Patent Publication Number: US-2013239951-A1

Title: Retrofittable tankless passive solar water heater

Description:
TECHNICAL FIELD 
     The present invention relates to a solar water heater for heating pressurized domestic water that does not require a separate large capacity hot water storage tank that occupies valuable living space; is passive because it does not have any pumps or other moving parts in normal operation, so that service life is increased; and is retrofittable to use any smaller capacity hot water tank from an existing electrical, gas, or other non-solar domestic hot water heater. 
     BACKGROUND ART 
     In most developed countries, domestic water is required to be pressurized by a central utility so that water flows through pipes when a faucet is turned on. In conventional solar hot water systems, domestic water is heated during the day by being pumped (using a separate circulation pump) up to a solar panel on the roof of a house, where it is heated, and then the heated water returns to the house through insulated pipes, and is stored in a large capacity insulated tank for future use, usually at night. This insulated solar hot water tank must be of large capacity because the solar panel only heats water during the day, when the sun is shining, and does not heat water at night. This hot water tank typically has a capacity based on expected hot water usage of 20 gallons for the first person, and 15 gallons per each additional person in the household. Because of this size requirement, typical solar hot water tanks have a capacity of 80 gallons or higher. Solar hot water tanks eventually corrode, often in less than 10 years, and when they leak, large quantities of water are released into the house, causing damaging flooding. Further, because of the need for insulation and large capacity, conventional solar hot water tanks occupy a large amount of valuable living space. 
     In conventional solar hot water systems, the panels on the roof are not well insulated, so that they cool down at night. In the morning, the panels are cold, and therefore cannot heat water. 
     One problem with designing solar hot water systems is that water expands when heated: 5 gallons of water increases by 1 pint in volume from a 100 degree Fahrehnheit rise in temperature. The maximum temperature in a properly operating solar heating system is about 180 degrees Fahrenheit, so this increase in volume can easily occur from the coldest temperature at dawn to the highest temperature in the afternoon. 
     A conventional non-solar hot water system typically uses electricity or natural gas to heat up water that is then stored in a small capacity tank. For example, common sizes for the tanks of conventional electric or gas water heaters are 30, 40 or 50 gallons. Because the heater can be turned on at any time (instead of heating only during the day, as is the case with solar heating), a conventional water heater can be turned on to heat more water when the water in the tank becomes colder. Thus, when a conventional solar water heater replaces a conventional non-solar hot water system, both the non-solar heater and the existing small capacity tank must be replaced, and a large insulated solar hot water tank must be installed instead. Usually the solar hot water tank is twice or three times the capacity of the existing small capacity tank, so different or additional storage space within the house must be found. Further, the cost for replacing a solar hot water tank is usually at least twice the cost for replacing a conventional heater&#39;s tank, due to the greater size and need for specialized solar contractor to perform the replacement (normal plumbers can replace conventional hot water heater tanks). 
     There are also tankless electric or gas heaters that can heat sufficient water instantaneously so that no storage tank is necessary. 
     Some companies combined solar water heaters (using conventional solar hot water storage tanks) with gas or electric tankless water heaters. For example, Bosch sold the Aquastar 1600PS propane solar tankless water heater to receive preheated water from a conventional solar heating system having a conventional solar hot water tank. 
     U.S. Pat. No. 980,505 to Emmet discloses a series of tubes with vacuum chamber jackets placed side by side, connected at their open ends to a chambered header through which fluid flows into and out of the tubes, absorbing heat as it goes. Page 2, lines 77-79, state that it is difficult to make an air-tight joint or seal between a metal vessel and an outer glass envelop. 
     U.S. Pat. No. 4,018,215 to Pei discloses a manifold for a solar energy collector assembly in which the working media is a liquid circulated through several tubular collectors in series. Col. 1, lines 48-54, indicate thermal expansion differences cause failure in glass to metal seals.  FIGS. 7 and 8  show a single-acting manifold. 
