GB2537145A - Double wicking solar heater - Google Patents

Abstract

A solar heat pipe that contains a double wick system that circulates the working fluid of a solar heat array; the double wick system comprises a primary wick 2 that encloses a secondary wick 3 that has a larger pore size to the primary wick. In use the primary wick uses capillary action to draw working fluid from the secondary wick 3 that is subsequently evaporated by the suns heat into a vapour. The vapour is allowed to move towards a heat exchanger via channels 1 in the outer surface of the primary wick 2, to a heat exchanger and upon condensing the working fluid transfers to the secondary wick 3 under gravity, which then transfers to the working fluid to the primary wick to continue the cycle. Preferably an adiabatic connector joins the heat pipe to the heat exchange, and maintains the separation of the vapour and condensed working fluid. Preferably the double wick system sits in a heat pipe that is a glass solar tube. The wicking material may be made from any suitable material or combination of materials such as sintered metals or ceramics; natural or synthetic fibres; granular or foam materials and the like.

Description

Double Wicking Solar Heat Tube Description

This invention relates to an improvement in thermal transfer with a solar heat array.

A heat pipe is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces.

At the hot interface of a heat pipe a liquid in contact with a thermally conductive solid surface turns into a vapor by absorbing heat from that surface. The vapor then travels along the heat pipe to the cold interface and condenses back into a liquid -releasing the latent heat. The liquid then returns to the hot interface through either capillary action, centrifugal force, or gravity, and the cycle repeats. Due to the very high heat transfer coefficients for boiling and condensation, heat pipes are highly effective thermal conductors.

Most heat pipes use a wick and capillary action to return the liquid from the condenser to the evaporator. The liquid is sucked up to the evaporator. similar to the way that a sponge sucks up water when an edge is placed in contact with a water pool. The wick allows the heat pipe to operate in any orientation, but the maximum adverse elevation (evaporator over condenser) is relatively small, on the order of 25 cm long for a typical water heat pipe.

Taller heat pipes must be gravity aided. When the evaporator is located below the condenser, the liquid can drain back by gravity instead of requiring a wick. Such a gravity aided heat pipe is known as a thermosyphon and has been widely adopted in solar heat collecting tubes In a thermosyphon, liquid working fluid is vaporized by a heat supplied to the evaporator at the bottom of the heat pipe. The vapor travels to the condenser at the top of the heat pipe, where it condenses. The liquid then drains back to the bottom of the heat pipe by gravity, and the cycle repeats.

The general principle of heat pipes using gravity_ commonly classified as two phase thermosiphons, dates back to the steam age and Angier March Perkins and his son Loftus Perkins and the "Perkins Tube", which saw widespread use in locomotive boilers and working ovens. Capillary-based heat pipes were first suggested by R.S. Gaugler of General Motors in 1942, who patented the idea, but did not develop it further.

George Grover independently developed capillary-based heat pipes at Los Alamos National Laboratory in 1963, with his patent of that year being the first to use the term "heat pipe", and he is often referred to as "the inventor of the heat pipe". He noted in his notebook: "Such a closed system, requiring no external pumps, may be of particular interest in space reactors in moving heat from the reactor core to a radiating system. In the absence of gravity, the forces must only be such as to overcome the capillary and the drag of the returning vapor through its channels." Grover's suggestion was taken up by NASA, which played a large role in heat pipe development in the 1960s, particularly regarding applications and reliability in space flight. This was understandable given the low weight, high heat flux, and zero power draw of heat pipes -and that they would not he adversely affected by operating in a zero gravity environment.

Heat pipes are now widely used in solar thermal water heating applications in combination with evacuated tube solar collector arrays. In these applications, distilled water is commonly used as the heat transfer fluid inside a sealed length of copper tubing that is located within an evacuated glass tube and oriented towards the sun. In connecting pipes, the heat transport occurs in the liquid steam phase because the thermal transfer medium is converted into steam in a large section of the collecting pipeline.

