PUMP DRAINED SOLAR WATER HEATING SYSTEM
A pump drained solar water heating system, includes a solar collector; a reversible pump; a reservoir for storing heat transfer fluid; an insulated water storage tank for storing the potable water; piping for fluidically connecting the pump to the solar collector, the heat transfer fluid reservoir; and a heat exchanger; and a controller for operating the pump to transmit fluid in a forward flow or reverse flow direction.
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Solar water heating systems are made up of components that collect solar energy, transfer the energy to the potable water via a heat exchanger, store the thermal energy, control system operations, deliver hot water where it is needed, and protect the system against freezing. Components can be combined in a variety of ways, but these six basic functions must be met, although some systems are simple enough that passive physics provide the control and motive forces to drive the fluids and heat transfer in the system.
Solar water heating systems can generally be described using the following four terms: direct or indirect and passive or active. Direct systems heat the potable water directly through solar exposure. These systems are typically used in regions with little freeze risk since the potable water is used as the heat transfer fluid and therefore exposed to temperature outside of conditioned space. Indirect systems use a secondary heat transfer fluid to collect and transfer the solar energy to the potable water. The heat transfer fluids used are typically freeze resistant and these systems are better suited for cooler climates. Passive systems use the differential density created by the thermal gradient of the water or heat transfer fluid to move fluids in the system and accumulate heated potable water for usage. Active systems employ pumps to circulate the water or heat transfer fluid for the purpose of heat exchange. Several different system configurations are currently used based on these concepts.
One simple configuration of solar water heater is a solar batch heater. The system consists of a south facing, roof-mounted tank enclosed in an insulated box covered with glass. Cold water is supplied to the bottom of the tank. When the water is heated by the sun, it rises to the top of the tank. The heated water is removed from a port located at the tank top. The mains supply water pressure is used to push the hot water from the tank as the cold is supplied to the bottom. The system is a direct passive system, meaning the potable water is heated directly and no water circulation is directed beyond that which is created by the thermal density driven gradient. Because of its relatively low cost and simplicity, the solar batch heater configuration works well for those living in moderate climates with good sunshine available.
Another relatively simple, passive solar water heater system is the thermosyphon system in which the water tank is located directly above the collector. Thermosyphon systems work on the principal of heat rising. In a direct system, the potable water enters the bottom of the collector and rises to the tank as it warms. The insulated tank at the top of the system stores the water prior to usage. In colder climates, an antifreeze solution, such as propylene glycol, is used in an indirect closed solar loop, which uses a heat exchanger to pass energy to the potable water. Since water is supplied directly to the roof mounted storage tank, design provisions must be made to avoid freezing of the exposed mains supply in the attic and roof.
A third solar water heating system is an active, direct system. This system is typically used in tropical settings where freezing never occurs. A solar storage tank is combined with a solar thermal collector. A pump is used to circulate the potable water through the collector and to the storage tank which serves as a preheat tank for a separate, existing water heater which includes an auxiliary heater for supplemental heating.
Another solar water heating system is an indirect, active system, which uses a pressurized heat transfer fluid to heat the potable water. In this system, incoming potable water is routed to the storage tank. Heat transfer fluid circulates from the collectors through a heat exchanger at the storage tank, and then is actively pumped back through the collectors. The potable water is heated via the heat exchanger at the storage tank. Supplemental heaters are provided in the storage tank or the storage tank serves as a preheat for an existing water heater.
Still another solar water heating system is an indirect, active drainback system. The heat transfer fluid is typically distilled water. When the system is not pumping, the solar collector is empty and the distilled water is stored in an appropriately sized reservoir tank located just above the storage tank. When the pump turns on, the distilled water is circulated from the reservoir back through the collector and heat exchanger, passing heat to the potable water in the storage tank. When the pump shuts off again, the distilled water drains back into the reservoir. As a result, a collector must always be higher than the storage tank, and there must be sufficient continuous slope in the piping to ensure complete drainage and prevent freezing of the heat transfer fluid.
A problem with the drainback system is it requires the fluid line to be configured to allow complete drainage due to gravity only. This configuration requires the installer to mount all plumbing so as to avoid trapping of any fluid which would potentially freeze and rupture the plumbing. Another problem with this drainback system is that larger pumps usually have to be used, especially if water is being pumped up two stories or more, since the drainback pump has to lift the distilled water to the height of the solar collector.
Thus, it is desirable to develop a new and improved indirect active system for a solar water heater, which overcomes the above-mentioned deficiencies and others while providing better and more advantageous overall results.
