Abstract:
A method for automatically initiating the flow of fluid in a pipe that may be blocked by frozen fluid in a portion of the pipe includes urging the flow of fluid through the pipe and energizing a heater disposed in and extending the pipe portion if no flow is detected in the pipe.

Description:
FIELD OF THE DISCLOSURE 
     The disclosure relates to a method for automatically initiating the flow of fluid through a pipe or the like in which a portion of the pipe may be exposed to freezing temperatures such that fluid flow through the pipe may potentially be blocked by frozen fluid in the pipe, and particularly, to a method for initiating the flow of water through a solar hot water heating system. 
     BACKGROUND OF THE DISCLOSURE 
     Fluid systems that urge a fluid to flow from one location to another are well known. The fluid in some fluid system applications may flow through an area exposed to freezing temperatures that may cause the fluid to freeze and block fluid flow. For example, solar water heating systems utilize a pump that urges water to flow through supply pipes extending between a solar heating assembly that heats the water by solar energy and a hot water tank that stores the heated water. The supply and return pipes, however, may be exposed to outdoor temperatures below freezing that cause the water in the supply pipes to freeze and prevent operation of the heating system. 
     Some users of solar water heating systems manually monitor the system for water freezing in the supply pipes and turn on heaters wrapped around the pipes as needed to melt the ice blocking the pipes, and then turn off the heaters well after the blockage is cleared. Requiring manual intervention and supervision of the hot water heating system is often impractical, particularly in residential applications where the homeowner may be away. Energy is wasted if the heaters are kept on longer than necessary. 
     Some conventional solar water heating systems avoid the problem of water freezing in the supply lines by draining the water from the pipes when freezing may occur. This increases system complexity and prevents use of the system during cold months. Other conventional solar heating systems replace water with another fluid, such as ethylene glycol, that does not freeze when exposed to outdoor temperatures. This solution is less energy efficient and is considerably more complicated and expensive than solar heating systems that use water as a heat transfer medium. 
     Accordingly, there is a need for an improved method for automatically initiating the flow of fluid through a pipe or the like in which a portion of the pipe may be exposed to freezing temperatures such that fluid flow through the pipe may potentially be blocked by frozen fluid in the pipe. The method should be relatively inexpensive to implement, not require manual intervention, and be relatively energy-efficient in clearing the blockage. The improved method should be useable with solar hot water heating systems. 
     SUMMARY OF THE DISCLOSURE 
     The disclosure is an improved method for automatically initiating the flow of fluid through a pipe or the like in which a portion of the pipe may be exposed to freezing temperatures such that fluid flow through the pipe may potentially be blocked by frozen fluid in the pipe. The method is relatively inexpensive to implement, does not require manual intervention, and is relatively energy-efficient in clearing the blockage. The improved method can be applied to solar hot water heating systems. 
     A method for initiating the flow of fluid through a pipe in which a portion of the pipe may be exposed to freezing temperatures such that fluid flow through the pipe may potentially be blocked by frozen fluid in the pipe portion in accordance with the present disclosure includes the steps of:
         (a) urging the flow of fluid through the pipe; and   (b) while urging the flow of fluid through the pipe, performing the following steps:   (c) detecting the absence or presence of fluid flow through the pipe;   (d) if no fluid flow is detected, heating a heater disposed in the pipe portion and extending the length of the pipe portion, thereby transferring heat from the heater to any frozen fluid blocking the pipe portion sufficient to initiate melting of a flow passage through the blockage;   (e) detecting the absence or presence of fluid flow through the pipe while the heater is transferring heat; and   (f) when fluid flow through the pipe is detected, stopping the heating of the heater.       

     The fluid flow detected may be slight at first. It is not necessary for the heater to melt all the frozen fluid, only enough to initiate flow—thereby reducing the energy expended to initiate flow. The heater is disposed in the pipe and is thereby in direct contact with the frozen fluid, increasing the efficiency of heat transfer from the heater to the frozen fluid and further reducing the energy expended to initiate flow. 
