For example, concerning lower rather than high solar collector stagnation temperatures. Stagnation temperature is approximately the temperature at which a solar collector will settle down in full sun if left there for an hour or more without heat being removed from it, and with the pump switched off. Stagnation temperatures in solar collectors vary from 130C to over 200C above the air temperature in which this occurs. Lower stagnation temperatures are inherently safer and therefore better practice than high stagnation temperatures since they offer far lower boiling risks and lower pressure risks. In addition, low pressure solar water heating systems which typically operate under 0.5 bar pressure are inherently better practice than systems operating at higher pressures. Although the UK and Ireland do not currently insist on thermostatic blender valves being installed, solar installations which include a thermostatic blender valve which limit the water leaving the hot water cylinder to 60C (by blending it with cooler water) are inherently safer than ones which do not, even though scalding can occur at temperatures as low as 45C. However, thermostatic blender valves may reduce the speed at which water flows from taps. This drawback can be reduced, but not eliminated, by selecting a blender valve which is designed for one pipe size larger than the pipes to which they will be fitted: for example on 22 mm pipes, a 28 mm blender valve can be fitted. In all solar installations, heating the cylinder regularly to 60C, right to the bottom is advisable. Twin coil conventional solar cylinders, have an “exclusive solar volume” (often tens of litres) at the bottom of the cylinder with the backup heating usually positioned higher up. Exclusive solar volumes may be inherently less safe, from a Legionella perspective, than conventional cylinders which have backup heating located closer to the bottom. This is because Legionella bacteria are killed at temperatures over 55C, however the bottoms of conventional twin coil solar cylinders may not reach this temperature for weeks at a time in winter, thus potentially allowing dangerous bacteria to proliferate to high levels. Avoiding solar cylinders with a separate pre-heat volume may be advisable in some circumstances, unless they can be regularly heated, right to the bottom to 60C, particularly in winter. Solutions to the above, if considered important, can include using a destratification pump regularly, or having backup heating coils which reach the bottom of the cylinder, which are, perhaps, intertwined with the solar coil, or using direct solar water heating in a normal cylinder which has its backup heating at the bottom. It should be noted here that in domestic installations the UK’s DTI do not require any of these particular Legionella control measures as part of grant aided solar. Panel weight can be a safety issue. When retrofitting solar panels to a roof, heavier panels are more likely to need structural calculations and roof reinforcement, so many panels are designed for lightness. Weight also has health and safety implications when it comes to lifting and handling panels on the ground and on the roof. |
|
