Desert farm, Big pump, Batteries?

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Re: Desert farm, Big pump, Batteries?

Postby Gordon-Loomberah » Tue May 16, 2017 7:37 pm

gfsben wrote:If it were possible to hook it up to bore-water on the condenser, or possibly just a big out door water tank as a passive heat loss device, then it could solve the issues of trying to dump heat into 45 degree desert temperatures in the middle of summer.... or it could just be an added complication. Perhaps a air-cooled chiller is simpler?


Before buying a chiller, you should get find out the COP/EER, as there is a huge range in efficiency between brands. Some only operate around COP =1 as far as I can tell, based on claimed water tank volume cooling ability and energy use for work done.

The nominal 2HP chiller I have can operate at better than COP=3, meaning I need much less energy to chill the ~13500l of water than with some other brands. It wont produce ice though.

One thing you will need to take into consideration is the reduced COP at higher temps. Since you only want to operate directly from PV, you'll be operating when the COP is lowest. Facing a big array to the east so you can start chilling as soon as the sun rises would be beneficial.
Another trick I use is to spray a fine mist of water over the condenser when it's 35C or more, which does improve efficiency.
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Re: Desert farm, Big pump, Batteries?

Postby Warpspeed » Wed May 17, 2017 9:21 am

Why has this not been done/ or done and well documented on the web? It seems quite doable. And now i search a bit deeper, i am finding a number of commercial offerings that are similar.

Keep digging my friend.
Its not that it has never been done, its just that its not a common thing to do off grid.

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Re: Desert farm, Big pump, Batteries?

Postby jaahn » Thu May 18, 2017 7:39 am

Hi Warpspeed :)
Where did you find those "cool" girls :D
gfsen, if you keep looking on the web using normal search engines, then you get a biased view that is set by what is common and what is paid advertising and what is trendy currently. You have to look out side the box to find something better than that.
IMHO librarys were the answer and for this sort of thing tech colleges and town librarys had lots of usefull old books. Sadly they are throwing them out these days. Everone seem to think all useful things may be found on the internet. HMMMMMMM perhaps, perhaps not.
Real innovators look everywhere and use their own brains in different ways ! I worked in a couple of third world countries where the locals existed by using their brains and hands and the natural world around them with almost no formal education. Sometimes overrated by vested interests as are computers. :roll:
Rant over Jaahn
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Re: Desert farm, Big pump, Batteries?

Postby gfsben » Fri May 19, 2017 3:26 pm

Thanks Gordon. I can see the sense in an easterly biased array. I wonder how much east of north and tilt angle would be optimum. I'm sure there is a calculator for that sort of thing, although probably not one that is optimised for cooling. Too many variables. Just needs a lot of experimenting.

I have been thinking about the various ways to build up a system, using a range of off the shelf equipment, but I keep running up against the problem that, is that none of the equipment is quite the right/simplest/most efficient way to achieve the ends.

Consider the efficiency and resolved engineering in a mass produced, brand name, inverter spit system. They can be had for $800. If this this type of production was coupled to eutectic/phase change storage, and the compressor run according to surplus PV output (on or off grid), that would have huge demand management and waste PV energy reduction benefits.

All the liquid chillers are still a bit expensive for my liking. The extra cost starts eating into the case against just running the existing drop-in off batteries, or not even that, just running it during the day like we are now.
I'm trying to think of ways to leverage a simpler, more mass produced heat pump, into cooling a thermal storage tank (sensible or latent heat).
How about just removing the fan off a half decent drop-in like this https://www.gumtree.com.au/s-ad/christmas-hills/fridges-freezers/coolroom-drop-in-refrigeration-unit/1100545617
and submerging the evaporator in an insulated tank of coolant solution, and fill the rest of the volume up with bottles of water or PCM http://pcpaustralia.com.au/pcm-range-products/pc0/?
The fins on the evap. would be superfluous and its not ideally designed for immersion, but it could work ok?
Maybe I could just ask the manufacturer to replace the evaporator/air handler, with simpler, open coil design evaporator.
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Re: Desert farm, Big pump, Batteries?

Postby Warpspeed » Fri May 19, 2017 3:45 pm

All you really need is the condensing unit.
You then get to choose between a sealed unit compressor or a belt driven compressor.
Belt drive opens up a few more possibilities for changing motor size and belt drive ratio which may be an advantage tuning the system once its installed and running.

The evaporator can be a simple as a standard twenty meter coil of copper pipe, submerged in your ice tank, fitted with an appropriate thermostatic expansion valve (purchased as a spare part).
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Re: Desert farm, Big pump, Batteries?

Postby Gordon-Loomberah » Fri May 19, 2017 4:44 pm

gfsben wrote: I can see the sense in an easterly biased array. I wonder how much east of north and tilt angle would be optimum.


You'd probably set the azimuth of an array for summer, since that's when you are going to need the most cooling. Do you need any cooling in winter?

You get about 87% of the solar radiation onto the panel cf face-on when it is pointing 30 degrees away from the sun, so tilting it up 30 deg or a bit more, facing East if you are wanting to maximise COP when running a chiller in the mornings, and a second array tilted at say 10 deg, facing a little W of north.

If you want even output all day, then the virtual tracker arrangement would be best, and probably facing the array directly E and W, ~30 deg above the horizon, to maximise output for the hotter months.
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Re: Desert farm, Big pump, Batteries?

Postby Warpspeed » Fri May 19, 2017 6:25 pm

The above link to the virtual tracker thread is pure gold.

I have both a north facing array, plus an east west virtual tracker. The virtual tracker produces almost constant power all day long, and starts and ends about an hour before and an hour after the main north facing system gets going. But the TOTAL power produced falls far below what the north facing array produces throughout the day, given similar panel areas.

