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Heating up to cool down

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It seems counterintuitive to rely on the sun to cool a building, but that's exactly what the West Central Research and Outreach Center is doing with a new solar-powered air conditioning system, one of the first systems of this kind in the country.

Renewable Energy Scientist Eric Buchanan is testing the performance of the system as part of a grant to the WCROC to test small-scale renewable energy through the Initiative for Renewable Energy and the Environment (IREE) with the University of Minnesota.

The solar system at the WCROC doesn't look like the typical large, flat solar panels. The evacuated tube system, installed in February 2009, uses a thermos-like system that traps visible sunlight that heats water inside a copper tube layered between two tubes of glass.

Buchanan says the evacuated tube system was developed in China and is used in half of the solar systems across the world - mostly in China, Australia, and Europe.

The WCROC's evacuated tube system is comprised of 40 panels that have 30 tubes each. The 1,200 tubes face directly south at a 45 degree angle - a match to Morris' latitude. The system has an advantage over traditional flat solar panels because it can heat to higher temperatures. The tubes also collect sun longer during the day because the cylindrical shape picks up more light.

The thermal energy in the evacuated tubes heats water in a bigger storage tank, which holds the water until it is needed in a heating or cooling system.

The WCROC's solar chiller - the air conditioning unit - uses the thermo-chemical process of adsorption. Through this process (explained in more detail at the bottom of the page), water is cooled inside a vacuum, which is then used to cool the building.

Installing a system like this one is not without challenges. One piece of advice Buchanan offers for individuals looking to invest in a solar heating or cooling system is to get a contractor that is experienced.

"The company we worked with had never put in a system this big, and had never put in a solar chiller," Buchanan explained. "They told us there are less than six solar chillers like this one in the country."

Because of some of the inexperience, Buchanan says the system might be slightly larger than the WCROC needs, and there have been some delays adding and tweaking parts of the system.

The Minnesota Solar Energy Industries Association (MnSEIA) offers a list of solar installers on their website,

The entire solar heating and cooling system is being monitored and tested so the WCROC can test the efficiency and cost of small-scale renewable energy systems.

"Right now, installers just put in a system that works, not necessarily one that is the most efficient," said Buchanan.

With information gleaned from the WCROC's own building, Buchanan and others will be able to develop mathematical models of solar energy systems that could be used by consumers to design their own solar systems and test whether using solar energy will be more cost-efficient in the long term for individuals.

"[Solar energy] isn't going to cost less right now," said Buchanan, "but there are other things to think about."

Buchanan emphasized that the WCROC is always open to the public, and that anyone is invited to visit, see the new system and ask questions.

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Solar Thermal Cooling

The basic principle behind solar thermal cooling is the thermo-chemical process of sorption: a liquid or gaseous substance is either attached to a solid, porous material (adsorption) or is taking in by a liquid or solid material (absorption).

This process relies on the principle that water molecules bind more efficiently to certain sorbent materials than to other water molecules. So, if two separate bowls - one containing water, and the other containing a sorbent - are put into a close space, the water will evaporate to get to the sorbent which absorbs or adsorbs the water. If the closed space is in a state of vacuum, the water will start boiling in order to produce vapor at the same speed that it is sorbed.

Boiling itself requires a lot of energy. If the energy is not supplied from outside the system it will be take from the water itself, which, as a consequence gets colder.

A cooling cycle can be created if the chilled water is used to provide air conditioning and the sorbed water is liberated from the sorbent by boiling it out with the heat from a solar-heated fluid. The heat transferred to the sorbent also needs to be removed, typically with a wet or dry cooling tower.