MIT develops new "reverse air conditioner" solar power system for the developing world


August 16, 2012

Matthew Orosz and Amy Mueller work with locals in Lesotho to implement their solar ORC system (Photo: STG International)

Matthew Orosz and Amy Mueller work with locals in Lesotho to implement their solar ORC system (Photo: STG International)

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Solar power would appear to be an obvious choice for the developing world, but as impoverished regions need systems that are simple, self-operating and cheap to build and maintain, this is generally not the case. The ability to provide heating in addition to electricity would also be beneficial because many communities need hot water has much as they need lights. An MIT team has developed a solution that meets these needs with a solar power system that is an air conditioner built backwards.

Mention of the developing world brings up images of deserts, jungles, veldts and other hot climates, but some poor regions lie in the temperate zone. In southern Africa, for example, it can get very cold in the winter time. MIT graduate Matthew Orosz noticed this while working for the Peace Corps in Lesotho where local clinics needed not only electricity, but access to hot water. “We’ve had nurses tell us they avoid washing their hands in the winter, because the water is so cold,” he said. “So hot water is very welcome.”

According to Orosz, 30,000 clinics and 60,000 schools worldwide lack electricity, but enjoy enough sunshine to meet their energy needs. In a paper to be published in the ASME Journal of Engineering for Gas Turbines and Power, Orosz describes how the MIT team that he helped establish, set up a non-profit organization called Solar Turbine Group (STG) to develop new solar power systems for the developing world.

The answer they came up with is a new type of solar power system, which was first installed at the at Matjotjo Village Health Clinic in Lesotho. The prototype looks pretty conventional with parabolic mirror troughs focusing sunlight on a tube. However, looks can be deceiving because this was neither a steam or hot water power system. Instead, STG developed a system that uses an Organic Rankine Cycle (ORC).

An ORC system is basically one in which the water is replaced with an organic fluid similar to that used in commercial air conditioners. In fact, the STG system not only uses air conditioner fluid, it is an air conditioner that runs in reverse.

An air conditioner uses a liquid with a very low evaporation temperature, such as ammonia or an organic chemical. In the air conditioner, a compressed liquid expands into a gas inside a coil. This expansion draws heat out of the coil and makes it very cold. This cold coil is what cools the warm air as it blows over the coil. The gas then goes through a compressor, which turns it back into a liquid, but makes it hot in the process, so it goes through another coil that cools it with outside air before sending it back to expand in the first coil. To put it very simply, electricity and hot air goes in and cold air comes out.

In the STG system, the cycle runs in reverse. Instead of cooling a house, the system uses the sun to heat the liquid and turn it into a hot gas. This expanding hot gas turns the generator and some of the waste heat also goes to making hot water. It’s basically a mechanical version of how Michael Faraday described an electric motor. Run electricity through a motor and it turns. Turn the motor and it makes electricity. By adding a chiller stage, the system can not only generate power and heat water, but it can also chill things as well.

The key to all of this is the system’s scroll expander. This is a backwards version of a common air conditioning part called a scroll compressor. This link to air conditioning technology isn’t coincidental. The team deliberately chose to use air conditioning parts to keep down costs and make maintenance easier on the grounds that it’s better to use off the shelf bits than have to order special parts or fabricate them yourself.

A scroll expander is made of two coiled metal sheets, one fitted inside the other like two rolls of paper. The inner one is rigged to turn eccentrically inside the other like an intentionally wobbly wheel. As the inner coil turns, it presses against and falls away from the outer coil. When used as a compressor, this pushes and squeezes the gas in the air conditioner into a smaller and smaller space until, by the time it reaches the center of the coil, it’s a fluid.

When used as an expander, the hot fluid at the center pushes the inner coil away, providing more space and allowing the fluid to evaporate and expand as it travels to the edge of the coil. As it travels, the inner coil spins and this turns the generator. Meanwhile, the gas is cooled by a condenser before returning to the mirror trough for reheating and the cycle continues.

Initially, the STG system required a skilled operator to handle voltage changes, but more recent versions employed a computerized control system. The system now runs itself and the only maintenance it requires is cleaning of the mirrors every six months.

As to the future of the STG system, the latest version is undergoing tests in Florida and the Lesotho clinic is closed for renovations during which the system there will be upgraded. The STG group plans to test five more units at African clinics and schools. However, the real key to the success of the system is if it works under local conditions.

Daniel Kammen, a professor of energy at the University of California at Berkeley, not a member of the project, said, “There are a number of exciting solar thermal technology options, including but not limited to that being tested by STG International. All hold promise. The challenge is not in the basic hardware, but in sustainable, viable field operation, but the jury is out until these facilities function in the field, operated by the local communities.”

