A very simple method suitable for making a fast estimate of the long-term performance of solar water heaters is presented here. It is intended to give an order of magnitude of the annual energy output of a solar water heater in any location, with relative uncertainty smaller than 10 %, when compared to a simulation program. This method was derived from a method published by the Centre Scientifique et Technique du Bâtiment (CSTB 1993) for regulation purposes in France. The use of this method is nevertheless limited to standard systems with a storage capacity corresponding to the daily load volume and with standard insulation.

Two parameters define the hot water demand and should be provided by the user: how many liters per day, e.g., 200 l, and the temperature, as a difference with the temperature of the cold water, e.g., 45 degrees C.

Other parameters are asked dealing with the water heater characteristics: type of collector, presence or not of a primary exchanger, and also the geometrical parameters of the collector: tilt (inclination from the horizontal plane) and azimuth (angle from the north-south direction). As a first approach, it is suggested to set tilt to the latitude value of the site, and azimuth to zero.

Once the geographical site is selected, a request to the SoDa databases is initiated and two parameters are extracted: the average clearness index and monthly mean of daily irradiation. Then a request is made to a specific server that runs the European Solar Radiation Atlas (ESRA) blackbox in order to evaluate the solar irradiation on collector plane.

The algorithm produces four parameters: the total daily hot water load (directly linked to the user demand), the daily energy output given by the water heater, the yearly energy output, and the solar fraction, which is the percentage of energy used from that available from the sun.

For a detailed description of the algorithm, see the ESRA handbook, volume 2, page 234, paragraph 5.3.4.1.

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