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.
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