Solar Heating From A Radiant Contractor's Perspective
As a heating contractor, you have probably noticed that solar thermal systems
have some things in common with hydronic heating systems: pumps moving hot water through pipes, electronic controls and phone calls in the middle of the night if it doesn’t work. This puts us in a good position to not only expand into offering solar systems, but gives us potential advantages over solar-only contractors.
I believe that solar thermal systems need to be simple. Years ago, when I first started fixing up heating systems to save energy, I was proud of some of the really sophisticated things I did. But when I came back later and saw my masterpieces disconnected or not working because nobody understood or took care of them, I learned the hard way that unless I stick to things that are very simple, I wasn’t saving anyone any energy.
For solar thermal, the level of system complication depends mostly on three questions:
- 1. How big will the system be - small, medium or large?
2. Will the system heat domestic water only or also heat the building?
3. How will the system be protected from freezing in the winter?
System SizingA large system heats all the domestic hot water plus meets some of the building’s heating needs. It is the most complicated because it needs fancy controls to sometimes heat domestic water and sometimes provide heat, all without ever overheating anything or allowing a back-up heater or boiler to heat the sky.
Because the large system meets so much of the load, it has the highest “solar fraction,” which is what solar-only contractors often focus on: the percentage of the overall energy load that the system can meet. A higher solar fraction, however, can mean the worst economics.
The large system has enough capacity to make heat in the winter, but much of its capacity goes unused the rest of the year. All summer, much of the equipment just sits there producing nothing but loan payments. It might take the longest time to pay for itself.
A medium system is large enough to heat all the domestic hot water without any back-up system, even under special circumstances, such as several overnight guests during a cloudy week. So it, too, will have a lot of capacity that costs money to buy but goes unused much of the time.
A small system has the capacity to meet some, but not all, domestic hot water needs on an average summer day. That means during the rest of the year, when there is less sun and the panels are surrounded by colder air, it needs help from a backup source of heat, such as a conventional water heater or boiler.
Since all of its capacity is used most days, however, there is no capacity that goes unused for long, which makes the payback better than for medium or large systems.
Once the size question is answered, the question about making just hot water vs. hot water and heat goes away: just heat some of the hot water. This can make selling the job a little tougher. But the economic reality is that a homeowner can save the most energy for the least money by installing the small system or buying a sealed combustion boiler, new windows, better insulation, etc.
Freeze ProtectionThe next question is how to protect the outdoor components of the system from freezing. One popular approach is to add glycol antifreeze to the system. Unfortunately, glycol is a carbohydrate, which bacteria can eat and produce an acidic byproduct which corrodes copper. And high temperatures can cook it into a sugar-like substance, which coats the critical heat-exchange surfaces of a solar thermal system, causing performance to drop off.
Overheating can damage glycol during an electricity blackout or equipment failure, which prevents the system from removing heat from the panels. Even when the system is working perfectly, it still needs to remove enough heat to prevent damage to the glycol.
What do you do with the heat when the storage tank is already hot? A common approach is a “dump load heat exchanger,” such as a radiator mounted outdoors, sometimes with a fan. If the fan fails or the pump doesn’t work when it should, the panels will overheat and cook the glycol.
If glycol is starting to sound complicated, consider that it also needs to be kept at a safe distance from drinking water, stored somewhere when the system is drained for maintenance, checked periodically to make sure it is still providing freeze protection, replaced every so many years and, eventually, disposed of in a safe manner.
My favorite freeze protection is a drainback system, which lets the water drain back from the panels whenever the system is not operating. This protects the outdoor components of the system from freezing without glycol or a dump load heat exchanger.
The Tank-And-A-HalfIf successful solar installation was as easy as choosing drainback and simplifying the system by choosing the small size, there must be a reason why solar-only contractors often choose much more complicated systems. I think the answer is that from their perspective, with solar as their only product, they have every reason to put in the system that will capture as much energy as possible. This is understandable, as is another trend common to solar-only contractors: leaving all existing water heating equipment intact.
As plumbing and heating contractors, we’re already responsible for providing uninterrupted heat and hot water, so we don’t need to limit ourselves to add-on solar systems. Instead, we can design and install systems that make the most sense overall, with the solar and conventional parts working together.
To keep things separate, solar-only contractors often add a solar-heated water tank in series with the existing water heater. They go to a lot of trouble to get the solar tank to handle the standby losses from the water heater, which can involve three-way valves, pumped loops and other complications. Or worse, some systems attempt to simply add heat to the existing water heater, which means either overheating the water and possibly scalding people or severely limiting the amount of heat the solar system can add.
Instead of choosing between one tank or two, we can install a tank with one heating coil near the top, heated by a back-up source such as a boiler, and a second coil near the bottom, which the solar thermal system uses to heat the bottom half of the tank.
Convection moves hot water from the bottom of the tank to the top whenever appropriate, which handles standby losses. When someone showers at night, the conventional heat source only heats half the tank, leaving the cold water in the bottom of the tank ready to be heated by the solar system.
Because this type of tank works so well for solar thermal, it should be seriously considered instead of a conventional indirect water heater.
More AffordableWe have other ways to make solar systems less expensive. The next time we have walls open to work on a bathroom, kitchen or heating system, we can run extra pipe that can be used for a future solar thermal system. Not only do these preparations save the owner money, but guess who the owner will call when it is time to install a solar system?
I believe it should be standard practice to install a real energy meter on each system and use it to measure actual production. Some solar controllers do measure temperature difference between the pipes to and from the panels, and multiply that difference by a flow that is assumed based on the pump operating hours.
However, I have seen that type of “meter” recording energy gain while the system was airbound and doing nothing but wasting electricity operating the pump. Real meters that measure actual water flow and temperature difference are what we need to use.
With building energy use perhaps the largest field of human endeavor in which almost nobody measures anything, it is important that we start measuring how much energy solar systems capture and use that information to learn what really works. At the same time, we can show our customers real, measured savings that endure year after year, with realistic payback periods.