Tank flow center for closed loop geothermal system

A closed loop geothermal system has a fluid circulation loop that includes in series: a heat pump, a ground loop heat exchanger, a tank flow center and a circulation pump. The tank flow center includes a tank, a first valve, a second valve and a tank bypass line all mounted in a common housing. The tank may include a refill inlet that opens through a top surface of the housing. A cap may be attached to close the refill inlet. The tank flow center has the regular geothermal operation configuration, a system flush configuration, and a refill configuration. The tank flow center may be elevated above the circulation pump a distance to produce a water column that is greater than a minimum cavitation avoidance head pressure for the circulation pump.

TECHNICAL FIELD

The present disclosure relates generally to closed loop geothermal heating and cooling systems, and more particularly to a tank flow center that permits easy servicing of the system and inhibits air from traveling from the ground loop heat exchanger to the circulation pump.

BACKGROUND

Closed loop geothermal systems typically consist of a heat pump (heating and/or cooling unit), a ground loop heat exchanger (which can be in many different configurations), and one or more pumps used to circulate the heat exchange fluid (which is usually water mixed with antifreeze). During initial system installation, the ground loop must be filled with liquid and purged of all air and debris. This is typically accomplished with an external pumping system known in the industry as a flush cart. The flush cart often consists of a one to two horsepower pump, valves for directing/controlling flow, a large supply container for supplying the flushing liquid, and hoses/fittings for coupling to the geothermal system. Many geothermal system manufacturers provide a packaged unit that consists of the necessary system circulating pump(s) along with valves and exposed ports for connection to a flush cart. The package units are often known in the industry as flow centers, flow controllers, loop pump modules, or pump packs. The valves are often three way, four position valves in the package center that enable the ground loop system to be isolated from the circulating pump(s) and the heat pump during initial system start up and any subsequent maintenance such as replacing a circulation pump. The valves and the exposed ports also allow a flush cart to be coupled to the geothermal system for filling, flushing and purging. Geothermal systems are often categorized as either being pressurized or unpressurized.

During initial system start up of traditional pressurized systems (after the filling, flushing and purging is complete) the return valve on the flush cart is closed, dead heading the pump against the closed valve. This drives up the pressure in the system as the pump draws a small amount of liquid from the flush cart reservoir and forces it into the system. The ground loop often consists of plastic pipe that flexes slightly allowing this additional liquid to enter the system. The flush cart operator watches the flush cart fluid supply container for a significant liquid level drop. If the water level in the supply tank drops significantly (maybe several inches or more) it may indicate that air is still in the system and is being compressed, and further flushing/purging may be necessary. Once the operator is confident that all air has been purged from the geothermal system, the flush cart is once again dead headed and the valves are turned to the operating position capturing the pressure in the system. This initial system pressure, which is provided by the flush cart pump, may be generally on the order of 40-60 psig. This trapped pressure provides the suction pressure required by the circulating pumps, and also allows for some pressure loss due to the relaxing of the plastic piping over time. If all air has been properly flushed from the system, and there are no leaks anywhere in the system, these pressurized geothermal systems will function for many years without any maintenance required.

This method of installation has been the dominate strategy since the geothermal industry was originally founded. However, if any air remains in the system and/or there is a small leak anywhere in the system, the system pressure will drop and the volume of trapped air will grow, due at least in part to the decrease in pressure. This build up of air can cause two modes of failure for the circulating pump(s). First, the air can migrate to the pump inlet and cause the pumps to become air locked. Since the loop fluid cools the circulating pump, an air locked pump will eventually overheat and fail. Second, if there is inadequate positive pressure on the suction side of the pump, cavitation can result and the pump will eventually fail.

Unpressurized systems have been gaining wider industry acceptance over the past several years. Unpressurized systems generally allow the installation to be less precise, thereby allowing a larger group of lesser skilled installers to offer closed loop geothermal heating and cooling systems to their customers. Since there is little to no system pressure relative to ambient pressure, piping connections do not necessarily have to be as leak proof as connections in their counterpart pressurized geothermal systems. Unpressurized systems typically allow for some air separation and often provide a strategy for make up water, rendering the necessity to purge air from the system not as critical as in pressurized installations. For instance, if there is a leak in a nonpressurized installation, the system owner (usually the homeowner) can easily add loop liquid to the system without the use of specialized tools or equipment. Nevertheless, although unpressurized systems are generally considered more forgiving than their pressurized counterparts, the same types of system failures can also occur. For instance, if too much air finds its way to the pump inlet, the pump can still become air locked and eventually overheat and fail. In addition, if there is inadequate fluid pressure at the suction side of the pump, cavitation can still result and the pump will eventually fail.

