|This section covers how you go about adapting the collector loop plumbing
and pump design to your particular collector size and to the elevation
distance between your tank and your collector.
These are the rules that you must carefully follow to have a drain back system that will drain reliably and give good freeze protection:
- The supply and return line between the solar storage tank and the collector must all have a continuous slope down toward the tank.
This allows all the water in the plumbing and the collector drain back to the tank when the pump turns off.
This is critical for freeze protection.
The slope should be at least 1/8 th inch per foot, but 3/16 th or a 1/4 inch is better.
Be careful to support non-rigid lines (eg PEX) often enough so that they can't sag enough to take the down slope out.
- The bottom of the collector must be above the water line in the tank so that water can gravity drain from the collector down to the tank.
- The collector absorber array must also be tilted such that it will also drain. The corner where the supply line enters must be the low corner of the collector -- see picture below.
- All of the lines to the collector should be at least 3/4 inch in diameter. Smaller diameter lines may not drain back reliably.
- The risers in the collector should not be smaller than half inch for good drain back. This rule is sometimes broken, but you are taking a bit of a chance.
- The return line must terminate above the water line where it comes back to the tank. That is, there must be an air gap between the end of the return line and the water line in the tank. Without this air gap, the collector will not drain, because there is no place for the air that is needed to replace the draining water to get in.
- The pump must be constructed in such a way as to allow water to flow back through it into the tank during the drain back. All of the common pumps will do this, but I would be suspicious of self-priming pumps.
Some pumps (eg Grundfos) come with a check valve installed to prevent reverse flow -- this must be removed.
Drain back systems are the simple and highly reliable and provide excellent freeze protection, BUT, you have to follow the rules above or all bets are off.
I have two drain back systems that have been operating for years through our -30F Montana winters with absolutely no problems at all.
In a nutshell, the pump must met two basic requirements:
- When the system starts up, the pump must be able to get the flow established by pumping water from the tank up to the top of the collector. This requires that the pump have enough pressure head at startup to get water all the way to the top of the collector and a bit extra to get some flow going. Once the collector and return line fill, the required pump pressure drops because the return line pressure recovery balances the supply line pressure requirement.
This startup pressure gets more and more important as the collector elevation above the tank increases.
For many systems this startup head will be the main factor in picking the pump.
- The 2nd requirement is that the once flow is fully established that the pump provides enough flow to remove the heat from the collector efficiently.
This pump sizing procedure for drain back systems will lead you through the whole process...
- Run the inlet pipe for the pump through the storage tank wall below the water level. This can be done with an EPDM lined tank by (very carefully) using a bulkhead fitting that is made for this purpose. There is an obvious opportunity for a leak here, but if done carefully, this approach can work fine. - The more commonly used method is the run the inlet pipe for the pump from the bottom of the tank, up over the side of the tank, and then down the outside of the tank to the pump. this is called a U-tube inlet because its in the shape of an inverted U. Once the U-tube has been filled with water and purged of air, it (and the pump) will retain their charge of water during drain back, and will start up fine the next time.Most of the pumps used in these systems require that there be some positive water pressure at the inlet of the pump. This is best accomplished by mounting the pump as low as possible so that the depth of the water in the tank provides the needed inlet pressure. This is why we mounted our pumps down as close to the floor as we could. Pumps can be sensitive to pressure losses in the U-Tube, so if your system is a relatively high flow rate system, you should go to 1 inch pipe for the U-Tube. When first starting up the system, it is very important to purge all the air out of the pump inlet line. If the pump makes noise while running, it very likely still has some air bubbles, and needs to be purged again. Note the valves on our system to allow the use of a garden hose for purging air and filling the pipes.
It is possible to use a PV panel driven pump to power this system. The obvious advantage is that it does not use any power from the AC mains, and this saves some money on electricity and some carbon emissions. I will say up front that I am not the biggest fan of the PV powered pump for this system for these reasons:
- There are some who say that its the perfect match because the PV panel can not only power the pump, but it can also be the pump controller. The idea is that the PV panel will only generate enough power to run the pump when the sun is out, and that's exactly when you want the pump to run. I believe that this does not work nearly as well as it would appear at first glance for these two reasons:
The PV panel does not know anything about the temperature of the water in the storage tank, and this is important. When the water in the tank is fairly hot already, the system needs to wait longer to turn the pump on -- basically there has to be more sun because the collector has to be hotter than the already pretty hot storage tank. On the other hand if the storage tank temperature is cool, then the pump can turn on earlier because the collector does not have to be that hot to warm the cool water from the tank up. A direct driven PV pump will turn start the pump when the sun reaches a certain level regardless of tank temperatures -- sometimes this will be just right, but other times it won't, and the consequence will be that the system might actually cool the water in the tank instead of heating (the hot tank situation), or some water heating energy will be missed because the tank water is cool and the pump could have turned on earlier. For a regular pump that is controlled by differential controller, this does not happen because the control knows the temperature of both the tank and the collector, and can turn the pump on at the right time.
