Electronics > Power/Renewable Energy/EV's
[Solar air conditioning, ice storage] Sources for DC refrigeration compressors?
Photon939:
I would like to install solar panels on my house at some point but I want to avoid the expense and bad deals for grid-tied solar power systems.
I work with HVAC equipment regularly for my job and have experience with custom refrigeration setups, brazing and refrigerant handling. I have found small compressors available for the camper/RV market but they aren't particularly cheap considering their small cooling capacity and poor efficiency.
Example: http://www.ebay.com/itm/Rotary-DC-Tiny-Compressor-for-for-Refrigerator-Air-Conditioner-R134A-150-300W-/181433530201
They are ideal as they directly run on DC (brushless motor) with speed control (capacity modulation based on available solar power)
The next step up is to use a regular variable frequency drive for AC induction motors with a normal compressor. but most AC compressors are not designed to be speed or capacity controlled and may not lubricate properly running at lower speeds. I would also need to keep the input voltages happy to the VFD and limit power output based on available sun.
Ideally I could use peak sun to generate ice to provide cooling at night. This technique is used in some large buildings to reduce peak energy demands and get cheaper power. In my case it would be making better use of solar generated power when I'm not likely to be at home anyway.
Someone:
You could probably do better than water as the storage fluid in the tank if you're going to build a double circuit system, and consider reversing the heat pump for heat storage in cooler months (nothing quite like a heated bathroom in winter). Ask around some farmers or agricultural suppliers, variable speed drives are used extensively for water pumping and this moved to solar years ago (you don't see new windmills anymore).
Photon939:
--- Quote from: Someone on March 22, 2016, 12:58:57 am ---You could probably do better than water as the storage fluid in the tank if you're going to build a double circuit system, and consider reversing the heat pump for heat storage in cooler months (nothing quite like a heated bathroom in winter). Ask around some farmers or agricultural suppliers, variable speed drives are used extensively for water pumping and this moved to solar years ago (you don't see new windmills anymore).
--- End quote ---
I have a Teco N3-205-C 5HP VFD that I can try to use for this purpose - I guess I can just keep an eye out for smaller 3 phase compressors on ebay.
I do plan on installing a reversing valve for reverse cycle operation in the winter but the reason for water is that the phase change from water to ice allows for a much greater energy storage capacity for a given volume of water (144 btu/lb) vs just 1 btu/°F/lb for sensible heating/cooling. Refrigeration COP drops with higher temperature delta.
According to the label it's looking for a minimum of 200vac so I would need somewhere between 200-282vdc to keep it happy.
A quick glance at some solar panels it seems most have a VMP around 18v which puts me at about 12 panels to achieve that voltage. Should be fairly doable. I'm more concerned about DC arcing once you get up to those voltages though. Circuit breakers usually can't handle much voltage on DC so I may have to get either special ones or high voltage fuses to protect the system.
Someone:
--- Quote from: Photon939 on March 22, 2016, 01:46:28 am ---I do plan on installing a reversing valve for reverse cycle operation in the winter but the reason for water is that the phase change from water to ice allows for a much greater energy storage capacity for a given volume of water (144 btu/lb) vs just 1 btu/°F/lb for sensible heating/cooling. Refrigeration COP drops with higher temperature delta.
--- End quote ---
Its an interesting optimisation problem, you can get more storage in the same volume by switching to propylene glycol antifreeze as the additional range has greater energy than the enthalpy of fusion of water, so your COP would need to plummet for the water to be able to store more energy. Cost/Environmental restrictions of course favour water :)
It looks like most agricultural applications are using DC pumps and not worrying about the speed they run at, so you can scratch that idea. Sad to say but with wanting to run the compressor at a constant speed (a good thing for COP) you're probably headed for a battery or capacitor storage between the panels and the inverter. Masterflux don't offer much detail on their inverters but that could make a neat 12/24/48V system with a MPPT and a capacitor bank offering a minimum on time of 5 minutes.
Photon939:
--- Quote from: Someone on March 23, 2016, 03:21:34 am ---It looks like most agricultural applications are using DC pumps and not worrying about the speed they run at, so you can scratch that idea. Sad to say but with wanting to run the compressor at a constant speed (a good thing for COP) you're probably headed for a battery or capacitor storage between the panels and the inverter. Masterflux don't offer much detail on their inverters but that could make a neat 12/24/48V system with a MPPT and a capacitor bank offering a minimum on time of 5 minutes.
--- End quote ---
Perhaps I should try cutting open a standard reciprocating compressor and see how fast it needs to run before the oil slinger starts working. I could also possibly use a bank of standard electrolytic capacitors to get a bit of energy storage, then program the MCU to do short bursts of higher speeds to make sure it stays lubricated.
On the refrigeration side I could use some solenoid valves with capillary tubing of different lengths on each for capacity control and keeping head pressure up when the compressor is running at lower speed.
So the refrigerant circuit would look something like this:
--- Code: ---Compressor - Condenser < Solenoid valve 1 - Longer captube > Evaporator (refrigerant to glycol heat exchanger) - Compressor
Solenoid valve 2 - Shorter captube
--- End code ---
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