Compressed air powered water pressure

[bigyabbie]

TL;DR Summary

Trying to design a simple system to provide sustained water pressure during blackouts without the use of a water tower

Hi there,
I have a few interesting problems that I have been exploring for a while now, but i quickly get beyond my level of knowledge of physics, so wondered if members of this Forum may be able to assist. One of the projects that we have been trying to solve is how to maintain a decent quality of water pressure for showers and toilets during blackouts without resorting to building a water tower.

I see from past threads that being clear and specific seems to help, so please let me explain some context. I am a disaster worker, working primarily in the coordination of shelter assistance in mega disasters, so i have a building background in the housing sector and post graduate studies in Resource and Environmental Science. Between disaster deployments I have been slowly building an eco-resort in a small village in a small village in South East Asia. So far we have ten rooms across half a dozen bungalows with organic gardens, living pools and a wholefood restaurant. We are trying to make the business as environmentally friendly and culturally sensitive as possible, but face a range of unique complexities.

One of the complexities we face is fairly regular power brown outs and black outs, which are obviously not too compatible with maintaining high quality services for tourists. To get around this we have installed small solar systems with batteries in each of the bungalows that directly run 12v lights, fans, usb chargers for phones, and the starters for instant gas hot water heaters. Protectionist state laws protecting the national power companies mean that there are significant challenges to installing larger solar systems. Decent quality inverters and lithium batteries are prohibitively expensive and importing almost anything is complicated and expensive. Legally (at least in theory) we would be required to be a certified licensed Energy Provider to go off the grid, and grid connected meters have only just been introduced offering horrendous power buy in and pay back rates and high monthly fees. Hence our solution of placing anything essential that is easy to run on 12v on small dispersed 'auxiliary' systems. We are still left right now with the problem of pumps and fridges, and hence for now are running a 5kva LPG Genset, but this has been reaching/exceeding the limit of its capacity and while we have moved more and more things to our dispersed solar system. We have very energy efficient fridges and freezers that hold their coolth quite well for a while, but I am stuck trying to figure out how to get around pumps for water pressure... hence this message

Clearly the obvious first answer is a water tower, and yes I do vacillate backwards and forwards about building one. There are however a few considerations that prevent me... Firstly it would need to be quite high to provide a decent amount of pressure, and secondly with a growing number of bungalows with baths, showers and toilets it would need to store quite a decent volume of water to get us through a decent black out. We estimate that if we used solinoids to block off the bathtubs during a blackout then we could survive on about 2,000L, though this will probably grow over time to around 3-4,000. Now this may not seem like much, but we have another complexity. Java where we are located is one of the most earthquake prone islands on earth, and we are in rice fields that are basically deep sandy volcanic mud, that is potentially liquefaction prone. So a water tower holding a few tonnes, 20-30m up would need to be pretty strong and require some fairly hefty investment in foundations. Another complexity is that as a foreigner I am not allowed to own land, but can only rent it. This makes a huge investment in foundations and towers seem fairly dubious compared to other solutions that I could take with me if our long term leases are ever not renewed. (This is one of the reasons all our bungalows are dismantle-able structures made from recycled teak, (built on engaged raft slabs which act as bell chambers for liquefaction proofing).

So my current thoughts are that I could use mains power to compress air into a reasonably larger tank at say 10-12 bar, and then use the compressed air to provide water pressure when the mains power is out.
The first option I see would be to simply run a Air Operated Double Diaphragm (AODD)pump in-line with our existing pressure pumps (or replacing it if the efficiency of running off compressed air is not too wildly different??). We could then simply maintain set the air pressure control valve to slightly below the standard pressure pump, and it would then effectively cut in as soon as the pressure dropped out in a power black out. (I presume we would need to install dampeners to smooth out the water supply).
The second option I have been considering is to buy some large 12" PVC piping and create our own pressurised water tank, again pressurised to just below the standard pressure in our system. This would sit in line in the system with water flowing through it to ensure it was not stale. As soon as the pressure in the water line dropped compressed air would enter at the top pushing water out the bottom until it was empty closing off with a float valve if it ran out. When power kicked back in the tank would then refill with air evacuating out a a pressure release valve set to a higher value that the air pressure in feed line, again using a simple float valve to close the the pressure vent when the tank was full.

