Patent Application
20130239951
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.
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'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.
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.
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'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.
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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
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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
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'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'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'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.
This invention is applicable whenever it is desired to provide solar heating of water without using a solar hot water tank.
