US20150153072A1

US20150153072A1 - Insulated Solar Thermal System

Info

Publication number: US20150153072A1
Application number: US14/292,702
Authority: US
Inventors: Mark W. Miles
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-12-03
Filing date: 2014-05-30
Publication date: 2015-06-04

Abstract

A solar thermal system is provided. The system comprises a solar thermal collector, comprising: a solar absorber to convert incident sunlight into heat; a transparent insulating component positioned relative to the solar absorber to allow sunlight to pass through to the solar absorber, and to reduce heat losses from the solar absorbed; wherein the transparent insulating component comprises a nano-porous matrix; and a heat exchanger coupled to the solar thermal collector.

Description

This application is a continuation-in-part of U.S. patent application Ser. No. 14/095,871, titled “Induced Flow Solar Thermal Collector and Method” filed on Dec. 3, 2013, the specification of which is hereby incorporated herein by reference.

FIELD

Embodiments of the invention relate to systems and methods to convert sunlight into heat, the transportation of the resulting heat, and the exploitation of the transported heat for some useful purpose.

BACKGROUND

Solar thermal systems are collections of components which, when integrated and configured in the appropriate fashion, can enable the collection of sunlight for conversion into heat, the transportation of the resulting heat to a point of use, and potential transfer of the heat to another medium. The transferred heat, or in some cases the original heat, can be exploited in a number of ways that are useful to human society and the many heat driven processes that support modern civilization.

SUMMARY

According to one aspect of the invention, there is provided a solar thermal system incorporating a transparent insulating medium comprising a nano-porous matrix:
which allows sunlight to pass into the solar thermal system while reducing losses through the medium which occur via conduction and radiation by virtue of the properties of the transparent insulating medium;
the heat from which is subsequently exploited to provide a useful function.
According to one aspect of the invention, there is provided a solar thermal system incorporating a transparent insulating medium comprising a nano-porous matrix:
which allows sunlight to pass into the solar thermal system while reducing losses through the medium which occur via conduction and radiation by virtue of the way in which a heat transfer fluid interacts within the transparent insulating medium;
the heat from which is subsequently exploited to provide a useful function.
According to one aspect of the invention, there is provided a solar thermal system incorporating a transparent insulating medium comprising a nano-porous matrix:
which allows sunlight to pass into the solar thermal system while reducing losses through the medium which occur via conduction and radiation by virtue of the heat transfer fluid being a gas or combination of gasses;
the heat from which is subsequently exploited to provide a useful function.
According to one aspect of the invention, there is provided a solar thermal system incorporating a transparent insulating medium comprising a nano-porous matrix:

- wherein the heat transfer fluid is transported through the system via thermally driven buoyancy forces;
- the heat from which is subsequently exploited to provide a useful function.

According to one aspect of the invention, there is provided a solar thermal system incorporating a transparent insulating medium comprising a nano-porous matrix:

- wherein components of the system can be fluidically isolated via automatic valves which actuate if a breach occurs at some point in the system;
- the heat from which is subsequently exploited to provide a useful function.
  Other aspects of the invention will be apparent from the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a solar thermal system incorporating a transparent insulating medium, in accordance with one embodiment of the invention.

FIG. 2 is a diagram illustrating a solar thermal system incorporating a transparent insulating medium and associated flows of a heat transfer fluid within the insulating medium, in accordance with one embodiment of the invention.

FIG. 3 is a diagram illustrating a solar thermal system incorporating a transparent insulating medium and an alternative positioning of the insulating medium and a solar absorber and associated flows of a heat transfer fluid within the insulating medium, in accordance with one embodiment of the invention.

FIG. 4 is a chart illustrating a list of gasses and their thermal conductivities which are candidates for heat transfer fluids.

FIG. 5 is an illustration of a solar thermal system incorporating a transparent insulating medium, configured to exploit buoyancy forces to drive the flow of the heat transfer fluid within the system, in accordance with one embodiment of the invention.

FIG. 6 is an illustration of a solar thermal system incorporating a transparent insulating medium, which includes automatic valves capable of fluidically isolating components within the system, in accordance with one embodiment of the invention.