I think that both these ideas would probably work, but I am no expert and am left with lots of questions that would be better answered prior to building either.
In the first option AODD pumps are relatively common in industry, so there is a baffling array of pumps available on the market, but I find it very difficult to work out exactly what I should specify. I am unclear as to exactly how much air this would use, my presumption is that if I wanted my water pressure to be at 3bar, then I would need to store 1 cubic meter of air compressed at 9bar to move 1 cubic meter of water. Is that correct or did I miss something obvious? Are these pumps noisy and or could I simply submerse it in one of my existing water tanks? As above I am also wonder if this system could in fact just replace my existing electric pressure pump all together? or would that be horrendously wasteful. As in, does a compressor consume far far more energy than the pump? Is there a particular type of compressor that would be better? We have both a few electric compressors, a larger one for our workshop, and a smaller silent oil free one we use for other applications, but could also obviously by another if needed.

With the second option, does it make sense? have I missed anything? I presume the air consumption calculations would be the same as the earlier example or again did i miss something. I presume with this solution I would need to use an oil free compressor or have an inline water and oil filter. I imagine this would be quite silent near the water tanks as an air hose could run back to the air compressor at our workshop. Did I miss anything obvious?

As I say i am no expert at any of this, but would love some advice or feedback before I go ahead with any of it. Maybe I am overlooking something and should just install a mega lead acid battery bank or something, I really don't know and as I say any advice would be appreciated
Thanks in advance

 

[anorlunda]

That sounds like a noble project. Good for you. I hope you get helpful advice here.

But first, I'm confused by your post. You worry about the limited capacity of a water tower tank, but tanks powered by compressed air instead of air also have limited capacity.

Simplicity (the KISS principle) ranks very high, especially in third world situations. Therefore I would want overwhelming evidence before abandoning the simple water tower.

Can you elaborate on the advantages of alternatives to the water tower?

 

[jedishrfu]

What about a distributed network smaller tanks, one tank for each bungalow filled when your pumps are working using gravity to supply water to the unit. You have to compute the amount of water each bungalow uses and double or triple the tank size to cover when and how long an outage occurs.

You could use small pumps to push out the water to the resident at the point needed ie shower, sink, etc...

In Taiwan, they used a scheme where water was heated via natural gas at the point it was dispensed hence no need for a hot water tank of any size.

 

[bigyabbie]

Hey Anorlunda, thanks for the rapid reply.
My concern about the water tower is that i need quite some height to get enough pressure, and that I really am in a highly tectonically volatile area, My village was completely destroyed by an earthquake in 2006 and has some pretty impressive shakes since. Additionally we are on very sandy volcanic soils being used for rice farming in the tropics, making a great recipe for soil liquefaction. This means the tower would need to be well engineered and well constructed, which not only makes it very expensive but very difficult given the challenges of local labour skills ans the height...
As I said it was my first option, but the more I looked at it the less i liked it.
perhaps it's just me and my warped sense of reality from working in disasters for a living, but to be frank I'd have trouble sleeping near such a tower

 

[bigyabbie]

What about a distributed network smaller tanks, one tank for each bungalow filled when your pumps are working using gravity to supply water to the unit. You have to compute the amount of water each bungalow uses and double or triple the tank size to cover when and how long an outage occurs.

You could use small pumps to push out the water to the resident at the point needed ie shower, sink, etc...

That would still require a tower at each bungalow, sure not as strong a tower but still just as tall, and in compiling the water in one place let's me average out the amount of water needed and use a lower volume in the air, as it is unlikely all bungalows would be full at the same time and use the showers and toilets at the same time. Smaller pumps at each site would still need electricity that we don't have unless i add more batteries in each bungalow, more solar cells etc

 

[Dullard]

I'm wondering:
You say that you're talking about a maximum of 4000L of water, and outages that are hours in duration. That isn't (on average) a very big pump. If the outage duration is 1 Hr and the full 4000L is consumed at a uniform rate (67 LPM), it's < 500W. A generator and one (or a staged array of) small 'backup' pump might be the cheapest and easiest way to go.

 

[anorlunda]

How much pressure do you need.

I lived for 12 years on a boat with no pressurized water, no hot water. We used foot pumps for sinks, and gravity for showers, and the stove to heat water in sinks and the sun to heat water for showers. The inconvenience was hardly noticeable. My point is that rural people looking for a backup solution, should not set big city water pressures as their standard.

