WO2009018016A2

WO2009018016A2 - Solar heat management in photovoltaic systems using phase change materials

Info

Publication number: WO2009018016A2
Application number: PCT/US2008/070762
Authority: WO
Inventors: Michael M. Mazor; Steven R. Ellerbracht; Robert J. Cleereman; Kevin E. Howard
Original assignee: Dow Global Technologies Inc.
Priority date: 2007-07-30
Filing date: 2008-07-22
Publication date: 2009-02-05
Also published as: WO2009018016A3

Abstract

This disclosure describes systems, apparatuses and methods in which a phase change material is used in conjunction with a photovoltaic device to manage the device temperature due to the heat energy received by the device while the device is generating electricity. The use of the phase change material increases the average operating efficiency of the device by reducing the temperature change that the device is subjected to during operation. Furthermore, the disclosure describes how a phase change material may be selected and incorporated into a PV module based on the anticipated operating conditions and/or the characteristics of the photovoltaic device.

Description

SOLAR HEAT MANAGEMENT IN PHOTOVOLTAIC SYSTEMS USING PHASE CHANGE MATERIALS

This application is being filed on 22 July 2008, as a PCT International Patent application in the name of Dow Global Technologies Inc., a U.S. national corporation, applicant for the designation of all countries except the US, and Michael M. Mazor, Steven R. Ellerbracht, Robert J. Cleereman and Kevin E. Howard, citizens of the U.S., applicants for the designation of the US only, and claims priority to U.S. Provisional Patent Application Serial No. 60/952,803, filed July 30, 2007 and is incorporated herein by reference.

Background

Non-polluting sources of energy are actively being sought as a replacement for the burning of fossil fuels. The generation of energy from solar radiation is one type of clean energy that is receiving significant attention. Solar energy collectors, such as photovoltaic cells, may be used in geographic regions to generate energy whenever there is adequate sunlight.

Whether a given solar energy collector is chosen over a conventional power source depends on the relative cost of the technologies. Thus, as solar collectors improve in efficiency, and thus produce power more cheaply, they become more attractive alternatives to power generation from burning hydrocarbons.

Summary

This disclosure describes systems, apparatuses and methods in which a phase change material is used in conjunction with a photovoltaic device to manage the heat energy received by the device while the device is generating electricity. The use of the phase change material reduces the amount of incident, or latent, heat thereby increasing the average operating efficiency of the device. In addition, the use of the phase change material reduces the total range of temperature change experienced by the photovoltaic device during a diurnal cycle. Furthermore, the disclosure describes how a phase change material may be selected based on the anticipated operating conditions and/or the characteristics of the photovoltaic device, and incorporated with the photovoltaic device to maximize the device's efficiency. In one aspect, the disclosure describes a device for generating electricity. The device includes at least one photovoltaic cell that generates electricity from sunlight. The photovoltaic cell has a first efficiency at a first temperature and a second efficiency at a second temperature, wherein the second efficiency is less than the first efficiency. In most cases, the second temperature is higher than the first temperature. The device further includes a phase change material in thermal contact with the photovoltaic cell, in which the phase change material has a phase change temperature selected based on the first temperature. In that way, when heating up or cooling down, the device is maintained at the phase change temperature for a relatively longer time than it would otherwise, due to the phase change effect. Therefore, because the phase change temperature is selected based on the more efficient first temperature, the device operates at more efficient temperature than it would otherwise. When the temperature is dropping (e.g. from a passing cloud) the phase change materials would hold the temperature steady, perhaps missing the advantages of a temperature reduction.

Another aspect of the disclosure is a solar energy generation system that includes a plurality of solar energy collectors. Each solar collector is characterized by a first energy generation efficiency at a first temperature greater than a second energy generation efficiency at a second temperature, both temperatures being within the normal range of operating temperatures experienced by the solar collectors in a day. The system also includes a plurality of phase change material cells in efficient thermal contact with one or more of the solar collectors. The cells contain a phase change material having a phase change temperature equal to the first temperature. The cells may be contained with some part of the solar collector or may be in a separate phase change material body that is attached to the solar collector.

Another aspect of the disclosure is a method for creating a photovoltaic assembly. The method includes selecting a photovoltaic cell having a first efficiency at a first temperature for use in the photovoltaic assembly. The method also includes selecting a phase change material having a phase change temperature at the first temperature. The temperature selection may be made so that the photovoltaic cell operates at the first temperature for as long as possible in order to increase the average efficiency of the photovoltaic cell during operation. In addition, the method includes creating the photovoltaic assembly having the photovoltaic cell in efficient thermal contact with a quantity of the phase change material. The method may further include estimating an average amount of solar energy that will be received by the photovoltaic cell during a day and selecting the amount of phase change material based on the estimated average amount of solar energy.

Yet another aspect of the disclosure is an article of manufacture having a body enclosing at least one volume containing a phase change material and having at least one photovoltaic cell on the body. Each photovoltaic cell has an energy collection surface that when exposed to sunlight generates electricity at an efficiency determined, at least in part, on the temperature of the photovoltaic cell. The photovoltaic cell further has a first efficiency at a first temperature and a second efficiency at a second temperature less than the first efficiency. In the article, the phase change material contained in the volume has a melting point equal to the first temperature. In that way, the article will be maintained at the first temperature for a longer period than at the second temperature when heated up or cooled down over a range of temperatures that includes the first and second temperatures. Therefore, the average efficiency of the photovoltaic will be improved. This statement reflects the fact that incident solar radiation tends to heat PV cells while cooling (seen as lower PV cell temperatures) occurs when incident solar radiation is blocked by terrestrial objects, atmospheric objects, or the earth itself.

These and various other features as well as advantages will be apparent from a reading of the following detailed description and a review of the associated drawings. Additional features are set forth in the description that follows and, in part, will be apparent from the description, or may be learned by practice of the described embodiments. The benefits and features will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

Brief Description of the Drawings The following drawing figures, which form a part of this application, are illustrative of embodiments systems and methods described below and are not meant to limit the scope of the disclosure in any manner, which scope shall be based on the claims appended hereto.

