Patent Application
20150292770
This invention relates to radiant energy management, and more particularly to a solar thermal collector system configured for use on buildings having a pitched roof construction.
Skylight systems have previously been provided that are capable of providing the majority of the lighting needs for various flat roof commercial buildings. In such systems, the skylight may convert the excess solar energy that is not needed for illumination into thermal energy that can be used for process hot water, space heating, and solar cooling. Solar cooling apparatus, such as absorption chillers and liquid desiccant dehumidifiers, typically require between 1.3 and 1.7 units of thermal energy to provide one unit of cooling to the building. An economically designed energy managing skylight system may employ skylights that cover only about 5 to 6% of the roof area. Even at relatively high thermal efficiencies, this only provides about one fourth of the thermal energy needed to cool the entire building area below the skylights.
In order to serve the entire building, and to make the best use of the roof as an energy resource, it is preferable to provide supplemental thermal energy in addition to what is generated by such energy managing skylights. It is possible to supplement the thermal energy using conventional solar thermal collectors. However, the cost of installation per unit of thermal energy generated by conventional thermal collectors is much higher than that generated by previously presented energy managing skylights, and the overall project economics can be significantly degraded. Another option is to use a fossil fuel such as natural gas as the supplemental heat source. This has the advantage of providing firm capacity during periods of low sunlight, but the expense of the gas backup degrades the project economics and the additional fossil fuel use works against one of the main product objectives of being a primary renewable energy source. Thus, there is an ongoing need for a lower-cost method of generating thermal energy at a sufficient temperature to drive solar cooling equipment that is optimized for the commercial rooftop and seasonal summer operation.
Another factor in the design of a solar collector for large commercial buildings is the potential use of the solar collector as a cooling device. In climates with high solar radiation during the day and relatively low humidity at night, a significant amount of building cooling can be achieved by using night sky radiant cooling techniques. One problem with night sky radiant cooling is that the typical cooling heat fluxes are only about 1/10 those which can be achieved during solar collection of sunlight. That is, the night sky radiant cooler rejecting heat at 80° F. into a clear night sky can radiate only a maximum of about 80 W per square meter, compared with maximum heat collection rates of 700 or 800 W per square meter in full sunlight. Also, the cooling effect is generally uncorrelated in time with the cooling loads. Therefore, for a night sky radiant cooling method to be cost-effective, it must either be of extremely low cost or must piggyback to provide a cooling function on top of an existing heat collection function, plus it ideally would have low cost thermal storage as well. Several techniques have been described for night sky radiant cooling, such as flushing the roof surface with water at night, and using relatively low efficiency solar collectors as radiators at night, but these techniques are not practical as yet. The fundamental problem with using solar collectors as radiant cooling devices is that the design of the collector is intended to thermally isolate the fluid from the ambient air and the radiant sky environment. A solar collector with two separate fluid paths that could do double duty as an efficient night sky radiant cooling device could make a significant contribution to cooling flat roof commercial buildings in sunny dry climates.
Moreover, the majority of solar panels on the market today are designed to optimize efficiency for a given amount of solar radiation flux and outside air temperature. The drive for higher efficiency results in the use of relatively expensive materials such as copper, aluminum, and glass, as well as optical treatments such as low emissivity absorber coatings and low reflective coatings for cover glass. The higher the thermal collection efficiency for the panel, the higher will be the stagnation temperature which occurs when the module is in full sunlight but there is no liquid flow to pull the heat away. Stagnation temperatures between 350 and 400° F. are not uncommon for good quality collectors. These high stagnation temperatures then drive the need for even more expensive materials and components to ensure that the panel does not damage itself in full sunlight.
The drive for higher efficiency and the need to withstand high stagnation temperatures increases the cost per unit area of the solar collector. The relatively high cost per unit area, plus the perceived need to generate the most energy in the winter, generally makes it imperative to orient the collector in an optimal position to capture the most possible sunlight to convert to thermal energy. On a commercial rooftop, this means tilting the collectors to an angle which roughly corresponds to the latitude of the building location, or in most of the United States between 25 and 50° to horizontal. Orienting the panel at such a relatively high angle to the horizontal roof brings many problems and costs to the system installation. Structural supports required to maintain the panel in position must be anchored to the roof structure or held in place with heavy ballast blocks. The relatively high profile of the solar panel drives wind loads which can be up to 75 pounds per square foot of collector area. These wind loads must be handled by the module, structural supports, the roof anchors, and the roof itself. On buildings with large flat roofs in relatively high wind areas, it is often not practical to even install large numbers of flat plate solar collectors due to the structural reinforcements that would be required to handle the wind loads. Evacuated tube collectors may have lower but still significant wind loads.