     In a preferred embodiment of the method, the heater is operatively connected to a controller, the controller also being operatively connected to a pump that urges fluid flow through the pipe. If the controller energizes the pump but then receives a signal indicating no fluid flow (thereby indicating the pipe is blocked by frozen fluid) the controller energizes the heater to initiate melting of the blockage. Once the controller receives a signal indicating fluid flow, the heater is turned off. 
     In preferred embodiments of the method the heater is formed as a resistance heating a wire, and heating the heater is accomplished by passing an electric current through the wire. If there are multiple pipe portions exposed to freezing conditions, there may be a respective wire or wire portion in each pipe portion, and heating the heater is accomplished by passing an electric current through each of the wires. The wires are relatively inexpensive and easy to install, and in most practical applications of the disclosure would not substantially impede fluid flow. 
     In some possible embodiments of the disclosure, the step of determining whether there is fluid flow is accomplished using a flow sensor. In other possible embodiments of the disclosure, the step of determining whether there is fluid flow is accomplished by taking temperatures at two different locations, each temperature related to the temperature of the fluid at that location. The magnitude of the temperature difference indicates whether or not fluid is flowing through the pipe (depending on system construction, blockage may be indicated if the magnitude of the temperature difference is less than some difference; in other system constructions blockage may be indicated if the magnitude of the temperature difference is greater than some difference). 
     The method of the present disclosure may be used to initiate flow in a solar water heating system of the type having a solar collector assembly that heats water by solar energy and a pipe system supplying water to the solar collector assembly and discharging water from the solar collector assembly, the pipe system having one or more pipe portions that may be exposed to freezing temperatures whereby ice formed in a pipe portion from such exposure may block the flow of water through the pipe portion. The heater is preferably formed as a respective wire extending through each pipe portion as previously described. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a representational view of a first embodiment solar heating system according to the disclosure; 
         FIG. 2  is a perspective view of a first solar collector; 
         FIG. 3  is a sectional view taken along line  3 -- 3  of  FIG. 2 ; 
         FIG. 4  is an enlarged sectional view of an absorber and water heating pipe shown in  FIG. 3 . 
         FIG. 5  is a sectional view along line  5 -- 5  of  FIG. 1 ; 
         FIG. 6  is a representational view of a second embodiment solar heating system; 
         FIG. 7  is a representational view of a third embodiment solar heating system; 
         FIG. 8  is a representational view of a fourth embodiment solar heating system; 
         FIG. 9  is a transverse sectional view through a second solar collector; 
         FIG. 10  is a partially broken away transverse sectional view through the solar collector of  FIG. 9  when the water in the water heating pipe is liquid; and 
         FIG. 11  is a view like  FIG. 10  when the water in the water heating pipe is frozen and has expanded. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The method of the present disclosure is illustrated and described with respect to a solar heating system shown in  FIG. 1 . System  10  includes a solar collector assembly  12 , and a heated water storage tank  14 . The system provides heated water for use in a building, typically a residential dwelling. The solar collector assembly  12  is mounted on the outside of the building and is positioned to receive solar energy. The tank  14  is located inside the building. 
     Cold water supply pipe  16  flows water to the bottom of tank from a water source. Heated water outflow pipe  18  flows heated water from the top of the tank for use in the building. Collector supply pipe  20  extends from the bottom of tank  14  to collector assembly  12 . Collector return pipe  22  extends from the collector assembly to the top of the tank. Pipes  20  and  22  are formed preferably from PEX tubing. Circulator pump  26  located in pipe  20  flows water from the bottom of the tank through pipe  20 , assembly  12  and pipe  22  back to the top of the tank. Water flowed through assembly  12  is heated by solar energy and heats the water in the tank. The portions  28  and  30  of pipes  20  and  22  outside of building wall  54  are connected to water heating pipe  56  running through collector assembly  12 . Pipes  20 ,  56  and  22  form water conduit  24  extending from the tank  14 , to assembly  12  and back to the tank. 
