Wells Concentrator / Distiller
Last modified on 20010806
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Dave's Rule of Thumb: "¼% efficiency rule" applied to Air Conditioning (Part One).

It would be nice to take a look at efficiency of a bunch of different types of solar collectors and be able to compare them for some uniform application.

For example, air conditioning is a big application, since during the peak of summer, about 65% of electric load goes to air conditioning. Is it possible to make a stab at an estimate as to how well the PV versus evacuated tube/absorption versus parabolic trough/absorption would work?

Which collector is the most efficient? Evacuated fixed (like THERMOMAX), or tracking trough with evacuated receiver (like SOLEL-150) or PV?

I made an estimate of the performance of both types (Solel-150 parabolic "mini trough" and Thermomax evacuated tube flat plate) using both manufacturer's data, and an operating temperature of 175°C. This is pretty high temperature by most systems standards, but represents the kind of temperature you need to deliver to run either double-effect lithium bromide, or single-effect air-cooled ammonia absorption chillers, or scroll/Rankine power conversion systems.

Surprisingly, I found that the average daily efficiency of both the SOLEL and the THERMOMAX would be about the same! They both would be expected to have an average daily efficiency on a sunny day of around 42%, based on an operating temperature of 175°C, which is about the temperature you need to run a single-effect ammonia absorption unit with air cooling.

Now, in comparing the performance of the absorption units, Rohr's ammonia unit, which runs at 175°C and is air-cooled, has a COP of around 0.6. Single-effect lithium bromide units run at only 100°C and have a COP of about 0.6, while Double-effect lithium bromide absorption units have a COP of around 1.0 and run closer to 120°C. They use water-cooled towers, though.

How can anybody evaluate which works best and under what conditions? In a simple five-page analysis I did (e-mail me if you want it), I found a simple rule of thumb calculations for both the evacuated fixed flat plate collector type from Thermomax, and the evacuated "mini-Kramer Junction" collectors made by SOLEL, the Solel-150.

The bottom line rules of thumb I found are as follows:
1. -Both Solel-150 and Thermomax would be expected to have an average sunny day efficiency of around 42% or so operating with the Rohr air-cooled ammonia absorption unit running at 175°C.

The "Dave's Rule of Thumb" can be summarized as "1/4% thermal collector efficiency drop per °C collector temperature increase OR per percent insolation decrease from 1000 W/m²".

The Rohr unit has a COP of about 0.6 (heat-to-cool), so that means the effective SYSTEM COP of the combined collector plus absorption unit would be around 0.42 x 0.6, or 0.25.

2. -In comparison, a 12% efficient (that's solar to AC power)PV system would , if coupled to a vapor compression cooler with a COP of 3 (electric to cool) would have a combined SYSTEM COP of 0.12 x 3 = 0.36.

3. -Running the solar collectors with LITHIUM BROMIDE absorption chillers would result in higher efficiency of the solar collectors, and a higher net system efficiency. Taking this rule of thumb to apply in this case means the solar collectors running at 100°C versus 175°C would mean a collector efficiency increase of 75°C x 1/4%/°C = 18.75%. The collectors would run at 42% + 19%, or about 61% efficiency.

4. -The SYSTEM COP of the solar collector-lithium bromide chiller system using the single-effect absorption chiller would then be about 0.61 x 0.6, or 0.366.

5. -Running the DOUBLE-EFFECT lithium absorption unit, which runs at 120°C, would mean a slightly lower solar collector efficiency. The DECREASE in the solar collector efficiency would be again found from my "rule of thumb", and the decrease in efficiency due to the 20°C increase in temperature over the single-effect unit temperature of 100°C would be about 20 / 4, or 5%. The collector efficiency would be around 61% - 5, or 56%. The net system COP would be around 0.56 x 1.0 = 0.56.

6. -In comparison, a RANKINE engine system direct-coupled to a vapor compression pump (assumptions: operating temperature 175°C, thermal-to-electric conversion of 18%, (and mechanical shaft power-to-refrigeration COP of 3.0) would have a solar collector efficiency the same as the solar collector efficiency for the ROHR ammonia unit, i.e. a solar collector efficiency of about 42%. The effective COP of the power conversion unit would be 0.18 x 3, or 0.54. The net system solar COP would be the product of the solar collector efficiency with the net COP, or 0.42 x 0.54 = 0.22.