The wind loads also drive significant structural requirements for the panel itself. Solar panels that are mounted in typical, inclined orientations must handle 1000 to 2000 pounds of total force on both the front and back of the panels in both positive and negative directions. This is a significant driver for the thickness of the glazing, the back cover plate, and of the framing, driving flat plate collectors to thicknesses of over three inches. Heavy duty anchors or heavy ballast blocks are usually required to secure the mounting framing to the steel deck and even to the underlying truss structure. Furthermore, mounting the panel at an inclined position exposes the back of the panel to ambient air, resulting in the need for thick insulation to prevent significant loss from the back of the collector. Finally, orienting the solar collector at an angle that is designed to maximize the collection over the course of the year does not result in a heat generation profile that corresponds with the thermal needs of the building if solar cooling is taken into account. To the contrary, about 1.5 times more daily thermal energy is required to cool a typical flat roof commercial building in the summer using heat-to-cooling systems than to heat it in winter with solar heat. Therefore, a horizontal or nearly flat solar collector installation angle would have many advantages, including providing a generation profile that is better matched to the building needs, reducing wind loading on the collector itself, and reducing wind loading that the collector places on the roof structure.
In summary, there are many other design considerations in addition to a high heat collection efficiency per unit area when providing heat to support year-round space conditioning on a flat roof commercial building.
Virtually all existing solutions make use of some form of fin and tube configuration. That is, solar energy is collected on a flat surface normal to the sun's rays, and the heat is conducted along the surface to a tube through which a working fluid flows. As the heat is conducted along the relatively thin absorber surface, there is a significant temperature drop between the absorbing surface and the working fluid. This temperature drop results in thermal losses of between 12 and 18 percent, because the higher temperature of the absorber surface compared to the fluid temperature results in higher losses to the environment. In addition, an efficient fin design requires creating a good thermal bond between the flat sheet and the fluid tube, which is a significant design challenge that drives up costs and creates failure points. The tubes can be clamped, brazed, soldered, or attached with thermal grease, all of which require substantial manufacturing resources. A solar collector configuration which eliminates heat flow transverse to the sun's rays will effectively have a fin efficiency of 1.0, with a significant overall performance improvement, and have none of the assembly issues.
Further, nearly all current designs place the absorber surfaces directly under the glazing. This causes a direct convective and radiative coupling between the two surfaces, accounting for the majority of the heat loss from the collector. The glazing is typically made of low-iron glass, which has a high light transmissivity of about 90% but which is heavy, is a very poor insulator and so does not maintain more than a few degrees temperature difference across it. Since the emissivity of the glass is high (0.9 or so), in order to limit the radiative heat loss, it is necessary for the absorber to have a low emissivity, along with a high absorptivity. This can be achieved using very thin black coatings such as black chrome, but applying such coatings requires specialized techniques such as vacuum deposition. In addition, many such coatings make use of toxic materials that require special handling, all adding considerable expense to the finished product. A collector design in which the absorber surface is somewhat insulated from the glazing, which uses a glazing with more insulating properties, and which can use simpler absorbing materials such as ordinary black paint, would be of lower cost, of higher efficiency, and more environmentally friendly.
Other solutions to reduce collector cost have recently become available that make use of lower cost materials than the classic flat plate or evacuated tube designs. The most common is the unglazed black plastic “pool heater” solar collector. In this collector design, solar radiation is absorbed directly by the black plastic tubes through which the coolant flows. Because there is a low insulation level between the coolant and the environment, this collector design is typically used for low temperature applications in warm climates such as heating water for swimming pools. There are several fundamental limitations which have thus far prevented the deployment of high efficiency, low cost, polymer (i.e., plastic) collectors.
The first problem is the low melting point of plastics that are sufficiently low in cost to be considered for use as collectors. Adding a glazing layer of glass or plastic material over the collector surface is of course the simplest way to increase the efficiency. However, even one glazing layer over a black plastic collector surface can allow the stagnation temperature to quickly exceed the softening point of the plastic.
Secondly, extruded panels with discrete flow channels must be connected to a header or manifold. A waterproof seal must be made between the irregular shape of the cross section of the end of the extrusion and the fluid carrying tube that forms the manifold. The seal is typically made by making the manifold of the same material as the extruded panel and welding the two materials together. The irregular shape of this welded joint makes the joint difficult to fabricate and prone to leakage with thermal cycles.
Further, all plastics have very low strength and stiffness relative to metals. This makes it difficult for plastic solar panels to contain typical aqueous heat transfer fluid that in ordinary solar thermal systems can reach pressures of 150 PSI. The fluid pressure requirements in ordinary solar thermal systems are driven in large part by the use of water-based coolants such as water glycol mixtures which can boil or create vapor bubbles that can rapidly increase the pressure in the fluid passages. A solar collector that makes use of low-cost and lightweight plastic materials, but that overcomes the issues of the low service temperatures and pressure containment, would have the advantages of low cost and weight without the disadvantages of current plastic collector designs.