FIGS. IA and IB illustrate an embodiment of a simple photovoltaic (PV) module that incorporates a phase change material (PCM) to passively manage the temperature of the PV module.

FIGS. 2 A and 2B illustrate another embodiment of a simple PV module that incorporates PCM to passively manage the temperature of the PV module.

FIGS. 3 A and 3B illustrate an embodiment of a roofing shingle incorporating a PV module and PCM to passively manage the temperature of the PV module.

FIG. 4 illustrates a theoretical representation of an example temperature profile of a PV module that incorporates PCM and which receives sufficient solar energy during a daylight period to reach the melting point of the PCM and with sufficient energy to completely melt the quantity of PCM with melting temperature ofT_mp .

FIG. 5 illustrates a theoretical representation of an example temperature profile of a PV module that incorporates PCM and which receives sufficient solar energy during a daylight period to reach the melting point of the PCM but insufficient energy to completely melt the quantity of PCM with melting temperature A _mp.

FIG. 6 illustrates an embodiment of a method for creating a photovoltaic device that incorporates PCM to manage the efficiency of the device during an energy collection period.

FIG. 7 illustrates an embodiment of a method of assembling and installing a PV module containing PCM. 008/070762

FIG. 8 illustrates an alternative embodiment of a method of assembling and installing a PV module containing PCM.

FIG. 9 is a graph plotting surface temperature over time for two different PV modules on two different days including a PV module that incorporates PCM and a PV module on a wood backing.

FIG. 10 is a graph plotting surface temperature over time for several different PV modules including a PV module that incorporates PCM and several other PV modules that incorporate different backings.

FIG. 11 is a graph plotting surface temperature over time for several different PV modules including a PV module that incorporates PCM and several other PV modules that incorporate different backings including an air gap.

FIG. 12 illustrates a simulation of the effect on temperature of changing the T_mp of the PCM and the effect of changing the relative mass (amount) of the PCM in a layer. FIG. 13 illustrates a simulation of the effect on temperature of changing the

T_mp of the PCM without changing the relative mass (amount) of the PCM in a layer.

Detailed Description

This disclosure describes systems, apparatuses and methods in which a phase change material is used in conjunction with a photovoltaic device to manage the heat energy received by the device while the device is generating electricity. As will be shown, the use of the phase change material increases the efficiency of the device. In addition, the use of the phase change material reduces the total range of temperature change experienced by the photovoltaic device. Minimizing the range of the thermal excursions experienced by the photovoltaic device may also improve the length of use, or lifetime, of the photovoltaic device. Furthermore, the disclosure describes how a phase change material may be selected based on the anticipated operating conditions and/or the characteristics of the photovoltaic device, and incorporated, through efficient thermal contact, with the photovoltaic device to maximize the device's efficiency. As used herein, efficient thermal contact means that the thermal resistance between the phase change material and the photovoltaic device through the point of contact is less than the thermal resistance between the 008/070762

phase change material and the photovoltaic device by an alternative heat transfer route through the ambient environment.

The following description of various embodiments is merely exemplary in nature and is in no way intended to limit the disclosure. While various embodiments have been described for purposes of this specification, various changes and modifications may be made which will readily suggest themselves to those skilled in the art, and which are encompassed in the disclosure.

Photovoltaic (PV) cells refer to solar energy collectors that change solar radiation (sunlight) into electricity. PV cells are typically small and may be combined into a single PV module that acts as a single source of electricity, e.g., like a battery or power supply. PV modules are known in the art including those available from Shell Solar, BP Solar, Kyocera, Astropower, Sharp, Photowatt, or Uni-Solar. For example, examples of commercially available PV modules and devices include the flexible solar panel modules sold by POWERFILM and rigid panels sold by INNOVATIVE SOLAR SOLUTIONS and SUNMAXX SOLAR CELLS.

PV modules come in a variety of configurations. For example, PV panels are a common end-user form of PV modules. Such panels may include a layer of PV cells within a protective shell. Flexible sheets are another form of PV modules. Such sheets may be rolled out and attached to a surface. Alternatively, PV modules may be provided in a form ready for packaging into an end-user form.

A Phase Change Material (PCM) is a substance with a melting point at a predetermined temperature (or within a small range of temperatures) of interest which is capable of storing or releasing large amounts of energy due to the change in phase between a solid phase and a liquid phase. Phase-change materials, such as paraffin waxes, provide large heat storage or release over the narrow temperature range of their melting point. The amount of energy stored or released due to the phase change is referred to as the latent heat of fusion or, simply, as the heat of fusion. When not changing phase, solid-liquid PCMs change temperature as they absorb or emit heat as any material; their temperature rises as they absorb heat and falls as they emit heat based on the specific heat capacity of the material. However, when PCMs reach the temperature at which they change phase (their melting point) they absorb (or emit) large amounts of heat without a significant change in temperature. After the phase change is complete, the PCM again acts as a conventional heat energy storage material, with a given amount of heat energy resulting in a substantially proportional change in temperature.

PCMs have been developed with different phase change temperatures, or melting points. For example, within the human comfort range of 20° to 3O⁰C, some PCMs are very effective at storing energy and can store 5 to 14 times more heat per unit volume than conventional materials such as water, masonry, or rock that do not have a phase change in that temperature range. Other PCMs have been developed for other ranges of temperatures. Many different PCMs with different melting points are currently known in the art. A short list of some known PCMs and their melting points is provided below in TABLE 1. Subject to design and manufacturing considerations, the systems and methods described herein may utilize any suitable PCM now known or later developed in order to achieve the desired effects. For the purpose of this disclosure, a material will be referred to as a PCM if it has a melting point between -40 and 150 degrees C and the heat of fusion is greater than the specific heat capacity of the material.

