Abstract:
A simple solar heating system incorporates a heat dissipater into a heat exchange circuit for bypassing solar collectors when either the temperature or the pressure in the heat exchange circuit exceeds preset limits. In the absence of electric controllers, fluid in the heat exchange circuit is caused to bypass the solar collectors using a valve which is controlled by either the temperature or pressure of the fluid. A solar photovoltaic panel energizes a circulating pump for increasing the rate of pumping as more solar energy is available at the PV panel and decreasing the rate as solar energy decreases.

Description:
FIELD OF THE INVENTION 
     Embodiments of the invention relate to solar heating systems and more particularly to systems for use with solar water heaters that are independent of the electrical utility grid and that substantially prevent overheating of fluids circulating therethrough. 
     BACKGROUND OF THE INVENTION 
     Collection of solar energy for use in heating fluids, such as water, is a well known concept with rudimentary systems originating in ancient times. Modern solar heating systems typically incorporate a solar collector that converts the sun&#39;s energy to thermal energy and utilize a variety of means to transfer the collected thermal energy into the fluid to be heated, such as for residential, commercial or industrial heating applications. 
     Solar water heaters may be combined systems or distributed systems. In the case of a combined system, a domestic water storage tank is typically mounted directly to the solar collector. Combined systems are generally not practical in colder climates as the hot water storage tank is cooled by the cold ambient air. In the case of a distributed system, the solar collector is typically located remote from the heated water storage tank, the storage tank being placed in a sheltered location to avoid heat loss to the atmosphere. Distributed solar water heaters are common. 
     “Direct” solar water heater systems circulate the domestic water to be heated through the solar collector. Direct systems are typically prone to scaling of the collector as a result of the domestic water passing therethrough. Further, direct systems require the collector to be drained when ambient temperatures fall below the freezing point of water (0° C.). Direct systems can be configured as either combined systems or distributed systems. 
     More sophisticated distributed systems known as “indirect” heating systems circulate a heat transfer fluid or working fluid between the solar collector and a potable water heat exchanger which transfers the solar heat from the working fluid into the potable water. The heat exchanger, such as a tubular coil, may be placed inside a potable water tank for transferring heat from the working fluid circulating through the coil to the water in the tank. Alternatively, the heat exchanger can be located external to the potable water tank, the potable water circulating on one side of the heat exchanger and the working fluid on the other side. Indirect systems typically use a working fluid that comprises agents to reduce scaling and an anti-freeze agent to avoid freezing of the working fluid. 
     Solar energy can only be harnessed when the sun is shining and some of the heat gained during the day is lost if the potable water or working fluid continues to circulate during nights or during periods of low solar potential. Consequently conventional solar systems require a means for stopping circulation of the working fluid during non-heating conditions. Some systems use a “drain-back” approach that drains the working fluid into a holding tank during the non-heating periods. Systems that don&#39;t “drain-back” require enough anti-freeze agent to ensure the working fluid does not freeze up and damage the piping or solar collector. 
     A significant issue with solar water heating systems is how to mitigate excessive heat. During periods when solar heating of the potable water exceeds the demand for heated potable water, heat will build up in the system. If means for releasing pressure are not provided, excess heat leads to boiling of the working fluid and the resultant pressure increases will rupture the piping or solar collector. Conventionally, overheating is addressed using a number of different mechanisms. “Heat dumps” dissipate excess heat to the atmosphere or through a ground loop or other location. Alternatively, the system is drained back and shut down or the system controller can be manually set to a “vacation” setting that diverts the heat from the potable water system. 
     Often systems are deliberately under-sized to avoid the overheating challenge. In this case, the solar collector system is sized such that its peak output will provide 90% of the minimum anticipated heat load. As the output of the solar collectors is seasonally dependent, this approach usually results in the solar water heating system contributing about half of the water heating requirement, the remainder being provided through conventional non-solar water heating systems and requiring a reliance on the electric utility grid or other external energy provider. Thus, it is clear in these cases that solar collection is not maximized. 