In fact, people who are adapted to foot pumps and gravity showers full time, have resilience and a degree of immunity to blackouts. It is analogous to using daylight from windows during the day rather than putting up blinds and depending on electric lights, and backup electric lights during blackouts.

Without analyzing your compressed air plans, I worry about reliability just because of complexity. You may get DIY tips here, but you are not likely to get an estimate of the reliability of the compressed air system. Mean time between failures, is part of that, but mean time to repair can be a much larger problem in third world settings.

I would also check Peace Corps or International Aid experience from around the world. These problems with electricity can't be unique. What has been tried other places successfully? unsucessfully?

 

[bigyabbie]

How much pressure do you need.

I lived for 12 years on a boat with no pressurized water, no hot water. We used foot pumps for sinks, and gravity for showers, and the stove to heat water in sinks and the sun to heat water for showers. The inconvenience was hardly noticeable. My point is that rural people looking for a backup solution, should not set big city water pressures as their standard.

I would also check Peace Corps or International Aid experience from around the world. These problems with electricity can't be unique. What has been tried other places successfully? unsucessfully?

We run an eco-resort and although we would be happy with low pressure our guests would not

I work full time in the international Aid sector and haven't found anything there yet

 

[bigyabbie]

I'm wondering:
You say that you're talking about a maximum of 4000L of water, and outages that are hours in duration. That isn't (on average) a very big pump. If the outage duration is 1 Hr and the full 4000L is consumed at a uniform rate (67 LPM), it's < 500W. A generator and one (or a staged array of) small 'backup' pump might be the cheapest and easiest way to go.

You are correct about the low need, our current pressure pump is way less than that. You may also be correct about a generator being an easy solution, but I really can't stand generators, they are so pollutive and such an anathema for an eco-resort. Compressed air seemed like an interesting alternative, that in the future I could potnetially then add in a wind powered compressor to the mix, or simply buy green power when it becomes available.

Thanks for the great input, though frustratingly nobody so far is actually addressing my questions :)

 

[chemisttree]

What about a distributed network smaller tanks, one tank for each bungalow filled when your pumps are working using gravity to supply water to the unit. You have to compute the amount of water each bungalow uses and double or triple the tank size to cover when and how long an outage occurs.

This is what is done in Mexico City. Water is rationed by interrupting service. So, everyone has a large tank on the roof that gravity feeds the home. When the water comes back on, the tank is refilled. The house always uses gravity-fed water from the tank so the process is very hands-off automatic. A single float valve controls it all.

 

[Rive]

We estimate that if we used solinoids to block off the bathtubs during a blackout then we could survive on about 2,000L, though this will probably grow over time to around 3-4,000.

I really have the feeling that you are just making it difficult, but whatever. What I would do:
- pressurized 4000l water buffer tank, with a valve on the input side
- high pressure air tank with pressure regulator, set below the pressure of the local water system: connected to the buffer
- when there is no pressure from the system, the air tank provides a reduced but constant pressure: the valve prevents back-flow
- when the pressure/power is back you should readjust the water level in the buffer tank and fill up the air reserves
- maintain the cleanness of the system will be the biggest hurdle.

 

[Dullard]

"Thanks for the great input, though frustratingly nobody so far is actually addressing my questions"

Upon review, I agree. Rive's answer (post #11) is about what I was thinking. I think there are several considerations that haven't been explicitly described - they are why no engineer would go down the road that you're discussing (given any other choice):

Your 4000L 'pressurized storage tank' will have to be a pressure vessel. That's a lot more expensive than (for example) an elevated tank at atmospheric pressure.

Your high pressure air tank will be a large pressure vessel - something like half of the water tank volume, if you want to use a conventional single stage air compressor, and larger if you want to keep the air pressures 'safe' WRT the storage tank and downstream plumbing.

 

[jedishrfu]

You have to think outside the box here. The compressed air solution may be harder to implement than the distributed water tanks solution.

 

[berkeman]

That would still require a tower at each bungalow, sure not as strong a tower but still just as tall

No, ground level water storage.

Thanks for the great input, though frustratingly nobody so far is actually addressing my questions :)

Hey, we are engineers and scientists here. Sometimes we hold off answering the literal question you are asking because it's not practical in the real world. We try to offer real-world advice and solutions.