     U.S. Pat. No. 4,033,327 to Pei discloses a solar energy collector apparatus having several double-wall glass tubular elements connected on opposite sides of an elongated module. The elements are sealed in oppositely facing metal cups and inside the opposite elements is a cross supply tube. The cups are connected by conduits for flow of a liquid through the collectors. 
     U.S. Pat. No. 4,043,318 to Pei discloses over-sized test tubes having inner and outer walls, with the space between evacuated. A working fluid circulates and is heated. Several of these energy collectors are connected into a manifold for circulation of working fluid. 
     U.S. Pat. No. 4,212,293 to Nugent discloses a solar energy collector apparatus in which several double-wall glass tube collectors, each with vacuum jacket, depend from opposite sides of an elongated manifold. Several modules are inter-connectable to desired capacity for a particular solar powered heating or cooling system. 
     U.S. Pat. No. 4,440,156 to Takeuchi discloses a solar heat collector including inner and outer substantially straight tubes being closed at one end and open at the other end sealed at their open ends with the space therebetween being evacuated. A hairpin pipe for circulation of fluid media is disposed within the inner tube and includes two substantially straight sections wherein both or at least one is in surface contact with the inner surface of the inner tube. 
     U.S. Pat. No. 4,554,908 to Hanlet discloses an electromagnetic energy collector assembly in which a cylindrical glass tube I sealed under vacuum at one end to an inner cylindrical energy absorber having a plurality of grooves on the exterior surface. 
     U.S. Pat. No. 5,931,156 to Wang discloses a heat-pipe type solar collector that includes a heat absorber portion adapted to absorb solar energy to evaporate a working fluid in heat tube elements; and heat release portion communicating with the heat absorber portion and having a body of a semi-annular or annular cross-section. At night, the working fluid portion flows to the heat absorber portion to generate a vacuum for heat insulating purposes, thereby maintaining the temperature in the water reservoir. 
     Dewars type vacuum tubes are tubes that are placed one within the other, joined at the neck, with the space between the tubes being evacuated. 
     However, the inventor is not aware of a tankless passive solar water heater retrofittable to an existing domestic hot water system using Dewars type large diameter vacuum tubes. 
     Accordingly, it is an object of this invention to provide a solar water heater that avoids the need for a separate solar hot water storage tank. 
     It is a further object of this invention to provide a solar water heater that is passive, that is, has no moving parts during normal operation, to provide a longer service life. 
     It is a still further object of this invention to provide a solar water heater that is retrofittable to use a preexisting conventional non-solar water heater and its small capacity tank. 
     It is a still further object of this invention to avoid the difficulties with existing glass to metal vacuum tubes, specifically the problem of maintaining a vacuum between materials with different thermal expansions. 
     DISCLOSURE OF THE INVENTION 
     The above and other objects are achieved by a tankless solar water heater that includes an insulated container (open at an upper end), for solar heating and insulating a solar heating fluid; a flexible insulated heat exchanger housing sealingly attached over the upper end; a check valve (a one way valve that allows fluid or air to escape, but not to enter) sealingly mounted at an uppermost location in the heat exchanger housing; and heat exchanger tubing at least partially contained within the heat exchanger housing, sealingly extending through heat exchange ports in the heat exchanger housing to exchange heat between the solar heating fluid and domestic pressurized water circulating through the heat exchanger tubing. The check valve releases air (and the solar heating fluid and any gas therefrom) when interior pressure in the container and the heat exchanger housing is greater than approximately 1 pound per square inch above ambient pressure, without admitting air into the container or the heat exchanger when the interior pressure is less than the ambient pressure due to volume reduction of the solar heating fluid from cooling. The heat exchanger housing contracts to accommodate the volume reduction of the solar heating fluid from cooling. The container and the flexible heat exchanger are sufficiently insulated to reduce cooling of the solar heating fluid in the heat exchanger housing to 1 degree Fahrenheit per hour at 130 degrees Fahrenheit. 
