Four out of ten solar systems were monitored for a period ranging between 5 months and 2 years, starting in April 2005 up to December 2007 [6] . One of the targets was to calculate the efficiency (n) of the solar system as a whole. This may be defined as:

П = — (Equation 1)

Hincl * A

Useful output energy (hot water) = m*c*AT

mass of hot water usage = volume of water * density of water

specific heat capacity of water = 4.2 kJ/kg K

difference in water temperature before and after sunshine during one day in-plane solar radiation (kWh/m2/day) area of the collector (m2)

Since solar radiation could not be measured on site, data was taken from the nearby Institute for Energy Technology (8 km away) and used for this project. Modelling was also used to calculate the expected solar radiation at the inclination of the tubes (45° to the horizontal), since the available data at the Institute was at an inclination of 36° to the horizontal. Figure 3 shows the comparison between the calculated and measured data at an angle of 36°, which proved to be sufficiently accurate, with a maximum difference of ±8% for clear skies.

Figure 4 shows a compilation of the efficiency results for four solar systems. Although all apartments had the same number of occupants (2 adults each), the efficiency of the solar systems varied dramatically between 30 and 80%. This was mainly caused by the different lifestyles and the amount of hot water consumed. Higher efficiencies implied that hot water was being used in larger quantities, giving the chance for cooler water to absorb more energy from the sun. On the other hand, lower efficiency systems were associated with careful users of hot water. It was interesting to note that all users were satisfied with the performance of their systems and had sufficient hot water without reverting to the electric back-up heater. This was purposely disconnected throughout the data acquisition period.

100 80

60

O’"

40 20 0 100 80

60

O’"

40 20 0

Fig. 4. Typical monthly efficiencies for 150-litre solar heaters, used by couples in four different apartments.

Consequently, it was concluded that the term ‘Efficiency" could confuse the users to understand the effectiveness of their system, as they would associate low efficiency with bad performance, which was not the case here. A new term had to be introduced to better describe the system’s performance as shown in the next section.