Patent Application
20140283815
Sunlight provides a source of renewable, clean, and freely available energy useful in a variety of applications, including water heating, space heating, electricity generation, and other areas. Devices for collecting and concentrating solar energy have been developed to take advantage of the ready availability of this energy source. For example, flat panel collectors are often used in low temperature applications such as space heating and domestic water heating. Concentrating collectors may be used in higher temperature applications such as electric power generation, industrial process heating, and other applications.
Conventional flat panel collectors have a number of disadvantages. For example, they are typically made of relatively expensive materials such as copper, aluminum, and glass due to requirements for strength and thermal conductivity. Flat panel collectors are also typically provided in relatively large form factors, for example up to four by eight feet, and thus may be difficult to integrate into architectural designs.
According to one aspect, a solar energy collector comprises an elongate plastic receiving tube configured to carry a heat transfer fluid, and an elongate clear plastic tubular sheath surrounding the receiving tube. The sheath is of a larger cross section than the receiving tube such that a generally annular air space is formed between the receiving tube and the sheath. The sheath has a front side configured to be disposed toward the sun and a back side opposite the front side. The solar energy collector further includes a reflective coating partially covering a portion of the inside surface of the tubular sheath at the back side of the tubular sheath and configured such that when sunlight is directed at the front side of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly, and a second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube. In some embodiments, the receiving tube is made of black polyethylene tubing. The sheath may be made of clear polycarbonate tubing. In some embodiments, both the receiving tube and the sheath have circular cross sections.
According to another aspect, a solar energy collector includes a plurality of elongate plastic receiving tubes configured to carry a heat transfer fluid. The solar energy collector also includes a plurality of elongate clear plastic tubular sheaths, each sheath surrounding a respective one of the plurality of receiving tubes, each sheath being of a larger cross section than its respective receiving tube such that a generally annular air space is formed between sheath and the receiving tube. Each sheath has a front side configured to be disposed toward the sun and a back side opposite the front side. A portion of the interior surface of the sheath at the back side is covered by a reflective coating configured such that when sunlight is directed to the uncoated portion of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly and second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube. The solar energy collector further includes an inlet manifold including an inlet opening for receiving the heat transfer fluid to be heated and a plurality of outlet openings, and an outlet manifold including a plurality of inlet openings and an outlet opening. Each of the plurality of receiving tubes is coupled between one of the plurality of outlet openings of the inlet manifold and one of the plurality of inlet openings of the outlet manifold. In some embodiments, the inlet and outlet manifolds are made of plastic. In some embodiments, the plurality of receiving tubes are disposed parallel to each other to form a rectangular collector unit. In some embodiments, the inlet manifold, the outlet manifold, the plurality of receiving tubes, and the plurality of sheaths are comprised in a first collector unit, and the solar energy collector comprises one or more additional collector units of like construction to the first collector unit, the inlet manifolds of the one or more additional collector units operatively coupled to the inlet manifold of the first collector unit and the outlet manifolds of the one or more additional collector units operatively coupled to the outlet manifold of the first collector unit. In some embodiments, gaps exist between adjacent members of the plurality of sheaths. The solar energy collector may have an aspect ratio of at least 3:1. The solar energy collector may have an aspect ratio of at least 5:1.
According to another aspect, a solar energy collection system includes a solar energy collector comprising an elongate plastic receiving tube surrounded by an elongate clear plastic sheath. The sheath is of a larger cross section than the receiving tube such that a generally annular air space is formed between the receiving tube and the sheath. The sheath has a front side configured to be disposed toward the sun and a back side opposite the front side and a reflective coating partially covering a portion of the inside surface of the tubular sheath at the back side such that when sunlight is directed at the front side of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly, and a second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube. The solar energy collection system further includes a supply of the heat transfer fluid to be heated by the solar energy collector. In some embodiments, the solar energy collector comprises a plurality of elongate plastic receiving tubes, each surrounded by a respective elongate clear plastic sheath, the plurality of sheaths and receiving tubes are arranged in parallel such that the solar energy collector is rectangular, and the solar energy collector comprises an inlet manifold and an outlet manifold to direct the heat transfer fluid through the parallel receiving tubes. The heat transfer fluid may be water comprising nanoparticles. The solar energy collection system may further include an ion generator to generate the nanoparticles. In some embodiments, the solar energy collection system further includes a tank for holding the supply of heat transfer fluid, a supply pipe for carrying heat transfer fluid from the tank to the solar energy collector, a return pipe for carrying the heat transfer fluid from the solar energy collector to the tank, a circulation pump for circulating the heat transfer fluid between the solar energy collector and the tank through the supply pile and the return pipe, and a controller that controls the operation of the circulation pump based at least in part on the temperature of the heat transfer fluid in the tank and the temperature of the solar energy collector. The controller may be configured to determine when the solar energy collector reaches a stagnation condition and in response to the determination, enter a stagnation remediation mode. In some embodiments, the solar energy collection system further includes a source of cooling fluid, wherein during the stagnation remediation mode, the cooling fluid is circulated through the solar energy collector without passing through the tank. In some embodiments, the solar energy collection system further includes a ground coupled heat exchanger, wherein in the stagnation remediation mode, the cooling fluid is circulated through the solar energy collector and the ground coupled heat exchanger without passing through the tank. In some embodiments, the ground coupled heat exchanger comprises a piping loop connected between the supply pipe and the return pipe, and the system further comprises a set of valves operated by the controller to isolate the tank during the stagnation remediation mode. In some embodiments, the solar energy collection system further includes a photovoltaic panel that supplies power to operate the controller and the circulation pump. In some embodiments, the solar energy collection system further includes a root zone heating loop, wherein the system circulates the heat transfer fluid through the root zone heating loop to heat the root zone of plants.