     Preferably, the insulated container comprises an outer rigid transparent tube and an inner opaque tube, with an insulating vacuum in the space between the tubes, commonly called an all glass solar vacuum tube, or a Dewar&#39;s type vacuum tube solar collector. 
     Preferably, the heat exchanger tubing is entirely contained within the heat exchanger housing. 
     Preferably, also, the check valve vents the solar heating fluid and gas therefrom in case of boiling. 
     In a preferred embodiment, the invention has a plurality of insulated solar heating tubes, preferably eight, each with a flexible insulated heat exchanger housing sealingly attached over each of the upper ends, with a check valve sealingly mounted in the uppermost portions of the heat exchanger housings. 
     Heat exchanger tubing is mounted at least partially within each of the heat exchanger housings, with the heat exchanger tubing being connected in series between heat exchanger housings. In this manner, when solar heating fluid filling the insulated tubes and the heat exchanger housings is heated by the sun, heat is exchanged between the solar heating fluid and domestic pressurized water circulating through the heat exchanger tubing. The flexible heat exchanger housings contract to accommodate volume reduction when the solar heating fluid cools at night, and the check valves releases solar heating fluid and any gas therefrom if interior pressure in the tubes and the heat exchanger housings exceeds ambient pressure by more than one pound per square inch, without admitting air into the tubes or the heat exchangers when volume of the solar heating fluid reduces due to cooling. The tubes and heat exchangers are sufficiently insulated to reduce cooling of the solar heating fluid in the heat exchanger housing to 1 degree Fahrenheit per hour at 130 degrees Fahrenheit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective exploded view of a presently preferred embodiment of the present invention. 
         FIG. 2  is a side elevational view of a vacuum tube according to the presently preferred embodiment of the present invention. 
         FIG. 3  is cutaway view of the vacuum tube of  FIG. 2  along the line A-A. 
         FIG. 4  is an exploded perspective view of a heat exchanger housing and heat exchanger according to the presently preferred embodiment of the present invention. 
         FIG. 5  is a side cutaway view of a presently preferred embodiment of the check valve of the present invention. 
         FIG. 6  is a side elevational view of the vacuum tube of  FIG. 2 , the heat exchanger housing and heat exchanger of  FIG. 4 , and the check valve of  FIG. 5 , in assembled configuration. 
         FIG. 7  is a top end view of the series of heat exchanger housings on the vacuum tubes depicted in  FIG. 1 . 
         FIG. 8  is a perspective assembled view of the presently preferred embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring to  FIG. 1 , shown is a perspective exploded view of the presently preferred embodiment of a tankless solar water heater according to the present invention  10 , comprising eight cylindrical vacuum tubes  20  with flexible heat exchanger housings  30  sealingly attached to the upper ends  22  of each of the vacuum tubes  20 . Preferably, the heat exchanger housings  30  are received, aligned, and retained in position in an insulated box  40  lined with polyurethane, fiberglass, or other insulating material (not shown) with an insulated lid  48 . Preferably also, the lower ends  24  of the vacuum tubes  20  are received, aligned, and retained in position by a preferably aluminum bottom support holder  60  (comprising a base  62  and top  64 , with split rubber hose bushings  66  and  68  to hold the vacuum tubes), and covered by a bottom cover  70 . The bottom support bracket  60  and box  40  are preferably joined together by angle members  80 . 
     Referring to  FIG. 2 , shown is a side elevational view of a vacuum tube  20 , comprising a transparent outer tube  22  and a coaxial opaque, preferably black (with a radiant energy absorbing coating) inner tube  24 . The space  26  between the outer tube  22  and inner tube  24  is evacuated, to provide thermal insulation for the inner tube  24 . Preferably the inner tube  24  has a capacity of approximately 3.5 gallons. 
     Referring to  FIG. 3 , shown is an end cutaway view of the vacuum tube of  FIG. 2  along the line A-A. As can be seen, the inner tube  24  is coaxial with the outer tube  22 , and the space  26  between them is evacuated. 