     Canadian Patent 1,080,566 to Cummings teaches a solar water heater incorporating a heat rejecting loop to attempt to cool the system. The system is complex and incorporates two separate fluid circuits; one comprising a heat absorbing loop fluidly connected to a heat rejecting loop and the second comprising a heat pickup loop thermally coupled to the solar panel to carry thermal energy away from the panel to the point of use. Circulation of fluid through at least the heat absorbing and heat rejecting loops is solely by gravity and thermal convective effects. 
     EP 04727915 to Torrens teaches a complex solar collection system in series with a hot water system. A heat dissipater circuit, which may comprise at least part of the panel framework, is used for cooling at least a portion of hot water exiting the solar panels when the water is overheated. The inlet to the heat dissipater is downstream from the solar panels and thus all of the fluid must first be heated and then at least a portion cooled for cooling the system. Torrens relies upon thermosiphon effects in the event of pump failure to ensure all of the water in the system is directed through the heat dissipater to prevent overheating. Applicant believes it is likely that there will be insufficient impetus for thermosiphon within the complex piping of Torrens, resulting in the possibility of overheating of the fluids therein despite the heat dissipation circuit. The Torrens system is particularly unsuitable for use where ambient temperatures fall below freezing as it is a direct system. 
     Apricus Solar Co. Ltd. (www.apricus.com/html/solar_heat_dissipator.htm) teaches a solar hot water system comprising a fin and tube heat dissipater connected downstream from solar collectors. The system as described utilizes an electrically powered controller and a solenoid valve operated by the controller, to direct overheated fluid from the solar collectors to the heat dissipater. Alternatively, it is mentioned that a thermostatic valve may be used. All of the fluid in the heat transfer circuit is first heated in the solar collector after which at least a portion of the fluid is directed to the heat dissipater for cooling after which the cooled fluid is mixed into the stream of overheated fluid. In cases of peak insolation, sufficient heat may not be released by the heat dissipater. Following heat dissipation, the temperature of the re-mixed working fluid may be inconsistent as the efficiency of the heat dissipater varies with atmospheric conditions. If excessive heat dissipation occurs the efficiency of the system is reduced. If insufficient heat dissipation occurs there remains a risk that the system will over-heat. 
     Current indirect-distributed systems typically utilize electronic control systems to activate pumps and valves to operate the system. The electronic controller utilizes preprogrammed logic to operate the valves and pumps as conditions determine when to circulate fluid to the solar collector, when to drain-back or load the working fluid, if applicable, when to circulate through an external heat exchanger and when to activate systems which handle excess heat, if available. The operating conditions are measured by electronic temperature and pressure sensors which are connected electrically to the electronic controller. Thus, these control and operating systems require electrical energy which is usually supplied from the electric utility grid. Loss of electrical energy will, at a minimum, cause loss of solar heating. It can also potentially cause damage to the system should the system overheat, result in injuries such as scalding and result in collateral damage to the building such as stained walls and floors caused by overflow of working fluid from ruptured lines and the like. 
     In order to deal with these problems, some systems provide a battery backup to enable the system and controller to operate for a period of time when the power goes out. In some cases, solar photovoltaic (PV) systems are available to supply the necessary electrical energy either directly to the solar heating system and controller or indirectly, such as through a battery pack. 
     In addition to requiring electrical energy to operate the solar heating system, electronic control methods are prone to component failure especially when considered in the context of the twenty-year life of a typical solar water heating system. Failure of the electronic control system can lead to piping or component damage and collateral damage similar to that which occurs with the loss of electrical energy. Battery systems also have a shorter life expectancy, usually in the five to ten year range. Failure to test and replace the battery system can lead to same type of damage seen with loss of electrical energy. 
     Ideally, what is required is a solar water heater system that is simple, efficient and requires no reliance on the electric utility grid or other external energy provider. The solar water heater system should be capable of meeting maximum demand during periods of low insolation without concern of overheating and the resulting potential damage to the systems and structures during periods of high insolation, and particularly during periods where there is also a low demand. 
