MIT develops new "reverse air conditioner" solar power system for the developing world
By David Szondy
August 16, 2012
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.