This invention relates to water circulation apparatus for destratifying a body of water and also to a method of circulating water.
This invention relates particularly to water circulation apparatus for destratifying a large body of stagnant water such as a dam, lake or water storage reservoir. It will therefore be convenient to describe the invention with reference to this example application. However, it is to be clearly understood that the invention is capable of broader application.
A large body of water such as a dam, lake or water storage reservoir is different to the ocean, and rivers in that it is not exposed to the same levels of mixing of the water. As a result, such bodies of water may be prone to stratification and lower regions of water can become anaerobic and/or anoxic. Typically, the problems arise during the summer months when the water near the surface becomes warmer than the lower regions of water, and as a result, the driving force for natural convection for circulation is not present.
As a result, the body of water tends to stratify into an upper warm layer known as the epilimnion and a deep colder layer the hypolimnion. The warm and cold layers are separated by a thermocline which acts as a barrier to normal convection flow. That is, natural convection occurs within each of the epilimnion and hypolimnion but not between the two. The result is a stratified body of water having relatively warm less dense oxygenated water near the surface and cold heavier deoxygenated water near the bottom of water. This problem does not occur during the winter months because the surface region of water becomes cooler than the deeper levels of water due to the cooler air and land temperatures which leads to this cooler water sinking and the deeper water rising. This process of natural convection tends to mix up the water and avoids stratification.
This stratification of water is undesirable because the colder anaerobic, anoxic layer towards the bottom of the body encourages the growth of toxic anaerobic organisms such as blue green algae. Further, it enables metal ions such as Mn, Fe, Ca, P and Cu to dissolve into the water where they act as a health hazard. For example, these metal ions can kill fish and other marine life and also render water unsuitable for use as domestic water. One way of addressing the problem is to treat water from such a stratified body of water prior to consumption to remove all the undesirable elements. However the economics and cost of doing this are clearly disadvantageous. Further, it does not address the underlying problem of avoiding the formation of stratification. It also does not address the damage caused to marine life and recreational swimmers by having harmful agents in the dam or lake in the first place.
One previous attempt to solve this problem involved introducing air bubbles into the body of water through diffusers positioned near the bottom of the reservoir. The idea was that the bubbles would oxygenate the lower levels of water and also encourage mixing and destratification of the body of water. However, the device was not effective and did not enjoy great success in overcoming the thermocline between the cold and warm layers of water. Further, the solution was expensive and unattractive commercially.
A large number of dams and water reservoirs around the country suffer from the problem outlined above and the problem has been widely recognised by municipal authorities and research workers. Clearly therefore any solution to this problem would be regarded as major breakthrough and would be highly advantageous to society generally.
According to one aspect of this invention, there is provided a water circulation apparatus for destratifying a body of water, the apparatus including: ducting having an upper end positioned towards the surface of the body of water and a lower end positioned towards the bottom of the body of water; and
pumping means for pumping water through the ducting.
Thus, in use the apparatus enables water to be pumped across a thermocline so as to enable water from the epilimnion containing dissolved oxygen to enter the hypolimnion. The oxygen reacts with dissolved metal ions precipitating them out and also introduces oxygen into the water and warms the water. The ducting reduces energy losses associated with pumping water between the surface of the body of water and the bottom of body of water.
Typically, the apparatus further includes a support wherein both the pumping means and the ducting are mounted on the support.
The pumping means may be arranged to pump water downwardly through the ducting from the upper end thereof to the lower end. While this direction of water pumping through the ducting is preferred, water may also be pumped upwardly through the ducting.
Advantageously, the pumping means includes an impeller having six to fourteen blades, preferably six to ten blades, each having a free tip and projecting radially outwardly from a hub.
Thus, the impeller in many respects resembles a wind turbine. It has a large diameter and it has relatively few large blades. Naturally, it is beneficial that the blades be as light as possible. In one embodiment, the blades comprise aluminium coated with a carbon fibre coating. Naturally, the blades may also be made of a material such as a composite.
Preferably, the ducting and impeller each have a diameter greater than 2 meters, more preferably a diameter of 2 to 6 meters, most preferably 3 to 5 meters.
By having a diameter greater than 2 meters, a large volume of water can be pumped through the ducting. In addition, the larger diameter has the effect that the energy losses will be smaller than would have been the case through a smaller ducting.
Preferably, the ducting is a draft tube of flexible lightweight material open at both ends and which is capable of contraction in an axial direction.
Typically, the ducting is made of inexpensive sheet plastic, e.g. PVC plastic. This is advantageous as the ducting is large in both diameter and length and by using inexpensive material, the overall cost of the ducting is kept reasonable. Further, the flexible sheeting enables the ducting to be folded into a compact shape which can be easily carried about. Further, it enables the ducting to be easily hoisted out of the water and onto a support. The ability of the ducting to be capable of contraction or telescoping in an axial direction is important as it will be necessary to vary the length of the draft tube for different applications. This is accomplished by simply collapsing the flexible plastic in an axial direction.
Preferably, the draft tube has a substantially circular cylindrical configuration. A particularly preferred embodiment is circular cylindrical with a region which flares open towards the upper end thereof.
While a cylindrical tube naturally provides an efficient shape, other cross sectional shapes, e.g. rectangular, hexagonal and pentagonal are not excluded.
In another embodiment, the draft tube tapers outwardly towards each end thereof, e.g. with an hourglass configuration. Applicant believes that a more pronounced tapering such as with an hourglass configuration flaring towards both ends of the ducting will have the effect of further lowering friction losses through the tube and therefore produce even better results.
Optionally, the impeller may be located proximate to the upper end of the ducting. While the impeller is preferably located towards the upper end of the ducting for obvious reasons of convenience, it is to be appreciated that it may also be located in other positions in the ducting. For example, it may be spaced a distance, e.g. a short distance, below the upper end of the ducting inside the ducting. It may also be positioned about midway along the length of the ducting or even at the bottom of the ducting. Applicant prefers not to use a submersible pump and this can be easily accomplished if the impeller is placed towards the upper end of the tube.
According to another aspect of this invention, there is provided a method of destratifying a large body of water, the method including pumping water from one of an upper region of a body of water towards the surface thereof and a lower region of the body of water towards the bottom thereof, to the other of the upper and lower regions thereby to transport water across a thermocline in the body of water to thereby destratify the body of water, wherein the water is pumped through a draft tube from said one region to said other region.
Preferably, the water is pumped from the upper region to the lower region at a velocity less than 1 meter/second, preferably less than 0.6 meter/second, through the ducting having a diameter greater than 2 meters.