SUMMARY OF THE DISCLOSUREA solar water heating system comprises a solar collector panel in fluid communication with a heat exchanger which is in thermal communication with the water storage tank. A reversible pump is used to circulate the heat transfer fluid between the heat exchanger and the solar collector panel. Under normal operation, the heat transfer fluid is circulated in a first direction filling the system and transferring heat energy from the collector to the water storage tank via the heat exchanger. When conditions warrant, such as for example to avoid freezing or overheating of the fluid, the pump reverses the flow to remove the heat transfer fluid from the collector to a storage reservoir.
Specifically, a positive displacement reversible pump allows for the normal flow operation and the ability to remove the fluid from the solar collector. The diameter of the pipes linking the collector panel, the heat exchanger and the pump is of the appropriate dimension to maintain a capillary action which allows the fluid to be pumped from the pipe under suction. The pipe diameter must also be sufficiently small to avoid entrainment of air into the fluid, which would prevent complete evacuation of the fluid.
One aspect of the disclosure is a solar water heating system, including: an insulated water storage tank for storing the water to be heated; a solar collector; a reversible pump; a reservoir for storing heat transfer fluid; a heat exchanger in thermal communication with the storage tank for transferring the thermal energy from the heat transfer fluid to the water in the tank; piping for connecting the pump, the collector, the heat transfer fluid reservoir, and the heat exchanger in fluid communication; and a controller with a back up or reserve power supply for briefly operating the controller and pump during a loss of power; wherein the controller senses certain system temperatures and selectively operates the pump in a forward flow or reverse flow direction as a function of the sensed temperatures.
Another aspect of the disclosure is that the system automatically removes the heat transfer fluid from the solar collector, upon detection of conditions that could otherwise expose the heat transfer fluid to temperature extremes which could degrade the fluid resulting in increased corrosion to the system components and possible leakage failure or otherwise damage the collector or other elements of the system.
Another aspect of the disclosure is that the system pumps the heat transfer fluid to evacuate the plumbing system, and avoids the disadvantages of a pipe installation which relies on gravity to drain the fluid from the collector and piping system.
Another aspect of the disclosure is that deionized water can be used as the heat transfer fluid. Deionized water has improved heat transfer properties, is less expensive and less toxic than traditional heat transfer fluids, such as propylene glycol. Using deionized water may also allow usage of a single wall heat exchanger between the heat transfer fluid and the potable water, depending on local plumbing code requirements.
Yet another aspect of the disclosures is the installation time for the system would be reduced because the installer would simply fill the reservoir with fluid and prime the system. In contrast, traditional closed systems require a secondary pump and a valve for pressurization of the system.
Still other aspects of the disclosure will become apparent upon a reading and understanding of the following detailed description.
Referring now to
The heat transfer fluid used in solar water heating systems is typically either water or a combination of propylene glycol and water. Water is typically the best heat transfer fluid for both holding and transferring heat. Deionized water can be used as the heat transfer fluid. Deionized water has improved heat transfer properties, is less expensive and less toxic than traditional heat transfer fluids, such as propylene glycol. Using deionized water may also allow usage of a single wall heat exchanger between the heat transfer fluid and the potable water. However, water does freeze. Propylene glycol, mixed with water, is the industry standard heat transfer fluid where freezing is a hazard. Since propylene glycol is nearly non-toxic, heat exchangers designed for this fluid need only be single wall construction, depending on local code requirements.
Conventional drain back solar water heater systems rely on gravity to remove the heat transfer fluid from the solar collector when freezing or overheating conditions arise. Referring again to
In
Referring now to
A pressure vacuum valve 30 is used where the fluid circuit opens into the heat transfer fluid reservoir. The purpose of the valve is to prevent unintended draining of the heat transfer fluid from the fluid circuit. When the pump is pumping heat transfer fluid from the reservoir to the collector, sufficient pressure is produced to open the pressure valve and allow flow into the reservoir. When the pump stops the valve closes preventing drainage from the fluid line. Under conditions which warrant evacuation of the heat transfer fluid from the lines and collector, such as freezing, excessive temperature or a power failure, the pump reverses and creates a pressure differential sufficient to open the vacuum valve and allow air to enter the system as the fluid is pumped from the collector and line to the reservoir. While the system will function without the pressure vacuum valve, use of such a valve to keep fluid in the lines when the pump is idle under conditions which do not warrant fluid evacuation reduces the thermal shock experienced when introducing fluid, which has cooled, into a heated collector during periodic cycling of the pump.
