DIY dehumidifier/air conditioner/heat pump water heater
 

I noted that even during the peak of Texas summer, the cold water is still too cold to shower with, especially in the morning. So why not use a heat pump to dehumidify the house (and provide a little cooling) in order to reduce the load on the main A/C units as well as provide "free" hot water during the summer?

My plan is to start out with a 1/2HP (or so) dehumidifier or window A/C, preferably one that uses R22 since I have quite a bit of ES22A. I'm thinking of replumbing the existing condenser as an additional evaporator in order to increase efficiency. But rather than just connect it in series or parallel with the evaporator, why not use an ejector like the A/C system in the Prius uses? ( 2010 Prius first with ejector-cycle air conditioning ) I'm not exactly sure how to design the ejector, can someone point me to some resources? (I'm planning on using a MIG welding tip for the nozzle unless it's going to be the completely wrong size.)

I'll have to add in a heat exchanger and a pump. I found the following two items:
Grundfos UP15-18B5 Open System Circulator Pump, 1/25 HP, Bronze, 115V, 1/2" Sweat Mount - eComfort.com
B3-12A 40 Plate Heat Exchanger 3/4" Male NPT 7.5"x2.9"
Is that pump going to be powerful enough for this application? It's expensive but so far, it's the cheapest good quality unit I have found. The heat exchanger is on the big side but that should just result in better efficiency. I'm going to build my own variable frequency drive for the pump. (The compressor, however, isn't going to be variable frequency since that doesn't add much benefit for my use.)

I'll use variable speed computer fans for the evaporator. (I could possibly use some fancy Deltas with Cindy Wu sensorless FOC, but that would be overkill so I'll most likely just use some Panasonics I have lying around.) I might also use a pair of radiators or heater cores and a fountain pump for precool/reheat in order to enhance dehumidification, but I would first get it working and add those in only if it doesn't work well enough without them.

And instead of using a TXV, I'm thinking of using a solenoid valve. But rather than the conventional solenoid valves where the pressure difference tries to close the valve (which would result in very unstable flow modulation), the pressure difference tries to open the valve like how some transmission valves work. For example, if the worst case pressure difference is going to be 200PSI, the spring would be sized such that it would begin to open at that pressure. By varying the voltage applied to the coil, the pressure difference that opens the valve (and thus the flow rate) can be varied. (I'm very good at electronics and microcontrollers, so don't worry about the control side of things.) The nozzle in the ejector provides some resistance in order to stabilize things.

Full speed ahead on your project! Sounds great.

My only caution would be that brazed plate HXs are really efficient, but given that you want to do an open loop project, a brazed plate HX will be prone to fouling, over time. This is due to the fact that in closed loop situations, there is a limited amount of 'foulants' (did I just coin a new word?) but in an open source the supply of foulants is unlimited.

Tube-in-tube (or similar) would be the correct choice here. Acquario and also Randen have successfully built their own tube-in-tube HXs... check them out.

-AC

I know the ejector has to be very precise in order to work, and you gain about 10% efficiency average and sometimes better by eliminating most of the throttling losses you get from a TXV or cap tube. I thought of getting one out of a junked Prius for my GSHP. Keep us posted on your success!

If you don't need potable water rated pumps, check out the Grundfos Alpha and Alpha2-self adjusting to maintain a given amount of head regardless of changing circumstances, and about $145 for iron and about $250 for stainless. They take from 5 to 45 watts.

Laing/Bell & Gossett make potable and non-potable ECM pumps-they come in several types that aren't PWM controllable but very efficient from 11-60 watts.

All of these are much more efficient than the usual Taco/B&G induction motor pumps. The Grundfos might already do what you want out of the box (unless the fun is in the designing, of course).

I installed OpenFOAM on my PC to try to simulate the ejector, but there sure is quite a learning curve! (I'm definitely going to ask some friends who have studied chemical or mechanical engineering.)

The pump does have to work with potable water, as does the heat exchanger. I just looked up the ECM pumps and they're not that much more expensive than the conventional ones, probably break even or better after considering the cost of an inverter. What I'm not sure about is whether or not they're easily hackable to allow speed control from a microcontroller. Looks like my best bet is to get one of the solar pumps and then emulate a solar panel.