     Referring to  FIG. 4 , shown is an exploded perspective view of a heat exchanger housing  30  and heat exchanger tubing  32  according to the presently preferred embodiment of the present invention. An adapter end plate  90 , preferably of stamped brass, is preferably sealingly attached to the upper end  34  of the heat exchanger housing  30  (see  FIG. 6 ), and has two heat exchanger ports  92 , a check valve port  94 , and an optional fill port  96  (which is plugged, capped or otherwise closed after the heating fluid fills the entirety of inner tube  24  and the heat exchanger housing  30 , with any dissolved or entrained air later being released through the check valve as described below). The heat exchanger housing  30  must be made of a flexible material, such as silicon, to be able to expand and contract to accommodate changes in volume due to heating and cooling, as described below. 
     Preferably the heat exchanger housing  30  is approximately 20 inches long and has a capacity of approximately 1.5 gallons, so that each combination of a vacuum tube  20  and heat exchanger housing  30  has a combined capacity of approximately 5 gallons. This will provide the ability to accommodate a change in volume of approximately 1 pint for the 5 gallons. Preferably the heat exchanger housing  30  is cylindrical and made from extruded silicone, not press molded, in order to provide easier collapsing or puckering when the volume of solar heating fluid cools down and contracts. The heat exchanger tubing  32  is preferably made of copper and is bent into two or three loops, with two preferred, to reduce costs. 
     Preferably each heat exchanger housing  30  contains approximately 10 feet of heat exchanger tubing  32 , holding about 0.1 gallon. The ends of the heat exchanger tubing  36  extend through the heat exchanger ports  92 , as shown in  FIG. 6 . 
     Referring to  FIG. 5 , shown is a side cutaway view of a presently preferred embodiment of the check valve  100  of the present invention. As can be seen, a copper pipe nipple  104  is brazed to and extends outwardly through the check valve port  94  in the upper portion  102  of the adapter end plate  90  shown in FIG.  4 . Although not shown, a similar copper pipe nipple is brazed to and extends outwardly through the fill port  96 , and the ends of the heat exchanger tubing  36  are also brazed to and extend through the heat exchanger ports  92  by approximately the same distance as the pipe nipples. Preferably, a silicone insert  106  is placed inside the nipple  104  to retain a check valve  110 , which has a barb end  112 . As can be seen, the barb end  112  is inserted and retained in the silicone insert  106 . Inside the check valve  110 , a chamber  114  contains an O ring at the bottom, and contains a stainless steel spring  116  that biases a stainless steel ball  118  against the O ring  119  to seal off the check valve  110 . If pressure inside the vacuum tube  20  and heat exchanger housing  30  overcomes the bias of the stainless steel spring  116 , then the contents of the heat exchanger housing (air, the solar heating fluid (preferably water) and gas from the solar heating fluid (preferably water vapor)) are vented through the purge outlet  118 . Preferably the stainless steel spring  116  and stainless steel ball  118  are selected so that the check valve  110  releases air, water and water vapor through the purge outlet  118  when the interior pressure in the vacuum tube  20  and the heat exchanger housing  30  is greater than approximately 1 pound per square inch above ambient pressure, without admitting air into the vacuum tube  20  or the heat exchanger housing  30  when the interior pressure is equal to or less than the ambient pressure due to volume reduction of the solar heating fluid, as described below. 
