Annual System Efficiencies for Solar Water Heating

In this paper, we investigate annual system efficiencies for solar water heating as a basis for extrapolating performance across a range of climates. Annual system efficiency is found to be remarkably insensitive to climate, based on TRNSYS simulations for 167 cities in the continental United States. If a single efficiency value from a representative climate (such as Topeka, Kansas) is used forall 167 cities, the root-mean-square (rms) error is about 0.5 percentage points of efficiency. Efficiencies increase slightly with decreasing ambient temperature; in the coldest climates efficiencies are approximately 1.2 percentage points higher than the 167-city mean, and in the hottest climates efficiencies are approximately 0.7 percentage points lower than the mean. Efficiencies increasewith increasing hot water loads; for the specified solar water heating system, the 167-city mean efficiencies are 33.4, 39.4, and 42.9% for hot water loads of 150, 225, and 300 liters/day, respectively. Thus, if an annual system efficiency is known for a particular system, load, and location, it is a simple matter to extrapolate performance to any (or all) other locations in the continentalUnited States.