In practice, many PCMs do not have a precise phase change temperature (T_mp) but rather most PCMs change phase over a some range of temperatures. Some PCMs may have a narrower range than others, but those skilled in the art are aware that the no large quantity of PCM melts and freezes at exactly any temperature. However, for the purposes of this disclosure, the common usage of characterizing PCMs by an average or representative phase change temperature will be used. However, the reader will understand that, depending on the actual PCM, there will be a range around the stated phase change temperature over which the PCM will be undergoing the phase change. 70762

TABLE 1 - Phase Change Materials

It has been proposed to incorporate devices that generate electricity from solar radiation, such as PV cells, into surfaces, such as roofs, walls, etc., that typically are exposed to solar radiation. One method of doing this is to shape or incorporate the collector into objects that are commonly used for covering those surfaces. Such objects include roofing tiles and shingles, exterior siding, flexible coverings and other objects commonly used in residential and commercial building construction on exposed surfaces. Many solar energy collectors, however, have different efficiencies (i.e., the ratio of electrical power produced from a given amount of solar radiation received) depending on the temperature of the collector. For example, the efficiency of some photovoltaic modules is inversely related to the temperature - the hotter the cell, the less efficient it is in producing electrical power from sunlight. In addition, some collectors may have an optimum temperature at which they operate at a peak efficiency.

During the course of a day, however, exposed surfaces of structures and objects exposed to direct sunlight may receive a significant and varying amount of solar radiation. This causes the temperature of those objects to vary widely through the diurnal cycle and over the course of the daylight period. This is particularly true of exterior building materials that are attractive for use as solar radiation collectors, some of which may be subjected to temperatures ranging from 20 degrees Celsius (deg. C) to more than 100 deg. C during the daylight hours when they could be generating electrical power. For example, for a roofing application in northern Michigan, a temperature range of about 30 to about 70 deg. C may be expected or determined by experimentation.

FIGS. IA and IB illustrate an embodiment of a simple PV module that incorporates PCM to passively manage the temperature of the PV module. The PV module 100 shown could be a roofing shingle, a piece of siding, a plank for use as decking or some other object designed for use as a surface covering for a structure. Each such object may be provided with its own mechanisms or attachment points, e.g., holes, tabs, interlocking ridges, roughened surface for good contact of adhesives, etc., for attachment to the structure. Such attachments may also include the electrical connections necessary to conduct the generated energy away from the module to the point of use or distribution network. Such attachment means are known to those skilled in the art. For the purposes of this disclosure, it will be understood that the module 100 could be shaped or designed in any form suitable for a particular use, and therefore they are not illustrated in FIGS. IA and IB. FIG. IA is a plan view of the PV module 100 and FIG. IB is a cross section view of the PV module 100 at the location indicated in FIG. IA. In the embodiment shown, a PV module 100 is provided that includes a PV layer 102 in thermal contact with some amount of PCM 106 enclosed within a volume defined by the PV layer 102 and a containing enclosure 104. By thermal contact, it is meant that as the PV layer 102 increases in temperature due to the receipt of solar energy, heat is transferred directly or indirectly from the PV layer 102 to the PCM 106. As such thermal contact could be a physical contact between a surface of the PV layer 102 and the PCM 106 so that heat can be transferred between the two via conduction if the PCM is in solid phase or via conduction and convection if the PCM is liquid phase and of low viscosity. In the embodiment shown, the PV layer 102 may be a single PV cell, a set of

PV cells, or some other solar energy collector. For example, the PV layer 102 may be a set of interconnected PV cells on a rigid silicon substrate. Alternatively, the PV layer 102 could be a flexible sheet of PV cells as described above. The PV layer includes a solar energy collection surface 108 that, when exposed to solar radiation, generates energy in the form of electricity, hi the embodiment shown, the solar energy collection surface 108 is opposite the surface of the PV layer 102 that is in contact with the PCM 106. However, in an alternative embodiment, if the PCM 106 and the enclosure 104 are transparent or substantially transparent or translucent then the solar energy collection surface 108 could be in direct contact with the PCM 106. As described above, the PCM 106 could be of any suitable type now known or later developed. However, as described below, the PCM 106 may be selected based on the properties of the PV layer 102 so as to form a more efficient energy collector for the environment that the PV module 100 is intended to operate. For example, a PCM 106 should be selected that has a melting point within the normal range of operating temperatures encountered by the PV module 100 during the normal cycle of operation, e.g., during the daylight period of the diurnal cycle.

