Solar Water Heating Systems – Definitions
Drainback Solar Hot Water System
Some of the words used in solar thermal system design may be confusing to the newcomer, so I thought I would go over some of them in this article.
We hear the words, pressurized, non-pressurized, drainback, draindown, open loop, closed loop. I have heard professionals confuse some of these terms. Let’s go through them one at a time and see if we can clarify their meanings. In the definitions below, we are referring to the collector loop only, not the DHW system, or space heating, or any other circuit.
Open Loop: An open loop system has water entering one end and leaving the other. It is continuously being dumped and replenished. Small DHW solar systems in the tropics work this way. Drinking water is circulated through the collectors and goes directly into the sink, dishwasher, bath, etc. Open loop systems are the cheapest solar system, and fairly efficient, since they have few components and no heat exchangers to waste the heat. However, they are prone to scaling from the minerals in the water, so the high efficiency can be short lived.
Closed Loop: The fluid in the collector loop is captive and stays in the circuit. If it isn’t open loop, it is closed loop. Closed loop systems can either be pressurized or not. All open loop systems are pressurized because they are hooked to the water mains.
Drain down: Open loop systems in marginal climates where freezing is very rare can be protected by shutting off the inlet water supply and opening a valve to drain all the water out of the system. These systems usually have solenoid valves operated by the solar controller to do the draining automatically. There have been many horror stories where thousands of these systems failed in a harsh winter. I don’t recommend them.
Drainback: This is the system illustrated in this blog. The name is confusingly close to the drain down system described above, but they are completely different in structure.
Drainback is a closed loop system, where the collector loop water is captive. The water drains out of the collectors whenever the pump stops into a reservoir. Drainback systems can be either pressurized or non-pressurized.
a) The pressurized kind are usually smaller systems for residential DHW, and use a standard water heater for the solar storage tank. We call this a “single tank” system. The collector loop contains a drain back reservoir, usually 10-20 gallons, an internal or external heat exchanger to put the heat into the water heater, pumps, controls, etc. A small pressurized drainback system looks similar to a glycol system, except the expansion tank is now a reservoir and the check valves and air vents are missing. Both require a heat exchanger to put the heat into the water heater. Single tank systems try to keep the heat from the electric elements from getting into the solar loop and artificially heating the water going to the collectors. This can kill the efficiency of the collectors. To do this, it is necessary to try to isolate the colder water at the bottom of the tank for the collector loop. The design should also keep circulation inside the tank to a minimum. To much mixing of the hot and cold portions destroys the temperature stratification between the (relatively) cold water at the bottom and the hot water at the top. “Mixing” in a single tank system is to be avoided as much as possible. These conditions limit the volume of water that can be considered solar storage and the volume of water that can be ascribed to the conventional water heater. For example, an 80 gallon system may be considered 40 gallons of solar storage and 40 gallons of electric water heater. Such small solar storage limits the total size of the system.
Note that single tank systems are not recommended for gas water heaters where the flame is at the bottom of the tank. This puts the heat source right where the colder water line to the collectors is supposed to be. Conventional gas water heaters should only be used in two tank solar systems (see below).
For larger systems, the pressurized drainback reservoir starts getting more and more expensive. Over 120 gallons, they have to be ASME boiler code rated. That is why you see so many 120 gallon water heaters strung together for storage on larger systems.
b) The non-pressurized type of drainback system eliminates the pressurized reservoir and input heat exchanger. Since the tank is non-pressurized, it does not have to be ASME certified and does not have to cost an arm and a leg for bigger sizes. The heat exchanger is moved from the collector side to the DHW side.
Non-pressurized drainback systems are “two tank” systems. The solar storage is the drainback tank, not the regular water heater. The advantage is the solar tank is heated by solar energy only. 100% of its volume can be considered as solar storage. There is no competition with electric elements, and gas water heaters work fine. There are no tedious methods needed to try to keep the conventional heating system from overriding the solar heating system. Stratification is less of a concern. With no heat exchanger blocking heat transfer, all the heat from the collectors goes into the tank and the efficiency of the collector circuit goes up significantly. Getting the heat out of the tank into the DHW with a demand side exchanger is straightforward.
Finally, non-pressurized type of drainback system can be scaled to any size system, from small to large. The circuit diagram is identical between a small residential system and a large commercial system.
Leave your thoughts in the comment area below!
Dr. Ben
Bradford White now has a natural gas backup solar tank available.
Here’s the description:
http://www.bradfordwhite.com/images/shared/pdfs/specsheets/507-B.pdf
This means that the old rule “Conventional gas water heaters should only be used in two tank solar systems” is now wrong.