The present disclosure is directed toward avoiding pump failures in geothermal systems in general, and especially unpressurized geothermal systems specifically.

SUMMARY OF THE DISCLOSURE

A closed loop geothermal system has a fluid circulation loop that includes in series: a heat pump, a ground loop heat exchanger, a tank flow center and a circulation pump. The tank flow center includes a tank, a first valve, a second valve and a tank bypass line mounted in a housing. The tank has a refill inlet that opens through a top surface of the housing. The tank flow center has a regular geothermal operation configuration in which a cap is attached to close the refill inlet, and a refill configuration in which the cap is detached from the refill inlet of the tank. The tank flow center also has a system flush configuration.

In another aspect, a method of installing a closed loop geothermal system includes mounting a tank flow center at an elevation above a circulation pump a distance to produce a water column that is greater than a minimum cavitation avoidance head pressure for the circulation pump. A heat pump, a ground loop heat exchanger, the tank flow center and a circulation pump are fluidly connected in series. The ground loop heat exchanger is flushed while the tank flow center is in a system flush configuration in which a tank of the tank flow center is bypassed via a bypass line. A refill inlet through a top of the housing of the tank flow center may be opened to partially fill the tank with liquid, and then the refill inlet is closed. The tank flow center is then changed from a system flush configuration to a regular geothermal operation configuration after the flushing and closing steps.

In still another aspect, a tank flow center includes a tank mounted in a housing that includes a refill inlet that opens through a top side of the housing. A removable cap is covering the refill inlet. A first valve is mounted to the housing and has an inlet port that opens outside the housing for fluid connection to a ground loop heat exchanger, a tank port fluidly connected to the tank, and a bypass port fluidly connected to a bypass line. A second valve is mounted to the housing and has an outlet port that opens outside the housing for fluid connection to a circulation pump, a tank port fluidly connected to the tank, and a bypass port fluidly connected to the bypass line.

DETAILED DESCRIPTION

Referring toFIGS. 1-4, a closed loop geothermal system10has a fluid circulation loop12that includes in series: a heat pump13, a ground loop heat exchanger14, a tank flow center16and a circulation pump18. The tank flow center16includes a tank20, a first valve30, and second valve40and a tank bypass line50all mounted in a common housing60. Preferably, all of the various components of tank flow center16are made from noncorrosive materials, such as plastic and the like. Tank20may include a refill inlet21that opens through a top surface61of housing60. The tank flow center16may have a regular geothermal operation configuration in which a cap22is attached to close refill inlet21, and valves30and40are positioned to route the circulating liquid through tank20while avoiding bypass line50. Tank flow center16may also have a system flush configuration in which valves30and40are positioned to route liquid through bypass line50while avoiding tank20. Finally, tank flow center16may have a refill configuration in which cap22is detached from refill inlet21to allow a user to add make up liquid into tank20to a desired level. When closed loop geothermal system10is installed in an unpressurized configuration, tank flow center16may be elevated above circulation pump18a distance19to produce a water column that is greater than a minimum cavitation avoidance head pressure for circulation pump18. For instance, tank flow center16may be mounted on a vertical wall surface17above circulation pump18.

Although not necessary, cap22can be a commercially available cap that includes an internal pressure relief valve24. For instance, the pressure relief valve may be set to open when pressure within system10exceeds some predetermined pressure such as 13-15 psig. In addition, cap22may also be equipped to include a vacuum relief valve to protect tank20from collapse in the event that pressure within system10drops below ambient pressure. In the illustrated embodiment, cap22includes a pressure relief valve24operable to open the tank when an absolute pressure differential on opposite sides of the cap exceed a predetermined pressure, whether that pressure differential be a vacuum or overpressurization. It is this feature of the invention that may allow tank flow center16to be mounted at a lower distance above circulation pump18providing a lower water column pressure on the inlet of the circulation pump, but compensating by pressurizing the system to a level below which the pressure relief valve24will open.