FIG. 7 is a diagram illustrating a generic energy output mechanism to be coupled to a solar thermal system incorporating a transparent insulating medium, in accordance with one embodiment of the invention.

FIG. 8 illustrates the interplay between advection and convection cells is a fluid medium.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details.
Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not others.
In one embodiment, a solar thermal system is disclosed. The system includes a collector which harvests incident sunlight and converts into heat by raising the temperature of a heat transfer fluid, an insulated piping mechanism which transports the heated fluid to a point of use, and a heat exchanger which transfers the heat from the heat transfer fluid to another heat transfer fluid or to the medium to be heated so that the heat can be exploited. In general the system is nominally airtight and under neutral or slightly positive pressure in order to minimize the inclusion of water vapor or other contaminants which may be present in the external environment.
FIG. 1 shows a schematic drawing of a solar thermal system 100, in accordance with on embodiment of the invention. As will be seen, the system 100 comprises a solar collector 102, heat exchanger 118, and inter-connect piping send and return paths 116 and 120, respectively. Light 114, from the sun 104, passes through nano-porous medium 108 before it is incident on solar absorber 110. Solar collector 102 is a device whose basic function is to convert sunlight into heat by raising the temperature of a heat transfer fluid that comes into thermal contact with the solar absorber 108 as it passes through the collector 102. Heat losses from the solar thermal system 100 are limited to some extent by the inclusion of the nano-porous medium 108, which has several properties which make it useful in this role. These include transparency to most of the spectrum of light emitted by the sun and some degree of absorptivity or reflectivity for light wavelengths longer than those emitted by the sun. The medium 108 may be in the form of a planar monolithic structure with a planar surface area ranging from fractions of a meter to a meter or greater and a thickness of fractions of a centimeter to a centimeter or greater, or a similarly sized and shaped matrix comprised of an aggregate of smaller particulates which may have sizes ranging from hundreds of microns to several millimeters or more in diameter. The particulates in the matrix may be of a smooth distribution in size within this range, or may have several distinct sizes. The geometries may be uniform and regular in nature or completely random.
The material comprising the nano-porous medium 108 may be in the form of an aerogel or aerogel like material made from a metallic oxide or other organic or inorganic materials whose fundamental structure consists of open or closed cellular pores whose diameter may range from tens of nanometers to tens of microns or more. The porous structure is such that it impedes the flow of a gas or gasses through the medium and therefore impedes the loss of heat from the solar thermal system. Other materials with similar properties including but not limited to masses of woven organic or inorganic fibers may be utilized and/or incorporated as well as long as some combination of the requisite transparency (i.e. transparent to the solar spectrum and absorbing and/or reflective for wavelengths longer than the solar spectrum) porosity and thermally insulating characteristics (i.e. less than 0.04 W/m K) can be achieved with the resulting medium. In this way heat losses 112 from the solar absorber 110 are mitigated thereby improving the efficiency of the solar thermal system.
During operation a cold heat transfer fluid is pumped via interconnect path 120 into collector 102 where it comes into contact with the solar absorber 110. The heat transfer fluid is nominally a gas or some combination of gasses, such as air, which have been selected due to their low-cost, their positive characteristics with respect to specific heat and thermal conductivity, and their benign nature from the standpoint of chemical reactivity and human health. In the embodiment shown, heat transfer fluid (introduced via the interconnect path 120 as described above) does not flow through the nano-porous medium 108 and only flows along a first fluid flow path 122 that exists only between the medium 108 and the absorber 110 and or within the absorber 110 as it gathers heat before exiting the collector 102.
The heated fluid leaves collector 102 via a second fluid flow path defined by inter-connect path 116 wherein it enters heat exchanger 118. Inter-connect paths 116 and 120 are airtight conduits which are insulated in some fashion to minimize heat losses to the environment. In one embodiment they comprise rolled sheets of galvanized steel which have been assembled and mounted to form a coaxial tube. In the annular space within the coaxial assembly resides an insulating material such as mineral wool, aerogel, or other material which is available and known by those skilled in the art. Heat exchanger 118 serves to transfer heat from the heat transfer fluid to a point of use. The phrase “point of use” is used to define the location wherein the heat begins to serve a useful function for example heat exchanger 118 may transfer the heat to water which is subsequently used for bathing or to an ammonia/water mixture which is subsequently used to drive an absorption chiller. After passing through heat exchanger 118, the heat transfer fluid returns to the collector.