Single tank systems ALWAYS win the efficiency and cost contests in every report I’ve ever seen. There is less area for heat loss, less dollars spent on tanks, less real estate occupied.
But here’s the most important advantage of a single tank system– if there is solar heat available, it rises up to the portion of the tank that is normally heated with the backup source. Now the heat loss of the backup tank is made up by solar, not gas or electric. In a two tank system, a recirculation pump or thermosiphon would be required to accomplish the same thing.
In theory, I like this new tank. It works simply by placing the temperature sensor higher up in the tank. That means the lower half of the tank will be cool until solar is available. What seems a little magical is how does the heat reach the top of the tank without heating the bottom of the tank, since the burner is still at the bottom?
Well, in an 80 gallon tank, let’s say the burner has heated all 80 gallons by 6am. Now everyone in the household takes their morning shower and uses 40 gallons. Since the thermostat is high up in the tank, it doesn’t see the new cold water that is now in the bottom of the tank. The burner doesn’t come on, and solar will heat up the bottom of the tank, and if there is excess solar heat for the day, it rises up and helps keep the burner off all night as well.
This effect is significant for tank-style natural gas water heaters, because they typically lose 30% -43% of their heat, so you definitely want to replace that heat with solar. Single tank design is the easiest and best way.
Kevin,
Thanks for the note and the reference to the Bradford White single tank system. From the looks of it, my guess is it is an improvement over previous efforts.
Let’s define single tank vs two tank systems up front for the readers. A two tank system has totally separate tanks. One is a “solar tank” with the collector pump and controls and DHW heat exchanger. The other is a standard water heater. Solar heat is transferred to the water heater two different ways. The first and simplest way is called “single pass”. The cold water goes through the heat exchanger coil in the solar tank to be preheated before going into the cold water inlet on the standard water tank. I like to think of this as having hot well water.
The second heat transfer method is a recirculator system, or “recirc” for short. The components consist of a pump on the solar heat exchanger that circulates solar heated water to the regular water heater. The pump is controlled by the same kind of delta T controller that runs the solar loop, with different set points. All these pieces cost money and labor and contribute to wasting heat from the extra piping. I don’t recommend the “recirc” system except in certain circumstances.
A single tank system uses sections of a single tank for solar storage (the bottom) and for conventional energy (the top). Preventing mixing of the solar heated water and the conventionally heated water occupies all the waking hours of the designers.
Two tank systems do not refer to the small package modules for drain back and glycol systems that have pumps, reservoir, exchanger and controls in one small package. These are still single tank systems because the little module has no thermal storage, it is only a pumping/exchanger package. Some modules have the exchanger built in and some use an exchanger in the water heater.
If the exchanger is in the module, then there are two pumps – one for the collector loop, and one for the exchanger to tank loop. This allows you to use cheaper standard water heaters. If the exchanger is in the tank, then only one pump is used from the exchanger through the collectors. This type system requires a specialized, much more expensive water heater with the heat exchanger built in. The Bradford White system is this type.
Since gas water heaters heat from the bottom, the heat source is right in the middle of the solar portion of the storage, sending artificially heated water to the collectors, greatly reducing their efficiency. This is the origin of the rule that you need to do two tank systems with a gas water heater. It also explains why the new Bradford White solar gas water heater is interesting.
This subject has been discussed since the early ’80s. Which is better – single tank or double tank? The forum http://www.heatinghelp.com/forum-thread/129646/2-tank-systems-Vs-One-tank-system-SDHW has lots of comments pro and con. Some manufacturers of water heaters put steel baffle plates across the center of the tank to minimize mixing of solar energy with conventional energy. We don’t want the conventional energy going into the solar collectors. (But we don’t mind solar heated water going up into the conventional water heat portion). Other manufacturers moved the elements around and cut the dip tube off.
We also need to understand up front that we are talking about residential size systems up to a max of 120 gallons, since that is the largest residential water heater made. If you assume 1/2 that volume could be designated as solar storage, then 60 gallons of storage relates to about 30-48 ft2 of collector to maintain the proper storage/collector area ratios. So, we are talking about residential size systems – no commercial stuff here.
With that as background, let’s look into the current discussion about single vs two tank systems.
I haven’t seen the studies showing a single tank being more efficient than two tank systems. The results are entirely dependent on the system designs tested, the size of the systems, and the testing methods. Are the two tank systems drainback or glycol, single pass or recirc (my guess)? Without a description of the system designs, I can’t conclude anything. The Bradford White gas water heater may be better than previous models, but the literature doesn’t say anything about solar performance, so that doesn’t allow us to jump to too many conclusions about how good it is. It still looks like it heats from the bottom. I will be happy to see some actual test results.
