Abstract:
A solar water heater includes a drainback unit with a drainback reservoir and an anti-airlock conduit for assuring that working fluid in the solar collectors is consistently drained from the solar collectors into a drainback reservoir once circulation the working fluid in the solar collectors has stopped. The drainback unit also provides rapid startup by positioning the heat exchanger outside of the drainback reservoir and by positioning the inlet and outlet of drainback reservoir in close proximity to each other.

Description:
CLAIM OF PRIORITY 
       [0001]    The present patent application claims priority from U.S. Provisional Application No. 61/254,380, filed Oct. 23, 2009 and U.S. Provisional Application No. 61/258,262, filed Nov. 5, 2009, both of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a solar water heater with a drainback unit to drain fluid from solar heat collectors connected to the solar water heater. 
       BACKGROUND OF THE INVENTION 
       [0003]    A conventional solar water heater includes one or more solar collectors for heating a working fluid, a storage tank for storing heated water, and a heat exchanger for transferring the heat in the working fluid to the water in the storage tank. Particularly, the conventional solar water heater has a solar heat exchanger circuit comprising the solar collectors, a solar hot fluid conduit, a solar heat exchanger coil in the heat exchanger, a solar cold fluid conduit, and a solar pump for circulating the working fluid through cold fluid line, through the solar collectors, through the hot fluid conduit, and through the solar heat exchanger coil when heat is being captured by the solar collectors. In addition, the conventional solar water heater has a storage tank heat exchanger circuit comprising the storage tank, cold water siphon conduit, a storage tank heat exchanger coil in the heat exchanger, a hot water return conduit, and a hot water pump for circulating water in the storage tank through the storage tank heat exchanger coil. 
         [0004]    During operation of such a conventional solar water heater, circumstances arise when the working fluid in the solar collectors should be drained from the solar collectors. In order to extend the life of the solar collectors, the working fluid is typically drained from the solar collectors any time that the solar pump is shut down, and the working fluid is not circulating through the solar collectors. The solar pump is shutdown when the requirement for heat for heating the water in the storage tank has ended or when heat is not available from the solar collectors because of the absence of sunlight. By draining the solar collectors, the solar collectors are protected from freezing and corrosion and the working fluid is protected from degradation. 
         [0005]    Draining the working fluid from the solar collectors requires a drainback unit that is part of the solar heat exchanger circuit and that includes a drainback reservoir for storing the working fluid and the necessary plumbing to allow the working fluid to drain from the solar collectors into the drainback reservoir. Conventionally, the heat exchanger is located within the drainback reservoir. When the conventional solar water heater frequently cycles between a heating operation with the solar pump running and a drain back operation with the solar pump shut off, the working fluid in the drainback reservoir typically remains at high temperature, and heat from the working fluid in the drainback reservoir is transferred to the heat exchanger located within the drainback reservoir. When the cycle time between the heating operation with a solar pump running and the drain back operation is long, such as overnight, the working fluid in the drainback reservoir cools, and the working fluid in the heat exchanger and the water in the heat exchanger both become cool. On restart, when heat is again available from the solar collectors, the solar collectors must heat all of the working fluid in the solar heat exchange circuit including all of the drainback working fluid before heat can be transferred by the heat exchanger to the water circulating in the heat exchanger. Consequently, such a conventional drainback unit experiences a substantial startup delay for the delivery of heat to the water in the water storage tank. 
         [0006]    In order for the working fluid to drain into the drainback reservoir, air must displace the working fluid in the solar collectors as the working fluid is drained. Under certain operating conditions, an airlock may develop in the plumbing between the solar collectors and the drainback reservoir thereby preventing the working fluid in the solar collectors from draining into the drainback reservoir. Under such circumstances, the trapped working fluid may freeze in the solar collectors thereby damaging the solar collectors, the trapped working fluid may corrode the solar collectors, or the heat transfer characteristics of the trapped working fluid may, when subjected to extreme cold, degrade. 
       SUMMARY OF THE INVENTION 
       [0007]    The drainback unit of the present invention addresses both the problem of startup delay described above and the problem of an airlock inhibiting the drain back of working fluid into the drainback reservoir once the solar pump has stopped circulating the working fluid in the solar collectors of the solar heat exchanger circuit. 
         [0008]    In order in order to assure that the working fluid in the solar collectors is consistently drained from the solar collectors, the drainback unit of the present invention has an anti-airlock conduit connecting air in the top of the drainback reservoir to the solar hot fluid conduit to allow the air in the drainback reservoir to bubble through the solar hot fluid conduit to the solar collectors while the working fluid drains by the force of gravity through the solar cold fluid conduit and through the solar pump into the bottom of the drainback reservoir. 
         [0009]    In order to speed the delivery of heat to the water in the storage tank up on restart of the solar pump, the heat exchanger of the drainback unit in accordance with the present invention is located outside of the drainback reservoir. Moreover, the drainback reservoir outlet conduit and the drainback reservoir inlet conduit are connected to the drainback reservoir near the bottom of the drainback reservoir and in close proximity to each other. Consequently, upon restart, the solar collectors need only heat the working fluid in the solar collectors, the solar hot fluid conduit, the solar cold fluid conduit a small layer of working fluid near the bottom of the drainback reservoir and before the heat exchanger can begin delivering heat to the water in the storage tank. 