In one embodiment, tank20may be formed from an HDPE translucent plastic and housing60may be shaped from opaque plastic to include a window62so that one may check the liquid level75in tank20without detaching cap22or otherwise altering system10. Thus, a homeowner may quickly confirm that the liquid level75in tank20is at a proper level with merely a glance at tank flow center16, and while geothermal system10is in operation. In an unpressurized configuration, liquid can be added to tank20while geothermal system10is in operation by detaching cap20and adding liquid to tank20up to a desired level indicated through window62. In one embodiment, tank20may have a capacity on the order of 2.5 gallons and window62may be sized in a range to show a preferred liquid level in tank20on the order of 2 gallons plus or minus half a gallon. Those skilled in the art will appreciate that the size and markings associated with window62may indicate to a homeowner that adequate liquid is in system10whenever the liquid level75appears in window62.

In order to inhibit air from escaping from tank20toward circulation pump18, tank20includes internal flow directing geometry23. Internal flow directing geometry23is shaped, sized and configured to inhibit air entering tank20from the ground loop heat exchanger14from escaping from the tank20to the circulation pump18. In the illustrated embodiment, this internal flow directing geometry23includes an internal flow directing tube27that connects on one end80to a tank port32of first valve30, and opens at its other end81into one side28of a baffle25that divides the lower portion of tank20as shown inFIG. 4. Since outgoing liquid from tank20leaves from the opposite side29of baffle25into tank port42of second valve40, air bubbles that arrive into tank20via flow directing tube27tend to percolate toward the top of the tank rather than being drawn into valve40. In addition, any debris swept up through the ground loop heat exchanger14may settle on floor26of tank20after arriving through flow directing tube27. Although the internal flow directing geometry23of the illustrated embodiment includes the flow directing tube27and a baffle25in tank20, those skilled in the art will appreciate that many other geometries are available and known to those of ordinary skill in the art that could be properly characterized as being shaped, sized and configured to inhibit air from escaping tank20toward circulation pump18. Thus, when valves30and40are positioned such that tank flow center16is in its regular geothermal operation configuration, liquid enters tank flow center through inlet55into inlet port31of valve30and out of tank port32into flow directing tube27. At the same time, liquid is drawn from tank20from side29into tank port42of valve40toward outlet port41and eventually out of tank flow center16through outlet56toward circulation pump18.

Access to change the configuration of first valve30and second valve40may be gained through the front side64of housing60by detaching a first removable cover37and a second removable cover47to reveal tool engagement surfaces33and44respectively. For instance, tool engagement surfaces34and44may have a hexagonal shape for receiving an allen wrench to rotate the internal valve member between different configurations to open different ports and provide different flow configurations through tank center16. For instance, when the tank flow center16is put into a system flush configuration, valve30would be rotated to close tank port32and open inlet port31directly to bypass port33. In addition, valve40would be rotated to close tank port42and open bypass port43directly to outlet port41. As such, liquid entering inlet55would flow directly through valve30, through bypass line50, through valve40and out of outlet56.

In one system configuration, a pump flow center90may include valves91and93with exposed ports92and94for fluid connection to an appropriate flush cart as described earlier. In some configurations, pump flow center90may also include a second circulation pump96that receives liquid from heat pump13and pumps the liquid through the ground loop heat exchanger14. Nevertheless, those skilled in the art will appreciate that two circulation pumps may not be necessary in many installation applications. Those skilled in the art will appreciate that tank20is preferably bypassed during the flushing operation in order to avoid overpressurizing tank20or channeling too much debris and the like into tank20during a system flush configuration.

In addition to those features previously described, tank flow center16may also include wall mounting flanges65that include fastener bores66to easily mount tank flow center16on a vertical wall surface17, as shown inFIG. 3. In addition, housing60may be equipped with one or more spray foam holes67that allow foam insulation70to be injected or sprayed into housing60between internal surface63and tank20after tank flow center16has been assembled. After the foam insulation70has been added, spray foam hole(s) may be closed with a suitable cover or the like. This aspect of the tank flow center16may permit steady and problem free operation in colder climates and avoid condensation issues inside of housing60.

INDUSTRIAL APPLICABILITY

The tank flow center16of the present disclosure finds general applicability in closed loop geothermal systems. The tank flow center16finds specific application in unpressurized systems, but can also find potential application in systems needing some pressurization in order to have adequate head pressure at the inlet to the circulation pump18. Nevertheless, those skilled in the art will appreciate that the valve opening pressure for the pressure relief valve24in cap22could be selected so that tank flow center16could be utilized in current pressurized closed loop geothermal systems with internal pressures exceeding 40 psig, for example, without departing from the scope of the present disclosure.