According to another aspect, a method of collecting solar energy includes providing a solar energy collector. The solar energy collector includes one or more elongate plastic receiving tubes each surrounded by a respective elongate clear plastic sheath. Each respective sheath is of a larger cross section than its respective receiving tube such that a generally annular air space is formed between the receiving tube and the sheath, the sheath having a front side configured to be disposed toward the sun and a back side opposite the front side and a reflective coating partially covering a portion of the inside surface of the tubular sheath at the back side such that when sunlight is directed at the front side of the sheath, a first portion of the sunlight transmitted through the sheath strikes the receiving tube directly, and a second portion of the sunlight transmitted through the sheath strikes the reflective coating and is redirected to the receiving tube. The method further comprises installing the solar energy collector in a location that receives sunlight, and passing a heat transfer fluid through the solar energy collector to be heated by the sunlight. In some embodiments, the solar energy collector has an aspect ratio of at least 3:1, and installing the solar energy collector in a location that receives sunlight comprises installing the solar energy collector in a location that cannot accommodate a collector of equal area having a significantly smaller aspect ratio. In some embodiments, the solar energy collector has an aspect ratio of at least 5:1, and installing the solar energy collector in a location that receives sunlight comprises installing the solar energy collector in a location that cannot accommodate a collector of equal area having a significantly smaller aspect ratio.
Embodiments of the invention provide a solar collector made of low-cost, lightweight polymer materials.
Collector unit 100 also includes four elongate receiving tubes 203, one for each of side tubes 201. Although other materials and sizes may be used, receiving tubes 203 may be made of flexible ½ inch black polyethylene pipe, which is manufactured in lengths up to hundreds of feet, and is typically packaged in rolls. The black color of the pipe may facilitate absorption of solar energy. Receiving tubes 203 may be connected to outlet manifold 102 in any suitable way, for example using crimp clamps 204 (only two of which are visible in
Collector unit 100 further includes four elongate clear plastic tubular sheaths 205, one surrounding each receiving tube 203. Sheaths 205 are a form of glazing and may be made, for example, of clear polycarbonate tubing or another suitable material that is transparent to sunlight or nearly so. In the example of
In
In
In some embodiments, the width of each sheath is smaller than the spacing between the receiving tubes, so that a gap exists between adjacent sheaths. This gap may allow airflow through the collector, and may reduce the effect of wind on the collector.
The use of a heat reservoir of this type for various purposes is discussed in co-pending U.S. patent application Ser. No. 12/481,745 filed Jun. 10, 2009 and titled “Integrated Energy System for Whole Home or Building”, the entire disclosure of which is hereby incorporated by reference herein for all purposes. Other related information may be found in U.S. patent application Ser. No. 12/481,741 filed Jun. 10, 2009 and titled “Thermoelectric Generator”, and U.S. patent application Ser. No. 12/481,750 filed Jun. 10, 2009 and titled “Automatic Configuration of Thermoelectric Generation System to Load Requirements”, the entire disclosures of which are hereby incorporated by reference herein for all purposes.
In one simple mode of operation, a controller 608 receives signals indicating the temperature of heat transfer fluid 602 in tank 603, and the temperature of collector 500. If heat transfer fluid 602 in tank 603 is not at its desired temperature, and collector 500 is at a temperature higher than the temperature in tank 603, controller 602 causes circulation pump 604 to run so that the available energy at collector 500 is used to heat tank 602. In some embodiments, a photovoltaic panel 612 may be used to power controller 608 and circulation pump 604.