         [0010]    Further objects, features and advantages will become apparent upon consideration of the following detailed description of the invention when taken in conjunction with the drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic drawing of a solar water heater with a drainback unit in accordance with the present invention. 
           [0012]      FIG. 2  is a perspective view of the drainback unit for the solar water heater in accordance with the present invention. 
           [0013]      FIG. 3  is a perspective elevation view of the drainback unit (cover removed) for the solar water heater in accordance with the present invention. 
           [0014]      FIG. 4  is a detailed section view of the anti-airlock conduit connecting the solar hot fluid conduit to the drainback reservoir of the drainback unit in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    Turning to  FIG. 1 , a solar water heater  10  in accordance with the present invention comprises a storage tank  16  for holding heated water, one or more solar collectors  12  charged with a working fluid, a heat exchanger  14  for transferring heat, during a heating operation, from the working fluid to the water in the storage tank, and a drainback unit  40  ( FIGS. 2 and 3 ) for collecting the working fluid when the heating operation has terminated. The storage tank  16  forms part of a storage tank heat exchanger circuit  22 , which comprises the storage tank  16 , a cold water siphon conduit  32 , a heat exchanger cold water inlet  28 , a storage tank heat exchanger coil  24  inside the heat exchanger  14 , a heat exchanger hot water outlet  30 , a storage tank water pump  26 , a hot water return conduit  34 , and an air vent  27 , all connected in series as shown in  FIG. 1 . 
         [0016]    The solar collectors  12  form part of a solar heat exchange circuit  46 , which comprises the solar collectors  12 , a solar hot fluid conduit  54 , a solar heat exchanger coil  48  inside the heat exchanger  14 , a drainback reservoir inlet conduit  62 , a drainback reservoir  44 , a drainback reservoir outlet conduit  60 , a solar fluid pump  50 , and a solar cold fluid conduit  52 , all connected in series as shown in  FIG. 1 . The drainback reservoir  44  and an anti-airlock conduit  56  connected between the solar hot fluid conduit  54  and the top of the drainback reservoir  44  comprise the drainback unit  40 . The anti-airlock conduit  56  of the drainback unit  40  is illustrated in greater detail in  FIG. 4 . As shown in  FIG. 3 , the heat exchanger  44  is physically located outside of the drainback reservoir  44 . Further, the drainback reservoir outlet conduit  60  and the drainback reservoir inlet conduit  62  are located adjacent to each other and near the bottom of the drainback reservoir  44 . 
         [0017]    With continued reference  FIG. 1 , the storage tank  16  receives cold water from a pressurized source of cold water (not shown) via cold water supply line  21  and storage tank inlet conduit  18 . Hot water in the storage tank  16  is delivered from the storage tank  16  to a water system (not shown) via a storage tank outlet conduit  20 , a mixing valve  19 , and a hot water output line  23 . In order to store the most amount of heat in the storage tank  16 , the water in the storage tank  16  is maintained at a temperature well above the temperature of the water required by the water system. The mixing valve  19  serves to reduce the temperature of the water flowing in the storage tank outlet conduit  20  by injecting cold water from the cold water supply line  21  to produce water in the hot water output line  23  that is of the appropriate temperature for use by the water system. The storage tank  16  is further equipped with a conventional temperature/pressure relief valve  25  to prevent over temperature or over pressure build up in the storage tank  16 . 
         [0018]    In order to heat the water in the storage tank  16 , solar energy is collected by the solar collectors  12  which in turn heat the working fluid in the solar heat exchanger circuit  46 . The working fluid may include among other fluids, water, glycol, glycol/water mixtures, alcohols, alcohol/water mixtures, and other heat transfer fluids known to those persons of ordinary skill in the art. During a water heating operation, the working fluid in solar heat exchanger circuit  46  is circulated by means of solar fluid pump  54  in the direction shown by the arrows in  FIG. 1 . Cool working fluid is drawn from the drainback reservoir  44  through the drainback reservoir outlet conduit  60  by the solar fluid pump  54  and forced through solar cold fluid conduit  52  into the solar collectors  12 . As the working fluid passes through the solar collectors  12 , the working fluid is heated and exits through solar hot fluid conduit  54 . A hot fluid temperature sensor  72  is connected to the solar hot fluid conduit  54  adjacent the solar collectors  12  to determine the temperature of the working fluid as it exits the solar collectors  12 . The working fluid then passes from the solar hot fluid conduit  54  into the solar heat exchanger coil  48  of the heat exchanger  14 . In the heat exchanger  14 , the working fluid gives up its heat to the storage tank heat exchanger coil  24 , and the working fluid, in a cooler state, exits the solar heat exchanger coil  48  and into the drainback reservoir  44  through drainback reservoir inlet conduit  62 . The working fluid in the drainback reservoir  44  is withdrawn through drainback reservoir outlet conduit  60  by the solar fluid pump  50  and the heating operation cycle continues. The drainback reservoir  44  is equipped with a pressure relief valve  45  to accommodate any overpressure condition that might exist inside the drainback reservoir  44 . 