     Supply and return pipes  20  and  22  extend from tank  14  inside the building, through the exterior building wall  54  to collector assembly  12  mounted on the exterior of the building, typically on the roof of the building. The outdoor portions  28  and  30  of pipes  20  and  22  run from wall  54  to assembly  12 . Portions  28  and  30  may extend from ground level up along a side of the building to assembly  12  mounted on the roof of the building. Pipes  20  and are surrounded by tubes of thermal insulation  32  and outer protective sheaths  34 . See  FIG. 5 . The insulation  32  minimizes thermal losses in pipes  20  and  22  and prevents condensation. Pipes  20  and  22  are typically formed from PEX tubing which is not injured by freezing of water within it. 
     Electric control circuit  36  operates system  10 . Circuit  36  is connected to flow sensor  38  located in collector supply pipe  20 , and to differential thermostat  40 . Circuit  36  activates and deactivates an electric resistance heating wire  42  located in each outdoor portion  28  and  30  of pipes  20  and  22 . 
     Thermostat  40  includes temperature sensor  44  in tank  14  and temperature sensor  46  on water heating pipe  56  in collector assembly  12 . Sensor  44  senses the temperature of the water in the tank. Sensor  46  senses the temperature of the water in pipe in the collector assembly. The differential thermostat  40  senses when the temperature at the heating pipe  56  exceeds the temperature of water in tank  14  by a predetermined amount using inputs from sensors  44  and  46 , and when this condition exists it sends a signal to control circuit  36 . 
     Power wire  48  extends from control circuit  36  through the wall of pipe  20  inside of building wall  54  and is connected to a resistance heating wire  42  in pipe portion  28  extending from wall  54  to housing  60 . 
     Power wire  50  extends from the end of the resistance heating wire  42  in pipe  20  adjacent the housing  60 , through the wall of the pipe, along the housing, through the wall of pipe  22  adjacent housing  60  and joins the end of a second resistance heating wire  42  in the outer end  30  of pipe  22 . Wire  42  extends along the interior of pipe  22  through wall  54  and inside the building. Power wire  52  extends from the end of wire  42  through the wall of pipe  22  and to control circuit  36 . When actuated, the two resistance heating wires  42  melt ice blockages in the exterior portions  28  and  30  of pipes  20  and  22  extending from wall  54  to collector assembly  12 . Alternatively, the power wires  48  and  50  may be lengths of double conductor wires with the wires joined at ends of the power wires and with the two lengths extending through the pipes at single locations in the pipes. 
     Solar collector assembly  12  includes four solar collectors  58  mounted side-by-side in housing  60 . A sheet transparent to solar energy overlies the top of the housing to provide thermal insulation and to prevent water and debris from entering the housing. If desired, solar collectors  144 , disclosed in  FIGS. 9-11 , may be used in place of solar collectors  58 . 
     Each solar collector  58  includes an elongate, semi-cylindrical mirror  62  and a vertical solar absorber  64  in the center of the mirror. Water heating pipe  56  extends along the center of the absorber  64  between a pair of like energy absorber plates  66 . 
     The pipe  56  is elliptical in cross section and has a short axis  70  and a long axis  68 . The pipe is preferably formed from thin walled, elastic metal which may be austenitic or Series 300 stainless steel. The pipe sidewall may be about 0.020 inches thick to permit elastic outward flexing of the sidewall of the pipe when water in the pipe freezes and increases in volume. The pipe short axis  70  is sufficiently less than its long axis  68  so that when water in the pipe freezes and expands about 9%, the increased volume of the ice in the pipe elastically expands the pipe but does not expand the pipe to a maximum, circular cross section. Expansion of the pipe does not permanently deform or crack the pipe. When ice in the pipe melts, the pipe elastically returns to its original shape. 
     Elliptical pipe  56  should have an axis ratio (length of long axis  68  divided by length of short axis  70 ) of 1.44 or greater to prevent freeze expansion of the pipe to a circular cross section. In practice, the pipe may have an axis ratio of at least 1.50 to limit flexing of the pipe when water freezes and to reduce the possibility of fatigue cracking. 