Further, solar thermal collector systems for use on pitched roof constructions, such as many residential constructions, which may by way of non-limiting example form a part of a solar hot water system for a residential application, can carry their own unique challenges and design requirements. Since heat-to-cooling systems are not as widely available in the cooling capacity range (3-5 tons) suitable for residential applications, the heat generated is typically valued more in winter rather than summer if used for space heating. Therefore, it is desirable to have the collector mounted at an angle to the horizon which is equal to or greater than the latitude of the location. Also, residential roofs are typically pitched at an angle between 20° and 40°. Therefore, a common collector mounting scheme is to put the collector mounted on the roof matching the roof pitch. As such, the concept of a horizontally mounted collector described above is not as applicable to residential systems, except in very warm climates where flat residential roofs are common; in these cases, a solar thermal collector system configured for flat roof constructions, such as those used for commercial system design, can be applied.
The market for residential solar hot water heating systems in the US approaches 100 million units, and currently less than 5% of US homes have any type of system installed. A study by the National Renewable Energy Laboratory (NREL) published last year concluded that if the installed cost of residential systems could be reduced by half from the current $10,000 per system, then solar hot water would be cost effective in 25% of US homes. The additional 20% of US homes represents millions of solar hot water systems, so the interest in developing lower-cost systems is highly justified.
Existing flat plate solar collectors typically include a glass glazing layer and a flat metal absorber sheet that conducts heat to coolant tubes welded or brazed to the absorber. The back of the panel is insulated with several inches of insulation, and a backing plate protects the back and adds structural rigidity. The material choices of glass and metal drive the weight of these types of panels to three to four pounds per square foot, making a typical 4′×8′ panel over 120 lbs. This high weight drives the need to attach the collector to the roof structure using hard mounting points, such that the panel sits several inches above the roof, which then makes it necessary for the panel itself to support wind and snow loads and transfer them to the mounting points. These structural features of the panel do nothing to collect heat. By contrast, unglazed solar pool heaters are light enough to sit directly on the roof itself, and so do not have the structural self-supporting requirements of typical glazed panels. Thus, a panel that has the low weight density of the plastic, unglazed pool heating panels, and the efficiency and temperature capability of the glazed panels would have advantages of both and the disadvantages of neither.
Typical residential solar thermal systems use a temperature sensor, a controller and a 120V mechanical pump, which not only drive material costs, but also constitute a significant portion of the installation costs since more highly skilled installers are required to run wiring and connect sensors. Usually, permits and inspections are required with such electrical installations, which further drives costs. In order to radically lower the cost of residential solar thermal systems, the system must use lighter and lower-cost raw materials to eliminate expensive components such as pumps, sensors, and controls, and be capable of being installed by relatively low-skilled labor.
Another feature of current residential solar systems is the use of a hot water tank for thermal storage. Although the hot water tank has the advantage that the storage medium itself (water) is almost free, it suffers several drawbacks. First, in order to store the energy, the water temperature must be raised and lowered as the heat is put into and discharged from storage. This means that the collector is operating most of the time at a higher temperature than the load requires, which reduces its efficiency. On the load side, the varying temperature of the water supplied by the tank requires that either supplemental energy be supplied, or that the water be cooled by a mixing valve before delivery to the domestic hot water user. All of these effects reduce the overall system efficiency compared to a system which uses constant temperature storage.
Another disadvantage of the hot water storage tank is that its extreme weight limits the locations in the residence in which it can be installed. Typical storage tanks are 70-90 gallons, weighing 500-700 lbs when full and so must be mounted on a floor, typically in the basement. Allowing for clearance around the tank, it occupies between 15 and 25 square feet of floor space. This is on the order of 1 percent of the area of a single family home in the US; using the median home value of $188,000, the area taken by the storage tank represents $1880 of opportunity cost to the home owner. A storage system that could operate at a constant temperature and occupy no floor space would have an advantage over current systems.
One solution to simplify residential solar hot water systems is to use thermosiphoning instead of pumped flow. The biggest factor limiting the widespread adoption of thermosiphon residential solar hot water systems is the fact that the addition of glycol to the water increases the viscosity to the point that there is not adequate thermosiphon potential to overcome the additional pressure drop. Therefore, thermosiphon residential solar hot water systems have only been found practical in very warm climates where freezing conditions are never encountered, such as subtropical and tropical regions. In the US, only southern Florida and southern California are candidates for such systems. A thermosiphon-based system which can operate in all climates including very cold climates would have tremendous advantages.
Thus, there remains a need in the art for a cost effective and efficient solar thermal collector system that may be used on pitched roof construction that will avoid the foregoing disadvantages of prior art solar thermal collector systems.
Disclosed herein is a solar thermal collector system that is particularly configured for installation on a pitched roof of a building. In accordance with aspects of a particular embodiment of the invention, the solar thermal collector system includes a solar thermal collector module having a glazing sheet at a top, exterior surface, and an absorber sheet within the module positioned below and spaced apart from the glazing sheet. At least the absorber sheet is fluidly connected to a fluid handling system, and carries a working fluid that is heated in the module by the sun and transfers such heat to thermal storage modules by the fluid handling system. The solar thermal collector module is preferably provided a thermally actuated valve that allows the working fluid to also flow through the glazing sheet, which results in self-regulation of the temperature of the module below a critical design temperature.
