Solar Thermal – What to do when the power goes out!


I spent 6 months living in Kathmandu back in the 90s.  It was commonplace for the power to go off each evening for 2 or more hours and to cope with the outages everyone had battery backups and gas-powered generators. 

But over here in Canada we have never needed battery backups or generators to keep things running.  The electrical system is far more reliable here than it was in Nepal in the 90s.  That being said, we do still get the occasional power outages but for the most part they are little more than an inconvenience.  The same cannot be said for solar thermal systems when the power goes out. 

When the power goes out in the middle of a sunny day, solar thermal systems will tend to overheat.  That is because the pumps run on electricity, and when the electricity goes out on a sunny day the fluid in the system can get up to boiling in next-to-no time.   When this happens, the pressure in the system can rise precipitously and usually ends up in a solar thermal “event”.  However, if the sun isn’t shining (at night or on a very overcast day) the likelihood of an event occurring is minimal.

An “event” occurs when either the system’s heat sensors or pressure sensors “blow” (when they sense temperatures or pressures higher than an established threshold) causing the hot water or fluid used to cool the solar collectors to flush out of the system and empty down the drain, thus reducing the pressure in the system and providing a failsafe against pressure or temperature damage to the system and surroundings.  Even with these safeguards, an “event” can be both expensive and nerve-racking.  The expense comes in when you have to replace your coolant and any other components that might have been damaged by the heat.  The nerves come in when you hear the valves blow and see hot fluids discharging into the drain (don’t touch them…they are really hot and they can give you serious burns).

So what can you do to protect yourself and your system from this type of event.  Well, there are only two things you can do to avoid an event when the power goes out on a sunny day.  The first is to have a backup energy system like a UPS (uninterruptible power source) that can take over and power the pump motor while the electricity is unavailable.  The second way to avoid an “event” when the power goes out on a sunny day is to get up on the roof (or wherever your solar array is installed) and cover the solar collectors or physically turn them so that they do not face the sun.  This effectively shuts off the solar collectors.  Unfortunately, this second option can’t be easily automated with current technology so if you aren’t around when the event occurs, it isn’t an option you want to consider. 

This only leaves the first option…installing a UPS.  So what is a UPS, where do you get one and how do you use it.  A UPS is very similar to a power bar that you are used to using to protect your computers and electronic gear from power surges.  It can “sense” and react to variations in current. The difference is that when you have a UPS, it includes some battery backup power along with the ability to sense current variations.  If the current surges, the UPS will cut it off in the same way that a power bar does.  If the current is cut off or reduced (as will happen in a power outage), the batteries in the UPS will immediately take over and keep your system controller and pump motor working until the power goes back on (or until you can cover the array if the outage is a long one).   

Wikipedia has a good article on how a UPS works and you can read it here.  They aren’t cheap, but they are a good investment if your power company has difficulty providing reliable power.

You can buy a UPS at just about any electronics store.  They come in various “sizes”.  The size of the unit is related to the length of time that it will run attached devices until the batteries run out.  The higher capacity the batteries, the longer the unit will keep things running. 

How do you use a UPS in your solar system?  Each solar thermal system has at least two devices that need power.  The first is the controller (a small computer that determines when to turn the pump on and where to direct the hot water) and the second is the pump motor itself.  Both have to be attached to the UPS for it to help you avoid an “event”.  Normally, you will plug the UPS into a wall socket and then plug the controller and pump into the UPS.  This will give these devices isolated and cleaned power when the electrical system is up-and-running, and backup power when an electrical outage occurs. 

In summary, power outages are bad and can cause events.  Uncontrolled events can cause damage and are dangerous.  Installing a UPS can reduce the risk of damage and danger associated with events.  As power companies struggle with failing infrastructure, this type of outage is bound to occur more often.  Be prepared … install and use a UPS on your solar thermal system!

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