     Pipe  56  need not have an elliptical cross section. The pipe may have two opposed long sides joined by short ends, or other cross sectional shapes which permit outward elastic flexing of the pipe wall when water in the pipe freezes. Typical domestic water pressure of less than 100 psi does not significantly deform pipe  56 . 
     Energy absorbing plates  66  are preferably formed from extrusions of high thermal conductivity metal, such as aluminum. Each plate includes a flat, two-sided absorber panel  72  extending away from pipe  56  in a direction along pipe long axis  68 . A pair of opposed solar energy transfer arms  74  at the inner edge of panel  72  extend to either side of pipe  56 . The arms  74  are connected to the panels  72  by beams  80 . The concave inner surfaces  76  of arms  74  are semi-elliptical in shape and are in surface-to-surface heat transfer contact with the outer surface of pipe  56 . See  FIG. 4 . A slot or slit  78  extends from arms into panel  72  to either side of beams  80  to permit elastic flexing of the beams and outward movement of arms  74  with expansion of pipe  56  when water in the pipe freezes. 
     The spacing between arms  74  before mounting of plates  66  on the pipe  56  is slightly less than shown in  FIG. 4  so that beams  80  are flexed outwardly slightly when the plates are mounted on the pipe and tight, surface-to-surface heat transfer connections are established between arms  74  and pipe  56 . The connections promote efficient flow of solar heat from the plates to the pipe to heat water in the pipe. 
     If desired, a thin layer of flexible, thermally conductive material can be provided in the interface between surfaces  76  of arms  74  and the outer surface of pipe  56 . The flexible, thermally conductive material may be silicon grease with thermally conductive particles. The material increases heat flow from the plates  66  to the pipe. 
     Water heating pipe  56  includes an inlet end  82  joined to pipe  20  and an outlet end  84  joined to pipe  22 . The pipe  56  includes four straight heat-absorbing lengths  86  each extending along one of the four side-by-side semi-cylindrical mirrors  62  in assembly  12 . The pipe also includes three semi-circular, 180-degree bends  88  between adjacent pipe lengths  86 . Mirrors  62  may have a diameter of eight inches so that the radius of curvature of bends  88  is four inches. 
     Pipe  56  may be formed from a continuous length of elliptical stainless steel piping. Tooling is used to bend segments of the pipe about axes parallel to the pipe long axis  68  to form bends  88 . Forming bends  88  by bending elliptical pipe around its long axis is easier than forming bends in a cylindrical pipe or bending elliptical pipe around an axis parallel to the short pipe axis. 
     Portions  28  and  30  of pipes  20  and  22  extend outside of the building and are exposed to temperatures which can form ice blockages in the PEX tubing. An ice blockage prevents flow of water through system  10  but does not injure the PEX tubing. Resistance heating wires  42  extend through the interiors of portions  28  and  30  of pipes  20  and  22  and are in direct contact with any ice blockage in the outdoor portions of the pipes. Heating of the resistance wires efficiently melts the ice blockage. 
     If desired, a resistance heating wire, or a number of resistance heating wires, may be mounted outside pipe portions  28  and  30 . Flowing electricity through a wire or wires mounted on the outside of the portions heats each pipe and melts an ice blockage in the pipe. 
     The semi-cylindrical mirrors  62  have highly reflective inner surfaces. Sunlight received by the mirrors is reflected inwardly against the vertical absorber  64 . The sides of absorber plates have heat-absorbing coatings to absorb heat from light reflected against the plates by the mirrors. Sunlight received by the mirrors is reflected against the absorber plates, independent of the angle at which the light strikes the mirrors. 
     Mirrors  62  need not be semi-cylindrical. The mirrors may have different shapes in order to reflect captured light onto plates  66 . 
     The operation of solar heating system  10  will now be described. 
