The Bubble Regeneration Cycle
Now the bubbles have filled the cavity space where they can remain in place for hours. The high expansion bubbles are very light and will not 'slump' or fall as they grow older, but will retain the same position in the cavity space. According to the soap chemistry the bubbles will have a certain life in a closed cavity space that is first completely full of bubbles. Bubbles that are next to air or pockets of air are affected by this proximity and the properties of the air. Bubbles that last a couple of hours in the cavity space may last only a few minutes in the open air. The bubbles will drain down gradually. The first stage of drainage, within the first 10 minutes changes the weight of the foam quickly, after which stage the bubble walls continue to thin slowly. They may become very difficult to see but still be intact. The initial, fresh bubbles may hide the disk of the sun, like a heavy cloud cover, but the later the disk of the sun would become clearly visible through the bubbles.
'Drier' bubbles have a higher insulation property than wetter bubbles since the water component is a thermal bridge for heat conduction. But more importantly we must be aware of the temperature of the bubble mass. The fresh bubbles have a temperature that is the same as the liquid supply temperature. This is due to the fact that the air component has such little heat/cold capacitance compared to the liquid component. In the dynamic Solaroof concept the bubbles are not only for insulation but are used to control the climate within the structure. The temperature of the building envelope will control the building interior temperature and therefore the control of bubble temperature is the key to this climate control process. Therefore, by controlling the bubbles in the cavity space of the roof and walls of the Tunnel we will control the climate in the crop production space. The control is possible through the renewal of the bubbles as required during the regeneration cycles. This method is very effective for utilizing low-grade solar energy that can be absorbed by the Liquid Solar System.
When we regenerate the bubbles the new bubbles bring low-grade energy into the roof/walls from the liquid storage tank and this energy is released very slowly because of the insulating properties of the bubble filled envelope. Therefore the bubbles are usually regenerated not because they have collapsed, leaving un-insulated voids, but because the bubbles have become too cold (on a winter night) or too hot (on a summer day). Regardless of the scenario for regeneration of the bubbles, the regeneration cycle will have similar characteristics and follow the same operational procedure.
The first requirement is to WAIT a certain time period after filling the roof cavity before starting a regeneration cycle. The very fresh bubbles are too wet and heavy to easily dislodge and therefore the new bubbles entering the flow-path will not move them. However, after the first stage of liquid drain-down from the bubbles, which may take 10 to 30 minutes, the old bubbles can be pushed ahead of the incoming fresh bubbles and the entire channel will be filling with new bubbles. The new bubbles should not channel through the old bubbles but will move them all together towards the intake of the blower where the old bubbles are consumed to make the fresh bubbles. A longer WAIT time is better, as the older the bubbles the more easily removed by the regeneration cycle. Ideally the new bubbles will move out all of the old bubbles across the entire roof slope. All of the old bubbles will move as one mass towards the inlet of the bubble generator as the new bubbles flow into the roof cavity. Ideally, all the roof and wall cavity areas could have old bubbles completely removed and replaced by fresh bubbles that have a controlled temperature. However if a substantial proportion of the building envelope can be controlled by the regeneration cycle then that will provide an adequate climate control function for most applications, including agricultural production.
The operation of the bubble machine is the same for bubble regeneration as it is for the first bubble generation cycle. The ON-CYCLE may require a longer time but the pump advance-ON and delayed-OFF will probably be the same. It is best to actually measure the time required in practice and then set the timer controls for the operation of the equipment. The new bubbles will remain in the cavity space, referred to as the DWELL time, as long as their properties hold up. When they have changed too much, becoming hotter or colder than desirable, another regeneration cycle will take place. The bubbles are best regenerated before they dissipate since if a void becomes established the new bubbles may then channel within the void space. There is a maximum DWELL time for the regeneration cycle when the old bubbles are still filling the cavity completely. This can be determined by trying out various durations for the DWELL time, until the longest time that gives a good result is found. Then a range can be established for the DWELL time that is longer than the WAIT time but not longer then the longest effective DWELL time. Within this range the automated regeneration of the bubbles is effective. The automated regeneration of bubbles is triggered by detecting that the bubbles have become too warm or too cold which could be achieved by sensing the radiant temperature of the roof. The shading system could also trigger the regeneration based on the brightness detected under the bubble filled roof.
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