These system COP numbers should give you a feel for the relative size of the solar collector systems required for a given cooling capacity. For example, if you wanted to cool a house or building with the least amount of area of solar collector, you would use the double-effect system coupled to either the SOLEL-150 or the THERMOMAX unit. Since its COP is around 0.56, you would get about 0.56 kW of cooling per thermal kW installed.

In comparison, if you wanted to cool the same building using PV and electric vapor compression refrigeration, the system COP is 0.36, and the relative scale factor for the PV area in comparison would be 0.56/0.36, or about 55% larger.

A system with a single-effect ammonia absorption unit, with its system COP of 0.25, would be bigger still, with a relative area of 0.56 / 0.25 = 2.24 times larger.

Biggest of all would be the Rankine system, with the relative size ratio of 0.56 / 0.22 = 2.54 times larger.

Later, I'll look at adding topping and bottoming cycles, and look at desiccant systems.

Missing from this analysis is, of course, are numerous "frills" associated with each system.

For example, the fact that both the PV system and the Rankine systems could be designed to produce electric power which can be fed into the grid at all times makes them more attractive than the absorption systems. On the other hand, the Rankine and the absorption systems are directly compatible with fuel-burning backup systems, which the PV is not.

Yet another very important difference is the temperature of the waste heat available from all systems. The ammonia absorption, Rankine and PV units, for example, reject heat at an assumed 50°C, which could be used by certain types of desiccant systems or for space heating or air conditioning.

All of the thermal systems could be used in conjunction with internal-combustion engine driven systems, or with microturbines, as both these types of distributed electric systems have waste heat which is potentially high enough to drive these systems. This is not the case with PV, of course.

Developing economic "rules of thumb" for evaluating the value of these differences is our next challenge.

-David Wells

Dave's Air Conditioning (Part Three).

So, O.K., Duane, you put the cat on the web pages. I'll comment.

The system shown is a combined scroll-turboexpander engine. This is yet another variation of a theme of mine, to develop solar engines based on existing mass-produced components, rather than to invent a completely new engine.

I've already sung out in praise of scroll engines in terms of their efficiency and ease of conversion into solar engines.

Now a word about the "turbo-compounded scroll" engine system.

Turbochargers are inexpensive devices consisting of two turbines on a single shaft. One turbine is an expander or engine, the other is a compressor or pump.

Typically, turbochargers are used with internal combustion engines in order to boost output power. The additional energy which is available in the combustion gas is captured by the expander turbine in the turbocharger. The power captured is applied to compressing air.

Turbochargers typically operate with a pressure ratio of about two to three, and at speeds of from 50,000 to 250,000 rpm. They are incredibly powerful little things, with output on the order of several tens of kilowatts with a device weighing a few kilograms.

They are inexpensive. A small turbocharger, new, costs around $350us retail, if you do your shopping right. That means the factory is making it for about half that.

The reason that turbochargers are good for "compounding" the solar engine cycle is due to the fact that the operating pressure range matches the second-stage expansion pressure (more on this later). After the gas is expanded by the scroll, it is typically at a pressure of a few atmospheres. Most positive-displacement engines do not work efficiently at this pressure. In an automobile engine, you don't even try to recover this energy, but rather just vent it out the exhaust pipe.

So, as a second-stage expander for a solar Rankine cycle, they are a good match. However, they are really only good at high power levels, meaning close to ten kilowatts or more, which means that they are only practical in larger systems.

The output of the turbocharger isn't shaft power. The output of the turbocharger is a compressed gas.

In the distillation system shown on the PowerPoint presentation, the gas being compressed is water vapor. Basically, the turbocharger is doing the same function that the centrifugal compressor does in a vapor compression distillation system.

I've structured my patent application to show simply a vapor being compressed, and left it as general as possible. There are some very interesting versions of this that I'll mention here.

First, suppose the gas being compressed is air. In this case, what you have is a medium-sized solar power station producing AC electricity at about 10% conversion efficiency from the scroll(the scroll expander is directly connected to an induction motor/generator, which is connected to the grid), and this same power station is producing compressed air at about 3 atmospheres gage pressure, the rate of production of the compressed air represents mechanical power output at a rate of about 10% or so IN ADDITION to the electrical power being produced.