At time of installation, an installer might consult the manufacturers suggestion for the desired head pressure at the inlet to the circulation pump18. Using appropriate calculations, that pressure can be converted into a desired mounting distance19at which tank flow center16should be above circulation pump18to provide the desired minimum pressure to avoid cavitation. The tank flow center then can be mounted using the included wall mounting flanges65with appropriate fasteners such as wood screws or the like. Next, the various fluid connections for the fluid circulation loop could be facilitated by fluidly connecting in series the heat pump13, the ground loop heat exchanger14, the tank flow center16and circulation pump18. Next, the tank flow center can be placed in its flush configuration to bypass tank20so that all of the liquid entering tank flow center travels through bypass line50. Next, the flush cart would be fluidly connected to ports92and94of the pump flow center90. The ground loop heat exchanger14would then be flushed by turning on the flush cart, with none of the flush liquid entering the tank20during flushing. In the illustrated embodiment, dead heading the flush cart into the tank20would be undesirable, as overpressurization might damage tank20. After flushing is complete, the flush cart is turned off and the valves91and93are returned to their normal operation configuration. In addition, valves30and40of tank flow center are returned to their normal operation configuration so that liquid flows through tank20and not through bypass line50. Next, the cap22may be detached from refill inlet21and the tank20filled to a desired level with clean debris free heat exchange liquid. For instance, tank20may be filled to an extent that places the liquid level75in the middle of window62. Next, the circulation pump18can be started and operated for several minutes. Additional loop fluid can be added to tank20if necessary. The system is then ready for operation and the cap22is returned to cover refill inlet21and the replaceable valve covers37and47may be returned to cover the tool engagement surfaces34and44of valves30and40, respectively.

The tank flow center16of the present disclosure provides numerous additional functions and benefits over current systems. Among these, are providing the necessary suction head pressure at the circulation pump due to appropriate mounting of the tank flow center16above the circulation pump18. This insures a flooded pump housing volute and avoids the pump experiencing low pressures that could result in cavitation. In addition, the internal flow directing geometry23allows air to be separated from liquid, prevents air from being drawn into pump18or ground loop heat exchanger14. This is accomplished in the illustrated embodiment by using a liquid path that returns the air/liquid mix on one side28of an internal baffle25while outgoing liquid leaves from the opposite side29of baffle25. In addition, heavy debris such as sand that enters tank20may settle on the floor26of the tank rather than being directed toward pump18, potentially damaging the same. In addition, tank20essentially functions as an expansion tank allowing the fluid level to rise and fall in the system10due to seasonal temperature fluctuations and the like. The valves30and40of tank flow center16also allow the ground loop heat exchanger14to be flushed with an external flush cart while bypassing tank20. The translucent construction of tank20along with the window62and housing60allows the liquid level in the tank to be monitored at all times without opening the system. In addition, the refill inlet21and the cap22allow liquid to be manually added to system10when it falls below a predetermined level, without the need of special tools or skills. Cap22also seals system10from the outside environment. This may prevent air from continuously mixing with the liquid and prevents evaporation of the liquid, which can include antifreeze such as ethanol and methanol. In addition, closure of the cap can limit the presence of oxygen and therefore help avoid bacteria growth with another common antifreeze, namely propylene glycol. In addition, almost all antifreeze solutions are known to cause corrosion problems in the presence of oxygen if they are mixed with a mineralized or contaminated water source. The cap22may include a pressure relief valve29that allows pressure and vacuum relief to protect the tank from excessive expansion and/or collapse. Nevertheless, the system can still be pressurized up to the valve opening pressure of the pressure relief valve24. The mounting flanges65also allow the tank flow center16to be easily wall mounted. In one version, the cap22and refill inlet opening are sized to be small to prevent a hand from being inserted into tank20. Foam insulation70between the interior surface63of housing60and tank20can also inhibit condensation within the housing and allow for colder weather operation. By making the components of tank flow center all plastic or composite construction, corrosion can also be avoided. In one specific embodiment, no mechanical fittings (threads) are used, and the internal transitions are fused and sealed with o-rings to preempt potential problems associated with threaded connections. Because the tank flow center16is modular, it can be replaced without replacing the pump flow center90, and the tank flow center16is retrofitable into current systems. Thus, by appropriate mounting height of tank flow center16, a previously pressurized system that has leak problems, may be remedied, and converted into a non-pressurized system according to the present disclosure. The easy access to the liquid in the tank through refill inlet21also allows the loop fluid to be tested, such as for antifreeze concentration. Finally, by appropriately shaping the connection ports on tank20as well as housing60it can be constructed to accommodate both left and right hand version installations.

LIST OF ELEMENTS

Title: Tank Flow Center for Closed Loop Geothermal System