I'm also not sure exactly what variant of ECM technology they use, since from my experience with computer fans, it makes a pretty big difference. My old Nidec ECM fan, once considered the most advanced fan available, is completely blown away by a new Delta. Nidec's original ECM technology is just a 5 level drive (very easy to implement in an 8 bit microcontroller) and makes a very distinctive buzz even at low speeds. Delta's Cindy Wu sensorless FOC and Nidec's new ECM technology (the two work in a very similar manner, both use 32 bit DSPs) drive sine waves to the motor at low speed and trapezoidal waves at high speed, as well as measure the voltages and currents coming back from the motor in order to get the phase just right.

For your information, I got curious and decided to scope a Delta fan with Cindy Wu sensorless FOC. Top trace is the phase to ground voltage, bottom trace is the neutral to ground voltage. (Unfortunately, I wasn't able to take it apart far enough to see the control logic. The real magic is going to be in the code inside the microcontroller anyways...) Modern ECM motors sure are complex! (As for a Delta fan being Y wired, that's just ironic...)
http://i42.tinypic.com/2ijsx9t.png

Here's a Delta fan someone else took apart, but it's much different from the one I have. Also there are the pictures inside a Sanyo fan and a (newer) Nidec. The interesting part is that the Sanyo has a resolver whereas the Deltas and Nidecs don't.
Pictures of A Dead Delta Fan and It's Internal Parts █ NO 56K-TONS OF PICS █ - Overclockers Forums

Laing D4 (aka mpc650) pcb mod
http://www.honigtopf.mynetcologne.de/D5internal.JPG
It looks like the Laing pumps have some sort of DSP drive. In the detailed picture, the VPP, DAT, and CLK testpoints means that it is most likely a 16 bit dsPIC. Not as powerful as the 32 bit C2000 and ARM processors in the Delta and Nidec fans, but still plenty of power to do good motor control. It looks like it should be pretty easy to tap a wire to control it from a microcontroller, but I'm not sure what kind of control range to expect. (Sensorless drives generally have difficulty at very low speeds.)

Assuming 5200BTU/hr of heat dissipation to the water (stock rating of A/C, would expect more after mods), 70F inlet water, and 140F outlet water (dishwashing mode), it translates into about 0.15 gallon/minute if I did my calculations right. The high pressure variable version of the Laing D5 seems to vary down to about 1 gallon/minute so it looks like the real problem might be the pump not being able to throttle low enough. I can easily workaround that by adding a bypass valve. There will be some efficiency loss but it would be tiny compared to the rest of the machine. I could start out with a bigger compressor but it seems wasteful when I would expect a 5200BTU/hr unit to easily keep up with all of my needs for hot water during the summer.

So are you going to build a batch-type water heater device or an on-demand type device? The intended use eludes me at this point. With (only) 5000 Btu of capacity, it seems you would need either a large HX with a long, somewhat restrictive counterflow water path to sufficiently extract enough heat or some kind of storage tank to use as a buffer.

It's going to use the existing water heater as a storage tank. There will be a wireless sensor for checking the water heater temperature. (Think of it as similar to the Airtap, but located distant from the water heater.)

I'll probably go with the higher powered version of the D5 so there'll be plenty of headroom for highly restrictive plumbing. The fact that the D5 series uses something very similar to the Cindy Wu algorithm (or that might be exactly what it uses) means it should be possible to throttle it down quite a lot. (The Cindy Wu algorithm is tuned for the quirks of small fan motors, but a centrifugal pump is very similar to a fan as far as the torque/speed curves go.)

Be sure and post lots of photos, it's very helpful to anyone wanting to do something similar.

-AC

I would definitely buy, beg, scrap or steal a prius evap assembly for this one. Ejector assemblies of any type are a research scientist's dream. They are a hacker's nightmare. Even firemen and sailors don't like them because they are finicky. And those designs are time tested.

How's this project coming? It shows lots of promise.

-AC