What you do with the compressed air is certainly up to you. For an example, you could use the compressed air to run a second air device, such as a gas turbine. If there were a solar farm of many of these little turbocharger/scroll units, you could have a single large gas turbine.

The gas turbine would be running more efficiently since the energy normally subtracted from the gas turbine's output by its compressor is now operating with compressed air at its inlet, reducing the work required. Yeah, this is hard to understand, but it works. For big power plants, this may be really attractive.

You could store the compressed air in an underground reservoir, such as a "salt dome". So, in effect, you have mechanical storage.

If you live near a body of water, you could send the compressed air to an air bag under the water. When the sun doesn't shine, you use the air to drive a turbine, producing electricity and cold air (yes, cold air--cold enough to use for air conditioning).

Lots and lots of alternative ideas and configurations.

And, again, so little time and money to go into any of it.

Although the turbocharger is "off the shelf", there is clearly a good amount of engineering work needed to really demonstrate an operational system. You'd have to have a good way to separate the oil from the vapor before sending it to the turbo. You'd need to work out the lubrication issues of the turbine, and solve the issues relating to sealing the compression gas (air, water vapor, or other) from the turbocharger lubricant.

This is a "sensible system" that should be developed. It is small, perfect for a community-sized power system. This sort of small, distributed solar energy plant is what our governments and research companies should be spending their time on, rather than huge central receiver heliostat systems or multimegawatt trough systems.

Solar is distributed. The "waste" heat is a resource, to be utilized with our buildings and cities.

Storing energy as compressed air under a body of water is pretty benign. Not much can go wrong, except the occasional surfacing of the air bag during a hurricane perhaps.

These kinds of possibly huge designs are the stuff that strong presidents take to heart and present to a nation as a part of a dream whose undertaking makes the nation stronger and smarter and richer.

Not being a political sort, though, I'll leave that last line alone, and hope that this won't tip off the group to go into yet another pointless political discussion.

-David Wells

Dave's "Water & Power from
Solar Energy" slide show converted from Power Point.

Power from
Solar Energy Systems
Northside Inc. is involved with systems that convert solar energy into both electric power, cooling, and distilled water. Here is a summary of the technology.

Page 2
Stationary Mirror/Moving
Absorber Concept
Here is a quick one-pager on the solar collector. As you can see, the idea is that it is a solar collector that has a very low profile to avoid high wind loads. The mirrors are nearly flat, and are made of common float glass mirror bowed on steel frames. The absorber moves. This kind of collector is perfect particularly for countries with low latitudes like Bolivia or Jamaica.
Because only the absorber moves, very low torques are needed for the drive motors, which are mass-produced satellite actuators.
The facets are simple. The gently curved tubes are mass produced. The mirror is laminated and shatterproof and hail resistant.

Page 3
What Are Options To
Clean Water?
Let’s focus here on water. How can the collector be used to make drinking water?
The competing technology is filtration (settling tanks, sand filtration, flocculation with aluminum sulfate) and chlorination (adding chlorine to kill the remaining germs).
Reverse osmosis involves filtering, then forcing the water through a permeable membrane. It’s often very cheap, and used for desalination.
Distillation involves using heat. There are a number of improvements on simple distillation. One is called “multi-effect”, and another is called “vapor compression”. Both are more efficient than simple distillation.

Page 4
Purification and

Page 5
Heating vs Chlorine
Required to Kill
Cryptosporidium Spores

Page 6
Energy Is Required for
Here are notes on magnitudes of the energy involved in heat sterilization.
The key thing here is the amount of water you get per kilowatt-hour of heat or electricity.
Now, in most locations in the U.S., electricity sells for about 5 cents to 10 cents per kilowatt-hour.
If you want thermally disinfect water and use electric or solar heat costing 10 cents per kWh to make it, you get 8.6 kg of water for each kilowatt-hour of electric heat you put in.
So, since water weighs 1 kg per liter, you pay 10 cents and you get 8.6 liters of water. That’s about 1.2 cents per liter.
The “thermally purified” water doesn’t have to get delivered at a high temperature, obviously. The heat in the water can be used to “pre-heat” incoming water by using a device known as a “heat exchanger”.
Depending on how complex this heat exchanger is, you could reduce the cost of this process by a factor of from 3 to 10 times or more.
For example, a heat exchanger costing around $150 could “recuperate” heat and reduce the cost per liter by a factor of five. It would recuperate the equivalent of 10 kW of heat. The value of the energy per hour at 10 cents per kWh is one dollar per hour. It very rapidly pays for itself!