Referring now to FIG. 2, reference numeral 200 generally indicates a variation on the solar thermal system 100. The system 200 illustrates how the flow of the heat transfer fluid can help to mitigate heat losses from the system, as will be explained. The system 200 is similar in all respects to the system 100 portrayed in FIG. 1. In the case of the system 200 however, the internal configuration of solar collector 102 is defined such that some portion of the heat transfer fluid entering the collector must pass through the insulated medium, as shown by flow paths 122 and 124, before coming into contact with the solar absorber 110. Some of the heat losses 112 from the solar absorber 110 actually raise the temperature of the insulating medium 108. As a consequence heat loss occurs via conduction through the medium 108 at a rate that is determined in large part by the properties of the insulating medium. This conduction is represented by flow path 126. Heat transfer fluid flowing through the insulating medium along flow path 124 is thus capable of capturing some of the heat lost from solar absorber 110 and being conducted by the insulating medium 108. This is by virtue of heat generated in the insulating medium 108 by conduction path 126 being collected by heat transfer fluid flowing along path 124 as it comes into physical contact with the insulating medium 108. This heat is thus returned to the solar absorber 110 which helps to minimize overall losses of the solar thermal system. A portion of the heat transfer fluid travels along flow path 122 without passing through the nano-porous medium 108. Thus, in this configuration the performance of the solar thermal system can be altered by changing the ratio of the volume heat transfer fluid flowing along path 124 to the volume heat transfer fluid flowing along path 122. This ratio can be set during the manufacture of collector, or could be varied during operation of the system by the inclusion of a set of flow rate control valves which have been installed in the collector during manufacture. Varying the flow rate of heat transfer fluid flowing along path 124 through the insulating medium 108 can provide a means of modulating the rate at which heat is lost through the use of advection. The term advection is used to refer to the countervailing flow of the heat transfer fluid flowing along path 124 against the flow of heat 126. Raising the advective flow rate reduces the rate of heat loss while lowering the advective flow rate increases the rate of heat loss. Varying the advective flow rate is also a means for controlling the efficiency of the solar thermal system and making it possible to adapt the system's operating characteristics to a variety of environmental conditions including but not limited to solar insolation, ambient temperature, and wind velocity.
Advection is difficult to realize in fluids that are heated in a non-uniform fashion. This is due to a number of factors but is driven in large part by the fact that non-uniform temperatures in the fluid will lead to the generation of convection cells. Convection cells are phenomena that result when differences in fluid temperature, which produce differences in fluid density, produce currents within the fluid which are rising and/or falling as a result of the differences in density. If the fluid is unconstrained in any way then forcing the fluid in the direction of a heat source, in an attempt to mitigate heat flow away from the source, is very difficult because convection cells prevent uniform flow. FIG. 8 is a simplified representation of this effect. Referring to FIG. 8, region 800 includes a fluid which is unconstrained and is heated at the bottom. Convection cell 802 is a representative of a cell of which many can manifest within the region 800. The circulation of fluid within each cell, from warm at the bottom to cool at the top, is driven by difference in density. With multiple cells in operation the overall flow through the region 800 is from hot fluid at the bottom to cold fluid at the top thus heat flows from bottom to top. Region 804 shows flow which has been imposed upon the fluid in the direction of arrow 806. While the overall flow of the fluid is from top to bottom, the convection cells still exist though they have been disrupted. The consequence is that heat still flows from bottom to top. Region 820 includes a porous medium distributed uniformly within it. The medium is such that the pore sizes are comparable to the mean free path of the fluid that is also present within the region. For gasses this is in the nanometer range depending on the pressure. The size of the unconstrained space within these pores is too small for convection cells to occur, or if they do they are severely mitigated. The pores do not, however, prevent an overall flow to be imposed upon the fluid. As a consequence advection can be achieved, and the net flow of heat from bottom to top can be mitigated, neutralized, or reversed depending on the flow rate and other properties of the medium and the fluid.