In accordance with a particularly preferred embodiment of the invention, a solar thermal collector system is provided having at least one solar thermal collector module comprising a glazing sheet forming a top surface of the module, an absorber sheet on an interior of the module and positioned below and spaced apart from the glazing sheet, the absorber sheet being configured to absorb heat from the sun and to transfer heat to a working fluid in the absorber sheet, a first manifold connected to a first end of each of the glazing sheet and the absorber sheet, and a second manifold connected to a second end of each of the glazing sheet and the absorber sheet, and at least one thermal storage module in fluid communication with each of the first manifold and the second manifold and configured to transfer heat from the solar thermal collector module to the thermal storage module.
The numerous advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying drawings in which:
The following description is of a particular embodiment of the invention, set out to enable one to practice an implementation of the invention, and is not intended to limit the preferred embodiment, but to serve as a particular example thereof. Those skilled in the art should appreciate that they may readily use the conception and specific embodiments disclosed as a basis for modifying or designing other methods and systems for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent assemblies do not depart from the spirit and scope of the invention in its broadest form.
In accordance with certain aspects of an embodiment of the invention, an improved solar thermal collection system is provided, including one or more solar thermal collection units 100 which may be situated, for instance, on the flat roof 500 of a commercial building, as shown in
As best viewed in
Referring again to
As shown in
Other advantages may also be realized through use of the above-described absorber configuration. For example, the tube and fin feature that is a cause of 12 to 18% of losses in almost every existing absorber design is eliminated. This is because the conduction path from the absorbing surface of the absorber sheet 110 to the fluid is only a few millimeters long and is perpendicular to the surface of the sheet, as opposed to a path that is several inches long and is in the plane of the sheet.
As mentioned briefly above, each manifold 200 and 300 is joined to each of glazing sheet 102 and absorber sheet 110. As shown in
Connected to each manifold 200 and 300 are fluid fittings 204 that allow a working fluid to enter at the bottom end of the module (which in typical installations will be the south end of the module), flow directly through the narrow space within at least absorber sheet 110 and in certain configurations glazing sheet 102, and exit via the upper manifold (which in typical installations will be the north end of the module), as discussed in greater detail below.
The north and south manifolds, 200 and 300, respectively, and the frame lattice 104, are supported around the perimeter by walls 108, which are also formed of multi-wall polycarbonate. However, these side walls 108 are much thicker (16 mm to 25 mm thick) to transfer the compressive loads to the bottom layer and provide thermal insulation to the air cavity below the glazing. Present in the design, but not shown in the cross-section are triangular-shaped walls that seal the east and west sides of the module 100 and support the glazing along the sides. The back, north wall 112 has the highest profile above the roof and thus is susceptible to the highest wind loading. Sloping the back, north wall 112 slightly (as shown in
With continued reference to
The modules 100 are mounted horizontally, directly in contact with the roof surface, which may offer one or more of the following advantages. First, the near-horizontal mounting may provide an optimal heat collection profile over the year, with more heat collected in the summer cooling season when it is needed to drive heat-to-cooling systems. Second, because the ballast holds the entire back of the module 100 firmly to the roof surface, there is no dynamic pressure resulting from wind coming under the module 100, which eliminates bending loads or uplift loads on the module structure. If only compressive loads need be designed for, much lower cost materials can be used in the design and, further, the amount of site engineering to determine wind loading on a particular installation is greatly reduced. Third, in the absence of airflow behind the collector, the module 100 is thermally coupled to the roof itself, so much less insulation is required than if the back of the panel were exposed to the environment. In this way, the module 100 essentially “borrows”, or makes double use of, the existing roof structure and insulation to reduce module cost without adding significant thermal or structural loading on the building. Fourth, no penetrations of the roof membrane are required in the installation process and so the likelihood of roof leaks is greatly reduced. Fifth, the operation can be performed by very low-skilled labor with no cutting, drilling, mating or resealing operations required.
Moreover, advantages of the design may be realized related to the weight of the module 100 and ballasting compared to typical ballasted solar thermal installations. Due to the density of the materials (glass, copper, aluminum), the area of a typical flat plate collector is often limited by the total lifting weight of the module. For example, a typical 4′×8′ (32 ft2) flat plate solar collector has a shipping weight of about 120 lbs (3.75 lb/ft2), which is the maximum that two workers could reasonably carry without roof top material handling equipment. For the system described herein, the use of lighter materials reduce the shipping weight area density by more than half to only 1.7 lbs/ft2, which means that a module twice the size (6′×10′) weighs less than 110 lbs. The same total installed collector area can then have half the lifting weight and half the number of individual modules, reducing installation time significantly.