The enclosure 104 could be of any material that is capable of holding the PCM 106 without leakage when in either phase during the operation of the PV module 100. For example, the enclosure 104 could be made of one or more metal, polymer, plastic or composite materials. The enclosure 104 could be rigid or flexible. The enclosure 104 may be bonded to the PV layer 106 in a conventional manner such as by adhesive or chemical bonding. The enclosure 104 could also be created as an integral part of the PV layer 102 or be integrated into the PV layer during manufacture so that in the finished PV module 100 PV layer 102 and the enclosure 104 are a unitary assembly. hi an embodiment, the enclosed volume containing the PCM 106 could be completely enclosed, hi an alternative embodiment, one or more vents could be provided, for example to allow for relief of pressure or to encourage air flow through and around the PV module 100. However, regardless of any such vents the design of the enclosure 104 should prevent the PCM from leaking out of the enclosed volume and prevent contamination of the PCM from environmental materials such as water vapor and air. For example, if contaminated with water vapor, many salt-based PCMs will drastically reduce the melting temperature of that type of PCM. hi the embodiment shown, the PV layer 102 is in thermal and physical contact with PCM 106. As described above, the PV layer 102 may be made of a PV cell or cells encapsulated in some barrier material so that the PCM 106 may not be in direct contact with the actual PV cells, but rather with the barrier, substrate or back sheet to which the PV cell(s) are attached, hi that embodiment, the PCM 106 may be in thermal contact with the PV cells in the PV layer through the encapsulating barrier, substrate or back sheet between the PCM 106 and the PV cell(s). In an embodiment, some or all of the material used for the encapsulating barrier, substrate or back sheet of the PV layer 102 may be selected to facilitate heat transfer between the PV cells in the PV layer 102 and the PCM 106. If the PCM 106 is encapsulated in its own package or enclosure, to facilitate connection of the PV layer 102 to the PCM 106, it may be preferable to incorporate a thermally conductive adhesive or contact material to facilitate thermal transport between the PV layer 102 and the PCM 106. hi an alternative embodiment, the PCM 106 may be fully enclosed within the enclosure 104 and the PV layer 102 is in contact with a portion of the enclosure 104. hi operation, the PV module 100 will increase in temperature proportionally as solar energy is received at the beginning of a day. The temperature increase will be affected by environmental considerations such as wind, ambient temperature, and amount of solar energy incident on the PV module 100. As discussed below, the PCM 106 can be selected so that the PCM 106 reaches its melting point at some time during the normal daylight energy generation period. As discussed above, after the melting point is reached the PV module 100 will stay at substantially the same temperature until enough additional heat has been received to change the phase of all the PCM 106. The reader will understand that the phrase "substantially" is used because, regardless of how low the thermal resistance between the PV layer 102 and the PCM 106, there may be some minor differential due to imperfect or delayed transfer of heat between the PV layer 102 and the PCM 106 (that may be facilitated by a thermally conductive contact material) and within the PCM 106. If all the PCM 106 has changed phase and the conditions are such that solar energy is still being received in an amount greater than the environmental conditions can remove, the temperature of the PV module 100 will resume increasing until such a point as the environmental conditions and/or incident solar energy change and the PV module 100 begins to cool down. While cooling down, the effect is reversed and the temperature of the PV module 100 will again be arrested as the PCM 106 changes phase through the emission of heat energy to the environment.

FIGS. 2 A and 2B illustrate another embodiment of a simple PV module that incorporates PCM to passively manage the temperature of the PV module. FIG. 2 A is a plan view of the PV module 200 and FIG. 2B is a cross section view of the PV module 200 at the location indicated in FIG. 2A. In the embodiment shown, the PV module 200 provided that includes a PV layer 202 in thermal contact with a PCM- containing enclosure 204. The enclosure 204 includes an amount of PCM 206 enclosed within a number of distinct volumes within the enclosure 204. In the embodiment shown, at the PV layer 202 increases in temperature due to the receipt of solar energy, heat is transferred to the enclosure 204 and therefore to the PCM 206.

As discussed above with reference to FIGS. IA and IB, the enclosure could be a rigid or flexible sheet containing a plurality of volumes of PCM 206, spaced regularly or irregularly throughout the enclosure. For example, the PV module 200 could be in the form of a long plank intended for use as a deck surface. The materials making up the different parts of the module 200 could be selected to approximate the cutting and drilling properties of wood allowing the PV module 200 to be used as a replacement for wood decking. Regardless of what length the module 200 was cut into, only a few of the volumes containing PCM 206 would be exposed and the module 206 would still exhibit the temperature control described herein. Electrical contacts could be provided at regular intervals on one or more surfaces of the module 200 so that regardless of what shape a module 200 was cut down to, some or all of the remaining PV layer 202 would act as an energy generator when exposed to sunlight.

FIGS. 3 A and 3B illustrate another embodiment of a simple PV module in the form of a roofing shingle that incorporates PCM to passively manage the temperature of the PV module. In the embodiment shown, two interconnected shingles 300 are illustrated as they would be installed on a roof 310 or other surface. In the embodiment shown, the shingles 300 include a PV element 302 attached to a surface of a shingle body 304. The shingle body 304 is shaped to enclose a PCM body 306 within a cavity 312 that is created when the shingle body 304 is attached to the roof 310.

In the embodiment shown, the PCM is contained in a PCM body 306 that is placed and retained in the cavity 312 of the shingle 300. The PCM body 306 may or may not contact the shingle body 304 at any given location. If there is no contact, as shown, heat must be transferred through the unfilled cavity space by the heating or cooling of the air within the cavity 312. Thus, even though the PCM body 306 and the shingle body 304 are not in direct physical contact, they are still in thermal contact.

In the embodiment shown, the PCM body 306 provided for each shingle 300 is illustrated as a sheet containing PCM-filled pockets 316 (two are illustrated). The PCM body 306 may be a flexible sheet or a rigid sheet. For example, in an embodiment, the PCM body 306 may be purchased as a long roll of flexible material containing pockets or cells filled with PCM. The body may be cut from the roll allowing it to be sized to fit in the cavity 312. FIG. 4 illustrates a theoretical representation of an example temperature profile 400 (illustrated with a solid line) of a PV module that incorporates PCM and which receives sufficient solar energy during a daylight period to exceed the melting point of the PCM. For comparison, a second temperature profile 408 (illustrated with a dashed line) is provided that illustrates the change in temperature that would be experienced by a conventional PV module that does not incorporate PCM. In the illustration, the temperature T of the PV modules is plotted on the x-axis 402 against the time, t, on the y-axis 404 during a theoretical daylight or other energy generation period. As shown, the temperature of the PV modules will increase as solar energy is absorbed. However, for the PV module with PCM, when the melting point, T_mp, is reached the PV module will remain at substantially that temperature until the PCM has changed phase and then the temperature increase resumes. The temperature of the PV module will then continue to increase until the amount of solar energy received is in equilibrium with the amount of energy being emitted to the environment, at which point the PC module with PCM temperature profile 400 will match that of no-PCM temperature profile 408. At some point in the diurnal cycle, the conditions will be such that the PV modules will begin to cool. During cooling the same PCM temperature effect will be observed with the PV module with the PCM again experiencing a period of time with substantially no temperature change as the PCM changes phase.