In solar thermal water heating applications, an individual absorber tube of an evacuated tube collector is up to 40% more efficient compared to more traditional "flat plate" solar water collectors. This is largely due to the vacuum that exists within the tube, which slows down convective and conductive heat loss. Relative efficiencies of the evacuated tube system are reduced however, when compared to flat plate collectors because the latter have a larger aperture size and can absorb more solar energy per unit area. This means that while an individual evacuated tube has better insulation (lower conductive and convective losses) due to the vacuum created inside the tube, an array of tubes found in a completed solar assembly absorbs less energy per unit area due to there being less absorber surface area pointed toward the sun because of the rounded design of an evacuated tube collector. Therefore, real world efficiencies of both designs are about the same.

Evacuated tube collectors reduce the need for anti-freeze additives since the vacuum helps slow heat loss. However, under prolonged exposure to freezing temperatures the heat transfer fluid can still freeze and precautions must be taken to ensure that the freezing liquid does not damage the evacuated tube when designing systems for such environments.

The heat pipes within solar arrays operate with a counter current flow which limits the distance and amount of power a heat pipe can transfer. Additionally, the pipe operates most effectively when incorporating a wicking system that adds to complexity and cost of the overall system. It is to overcome these limitations that the invention is aimed at.

A loop heat pipe is a passive two-phase transfer device related to the heat pipe. It can carry higher power over longer distances by having co-current liquid and vapor flow, in contrast to the counter-current flow in a heat pipe. This allows the wick in a loop heat pipe to he required only in the evaporator and compensation chamber.

The general structure of a loop heat pipe consists of an evaporator, a condenser, a compensation chamber, and vapor and liquid transport lines. Only the evaporator and the compensation contain wicks; the rest. of the loop is made of smooth wall tubing.

The wick in the evaporator is made with fine pores for purpose of developing a capillary pressure to circulate fluid around the loop, while the wick in the compensation chamber is made with larger pores for purpose of managing fluid ingress and egress. The operating principle of the LHP is as follows. As heat is applied to the evaporator, liquid is vaporized and the menisci formed at the liquid/vapor interface in the evaporator wick develop capillary forces to push the vapor through the vapor line to the condenser. Vapor condenses in the condenser and the capillary forces continue to push liquid hack to the evaporator. The waste heat from the heat source provides the driving force for the circulation of the working fluid and no external pumping power is required. The two-phase compensation chamber stores excess liquid and controls the operating temperature of the loop.

The compensation chamber is located close to the evaporator. In fact, the compensation chamber is usually made as an integral part of the evaporator, and a secondary wick is used to connect the two elements. Liquid returning from the condenser always flows through the compensation chamber before it reaches the evaporator. The secondary wick provides a liquid link between the compensation chamber and the evaporator so that the evaporator will always he replenished with liquid. There are two major advantages of such a design. First, the loop can be started by directly applying power to the evaporator without the need of preconditioning. Second, the evaporator is tolerant of vapor bubbles in its liquid core. Because the primary wick is made of metal powder with a high thermal conductivity, liquid evaporation usually takes place inside the evaporator core and vapor bubbles are present there in most operation. To prevent vapor bubbles from accumulating inside the evaporator core, the secondary wick design incorporates vapor arteries which allow vapor bubbles to vent to the compensation chamber. Regardless whether or not vapor bubbles are present, the evaporator core can be considered as an extension of the compensation chamber, and both have the same absolute pressure during steady operation.

The loop heat pipe (LHP) was invented in Russia in the early 1980's. It is a two-phase heat transfer device that utilizes the evaporation and condensation of a working fluid to transfer heat, and the capillary forces developed in fine porous wicks to circulate the fluid. The LHP is known for its high pumping capability and robust operation because it uses finepored metal wicks and the integral evaporator/hydroaccumulator design. It has gained rapid acceptance in recent years as a thermal control device in space applications. However, despite the advantages of the closed loop system over the simple thermosiphon type heat pipe for solar applications the relative complexity of construction has prevented its uptake. The present invention takes the principles of the closed loop system and applies it to the solar array

