The ideal scenario is if we had a system that could maintain a room (lets say at least 20 cubic meters capacity to anywhere between 0-20oC (depending on what is to be stored) constantly.
However neither of the systems we have investigated so far are able to meet these specifications.
Simple evaporation of water (as used in the original Desert Fridge ) can somewhat extend the life of some products (such as tomatoes because they happen to have an optimum storage temperature of 17-21oC) but it is limited to the wet bulb temperature so will never be able to store most other vegetables for any significant length of time. Also it requires a hot AND dry climate to operate which is fine in some places at certain time for example in Gambia it works well in the dry season but not the wet season.
The solar ammonia absorption ice maker could get the temperature down to freezing (or even below i think) which would be enough to store anything but the cooling cycle would only operate at night. Even if we used the suggested modification to allow continuous cooling this would only be “continuous” during the day (since the heat cycle would not run at night) and again this would require a condusive climate for evaporation of water which can not be relied on.
Einstein’s fridge could work continuously and deliver the required cooling but required burning gas a a heat source which would be too expensive to run in the long term. If we used solar power as the heat source it would not be continuous and reliable.
Using a thermal storage fluid could help by storing surplus heat, by storing the thermal fluid in an insulated container, and returning it, by pumping it to the generator when required.
Also we must consider the use of a fan (run on a car battery) inside a room of this capacity to increase the rate of heat transfer from the immediately cooled (by conduction) section to the remaining air (by convection). Now the speed of this fan could also be controlled to facilitate continuous constant cooling - when the sun is delivering surplus power, more/cooler ice (or whatever coolant) is produced so the fan could run on lower power and visa-versa.
Another way of tackling the problem of the unreliability of solar power is to use kerosene or any other locally available cooking gas as a backup heat source.
Maybe the best way to proceed is to design a hybrid system combining the best of all these ideas. For example lets consider something like this....
This would require an electronic control system (again run on the car battery) would be required to control any combination of (whichever works out best) the gas burner, the fan and the thermal fluid, facilitating the primary solar power, to maintain the temperature in the cold store constantly at the required temperature 24/7! Some may say it defeats the object of the original simple Desert Fridge, but im sure with a bit (actually a lot) of clever engineering we could adapt this process to make it feasible for application in developing countries.
However neither of the systems we have investigated so far are able to meet these specifications.
Simple evaporation of water (as used in the original Desert Fridge ) can somewhat extend the life of some products (such as tomatoes because they happen to have an optimum storage temperature of 17-21oC) but it is limited to the wet bulb temperature so will never be able to store most other vegetables for any significant length of time. Also it requires a hot AND dry climate to operate which is fine in some places at certain time for example in Gambia it works well in the dry season but not the wet season.
The solar ammonia absorption ice maker could get the temperature down to freezing (or even below i think) which would be enough to store anything but the cooling cycle would only operate at night. Even if we used the suggested modification to allow continuous cooling this would only be “continuous” during the day (since the heat cycle would not run at night) and again this would require a condusive climate for evaporation of water which can not be relied on.
Einstein’s fridge could work continuously and deliver the required cooling but required burning gas a a heat source which would be too expensive to run in the long term. If we used solar power as the heat source it would not be continuous and reliable.
Using a thermal storage fluid could help by storing surplus heat, by storing the thermal fluid in an insulated container, and returning it, by pumping it to the generator when required.
Also we must consider the use of a fan (run on a car battery) inside a room of this capacity to increase the rate of heat transfer from the immediately cooled (by conduction) section to the remaining air (by convection). Now the speed of this fan could also be controlled to facilitate continuous constant cooling - when the sun is delivering surplus power, more/cooler ice (or whatever coolant) is produced so the fan could run on lower power and visa-versa.
Another way of tackling the problem of the unreliability of solar power is to use kerosene or any other locally available cooking gas as a backup heat source.
Maybe the best way to proceed is to design a hybrid system combining the best of all these ideas. For example lets consider something like this....
This would require an electronic control system (again run on the car battery) would be required to control any combination of (whichever works out best) the gas burner, the fan and the thermal fluid, facilitating the primary solar power, to maintain the temperature in the cold store constantly at the required temperature 24/7! Some may say it defeats the object of the original simple Desert Fridge, but im sure with a bit (actually a lot) of clever engineering we could adapt this process to make it feasible for application in developing countries.
We could even use firewood or cow dung as a back up fuel.
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