FIG. 5 illustrates a theoretical representation of an example temperature profile 500 of a PV module that incorporates PCM and which receives sufficient solar energy during a daylight period to reach the melting point of the PCM but insufficient energy to exceed the melting point. Again, for comparison purposes, a second temperature profile 508 (illustrated with a dashed line) is provided that illustrates the change in temperature that would be experienced by a conventional PV module that does not incorporate PCM. hi the example temperature profile 500, the temperature T of a PV module on the x-axis 502 is plotted against the time, t, on the y-axis 504. As shown, the temperature of the PV module increases until the melting point, T_mp, is reached. However, in this example, the amount of solar energy received before the PV module begins to cool down at the end of the daylight period is not sufficient to change the phase of all of the PCM in thermal contact with the PV layer. Thus, the PV module remains at the melting point temperature until such a point in the cycle that it is cooling down.

FIGS.4 and 5 illustrate several effects of incorporating the PCM on the PV module. One effect is that of reducing the overall temperature change experienced by the PV module during an operational period. This effect benefits the longevity of the PV module by reducing the amount of temperature induced stress it experiences. Li addition, an added benefit is that less heat is transmitted to the structure to which the PV module may be attached. If that structure is a house or other temperature controlled building, the PCM-equipped PV module actually reduces the amount of heat that would otherwise be transmitted to the house.

A second effect is that during the operational period the PCM-equipped PV module experiences a relatively larger amount of time at or near the melting point temperature of the PCM than any other temperature during a period of heating or cooling. Noting that the efficiency of the PV cells in the PV layer 102 are temperature dependent, this effect allows a manufacturer to create a PCM-equipped PV module that preferentially experiences a larger percentage of time at or near a preferred efficiency, hi addition, since the diurnal cycle is relatively predictable, it is also possible to select a PCM so that the PV module is at its peak efficiency at a preferred time in the cycle, e.g., during the time at which the most convertible solar energy is incident on the PV module.

FIG. 6 illustrates an embodiment of a method 600 for creating a photovoltaic device that incorporates PCM to manage the efficiency of the device during an energy collection period. In the embodiment shown, the method 600 begins with the selection of a PV to be used for the device in a PV selection operation 602. The PV selection operation 602 may take into account such factors as availability, durability, overall efficiency, price and suitability for the desired end use.

The method 600 also includes determining the operating conditions under which the PV device will be used, hi the determination operation 604, the likely locations and configurations under which the device will be used may be investigated and the operating conditions for which the device will be subjected will be determined. For example, if the device is to be used as a roofing shingle in the southwestern United States, the conditions that a roofing shingle may be subjected to during a normal day or year may be identified, either through experiment or by other research. Such conditions as the anticipated total amount of solar energy received during a diurnal cycle, the resulting heat load on the shingle due to the received solar energy, and the point within the diurnal cycle in which the most solar radiation is incident on the shingle. Other operating conditions may also be identified.

For example, in an embodiment the operating condition determination 604 may include determining the total heat load (i.e., the total amount of heat energy) that the device will be subjected to during a day and also determine the temperature range that the device would be subjected to. As discussed later below, the heat load information may be used to determine the total amount of PCM to be used in the device in order to achieve a desired temperature profile, such as the theoretical profiles shown in FIGS .4 or 5. The method 600 further includes characterizing the performance of the selected PV over the range of operating conditions in a characterization operation 606. In an embodiment, the characterization 606 includes testing the efficiency of the selected PV to determine how the energy generation efficiency of the PV changes throughout the range or ranges of the operating conditions evaluated. The information obtained from the characterization is then used in a selection operation 608 to select a target operating temperature. The selection operation 608 may be a simple selection based on a single factor or a complicated statistical determination based on multiple factors. For example, in one simple embodiment, if a peak efficiency is identified for the PV within the range of operating temperatures, the temperature corresponding to that peak efficiency may be chosen as the target operating temperature. As another example of a simple determination, if it is determined that the efficiency of the PV continually declines over the range of operating temperatures as the temperature increases, a PCM may be chosen with a melting point at or near the lower end of the operating temperature range. More complicated analyses may also be performed. For example, analyses that model the total output of generated energy based on the anticipated operating conditions may be used to identify the optimum target temperature based on the overall total energy generation for the diurnal cycle.

After the target operating temperature has been identified, the PCM is selected in a PCM selection operation 610. As shown above, there are many known PCMs with different melting points and one or more may be selected based on how close their melting points are to the target operating temperature. In addition, a PCM may be created so that its melting point specifically corresponds to the target operating temperature and that may have other properties suitable for the device. A PCM may also be selected based on other factors or a combination of factors such as price, durability, suitability for use, compatibility with other materials to be used in the device , etc.

The selection operation 610 may also include selecting the PCM based on additional selection criteria. For example, some PCMs are considered hazardous materials and, depending on the target market of the PV assembly, may not be appropriate. PCMs may also be selected based on other considerations including the difference in density/volume between the two states of the PCM (i.e., the different in density between liquid state and the solid state or the liquid state and the gas state), the viscosity of the liquid state of the PCM, the ability of large volumes of a PCM to uniformly distribute thermal energy throughout the volume, stability (e.g., the ability of the PCM to go through multiple phase change cycles without degradation) and/or flammability of the PCM. Another factor to be considered in the selection operation will be the relative economics of the different PCMs. For example, a slightly less efficient PCM for a PV device may be chosen over a more efficient PCM if the cost differential is significant. PCMs selected will generally have a phase change temperature (T_mp) within a range that the PV device is expected to encounter during normal daylight operation based on its intended end use/target market. Broadly, PCMs having a phase change temperature within the range of about -40 to about 150 degrees C maybe suitable for different geographic locations. For example, for uses at high altitude or in higher latitude/colder climates a lower phase change temperature may be more suitable and for uses near the equator in desert or other high temperature applications a higher PCM phase change temperature may be selected. More narrowly, for usage in less extreme environments a PCM phase change temperature between 25 to 75 degrees C may be more suitable. For example, for common rooftop applications in North America a range of PCM phase change temperatures may be between 40 and 60 C. The method 600 may also include calculating the amount of PCM to be used in order to achieve a preferred temperature profile in a quantity calculation operation 612. For example, if a temperature profile as shown in FIG. 5 is desired, after the temperature is selected in the target temperature selection operation 608 and the PCM is selected in the PCM selection operation 610, if the total heat load on the device in a diurnal cycle is known (e.g., from the operating condition determination operation 604), the amount of PCM necessary to prevent the PV device from exceeding the target temperature can be calculated using the following example equation:

