature level (8temperature level (800ºC ºC --250ºC). The most 250ºC). The most important of them are: heat production for important of them are: heat production for industrial processes, solar cooling and air industrial processes, solar cooling and air conditioning, solar drying, distillation and conditioning, solar drying, distillation and desalinationdesalination [3]. [3].

2.Components of Solar Industrial Process 2.Components of Solar Industrial Process Heat SystemHeat System

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

ComponentsComponents

CollectorsCollectors PumpPump Heat exchangerHeat exchanger Storage tanksStorage tanks

3.Solar 3.Solar CollectorCollector TechnologyTechnology

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

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

3.1 Flat Plate 3.1 Flat Plate CollectorsCollectors

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

3.2 Compound Parabolic Concentrator 3.2 Compound Parabolic Concentrator (CPC)(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 within a certain angle into the collector aperture in direction to the absorber after one or more reflections. Compound parabolic concentrator is shown in figure 1.

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

3.3 3.3 Evacuated Tubular CollectorsEvacuated Tubular Collectors

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

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

3.4 Parabolic Troughs 3.4 Parabolic Troughs CollectorsCollectors

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

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

3.5 Solar Ponds3.5 Solar Ponds

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

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

almost any climatealmost any climate [5].[5].

Types of Solar pondsTypes of Solar ponds

* Nonconvecting ponds,* Nonconvecting ponds, which reduce heat loss by preventing which reduce heat loss by preventing

convection from occurring within the convection from occurring within the pond.pond.

* Convecting ponds,* Convecting ponds, which reduce heat loss by hindering which reduce heat loss by hindering

evaporation with a cover over the surface evaporation with a cover over the surface of the pond [5].of the pond [5].

4. Industrial Process Heat Systems4. Industrial Process Heat Systems

The economic and technical feasibility of any solarThe economic and technical feasibility of any solar industrial process heat industrial process heat (SIPH) (SIPH) system depends on system depends on four factors four factors [1]. [1]. Heat must be supplied in sufficient quantity,Heat must be supplied in sufficient quantity, Heat must be of adequate quality, i.e. at an Heat must be of adequate quality, i.e. at an

appropriate temperature,appropriate temperature, Heat must be transferred directly from the solar Heat must be transferred directly from the solar

collector to the process where it is to be used, collector to the process where it is to be used, andand

Solar energy must be used profitable. Solar energy must be used profitable.

Each industrial plant has unique requirement Each industrial plant has unique requirement and hence the SIPH system is to be carefully and hence the SIPH system is to be carefully designed. Because of the specific intermittent designed. Because of the specific intermittent nature of solar radiation, SIPH must be backed nature of solar radiation, SIPH must be backed up with alternate fossil-fuel system so that the up with alternate fossil-fuel system so that the industry gets uninterrupted supply of process industry gets uninterrupted supply of process heat. Generally SIPH has one of the following heat. Generally SIPH has one of the following two possible modes :two possible modes :

Solar Augmentation without energy Solar Augmentation without energy storage, andstorage, and

Solar Augmentation with energy storageSolar Augmentation with energy storage [1][1]..

4.1 Industrial Solar System Without Heat Storage4.1 Industrial Solar System Without Heat Storage

In most of the industries heat demand is so high that there In most of the industries heat demand is so high that there is no need to store heat. Eliminating storage cost it is is no need to store heat. Eliminating storage cost it is possible to build a low cost solar system. possible to build a low cost solar system. The simplest The simplest case is an industrial solar system supplying heat for a case is an industrial solar system supplying heat for a process with a continuous operation and a load alwaysprocess with a continuous operation and a load 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]. Figure 4 shows solar system without storage.

FigFig. 4. 4 :: Solar Solar SSystem ystem WWithout ithout SStoragetorage [4]. [4].

4.2 Industrial Solar System With Heat Storage4.2 Industrial Solar System With Heat Storage

If, as it is mostly common, the industrial process operates If, as it is mostly common, the industrial process operates only 6 or 5 days a week and it isonly 6 or 5 days a week and it is idle during the idle during the weekend, the system can be designed considering weekend, the system can be designed considering storage of the energystorage of the energy collected during these weekend-collected during these weekend-breaks. The collected energy will be used during the breaks. The collected energy will be used during the restrest of the days of the week.of the days of the week.