If you are running an energy storage system (battery, pumped water storage, or ice) a north facing system should give you more bang for your solar panel buck, provided the system can use all of the massive short mid day peak that will be produced.

For systems that do not use a battery, maximum extended hours of solar can be very beneficial, and much more useful than a huge narrow mid day power peak where there will hardly ever be enough load to usefully use the power available.

If you have an ice storage capable of supporting at least 24 hours of cool room load, there is no real advantage in trying to get the system running an hour earlier, if that reduces the total system capacity. The COP will definitely be better in the early morning cool, but I doubt if that will make up for the mid day shortfall of east and west facing panels.

Solar panels are now so cheap anyway, that the whole situation really ceases to be a problem, unless you are really restricted for installation space.
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Re: Desert farm, Big pump, Batteries?

Postby davidg » Sat May 20, 2017 12:49 am

Having conducted a bit more research on plastic tanks, I find that bushman tanks test theirs at -40C so a if saturated solution of lets say brine was used in some way that allows for an ultimate freezing point of -21.1°C (solid) so there's a thought for saving a few dollars in tank storage, I generally thought it could be an issue, it appears I stand corrected.
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Re: Desert farm, Big pump, Batteries?

Postby davidg » Sat May 20, 2017 12:53 am

Having conducted a bit more research on plastic tanks, I find that bushman test their tank at -40°C so if a saturated solution of lets say brine was used in some way in containers or directly that allows for an ultimate freezing point of -21.1°C (solid) so there's a thought for saving a few dollars in tank storage, I generally thought it could be an issue. I found a number of references to "brine" being used in large refrigeration systems so it must work.

I stand corrected. :)

It might be worth considering, to reduce the possible corrosion issues the liquid being used to transfer cold could be glycol based with the "coil" in the brine being polyethylene pipe not the most efficient so would need more pipe to produce the correct heat exchange ability, should keep the cost reasonable and possible corrosion, while using only a small amount of glycol, which should keep the cost down a fair bit.

More about them can be found in their FAQs
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Re: Desert farm, Big pump, Batteries?

Postby Warpspeed » Sat May 20, 2017 8:47 am

I am not so sure an "ultra cold" ice storage is such a good idea. It would significantly degrade the compressor COP trying to freeze down to a much lower temperature.

Typical cool room temperatures are usually kept just a very few degrees above freezing, and if you have a large exposed area of ice or chilled water at 0 Celsius, it should be able to meet that requirement if aided by some form of forced air circulation.
In fact, many items kept in cool rooms must never be allowed to freeze. So normal ice offers a pretty simple safe method to prevent freezing damage to the product.

If the system struggles with 0C ice, you could use weak brine to lower the ice temperature slightly as a demon tweak, but I don't think initially designing for the lowest achievable storage temperature is going to be very productive.

The closer together the evaporating and condensing temperatures, the higher the coefficient of performance for the refrigeration system. So its just as important on the evaporator side.
Just an immersed bare copper coil at the bottom of the tank should be all that is required. Providing a slightly larger volume of ice should not present any great problem.

I have been thinking of how I would go about doing all this myself, its a particularly interesting engineering exercise.
The electronic engineering design side of things is what I used to do before I retired. The refrigeration side of things I have had some limited exposure to, but feel far less comfortable with nailing down the precise thermodynamics, you really need to find a tame HVAC engineer (heating ventilation and air-conditioning).

Engineering is all about familiarity with relationships, and working through the numbers, usually from published data and specifications.

I can go into it more deeply if anyone is interested, but the basic idea would be to hook up a belt driven compressor to a variable frequency drive, fed directly with high voltage dc from a large series connected solar panel array.

Modern VFDs all now use microprocessors and its virtually certain to already have internally a programmable PID control loop that is user programmable to vary motor speed to maintain some input parameter constant.
That input parameter could be solar panel voltage.

The idea would then be that the motor speed and power drawn from the solar panels would be varied up and down to keep the panel voltage at the optimum MPPT voltage value.

At sunrise the motor would start to creep, then gain speed to some maximum at mid day, then slow down as the sun sets. Speed would go up and down as clouds pass overhead, but always the compressor optimally loads the solar system. It should be that simple.

There are only two potential problems that should be fairly easy to get around.

The first is that VFDs all have very comprehensive monitoring and protection, for both the drive itself and the motor. Any drive greater than about 2Hp will have an input that normally requires 415v three phase power.

While feeding dc straight into the three phase rectifier presents no particular problem in itself, the protective software will probably see that as failure of one incoming phase, and it will shut down the drive and likely come up with an error code or error message of some kind that stops the fun.

So someone will need to get into the drive hardware and modify whatever input voltage sensing there is to fool the microprocessor into thinking there are three incoming ac phases of equal voltage. Probably not difficult, but it needs someone that knows what they are doing to work it all out.
Once that problem is overcome its full steam ahead with the VFD, once all the internal functions have been correctly programmed into it, which is not a minor task.

The second problem could be potential overheating of the induction motor when run at low speed and high torque, due only to the fact that the fan on the end of the motor shaft not providing sufficient cooling.
That can easily be fixed by fitting an external cooling fan that always runs at full speed powered externally from a constant power source (12v dc?).

Its the motor cooling problem that worries me with sealed compressor units. I think that may create an insurmountable problem. At least with belt drive and a standard TEFC induction motor, its very easy to fix.
The motor manufacturers often can supply a motor with an optional external fan, specifically for low speed high torque VFD applications. Or you can do it yourself.
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