Detailed Description of the Invention Drawings

Fig 1 -radial cross section of double wick Fig 2 -tangential cross section of double wick Fig 3 -double wick in commercial heat tube Fig 4 -cross section of adiabatic connector Fig 5 -cross section of the adiabatic connector across the line AB in fig 4 Fig 6 -Schematic of operation Detailed description of drawings FIGURE 1 shows a radial cross section of the double wick structure where 2 is the primary wick having a smaller pore size than the secondary wick 3 and incised by vapor channels 1 tangential to the wick structure FIGURE 2 -shows a tangential cross section of the wick structure with the primary wick 2 encasing secondary wick 3 with a communications port 5 left open at the top of the structure for connection to the rest of the device and being scored with vapor channels 4 that are radial to the wick structure FIGURE 3 -shows a tangential cross section of the double wick structure 7 inserted into a commercial available solar heat tube 6 FIGURE 4 -shows cross section of adiabatic connector which is made up of three zones. A straight zone 10 connects with the heat exchange head with a seal 9 and with a conical zone 11 the bottom of which is perforated by vapour passage holes 8 and connected to another straight zone 12 which has the same diameter as the secondary wick 3 and is long enough to pass through the communications port 5 and directly connect with the secondary wick 3 when the structure is assembled. The connector is sealed against the commercial tube 6 by a second seal 13 around the conical zone 11. The straight zone 12 is filled with a wick material 14.

FIGURE 5 -Shows across section of the adiabatic connector across the line AB in fig 4 to indicate the position of the vapour transport holes 8 in relation to the rest of the connector.

FIGURE 6 -Shows a schematic of operation which will be discussed below.

The invention breaks down into four major components that incorporate the operating principles of the closed loop heat pipe within a solar heat array.

Part 1 -collector tube The collector tube is filled with a double wicking system divided into a primary and secondary wick. The secondary wick is enclosed by the primary wick with the exception of the top end which is left open to communicate with the liquid return route. The secondary wick has a larger pore size than the primary wick allowing for free flow of the operating fluid. The primary wick has a finer pore structure and is grooved both radially and tangentially to allow for the free flow of vapor. The wick structure is shown in figure 1 and fits tightly against the side walls of the commercial solar tube. Almost any suitable material can be used for the wick material including cotton, carbon fibre, reticulated carbon foams, packed granular materials like frit or suitable synthetic foams for example a phenolic foam which is a preferred example as it has high absorhacy, rigid structure and is easily workable. The foam is formed into a bar of suitable diameter for the commercial tube with a central bore hollowed out, grooved racially and tangentially and filled with the secondary wick of, for example, fine glass hit which pores easily and forms a tightly packed inert wick structure. The double wick is then inserted into the commercial solar tube.

Part 2 -Adiabatic connector The adiabatic connector performs many of the functions associated with the closed loop system. Primarily it connects the solar tube, double wick and heat exchange head together and seals the unit. It forms an adiabatic zone and allows for communication between the heat head and solar tube but separates the liquid and vapour flow making the system a double loop. The connector shown is a short stem connector but there is no real limit on the length of the adiabatic connector and is only shown short as a means of constructing a compact system for installation in a domestic location.

Part 3 -Heat exchange head The heat exchange head hasn't been shown as it consists of quite simply a one end closed metal tube, preferably copper but any suitable material, that may or may not have heat exchanger fins attached and may or may not be immersed in a secondary fluid, like water or oil for transfer of heat from the head into a useful form. The head performs the function of condensor and compensation chamber.

Part 4 -Working fluid A wide variety of working fluids are possible though water is the simplest and cheapest. The system operates at a slight over pressure and operating temperature range can he adjusted by adjusting pressure or working fluid or co fluids Operation The system operates when the sun striking the surface of the collector tube raises the internat tube temperature. This causes the liquid on the surface of the primary wick to evaporate, pass through the radial and tangential channels to the vapor communication ports of the adiabatic connector. As the working fluid evaporates from the surface of the primary wick the capilliary action of the wick draws more fluid from the cetral secondary wick through the primary wick to he heated.

The vapour passes through the communications ports of the adiabatic connector into the heat exchange head where it condenses and gives up its heat to the surrounding metal walls of the heat exchange head which is the transferred to the secondary fluid via fins or any other suitable structure The fluid then returns by gravity to the secondary wick where the cycle begins again.

The above embodiment is meant only to describe the operation of the system and other form factors should be readily apparent to those skilled in the art, for example a flat panel structure, specifically made encasements or non commercial tubes, nor should the choice of working fluid e seen as limiting as many fluids will perform the task for example, methanol, ethanol. Glycols, ammonia,etc