Storage may also be necessary if there are strong Storage may also be necessary if there are strong fluctuations of the process heatfluctuations of the process heat demand during the demand during the operational periods (demand peaks, short breaks of operational periods (demand peaks, short breaks of operation)operation) [7]. [7].

FigureFigure 5 5 showsshows ssolar system with heat storageolar system with heat storage..

FigFig.. 5 : 5 : Solar Solar SSystem ystem WWith ith HHeat eat SStoragetorage [7]. [7].

5. Industrial Process Heat System Design5. Industrial Process Heat System Design

The process heat in various industries is supplied The process heat in various industries is supplied generally in the following three modesgenerally in the following three modes [1]. [1].

Process hot water,Process hot water, Hot air,Hot air, andand Process Process ssteam. team.

5.1 Hot Water Industrial Process Heat System5.1 Hot Water Industrial Process Heat System

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

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

Fig. 6 :Fig. 6 : SchematicSchematic D Diagram iagram oof f T The he SSolar olar EEnergy nergy SSystemystem [4].[4].

5.2 Hot Air Industrial Process Heat System5.2 Hot Air Industrial Process Heat System

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

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

5.3 Steam Industrial Process Heat System5.3 Steam Industrial Process Heat System

In industries the largest share of process heat (two thirds In industries the largest share of process heat (two thirds of all industrial process heat) is met by steam. of all industrial process heat) is met by steam. Significantly different approaches is used for Significantly different approaches is used for producing steam using solar energy then that for air or producing steam using solar energy then that for air or water process heating. Following three possible ways to water process heating. Following three possible ways to supply steam with solar collectors are triedsupply steam with solar collectors are tried : : Circulation of pressurized water in the collectors Circulation of pressurized water in the collectors

with subsequent flashing to steam in a flash tank,with subsequent flashing to steam in a flash tank, Use of high temperature fluid in the collectors with Use of high temperature fluid in the collectors with

heat transferred to an unfired boiler, andheat transferred to an unfired boiler, and Boiling of water in collectorsBoiling of water in collectors [1][1]..Figure 8Figure 8 shows s shows schematic diagram of the solar process chematic diagram of the solar process

steam system using a flash tank.steam system using a flash tank.

FigFig.. 8. Schematic 8. Schematic DDiagram of iagram of TThe he SSolar olar PProcess rocess SSteam team SSystem ystem UUsing sing

AA FFlash lash TTank [1].ank [1].

6. 6. Guidelines for Evaluation and Guidelines for Evaluation and System DesignSystem Design

66.1 Feasibility Analysis.1 Feasibility Analysis

66.1.1 Selection of Appropriate Interfaces for .1.1 Selection of Appropriate Interfaces for the Coupling of a Solar Systemthe Coupling of a Solar System

First of allFirst of all most appropriate interfaces most appropriate interfaces (processes) of coupling a solar system to the (processes) of coupling a solar system to the existing heat supply have to be selected. The existing heat supply have to be selected. The selection criteria are the followingselection criteria are the following [7]. [7].

Low temperature level: Solar heat at Low temperature level: Solar heat at temperatures above 150 temperatures above 150 C is technically C is technically feasible but not economically reasonable at feasible but not economically reasonable at present system costs. Applications at low present system costs. Applications at low temperature (<60 temperature (<60 C) are best, C) are best,

Continuous Continuous ddemand (otherwise storage is emand (otherwise storage is needed), andneeded), and

Technical possibility of introducing a heat Technical possibility of introducing a heat exchanger for the solar system in the existing exchanger for the solar system in the existing equipment or heat supply circuitequipment or heat supply circuit [7]. [7].