     Sunlight is reflected by mirrors  62  against both sides of absorbers  64  to heat plates  66 . Heat from the plates flows to pipe  56  to heat water in the pipe. When the temperature in pipe  56 , as determined by sensor  46 , exceeds the temperature of the water in tank  14 , as determined by sensor  44 , by a predetermined difference, which may be 30 degrees F., the differential thermostat  40  sends a signal to control circuit  36  and the circuit actuates circulator pump  26 . Pump  26  flows water from the bottom of tank  14  through the collector assembly  12  for solar heating and flows the heated water from the assembly into the top of the tank to heat the tank water. When the temperature of the water in the collector assembly no longer exceeds the temperature of the water in the tank by the predetermined difference, the control circuit  36  turns off circulator pump  26 . 
     If the temperature outside the building wall  54  falls below freezing, water in the outside portions  28  and  30  of pipes  20  and may freeze, despite the fact that water in pipe  56  in collector assembly  12  is heated above freezing and may be warmer than water in tank  14 . In this event, an ice blockage prevents solar heating of water in tank  14 . 
     When water freezes to block pipe portion  28  or  30 , pump  26  will run but water will not flow through pipe  20 , pipe  56  and pipe  22 . The absence of flow while pump  26  is running is detected by direct flow sensor  38  which sends a signal to circuit  36 . Circuit  36  then flows electricity through power wires  48  and  52  to heat the resistance wires  42  in pipe portions  28  and  30 . One of the wires  42  extends past the ice blockage. Heat from the wire  42  melts the ice blockage to reestablish flow of water through the pipe  56  by pump  26 . 
     The resistance heating wires  42  are activated until sensor  38  detects reestablished flow of water, which may be slight at first. Once flow has been reestablished, a signal from flow sensor  38  actuates circuit  36  to deactivate the wires  42 . Water is flowed past the remaining ice to rapidly melt the ice and reestablish normal operation of system  10 , despite an outdoor temperature below freezing. 
     Flow sensor  38  detects decreased flow or no flow due to an ice blockage. The sensor may include a vane or a rotary turbine wheel located in supply pipe  20  and a detector responsive to movement of the vane or wheel. Other types of flow sensors may be used if desired including differential pressure flow sensors, ultrasonic flow sensors, calorimetric flow sensors, and the like. 
       FIG. 6  illustrates a second embodiment solar heating system  90  which is like solar heating system  10 . Reference numbers shown in  FIG. 5 , which are identical to reference numbers shown in  FIG. 1 , describe components of system  90  identical to the components of system  10 . System  90  includes a solar collector assembly  12 , water storage tank  14 , water supply and return pipes  20  and  22 , including insulated portions  28  and  30 , pump  26  and differential thermostat  40 , as previously described. Control circuit  36  is connected to the differential thermostat  40  and to pump  26 . 
     System  90  includes a second differential thermostat  92  connected to temperature sensor  94  located in the return pipe  22  inside of exterior wall  54  and to temperature sensor  96  located in supply pipe  20  inside of exterior wall  54 . System  90  does not use a flow sensor  38  and does not sense flow using moving parts. 
     During normal operation of solar heating system  90 , pump  26  is actuated to circulate water through the solar collector  12  and flow the heated water back to tank  14 , as previously described. Temperature sensor  94  detects decreased temperature in pipe  22  due to decreased flow and is an indirect flow sensor. If an ice blockage exists in portion  28  or  30  of pipe  20  or  22  the blockage will prevent flow of heated water from collector assembly  12  to tank  14 . The temperature of the water in pipe  22  will not rise. When this condition exists the temperature difference between the water in pipe  22  will not greatly exceed that in pipe  20 , as determined by sensors  94  and  96 . When this difference is below a predetermined amount, which may be 20° F., differential thermostat  92  sends a signal to control circuit  36  to actuate the resistance heating wires  42  in the exterior portions  28  and  30  of pipes  20  and  22  to melt the ice blockage, as previously described. 