Referring now to FIG. 3, another embodiment of the solar thermal system is shown, where it is indicated generally by reference numeral 300. This embodiment also uses the flow of the heat transfer fluid to mitigate thermal losses. In this case thermal losses which flow via paths 126 from both sides of the solar absorber 110, can be mitigated by advective flows 124 and 128. This can be achieved by embedding the solar absorber fully within the insulating medium 108 as shown in this figure, and adjusting the internal configuration of the collector 102, so that the heat transfer fluid must flow through all sides of the insulating medium 108, before coming into contact with the solar absorber 110. The heat transfer fluid can also follow flow path 122. Thus in this configuration the performance of the solar thermal system can be altered by changing the percentages of the volume heat transfer fluid flowing along paths 124, 128 and 122. These percentages can be set during the manufacture of collector, or could be varied during operation of the system by the inclusion of a set of flow rate control valves which have been installed in the collector during manufacture. Varying the advective flow rates 124 and 128 has the same operational impacts for this system as the solar thermal system 200 illustrated in FIG. 2.
Referring now to FIG. 4, a table is shown which reveals the thermal conductivities of several candidate gasses for the heat transfer fluid. As can be seen there are a number of gasses with thermal conductivities which are lower than air. The utilization of one of these candidate gasses could consequently result in the improvement of the efficiency and operational temperature of the solar collector. In general all of the gasses including air are of interest because they are nontoxic and not subject to freezing. Additionally they are lower in thermal conductivity than their liquid counterparts (i.e. water, glycol/water mixtures, thermal oils, etc.) and as such experience lower conductive losses as they are transported through various portions of the solar thermal system.
Referring now to FIG. 5 an embodiment of the solar thermal system is shown, where it is indicated generally by reference numeral 500. For this embodiment, the heat exchanger 118 resides at a location which is physically higher than the collector 102. One of the properties of a gas or a combination of gasses, like most fluids, is that as the temperature of the gas increases its density decreases thus it tends to rise in the presence of a gas which is at a lower temperature and thus at a higher density. Conversely the gas with the higher density tends to sink in the presence of a gas with a lower density. This characteristic can be utilized to passively drive the flow of the heat transfer fluid within the system by thermal buoyancy forces if the point of use heat exchanger 118, is physically located above the collector. In such a configuration hot heat transfer fluid in inter-connect path 116, because it is less dense and therefore lighter, will rise towards the heat exchanger 118, while cold heat transfer in inter-connect path 120, because it is more dense and therefore heavier, will be sink towards the collector 102. The flow rates are determined by factors which include the temperature difference between the gasses in the two inter-connect legs, the diameter of the inter-connect piping, the quality of the interior walls of the piping, as well as the difference in height between the collector and the heat exchanger, the vertical height and tilt of the collector and other factors. During the design of the installation of the solar thermal system one or more of these factors may be optimized to enhance the performance of the system based on inputs which include geographic location of the system, the prevailing weather patterns, and external constraints which limit the physical orientation of the components of the system. The flow may be supplemented by an externally driven pump or fan and possibly facilitated by the incorporation of one way valves to restrict the flow to one direction.
Referring now to FIG. 6, there is shown a solar thermal system 600 which is similar to the systems 100, 200, 300, 500, and 600 save for the inclusion of a system of valves located within, adjacent to, or in close proximity to the collectors such that if one of the collectors or some other component loses its airtight nature the collector or component can be automatically fluidically isolated from the rest of the system. The term fluidic isolation refers to preventing the flow of the heat transfer fluid from one component to the other. In this case the valves act in an automatic fashion to isolate a collector, for example, from the remainder of the system. For example cutoff valves 128 and 130 during normal operation of the system allow the heat transfer fluid to flow freely throughout the system. However in the case that the collector 100 might break and lose its airtightness then valves 128 and 130 would automatically engaged and fluidically isolate the collector from the rest of the system preventing any contaminants from the environment from entering the system through the breaks or leaks in the broken collector. Other components of the system my develop leaks during the course of operation and therefore the system can be designed to incorporate valves at the appropriate locations to compensate if so desired. This automatic response could be driven by a passive mechanical response that drives the valve into a different state say, for example, on the basis of changes in pressure. It could also be driven electronically when an electronic sensing element notes the break in the collector and sends a signal to actuate the valves.