The piping 18 is the conduit through which heat transfer fluid travels between the collector 12 and the water storage tank heat exchanger 19. The piping extends around the exterior of the tank in a coil wrap heat exchanger configuration 19. The piping must be properly sloped to allow for proper flow, dropping at least ¼ inch per linear foot. A diameter of the system line pipe also defines the ability of the pipe to maintain a capillary action which allows the fluid to be pumped from the pipe under suction. The pipe diameter must be sufficiently small to avoid entrainment of air into the fluid, which would prevent complete evacuation of the fluid. A pipe diameter on the order of ⅜ inches provided satisfactory results in the embodiment of
The storage tank heat exchanger could also be implemented in a variety of alternative configurations to provide an efficient heat exchange relationship between the fluid and the water in the storage tanks.
Pipe insulation is also used for energy performance and freeze protection. Suitable insulation includes elastomeric insulation which has an R-value of 3.5 per inch, is used for heating and cooling systems, and is preferred for exterior and interior applications. Fiberglass insulation, with an R-value of 3 per inch, is suitable for interior applications. Preferably, such insulation is applied to all exposed lengths of pipe.
As hereinbefore briefly described, pump 16 is selectively operated in a first direction to circulate the heat transfer fluid through the heat exchanger to heat the water in the tank, and also under certain circumstances to operate in a reverse direction to drain the fluid from the collector. Referring again to
When the controller determines that the temperature of the water in the tank as sensed by sensor 24 is less than the Maximum Water Storage Temperature Reference value and the temperature differential between the sensed temperature of the fluid in the collector sensed by sensor 22 and the sensed temperature of the water in the tank sensed by sensor 24 exceeds a pre-set “Pump On” reference value, the controller turns on the pump for operation in the first or heat exchange direction. Unless overridden due to other circumstances to be hereinafter described, once turned on, the pump remains on until the temperature of the water in the tank as sensed by sensor 24 is no longer less than the maximum storage tank water reference in order to prevent overheating of the water in the storage tank, or until the sensed temperature differential between the fluid in the collector and the water in the tank drops to a reference “Pump Off” value, at which time the controller turns off the pump. These reference differential temperature values are pre-set by the installer in accordance with the specific characteristics of the system to enable the system to heat efficiently without cycling too frequently. A reference Pump On value on the order of 18 degrees F. and a reference Pump Off value on the order of 9 degrees F. should work satisfactorily for typical installations but other values could be similarly employed. A Maximum Water Storage Temperature Reference value on the order of 160 degrees F. is suitable for typical installations but other values could be similarly employed.
In addition to controlling the pump in response to the temperature differential between the temperature of the fluid in the collector and the temperature of the water in the tank for efficient heating, the controller also operates to reverse the direction of the pump to drain the fluid from the collector to the reservoir under certain conditions to avoid damage to the system. For example, the controller is arranged to operate the pump in the reverse direction (that is, in the direction opposite to the direction for circulating the fluid through the heat exchanger) to drain the collector in response to the detection by sensor 22 of a temperature of the fluid in the collector greater than a pre-set Maximum Collector Reference Temperature to prevent overheating of the fluid in the collector which could damage the collector or and/or cause deterioration of the fluid. A value for the Maximum Collector Reference Temperature that is high enough for efficient heating of the water in the storage tank, but low enough to avoid exceeding the boiling point of the fluid is desirable. Similarly, the controller operates the pump in the reverse direction in response to the detection by sensor 22 of a fluid temperature less than a pre-set Minimum Collector Reference Temperature to prevent damage due to freezing of the fluid in the collector. A Maximum Collector Reference Temperature value on the order of 210 degrees F. and a Minimum Collector Reference Temperature value on the order of 35 degrees F. are suitable for typical installations, but other values could be similarly employed to avoid overheating or freezing of the fluid in the collector. It is also desirable to drain the fluid from the collector in the event of a power failure. Upon detection of such a failure, the system is arranged to activate a reserve power source operative to run the pump in the reverse direction to drain the fluid from the collector to the reservoir. In the illustrative embodiment operation of the pump in the reverse direction is timer controlled, that is, once operation in the reverse direction is initiated, the pump continues to run in that direction for a predetermined time sufficient to assure that the fluid has been sufficiently drained from the collector. Alternatively, a float switch or other level sensing arrangement could be used to turn of the pump off when the fluid level in the reservoir reaches a level signifying that the collector has been adequately drained.
In the illustrative embodiment, the pump 16 is driven by a reversible dc motor.
An alternative back up power supply circuit is illustrated in
Referring now to
Inquiry 66 determines if the temperature of the fluid in the collector sensed by sensor 22 (“Collector temp”) either exceeds the pre-set Maximum Collector Reference Temperature value (“max temp”) or is less than the pre-set Minimum Collector Reference Temperature value (“min set temp”). If “Yes” the pump is run in the Reverse Direction (Block 68) to evacuate the collector by draining the fluid from the collector to the reservoir and the system returns to Inquiry 62.