     Melting of the blockage and flow of heated water through pipe  22  which will raise the temperature of the water in the pipe. When the temperature of the water in pipe  22  exceeds the temperature of the water in pipe  20 , as again determined by temperature sensors  94  and  96 , by the predetermined amount, the differential thermostat  92  sends a signal to control circuit  36  to deactivate the resistance heating wires in pipe portions  28  and  30 . Flow of water through the exterior portions  28  and  30  of pipes  20  and  22  melts any remaining ice in the blockage to reestablish normal operation of system  90 . 
       FIG. 7  illustrates third embodiment solar heating system  98  having three series connected solar heating assemblies  100 ,  102  and  104  which replace the single assembly  12  used in the systems of  FIGS. 1 and 6 . The assemblies are each identical to solar collector assembly  12 . The three collector assemblies are connected to water supply pipe  20  and water return pipe  22  of a solar heating system  10  or  98  located inside of building exterior wall  54 . These alternative interior components are not illustrated in  FIG. 7 . 
     The outer insulated end  106  of pipe  20  extends from wall  54  to collector assembly  100  and is connected to the inlet end of water heating pipe  56  in assembly  100 . Pipe end  106  is surrounded by insulation and a sheath, as previously described. The outlet end of water heating pipe  56  in assembly  100  is connected to an insulated pipe  108  extending from assembly  100  to assembly  102 . Pipe  108  is connected to the inlet end of pipe  56  in assembly  102 . The outlet end of pipe  56  in assembly  102  is likewise connected by insulated pipe  110  to the inlet end of the pipe  56  in assembly  104 . The outlet end of pipe  56  in assembly  104  is connected to the insulated outer end  112  of pipe  22  which extends to wall  54 . The control wiring  114  for temperature sensor  46  in assembly  104  extends through wall  54  to the differential thermostat  40  for system  98 . Resistance heating wires (not illustrated) extend through outdoor pipes portions  106 ,  108 ,  110  and  112  and are connected to power wires  48  and  50 . The resistance heating wires are actuated to melt ice blockages as previously described. Pipes  20 ,  56 ,  108 ,  110  and  22  form a single passage water conduit  99  extending from tank  14 , through the assemblies  100 ,  102  and  104  and back to the tank. 
     The system  98  operates essentially like the systems  10  and  90 . Pump  26  circulates water through the three solar collector assemblies  100 ,  102  and  104  and water is solar heated. The temperature sensor  46  determines the temperature of the water in assembly  104 , which typically is higher than the temperatures of the water in assemblies  100  and  102 . This temperature is used to determine whether the water returned through pipe  22  is sufficiently hot to heat the water in tank  14 . 
       FIG. 8  illustrates a fourth embodiment solar heating system  118  which includes three parallel connected solar collector assemblies  120 ,  122  and  124 . The assemblies are each identical to solar collector assembly  12 . The three collector assemblies are connected to water supply pipe  20  and water return pipe  22  of system  10  or system  98  located inside building exterior wall  54 . These components are not illustrated in  FIG. 8 . 
     Water supply pipe  20  extends outwardly of wall  54  and includes insulated outdoor supply branches  126 ,  128  and  130 . The branches are connected respectively to the inlet ends of the water heating pipes  56  in the three assemblies. The outlet ends of the water heating pipes  56  in the assemblies are connected to insulated return branches  132 ,  134  and  136  of return pipe  22 . All of the portions of the supply and return pipes located outwardly of wall  54  and connected to the three assemblies are surrounded by insulation and protective sheeting, as previously described. A single temperature sensor  46  is connected to a water heating pipe  56  in collector assembly  122 . The sensor may be attached to the water heating pipe in any of the collector assemblies. Insulated resistance heating wires (not illustrated) are extended through the supply and return branches of pipes  20  and  22 . When an ice blockage is sensed, the wires are actuated to melt the blockage, as previously described. Pipes  20 ,  56  and  22  form a plural passage water conduit  116  extending from tank  14 , through assemblies  120 ,  122  and  124  and back to the tank. 
     During operation of system  118 , pump  26  flows water from tank  14  through the three collector assemblies and flows the heated water directly back to the tank. The system  118  operates like system  10  or  98 , as previously described. 