Referring now to FIG. 7, a diagram is shown to illustrate the useful exploitation of heat generated by the solar thermal systems shown and described herein. It is deliberately high level and generic so that it may encompass the many variations in the kinds heat exploitation. In this case the point of use heat exchanger 118, which is identical to heat exchangers shown in the previous figures, is thermally coupled to heat function 134, via distribution piping 132 and 136. Heat function 134 is a generic term mean to describe the location where the heat is actually put to use. Distribution piping 132 and 134 represents the network of pipes which distributes the heat to where it is put to use. For example in a space heating application this network might correspond to the array of HVAC ducts within a building. In some cases the piping is not required and the heat exchanger is thermally coupled directly to the heat function. “Heat function” is the term used to represent a mechanism or manner in which the heat is utilized. There are innumerable ways in which heat from a solar thermal system may be exploited and as such it is impossible to document all of them completely. However a number of representative examples can be described which capture the range of possibilities and the following descriptions are not in any way meant to suggest limitations on the applications for the solar thermal system.
In one embodiment function 134 may comprise the heating of water for residential or commercial use ranging from personal bathing and washing of clothes to the sterilization of containers or other items used in industrial processes.
In one embodiment function 134 may provide heat to heat the air within a residence or commercial space (space heating) in order to provide for a suitably comfortable environment for people who might occupy the space or items which are being stored there and which must be maintained at a certain minimum temperature. Greenhouses whose internal temperature must be maintained in cooler climates could benefit as well as any industrial processes that use air as a means to provide heat (clothes drying, fruit drying, baking, and cooking, etc.)
There are many refrigeration and cooling processes which utilize heat to drive their operation. In this case function 134 serves to ultimately provide a cooling resource that can be useful in maintaining the interior temperature of a residence or a commercial building. The cooling or refrigeration can also be used to maintain the temperature to keep certain perishable items from degrading which includes many kinds of foods, ice, medicines, etc. that must be kept chilled or frozen. Thermal cooling processes include but are not limited to adsorption, absorption, rankine driven compressors, and vapor ejection systems.
Most of the electricity produced in the world today is derived from a heat engine which drives an electrical generator. Function 134 could serve to ultimately provide electricity by coupling the heat to such a heat engine. There are many heat engines which could exploit this heat including steam and organic vapor driven rankine cycles based on turbines or pistons, stirling engines, among others.
Similarly function 134 could serve to provide mechanical power via one or more of the heat engine concepts described above. Mechanical power is of use in many situations which rely on rotational machinery ranging from fluid pumps for water and irrigation to vapor compressors for refrigeration and the transport of natural gas through pipelines.
Water distillation or the distillation of many fluids often relies on a source of heat to drive an evaporative process. Consequently function 134 could provide the basis to generate steam in a flash water distillation process for the desalination of water or perhaps heat for use in a whiskey distillery or hydrocarbon cracking process.
Any of the above mentioned applications may derive the bulk or a fraction of their required heat from another source, perhaps the combustion of a hydrocarbon for example, or a geothermal source. In such cases function 134 could provide a supplemental or primary source of heat that would work in conjunction with one or more additional sources to reduce the heat required from these sources.
Any of the above mentioned applications may derive the bulk or a fraction of their required heat from a thermal storage component. There are many techniques for storing heat which range from latent heat storage in a medium such as water or oil, to the heat of transformation which includes but is not limited to processes such as the reversible conversion of water into ice and the reversible conversion of a solid salt into a liquid, and the reversible adsorption of a liquid onto the surface of an desiccant. In such a situation function 134 may provide heat which may be stored directly or indirectly in a storage component. The term “indirectly” is meant to describe heat that is used to drive a conversion process, for example a freezer, which then freezes a medium, for example water, and thereby is able to indirectly store the heat in the form of ice. The indirectly stored heat may then be used by the application at a time or date where its utilization is optimized. Heat function 134 may also provide heat simultaneously to a storage mechanism and for immediate use in any of the above described classes of applications and functions.
Function 134 may incorporate one, all, or some combination of the applications and functions described above depending on the nature and complexity of the energy needs of the customer. A residence, for example, may require water and space heating throughout the year but only require space heating in the winter and space cooling in the summer. Because bathing generally occurs during the mornings or evening a thermal storage component may be required to store heat that is collected during the day to be utilized at those times.