Further, prior known flat plate modules secured to a flat roof using the ballast technique typically require a ballast weight up to two times the collector weight. Thus, up to 360 lbs for each module (120 lbs for the collector, 240 lbs for the ballast) must be lifted to and supported by the roof; this load is also concentrated at the base of the supporting structure where the ballast is located, resulting in concentrated loads of 20-30 lb/ft2. The system described herein derives 80% of its ballast weight from water obtained locally and brought to the roof via water pressure, at virtually no labor or material cost. Further, the ballast is evenly distributed over the area of the module, which places the center of mass of the ballast very low in the center of the module, which is the optimal location to resist rotational moments due to wind. This also results in a peak roof loading of less than 6 lbs/ft2, or ⅓ to ⅕ that of the current designs. Also, since the ballast is contained in hundreds of separate containers formed by each of the separate cells formed by the walls of the multi-wall polycarbonate sheets, the ballast sheet can withstand minor cuts and punctures with substantial loss of ballast weight. If the super absorbent polymer is employed, any leaks would be impossible because the water would be locked in a solid state. In summary, the physical installation of the modules of the system described herein requires half the number of modules, imposes less than half the total weight on the roof, and requires no lifting or handling of ballast material. The installation is achieved by simply placing the modules 100 at the desired location and filling them with water.
One potential challenge presented by any in-roof installation is the obstruction of rainwater drainage. For example, in the current system, drainage around an installed module 100 may present a challenge. Skylights or curbs on flat rooftops often have “crickets” installed around them that raise the roof membrane surface slightly on the side that faces the uphill grade to guide water around the obstacle. This could be done in the case of the system described herein if necessary, although it would possibly increase installation costs. This also might not be preferred if a row of modules is placed across a drainage path. An alternative would be to install one or more additional sheets of twin wall polycarbonate underneath the module which have their open cells facing the uphill grade. This would allow standing water to drain under the module without exposing the bottom of the module to wind loads.
Optionally, module 100 may be provided without bottom layer sheet 116 configured to receive water for ballast. For example, as shown in
Similarly, and with reference to
In operation, the sun's rays pass through top glazing sheet 102 and through absorber sheet 110 itself (and the fluid in the absorber sheet), and strike the black paint on the back surface 111 of absorber sheet 110. This heats up the lower half of the absorber sheet 110. A working fluid is circulated directly through the twin wall absorber sheet 110 from the lower south manifold 300 to the upper north manifold 200, which working fluid picks up the heat and causes it to rise in temperature as it flows. The fluid exits the upper north manifold 200 at a higher temperature, thus collecting solar heat.
Modules 100 are also preferably configured to passively manage their temperature to within the limits of the plastic material that forms the module 100. In this regard, a review of some background on panel temperature regulation is appropriate.
There are two classes of solutions to address the problem of overheating solar collectors. The first class of solutions could be called “open loop,” in which the collector is designed such that the collector materials cannot reach their maximum allowed temperatures even under worst case conditions of full sun, hot environment, and no flow. This has the disadvantages of either 1) putting a limit on the thermal efficiency of the panel in order to maintain the temperature below service limits, or 2) disallowing the use of lower cost materials with lower service temperatures not only for the collector itself, but for the piping and downstream components which may be subject to the stagnation temperature conditions.
The other class of solutions are closed loop, which have some type of additional cooling function that is activated when the temperature of the panel approaches its limit. The closed loop solution is preferable to the open loop because it does not place the same limits on the thermal efficiency of the panel or downstream components such as seals, fluid lines, and valves. The most desirable temperature regulation system is passive (i.e., one that does not require any active electrical or mechanical components), based on fundamental physical properties of the material as opposed to emergent properties of complex designs.
In the system described herein, the material with the lowest service temperature needing to be protected is the center polycarbonate sheet that forms absorber sheet 110, which has a maximum allowable temperature of about 260° F. The configuration for self-regulation of panel temperature is shown in the schematic views of
The previous paragraph describes the temperature regulation configuration shown in
Any fluid may generate thermosiphon pressure under the right conditions; however, three properties of the particular working fluid described herein are significant to achieving meaningful flowrates: high density, high coefficient of thermal expansion, and low viscosity. The preferred working fluid used in the system described herein is PDMS (described in greater detail below), which has a very high coefficient of thermal expansion, roughly 5 times that of water, combined with a density that is only 15% less than water. The viscosity is much higher than water at room temperature, but at the operating temperatures of a solar thermal system, it is comparable to that of water. This makes this thermal fluid particularly suited to thermosiphon applications, because the pressure available due to the thermosiphoning effect is proportional to the coefficient of thermal expansion of the liquid, the density of the liquid, and the temperature difference between the hot and cold fluid lines.
Two additional configurations for achieving the thermosiphon flow are shown in
Likewise, as shown in
As mentioned above, the thermally actuated valve is mounted in the upper manifold 200 on the hot end of the module 100. This will either be the end of the module that is oriented toward the north in the box thermosiphon design, or the upper end of the module using either of the other two configurations discussed above. The main function of the valve is to hydraulically isolate the upper glazing sheet 102 from the lower absorber sheet 110 when below the critical temperature, and to allow flow between the two sheets when above the critical temperature. The thermosiphon pressure is very small (about 0.015 psi) compared to pumped flow pressures (about 10 psi), such that it may be difficult to support axial flow along the manifold because the small cross-sectional area of the manifold compared to the upper glazing sheet 102 and the lower absorber sheet 110 would result in excessive pressure drop. Therefore, in the thermosiphon case, the valve is designed to allow flow to enter from the lower absorber sheet 110 and pass vertically through the valve to enter the upper glazing sheet 102.