     SUMMARY OF THE INVENTION 
     A self-controlled solar heating system and method of use is independent of the electrical utility grid or external energy provider and operates substantially without risk of overheating during periods of maximum insolation, despite being sized for maximum solar energy absorption. When the temperature or pressure of a fluid in the heat exchange circuit exceeds a preset operating maximum, some of the fluid is caused to automatically bypass the solar collectors to enter a heat dissipater. Fluid in the system is pumped at a rate relative to the amount of solar energy available using a solar powered pump. 
     In one broad aspect of the invention, apparatus for maximizing thermal energy collection in a solar collection system independent from the electric utility grid or external energy provider comprises: one or more solar collectors; a heat exchange circuit having fluid therein and being thermally connected between the one or more solar collectors and a point of use; a solar powered pump for substantially continuously pumping the fluid through the heat exchange circuit during solar energy collection; a heat dissipater fluidly connected to the heat exchange circuit and having an inlet upstream from the one or more solar collectors and an outlet downstream from the one or more solar collectors; and a valve positioned downstream from the heat dissipater which, when closed in response to a condition being at or below a maximum preset operating condition, prevents fluid from entering the heat dissipater; and when opened in response to the condition exceeding the maximum preset operating condition, permits at least a portion of the fluid in the heat exchange circuit to bypass the one or more solar collectors to flow through the heat dissipater for cooling the at least a portion of the fluid, the cooled fluid being returned to the heat exchange circuit thereafter through the outlet for maintaining the working fluid at or below the maximum preset operating condition. 
     In another broad aspect of the invention, a method for maximizing thermal energy collection in a solar collection system independent from the electric utility grid or external energy provider comprising one or more solar collectors, and a heat exchange circuit having fluid therein, the heat exchange circuit being thermally connected between the one or more solar collectors and a point of use, the method comprising: continuously pumping fluid through the heat exchange circuit and the one or more solar collectors during solar energy collection using a solar-powered pump to heat the fluid and when a condition of the heated fluid exceeds a maximum preset operating condition; bypassing at least a portion of the continuously pumped fluid around the one or more solar collectors through a heat dissipater for producing a cooled fluid; and recombining the cooled fluid with the heated fluid in the heat exchange circuit downstream from the solar collector for cooling the heated fluid for maintaining the working fluid at or below the maximum preset operating condition. 
     In embodiments of the invention, the valve which opens to flow fluid to the heat dissipater and bypass the solar collectors is actuated by either temperature or pressure. 
     In embodiments of the invention the system can be either an indirect system, wherein the working fluid flowing through the heat exchange circuit is a fluid such as glycol, or a direct system wherein the fluid which is circulated through the heat exchange circuit is the fluid to be used at the point of use, such as potable water. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1D  are schematics illustrating flow paths of some prior art, more complex, solar heater systems; 
         FIG. 2A  is a schematic illustrating a solar water heater system according to an embodiment of the invention; 
         FIG. 2B  is a schematic illustrating flow paths in the solar water heater system of  FIG. 2A ; 
         FIG. 3  is a schematic illustrating flow of working fluid through the solar water heater system of  FIG. 2A , during a normal heating cycle; 
         FIG. 4  is a schematic illustrating flow of working fluid through the solar water heater system of  FIG. 2A , at a maximum operating condition of the working fluid in the system; and 
         FIG. 5  is a graphical representation of working fluid temperature in degrees Centigrade, BTU&#39;s generated and BTU&#39;s dissipated using an embodiment of the invention and monitored through a mid-day period where solar energy is at a maximum. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the invention comprise self-controlling solar heating systems having one or more solar collectors, an excess heat dissipater, and an elegant yet simple heat transfer circuit fluidly connected between the solar collectors and a point of use. A variable speed direct current (DC) pump circulates fluid through the heat transfer circuit, powered by a photovoltaic panel so as to remove any reliance upon an external energy provider. The pumping rate of the fluid is self-controlling and relative to the amount of solar energy available. 
     The heat dissipater is fluidly connected around the solar collectors. Without a need for a sophisticated controller, upon overheating, a valve directs at least a portion of the fluid to flow through the heat dissipater for bypassing the solar collectors and cooling the fluid. The valve opens when the fluid exceeds a preset threshold condition in the fluid which is indicative of overheating. The valve is positioned in the heat transfer circuit downstream from both the solar collectors and the heat dissipater. As the valve opens, at least a portion of the fluid bypasses the solar collector, fluid flowing both through the heat dissipater and the solar collectors. Cooled fluid from the heat dissipater mixes with heated fluid exiting the solar collectors for maintaining the working fluid below the preset threshold operating condition, being either temperature or pressure. 