Q ⁼ (Cdevice nidevice + CpcM HIPCM) (T_mp - T_1nJ_n) + (mpcM hf) hi the equation above, Q is the total heat load for a device (or a given surface area of the device), C_devi_ce is the average specific heat of device, md_eviceis the mass of the device (or mass per surface area of device) + C_PCM is the average specific heat of the PCM in solid phase, mpc_M is the mass of PCM (or mass per given surface area), T_mp is the melting point of the PCM, T_1nJn is the minimum operating temperature, and h_f is the latent heat of fusion of the PCM.

The example equation has one unknown, the mass of PCM, and can be solved to determine how much PCM should be included to hold the device temperature at the melting point of the PCM. More complicated equations may be developed based on heat transfer engineering principles that take into account other factors such as for example how the heat is transferred between the PV cells and the PCM. Equations with a temporal component may also be developed in order to account for such things as the peak temperature occurring at a time different than the time of peak energy generation by the PV.

Method 600 ends with the assembly of the PV device containing a selected amount of the selected PCM. Such assembly may take many forms, some of which are discussed further below with reference to FIGS. 7 and 8, such as creating a unitary PV module or assembling a PCM-containing body and a separate PV body and attaching the two. For example, a unitary PV module could be created in which the PCM is incorporated into the substrate on which the PV cells are formed. However, in general, the assembly may include such actions as creating a body containing the amount of the phase change material and attaching one or more PV cells to the body. As discussed above, the body and PV could be shaped as needed for the desired end use and provided with suitable attachment points. The body could contain the PCM in a single volume or in multiple volumes. The body could fully enclose the PCM or only bound one portion of volume so that the PCM is in direct contact with the PV layer after final assembly.

The embodiment of the method shown in FIG. 6 is discussed above in a generalized manner. One skilled in the art may alter the order of the operations discussed or may combine or eliminate operations to suit particular needs. For example, a manufacturer may be limited in its selection of PV, PCM or total mass of PCM by other design criteria. The method above, then, may be modified in order to determine the best way to create the PV device within those limitations. For instance, if a manufacturer is limited to a specific PCM, the manufacturer may adjust the order of the operations of the method above so that the PV is selected based on its suitability for use with the PCM. Likewise, if a manufacturer is limited to a specific mass of PCM, the manufacture may adjust the order of the operations of the method above so that the PCM is selected based on a desired temperature profile.

FIG. 7 illustrates an embodiment of a method of assembling and installing a PV module containing PCM. In the method 700 shown, the PCM-containing body is assembled in a body assembly operation 702. As described above, the PCM- containing body may be shaped as desired for the proposed end use of the PV assembly.

The body and PV layer are then attached in an attachment operation 704. The body and PV could be attached by any suitable method now known or later developed as long as the two are in thermal contact. Such attachment methods include bonding the two together with adhesives, thermal bonding, or fixing the two components together with permanent or removable fasteners. The PV used could be any PV module commercially available and need not be a PV module developed specifically for use in the PV assembly. Thus, using the methods described herein, the efficiency of a pre-existing PV module could be enhanced after its manufacture. The resulting PV assembly is then shipped and installed at the location of its intended use in an installation operation 706. Such installation may include attaching the PV assembly to the surface using attachment points provided on the body or the PV module. For example, tabs may be provided for fasteners to allow the PV assembly to be fastened to a roof or wall via nails, screws or other common construction methods.

FIG.8 illustrates an alternative embodiment of a method of assembling and installing a PV module containing PCM. hi the method 800 shown, the PCM- containing body is assembled in a body assembly operation 802. As described above, the PCM-containing body may be shaped as desired for the proposed end use of the PV assembly.

The body and PV layer are then installed on the structure and attached in two separate attachment operations 804, 806. A first attachment operation 804 attaches the PCM-containing body to the structure. For example, one or more bodies in the form of sheets or planks could be attached to cover a roof or wall of a house, commercial building, or vehicle, hi an embodiment, for example, if the body is impermeable it could be used in place of the waterproof sheeting now commonly used between the plywood of a roof and protective shingles on the exterior of the roof.

After the body is installed on the structure, the PV is then attached so that the body and PV are in thermal contact. The PV could be attached to the structure, to the PCM-containing body or both depending on the method of attachment. Continuing the example from above, the PV module is then attached to the PCM- containing material previously attached to the roof or wall. Again, the PV used could be any PV module commercially available and need not be a PV module developed specifically for use in the PV assembly. The body and PV could be attached by any suitable method now known or later developed as long as the two are in thermal contact. Such attachment methods include but are not limited to bonding the two together with adhesives, thermal bonding, or fixing the two components together with permanent or removable fasteners. EXAMPLES Experiments were performed to determine the effectiveness of using PCM to manage the temperature of a PV module. In the experiments, a commercially available flexible PV sheet was placed on top of a sheet containing PCM. The PCM was calcium chloride with a melting point of approximately 80 deg. Fahrenheit (F). Three other identical PV sheets were located next to the PV on the PCM for use as control samples. One was placed on a wood body, one was placed on a piece of insulation, and one was placed over an air channel so that the back of the PV sheet was open to the ambient air. The four PV sheets were then exposed to 4 diurnal cycles and the surface temperature of each of the PV sheets was measured at different times throughout each diurnal cycle. A table of the results is provided below in TABLE 2. These results are presented graphically in FIGS. 9-11. TABLE 2 - Surface temperature measurements