Page 7
How Much Energy Is
Required for Simple
Here are notes on magnitudes of the energy involved in each process.
The key thing here is the amount of water you get per kilowatt-hour of heat or electricity.
Now, in most locations in the U.S., electricity sells for about 5 cents to 10 cents per kilowatt-hour.
If you want distilled water and use electric or solar heat costing 10 cents per kWh to make it, you get 1.6 kg of water for each kilowatt-hour of electric heat you put in.
So, since water weighs 1 kg per liter, you pay 10 cents and you get 1.6 liters of water. That’s about 6 cents per liter.
Cheaper than Pepsi, but still not very cheap.

Page 8
How Much Energy Is
Required For Single-Effect
CARS Condensation?
“CARS” is short for “condensation absorption refrigeration system”, and idea where cooling from an absorption refrigerator is used to condense water out of the air.
A “kWh” is a “kilowatt hour”. So if you have a kilowatt-hour of heat, using absorption cooling, you could make at best only about 0.4 kWh of cooling.
Since, from the previous page, you learned that it takes 1.6 kWh of heat to distill one kg of water, and since you need 1.6 kWh of “cool” to condense a kg of water using the “CARS” process, you conclude that you could get 0.64 kg of water for each kWh of heat you put into the “CARS” system.
So, assuming the same 10 cents per kilowatt hour of solar heat, you would be paying about 16 cents per liter of water in this approach.
That’s not very efficient. Let’s compare it to the other methods.

Page 9
What Is Vapor
Compression Distillation?
The absolute most efficient way to distill water is through a process called “Vapor Compression Distillation”.
In this process, you have to have ELECTRICITY or SHAFT MECHANICAL POWER available, NOT heat as with multi-effect steam distillation.
The process works like a “heat pump”, and that means it works like this:
First, you start with water in two chambers. The chambers are hot, so the water in the chambers is at a temperature nearly or greater than atmospheric pressure. You have a centrifugal compressor. Find it in the drawing.
When you spin the centrifugal compressor, it forces its inlet to a low pressure, and its outlet to a higher pressure.
Now, the outlet water vapor at a higher pressure is forced to condense in a chamber, shown in blue. This condensation process gives off heat. The heat is then used to boil the cooler water( shown for some reason in RED), and this cooler water evaporates and goes into the inlet of the pump.
Systems like this can be VERY, VERY efficient. In fact, for each kWh of mechanical shaft power you put in, you can “pump” the equivalent of twenty three times the heat
So, in comparison to simple distillation, that same 1.6 kWh to make 1 kg of water now gets you TWENTY THREE kg of water. Efficient!!
At 10 cents per kilowatt-hour of electricity, that’s 0.4 cents per liter!

Page 10
Which Distillation Method
is More Efficient?
High efficiency translates into low cost per gallon.
Here’s a graph I like. It shows the amount you’d pay for the various processes.

“Multi-effect” distillation is a process I haven’t covered with a detail graph. I will go over that later when I get a graph to show it.

Page 11
Lowest-Cost Solar Vapor
Compression Distiller
Now, in the previous graphs, I was talking about the “Vapor Compression Distillation” system.
It requires SHAFT POWER or ELECTRIC POWER to work.
But with a solar thermal collector, you don’t have shaft power or electric power. You have heat.
How do you convert the heat from the solar collector into the shaft power required for the vapor compressor distillation system?
There are a number of ways to do it.
Here’s the first way, which is pretty simple. It uses a thing called a “vapor jet compressor”. The vapor jet compressor is a neat, cheap, simple device(no moving parts!!). All it is is a steam fitting. This little steam fitting has on it a nozzle. The nozzle takes the high pressure steam that you create in the solar collector into a supersonic high speed steam jet.
Combined with a vapor compression distillation system, it does the same job that the centrifugal compressor does.

Go back to that drawing, and then to this drawing to see how it fits in.

As far as efficiency goes, the steam jet is not that great, but it is very cheap!