To assemble the valve 400, two side rails 406 are first inserted into the preferably square polycarbonate tube of manifold 200. Each rail may have preferably two grooves 408 cut along its length facing the interior walls of the manifold 200, and each groove 408 preferably holds a linear rubber gasket 410. A lower valve plate 420 is inserted into an interior groove 412 on the inside of each rail 406, which forces the two rails 406 towards the interior walls of the manifold 200, which in turn seats the linear rubber gaskets 410 and fluidly separates the upper and lower cavities on the interior of manifold 200. A second upper valve plate 430 is inserted just above the lower valve plate 420; this upper valve plate 430 is sized so as to provide sufficient clearance between the outer edges of upper valve plate 430 and the interior walls of side rails 406 so that it easily slides axially in the manifold 200. In addition, it operates in a bath of PDMS, otherwise known as “silicone oil” (i.e., the working fluid), which is an excellent lubricant. A thermal actuator 440 may be provided just below the lower valve plate 420, which may be used to move the upper valve plate 430 based on temperature. The function of each of these components is described below.
The actuator 440 is preferably configured as a long cylindrical rod immersed in the working fluid in the lower portion of the manifold 200. The fluid exiting the upper, hottest portion of the absorber sheet 110 bathes the actuator 440 in fluid so that the temperature of the rod is within a few degrees of the hottest fluid in the module 100. The rod is preferably made of a material that has a relatively high coefficient of thermal expansion, and which also maintains adequate stiffness at the upper service temperature. Polyvinyldiene fluoride (PVDF), and polytetrafluoroethylene (PTFE) are suitable materials, with a linear expansion of about ¼″ to ⅜″ between 180° F. and 230° F. for a 48″ to 60″ rod. PVDF is preferred because it has a higher coefficient of thermal expansion and it maintains a higher compressive modulus at the service temperature. As shown in the close-up view of one end of actuator 440 of
When the average temperature of the rod 442 is below the critical temperature, the end of the rod 442 is retracted inside the copper tube sheath 444 as in the left view of
As shown in the schematic view of the thermally actuated valve 400 of
The valve body consists of lower valve plate 420 and upper valve plate 430, which again are mounted so as to extend horizontally across the manifold 200. The bottom valve plate 420 is fixed and, as mentioned above, is mounted such that it has a compression fit on the sides with side rails 406 so that no flow can go around the bottom valve plate 420. The bottom valve plate 420 has a regular series of slots 422 cut into it, as shown in the schematic view of
In the simplest implementation, the slot pattern of the upper and lower plates 430 and 420, respectively, are identical. This allows for function of the valve with relatively simple fabrication techniques. One challenge with this configuration, however, is that once the critical temperature is reached, all of the slots open at the same time. Because the temperature of the fluid in the upper portion of manifold 200 can be much lower than the temperature of the fluid in the lower portion of manifold 200, a sudden influx of cool fluid could quench the actuator 440, causing it to suddenly contract and resulting in instability. By having a variable spacing of the slots 432 in the upper plate 430 as shown in
The thermally actuated valve has the advantage of being housed entirely in the upper north manifold 200, and if the end plate of the manifold 200 is removable, the valve can be maintained in the field without further disassembly of the module.
Thermally actuated valve 400 may optionally be provided without actuator 440, in which case opening of the thermally actuated valve 400 is dependent solely upon expansion of upper valve plate 430 and/or bottom valve plate 420. More particularly, material that undergoes thermal expansion expands in every dimension. Because the width of the thermally actuated valve 400 is much smaller than its length, the expansion may be approximated as being linear. If one end of a long piece of solid material is fixed, the axial deflection of any one point is proportional to its distance from the fixed end. That is, a point on a rod that is close to a fixed end of the rod will not deflect much at all, while a point on the opposite end will deflect the most. In order to achieve an even amount of flow across the whole length of the thermally actuated valve 400, it is important that the valve open area be relatively constant over the length. In a configuration that does not make use of actuator 440, both the upper valve plate 430 and the lower valve plate 420 are fixed at opposite ends, as shown in the schematic view of the valve of
Below the critical temperature, and as shown particularly in
This configuration has the advantage of requiring only two moving parts: the upper valve plate 430 and the lower valve plate 420 themselves. The elimination of the thermal actuator 440 opens up more of the lower half of the manifold 200 for fluid flow, reducing the pressure drop and reducing pumping power.
The selection of the particular fluid to serve as the thermal working fluid is important in this ultra-low-cost solar collector design. The working fluid must have low viscosity, adequate thermal conductivity and heat capacity, be non-toxic, have very low vapor pressure at high temperatures, and most importantly be chemically compatible with the polycarbonate material. One fluid that meets all these requirements is polydimethyl siloxane, or PDMS, commonly known as silicone oil. It is also colorless, odorless, and non-toxic, and also provides good lubrication of the thermal valve sliding surfaces.