The experimental data in TABLE 2 shows that the PV over PCM was consistently lower in temperature than the placing the PV over wood or insulation. FIGS. 12 and 13 illustrate a computer simulation of the temperature at the surface of a PCM layer under different scenarios. The simulation tracked that described in C. Halford and R. Boehm, "Modeling of Phase Change Material Peak Load Shifting," ISEC2005-76035, 2005 International Solar Energy Conference using a PCM of CaCl₂.»6H₂O having a mass per exposed area of 6.764kg/m² (1.4 lbs/ft²). In the simulation, a set of basic conditions such as amount of solar radiation received over time and the ability of a PCM layer to release energy to the surrounding environment as a function of temperature were created. Standard principles of heat transfer were then used to simulate how the temperature of different PCM layers would change during a diurnal

FIG. 12 illustrates a simulation of the effect on temperature of changing the T_mp of the PCM and the effect of changing the relative mass (amount) of the PCM in a layer, hi FIG. 12, the solid thin line 1200 illustrates the temperature for a given amount of PCM having a T_mp of 180 degrees F (about 80 degrees C). This system does not change phase due to solar radiation during the course of the day and is provided for comparison purposes. The temperature of this system rises and falls in accordance with the solar radiation received and the ability of the system to release heat to the surrounding environment.

The second temperature profile 1202 is illustrated by a solid, but thicker, line. This profile corresponds to a PCM layer with a T_mp of 80 degrees F (about 25 degrees C) but otherwise under the same conditions as the first temperature profile discussed above. The simulation showed that this system required about 6 hours to change phase but did not reduce peak temperature. This illustrates that the system could be used to cause a PV module to spend a significant amount of time at the phase change temperature even though the overall temperature change experienced by the system was essentially unaffected.

The third temperature profile 1204 is illustrated by a dashed thick line. This profile corresponds to a PCM layer with a T_mp of 80 degrees F and 3 times the quantity of PCM of either the first or the second PCM layers discussed above. Otherwise, all conditions were the same. The simulation showed that this PCM layer required about 9 hours to melt and reduces peak temperature by about 10 F. However, as shown the PCM layer spent event more time at or near the T_mp of 80 degrees F than the prior two systems.

The fourth temperature profile 1206 is illustrated by a line of open dots. This profile corresponds to a PCM layer with a T_mp of 80 degrees F and 4 times the quantity of PCM of either the first or the second PCM layers discussed above. Under the conditions of the simulation, this PCM layer never completely changes phase and spends the entire diurnal cycle at approximately 80 degrees F.

FIG. 13 illustrates a simulation of the effect on temperature of changing the T_mp of the PCM without changing the relative mass (amount) of the PCM in a layer. The first temperature profile 1300 is illustrated by a thin solid line and corresponds to a PCM layer having a a T_mp of 180 degrees F. This system does not receive enough energy to change phase during the daylight period and exhibits the characteristic sinusoidal temperature profile. The second temperature profile 1302 is illustrated by a thick, solid line. This profile corresponds to a PCM layer with a T_mp of 80 degrees F but otherwise under the same conditions as the first temperature profile 1300 discussed above. The simulation showed that this system required about 6 hours to change phase but did not reduce peak temperature. This illustrates that the system could be used to cause a PV module to spend a significant amount of time at the phase change temperature 008/070762

even though the overall temperature change experienced by the system was essentially unaffected.

The third temperature profile 1304 is illustrated by a thick, dashed line. This profile corresponds to a PCM layer with a T_mp of 90 degrees F (about 30 degrees C) but otherwise under the same conditions as the first temperature profile 1300 discussed above. The simulation showed that a portion of the PCM in this system melted and then re-froze during the hours of 9 and 20. After hour 21, the PCM was entirely solid.

Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

Those skilled in the art will recognize that the methods and systems of the present disclosure may be implemented in many manners and as such are not to be limited by the foregoing embodiments and examples. Moreover, the scope of the present disclosure covers conventionally known manners for carrying out the described features and functions, as well as those variations and modifications that may be made to the materials and shapes of the components described herein as would be understood by those skilled in the art now and hereafter. 62

While various embodiments have been described for purposes of this disclosure, such embodiments should not be deemed to limit the teaching of this disclosure to those embodiments. Various changes and modifications may be made to the elements and operations described above to obtain a result that remains within the scope of the systems and processes described in this disclosure. For example, in addition to PCM, a PV module may be designed with vents, passages or other exterior shapes to facilitate the flow of ambient air through, over, or under the PV module when it is installed. As another example, a PCM-containing body may be made from a substance such as a polymer or plastic that has some particles of PCM distributed throughout the body. For example, a substrate on which PV cells are etched or otherwise installed may be made from a material within which PCM particles of some size are distributed. If the PCM particles are uniformly distributed with the substrate, then a desired amount of PCM material can be easily obtained for a PV module by utilizing a proportional amount of substrate for the PV module. As yet another example, when determining the target temperature and PCM quantity for a PV module, the determination could be made on a site by site basis or on the basis of the geographic or climatic region basis. In this way, different PV modules having different amounts or even types of PCM could be designed to be optimized for different locations. Numerous other changes may be made that will readily suggest themselves to those skilled in the art and which are encompassed in the spirit of the invention disclosed and as defined in the appended claims.

Claims

What is claimed is:

L A device for generating electricity comprising: at least one photovoltaic cell that generates electricity from sunlight, the at least one photovoltaic cell having a first efficiency at a first temperature and a second efficiency at a second temperature, wherein the second efficiency is less than the first efficiency; and a phase change material in thermal contact with the photovoltaic cell, the phase change material having a phase change temperature selected based on the first temperature.

2. The device of claim 1 wherein the first and second temperatures are within a specified operating temperature range for the device and the first efficiency is a desired efficiency within the specified operating temperature range of the device and the phase change temperature is the first temperature.

3. The device of claim 2 wherein the first efficiency is the highest efficiency of the at least one photovoltaic cell within the anticipated operating temperature range of the device.

4. The device of claim 1, 2 or 3 further comprising: the phase change material is contained within at least one enclosed volume, at least a portion of each enclosed volume bounded by at least one of the photovoltaic cells.