In some embodiments, heat transfer fluid 602 is water. Because collector 500 is mounted on roof 601 and exposed to the elements, water in collector 500 could be subjected to freezing temperatures. Preferably, tank 603 is located in a conditioned space, and system 600 operates as a “drain back” system. A volume of air may be included in the system, sufficient to fill collector 500 and the portions of pipes 605 and 606 that are above roof 601 and subject to freezing. When circulation pump 604 is not running, for example when the collector temperature is very low due to cold weather and is lower than the temperature of tank 603, heat transfer fluid 602 can drain back into tank 603 by the action of gravity, so that collector 500 and the upper portions of pipes 605 and 606 fill with air, and freezing of water in collector 500 is prevented. Preferably, collector 500 is mounted in such a way that corner 609 is the lowest portion of collector 500, so that heat transfer fluid 602 can drain as completely as possible from collector 500 by gravity.
Optionally, a vacuum breaker valve may be placed collector 500, for example at topmost corner 613, to admit air to collector 500 during drain back, to speed the drain back of heat transfer fluid 602.
Traditionally, solar collectors have been made of materials that are highly thermally conductive, for example aluminum and copper, in order to promote the efficient transfer of heat from the collectors to the heat transfer fluid used in them. However, collector 500 is made primarily of plastic materials, which have much lower thermal conductivity than traditional collector materials. For the purposes of this disclosure, to be made primarily of plastic materials means that the receiving, fluid distribution, and glazing components of the collector are made of polymer materials. Some metal parts may be used in a collector that is made primarily of plastic materials, for example for reflective coatings, clamps, mounting hardware, and other incidental functions. The primarily plastic construction of a collector in accordance with embodiments of the invention may afford advantages in material cost, shipping cost, and ease of assembly of the collector. In addition, the light weight of the collector may enable mounting lighter mounting hardware, and mounting the collector in locations that would not support a heavier collector made of traditional materials.
Even though made primarily of plastic, a model collector made according to an embodiment of the invention has proved surprisingly effective, especially in light of the low cost of its components. For example, a system similar to system 600 was constructed including a collector made the manner shown in
The performance of a system embodying the invention can be further improved by enhancing the heat transfer characteristics of heat transfer fluid 602. One especially effective and economical technique for enhancing the thermal conductivity and convective heat transfer characteristics of water is to convert the water to a nanofluid having suspended nanoparticles. Techniques for generating nanoparticles in water are described in co-pending U.S. patent application Ser. No. 13/035,479 filed Feb. 25, 2011 and titled “Thermoelectric Generator Using Nanofluid”, and U.S. Provisional Patent Application No. 61/680,671 filed Aug. 7, 2012 and titled “Efficient Energy Collection and Distribution”, the entire disclosures of which are hereby incorporated by reference herein for all purposes.
In one embodiment as illustrated in
The test results described above for the model system were obtained using pure water as the heat transfer fluid. The same system was tested again with the addition of nanoparticles, and its performance improved significantly. The system was operated in the same manner as described above, on three sunny days in late winter of 2013 in Longmont, Colo. When operated using water including nanoparticles as the heat transfer fluid, the system delivered an average of 8672 BTUs to the tank, or 808 BTUs per square foot of collector area. The addition of nanoparticles thus at least partially counters the effect of the lower conductivity of the collector materials.
Other arrangements are possible. For example, if it is desired to use a system similar to system 600 to heat domestic hot water (DHW), then the heat transfer fluid circulated through the collector may be isolated from the domestic hot water tank, using a heat exchanger to transfer heat to the domestic hot water tank. An example of such a system 700 is shown in
Another phenomenon that may be of particular interest in a plastic solar collector is stagnation of the collector. Stagnation refers to the condition where the heat transfer fluid is not circulating through the collector, but the collector is in sunlight. This situation may occur, for example, if tank 603 has reached its desired temperature and controller 608 shuts off pump, even though collector 500 is at a higher temperature than tank 603. Without the circulating heat transfer fluid to carry heat away from collector 500, collector 500 may reach such high temperatures that collector 500 may be damaged.
In conventional systems having collectors made of metal parts, simply draining back the heat transfer fluid may be sufficient to provide adequate protection, as the metal parts may be able to withstand the temperatures reached by the empty collector. However, the plastic materials used in a collector according to embodiments of the invention may have a relatively low melting temperatures, and may be subject to thermal expansion and warping whether filled or empty.
In some embodiments, protection from damage due to stagnation may be provided by circulating a cooling fluid through the collector when the storage tank is at its full temperature, but the collector is in sunlight.
