20050608
This is the first Agro Best blog entry and there will be an ongoing use of this space to create a record of this pioneering project in Surinam.
Today I (Sola Roof Guy) have sent an email to Steve and Tuisco as below:
Richard Nelson [solaroof@yahoo.com] wrote:
Steve's questions in Bold
my answers immediately following
Hi Steve, Tuisco,
This is just a quick reply to assure you that I will be able to respond in detail to your very good outline of key questions below. I was out all day and evening today in meetings with very favorable results; tomorrow also most of the day. The good news is that next week I will start to work with a very able team at Nottingham University and we are building a structure with the bubbles, water cooling and under-pressure chilling systems. This will be helpful for additional input into the design issues for your project. Do not worry about the inlet and outlet - these are very small panels that are created at the end walls. An exhaust panel at the blower end where the bubble generator would have been and an inlet at the other end that is designed to let air in after the under pressure reaches a specified value.
In general your drawing that shows the placement of the sprinkler nozzles looks good. There might be some issues concerning the flatness of the roof and the shape that the inner film will take because of the low pressure in the cavity space. This might create some dripping problems but this is not so serious. If we get lots of condensation then we should be very pleased because this shows that the skin is staying cool relative to the inside air temperature.
How cool or chill we can make the roof (inner skin) depends on what temperature the droplet attain and the cooling film. The large surface area of the liquid film and the droplets also gives a great chilling potential. Air in the cavity space will absorb evaporation and then it is exhausted at a specific rate. This can be calculated. Our Nottingham team can handle such engineering. Outside air coming into the cavity transitions from the outside ambient pressure and humidity to a low pressure as it flows into the cavity space. In addition there is a dynamic pressure drop which results from flowing air. The flowing air has a lower pressure than static air.
This means that the air in the cavity will have a reduced density and can absorb more vapor. It will become saturated before exhausting it from the roof cavity. Thus it first has a decrease in RH and then absorbs evaporation to reach 100% RH. This is the chilling power of the system.
Thus keeping airflow in the greenhouse space will also result in a decrease in RH. How can de optimal air flow is determined to decrease the RH for any given space? Other than the relational variables: Temp, RH, and CFM are there other variables to account for.
There is no question that air flow will result in more evaporation or transpiration. It is common sense to know that even if the air has the same temperature that it will cool more by evaporation by moving. This is the main effect of fans. The leaf canopy will transpire and transfer more moisture to the air if it is moving around. If we can get the cooling that we need from the chilling in the roof cavity, then it is best to not ventilate but to have CO 2 enriched, closed atmosphere in the greenhouse. But it will improve the situation by having some air flow around the plants rather then having still air. So circulation fans that create currents of air flow within the greenhouse would be good. This can also keep the CO 2 mixed and distributed, using a "jet fan" and ducting with holes along its length for air mixing and air motion in the leaf canopy.
Ventilation with outside air coming into the greenhouse and distributed for CO 2 from the ambient atmosphere is a necessary backup system. An air change of the greenhouse per ten minutes should be possible. The enriched atmosphere will produce much higher growth and yield - typically doubling the productivity.
Plants cannot grow well at 90% to 100% RH - but there are no plants in the cavity space. Perhaps algae - but algae are okay in a saturated environment. You say the conventional evaporative cooling exhausts air at 80 to 90% RH but than you also say that the inlet air is already at 80 to 90% so effectively there is no cooling possible. But I am proposing that the air going into the cavity will decrease from the outside RH to a lower value of 60 to 70% and then will absorb all the way to saturation at 100% thus that can provide much more chilling action.
Okay, I can understand that. My question is can you put a number to the word chilling. Say for example a 60% RH results in a 20 C chilling?
Steve, you need to understand that we are exploring this direction because of your decision. I have cautioned that my experience with these designs (remember I did my research in Canada) was only in laboratory situations and experimental development. The water cooling that uses ambient cold water resources works very well. But now we are proposing to operate in a similar mode when we have no cold water resources at hand, and in a hot and humid climate. This is the biggest challenge and this is a real breakthrough if we can demonstrate a system that works in such an extremely challenging climate! As an inventor I work with vision and usually I have been correct. However, these designs are based on "untested" but "reasonable" assumptions.
Also, it might be necessary to operate the chilling process in the roof cavity during the night to produce a reserve of chilled water (this could be the growing water for some floating raft production area) that will help to lower the peak water temperature, if it is too high. It depends on how strong the process is. We will not be using the most advanced process, because that process needs more R&D. In the future the chilling system will be a closed cycle that will generate dry air and then chill, making saturated air, then precipitate and separate the moisture from the flowing air and send it back around the cavity space to chill once more.
But in the present - as a step in the right direction - we use the outside air for chilling and this has a high RH when it comes into the cavity space. This is not ideal but it is still better than the other evaporative cooling mechanisms that could be used. It makes a high humity in the roof cavity - not the growing space - and that is good. It will prevent a "hot roof" (a dry cavity space or space between glazing and shade curtain will get very hot) and so has immediate benefits. It will directly absorb about 20% of the IR solar spectrum and that is a benefit to the plants. It will indirectly cool and dehumidify the interior space and this is better than the conventional evaporative cooling result, which creates a high humidity in the growing space.
The more vapor is lost and rejected by the exhaust blower the greater is the chilling action.
Thus CFM and the vapor rate are the key determining variables (factors) for chilling?
Yes, and the incoming RH of the inlet air is a deduction from the chilling action.
This makes the water film colder than the supply temperature even while the solar heat gain is also rejected.
WOW, this eliminates the vapor rate as determining factor for chilling?
I should be more cautious - even if the cooling liquid system was gaining temperature duing the peak hours - this would be okay if the Liquid Thermal Mass system was designed for this and it would be chilled back to the starting point (morning) temperature in a diurnal cycle. But once the closed cycle chilling is developed I believe that the system could operate to create cool days and warm nights if that is what you want. But the standard would be that a night set back temperature is set as the base operating temperature of the liquid thermal mass system and then a "programable" warm up temperature swing is used as a standard operating pattern.
But at night there is no gain so therefore the cooling is more effective.
OK, got it, so by increasing the CFM during the daytime when heat gain is highest the same cooling rate can be achieved as is the case at night when there is no heat gain. Thus we can keep GH temp constant day and night by monitoring CFM
Yes, the "actual heat rejection" rate is a factor of how much "net" humidiy is rejected per hour and how much solar thermal gain is received. So you would have a thermal balance. Ambient conductive gain and gain from air change are also smaller factors.
But night chilling must be stored for daytime effectiveness. This requires a large liquid thermal mass such as we get with the shallow pond (flotation raft) growing system. So if you have such a growing system or some other liquid thermal mass.
to be continued.....
That is some quick thoughts - but we will get into the details tomorrow evening and Sunday.
Sorry for the delay in getting back to you on this.
Keep on smiling, Rick