Moreover, the fluid type used as the working fluid may also offer performance advantages. Most solar thermal working fluids are water-based with some type of anti-freeze added. The PDMS fluid has two main advantages compared to water. First, water is not compatible with polycarbonate at elevated temperatures since the water can penetrate the plastic and disrupt the chemical bonds. Second, the vapor pressure of water rises rapidly with temperature, which requires that the fluid passages be made of high tensile strength materials such as metals and that the cross section must generally be round to contain pressures as high as 150 psi. PDMS fluid has a very low vapor pressure even at elevated temperatures, enables the use of fluid channels that are made of plastic, and also of arbitrary cross section. In the system disclosed herein, the flow is essentially planar across the very thin absorber sheet 110. The hydraulic diameter in the direction of the flow is one to two orders of magnitude greater than that corresponding to a typical serpentine flow solar collector or a pool heater with small flow passages. The larger hydraulic diameter implies greatly reduced flow velocities that then reduce the pressure drop across the module by an order of magnitude compared to typical solar panels, resulting in lower pumping loss and further reductions in static pressure. The lower velocity of the fluid across the absorber surface does also reduce the heat transfer coefficient between the absorber surface and a working fluid. However, the much larger heat transfer area more than makes up for the lower heat transfer coefficient, so that the temperature differences between the fluid and the wall are comparable to standard collectors employing the serpentine tube arrangement.
A residential solar thermal system is shown in
As described above, the thermosiphon temperature regulation system could be provided in three slightly varied configurations in a flat roof, typically commercial installation. Because residential applications in general begin with a naturally sloping roof, the third option described above for thermosiphon temperature regulation systems is preferable (i.e., having a sloped solar thermal collector module), as shown in detail in
Thermal storage and heat transfer to the process water is accomplished in discrete thermal storage modules 600, as shown in greater detail in
The storage medium inside the modules is not water, but a phase change material (PCM) such as palmitic acid or paraffin which is selected to have a fusion temperature (melting point) a few degrees higher than the desired delivery temperature of the hot water to the residence. The use of the phase change material may provide one or more of the following advantages. First, the energy density (BTUs of thermal storage per-unit of mass and volume of storage media) is several times that of liquid water. Second, the energy is stored in an isothermal manner. That is, the storage medium does not heat up appreciably as more heat is added. This means that the solar thermal collector 100 is essentially running at the same temperature for the entire day and is collecting heat at a temperature that is close to that provided to the load. When the energy is transferred to the hot water system in the residence, similarly, all of the heat is released at close to the same temperature. This significantly reduces the need for supplemental heat to make up for what the solar system can't provide. By contrast, a system that uses simple sensible (temperature change) heat storage requires a gradually increasing amount of supplemental heat to maintain the desired outlet temperature as the reservoir is exhausted. (Alternatively it could be collected and stored at a temperature above that which is required for the load and mixed with cooler water before being provided to the users; however, this is thermodynamically inefficient as it requires solar collection temperatures at higher levels, which reduces collector efficiency.) A storage system using phase change material can provide hot water to the load in a more binary fashion. That is, it can provide heat at a constant temperature until the reservoir is exhausted, so that no supplemental heat is required until that point.
Maintaining the thermal storage media in a location physically above the collector (as shown in
One of the main challenges in using phase change thermal storage media is getting the heat into and out of the material. The conductivity of most PCMs is very low, and the liquid phase relatively viscous, so either the material must be encapsulated in very small enclosures or some type of conductivity enhancement must be added to the material. The solutions set forth herein provide three separate methods to overcome this problem while using the same conductive media to provide the heat transfer from the solar working fluid to the process water.
The top drawing of
The heat is conducted across the wall of the storage cylinder 604 and to the storage media 602. The storage media is a phase change material having a melting point of preferably about 140 F. When the PCM is in a liquid phase, the heat transfer from the walls of the storage cylinder 604 to the PCM is more efficient due to natural convection flow of the liquid material. Most of the suitable phase change materials, however, have very low conductivities in the solid state, such that cooling of the PCM to its solid state requires some additional materials for extracting the heat from the bulk solid PCM. Various techniques are described in the literature to improve the bulk conductivity of the PCM, such as adding aluminum wool or expanded graphite to the material. However, these methods make inefficient use of the conductive material since they improve the conductivity in all directions, and in our case only the radial direction of heat conduction is of interest. A uni-directional conductivity enhancement, as shown in
The radial heat transfer configuration may offer one or more of the following advantages. First, it may offer much more heat transfer area on the outside of the cylinder 604 than the inside. This is important because in the thermosiphon case, there is very little flow velocity on the outside of the cylinder 604, such that the heat transfer coefficients are an order of magnitude lower on the outside of the cylinder 604 than the inside, which has water flowing at up to several feet per second. Second, the required heat rate on the discharge cycle (about 10 kW) is higher than the charging cycle (about 3 kW). The radial configuration of the fins 610 yields a much higher density of conductors near the center of the cylinder, which gives an overall higher conductivity through the PCM 602, supporting higher discharge rates.