     Referring to  FIG. 6 , shown is a side elevational view of the vacuum tube  20 , the heat exchanger housing  30 , heat exchanger tubing  32 , and check valve  100 , in assembled configuration, showing the outer transparent tube  22 , the inner opaque tube  24 , the evacuated space  26  between those tubes to form a vacuum, the heat exchanger housing  30 , the heat exchanger tubing  32  (illustrated with only two loops in this embodiment), the heat exchanger ports  36 , and the adapter end cap  90 , showing the heat exchanger ports  92 , check valve port  94  and optional fill port  96 . The adapter end cap  90  is retained inside the heat exchanger housing  30  by a stainless steel clamp  97 . As can be seen, the heat exchanger housing  30  overlaps the upper portion  22  of the vacuum tube  20  and is then clamped by a stainless steel clamp  120 . The inner tube  24  is joined to the outer tube  22  at a lip  28 . Preferably the inner diameter of the inner opaque tube  24  is approximately 3.5 inches and the outer diameter of the outer transparent tube is approximately 4.75 inches. 
     Referring to  FIG. 7 , shown is a top end view of the heat exchanger housings  30  on the top ends of the series of vacuum tubes  20  (not shown). As can be seen, the ends  36  of the heat exchanger tubing  32  (not shown) extending from the heat exchanger ports  92  are connected in series, so that water circulates through all of the heat exchanger tubing of each heat exchanger housing  30 . 
     Referring to  FIG. 8 , shown is a perspective assembled view of the presently preferred embodiment of the present invention, which shows more clearly how the heat exchange tubing  32  of each of the heat exchange housings  30  is connected in series, and how each heat exchange housing  30  is clamped onto each vacuum tube  20  with a stainless steel clamp  120 , and how the adapter end plates  90  are retained inside the top end of the heat exchanger housings  30  by stainless steel clamps  97 . 
     Operation of the tankless water heater of the present invention will now be explained. After assembly of all the components of the embodiment 10 shown in  FIG. 1 , each of the inner tubes  24  and heat exchange housings  30  is completely filled with a heating fluid, preferably water, optionally through fill ports  96 . The tankless solar water heater is then placed in the sun at an appropriate orientation for maximum solar exposure, and with the check valves  100  at the highest points. The pressurized domestic water supply is then connected to the ends of the series-connected heat exchanger tubing  32 , so that the domestic water circulates through the heat exchanger tubing  32  under the normal domestic pressure. Thus, no pump is necessary, so that the system is not vulnerable to shutdown because of a pump failure. 
     The radiant energy of the sun will heat the opaque inner tubes  24 , which will heat the solar heating fluid inside, up to a maximum of approximately 180 degrees Fahrenheit. Preferably, an optional anti-scalding valve is provided to prevent water of this maximum temperature from entering the shower, bath, sink or other fixture. The transparent outer tubes  22  form a vacuum  26  around the opaque inner tubes  24 , so that the solar heating fluid is insulated against heat loss, much like a Thermos bottle. Because heat rises, the heated solar heating fluid will rise to the top of the vacuum tubes  20  and into the heat exchanger housings  30 . Initially, this heating will drive out air that has been entrained in the solar heating fluid, which will then create outward pressure on the check valve  100 , which overcomes the urging of the stainless steel spring  116  and stainless steel ball  118  against the O ring  119 . The air will then vent through the purge outlet  118 . The solar heating fluid will also expand as it heats up, and may also generate gas, and this will similarly be vented through the purge outlet. After the solar heating fluid reaches its maximum temperature of about 180 degrees Fahrenheit, it will start to cool down when the sun starts to go down. This cooling will cause the solar heating fluid to contract, which will create negative pressure in the vacuum tube  20  and heat exchange housing  30 . This negative pressure will urge the stainless steel ball  118  against the O ring  119  even more strongly, so that the check valve  100  will close even tighter. Because the heat exchanger housing  30  is flexible, it will contract by puckering inward to accommodate the volume reduction caused by this cooling. 
     After perhaps a few weeks of operation, all entrained air and excess solar heating fluid will be driven out of the vacuum tube  20  and heat exchanger housing  30  by the expansion from the maximum temperature achieved. The check valve  110  will then effectively remain shut indefinitely, and the heat exchange housing  30  should now fill to its maximum capacity only when it again achieves the highest temperature. At this point, the system is completely closed to the atmosphere, except that, in the unlikely event of boiling of the solar heating fluid, the check valve  100  will open. 