     As atmospheric conditions vary, the valve modulates between open and closed so as to alter the rate of fluid flowing through the heat dissipater. Thus the system automatically and efficiently maintains the working fluid at as close to the maximum operating condition without exceeding the preset threshold. 
     In order to understand the simplicity and the reliability of the overheating protection accomplished without reliance upon the electric utility grid or external energy provider and according to embodiments of the invention, it is first necessary to understand the flow paths of some complex prior art systems which also employ heat dissipaters. 
     PRIOR ART SYSTEMS 
     As shown in simplified schematic  FIG. 1A , Canadian Patent 1,080,566 to Cummings teaches two separate fluid circuits. A first circuit  10  within a solar panel  12  comprises both a heat absorbing loop  14  and a heat dissipating loop  16 . A second circuit or heat-pickup loop  18  picks up heat within the panel  12  and circulates fluid therein outside the solar panel  12  to a point of use  20 , typically a heat exchanger in a water tank. A thermally actuated valve  22  is positioned between the heat-absorbing loop  14  and the heat-dissipating loop  16 . When the temperature in the heat-absorbing loop exceeds a preset threshold, the valve  22  opens for directing the fluid to the heat-dissipating loop  16 . The system relies solely upon differential buoyancy and thermosiphon effects to circulate fluid through at least the first circuit  10 . Applicant believes that the heat-dissipating loop  16  acts to cool the solar panel  12 , but is less effective in cooling the working fluid. Further Applicant believes that there are potential efficiency losses if the heat-pickup loop  18  loses heat to the first circuit  10 . 
     Simplified schematics,  FIGS. 1B and 1C , illustrate two embodiments taught by EP04727915 to Torrens. Torrens utilizes a heat exchange circuit  30  to circulate water through a series of solar panels  12 . In a first embodiment ( FIG. 1B ) a thermostatic T-valve  32  is positioned downstream from the solar panels  12  and upstream from a heat dissipater  34 . In this embodiment, all of the fluid in the heat transfer circuit must pass through the solar panels  12  and be heated therein. Thereafter, a portion of the flow is directed to the heat dissipater  34  for cooling therein. Torrens employs a check valve  36  between the heat dissipater  34  and the heat exchange circuit  30 . Torrens states that the check valve  36  is designed to ensure fluids exiting the heat dissipater  34  return to the heat exchanger circuit  30  upstream from a point of use  38  for cooling the fluid therein, when a pump  40  which circulates fluid in the heat exchange circuit  30  is functioning. Applicant believes it likely that there is more resistance to flow through the heating load and therefore, unless a very high resistance check valve is used, fluids are likely to bypass the heating load through the check valve. In this case there is reduced flow of fluids though the heating load reducing the energy available to the working load and ultimately reducing the efficiency of the overall system. 
     Torrens also illustrates that in the event of a pump failure or loss of electricity, hot fluid exiting the solar panel is directed by thermosiphon through the check valve  36  and back to the series of solar panels  12 , bypassing the point of use  38 . If a high resistance check valve  36  is used to prevent bypass of the heating load in regular use, it is more unlikely that there would be significant impetus for fluids to flow through the check valve  36  by thermosiphon in the event that the pump  40  or the electricity fails. 
     In an alternate embodiment ( FIG. 1C ), where the heat dissipater  34  is part of the solar panel framework, Torrens does not use a check valve  36  but instead separates a first solar collector  12 ′ from the remaining solar collectors  12  using a thermostatic valve  32  and in the event of a need to cool water exiting the remaining solar collectors  12 , passes water through the heat dissipater  34  for recycling through the remaining solar collectors  12 . Applicant assumes that in the event of a pump failure, maintaining the first solar panel  12 ′ cooler than the remaining solar panels  12  permits some thermosiphon effect, however it is unclear if the system would operate as described. 