The video below explains the STG solar ORC system.

Source: MIT

About the Author
David Szondy David Szondy is a freelance writer based in Monroe, Washington. An award-winning playwright, he has contributed to Charged and iQ magazine and is the author of the website Tales of Future Past. All articles by David Szondy

"The gas then goes through a compressor, which turns it back into a liquid, but makes it hot in the process, so it goes through another coil that cools it with outside air before sending it back to expand in the first coil."

Not quite; The gas goes through the compressor and is compressed into a superheated vapour which enters the condenser and as it cools, becomes a liquid before sending it back to the evaporator.

(You can't compress a liquid)


It seems to me that if you take a solar water heater, an absorption refrigerator, and a Sterling cycle engine you could produce hot water, refrigeration, and electricity. Use the waste heat from the refrigerator's condenser to power the engine.

re; Mic

Not all liquids are incompressible.


Um no thats the refrigeration cycle this is reverse of that. It uses the properties of the refrigerant you don't actually compress the refrigerant. It uses heat to turn the refrigerant from a liquid to a vapor and that expansion powers the turbine. Then you cool the refrigerant back to a liquid state and use a liquid pump to push it back to be expanded and spin the compressor backwards. You need a temperature differential to make it work. This isn't new technology. This is used in waste heat recapture and geothermal applications. What makes this truly astounding and hats of to the STG team is they found a way to do it with off the shelf technology scroll compressors are everywhere. Before it required the turbine to be machined out of a single peice of aluminum. The problem is they leak. But a compressor holds the refrigerant better and reduces maintenance.

Rocco Bradford

What's wrong with an off-the-shelf standard energy efficient heat pump such as the Mitsubishi Ecodan? We are installing loads of these combined with solar photovoltaics. Tried and tested technology with major backup. Can run on off-grid battery systems also. We have such a system operating in the Island of Sark, Channel Islands from an inverter.

Paul Fletcher CEO

re; Paul Fletcher CEO

We can start with this is not an air conditioner it is an exotic steam engine driving a generator.

But then there is that you are using AC motors with a DC power supply.


We could call such a device a "heat pump."

Iman Azol

why not just hot water panels and a storage tank? ? wle


Solar PV panels were first widely used in Papua New Guinea and not the USA or Europe. The cost of getting fuel to locations where power was provided by a generator was easily offset the one time cost of installing PV panels instead. A PV system producing electricity is as simple as anyone can imagine and takes no skills to install and requires no maintenance other than washing off the panels once a month.

The most efficient use of solar is to directly heat water and this can be done with a 100% passive system that requires no electricity. Leave it to MIT students to create a complex device that requires a computer to keep it operating and a power system to keep the computer running.

Want to see how this should be done read about the designs of Victor Papenek and his students. Start with his book, Design for the Real World.


Great idea but A/C scroll compressors only have a 3-1 expansion so this unit can't be very eff, no more than 15%.

If they designed a much higher expansion motor to extract more power of say 30%, then the other componants can be 50% of the size, lowering costs greatly.

A lower tech uniflow steam unit at 30% eff that they can fix, rebuild and maintain and even produce them would give not only energy but jobs too.


I commend the effort, goals and objectives in helping the developing world. But does it really help the developing world? To really help them and advance our own "developed world" why not deploy modern technology like Mr. Fletcher suggests as the the KISS rule applies in both developed and developING worlds.

Maybe I missed something but they didn't really point out where they acquired the mirrors, pipes motors, fans and all the other moving parts to make this apparatus work. A parabolic mirror can't be found under a rock so building one that is efficient, hauling it over rough terrain without damaging it and maintaining it for any length of time will cost a lot more work than to just use suitable technology. The probability of error increases as a square of the number of parts. (sorry)

My suggestion, in due consideration of the big picture, the most effective way to deploy heating and refrigeration to the developing world and to indigenous people might be to use the peltier system which is solid state and with no moving parts. The bill of materials for this project will fit on a bar napkin and can be hauled on the ground or frankly dropped by an aircraft to the "remote location" with relatively low low probability of damage. It and could be assembled with Swiss army knife.

One must consider all the parts and tools required to put it together, probability of a mistake in doing so, then where all this "stuff" will really come from and what happens if (when) a human is accidentally exposed to the "organic" refrigerant.

Why not try a small camping frig, 12V battery and PV panel or two. As demand mandates, scale that up to meet your requirement. The probability of it getting destroyed in route is much lower.

