There are places on the earth that ambient temperature is very low during winter (say, -35 C). At that time, inside the supermarket, the temperature is still maintained at a high temperature for human comfort. I am thinking maintaining temperature control for vegetables or fruits in this kind of situation. Does any one know any existing system in the world that directly using cold air outside for food temperature control? or pumping hot fluid (could be liquid CO2) outside and get itself cooled and then send back for temperature control?
I know that conventional refrigeration system does this by intentionally increasing condensing temperature such that the vapor compression refrigeration cycle can still be established. I think it is kind of energy waste.
during winter (say, -35 C), suppose a glycol loop include a pump, outdoor dry cooler, indoor heat exchanger, and some accessories etc. maybe it can directly pick up cooling capacity from outside for maintaining temperature control for vegetables or fruits.
Dr. yin cai posted on 2009-04-13 04:07:31: during winter (say, -35 C), suppose a glycol loop include a pump, outdoor dry cooler, indoor heat exchanger, and some accessories etc. maybe it c ... Jump to Post
it is a good idea. During summer, switching this secondary loop to vapor compression system should be OK.
Now, I am wondering how to do the switching? depending on ambient temperature alone?
by this suppose, there are two heat exchanger in a glycol loop, one is dry cooler which obtain cooling capacity from outside-air to maintaining middle temperature range in winter, and in summer, anther one could be a plate heat exchanger(or other) which is connected in your refrigeration system that gain cooling capacity from CO2.
to wittch on alternatively one could be depending on HP pressure of system or ambient temperature, I think.
it means,the indoor heat exchangers to maintaining middle temperature range of cabinets may be suitable for all seasons.
Dear Degang,
actually there exists already a good solution by secondary cricuit.
I will send you some info about it in the next days by inernal company mail.
Kind regards, Matthias
Dr Degang, you are basically describing a heat pipe operation. If you have a situation where the evaporating temperature and pressureare higher than the possible condensing temperature and pressure, you can use a heat pipe.
For vegetables you may use an evaporating temperature of -5 to -7 to be able to maintain good humidity. At -7 degrees C evaporating temperature the saturation pressue of CO2 is about 29 bara. If the vapour at this pressure is allowed to enter an outside aircooled cooler with an air temperature of -35 deg C, one could condense the - 7 deg CO2 vapour easily at say - 12 degrees C at a saturation pressue of about 25 bara, which is 4 bar lower than the 29 bara saturated vapour pressure at an evaporarting temperature of - 7 degrees C. Then one does not need a compressor. All one needs is a liquid CO2 pump to pump liquid CO2 at a temperature of -12 degrees C back to the evaporator.
The air cooled condenser can be the gas cooler of a 2 stage transcritical CO2 system. The change over back to compression can be automatic from the ambient temperature or as the temperature of the vegetable case stars to rise. There are not many places where the temperature is consistently low at - 35 degrees C, but I estimate that the heat pipe operation sketched above would be workable in an ambient temprature as high as -15 degrees C. The attraction would be that some system components would be components in a 2 stage transcritical system.
There may also be other considerations such as CO2 heat pumping for store heating. Finding an optimum solution for all seasonal requirements using a two stage transcritical CO2 system would be an interesting applied engineering exercise.
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