     As illustrated in  FIG. 1D , Apricus Solar Co. Ltd., teaches a system having solar panels  12  and a heat exchange circuit  50 , incorporating a fin and tube heat dissipater  52  connected downstream from the solar collectors  12 . An electrically powered controller and a solenoid valve, operated by the controller, direct overheated fluid from the solar collectors  12  to the heat dissipater  52 . Alternatively, it is mentioned that a thermostatic valve  54  may be used. All of the fluid in the heat transfer circuit  50  is first heated in the solar collectors  12  after which at least a portion of the fluid is directed to the heat dissipater  52  for cooling after which the cooled fluid is mixed into overheated fluid in the heat exchange circuit  50  for cooling the fluid therein. 
     EMBODIMENTS OF THE INVENTION 
     Embodiments of the invention are described herein in the context of a domestic hot water heater. As those of skill in the art will appreciate however, the system as described can be used to collect solar energy for a variety of purposes and therefore the system is not limited solely for use as a water heater for domestic hot water use. Other uses may include pool heating, space heating via hydronics, forced-air, radiant or other fluid-based space heating processes, process water or fluid heating for industrial, refining, processing, smelting or commercial operations; heating of cleaning water for commercial laundries, car washes or similar uses; or any other heating or pre-heating application, either directly or indirectly from the solar heated fluid. 
     In an embodiment of the invention, as shown in  FIGS. 2A ,  2 B,  3  and  4 , the solar water heater system  110  comprises one or more solar collectors  112 , such as evacuated-tube heat-pipe collectors, flat plate solar collectors or any other type of solar thermal collector or panel, for absorbing solar energy from the sun. 
     A heat transfer or exchange circuit  114  is connected for circulating a working fluid between the solar collectors  112  and a point of use  115 . The solar water heater system  110  can be either an indirect system or a direct system. 
     Embodiments of the solar heater system  110 , whether direct or indirect, further comprise a heat dissipater  120  which is connected to the heat exchange circuit  114 . The heat dissipater  120  forms a bypass around the solar collectors  112 , connected at an inlet  121  upstream of the solar collectors  112  and connected at an outlet  122  downstream thereof. The heat dissipater permits at least a portion of the circulating fluid in the heat exchange circuit  114  to bypass the solar collectors  112  when a condition of the fluid related to overheating exceeds a preset threshold. The flow of fluid to the heat dissipater is modulated to maintain the condition of the fluid at or below the maximum preset condition. 
     In embodiments of the invention, the condition of the fluid is generally either the temperature of the fluid or the pressure of the fluid. 
     In embodiments of the invention, the solar collectors  112  are sized to absorb the maximum solar energy available and the heat dissipater  120  is sized accordingly. 
     The system  110  is further described herein in the context of an indirect system using temperature as the condition indicative of overheating. Those of skill in the art would appreciate however that the description is equally applicable in the case of a direct system or where pressure is the condition of the fluid indicative of overheating. 
     Accordingly, using temperature as the condition, a thermally-actuated control valve  124  is positioned in the heat exchange circuit  114  downstream from the solar collectors  112 . In one embodiment, the valve  124  is located at the conjunction of the heat dissipater  120  and the solar collectors  112 . As shown, the valve  124  can be a three-way valve, receiving fluid from the solar collector or both the heat dissipater and the solar collectors for discharge back to the heat exchange circuit  114 . 
     If the temperature of the working fluid exiting the solar collectors  112  reaches a preset threshold, the valve  124  opens to fluidly connect with the heat dissipater  120  to permit at least a portion of the circulating working fluid to exit the heat dissipater  120 . Accordingly, at least a portion of the fluid which would otherwise enter the solar collectors  112  instead enters the heat dissipater  120  for producing a cooled fluid. The cooled fluid is thereafter mixed with the heated fluid exiting the solar collectors  112 . When the temperature of the working fluid is below the preset threshold or maximum preset operating temperature, such as when sufficient heat has been utilized at the point of use  115  or has been dissipated from the working fluid, the valve  124  closes to again direct the entirety of the flow of working fluid through the solar collectors  112 . The valve  124  modulates between open and closed. As previously noted, changes in atmospheric conditions can alter heat dissipation from the heat dissipater  120 . As the temperature of the heated working fluid changes in response to changes in heat dissipation, the flow of fluid entering the heat dissipater  120  is automatically changed or modulated by the thermally-actuated control valve  124  so as to maintain the temperature of the working fluid at the maximum preset operating temperature. 