The arguments in favor of a single tank system are…
1. Less total heat loss from less surface area vs having two tanks.
Ans: This is not necessarily true. Consider an 80 gallon single tank system. Assume that 40 gallons of that can be considered solar storage and 40 gallons standard water heater. One could easily substitute a 40 gallon water heater and a 40 gallon solar tank and have exactly the same surface area. In fact, I used to make an 80 gallon solar tank about the size of a washing machine (cube). You could put a 40 gallon standard water heater on top and have a package no bigger than a 120 gallon standard system. In addition, the solar got 80 gallons of storage, not 60 as would be the case in a single tank 120. We look forward to making that same tank as we get the factory cranked up to a higher level.
2. Solar energy contributes to the standby losses, holding off the regular heat source.
Ans: The solar heat rises to the top and can hold off the standard heating elements. This is true. However, I have seen many two tank drainback systems (in NC, including mountains) where the standard water heater is turned off and bypassed completely during the summer months. No standby heat is consumed at all. One example is a family of 6 in Durham, NC. They turned off the standard water heater about June and didn’t turn it back on until late September. I presume you could do this with a single tank system also, but this one has a solar tank of 130 gallons, and water heater of 80 gallons, so this system can’t be assembled from current components at the plumbing supply house.
I don’t think having the solar feeding standby losses is all that smart. I would prefer to cut the standby losses in the first place with an extra insulation blanket, than to waste solar energy.
3. Transferring solar heat into a regular water heater with a pump and controls (the recirc option) is expensive and wasteful of energy.
Ans: This is true, which is why I don’t recommend it except is certain circumstances.
There are probably cases where a single tank system may be the only option, mainly due to equipment availability. I am glad they work as well as they do.
However, I still like the two tank option. Think about it. All of the design effort put into the single tank solar systems: that is, moving the heating elements, arranging the collector heat exchanger, positioning the dip tube and ports – all of that effort is meant to make the single tank system look like a two tank system. Why not just stack a solar tank underneath a regular water heater?
We haven’t discussed cost. I don’t know what the Bradford White solar unit costs. I couldn’t find it on the web. But I did find a Bradford White gas water heater of 72 gallons with the same level of equipment, without the solar coil. It cost $3000 and is guaranteed only for 6 or 7 years. The logic here escapes me – $3000 replacement every 6-10 years plus labor? How about a $600 water heater and a drain back solar tank. It is typical for a non-pressurized drainback system to last 20 years or more. Only the replaceable exchanger touches fresh water. They sure don’t cost $3000 to replace if the region has bad water.
So, I haven’t seen convincing data that a well designed two tank drainback system is less efficient than a single tank system. I also have many years of observations to the contrary. I really think it boils down to design.
Thanks for writing, Kevin, your comments are always appreciated and bring up good points.
Dr. Ben
“Why not just stack a solar tank underneath a regular water heater?”
That’s a great way to do it, since thermosiphons are very smart and free to install and to run. Most basement mechanical rooms don’t have the headroom, however, especially when there’s a flue vent above the tank. Your cubic tank may fix that problem, so put me on the list for one when they are ready.
The B-W 507-B is less than $1500 (with heat exchanger) but you are absolutely correct: A gas fired water heater always needs regular replacement, so why use an expensive one?
You can look at the OG-300 data at the SRCC for apples to apples comparisons of one-tank and two-tank systems. Meanwhile, I’ll track down the reports I’ve seen.
My comments about single tank reports were only about electric single tank systems. Single tank gas fired systems were generally thought to be unfeasible.
Here’s an article that references a report by the Oregon DOE:
http://homepower.com/view/?file=HP124_pg42_Patterson
“In a single-tank system, the solar collectors
can heat water well above the electric element’s thermostat
setting of 120°F (130°F to 150°F is common). Because of this,
the tank water may remain above 120°F even if it cools by
5°F to 10°F overnight. The result? The element in the singletank heater doesn’t need to kick on at all, yielding a 10% to 20% savings advantage in standby losses compared to two tank systems”
The article correctly states that the standby losses of a gas fired tank are even larger than an electric tank.
I called Bradford-White to find out if they had any performance results for their 507-B. I’m worried, as you are, that gas will increase the temperature of the lower part of the tank and destroy the collector efficiency. The engineer I talked to didn’t know of any reports, but he said “Velux thinks it works well.” So, the jury is out until some test results come in.