Page 12
Vapor Jet Pump
Vapor jets are SIMPLE because there is no rotating part.
Again, it’s just a little brass fitting!
So, as a result, this method is LOW IN COST.
In effect, the little vapor jet compressor does two jobs:
-It acts like an engine, converting steam into mechanical motion
-It acts like a centrifugal compressor, pumping low pressure gas from a lower to a higher pressure.
There is a down side, though.
In comparison to other engines, it is below 50% conversion efficiency in comparison to “Ideal Carnot Efficiency”.
Other engines would have a greater efficiency, close to 80 or 90% of Carnot efficiency.
So, keep in mind that you are trying to get the most performance out of the solar collector.
While the “vapor jet” is cheap, it is perhaps a little too inefficient to justify its use with a solar collector system. But it IS a viable way to go, and it is SIMPLE and RELIABLE. For each 1.6 kWh of heat, you could generate effectively four kg of water. Cost per liter of water is then about 1.5 cents.

Page 13
System “SSTEPS”
Now, here’s a pretty complicated-but EFFICIENT-alternative: the “SSTEPS” system. It’s a system that I’ve got a patent pending on, but it hasn’t been built just yet. Let’s see how it works.
In this system, dubbed the “SSTEPS” system, the heat from the solar collector is used to boil a material (not steam. It’s a “freon”).After being boiled, the freon gas is sent into a “Scroll expander”. This is simply a modified air conditioning compressor! There are two ways to make this compressor: either from an existing hermetically-sealed air conditioning compressor, or from an automotive air conditioning compressor. ANYWAY, the interesting thing is that ELECTRICITY is produced from this system. It’s not that highly efficient--only about 10%--but, heck, that is electricity that you have to do with what you want!Next, the outlet gas from the scroll expander is sent back to the solar collector, and “re-heated”. The vapor pressure is much lower, only around 50 psi at that point.
After coming back from the solar collector, it goes into another mass-produced engineering marvel called a turbocharger. A turbocharger is TWO turbines combined back-to-back: one acts as an engine, the other as a compressor. It spins at an incredible 50,000 rpm, and virtually sings as it converts the mechanical energy of the compressed gas into shaft power--that then is converted by the compressor into useful work: it is so much more efficient than the vapor jet, that the turbocharger generates about three times more water as a vapor compression distillation system than the vapor jet.

Page 14
Cost per Liter of
Distilled Water from
So although the SSTEPS is complex, it is efficient. It is estimated to be twelve times more efficient than distillation.
In terms of cost of water, the system delivers water at a cost of about a half a cent per liter. But note: this is based on the cost of the SOLAR COLLECTOR HEAT being about 10 cents per kWh. It does NOT include the cost of this conversion system. However, it is estimated that the cost of this system could be very low.
The important point: The SSTEPS is an ELECTRIC POWER GENERATION SYSTEM that also happens to provide water. Another way to look at it: It is a water system that also happens to provide electricity.
Not to confuse things even more, but here is another point:
The system can be used to produce cooling for a building as well. How? The vapor compression unit shown is pumping water vapor for the distillation process. But it doesn’t have to be limited to just pumping this type of vapor. It could also pump a refrigerant gas. The refrigerant gas could be freon.
Confusing? Let’s drop that point for now and go on.
In summary: the SSTEPS is, again, a system made up of off the shelf automotive parts that are being mass-produced. It’s an efficient system for making distilled water. Its efficiency is similar to simple sterilization, and it can make electricity too. It’s complex, will require development--but worth it!

Page 15
Northside Water &
Power Summary
In conclusion, you’ve seen the technical concepts of various ways to make purified water from solar heat.
You’ve seen the simplest system, sterilization, is efficient and simple, and it should be the type of system we put in the field in phase 1.
The Phase 1 system will prove out the solar collector, and do the useful function of making sterilized water. It’s simple, should be very reliable.
In the future, the same solar collector can be coupled to the Phase 2 system: the SSTEPS.
SSTEPS will not only make DISTILLED water--but electricity too.
This system will be more complex, and there will be a much greater “learning curve” to overcome. However, the big advantage is that it will produce electricity as well as water, and with the same low-cost solar collector. Thus, a Phase 1 system could be upgraded to be a SSTEPS.
The SSTEPS will produce electricity at a cost far lower than today’s solar cell panel prices. The social benefit will be enormous.