We run an eco-resort and although we would be happy with low pressure our guests would not

What kind of eco-clients do you cater to? Posers who want luxury with rustic appearance? What self-respecting eco-tourist wouldn't appreciate a foot pump sink in your units that can be used when the power is out? My goodness...

I lived for 12 years on a boat with no pressurized water, no hot water. We used foot pumps for sinks, and gravity for showers, and the stove to heat water in sinks and the sun to heat water for showers. The inconvenience was hardly noticeable.

 

[256bits]

You said

What kind of eco-clients do you cater to? Posers who want luxury with rustic appearance? What self-respecting eco-tourist wouldn't appreciate a foot pump sink in your units that can be used when the power is out?

Seems compatible with the OP has been saying,

We are trying to make the business as environmentally friendly ... as possible,

But, what's in a name...

we run an eco-resort and although we would be happy with low pressure our guests would not

Is that just a guess on the clientele preferences for roughing it when the time comes.
If the resort is to be run with some luxury, than that would make sense as it would be different than just camping out.

One thing I do see an explanation of is where is the source of water in the first place? There must be a hookup with a town supply. Brownouts have a affect on the water pressure?

 

[Spinnor]

Have a large tank of water on the ground that is keep topped off when power is on and which is at atmospheric pressure and which acts as your backup supply. When power is out a relatively low power water pump pumps water from the storage tank into your existing water system to maintain water pressure. Pump would be powered by your existing generator and as it is relatively low power you should be able to run it. Many details left out but hopefully you get the idea. You may need to add more water pressure tanks. This could all be set up so that everything was automatic.

 

[Baluncore]

I think you need to avoid compressing air to run a water pump.

There should be little need to pump air to pressurise the system once it is operating. As an example. Start with a 6 m³ pressure tank/accumulator. Introduce water when available at pressure to the mid-height point of the tank, through a ballcock and non-return valve. Remove water when needed from the bottom through a float valve that closes to prevent the escape of air if the tank runs empty of water.
Pressurise the tank once with air to the minimum water pressure required. Then allow the tank to half-fill with water from an available pressure supply, until the ballcock closes. That will double the air pressure in the tank. As water is used the pressure will fall back to the minimum.

The cost is going to be the large volume high pressure storage tank = a cylinder with spherical ends.
There is another source of water pressure available. Pay local labour a premium when needed, to operate a pedal powered water pump that maintains water pressure in a small accumulator.

Are you near a 2 bar = 20 metre high hill? That would eliminate compressed air and the tower, replacing it with a low pressure tank and a single long pipe. Air pressure is then not required.

 

[Merlin3189]

I'm surprised by your 30m high tower. Perhaps the terrain varies in height and the tower is not to be at a high spot? And I suppose your communal tower needs to provide for pressure drop in distribution pipes.

Otherwise, tanks on each bungalo would need be no more than 3m above the highest outlet. I say that because that's been the head on our system for 40 years and has more than enough pressure for all our needs, including a high flow mixer shower. ( I see from a quick look on a plumbing site that they have showers which claim to work down to 1m head, but I've no experience with that and doubt whether that would be as high flow as ours.)

A tank in each property could be taken away and moved with it if needed, so no lost investment at the end of the lease.
Individual property tanks give automatic rationing, so that everyone gets a reasonable supply and no one's excessive consumption (or accidentally leaving the tap on) would stop anyone else from getting their due.

 

[BruceM]

I think you need much less water pressure than you might think. I live on 12 PSI and at faucets and shower you can't notice a shortage due to carefully selected fixtures and oversized supply pipes.

Background-
I've been living off grid for the last 12 years on a 120VDC PV/battery bank system I designed and built myself. I designed an ultra low conducted EMI sine (5 step) inverter I use only for washer, well pump to storage, and compressing air to my 500 gallon storage tank, which is used for running my shop. A 6/1 Lister CS provides backup compressed air and AC power.

I have a 2000 gallon water storage tank on a roughly 30 foot high hill next to the house site, which provides 12 psi to the home and shop site. By using oversized (1 1/4") pipe to the site, and 3/4" ID inside the house to very close to the fixtures, and picking fixtures with high flow, it works very well and a casual user does not realize the pressure is low. I picked fixtures by blowing through them and turning the valves at the hardware store to find the least restrictive valve bodies. Many faucet fixtures in the US will have flow restricting plastic bits at the spout which may be removed while retaining some aeration. For my exterior hydrants, fed by 1 1/4 ID PVC, a 25 foot length of 5/8 ID garden hose works OK with a spray nozzle, but I use 3/4 ID hose for 50 foot. In short, jacking up to high pressures just so you can use tiny pipe and restricted fixtures is a waste of energy. 12PSI is quite workable. I suggest turning off the pump of the existing system and bleeding it down to lower pressure and testing your existing fixtures with restrictors removed.