This same problem can happen again as the sun is going off the panel in the afternoon.
The other problem that can occur is due to the fact that the efficiency PV panels and thermal panels have opposite dependencies on temperature. PV panels get more efficient as it gets colder, and solar heating panels get less efficient as it gets colder. This means that if the PV panel is setup so it starts the pump up at just the right time when its (say) 50F out, then on a morning when its (say) 20F out it will start the pump to early because the PV panel is more efficient and generates the pump startup current with a lower sun level, while the solar thermal panel is less efficient and actually need a higher sun level to get to a temperature that is right for startup.
So, for those two reasons I don't think its a good idea to use a PV panel directly driving the pump. Its not that it does not work, but its not (I think) an efficient way to go, especially in cold climates.
I'm not alone in thinking this as evidenced by the appearance of several differential controllers that are specifically made for systems that run a pump off a PV panel. These controllers run on PV power, and the combination of one of these controllers a PV panel and a suitable pump does overcome the two problems mentioned above, but this is getting to be a pretty pricey solution.
Other reasons things to consider for PV pumps:
- You will likely pay a fair bit more for the PV solution, and I think that if you put this extra money into just making your collectors larger you will come out far ahead in net energy saving. If it costs an extra $200 for the PV pump solution, this could be used to add nearly 30 sqft of additional collector, which would generate an additional 1300 watts of power under full sun conditions. That's a whole lot of power compared to what it costs to run a regular AC pump.
- If you have or plan to do a grid-tie PV system, I think its more cost effective and simpler to just add a little capacity to that system to drive your solar heating system pump. This is not only cheaper and simpler, but it will get benefit from times when the sun is out but the solar heating system is not running -- say in the summer when the tank is already heated to its maximum temperature.
- If you pick an efficient pump that is powered off the AC mains, the energy it uses compared to the energy the solar heating system produces is just very small. However, pump power can become significant if you pick a low efficiency pump, or one that is much larger than is really needed.
- If the collectors are well above your tank (for example your tank is in the basement and the collectors are on the roof), then it will be hard to find a pump that will provide the startup head to get the flow going and still be a pump that can be powered by PV.
Or, if your solar heating collector array is large and requires a fairly high flow rate, it may be difficult to find a pump that can be PV driven that is large enough.
If you do decide you want to drive the pump from a PV panel, then I would keep these things in mind in selecting the PV panel and pump and controller:
- For the reasons mentioned above, it would be good to use a differential controller -- see the Controls Section for some examples of ones that work with PV...
- For a drain back system, the pump needs to be able to put out enough head at startup to get the water up to the top of the collector. This takes more power from the PV panel at startup than is needed at startup for a closed loop system. So, its good to make sure that the PV panel larger enough to provide the needed power in the morning. A linear current booster can also be used to help the pump get going on less sun.
- Also, bear in mind that not all DC pumps are suitable for running direct from a PV panel. The pump has to be able to tolerate a fairly wide range of voltages. Some pumps (eg the Laing D5 pumps) are made to be driven by PV and even have hardware/software in their electronics to maximize the PV panel output (MPPT).
This all sounds a bit complicated, and it is more difficult to bring it all together on a drain back system, but its possible -- I ran the $1K solar water heating system from a March DC pump and a 16 watt PV panel successfully for quite a while.
If you find for any reason that a drain back system will not work in your situation, then you can probably switch over to a closed loop system.
Going to a closed loop system can make sense if:
- You cannot locate your storage tank below your collectors.
- Your collectors are so far above your storage tank that you can't find a pump with enough startup head to get a drain back system started.
- It is not possible to install the plumbing from the collectors to the storage tank with a continuous down slope.
To build our system as a close loop system, you need to make the following changes:
- Use non-toxic antifreeze in the collector loop plumbing (this provides your freeze protection). The antifreeze must be checked each year, and replaced when needed.
- Add a heat exchanger to transfer heat from the collector loop to the solar storage tank. In most cases, this can be a coil of copper pipe immersed in the solar tank that carries the collector loop fluid.
The heat exchanger could be similar to the ones used in commercial drain back tanks like this one...
One company appears to use a heat exchanger sizing rule of about 20 sqft of collector area per 1 sqft of heat exchanger area...
- Add an expansion tank, pressure/temperature relief valve, and fill valves to the collector loop.
A good source of information for installing closed loop systems is the Home Power article on closed loop systems that is listed in the Basics section of this page ...
Gary Feburary 16, 2011