5. The device of any of claims 1-4 further comprising: the at least one photovoltaic cell having a solar energy collection surface attached to a substrate; and the phase change material in efficient thermal contact with the substrate.

6. The device of claim 5 further comprising: the phase change material contained within at least one enclosed volume, at least a portion of each enclosed volume bounded by the substrate.

7. The device of claim 6 wherein each enclosed volume is within the substrate of the at least one photovoltaic cell.

8. The device of any of claims 1-7 wherein the phase change material is selected from Na₂SO₄ • 10H₂O/NH₄Cl/KCl, PE Glycol, CaCl₂/CaBr₂/6H₂O/KBr,

CaCl₂/CaBr₂/6H₂O, Na₂SO₄ • 10H₂O/NaCl, PE Glycol, CaCl₂ • 6H₂O, Na₂SO₄ • 10H₂O, CaBr₂ • 6H₂O, Neopentyl glycol, Na₂S₂O₃ • 5H₂O, Paraffin wax, MgCl₂/Mg(NO₃)₂/6H₂O, NaCO₂CH₃ • 3H₂O, Na₄P₂O₇ • 10H₂O, Mg(NO₃)₂ • 6H₂O, NH₄A1(SO₄)₂ • 12H₂O, MgCl₂ • 6H₂O, Polyethylene, Tetradecane, Pentadecane, Hexadecane (cetane), Heptadecane, Octadecane, Nonadecane, Eicosane, Heneicosane, Docosane, Tricosane, Tetracosane, Pentacosane, Hexacosane, Heptacosane, Octacosane, Nonacosane, Triacontane, and micro-encapsulated long straight-chain paraffmic hydrocarbons having a formula C_n H_2n+₂, where n ranges from 10 to 30.

9. The device of any of claims 1-8 wherein the phase change temperature is between -40 and 150 degrees Celsius.

10. A solar energy generation system comprising: a plurality of solar energy collectors, each solar energy collector having a first energy generation efficiency at a first temperature and a second energy generation efficiency at a second temperature, the first energy generation efficiency greater than the second energy generation efficiency; a plurality of phase change material cells in thermal contact with one or more of the solar energy collectors, the cells containing a phase change material having a phase change temperature equal to the first temperature.

11. The solar energy generation system of claim 10 further comprising: the plurality of solar energy collectors are electrically and physically interconnected into an energy collecting surface.

12. The solar energy generation system of claim 11 wherein the energy collection surface is incorporated into an exterior surface of a structure.

13. The solar energy generation system of claim 11 wherein the energy collection surface is incorporated into an exterior surface of a vehicle.

14. The solar energy generation system of claim 11 wherein the energy collection surface is incorporated into a roofing shingle.

15. The solar energy generation system of any of claims 10-14 wherein the energy collection surface is incorporated into a plank.

16. The solar energy generation system of any of claims 10-15 wherein the energy collection surface is incorporated into a flexible sheet.

17. A method for creating a photovoltaic assembly comprising: selecting a photovoltaic cell having a first efficiency at a first temperature for use in the photovoltaic assembly; selecting a phase change material having a phase change temperature at the first temperature; and creating a photovoltaic assembly having the photovoltaic cell in thermal contact with a quantity of the phase change material.

18. The method of claim 17 further comprising: estimating an average amount of solar energy that will be received by the photovoltaic cell during a day; and selecting the amount of phase change material based on the estimated average amount of solar energy.

19. The method of claim 17 or 18 further comprising: determining a range of operating temperatures for the photovoltaic assembly based on the projected use of the photovoltaic assembly; characterizing the energy generation efficiencies of the photovoltaic cell within the range of operating temperatures; and selecting, based on the energy generation efficiencies of the photovoltaic cell within the range of operating temperatures; the first temperature as the desired operating temperature for the photovoltaic assembly within the range of operating temperatures.

20. The method of any of claims 17-19 wherein creating the photovoltaic assembly further comprises: attaching the photovoltaic cell to a body containing the amount of the phase change material.

21. The method of any of claims 17-20 wherein creating the photovoltaic assembly further comprises: assembling a phase change material body having one or more enclosed volumes containing phase change material; and attaching the photovoltaic cell to the phase change material body.

22. The method of claim 21 wherein the attaching operation is performed as part of installing the photovoltaic cell on a structure.

23. The method of claim 22 further comprising: attaching a first layer including the phase change material body to a surface of the structure; and attaching the photovoltaic cell to the surface so that the first layer is between the photovoltaic cell and the surface and in thermal contact with the photovoltaic cell.

24. The method of claim 23 wherein the phase change material body is a sheet containing one or more enclosed volumes of phase change material and attaching a first layer comprises: attaching at least one sheet to the structure.

25. The method of claim 24 wherein attaching the photovoltaic cell further comprises: fastening the photovoltaic cell to the structure.

26. The method of claim 24 wherein attaching the photovoltaic cell further comprises: attaching the photovoltaic cell to an exposed surface of the sheet.

27. The method of claim 25 wherein attaching the photovoltaic cell further comprises: bonding the photovoltaic cell to the sheet.

28. An article of manufacture comprising: a body enclosing at least one volume; at least one photovoltaic cell on the body, each photovoltaic cell having an energy collection surface that when exposed to sunlight generates electricity at an efficiency determined, at least in part, on the temperature of the photovoltaic cell, the photovoltaic cell having a first efficiency at a first temperature and a second efficiency at a second temperature less than the first efficiency; and a phase change material contained in the volume having a melting point equal to the first temperature.

29. The article of claim 28 wherein the article is a roofing shingle.

30. The article of claim 28 wherein the article is a flexible sheet.

31. The article of claim 28 wherein the first temperature is between -40 degrees Celsius and 150 degrees Celsius.

32. The article of claim 28 wherein the first temperature is between 25 degrees Celsius and 75 degrees Celsius.

33. The article of claim 28 wherein the first temperature is between 40 degrees Celsius and 60 degrees Celsius.