In system 800, controller 608 monitors the temperature of heat transfer fluid 602 in tank 603, and also the temperature of collector 500. When stagnation is detected, for example when tank 603 is at its maximum desired temperature and collector 500 exceeds a safe operating temperature, controller 608 enters a stagnation remediation mode. In one example embodiment, controller 608 may enter the stagnation remediation mode when collector 500 exceeds 120° F., although other criteria may be used.
In the stagnation remediation mode, circulation pump 604 is shut off. Preferably after a short delay, two normally-open valves 802 and 803 are switched closed under control of controller 608, to isolate tank 603. Second pump 804 is then started. Piping loop 801 has been previously filled with water, so that water is drawn by second pump 804 from piping loop 801, travels up pipe 605 to collector 500 where it is heated and cools the collector, and travels down pipe 606. Because normally-open valve 802 is closed, the water travels through pipe 805 back to piping loop 801 to give up heat and circulate for another cycle through collector 500 if needed. It is envisioned that only a few minutes of cooling will be required at any one time to bring collector 500 back to a safe temperature, and that the energy consumption of second pump 804 and valves 802 and 803 will therefore be small.
For added security, an additional, redundant pump 806 may be installed in parallel with second pump 804. Controller 608 may sense whether pump 804 is operating properly and if not, redundant pump 806 would be started, to ensure that collector 500 is properly cooled. In some embodiments, emergency battery power could be used to operate the system during power outages.
In other embodiments, for example for installations where it is not possible to install a collector in such a way that it can drain back completely, the stagnation remediation mode may also be used for freeze protection. That is, heat transfer fluid could be circulated through piping loop 801 and through the collector in times of freezing temperatures. Instead of cooling the collector to prevent damage from stagnation, the circulating fluid would be heated by the ground loop and would warm the collector to prevent damage from freezing. In some embodiments, no additional hardware would be necessary. The additional control logic could simply be programmed into controller 608.
While controller 608 is described above as being programmable, the system may also be implemented by a set of temperature sensors and logical connections between them, without requiring a programmable element such as a microprocessor. For example, a differential temperature sensor may compare the temperatures of the collector and tank, and generate a logic level indicating whether the collector is warmer or cooler than the tank. That signal would be routed to a drive circuit for circulation pump 604, and circulation pump 604 would run when the collector is warmer than the tank, but would not run otherwise. The drive signal could be further gated by other logic signals. For example, another differential sensor could compare the tank temperature to a desired upper limit value, and generate a logic level indicating whether the tank is at the desired temperature or not. If so, the drive signal to circulation pump 604 could be overridden so that circulation pump 604 would not run in this state. Other gating signals would be derived to indicate stagnation (collector above a stagnation limit temperature), freeze danger (collector below a threshold temperature), or other conditions. It is intended that the term “controller” encompass both programmable devices and control logic implemented by discrete components.
In one example embodiment, controller 608 may be programmed or otherwise configured to implement the following control scheme:
While collector 500 shown in
For example,
A collector such as collector 900 or another collector according to embodiments of the invention may be installed in places where a conventional collector panel could not. For example, collector 900 may be installed on top of a fence or wall, on a building ledge, on a building eave, or in another location that has good sun exposure but would not be convenient for mounting another kind of collector, for example a traditional flat panel collector having an aspect ratio of about 2:1. Collector 900 may be somewhat flexible, and could include right-angle or other angular bends through the use of simple and readily available fittings. Thus, solar energy may be utilized in an unobtrusive manner.
It may be especially helpful for such a system to include the stagnation remediation capability described above and shown in
Many other collector configurations are possible as well. For example, long narrow collectors having two, three, four, or another number of parallel receiving tubes and sheaths could be used, depending on the size and shape of the area available for mounting the collector. Such collectors may have aspect ratios much larger than the typical conventional flat panel collector, for example 3:1, 4:1, 5:1, 10:1, 20:1, 50:1, 100:1, 200:1, 500:1, or another aspect ratio, and may be installed in spaces that could not accommodate a collector of equal area having a significantly smaller aspect ratio. For the purposes of this disclosure, a significantly smaller aspect ratio is any aspect ratio less than about 50 percent of the aspect ratio of the collector in question.
As was mentioned earlier, one application of a solar energy collection system according to embodiments of the invention is root zone heating of plants. Research has shown that heating the soil in the root zone of plants can promote improved growth and productivity of the plants, and enables cultivation of plant species in much colder ambient environments than the plants' native environments.
The invention has now been described in detail for the purposes of clarity and understanding. However, it will be appreciated that certain changes and modifications may be practiced within the scope of the appended claims. It is intended that all workable combinations of the elements and features described herein are also considered to be disclosed.
