The job is only half done if all openings don't have weather stripping. All portals-doors and windows-need double or triple panes, to make an insulating dead-air space. (A sheet of plastic stretched over a window opening, even on the inside, will make an air space of sorts.) Double-pane windows have a dual benefit: they will help keep out the summer heat, as well as keep in the winter heat. A side benefit is that your home will be quieter.
Remote temperature and pressure sensors for solar energy equipments, discussed earlier, should be routed to a central location where they can be monitored. (Southern California Edison used to run 1/8 inch lines to the lab or central collection points. Today they use electrical sensors and run the wires to a control center.) These are worth more than you can imagine; they're an early warning system for change in any part of your solar heating system. It's advisable, both for control and safety, to have an alarm system. Certain key points should be checked by thermistors set to alert you by light and/or sound when one reaches its maximum safe point. This should be a point about 10% below your design maximum-well below the point at which safeties begin to lift, or collectors trip out-of-service. An alarm should give you time to bring the units under control without having to do anything too drastic. (Start or speed up a pump; throw a sheet of white plywood over a collector, etc.)
Your solar power system should be so designed that you can remove one or more of your collecting devices from use when your total system approaches capacity. In an automatically controlled system, it isn't difficult to have a progressive shut down-a cool down sequence. This is a program which, manual or automatic, has plateaus. It can be stopped, allowed to rest awhile, then proceed with shutdown, or reverse, and return to full operation:
Assume a system with a design-maximum temperature of 2500F, with safety valves that lift at 3440 (125# psia), and collectors in four banks. Each collector has its own header, connected to the other headers before all are piped to storage. When a thermometer indicates the storage vessel has reached 2300F, things happen: First, an alarm rings, and a warning light illuminates that gauge. Next, the solenoid which keeps the first bank of collectors in operation is killed; a spring takes that bank out-of-service. (By masking, de-focusing, etc.) If the temperature continues to rise, at 2400F, a second alarm sequence is initiated, and the second set of collectors is taken out-of-service. If you have designed a heatspilling mode, such as a coil routed to a swimming pool or hot tub, this should cut in now. You need to get rid of heat before it damages your components or injures your whole solar energy system.
The third alarm takes out the third set of collectors at 2600F. This is 100 over the designed temperature of storage, but within the 25% safety margin for the vessel. (A 25% safety margin, at least, should be allowed, from design through finished product.) At 3000F. the system goes to full alarm, all collectors set outof- service. And with any luck, you will never need it. Masking, which we touched earlier, and commented on under Safety, is the last step in controlling excess heat. If this doesn't do it, then it may be necessary to cover the collecting devices, or tip them out of focus. Masking is just what it sounds like. Something goes between the collecting surface and the Sun and keeps the rays from striking the collectors. Some heat will still radiate through, depending on the quality and style of the mask. Masking may be a piece of sail cloth, or a piece of plywood with the sunward side painted white or silver; it may be a piece of rigid insulation board. The actual de-focusing of a parabolic collector is the step after masking, although some designers consider the two as steps in masking.
Masking is not always full; but may partially shade all of a collector1s surface, or fully cover part of a collector. It may adjustable, as a bank of shutters, either tied to a heat sensor (thermistor) or by a solenoid you trigger.
