RENEWABLE ENERGY SYSTEMS MONITORING
system against future shading factors. PV systems may also need cleaning.
Where they are installed at shallow angles (less than 20°), they may not benefi t from self-cleaning. Algae or moss growth can be common. And because they are located on roofs, it may not be easy to keep a visual check on the need for cleaning. They can also suffer from bird damage or fouling, or damage from objects blown in strong winds. All of these are unpredictable and possible almost anywhere. PV systems also suffer from inverter
failure. Multiple inverter systems might experience partial component failures that could go unnoticed for a long time. Solar thermal systems also suffer from shading, but this is less of a problem than with PV. More signifi cantly, solar thermal systems need to be viewed as any other hydronic thermal system in terms of their maintenance and operating requirements. Solar collectors regularly experience temperatures in excess of 100°C, and the primary circuits must be adequately pressurised. Any loss of working fl uid and pressure can result in vaporisation and fl ow failure. Primary fl ow is often initiated by a differential temperature being sensed
Badly sited installations don’t help
between the collectors and the thermal store. If the controls fail or drift out of calibration, the system won’t work properly. Some systems use heat dissipation
circuits that operate in times of low or no demand. These introduce another set of control elements (sensors, valves and
actuators) that can cause problems. Heat pumps (ground or air) are considered renewable due to the high ratio of ambient energy transfer to fossil fuel input. However, there is only a true carbon advantage of using heat pumps over gas-fi red condensing boilers if the average coeffi cient of performance
This calculation uses solar irradiation data, which may not always be available for a particular site. Sunshine hours may be a good alternative, provided a good relationship can be established for that site. Another, and perhaps better, alternative is to use the original predicted monthly energy yields. At the feasibility/design stage it is likely that the consultant will have used some modelling to predict the yields in order to forecast carbon savings and fi nancial benefi ts. These predicted (or target) values can be used in place of the regression analysis equation to produce the CUSUM graph. Figure 4 shows this for the system presented in Figures 1 and 2. Notice similar trends with Figure 2, showing when the system began to deteriorate signifi cantly. Using this method can still provide a tool for early corrective action. Again, contrast this with Figure 5, which shows the CUSUM against consultant predictions for the well-performing system shown in Figure 3. This shows that the system consistently out-performs predictions, and the actual rate of return can be recalculated
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and compared with the original investment decision. Reporting such successes can be every bit as important as spotting and fi xing failing systems.
Other technologies The same principles shown here can be adopted for the other technologies discussed in this article. However, the variables used to measure performance may change, as may the calculations for reporting savings. For example, the performance of a heat pump will depend on the heat demand (and therefore degree-days), and its comparison to a hypothetical alternative plant (for example, a gas-fi red boiler). Solar thermal systems present other issues, as there may be a mismatch between supply (solar irradiation) and demand (hot water/occupancy levels), which need to be accounted for. Measurement and verifi cation systems, therefore, need to be set up with care, and a good understanding of the monitored technology and its intended use is essential.
Figure 4: CUSUM chart based on actual minus predicted monthly electricity yields from consultant feasibility study
Figure 5: CUSUM chart for a well performing system measured against consultant predictions
August 2012 CIBSE Journal 41
Courtesy of NHBC Foundation
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