This fourth edition emphasizes solar system design and analysis using simulations. The
design of many systems that use conventional energy sources (e.g., oil, gas, and electricity)
use a worst-case environmental condition—think of a building heating system. If the
system can maintain the building temperature during the coldest period, it will be able to
handle all less severe conditions. To be sure, even building heating systems are now using
simulations during the design phase. In addition to keeping the building comfortable during
the worst conditions, various design choices can be made to reduce annual energy use.