     Thus, as shown in  FIG. 3 , in normal operation, when the temperature of the working fluid exiting the solar collectors is below the maximum preset operating temperature, the thermally-actuated control valve  124  remains closed and none of the working fluid is circulated through the heat dissipater. The temperature of the working fluid is substantially continuously monitored by the thermally actuated control valve  124 . 
     As shown in  FIG. 4 , when the temperature in the working fluid approaches the preset maximum operating temperature, the valve  124  begins to open, permitting at least a portion of the fluid to pass through the heat dissipater  120  to be cooled. The cooler fluid exits the heat dissipater  120  and mixes or recombines with the heated working fluid exiting the solar collectors  112  to maintain the temperature of the working fluid in the heat exchange circuit  114  at the preset maximum operating temperature. In order to permit maximum solar energy absorption by the solar collectors  120  without risk of overheating, the valve  124  is capable of opening fully to split the flow of working fluid between the solar collectors  112  and the heat dissipater  120 . With the valve  124  fully open, the heat gain by the solar collectors  112  is balanced with the heat loss from the heat dissipater for maintaining the temperature of the working fluid at about or below the preset maximum operating temperature. 
     In embodiments of the invention, the heat dissipater  120  comprises a fin and tube radiator for exchanging heat from the working fluid to the atmosphere. Alternatively, the heat dissipater  120  could comprise a flat plate radiator, a ground loop or other type of heat sink to absorb the excess heat from the working fluid. 
     With reference again to  FIG. 4 , during periods of high solar energy or insolation, such as midday and when the working fluid temperature approaches the preset maximum temperature, in this case 90° C., the thermally-actuated control valve  124  opens to bypass a slipstream of fluid through the heat dissipater  120  and remove heat from the working fluid.  FIG. 5  represents a test of solar collectors, a fin and tube radiator for dissipating heat and a thermally-actuated control valve. There was no heating load on the system. 
     The heat exchange circuit  114  further comprises a variable speed DC pump  130  for pumping fluids through the heat exchange circuit  114 . In order to remove any reliance upon the electric utility grid, the pump  130  is powered by a photovoltaic (PV) array or panel  132 . The PV panel  132  may be integrated with the solar collectors  112  or may be remote from the solar collectors  112 . Use of the PV panel  132  to create solar electricity causes the variable speed DC pump  130  to circulate the working fluid proportionately to the solar conditions. In other words, when solar energy or insolation is at a maximum, the pump circulates fluid more quickly through the heat exchange circuit  114  and, when solar energy is very low or not available, such as at night or in other low light conditions, the pumping slows or stops, effectively shutting down the system  110 . 
     The heat exchange circuit  114  further comprises an expansion tank  140  to accommodate increases in volume of the working fluid with increased temperature. A pressure relief valve (PRV)  142  is incorporated for releasing working fluid from the heat exchange circuit  114  in the event of a failure of the thermally-actuated control valve  124 , the pump  130  or a vapor lock within the system  110 . Should the pressure in the heat exchange circuit  114  exceed a preset pressure, generally as a result of expansion beyond the capacity of the circuit  114 , including the expansion tank  140 , the PRV  142  opens and the working fluid is discharged. Such conditions can occur if the working fluid begins to vaporize. 
     To avoid collateral damage, the environmentally-friendly working fluid is discharged such as onto the roof or through conduits (not shown) which direct the working fluid away from structures which may be damaged thereby. 
     As shown in  FIG. 2A , and in embodiments where the system is an indirect system, the heat transfer or exchange circuit  114  is connected between the solar collectors  112  and a heat exchanger  116  which is typically at the point of use  115 . A working fluid, such as glycol, circulates through the exchange circuit  114 . In the case of a water heater, the heat exchanger  116  is typically internal to a hot water tank  118  for exchanging heat between the working fluid and potable water contained in the hot water tank  118 . Alternatively, the heat exchanger  116  can be external to the hot water tank  118 . When intended for use in climates where ambient temperatures are low, such as in northern climates where temperatures may be below freezing, the working fluid comprises suitable amounts of antifreeze. 