         [0019]    The water in the storage tank heat exchanger circuit  22  as previously stated is heated in the heat exchanger  14  when the working fluid in the solar heat exchanger coil  48  gives up its heat to the storage tank heat exchanger coil  24 . The water in the storage tank heat exchanger circuit  22  is circulated by means of the storage tank water pump  26 . The storage tank water pump  26  circulates the water in the storage tank heat exchanger circuit  22  by drawing the water in the storage tank  16  through the cold water siphon conduit  32 , the heat exchanger cold inlet  28 , the storage tank heat exchanger coil  24  of the heat exchanger  14 , and the heat exchanger hot outlet  30 . The storage tank water pump  26  then forces the heated water through the hot water return conduit  24  and back into the storage tank  16 . An air vent valve  27  is provided on the hot water return conduit  24  adjacent the storage tank  16  to allow for the exhaustion and intake of air to and from the storage tank  16  as the water level in the storage tank  16  rises and falls. A storage tank temperature sensor  70  is connected to the storage tank  16  to sense the temperature of the water in the storage tank  16 . 
         [0020]    The operations of the storage tank water pump  26  and the solar fluid pump  50  are controlled by the control module  74 . The control module  74  monitors the temperature of the working fluid in the solar hot fluid conduit  54  as sensed by the hot fluid temperature sensor  72  and the temperature of the water in the storage tank  16  as sensed by the storage tank temperature sensor  70 . When the temperature of the working fluid exceeds the temperature of the water in the storage tank, typically by a differential of 16° F., the control module  74  recognizes that the solar collectors  12  are producing sufficient heat to begin heating the water in the storage tank  16 . At that point, both the storage tank water pump  26  and the solar fluid pump  50  are turned on, and a heating operation is commenced. Once the differential between the hotter working fluid in the solar collectors  12  and the water in the storage tank  16  drops to a predetermined differential value, typically 6° F., the control module  74  recognizes that the water temperature is sufficiently high, the storage tank water pump  26  and the solar fluid pump  50  are shut off, and the heating operation ceases. The control module  74  also monitors both the working fluid temperature and the water temperature, by means of the hot fluid temperature sensor  72  and in the storage tank temperature sensor  70 , for safe maximum temperatures. 
         [0021]    Once a heating operation has ceased, the working fluid should be drained from the solar collectors  12  and stored in the drainback reservoir  44 . As the working fluid drains from the solar collectors  12  through the solar cold fluid conduit  52  and the solar fluid pump  50  and into the drainback reservoir  44 , air must flow into the solar collectors  12  through the solar hot fluid conduit  54 , or the solar collectors  12  become airlocked, and the working fluid will not drain from the solar collectors  12 . In order to supply vacuum relief air to the solar collectors  12  through the solar hot fluid conduit  54 , the present invention includes an anti-airlock conduit  56  connected between the top of the drainback reservoir  44 , where there is a supply of air, and the solar hot fluid conduit  54  as shown in detail in  FIG. 4 . The anti-airlock conduit  56  has a cross sectional area that is less than half the size of the cross-sectional area of the solar hot fluid conduit  54 . Through testing, the preferred ratio of the cross-sectional area of the anti-airlock conduit  56  to the cross-sectional area of the solar hot fluid conduit  54  is approximately 1:36, which is the ratio resulting from the use of a ¾ inch solar hot fluid conduit  54  and a ⅛ inch anti-airlock conduit  56 . The proper ratio of the cross-sectional areas ensures that sufficient air can bubble up through the solar hot fluid conduit  54  during a drain back operation to displace the working fluid that drains from the solar collectors  12  through the solar cold fluid conduit  52 . By the same token, the proper ratio of the cross-sectional areas ensures that during a heating operation, significant amounts of working fluid circulating through the solar hot fluid conduit  54  are not diverted into the drainback reservoir  44  instead of into the heat exchanger  14 . 
         [0022]    By locating the heat exchanger  14  outside of the drainback reservoir  44  and by placing the drainback reservoir inlet conduit  62  from the solar heat exchanger coil  48  adjacent the drainback reservoir outlet conduit  60 , the delay in providing heat at the startup of a heating operation after a drain back operation can be minimized. Particularly, where the working fluid has been drained from the solar collector  12  overnight or for an extended period of time, the temperature of the working fluid in the drainback reservoir  44  will be cold. Consequently, the amount of time required for startup depends on how quickly the circulating working fluid can be heated in the solar collectors  12 . The startup time depends on the amount of working fluid that must be heated by the solar collectors  12 . By limiting the amount of working fluid to only the working fluid in the solar collectors  12 , in the solar cold fluid conduit  52 , in the solar hot fluid conduit  54  and in small layer of working fluid in the drainback reservoir  44  adjacent the drainback reservoir outlet conduit  60  and the drainback reservoir inlet conduit  62 , the startup delay can be minimized 
         [0023]    While this invention has been described with reference to preferred embodiments thereof, it is to be understood that variations and modifications can be affected within the spirit and scope of the invention as described herein and as described in the appended claims.