1.3. What is the energy production?

In 3.1 the difference between the input and output method is shown. This gives a large difference in the figures. An example for the Netherlands shows the following figures:

Global insolation: 3.6 GJ/m2/year

Insolation under optimum angle 4.0 GJ/m2/year

Collector output 2.0 GJ/m2/year (assuming 50% average collector efficiency)

System output 1.0 GJ/m2/year (average for 120 measured systems)

Energy saved 1.5 GJ/m2/year (65% efficiency for a reference hot water system)

It shows that the production of solar thermal systems varies between 4.0 GJ/m2/yr (1111 kwh) and 1.0 GJ/m2/yr (278 kwh) if you vary between the input method and the output method.

To make the figures independent of the insolation a formula was proposed by Jan Erik Nielsen from Estif.

E = C * A [m2] * G [GJ/m2]

C = a coefficient dependant on the application.

A = average installed collector area in the monitoring year.

G = the global radiation for the optimal collector orientation for the monitoring year.

For the input method a coefficient of 80% was proposed in 2007, but new information shows that this would be considered too high. It would be logical to take the average collector area for the typical application. For glazed collectors and hot water systems the coefficient would be around 0.50.

For the output method the coefficient can be calculated from monitoring data. Examples from the Therra monitoring project are [12]:

Output coefficient C:

The Netherlands: 0,25 (50 systems)

France: 0,29 (large systems)

France 0,14 (120 small systems)

There is a large difference in the performance. Especially large systems show a lower performance (but a higher solar fraction). More monitoring data should be analysed before an average default value can be given.