Off Grid Solar Air Conditioning

[Buenijo]

I am interested in going off grid. Unfortunately, I live in a very hot and humid region, and going without a/c is not a viable option for me. Therefore, I've considered some configurations that might make it practical. The biggest problem seems to be the size of the battery bank required. Both the short and long term costs of batteries seem to be the bane of off grid PV systems. The scenario is to use a small highly insulated enclosure to contain a thermal mass cooled by conventional vapor compression a/c units. A small battery system is provided to act primarily as a voltage regulator. Air is circulated through the enclosure and then through the home using a small fan on a thermostat. NOTE: I am considering here a very small well-insulated home.

The simplest approach I've considered would be to use water as the thermal mass, and use multiple small window a/c units. These are not particularly efficient, but they are readily available and inexpensive. Furthermore, I like the redundancy that this configuration provides (i.e. multiple small units), and it's a better fit for a small battery system. What I've considered is that it must be possible to design the system such that the units power on and off sequentially. It seems this can be devised by using battery voltage as an input. I have no formal electrical training. Yet it seems that this could be configured using solenoid switches. Could a load diversion charge controller (i.e. shunt regulator) be used to energize the solenoid circuit with the primary contacts used for the compressor motor? Could several solenoid switches be made to operate motors sequentially by placing rheostats in the solenoid circuits (to effectively adjust set points)? The idea is that battery voltage rises as it's charged from the PV array, and above a certain setpoint some current is shunted to a secondary circuit that contains multiple solenoids wired in parallel and each with a separate reheostat. When one solenoid is energized the compressor motor is energized off the inverter that's placed on the battery. This would draw down the battery voltage. If the PV array output is high enough, then battery voltage would continue to rise until one or more units come on line. Conversely, the units would power down sequentially in a similar fashion.

Any engineers out there please chime in with your thoughts as to why this is (or is not) a viable idea. If not, then please suggest a way to make it so.

Mark

[Sunking]

I am interested in going off grid. Unfortunately, I live in a very hot and humid region, and going without a/c is not a viable option for me.

Mark OK sound slike me in TX. Solar is not an option for cooling unless money and space is of no concern. It is that simple.

[Buenijo]

Mark OK sound slike me in TX. Solar is not an option for cooling unless money and space is of no concern. It is that simple.

Of course, I have no doubt... the thermal mass would take a lot of space, and the cost of the system would be very high. Still, consider this perspective: Let's say someone wants to move to a remote location where electricity and gas are not available (I'm considering a small modular home that would consume about 15 kwh each day when a/c is required, and only about 5 kwh each day otherwise - a mean of 10 kwh each day). A Diesel generator would cost about $35,000 over a 20 year period when fuel costs are considered (not including the likelihood of fuel costs being higher in the future). It seems that a PV system can be configured for less than this price to deliver the same results, particularly if the size of the battery can be reduced with a thermal mass.

[Mike90250]

I am interested in going off grid.

Grid tie, with net metering, is the way to accomplish this. Adding batteries will only waste 50% of the power you generate.

[Buenijo]

Grid tie, with net metering, is the way to accomplish this. Adding batteries will only waste 50% of the power you generate.

Yes, that would be ideal... but I'm considering a very remote site. I had estimated a 5000 watt PV array to generate an average net output (i.e. from an inverter powered by a battery system) of 15 kwh a day. The region has an average solar insolation of 5.5 kwh/m^2 each day. Is this too optimistic?

[Sunking]

YThe region has an average solar insolation of 5.5 kwh/m^2 each day. Is this too optimistic?

To optimistic and flawed logic. With a battery system you cannot use yearly average insolation. You have to use worse case of the shortest months in the year. If you use 5 hours, and your December insolation is 3 hours, welcome to being dark most of winter with dead batteries that need replaced.

[vinniethePVtech]

DC powered Heat Pump no inverter required, Minimizes AC conversion losses.

[Buenijo]

To optimistic and flawed logic. With a battery system you cannot use yearly average insolation. You have to use worse case of the shortest months in the year. If you use 5 hours, and your December insolation is 3 hours, welcome to being dark most of winter with dead batteries that need replaced.

Please note the insolation figure I listed is the average for the summer months. I'm hoping for 15 kwh only for most days during the summer months (to power the a/c unit primarily). I require only 5 kwh each day when a/c is not required. During the winter months the average insolation is 3 kwh/m^2 per day. Certainly I would protect the batteries from excessive discharge and provide a small generator as a back up.

So, assuming the insolation figure listed for the summer months, will a 5000 watt array generate 15 kwh (net)? If not, then what size array is required?

[louiedonovan55]

reply

Hello, I have this question, Is there such an air conditioning system that can be powered by solar energy? Air conditioner such as the split system like those small units.

Mod note - link removed

[Shockah]

DC powered Heat Pump no inverter required, Minimizes AC conversion losses.

Sounds great!... until you ask how many watts that unit will consume in a 24 hour period...?

I doubt 4 12v batteries (x20%) has enough capacity to cycle that unit over 24 hours.
800watts of Solar Panels?... That covers less than 2100wh per day.
@ 18,000btu, that A/C unit is consuming more than 2100wh per day.

[russ]

Sounds great!... until you ask how many watts that unit will consume in a 24 hour period...?

I doubt 4 12v batteries (x20%) has enough capacity to cycle that unit over 24 hours.
800watts of Solar Panels?... That covers less than 2100wh per day.
@ 18,000btu, that A/C unit is consuming more than 2100wh per day.

That really is a wild ad - I suppose they find some suckers.

[jon_r]

It's best if you run the AC while the sun is shining and not use batteries at all (or perhaps just for motor starting peak loads). To do this and still have cooling at night, you need storage. Water or ice is ideal. So you need something to refrigerate/freeze water and then a simple circulation system with fan coils. For small systems, a chest freezer might serve as the cooler and the storage tank.

If you have a small, very well insulated and air sealed space, solar AC is very feasible.

[russ]

It's best if you run the AC while the sun is shining and not use batteries at all (or perhaps just for motor starting peak loads). To do this and still have cooling at night, you need storage. Water or ice is ideal. So you need something to refrigerate/freeze water and then a simple circulation system with fan coils. For small systems, a chest freezer might serve as the cooler and the storage tank.

If you have a small, very well insulated and air sealed space, solar AC is very feasible.

Pray tell us how you propose to do this? You are blowing smoke!

[jon_r]

If you had a specific question and could learn to be polite, I might answer that.

[russ]

If you had a specific question and could learn to be polite, I might answer that.

Answer it and about other posts or you are banned. We do not allow people to show up and make silly claims.

The polite part - I reserve for those that deserve it.