PS: Some indigenous people are that way by choice. Many have a higher quality of life than we in the "developed world" do.

Jody Price

At some stage of the process there is going to be cooling of the ambient air, which should result in condensation of atmospheric moisture. If it isn't already there, why not fit the cooling cools with water traps, then allow to condensate to feed (under gravity) through a U-shaped pipe with aquarium grade sand in one side and activated charcoal granules in the other? This should give a small amount of potable water for very little cost. If further treatment is necessary a small ultra-violet light tube (again from an aquarium filter system) could be used to sterilise the water, and this could run off a photovoltaic array and battery attached to the parabolic dish which tracks the sun. With extra power from more PV panels, a small pump could be powered to drive the water through a reverse osmotic filter system. The water which results from this system would probably be of higher quality than that of most "first world" cities.


They sure go through a lot of technobabble to avoid calling it what it is, a heat pump, and try to make it seem like a new thing.

Simple solar thermal heating should be enough to provide hot water for use in Lesotho and other places with similar climate.

Buy up the big old C-Band TV dishes sitting idle, spray them with mirror paint like Alsa Mirra Chrome or Spaz Stix Ultimate Mirror Chrome or use one of the silvering systems like Spectra Chrome.

Those all use an extremely tough clear coat to protect the shiny finish.

With a 6 to 10 foot parabolic mirror in the tropics, a 500 gallon water tank, a pump and a collector at the focus - you have the elements to build a very good water heating system.

30 or so years ago The Mother Earth News ran some articles on such a system, but instead of a dish the builder used many pieces of flat mirror, all angled to focus on the collector, which could hit over 1,000 degrees Fahrenheit. It was crucial to keep water flowing through or the collector would melt. That inventor used it to run a steam engine connected to a generator. Look for the article in the magazine's website archives.

There have also been experiments combining solar thermal with photovoltaic, using a solar cell at the focus and keeping it from melting via a lot of water flowing through a heat sink. The water could be stored in a tank for heating and/or hot water or allowed to flash into steam for running a steam engine or turbine.

This Solar ORC system has more steps in the process and thus is less efficient than simply solar heating the water and storing a lot of it to maintain the desired temperature through the thermal mass of the water.

Gregg Eshelman

Why is all this "innovation" always aimed at the third world? Time and again I see systems that are described as developed by some educational facility that I would use here in beautiful outback Oregon, (I.E. Solar refrigeration) that is not available to the American public. Yes, I understand it is a dire need in deepest, darkest Africa, but if it were marketed here to us off grid freaks, the proven technology would lead to lower costs and off the shelf systems for anyone around the world instead of one at a time projects in politically correct somewhere.

John Cogar

It would be interesting to read a concise history of this development, as the fundamentals are not new. Two or 3 years ago I recall reading about some novelty in making the system as simple as possible. For example, I want to say they were applying for a patent to use an automotive steering pump for fluid transport. The idea was to use commonplace and off the shelf components. But the curious part is the use of parabolic troughs as they are the most sensitive of the solar technologies to the radiation source (intensity and direction) making them least robust, depending on local conditions. It is the defacto method in use for most solar cooling or A/C system installed to date, which haven't become popular in more developed regions due to uncompetitive costs. That is partly due to the capacity factor, owing to the nature of the concentrating tech.

John Sullivan

Noble effort and intentions, but not in keeping with the KISS principle. Photovoltaic panels and inverters, plus simple solar water heaters would do the same job without complication.

Also note that this is not a new invention. United Technologies, using components from Carrier commercial scale air-conditioning systems "invented" and commercialized their PureCycle200 ORC power system several years ago!

Dr Dick

The first refrigerators used natural gas to heat ammonia, similar to what they are doing here (only backwards?). My mom had a similar system in her home until about 2 years ago. It had two loops, one threw off hot water, the other threw off cold water. The cold water loop went to a heat exchanger in her heater to cool the house, the hot loop was vented outside. The hotter it was, the more it ran itself (was going to say use less energy, but it had more work to do cooling the house).


This system has a great advantage of PV and passive solar water heating a small combustion boiler can power it as well. This can be fueled by methane from an anaerobic digester, dried dung, bio contaminated medical waste, biomass, or in a pinch gasoline and diesel.

Parabolic mirror troughs generate higher temperatures than flat panel collectors.

How big of a fire do you have to build to get useful amounts of electricity from a PV panel?


This is a great invention. I think it perfectly fits here in the Philippines where the climate condition is quite abnormal - it 's hot and rains as well during sumer, and vice versa. I wonder how much this costs.

Mortisha Brown
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