     Solar heating systems  98  and  118  use plural solar collector assemblies in order to increase the capacity and performance of the system. The use of a number of small individual solar collector assemblies facilitates manufacture, transportation and the mounting of the assemblies on the roof of a dwelling. Frequently smaller assemblies can be mounted advantageously where it is impossible to mount a large assembly having the same heating capacity. 
       FIGS. 9-11  illustrate a second solar collector  140  which may be used in the solar collector assemblies  12  of the previously described embodiments in place of previously described solar collector  58 . Solar collector  140  includes an elongate semi-cylindrical mirror  142 , like mirror  62 , and a vertical solar absorber  144  located in the center of the mirror, in the same position as solar absorber  64  is positioned in mirror  62 . Compare  FIGS. 2 and 9 . 
     Solar absorber  144  includes two like elongate absorber plates  146  and an elongate, flat water pipe  148  held between the plates. Each plate  146  includes elongate strips  150  at the top and bottom of the collector. Strips  150  have uniform thickness. The plates each include an elastically deformable pipe contact energy transfer portion  152  extending between the strips  150 . Flat recesses  154  on the inner surfaces of portions  152  form a flat pocket  156  extending along the portions  152  between plates  146 . Outer surfaces  155  of portions  152  are concave. The thickness of each energy transfer portion  152  decreases smoothly from a maximum thickness adjacent strips  150  to a minimum thickness at the centers of the strips. Plates  146  form from heat-absorbing metal which may be high thermal-conductivity aluminum. Plates  146  are secured together by threaded fasteners passing through holes  147  and spaced along strips  150 . The fasteners may be formed from stainless steel and are preferably spaced from the aluminum in strips  150  by dielectric separators to prevent electrolytic corrosion. 
     Flat water pipe  148  has opposed wide and normally flat sidewalls  158  and opposed rounded and narrow sidewalls  160 . The pipe is fitted in flat pocket  156  with sidewalls  158  abutting the flat pocket  154 . Narrow, rounded sidewalls  160  are spaced inwardly from the top and bottom of flat pocket  156 . 
     Water pipe  148  is preferably formed from the same metal as pipe  56 , previously described. Likewise, the pipe sidewall may have a thickness of 0.020 inches to permit outward flexing of the flat sidewalls  158  when water in the pipe freezes. The pipe  148  may have a width of three inches and a spacing between the flat sidewalls of 0.150 inches. The long, closely spaced sidewalls  158  permit elastic outward expansion of the pipe when water freezes. 
     The pipe may be flattened slightly when the strips  150  are secured together in order to assure close surface-to-surface interfaces between the pipe and the plates. Rounded sidewalls  160  may be stressed. If desired, a thin layer of flexible, thermally conductive material may be provided in the interface between the pipe and the absorber plates, as previously described. 
     During normal operation of solar collector assembly  140 , mirror  142  reflects solar energy onto absorber  144  and heat is transferred from the absorber plates  146  to water in pipe  148 , as previously described. 
     In the event that water in pipe  148  freezes and expands, the increased pressure in the pipe forces the flat sidewalls  158  outwardly and flexes the portions  152  outwardly as shown in  FIG. 11 . Maximum deflection occurs at the centers of the portions, where the portions  152  are thinnest. Reduced deflection occurs to either side of the centers of the portions. During freezing, the portions  152  and pipe flat sidewalls  158  are bowed outwardly within their elastic limits and without permanent deformation. This means that when water in the pipe melts, the pressure in the pipe decreases and the pipe contact portions and flat pipe sidewalls return elastically to their original positions shown in  FIG. 10 . Freezing of water in the pipe does not injure the solar absorber or impair its efficiency in flowing solar energy to water in the pipe. 
     Although disclosed embodiments have been illustrated using water as the working fluid that may freeze, this is not intended to be limiting. The disclosed method is also applicable to fluid systems that utilize other, different working fluids (including without limitation fluids that have freezing points that are different than the freezing point of water). Other heater constructions or sources of energy for heating the heater may be used as is already known in the heating arts.