The storage module 600 may also be configured as shown in
Still further, the storage module may be configured as shown in
First, it is much easier to obtain aluminum sheet in thicknesses that are smaller than either the thickness of an extrusion or the diameter of wire. Sheet is readily available in thicknesses of 5 to 10 thousandths of an inch, while extrusions below 30 thousandths are difficult. Wire can be drawn to diameters as small as 10 mils, but the stiffness is very low and the wall contact is not strong. The second reason is that the fin assembly can be fabricated in a single piece with simple sheet metal bending processes and no other fastening hardware is required to maintain it in position.
The process water to be heated flows through pipe 612 extending through the center passage of the heat transfer structure, and in contrast to the flow arrangement of the solar working fluid (which flows through the open space 606), the process water (domestic hot water) flows through the storage modules in series. The flow is in series because, firstly, there is plenty of available pressure drop on the waterside to allow flow in series through the storage modules. Secondly, there is less heat transfer area available on the inside than on the outside of the annular storage media space, such that the water must flow through a longer path to achieve the required heat gain.
The course of a thermal cycle carried out by the foregoing system can be described as follows:
1. The cycle starts with a thermal storage medium in a solid state and the working fluid at ambient temperature.
2. As the sun heats the fluid in the solar thermal collector module 100, a thermosiphon pressure difference [or turning on a mechanical pump] causes the hot fluid to flow from the collector module 100 into the outer fluid area of the storage module 600.
3. The heat conducts from the outer shell 604 through the axial wires or fins 610, and the heat storage medium 602 begins to melt at the wall and around the wires or fins 610.
4. As more heat is collected from the collector module 100 and transferred to the heat storage medium 602, the system remains in a quasi-steady-state with the heat being collected at a temperature which is only about 10° above that at which it is being stored. Thus, the solar collector operates at its most efficient possible operating point throughout the day while melting the PCM 602.
5. Towards the end of the day as the insolation is reduced, the temperature of the collector module 100 drops below the phase change temperature, the thermosiphon pressure drops to zero, and the flow stops (or is turned off).
6. When hot water is desired inside the residence, cold water enters one end of the first thermal storage module 600. The cold water is warmed by releasing the heat of fusion of the phase change thermal storage media 602 near the entrance of the cold water pipe 612 and then continues isothermally through the remainder of the storage modules 600. As the water continues to flow, a thermal wave is induced in the thermal storage media 602 with the freezing edge of the phase change material continuing along the direction of the flow of the process water. In this way, the thermal storage media 602 is able to provide water to the load at a nearly constant temperature until the heat storage cylinders begin to be exhausted. When the thermal storage media 602 in the later cylinders becomes solid, the outlet temperature begins to drop.
7. If there is no flow of the process water for several days, and the thermal storage media 602 is completely in liquid phase, there will be very little temperature change in the solar working fluid as it flows through the storage medium, and the collector module 100 will begin to stagnate. At this point, the passive thermosiphon temperature regulation mechanisms previously described will initiate the self cooling process.
This solar energy storage system is optimized for an attic-mounted thermosiphon application in that the material cost is greater than that associated with a standard water tank, but the advantages in terms of ease of installation, improvement in efficiency of the solar collection, and the reduced opportunity cost of the space that the storage equipment occupies are expected to outweigh the higher material costs.
The phase change material thermal storage media 602 has an upper allowable service temperature, and the temperature self-regulation of the collector module 100 is required for the system to operate successfully. Nevertheless, this storage system design could also be paired with a conventional solar thermal system provided the proper temperature safeguards are in place.
The use of modular phase change thermal storage units 600 provides the opportunity to make the system as a whole more thermodynamically efficient by using different phase change materials in different modules to optimize the heat collection temperature while maintaining a more constant outlet temperature to the load. This is illustrated in
In the second configuration of
A potential disadvantage of this configuration is a result of filling the several types of thermal storage cylinders sequentially. If the solar collection period is interrupted during the day, only the low temperature cylinders would be filled, resulting in a supply temperature below the desired delivery temperature. A further optimization of this concept would apply in the case where more than one collector is used to gather solar heat, as shown in the schematic view of
Having now fully set forth the preferred embodiments and certain modifications of the concept underlying the present invention, various other embodiments as well as certain variations and modifications of the embodiments herein shown and described will obviously occur to those skilled in the art upon becoming familiar with said underlying concept. It should be understood, therefore, that the invention may be practiced otherwise than as specifically set forth herein.
This application is based upon and claims benefit of copending U.S. Provisional Patent Application Ser. No. 61/928,111 entitled “Systems and Methods for Solar Heating and Cooling of Buildings,” filed with the U.S. Patent and Trademark Office on Jan. 16, 2014 by the inventor herein, the specification of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61928111 | Jan 2014 | US |