     In embodiments where the system is a direct system, the heat transfer circuit  114  is fluidly connected to the hot water tank  118  and potable water from the tank is the working fluid being circulated through the heat transfer circuit  114 . Direct systems may be limited for use in climates where the temperature remains above freezing as antifreeze cannot be mixed with the potable water flowing therethrough. 
     Example 
     In an indirect solar water heater system, according to an embodiment of the invention and as shown schematically in  FIG. 2A , a solar collector  112  comprising 30, 58 mm×1800 mm evacuated tubes, rated at a maximum thermal output of about 7400 BTU per hour, available from Jiangsu Sunrain Co. Ltd., was mounted to the roof of a structure. The collector assembly was mounted at an angle of about 70 degrees from horizontal to ensure solar gain was maximized during the winter and minimized during the summer as is known in the art. 
     The solar collector  112  was thermally and fluidly connected to a 300 L (80 USG) hot water tank  118  located within the structure, using ¾″ cross-linked polyethylene (PEX) pipe and fittings, for forming the heat exchange circuit  114 , through which a working fluid was circulated. All piping in the system was insulated to reduce energy losses. 
     The working fluid for circulation through the heat exchange circuit comprised distilled water mixed at about 50% with non-toxic propylene glycol to ensure the fluid would not freeze at −40° C. temperatures. An expansion tank  140 , having a volume sufficient to contain about 2.5% of the volume of the fluid at 20° C., was fluidly connected to the heat exchange circuit  114 . The pressure within the heat exchange circuit  114  was maintained at a lower pressure than that in the hot water tank  118  to avoid glycol from entering the domestic hot water system in the event of a leak in the internal heat exchanger  116 . 
     Fill and drain valves  150  were incorporated into the heat exchange circuit  114  to facilitate loading the circuit  114  and to permit periodically checking the pH and strength of the glycol/water mixture. 
     A pressure relief valve (PRV)  142  having a preset threshold of about 50 psi was connected to the heat exchange circuit  114 . Should the pressure within the circuit  114  exceed the preset threshold, for example as a result of a failure in the system, the PRV  142  would open and the working fluid would be released to the roof of the structure. 
     An 8 foot length of fin and tube radiator  120 , sized to exceed the maximum BTU rating of solar collector  112  by about 5% or about 40 BTU per hour, was connected to the heat exchange circuit  114  to bypass the solar collector  112 . An inlet  121  to the radiator  120  was upstream from the solar collector  112  and an outlet  122  from the radiator  120  was downstream from the solar collector  112 . 
     A bimetallic thermal by-pass valve  124 , available from Caleffi Hydronics Solutions (Part #309460) was connected to the heat exchange circuit  114  downstream from the solar collector  112  and the heat dissipater  120 . The preset maximum operating temperature of the valve was 90° C. (200° F.) for diverting flow from the heat exchange circuit  114  to the inlet  121  of the heat dissipater  120  and bypassing the solar collector  112  if the temperature in the working fluid exceeded 90° C. 
     A variable DC Pump  130 , such as a 12 VDC, 20 watt, 8 liter/min pump, such as an El CID pump available from Ivan Labs Inc. or an Ecocirc pump available from Laing Thermotech, Inc., was used to pump the working fluid through the heat exchange circuit  114 . The pump  130  was powered by a 25 watt photovoltaic panel available from Fuzhou Pingchi Import &amp; Trading Co of China. 
     An anti-scald valve  160 , such as a Danfoss ESSBE 065B8870 valve available from Danfoss Hydronic Heating North America, was placed on an outlet of the hot water tank  118  to act as a safety device for ensuring water exiting the tank  118  would not exceed a safe temperature, in this case from about 50° C. (122° F.) to about 60° C. (140° F.) where higher temperatures are required for appliances such as dishwashers and clothes washer.