66.1.2 Influence of the Working Temperature.1.2 Influence of the Working Temperature

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

66.1.3 Continuity of the Load and Storage.1.3 Continuity of the Load and Storage

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

6.2 Guidelines for System Design6.2 Guidelines for System Design

6.2.1 Solar Collector Field6.2.1 Solar Collector Field While selecting collector type, operating While selecting collector type, operating

temperature is most imported aspect. temperature is most imported aspect. Other Other aspects such as the possibility of roof integration aspects such as the possibility of roof integration or system size have to be considered as wor system size have to be considered as well . ell . By By an adequate design of flow rates, pipe diameters an adequate design of flow rates, pipe diameters and pipe insulation, the electricity consumption and pipe insulation, the electricity consumption for fluid circulation can be below 1% of the for fluid circulation can be below 1% of the overall heat gains. Thermal losses in the piping overall heat gains. Thermal losses in the piping and storage should not be above 5% of the overall and storage should not be above 5% of the overall heat gains for medium and large size systemsheat gains for medium and large size systems [3] [3]. . Table 1Table 1 shows t shows the selection he selection criteria of collector criteria of collector type for different applications.type for different applications.

Table 1.Table 1. T The selection he selection criteria of collector type for different criteria of collector type for different

applications [6].applications [6].

6.2.2 6.2.2 StorageStorage

Short-term heat storage is recommended whenever a Short-term heat storage is recommended whenever a mismatch between available solar radiation and heat mismatch between available solar radiation and heat demand occurs. For short-term storage (several hours) demand occurs. For short-term storage (several hours) storage volumes about 25 liter /storage volumes about 25 liter /mm22 are recommended. are recommended. Short-term storage may even be recommended for Short-term storage may even be recommended for continuously operating process, in order to lower the continuously operating process, in order to lower the mean working temperature of the solar system and mean working temperature of the solar system and thereby improving its efficiency, especially if low cost thereby improving its efficiency, especially if low cost solar collectors with high thermal loss coefficients are solar collectors with high thermal loss coefficients are used. The larger the system’s size the more effective the used. The larger the system’s size the more effective the heat storage over longer periods (e.g. weekends)heat storage over longer periods (e.g. weekends) [3]. [3].

7.Conclusion7.Conclusion

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

And primary sources of this thermal energy are fossil And primary sources of this thermal energy are fossil fuels like coal, lignite, oil and gas. But upon fuels like coal, lignite, oil and gas. But upon combustion, these fuels release large quantities of combustion, these fuels release large quantities of pollutants such as suspended particulate matter, SOpollutants such as suspended particulate matter, SO22, ,

NONOxx, CO, CO22 and CO. Thus, there is an urgent need to find and CO. Thus, there is an urgent need to find

alternative technologies that not only address ever-alternative technologies that not only address ever-worsening energy situation but also are enviromentally worsening energy situation but also are enviromentally benign. Solar technology is one of such options. It offers benign. Solar technology is one of such options. It offers various cost-effective enduses without endangering the various cost-effective enduses without endangering the environment. By virtue of having built-in energy environment. By virtue of having built-in energy storage, it can be used irrespective of time and season. storage, it can be used irrespective of time and season. In industry, where there is a demand of thermal energy In industry, where there is a demand of thermal energy in a number of energy intensive processes, SIPH can in a number of energy intensive processes, SIPH can

offer cost-effective solutions.offer cost-effective solutions.

ReferencesReferences [1] Advances in Solar Energy Technology, Garg H.P, [1] Advances in Solar Energy Technology, Garg H.P,

Volume 2 (Industrial Application of Solar Energy), D.Reidel Volume 2 (Industrial Application of Solar Energy), D.Reidel Publishing Company, 1987Publishing Company, 1987

[2] http://www.teriin.org/division/eetdiv/reta/docs/abs02.htm [2] http://www.teriin.org/division/eetdiv/reta/docs/abs02.htm [3] [3] Poship Final Report :Poship Final Report : http://http://www.aiguasol.com/poship.htmwww.aiguasol.com/poship.htm [4] [4] http://www.http://www.nrelnrel..govgov//cleanclean__energyenergy//solarprocessheatsolarprocessheat..htmlhtml [5] [5] http://www.http://www.eereeere..energyenergy..govgov//consumerinfoconsumerinfo//factsheetsfactsheets /aa8. /aa8.

htmlhtml [6] [6] http://www.http://www.solarnetixsolarnetix.com/.com/vacuumtubesolarvacuumtubesolar..htmhtm [7] [7] http://www.eere.energy.gov/consumerinfo/factsheets http://www.eere.energy.gov/consumerinfo/factsheets

/aa8.html /aa8.html