Interesting compressor for solar refrigeration and mini A/C
Thermaltake Xpressar - World's First DC Inverter Type Micro Refrigeration Cooling System
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sure it only draws 50 watts... but for that energy use how much does it cool? if I remember correctly my fridge only draws around 150-180 watts while it is running.
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A CPU used in such a system dissipates a lot of heat. Maybe about 100-150w overclocked.
I think it would work very well for a mini solar A/C intended to be used on the days when even a powerful fan isn't quite enough. |
I've seen mini-fridges with a 55W compressor, but they were AC. I don't know about the ones designed for automotive/RV use, they may be DC powered, or DC-inverter-AC (much less efficient.
A DC powered compressor is harder to find, but it does have its good sides. A few years back I remember reading in a solar magazine about a DC powered heat pump, designed for solar systems. If I find it, I'll update this post. UPDATE: I found it in Solar Today, Jul/Aug 2001, p.51-52. The heat pump was SUNStation by Sun Utility Network, Inc., L.A. Cal. It appears that the company no longer exists, since googling "SUNStation" doesn't produce any results, and their page www.sunutility.com can't be found. Googling for DC heat pumps shows that there are 12/24V A/c and split models for truck drivers. May be enough to help heat/cool one room. |
DC compressors are often called inverter drive compressors. In general, about the only difference from a regular compressor is that the stator is 3 phase since that is optimal for control purposes. (There are exceptions, such as the "Tesla Turbocompressor" prototype my friend is working on, which has a switched reluctance motor that runs as fast as 75kRPMs! It uses distilled water as a refrigerant due to the high flow rates and low operating pressures.) But in theory, it is possible to operate a regular compressor motor as a 2 phase motor.
Direct-to-line motors are difficult to optimize for efficiency under varying load conditions since the torque angle can never be optimized over a range of load torques. Inverter drive motors are therefore often more efficient. Then reductions in delta T in HVAC systems at partial capacity increase efficiency even further. In fact, it is possible to increase the efficiency of an A/C by downsizing the compressor, without changing the technology used. How to Add or charge Freon or Refrigerant to your air conditioning equipment Quote:
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Mike, so are you saying that adding a larger heat exchanger (on one or both sides?) increases the efficiency of a heat pump?
I read that DC is better than AC for ventilation fans because it's easier to control speed, plus it almost eliminates noise (no humming or vibrations). I believe it was also slightly more efficient, but I'm not sure about that one. I guess that pumping air in a duct is not the same as compressing refrigerant, so just replacing the AC part with DC won't work? I think the varying load problem is solved in new units by plumbing 2-3 smaller compressors in parallel, each one being turned on only when needed. This, and using a spiral compressor instead of the older piston style, supposedly greatly increases efficiency. |
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I have a computer fan with an ECM (electronically commutated, same as inverter drive) motor. It is much more efficient than an AC desk fan of roughly the same size. Quote:
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HGT1S12N60A4S9A Fairchild Semiconductor IGBT Transistors |
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Changing fan speed will change the rate of heat exchange, which will reduce efficiency of compressor-to-heat-exchanger-area combination. So, ideally when changing fan speed, you also change compressor rate or surface area (to maximize efficiency of meeting the load.) However, there are other factors to system design. Humidity control is important to comfort and health. The indoor coil can provide dehumidification to keep levels below a maximum. Rate of dehumidification depends on coil area, temperature, and air flow. Air speed at ducts affects temperature stratification and air distribution noise levels. Typically, your ductwork is sized to provide a certain exit velocity for a specific flow rate. Changing the fan speed throws off duct design, unless your ductwork contains active components to maintain exit velocities (very unlikely.) So ideally the fan speed is constant, and compressor rate and surface area are variable. I'm not familiar with variable surface area systems, but I would guess a few rows of coils with electronic valves could work. The compressor rate and surface area change to meet the load at maximum efficiency, and to dehumidify at a desired rate. I suppose you could say, to meet the sensible and latent loads appropriately.. but I find it intuitive to clearly distinguish between latent load and total load. |
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As such, the fan speed could be reduced with lower delta T to even further improve efficiency. Certainly there are algorithms to optimize all of these factors together to maximize efficiency while meeting the design requirements. It just requires all the components to work together, flexibly, and be designed properly. |
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My friend is not planning for that feature in her design, since the areas where hybrid A/C works the most efficiently are the areas that are naturally low in humidity. |
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