Pressure vessels in larger sizes will likely be very costly. HDPE lined fiberglass tanks with air bladders will last much longer.

My neighbor has a higher hill and gets about 18 psi, which is luxuriously high, allowing some sloppy 1/2" ID plumbing without problems. His garden hose performance (with 1 1/4 supply to freeze proof hydrant) is noticeably more energetic than mine but he still needs 3/4" ID hose for 50 foot length or more.

I applaud your project and the existing minimalist approach of direct DC (12v) power use. With the use of LED lighting, that can be quite practical (and simple).

One last thought- for an off grid neighbor with a brain injury and sound sensitivity, we used a Lister CS (India made clone) in an insulated (for sound) shed with a leach field type muffler where the exhaust is run through a ditch with 3 inch cinders normally used in septic systems. This completely eliminates exhaust noise and smell. I was astounded at how well this worked, and was relatively simple to construct. I mention this as it might make a backup generator more acceptable to your eco resort clients.

Best Wishes,
Bruce

 

[Tommyboyblitz]

As someone stated, simplicity is going to make it cheaper and less problematic with reliability. Pressureising a system would require seals that probably won't last long being always under pressure waiting for the blackouts.

A water tower is extremely simple and efficient. Doesnt require any sort of big change or special pumps or materials. Theres not a huge amount to go wrong with it.

 

[Rive]

Pressureising a system would require seals that probably won't last long being always under pressure waiting for the blackouts. ...A water tower is extremely simple and efficient.

A water tower is a pressurized system.

 

[Tommyboyblitz]

A water tower is a pressurized system.

But its using gravity to pressurised the system. If yous start adding compressed air to a tank it's entirely different

 

[Rive]

Regarding the pressure itself, it is the very same.

 

[berkeman]

A water tower is extremely simple and efficient. Doesnt require any sort of big change or special pumps or materials. Theres not a huge amount to go wrong with it.

Looks like you missed this comment by the OP in his 2nd post in the thread...

My concern about the water tower is that i need quite some height to get enough pressure, and that I really am in a highly tectonically volatile area, My village was completely destroyed by an earthquake in 2006 and has some pretty impressive shakes since. Additionally we are on very sandy volcanic soils being used for rice farming in the tropics, making a great recipe for soil liquefaction. This means the tower would need to be well engineered and well constructed, which not only makes it very expensive but very difficult given the challenges of local labour skills ans the height...

 

[Tommyboyblitz]

Regarding the pressure itself, it is the very same.

Sealing water pressure is a lot easier than sealing air pressure, especially when mixing air and water together, adding a hot country it's very prone to fail.

 

[Tommyboyblitz]

Then the next best option would be to use a pump, keeping a large tank air tight sealed would be very unrelaible especially in an earthquake prone location and the tropical climate won't help much either.

I use a 12v pump at my workshop, I have an old oil burner and use it to heat up water and pump it though pipes at my workshop. A couple of old car batteries along with a solar panel and I rarely have to charge them either and in the UK we don't get a huge amount of sun. You would probably need larger or more pumps with more batteries but the system could be kept topped up with solar or mains supply for when you need it. You will also find you can use the batteries maybe for emergency lighting also when the power is out at night etc.

 

[Joe591]

Hi
your first suggestion of having a water tank that you pressurise with air pressure might end up being a lot more expensive than you think. YOu will basically need two pressure vessels. The first pressure vessel will be the water tank and the second pressure vessel will be the air receiver. If you are having a hard time picturing the water tank as a pressure vessel just ask yourself what it is filling up with as it's water level is dropping. That's right, it's filling up with compressed air. I live in south africa, so I quote south african law. In these here parts a 2000 Liter tank with 4 bar water pressure is classified as "SEP", which means that it needs to be built to sound engineering practise. Not a big deal if you know what you are doing. A 2000 liter vessel under 4 bar air pressure is a category 3 pressure vessel. that is a big deal. The reason why I'm making this distinction is that there was somebody that made a statement in a previous comment. He said something to the effect of just use air to pressurise the water tank to the same pressure that the local municipal water pressure would pressurise it to. It's not that simple. Air pressure and water pressure are two completely different things. Water pressure is not dangerous. Air pressure is incredibly dangerous.