In response to a “No” at Inquiry 66 the system moves to Inquiry 70 which determines if a loss of power has been detected. If “Yes” the reserve power supply is activated (Block 72) and the pump is operated in the reverse direction to drain the fluid from the collector to the reservoir (Block 68). For example, a battery 29 used to provide power needed to evacuate the heat transfer fluid from the system during a power outage, is switched into the system to energize the pump.
In response to a “No” to Inquiry 70 the system continues to Inquiry 76 which determines if the temperature differential between the temperature of the fluid in the collector, sensed by sensor 22, and the temperature of the water in the storage tank sensed by sensor 24 (“Collector-tank temp delta”) is greater than the Pump On value (“set pump on value”). If “Yes”, the system operates the pump in the forward direction (heat exchange direction, to circulate the fluid from the collector through the heat exchanger to heat the water in the storage tank (Block 78) and the system returns to Inquiry 62.
If the response to Inquiry 76 is “No”, the system proceeds to Inquiry 80 which determines if the temperature differential between the temperature of the fluid in the collector, sensed by sensor 22, and the temperature of the water in the storage tank sensed by sensor 24 (“Collector-tank temp delta”) is less than the Pump Off value (“set pump off value”). If “Yes”, the controller then stops or deactivates the pump (Block 82, and the system returns to Inquiry 62. If the response to Inquiry 80 is “No” the system returns to Inquiry 62.
The system removes the heat transfer fluid from the collector, thus avoiding exposure to excessive temperature which degrades the fluid resulting in increased corrosion to the system components and possible leakage failure. Pumping the heat transfer fluid to evacuate the plumbing system, avoids the disadvantages of a pipe installation which relies on gravity to drain the fluid from the collector and piping system.
The disclosure has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. For example, in order to be rapidly responsive to the user needs for hot water, a thermistor or a bimetal controlled electric heating element might be provided in the upper region of the water storage tank to supplement the solar heater. It is intended that the disclosure be construed as including all such modifications and alterations.
Claims
1. A solar water heating system, comprising:
- a solar collector;
- a reversible pump;
- a reservoir for storing heat transfer fluid;
- a heat exchanger for transferring heat from the heat transfer fluid to potable water;
- an insulated water storage tank storing said potable water;
- piping for connecting said pump, said collector, said heat transfer fluid reservoir, and said heat exchanger in fluid communication; and a
- controller for operating the pump to transmit fluid in a forward flow direction for heating the water and in a reverse flow direction to drain the fluid from the collector to the reservoir.
2. The solar water heating system of claim 1, further comprising a sensor connected to said collector for determining temperature of the fluid in said collector.
3. The solar water heating system of claim 2, further comprising a second sensor connected to said water storage tank for sensing temperature of the water in said storage tank.
4. The solar water heating system of claim 2, wherein said controller is connected to said first sensor.
5. The solar water heating system of claim 3, wherein said controller is connected to said second sensor.
6. The solar water heating system of claim 5, wherein when said controller detects a temperature differential, in the range of 9° F. to 18° F. between the temperature sensed by said first and second sensors, said controller activates said pump in said forward direction.
7. The solar water heating system of claim 6, wherein when said controller detects a temperature differential between said first and second sensors which is less than 9° F., said controller deactivates said pump.
8. The solar water heating system of claim 5, wherein said second sensor is operative to detect a temperature greater than a maximum reference water temperature and said first sensor is operative to detect a temperature greater than a maximum reference collector fluid temperature and said controller is operative in response to said detection by said first sensor or said second sensor to activate a pump flow reversal to evacuate the heat transfer fluid from the system to the reservoir.
9. The solar water heating system of claim 4, wherein said first sensor detects a minimum set level which activates a pump flow reversal which evacuates the heat transfer fluid from the system to the reservoir.
10. The solar water heating system of claim 1, further comprising a reserve power supply for energizing the system and wherein said controller is operative in response to a loss of externally supplied power to activate said reserve power supply and to operate the pump in a reverse direction to evacuate the heat transfer fluid from the system to the reservoir.
11. The solar water heating system of claim 1, wherein said heat transfer fluid storage reservoir comprises a baffle to control fluid drainage into said reservoir.
12. The solar water heating system of claim 1, further comprising a pressure-vacuum valve adjacent said heat transfer fluid storage reservoir.
Type: Application
Filed: Nov 1, 2010
Publication Date: Apr 19, 2012
Applicant:
Inventor: John Joseph Roetker (Louisville, KY)
Application Number: 12/917,051
International Classification: F24J 2/40 (20060101); F24J 2/04 (20060101);