Solar Basics
Here you'll find information about the different types of
water heating systems. In the future, we will beef this page
up a bit with some more basic terminology and access to more
educational resources.
For now, you can ASK THE SOLAR
MAN your specific questions via email. Please
send your inquiries via our General
Inquiry Online Contact Form.
He wrote the article below for a Basic overview of
solar water heating systems produced by AET.
Active Systems
Active Systems require a pump to circulate the water from
the solar storage tank through the collectors. There are two
general categories of active solar water heating system –
open loop and closed loop systems.
In these types of systems the water stored in the solar storage
tank is circulated directly through the solar collectors where
it is heated and returned to the storage tank. Open loop systems
are very basic and were the first to appear on the market
many years ago. The concept is very simple. A pump controller
(called a differential controller) will turn the pump on when
the collectors are hotter than the stored water and shut it
off when they reach the same temperature. The benefits to
this system are cost and ease of installation, but the drawbacks
often outweigh these benefits. First, there is no totally
reliable form of freeze protection other than manually draining
the collectors. Therefore they should only be considered in
areas that do not freeze, ever. A second problem is that there
is no high limit protection. By this I mean that without daily
consumption of hot water, the water in the storage tank and
solar collector can boil. If this occurs, relief valves (when
operating properly) will release. This can be messy and a
waste of energy. Finally, in open loop systems, the solar
collectors are susceptible to clogging from mineral deposits
in areas of problem water. Figure 1 depicts a typical open
loop system.
Figure 1. The open loop system.
Unlike open loop systems, closed loop systems do not directly
heat the water in the storage tank. By this I mean the water
in the tank does not circulate directly through the solar
collectors. There are two types of closed loop systems –
the glycol indirect system and the drainback system. In this
type of system a separate fluid is circulated through the
collectors and a heat exchanger is used to transfer this heat
to the water in the storage tank. This heat exchanger can
either be built into the storage tank, or separate from it.
If separate an additional pump is required to circulate water
from the storage tank through the exchanger. The heat exchanger
fluid is typically propylene glycol. Additional items such
as an expansion tank, pressure gages, and fill valves are
also required.
Glycol Indirect System
Like the open loop system, the glycol closed loop system has
been around a very long time. It also alleviates two of the
three problems encountered in an open loop system. By using
glycol as the heat transfer fluid the chance of freeze damage
is eliminated. Additionally it eliminates the problem of fouling
the collectors in areas that encounter hard water. There still
remains the problem of high limit protection and the potential
of failure of the additional components added (expansion tank,
pressure gages, etc). Figure 2 depicts a typical indirect
glycol closed loop system.
Figure 2. Indirect Glycol Closed Loop
System.
The drainback system was first introduced in
a large scale back in the 1980’s. They have been proven
very reliable and are more efficient than the indirect glycol
system. This is a non-pressurized closed loop system using
water as the heat transfer fluid. A small drainback reservoir
is installed in the collector loop. When this system is filled
with water it is only filled to the top of reservoir. Since
it is located below the collectors, they remain dry when the
pump is not circulating. When the collectors are hotter than
the water in storage, the pump circulates the water in the
reservoir through the collectors where it is heated. The heat
from this water is then transferred to the solar storage tank
through a heat exchanger located either in the storage tank
or drainback reservoir. When the collectors approach the same
temperature as the water in the storage tank or this water
has reached a preset temperature, the pump shuts off and all
the water drains back the reservoir. The drainback system
eliminates all the problems inherent in the other types of
systems. First, freeze protection is based on gravity. Second,
the pump is shut off when the storage tank reaches its high
limit setting and all water is drained from the collectors.
And finally, the problems encountered in the collectors in
hard water areas are eliminated. The drainback system also
uses fewer components than any of the other types of active
system, thus the potential of component failure is drastically
reduced. Figure 3 depicts a typical drainback system.
Figure 3. Drainback
Closed Loop System.
Date written: December 8, 2003
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