What colours are prison clothes in Taiwan? Is it a bold fashion statement? Would you like to make a bold fashion statement? If you build something that is illegal and somebody dies, where do you go? These questions are for introspection...

A category 3 pressure vessel involves a lot of paperwork and legalities and pressure vessel codes and government inspectors and Approved Inspection Authorities, designs that have to be stamped by professional engineers and verified by Inspection Authorities after the Professional Engineer has signed it off. Even the sheet material that you buy has to conform to a pressure vessel code. This is not something that you can DIY your way through. If you really want to go this route get a quote from a reputable engineering company in your vicinity. Make sure you sit down before you open it though.
You're better off building a water tower. If the first one collapses due to an earthquake, just build another one. Get a quote for the pressure vessels and then get a quote for the water tower and calculate how many water towers you can build for one compressed air system as has been mentioned in the opening paragraph. I dare you...
How many stars does your accomodation have? 5 star, 2 star?

 

[ryan booker]

Did i miss something or has nobody mentioned a ram pump? Throw a water tank on the roof of each bungalow and you’ll have 40 psi no problem. A ram pump basically turns flow into pressure, so 10’ head could give you like 70 psi, but use the excess pressure to either pump the waste to a lower tank for treatment or to generate power with inline micro hydro generators.

 

[berkeman]

Did i miss something or has nobody mentioned a ram pump? Throw a water tank on the roof of each bungalow and you’ll have 40 psi no problem. A ram pump basically turns flow into pressure, so 10’ head could give you like 70 psi, but use the excess pressure to either pump the waste to a lower tank for treatment or to generate power with inline micro hydro generators.

Welcome to PF.

It's been a while since I read through this old thread, but I don't think the bungalo roofs are built assuming the extra weight of any significant water storage. Maybe out near the edges where the load bearing walls can supply more support, but you also have the seismic issues that the OP mentioned.

Also, could you maybe post a sketch of what you are proposing with the ram pump?

 

[hutchphd]

Did i miss something or has nobody mentioned a ram pump?

A ram pump is not a viable solution here. It uses a lot of low head water to pump a little (maybe 5%) of the flow to high pressure. Where they are appropriate they work very well...my brother in Maine has supplied his house for 45 years this way and it is like free money.

 

[berkeman]

A ram pump is not a viable solution here. It uses a lot of low head water to pump a little (maybe 5%) of the flow to high pressure. Where they are appropriate they work very well...my brother in Maine has supplied his house for 45 years this way and it is like free money.

That's very interesting. Could you maybe post a diagram of how that works? (Or recruit your brother to draw the diagram...). Thanks

 

[hutchphd]

When he built it (1970's) I had never heard of such a thing and was suitably impressed by his engineering diligence (not bad for an MD!). They are decidedly not new, but are very clever. Now there are lots of good explanations online:


The downhill flow builds up until turbulence causes a flap valve to slam shut and it squirts the water hammer into a little local pressurized holding tank via a one way valve. Repeat.
He rebuilds it every couple of years and in between it just thumps away like a slightly metallic heartbeat in his little semi-underground spring house. His house tank is about 80 feet above the ram pump. Any overflow pumped waters the garden.

 

[sophiecentaur]

I searched this thread for the word "safety" and I couldn't find it. Has no one considered the energy that's stored in compressed air, even at moderate pressures? You have the choice between a small air reservoir at frighteningly high pressure or a larger one with still worrying pressure.

If there is enough room for a water tank then why not use the sort of pump(s) that can be bought in any plumbing supplier's. They have adjustable output pressure. When you leave the system and go home at night there's no problem with leaks and your average plumber can fix any problems.

 

[timMoore]

have you checked out any energy storage options to prevent the blackouts/brownouts? If you want to use compressed air and water there are a few energy storage options. how much energy does your resort use? I read about a new system which uses compressed air to blow out high pressure water through a pelton turbine to generate electricity. This would increase the value of your solar panels too by maximizing your overall consumption.

 

[berkeman]

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