     The heat exchanger tubing  32  is preferably connected in series, so that domestic hot water flows through approximately 10 feet of heat exchanger tubing, therefore becoming heated almost instantaneously. 
     This construction allows the elimination of a solar hot water tank because the vacuum tubes  20  and heat exchanger housings  30  are insulated, so they maintain the temperature of the solar heating fluid for much longer than conventional uninsulated solar panels. Heat loss at the maximum temperature of 180 degrees Fahrenheit will occur at a rate of about 2 degrees Fahrenheit per hour. Thus, from the maximum daytime temperature of about 180 degrees Fahrenheit, usage for heating water will cause the solar heating fluid to cool to about 130 degrees Fahrenheit in the evening. This is still a very substantial temperature, because the maximum temperature desired to avoid scalding is about 120 degrees Fahrenheit. 
     At a temperature of about 130 degrees Fahrenheit, the solar heating fluid will cool at about 1 degree Fahrenheit per hour. Thus, even throughout the night, the solar heating fluid will maintain a satisfactory temperature. At this rate, by the time the solar heating fluid cools down below 100 to 120 degrees Fahreneheit, which is quite usable for domestic hot water purposes, the sun will rise and warm the solar heating fluid again. Indeed, the US Consumer Products Safety Commission&#39;s Document 5098 entitled “Tap Water Scalds” recommends that hot water heaters be set to a maximum temperature of 120 degrees Fahrenheit, but points out that a five minute exposure to water at this temperature could result in third degree burns. 
     Without a flexible, high temperature housing and check valve, it would be necessary to incorporate external expansion tanks, float valves and pressure relief valves for operation. In areas where the temperature can fall below freezing, these exterior components could freeze or suffer freeze damage. 
     Although a single set of 8 vacuum tubes can be used, it is preferred that 2 or 3 sets of 8 tubes each be used in households with 2 or more people. 
     This construction is advantageous for servicing, because the components are all individually replaceable. For example, if a vacuum tube  20  breaks, if a heat exchanger housing  30  fails, or if heat exchanger tubing  32  leaks, each can be quickly removed and replaced. The adapter end cap  90  and check valve  100  are removable as well. 
     The construction is low profile and provides very good weight distribution for support on roof structures. 
     This invention is preferably retrofittable to existing conventional non-solar hot water heaters, using their existing smaller capacity tanks for additional solar hot water storage, and their electrical or gas heaters as backup heaters for prolonged cloudy periods. In this arrangement, solar heated water from the invention would flow into the existing tank and would be usable directly. Because the water heats almost instantaneously in the series-connected heat exchanger tubing  32 , as described above, the invention can continue to heat water as long as the working fluid (preferably water) in the vacuum tubes  20  and heat exchanger housings  30  remains hot enough. As explained above, the fluid in the vacuum tubes  20  and heat exchanger housings  30  remains hot enough overnight, until the sun heats them again. If, however, there is a prolonged period of cloudy weather, then the existing heater can warm the water in the tank. 
     Further, this invention can supply solar preheated water into the tank of an existing conventional non-solar hot water heater, which can dilute the preheated water&#39;s temperature to reduce the chance of scalding, and also act as a backup in case there are prolonged cloudy periods that prevent adequate solar heating. It is preferred that the aggregate capacity of all the vacuum tubes  20  and heat exchanger housings  30  be approximately twice as much as the capacity of the conventional heater&#39;s tank. 
     While the present invention has been disclosed in connection with the presently preferred best mode described herein, it should be understood that the best mode includes words of description and illustration, rather than words of limitation. There may be other embodiments which fall within this spirit and scope of the invention as defined by the claims. Accordingly, no limitations are to be implied or inferred in this invention except as specifically and as explicitly set forth in the claims. 
     INDUSTRIAL APPLICABILITY 
     This invention is applicable whenever it is desired to provide solar heating of water without using a solar hot water tank.