The SWEC (Sub-wet bulb evaporative chiller) is a unit that is used in our home to cool water through evaporative cooling. Water is cooled through evaporation when a phase change occurs and the water at the surface absorbs enough energy from its surroundings to become a gas instead of a liquid. That energy absorbed comes from the air and water around it, therefore cooling the rest of the water while only a little is lost when it becomes water vapor. The cooled water produced by the SWEC is stored in a cold water reservoir for use in the radiant system when the home requires active cooling. The SWEC is powered by a typical wall socket providing 110V so residents merely have to plug it in and then sit back and relax.

Evaporative chillers are much more efficient than most common air conditioners. They achieve COPs of about 10, compared to a COP of 5 for a heat pump which is the backup option for generating cold water in Casa del sol. The COP is a measure of efficiency calculated by amount of energy produced divided by the amount of energy used. This means that for 1 Watt of power, you can generate 10 Watts of cooling with an evaporative chiller. But the SWEC beats even this impressive figure with a COP close to 20 under certain conditions. By using the SWEC to pre-chill the cold water, we use a fourth of the energy used by the heat pump to generate that much cooling.

The SWEC benefits residents of Casa del sol by significantly decreasing the amount of electricity required to cool a home. Given the warm Southern California climate, a large portion of people’s electric bill is spent of air conditioning. By using the more efficient SWEC, we are able to cool the home with much less power which means the residents don’t have to buy from the power company–they are able to produce all the power they need with solar panels.

By Teagan Barnes, Solar Decathlon