Claims

1. A solar thermal system, comprising:

a solar thermal collector, comprising

a solar absorber to convert incident sunlight into heat;

a transparent insulating component positioned relative to the solar absorber to allow sunlight to pass through to the solar absorber, and to reduce heat losses from the solar absorber; wherein the transparent insulating component comprises a nano-porous matrix; and

a heat exchanger coupled to the solar thermal collector.

2. The solar thermal collector of claim 1, wherein the nano-porous matrix is defined by an aerogel.

3. The solar thermal collector of claim 1, further comprising:

a first fluid flow path defined in the solar thermal collector for a heat transfer fluid to be introduced into the solar thermal collector for heating by the solar absorber; and

a second fluid flow path defined in the solar thermal collector for removal of heated heat transfer fluid from the solar thermal collector; wherein the first fluid flow path comprises an advective flow path through the transparent insulating component whereby at least some of the heat transfer fluid entering the collector flows through the transparent insulating component en route for the solar heat absorber.

4. The solar thermal system of claim 3, wherein the first fluid flow path further comprises a non-advective flow path that is disposed between the transparent insulating component and the solar absorber.

5. The solar thermal system of claim 4, wherein a ratio of a volume of fluid flowing through the advective flow path to a volume of fluid flowing through the non-advective flow path is set to minimize heat losses from the solar collector through advection.

6. The solar thermal system of claim 5, wherein said ratio can be set dynamically during operation of the system.

7. The solar thermal system of claim 3, wherein the solar absorber is embedded in the transparent insulating component.

8. The solar thermal system of claim 1, wherein the heat transfer fluid is driven between the solar thermal collector and the heat exchanger by thermal buoyancy forces.

9. The solar thermal system of claim 1, further comprising at least one valve for fluid isolation of the solar thermal collector from the heat exchanger in cases where a structural integrity of the solar thermal collector is compromised.

10. The solar thermal system of claim 1, coupled to a heat function.

11. The solar thermal system of claim 10, wherein the heat function is selected from the group consisting of a heating function, a cooling function, an electricity generation function; mechanical power function and a heat storage function.

12. A solar thermal system, comprising:

a solar thermal collector, comprising:

a solar absorber to convert incident sunlight into heat;

a transparent insulating component positioned relative to the solar absorber to allow sunlight to pass through to the solar absorber, and to reduce heat losses from the solar absorber;

a first fluid flow path defined in the solar thermal collector for a heat transfer fluid to be introduced into the solar thermal collector for heating by the solar absorber; said first fluid flow path comprising an advective flow path through the transparent insulating component whereby at least some of the heat transfer fluid entering the collector flows through the transparent insulating component en route for the solar heat absorber; and

a second fluid flow path defined in the solar thermal collector for removal of heated heat transfer fluid from the solar thermal collector; and

a heat exchanger coupled to the solar thermal collector.

13. The solar thermal system of claim 12, wherein the transparent insulating component comprises a nano-porous matrix.

14. The solar thermal system of claim 13, wherein the nano-porous matrix is defined by an aerogel.

15. The solar thermal system of claim 14, wherein the first fluid flow path further comprises a non-advective flow path that is disposed between the transparent insulating component and the solar absorber.

16. The solar thermal system of claim 15, wherein a ratio of a volume of fluid flowing through the advective flow path to a volume of fluid flowing through the non-advective flow path is set to minimize heat losses from the solar collector through advection.

17. The solar thermal system of claim 16, wherein said ratio can be set dynamically during operation of the system.

18. The solar thermal system of claim 12, wherein the solar absorber is embedded in the transparent insulating component.

19. The solar thermal system of claim 12, coupled to a heat function selected from the group consisting of a heating function, a cooling function, an electricity generation function; mechanical power function and a heat storage function.

20. The solar thermal system of claim 12, wherein the heat transfer fluid is driven between the solar thermal collector and the heat exchanger by thermal buoyancy forces.